PDCD1 and neoplasm: In addition, human cartilage glycoprotein-39 (HC-gp39) derived from M2-like TAMs can induce the expression of growth differentiation factor 15, which is secreted by tumor cells and promotes the expression of phosphatidylinositol 3-kinase (PI3K), protein kinase B (AKT) and extracellular regulated protein kinase (ERK) activation, which in turn upregulates programmed cell death protein 1 and its ligand, leading to tumor immune escape22.