A SARS-CoV-2 RBD vaccine fused to the chemokine MIP-3 elicits sustained murine antibody responses over 12 months and enhanced lung T-cell responses

Background: Previous studies have demonstrated enhanced efficacy of vaccine formulations that incorporate the chemokine macrophage inflammatory protein 3 alpha (MIP-3 alpha) to direct vaccine antigens to immature dendritic cells. To address the reduction in vaccine efficacy associated with a mutation in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) mutants, we have examined the ability of receptor-binding domain vaccines incorporating MIP-3 alpha to sustain higher concentrations of antibody when administered intramuscularly (IM) and to more effectively elicit lung T-cell responses when administered intranasally (IN). Methods: BALB/c mice aged 6-8 weeks were immunized intramuscularly or intranasally with DNA vaccine constructs consisting of the SARS-CoV-2 receptor-binding domain alone or fused to the chemokine MIP-3 alpha. In a small-scale (n = 3/group) experiment, mice immunized IM with electroporation were followed up for serum antibody concentrations over a period of 1 year and for bronchoalveolar antibody levels at the termination of the study. Following IN immunization with unencapsulated plasmid DNA (n = 6/group), mice were evaluated at 11 weeks for serum antibody concentrations, quantities of T cells in the lungs, and IFN-gamma- and TNF-alpha-expressing antigen-specific T cells in the lungs and spleen. Results: At 12 months postprimary vaccination, recipients of the IM vaccine incorporating MIP-3 alpha had significantly, approximately threefold, higher serum antibody concentrations than recipients of the vaccine not incorporating MIP-3 alpha. The area-under-the-curve analyses of the 12-month observation interval demonstrated significantly greater antibody concentrations over time in recipients of the MIP-3 alpha vaccine formulation. At 12 months postprimary immunization, only recipients of the fusion vaccine had concentrations of serum-neutralizing activity deemed to be effective. After intranasal immunization, only recipients of the MIP-3 alpha vaccine formulations developed T-cell responses in the lungs significantly above those of PBS controls. Low levels of serum antibody responses were obtained following IN immunization. Conclusion: Although requiring separate IM and IN immunizations for optimal immunization, incorporating MIP-3 alpha in a SARS-CoV-2 vaccine construct demonstrated the potential of a stable and easily produced vaccine formulation to provide the extended antibody and T-cell responses that may be required for protection in the setting of emerging SARS-CoV-2 variants. Without electroporation, simple, uncoated plasmid DNA incorporating MIP-3 alpha administered intranasally elicited lung T-cell responses.