PDCD1 and rheumatoid arthritis: An important function of PD-1 in maintaining the peripheral self-tolerance and prevention of autoimmunity is further supported by the reported association between single nucleotide polymorphisms (SNPs) in the human PD-1 gene with susceptibility to systemic lupus erythematosus (SLE) (27, 28) and rheumatoid arthritis (27, 29).