Progress in research of Trp-IDO1,2/TDO2-Kyn metabolic pathway in the pathogenesis of rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune disease characterized by excessive activation of autoreactive T cells and B cells, abundant production of autoantibodies and multiple joint involvement. Under the influence of heredity and environment, the disorder of innate immunity and adaptive immunity is the fundamental cause of the disease. In recent years, with rapid development of immunometabolism, milestone has been made in regulating the differentiation and function of immune cells through different energy metabolism pathways and related molecules. Many studies have shown that Trp-IDO1,2/TDO2-Kyn metabolic pathway mediates the pathogenesis and development of autoimmune diseases such as RA. This review summarizes the role of tryptophan (Trp), kynurenine (Kyn) and other metabolites in this metabolic pathway, as well as the role of rate-limiting enzymes indoleamine 2,3-dioxygenase 1 (IDO1), indoleamine 2,3-dioxygenase 2 (IDO2) and tryptophan-2,3-dioxygenase 2 (TDO2) in mediating RA inflammatory immune response and synovitis inflammation. This provides an important basis for elucidating the new pathological mechanism of RA and discovering new drug targets.