When evolution first fashioned multicellular organisms, a problem arose. The main goal of unicellular organisms like bacteria and protozoans is to reproduce; to leave as many descendants as possible to ensure that their genes are passed to the next generation. While that aim continued with the advent of multicellularity, the individual cells in a multicellular creature are constrained. They are parts of a community and cannot go off, willy-nilly, reproducing on their own. Their ultimate job is to make sure that the genes carried by germ cells, sperm and eggs, not their own genes, are propagated. They live to sacrifice their own welfare for the greater good. As a part of this behavior, they must interact with their fellows, adjusting their rate of proliferation to match their neighbors so that tissues and organs form and function properly. Moreover, as cells die, new ones must replace them. That process too, tissue renewal, must be carefully controlled.
In order for organisms to regulate the growth of their tissues and organs, cells must communicate. They must inform each other when to divide, when to stop, even when to die. The major signalling molecules in this process are called "growth factors", and they generally are proteins. But proteins are much too large to cross cell membranes. How can a molecule like a protein affect the behavior of a cell if it can't pass through the cell membrane?
We've already seen the answer. Cells have proteins embedded in their membranes, called receptors, that function to pass information from outside to inside. How growth factors achieve this aim was first worked out by studying the mechanism of action of a growth factor, called EGF (for "epidermal growth factor"). EGF, as its name implies, serves to stimulate the proliferation of a variety of epidermal cells. It works by interacting with a receptor molecule, called logically enough, the EGF-receptor.
The figure above right shows the EGF-receptor protein before it has bound a molecule of growth factor. The yellow rectangle is supposed to represent a portion of the cell membrane. The receptor protein chain is shown as a dashed line. The portion of the receptor outside the cell is drawn in green and labelled "Binding Domain" (A domain is a portion of a protein that folds up independently of the rest, often performing a specific function. Domains will play a prominent role in future postings). EGF can affix to the binding domain. A small area of the protein, drawn in red, traverses the membrane (the "Transmembrane Domain"). A third section of the receptor that lies inside the cell is called the "Kinase Domain". It is shown as a blue bulge. Its function is to enzymatically transfer phosphate groups from ATP or GTP to another protein. Below the kinase domain is a tail of amino acids, that is located at the terminus of the protein.
As mentioned in a previous post in conjunction with immune receptors, the EGF receptor is free to move laterally within the membrane. Occasionally, it contacts another similar receptor and briefly forms a dimer (see the second illustration at the right and compare it with the one above). As shown, these transient dimers are good at binding EGF. When EGF binds it both stabilizes the association between the two receptors and activates the two closely opposed kinase domains. They add phosphate groups onto specific amino acids (tyrosines) in the tails of their adjoining partners. We'll delve into the consequences of this transphosphorylation in the next post, but suffice it to say, they are profound, allowing the receptor to set up a cascade of reactions that promote cell growth.
There are many growth factors that interact with their specific receptors and operate in a manner similar to the EGF receptor. Weinberg claims that DNA sequencing of the human genome has identified 59 proteins that bear close resemblance and presumably have similar functions. If that isn't enough, there are a host of other unrelated receptors that also send growth signals from the outside of cells to the inside. Some bear kinase domains and transfer phosphates to proteins, other use alternative mechanisms for signalling. A detailed description of these other receptors is beyond the scope of this blog. However, a key take home lesson is that many cancers harbor errors in receptors in general, errors that contribute to the malignancy of cells. I'll discuss this further in the upcoming posts.
After many cancer-causing genes - oncogenes - were found in tumor viruses and their cellular counterparts - proto-oncogenes - identified in normal cells, it seemed possible that mutations in proto-oncogenes might be responsible for cancer. The idea was that these genes could become corrupted and, instead of performing their normal function, could cause proliferation to run amok. If so, all tumors, not only those that were induced by viruses, should bear such mutated genes.
The technique used to pick out oncogenes from among the thousands of genes in the genome appeared daunting. But a technique was soon developed that did the job (see figure). DNA was extracted from tumor cells, fragmented, and introduced into a line of mouse cells growing on a Petri plate. The recipient cells were carefully chosen. They had to grow well in culture and readily take up foreign DNA and integrate it into their chromosomes. If a cell happened to take up a piece of DNA carrying an oncogene, it would take on some of the characteristics of a cancer cell. Namely, it would change shape and its progeny would pile up on one another, forming little clumps among a flat background of normal cells.
But were the cells in these clumps really cancerous? To find out, scientists injected the cells into mice as Rous had done (albeit with chickens) decades previously. And, like Rous, they found that the injected cells did, in fact, produce tumors. With this information in hand, research now turned to far bigger and more difficult question: How do oncogenes turn normal cells into cancerous ones?
The answer to that question is complex. For example, a gene called myc, named for its discovery in an avian myelocytomatosis virus, was found to be a potent oncogene. It's cellular counterpart, c-myc, can become cancer-causing by several mechanisms. One way is by an increase in the number of genes though a process called gene amplification. Apparently, the presence of multiple copies of the myc gene results in an increase in the protein that myc specifies. Since the myc protein is a growth promoter, the result is an acceleration of cell proliferation and cancer. Gene amplification seems to be a general phenomenon. Amplification of a variety of oncogenes beside myc is known to be associated with at least 20 different malignancies including breast, colon, lung and pancreatic cancer.
Another way of turning myc into an oncogene is by modifying the sequences that control its transcription. Normally, genes are regulated by nearby DNA regions that manage the rate of RNA synthesis. In several cancers, myc comes under the control of sequences that increase the rate of transcription, thereby flooding the cell with growth promoters, causing unregulated proliferation.
Still another mechanism of perverting a proto-oncogene is via a change in the amino acid sequence of the protein that it specifies. The ras gene offers an instructive example. The ras gene, named for the rat sarcoma virus in which it was first found, plays an important role in normal cells. However, it can become a cellular oncogene and can be detected as such via the assay described above. DNA sequencing revealed that a single base change, a G to a T, was responsible for turning Dr. Jekyll into Mr. Hyde. The mutation results in a single amino acid change, a glycine into a valine, and that is sufficient to make a difference. The ras gene has been found to be so altered in 90% of pancreatic and 45% of colorectal cancers.
In the next post, I'll begin to delve into the intricacies of how cellular growth is controlled by signals from their surroundings.
The genes that were associated with tumor formation were first found in viruses. So called "tumor viruses" don't kill their hosts directly. Instead they force the cells that they infect to multiply uncontrollably. It turns out that virus-induced cancers are rare, accounting for less than one fifth of all malignancies. Nevertheless, scientists in the 1970's recognized that they might provide clues to the mechanism of carcinogenesis in general. The idea was that if one could understand how a gene in a virus led to cancer then that knowledge could be more widely applied. Ultimately, these studies resulted in the discovery of oncogenes.
Peyton Rous and the Plymouth Rock Hen
The history of the connection between viruses and cancer actually begins in the second decade of the twentieth century. One day a chicken farmer arrived at the Rockefeller Institute in Manhattan bearing a Plymouth Rock hen in her arms. The bird had a large mass (a sarcoma) growing out of its right breast muscle. It isn't clear what the farmer expected from her visit to that venerable institution but she and her bird were shown into Peyton Rous' laboratory. Weinberg writes that Rous "dispatched" the hen but that's not quite correct. Rous etherized the chicken and removed the mass. He sliced it into fragments and injected a few into the chicken's other breast and several into its peritoneal cavity. The hen died a month later. By that time, the cancer had grown in the injected locations and was likely responsible for the fowl's demise. Injections of fragments of the tumor into similar locations in closely related chickens also caused tumors. By continually transferring pieces of tumor from one chicken to another, the cancer could be propagated indefinitely.
These experiments had shown that the agent promoting the cancer was transmissible. Rous wondered what the biochemical nature of the agent was. Could it have been the tumor cells themselves? Or chemicals emanating from the tumor? Could it have been bacteria or something more exotic?
To find out, Rous took a relatively straightforward approach. He took the tumor, ground it up in a weak salt solution, and passed the resulting slurry through a filter. The filter that he used was sufficiently fine that it didn't allow anything bacteria-sized or bigger to pass through. Subsequently, he injected the filtrate (the solution that came through), into the breasts of susceptible chickens. The result: The chickens got tumors! He concluded that the cancer causing agent was smaller than a bacterium and probably a virus (he didn't use the term because it wasn't popular at the time), or, less likely, some chemical given off by the tumor.
The scene not shifts ahead several decades to a laboratory at Cal Tech in Pasadena, California. There two scientists found that Rous' virus, now called RSV (for Rous Sarcoma Virus) could be used to infect chicken cells grown in a Petri dish. The infected cells acted in several ways like they had become cancerous. They didn't stop growing after they covered the entire dish in a single layer. Instead, they piled on top of one another, unlike normal chicken cells. They acquired a distinctive rounded shape that differed from normal cells. And they grew indefinitely. What was it in the virus that caused these dramatic changes?
RSV and similar viruses have a very small genome consisting of less than five genes (RSV is an RNA virus, a retrovirus, that reproduces by making a DNA copy of its genome and integrating it into the chromosome of its host). It seemed to researchers that if a viral gene was responsible for transforming normal cells into cancerous ones, it would be a simple task to identify which viral gene was responsible for the deed. The genes that specify the viral coat were ruled out as were the genes responsible for converting the RNA into DNA and integrating a DNA copy of the virus into the chicken genome. What was left was a gene that researchers called src (pronounced "SARK") for "sarcoma" named for its probable role in cancer formation.
All now seemed clear. The scientists involved reasoned that the virus attaches its DNA to the chicken chromosome, and the newly introduced src gene somehow causes the infected cells to become cancerous. To demonstrate that their reasoning was correct, they infected some chicken cells with the virus and using a well worn molecular technique assayed the cells for the src gene. As expected, they found it. However, as a control, they looked for the src gene in uninfected cells. To their astonishment, it was there too.
What soon became clear was that every animal they looked at harbored a src gene. Mammals, fish, insects, even sponges had it. Or at least a close relative, genes that had almost the same sequence. What was going on? To shorten a long explanation, it appears that the cellular version of the src gene, called c-src to distinguish it from its viral cousin (v-src), is an important gene that regulates cell growth in a great variety of organisms. Somehow the RSV virus had picked up the gene and corrupted (mutated) it, causing it to promote unwarranted proliferation after viral infection. The implications of this interpretation were clear: perhaps some cancers were caused by mutations in the normal c-src gene. Perhaps, by looking in other tumor viruses, other genes could be identified that had cellular counterparts.
And so it was so. The myc gene, a gene present in another avian RNA virus, was found to induce bone marrow cancers in chickens. And in time more than 30 similar genes were found in viruses that infect chickens, mice, cats, and monkeys. The cancer causing genes in these viruses were termed "oncogenes", and their normal equivalents "proto-oncogenes". Scientists predicted that these proto-oncogenes could be converted into oncogenes by mutations.
These observations opened up a key question: How do oncogenes, either of cellular or viral origin, change a cell from normal to malignant? That is the subject of the next post.
We all begin life as a single cell - a zygote - the result of fertilization of one of our mother's eggs by a sperm from our father. In time, and after many cell divisions, a multitude of different cell types form, each destined to carry out a specific function, each with distinct physical and biochemical properties. This process, whereby a cell becomes committed to a specific identity is called differentiation. The mechanism of differentiation has been worked out: A cell becomes differentiated by transcribing a limited portion of its genetic endowment. Each of the two hundred or so different cell types employs a specific and limited set of genes.
