Abstract

BackgroundHuman respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract illness (LRTI) worldwide, particularly in infants, the elderly and immunocompromised individuals. Despite this substantial disease burden, no licensed pediatric vaccines are currently available for infants aged ≥ 6 months, highlighting an urgent need for effective immunization strategies.MethodsLive-attenuated vaccines (LAVs) were developed and characterized based on RSV subgroup A strain Long (wtRSV) by deleting the M2-2 gene alone or by further introducing silent mutations into the small hydrophobic (SH) gene alongside a 112-nucleotide deletion within its downstream untranslated region. Subsequently, the in vitro and in vivo characteristics of these constructed recombinant RSV strains were evaluated.ResultsIn HEp-2 and Vero cells, the recombinant strains RLΔM2-2 and RLΔM2-2112 displayed reduced replication capability compared with the parental wtRSV. In BALB/c mice, RLΔM2-2-infected groups demonstrated significantly lower lung viral loads and less pronounced body weight loss than those infected with RLΔM2-2112. Following intranasal immunization, RLΔM2-2 robustly induced systemic and pulmonary humoral and cellular immune responses including preF-specific antibodies, neutralizing antibody responses, and RSV-specific CD8+ T-cell responses characterized by a Th1-biased immune profile. Furthermore, the vaccine conferred protection against subsequent RSV challenge with no signs of enhanced respiratory disease (ERD).ConclusionThese results support the application of M2-2 deletion strategy to the parental RSV strain Long backbone, providing a solid foundation for further evaluation and development of RSV LAVs.