LAUSR

Introduction Adipose-derived mesenchymal stromal cells (ASCs) exhibit anti-inflammatory characteristics and reduce development of joint pathology after injection into murine experimental inflammatory osteoarthritis (CiOA) joints.1,2 This protection is only achieved when ASCs are applied in early CiOA. This early, but not the late phase of CiOA, is characterised by strongly elevated levels of S100A8/A9 and interleukin-1 beta (IL-1β),3 suggesting that the inflammatory environment mediates the protective effect of ASCs. Objectives To examine the mechanism behind ASC-mediated amelioration of CiOA pathology. Methods CiOA was induced by intra-articular collagenase injection. Knee joint sections were stained with haematoxylin/eosin, the PMN-specific antibody NIMP-R14, or CD271 to locate ASCs. Gene expression and protein release of chemokines by IL-1β- or S100A8/A9-stimulated ASCs were assessed with qPCR and Luminex. Migration of MACS-isolated PMNs towards ASC-conditioned medium through Transwell membranes was examined using flow cytometry. ASC-PMN co-cultures were analysed with microscopy and Luminex. Phagocytic capacity of PMNs was measured with fluorescent labelled zymosan particles. Results Intra-articular ASC injection on day 7 of CiOA (when IL-1β and S100A8/A9 levels are highest) strongly attracted particularly PMNs, which clustered around ASCs in the synovium 6 hour after injection. IL-1β-stimulation of ASCs in vitro strongly increased protein expression of PMN-attracting chemokines KC, CXCL5, and CXCL7, whereas S100A8/A9 did not. Migration of PMNs towards conditioned medium of IL-1β-stimulated ASCs (IL-1β-CM) was significantly enhanced (two-fold increase) when compared to CM of non-stimulated ASCs (NS-CM). After 6 hour co-culturing ASCs with PMNs, both the number of ASCs clustering with PMNs and clustered PMNs per ASC were significantly increased after IL-1β-stimulation. Interestingly, association of PMNs with ASCs significantly diminished the release of KC protein by ASCs (69% lower after 24 hour), and also strongly reduced the release of S100A8/A9 protein by the PMNs. Finally, phagocytosis of zymosan by PMNs was strongly enhanced after priming with IL-1β-CM. Conclusions Local application of ASCs in inflamed CiOA joints results in attraction and clustering of PMNs with ASCs in the synovium, which is likely mediated by IL-1β-induced up-regulation of chemokine release by ASCs. This results in lowered levels of pro-inflammatory S100A8/A9 and enhanced phagocytic capacity of PMNs, enabling the clearance of debris to cease synovitis and promote tissue repair. References . Ter Huurne M, et al. Arthritis Rheum2012November;64(11):3604–13. . Schelbergen RF, et al. Osteoarthritis Cartilage2014August;22(8):1158–66. . Schelbergen RF, et al. Ann Rheum Dis2016January;75(1):218–25. Acknowledgements This research was supported by ADIPOA2. Disclosure of interest None declared