Immunocontraception: Filamentous Bacteriophage as a Platform for Vaccine Development

Abstract

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.

Keywords: Animal population control; bacteriophage; contraception; phage immunogenicity; phage-peptide fusions; vaccine..

Fig. (1)

Structure of filamentous phage. (A) Microphotograph of filamentous phage particles. (B) Schematic structure of Ff wild type phage particle. Ff phages consist of a tubular protein coat surrounding a single stranded circular DNA. Phage coat is composed of five proteins (pIII, pVI, pVII, pVIII, and pIX). Proteins III, VI, VII, and IX, present in low copy numbers, constitute the minor coat proteins. Proteins III and VI form a cap on one end of the phage virion, whereas proteins pVII and pIX cap the other end of the particle. Protein VIII is the major phage coat protein. It is present in high copy numbers (2700 in wild type phage). Protein VIII surrounds the phage DNA and forms the tubular body of the particle. (C) Major types of Ff filamentous phage vectors used for peptide display. Phage vectors 33 and 88 contain two respective genes, one encoding a recombinant protein and one encoding wild type protein. Therefore, the resulting phage particles display fusion proteins interspersed with wild type proteins. Phage vectors 3 and 8 contain gene encoding a fusion protein, but no gene for the wild type protein. As a result, all copies of their corresponding proteins are modified with fusion peptides.

Fig. (2)

Immune cell responses to filamentous phage. (1) Dendritic cells uptake phage particles, degrade them, and present phage-derived peptides as complexes with MHC II molecules to naïve antigen-specific T helper (Th) cells. (2) Th cells become activated and undergo transformation into Th1 or Th2 effector cells. (3) Activated Th1 cells secrete Th1-type cytokines to upregulate cell-mediated immune responses such as activation of macrophages or cytotoxic T (Tc) cells. (4) Antigen-specific activated (effector) Tc recognize MHC I:phage-derived peptide complexes presented on target cells and destroy these cells. (5) Th2-type cytokines induce antigen-specific B cells, which (6) undergo transformation into antibody secreting plasma cells. Antibodies secreted by plasma cells neutralize antigens similar to B cell epitopes on phage particles.

Fig. (3)

Sperm-oocyte interaction in in vitro fertilization system in the presence of porcine serum. (A) Serum collected from a control pig (not immunized). (B) Serum collected from a pig immunized with phage particles that stimulated production of anti-sperm antibodies. Dim circles are pronuclei in oocytes. Small bright elongated dots (indicated with arrows) are sperm cells. Cellular DNA is blue (staining with Hoechst dye). (The color version of the figure is available in the electronic copy of the article).