The Immunogenicity of Bacillus Anthracis Protective Antigen Domains and the Efficacy of Elicited Antibody Responses Depend on host Genetic Background

Department of Microbiology and Immunology, and Department of Medicine, Division of Infectious Diseases, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, New York, 10461

^* To whom correspondence should be addressed. Email: casadeva{at}aecom.yu.edu.

	Abstract

Neutralizing antibodies to Bacillus anthracis protective antigen (PA), a component of anthrax toxin, mediate protection against anthrax. PA is antigenically complex and can elicit protective and non-protective antibodies. Furthermore, vaccinated individuals demonstrate considerable variability in antibody responses to PA. To explore the relationship between PA structure and antigenicity, we produced E.coli expressing full-length PA (PA1-4), domains 2-4 (PA2-4), domain 1, (PA1), and domain 4 (PA4) and evaluated their immunogenicity and protective efficacy in four mouse strains (A/J, Balb/c, C57BL/6, and Swiss Webster). Immunization with PA1-4 resulted in significantly higher lethal toxin-neutralizing antibody titers than immunization with any recombinant protein fraction of PA. The magnitude and neutralizing capacity of the antibody response to full-length PA and its fragments varied depending on the mouse strain. We found no correlation between antibody titer and neutralizing titer for A/J and Swiss Webster mice. In C57BL/6 mice antibody titers and neutralization capacity correlated for two of four recombinant PA (rPA) domain proteins tested, while Balb/c mice displayed a similar correlation only with one. By correlating the reactivity of immune sera with solvent exposed linear peptide segments of PA we tentatively assign the presence of four new linear B cell epitopes in PA amino acids 121-150, 143-158, 339-359, and 421-440. We conclude that genetic background of the host determines the relative efficacy of the antitoxin response. The results suggest that the variability observed in vaccination studies with PA-derived vaccines is a result of host heterogeneity and implies a need to develop other antigens as vaccine candidates.