RHOA Deletion Downregulates CD19 and Promotes Dysfunctional Immune Microenvironments in CAR-T Resistant Large B-cell Lymphomas.

CD19-directed chimeric antigen receptor (CAR)-T cells are breakthrough therapies for large B-cell lymphomas, but fewer than half of patients achieve durable responses. We previously showed RHOA deletions are enriched in progressing cases, but the role of RHOA deficiency is poorly defined despite frequent occurrence in newly diagnosed disease. Here we show RHOA loss downregulates CD19, promoting cell-intrinsic resistance to CAR-19 in vitro and in vivo. CD19 decline, however, provides an incomplete explanation of resistance and would not explain high frequency of the deletion in newly diagnosed cases. We therefore performed single-cell assessments of immunocompetent RHOA-deficient lymphoma microenvironments, revealing remarkable concordance with features in CAR-19-resistant patients. Despite increased microenvironmental production of interferon gamma (IFNγ), RHOA-deficient tumor cells themselves show impaired response to the cytokine including failed induction of Class I MHC molecules. Overall, we describe for the first time how a single-gene alteration recurrent in CAR-19-resistant lymphoma contributes to treatment failures.