Objective. T helper 17 (Th17) and mast cells produce IL-17A in RA and critically contribute to the pathogenesis of RA. However, the complete IL-17 cytokine profile in RA is unknown. The aim of the study was to systematically study the expression of IL-17 family cytokines in RA. Methods. The expression of all IL-17 cytokines in RA synovium and pannus as well as in the synovium of OA was determined using quantitative RT-PCR (qRT-PCR). IL-17A and IL-17B were immunostained. Peripheral blood

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... ils were analysed for IL-17B. The effect of IL-17B alone or in combination with TNF-a was tested in vitro on fibroblasts and endothelial cells. Results. In all tissues IL-17B was the most expressed IL-17 family cytokine, found in lining but most strongly expressed in human neutrophil elastase containing polymorphonuclear cells. This pattern was distinct from that of IL-17A, which was found in mast cell tryptase immunoreactive cells. Circulating neutrophils contained IL-17B, verifying the in vivo results. Fibroblasts up-regulated the expression of IL-17RB, a putative receptor of IL-17B, after TNF-a stimulation. IL-17B significantly enhanced TNF-a-induced production of G-CSF and IL-6 in fibroblasts. Conclusion. IL-17B, which is present in synovium, may contribute to the pathogenesis of RA. IL-17B can enhance the effects of TNF-a on the production of cytokines and chemokines that control immune cell trafficking and neutrophil homeostasis in the inflamed tissues.