Introduction Osteoarthritis (OA) is characterised by cartilage breakdown and ectopic bone formation in joints. Recent studies have shown that low grade synovial inflammation, reflected by inflammatory factors like interleukin-1 beta (IL-1β), contributes to joint pathology. Recently we found that adipose derived mesenchymal stem cells (ASCs) exhibit anti-inflammatory characteristics¹ and reduce joint pathology after local application into mouse knee joints with experimental OA.² This anti-inflammatory effect is only observed after intra-articular injection in early but not late phase OA, suggesting that the effect may be mediated by pro-inflammatory mediators.¹

Objective To study the effect of IL-1β on the anti-inflammatory potency of ASCs in early OA.

Methods Experimental OA was induced by injection of collagenase into murine knee joints (CIOA). Total knee joints were stained with haematoxylin/eosin and the PMN specific antibody NIMPR14. ASCs were isolated from adipose tissue and stimulated with IL-1β or interferon-gamma (IFN-γ). Gene expression in synovium and stimulated cells were analysed using qPCR. Protein levels of chemokines and cytokines were measured in the supernatant and washouts using Luminex. ASCs were co-cultured with MACS isolated bone marrow (BM-) PMNs and analysed using histology, qPCR and Luminex.

Results Injection of ASCs into day 7 CIOA knee joints (when synovitis is highest) caused a strong influx of immune cells into the joint cavity shortly after injection (6 h), which had largely disappeared after 24 h. Immunohistochemistry revealed that particularly PMN-like cells were attracted. Synovial gene expression of neutrophil attracting chemokines KC, CXCL5, and CXCL7 was increased. In line with this, IL-1β stimulated ASCs injected in naive knee joints also resulted in massive influx of PMN-like cells.

IL-1β and IFN-γ (as a positive control) stimulation of ASCs in vitro strongly enhanced gene expression of KC, CXCL5, and CXCL7 and protein levels of KC. Finally, we co-cultured ASCs with BM-PMNs in the presence of IL-1β or IFNγ. After 3 h, a clear clustering of neutrophils around ASCs was observed which significantly decreased protein levels of KC (-69% after 24 h; -76% after 48 h).

Conclusion ASCs attract PMN-like cells when injected locally into a day 7 CIOA knee-joint expressing low levels of IL-1β. In vitro, IL-1β stimulated ASCs show an increase in chemokine expression, leading to attraction and clustering with neutrophils and significantly decreased levels of pro-inflammatory factors like KC. The anti-inflammatory effect of locally applied ASCs into OA joints showing synovitis may be triggered by IL-1β and attraction of PMN-like cells.

This research was supported by the Dutch Arthritis Association.

References

1. ter Huurne M, Schelbergen R, Blattes R, et al . Arthritis Rheum 2012; 64 :3604–13.

2. Schelbergen RF, van Dalen S, ter Huurne M, et al . Osteoarthritis Cartilage 2014; 22 :1158–66.