Enhancing Immune Responses against SARS-CoV-2 Variants in Aged Mice with INDUK: A Chimeric DNA Vaccine Encoding the Spike S1-TM Subunits

Currently available vaccines against COVID-19 showed high efficacy against the original strain of SARS-CoV-2 but progressively lower efficacy against new variants. In response to emerging SARS-CoV-2 strains, we propose chimeric DNA vaccines encoding the spike antigen, including a combination of selected key mutations from different variants of concern. We developed two DNA vaccines, pVAX-S1-TM-D614G and pVAX-S1-TM-INDUK (INDUK), encoding the SARS-CoV-2 S1 spike subunit in fusion with the transmembrane region that allows protein trimerization as predicted by in silico analysis. pVAX-S1-TM-D614G included the dominant D614G substitution, while the chimeric vaccine INDUK contained additional selected mutations from the Delta (E484Q and L452R) and Alpha (N501Y and A570D) variants. Considering that aging is a risk factor for severe disease and that suboptimal vaccine responses were observed in older individuals, the immunogenicity of pVAX-S1-TM-D614G and INDUK was tested in both young and aged C57BL/6 mice. Two vaccine doses were able to trigger significant anti-SARS-CoV-2 antibody production, showing neutralizing activity. ELISA tests confirmed that antibodies induced by pVAX-S1-TM-D614G and INDUK were able to recognize both Wuhan Spike and Delta variant Spike as trimers, while neutralizing antibodies were detected by an ACE2:SARS-CoV-2 Spike S1 inhibitor screening assay, designed to assess the capacity of antibodies to block the interaction between the viral spike S1 protein and the ACE2 receptor. Although antibody titer declined within six months, a third booster dose significantly increased the magnitude of humoral response, even in aged individuals, suggesting that immune recall can improve antibody response durability. The analysis of cellular responses demonstrated that vaccination with INDUK elicited an increase in the percentage of SARS-CoV-2-specific IFN-γ producing T lymphocytes in immunized young mice and TNF-α-producing T lymphocytes in both young and aged mice. These findings not only hold immediate promise for addressing evolving challenges in SARS-CoV-2 vaccination but also open avenues to refine strategies and elevate the effectiveness of next-generation vaccines.