Progress in the Development of Subunit Vaccines against Malaria

Malaria is a life-threatening disease and one of the main causes of morbidity and mortality in the human population. The disease also results in a major socio-economic burden. The rapid spread of malaria epidemics in developing countries is exacerbated by the rise in drug-resistant parasites and insecticide-resistant mosquitoes. At present, malaria research is focused mainly on the development of drugs with increased therapeutic effects against Plasmodium parasites. However, a vaccine against the disease is preferable over treatment to achieve long-term control. Trials to develop a safe and effective immunization protocol for the control of malaria have been occurring for decades, and continue on today; still, no effective vaccines are available on the market. Recently, peptide-based vaccines have become an attractive alternative approach. These vaccines utilize short protein fragments to induce immune responses against malaria parasites. Peptide-based vaccines are safer than traditional vaccines, relatively inexpensive to produce, and can be composed of multiple T- and B-cell epitopes integrated into one antigenic formulation. Various combinations, based on antigen choice, peptide epitope modification and delivery mechanism, have resulted in numerous potential malaria vaccines candidates; these are presently being studied in both preclinical and clinical trials. This review describes the current landscape of peptide-based vaccines, and addresses obstacles and opportunities in the production of malaria vaccines.

Bivalent mucosal peptide vaccines administered using the LCP carrier system stimulate protective immune responses against Streptococcus pyogenes infection

Despite the broad knowledge about the pathogenicity of Streptococcus pyogenes there is still a controversy about the correlate of protection in GAS infections. We aimed in further improving the immune responses stimulated against GAS comparing different vaccine formulations including bis-(3',5')-cyclic dimeric adenosine monophosphate (c-di-AMP) and BPPCysMPEG, a derivative of the macrophage-activating lipopeptide (MALP-2), as adjuvants, respectively, to be administered with and without the universal T helper cell epitope P25 along with the optimized B cell epitope J14 of the M protein and B and T cell epitopes of SfbI. Lipopeptide based nano carrier systems (LCP) were used for efficient antigen delivery across the mucosal barrier. The stimulated immune responses were efficient in protecting mice against a respiratory challenge with a lethal dose of a heterologous S. pyogenes strain. Moreover, combination of the LCP based peptide vaccine with c-di-AMP allowed reduction of antigen dose at the same time maintaining vaccine efficacy.

The application of self-assembled nanostructures in peptide-based subunit vaccine development

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Smaller polymer-peptide conjugates-based nanoparticles are often more immunogenic.

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Lipid-antigen conjugates-based nanoparticles can interact with immune receptors.

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Peptides with β-sheet conformation usually form nanofibers.

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α-Helical and random coil peptides tend to self-assemble into nanoparticles.

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Peptide-based nanostructures are usually poorer inducers of immune responses.

Peptide based-vaccines are becoming one of the most widely investigated prophylactic and therapeutic health care interventions against a variety of diseases, including cancer. However, the lack of a safe and highly efficient adjuvant (immune stimulant) is regarded as the biggest obstacle to vaccine development. The incorporation of a peptide antigen in a nanostructure-based delivery system was recently shown to overcome this obstacle. Nanostructures are often formed from antigens conjugated to molecules such as polymers, lipids, and peptide, with the help of self-assembly phenomenon. This review describes the application of self-assembly process for the production of peptide-based vaccine candidates and the ability of these nanostructures to stimulate humoral and cellular immune responses.

Group A Streptococcal vaccine candidate: contribution of epitope to size, antigen presenting cell interaction and immunogenicity

Aim: Utilize lipopeptide vaccine delivery system to develop a vaccine candidate against Group A Streptococcus. Materials & methods: Lipopeptides synthesized by solid-phase peptide synthesis-bearing carboxyl (C)-terminal and amino (N)-terminal Group A Streptococcus peptide epitopes. Nanoparticles formed were evaluated in vivo. Results: Immune responses were induced in mice without additional adjuvant. We demonstrated for the first time that incorporation of the C-terminal epitope significantly enhanced the N-terminal epitope-specific antibody response and correlated with forming smaller nanoparticles. Antigen-presenting cells had increased uptake and maturation by smaller, more immunogenic nanoparticles. Antibodies raised by vaccination recognized isolates. Conclusion: Demonstrated the lipopeptidic nanoparticles to induce an immune response which can be influenced by the combined effect of epitope choice and size.

