PDCD1 and neoplasm: CAR-T, Chimeric Antigen Receptor T; CTLA-4, cytotoxic T-lymphocyte-associated protein 4; CXCR2, chemokine receptor 2; DCs, dendritic cells; ECM, extracellular matrix; MDSCs, myeloid-derived suppressor cells; NK, natural killer; PD-1, programmed cell death protein 1; PD-L1, programmed death-ligand 1; TAMs, tumor-associated macrophages; TILs, tumor-infiltrating lymphocytes; TNBC, triple-negative breast cancer; TME, tumor microenvironment; VEGF, vascular endothelial growth factor.