Impact of CD40 Ligand, B Cells, and Mast Cells in Peanut-Induced Anaphylactic Responses 1 Jiangfeng Sun, 2 * Katherine Arias, 2 * David Alvarez,* Ramzi Fattouh,* Tina Walker,* Susanna Goncharova,* Bobae Kim,* Susan Waserman, † Jennifer Reed, ‡ Anthony J. Coyle,* ‡ and Manel Jordana 3 * The effector immune mechanisms underlying peanut-induced anaphylaxis remain to be fully elucidated. We investigated the relative contribution of Igs, mast cells (MCs), and FcRI in the elicitation of anaphylaxis in a murine model. Assessment of peanut hypersensitivity reactions was performed clinically and biologically. Our data show that wild-type (WT; C57BL/6 strain) mice consistently developed severe anaphylaxis (median clinical score: 3.5/5), an 8°C drop in core body temperature, and significantly increased plasma levels of histamine and leukotrienes. CD40 ligand- and B cell-deficient mice presented evidence of allergic sensitization as demonstrated by production of Th2-associated cytokines by splenocytes and a late-phase inflammatory response that were both indistinguishable to those detected in WT mice. However, CD40 ligand- and B cell-deficient mice did not exhibit any evidence of anaphylaxis. Our data also show that MC-deficient (Kit W /Kit W-v ) mice did not suffer, unlike their littermate controls, anaphylactic reactions despite the fact that serum levels of peanut-specific Igs were similarly elevated. Finally, FcRI- deficient mice experienced anaphylactic responses although to a significantly lesser degree than those observed in WT mice. Thus, these data demonstrate that the presence of peanut-specific Abs along with functional MCs comprise a necessary and sufficient condition for the elicitation of peanut-induced anaphylaxis. That the absence of FcRI prevented the development of anaphylaxis only partially insinuates the contribution of an IgE-independent pathway, and suggests that strategies to impair MC degranulation may be necessary to improve the efficacy of anti-IgE therapy. The Journal of Immunology, 2007, 179: 6696 – 6703. A naphylaxis is an acute, life-threatening, allergic reaction. Food hypersensitivity is the leading cause of anaphylac- tic episodes treated in hospital emergency departments in western countries (1–3). Food allergy affects 6–8% of children under 4 years of age and 2% of the U.S. population beyond the first decade of life (4 – 6). It is estimated that allergy to peanut and tree nut accounts for 80% of the fatal and near-fatal food hy- persensitivity reactions (7, 8). Of concern, the prevalence of child- hood peanut allergy (PA) 4 has doubled between 1997 and 2002, and although most food allergies are outgrown, peanut hypersen- sitivity is usually lifelong (9, 10). At this time, there are no proven effective therapies for PA and, unlike other allergic diseases, im- munotherapy strategies are presently not available. The recent development of murine models of peanut-induced ana- phylaxis (PIA) that mimic physiologic and immunologic features of human PA has facilitated both understanding of the underlying mech- anisms and evaluation of potential therapeutic approaches (11–13). An important focus of these models has been on the role of Th2 responses in the regulation of sensitization (14 –16) and, subsequently, therapeutic strategies to redirect immune responses away from Th2 (17, 18). Comparatively, studies investigating effector events that me- diate systemic anaphylaxis have received less attention. We have developed a murine model of PIA and investigated the involvement of key immunological components in the elicitation of peanut hypersensitivity reactions. Our data show that oral sen- sitization to peanut in C57BL/6 mice generated peanut-specific Th1- and Th2-associated Igs and cytokines and provoked Th2- effector responses, namely systemic anaphylactic shock and late- phase allergic responses upon in vivo peanut recall. Examination of secondary lymphoid organs provided evidence of pervasive im- mune activation that was associated with the development of late- phase respiratory and cutaneous responses upon airway and skin re-exposure to peanut, respectively. In addition, our data demon- strate that CD40 ligand (CD40L)-, B cell- or mast cell (MC)-de- ficient mice failed to mount measurable anaphylactic responses, indicating that the presence of peanut-specific Igs along with func- tional MCs is necessary and sufficient for the elicitation of a pea- nut-induced anaphylactic event. Materials and Methods Animals Female C57BL/6 mice (6 – 8 wk old) were purchased from Charles River Laboratories. B cell-deficient (B6.129S2-Igh-6 tm1Cgn /J), CD40L-deficient (B6.129S2-CD40lg tm1Imx /J), MC-deficient (WBB6F1/J-kit W /Kit W-v ), and *Division of Respiratory Diseases and Allergy, Centre for Gene Therapeutics, De- partment of Pathology and Molecular Medicine and † Department of Medicine, Mc- Master University, Hamilton, Ontario, Canada; and ‡ Department of Inflammation and Autoimmunity, MedImmune, Gaithersburg, MD 20878 Received for publication April 27, 2007. Accepted for publication August 30, 2007. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1 This work was supported by the Food Allergy Initiative organization (New York, NY). M.J. holds a Senior Canada Research Chair on the Immunology of Respiratory Disease and Allergy. D.A. and R.F. held Canadian Institutes of Health Research Doctoral Awards. 2 J.S. and K.A. contributed equally to this work. 3 Address correspondence and reprint requests to Dr. Manel Jordana, Department of Pathology and Molecular Medicine and Division of Respiratory Diseases and Allergy, Centre for Gene Therapeutics, Michael G. DeGroote Centre for Learning and Dis- covery, Room 4013, McMaster University, 1200 Main Street West, Hamilton, On- tario, L8N 3Z5, Canada. E-mail address: jordanam@mcmaster.ca 4 Abbreviations used in this paper: PA, peanut allergy; PIA, peanut-induced anaphy- laxis; CD40L, CD40 ligand; PP, peanut protein; PL, peritoneal lavage; CPE, crude peanut extract; CT, cholera toxin; LN, lymph node; BAL, bronchoalveolar lavage; GI, gastrointestinal; i.d., intradermal; i.n., intranasal; DIG, digoxigenin; DC, dendritic cell. Copyright © 2007 by The American Association of Immunologists, Inc. 0022-1767/07/$2.00 The Journal of Immunology www.jimmunol.org