Previous studies have shown that dysregulation and deficiency of the PD-1/PD-L1 axis can cause human autoimmune diseases [12,13,14,15]; for example, the PD-1/PD-L1 axis plays an important role in the maintenance of immune tolerance in pancreatic islet cells for the prevention of type 1 diabetes mellitus [16]. Here, PDCD1 is linked to autoimmune disease.