As a T-cell immune checkpoint, the function of the PD-1-PD-L1 axis in T cells has been well elucidated; however, recent studies have shown that this axis also functions in the regulation of the phagocytic ability of TAMs.9 PD-1 is expressed not only in T cells in peripheral tissues but also in B cells, activated monocytes, DCs and NK cells.187,188 TAMs express high levels of PD-1 compared to splenic macrophages or circulating monocytes, and PD-1 expression increases with tumor volume after engraftment. This evidence concerns the gene PDCD1 and neoplasm.