ICOS is essential for the development of experimental autoimmune myasthenia gravis Benjamin G. Scott a , Huan Yang a , Erdem Tu ¨zu ¨n a , Chen Dong b , Richard A. Flavell c , Premkumar Christadoss a, * a Department of Microbiology and Immunology, The University of Texas Medical Branch, 301 University Boulevard, 3.142 MRB, Galveston, TX 77555-1070, USA b Department of Immunology, University of Washington School of Medicine, Seattle, WA 98195-7650, USA c Section of Immunobiology, Howard Hughes Medical Institute, Yale University School of Medicine, New Haven, CT 06520-8011, USA Received 12 March 2004; accepted 19 April 2004 Abstract Lymphocyte costimulation via the inducible costimulatory molecule (ICOS) is required for effective humoral immunity development. Following immunization with Torpedo acetylcholine receptor (AChR), ICOS gene knockout (KO) mice were highly resistant to clinical experimental autoimmune myasthenia gravis (EAMG) development, had less serum AChR-specific immunoglobulins (Igs), and exhibited a diminutive germinal center (GC) reaction in secondary lymphoid tissues. Lymphocyte proliferation and both Th1 and Th2 differentiation in response to AChR and the AChR dominant a146– 162 peptide were inhibited by the ICOS gene deficiency. ICOS-mediated lymphocyte costimulation is thus vital to the induction of T cell-mediated humoral immunity to AChR and the development of clinical EAMG. D 2004 Elsevier B.V. All rights reserved. Keywords: Experimental autoimmune myasthenia gravis; Myasthenia gravis; ICOS; B7RP-1; Costimulation; Autoimmunity 1. Introduction Myasthenia gravis (MG) is an autoimmune disorder characterized by muscle weakness and fatigue resulting from defective neuromuscular transmission. In the majority of MG patients, autoantibodies bind to the nicotinic acetylcholine receptors (AChRs) of the neuromuscular junction (NMJ) and activate the complement cascade, ultimately resulting in a net reduction of neuromuscular AChR (Patrick and Lindstrom, 1973; Drachman, 1994; Engel et al., 1977; Tsujihata et al., 1989; Tuzun et al., 2003) . Experimental autoimmune myasthenia gravis (EAMG) accurately models the pathogenesis of MG and is induced in the C57BL/6 strain of mice following two immunizations with AChR emulsified in CFA (Christadoss et al., 2000). AChR peptides are presented to CD4 + T helper cells by antigen-presenting cells (APC) in an MHC class II-restricted manner. Mice deficient in MHC class II + APC and CD4 + T helper cells are resistant to EAMG development and display defective AChR-specific lympho- cyte proliferation and antibody (Ab) production (Christa- doss and Dauphinee, 1986; Kaul et al., 1994a,b; Zhang et al., 1996). Optimal helper T cell activation requires the integration of at least two key signaling events (Salomon and Bluestone, 2001). Signal one results from antigen-specific TCR engagement by MHC class II-bound peptide pre- sented by APC. Signal two results from the engagement of costimulatory molecules expressed by the APC and the T cell. CD28 is the most extensively investigated costi- mulatory molecule expressed by T cells, and it binds the ligands CD80 (B7-1) and CD86 (B7-2) expressed by APC. The use of CD28 knockout (KO) and CD40L KO mice has demonstrated a differential requirement for the CD28-B7 and CD40L-CD40 costimulatory pathways in EAMG (Shi et al., 1998). B7-1 KO and B7-1/B7-2 double-KO mice were resistant to clinical EAMG develop- ment and produced less serum AChR-specific immuno- 0165-5728/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jneuroim.2004.04.019 Abbreviations: APC, antigen presenting cell; Ab, antibody; AChR, acetylcholine receptor; BTX, a-bungarotoxin; EAMG, experimental autoimmune myasthenia gravis; Ig, immunoglobulin; MG, myasthenia gravis; MAC, membrane attack complex; NMJ, neuromuscular junction. * Corresponding author. Tel.: +1-409-772-5857; fax: +1-409-747- 6869. E-mail address: pchrista@utmb.edu (P. Christadoss). www.elsevier.com/locate/jneuroim Journal of Neuroimmunology 153 (2004) 16 – 25