Phage display of a CTL epitope elicits a long-term in vivo cytotoxic response

Antigen Delivery Systems: Past, Present, and Future

The COVID-19 pandemic has increased demand for safe and effective vaccines. Research to develop vaccines against diseases including Middle East respiratory syndrome, Ebolavirus, human immunodeficiency virus, and various cancers would also contribute to global well-being. For successful vaccine development, the advancement of technologies such as antigen (Ag) screening, Ag delivery systems and adjuvants, and manufacturing processes is essential. Ag delivery systems are required not only to deliver a sufficient amount of Ag for vaccination, but also to enhance immune response. In addition, Ag types and their delivery systems determine the manufacturing processes of the vaccine product. Here, we analyze the characteristics of various Ag delivery systems: plasmids, viral vectors, bacterial vectors, nanoparticles, self-assembled particles, natural and artificial cells, and extracellular vesicles. This review provides insight into the current vaccine landscape and highlights promising avenues of research for the development and improvement of Ag delivery systems.

Arming Filamentous Bacteriophage, a Nature-Made Nanoparticle, for New Vaccine and Immunotherapeutic Strategies

The pharmaceutical use of bacteriophages as safe and inexpensive therapeutic tools is collecting renewed interest. The use of lytic phages to fight antibiotic-resistant bacterial strains is pursued in academic and industrial projects and is the object of several clinical trials. On the other hand, filamentous bacteriophages used for the phage display technology can also have diagnostic and therapeutic applications. Filamentous bacteriophages are nature-made nanoparticles useful for their size, the capability to enter blood vessels, and the capacity of high-density antigen expression. In the last decades, our laboratory focused its efforts in the study of antigen delivery strategies based on the filamentous bacteriophage 'fd', able to trigger all arms of the immune response, with particular emphasis on the ability of the MHC class I restricted antigenic determinants displayed on phages to induce strong and protective cytotoxic responses. We showed that fd bacteriophages, engineered to target mouse dendritic cells (DCs), activate innate and adaptive responses without the need of exogenous adjuvants, and more recently, we described the display of immunologically active lipids. In this review, we will provide an overview of the reported applications of the bacteriophage carriers and describe the advantages of exploiting this technology for delivery strategies.

Immunocontraception: Filamentous Bacteriophage as a Platform for Vaccine Development

BACKGROUND

Population control of domestic, wild, invasive, and captive animal species is a global issue of importance to public health, animal welfare and the economy. There is pressing need for effective, safe, and inexpensive contraceptive technologies to address this problem. Contraceptive vaccines, designed to stimulate the immune system in order to block critical reproductive events and suppress fertility, may provide a solution. Filamentous bacteriophages can be used as platforms for development of such vaccines.

OBJECTIVE

In this review authors highlight structural and immunogenic properties of filamentous phages, and discuss applications of phage-peptide vaccines for advancement of immunocontraception technology in animals.

RESULTS

Phages can be engineered to display fusion (non-phage) peptides as coat proteins. Such modifications can be accomplished via genetic manipulation of phage DNA, or by chemical conjugation of synthetic peptides to phage surface proteins. Phage fusions with antigenic determinants induce humoral as well as cell-mediated immune responses in animals, making them attractive as vaccines. Additional advantages of the phage platform include environmental stability, low cost, and safety for immunized animals and those administering the vaccines.

CONCLUSION

Filamentous phages are viable platforms for vaccine development that can be engineered with molecular and organismal specificity. Phage-based vaccines can be produced in abundance at low cost, are environmentally stable, and are immunogenic when administered via multiple routes. These features are essential for a contraceptive vaccine to be operationally practical in animal applications. Adaptability of the phage platform also makes it attractive for design of human immunocontraceptive agents.

Bacteriophages and phage-inspired nanocarriers for targeted delivery of therapeutic cargos

The main goal of drug delivery systems is to target therapeutic cargoes to desired cells and to ensure their efficient uptake. Recently a number of studies have focused on designing bio-inspired nanocarriers, such as bacteriophages, and synthetic carriers based on the bacteriophage structure. Bacteriophages are viruses that specifically recognize their bacterial hosts. They can replicate only inside their host cell and can act as natural gene carriers. Each type of phage has a particular shape, a different capacity for loading cargo, a specific production time, and their own mechanisms of supramolecular assembly, that have enabled them to act as tunable carriers. New phage-based technologies have led to the construction of different peptide libraries, and recognition abilities provided by novel targeting ligands. Phage hybridization with non-organic compounds introduces new properties to phages and could be a suitable strategy for construction of bio-inorganic carriers. In this review we try to cover the major phage species that have been used in drug and gene delivery systems, and the biological application of phages as novel targeting ligands and targeted therapeutics.

