Inhibitor of differentiation-2 protein ameliorates complete Freund's adjuvant-induced arthritis and inhibits STAT3 phosphorylation in the synovium

Rheumatoid arthritis (RA) is a chronic autoimmune disease causing joint inflammation, dysfunction, and deformity, along with systemic inflammatory manifestations. Inhibitor of differentiation-2 (ID2) is a transcription factor containing a helix-loop-helix (HLH) structure. Studies suggest that ID2 regulates innate and adaptive immunity and inhibits the differentiation of osteoclasts. However, the effects and underlying molecular mechanisms of ID2 on rheumatoid arthritis (RA) remain unclear. In the present study, we found that exogenous supplementation of human recombinant ID2 (hID2) protein significantly reduced paw swelling and arthritis index scores in adjuvant-induced arthritis (AIA) rats, and improved ankle joint pathology. Analysis of pro-inflammatory factor levels in peripheral blood mononuclear cells and synovial tissues indicated that hID2 attenuated inflammatory responses in AIA rats. Furthermore, RNA sequencing demonstrated that hID2 down-regulated the JAK-STAT pathway, and the phosphorylation of its key molecule, Signal Transducer and Activator of Transcription 3 (STAT3), was inhibited in synovial tissues. Additionally, the expression of chemokine-related genes was noticeably down-regulated in synovial tissues, though further investigation is needed to understand the underlying mechanisms. Overall, these findings suggest that hID2 effectively attenuated the inflammatory response and joint destruction in AIA rats, highlighting the potential of hID2 as a therapeutic agent for the treatment of RA.