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 <i>Escherichia coli</i> 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-<i>γ</i> 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.