CD8A and breast cancer: Studies on pre-clinical BC models have reported that Granulocyte-macrophage colony-stimulating factor (GM-CSF) secreted by tumor cells promotes programmed death ligand-1 (PD-L1) overexpression on TAM cell surface, thereby deactivating CD8+ cytotoxic T cells via the immunosuppressive PD-1/PD L1 interaction (35).