Omalizumab inhibits acceleration of FcRI-mediated responsiveness of immature human mast cells by immunoglobulin E Yoshimichi Okayama, MD, PhD *; Jun-ichi Kashiwakura, PhD *; Tomomi Sasaki-Sakamoto, BSc *; Kenji Matsumoto, MD, PhD † ; Noriko Hashimoto, BSc † ; Kazumitsu Ohmori, MD, PhD ‡ ; Toshiaki Kawakami, MD, PhD § ; Hirohisa Saito, MD, PhD † ; and Chisei Ra, MD, PhD * * Division of Molecular Cell Immunology and Allergology, Advanced Medical Research Center, Nihon University Graduate School of Medical Science, Tokyo, Japan † Department of Allergy & Immunology, National Research Institute for Child Health and Development, Tokyo, Japan ‡ Department of Thoracic Surgery, Nihon University Graduate School of Medical Science, Tokyo, Japan § Division of Cell Biology, La Jolla Institute for Allergy and Immunology, La Jolla, California ARTICLE INFO Article history: Received for publication October 19, 2011. Received in revised form December 19, 2011. Accepted for publication January 16, 2012. ABSTRACT Background: A large body of evidence has demonstrated that treatment with omalizumab is clinically effective for the management of moderate to severe allergic asthma, emphasizing the importance of IgE in the pathogenesis of allergic asthma. We hypothesized that IgE accelerates FcRI-mediated responsiveness of “immature” human mast cells (MCs) and that omalizumab downregulates the acceleration. Objectives: To examine when MC progenitors acquired the ability to degranulate following FcRI aggrega- tion, whether IgE accelerates the responsiveness of immature MCs following FcRI aggregation, and whether omalizumab regulates such an acceleration. Methods: Gene expression was examined using a microarray and quantitative reverse transcription polymerase chain reaction. Protein expression was investigated using FACS. Histamine release was examined using an EIA. Results: The time-course analysis of the mRNA expression of MC-related genes, including FcRI, in Kit + sorted cells during the differentiation and histamine experiments revealed that the expression level of FcRI in 5 week (w)-cultured MCs was not sufficient to induce degranulation following FcRI aggregation but that 5 w-cultured MCs were fully responsive to calcium ionophore. By addition of IgE in culture medium FcRI expression level and FcRI-mediated histamine release of 5 w-cultured MCs were significantly increased compared with those without addition of IgE, whereas the expression level of tryptase and number of MCs was not affected. Omalizumab significantly inhibited IgE-dependent enhancement of FcRI expression level and FcRI-mediated histamine release. Conclusions: High levels of IgE in the microenvironment in vivo may upregulate the responsiveness of immature MCs to allergens. Omalizumab may inhibit the IgE-mediated responsiveness of not only mature MCs, but also immature MCs.  2012 American College of Allergy, Asthma & Immunology. Published by Elsevier Inc. All rights reserved. Introduction Mast cells (MCs) play a central role in immunoglobulin E (IgE)- dependent allergic diseases such as asthma, and rhinoconjunctivi- tis through the release of a variety of vasoactive and bronchospastic autacoid mediators and functionally diverse proteases, chemo- kines, and cytokines. 1 Mast cells normally do not mature before leaving the bone marrow (BM) but circulate through the vascular system as immature progenitors that then complete their develop- ment peripherally within connective or mucosal tissues. 1 Human CD34 positive ( + ) progenitor cells derived from BM, umbilical cord blood (CB), or peripheral blood (PB) develop into tryptase + , Kit + , FcRI + MCs when cultured in the presence of human stem cell factor (SCF) and IL– 6. 2–4 Ishizaka et al described that human cul- tured MCs are recognized as mature MCs if they have amounts of histamine that are comparable to those of tissue-isolated MCs, contain tryptase, exhibit a morphologically mature appearance when observed using electron microscopy, and are capable of re- leasing histamine after the aggregation of FcRI. 5 In the culture system, BM-, or PB-derived MCs cultured for more than 10 weeks seem to be mature according to the definition. 2–6 Although human Reprints: Yoshimichi Okayama, MD, PhD, Division of Molecular Cell Immunology and Allergology, Advanced Medical Research Center, Nihon University Graduate School of Medical Science, 30-1 Oyaguchikami-machi, Itabashi-ku, Tokyo 173-8610, Japan; E-mail: okayama.yoshimichi@nihon-u.ac.jp. Drs Okayama and Kashiwakura contributed equally to the manuscript. Disclosures: Authors have nothing to disclose. Funding Sources: This work was supported in part by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government (Project No. (C) 20591195 and (C) 23591470, awarded to Y.O., and (B) 22390202, awarded to C.R.), the Nihon University Multidisciplinary Research Grant for 2010 (awarded to Y.O.), and the Matching Fund Subsidy for Private Universi- ties from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government (to C.R.) and Novartis Pharma AG, Basel, Switzerland (to Y.O.). Ann Allergy Asthma Immunol 108 (2012) 188 –194 Contents lists available at SciVerse ScienceDirect 1081-1206/12/$36.00 - see front matter  2012 American College of Allergy, Asthma & Immunology. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.anai.2012.01.009