Normally, differentiation is irreversible. Liver cells don't become muscle cells, and nerve cells don't become skin cells. So it was surprising to the scientific community that cells don't lose genetic information during differentiation, despite the fact that much of their genomes goes unused. It's now firmly established that every cell, regardless of its differentiated state carries the same complement of DNA as a zygote. It certainly appears to be an inefficient design strategy since every time a cell replicates its DNA it uses a considerable amount of energy to insert the correct base in the right order into the newly forming strands. If a cell eliminated all the DNA that it didn't need during differentiation, it would save a lot of wasted energy.
But carrying a zygote's worth of DNA in every differentiated cell has another, more serious, drawback. It opens up the possibility that a rogue cell could make use of genes that it normally has no access to and thereby behave inappropriately. One such abnormal behavior is called cancer. Cancer begins with a single cell. Somehow one of its genes has mutated (acquired a change in DNA sequence). Suddenly, instead of behaving as a neighborly member of a multicellular society it begins to act selfishly, proliferating abnormally. New mutations occur as the cells divide. Then evolution via natural selection takes over. Offspring of the original mutated cell that are best able to survive and reproduce are favored over those cells that are less capable. Soon a tumor forms. It it not removed, it may continue to grow. Additional mutations may occur that allow the cells to escape the original mass of tumor cells and travel through the lymph system or blood vessels to other sites, a process called metastasis. Without medical intervention, the outcome may be dire.
Types of Cancer
More than 80% of cancers arise in epithelia - tissues that "line the outer surfaces of organs and blood vessels throughout the body, as well as the inner surfaces of cavities in many internal organs." (Wikipedia). Cancers that originate in epithelial tissues are called carcinomas, and their prevalence in this type of tissue probably reflects the fact that epithelial cells divide rapidly. Breast, prostate, lung, colon, liver, stomach, and pancreatic cancers are carcinomas. Only about 1% of cancers arise from connective tissues (sarcomas) and a little less than 10% from blood forming tissues (leukemia, lymphoma, multiple myeloma). In addition, there are cancers of nervous tissue, pigmented cells of the skin (melanoma), and even eggs and sperm. Practically any cell capable of cell division can switch from a normal member of the cellular community to one that pursues its own selfish interests and proliferates uncontrollably.
Carcinomas, sarcomas, and the other members of the cancer panoply are caused by errors in genes - mistakes in the sequence of DNA. But what causes these errors, these mutations? I think that many people when asked this question would answer that most mutations are caused by harmful agents that originate from industrial processes. Air pollution, atomic reactors, artificial chemicals added to our foods, pesticides, and the like would be examples that many would cite. After all, they would say, haven't cancer rates been rising in the modern era?
Cancer is now more prevalent than it was in previous centuries, but that's because we're living longer and cancer is a disease primarily of the old. The fact is that there is strong evidence that the factors listed above are not the primary drivers of carcinogenesis, One surprising case in favor of this view is cited in chapter 20 of "Molecular Biology of the Cell". They argue that in the absence of agents that cause mutations (mutagens) an error in DNA sequence occurs on the average once per cellular division. You can calculate that this means that that on average any given gene will experience a change in sequence once every million cell generations. Alberts et al estimate that in a lifetime a normal human will experience 10 quadrillion divisions (that's a one with 15 zeroes). Dividing the number of divisions by the rate of mutation results in the amazing estimate that every gene in our body will be subject to a change in sequence ten billion times! All this without any external mutagens. If all it took was a single mutation in a growth promoting gene, cancer would be be much more common. The reason it isn't is that more than one event must occur independently in the same cell. That's one reason that cancer rates increase with age.
Of course I'm not saying that external agents aren't ever responsible for cancer. On the contrary. The chemicals in cigarette smoke and the ultraviolet radiation from the sun are major contributors to lung and skin cancer. Asbestos and X-rays are proven carcinogens. There are viruses that have been implicated in cancer. And even some common components of our diet, like burnt toast and grilled meats, are known mutagens and are suspected carcinogens. In addition, as I'll discuss later, genes that are defective in repairing DNA errors can increase the rate of cancers significantly.
In summary, cancer is a disease cause by mutations in genes. While cancer may begin with a single mutation, it takes several errors in multiple genes for the disease to progress to where it is harmful. Which genes? Next time.
I began reading Robert Weinberg's book, "The Biology of Cancer", second edition, a few days ago. It was an illuminating experience. It's a formidable book of over 900 pages that summarizes much of the enormous amount of material that was known about the molecular biology of cancer as of four years ago. But instead of being a dry accounting of the facts, it reads almost like a mystery novel. In many chapters, Weinberg feeds us the data and experimental facts that were known at a given time, and then presents alternatives explanations from which to choose. Which one turned out to be right? And why? I really like this approach. It got me involved. In addition, Weinberg regularly intersperses questions throughout, many of which remain unanswered by the scientific community. To my mind, pointing out what the scientific community doesn't know is just as important as describing what is known. As a former research scientist, I started thinking about experiments designed to address some of these issues.
Weinberg writes beautifully. He also takes the time to explain appropriate techniques and concepts that other authors skip over. And the book isn't as long as the page numbers suggest. It is filled with numerous tables, illustrations and photos. All in all, it is one of the best textbooks in molecular biology that I've ever encountered. Of course, it isn't for everyone. While there's an introductory chapter that attempts to cover the basics of genetics, biochemistry, and molecular biology, beginners without a reasonable foundation in these fields will find the remainder of the book tough going. And even for people with a good understanding of these matters, many of the subjects covered are extraordinarily complex. Weinberg makes a valiant effort to cope, but as he writes in the introduction to chapter six: "The present chapter will perhaps be the most challenging of all chapters in this book. The difficulty comes from the sheer complexity of signal transduction biochemistry, a field that is afflicted with many facts and blessed with only a small number of unifying principles. So absorb this material in pieces, the whole is far too much for one reading." Nevertheless, if you have the time and some background, I highly recommend "The Biology of Cancer". It's the definitive book on the subject.
In the remainder of this post, I've set out my agenda for subsequent entries in this blog. My aim will be to more or less follow the order of chapters in Weinberg's book culminating with his chapter 15 on immunology and immunotherapy. From there, I hope to discuss more recent ways the immune system has been used to fight cancer. Here's what I hope to cover (however, I reserve the right to add or delete topics at my discretion):
Just as it was with immunology, my ignorance of cancer biology was, and is, profound. Accordingly, I looked around for both an introductory text to provide an overview of the subject and a more comprehensive source to fill in details. While it probably isn't suitable for someone with a limited background in modern biochemistry, Alberts et al's book, "Molecular Biology of the Cell", has a chapter appropriately entitled "Cancer" that nicely summarizes the field. Like the bulk of the book, it's beautifully written and, so far, it has proven invaluable. However, I needed a more exhaustive account of the subject. At 960 pages and $162 (for the paperback version), Robert Weinberg's tome, "The Biology of Cancer", second edition, is massive both in volume and cost. By all accounts, it's the definitive authority on the subject. But it seemed to be a bit much, so I opted for a less weighty book, and chose "Molecular Biology of Cancer", 4th edition, by Lauren Pecorino. It had gotten good reviews on Amazon, had a newer publication date, was less expensive, and promised more or less the same information as Weinberg in a smaller package. Unfortunately, after perusing a few chapters, I didn't care for the author's style. So, just this morning, I ordered the Weinberg book. It'll arrive the day after tomorrow. I'll have additional comments about it in the next post.
While waiting for Amazon to deliver Weinberg's book, let me introduce some important general principles.
I'll elaborate on these themes in subsequent posts, thereby providing enough of a decent introduction so that I can discuss how cancer and the immune system interact. Ultimately, I plan on getting to how the scientific community hopes to engineer the cells of the immune system to serve as therapeutic agents against cancer.
As aside: Just this morning (Oct 1, 2018) it was announced that the Nobel Prize had been awarded to James P. Allison and Tasuku Honjo "for their discovery of cancer therapy by inhibition of negative immune regulation". I hope to add their studies to the list of topics that I will cover eventually.
For a summary of the entirety of the immune system I recommend the first chapter in each of the three books on immunity that I mentioned in an earlier post: "Undergraduate Immunology" by C. Erridge, "How the Immune System Works" by L. Sampayrac, and "Cellular and Molecular Immunology" by Abbas et al. Another helpful resource is a website called "Bite Sized Immunology". It has a ton of short articles on most of the topics I've covered in this blog. For those looking to get an overview of the adaptive cellular system, there's a well written summary of the T cell response in an excellent paper that appeared in the journal "Advances in Physiology Education" by Nathan Pennock and others.
Here's my version of the major points covered so far.
Now I'm going to switch subjects. It's been a long time since the initial posting, but recall that the ultimate aim of this blog is to understand how the immune system can be engineered to react against cancer cells. Hence the next postings on cancer.
CD4 and CD8
In order for activation to take place, a T cell not only has to bind to the peptide carried by a presenting cell (mostly dendritic cells), it also must attach to the MHC itself. This feat is accomplished by one of two proteins found on the surface of T cells. They're called CD4 and CD8. CD4 only fastens to MHC II molecules, and CD8 only to MHC I's. Both CD4 and CD8 help to hold the dendritic cell and T cell tightly together. In time, in the various lymph nodes throughout the body, the bond between these two kinds of cells increases even further as a result of the recruitment of additional co-stimulatory molecules. Their embrace groups the intracellullar parts of the T cell receptors, thereby initiating a cascade of chemical signals that ultimately gain access to the nucleus and cause the T cells to propagate, leave the lymph nodes, and perform their functions.
Just a few words about the nature of this "cascade". In a previous post in conjunction with a discussion of complement, I discussed proteolytic cascades, where one protein cleaves an inactive target, thereby turning it into an active proteolytic enzyme. In turn, this newly activated enzyme goes on to perform the same operation on a downstream protein. and on and on. The cascade that is initiated by activation of T cells operates in a similar manner, but instead of sequential proteolysis is makes use of successive phosphorylations. It turns out that many proteins can be controlled by the addition and removal of phosphate groups. These act as kind of on/off switches. The enzymes that perform the addition of the phosphates are called "kinases". In the T cell activation cascade, one inactive kinase gets a phosphate added, thereby becoming active. In turn, the newly awakened kinase adds a phosphate to another inactive enzyme, turning it on. And so on. Such phosphorylation cascades are very commonly used when a cell needs to transmit a signal from its exterior to its nucleus. In this way, it can respond to external events by switching appropriate genes on and off.
Most textbooks divide T cells into two major categories: helper and cytotoxic T cells. Cells with CD4 on their surface are destined to become helper T cells, while CD8 bearing cells become killers (I'll refer to cytotoxic T cells as killer T cells from now on. The name presents such a powerful image I just can't resist. Other names that these cells go by include CTL's and CD8+ cells. However, don't confuse killer T cells with natural killer cells that are part of the innate immune system).
Killer T Cells
Once activated, killer T cells and helper T cells have different missions. Killer T cells, after they have undergone many rounds of proliferation, move into the blood stream and search for cognate peptides presented on MHC I proteins. Since virtually all cells have MHC I's, any cell infected by a virus will display viral peptides on its surface and be vulnerable to passing killer T's. Like natural killer cells and professional hit men, killer T cells destroy their victims neatly, without leaving a mess and without too much collateral damage. They cozy up to their targets and inject special enzymes through the cell membrane into their quarry's cytoplasm, causing the victim to commit suicide (apoptosis). Alternatively, receptors on their surface can bind special proteins on the surface of their targets that in turn will elicit a suicide response.
