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Summary The anti-FcRH5/CD3 T cell-dependent bispecific antibody (TDB) targets the B cell lineage marker FcRH5 expressed in multiple myeloma (MM) tumor cells. We demonstrate that TDBs trigger T cell receptor activation by inducing target clustering and exclusion of CD45 phosphatase from the synapse. The dimensions of the target molecule play a key role in the efficiency of the synapse formation. The anti-FcRH5/CD3 TDB kills human plasma cells and patient-derived myeloma cells at picomolar concentrations and results in complete depletion of B cells and bone marrow plasma cells in cynomolgus monkeys. These data demonstrate the potential for the anti-FcRH5/CD3 TDB, alone or in combination with inhibition of PD-1/PD-L1 signaling, in the treatment of MM and other B cell malignancies., Highlights • Prevalence of FcRH5 expression in multiple myeloma is 100% • Anti-FcRH5/CD3 TDB redirects T cells to kill myeloma cells • Target clustering and CD45 exclusion activate T cells • Anti-FcRH5/CD3 TDB is a highly efficacious immunotherapy for myeloma, Li et al. report that the size and epitope location of the target play a key role in the efficiency of T cell activation induced by T cell-dependent bispecific antibodies (TDBs). They develop a TDB targeting FcRH5 expressed in all multiple myeloma tumor cells and show its potential in treating this disease.