PDCD1 and systemic lupus erythematosus: A growing number of studies indicate that the PD-1/PD-L1 axis, as a brake on T cell activation, participates in the pathogenesis of autoimmune diseases, including systemic lupus erythematosus (SLE) (Prokunina et al., 2002), rheumatoid arthritis (RA) (Kong et al., 2005), type 1 diabetes (T1D) (Wang et al., 2005), autoimmune hepatitis (AIH) (Curran and Sharon, 2017), and MS (Kroner et al., 2005).