Virus-like Particles for Vaccine Development

VLPs are nanoparticles formed by the self-assembly of viral structural proteins that mimic viral particles. VLPs do not contain genetic material, therefore they cannot replicate and are not infectious. This significantly enhances their safety compared to attenuated live vaccines or recombinant viral vectors, as unlike viral vector vaccines, they do not synthesize additional copies of immunogens.

Classification of VLPs

VLPs can be divided into two categories: non-enveloped VLPs and enveloped VLPs. Non-enveloped VLPs lack an external lipid envelope, while enveloped VLPs have a membrane envelope derived from host cells that can incorporate glycoprotein antigens into the lipid membrane. Both non-enveloped and enveloped VLPs can be single-layered or multi-layered and are assembled from one or multiple proteins. The HPV VLP vaccine is an example of a simple non-enveloped VLP, consisting of a single layer. Influenza VLPs are a mature example of enveloped VLPs.

Immunological Properties of VLPs

VLPs carry specific viral epitopes of their own or foreign antigens (chimeric VLPs) and can safely stimulate humoral and cellular immune responses, inducing strong immunogenicity. VLPs typically exhibit antigenic similarity to their source viruses. They present antigens in an organized and highly repetitive manner, thereby eliciting effective humoral and cellular immune responses due to optimal stimulation of B cells by particles with repetitive surfaces. Additionally, VLPs have the ability to activate T-helper cells because they naturally encode T-helper cell epitopes that activate cellular immune responses. The immune system can be further activated by loading VLPs with specific pathogen-associated molecular patterns (PAMPs), such as single-stranded RNA or CpG oligodeoxynucleotides (CpG-ODN). This characteristic enhances the immunogenicity of VLPs even at lower doses.

Versatility and Applications of VLPs

Proteins from over 100 viruses across 35 viral families (including both enveloped and non-enveloped viruses) have been demonstrated to assemble into VLPs. This explains why the use of VLPs provides a versatile emerging platform for creating vaccines, offering an alternative to using viral vectors. In recent years, the application of this technology has significantly grown due to the diversification of designed vaccine types and their clinical applications. Compared to subunit vaccines, typically lower antigen doses are sufficient to induce similar protective responses. The spontaneous assembly of viral proteins into VLPs ensures rapid and efficient production, enabling economical large-scale manufacturing of VLPs. Beyond their use in preventing infectious diseases and cancer vaccines, VLPs can also serve as carriers for drugs, dyes, or nanomaterials in nanomedicine.

Production Systems for VLPs

VLPs are developed by cloning viral structural genes into expression vectors and then expressing these genes in expression systems. Approximately 30% of VLPs are produced in bacteria, although different expression systems are also used, such as baculovirus-insect cells, mammalian cells, and plants. The E. coli expression system is known for its ease of use and cost-effectiveness. However, its application in producing non-enveloped VLPs is limited due to the lack of post-translational modification (PTM) systems. Production in yeast features low production and maintenance costs but has limited ability to introduce PTMs (e.g., glycosylation, phosphorylation). Conversely, baculovirus-insect cell and mammalian cell systems have the advantage of being able to induce more complete PTM modifications, thereby facilitating the production of both enveloped and non-enveloped VLPs, including the assembly of multi-protein VLPs. Additionally, plant expression systems have emerged as versatile and promising platforms for reducing protein production costs.

Commercial VLP Vaccines

The first VLP-based vaccine, Recombivax HB, was approved for human use in 1986. It is a yeast-produced hepatitis B vaccine based on VLPs, first developed in 1982. Since then, several VLP-based vaccines have been commercialized. Engerix-B is another vaccine against hepatitis B virus, also produced in yeast. Gardasil and Gardasil 9 are two other commercialized VLP-based vaccines produced in yeast, carrying the L1 protein of HPV. Recent studies have successfully produced HIV VLPs carrying Gag as well as p17 and p24 proteins. The recently approved two malaria vaccines, RTS, S/AS01 (Mosquirix) and R21/Matrix M, utilize hepatitis B surface antigen (HBsAg) VLPs as a platform, displaying malaria epitopes on the surface of VLPs formed in yeast.

Hecolin is the only vaccine that effectively prevents hepatitis E. It is the first VLP vaccine produced in bacteria carrying the truncated capsid protein p239 of hepatitis E virus. Phase III clinical trials have confirmed the high immunogenicity and efficacy of Hecolin, demonstrating its ability to induce significant titers of hepatitis E virus antibodies. Furthermore, the utilization of the E. coli expression system facilitates cost-effective production. Hecolin received regulatory approval in China in 2011. Recently, it has been approved as the third-generation hepatitis B vaccine PreHevBio, produced in mammalian cells. Vaccines targeting malaria, influenza, HPV, and West Nile virus are other examples of VLP candidates produced in bacteria. The baculovirus/insect cell system is the preferred system for VLP production because it facilitates efficient expression of multi-protein VLPs with various PTMs. For example, the HPV vaccine Cervarix contains L1 capsid proteins of HPV 16 and HPV 18.

Various VLP-based vaccines targeting viruses such as SARS-CoV-2, influenza A, Ebola, adenovirus 7, HPV, HIV-1, hepatitis B, SARS-CoV, dengue, rabies, rotavirus, norovirus, and malaria have recently been approved or are being tested in clinical trials.

Reference

Henríquez R, Munoz-Barroso I. Viral vector-and virus-like particle-based vaccines against infectious diseases: A minireview. Heliyon, 2024, 10(15).