Helper T Cells
CD4+ helper T cells play a more complicated role. They secrete a variety of cytokines that instruct other cell types, B lymphocytes, other T cells, macrophages, and dendritic cells, on ways to fight infection. There are three major types of helper T cells: Th1's, Th2's, and Th17's, each of which secretes a different spectrum of cytokines, and each of which is specialized to combat different foes. For example, Th1 helper T cells secrete a variety of cytokines that activate macrophages, which, in turn, gobble up cells infected with intracellular pathogens. In addition, they interact with B cells, causing them to switch to making IgG antibodies. Th2 helper cells, which master in the ability to defend against parasitic attacks, secrete other cytokines that among other effects, causes class switching in B cells to IgE. Th17's, only recently discovered, seem to focus on fungal invaders.
Remarkably, it turns out that the helper T cells are instructed to assume their various roles by dendritic cells. The numerous receptors that I previously described in the innate immunity section that are located on dendritic cells cause them to secrete specific cytokines appropriate to the triggering pathogen. In turn, these cytokines cause the helper T cells to become committed to one or another of the tracks described above.
After T cells have been activated they proliferate and go off to do their job. After their mission is completed, most get no social security or pension and just wither away via apoptosis. A fraction remains in the site where they first encountered the invader. If another pathogen of the same kind infects again, they are already activated and resume their attack Another fraction of activated T cells take up residence in lymph nodes. If they encounter the same invader again, they are much more readily activated than naive T cells, and seek out their quarry with increased efficiency. This immunological "memory" is one of the hallmarks of the adaptive immune system.
That's it. That's what I've learned about the immune system. Of course, all the information that I've covered is just a tiny fragment of what is known. But still, it's a lot to learn and remember. I'll present a brief overview in the next post. After that, I'll begin a discussion of cancer and the immune system response.
T Cell Receptors
T cells detect their targets by way of their T cell receptors. I've shown a cartoon of this structure in a previous post. Recall that it consists of two chains, alpha and beta, that each bear a variable and constant region. Like their B cell counterparts, T cell receptors can assume enormous diversity in sequence due to random recombination of their variable gene segments. However, the T cell receptor doesn't refine its binding site after activation like the B cell receptor. Nor is there switching of constant regions. And, as I've already mentioned, the T cell receptor only binds to peptides that are affixed to the MHC. A model of the T cell receptor bound to a peptide that is being presented by a MHC is shown at the right. The molecular modelling program, Chimera, was used to generate the picture.
There are several other parts to the T cell receptor aside from the peptide recognition chains shown in blue in the figure above. In fact there are six additional protein chains that associate with the alpha and beta chains. They function to signal the inside of the cell that the recognition part has found a target. When properly stimulated, they signal to the cell nucleus to begin synthesizing the appropriate molecules that "activate" the various kinds of T lymphocytes so that they can begin fulfilling their functions. I'll discuss what these functions are in a subsequent post. For now I'll discuss the mechanism of the activation process.
The first step in T cell activation is binding of its receptor to a MHC borne antigen. Previously I mentioned that almost all cells have MHC I proteins on their surface and many have MHC II's. But naive T cells, ones that haven''t ever encountered their cognate antigens, can not be activated by any old MHC-bearing cell. Activation requires that they interact with what immunologist have quaintly termed a "professional" antigen presenter. There are three kinds of these professional cells, but one is most important. And it's one that we've encountered previously in the innate immune system: the dendritic cell.
Dendritic cells are located all over the body. In the absence of a microbial invader they assume a resting state, and, as such, are not very good at activating T cells. But they keep one eye open for trouble. Using their pattern recognition receptors they can detect an attack that is directed on them. Or they can react to an indirect attack on a neighbor by responding to chemicals given off by cells that are in distress. They also can respond to cytokines secreted by macrophages and neutrophils (remember them?). All of these situations result in dendritic cell activation. Yes, that's right. Dendritic cells must be activated before they, in turn, can activate T cells.
The results of this activation are profound. The dendritic cells begin a journey to the nearest lymph node. As they move they transfer additional peptide-loaded MHC I and II molecules to their cell surfaces. In addition, dendritic cells begin to mobilize several surface proteins (co-stimulators) that are complementary to receptors on the surface of T cells. When they reach their destination they encounter numerous T cells. If a T cell with the appropriate receptor encounters an activated dendritic cell presenting a corresponding peptide and co-stimulators, the two cells will cozy up to each other. It is the junction between the two cells when they are in close proximity that activates the T cell. A portrait of their embrace is shown.
There is one additional feature of T cell/dendritic cell interaction that I haven't explained. It has to do with the protein labelled "CD4 or CD8" in the diagram. I will discuss this matter along with the role of activated T cells in the next post.
In order for T cells to recognize proteins located inside of cells as antigens they must be properly "presented". What does that mean? In brief, presentation consists of two steps. First the protein must be split into fragments of the proper size. And second, the fragments have to be be positioned on the cell surface so that the T cells can interact with them. Two impressive molecular machines and a cell vacuole achieve these objectives.
As I mentioned a while back, all of our proteins are being degraded and replaced all the time. Some "turn over" faster than others. In particular, defective proteins, ones that carry errors in sequence, are particularly short lived and are chopped up into pieces rapidly. (Amazingly, I've seen several references including this one that estimate that 30 - 70% of proteins normally synthesized by cells are defective.) Viral proteins that are made within cells also degrade quickly. There is a mechanism that I won't describe that marks these, and other, proteins for discard. But the machine that actually does the dirty work, the one that breaks the protein apart, is called a "proteasome". I show two views of proteasomes on the right. The one labelled "A" comes from a website of the U.S. Department of Energy Genome Programs. It shows a cartoon version of a proteasome caught in the act. The green snake-like line at the top is meant to represent a protein as it undergoes digestion. It enters at one end of what looks like an in-sink garbage disposal and gets cut into pieces in a hollow in the middle. Out comes protein fragments - peptides - consisting of ten to a few dozen amino acids. The second image, "B", is a portrait of the central portion of the proteasome as depicted by the molecular modelling tool, Chimera.
All human cells carry these machines inside them, but the cells of the immune system carry a modified proteasome in which some of the components have been replaced. The protein fragments that are produced by these specialized machines are more well suited for presentation to the major histocompatibility complex.
What about viral proteins that haven't been synthesized in cells and are simply floating around in the extracellular fluid? Nature has devised an entirely different mechanism for dealing with this situation. While I wasn't aware of it, cells continually take sips of the liquid surrounding them. They enclose a tiny drop in transfer it into their interior surrounded by a pinched off portion of the cell membrane. These vesicles, called endosomes, that result from this action slowly change their internal composition, accumulating a host of digestive enzymes and becoming more acidic. In this way, they become capable of breaking down any proteins that may have been captured into fragments that are suitable for presentation.
It is the proteins of the MHC that bind these protein fragments and display them on the surface of cells so that they can be detected by T lymphocytes. But, as usual, there are some complications. For one, there are two kinds of T cells: Cytotoxic T lymphocytes and helper T lymphocytes. For another, there are two corresponding MHC's: MHC I and MHC II.
Let me tackle MHC I and cytotoxic T cells first. MHC I displays the fragments generated by the proteasome on the surface of cells. The MHC I is ideally suited for this purpose as shown in the illustration above. The pictures were created using Chimera, the molecular modelling program that I alluded to earlier. On the left is a view of MHC I from above. The protein fragment held on a platform near the top of the molecule is clearly shown in green. It looks to me like an Incan sacrificial animal on an alter, truly a presentation to the gods. The picture on the left shows the same molecule as seen from the side. Neither view is of the complete molecule. There's an additional part of the protein that is not shown. It is located near its base. It traverses the cell membrane and extends a small segment into the cell for anchoring the MHC to the surface.
Here's the way presentation works in more detail. Proteins that are marked for destruction enter the proteasome where they are enzymatically cut into pieces. These protein fragments are transported into the endoplasmic reticulum, a network of membrane-bound sacs that occupies a good proportion of most cells. Here the MHC I proteins bind the peptides, one peptide to one MHC molecule. After a series of additional steps, the MHC-peptide complex is transported to the cell surface where it hang outs, waiting for a wandering T cell to recognize the peptide that the MHC carries. But it doesn't wait long. Old MHC/peptide complexes are constantly being degraded. New ones carrying some other peptide take their place. The process ensures that the internal protein composition of the cell is continually being sampled.
Now, I learned in graduate school that proteins bind substances with exquisite specificity. That's true of enzymes and of antibodies, for example. But the MHC's are exceptional in that they bind a wide variety of protein fragments. But not every peptide will adhere equally well. To bind to MHC I, peptides have to be about 10 amino acids long and there are some additional requirements as to which amino acids are at their ends.
There's more. I've said that each human has are six MHC I genes and therefore six corresponding MHC I proteins. Because of their diversity in sequence (there are over 1,000 MHC variants in the human population), the chances are that each of our MHC I genes specifies a different MHC protein. And, for the same reason, unrelated people will carry different MHC's. It turns out that each of these various MHC's differs in its ability to bind specific peptides, a fact that has an important consequence. If there is a viral attack on the human population, the wide range of specificities of the MHC's means that some individuals will have an MHC that can successfully bind one of the viral peptides and thereby fend off disease. Of course, that means that some people will be less able to do so.
After the MHC I rises to the cell surface it can interact with a cytotoxic T cell. I'll pursue this matter further in the next post.
Mature B lymphocytes secrete antibodies in great quantity into the blood stream. In aggregate they are capable of synthesizing proteins that are capable of binding to and dealing with almost any molecular intruder. But they have one great limitation: they can't see inside of cells. Antibodies can bind to the surface of foreign microbes, but when invaders, like viruses, penetrate the cell membrane and take up residence within, they are hidden from attack. Evolution has, of course, devised an appropriate solution. It has developed the second arm of the adaptive immune system: cellular immunity. The major players in this process are two kinds of T lymphocytes, so named because they mature in the thymus rather than the bone marrow.
T and B lymphocytes share many properties but also exhibit considerable differences. One critical distinction between the two cell types is that T cells don't secrete antibodies. In fact, T cell don't make antibodies at all. However, like B cells they have receptors that have constant and variable regions that derive from genes whose segments have been randomly rearranged during their development. Utilizing this mechanism, millions of T cell clones are generated, each bearing a different receptor. Because of the great diversity of binding sites in the T cell population, it would seem like T lymphocytes would be capable of recognizing many different molecules. Well, they do and they don't. T cells are specialists. They can't bind to molecules just floating in the blood. What's more, they're restricted to the kinds of molecules to which they can bind. They only (with a few exceptions) recognize small fragments of proteins. And only protein pieces that are properly "presented" to them, meaning that they only can bind peptides that are in the grasp of a special apparatus on the surface of another cell. I'll begin this post on cellular immunity with a short description of the gene that specify the proteins that act as the presenters, MHC I and MHC II. In the next post, I'll tackle the presentation mechanism itself.
MHC - Genes
I wrote briefly about the major histocompatibility complex in a previous post in connection with the natural killer cells of the innate immune system. Because the MHC plays a much bigger role in the workings of T cells, I'll expand upon my previous discussion here.
