title: A heterologous helper T-cell epitope enhances the immunogenicity of a multiple-antigenic-peptide vaccine targeting the cryptic loop-neutralizing determinant of Bacillus anthracis protective antigen.
abstract: We previously showed that a multiple antigenic peptide (MAP) displaying amino acids (aa) 305 to 319 from the 2beta2-2beta3 loop of protective antigen (PA) can elicit high-titered antibody that neutralizes lethal toxin (LeTx) in vitro and that this loop-neutralizing determinant (LND) specificity is absent in PA-immune rabbits. Some immune rabbits were, however, nonresponders to the MAP. We hypothesized that the immunogen elicited suboptimal major histocompatibility complex (MHC) class II-restricted T-cell help and that introduction of a functional helper T-cell epitope would increase MHC-restricted responsiveness and the magnitude and affinity of the antibody responses. In the current study, we characterized the T- and B-cell responses to LND peptides in mice, then designed second-generation MAP immunogens for eliciting LND-specific immunity, and tested them in rabbits. The 305-319 sequence was devoid of helper T-cell epitopes in three strains of mice; however, a T-B peptide comprising aa 305 to 319, colinearly synthesized with the P30 helper epitope of tetanus toxin, elicited robust LeTx-neutralizing immunity in mice. T-B MAPs displaying B-cell epitopes 304 to 319 (MAP304) or 305 to 319 (MAP305) elicited high-titer, durable antibody responses in rabbits which exhibited potent neutralization of LeTx in vitro. All MAP304-immune rabbits demonstrated neutralization titers exceeding that of hyperimmune sera of rabbits immunized with PA in Freund's adjuvant, with peak neutralization titers 23-, 6-, and 3-fold higher than that of the PA antiserum. Overall, immunization with MAPs containing the P30 epitope elicited higher antibody and toxin neutralization titers and peptide-specific affinity than immunization with an LND MAP lacking a helper epitope. P30-containing MAP304 represents a promising LND-specific vaccine for anthrax.
authors.affiliation: Division of Hematology-Oncology, Department of Internal Medicine, University of Michigan Medical School, Ann Arbor,Michigan 48105, USA. joscher@umich.edu
authors.forename: Jon
authors.initials: J
authors.lastname: Oscherwitz
authors.forename: Fen
authors.initials: F
authors.lastname: Yu
authors.forename: Kemp B
authors.initials: KB
authors.lastname: Cease
mesh_terms: D000818:Animals; D022122:Anthrax Vaccines / Q000235:genetics / Q000276:immunology*; D057134:Antibodies, Neutralizing / Q000097:blood; D000942:Antigens, Bacterial / Q000235:genetics / Q000276:immunology*; D000992:Antitoxins / Q000097:blood; D001408:Bacillus anthracis / Q000235:genetics / Q000276:immunology; D001427:Bacterial Toxins / Q000235:genetics / Q000276:immunology*; D018984:Epitopes, T-Lymphocyte / Q000235:genetics / Q000276:immunology*; D005260:Female; D051379:Mice; D008815:Mice, Inbred Strains; D011817:Rabbits; D013744:Tetanus Toxin / Q000235:genetics / Q000276:immunology; D022223:Vaccines, Subunit / Q000235:genetics / Q000276:immunology
chemical_list: D022122:Anthrax Vaccines; D057134:Antibodies, Neutralizing; D000942:Antigens, Bacterial; D000992:Antitoxins; D001427:Bacterial Toxins; D018984:Epitopes, T-Lymphocyte; D013744:Tetanus Toxin; D022223:Vaccines, Subunit; C030325:anthrax toxin