Background Pathological bone formation associated with spondyloarthropathies (SpA) is a major cause of structural tissue damage causing permanent disability. A paucity of in vitro models that faithfully replicate human skeletal biology has impeded research into the cellular and molecular triggers for this osteoimmunological phenomenon. Nevertheless, clinical and animal studies have defined IL-17 signalling as a key regulator of SpA disease; however, the role of IL-17 in bone pathology is unclear. IL-17-producing γδ-T cells have a critical function in periosteal bone formation for fracture repair1. The periosteum has also been implicated in pathological bone formation during SpA disease progression2.
Objectives To investigate IL-17 signalling in the context of pathological bone formation using a biomimetic human periosteum derived stem cell (hPDSC) model of osteogenic differentiation.
Methods hPDSCs were obtained through enzymatic digestion of periosteal biopsies from patients undergoing orthopaedic surgery. Expanded cultures were then treated with recombinant human IL-17A, IL-17F, or both over 96h. The expression of gene markers was then evaluated. Alternatively, hPDSCs were stimulated using a biomimetic protocol in combination with IL-17A and IL-17F, or human T-cell supernatants (SNs) (as a surrogate disease-like inflammatory milieu). Antibodies with strong-affinity to IL-17A, IL-17F, or bimekizumab (a humanised monoclonal IgG1 antibody with strong affinity for both IL-17A and IL-17F) were used to define the role of these cytokines in the SNs. Expression of osteogenic markers and matrix mineralisation was assessed to define in vitro bone formation.
Results Under basal conditions IL-17A and IL-17F significantly up-regulated IL-6 expression. Additionally, IL-17A and IL-17F transiently enhanced the expression of the osteogenic transcription factor RUNX-2. When IL-17 cytokines were combined in a biomimetic differentiation protocol, both IL-17A and IL-17F promoted osteogenic differentiation. Importantly, IL-17F enhanced the expression of most osteogenic markers to a greater extent than IL-17A alone following 9 days' exposure. Conversely, IL-17A treatment resulted in elevated in vitro mineralisation vs IL-17F. Th17 and γδ-T cell SNs potently enhanced hPDSC osteogenic differentiation and mineralisation. Whilst IL-6 expression and in vitro bone formation were blocked by neutralisation of IL-17A or IL-17F, dual neutralisation of IL-17A and IL-17F in the inflammatory milieu exhibited the greatest effect on most of the tested parameters.
Conclusions These data show that both IL-17A and IL-17F enhance in vitro osteogenic differentiation and bone formation from hPDSCs. The source of these cytokines has not been established but is likely to involve entheseal resident γδ-T cells. We propose that following their release, IL-17A and IL-17F drive pathological bone formation resulting in enthesophytes at the enthesis/periosteum interface. Current therapeutics display limited efficacy in blocking enthesophyte formation, hence inhibition of both IL-17A and IL-17F offers an attractive therapeutic strategy to prevent this debilitating feature of SpA.
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Disclosure of Interest M. Shah Grant/research support from: UCB Pharma, Employee of: UCB Pharma, A. Maroof Employee of: UCB Pharma, R. Al-Hosni: None declared, P. Gikas: None declared, N. Gozzard Employee of: UCB Pharma, S. Shaw Employee of: UCB Pharma, S. Roberts Employee of: UCB Pharma