CD4 and neoplasm: Binding of PD-L2 to PD-1 significantly inhibited TCR-mediated CD4+ T cell proliferation and cytokine production, leading to the discovery of the overlapping functions of PD-L1 and PD-L2 (53).While initially thought to be expressed primarily in macrophages in the presence of interleukins, recent studies have shown that PD-L2 is expressed in various tumor cells depending on the tumor microenvironment (54).