The MHC refers to two entities, and the distinction is not always clearly noted. First, there are the MHC proteins, a subject that I'll get into in the next post. And second, there is the region of the sixth human chromosome that codes for them - the major histocompatibility complex. As a geneticist, I was surprised to learn that the chromosomal MHC spans an enormous distance, some three million five hundred base pairs. That's nearly the size of an average bacterial genome! Located in this region are over 200 genes, about half having a known function in immunity (the role of many has not yet been determined). The location of the six MHC genes and several others are shown in the figure above. Notice that three of these genes specify the MHC I proteins (in blue), and three the MHC II's (in yellow).
Perhaps the most intriguing property of the MHC I and II genes is their enormous variation in sequence. Of course that's reflected in the proteins they code for as well. I've already written about the variation in antibody genes and T cell receptor genes. But the variety of the MHC's is a different story. Because we inherit two six sixth chromosomes (one from our mother and one from our father), we humans have 12 MHC genes in all. We're born with these genes and, unlike antibody genes, we retain the same sequence and number throughout our lives. The variation that I'm writing about occurs within the human population. That is, if you were to determine the sequence of the MHC genes in thousands of unrelated individuals, the probability is that few would be the same.
Now to be clear, almost all genes show variability from person to person. For example, there are about 1,000 known variants in the beta hemoglobin genes (yes, there's more than one) within the human population. But most of these differences in DNA sequence are rare. Most people bear only one form of the gene. The MHC genes are different. More than 10,000 variants are known, and they're widely distributed among the populace. According to Abbas et al., the MHC genes are the most variable found in "any mammalian genome".
What intrigues me as a geneticist is how this variability is maintained. In most cases, if a change of sequence occurs in a gene – a mutation – it will either be more favorable than the existing sequence or not. If so, it will tend to replace the existing gene in the population over time, eventually becoming the dominant form. If not, it will be selected against and tend to disappear. (There may, of course, be mutations that are neutral, neither favorable or harmful. These will replace the prexisting gene at a random rate partially dependent on how often they appear in the population). This maintenance of enormous variability almost certainly has to do with what the MHC proteins do and how they do it, subjects that I'll take up in the next post.
An antibody switches its class due to rearrangements in the constant region of the heavy chain gene. Intervening DNA segments are removed and new ones are appended to the variable regions (see the figure entitled "Class Switching" below). The various constant region segments are given Greek letter names. The names of the antibodies that result use the English equivalents. As a result, there are five major classes to which an antibody can belong: IgM (constant region C-mu), IgG (constant region C-gamma), IgD (constant region C-delta), IgA (constant region C-alpha), and IgE (constant region C-epsilon). You may have noticed a discrepancy. There are nine constant regions in all, but only five classes. That's because several of the constant region segments come in subclasses. There are four C-gamma constant regions, numbered 1-4 and two C-alpha's. Each of these subclasses plays a slightly different role, a complication that's outside the scope of this discussion. Again, remember that the binding site of an antibody from any given B cell (and its descendants) remains the same regardless of the class of the constant region.
Antibodies start out by elaborating IgM's, the complicated pentamers pictured in the last posting. IgM antibodies are good at initiating the cascade of reactions that activate complement. I've already described how the innate immune system can activate the complement pathway. An alternative pathway to initiating complement activation is through IgM antibody binding. The initial steps are different, but the end result is the same: a complement cascade that can destroy invading microbes to which IgM has bound.
IgG antibodies represent a large fraction of all the antibodies in the blood. Their "Y" shape should be familiar to you by now. They're important in several ways. By binding to the outer walls of microbes, they mark pathogens for phagocytosis by cells of the innate immune system. Similarly, surface receptors on natural killer cells bind to them, marking those cells covered with antibody molecules for destruction. They also bind to viruses and some toxic products of bacteria, rendering them impotent. Like IgM's, IgG molecules can also spark the complement activation cascade, but they are less capable of doing so.
IgA antibodies are dimers - similar to two IgG monomers linked together. They are the most abundant of all antibodies - an average adult human makes two to three grams per day. Most of it is secreted into the gut where it acts to bind to pathogens. Apparently the structure of IgA makes it resistant to the degradative enzymes and harsh condition that are present there.
IgE antibodies resemble IgG molecules but, of course, have their own distinctive heavy chain tail. They are produced in response to a variety of parasitic infections and bind to their quarry in great numbers, cloaking it like snowflakes adhering to a parked car after a winter storm. Receptors to IgE antibodies on mast cells bind to the this surface coating. The newly recruited mast cells respond by releasing their toxic contents in the close vicinity of the parasite. In addition to the poisoning of their quarry, the result may be dilation of nearby blood vessels, contraction of smooth muscles, and fluid retention. These reactions are appropriate if some worm has infiltrated our body, but IgE is also expressed upon exposure to less harmful challengers. For example, pollen can elicit an IgE reaction and subsequent mast cell response. Our runny noses and watery eyes are evidence of the many pollens in the air interacting with our immune system. Another stimulant of IgE production is bee or wasp venom. Upon repeated stings, the body may respond massively, releasing mast cell contents in many tissues simultaneously. The result may be anaphylactic shock, a very dangerous condition.
Most activated B cells become plasma cells, highly productive workshops for synthesizing massive amounts of antibodies. They perform their duties with great energy, but when they have accomplished their job, when the invader has been vanquished, they die to be replaced by new B cells with different specificities. But by mechanisms unknown, a few B cells activated by helper T cells, undergo profound changes. They stop secreting antibody. They change their surface proteins. They remain alive, sometimes for many decades. With the help of T cells, they undergo further class switching and hypermutation, thereby becoming capable of secreting even more potent (and appropriate) antibodies. And they patrol the blood stream and lymph nodes, looking for some substance that matches their antigen recognition site. When they encounter such a molecule, they respond with force, much more powerfully then the initial antibody response.
Edward Jenner was the first to take advantage of this immunological memory in England the late 18th century. We continue to use more sophisticated versions of his technique, now called "immunization", to protect populations from a great variety of diseases.
CD stands for "Clusters of Differentiation". Also "Certificates of Deposit" and "Compact Disc", but those subjects are outside the bounds of this blog. The CD nomenclature is principally applied to cells of the immune system and has a long back story that begins with entities called monoclonal antibodies - a topic that I could greatly expand on. However, I'll try to make it short.
When a foreign protein enters our bodies, we respond with a burst of antibody production. Many different antibodies may bind to this single molecule. That's because, as I mentioned in the last post, proteins are complicated with many nooks, crannies, cavities, and protrusions. A single antibody is capable of recognizing and binding to only one of these surface features as shown in the illustration on the right. It is for this reason that the antibody response is called "polyclonal", meaning that many different B cells, each with a distinct recognition site, bind to the antigen, each eventually elaborating different antibodies.
To make it "monoclonal", restricting the response to one B cell and its descendants, you would have to isolate a single B cell, place it in a culture vessel, and have it reproduce many times to yield a clone of identical progeny. While this approach is feasible in theory, and actually was done in practice, its utility depends on the ability for B cells to reproduce indefinitely in a culture vessel, something that it is incapable of. That's because all differentiated human cells have a limited lifespan. Most cancer cells, by contrast, are immortal, and can grow in culture forever. In 1975, Cesar Milstein, an Argentinian immigrant, and Frederick Kohler, a German postdoctoral fellow in Milstein's laboratory at the Medical Research Council in the United Kingdom, fused a mouse B cell with a cancerous plasma cell, to create a hybrid that was immortal and capable of spitting out loads of antibody. For their efforts, they shared two-thirds of a Nobel Prize in 1984.
Why was their achievement so important? Monoclonal antibodies can be produced in great quantities and have an exquisite specificity. As such, they can act as "magic bullets" and home in on a specific therapeutic target. Since 1985, more than 73 monoclonals have been approved by governmental agencies, 33 of these in the last four years alone. Rheumatoid arthritis, multiple sclerosis, psoriasis, asthma, and several kinds of cancers are some of the diseases that are being treated by these reagents. Six out of 10 of the best selling drugs in the world are monoclonals. They're also extremely useful in diagnosis, acting as probes that allowing scientists to identify specific cells. (For a more extensive discussion of monoclonal antibodies, I recommend "The Lock and Key of Medicine: Monoclonal Antibodies and the Transformation of Healthcare" by Lara V. Marks, Yale University Press, 2015).
And that's where clusters of differentiation comes in (did my digression distract you?). As monoclonal antibodies became more widely used, a number of laboratories used them to characterize the cell surface proteins of a variety of immune cell types. Many of these cell types were indistinguishable microscopically They could only be told apart by the monoclonal antibodies that bound to the antigens that they bore on their surface. With many laboratories working with many monoclonals and a host of cell types, it soon became a mess. For example, lab A identified an immune cell that reacted with a specific monoclonal antibody that they had prepared. Lab B found a similar looking cell but used a different antibody. Were the two cells the same? Were the two antibodies binding to the same antigen? No one knew. There was a further contribution to uncertainty. Each laboratory gave its own favorite name to the cells they had characterized and the antigens that they had detected. Confusion reigned.
In 1982 immunologists met together in Paris in an effort to resolve this chaos. The meeting was called the "First International Workshop and Conference on Human Leukocyte Differentiation Antigens" and the result was the CD nomenclature. The term "Workshop" is appropriate. Each monoclonal antibody was tested in the lab. When two or more were found to bind to the same molecule, a CD number was assigned to that antigen. Since 1982, nine additional workshops have been organized. The last was held in Australia in 2014. Over that time, some 370 CD numbers have been allocated.
(I admit to having been bothered by the term "clusters of differentiation", both the "clusters" and "differentiation" parts. People using the term in different ways just added to my confusion. I eventually learned that surface proteins are often referred to as differentiation antigens. The cluster part of the term comes from the fact that the same molecule, most often a protein, may be bound by a group/cluster of different monoclonals (remember that proteins present a variety of different surface features and therefore may be recognized by different antibodies). The important point to remember is that CD's refer to surface proteins that can be used to classify different cell types. An article in Wikipedia states that some prefer to call CD's "Classification Determinants", a better name in my opinion.)
You'll recall that the heavy chain gene bears multiple constant regions. When a B lymphocyte is first activated, it caries a Cm segment on its heavy chain, and produces antibody of the IgM class (some IgD may also be made, but only in small quantities). IgM antibodies look a lot different than the ones that I've described previously. They are pentamers, consisting of five Y shaped antibody molecules bound together (see the figure). Later on, as the B cell matures, it can change its heavy chain constant region. By cutting out the other C regions, it can append one of the other C regions onto the heavy chain gene. The antibodies produced as a result of this switch may be IgG, IgE or IgA depending on which C segment remains abutted to the remainder of the gene. But remember, the variable regions, the antigen binding site, remains as it was, meaning that the resultant antibody retains its specifity.
You may wonder why "class switching" occurs, especially since switched antibodies continue to bear the same variable region. The answer is that the various classes of antibodies have different functions. I'll discuss these functions as well as memory cell formation in the next positing.
T Cell Dependent B Cell Activation
I've had some difficulties with this topic. Abbas et al's book describes the subject in great detail and I've found it hard to follow. Sompayrac devotes only a paragraph to it. The clearest, and simplest, explanation comes from Erridge's book, supplemented by material from Wikipedia. Here's what I learned.
