CD28 and plasma cell myeloma: Importantly, they seem to play a dual role in the MM BM microenvironment: on the one hand, DCs activate cytotoxic CD8+T cells against tumor cells through engulfment of apoptotic plasma cells and, on the other hand, DCs protect myeloma cells against CD8+T cell killing by downregulation of proteasome subunits in a contact-dependent manner involving the CD28-CD86/CD80 axis [63,64].