Virus-like particles derived from bacteriophage MS2 as antigen scaffolds and RNA protective shells

Abstract

The versatile potential of bacteriophage MS2-derived virus-like particles (VLPs) in medical biotechnology has been extensively studied during the last 30 years. Since the first reports showing that MS2 VLPs can be produced at high yield and relatively easily engineered, numerous applications have been proposed. Particular effort has been spent in developing MS2 VLPs as protective capsules and delivery platforms for diverse molecules, such as chemical compounds, proteins and nucleic acids. Among these, two are particularly noteworthy: as scaffolds displaying heterologous epitopes for vaccine development and as capsids for encapsulation of foreign RNA. In this review, we summarize the progress in developing MS2 VLPs for these two areas.

Plain language summary

Hollow, nanosized protein particles have many potential uses. If they can be appropriately engineered, they may for example be able to carry therapeutic cargoes to diseased cells or be used as a vaccine where appropriate antigens are mounted on their external surface. Many viruses offer a ready-made protein particle, the capsid, which can be made hollow by exclusion of the viral genetic material. MS2 is a virus that targets bacteria – a bacteriophage – which is well characterized and has been developed over many years for a number of applications. It has particular promise for development as a vaccine and for RNA delivery, both of which are reviewed here.

Graphical abstract

Bacteriophage MS2 has features and properties that make it attractive for a range of biotechnological and biomedical applications. Figure created with Biorender.com.

Executive summary

MS2 virus-like particles as scaffolds displaying antigenic peptides

• A number of virus-like particles (VLPs) are known to be effective immunogens and in some cases have been developed into clinically approved vaccines.

• MS2 is an icosahedral ssRNA bacteriophage around 26 nm in diameter.

• Immunogenic epitopes can be genetically engineered into the externally displayed AB loop of the capsid proteins.

• MS2 VLPs displaying such epitopes have been shown to be capable of eliciting an immune response in mice.

• Genetic fusion of identical coat protein monomers to form fused dimers stabilizes the resulting VLP but only one A/B loop of the resulting dimer can tolerate an insertion if stability is to be maintained.

• Proof-of-principle studies have been carried out for MS2 modified to act as vaccines against HPV, foot-and-mouth virus and Chlamydia trichomatis.

• The ability of MS2 VLPs to encapsidate their own encoding nucleic acid means that CPs with fused peptide can be used to ‘biopan’ a peptide library for binding to targets (e.g., antibodies) wherein the sequence encoding binding peptides can be easily recovered.

MS2 VLPs as capsids for encapsulation of foreign RNA

• MS2 encapsidates its own RNA by binding to the ‘pac site’, a stem-loop structure.

• MS2 VLPs can encapsidate foreign RNA sequences by fusion to a pac site sequence.

• Fusion of target RNA to a copy of the pac site sequence with expression from the same transcript as the MS2 CP and the maturase avoids nonspecific RNA packing. The RNA is protected and stabilized and is known as armored RNA.

• Armored RNA can be used as internal control standards in real-time-PCR.

• Armored RNAs able to protect longer RNA sequences were developed through modification of expression vector strategy and pac site.

• Both in vivo and in vitro RNA packaging approaches have been developed.

• Foreign sRNAs and mRNA can be encapsidated within MS2 VLPs.

• MS2 VLPs decorated with cell-penetrating peptides can deliver miRNAs to cells.

• mRNA, being longer, is more challenging to package but this has been demonstrated.

Keywords: :

• bionano
• capsid
• drug delivery
• phage
• protein cage
• protein engineering
• RNA delivery
• RT-PCR
• vaccines

Financial disclosure

A Naskalska acknowledges the support of grant no. UMO-2020/37/B/NZ6/03878 awarded by the National Science Centre (NCN) Poland. JG Heddle was funded by a Leverhulme International Professorship awarded by the Leverhulme Trust. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

Competing interests disclosure

The authors declare the following competing financial interest: JG Heddle is the founder of and holds equity in nCage Therapeutics LLC, which aims to commercialize protein cages for therapeutic applications. The authors have no other competing interests or relevant affiliations with any organization or entity with the subject matter or materials discussed in the manuscript apart from those disclosed.

Writing disclosure

No writing assistance was utilized in the production of this manuscript.

Additional information

Funding

A Naskalska acknowledges the support of grant no. UMO-2020/37/B/NZ6/03878 awarded by the National Science Centre (NCN) Poland. JG Heddle was funded by a Leverhulme International Professorship awarded by the Leverhulme Trust.