Vaccine 25 (2007) 1780–1788 Analysis of anti-protective antigen IgG subclass distribution in recipients of anthrax vaccine adsorbed (AVA) and patients with cutaneous and inhalation anthrax V.A. Semenova ∗ , D.S. Schmidt, T.H. Taylor Jr., H. Li, E. Steward-Clark, S.D. Soroka, M.M. Ballard, C.P. Quinn Centers for Disease Control and Prevention, Atlanta, GA, USA Received 24 March 2006; received in revised form 6 November 2006; accepted 13 November 2006 Available online 27 November 2006 Abstract The anti-PA IgG1, IgG2, IgG3, and IgG4 subclass responses to clinical anthrax and to different numbers of anthrax vaccine adsorbed (AVA, BioThrax ® ) injections were determined in a cross-sectional study of sera from 63 vaccinees and 13 clinical anthrax patients. The data show that both vaccination with three AVA injections and clinical anthrax elicit anti-PA IgG1, IgG2, and IgG3 subclass responses. An anti-PA IgG4 response was detected in AVA recipients after the fourth injection. The anthrax lethal toxin (LTx) neutralization efficacy of sera from recipients who received 4 to ≥10 AVA injections did not vary significantly in relation to changes in distribution of anti-PA IgG1 and IgG4 subclasses. © 2006 Elsevier Ltd. All rights reserved. Keywords: AVA; BioThrax ® ; Anti-PA antibody; IgG subclasses; Anthrax; Bacillus anthracis 1. Introduction Protective antigen (PA) is the pivotal protein of the anthrax toxin complex and the principal immunogen of anthrax vac- cines, including anthrax vaccine adsorbed (AVA, BioThrax ® ; BioPort Corp., Lansing, MI) [1–4]. Studies in animal mod- els indicate that the immune response to PA is central to protection against Bacillus anthracis [3–5]. The anti-PA immunoglobulin G (IgG) antibody concentrations and dilu- tional titers are, therefore, the most commonly reported marker of human immune responses to anthrax vaccines and B. anthracis infection [6–8]. Previous reports on the human immune response to the licensed UK anthrax vaccine and ∗ Corresponding author at: Microbial Pathogenesis and Immune Response Laboratory, Meningitis and Special Pathogens Branch, Division of Bacterial and Mycotic Diseases, Centers for Disease Control and Prevention, Mail Stop D-01, 1600 Clifton Road, Atlanta, GA 30333, USA. Tel.: +1 404 639 4390; fax: +1 404 639 4550. E-mail address: vsemenova@cdc.gov (V.A. Semenova). B. anthracis infection showed that there is a difference in anti-PA IgG subclass distributions between vaccine recipi- ents and clinical patients [9]. However, quantitative analyses of human IgG subclass responses to AVA, recombinant PA (rPA) based vaccines or inhalation anthrax and their anthrax lethal toxin (LTx) neutralization efficacies have not been reported. Studies in guinea pigs have correlated the func- tional characteristics and anti-PA IgG subclass distribution in that genus with survival against virulent B. anthracis chal- lenge [10]. Based on these observations and the knowledge that each of the four human IgG subclasses has a unique effector function relevant to the clearance and elimination of a foreign antigen [11,12], we hypothesized that determina- tion of changes in the anti-PA IgG subclass distribution in response to AVA vaccination and human clinical anthrax will help characterize the humoral antibody responses to repeated antigen exposure during the full regimen of AVA vaccination at 0, 2, and 4 weeks and 6, 12, and 18 months. These data will provide new information on serological responses to PA that may be used as a marker for differentiation between 0264-410X/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.vaccine.2006.11.028