Mucosal boosting of chimpanzee adenovirus-vectored vaccine encoding SARS-CoV-2 RBD-heterodimer provides robust protection in mice

Abstract

ABSTRACT The major route of COVID-19 vaccination currently is via intramuscular injection. Data from clinical trials and real-world studies have demonstrated its effectiveness in preventing severe illness and death caused by SARS-CoV-2 infection. However, its protective efficacy against SARS-CoV-2 infection and transmission in situ remains relatively low. Given that SARS-CoV-2, especially the Omicron variant and its sub-variants, primarily infects and replicates in the human upper respiratory tract, mucosal immune responses are crucial for preventing viral infection. Therefore, we constructed a chimpanzee adenovirus (AdC68)-vectored vaccine expressing the Delta-XBB receptor-binding domain (RBD)-dimer and comprehensively compared the immune responses induced by intramuscular injection, intranasal administration, or aerosol inhalation. Our results revealed that aerosol inhalation of the recombinant AdC68 vaccine induced robust systemic and mucosal immune responses and immune memory, particularly activating memory T cells in the lungs with a long duration in the mouse model. Additionally, we assessed long-term protection against a SARS-CoV-2 XBB.1 challenge after ~6 months following a booster vaccination with AdC68-Delta-XBB via different immunization routes. We found that, compared with the intramuscular route, aerosol inhalation provided significantly better protection, without detectable replicating virus in the nasal tissue. This study demonstrates that the AdC68-Delta-XBB vaccine induces robust mucosal immune responses via aerosol inhalation vaccination and prevents SARS-CoV-2 infection in mucosa. IMPORTANCE Immunity induced by first-generation COVID-19 vaccines administered by intramuscular injection is highly effective against severe disease and death but is limited in its ability to prevent viral infection and transmission. A more cost-effective and practical vaccine delivered by the respiratory route is needed to better understand mucosal immune responses and to assess protective efficacy. This study evaluated the immune responses and protective efficacy elicited by intramuscular injection, intranasal administration, or aerosol inhalation of AdC68-Delta-XBB in a mouse model and demonstrated that the aerosol inhalation approach is particularly advantageous for robustly stimulating both systemic and mucosal immune responses. These findings will help guide future clinical development and provide a basis for developing vaccines against other respiratory pathogens. Immunity induced by first-generation COVID-19 vaccines administered by intramuscular injection is highly effective against severe disease and death but is limited in its ability to prevent viral infection and transmission. A more cost-effective and practical vaccine delivered by the respiratory route is needed to better understand mucosal immune responses and to assess protective efficacy. This study evaluated the immune responses and protective efficacy elicited by intramuscular injection, intranasal administration, or aerosol inhalation of AdC68-Delta-XBB in a mouse model and demonstrated that the aerosol inhalation approach is particularly advantageous for robustly stimulating both systemic and mucosal immune responses. These findings will help guide future clinical development and provide a basis for developing vaccines against other respiratory pathogens.