The abundant synovial expression of the RANK/RANKL/osteoprotegerin system in peripheral spondylarthritis is partially disconnected from inflammation

Objective. Spondylarthritis (SpA) and rheumatoid arthritis (RA) have different patterns of bone damage, with more pronounced bone erosions in RA. The RANK/RANKL/osteoprotegerin (OPG) system plays a central role in bone resorption by promoting the maturation and activation of osteoclasts. To assess the potential role of this system in the distinct bone phenotype, we studied the synovial expression of these mediators in SpA and RA peripheral synovitis. Methods. Synovial biopsy specimens were obtained from the actively inflamed peripheral joints of 35 patients with SpA and 19 patients with RA. Paired synovial biopsy samples were obtained from 24 patients with SpA after tumor necrosis factor alpha (TNF alpha) blockade. Synovial tissue sections were immunostained for RANKL, OPG, RANK, and TRAP and assessed by semiquantitative scoring and digital image analysis. Results. After extensive validation of the reactivity and specificity of the antibodies, we demonstrated the abundant expression of RANKL and OPG in SpA synovitis. RANKL was expressed by both fibroblast-like synoviocytes and sublining T lymphocytes. RANK-positive osteoclast precursors but no mature TRAP-positive osteoclasts were present in the inflamed tissue. The expression of these mediators was not different between patients with nonpsoriatic SpA, patients with psoriatic SpA, and patients with RA, was not related to the degree of systemic or local inflammation, and was not significantly modulated by highly effective treatment with TNF alpha blockers. Only the subset of patients with the best systemic response to TNF alpha blockade had decreased RANKL expression in the intimal lining layer. Conclusion. The relative protection against bone erosions in SpA cannot be explained by qualitative or quantitative differences in the synovial expression of RANKL, OPG, and RANK. The abundant expression of these factors in SpA peripheral synovitis is largely disconnected from systemic and local inflammation.