The antigens that invoke a T-dependent activation of B cells are mostly proteins. That's because proteins are not repetitive molecules in the same way that polysaccharides are. That is, their surface consists of a great variety of distinct three dimensional features. A different receptor binds to each one. Therefore, when a B cell encounters a protein antigen, it can't cluster its B cell receptors in the same way that it does when binding a polysaccharide. Instead, when a B cell receptor binds to a protein, it internalizes it, meaning that it draws it into its interior. Subsequently it processes the protein into small fragments that it displays on the cell surface (much more on this later when I talk about T cell functions). These fragments can be recognized by helper T cells. Upon recognition, the T cells become active, secrete cytokines, and, at the same time, express a ligand on their surface called CD40L. B cells have a receptor for this ligand called CD40, with the result that the T cell and B cell bind to each other. This in turn causes the B cells to become active. They begin dividing and churning out antibody. This process is complex and takes place over several days. The cells make good use of this time. They produce antibodies that are much better at binding antigen and more versatile than those produced via the T cell independent pathway, employing a mechanism that I'll describe below.
What's with the CD nomenclature? You promised to trip light on the jargon and not use abbreviations. I'm glad you asked because it offers me the opportunity to discuss two other topics of interest. However, because the explanation is lengthy, I'll put it off until the next post.
Recall that antibodies are constructed from proteins that are specified by heavy and light genes. The heavy chain genes carry nine multiple constant regions with names like Cm, Cd, Ca, etc.. Newly activated B lymphocytes synthesize heavy chains bearing a protein coded for by the Cm region. Therefore the antibodies that they produce are said to members of the IgM class, where the "Ig" part indicates "immunoglobulin" and the "M" stands for the Cm constant region. Again, I'm going to put off a discussion of how the different classes of antibodies differ from one another and what roles they play for later. For now it's important to know that the class of an antibody is dependent on the heavy chain it carries, and that while undergoing T cell dependent activation, the B cell can switch the class of its heavy chain (Heavy chain switching doesn't occur in T cell when the B cell is activated in a T cell independent manner). This switching occurs via DNA rearrangement, causing the VDJ variable region of the heavy chain to become joined to the constant region. During the switch, rearrangement the variable region remains as it was, retaining its specificity for whatever antigen promoted the activation of the B cell. Any of the nine constant regions can be appended to the variable region, resulting in the production of different class of antibody. Each of these classes play a different role as I'll explain next time.
However, class switching isn't the only change that occurs to the antibody genes in T cell dependent activated B cells. The variable region also undergoes a change in DNA. As a B cell divides, some of the variable region DNA sequence begins to change (mutate) at a rate at least 1,000 times that of normal genes. The end result may be a variable region that differs by as much as 5% from that with which it began. Since these changes are more or less random, any given cell may carry an improved antibody (one which binds more tightly to its cognate antigen) or one that isn't any better at binding than it started out with, or one that it is worse or loses its affinity for the antigen altogether. If that's the case, how does hypermutation improve the immune response?
The answer is: via selection. Those B cells that have managed to acquire an improved antibody via hypermutation continue to interact with T cells, and they signal via CD40L to remain alive. In the absence of a good T cell interaction, the B cells will cease proliferating and die. In this way, B cells with "improved" antibodies will dominate the population.
Next time, I'll discuss the benefits of class switching, what "CD:" means, and immunological memory.
Millions of B cells that have passed through multiple checkpoints in the bone marrow will now enter lymph nodes. They're explosive devices, like naval mines, each capable of destroying a specific microbe or microbial product that they've targeted by virtue of the receptors bound to their surfaces. Most, however, will never encounter their specific prey. After a few months these unfulfilled B cells die, to be replaced by new generations that are constantly being spawned in the bone marrow.
B cells that have found and bound a target molecule first need to become activated, meaning that they have to undergo some drastic changes in order to transform into death dealing devices. Activation causes a burst of proliferation and the ability of individual cells to churn out soluble antibodies in enormous numbers (according to Abbas et al, a B cell can give rise to 5,000 descendants that can collectively synthesize a trillion antibody molecules in a week). The change is so dramatic that immunologists even give fully activated cells their own distinct name - "plasma cells". It's important to understand that a plasma cell continues to bear the same rearranged gene before and after it has met its cognate antigen. As I'll elaborate, this means the antibodies it produces are essentially the same as those borne by the receptor affixed to its surface (although, intriguingly, they may be improved under some circumstances).
But even after all it's been through, binding to an antigen is a necessary but not a sufficient action for activation of B cells to occur. One of two possible additional steps is needed. One route to activation requires the participation of a T cell, the second type of adaptive immune cell that I'll discuss in many subsequent posts. The other utilizes parts of the innate immune system. This second activation pathway is called "T cell independent". I'll discuss it next.
T cell Independent Activation
The antigens that generate the T cell independent activation pathway are generally repetitive chemical units, polysaccharides or lipids, borne on the surface of bacteria. As described in the last post, binding to these molecules causes the B cell receptors to cluster, priming the cells for activation. The actual activation process can be promoted by cytokines secreted by cells like macrophages that have detected an invader. Alternatively, a B cell that has detected a microbe can be activated by the simultaneous binding of its own Toll-like receptors to the antigen. Yet another source of co-activation are components of the complement system that are bound to a microbe. These are recognized by special receptors on the B cell surface. The last two of these routes to activation are illustrated in the cartoon on the right. You'll notice that all these routes to T cell independent activation involve participation of the innate immune system, emphasizing the close relationship between the two.
From what I can gather from the sources that I have access to, B cells activated via the T cell independent pathway don't marshal as robust an immune response as is provoked by the T cell dependent route. While B cells activated via the T cell independent mechanism proliferate and secrete antibodies, they appear incapable of undergoing two processes that I'll discuss in a coming post: class switching and somatic hypermutation. They also can't become memory cells, another topic that I'll have to put off for another day . However, T cell independent activation of B cells is fast. And it is often sufficient to meet the challenge of some bacterial infections.
T cell Dependent Activation
T cell dependent activation is generally promoted by proteins. That's because proteins don't often bear repetitive antigenic groups. B lymphocytes take up some proteins from invasive microbes, process them, and present them to helper T cells. In turn, the T cells help complete B cell activation. The B cells activated in this way can undergo class switching and hypermutation, the terms I introduced above. How all this unfolds and what it means will be the subject of the next post.
A B lymphocyte must feel like a student in an uncompromising boarding school. It is forced to endure a series of tests. If it passes them it gets to advance to the next grade. But if it fails, the consequences are grave. In fact, the grave. I've described a number of these high stakes trials in the last post. When a cell has overcome them, and while still in the bone marrow, it makes use of its newly rearranged antibody gene to synthesize B cell receptors, proteins that embed in the cell membrane. The cartoon at the right shows one of these. Notice that the molecule traverses the cell membrane with only a tiny portion (three amino acids) of the heavy chain sticking into the cell. Notice also that most of the receptor looks just like a typical antibody as depicted in a previous post. It bears at one end an antigen binding site and at the other a tail consisting of heavy chains specified by one of the constant segments of the heavy chain gene. Once in the membrane, it pairs with two other transmembrane proteins, Ig-alpha and Ig-beta. And there it must sit awaiting yet another trial.
Why is another hurdle placed before a B lymphocyte before it can leave the bone marrow and take up residence in sites where it can detect invaders? We've already seen that antibodies can take on millions of different configurations in its variable region, so many that it can bind to virtually any molecule. Of course, that means that molecules that are not foreign, not invading microbes, can also be detected. If that happens, the immune system may attack substances that are naturally part of the body (referred to as self molecules) resulting in potentially serious autoimmune diseases. The last step in B cell maturation helps to ensure that that doesn't occur. If a B cell carries a receptor that strongly reacts with one of the surrounding molecules in the bone marrow, it is directed to commit suicide. Only those cells that don't encounter a molecule to which it can bind pass this test. If a cell does, it exits the bone marrow, and heads toward more appropriate pastures. A summary of the checkpoints that B cell must endure is shown in the figure at the right.
Two questions. First, how does the B cell receptor get positioned on the cell surface? Second, since its binding site is outside the cell, how does it communicate to the interior that it has detected a molecule to which it can bind?
The answer to the first question is relatively simple. The role that a particular antibody plays is dependent on the constant region of the heavy chain. Recall that the heavy chain gene bears nine different constant segments. Any one can potentially join onto the rearranged variable segments but at this stage of B cell development, either a special C-mu or C-delta segment attaches (it does so via RNA splicing, not DNA rearrangement). These heavy chains are special in that they bear a short tail of amino acids. The resultant protein is directed to the cell membrane because of the chemical nature of this end piece. It binds there, sticking just three amino acids across the membrane into the interior of the cell to help hold it in place. In addition, two other proteins, Ig-alpha and Ig-beta, join the receptor at the cell surface, aiding in its attachment to the membrane.
One clarification. B cell receptors are bound to the cell membrane not like notes tacked to a billboard but like rafts on a pond. Like rafts, they cannot sink or rise. Instead they float freely on the surface. The fact that B cell receptors can move about helps to provide an answer to the next question.
How does a cell pass information from the outside to the interior? The answer involves a discussion of "signal transduction", a term defined by an anomymous Wikipedia author as follows: "Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events, ... which ultimately results in a cellular response. Proteins responsible for detecting stimuli are generally termed receptors..." When a B cell receptor has bound an antigen in its variable region, it utilizes the two transmembrane proteins previously described, Ig-alpha and Ig-beta, to transmit a signal to the interior. The details of signal propagation are Rube Goldbergian in complexity. They're critical to learn for those carrying on research in the field, but aren't if sufficient general interest for me to want to either learn or teach it. However, the initial events are fascinating and relatively straightforward in principle.
Most invading microorganisms are surrounded by a picket fence of sorts that consists of multiple copies of the same molecule linked together to form a protective barrier. Each B cell receptor can bind one of these molecules. And, since there are many B cell receptors on any given B lymphocyte, many receptors may bind at the same time to adjoining "pickets". Since receptors can move about on the cell surface, they may clump together as shown in the figure at the right. This cluster of receptors brings the Ig-alpha and Ig-beta proteins of adjacent molecules close together. And when they are in proximity they enzymatically alter each other, causing a chain of enzymatic reactions that end up signaling to the cell that something wicked is afoot. These signals help activate the B cell, causing it to become an antibody synthesizing powerhouse, a topic for next time.
Adaptive immunity is so complex it's difficult to know where to begin. Do I start with the cells or the molecules? Actually, I've decided to do neither. Since my main interest is in genetics, I'm going to lead with genes. In particular, the genes for antibodies.
Recall that antibodies are proteins expressed by B lymphocytes (small white blood cells) that consist of four chains of two types: heavy and light. There's one kind of heavy chain and two light ones (called lambda and kappa). Since DNA sequences encode proteins, that must mean that there are must be three genes that are responsible for specifying antibody molecules. And there are. All big ones. The heavy chain gene is found on the 14th chromosome in humans. It's very long, about 1.24 million bases. One of the light chain genes, kappa, located on chromosome two, is even longer, at 1.8 million bases. The other light chain gene, lambda, is found on the 22nd chromosome and stretches over one million bases. (I'm omitting the fact that humans have two of each chromosome, and therefore twice as many antibody genes as I've described. The immune system deals with these two sets of genes in an interesting way. Stay tuned).
But there's a problem. I've already noted that there are at least millions of different antibodies. Put another way, there are millions of antibody proteins each with a distinct amino acid sequence. You'll recall from elementary molecular biology that the sequence of a protein is specified by a gene. That must mean that there must be millions of antibody genes. Which is it? Just three or millions of genes? The fact is that if there were millions of antibody genes and each one was more than a million bases long, the genome would need to be a thousand times bigger than it is. On the other hand, how can three genes dictate the sequence of millions of proteins?
