PDCD1 and osteosarcoma: In a study of gastric adenocarcinoma “M2”-like macrophage infiltration was correlated with PD-L1 expression [34] while in a mouse model, anti-PD-1 therapy led to macrophage reprogramming from “M2”-like to “M1”-like phenotype and to a subsequent regression of osteosarcoma lung metastases [35].