Intranasal S-2P and lentinan formulation confers broad protection against SARS-CoV-2 VOCs via IFN-γ-dominant mechanisms

Effective respiratory mucosal vaccines remain urgently needed to mitigate the rapid mutation and transmission of SARS-CoV-2. Here, we demonstrated that the spike protein (S-2P) of ancestral SARS-CoV-2 acted as a self-adjuvanted antigen for intranasal immunization, inducing robust systemic and mucosal immunity via integrin- and STING-dependent pathways. In contrast, H1N1 influenza hemagglutinin (HA) failed to generate measurable serum IgG or mucosal IgA following intranasal immunization. In mice, intranasal S-2P vaccination conferred complete protection against lethal ancestral SARS-CoV-2 challenge and partial cross-protection against heterologous Omicron variants, with both effects being IFN-γ- and CD8 + T cell-dependent. Co-administration of S-2P with the clinical immunomodulator lentinan (LNT) achieved complete protection against Omicron variants, mediated by IFN-γ but largely independent of CD8 + T cells. These findings establish S-2P + LNT as a safe, broad-spectrum mucosal vaccine candidate against emerging SARS-CoV-2 variants and reveal novel protection mechanisms beyond neutralizing antibodies and T cell immunity.