Multivalent nanoparticle-based vaccines protect hamsters against SARS-CoV-2 after a single immunization

The ongoing COVID-19 pandemic highlights the urgent need for effective vaccines. While various vaccine platforms (nucleic acid-based, viral vector-based, and subunit vaccines) are under development, focusing on the SARS-CoV-2 spike (S) protein, concerns remain regarding the need for multiple doses and widespread vaccine distribution. Nanotechnology offers potential improvements, with nanoparticles like virus-like particles (VLPs) being ideal scaffolds for multivalent antigen display. Multivalent display enhances immunogenicity by effectively clustering B-cell receptors. Previous research showed improved immunogenicity with nanoscale scaffolds, but often required multiple immunizations and lacked testing of protective efficacy against SARS-CoV-2 challenge. This study aimed to create a general platform for nanoparticle-based antigen display to provide protection against SARS-CoV-2, addressing the limitations of existing approaches.

The literature review section emphasizes the severity of the COVID-19 pandemic and the importance of developing effective vaccines. It highlights the various vaccine platforms under development, all focusing on eliciting an immune response against the SARS-CoV-2 spike protein, which is crucial for viral entry. The review underscores the advantages of using nanotechnology and VLPs for antigen presentation, citing their ability to mimic natural viruses in size and geometry and the enhanced immunogenicity resulting from multivalent antigen display. A relevant study is mentioned, demonstrating that antigen display on a nanoscale scaffold resulted in increased immunogenicity, though it employed a prime-boost immunization strategy and lacked assessment of protective efficacy against SARS-CoV-2 challenge in the animal model. This gap motivates the current research which aims for a single-dose effective vaccine.

The study developed a VLP-based platform using the MS2 bacteriophage coat protein. A single-chain MS2 coat protein dimer with an Avitag insert was generated, enabling site-specific biotinylation. This was co-expressed with BirA in E. coli, purified using affinity chromatography and size exclusion chromatography (SEC). A prefusion-stabilized variant of the SARS-CoV-2 S protein (S-2P and HexaPro) was also created with an Avitag and his-tag, expressed in Expi293f cells, purified using IMAC, and biotinylated. Biotinylated S protein was then coupled to streptavidin (SA)-coated MS2 VLPs to create VLP-S2P and VLP-S6Pro. The resulting VLPs were characterized using SDS-PAGE, analytical SEC, dynamic light scattering (DLS), and negative-stain transmission electron microscopy (NS-TEM). Binding of ACE2-Fc and CR3022 antibody was assessed by ELISA to confirm proper protein folding. Cryo-electron microscopy (cryo-EM) was used to visualize VLP-S6Pro structure. Syrian hamsters were immunized with a single dose of VLP-S2P or VLP-S6Pro. Antibody responses (IgG and neutralizing antibodies) were measured by ELISA. Hamsters were then challenged with SARS-CoV-2, and viral loads in lungs and nasal turbinates were quantified. Body weight changes were also monitored.

The researchers successfully generated VLPs displaying multiple copies of the SARS-CoV-2 spike protein. Characterization confirmed the expected structure and size of the VLPs, with approximately 27 streptavidin molecules and approximately 50 S2P or S6P molecules per VLP. ELISA showed that the S protein on the VLPs maintained the correct conformation and retained binding to ACE2 and CR3022, indicating correct folding. A single immunization with VLP-S2P or VLP-S6Pro elicited high neutralizing antibody titers in Syrian hamsters. Critically, hamsters immunized with either VLP construct showed complete protection against SARS-CoV-2 challenge, with no detectable infectious virus in their lungs three days post-infection. The nasal turbinates also showed significantly lower viral loads compared to control groups (PBS and MS2-SA VLPs alone). Body weight changes following infection supported these findings; the vaccinated hamsters displayed no significant weight loss compared to controls.

The results demonstrate the effectiveness of a single-dose nanoparticle-based vaccine against SARS-CoV-2. The complete protection observed in hamsters, as evidenced by the absence of detectable virus in the lungs, is a significant achievement. The high neutralizing antibody titers elicited by the vaccine likely contribute to this protective effect. The study addresses the need for simpler, single-dose vaccination strategies that are crucial for global pandemic response and control. The use of a readily scalable platform like the MS2 bacteriophage coat protein holds promise for future vaccine development against SARS-CoV-2 variants and other emerging pathogens. While the current study employed hamsters, further research including clinical trials is needed to confirm efficacy and safety in humans. The platform’s adaptability to incorporate different spike protein variants is also beneficial in addressing emerging variants of concern.

This study successfully developed a highly effective, single-dose nanoparticle-based vaccine that provided complete protection against SARS-CoV-2 in a hamster model. The use of a well-characterized, scalable platform (MS2 VLPs) offers significant advantages for rapid vaccine development and adaptation to emerging viral variants. Future studies should focus on optimizing the manufacturing process for clinical translation and evaluating the vaccine's efficacy and safety in human clinical trials. The platform's adaptability suggests broad potential for combating current and future pandemics.

The study used a hamster model, which may not perfectly reflect human immune responses. While the vaccine showed complete protection in this model, further studies are necessary to confirm efficacy and safety in humans. The scalability and manufacturing costs of this nanoparticle-based platform for large-scale production need further investigation to ensure clinical viability. Finally, the study's focus on a single viral variant warrants further exploration of its effectiveness against emerging variants of concern.