AB0070 INHIBITION OF STAT3 IN PBMCs FROM RHEUMATOID ARTHRITIS PATIENTS: CLUES TO UNDERSTAND SELECTIVITY OF JANUS KINASE INHIBITORS

Background The Janus kinase (Jak) - signal transducer and activator of transcription (STAT) pathway has 4 Jak proteins and 7 STAT factors that mediate intracellular downstream of cytokine receptors. Targeted small-molecule therapies with different bond affinity to Jak proteins have been demonstrated effective in rheumatoid arthritis (RA) treatment, but the clinical significance of selective inhibition remains unclear. Objectives To explore the effect of selective inhibition of Jak-STAT pathway in peripheral blood mononuclear cells (PBMC) from RA patients compared to healthy donors (HD). Methods In vitro Jak inhibition of the subunit 3 of phosphorylated (p) than activated STAT was measured by flow cytometry in peripheral blood mononuclear cells (PBMC) from RA patients with active disease (DAS28>5.1) naïve to any DMARDs (n.5) and HD (n.5), following recombinant human 0.1 ng/ml IL-6 (Peprotech – NJ, USA) stimulation. After blood separation, PBMC were overnight incubated with IC50 concentrations of selective Jak1-, Jak2-, Jak3- and Tyk2-inhibitors (Biovision Inc. – CA, USA) with or without IL-6 stimulation. Mean fold-increase of pSTAT3 was then compared in presence of different compounds stimulation. Results Mean pSTAT3 activity after overnight incubation was significantly higher in RA patients compared to HD (37%; 95CI 8.2-56.7 vs 17.9%; 95CI 4.6-21 – p=0.01). After IL-6 stimulation, a 2-fold and a 1.4-fold increase in pSTAT3 levels was observed in PBMC from RA patients and HD, respectively. In unstimulated PBMC from HD Jak-inhibitors didn’t significantly reduced pSTAT3 activity. In CD14+ cells from RA patients, pSTAT3 activity was reduced with no differences between all four selective Jak-inhibitors, while in CD4+ cells only Jak1-inhibition was able to reduce by 40% pSTAT3 activity. After IL-6 stimulation, the co-culture with Jak1- or JaK3- selective inhibitors was able to significantly reduce pSTAT3 levels in CD4+ lymphocytes, by an average of 20%. While in CD14+ monocytes Jak1-, Jak2- and Jak3- selective inhibitors were able to reduce pSTAT3 activity by a mean of 30%. Tyk-2 selective inhibitor did not interfere with STAT3 activation by IL-6 stimulation of PBMC from RA patients and HD. Conclusion Jak/STAT3 activity of PBMC from RA patients with active disease may be differently modulated by specific inhibitors. Selectivity of Jak-inhibitors seems more relevant in lymphocytes after IL-6 stimulation. These preliminary findings may explain discrepancies in effectiveness of selective Jak-inhibitors and pave the way for different choices in clinical practice. References [1]Tanaka Y, et al. Nat Rev Rheumatol. 2022 Jan 5:1–13. [2]Traves PG, et al. Ann Rheum Dis. 2021 Jul;80(7):865-875. [3]Choy EH. Rheumatology (Oxford). 2019 Jun 1;58(6):953-962. Disclosure of Interests None declared