Chimeric antigen receptor-mediated T cell (CAR T cell) therapies have revolutionized the treatment of hematologic malignancies and solid tumors. This therapy uses T cells, typically harvested from patients, that are engineered to express chimeric antigen receptors (CAR) specific to tumor cell antigens. CD19-targeting CAR T cell therapy was the first immunotherapy shown to effectively treat acute lymphoblastic leukemia, but a subset of patients relapse due to loss or poor engraftment of CAR T cells. Here we highlight advances in CAR T cell therapy to improve the quality of the immunotherapy product ex vivo for more effective responses in vivo.
T cells from peripheral blood mononuclear cells (PBMCs) are the primary cellular product used for CAR T cell therapy, but several steps must be carried out ex vivo to ensure that enough cells are made for therapeutic efficacy. The best treatment outcomes have been linked to high levels of CAR T cell engraftment and persistence upon transfer into a patient. Ex vivo culture methods have been optimized to expand T cells, and cultures using less differentiated T cells, like stem cell memory T cells or naïve-like T cells, have been linked to better persistence upon transfer. Recent studies are characterizing phenotypic features of T cells that preserve “stemness” upon ex vivo culture but still allow for expansion and expression of CAR T receptors. Reduced culture time has been one of most effective methods for improve engraftment and antitumor responses but is limited by the number of cells yielded by this minimal manipulation process.
The successful engraftment and persistence of CAR T cells depends greatly on where cells migrate upon transfer into patients. Preservation of stem cell-like features correlates with better engraftment, especially if donor-derived cells are not available and autologous cell sources must be used. In vivo imaging studies have been very helpful in understanding CAR T cell dynamics and anti-tumor responses. Studies in mouse models have indicated that CAR T cells can get trapped in tissues, including lungs, which can limit access to tumor targets. Analysis of CAR T cells with tumor cells also revealed extensive functional heterogeneity, including CAR-T cells that can interact with tumor cells but not exert cytotoxic effector functions. A recent first-in-human study examined the biodistribution of radioisotope-labelled CAR T cells and confirmed that these cells rapidly distribute to tumor tissue but also are taken up by the liver and spleen and can persist systemically for up to two weeks. As new in vivo imaging studies are carried out, these insights will inform how CAR T cell products are made and delivered and are likely to improve treatment outcomes.
Advances in ex vivo methods for CAR T cell preparation are already improving outcomes, and these methods are broadly applicable to donor-derived or autologous T cell products. In vivo imaging methods are also delineating characteristics of CAR T cells that improve tumor targeting or result in misdirected tissue homing. Future in vivo and ex vivo studies are well-poised to further advance CAR T cell applications.
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