PDCD1 and neoplasm: Tumor vascular normalization relieves hypoxia and reduces the secretion of VEGF, thus reducing the recruitment of immunosuppressive cells such as MDSCs and Tregs, and also reduce the expression of PD-1, PD-L1, CTLA-4, TIM-3 and other immune checkpoint molecules on the surface of immunosuppressive cells (27, 28).