Macromolecular self-association of a synthetic peptide derived from the hepatitis B surface antigen: construction of a quaternary epitope

A review of multiple approaches towards an improved hepatitis B vaccine

BACKGROUND

Hepatitis B is a DNA virus that can cause liver inflammation, cirrhosis, and cancer in chronically infected and symptomatic carriers. Antiviral treatments are usually limited in their effectiveness in treating the disease states. Vaccination against hepatitis B in pediatric and adolescent populations has proven to be a generally effective means for preventing diseases that could be potentially caused by this virus. Some 5 - 10% of the vaccinees do not develop protective immunity against the virus. Therefore, a significant amount of effort has been made in many research laboratories across the world to increase the potency of the vaccine by various innovative means, e.g., increasing the immunogenicity of the antigen or through introduction of novel adjuvants that elicit strong humoral and cell-mediated immune responses.

OBJECTIVES/METHODS

The objective of this review is to highlight publications of significant developments that have been made over the past decade and efforts that are continuing towards producing an improved vaccine. A number of patents that protect novel hepatitis B vaccine formulations, including those claiming novel hepatitis B core antigen formulations and combinations of a vaccine with small molecule therapeutics, are discussed.

CONCLUSION

There have been promising developments in the area of new adjuvants and delivery systems. The practical need for reducing the total number of childhood vaccinations has driven development of, and patent filings on, multivalent and combination vaccine formulations in which the hepatitis B vaccine is included as one component. Efforts and some advances have also been made in the critical area of therapeutic application of the vaccine. The existence of a large population of already infected patients and the inadequacy of most of the current antiviral drugs against hepatitis B diseases have also inspired efforts to produce a vaccine that would be efficacious in clearing an exiting infection.

Core particles of hepatitis B virus as carrier for foreign epitopes

To be effective as vaccines, most monomeric proteins and peptides either require chemical coupling to high molecular weight carriers or application together with adjuvants. More recently, recombinant DNA techniques have been used to insert foreign epitopes into proteins with inherent multimerization capacity, such as particle-forming viral capsid or envelope proteins. The core protein of hepatitis B virus (HBcAg), because of its unique structural and immunological properties, has gained widespread interest as a potential antigen carrier. Foreign sequences of up to approximately 40 amino acid residues at the N terminus, 50 or 100 amino acids in the central immunodominant c/e 1 epitope region of HBcAg, and up to 100 or even more residues at the C terminus, did not interfere with particle formation. The humoral immunogenicity of inserted epitopes is determined by the immunogenicity of the peptide itself and its surface exposure, and is influenced by the route of application. The probably flexible and surface-exposed c/e1 region emerged as the most promising insertion site. When applied together with adjuvants approved for human and veterinary use, or even without adjuvants, such chimeric particles induced B and T cell immune responses against the inserted epitopes. In some cases neutralizing antibodies, cytotoxic T cells and protection against challenge with the intact pathogen were demonstrated. Major factors for the potentiated immune response against the foreign epitopes are the multimeric structure of chimeric HBcAg that results in a high epitope density per particle, and the provision of T cell help by the carrier moiety. Beyond its use as subunit vaccine, chimeric HBcAg produced in attenuated Salmonella strains may be applicable as live vaccine.

Comparative biology and pathogenesis of AIDS and hepatitis B viruses: related but different

AIDS (HIV) and hepatitis B viruses are remarkably similar in their sharing of reverse transcription, in their ancestral origins and common genetic elements, and in their modes of transmission. Both are hypermutable and exist as quasispecies due primarily to errors in reverse transcription, though there is severe restriction in the replicative competence of most hepatitis B mutants. They differ in the lack of an integrase in hepatitis B virus and in their pathogenesis in the infected host. HIV survives mainly by antigenic variability, immune evasion, and impairment of immune function though viral regulatory control elements seek to restrict fatal damage to the host. Hepatitis B virus survives primarily by mutation of e antigen/core genes that directly obviates cytotoxic T cell destruction of infected liver cells, or indirectly limits destruction of infected cells through induction of anergy in the cytotoxic T cell response. Most persons infected with hepatitis B virus recover completely while recovery from HIV infection is rare if ever. Hepatitis B is highly preventable by vaccine while HIV vaccine is still seeking a meaningful immunoprophylactic target. AIDS and hepatitis B represent an extreme example, among the viruses of man, in their close similarities but distinct differences. In depth details and perspectives are presented in this review.

