PDCD1 and plasma cell myeloma: Previous studies found increased expression of markers associated with T cell exhaustion, such as TOX, PD-1, and cytotoxic T lymphocyte antigen 4 (CTLA-4), in the BM of both newly diagnosed and treated patients with myeloma when compared with healthy donors (7, 27–29), and an increase in Tregs has been linked to adverse clinical features and elevated risk of progression (30).