Intranasal CD40-targeted recombinant MERS-CoV S1 protein is superior to intramuscular immunization in eliciting systemic and mucosal immune responses in mice

The Middle East respiratory syndrome coronavirus (MERS-CoV) is a global One Health challenge with a potential pandemic threat, with no approved vaccines or antiviral drugs available to date. Thus, there is an urgent need for a safe and effective vaccine. Herein, we developed a recombinant subunit self-adjuvanted fusion protein vaccine targeting the MERS-CoV S1 subunit to CD40-expessing APCs (S1-F/CD40L). We found that intramuscular injection of S1-F/CD40L in conjunction with Alum and CpG was superior to Alum or CpG alone in terms of systemic Ag-specific humoral and cellular responses and Th1-dominant phenotype. Furthermore, the immunogenicity of co-adjuvanted S1-F/CD40L was compared to that of S1 alone via both intramuscular and intranasal immunization. Two intramuscular and intranasal doses of S1-F/CD40L and S1 were immunogenic in eliciting systemic and mucosal humoral and cellular immunity, including IgG, IgA, neutralizing antibodies (nAbs), and T cell responses in mice, with a greater response in the CD40-targeted S1 group. Intranasal vaccination with S1-F/CD40L induced systemic humoral and cellular immune responses comparable to those induced by intramuscular vaccination, including binding and nAbs and T cell responses. Importantly, intranasal vaccine was able to elicit significantly higher local mucosal humoral and cellular immune responses in mouse lungs and markedly elevated circulating IgA levels compared to intramuscular vaccination. Collectively, our results suggest that the S1-F/CD40L vaccine co-adjuvanted with Alum and CpG can be used as an effective and safe mucosal candidate vaccine against MERS-CoV. Furthermore, these data demonstrate that the incorporation of CD40L as APCs targeting ligand and molecular adjuvant enhances immunogenicity, thus offering a promising platform that could be explored further to respond to future emerging pathogens and possible outbreaks.