The answer to this conundrum is that antibody genes have some unique properties that allow a single DNA sequence to specify more than one protein. In simplest terms, it accomplishes this task by randomly mixing and matching gene segments, taking a piece from one region and pasting it onto a piece from another and deleting the region between the two. (There is another mechanism for generating multiple proteins from a single gene: alternative splicing. That process is entirely different than the one described here. Alternative splicing occurs in the RNA transcript. Antibody diversity in the variable regions of light and heavy chains is generated by changes in the DNA).
At the right is a diagram of the human heavy chain gene. It's composed of a series of different segments. There are 40 or so "V" or variable segments, each about 300 bases long. Adjoining them are about two dozen relatively short "D" or diversity segments. Next come six "J" or joining stretches, each some 3 to 6 dozen bases long. Finally, some nine or so "C" or constant regions make up a region near the end of the gene. Both the kappa and lambda light chains are similarly structured although light chains lack D sequences and have different numbers of V, J, and C regions.
In a complicated series of enzymatic reactions, these gene pieces are randomly assembled together into a different functional gene in each antibody producing cell (see three of the possible arrangements in the figure above). Notice that substantial portions of the gene are discarded in this process. It's also important to emphasize that these rearrangements only occurs in B lymphocytes. In all other cells in the body the heavy chain and two light chain genes remain unmodified.
The events leading to antibody diversity begin with the joining of one of the D segments to one of the J's. Intervening DNA is thrown away. Subsequently, a V segment joins the group, again with the removal of the bases in between. The J's remain attached to the C regions. The result is an edited gene consisting of one V, D, and J region, the three of which code for the variable portion of the heavy chain, followed by the constant region segments.
How many different antibody producing cells could result from these operations? Simple probability tells us that all the possible combinations in the variable region can be calculated by simply multiplying the number of potential V, D, and J segments. This comes to 45 X 23 X 6, or about 6,000. There's a lesser number of combinations possible from the V-J joining in light chains, say 200. Multiplying 6,000 by 200 yields about a little more than a million. While this is an impressive number, it doesn't take into account the fact (see below) that many combinations of heavy and light chains don't yield functional antibodies. This leaves us short of the millions of different antibodies that is claimed for the immune system. Quite a bit of added diversity comes from the results of joining the D to J and V to DJ segments together. Because this process is imprecise, bases are added, lost, and changed at random at the junctions. While inefficient, often leading to antibodies that can't possibly bind to anything, it results in a tremendous increase in the number of DNA sequences. As we'll see, the immune system employs an evolutionary process that rids the body of defective antibodies and selects for the ones that work well.
As I've noted, all these gene rearrangements occur only in B lymphocytes, the cells responsible for antibody production. However, somewhat similar changes to DNA sequence occur in T cells, the other major player in the adaptive immune system, the one responsible for cellular immunity. In T cells, the proteins analogous to antibodies are the T cell receptors. There are two genes responsible for the synthesis of 95% of these proteins (I've omitted the two other genes that code for the minor forms of the T cell receptor for simplicity's sake). The T cell receptor beta chain gene, about 600,000 bases long, lies on chromosome 7. The T cell receptor alpha chain gene, about a million bases long, is located on chromosome 14. Like the antibody genes, the T cell receptor genes carry repeated V, D, and J segments (D is absent from the alpha chain sequence) and these are rearranged in a similar manner to produce many millions of T cell receptors. A cartoon portrait of the T cell receptor is shown at the right. As in antibodies, these receptors bind to their targets via the variable regions shown near the top of the figure.
Back to B lymphocytes... Their development occurs in the bone marrow in humans and in a specialized structure called the bursa of Fabricus in birds, hence the "B". One of the first events in a B cell's life is the rearrangement of the heavy chain's gene on one chromosome as described above. Remarkably, if this process is successful, the heavy gene on the other chromosome doesn't undergo rearrangement. If not, it does. In either case, only one of the two chromosome 14's bearing the heavy chain gene participate in antibody formation. In those cases where both chromosomes can't specify functional proteins, the cell commits suicide, programmed cell death.
If a pre-B cell survives this checkpoint, it somehow tells the kappa light chain to begin rearranging the segments on its gene. Again, this process is restricted to only one of the two chromosomes. If both chromosomes fail to make a complete kappa light chain protein, only then will the lambda light gene come into play. This mechanism ensures that the B cell will only bear one of the two light chains. If both kappa and lambda chains aren't functional, the cell will die. In summary, a B lymphocyte gets six shots at survival. Two come when each of the sister chromosomes bearing the heavy chain chains rearrange, and four after rearrangement of the kappa and lambda light chain genes.
A surviving B cell will now transcribe its rearranged gene to form an RNA. The protein specified by this processed transcript is capable of binding an antigen. But before it can do so, it must survive an additional test. I'll put that off for the next post.
Before entering further into the labyrinth of adaptive immunity, I'm pleased to share yet another text that has helped me explore the subject. It's called "Undergraduate Immunology: A Textbook for Tablets and other Mobile Devices". The author is Clett Erridge, a Senior Lecturer at Anglia Ruskin University in England. I downloaded it from Amazon to a Kindle app on my iPad. My impression so far, after perusing a half dozen chapters, is that it's aimed at the same audience as Lauren Sompayrac's book. And it looks good. I particularly liked the organization of Chapter 6, in which he describes the limits of innate immunity and writes how they are addressed by the adaptive system. I'm going to use a variant of his approach in what follows as a way of introducing the features of adaptive immunity.
Question 1 - The innate system can only respond to a fixed number of evolutionarily conserved features. If a microbe has developed a way of hiding these features or creating new ones, the innate system is largely helpless. How does adaptive immunity avoid this seeming intractable problem?
The adaptive system gets around this issue by brute force. It creates millions of sentinel cells each bearing a different detector/receptor on its surface. It does so randomly. The system doesn't make use of prior knowledge about what kind of invader is threatening. The idea is to synthesize so many receptor shapes that one is bound to be complementary to whatever comes along. As you might imagine, this process is extraordinarily wasteful. The overwhelming bulk of cells never encounter a complementary shape to the receptor that it carries.
Question 2 - With millions of different cells each with a specific receptor, how can any given cell respond effectively?
This has an easy answer. Once a cell finds something that it can bind to, it responds by proliferating, making many identical copies of itself. These copies are a clone.
Question 3 - Receptors are proteins. Proteins are specified by genes. If an organism is going to make millions of different receptors, it will require millions of genes. But it's known that there are only tens of thousands of genes in humans and other vertebrates. How do you synthesize millions of proteins with only a limited set of genes?
The immune system is ingenious. It mixes and matches pieces from a relatively small number of gene segments in millions of combinations to build up a myriad of receptors, each with a different sequence. More on this next time.
Question 4 - But this strategy is sure to create receptors that bind to molecules that don't pose danger. How does the immune system avoid making antibodies to one's self?
Recognizing that reactions against self is a serious problem (it can lead to autoimmune diseases), adaptive immunity rids itself of cells whose receptors target the host. How this is accomplished is the subject of another post.
Question 5 - What about viruses and microbes that infiltrate the interior of cells? How does the adaptive immune system deal with these agents whose fingerprints are hidden?
It makes use of the major histocompatibility complex (the MHC) that I described earlier in connection with natural killer cells. Remember, the MHC displays little pieces of internal proteins on the surface of cells. These little pieces can represent viral or bacterial proteins that have invaded a cell.
Question 6 - What about memory, the ability of the adaptive response to react more strongly to a second attack, one that may have occurred many years previously?
Some cells from a clone that reacted to a foreign antigen are set aside. They're long lived and ready to proliferate rapidly if challenged with the same antigen again.
I'll address these matters in more detail in subsequent posts.
The adaptive immune response, at least the version practiced by humans, appears to be a novel development that is restricted to vertebrates. Innate immunity, on the other hand, is found in all organisms, but only vertebrates seem to have developed a sophisticated mechanism that is able to recognize nearly every molecule that enters their domain, an apparatus that remembers past events and can marshal an "improved" response upon encountering an invader for a second (and later) time. This state of the art form of immunity is widely distributed in the vertebrate kingdom, and even "primitive" fish, like sharks and rays, share this same system with their more "advanced" cousins.
But is it true that only vertebrates have managed to develop an adaptive immune system? Perhaps not. A recent article suggests that fruit flies, my favorite organism, seem to make use of an adaptive response of sorts when faced with viral infection. Although the mechanism that they use for defense against this kind of attack is quite different than that of vertebrates, it does result in a kind of immunological memory that is characteristic of vertebrate immunity. There are other examples. For instance, there is some evidence that snails can synthesize a diverse group of proteins that that have antibacterial activity. Even bacteria and archaea, organisms that are considered near the bottom of the evolutionary tree, make use of CRISPR to mount a specific defense against invading viruses, incorporating viral DNA into their genomes so that their response can target a specific marauder.
One other fascinating (to me) point is worth discussing before I get off the subject of the possible diversity of adaptive immunity. There is a tiny group of living vertebrates, one that includes hagfish and lampreys, that do adaptive immunity differently. These jawless fish (all other vertebrates have jaws and are placed in a separate superclass) don't employ immunoglobulins (see below for a definition of this term) in their immune system. Instead, they make use of an entirely different protein in the construction of the molecules they use for defense. What this indicates to me is that the adaptive response can be fashioned in quite different ways, and that it may very well be that the scientific community may have missed finding adaptive immune systems in groups other than vertebrates because they may be so different than the systems we know about. Of course that's only a guess from a relatively uninformed observer.
OK. Let's dig in. Adaptive immunity can be divided into two parts. There's the humoral response and there's cellular immunity. I wondered about the first term. It doesn't mean that this branch of immunity is funny. The word "humoral" has roots that go back to the fourteenth century when physicians thought that one's state of health was dictated by bodily fluids, the humors. The word now means fluid-based. In the current context it indicates that the responsible parties for humoral immunity occur in the non-cellular portion of the blood.
The main players in the humoral system are antibodies, a term that was coined in 1891 by the legendary physician/microbiologist/immunologist and Nobel Prize winner, Paul Ehrlich. Antibodies are proteins, immunoglobulins, that are specialized for sticking to entities called "antigens" - meaning any molecule that upon capture by an antibody can induce an immune response. A cartoon version of an antibody shown in the act of binding to an antigen is depicted at the right. Below it is a more detailed and realistic picture sans antigen. Notice that this particular antibody consists of four chains, two longer "heavy" chains, and two shorter "light" chains, all of which bind rather tightly together to make a functional molecule.
I should say a bit about the bottom part of the illustration at the right. What you're looking is a three dimensional rendering of the surface of an antibody. It was produced using two resources. The first is a collection of all the "solved" protein (and nucleic acid) structures in the universe. Solved in this case means that the position in three dimensions of virtually all the atoms in the molecule have been worked out. These coordinates, essentially a series of numbers assigned to each atom, are stored in the PDB, the Protein Data Bank, a public facility managed by Rutgers University and the University of California, San Diego. I searched the PDB for an antibody and found the one pictured. In order to display the structure and to manipulate it, I made use of a second public resource, "UCSF Chimera", a computer program from the University of California in San Francisco. It takes the raw data from a PDB file and creates a three dimensional depiction that can be manipulated on a computer screen. There are several such programs, but I've gotten to know Chimera fairly well and I use it often despite the fact that it rather complicated. If you'd like to explore the structure of antibodies and other proteins using a simpler program, look up "molecular modeling software" in a search engine. Or simply go to the PDB site (the URL is above) and use the modeling program found there.
