Bacteriophage T4 nanoparticles for vaccine delivery against infectious diseases

Abstract

Subunit vaccines containing one or more target antigens from pathogenic organisms represent safer alternatives to whole pathogen vaccines. However, the antigens by themselves are not sufficiently immunogenic and require additives known as adjuvants to enhance immunogenicity and protective efficacy. Assembly of the antigens into virus-like nanoparticles (VLPs) is a better approach as it allows presentation of the epitopes in a more native context. The repetitive, symmetrical, and high density display of antigens on the VLPs mimic pathogen-associated molecular patterns seen on bacteria and viruses. The antigens, thus, might be better presented to stimulate host's innate as well as adaptive immune systems thereby eliciting both humoral and cellular immune responses. Bacteriophages such as phage T4 provide excellent platforms to generate the nanoparticle vaccines. The T4 capsid containing two non-essential outer proteins Soc and Hoc allow high density array of antigen epitopes in the form of peptides, domains, full-length proteins, or even multi-subunit complexes. Co-delivery of DNAs, targeting molecules, and/or molecular adjuvants provides additional advantages. Recent studies demonstrate that the phage T4 VLPs are highly immunogenic, do not need an adjuvant, and provide complete protection against bacterial and viral pathogens. Thus, phage T4 could potentially be developed as a "universal" VLP platform to design future multivalent vaccines against complex and emerging pathogens.

Keywords: Bacteriophage T4; DNA packaging; Phage assembly; Phage display; Vaccines; Virus like particle.

Fig. 1.

Schematic diagram showing potential ways the VLP-based vaccines can stimulate innate and adaptive immune responses. The basic framework of innate and adaptive responses by the host immune system was adapted from Desmet et al. [36]. See text for details of specific advantages provided by VLP vaccines.

Fig. 2.

Structural model of bacteriophage T4. The enlarged capsomer shows the major capsid protein gp23* (cyan; “*” represents the cleaved form) (930 copies), Soc (blue, 870 copies), and Hoc (yellow; 155 copies). Yellow subunits at the five-fold vertices correspond to gp24*. The unique portal vertex (not visible in the picture) connects the head to the tail.

Fig. 3.

Various assembled states of the major capsid protein gp23*; monomer (A), hexameric capsomer (B), and three capsomers (C) of the hexagonal capsid lattice showing Soc subunits (magenta) bound at the interfaces of adjacent capsomers. Hoc monomers (orange) are located at the center of each capsomer. The structures are derived from the cryo-EM structure of the isometric phage T4 capsid [84]. The monomer (A) shows key domains of gp23* that associate through an intricate network of interactions to form the capsomer (B) and the capsid lattice (C). The structure is reinforced by trimeric Soc clamps at the quasi three-fold axes forming a molecular cage around the capsid (D). The capsid subunits are masked in E to depict the Soc molecular cage.

Fig. 4.

The bacteriophage T4 DNA packaging machine. (A) Structural model of the phage T4 DNA packaging machine. (B) Ribbon model of the pentameric motor (cyan) assembled at the dodecameric portal vertex (dark red). The motor fills the capsid with the phage genome. ~171 kb DN, utilizing the energy from ATP hydrolysis.

Fig. 5.

Schematic of phage T4 in vitro display system. The affinity-purified Soc-flised antigen (s) (A) are assembled on purified hoc⁻soc⁻ T4 phage (B) by mixing the two at 4 °C for 45 min to generate the VIPs [138, 139]. The capsid can be displayed with one antigen (C) or a mixture of different antigens (D; shown in different colors). The same principle is used for the display of Hoc-fused antigens or targeting molecules [86, 92].

Fig. 6.

Assembly of anthrax toxin complexes and plague antigens on phage T4. Negatively stained images of hoc⁻soc⁻ T4 phage (A) decorated with anthrax toxin complexes by in vitro display (B). Cryo-electron micrograph of (B) showing rings of anthrax toxin complexes decorating the T4 capsid (C). (D) Cryo-electron micrograph of phage T4 decorated with F1mutV fused to Soc. Due to the small size of Soc-F1mutV relative to anthrax toxin complex, the displayed antigen molecules are seen as a layer of fuzzy projections around the perimeter of the capsid. Some of the displayed anthrax toxin complexes and F1mutV molecules are marked with arrows.

Fig. 7.

Schematic of the assembly of prime-boost T4 nanoparticles. The DNA packaging machine is assembled by binding of gp17 motor at the portal of hoc⁻soc⁻ phage T4 empty head (the cut-out of the head shows both the exterior and the interior) (A). Using the energy from ATP hydrolysis the motor packages DNA molecules into the head (B). Soc-fused antigens (C) and Hoc-tused targeting molecules (D) are then displayed on the heads.

Fig. 8.

Properties of various bacteriophages used in the development of VLP vaccines.