SUMMARY Cardiac dysfunction is among the most common extrapulmonary complications of severe influenza virus infections. Although the heart complications of influenza virus infection are a clearly recognized clinical problem, they are poorly studied in terms of pathogenic mechanisms. We lack basic scientific understanding of 1) whether influenza virus directly or indirectly causes heart damage, 2) what viral features facilitate cardiac tropism of certain strains, 3) how virus disseminates from the lung specifically to the heart in the absence of viremia, and 4) how the immune system influences heart pathogenesis. The major barrier to mechanistic research in this area has been the lack of tractable animal models that recapitulate significant cardiac dysfunction in severe influenza. We have overcome this obstacle by developing interferon-induced transmembrane protein 3 (IFITM3) KO mice as a model of severe influenza virus infection that includes viral replication in the heart and significant cardiac electrical dysfunction, inflammation, and fibrosis. Importantly, IFITM3 is an antiviral protein of the innate immune system in which common deficiencies in the human population render individuals more susceptible to severe infections, making IFITM3 KO mice a relevant and informative model. We will make use of this groundbreaking model in the following three Aims. In Aim 1, we will determine whether virus dissemination and replication directly in heart tissue is required for this pathology or whether influenza virus indirectly induces heart dysfunction through systemic inflammation from the severely infected lung. To address this fundamental controversy in the field, we have developed a novel and innovative recombinant influenza virus strain with specific inability to replicate in heart cells while being fully replicative in the lung. In Aim 2, we will identify viral features that endow specific virus strains with cardiac tropism by using a candidate viral gene approach with reassortant viruses, as well as an adaptive passaging approach. In Aim 3, we will solve the mystery of how influenza virus moves specifically from the lung to the heart in the absence of viremia and other organ infections. For this, we will test whether infection of migratory immune cells facilitates virus trafficking to the heart. We will further investigate how IFITM3 expression in hematopoietic immune cells influences cardiac dissemination of virus and cardiac dysfunction during infection. Overall, our research will answer fundamental questions in the influenza field, and will reveal new strategies for combatting influenza-associated cardiac dysfunction.