The Impact and Modulation of Microenvironment-Induced Immune Resistance Against CAR T Cell and Antibody Treatments in Multiple Myeloma

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The tumor microenvironment of multiple myeloma (MM) is known to play a critical role in disease pathogenesis, MM cell survival, and drug resistance. In addition, we have previously demonstrated the impact of the microenvironment also on immunotherapies. We discovered that bone marrow mesenchymal stromal cells (BMMSCs) protected MM cells from HLA restricted CD4 and CD8 cytotoxic T cells and NK cell-mediated daratumumab dependent cytotoxicity through a cell-adhesion mediated immune resistance (CAM-IR). This CAM-IR corresponded with the upregulation of anti-apoptotic proteins Survivin and Mcl-1 in MM cells and could be modulated by small inhibitors of these molecules. We now extended our studies to investigate the impact of the microenvironment on novel immunotherapeutic approaches such as chimeric antigen receptor-transduced T cells (CAR T cells) and death receptor mediated antibody treatments, which can induce direct apoptosis in MM cells. We also investigated the possibility to modulate CAM-IR with small molecule inhibitors of Survivin, XIAP, and Mcl-1. To this end, we tested a panel of MM reactive CAR T cells directed against CD38, BCMA, and CD138, with different target affinities, for their potential to kill MM cells in the absence or presence of BMMSCs. We observed no effect of BMMSCs on the cytotoxic capacity of BCMA- and CD38-targeting CAR T cells with high affinity for the target and that were capable of inducing high levels of MM cell lysis at very low effector to target ratios. In contrast, BMMSCs effectively protected MM cells against killing by BCMA- and CD38-targeting CAR T cells with relative low affinity, or CD138-targeting CAR T cells that were less powerful in their lytic activity. Taken together, we discovered a significant inverse correlation between the lytic capacity of the CAR T cells and the extent of BMMSC-mediated protection. Additionally, we found that BMMSCs protected MM cells from apoptosis induction by death receptor 5 (DR5; or TRAIL receptor 2) antibodies. In all cases of BMMSC-mediated protection against CAR T cells as well as DR5 antibodies or daratumumab, the protection could be abrogated by an inhibitor of Survivin, XIAP, and Mcl-1. This indicates that BMMSC-mediated protection against CAR T cell or antibody-mediated lysis is indeed associated with an upregulation of anti-apoptotic proteins, similarly to the well-described cell adhesion mediated drug resistance. In conclusion, our results confirm the potential negative impact of the tumor microenvironment in the development of an adaptive resistance of MM cells against immunotherapies. Our data further suggest that this microenvironmental shielding of MM cells can be overcome either by increasing the avidity of immune killer cells or through combination of immunotherapy with inhibitors of anti-apoptotic mediators. Figure 1 Disclosures Li: Canget Bio Tekpharma LLC: Membership on an entity's Board of Directors or advisory committees. Zweegman:Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Takeda: Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding. Themeli:Covagen: Consultancy. Van De Donk:Janssen Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding; Bristol-Myers Squibb: Membership on an entity's Board of Directors or advisory committees, Research Funding; AMGEN: Membership on an entity's Board of Directors or advisory committees, Research Funding; Takeda: Membership on an entity's Board of Directors or advisory committees; Servier: Membership on an entity's Board of Directors or advisory committees; Celgene Corporation: Membership on an entity's Board of Directors or advisory committees, Research Funding; Bayer: Membership on an entity's Board of Directors or advisory committees; Roche: Membership on an entity's Board of Directors or advisory committees. Mutis:Aduro: Research Funding; Celgene: Research Funding; BMS: Research Funding; Amgen: Research Funding; Onkimmune: Research Funding; Janssen Pharmaceuticals: Research Funding; Novartis: Research Funding.