Abnormal kynurenine (Kyn) metabolism has been closely linked to the pathogenesis of rheumatoid arthritis (RA). The aims of this study were to investigate the role of tryptophan 2,3-dioxygenase 2 (TDO2), a rate-limiting enzyme that converts tryptophan (Trp) to Kyn, in regulating fibroblast-like synoviocyte (FLS)-mediated synovial inflammation in autoimmune arthritis.
The expression of TDO2 was determined by immunohistochemistry, confocal laser scanning fluorescence microscopy, imaging flow cytometry, and Western blot. TDO2 activity was tested by high performance liquid chromatography and colorimetric assay. TDO2 small interfering RNA (siRNA) and TDO2 inhibitor 680C91 were used to inhibit TDO2 in AA-FLS function in vitro. A rat model of adjuvant-induced arthritis (AA) was used to evaluate the in vivo effect of allopurinol (ALLO), a TDO2 inhibitor.
TDO2 expression was strongly increased in synovial tissue and FLS of RA and AA. Immune cells were found to express high amount of TDO2 proteins at the peak stage of AA. Pharmacological inhibition or knockdown of TDO2 in AA-FLS resulted in a reduced proliferation, secretion, migration and invasion. Kyn restored the inhibitory effect of TDO2 inhibition on activation of AA-FLS. ALLO treatment ameliorated the arthritis severity and decreased the activity of TDO2.
Our results suggest that elevated TDO2 expression may contribute to synovial inflammation and joint destruction during arthritis. Therefore, targeting TDO2 activity and the Kyn pathway of Trp degradation may represent a potential therapeutic strategy in RA.

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