Rheumatoid arthritis (RA) is an autoimmune and inflammatory disease in which many cytokines have been implicated. In particular, IL-15 is a cytokine involved in the inflammatory processes and bone loss. The aim of this study was to investigate the existence in synovial fluid of soluble IL-15Rα, a private receptor subunit for IL-15 which may act as an enhancer of IL-15-induced proinflammatory cytokines. Soluble IL-15Rα was quantified by a newly developed enzyme-linked immunosorbent assay (ELISA) in samples of synovial fluid from patients with RA and osteoarthritis (OA). The levels of IL-15Rα were significantly increased in RA patients compared to OA patients. Also, we studied the presence of membrane-bound IL-15 in cells from synovial fluids, another element necessary to induce pro-inflammatory cytokines through reverse signaling. Interestingly, we found high levels of IL-6 related to high levels of IL-15Rα in RA but not in OA. Thus, our results evidenced presence of IL-15Rα in synovial fluids and suggested that its pro-inflammatory effect could be related to induction of IL-6. 1. Introduction Rheumatoid arthritis (RA) is a chronic autoimmune disease in which imbalances in pro- and anti-inflammatory cytokines promote induction of autoimmunity, inflammation and joint destruction . IL-15 is a proinflammatory cytokine associated with several autoimmune diseases, particularly rheumatoid arthritis. [2, 3]. Three different functional forms of IL-15 have been identified: the soluble cytokine , IL-15R-independent membrane-bound IL-15 [5, 6] and membrane-IL-15 anchored through IL-15Rα . IL-15Rα is a unique high affinity private α chain that together with the IL-2 receptor, IL-2Rβ chain and the IL-2Rγ chain subunits constitute a trimeric receptor for IL-15 on cell membranes. Also, IL-15Rα may be secreted as a functional soluble molecule (s-IL-15Rα) and could behave as an agonist by forming a complex with IL-15 which is 100-fold more efficient than the noncomplexed soluble cytokine or as an antagonist for IL-15 [8–10]. In addition, s-IL-15Rα may activate a reverse signaling through interaction with IL-15R-independent membrane-bound IL-15, activating MAPKs (mitogen-activated protein kinases) and increasing production of several proinflammatory cytokines such as IL-6, IL-8, and tumor necrosis factor α [5, 6, 11]. This bidirectional signaling has also been described for most members of TNF ligand family contributing to multiple stages of immune regulation . Soluble IL-15 has been detected in synovia of patients with RA mainly expressed by macrophages,
H. P. Carroll, V. Paunovi？, and M. Gadina, “Signalling, inflammation and arthritis: crossed signals: the role of interleukin-15 and -18 in autoimmunity,” Rheumatology, vol. 47, no. 9, pp. 1269–1277, 2008.
J. D. Burton, R. N. Bamford, C. Peters et al., “A lymphokine, provisionally designated interleukin T and produced by a human adult T-cell leukemia line, stimulates T-cell proliferation and the induction of lymphokine-activated killer cells,” Proceedings of the National Academy of Sciences of the United States of America, vol. 91, no. 11, pp. 4935–4939, 1994.
G. G. Neely, S. Epelman, L. L. Ma et al., “Monocyte surface-bound IL-15 can function as an activating receptor and participate in reverse signaling,” Journal of Immunology, vol. 172, no. 7, pp. 4225–4234, 2004.
E. Mortier, A. Quéméner, P. Vusio et al., “Soluble interleukin-15 receptor α (IL-15Rα)-sushi as a selective and potent agonist of IL-15 action through IL-15Rβ/γ: hyperagonist IL-15·IL-15Rα fusion proteins,” The Journal of Biological Chemistry, vol. 281, no. 3, pp. 1612–1619, 2006.
E. Mortier, J. Bernard, A. Plet, and Y. Jacques, “Natural, proteolytic release of a soluble form of human IL-15 receptor α-chain that behaves as a specific, high affinity IL-15 antagonist,” Journal of Immunology, vol. 173, no. 3, pp. 1681–1688, 2004.
M. P. Rubinstein, M. Kovar, J. F. Purton et al., “Converting IL-15 to a superagonist by binding to soluble IL-15Rα,” Proceedings of the National Academy of Sciences of the United States of America, vol. 103, no. 24, pp. 9166–9171, 2006.
