A Live Single-Cycle RSV Vaccine with Immune Responses Focused on Prefusion F and the Central Conserved Region of Attachment Protein G

Respiratory syncytial virus (RSV) is a major cause of severe lower respiratory tract disease in infants and children, and a pediatric vaccine is not available. Among various approaches, live vaccines are attractive as they induce broad multi-antigenic responses and have never been associated with vaccine-enhanced disease in RSV-naïve children. We previously reported a live prefusion F (preF) expressing single-cycle RSV vaccine, which was deficient in cell–cell transmission and yet induced protective responses in mice. To improve efficacy, we focused the anti-attachment protein (G) response on the relatively conserved G central region (GCR). Anti-GCR antibodies were shown to cross-protect against different RSV strains, and the GCR contains a receptor-binding domain. Focusing on the GCR was done by replacing the G open reading frame with that of a hybrid protein displaying the GCR, while also retaining stabilized preF expression. The resulting virus, RSV-preF-GCR, was readily produced in complementing cells, expressed preF and GCR at the infected cell surface, and maintained its single-cycle phenotype. In mice, intranasal prime-boost vaccination induced serum anti-preF and moderate levels of anti-G antibodies, which were specific for the GCR. RSV-preF-GCR also significantly reduced lung pathology after challenge with wild-type RSV. However, relative to a vaccine expressing full-length G, in vitro neutralization by RSV-preF-GCR-induced antibodies was poor, and lung IgA responses were unexpectedly low. Thus, RSV-preF-GCR is promising in uniquely combining single-cycle replication and induction of both anti-preF and anti-GCR immunity but likely requires efficacy improvements to enhance anti-G antibody levels and induce mucosal immunity.