Our results demonstrate that blocking CD84 reduces PD-L1 expression on cells in the MM microenvironment and reduces PD-1 expression — in addition to other T cell exhaustion markers such as 2B4, CTLA-4, KLRG-1 and LAG-3 — on T cells, resulting in elevated T cell immunity, leading to a significant decrease in tumor load both in an in vivo mouse model and in patient-derived samples. Here, CTLA4 is linked to Miyoshi myopathy.