Antibodies are unique. All other proteins bear a specific sequence of amino acids, the monomeric units from which proteins are constructed. The amino acid sequence of a protein is dictated by a corresponding gene. In any particular organism, for any specific protein, there may be minor differences in the gene that codes for the amino acid sequence, but in general these are limited to one or two substitutions. Antibodies by contrast, occur in millions of forms, each with a different amino acid sequence, each one with the capability of binding a different antigen, each one dictated by the sequence of a different gene. The complex story of how this vast assembly of proteins is generated is the subject of the next posting.
Let me try to bring together the salient points that I've learned so far about innate immunity.
Point 1 - There are two parts to the immune system: innate and adaptive immunity. What's the difference? For one thing, innate immunity is older in evolutionary terms than the adaptive system. Organisms up and down the tree of life utilize various versions of it. It's also faster to respond to attack, taking minutes and hours rather than days. But the main difference between the two systems is that the adaptive defense is preprogrammed. It has an evolutionary designed repertoire of defensive agents and can make no more (at least not in less than millions of years). In general, if confronted with a threat that it hasn't anticipated, it isn't able to respond. The adaptive system, which seems restricted to vertebrates, has adopted a different strategy that I'll expand upon in later posts. But briefly, its game plan is to construct millions of different weapons at random without worrying about whether they'll be effective or not. One other difference between the two systems. Innate immunity apparently hasn't any memory, a second attack by the same agent doesn't result in a heightened response. That's not true of the adaptive system.
Point 2 - The systems are distinguishable but inseparable. That's something that I haven't emphasized enough, but is extremely important. Although most of my references divide immunity in two, they also warn that there exists no real barrier between them. One often works in conjunction with the other. I'll try to point out where this occurs in later posts.
Point 3 - The various components of the innate system make use of receptors to distinguish between the harmful and harmless. Receptors are proteins that lie in the membranes of cells, with portions protruding inside and out, one end binding to intruders. Once an enemy has been detected, receptors pass signald across the membrane, thereby releasing a cascade of events that prime the cell for an appropriate response.
Point 4 - Macrophages are sentinel cells. They are long term residents in many tissues and send out signals upon a microbial invasion. Upon activation, they ingest microbes and cells that are killed or injured. And they secrete cytokines to alert other immune components to come to their aid.
Point 5 - Neutrophils are the major phagocytic cells of the body. They more swiftly move in the bloodstream, awaiting a signal to slow down. When macrophages send out an alarm (through the release of releasing cytokines), they slow down, stop, and leave the blood stream and become killing machines.
Point 6 - The complement system consists of blood borne proteins that contribute to the innate and adaptive immune response, It also is responsible for the direct killing of bacteria and some viruses. Complement makes use of proteolytic cascades to initiate and amplify its effects.
Point 7 - Natural killer cells target viruses, injured cells, and tumors. Unlike the other cellular components of innate immunity, its main function is to detect invaders that have gotten inside cells. It does its job by making use of the major histocompatibility complex, a system for displaying fragments of the proteins located in the interior of the cell on its outer membrane.
Point 8 - Mast and dendritic cells are sentinels. Mast cells rapidly release toxic chemicals upon activation. Dendritic cells are the major link between innate and adaptive immunity.
That's a lot of food to ingest in a relatively brief sitting. Take a deep breath. The next topic is even more filling.
I'll end this journey through the innate immune system with a discussion of two more cell types: mast and dendritic cells. That's not say that there aren't additional innate components that I could describe. Leafing through Abbas et al. I discovered several others, both cellular and molecular, but as far as I can tell they are invoked rarely or play relatively minor roles. For example, eosinophils are white blood cells that help fight off parasites. And basophils look like, and function similarly to, mast cells, although they originate from different precursors. They're present in small numbers and its unclear how much they contribute to the overall innate immune response.
Born in the bone marrow and housed in the skin and the lining that covers the surface of our internal organs, mast cells are sentinels on the lookout for a variety of invaders. Their main claim to fame is that upon activation, they quickly disgorge a variety of granules located within their cytoplasm into the surrounding extracellular space. Among the chemicals contained in these granules are histamine and heparin. The latter is a potent anticoagulant. The former is a small molecule that increases the permeability of capillaries so that other innate cells can move from the blood into the surrounding tissue. Histamines also promote itching, and the increased permeability they induce causes runny noses and watery eyes. We take antihistamines to relieve these conditions. (How antihistamines operate is beyond the scope of this blog. That's another way of saying that I have no idea how they work).
One of the hallmarks of the mast cell response is rapidity. When they're challenged by a culprit that they recognize (recognition can be mediated by the kind of innate receptors that I discussed in a previous posting), they can release their load of granules within minutes. But they also can associate with some proteins that are part of the adaptive immune system. This allows them to respond quickly and specifically to a variety of substances that come in contact with our skin, respiratory tract, and digestive system and that the innate system doesn't recognize. We're all familiar with this kind of response. It's called an allergic reaction. I hope to get to the topic of allergy in a later post.
I've left these cells for last not because they're unimportant. On the contrary, I saved them until now because they are a critical bridge between the innate immune system and the adaptive immune system that I'm going to discuss in the remainder of these posts. Named for the tree-like projections of their cytoplasm (dendron is the Greek word for tree), dendritic cells are found in many tissues, but are principally located in the skin and respiratory tract lining. They come in two varieties. One type is a sentinel cell similar to macrophages, but with an even larger range of receptors, including many toll-like receptors that I discussed previously. They are particularly adapt at identifying viruses and their byproducts. When they do come across evidence of a virus, they produce and secrete a set of cytokines that have potent antiviral activity.
The other type of dendritic cell is specialized for antigen presentation. Making use of the major histocompatibility system discussed previously in association with natural killer cells, it is the principle way of letting the adaptive immune system know the exact chemical nature of an intruder. I'll define the terms "antigen" and "antigen presentation" in later posts.
OK. It's time to sum up what I've learned about the innate immune system. I'll leave this important task to the next post.
If this blog seems like a Ginsu knife commercial – "Wait there's more!" - you're right and you're following along nicely. Besides macrophages, neutrophils, and the complement system that I've already described, there are several other key players involved in the innate immune system. One such is the natural killer cell, whose name seems to come straight out of the "Silence of the Lambs". It is so designated because it's similar to the killer T cell of the adaptive immune system, which I'll discuss later. But it kills naturally, that is, without making use of the adaptive immune system's exquisite specificity and diversity.
Let me begin by offering an overview of the properties of this important player.
The third and fourth points are fascinating, new to me, and I elaborate on them below. Along the way, I'll cover some basic molecular biology that is worth learning about.
Killer T cells seem to have arisen because organisms seem to face an intractable problem: How to they defend against viruses, entities that propagate inside of cells? The defenses that I've described so far, macrophages, neutrophils, and the complement system, largely attack invaders by detecting molecules on their outer membranes. Once a foreign microbe has penetrated this barrier it would seem to be invisible and therefore invulnerable. Natural killer cells get around this problem by employing an ingenuous strategy that makes use of a cellular component that most of are unaware: the major histocompatibility complex (often referred to as the MHC. I'll use that designation despite my dislike of initialisms because of the length of the name of the molecules to which it refers (it's awkward to keep typing such a long phrase) and because you'll come across it in many contexts. In particular, I'm going to come back to a discussion of the MHC in a later blog when I cover the adaptive immune system and will be sure to remind you of its meaning there.
I'll begin my discussion of the MHC with a fact that many of my students disbelieve ("fake news"?). We are not the same person that we were years ago, months ago, even minutes ago. That's because our innards are continually turning over. By that I mean, our cells are dying all the time and being replaced with new ones. Just to give one example, our red blood cells have an average lifetime of about four months. Given their numbers, that means that we're losing about 2 million cells a second. And, of course, they're being replaced at the same rate. Other cells are being recycled similarly, some even faster. Furthermore, what many people don't appreciate is that the internal components of all of our cells, in particular our proteins, are also constantly being destroyed and renewed. In fact, there is a specific organelle (it's called the proteosome in case you wanted to look it up and learn more about it) that is found in all cells that is responsible for chopping up our proteins so that new ones can take their place.
What's this got to do with the MHC? Well, one form of the MHC, MHC I, is found in all cells with a nucleus (not human red blood cells because they've lost their nuclei during maturation). It has the responsibility for grabbing on to protein fragments from degraded proteins and displaying them on the cell surface. There's much more to this process, as you might imagine, but suffice it to say that cells with a MHC have on their surface a sampling of small bits of almost all their internal proteins, proteins that have been degraded but whose fragments have been captured by the MHC. These samples are less than 10 amino acids long and are waving about the outside of cells, advertising what's inside.
Natural killer cells have receptors on their surface that bind to the proteins of the MHC. Their response: They are inhibited and don't attack. Because normal cells bear an excess of these proteins, they're left alone. On the other hand, cells that that have been infected by a virus or have been stressed in other ways or are cancerous often have fewer MHC molecules on their surface. Moreover, stressed, injured, and cancerous cells may display a variety of other ligands on their surface, some of which may be recognized by the natural killers and which are excitatory and activating. When a natural killer cell encounters a cell bearing a decreased number of MHC molecules, it can attack. Whether it does so depends on the number of inhibitory MHC proteins on the surface of the probed cell versus the quantity of excitatory molecules. The take home lesson is that natural killer cells attack targets that are missing something, not because they bear a positive call to arms. The figures above illustrate the phenomenon.
Once a natural killer cell has targeted a cell for destruction, it signals it to commit suicide. Again, cell suicide (programmed cell death) may be a phenomenon of which most people are unaware. However, it has been recognized for decades. It's so old that I even learned about it as a graduate student. Rather then extend this posting past the patience of my readers, I'll forgo discussing the subject. Instead, for those interested, look up apoptosis in the usual references.
Natural killer cells make use of a kind of borer to poke holes in the membrane of cells so that it can inject suicide signals into their quarry. Alternatively, some of their surface proteins may interact with proteins on the outer membrane of targeted cells that also signal programmed cell death. In both cases, the affected cell dies "neatly". In the words of Alberts et al., "The cell shrinks and condenses. The cytoskeleton collapses, the nuclear envelope disassembles, and the nuclear DNA breaks up into fragments. Most importantly, the cell surface is altered, displaying properties that cause the dying cell to be rapidly phagocytosed, either by a neighboring cell or by a macrophage" (Molecular Biology of the Cell, 4th edition, Alberts et al, free to read online). In addition, natural killer cells secrete cytokines that activate macrophages.
All in all, the natural killer cell is a potent adversary for invading microbes. In the future, biologists may engineer it to take advantage of its properties and use them for medical purposes. In fact that's already happening. A paper published in late June, 2018 describes the how natural killer cells can be used to target cancer by engineering a unique receptor on it their surface. Here's the reference for those interested and willing to wade through the technical details.
More about the innate immune system in the next post.
In today's blog I'm going to discuss three matters: 1) additional consequences of complement activation (other than drilling holes in bacteria); 2) ways that the complement system is regulated; and 3) hereditary disorders associated with defective complement components. To enliven matters, I'll briefly rant about the cost of trying to read journal articles on line. I'll also tell you about another book that I purchased. Here goes.
