High-dose FcRH5xCD3 T-cell-engaging bispecific antibody (TCB) overcomes Treg-mediated suppression in-vitro in multiple myeloma

T-cell-engaging bispecific antibody (TCB) redirect cytotoxic T-cells to recognize and eliminate multiple myeloma (MM) cells, yet their activity may be influenced by the immunosuppressive tumor microenvironment, including regulatory T-cells (Tregs). Previous studies have suggested that Tregs may influence TCB-mediated cytotoxicity. To test this directly, we developed a human in-vitro co-culture model consisting of healthy donor-derived primary PBMCs, MM cell lines, and ex-vivo activated autologous Tregs, treated with a FcRH5×CD3 T-cell-engaging bispecific antibody analogous to cevostamab which is currently in clinical development. Myeloma cell killing was quantified by multiparametric flow cytometry across a dose range of TCB concentrations. At low FcRH5×CD3 doses, addition of Tregs significantly reduced MM cell killing, T-cell proliferation (Ki-67), and cytokine production (Granzyme B), consistent with Treg-mediated suppression. However, at higher TCB concentrations, cytotoxicity, cytokine production, and T-cell activation were restored, consistent with the idea that higher FcRH5×CD3 levels can overcome Treg-induced inhibition. Importantly, depletion of the added Tregs using an anti-CD25 depleting monoclonal antibody (analogous to CD25-targeting Treg depleters under clinical investigation) reversed the reduction in MM cell killing observed at low TCB concentrations, confirming that the inhibitory effect was Treg-dependent. This dose-dependent pattern was consistent across multiple donors, demonstrating that potent TCB activity effectively overrides Treg suppression. These findings suggest that achieving sufficient T-cell activation through optimal FcRH5 TCB dosing could counteract Treg-mediated immunosuppression in patients, supporting dose selection strategies that maximize efficacy within the immunoregulatory landscape of multiple myeloma.