Strategies for mapping and imitating viral B-cell epitopes

Identification of viral B-cell epitopes is of importance in the selection of peptides for inclusion into subunit vaccines, the development of virus-specific serological tests and understanding the interaction of antibodies with viruses at a molecular level. B-cell epitopes can often be determined unequivocally by X-ray analyses of antibody-antigen complexes. This technique is, however, time-consuming and alternative strategies have now been developed for identifying epitopes. This article provides an overview of approaches which are currently available for mapping and imitating B-cell epitopes.

Immunological characteristics of a recombinant hepatitis B virus-derived multiple-epitope polypeptide: a study in polyvalent vaccine design

Immunological properties of a model polyepitope immunogen (MEP-1) consisting of selected determinants from envelope proteins of hepatitis B virus (HBV) were examined. Immunization with MEP-1 induced high-titre antibodies in a variety of murine strains and in rabbits although an overall hierarchy of B-cell immunodominance was observed as pre-S1-derived > S-derived > pre-S2-derived segments. Anti MEP-1 antibody responses in all hosts were found to be exclusive for HBV-derived sequences in the absence of any fraction directed against the various interepitope junctions. With panels of overlapping peptides it was observed that the anti-pre-S1 and anti-pre-S2 components of anti-MEP-1 antibodies were, in all animals tested, of the desired specificity from the standpoint of potential virus-neutralizing ability. The MEP-1 segment representing residues 124-127 of the major protein of hepatitis B surface antigen (HBsAg) was found to elicit a conformation-specific antibody response. Furthermore, this subpopulation was either predominantly or exclusively against the Met133-Lys141-dependent group-specific epitope. Finally, the HBV sequences in MEP-1 were shown to retain their Th-cell activities. These studies suggest that MEP-1 provides a useful tool in the study of polyvalent vaccine design.

A self-associating hepatitis B surface antigen-derived peptide that is immunogenic in alum

We previously described an oligomeric synthetic peptide derived from the hepatitis B surface antigen that displayed a limited tendency to form self-associating macromolecular structures in solution. Here it is demonstrated that amino-terminal myristylation of this peptide results in near quantitative aggregation of the oligomeric peptide. The myristylated peptide is highly immunogenic when used in conjunction with alum as adjuvant in both the rabbit and rhesus monkey models. The antibody response generated by peptide also cross-reacted with native antigen and was long-lasting. Collectively the results described in this and previous reports offer an attractive new approach for generating immunogenic peptide mimetics of conformational epitopes that may find application as vaccines.

Towards vaccines against human papillomavirus type-16 genital infections

Human papillomavirus type-16 (HPV-16) is strongly associated with cervical carcinoma and cervical intraepithelial neoplasia. It may soon be possible to develop prophylactic vaccines designed to induce neutralizing antibodies to HPV-16 virions in genital secretions and therapeutic vaccines to induce cytotoxic T-cell responses against HPV-16 early proteins in cervical intraepithelial neoplasia and cervical cancers. Although significant advances have been achieved, problems remain before such vaccines can be used routinely.

Comparison of immune responses to a native viral antigen and a synthetic peptide derived from it: implications for vaccine development

Murine immune responses to the hepatitis B surface antigen (HBsAg) and a synthetic peptide derived from it were compared at the humoral level. Six of nine strains used responded to either peptide or HBsAg, though restriction profiles were not superimposable. Two of three strains non-responsive to HBsAg produced an antibody response on immunization with peptide which was cross-reactive with both peptide and HBsAg. In in vitro lymphocyte stimulation assays, lymphocyte from all six peptide-immunized mouse strains could be induced to proliferate on challenge with HBsAg. However, of the HBsAg-immunized groups, lymphocytes from only three of six responder strains proliferated on in vitro HBsAg challenge. Cumulatively, these results suggest that a vaccine formulation that includes both protein antigens and synthetic peptides derived from these proteins may be more effective at eliciting an immune response in a broader cross-section of target population.