Immunogenicity of SARS-CoV-2 vaccine prototype based on virus-like particles of hepatitis B core antigen from genotype G and interleukin 12 expressing Semliki Forest virus as a genetic adjuvant.

Virus-like particles (VLPs) based on hepatitis B core antigen (HBc) represent an immunogenic and modular platform for epitope presentation. In this study, the VLPs formed by the modified recombinant hepatitis B core antigen from genotype G (HBc/G) were used as carriers for the presentation of a receptor-binding motif (RBM) of the Delta variant of Severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2). The RBM was inserted at the C-terminus of the modified HBc/G extended by the addition of a second, specifically modified C-terminal domain of HBc/G. All arginine residues in the extra domain were replaced with glycine, resulting in a 'two-tailed' HBc/G-Gly vector. The resulting HBc/G-Gly-RBM construct successfully formed regular VLPs in Escherichia coli and elicited specific antibody responses in mice. Despite the moderate immunogenicity of the RBM insert compared with the HBc carrier, sera from RBM-VLP-immunized animals exhibited neutralizing activity against MLV particles pseudotyped with the SARS-CoV-2 Delta spike and showed cross-reactivity with receptor-binding domains from the Wuhan and Omicron variants. To enhance the immune response, a replication-deficient Semliki Forest virus (SFV) vector expressing IL-12 was evaluated alone and in combination with the squalene-based adjuvant AddaVax. The co-administration of SFV-IL12 and AddaVax modestly improved virus neutralization rates and promoted a Th1 response, characterized by increased IgG2a production and IFN-γ secretion. These findings demonstrate the feasibility of combining classical and genetic adjuvants with the HBc-based VLP platform and provide preliminary insights for further optimization toward more potent and protective SARS-CoV-2 vaccine candidates.