N. J. Chen, M. W. Huang, and S. L. Hsieh, “Enhanced secretion of IFN-γ by activated Th1 cells occurs via reverse signaling through TNF-related activation-induced cytokine,” Journal of Immunology, vol. 166, no. 1, pp. 270–276, 2001.
I. B. McInnes, J. A. Gracie, M. Harnett, W. Harnett, and F. Y. Liew, “New strategies to control inflammatory synovitis: interleukin 15 and beyond,” Annals of the Rheumatic Diseases, vol. 62, no. 2, pp. 51–54, 2003.
N. Oppenheimer-Marks, R. I. Brezinschek, M. Mohamadzadeh, R. Vita, and P. E. Lipsky, “Interleukin 15 is produced by endothelial cells and increases the transendothelial migration of T cells in vitro and in the SCID mouse-human rheumatoid arthritis model in vivo,” Journal of Clinical Investigation, vol. 101, no. 6, pp. 1261–1272, 1998.
I. B. Mcinnes, B. P. Leung, R. D. Sturrock, M. Field, and F. Y. Liew, “Interleukin-15 mediates T cell-dependent regulation of tumor necrosis factor-α production in rheumatoid arthritis,” Nature Medicine, vol. 3, no. 2, pp. 189–195, 1997.
S. Ferretti, O. Bonneau, G. R. Dubois, C. E. Jones, and A. Trifilieff, “Il-17, produced by lymphocytes and neutrophils, is necessary for lipopolysaccharide-induced airway neutrophilia: IL-15 as a possible trigger,” Journal of Immunology, vol. 170, no. 4, pp. 2106–2112, 2003.
W. A. Verri, T. M. Cunha, S. H. Ferreira et al., “IL-15 mediates antigen-induced neutrophil migration by triggering IL-18 production,” European Journal of Immunology, vol. 37, no. 12, pp. 3373–3380, 2007.
M. E. Miranda-Carús, M. Benito-Miguel, A. Balsa et al., “Peripheral blood T lymphocytes from patients with early rheumatoid arthritis express RANKL and interleukin-15 on the cell surface and promote osteoclastogenesis in autologous monocytes,” Arthritis and Rheumatism, vol. 54, no. 4, pp. 1151–1164, 2006.
Y. Ogata, A. Kukita, T. Kukita et al., “A novel role of IL-15 in the development of osteoclasts: inability to replace its activity with IL-2,” Journal of Immunology, vol. 162, no. 5, pp. 2754–2760, 1999.
S. Djaafar, D. D. Pierroz, R. Chicheportiche, X. X. Zheng, S. L. Ferrari, and S. Ferrari-Lacraz, “Inhibition of T cell-dependent and RANKL-dependent osteoclastogenic processes associated with high levels of bone mass in interleukin-15 receptor-deficient mice,” Arthritis and Rheumatism, vol. 62, no. 11, pp. 3300–3310, 2010.
P. Emery, E. Keystone, H. P. Tony et al., “IL-6 receptor inhibition with tocilizumab improves treatment outcomes in patients with rheumatoid arthritis refractory to anti-tumour necrosis factor biologicals: results from a 24-week multicentre randomised placebo-controlled trial,” Annals of the Rheumatic Diseases, vol. 67, no. 11, pp. 1516–1523, 2008.
E. W. Thurkow, I. M. Van Der Heijden, F. C. Breedveld et al., “Increased expression of IL-15 in the synovium of patients with rheumatoid arthritis compared with patients with Yersinia-induced arthritis and osteoarthritis,” Journal of Pathology, vol. 181, no. 4, pp. 444–450, 1997.
T. Musso, L. Calosso, M. Zucca et al., “Human monocytes constitutively express membrane-bound, biologically active, and interferon-γ-upregulated interleukin-15,” Blood, vol. 93, no. 10, pp. 3531–3539, 1999.
H. Kobayashi, S. Dubois, N. Sato et al., “Role of trans-cellular IL-15 presentation in the activation of NK cell-mediated killing, which leads to enhanced tumor immunosurveillance,” Blood, vol. 105, no. 2, pp. 721–727, 2005.
T. Matsumoto, T. Tsurumoto, and H. Shindo, “Interleukin-6 levels in synovial fluids of patients with rheumatoid arthritis correlated with the infiltration of inflammatory cells in synovial membrane,” Rheumatology International, vol. 26, no. 12, pp. 1096–1100, 2006.