The development of peptide-based subunit vaccines against Streptococcus pyogenes, Necator americanus and Schistosome mansoni

Abstract

Immunization via the use of whole pathogen (or its biological component) has been one of the most successful public health interventions. Historically, Edward Jenner pioneered the modern vaccination in the late 17th century and since; vaccine research has been evolving rapidly. The practice of conventional vaccination, with attenuated or killed pathogen, was accepted worldwide and with this strategy, protections against many diseases were established. Nevertheless, not all diseases can be prevented via the conventional vaccination approach. For example, there is no vaccine against group A streptococcus (GAS), hookworm or schistosome infection, although recent statistics shows that these diseases affect almost a billion people worldwide (i.e. high demand for vaccines). The conventional vaccination via the use of weakened or killed GAS pathogen could result in the development of fatal autoimmune diseases (i.e. rheumatic fever and rheumatic heart disease), while vaccination using whole hookworm or schistosome parasite failed to develop protection in human, despite these vaccines were successful in animal (murine and/or canine) models. Therefore, synthetic vaccination approach via the use of minimal pathologic component (subunit peptide) necessary to stimulate immune response has the potential to address the drawbacks of using conventional vaccination approach. However, as peptides are poor immunogens on their own, the approach necessitates the use of adjuvants (immunostimulant) or carrier molecules to boost the vaccines’ immunogenicity. In this PhD research project, novel carrier systems/delivery platforms were utilized for the development of synthetic (subunit) vaccines against three pathogens: Streptococcus pyogenes (GAS), Necator americanus (hookworm) and Schistosoma mansoni (schistosome). Two types of acrylate polymers, linear- and dendritic-poly(t-butyl)acrylate (PtBA), were used for the GAS vaccine development project while the lipid core peptide (LCP) systems were used for the development of self-adjuvanting vaccines against hookworm and schistosome infections. This research work mainly involved the design of peptide epitope, the synthesis of the subunit peptide and the delivery platforms, the conjugation peptides to the delivery platforms and immunological evaluation of the vaccine candidates in (inbred) mice model. In summary: • The synthetic pathway to produce self-assembled polymer-peptide hybrid vaccines via copper catalysed azide-alkyne cycloaddition (CuAAC) reaction was established for the development of vaccine candidates against GAS. The candidates were able to form distinct nanoparticles with sizes of 500 nm and 20 nm, for the linear- and dendritic- PtBA, respectively. The conjugates were able to stimulate IgG secretion against J14 peptide after a single-dose immunization, without the use of additional adjuvant. • Vaccines that are able to stimulate the humoral response (production of neutralizing antibodies) are vital for the development of therapeutics against hookworm infection. Alternative approach in designing the minimal B-cell peptide epitopes against hookworm aspartic protease (APR-1) was established using bioinformatics tools (i.e. computational methods). The epitopes were conjugated to two LCP carriers via stepwise solid peptide synthesis (SPPS) and CuAAC reactions. The vaccine candidates were able to self-assemble into nanoparticles and their potency in eliciting IgG antibodies were established. • Prior research using LCP vaccine against schistosome parasite was able to produce neutralizing IgG antibodies against the parasite’s cathepsin D protein. Unfortunately, inconsistencies in the antibody production were observed between mice. Based on the preliminary results, novel vaccines were designed to include a T helper epitope and synthesized via SPPS. Additionally, a new B cell epitope was designed using bioinformatics tools and conjugated to the LCP system bearing a T helper epitope. The improved and new vaccine candidates efficiently elicited high antibody titres, comparable to the strong but toxic Freund’s adjuvant. The poor immunogenicity of peptide-based vaccines was overcome with the application of the self-adjuvanting PtBA and LCP carrier systems. The subunit peptides conjugated to the carriers have been shown to be able to produce strong antibody response without the use of additional toxic adjuvant. These findings suggested that the novel carrier systems have great potentials as self-adjuvanting vaccine carriers for the development of subunit peptide-based vaccines.