I intended to finish this blog with the last entry. But here I am back again. What happened? I was busily transposing blog entries into a book that I've been writing, and, after week or two, reached the chapters on the innate immune system. When I began reviewing articles relevant to natural killer (NK) cells, I discovered several new (to me) developments. Like CAR-T's, scientists have begun genetically engineering NK cells, adding CARs into their genomes! It's an exciting happening because CAR-NKs offer a number of potential advantages over CAR-T cells for cancer therapy.
Let me review. Recall that natural killer cells attack tumors and cells infected by viruses. They distinguish their objectives from their healthy counterparts by weighing a variety of inhibitory and activating signals that emanate from the target cell's surface. Most healthy cells are decorated with MHC I proteins on their outer membranes that act as inhibitors of NK cell activation and are the primary signals preventing attacks. When a cell gets stressed or becomes cancerous or gets infected by a virus, it may lose these "stop" signals. It then becomes vulnerable to NK assault. While MHC I molecules and other proteins inhibit NK cell activation, there are a great variety of excitatory ligands that are found on virus infected cells and tumors that interact with receptors on the NK surface. The NK cell integrates these activating and inhibitory stimuli and, depending on their relative weights, either ignores a target or attacks it. In addition, a number of cytokines stimulate NK function, activating them independent of other signals. Interleukin 15 is a prime example. NK cells can also recognize and destroy cells that are coated with antibody molecules. You may recall that when a natural killer cell is activated, it releases a barrage of granules into the immediate environment of its victim, causing apoptosis and cell death. In addition, NK cells let loose a variety of cytokines that have a similar effect on their targets. Moreover, these molecules cause the activation of some of the other cells of the innate immune system, heightening their potency.
I've already discussed CAR-T cells as powerful therapeutic agents. They carry T cell receptors in addition to the engineered CARs that are added to their armory, and must be derived from a patient's own blood. If they aren't, if they're taken from another individual, the receptors may react with the patient's own cells with dire consequences. But such so called "autologous" treatments are expensive, both in terms of time (several weeks for cell growth, purification, and treatment) and money (upwards of $400,000). Often very ill patients can't delay treatment or afford the expense. Because NK cells don't carry T cell receptors, they offer the possibility of an off the shelf remedy, one that is immediately available, standardized, and obtainable at a more reasonable price compared to their T cell equivalents. In addition NK cells engineered with CARs possess other advantages. Patients often relapse because tumors change their dress and avoid the CAR that is aimed at them. Because NK cells retain their original receptors, they offer the possibility of toxicity via a pathway other than the one that they were engineered for.
But NK cells aren't without deficits when it comes to tumor therapy. First, they have a relatively short lifetime after transfer into patients. Second, they don't seem to be as effective as killer T cells in penetrating solid tumors. Third, tumor cells can evade them by upregulating inhibitory receptors such as MHC I's. Fourth, they seem to be inhibited by inhibitory cytokines and other molecules secreted in the immediate environment of tumor cells. And last, it isn't clear what's the best source for these cells. Scientists have gone to great lengths in order to address these difficulties.
NK cells taken directly from the blood of disease free individuals don't grow well in culture and therefore aren't very suitable for use as off the shelf reagents for therapy. Three alternatives are available. There are several NK tumor cell lines that can grow in culture indefinitely. These suffer from the disadvantage that they are derived from cancers and pose a potential risk. In clinical trials, such cells are irradiated to curb their proliferative capacity. Of course, this treatment also limits their persistence and, at least theoretically, makes them less effective cancer fighters. Another potential source of NK cells are embryonic stem cells or induced pluripotent stem cells. These cells must be treated with agents that direct them from an undifferentiated state to one that is similar to, or identical to NK cells. They too can be cultured indefinitely. NK cells derived from umbilical cord blood are a third possibility. Frozen cord blood is widely available (it's taken from the placenta and umbillicus after birth and would otherwise be discarded) and newly developed methods to isolate sizable quantities of NK cells from it promise commercial scale accessibility.
A clinical trial designed to test the efficacy of cord blood derived CAR-NK cells is currently underway at the MD Anderson Cancer Center in Houston. Designed to study the efficacy of CAR-NK cells against several varieties of leukemia and lymphoma, the phase I/II trial is currently recruiting patients. As far as I can determine, it's the furthest along of any study involving CAR-NK cells with final results to be reported in 2022. I discuss it here in some detail as an example of the great promise of the many efforts being made to use NK cells for cancer therapy.
The CAR-NK cells being tested in Houston have been ingeniously engineered to address some of the deficiencies noted above (See Liu et al, Leukemia 32: 520-531 (2018) for details). The CAR that has been introduced into these cells carries an antibody domain against an antigen found on the surface of leukemic cells and a typical transmembrane and activation domain. These parts of the CAR are similar to those employed against leukemia in CAR-T cells described previously that had so much success in clinical trials and which has been approved by the FDA. Liu et al have added two additional components to the intracellular portion of the CAR – the gene for interleukin 15 and an inducible suicide gene called iC9. When these engineered CAR-NK cells encounter their target, the interleukin 15 gene becomes active and stimulates their proliferation and survival. The suicide gene is there for safety. If the CAR-NK cells become too active and begin to cause damage, they can be quickly eliminated by activation of iC9 by a specific pharmacological agent.
CAR-NK cells are only one of many novel immunological therapies being tested in the medical community's efforts to combat the scourge of cancer. Some of the others that I've noted, including checkpoint inhibitors and CAR-T cells have also shown evidence of efficacy. It is possible that a combination of these therapies may work better than any individual one. And, while it is possible to test many such combinations in mice and on cells in culture, real world testing – in people – is extremely expensive and time consuming. Simply finding enough patients willing to undergo a clinical trial so that a statistical significant result can be achieved is a challenge. As an outsider looking in, I'm impressed by the rapidity of progress in the understanding of both cancer and immunology, as well as the talent, creativity and ingenuity of the practitioners in these fields. But I'm distressed at the enormous difficulties facing the community in trying to bring their findings into the clinic.