Background/Purpose: IL-15, known for its effects on survival of lymphocyte subsets, plays a role in synovitis of Rheumatoid Arthritis.  We reported that IL-15 is also detectable in synovial fluid (SF) of patients with knee osteoarthritis (OA), and correlates with levels of matrix metalloproteinase (MMP)-1 and MMP-3.  Serum levels have been associated with development and progression of radiographic OA changes, suggesting a possible role in cartilage loss.   We investigated IL-15 and IL-15 receptor staining in synovium and cartilage, and tested whether cartilage can respond directly to IL-15. 

Methods: Synovial membrane (SM) and cartilage samples from five organ donors collected within 24 hours of death were formalin-fixed and paraffin embedded.   Additional SM biopsies from four patients with early-stage knee OA were also collected.  Six-micron sections were stained for IL-15 and IL-15Rα using immunoperoxidase technique and polyclonal antibodies directed against IL-15 (rabbit anti-human IL-15, Abcam) or the IL-15Rα chain (goat anti-human IL-15 Rα, Santa Cruz Biotechnology).  Non-immune rabbit IgG and goat IgG were used as negative controls.  For explants culture, 4mm cartilage punch biopsies were prepared from the tibial surfaces of an additional organ donor; 2 explants per well were placed in a 24 well plate with DMEM (+100U/ml Penicillin-Streptomycin).  After 24 hours, media was replaced with 1 ml of fresh media or media+ rhuIL-15 (100ng/ml, Peprotech, NJ) or TNFα and Oncostatin M (100ng/ml each, R & D Systems, MN ).   Every two days for 12 days and again at 15 days, culture supernatants were collected and replaced with fresh media +/- cytokines.   Consecutive supernatants were analyzed for MMP-1, -3 and -9 content using a human MMP 3-Plex Immunoassay (Meso Scale Discovery).

Results: IL-15 and IL-15Rα staining was observed in SM mononuclear infiltrates in the OA patients.   Il-15 Rα staining was also identified in the synovial lining and sublining vessels in both patients and donors.  Positive staining for IL-15 and IL-15Rα was observed in chondrocytes from all layers (superficial to deep).  In vitro, IL-15 increased MMP-1 and MMP-3 release from cartilage explants compared to media controls at each time point up to 15 days.  Maximum MMP-1 release reached 89.5+/- 5.1 ng/ml at day 15 (vs. media control 0.002 +/- 0.003 ng/ml, p<0.05) while MMP-3 release reached 665.7 +/-13 ng/ml by day 12 (vs. media 12.2+/- 0.2 ng/ml, p < 0.001).  Lesser magnitude MMP-9 release from cartilage was observed in response to IL-15, with peak concentration of 962.7 +/- 1.8 pg/ml (p<0.05 vs.control) at day 15. TNFa and OSM induced substantial MMP-1, -3 and -9 release from explants, often >10 times higher than IL-15 induced levels. 

Conclusion: Both IL-15 and IL-15Rα are expressed in SM and cartilage indicating that both tissues may respond to this cytokine in vivo.  IL-15 induced  MMP-1, -3, and -9 release from cartilage explants compared to control but of lesser magnitude than levels induced by TNFa + OSM.   This is the first demonstration that cartilage is responsive to IL-15, and suggests a role for IL-15 in cartilage catabolism in various forms of arthritis including OA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/articular-cartilage-expresses-the-il-15-receptor-alpha-chain-and-responds-to-il-15-with-increased-matrix-metalloproteinase-release/