When a bacterium becomes covered with C3b (the larger of C3's two proteolytic cleavage products) as a result of activation of the complement cascade, specific receptors on macrophages and neutrophils recognize the ornamented microbe and allow these white blood cells to bind to the invader and "swallow" it up (phagocytosis). At the same time, the small proteins released by cutting C3 and C5 (C3a and C5a, remember?) act as cytokines to stimulate the inflammatory response, activating cells on the inner walls of capillaries. These endothelial cells increase expression of the protein selectin, thereby attracting neutrophils to the area via a mechanism discussed in a previous post. The neutrophils, in turn, may be stimulated sufficiently that they produce a series of powerful poisons that can kill bacteria (and if left unchecked, surrounding cells).
Another cellular player in the innate immune system, one which I haven't mentioned as yet, the mast cell, is also stimulated by these cytokines. It produces substances (histamines, if you must know) that cause blood vessels to swell and become more permeable, releasing plasma and blood cells into the surrounding tissue, thereby causing swelling and redness. Activated complement also interacts with the adaptive immune system, a topic that I'll get to in a future post. All these functions of complement, in addition to its ability to kill intruders by boring into their membranes, make complement a powerful force for defense against microbes. So powerful that it must be carefully regulated so that it doesn't get out of control.
One reaction that you don't want is to have the complement cascade activated so that it kills or injures normal cells. What's more, once a bacterial or other microbial invasion has been arrested, you'd like the complement reaction to stop. These two tasks are the concern of a dozen or so proteins, some of which circulate in the blood, others are located on the surface of cells. One of the most important of the complement regulators acts to break up the C3Bb complex. Another interferes in the last part of the cascade to block the formation of the apparatus that pokes holes in the membrane of bacteria. The others function at the early step in all three pathways of complement activation to limit the proteolytic cascade. Some of these regulatory proteins are found on the surface of almost all normal cells, thereby protecting them from attack by complement, limiting complement action to non-self cells. Mutations in the genes for some of these regulatory proteins result in several disorders including age related macular degeneration. For those of you who want to dive into the depths of the subject of complement regulation (and for a more detailed overview of the complement process in toto), I recommend this article by Marina Noris and Giiuseppe Remuzzi. It's not meant for beginners, but I found it less technical than others that cover the same subject.
If I give you the impression that everything is known about complement, you would be mistaken. Of course, a lot is known, and I haven't even pierced the skin of the subject. But my impression is that in coming years, much more will be learned and therapies that take into account the complement system will increasingly be important clinically. As of today, the impact of the defects in the system on disease can be best summarized in the following table taken from a review article by J. Vidya Sarma and Peter A. Ward. It provides a good summary of the major disorders associated with defects in the complement system.
Since the table contains many terms that I haven't covered, let me help interpret. The fifth row indicates that defects in complement's main components, including C3, C5, and C6 to 9 (ones that I've described) results in a susceptibility to meningitis (caused by bacteria of the genus Neisseria). Gonorrhea is another disease that is caused by an organism in the same genus and people with defects in these same components are also susceptible (not shown in the table). Another serious and common illness is lupus (as indicated in the row above). It is often associated with defects in the major components of two of the other branches of the complement system. Most of the other deficiencies in the table are in regulatory factors. They generally cause an increased susceptibility to infections of various sorts.
While other human disorders have been reported to be associated with defects in complement, they tend to be rare and, when they do occur, often resolve themselves without too much harm. To my mind, and remember, this is from an uninformed source, that may be the consequence of two factors. First, it may be that the human body is very good at compensating for defects. That in turn may arise from the fact that many physiological systems are redundant, and backups may come into play in the event of a deficit somewhere else. Second, it may be that we don't know enough about how complement acts. Subtle health issues may arise from problems associated with the complement system and we may not be aware of them. To that point, several articles that I've come across have hinted at a role for complement in Alzheimer's Disease, atherosclerosis, and cancer. Look out for news concerning the association of complement and these and other disorders in the future.
Now for a rant. I've presented references to two review articles above. Both are available on the web for free. But for many other articles, access is restricted and there's a charge to read them, sometimes a considerable one. As it happens, I can access almost all papers of interest gratis because I am an Emeritus Professor at Rutgers. The position entitles me to few benefits (no salary is one), but it does allow me to utilize the University library system to read journals that the library subscribes to on line. Since Rutgers is a major research university, it has to subscribe to almost all the important journals.
But what about my students? What about physicians? What about ordinary people who want to explore a subject in depth? They'll have to pay, sometimes $10 or more per article. Or they could subscribe to the journal. But subscription costs are extraordinarily high for many journals, so high that only major libraries can afford them. The publishers claim that these fees are justified, but my understanding is that journal publishers have an extraordinarily high rate of return on their investments, higher than almost any other industry. In response to the high cost of subscriptions, many "open-access" journals have appeared. I hope that this trend continues.
One last item. I've obtained another general reference, a short soft cover book. It's called "The Innate Immune System: A Compositional and Functional Perspective" by Tom P. Monie of Cambridge University. So far I find it well written, up to date, and a little less technical than the text by Abbas et al.
OK. I've learned, and I have informed you, that the complement system works via a proteolytic cascade. One proteolytic enzyme clips and activates another. In turn, the newly active enzyme cuts the next inactive enzyme in line and activates it. It's a positive feedback loop, an avalanche, a nuclear chain reaction. And, like the Chernobyl disaster, it can get out of control. There are, in fact, many pathologies that can result from a too energetic or a too prolonged complement reaction. I'll briefly mention some of these conditions in the next post. For now, I'll describe what I learned about some of the details of how the complement system gets going and one of the many major functions it serves when its fully deployed.
At the heart of complement initiation is a protein called C3. It's made in the liver and present in high concentrations in the blood. It cruises along without doing much, but it's a time bomb waiting to be set. As mentioned in the previous post, there are three ways for turning the complement cascade on, all involving the activation of C3. One mechanism is via the adaptive immune system and I'll not discuss it here. One of the other ways that the system is activated takes place via the innate immune system and is called the "alternative pathway".
The Alternative Pathway
Thank goodness that the scientists who work on the complement system use a system of nomenclature that is rational and relatively easy to understand. They simply add a lower case letter as a suffix to the name of the uncut protein when they give a name to the cleavage products. For example, when C3 is cut (or breaks) into two pieces, the fragments are called C3a and C3b. The lower case "a" always designates the smaller of the two pieces; the larger one, of course, bearing a "b" suffix. Two other examples just to be clear: There's a protein called "B" that is sliced into Ba and Bb, and another called C5 cut into C5a and C5b. (Further reading revealed that the nomenclature isn't as consistent as I thought. There is a C2 protein in the innate pathway. When it gets cleaved its larger fragment is called C2a rather then the other way around. So much for consistency. Go figure.)
Here's how the complement cascade gets activated. As C3 circulates in the blood, a tiny percentage is continually breaking apart spontaneously into C3a and b. When that happens, C3b changes its shape and become capable of binding to an invading microbe. Most of the time, there will be no intruders to bind to and C3b will rapidly become inactive. In this way the complement cascade never initiates. However, when a foreign organism is present, C3b can bind to it. C3b become more stable and it changes its shape so that it now becomes the subject of binding by another protein, called "B", also dissolved in the blood.
Now comes the Rube Goldberg part. When B binds to C3b, another proteolytic enzyme called "D", beaks it apart into, of course, Ba and Bb. The result is that the microbe now bears on its surface a complex of C3b and Bb. This duo now transforms into a proteolytic enzyme with the particular property of being able to cut more C3. The result is a large amount of C3b bound to the surface of the invader (What about the C3a? It gets released and has an important function, but more on that in next post).
But wait, there's more. Some of the C3b formed as a result of this activity binds to the C3bBb complex forming another proteolytic enzyme with the tripartite composition C3bBbC3b. This new enzyme is capable of cutting yet another complement component called "C5" (naturally into C5a and C5b). In turn, C5b attaches to C6, C7, C8, and C9 to form a device that opens up holes in invaders like bacteria and destroys them. Wow!
It's clear that an illustration is in order since a written description is hard to follow.
The nomenclature is pretty straightforward, but there appear to be some terms missing. Where are C1, C2, and C4? Simple. They're in the other two pathways. And what's the role of the small fragments, C3a, Ba, and C5a? I'll cover that question, the other functions that complement carries out, and the pathology that results when the complement system goes awry, in the next post.
The next component of the innate immune system that I learned about was the complement system. To get a good grasp of how it works, you'll need to study the picture on the right. What you're looking at is a cartoon of a self-operating napkin, a "Rube Goldberg Machine". Goldberg was a prominent cartoonist in the early to middle part of the twentieth century. He "invented" and displayed the schematics for a series of complicated and impractical machines that worked by one component triggering the next, one after another, in a long series of reactions. It turns out that the immune system (and many other biochemical and physiological processes) work in a similar manner, but are even more complicated. That's because most of Goldberg's machines utilize a linear sequence (A -> B -> C -> D and so on), while biological processes may require multiple inputs at B, C, or D in order to proceed to the next step. The complement system in particular not only utilizes multiple inputs, but it also includes steps that feedback upon itself. It also makes use of amplification, where a small input results in a larger result. I expand upon these concepts below.
While trying to master the intricacies of the complement system, I began to think more upon an issue that I'd faced in the past and will undoubtedly have to address in the future. It is: How much detail should I dive into in discussing this subject, or for that matter, any subject? It seems like a teacher has to answer a host of questions in coming to a decision about this problem. How much time (or space) should be devoted to this matter? What's the attention span of the students? How much material can a student retain? Isn't it more important to leave students with an understanding of the major principles, rather than muddy the waters with details? But aren't the details interesting in themselves? For what purpose are the students learning this material?
I'm not sure that I have any good answers to any of these questions. In what follows, I'll first try to elucidate the major take home lessons. For some this may be sufficient. Then I'll discuss the subject in more detail. It may be that you might want to just skim this material.
In contrast to the cell-based innate reactions that I've covered previously, the complement system consists of 30 or so proteins that are manufactured largely in the liver and that circulate in the blood. Credit for discovering complement is widely attributed to the Belgian-born microbiologist, Jules Jean Baptiste Vincent Bordet, who spent most of his research career working at the Pasteur Institute. In the very last part of the nineteenth century he found that he could separate the immune response into two parts, one of which was due to the action of antibodies. The other, easily destroyed by heat, "complemented" the first, aiding in its ability to attack invading microbes. This work, the beginning of a distinguished career, resulting in his being awarded the Nobel Prize in 1919.
The complement system is complex, but here are the major principles. In the next post, I'll delve into the details.
Living things of all sorts commonly make use of proteolytic cascades to perform many functions. For example, blood clotting and programmed cell death (also called apotposis) are two other processes that utilize this mechanism to accomplish their tasks.
What is it? Some enzymes are synthesized in a form in which they are inactive. In particular, proteolytic enzymes (enzymes that cleave proteins) are often synthesized in this way because otherwise they may cause injury to innocent cell bystanders. In a proteolytic cascade, a signal to inactive enzymes may cause them to become activated, often by another proteolytic enzyme, thereby acquiring enzyme activity themselves, and thus starting a chain reaction. In turn, these newly activated enzymes may cleave another set of inactive proteolytic enzymes thus continuing the chain reaction, and amplifying the original signal. I'll describe the actual players in the proteolytic cascade that characterizes the complement system in the next post.
I've discussed the inflammatory response previously without specifically defining it. It occurs when macrophages detect a foreign attack an injury occurs. They and surrounding cells respond by secreting cytokines. In turn, these recruit neutrophils to the site of infection, cause an increase in blood flow, and increase the permeability of small blood vessels. Inflammation is marked by redness, swelling, an increase in temperature at the site, and pain.
In the next post, I'll expand upon these fundamental principles.