PDCD1 and cancer: In addition, VEGF‐A, a proangiogenic factor, which regulates angiogenesis in cancer by activating vascular endothelial growth factor receptor 1 (VEGFR‐1) and VEGFR‐2, can induce T cell exhaustion, reflected by the expression of programmed cell death protein 1 (PD‐1), cytotoxic T‐lymphocyte associated protein 4 (CTLA‐4) and T‐cell immunoglobulin mucin‐3 (Tim‐3).30