The influenza A viral hemagglutinin (HA) protein binds to sialic acid (SA) groups of cellular surface glycoproteins to achieve viral attachment and entry. The SA binding specificity of HA is one of the major determinants for controlling viral tropism and host specificity. Fibronectin (FN) is a ubiquitinious glycoprotein secreted on cell surface, either circulating in plasma, or as one of the best characterized components of the extra cellular matrix. With its binding properties towards different types of molecules and pathogens, it has been utilized by different bacterial and viral pathogens for binding, entry, propagation and pathogenesis. The binding affinity and region of plasma FN to influenza A viral glycoprotein was identified in early 1980s. Evidence also suggests the binding is SA associated. FN associates with different viral pathogens. However, evidence of FN direct involvement in influenza A pathogenesis remains unknown. The objective of this thesis is to test the involvement of cellular FN in influenza A viral infection. To perform the study, FN siRNA and anti-FN antibody were applied. This study demonstrated possible involvement of FN in the replication of human H1N1 and highly pathogenic avian H5N1 viruses. It also discovered that FN is very important for the replication of H1N1 virus, but not H5N1 virus. Interestingly, the result suggested that FN does not affect the initial virus-host binding, but it has an effect on post-attachment events. Key amino acid positions controlling the SA binding specificity of seasonal human or avian influenza A viruses have been identified in the HA. In this thesis, reverse genetics and mutagenic work identified that viruses with a α2,3-linked SA (SA α2,3) binding preference were not inhibited by anti-FN antibody, while viruses with a α2,6-linked SA (SA α2,6) specificity were severely inhibited. This surprising finding of SA binding preference related FN involvement in post-attachment event led to the further investigation on the structural involvement of FN and viral entry pathway analysis. The 9th and 10th of type III repeating units of FN form the cell-binding domain of the protein for cell attachment. From site specific antibody inhibitory studies, the cell binding region of FN near the synergy adhesion site(SAS) and Arg-Gly-Asp-Ser(RGDS) cell adhesion signal was identified to be important for the replication of viruses that have a α2,6 SA binding preference, but it was also found to be independent of α5β1 integrin receptor. After attaching to a host cell, the virus was internalized in an endosome via clathrin- or caveolin- mediated endocytosis. By application of pathway inhibitors, the FN association with viral entry pathway was evaluated. Though this study failed to identify a single specific FN mediated viral entry pathway, this pathway study indicated the possibility of FN various involvement in influenza viral entry. The study indeed indicated that viruses have difference SA binding preferences are different in their choices in viral entry pathways. This thesis did not only introduce cellular FN as a novel host factor, but also identified possible target and brought new light in the control of influenza A viral infection. / published_or_final_version / Public Health / Doctoral / Doctor of Philosophy