STA‐21 regulates Th‐17/Treg balance and synovial fibroblasts functions in rheumatoid arthritis

Abstract JAK/STAT signaling pathway plays a significant role in cytokines and growth factors signaling involved in the pathogenesis of rheumatoid arthritis (RA). STAT3 is a major downstream signaling mediator of important pro‐inflammatory cytokines involved in Th‐17 cell differentiation playing a significant role in regulating Th‐17/ Treg balance and the development of autoimmune diseases, especially RA. Studies also have reported the role of the STAT3 pathway in inflammatory and destructive functions of synovial fibroblasts (SFs) in RA. STA‐21 is a small molecule inhibitor that can inhibit STAT3 activation impairing the expression of STAT3 target genes. In this study, we tested whether a STAT3 inhibitor, STA‐21, can alter Th‐17/Treg balance and SF functions in RA. Peripheral blood mononuclear cells (PBMC) and SFs were isolated from 34 RA patients undergoing orthopedic surgery and 15 healthy controls to investigate in vitro effects of STA‐21. The main assays were MTT assay, PI staining, reverse transcription‐PCR (RT‐PCR), flow cytometric analysis, and ELISA. Results showed that STA‐21 reduced the proportion of Th‐17 cells and the expression of STAT3 target genes, RORγt, IL‐21, and IL‐23R involved in Th‐17 cells differentiation while it conversely increased the proportion of Treg cells, which theoretically may result in suppression of inflammation. We found that STAT3 activation and its target gene expression increased in RA‐SFs. In addition, results showed that STA‐21 can reduce the expression of STAT3 target genes related to cell proliferation, apoptosis, and inflammation leading to a decrease in proliferation and conversely increase in apoptosis of RA‐SFs. Overall, our findings provide evidence that STA‐21 can reduce inflammatory immune processes conducted by T cells and RA‐SFs in RA, suggesting that this compound is a suitable option for clinical studies in RA.