IL-33 Promotes Rheumatoid Arthritis Progression by Enhancing Pro-inflammatory Macrophage Development in the Synovial Microenvironment

Rheumatoid arthritis (RA) is a chronic autoimmune disease primarily characterized by persistent synovial inflammation, hyperplasia, and joint destruction. Despite advancements in current treatment options, approximately 80% of RA patients fail to achieve optimal therapeutic outcomes, and the precise pathogenesis of RA remains unclear. Therefore, identifying new therapeutic targets is crucial for improving the management of RA. This study aims to explore the role of IL-33 in the synovial microenvironment of RA, particularly its key function in promoting the differentiation of pro-inflammatory macrophages, providing new experimental evidence to address the persistent synovial inflammation in some treatment-resistant RA patients. We measured the levels of IL-33 in serum and synovial fluid from matched RA patients and found that, in both the active disease group and the remission group, the synovial fluid IL-33 levels were significantly higher than those in serum. Furthermore, synovial fluid IL-33 levels were more strongly correlated with disease activity (Disease Activity Score 28, Erythrocyte Sedimentation and key autoimmune markers (anti-CCP antibodies, rheumatoid factor, immunoglobulins IgG, immunoglobulins IgM, Anti-keratin antibody). These results suggest that IL-33 plays a central role in the immune dysregulation of the synovial microenvironment in RA patients. Further multiplex immunohistochemical analysis revealed a significant increase in IL-33, ST2, and CD86-positive macrophages in the synovium of patients with active RA, with the proportion of CD86+ macrophages closely correlated with disease activity and the IL-33 concentration in the synovial fluid. Single-cell RNA sequencing data further indicated that IL-33 plays an important role in the development of pro-inflammatory macrophages in the synovium during the active phase of RA. KEGG enrichment analysis and cell experiments demonstrated that the IL-33/ST2 signaling pathway promotes the differentiation of monocytes into a pro-inflammatory macrophage phenotype through the MAPK/NF-κB pathway. The pro-inflammatory macrophages secrete IL-33 and TNF, further exacerbating synovial inflammation, creating a vicious cycle that leads to disease persistence. These findings provide new evidence for the progression and treatment of synovial disease in RA, highlighting the critical role of IL-33 in the synovial microenvironment, particularly in assessing disease activity and treatment evaluation in RA patients.