Tyrosine-Based Cross-Linking of Peptide Antigens to Generate Nanoclusters with Enhanced Immunogenicity: Demonstration Using the Conserved M2e Peptide of Influenza A

Generation of Broad Protection against Influenza with Di-Tyrosine-Cross-Linked M2e Nanoclusters

Tyrosine cross-linking has recently been used to produce nanoclusters (NCs) from peptides to enhance their immunogenicity. In this study, NCs were generated using the ectodomain of the ion channel Matrix 2 (M2e) protein, a conserved influenza surface antigen. The NCs were administered via intranasal (IN) or intramuscular (IM) routes in a mouse model in a prime-boost regimen in the presence of the adjuvant CpG. After boost, a significant increase in anti-M2e IgG and its subtypes was observed in the serum and lungs of mice vaccinated through the IM and IN routes; however, significant enhancement in anti-M2e IgA in lungs was observed only in the IN group. Analysis of cytokine concentrations in stimulated splenocyte cultures indicated a Th1/Th17-biased response. Mice were challenged with a lethal dose of A/California/07/2009 (H1N1pdm), A/Puerto Rico/08/1934 (H1N1), or A/Hong Kong/08/1968 (H3N2) strains. Mice that received M2e NCs + CpG were significantly protected against these strains and showed decreased lung viral titers compared with the naive mice and M2e NC-alone groups. The IN-vaccinated group showed superior protection against the H3N2 strain as compared to the IM group. This research extends our earlier efforts involving the tyrosine-based cross-linking method and highlights the potential of this technology in enhancing the immunogenicity of short peptide immunogens.

Empowering canonical biochemicals with cross-linked novelty: Recursions in applications of protein cross-links

Diversity in the biochemical workhorses of the cell-that is, proteins-is achieved by the innumerable permutations offered primarily by the 20 canonical L-amino acids prevalent in all biological systems. Yet, proteins are known to additionally undergo unusual modifications for specialized functions. Of the various post-translational modifications known to occur in proteins, the recently identified non-disulfide cross-links are unique, residue-specific covalent modifications that confer additional structural stability and unique functional characteristics to these biomolecules. We review an exclusive class of amino acid cross-links encompassing aromatic and sulfur-containing side chains, which not only confer superior biochemical characteristics to the protein but also possess additional spectroscopic features that can be exploited as novel chromophores. Studies of their in vivo reaction mechanism have facilitated their specialized in vitro applications in hydrogels and protein anchoring in monolayer chips. Furthering the discovery of unique canonical cross-links through new chemical, structural, and bioinformatics tools will catalyze the development of protein-specific hyperstable nanostructures, superfoods, and biotherapeutics.

Chemical and biological conjugation strategies for the development of multivalent protein vaccine nanoparticles

The development of subunit vaccine platforms has been of considerable interest due to their good safety profile and ability to be adapted to new antigens, compared to other vaccine typess. Nevertheless, subunit vaccines often lack sufficient immunogenicity to fully protect against infectious diseases. A wide variety of subunit vaccines have been developed to enhance antigen immunogenicity by increasing antigen multivalency, as well as stability and delivery properties, via presentation of antigens on protein nanoparticles. Increasing multivalency can be an effective approach to provide a potent humoral immune response by more strongly engaging and clustering B cell receptors (BCRs) to induce activation, as well as increased uptake by antigen presenting cells and their subsequent T cell activation. Proper orientation of antigen on protein nanoparticles is also considered a crucial factor for enhanced BCR engagement and subsequent immune responses. Therefore, various strategies have been reported to decorate highly repetitive surfaces of protein nanoparticle scaffolds with multiple copies of antigens, arrange antigens in proper orientation, or combinations thereof. In this review, we describe different chemical bioconjugation methods, approaches for genetic fusion of recombinant antigens, biological affinity tags, and enzymatic conjugation methods to effectively present antigens on the surface of protein nanoparticle vaccine scaffolds.

Effect of Antigen Structure in Subunit Vaccine Nanoparticles on Humoral Immune Responses

Subunit vaccines offer numerous attractive features, including good safety profiles and well-defined components with highly characterized properties because they do not contain whole pathogens. However, vaccine platforms based on one or few selected antigens are often poorly immunogenic. Several advances have been made in improving the effectiveness of subunit vaccines, including nanoparticle formulation and/or co-administration with adjuvants. Desolvation of antigens into nanoparticles is one approach that has been successful in eliciting protective immune responses. Despite this advance, damage to the antigen structure by desolvation can compromise the recognition of conformational antigens by B cells and the subsequent humoral response. Here, we used ovalbumin as a model antigen to demonstrate enhanced efficacy of subunit vaccines by preserving antigen structures in nanoparticles. An altered antigen structure due to desolvation was first validated by GROMACS and circular dichroism. Desolvant-free nanoparticles with a stable ovalbumin structure were successfully synthesized by directly cross-linking ovalbumin or using ammonium sulfate to form nanoclusters. Alternatively, desolvated OVA nanoparticles were coated with a layer of OVA after desolvation. Vaccination with salt-precipitated nanoparticles increased OVA-specific IgG titers 4.2- and 22-fold compared to the desolvated and coated nanoparticles, respectively. In addition, enhanced affinity maturation by both salt precipitated and coated nanoparticles was displayed in contrast to desolvated nanoparticles. These results demonstrate both that salt-precipitated antigen nanoparticles are a potential new vaccine platform with significantly improved humoral immunity and a functional value of preserving antigen structures in vaccine nanoparticle design.

Peptide Cross-Linking Using Tyrosine Residues Facilitated by an Exogenous Nickel-Histidine Complex: A Facile Approach for Enhancing Vaccine-Specific Immunogenicity

An improved method for the generation of peptide vaccines using di-tyrosine cross-linking is described. The conserved ion channel peptide, M2e, of influenza A virus was modified with the addition of small tyrosine-rich regions (GYGY-) at both the N- and C-termini and extensively cross-linked via tyrosine-tyrosine linkages to form peptide nanoclusters. The cross-linking was catalyzed using exogenous nickel(II) ions complexed to an exogenous glycine-glycine-histidine peptide in the presence of an oxidizer. Mice that were intranasally or intramuscularly immunized with the M2e-vaccine nanoclusters induced comparable levels of M2e-specific serum antibodies. Vaccination via the intranasal or intramuscular route protected mice from subsequent lethal challenge with an influenza A virus. In comparison to our previous approach, where a histidine-rich tag was added into the peptide structure, the use of exogenous histidine reduced irrelevant off-target immune response. Additionally, the purity of the resulting nanoclusters is an attractive feature, making this approach appealing for vaccine development.

Biopharmaceutical nanoclusters: Towards the self-delivery of protein and peptide therapeutics

Protein and peptide biopharmaceuticals have had a major impact on the treatment of a number of diseases. There is a growing interest in overcoming some of the challenges associated with biopharmaceuticals, such as rapid degradation in physiological fluid, using nanocarrier delivery systems. Biopharmaceutical nanoclusters (BNCs) where the therapeutic protein or peptide is clustered together to form the main constituent of the nanocarrier system have the potential to mimic the benefits of more established nanocarriers (e.g., liposomal and polymeric systems) whilst eliminating the issue of low drug loading and potential side effects from additives. These benefits would include enhanced stability, improved absorption, and increased biopharmaceutical activity. However, the successful development of BNCs is challenged by the physicochemical complexity of the protein and peptide constituents as well as the dynamics of clustering. Here, we present and discuss common methodologies for the synthesis of therapeutic protein and peptide nanoclusters, as well as review the current status of this emerging field.