Identifying and Overcoming Resistance to CAR T Cell Therapy in Solid and Liquid Tumors

Abstract

Understanding that the immune system can be harnessed and redirected to target cancer has revolutionized oncology treatment in the last decade. In addition to monoclonal antibodies, cytokine treatment, and the advent of engineered protein therapeutics, we have now advanced cellular and genetic engineering to a state where chimeric antigen receptor (CAR) transduced T cells from autologous cancer patients can be cultured ex vivo and administered back to patients, irradicating them of cancer. Despite the major clinical success of CAR T cell therapy which has led to several FDA approvals, there is known resistance to these treatments. In liquid tumors such as leukemia, lymphoma, and multiple myeloma, for which CAR T cells are approved, a well characterized mechanism of relapse is tumor loss of antigen. The first to be identified was CD19 loss in B cell leukemia and lymphoma. While CD19-directed CAR therapies have now been enhanced with many secondary target antigens in combination or bispecific format in clinical trials, B cell maturation antigen (BCMA) CAR therapy has only recently identified cases of antigen-negative relapse. As a result, there are few dual targeting CAR T cells for the treatment of multiple myeloma. We designed a new CAR T cell against multiple myeloma target transmembrane activator and CAML interactor (TACI) derived from a novel antibody sequence. In addition, we developed tandem bispecific CAR T cells that target both BCMA and TACI which were effective in vitro and in vivo even with only one antigen present. These therapeutics may improve currently approved BCMA CAR T cell therapy to overcome relapse due to antigen loss. CAR T cell therapy has had far less success in treating solid tumors. In addition to antigen loss, relapse in solid tumors has a variety of other sources, including an immunosuppressive tumor microenvironment. While these have been pursued clinically with dual targeting and armored CAR T cells in attempts to overcome this resistance, we hypothesized there were additional intrinsic mechanisms by which individual solid tumor cells evaded CAR T cell mediated killing. We conducted a whole genomewide knockout screen in glioblastoma and discovered that loss of interferon-γ receptor (IFNγR) signaling led to resistance to CAR T cell cytotoxicity. With further exploration, we observed that this was true of solid, but not liquid tumors. In glioblastoma, IFNγ was required to upregulate adhesion molecule ICAM-1 to increase CAR T cell binding duration and avidity required for maximal cytotoxicity. CAR T cell killing of liquid tumors like leukemia did not depend on this IFNγR/ICAM-1 pathway. While CAR T cells appeared to behave similarly in the presence or absence of tumor IFNγR signaling, solid and liquid tumors cells responded differently to CAR T cell cytotoxicity. To extend the benefits of CAR T cell therapy to more patients, it is essential that we adapt CAR T cell design based on the targets and tumors of interest. We demonstrate that antigen loss in multiple myeloma can be overcome by targeting a second antigen. Our investigation into intrinsic mechanisms of tumor resistance highlights an important difference between solid and liquid tumors in their response to CAR T cells. This research suggests that targeting solid tumors may require different CAR T cell design and engineering to have similar levels of success that we have observed with liquid tumors. An understanding of how tumor cells respond to CAR T cell cytotoxicity is critical to develop new therapeutics to expand response across tumor types.