Though CAR-Ts and TCR-Ts both present industry with the same daunting challenge of extremely rigorous cell culture of an unprecedented kind, CAR-Ts have a couple of inherent advantages that make them at present an easier industry deliverable compared to TCR-T.
A brief dive into what CAR-Ts and TCR-Ts are before getting to those advantages that currently give CAR-Ts an edge.
- The CAR-T is an entirely man-made (i.e., unnatural) hybrid entity that doesn’t exist in nature, made up of the variable portion of a monoclonal antibody (mAb) molecule genetically engineered onto various bits and pieces of invariable trans- and intracellular portions of the 𝛼𝛽 T cell receptor plus bits and pieces from other molecules deemed critical for T lymphocyte activation, all cloned into a viral vector as a single piece.
- The TCR in a TCR-T lymphocyte is the entire T cell receptor (TCR) derived from a Tumor-infiltrating lymphocytes – Wikipedia, specifically from a CD8+ TIL, and then cloned into a viral vector.
In both cases, T lymphocytes from patients are isolated using apheresis, purified, their endogenous TCRs activated in a standardized manner and the cells then incubated with a viral vector encoding either CAR or TCR (prior T lymphocyte activation makes them easier to transduce with viral vectors). During this process, the T cells expand greatly, reaching volumes of several liters. They’re then washed, concentrated and cryopreserved, ready to be thawed and infused back into the patient (a summary of the process below from Ping, Yu, Chaojun Liu, and Yi Zhang. “T-cell receptor-engineered T cells for cancer treatment: current status and future directions.” Protein & cell 9.3 (2018): 254-266. T-cell receptor-engineered T cells for cancer treatment: current status and future directions; process similar for CAR-Ts. Just substitute CAR for the TCR part).
CAR-T and TCR-T have some superficial similarities.
- Both are genetically engineered molecules designed to specifically target cancer cells.
- Both are transduced into T lymphocytes to be expressed on their cell surface.
- Their targets can be cancer-specific or -associated antigens; former’s better because it’s safer as it limits off-target toxicity.
- Both are personalized medicine in that they are engineered into a cancer patient’s own T lymphocytes.
- Both represent attempts to improve a patient’s own ability to specifically target and kill tumor cells by artificially expanding their T lymphocyte repertoire against antigens expressed specifically or preferentially by their cancer.
However, a couple of major advantages plus a minor one make CAR-T easier for industry, at least at present.
- CARs have an off-the-shelf component wherein the same mAb piece is engineered into T lymphocytes from multiple patients that have the same tumor. CAR-Ts thus piggyback off of the much more mature biotech industry in mAbs that now has an ever-expanding track record of developing and validating them.
- CAR-Ts are MHC-agnostic, being neither dependent on nor restricted to major histocompatibility molecules.
- TCR-Ts are restricted to TCRs cloned out of TILs found in tumor masses – a much more technically challenging task since access to enough material is often inherently limiting.
- TCR-Ts work best when restricted to the MHC of the patient it was derived from.
- Additional limitation of TCR-T requirement for MHC is that many tumors are known to down-regulate their cell surface MHC expression to evade being targeted by T lymphocytes.
- CAR-T is usually engineered into T lymphocytes which includes both CD4+ helper and CD8+ cytotoxic T lymphocytes while TCR-T is engineered into CD8+ cytotoxic (killer) T lymphocytes specifically. Lymphocytes for TCR-T transduction thus require a more stringent cell purification process.
Such obvious industrial application advantages for CAR-T beg the question of why invest in TCR-T at all?
An antigen could be anything present inside a cell, on its surface or secreted by it. Validated cancer-specific antigens are crucial limiting factors for both CAR-T and TCR-T. However…
Where CAR-T is a blunt instrument limited to binding intact portions of cancer antigens expressed on their cell surface, TCR-T is akin to a precision laser that binds, within MHC molecules, processed peptides derived from a vastly expanded universe of antigens including those expressed within and not just on the surface of cancer cells.
An ideal target antigen for cancer immunotherapy is one that
- Cancer cells alone express. The more a target antigen overlaps between cancerous and healthy cells, higher the risk of off-target toxicity that, depending on the degree of overlap, could even prove lethal.
- Cancer cells need for survival. Targets that cancer cells could easily mutate away from lead to cancer resistance.
- Is widely expressed. Fewer patients that express a specific cancer antigen, greater the need for personalized immunotherapy and higher the cost.
Key difference between CAR-T and TCR-T reflects the key difference between B and T lymphocytes in terms of the antigen pools they sample, which is inherently larger for T lymphocytes.
- Antibodies, i.e., B cell receptors, secreted by B cells directly bind antigens they are specific for. These tend to be antigens either expressed on cell surfaces or secreted. Of the two, obviously secreted cancer antigens aren’t therapeutically useful for targeting.
- T cell receptors bind relatively tiny stretches of peptides derived from antigens when presented within MHC molecules on the surface of antigen-presenting cells, ~15 to 22 amino acids long for CD4+s and ~8 to 14 amino acids long for CD8+s.
