PDCD1 and plasma cell myeloma: The PD-1/PD-L1 pathway is involved in cancer leak from immunosurveillance; PD-1 was expressed on effector T cells and on exhausted T cells in the tumor microenvironment, while PD-L1 was reported on the cell surface in different cancers, such as breast, bladder, colon, lung, kidney, ovary, melanoma, glioblastoma, and multiple myeloma [78,79].