IL-34-mediated FLS-macrophage crosstalk drives RANKL-dependent bone erosion in rheumatoid arthritis

In rheumatoid arthritis (RA), a self-reinforcing feedback loop between fibroblast-like synoviocytes (FLS) and macrophages fuels chronic inflammation and joint destruction. Elevated interleukin-34 (IL-34) levels in RA patients are known to correlate with disease severity, but the precise mechanisms by which IL-34 contributes to osteoclastogenesis and bone erosion through FLS-macrophage crosstalk remained unclear. Understanding this pathway is critical for developing targeted therapies that can disrupt the pathogenic circuit driving RA progression.

Researchers first assessed IL-34 expression in RA patient serum and synovial samples, correlating levels with disease severity and bone erosion. Primary FLS were then transfected with IL-34 overexpression plasmids or shRNA to modulate IL-34 levels, examining FLS proliferation, migration, invasion, and apoptosis. Conditioned medium (CM) from these FLS was incubated with THP-1-differentiated macrophages to evaluate macrophage migration and polarization. Separately, FLS, M1 macrophages, and their cocultures were treated with recombinant human IL-34 (rhIL-34), and their CM was incubated with peripheral blood mononuclear cells (PBMCs) to determine RANKL concentration and osteoclast differentiation. Finally, a collagen-induced arthritis (CIA) mouse model was treated with an IL-34 neutralizing antibody, with endpoints including arthritis score, paw swelling, histopathological analysis, and micro-CT analysis.

Serum IL-34 levels were significantly upregulated in RA patients and correlated with disease severity and bone erosion. In vitro, IL-34 overexpression in FLS facilitated proliferation, migration, and invasion, while inhibiting apoptosis. Conversely, IL-34 silencing showed the opposite results. Conditioned medium from IL-34-overexpressing FLS promoted macrophage recruitment and M1 polarization. Furthermore, rhIL-34-stimulated FLS-macrophage cocultures exhibited higher RANKL production and enhanced osteoclast differentiation from PBMCs. Importantly, the RANKL inhibitor denosumab abolished these effects, confirming RANKL's critical role.

In the CIA mouse model, IL-34 blockade with a neutralizing antibody attenuated RA severity, synovial hyperplasia, and bone erosion, demonstrating a direct therapeutic impact. These findings highlight IL-34 as a key mediator in the FLS-macrophage axis driving RA pathogenesis.