Targeting monocytic‐myeloid suppressor cells through CSF1R‐blockade enhances CD19‐CAR T‐cell response in DLBCL

Introduction: Adoptive immunotherapies such as chimeric antigen receptor (CAR) T cell therapy have strongly improved the outcome of patients with diffuse large cell lymphoma (DLBCL) that are relapsed or refractory after standard chemo-immunotherapy. However, approx. 50% of patients with DLBCL are not durably responding to CAR T cell therapy. CAR T cell expansion is associated with durable responses. However, the molecular mechanisms that mediate suppression of CAR T cells leading to resistance still remain elusive. Methods: We performed bulk RNA sequencing of DLBCL patients before CAR T cell therapy. We performed gene set enrichment analysis (GSEA) using signatures derived from published genes associated with an immunosuppressive lymphoma microenvironment. To investigate CAR T cell therapy in an immunocompetent autochthonous mouse model we established murine CD19-redirected CAR-T cells that were generated using splenic T cells isolated from mice harbouring a C57BL/6N background. We treated DLBCL derived from PPMBC (PRDM1-KO, Myd88 + BCL2 overexpression, CD19: Cre) mice with this murine CD19 CAR-T cells in combination with an anti-CSF1R targeted antibody compared to controls. Results: We found increased immunosuppressive metabolic characteristics in CD19 CAR T refractory DLBCL patients indicated by increased glycolysis, hypoxia and elevated reactive oxygen species (ROS) in GSEA analysis. This metabolic signature was associated with an enriched monocytic-myeloid cell signature and a lack of expansion of effector T cells in CD19 CAR T refractory DLBCL. CSF1-CSFR1 (CD115) signaling is one major pathway that mediates the differentiation of myeloid derived cells into immunosuppressive MSCs. CD115-positive MSCs are important immune regulators in the tumor microenvironment that mediate inhibition of T cells and induce the proliferation of Tregs. We therefore hypothesize that combined treatment with a CSFR1 inhibitor will enhance CAR T cell expansion and thus improve CAR T cell response. We show that CSF1R blockade shifts the immunosuppressive lymphoma microenvironment into an proinflammatory environment in CD19 CAR T cell treated mice with DLBCL and abrogation of the expansion of lymphoma associated myelo-monocytic suppressor cells (LAMMs). This shift into an immunosupportive lymphoma microenvironment was accompanied with an increase of T cell expansion within the tumor and expansion of CD19-CAR T cells. We next evaluated whether the combination of CD19-CAR T cell therapy with CSF1R inhibition improves therapeutic outcome PPMBC DLBCL mice. Strikingly, we show that CSF1R Inhibition displays synergistic efficacy in combination with CD19 CAR T cell therapy. Conclusions: Our data strongly indicates that CSF1R inhibition improves CD19-CAR T expansion, promotes an immunosupportive microenvironment and could enhance CD19-CAR T cell therapy efficacy in patients with DLBCL. Keywords: Aggressive B-cell non-Hodgkin lymphoma, Cellular therapies, Microenvironment No conflicts of interests pertinent to the abstract.