B7-H3 nanobody-based CAR T cells control multiple myeloma growth, while dual BCMA/B7-H3 CAR T cells overcome antigen escape

Background: CAR T cell therapy targeting BCMA has shown remarkable efficacy in multiple myeloma (MM), but relapses occur due to T cell exhaustion and the emergence of BCMA-negative subpopulations. Novel targets are needed to overcome antigen escape. Methods: B7-H3 (CD276) expression was assessed on primary MM patient samples. We engineered nanobody-based CAR T cells (nanoCARs) targeting B7-H3 and evaluated their cytotoxicity and cytokine production in vitro, including against patient-derived myeloma cells. Anti-tumor activity was tested in two different MM xenograft models. Dual CAR T cells (BCMA/B7-H3) and CARpooling (mix of BCMA and B7-H3 CAR T cells) were also tested for efficacy in antigen escape models. Results: B7-H3 expression was detected on plasma cells in 60% of MM patients. B7-H3 nanoCAR T cells exhibited strong antigen-specific cytotoxicity and effector cytokine secretion, including against primary MM cells. In vivo, they reduced tumor burden and improved survival. Dual (BCMA/B7-H3) CAR T cells and CARpooling effectively eliminated heterogeneous tumor populations with mutually exclusive BCMA or B7-H3 expression. These findings show that BCMA/B7-H3 targeting may be a strategy to overcome antigen escape mechanisms. Conclusion: B7-H3 is a promising immunotherapy target in MM. B7-H3-specific and dual-targeting nanoCAR T cells could offer a strategy to prevent antigen escape and improve treatment durability.