Review articlePD-1/PD-L and autoimmunity: A growing relationship

- Growing studies show that impaired PD-1:PD-L function plays an important role in a variety of autoimmune diseases.
- The role of the PD-1:PD-L pathway in the prevention of disorders caused by immune system abnormal activation has become evident recently.
- Sex hormones are capable of PD-1 expression modification, and therefore, involved in autoimmune disease.
- Single nucleotide polymorphisms of the gene encoding the PD-1, namely PDCD1, can affect the expression of PD-1 or interfere in function of protein; thus are linked to autoimmune disorders.
- Biologics like antibodies and modulated dendritic cells have been observed promising therapeutics in the treatment of autoimmune disorders, considering the function of PD-1:PD signaling.

Programmed death 1 (PD-1) and its ligands, namely PD-L1 and PD-L2, are one of the key factors responsible for inhibitory T cell signaling, mediating the mechanisms of tolerance and providing immune homeostasis. Mounting evidence demonstrates that impaired PD-1:PD-L function plays an important role in a variety of autoimmune diseases such as Type 1 diabetes (T1D), encephalomyelitis, inflammatory bowel diseases (IBD), Rheumatoid Arthritis (RA), autoimmune hepatitis (AIH), Behcet's disease (BD), myasthenia gravis (MG), autoimmune uveitis (AU), Sjögren's syndrome (SjS), systemic lupus erythematosus (SLE), systemic sclerosis (SSc), myocarditis, and ankylosing spondylitis (AS). By investigating the candidate genes, genome-wide association studies, and identification of single nucleotide polymorphisms (SNPs) in PD-1 gene in humans, it has been shown that there is a higher risk in relevant genetic associations with developing autoimmune diseases in certain ethnic groups. In this review we have tried to present a comprehensive role of PD-1:PD-L in all recently studied autoimmune diseases.