Effectiveness of a hybrid vaccine against COVID-19 and influenza

By Bhavana KunkalikarReviewed by Aimee MolineuxJun 16 2022

In a recent study published in Vaccines, researchers assessed the efficiency of a hybrid vaccine against influenza and coronavirus disease 2019 (COVID-19) viruses.

Study: Influenza Virus-like Particle-Based Hybrid Vaccine Containing RBD Induces Immunity against Influenza and SARS-CoV-2 Viruses. Image Credit: insta_photos/Shutterstock

People worldwide are exposed to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) as well as the influenza virus. Therefore the development of a two-in-one vaccine that could elicit protection against both these infections is a subject of great interest.

About the study

In the present study, researchers developed a hybrid vaccine using influenza virus-like particle (VLP) vaccine incorporated with glycosylphosphatidylinositol (GPI)-anchored SARS-CoV-2 receptor-binding domain (RBD) fused to granulocyte-macrophage colony-stimulating factor (GM-CSF) fusion protein.

The team developed a fusion protein gene by fusing deoxyribonucleic acid (DNA) sequences that are specific to the SARS-CoV-2 spike (S) protein RBD domain, GPI-anchor signal derived from human CD59, and mouse GM-CSF. Flow cytometry was subsequently performed to demonstrate the expression of the fusion protein on the transfected CHO-S cells. The team also assessed whether the anti-RBD antibodies present in the convalescent sera collected from SARS-CoV-2-infected patients identified the RBD of the fusion protein by performing an enzyme-linked immunosorbent assay (ELISA). 

The team subsequently evaluated whether the fusion protein had two functions as GM-CSF and while also binding to human ACE2. Furthermore, the researchers developed the VLP hybrid vaccine by incorporating the influenza VLP with GPI-RBD-GM-CSF and GPI-interleukin (IL)-12 via protein transfer. Flow cytometry was also conducted using anti-GM-CSF antibodies and fluorochrome-conjugated anti-IL-12 to verify the dual incorporation of the GPI-RBD-GM-CSF and GPI-IL-12 onto VLPs. The retention of function by VLP-incorporated GM-CSF was verified by culturing mouse bone marrow-derived dendritic cells (BDMC), followed by the measurement of BMDC proliferation using XTT assay.        

The team tested the induction of antibody response by the GPI-RBD-GM-CSF fusion protein by immunizing BALB/c mice with either the fusion protein or VLPs incorporated into the fusion protein along with GPI-IL-12. VLP without cytokines or commercially available RBD was used as control. The team administered a booster dose to the mice population two to four weeks after the first dose. Blood samples were obtained from the mice every two to four weeks in order to estimate antibody titer, virus neutralization assays, and ACE2 binding inhibition. 

Results

The study results showed that almost 76% of the fusion protein was secreted from the cell surface of the RBD-GM-CSF fusion protein, which suggested that the fusion protein was anchored into the cell membrane through a GPI tail. The RBD in the fusion protein exhibited ACE2 binding activity while RBD-specific neutralizing antibodies could bind to GPI-RBD-GM-CSF present on the CHO-S cell transfectants.

Antibodies found in the sera of COVID-19 patients could efficiently bind to the GPI-RBD-GM-CSF. Moreover, ELISA confirmed that RBD from the purified fusion protein retained its receptor binding activity. Additionally, the XTT assay showed that GPI-RBD-GM-CSF that was incorporated in the VLP vaccine could effectively stimulate BMDC proliferation.

The team observed that the VLP hybrid vaccine elicited a significant antibody response after the administration of the GPI-RBD-GM-CSF booster dose. The antibody response elicited by the fusion protein was similar in both the intramuscular and subcutaneous routes of vaccine administration. The antibodies selectively bound to the CHO-S cells which expressed the fusion protein but not to the untransfected CHO-S cells. This indicated the high specificity of the antibody response to the fusion protein.

The team also noted that the purified GPI-RBD-GM-CSF fusion protein-induced antibody levels that were comparable to those elicited by the VLP hybrid vaccine. However, the neutralizing antibody titers were considerably low in the mice that were immunized by only the GPI-RBD-GM-CSF protein without any VLP. This suggested that fusion protein that was incorporated with VLP elicited a more robust neutralizing antibody response as compared to that of the soluble fusion protein.

Furthermore, the purified GPI-RBD-GM-CSF fusion protein alone or when incorporated with VLPs induced a substantial antibody response. On its own, the fusion protein induced a primarily Th2 type immunoglobulin 1 (IgG1) response while the hybrid VLP vaccine elicited both IgG1 and IgG2a responses. Notably, the incorporation of GPI-IL-12 into the hybrid VLP vaccine did not affect the IgG2a response.

The team also observed that the hybrid vaccine by itself and the hybrid vaccine without IL-12 elicited robust antibody responses against the RBD as well as the influenza VLP antigens. These antibodies could sufficiently neutralize live virus infection when assessed via a plaque reduction neutralization titer (PRNT) assay. Moreover, substantially reduced viral replication was observed in the hybrid vaccine-treated mice as compared to the VLP-treated ones.

Overall, the study findings showed that the delivery of SARS-CoV-2 RBD protein via influenza VLPs along with cytokine adjuvants served as an effective platform for the development of multivalent vaccines. The researchers believe that the usage of immobilized cytokines as an adjuvant will help elicit anti-viral immunity with minimal adverse effects.