CD274 and neoplasm: It was reported that a proliferation-inducing ligand (APRIL), secreted by eosinophils, osteoclasts and myeloid cells in the MM microenvironment, could bind B-cell maturation antigens (BCMAs) expressed on MM cells, resulting in tumor cell survival as well as the upregulation of PD-L1 expression via the phosphorylation of MEK1/2 [71,72].