Dual bispecific protein engager–armed T cells targeting BCMA and B7-H3 overcome antigen heterogeneity in multiple myeloma

Multiple myeloma (MM) remains an incurable plasma cell malignancy, with most patients ultimately progressing to relapsed or refractory MM (RRMM) despite therapeutic advances. Antigen heterogeneity and immune escape, particularly the loss of B-cell maturation antigen (BCMA), pose major challenges to effective immunotherapy. Notably, BCMA loss driven by alterations in TNFRSF17 occurs in a substantial subset of RRMM, thereby limiting the durability of BCMA-targeted therapies. In parallel, B7-homolog H3 (B7-H3), frequently overexpressed in MM, has been implicated in tumor progression, immune evasion, and therapeutic resistance. To address antigen heterogeneity, we developed dual-targeting bispecific protein engager (BiPE)-armed T cells (dual BATs) directed against BCMA and B7-H3. Dual BATs were generated by simultaneously arming T cells with two BiPEs - anti-BCMA × anti-CD3ε and anti-B7-H3 × anti-CD3ε -engineered to engage distinct, non-overlapping CD3ε epitopes. Functional characterization demonstrated that dual BATs mediated antigen-specific T-cell activation, cytokine secretion, proliferation, and cytotoxicity across MM models with diverse antigen expression profiles. Importantly, dual targeting broadened antigen recognition rather than enhancing intrinsic cytotoxic potency, with functional responses primarily governed by antigen density, the binding characteristics of individual BiPE arms, and the balance of CD3 engagement. Collectively, these findings support dual BiPE-armed T cells as a flexible and adaptable strategy to overcome antigen heterogeneity and underscore the critical role of relative antigen expression level in optimizing T-cell-redirecting immunotherapies for MM.