A hepatitis B virus core antigen-based virus-like particle vaccine expressing SARS-CoV-2 B and T cell epitopes induces epitope-specific humoral and cell-mediated immune responses but confers limited protection against SARS-CoV-2 infection.

The hepatitis B virus core antigen (HBcAg) tolerates insertion of foreign epitopes and maintains its ability to self-assemble into virus-like particles (VLPs). We constructed a ∆HBcAg-based VLP vaccine expressing three predicted severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) B and T cell epitopes and determined its immunogenicity and protective efficacy. The recombinant ∆HBcAg-SARS-CoV-2 protein was expressed in Escherichia coli, purified, and shown to form VLPs. K18-hACE2 transgenic C57BL/6 mice were immunized intramuscularly with ∆HBcAg VLP control (n = 15) or ∆HBcAg-SARS-CoV-2 VLP vaccine (n = 15). One week after the 2nd booster and before virus challenge, five ∆HBcAg-SARS-CoV-2 vaccinated mice were euthanized to evaluate epitope-specific immune responses. There is a statistically significant increase in epitope-specific Immunoglobulin G (IgG) response, and statistically higher interleukin 6 (IL-6) and monocyte chemoattractant protein-1 (MCP-1) expression levels in ∆HBcAg-SARS-CoV-2 VLP-vaccinated mice compared to ∆HBcAg VLP controls. While not statistically significant, the ∆HBcAg-SARS-CoV-2 VLP mice had numerically more memory CD8+ T-cells, and 3/5 mice also had numerically higher levels of interferon gamma (IFN-γ) and tumor necrosis factor (TNF). After challenge with SARS-CoV-2, ∆HBcAg-SARS-CoV-2 immunized mice had numerically lower viral RNA loads in the lung, and slightly higher survival, but the differences are not statistically significant. These results indicate that the ∆HBcAg-SARS-CoV-2 VLP vaccine elicits epitope-specific humoral and cell-mediated immune responses but they were insufficient against SARS-CoV-2 infection.

Expression and Identification of Virus-like Particles Presenting Novel Coronavirus RBD Antigen

Objective: Hepatitis B virus core protein HBc was used as vector to construct virus-like particles expressing novel coronavirus spike protein receptor binding domain RBD, and their immunogenicity was identified, which provides a new idea for the development of COVID-19 vaccines. Methods: The amino acid coding sequence 78 and 81 of hepatitis B virus core protein HBc (1-183 aa) were inserted into novel coronavirus spike protein receptor binding domain RBD and ligated by flexible linker (G4S) 3. After sequence optimization, the fusion gene was cloned into prokaryotic expression vector pET-28a (+) and transformed into expression strain Rosetta. After induced expression in self-inducing medium, the virus-like particles (VLPs) were purified by sucrose density gradient centrifugation. VLPs were detected and identified by SDS-PAGE, Western blot and transmission electron microscope. BALB/c mice were immunized subcutaneously with the prepared VLPs in equal proportion with adjuvant. The specific antibodies in the serum of the mice were analyzed by ELISA to verify the immune effect of HBc-RBD VLPs. Results: Escherichia coli can express partially soluble VLPs in self-inducing medium. VLPs could be observed by transmission electron microscope after purification by sucrose density gradient centrifugation. Mice immunized with HBc-RBD VLPs produced specific antibodies against RBD antigen. Conclusion: VLPs displaying RBD antigen were successfully expressed in prokaryotic expression systems, and their immunogenicity was preliminarily verified by mouse experiment, which provides a new direction for the research and development of novel coronavirus vaccines. © 2022, China Biotechnology Press. All rights reserved.