PDCD1 and neoplasm: In addition, metabolism of arginine leads to the production of nitric oxide and reactive oxygen species suppressing T-lymphocyte receptor and HLA communication, further inhibiting T-cell activation.76 Furthermore, their expression of PD-L1 that binds to PD-1 on T lymphocytes interferes with T-cell-mediated immune mechanism, promoting a pro-tumor response77 (Fig. 1).