In previous studies, we found that smoking may participate in the pathogenesis of rheumatoid arthritis (RA) via the aryl hydrocarbon receptor (AhR) pathway. However, when we conducted a subgroup analysis, the expression of AhR and CYP1A1 in healthy people was higher than that in RA patients. We considered that endogenous AhR ligands may exist in vivo that activate AhR to play a protective role. Indole-3-pyruvic acid (IPA) is a tryptophan (Trp) metabolite produced by the indole pathway and serves as a ligand of AhR. This study aimed to reveal the effect and mechanism of IPA in RA.

IPA is a protective factor for RA; it can restore the Th17/Treg cell balance through the AhR pathway, which can alleviate RA.

A total of 14 patients with RA and 14 healthy volunteers were enrolled. The differential metabolites were screened with liquid chromatography-mass spectrometry (LC-MS) metabolomics technology. We also treated peripheral blood mononuclear cells (PBMCs) with IPA to evaluate the effect on the differentiation of T helper 17 (Th17) cells or regulatory T (Treg) cells. To determine whether IPA can be used to alleviate RA, we administered IPA to rats with collagen-induced arthritis (CIA). Methotrexate was used as a standard drug for CIA.

When the dose reached 20 mg/kg/d, the severity of CIA was significantly reduced. In vitro experiments verified that IPA inhibited the differentiation of Th17 cells and promoted the differentiation of Treg cells, but this effect was weakened by CH223191. Conclusions: IPA is a protective factor for RA; it can restore the Th17/Treg cell balance through the AhR pathway, which can alleviate RA.