CD8A and neoplasm: Furthermore, either a hypoxic environment or the vascular endothelial growth factor (VEGF) secreted by tumor-associated cells induce PD-L1 upregulation and reduce the expansion of antitumor cells, such as CD8 + T-lymphocytes, natural killer (NK) cells, and M1-polarized TAMs, inducing the expansion of tumor-tolerant populations, such as M2-polarized TAMs, myeloid-derived suppressor cells (MDSC), and T-regulatory (Treg) cells, resulting in tumor immunoresistance [31,32,33].