Mucosal immunization with an adenoviral vector expressing a prefusogenic F protein protects the upper and lower respiratory tracts of cotton rats against live respiratory syncytial virus challenge

Respiratory syncytial virus (RSV) is a major cause of severe respiratory disease in infants, young children, and the elderly. Current licensed prefusogenic F-based RSV vaccines induce systemic immune responses through intramuscular injection. However, mucosal immunization will be required to elicit local sterilizing immunity to prevent virus replication in the nasopharynx and the lungs as well as virus shedding and transmission. Adenovirus vectors are a promising platform to develop RSV vaccines, but further evaluation of mucosal adenovirus-based prefusion F (preF) vaccines is still needed. In this study, we constructed a recombinant chimpanzee adenovirus serotype 68 vector expressing a second-generation disulfide-stabilized (DS2)trimeric preF protein (ChAd68-PreF). In mice, intranasal immunization with ChAd68-PreF induced dose-dependent increase in RSV-specific secretory IgA (sIgA) in the lower respiratory tract, demonstrating its capacity to elict mucosal immunity. Subsequently, in cotton rats, intratracheal instillation of ChAd68-PreF induced high cross-neutralizing antibody titers against RSV A and RSV B. After RSV A2 challenge, ChAd68-PreF vaccinated animals showed no detectable viral replication in the nose nor in the lungs. Moreover, ChAd68-PreF vaccination did not lead to enhanced respiratory disease (ERD) in cotton rats. These results demonstrate that mucosal delivery of ChAd68-PreF confers potent protective immunity against RSV with good safety profile, warranting further clinical development as a mucosa-targeting RSV vaccine for vulnerable populations.