The interleukin-20 receptor axis in early rheumatoid arthritis: novel links between disease-associated autoantibodies and radiographic progression

BACKGROUND: Rheumatoid arthritis (RA) is often characterized by the presence of rheumatoid factor, anti-citrullinated protein antibodies, and bone erosions. Current therapies can compromise immunity, leading to risk of infection. The interleukin-20 receptor (IL-20R) axis comprising IL-19, IL-20, and IL-24 and their shared receptors activates tissue homeostasis processes but not the immune system. Consequently, modulation of the IL-20R axis may not lead to immunosuppression, making it an interesting drug target. We evaluated the role of the IL-20R axis in RA and associations between plasma cytokine levels and clinical disease.

METHODS: Plasma IL-19, IL-20, and IL-24 levels were measured in early RA patients during a treat-to-target strategy by enzyme-linked immunosorbent assays. The IL-20R1 and IL-22R1 levels in paired peripheral blood mononuclear cells and synovial fluid mononuclear cells from a different cohort of RA patients were evaluated by flow cytometry and confocal microscopy. Monocytes/macrophages were stimulated with heat-aggregated human immunoglobulin immune complexes and immune complexes containing citrullinated fibrinogen, and osteoclasts were incubated with IL-19, IL-20, and IL-24.

RESULTS: The plasma concentrations of IL-20 and IL-24 (but not IL-19) were increased in early RA patients compared with healthy controls (both P < 0.002) and decreased after 6 months of treatment (both P < 0.0001). The expression of IL-22R1 (but not IL-20R1) was increased on monocytes from RA synovial fluid compared with monocytes from both RA and healthy control peripheral blood. The plasma concentrations of IL-20 and IL-24 were increased in rheumatoid factor and anti-citrullinated protein antibody positive compared with negative early RA patients (all P < 0.0001). Immune complexes stimulated the production of the IL-20R cytokines by monocytes/macrophages. Increased baseline plasma concentrations of IL-20 and IL-24 were associated with Sharp-van der Heijde score progression after 24 months (Spearman's rho = 0.19 and 0.26, both P < 0.05) in the early RA patients. The IL-22R1 was expressed by osteoclast precursors and in multinucleated osteoclasts. IL-20 and IL-24 increased the secretion of monocyte chemoattractant protein 1 by these cells.

CONCLUSIONS: This study suggests that IL-20 and IL-24 link RA-associated autoantibodies with radiographic progression via the IL-22R1. Modulation of this axis holds promise as feasible anti-erosive treatment modalities in seropositive RA.