THU0060 T cell-derived IL-17A and IL-17F drive bone formation from human periosteal stem cells: implications for enthesophyte formation
Authors: M Shah, A Maroof, R Al-Hosni, P Gikas, N Gozzard, S Shaw, S Roberts
Journal: Annals of the Rheumatic Diseases (2017)

<h3>Background</h3> Pathological bone formation associated with spondyloarthropathies (SpA) is a major cause of structural tissue damage causing permanent disability. A paucity of <i>in vitro</i> 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 repair<sup>1</sup>. The periosteum has also been implicated in pathological bone formation during SpA disease progression<sup>2</sup>. <h3>Objectives</h3> 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. <h3>Methods</h3> 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 <i>in vitro</i> bone formation. <h3>Results</h3> Under basal conditions IL-17A and IL-17F significantly up-regulated <i>IL-6</i> expression. Additionally, IL-17A and IL-17F transiently enhanced the expression of the osteogenic transcription factor <i>RUNX-2</i>. 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 days9 exposure. Conversely, IL-17A treatment resulted in elevated <i>in vitro</i> mineralisation vs IL-17F. Th17 and γδ-T cell SNs potently enhanced hPDSC osteogenic differentiation and mineralisation. Whilst <i>IL-6</i> expression and <i>in vitro</i> 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. <h3>Conclusions</h3> These data show that both IL-17A and IL-17F enhance <i>in vitro</i> 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. <h3>References</h3> Ono, et al. Nat Commun 2016;7:10928. Lories, et al. Arthritis Res Ther 2009;11:221. <h3>Disclosure of Interest</h3> 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