Beyond phage display: non-traditional applications of the filamentous bacteriophage as a vaccine carrier, therapeutic biologic, and bioconjugation scaffold

For the past 25 years, phage display technology has been an invaluable tool for studies of protein-protein interactions. However, the inherent biological, biochemical, and biophysical properties of filamentous bacteriophage, as well as the ease of its genetic manipulation, also make it an attractive platform outside the traditional phage display canon. This review will focus on the unique properties of the filamentous bacteriophage and highlight its diverse applications in current research. Particular emphases are placed on: (i) the advantages of the phage as a vaccine carrier, including its high immunogenicity, relative antigenic simplicity and ability to activate a range of immune responses, (ii) the phage's potential as a prophylactic and therapeutic agent for infectious and chronic diseases, (iii) the regularity of the virion major coat protein lattice, which enables a variety of bioconjugation and surface chemistry applications, particularly in nanomaterials, and (iv) the phage's large population sizes and fast generation times, which make it an excellent model system for directed protein evolution. Despite their ubiquity in the biosphere, metagenomics work is just beginning to explore the ecology of filamentous and non-filamentous phage, and their role in the evolution of bacterial populations. Thus, the filamentous phage represents a robust, inexpensive, and versatile microorganism whose bioengineering applications continue to expand in new directions, although its limitations in some spheres impose obstacles to its widespread adoption and use.

HIV-1 Gag p17 presented as virus-like particles on the E2 scaffold from Geobacillus stearothermophilus induces sustained humoral and cellular immune responses in the absence of IFNγ production by CD4+ T cells

We have constructed stable virus-like particles displaying the HIV-1 Gag(p17) protein as an N-terminal fusion with an engineered protein domain from the Geobacillus stearothermophilus pyruvate dehydrogenase subunit E2. Mice immunized with the Gag(p17)-E2 60-mer scaffold particles mounted a strong and sustained antibody response. Antibodies directed to Gag(p17) were boosted significantly with additional immunizations, while anti-E2 responses reached a plateau. The isotype of the induced antibodies was biased towards IgG1, and the E2-primed CD4+ T cells did not secrete IFNγ. Using transgenic mouse model systems, we demonstrated that CD8+ T cells primed with E2 particles were able to exert lytic activity and produce IFNγ. These results show that the E2 scaffold represents a powerful vaccine delivery system for whole antigenic proteins or polyepitope engineered proteins, evoking antibody production and antigen specific CTL activity even in the absence of IFNγ-producing CD4+ T cells.

Triggering DTH and CTL activity by fd filamentous bacteriophages: role of CD4+ T cells in memory responses

The ability of fd bacteriophage particles to trigger different arms of the immune system has been previously shown by us with particular emphasis on the ability of phages to raise CTL responses in vitro and in vivo. Here we show that fd virions in the absence of adjuvants are able to evoke a DTH reaction mediated by antigen specific CD8+ T cells. In addition, we analyzed the induction of CTL responses in mice depleted of CD4+ T cells, and we observed that short-term secondary CTL responses were induced in the absence of CD4+ T cells while induction of long-term memory CTLs required the presence of CD4+ T lymphocytes. These results examine the cellular mechanism at the basis of fd efficiency and provide new elements to further validate the use of fd particles for eliciting and monitoring antigen-specific CTLs.

Antigenic properties of HCMV peptides displayed by filamentous bacteriophages vs. synthetic peptides

Several efforts have been invested in the identification of CTL and Th epitopes, as well as in the characterization of their immunodominance and MHC restriction, for the generation of a peptide-based HCMV vaccine. Small synthetic peptides are, however, poor antigens and carrier proteins are important for improving the efficacy of synthetic peptide vaccines. Recombinant bacteriophages appear as promising tools in the design of subunit vaccines. To investigate the antigenicity of peptides carried by recombinant bacteriophages we displayed different HCMV MHCII restricted peptides on the capsid of filamentous bacteriophage (fd) and found that hybrid bacteriophages are processed by human APC and activate HCMV-specific CD4 T-cells. Furthermore we constructed a reporter T-cell hybridoma expressing a chimeric TCR comprising murine alphabeta constant regions and human variable regions specific for the HLA-A2 restricted immunodominant NLV peptide of HCMV. Using the filamentous bacteriophage as an epitope carrier, we detected a more robust and long lasting response of the reporter T-cell hybridoma compared to peptide stimulation. Our results show a general enhancement of T-cell responses when antigenic peptides are carried by phages.