Tryptophan metabolism via the kynurenine pathway in selected rheumatic diseases: A review of the literature

In chronic inflammatory diseases, as a result of pro-inflammatory cytokines, the enzyme indoleamine 2,3-dioxygenase (IDO) is excessively activated, resulting in increased tryptophan metabolism via the kynurenine pathway (KP). Both IDO and metabolites of the KP affect cells of the immune system, mainly T lymphocytes. In this way, they exert an immunosuppressive effect, reducing inflammation. Also in rheumatic diseases such as rheumatoid arthritis (RA), osteoporosis, osteoarthritis and ankylosing spondylitis, there is excessive activation of the KP. This publication reports on disorders of tryptophan metabolism found in the above-mentioned disease entities.