CD4 and plasma cell myeloma: In a murine myeloma model using 5T33 cells, in which CD138+PD-L1+ myeloma cells were found in the bone marrow and spleen, the percentages of PD-1+ cells in CD8+ and CD4+ T cells in the spleen were 20–60% and 18–40% in the advanced stage, respectively, although those percentages were only 5–10% in control mice.