In a recent study posted to the bioRxiv* pre-print server, a team of researchers developed a hybrid vaccine for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and influenza viruses using influenza virus-like particles (VLP).

Study: Influenza virus-like particle-based hybrid vaccine containing RBD induces immunity against influenza and SARS-CoV-2 viruses. Image Credit: BaLL LunLa/Shutterstock

All the currently used coronavirus disease 2019 (COVID-19) vaccines, including Pfizer/BioNTech, Moderna, Johnson & Johnson, AstraZeneca target the full-length spike (S) protein of SARS-CoV-2 that binds to human angiotensin-converting enzyme 2 (hACE2) protein for entry into host cells.

Cytokines increase the vaccine efficacy by activating immune cells, and in the present study, two cytokines were evaluated for their potential as biological adjuvants – GM-CSF and interleukin-12 (IL-12). GM-CSF has been used as an adjuvant in the FDA-approved prostate cancer vaccine, Provenge®, by Dendreon. Also, it induces a robust immune response primarily through the maturation and differentiation of antigen-presenting cells (APCs), such as dendritic cells.

Similarly, the second cytokine IL-12 induces a Th1 T-cell response with a promising clinical benefit in cancer patients. Preclinical and clinical trials have shown that recombinant soluble IL-12 (as an adjuvant) also enhances immune response and alleviates unfavorable side effects and systemic toxicity in cancer, viral hepatitis, and influenza patients.

About the study

In the present study, researchers used an influenza VLP to develop a two-in-one hybrid vaccine against SARS-CoV-2 and influenza A. They formed a fusion protein using granulocyte-macrophage colony-stimulating factor (GM-CSF) adjuvant and glycosylphosphatidylinositol (GPI)-anchored SARS-CoV-2 S receptor-binding domain (RBD), expressed it in CHO-S cell lines, and then purified and incorporated it onto influenza VLPs to develop the hybrid vaccine.

The protein transfer approach allowed the anchoring of cytokines to the VLPs surface, thus limiting their systemic toxicity at the vaccination site. Another advantage of using a protein transfer approach was that it required only low amounts of VLP for optimum antiviral response in mice.

The researchers used a GPI-anchor to engineer a membrane-bound form of cytokines that permitted the incorporation of purified GPI-anchored proteins into the lipid bilayer of influenza VLPs by a simple protein transfer technique. This approach helped researchers in presenting multiple viral-specific antigens to the immune system to mount a robust immune response.

Findings

The results showed that the hybrid (or two-in-one) vaccine strategy was quite promising, and multivalent vaccines could quite effectively prevent both influenza A and SARS-CoV-2 infections.  

The hybrid vaccine induced a robust antibody response in mice against both influenza A H1N1 virus and mouse-adapted SARS-CoV-2 virus. Additionally, vaccinated mice showed decreased viral load, significantly lower lung viral titers, and less weight loss when challenged with mouse-adapted SARS-CoV-2, compared to naive mice administered with plain VLP.

After three months of receiving the booster dose, mice were still well protected against the H1N1 virus, suggesting that the hybrid vaccine-induced antibody and T cell responses against influenza were long-lasting. Subsequently, neutralizing antibody titers remained high even after six months of vaccination, confirming the durability of immune responses.

In mice, although viral titers in the lungs decreased, vaccination prevented lethality and weight loss, suggesting that the hybrid vaccine containing GPI-RBD-GM-CSF with cytokine adjuvants protected from severe SARS-CoV-2 infection.

Furthermore, purified vaccine (without VLP) induced long-lasting (up to one year) antibody response in immunized mice after a year of receiving a booster dose. However, these antibodies, mostly IgG1, could not neutralize the live virus though they blocked ACE-2 binding to SARS-CoV-2 S. 

Hybrid vaccine (VLP with GPI-RBD-GM-CSF fusion protein and GPI-IL-12), on the other hand, induced both IgG1 and IgG2a isotypes and blocked SARS-CoV-2 virus infection. These findings suggested that the Th1 type response induced by the hybrid vaccine was more protective than the Th2 type response induced by the purified GPI-RBD-GM-CSF.

In the hybrid vaccine, where the adjuvant and antigen source were physically linked, a simultaneous presentation to the host immune cells occurred. This enhanced immune reactivity and increased vaccine efficacy, compared to the vaccination approach wherein antigen and adjuvant mixture remained unconjugated. Further, IL-12 and GM-CSF targeted dendritic cells, by binding to IL-12 and GM-CSF receptors, thereby enhancing antigen uptake, presentation, and subsequently T cell responses.

Conclusions 

To summarize, the present study demonstrated the remarkable effectiveness of a vaccine platform using influenza VLP-based delivery of SARS-CoV-2 RBD protein in combination with cytokine adjuvants to develop hybrid vaccines. These vaccines showed promising efficacy against all the variant strains currently circulating during the ongoing SARS-CoV-2 pandemic. 

The fusion protein vaccine design also allowed the creation of fusion proteins with new variant sequences that might be quickly purified using anti-GM-CSF monoclonal antibody (mAb) affinity chromatography. In the future, the use of immobilized cytokines as adjuvants will pave a safer way to induce antiviral immunity with minimal side effects.

*Important notice

bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Written by

Neha Mathur

Neha Mathur has a Master’s degree in Biotechnology and extensive experience in digital marketing. She is passionate about reading and music. When she is not working, Neha likes to cook and travel.