Deficiency of PD-1 or blockade of PD-1/PD-L signaling results in the development or exacerbation of autoimmune diseases in mouse models of lupus-like glomerulonephritis/arthritis, cardiomyopathy, type I diabetes, experimental autoimmune encephalomyelitis (EAE), and autoimmune enteritis (Nishimura et al., 1999, 2001; Salama et al., 2003; Fife et al., 2009; Reynoso et al., 2009). Here, PDCD1 is linked to arthritic joint disease.