Are cancer cell-specific antigens just too technically difficult to discover or too few in number? Unclear at present. However, though either answer similarly impacts both CAR-T and TCR-T, simply expanding the pool of targetable antigens lends value to TCR-Ts. Though dearth of available cancer-specific antigens to use as immunotherapy targets affects TCR-Ts as well, that limitation is mitigated by the fact that TCRs inherently access a much larger pool of antigens, which improves the chances of finding promising ones.
Some weaknesses of the CAR-T approach are,
- CAR-T construction requires not just any mAb but one that specifically binds an antigen expressed specifically or preferentially on the cell surface of cancer cells.
- Few validated cancer-specific surface antigens means few validated mAbs available to specifically target them, which in turn limits mAb candidates available for constructing CARs.
- Success thus far limited to B lymphocyte tumors and even there, it’s not 100%. Such success could be more happenstance than acknowledged since T and B lymphocytes evolved to interact with each other anyway so CAR-T lymphocytes would be able to interact with B lymphocytes regardless the CAR construct grafted onto them willy-nilly, though obviously it greatly improves their efficiency in doing so such that CAR-Ts are able to eliminate cancerous B lymphocytes in some types of B cell cancers in some patients.
- Given the dearth of validated targets, i.e., confirmed cancer cell-specific surface antigens, and given the myriad technical difficulties inherent to penetrating tumor tissues reliably and reproducibly, CAR-T’s therapeutic promise is very much still at the starting gate for other types of tumors, especially solid tumors.
- The CAR-T construct cares nothing at all about the unique and variable TCR each individual T lymphocyte expresses on its cell surface. It just seeks to capitalize on an artificially constructed T lymphocyte activation apparatus, to activate CD8+ cytotoxic (killer) T lymphocytes to kill cancer cells that happen to activate them.
- CAR-T lymphocytes begin their therapeutic goal not when activated by an antigen that binds their ‘natural’ unique TCR but rather when one binds the CAR they are genetically engineered to express.
- To be an efficient cancer cell killer, a CAR-T needs to use all that activation apparatus once its CAR has bound to a cancer antigen.
- Better a CAR recapitulates the signaling machinery of TCR activation, greater the likelihood that the CAR-T lymphocyte bearing it gets properly activated to kill a cancer cell.
- The original CAR constructs left much to be desired in terms of their trans- and intracellular portions so much so that various labs are already working on the 4th generation of CAR-T constructs.
The latter disadvantage may well dog the heels of TCR-Ts as well though to a much lesser extent since the entire TCR is cloned intact. Less guesswork on what’s necessary and sufficient for T cell activation. A lot depends on the criteria used to select the TCR that goes into constructing a TCR-T. Greater the functional avidity maturation of the selected TCR, better its chances of triggering a strong and effective cancer-killing response in the CD8+ TCR-Ts that bear it.
The table below summarizes well the advantages and disadvantages of each approach (from The Other T-Cell Therapy).
Other literature also consulted for this answer:
Wang, Xiuyan, and Isabelle Rivière. “Clinical manufacturing of CAR T cells: foundation of a promising therapy.” Molecular Therapy-Oncolytics 3 (2016): 16015. Clinical manufacturing of CAR T cells: foundation of a promising therapy
Levine, Bruce L., et al. “Global manufacturing of CAR T cell therapy.” Molecular Therapy-Methods & Clinical Development 4 (2017): 92-101. Global Manufacturing of CAR T Cell Therapy
June, Carl H., et al. “CAR T cell immunotherapy for human cancer.” Science 359.6382 (2018): 1361-1365. CAR T cell immunotherapy for human cancer
Vormittag, Philipp, et al. “A guide to manufacturing CAR T cell therapies.” Current opinion in biotechnology 53 (2018): 164-181. http://discovery.ucl.ac.uk/10050033/1/Veraitch_20180109_CAR%20T%20cell%20manufacturing%20Review%20-%20Revision.pdf
Eyles, Jim E., et al. “Cell therapy products: focus on issues with manufacturing and quality control of chimeric antigen receptor T‐cell therapies.” Journal of Chemical Technology & Biotechnology 94.4 (2019): 1008-1016. Cell therapy products: focus on issues with manufacturing and quality control of chimeric antigen receptor T‐cell therapies
Tanna, Jay G., et al. “Critical testing and parameters for consideration when manufacturing and evaluating tumor–associated antigen-specific T cells.” Cytotherapy 21.3 (2019): 278-288. https://www.researchgate.net/profile/Patrick_Hanley4/publication/332062380_Critical_testing_and_parameters_for_consideration_when_manufacturing_and_evaluating_tumor-associated_antigen-specific_T_cells/links/5cb15975299bf12097625305/Critical-testing-and-parameters-for-consideration-when-manufacturing-and-evaluating-tumor-associated-antigen-specific-T-cells.pdf
Zhang, Jianxiang, and Lingyu Wang. “The emerging world of TCR-T cell trials against cancer: a systematic review.” Technology in cancer research & treatment 18 (2019): 1533033819831068. https://journals.sagepub.com/doi/pdf/10.1177/1533033819831068