Influenza A virus is an important human respiratory pathogen that causes world-wide yearly epidemics and occasional pandemics associated with significant morbidity and mortality. We have identified the NS 1 protein encoded by this virus as an important virulence factor associated with inhibition of type I IFN production in infected cells. However, little is known about the role of the NS 1 IFN antagonistic activity in the evasion of innate immune responses in the main target cells of viral infection in humans, i.e. human epithelial respiratory cells, as well as in human cells specialized in detecting viral infections and initiating specific immune responses, i.e. dendritic cells (DCs). Preliminary evidence suggests that the NS 1 protein of influenza A virus not only attenuates production of IFNalpha/beta in DCs, but also impairs optimal activation of these critical antigen presenting cells in response to virus infection. We are now planning to investigate the interactions of human influenza viruses with human epithelial cells and human DCs with a focus on the role of the NS 1 protein in modulating the outcomes resulting of these interactions. This will be done in close collaboration with the technology section of this center grant. Primary isolates of human influenza A viruses and recombinant human influenza viruses containing defined mutations in the NS 1 gene will be used to investigate the role of NS1 and NS1 domains in attenuating innate immune responses in primary human respiratory epithelial cell cultures and in DCs, as well as in modulating DC maturation and interactions between DCs and T cells. As a result, we hope not only to learn about how influenza viruses induce and/or prevent innate and specific immune responses in humans, but also to better understand the role of DCs and of the type I IFN response in modulating innate and specific immunity against viruses in humans. Our studies are also likely to advance our knowledge of the molecular determinants of pathogenicity and immunogenicity of influenza viruses in humans, potentially allowing for the rational design of novel vaccines and antiviral therapies.