Lymphocytes and synovial fluid fibroblasts support osteoclastogenesis through RANKL, TNFα, and IL‐7 in an in vitro model derived from human psoriatic arthritis

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Abstract
Psoriatic arthritis (PsA) is an inflammatory joint disease, characterized by extensive bone resorption, whose mechanisms have not been fully elucidated. Thus, in the present study we investigated the involvement of RANKL, TNFα, and IL-7 in the osteoclastogenesis of PsA patients. In vitro osteoclastogenesis models, consisting of unfractionated and T-cell-depleted mononuclear cells from peripheral blood (PBMCs) and synovial fluid (SFMCs) of 20 PsA patients as well as from healthy donors were studied. Freshly isolated T and B cells from PBMCs and T cells and fibroblasts from SFMCs of PsA patients were subjected to RT-PCR to detect the levels of RANKL, TNFα, and IL-7. Osteoclastogenesis was studied in the presence of RANK-Fc, anti-TNFα, and anti IL-7 functional antibodies. We demonstrate that lymphocytes and fibroblasts support osteoclast (OC) formation in PsA patients through the production of osteoclastogenic cytokines. In particular, OC formation was completely abolished in unstimulated T cell-depleted PBMC cultures, and reduced by approximately 70% in unstimulated T cell-depleted SFMC cultures. Freshly isolated T cells from PBMCs and SFMCs of PsA patients overexpressed RANKL and TNFα, while fibroblasts from synovial fluid produced only RANKL. We show that the presence of RANK-Fc and/or anti-TNFα functional antibodies reduced OC formation. Moreover, T and B cells from PBMCs as well as T cells and fibroblasts from SFMCs expressed IL-7 mRNA. Finally, the anti-IL-7 functional antibody significantly reduced osteoclastogenesis. Our results suggest that fibroblasts, B and T lymphocytes support OC formation by producing RANKL, TNFα, and IL-7, contributing to the aggressive bone resorption in PsA patients. Copyright © 2007 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.