Macrophages are the primary effector cells in IL-7-induced arthritis

Synovial macrophages are crucial in the development of joint inflammation and bone damage;however, the pathways that controlmacrophage remodeling in inflammatory M1 cells or bone-eroding osteoclasts are not fully understood. We determined thatelevated IL-7R/CD127 expression is the hallmark of rheumatoid arthritis (RA) M1 macrophages and that these cells are highlyresponsive to interleukin-7 (IL-7)-driven osteoclastogenesis. We established that lipopolysaccharide (LPS), interferon-γ (IFNγ), andtumor necrosis factor-α (TNFα), the classic M1 macrophage mediators, enhance IL-7R expression in RA and murine macrophages.The local expression of IL-7 provokes arthritis, predominantly through escalating the number of F480^(+)iNOS^(+) cells rather than CD3^(+)T cells. Ectopic LPS injection stabilizes IL-7-induced arthritis by increasing myeloid IL-7R expression, in part via IFNγ induction.Hence, in RAG−/− mice, IL-7-mediated arthritis is suppressed because of the reduction in myeloid IL-7R expression due to the lackof IFNγ. Moreover, the amelioration of IL-7-induced arthritis by anti-TNF therapy is due to a decrease in the number of cells in theunique F480^(+)iNOS^(+)IL-7R^(+)CCL5^(+) subset, with no impact on the F480^(+)Arginase^(+) cell or CD3^(+) T cell frequency. Consistent with thepreclinical findings, the findings of a phase 4 study performed with RA patients following 6 months of anti-TNF therapy revealedthat IL-7R expression was reduced without affecting the levels of IL-7. This study shifts the paradigm by discovering that IL-7-induced arthritis is dependent on F480^(+)iNOS^(+)IL-7R^(+)CCL5^(+) cell function, which activates TH-1 cells to amplify myeloid IL-7Rexpression and disease severity.