Furthermore, PD-1 tends to promote the polarization of macrophages to M2 polarization, most PD-1+ TAMs are M2-like macrophages, which are regarded as the protumor population in the tumor microenvironment (TME).9,19,189 Bone marrow transplantation experiment shows that most PD-1+ TAMs originate from circulating leukocytes rather but not resident immune cells.9 And PD-1+ TAMs show a reduced capacity for phagocytosis in contrast to PD-1-TAMs, indicating that PD-1 on TAMs inhibits phagocytosis. Here, PDCD1 is linked to neoplasm.