SARS-CoV-2 spike protein is a self-adjuvanted antigen for mucosal immunization and confers broad protection against lethal challenge with SARS-CoV-2 via intranasal vaccination

Effective respiratory mucosal vaccines are urgently needed to control the rapid mutation and spread of SARS-CoV-2. In this respect, the most focused virus vector-vaccine and adjuvanted recombinant vaccine strategies face safety and effectiveness concerns. Here, we revealed that the spike protein (S-2P) of the original SARS-CoV-2 strain is a self-adjuvanted antigen for intranasal immunization that can elicit potent systemic (serum IgG neutralizing antibodies and splenic T-cell responses to the S1 and S2 proteins) and mucosal immunity (respiratory tract IgA and T-cell responses) in the absence of an adjuvant. In contrast, intranasal immunization with hemagglutinin (HA) of influenza H1N1 virus failed to induce detectable serum IgG and mucosal IgA antibodies. Furthermore, intranasal immunization with S-2P in K18-hACE2 mice provided complete protection against lethal challenge with the original SARS-CoV-2 strain and 60% or 40% survival against the Omicron BA.5 and EG.5, respectively. The immune responses induced by intranasal immunization with S-2P were significantly enhanced by lentinan (LNT), an immunomodulator used in the clinic, completely protected the mice from Omicron BA.5 and Omicron EG.5 challenge and conferred additional protective mechanisms independent of CD8 + T cells. Compared with HA, S-2P robustly activated type I IFN signaling in vitro and in vivo , and importantly, S-2P significantly enhanced the antibody response to HA when it was simultaneously intranasally vaccinated with HA. Mechanistically, integrins and STING are critically involved in S-2P-eliciting immune responses via intranasal vaccination. Our findings demonstrate the potential of S-2P plus LNT as a safe and broad-spectrum mucosal vaccine for protection against SARS-CoV-2 variants.