A Novel Platform to Generate Synthetic Vaccine Candidates

Date

2012-07-10

Authors

Case, Allison Carroll

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Abstract

Vaccination remains the optimal means to prevent infectious disease by inducing antibodies that confer protective immunity against the pathogen in question [1-3]. However, there remain viruses against which no effective vaccines exists including human immunodeficiency virus (HIV), West Nile Virus (WNV) and hepatitis C virus (HCV). These viruses and others evade the immune response by undergoing rapid mutations in immunodominant epitopes [4-6]. In addition, although they usually express conserved epitopes that are important for inducing neutralizing antibodies, in many cases these are not immunodominant. Traditional techniques in vaccine development have not been able to overcome these barriers for these and other viruses. Subunit and peptide vaccines are very safe but it is often difficult to identify the key epitopes needed to make them effective.
New approaches to developing safe vaccines that induce broadly neutralizing antibodies are needed. Therefore, the long term goal of this project was to generate vaccine candidates for any virus for which a neutralizing antibody existed or could be made without prior knowledge of the protective epitope(s). Furthermore, we desired a way to administer these vaccine candidates safely and before exposure so as to induce neutralizing antibodies. To accomplish these goals, we began with the development of a platform to generate synthetic vaccine candidates. This platform consisted of 1) libraries of B cell epitopes or “shapes” prepared by displaying peptoid sequences on beads, 2) neutralizing monoclonal antibodies (MAbs) to select the peptoids that bound to the antibody’s antigen-combining site, and 3) protein G dynabeads (PGDs) and a magnet to bind and isolate antibody bound peptoid beads. Any sequences identified in the platform as potential B cell mimetics were further evaluated in two validation assays. The first consisted of a “color screening” assay to determine that the isolated on-bead peptoids were bound by antibody. The second confirmed that these peptoids would fail to be bound by antibody if an excess of the native antigen was added (i.e. that peptoid sequences were bound by the antibody’s binding sites). The major accomplishments to emerge from this study were 1) the creation of an optimized magnetic screening platform for the isolation of peptide B cell epitopes from an on-bead library, 2) a magnetic screening platform optimized for the isolation of peptoid B cell epitopes from a peptoid library, and 3) the identification of potential peptoid B cell epitope mimetics of FLAG peptide from a peptoid library using a MAb. Taken together, a sensitive, specific, and reproducible platform to identify vaccine candidates from a peptoid library was created. This platform is particularly important for viruses like HIV, HCV, and WNV where mutation makes foreknowledge of conserved, neutralizing epitopes difficult. Once sufficiently large and diverse libraries are created, the B cell epitope mimetics (vaccine candidates) identifiable by this platform will have several advantages over their peptide counterparts. These peptoid-based vaccines are “safe” as there is no potential for reversion, they are less expensive and faster to synthesize than peptides, they are not dependent on the twenty amino acids, and the B cell epitopes identified with this platform can be conjugated to carrier in such a way that the multivalency and immunodominance can be controlled making this platform advantageous both to the generation of new vaccine candidates and in reformulating current vaccines.

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