IL-35 promotes synovial fibroblast senescence via activation of cGAS-STING-TBK1-IRF3 pathway in rheumatoid arthritis

Interleukin-35 (IL-35) exerts immunosuppressive effects in rheumatoid arthritis (RA), but its direct impact on synovial fibroblasts (SFs) remains unclear. This study aimed to redefine the role of IL-35 in RA by uncovering its therapeutic mechanism through the induction of STING-dependent synovial fibroblast senescence. We first observed dysregulated IL-35 signaling in RA patients. In a collagen-induced arthritis (CIA) mouse model, recombinant IL-35 treatment effectively alleviated disease severity, reducing clinical scores, joint swelling, and pro-inflammatory cytokines. The pivotal discovery emerged from in vitro experiments: IL-35, but not TNF-α, directly promoted cellular senescence in RA synovial fibroblasts (RASFs). Mechanistically, IL-35 signaled through the glycoprotein 130 (GP130) receptor to activate the cGAS-STING-TBK1-IRF3 pathway, leading to upregulated senescence markers (p16, p21, p53). This STING activation was essential, as its inhibition abolished the pro-senescent effect. Crucially, in vivo knockdown of endogenous IL-35 exacerbated arthritis, and this aggravation was rescued by co-treatment with an IRF3 agonist. Our findings establish a novel protective axis in which IL-35 activates the STING pathway to drive RASFs into a senescent state, thereby inhibiting their pathogenic activity and ameliorating RA progression. Our work identifies IL-35 as a unique cytokine that confers protection by driving STING-dependent senescence in RASFs, highlighting this axis as a novel therapeutic target for RA.