PDCD1 and plasma cell myeloma: CD8+ T cells from myeloma patients express multiple immune checkpoint receptors, including programmed cell death protein 1 (PD-1), cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), T cell immunoglobulin mucin-3 (TIM-3), lymphocyte activation gene 3 (LAG-3), and, recently, T cell immunoglobulin and ITIM domain (TIGIT) [27,28].