Self-assembling protein nanoparticles in the design of vaccines

Abstract

For over 100 years, vaccines have been one of the most effective medical interventions for reducing infectious disease, and are estimated to save millions of lives globally each year. Nevertheless, many diseases are not yet preventable by vaccination. This large unmet medical need demands further research and the development of novel vaccines with high efficacy and safety. Compared to the 19th and early 20th century vaccines that were made of killed, inactivated, or live-attenuated pathogens, modern vaccines containing isolated, highly purified antigenic protein subunits are safer but tend to induce lower levels of protective immunity. One strategy to overcome the latter is to design antigen nanoparticles: assemblies of polypeptides that present multiple copies of subunit antigens in well-ordered arrays with defined orientations that can potentially mimic the repetitiveness, geometry, size, and shape of the natural host-pathogen surface interactions. Such nanoparticles offer a collective strength of multiple binding sites (avidity) and can provide improved antigen stability and immunogenicity. Several exciting advances have emerged lately, including preclinical evidence that this strategy may be applicable for the development of innovative new vaccines, for example, protecting against influenza, human immunodeficiency virus, and respiratory syncytial virus. Here, we provide a concise review of a critical selection of data that demonstrate the potential of this field. In addition, we highlight how the use of self-assembling protein nanoparticles can be effectively combined with the emerging discipline of structural vaccinology for maximum impact in the rational design of vaccine antigens.

Keywords: Ferritin; HBV; HCV; HIV; HPV; Influenza; Lumazine synthase; Malaria; Structural biology; Virus-like particle (VLP).

Fig. 1

Multivalent nanoparticles favor the generation of potent, long-lived immunoprotection in germinal centers. Recombinant nanoparticles loaded with the desired antigen are designed thoroughly to present multiple copies of a pathogen epitope in a highly ordered manner on the surface of a self-assembling nanoparticle. As opposed to single recombinant antigens that provide brief half-life 1:1 interactions with the BCRs (A), the polydentate nature, i.e. avidity, of the interaction with the nanoparticle enables tighter and prolonged bindings: the dissociation of one antigen molecule can be compensated by the binding of a new antigen molecule or re-association with a new BCR (B). This scenario enables the clustering of BCRs for multiple and simultaneous engagement with the antigen epitopes. Thus, the B-cell traps the antigen-loaded nanoparticle to establish a durable, localized and strong recognition that translates into B-cell intracellular signaling, internalization and processing of the antigen for presentation, via molecules of the MHC complex, to the T follicular helper cells (Tfh) within the germinal centers. This new recognition evokes the secretion of regulatory cytokines by the Tfh cell and ultimately the evolution of B cells into plasma cells that can secrete antigen-specific neutralizing Abs.

Fig. 2

A flow diagram illustrating how human immunology, B-cell cloning, epitope mapping, structural vaccinology, and nanoparticle design can be combined in order to generate next-generation antigen-nanoparticle vaccines. Iterative cycles of structure-based antigen design (SBAD) can be performed to optimize the candidate antigens.

Fig. 3

Generation of chimeric nanoparticles with surface-exposed arrays of immunogenic epitopes. Recombinant DNA technology can be used to make genes that encode self-assembling polypeptides fused with the desired immunogenic epitope for subsequent production in a chosen cell expression system. The chimeric polypeptide then self-assembles within the cell, with an ordered pattern of surface exposed epitopes. Here, we depict a model of ferritin shown as a grey isosurface (PDB: 3bve) that self-assembles in eight identical units, each composed of trimeric ferritin. On the left panel, one of the trimers can be visualized within one of the nanoparticle units. The monomers are colored in orange, green, and cyan. Given the intrinsic propensity of ferritin to self-assemble into a highly symmetric and ordered quaternary architecture, the chimeric nanoparticle is generated with the HA epitope (here shown on the right panel in orange, green, and cyan in cartoon-tube format (PDB: 3sm5)) incorporated and projected as a matrix of ordered and surface exposed epitopes ready for their recognition by BCRs, as described recently , . The figure was prepared using Pymol software (The PyMOL Molecular Graphics System, Version 1.7.6.2, Schrödinger, LLC).