PROJECT SUMMARY Influenza infection remains an international health concern despite global surveillance, effective vaccines, and antiviral therapy. Secondary bacterial infections are emerging as an important cause of human disease to influenza, however little is known regarding how influenza increases susceptibility to bacterial infection. Pneumococcal infection is the most common cause of secondary bacterial infection postinfluenza, and extrapulmonary pneumococcal infections are emerging as leading causes of acute cardiac and renal disease in humans. Significant gaps in our knowledge around secondary pneumococcal infection exists, none larger than the role of the lung epithelium in bacterial host defense following influenza. This application will mechanistically deconstruct how influenza enhances susceptibility to pneumococcal infection in the lung epithelium, and will fill important gaps in our current knowledge regarding influenza secondary bacterial infection. Our laboratory's expertise in experimental lung infection and primary differentiated lung epithelial culture models puts us uniquely positioned for this investigation. Here we put forth three mechanistically driven aims supported by published and preliminary data clearly supporting our scientific premise that influenza- driven changes in epithelial ion dysregulation confer airway surface fluid acidification that enhances susceptibility to pneumococcal infection. Specifically, Aim 1 will elucidate the critical role of the lung epithelium in susceptibility to pneumococci after influenza. Aim 2 will mechanistically determine epithelium ion dysfunction leading to acidification the airway epithelial surface fluid as the underlying molecular mechanism. Lastly, Aim 3 will elucidate the impact of increased pneumococcal infection postinfluenza in vivo in pneumonia and disseminated extrapulmonary disease, while exploring interventions of repurposed FDA approved drugs. Collectively, this proposal will put forth an entirely novel underlying mechanism for influenza-mediated susceptibility to pneumococcal infection, will establish a human experimental model to study influenza- pneumococcal coinfections, and evaluate pharmacologic interventions that could be implemented rapidly for prophylaxis to secondary pneumococcal infections to influenza.