Lipid core peptide targeting the cathepsin D hemoglobinase of Schistosoma mansoni as a component of a schistosomiasis vaccine

The self-adjuvanting lipid core peptide (LCP) system offers a safe alternative vaccine delivery strategy, eliminating the need for additional adjuvants such as CpG Alum. In this study, we adopted the LCP as a scaffold for an epitope located on the surface of the cathepsin D hemoglobinase (Sm-CatD) of the human blood fluke Schistosoma mansoni. Sm-CatD plays a pivotal role in digestion of the fluke's bloodmeal and has been shown to be efficacious as a subunit vaccine in a murine model of human schistosomiasis. Using molecular modeling we showed that S. mansoni cathepsin D possesses a predicted surface exposed α-helix (A₂₆₃K) that corresponds to an immunodominant helix and target of enzyme-neutralizing antibodies against Necator americanus APR-1 (Na-APR-1), the orthologous protease and vaccine antigen from blood-feeding hookworms. The A₂₆₃K epitope was engineered as two peptide variants, one of which was flanked at both termini with a coil maintaining sequence, thereby promoting the helical characteristics of the native A₂₆₃K epitope. Some of the peptides were fused to a self-adjuvanting lipid core scaffold to generate LCPs. Mice were vaccinated with unadjuvanted peptides, peptides formulated with Freund's adjuvants, or LCPs. Antibodies generated to LCPs recognized native Sm-CatD within a soluble adult schistosome extract, and almost completely abolished its enzymatic activity in vitro. Using immunohistochemistry we showed that anti-LCP antibodies bound to the native Sm-CatD protein in the esophagus and anterior regions of the gastrodermis of adult flukes. Vaccines offer an alternative control strategy in the fight against schistosomiasis, and further development of LCPs containing multiple epitopes from this and other vaccine antigens should become a research priority.

Peptide-based subunit vaccine against hookworm infection

Hookworms infect more people than HIV and malaria combined, predominantly in third world countries. Treatment of infection with chemotherapy can have limited efficacy and re-infections after treatment are common. Heavy infection often leads to debilitating diseases. All these factors suggest an urgent need for development of vaccine. In an attempt to develop a vaccine targeting the major human hookworm, Necator americanus, a B-cell peptide epitope was chosen from the apical enzyme in the hemoglobin digestion cascade, the aspartic protease Na-APR-1. The A(291)Y alpha helical epitope is known to induce neutralizing antibodies that inhibit the enzymatic activity of Na-APR-1, thus reducing the capacity for hookworms to digest hemoglobin and obtain nutrients. A(291)Y was engineered such that it was flanked on both termini by a coil-promoting sequence to maintain native conformation, and subsequently incorporated into a Lipid Core Peptide (LCP) self-adjuvanting system. While A(291)Y alone or the chimeric epitope with or without Freund's adjuvants induced negligible IgG responses, the LCP construct incorporating the chimeric peptide induced a strong IgG response in mice. Antibodies produced were able to bind to and completely inhibit the enzymatic activity of Na-APR-1. The results presented show that the new chimeric LCP construct can induce effective enzyme-neutralising antibodies in mice, without the help of any additional toxic adjuvants. This approach offers promise for the development of vaccines against helminth parasites of humans and their livestock and companion animals.

Strategies for intranasal delivery of vaccines

The vast majority of human pathogens colonize and invade at the mucosal surfaces. Preventing infection at these sites via mucosally active vaccines is a promising and rational approach for vaccine development. However, it is only recently that the stimulation of local immunity at the mucosal surfaces has become a primary objective in addition to inducing systemic immunity. This review describes vaccine formulations designed for mucosal delivery to the nasal-associated lymphoid tissue, via intranasal administration. The association of antigens with mucosal adjuvants and delivery systems is emphasised.