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

Multiple myeloma (MM) develops in a 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. MM cells cultured under hypoxia (1% O₂) 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. In conclusion, hypoxia diminishes BCMA expression and enhances secretion of immunosuppressive factors, including sEVs, thereby limiting the efficacy of BCMA CAR T cell therapy in MM.