Age-related pharyngeal microbiome and host transcriptomic signatures underlying fever responses in RSV bronchiolitis.

Respiratory syncytial virus (RSV) bronchiolitis is the leading cause of hospitalization in infancy and exhibits pronounced age-dependent clinical heterogeneity. Fever becomes increasingly prevalent with age, yet whether febrile representation reflects a uniform inflammatory and immune phenotype across infancy remains unclear. In this prospective cohort of infants hospitalized with RSV bronchiolitis, we performed an integrated analysis of clinical features, pharyngeal microbiome composition, host transcriptomic profiles, and host-microbe interaction networks, with particular attention to age-related variation in fever-associated patterns. Clinically, fever prevalence exhibited a strong age-dependent increase across infancy. Correspondingly, canonical correspondence analysis identified age and fever as dominant gradients related to variation in both pharyngeal microbiome composition and host gene expression. Although no significant age-dependent correlations were observed at the global microbial and host transcriptomic levels in the fever-age interaction model, distinct patterns of microbial and host responses related to fever were observed across different age groups. Specifically, ranked gene set enrichment analysis indicated that febrile infants in early infancy showed relative attenuation of host defense-related programs, whereas older infants showed stronger enrichment of antiviral and inflammatory effector pathways, with more selective regulatory and signaling-associated patterns in late infancy. Integrated host-microbe network analysis further delineated a coherent developmental trajectory of fever-associated interaction architectures, evolving from densely interconnected regulatory networks in early infancy to modular, selectively coupled, host-centered configurations with advancing age. Together, febrile responses in RSV bronchiolitis should not be interpreted as a uniform biological phenotype across infancy and support age-aware interpretation of fever in pediatric RSV infection.