ARTHRITIS & RHEUMATISM Vol. 60, No. 9, September 2009, pp 2782–2793 DOI 10.1002/art.24750 © 2009, American College of Rheumatology Defective Phosphorylation of Interleukin-18 Receptor  Causes Impaired Natural Killer Cell Function in Systemic-Onset Juvenile Idiopathic Arthritis Wilco de Jager, Sebastiaan J. Vastert, Jeffrey M. Beekman, Nico M. Wulffraat, Wietse Kuis, Paul J. Coffer, and Berent J. Prakken Objective. Systemic-onset juvenile idiopathic ar- thritis (JIA) is an autoimmune disease characterized by arthritis and systemic features. Its pathogenesis is still largely unknown. It is characterized immunologically by natural killer (NK) cell dysfunction and cytokine signa- tures that predominantly feature interleukin-1 (IL-1), IL-6, and IL-18. Since IL-18 can drive NK cell function, we examined how the high plasma levels of this cytokine are related to the documented NK cell failure in these patients. Methods. The phenotype and function of NK cells from 10 healthy control subjects, 15 patients with poly- articular JIA, and 15 patients with systemic-onset JIA were characterized by staining and functional assays in vitro. IL-18 ligand binding was visualized by fluores- cence microscopy. Phosphorylation of several MAP ki- nases and the IL-18 receptor  (IL-18R) were visual- ized by Western blotting. Results. IL-18 from the plasma of systemic-onset JIA patients stimulated the activation of NK cells from healthy controls and bound its cognate receptor. How- ever, NK cells from systemic-onset JIA patients failed to up-regulate cell-mediated killing molecules, such as perforin and interferon-, after IL-18 stimulation. Fur- thermore, treatment with IL-18 did not induce the phosphorylation of receptor-activated MAP kinases in NK cells. Alternate activation of NK cells by IL-12 induced NK cell cytotoxicity. We observed no additive effect of IL-18 in combination with IL-12 in systemic- onset JIA patients. Immunoprecipitation of IL-18R showed that NK cells from systemic-onset JIA could not phosphorylate this receptor after IL-18 stimulation. Conclusion. The mechanism of the impaired NK cell function in systemic-onset JIA involves a defect in IL-18R phosphorylation. This observation has major implications for the understanding and, ultimately, the treatment of systemic-onset JIA. Systemic-onset juvenile idiopathic arthritis (JIA), an autoimmune disease characterized by arthritis and systemic features, such as spiking fever, skin rash, gen- eralized lymphadenopathy, hepatosplenomegaly, and serositis (1), was first described by the British pediatri- cian George F. Still in 1897 (2). The pathogenesis of systemic-onset JIA differs from that of other types of JIA in several respects, such as the lack of association with HLA type (3) and the absence of autoantibodies or autoreactive T cells (1). In fact, systemic-onset JIA has similarities to autoinflammatory diseases, as exemplified by a central role of the innate immune system and by the cytokines involved (e.g., interleukin-1 [IL-1], IL-6, and IL-18) (4). These cytokines are thought to be responsi- ble for at least part of the clinical symptoms of the disease. Moreover, the blocking of both IL-1 and IL-6 has been shown to be efficacious in the treatment of systemic-onset JIA (5–7). Besides the unrestrained production of cytokines, Drs. de Jager, Wulffraat, and Kuis’ work was supported by the Dutch Arthritis Foundation (Reumafonds). Drs. Vastert and Beek- man’s work was supported by the Dutch Scientific Organization (a VIDI innovational research grant and grant NWO917.36.316, respec- tively). Dr. Prakken’s work was supported by the Dutch Arthritis Foundation (Reumafonds) and by the Dutch Scientific Organization (VIDI innovational research grant). Wilco de Jager, PhD, Sebastiaan J. Vastert, MD, Jeffrey M. Beekman, PhD, Nico M. Wulffraat, MD, PhD, Wietse Kuis, MD, PhD, Paul J. Coffer, PhD, Berent J. Prakken, MD, PhD: Centre for Cellular and Molecular Immunology, University Medical Centre Utrecht, Utrecht, The Netherlands. Address correspondence and reprint requests to Wilco de Jager, PhD, Department of Pediatrics, Centre for Cellular and Molec- ular Immunology (KC01.069.0), University Medical Centre Utrecht, PO Box 85500, 3508 GA Utrecht, The Netherlands. E-mail: wjager@umcutrecht.nl. Submitted for publication January 29, 2009; accepted in revised form May 17, 2009. 2782