Project Summary/Abstract Respiratory syncytial virus (RSV) is the major cause of bronchiolitis and pneumonia in young children and the leading cause for hospitalization of infants. A subset of RSV-infected children develops severe protracted lung inflammation that leads to respiratory failure and even death but effective therapies for RSV are lacking. Our preliminary data for this proposal implicates a role for NK cells in the pathobiology of RSV infection. We have determined that NK cells are abundant in the airways of RSV-infected children with profound hypoxemia and that NK cell cytotoxicity is impaired in pediatric RSV. NK cells are pivotal mediators of innate viral immunity and are also key cellular effectors of inflammation resolution to clear inflamed tissues of virus-infected and activated cells through immune synapse formation and lytic granule delivery to trigger targeted apoptosis. Disrupted inflammation resolution can perpetuate tissue damage and chronic inflammation; hence, NK cell resolution function is critical to prevent protracted lung inflammation. Resolution of inflammation is an active process mediated in part by specialized pro-resolving mediators (SPMs), a family of endogenous fatty acid- derived molecules that counter inflammatory signals and accelerate resolution through cell-specific actions. We recently identified that SPMs mitigate RSV-induced lung inflammation in mice, but whether RSV impacts SPM resolution pathways in humans remains unknown. In preliminary data for this proposal we identify for the first time that NK cells in human RSV express receptors for the SPMs resolvin D2, resolvin E1, and lipoxin A4. We have determined that the SPM lipoxin A4 up-regulates NK cell cytoskeletal proteins to promote lytic granule trafficking and augment cytotoxicity, distinct from steroids that disrupt NK cell cytoskeletal dynamics and impair cytotoxicity. This proposal will test our central hypothesis that RSV infection alters airway and circulating NK cell immunophenotype and disrupts cytoskeletal signaling pathways to impede NK cell resolution functions that can be restored by exogenous SPMs. To test this hypothesis in pediatric RSV+ ICU patients, we will pursue the following specific aims: Aim 1: Determine NK cell immunophenotypes and effector functions in pediatric RSV infection; Aim 2: Define mechanisms for disrupted lytic granule trafficking that governs NK cell cytotoxicity; and Aim 3: Establish circuits of defective inflammation resolution linked with pediatric RSV infection. This proposal brings an innovative approach to the study of RSV by leveraging human pediatric biospecimens to determine a novel NK cell–SPM immunoregulatory axis and utilizes cutting-edge mechanistic immunology techniques to identify and probe novel targets in RSV. This proposal has relevance to public health as an unprecedented post-pandemic surge of pediatric RSV infection is currently overwhelming hospitals and pediatric ICUs and identifying new im...