IDO1 and plasma cell myeloma: In addition, the cellular contact of myeloma plasma cells with BM immature DCs, through CD47–thrombospondin-1 interaction, leads to spontaneous DC fusion and trans-differentiation into osteoclasts (131, 132), which, beside their role in bone lesions, promote suppressive immune BM microenvironment inducing T cell apoptosis by the overexpression of ICP molecules and the release of IDO and APRIL (129).