To determine the in vivo role of CD84 in MM, 5TGM1 murine MM cells and MM.1S human myeloma cells were i.v. injected into syngeneic immunocompetent KaLwRij mice (Supplemental Figure 5A and Figure 10A) or into immunodeficient NSG mice (Supplemental Figure 5B), respectively, leading to a significant increase in MDSC numbers in the PB (Figure 10A and Supplemental Figure 5C), to increased CD84 expression (Figure 10B and Supplemental Figure 5D) on MDSC cell surface, and to increased PD-1 cell surface expression on BM CD8+ T cells (Supplemental Figure 5E). This evidence concerns the gene PDCD1 and plasma cell myeloma.