Hypoxia promotes BCMA loss and a suppressive secretome thereby hindering CAR T cell therapy in multiple myeloma

INTRODUCTION: Multiple myeloma (MM) develops in the hypoxic bone marrow (BM) microenvironment, which alters tumor behavior and immune responses. While hypoxia is known to directly suppress immune function, its effect on immunotherapy-relevant antigen expression and the MM secretome remains underexplored. Here, we investigated how hypoxia affects BCMA expression and BCMA-targeted CAR T cell responses. METHODS: MM cells were cultured under normoxia (21% O₂) or hypoxia (1% O₂). BCMA surface and total expression were analyzed. Anti-BCMA CAR T cells were co-cultured with normoxic or hypoxic MM cells to assess cytotoxicity and cytokine release. Conditioned media and small extracellular vesicles (sEVs) were isolated, quantified, and RNA-profiled. RESULTS: MM cells cultured in hypoxia showed reduced BCMA surface and total protein expression, resulting in reduced CAR-mediated signaling. Importantly, the hypoxic tumor secretome further reduced BCMA levels and significantly impaired CAR T cell killing and cytokine production, which was partially reversible by γ-secretase inhibition. To dissect the suppressive nature of the hypoxic secretome, we identified an increase in small extracellular vesicle (sEV) release under hypoxia. RNA profiling of sEVs revealed a hypoxia-induced RNA signature with potential immunomodulatory roles. CONCLUSION: This study shows that hypoxia diminishes BCMA expression and enhances secretion of immunosuppressive factors, including sEVs, thereby limiting the efficacy of BCMA CAR T cell therapy in MM.