ARTHRITIS & RHEUMATISM Vol. 63, No. 2, February 2011, pp 422–433 DOI 10.1002/art.30147 © 2011, American College of Rheumatology Octacalcium Phosphate Crystals Induce Inflammation In Vivo Through Interleukin-1 but Independent of the NLRP3 Inflammasome in Mice Sharmal Narayan, 1 Borbola Pazar, 1 Hang-Korng Ea, 2 Laeticia Kolly, 1 Nathaliane Bagnoud, 1 Ve ´ronique Chobaz, 1 Fre ´de ´ric Liote ´, 2 Thomas Vogl, 3 Dirk Holzinger, 3 Alexander Kai-Lik So, 1 and Nathalie Busso 1 Objective. To determine the mechanisms involved in inflammatory responses to octacalcium phosphate (OCP) crystals in vivo. Methods. OCP crystal–induced inflammation was monitored using a peritoneal model of inflammation in mice with different deficiencies affecting interleukin-1 (IL-1) secretion (IL-1 –/– , IL-1 –/– , ASC –/– , and NLRP3 –/– mice) or in mice pretreated with IL-1 inhibi- tors (anakinra [recombinant IL-1 receptor antagonist] and anti–IL-1). The production of IL-1, IL-1, and myeloid-related protein 8 (MRP-8)–MRP-14 complex was determined by enzyme-linked immunosorbent as- say. Peritoneal neutrophil recruitment and cell viability were determined by flow cytometry. Depletion of mast cells or resident macrophages was performed by pre- treatment with compound 48/80 or clodronate lipo- somes, respectively. Results. OCP crystals induced peritoneal inflam- mation, as demonstrated by neutrophil recruitment and up-modulation of IL-1, IL-1, and MRP-8–MRP-14 complex, to levels comparable with those induced by monosodium urate monohydrate crystals. This OCP crystal–induced inflammation was both IL-1– and IL-1–dependent, as shown by the inhibitory effects of anakinra and anti–IL-1 antibody treatment. Accord- ingly, OCP crystal stimulation resulted in milder in- flammation in IL-1 –/– and IL-1 –/– mice. Interestingly, ASC –/– and NLRP3 –/– mice did not show any alteration in their inflammation status in response to OCP crys- tals. Depletion of the resident macrophage population resulted in a significant decrease in crystal-induced neutrophil infiltration and proinflammatory cytokine production in vivo, whereas mast cell depletion had no effect. Finally, OCP crystals induced apoptosis/necrosis of peritoneal cells in vivo. Conclusion. These data indicate that macro- phages, rather than mast cells, are important for initi- ating and driving OCP crystal–induced inflammation. Additionally, OCP crystals induce IL-1–dependent peri- toneal inflammation without requiring the NLRP3 in- flammasome. Basic calcium phosphate (BCP) crystals including hydroxyapatite, carbonated apatite, tricalcium phos- phate, and octacalcium phosphate (OCP) have long been associated with rheumatic syndromes. BCP crystal deposition occurs most frequently in soft tissue, muscle, and articular sites and can manifest with acute inflam- mation and tissue degradation. Indeed, the presence of BCP crystals in synovial fluid is more common in Supported by the Fonds National Suisse de la Recherche Scientifique (grant 310030-130085/1) and the Jean and Linette Warn- ery Foundation. Drs. Ea and Liote ´’s work was supported by grants from the Fondation pour la Recherche Me ´dicale, the Association pour la Recherche en Pathologie Synoviale, and the Association Rhuma- tisme et Travail. Dr. Holzinger’s work was supported by a grant from the German Ministry of Education and Research (BMBF project AID-NET). 1 Sharmal Narayan, PhD, Borbola Pazar, MD, PhD, Laeticia Kolly, PhD, Nathaliane Bagnoud, MSc, Ve ´ronique Chobaz, Alexander Kai-Lik So, PhD, FRCP, Nathalie Busso, PhD: Centre Hospitalier Universitaire Vaudois and University of Lausanne, Lausanne, Swit- zerland; 2 Hang-Korng Ea, MD, PhD, Fre ´de ´ric Liote ´, MD, PhD: INSERM UMR-S606, Ho ˆpital Lariboisie `re, Assistance Publique- Ho ˆpitaux de Paris, and Universite ´ Paris Denis Diderot, Paris, France; 3 Thomas Vogl, PhD, Dirk Holzinger, MD: University of Mu ¨nster, Mu ¨nster, Germany. Drs. Narayan and Pazar contributed equally to this work. Address correspondence to Nathalie Busso, PhD, Division of Rheumatology, DAL, Laboratory of Rheumatology, CHUV, Nestle ´ 05-5029, 1011 Lausanne, Switzerland. E-mail: Nathalie.Busso@ chuv.ch. Submitted for publication May 3, 2010; accepted in revised form November 4, 2010. 422