Modulation of IL-6 receptor/STAT3 downstream signaling in rheumatoid arthritis patients

Interleukin-6 (IL-6) is a relevant cytokine in rheumatoid arthritis (RA) pathogenesis, potentially activating Janus kinases (JAK)-1, -2, and tyrosine kinase 2 (TYK2), and thus, three signal transducer and activator of transcription (STAT)-1, -3 or - 5 pathways. This pilot study aims to explore differences in phosphorylated (p)STAT3 levels among patients with RA, those not classified as RA (nRA), and healthy donors (HD), providing some clues on the relative contribution of each JAK protein to the downstream of the IL-6-induced STAT3 pathway. Clinical data and blood samples from 80 subjects (41 RA, 14 nRA, and 25 HD) were collected. The activity of the JAK-STAT3 pathway was assessed by Western Blot and Real Time-PCR analysis for the quantification of STAT3 in peripheral blood mononuclear cells (PBMC). Furthermore, the impact of JAK-1, -2, and TYK2 inhibitors on pSTAT3 was assessed in vitro by FACS, with and without IL-6 stimulation in RA patients naïve to treatment with DMARD and steroids. The pSTAT3 (%) was significantly higher in PBMC from RA compared to nRA patients and HD. Furthermore, pSTAT3 (%) was significantly associated with inflammation and disease activity (ESR, CRP, and DAS28). The JAK-1 inhibitor was more effective in reducing pSTAT3 expression in CD14^pos cells of RA patients, while the JAK-2 selective compound was more effective in CD4^pos cells of RA patients. On the contrary, the TYK2 selective agent showed no significant effects. This study highlights the importance of the JAK/STAT3 pathway in RA. Some differences among various JAK proteins have been pointed out, with JAK1 and JAK2 standing as the most relevant mediators of the STAT3 pathway in this in-vitro model after IL-6R activation.