PDCD1 and immune system disorder: Previous studies have shown that Tregs can express soluble immunosuppressive cytokines or factors such as transforming growth factor-β (TGF-β), galectin-1 (Gal-1), IL-35, IL-10, prostaglandin E2 (PGE2), and adenosine or act through cell–cell contacts via high levels of cell surface molecules, including programmed cell death protein 1 (PD-1), programmed cell death ligand 1 (PD-L1), cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), lymphocyte-activation protein 3 (LAG-3), CD39/73, and neuropilin 1 (Nrp1), inducing immune dysfunction (16, 17) (Table 1).