ARTHRITIS & RHEUMATISM Vol. 56, No. 7, July 2007, pp 2455–2463 DOI 10.1002/art.22741 © 2007, American College of Rheumatology Pathogenic Effects of Antimyeloperoxidase Antibodies in Patients With Microscopic Polyangiitis Philippe Guilpain, 1 Am´ elie Servettaz, 1 Claire Goulvestre, 1 Sandrine Barrieu, 1 Didier Borderie, 2 Christiane Ch´ ereau, 1 Niloufar Kavian, 1 Christian Pagnoux, 1 Loı ¨c Guillevin, 1 Bernard Weill, 1 Luc Mouthon, 1 and Fr´ ed´ eric Batteux 1 Objective. Microscopic polyangiitis (MPA) is a small-vessel vasculitis associated with antimyeloperoxi- dase (MPO) antibodies in 70% of patients. Anti-MPO antibodies can trigger the release of MPO by neutro- phils and monocytes, but their involvement in the pathogenesis of MPA is still questioned. The aim of this study was to investigate whether anti-MPO antibodies can activate MPO to generate an oxidative stress that is potentially deleterious to the endothelium. Methods. MPA sera, purified IgG from MPA sera, normal control sera, and purified IgG from normal sera were incubated with MPO coated onto microtitration plates. The peroxidase activity of MPO was evaluated by adding o-phenylenediamine. Production of hypochlor- ous acid (HOCl) was determined by chemiluminescence. The cytotoxic properties of byproducts of MPO activa- tion were tested on endothelial cells in culture. Results. MPA sera with anti-MPO antibodies were found to activate MPO in vitro (P < 0.0001 versus normal sera) and to generate HOCl (P < 0.001), as did IgG purified from MPA sera (P < 0.05). MPA sera without anti-MPO antibodies and MPA IgG absorbed on MPO did not show these activities. The byproducts of MPO activation by MPA sera exerted a strong cytolytic activity on endothelial cells in culture (P < 0.01). Both HOCl production and endothelial lysis were abrogated by N-acetylcysteine (NAC), an antioxidant molecule (P < 0.05 and P < 0.0001, respectively). Conclusion. Anti-MPO antibodies could play a pathogenic role in vivo by triggering an oxidative burst, leading to severe endothelial damage. Treatment of MPA patients with NAC might be proposed in an attempt to abrogate these deleterious phenomena. Microscopic polyangiitis (MPA) is a small-vessel vasculitis with frequent cutaneous, pulmonary, renal, and neurologic involvement. Antineutrophil cytoplasmic autoantibodies (ANCAs) are detectable in the sera of 70% of MPA patients and are mainly directed toward myeloperoxidase (MPO) (1,2). MPO is a 118-kd cationic protein that is present in primary azurophilic granules of polymorphonuclear neutrophils (PMNs) and monocytes. MPO uses hydrogen peroxide and chloride ions to generate hypochlorous acid (HOCl), a highly deleterious reactive oxygen species (ROS) that contributes to oxi- dative burst from PMNs in response to microbial infec- tions (3). The pathogenic role of anti-MPO IgG antibodies has recently been suggested by studies of the passive transfer of anti-MPO antibodies into Rag2 –/– mice, which was followed by the development of necrotizing crescentic glomerulonephritis and systemic vasculitis mimicking the disease in humans (4). However, the mechanism of the pathogenicity of anti-MPO antibodies is not completely understood. Some reports have sug- gested that anti-MPO antibodies can activate PMNs and monocytes and lead to the extracellular release of MPO and oxidative burst (5,6). We have hypothesized that, as already shown for proteinase 3 (PR3) (7,8), serum Dr. Guilpain’s work was supported by a grant from the Fondation pour la Recherche Me ´dicale. Dr. Servettaz’s work was supported by a grant from the Laboratoire Franc ¸ais du Fractionne- ment et des Biotechnologies. 1 Philippe Guilpain, MD, Ame ´lie Servettaz, MD, Claire Goul- vestre, PharmD, Sandrine Barrieu, PharmD, Christiane Che ´reau, PharmD, Niloufar Kavian, PharmD, Christian Pagnoux, MD, Loı ¨c Guillevin, MD, Bernard Weill, MD, PhD, Luc Mouthon, MD, PhD, Fre ´de ´ric Batteux, PhD: Universite ´ Paris Descartes, and Ho ˆpital Co- chin, Assistance Publique Ho ˆpitaux de Paris, Paris, France; 2 Didier Borderie, PharmD, PhD: Ho ˆpital Cochin, Assistance Publique Ho ˆpi- taux de Paris, Paris, France. Address correspondence and reprint requests to Fre ´de ´ric Batteux, PhD, Laboratoire d’Immunologie, Pavillon Hardy, Ho ˆpital Cochin, 27 Rue du Faubourg St. Jacques, 75679 Paris Cedex 14, France. E-mail: frederic.batteux@cch.aphp.fr. Submitted for publication July 11, 2006; accepted in revised form April 3, 2007. 2455