CAR-T cells, T-cells that carry a chimeric antigen receptor, were first concocted by Zelig Eshhar, an immunologist at the Weizmann Institute in Israel. His invention, now a quarter of a century old, was a triumph of genetic engineering that utilized knowledge fueled by decades of basic research in immunology. Since its introduction it has produced some dramatic successes in treating a variety of blood cancers. While treatment of solid tumors has lagged somewhat behind, hopes are high that it will prove equally effective as the procedure improves.
Because CAR-T developments are progressing so rapidly, even the latest textbooks are somewhat out of date. While Abbas et al. allots a few well written pages to the technique, I found two review articles published in 2018 that offer up more timely information. Both are freely available on line ("CAR T cell immunotherapy for human cancer" by Carl June et al, Science 359:1361-1365 (2018) and "Concise Review: Emerging Principles from the Clinical Application of Chimeric Antigen Receptor T Cell Therapies for B Cell Malignancies", by M. D. Jain and M. L. Davila, Stem Cells 36:36-44 (2018). While neither is directed at beginners, if you skip the hairy parts you can still get a good feel for the recent developments in the field and where it's going in the near future.
To best understand how CAR-T works, I found it useful to review the structure of the T cell receptor (see the figure at the right) and how killer T cells operate. Notice that the antigen (that's the small bright yellow structure) recognized by the T cell is presented to it by the major histocompatibility complex (MHC I) located on an antigen presenting cell (APC), often a dendritic cell. Recognition of the MHC by the APC requires the CD8 receptor. The T cell receptor undergoes a conformational change upon antigen binding and thereby signals its success to a complex of proteins (CD3). In turn, in conjunction with a few costimulatory proteins, this sets off a chain of reactions that ultimately result in activation of the T cell.
By contrast, consider the chimeric receptor devised by Dr. Eshhar. It is completely missing the part of the the T cell receptor that binds an antigen. Instead, it has exchanged it for a strange looking antibody composed of a variable light chain and a variable heavy chain tethered together by a string of amino acids. The idea is that this engineered antibody will act as a receptor and bind to a specific protein on the outside of a targeted cancer cell. Linked to the antibody on the same protein chain is a short span of amino acids that is designed to bridge the cell membrane. And following this transmembrane segment, still on the same chain, are several regions – labelled costimulatory domains in the figure – that are intended to enhance cell division, increase cytokine production, and prevent cell death in the T cells. Finally, the CAR carries a portion of the CD3 complex that is designed to transmit the signal that occurs upon antigen binding to the nucleus to activate the T cell. The result is truly a chimera - a melding of different proteins on one chain. Note that the CAR bypasses activation via the MHC, advantageous because many cancers lose MHC expression in the course of their development in an effort to bypass immunosurveillance.
Here's how the CAR is supposed to function. Its antibody portion has been engineered to stick out of the cell membrane and bind to a specific antigen on a cancer cell (the CAR's that have gotten FDA approval for therapy bind specifically to a protein called CD19, a common antigen on the surface of B cells). This binding causes a change in the conformation of the receptor that is communicated to the portion sticking into the cell, causing activation, cell division, and the unleashing of the killing potential of the T cell. Notice again that unlike in normal T cells, activation requires neither an antigen presenting cell nor a MHC.
And here's how it works in practice (this section and the next comes from a paper published just a few days before this was written – "Chimeric Antigen receptor (CAR T) Therapies for the Treatment of Hematologic Malignancies: Clinical Perspective and Significance", Boyiadzis et al; Journal for ImmunoTherapy of Cancer 6:137 (2018)). Blood is taken from a patient with leukemia or some other blood cancer and transferred to a facility where T lymphocytes are purified and placed into tissue culture. The cells are stimulated to divide with a variety of reagents. After they're grown to a reasonable number, some mechanism is used to introduce the chimeric gene into their genomes. T cells, now bearing a CAR directed against the cancer are transfused back into the patient where they, hopefully, kill the cancer. Time from initial blood collection to infusion back into the patient: three weeks.
Two CAR T constructs have been approved by the FDA. Both target CD19. The first, approved in August 2017, is intended to treat acute lymphocytic leukemia in young patients up to 25 years old who have not responded to treatment or who are in relapse. Approval was given on the basis of a clinical trial involving 75 patients aged three to 25. The results stunned the scientific community who were used to incremental progress. Fully 81% of the patients were in remission by the end of the trial! The same CAR-T has been recently approved (May, 2018) for treatment of older individuals with diffuse large B-cell lymphoma after the preceding encouraging results. A second, slightly different CAR-T, was approved by the FDA in October, 2017. It targeted patients with diffuse large B-cell lymphoma who had relapsed after two prior treatments or had failed to respond to conventional therapy. Again, the phase II trial results on which the approval were based were dramatic, with an 18 month survival rate of over 50%. These two CAR-T's are just the beginning. A multitude of clinical trials are ongoing. Encouraging results have been forthcoming for treatment of multiple myeloma, acute myeloid leukemia, and Hodgkin lymphoma.
These extremely promising results are the good news. They indicate that using CAR-T therapy can be effective against leukemia, lymphoma, and other cancers of white blood cells. They validate the general strategy of using modified T cells to attack and rid the body of malignant cells. They're breakthrough treatments and represent the first personalized gene therapy treatment that has gotten FDA approval. In the next post, I'll discuss the bad news: CAR-T cell toxicity, cost, and failure to offer effective treatment of solid tumors. I'll end with a discussion of the prospects for further developments for CAR-T therapy.