PDCD1 and cancer: Consequently, the blockade of the PD-1/PD-L1 interaction by various mAbs, i.e., immune checkpoint inhibitors (ICIs) [6,21,22,23], which target PD-1 (e.g., nivolumab, pembrolizumab and cemiplimab), PD-L1 (e.g., atezolizumab, avelumab and durvalumab) or CTLA-4 (ipilimumab and tremelimumab) [23,24,25], is considered a milestone in cancer treatment.