PROJECT 1 ABSTRACT - RSV and asthma: Defining the influence of host and exposure variation on disease development Respiratory syncytial virus (RSV) lower respiratory tract infection (LRI) is strongly and consistently associated with asthma. While this has been repeatedly demonstrated for decades, rather limited progress has been made on further understanding the causal association between RSV LRI and wheezing illnesses over the more than 50 years since the observation was first published. In our prior funding cycle we have demonstrated the relationship between a severe infection with RSV and asthma is confounded by a shared genetic predisposition to both conditions. We have also demonstrated that infant RSV infection, not only severe infection, is associated with increased asthma risk, and that missing RSV infection during infancy significantly protects from asthma development. To understand pathways through which RSV contributes to asthma development we have demonstrated that infant RSV infection alters the developing nasal microbiome and type 1 anti-viral responses, and we have identified specific RSV strains associated with increased asthma risk. This U19 renewal expands upon this series of significant findings to answer key questions to assess causality and inform targetable asthma prevention. Our overarching hypothesis is that age-dependent effects of infant RSV infection contribute to chronic respiratory disease through altering airway epithelial DNA methylation (DNAm), and airway epithelial metabolism and developmental programming. Further, we hypothesize that identifying gene x RSV interactions will explain individual variability in asthma susceptibility following infant RSV infection. To test these hypotheses we will use a combination of human natural quasi-randomization studies of infant RSV infection specifically designed to assess the impact of infant RSV infection on subsequent respiratory health and the airway epithelium, and in vitro models of RSV infection of nasal airway epithelial cells (NAECs). The overarching objectives of the 3 aims that will test these hypotheses are: 1) to determine whether the age of first infant RSV infection is associated with risk of subsequent incident recurrent wheeze and asthma; 2) to delineate the longitudinal effects of RSV on airway epithelial cell differentiation and metabolism throughout infancy and childhood; 3) to evaluate host gene ´ infant RSV infection interactions and RSV-dependent NAEC DNAm longitudinally to identify changes and temporal stability of RSV-dependent DNAm marks and their association with recurrent wheeze and asthma. This proposal has the potential to greatly advance our understanding of the mechanisms and developmental processes underlying the effect of RSV on recurrent wheeze and asthma, and to identify novel targetable pathways for prevention of acute RSV morbidity, childhood recurrent wheeze and asthma.