CD274 and plasma cell myeloma: Checkpoint signaling is an important immunosuppressive component of the myeloma microenvironment [65] and, indeed, in preclinical investigations, the anti-myeloma activity of the FcRH5/CD3 bsAb was limited by PD-1/PD-L1 signaling, however, the PD-L1 blockade significantly enhanced myeloma cell elimination in vitro [48].