ABSTRACT To infect airway epithelial cells, influenza A virus (IAV) must penetrate the secreted mucus and underlying periciliary layer barriers – both comprised, in large part, of mucin glycoproteins that can be secreted or tethered to the apical surface of cells. The importance of tethered mucins in various disease states and acute infection is increasingly being realized. Specifically, tethered mucin 1 (MUC1) has been implicated in various aspects of both bacterial and viral infections. However, these reports do not yield consensus on MUC1 function during pathogenic insult and while it seems logical to assume that MUC1 expression would benefit the host, our preliminary data indicate that MUC1 facilitates IAV infection. As MUC1 is expressed by multiple cell types that influence IAV pathogenesis, including airway epithelial cells and macrophages, and has been proposed to act as both a structural barrier and signaling molecule, we hypothesize that MUC1 facilitates IAV infection through multiple mechanisms, involving both physical and functional attributes of MUC1, and multiple cell types. We will test this hypothesis by pursing two specific aims: 1) Define the contribution of MUC1 expression in specific cell compartments to IAV pathogenesis and 2) Define MUC1 molecular interactions and intracellular dynamics in human airway epithelium during IAV infection. Aim 1 will generate a MUC1-expression atlas in the lung during infection, determine the impact of MUC1 expression on IAV infection of both human and mouse primary airway epithelial cells, and define the contribution of MUC1 expression in specific cell compartments to the outcome of infection in vivo. Aim 2 will interrogate the physical interaction between IAV and MUC1 in physiologically relevant models of airway epithelium and define the impact of IAV and interferon on MUC1 expression, activation, and interacting partners through mechanistic studies and comprehensive mass spectrometry-based technology. This research is innovative in its use of novel in vitro and in vivo models with altered MUC1 expression to dissect the interplay between IAV and MUC1 at both the molecular and organismal level. Our results will be significant as defining MUC1 function in the context of IAV infection will advance our understanding of tethered mucin biology and reveal mechanisms of how IAV negotiates specific mucin molecules to efficiently infect the respiratory tract.