IL-34-mediated fibroblast-like synoviocyte-macrophage crosstalk drives bone erosion in rheumatoid arthritis through RANKL-dependent osteoclastogenesis.

Background: The crosstalk between fibroblast-like synoviocytes (FLS) and macrophages forms a self-reinforcing vicious feedback loop in the rheumatoid arthritis (RA) immune microenvironment. Upregulated interleukin-34 (IL-34) levels in RA patients correlate with RA severity. This study investigated the roles and mechanisms of IL-34-mediated FLS-macrophage crosstalk in osteoclastogenesis-associated bone erosion in RA. Methods: IL-34 expression in the RA serum and synovial samples and its correlation with disease severity and bone erosion were assessed. Primary FLS were transfected with IL-34 overexpression plasmids or short hairpin RNA (shRNA) targeting IL-34, and cell proliferation, migration, invasion and apoptosis were examined. Then, conditioned medium (CM) from these FLS were incubated with THP-1-differeniated macrophages to evaluate macrophage migration and polarization. FLS, M1 macrophages, and their cocultures were respectively treated with recombinant human IL-34 (rhIL-34), and these CM were incubated with peripheral blood mononuclear cells (PBMCs). Receptor activator of NF-κB ligand (RANKL) concentration and osteoclast differentiation were determined. Additionally, a collagen-induced arthritis mouse model was established and treated with IL-34 neutralizing antibody. The arthritis score, paw swelling, histopathological analysis micro-CT analysis were conducted. Results: Upregulated serum IL-34 level in RA patients was correlated with disease severity and bone erosion. IL-34 overexpression facilitated proliferation, migration and invasion and inhibited apoptosis in FLS. Moreover, CM from IL-34-overexpressing FLS promoted macrophage recruitment and M1 polarization. In contrast, IL-34 silencing showed the opposite results. rhIL-34-stimulated FLS-macrophage coculture exhibited higher RANKL production and enhanced osteoclast differentiation. However, RANKL inhibitor denosumab abolished these effects. IL-34 blockade attenuated RA severity, synovial hyperplasia, macrophage M1 polarization, inflammatory infiltration, and bone erosion in CIA mice. Conclusion: IL-34 accelerates RA progression by regulating FLS-macrophage crosstalk to amplify bone erosion through RANKL-dependent osteoclastogenesis. We suggest that IL-34 blockade as a promising therapeutic strategy for concurrently alleviating synovial hyperplasia, inflammation, and bone erosion in RA.