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| The Flu Virus |The Flu and Global Health | The Epitope-based Approach | The Immune System | Links | Publications | | |||||
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Overview The identification of specific epitopes derived from infectious pathogens and tumors has significantly advanced the development of peptide-based vaccines. Improved understanding of the molecular basis of antigen recognition and HLA binding motifs has resulted in the development of rationally designed vaccines based on algorithms predicting the peptide’s binding to human HLA. This is supported by technological achievements that further encouraged the development of this approach, including the use of transgenic animals that enable a rapid screening of vaccine candidates. Indeed, many studies show the immunological efficacy of peptide-based vaccines against infectious diseases in animal models as well as in clinical studies, which demonstrated the responses to peptide vaccines against infectious diseases including malaria, Hepatitis B and HIV. The use of synthetic peptides in vaccines offers practical advantages such as relative ease of production, chemical stability, and an avoidance of any infectious or oncogenic potential hazard. An additional aspect to be considered is the similarity between the epitope sequence and any sequence of human proteins, in order to avoid autoimmune conditions. This should and can be avoided at the design phase of the new peptide-based vaccine. Peptides may also allow better manipulation of the immune responses through the use of epitopes designed for stimulating particular subsets of lymphocytes. Most importantly, peptide vaccines appear to be effective in generating immune responses to self-proteins which might be relevant for the development of tumor vaccines. Peptides used in such vaccines are modified, and hence differ from the target self protein. Clinical human trials of single peptides have demonstrated that cancer patients respond to self-tumor antigens, and some studies have shown early positive clinical results. Current efforts in the field are focused on improving the immunogenicity of individual HLA binding peptides as well as developing multiple peptide vaccines for the prevention and treatment of human malignancy. According to analysts, the cancer vaccine market is expected to reach $8 Billion USD by 2012. Immunological studies in which a single conserved B cell epitope (HA 91-108) was evaluated for its reactivity and efficacy in mice showed partial protection. This epitope, located next to the viral binding site to the host cell’s membrane, is conserved due to its functional role and the non changing nature of the host cell receptor. The partial protection conferred by this single epitope was enhanced by the addition of T cell epitopes from the inner nucleoprotein to the vaccine formulation. The improved efficacy and the cross strain protection observed were obtained by inducing the cellular arm of the immune system. Special attention should be given to the selection of CTL epitopes within the epitope-based vaccines due to the large degree of MHC polymorphism and the need to know the HLA frequencies in the target population. However, it is now known that HLA Class I molecules can be divided into several families, or supertypes, based on similar peptide-binding repertoires. A vaccine should include several T cell epitopes that are specific to the prevalent HLA genotypes in order to confer protection to the vast majority of the population. The epitope based concept, in which a combination of B- and T-cell epitopes are used to confer protection against viral infection, was also utilized in studies on a vaccine against Respiratory Syncytial Virus (RSV). The combination of RSV-specific humoral and cellular immunity induced by the peptide cocktail was more effective at clearing RSV than peptide-induced humoral or cellular immunity alone. In the case of influenza, different approaches have been considered for the presentation of peptides to the immune system, including the use of protein conjugate or proteosomes, live recombinant salmonella and eventually, the use of the recombinant flagellin, which has been effective. The flagella which comprise polymeric flagellin are highly immunogenic and encompass additional characteristics. The epitope-based vaccine using flagellin as a carrier was successfully tested in several animal models. The results obtained in these experiments illustrate the efficacy of the vaccine against different strains of influenza virus, including the highly pathogenic H5N1 avian strain. Cumulative results from the literature have shown that peptides directed mainly towards the Hemagglutinin (HA) and nucleoprotein (NP) could be successfully employed in various studies, leading to viral clearance and increased survival. M1 epitope is highly conserved in all Influenza A strains and was also included in BiondVax’s vaccine. This combination has been shown to significantly enhance the protective effect of vaccination against either sub-lethal or lethal challenge with the influenza H3N2 virus. Whereas the B cell epitope alone conferred only a partial protection (<20%), immunization with the B cell epitope together with the CTL epitope resulted in approximately 60% survival. A combination of B cell epitope together with CTL and T helper epitopes protected all the mice (100%) from a lethal challenge. The following section describes influenza epitopes that were utilized in epitope-based vaccines, as well as the flagellin that acts as a carrier and adjuvant for these epitopes. 
Flagellin: A Carrier with a Built-In Adjuvanticity The concept of utilizing flagellin as a carrier is well documented in the scientific literature. Attenuated Salmonella strains were used to express and present foreign epitopes from various pathogenic proteins such as F1 antigen of Yersinia pestis, cholera toxin, malaria circumsporozoite protein, Hepatitis B surface antigen, Tetanus toxin and Streptococcal M protein on its flagellin. Immunization with the whole recombinant bacteria induced a specific immune response directed against these foreign antigens. The intensive response to the bacteria and more specifically to the flagellin is mediated by toll-like receptors. The Toll-like receptor (TLR) family recognizes bacterial components such as LPS (TLR4), Hsp 60 (TLR1), CpG oligodeoxynucleotides (ODN) (TLR9), as well as flagellin that are associated specifically with the TLR5. Stimulation through these receptors leads to a cascade of events resulting in dendritic cells and NK cells activation, followed by cytokines secretion, thereby linking innate and adaptive immunity. Consequently, the incorporation of TLR-ligands into vaccines could result in more potent and efficacious vaccines. Previously published data indicate that immunization of mice with the recombinant-flagellin fused to several proteins resulted in potent antigen-specific T and B cell responses that were equal to or better than responses induced by the same proteins emulsified in Complete Freund's adjuvant. These responses included rapid and consistent antibody responses as well as the development of protective CD8+ T cell responses upon challenge with virulent Listeria monocytogenes. The contribution of flagellin to the adjuvant effect in the case of peptide vaccines stems also from the prolonged exposure of the immune system to the peptide when it is presented on the flagellin. Degradation of peptides in the body occurs within few minutes, whereas flagellin can be detected in the blood up to 12 hours post intramuscular administration. Research performed at the Weizmann Institute of Science indicated that pre existing anti-flagellin antibodies had no significant effect on the efficacy of a vaccine containing flagellin. Similarly, pre exposure to the salmonella administered orally did not lead to carrier suppression and hence did not affect the protective response of a recombinant flagellin-based vaccine expressing a foreign antigen. It should be noted that the salmonella bacteria used for the preparation of the recombinant epitope-flagellin construct is non-virulent vaccine strain bacteria. In addition, this universal flu vaccine does not induce allergic responses. These characteristics support the use of flagellin as an adequate carrier with built-in adjuvanticity. 
An epitope based vaccine against Influenza Ben-Yedidia T, Arnon R. Expert Rev Vaccines. 2007 Dec;6(6):939-48. |
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