| The focus of this dissertation was to understand how the innate immune receptor, Toll-like receptor 5 (TLR5), recognizes bacterial flagellin, and the consequences of this recognition event for host defense against flagellated bacterial pathogens. We first determined that the site on flagellin recognized by TLR5 was highly conserved among flagellin proteins from a wide variety of bacteria, and that this site was required for stacking of flagellin monomers in the flagellar filament. Mutations in this region reduced bacterial motility, indicating that TLR5 recognizes a highly conserved site on flagellin required for bacterial fitness. We next examined the region on TLR5 that was important for flagellin recognition. Using mouse and human TLR5 differences in flagellin recognition, we identified a central region in the TLR5 extracellular domain. We modeled the molecular structure of TLR5 based on other leucine-rich repeat proteins and, through a series of point mutations, identified a conserved concavity on TLR5 that is responsible for flagellin recognition. In combination, these analyses comprise the most detailed molecular information about the interaction of a TLR with its agonist to date, and have also suggested general principles of TLR-ligand recognition that will inform further studies in this area. We subsequently investigated the role of TLR5 recognition in vivo. We identified two groups of bacteria, including the important human pathogens, Helicobacter pylori, Campylobacter jejuni and Bartonella bacilliformis , that possess alternate flagellin sequences which simultaneously permit motility and TLR5 evasion. This suggested that TLR5 exerted strong selective pressure during the evolution of these bacteria. Finally, using TLR5-deficent mice, we demonstrated a unique role for TLR5 in defense to Escherichia coli urinary tract infection. The in vivo studies demonstrate, for the first time, the important and non-redundant role for TLR5 in defense against flagellated bacteria. Collectively, this dissertation maps in detail the proinflammatory activity of bacterial flagellin and demonstrates the biological importance of host recognition of bacterial flagellin. It is our hope that this information will be used in the future to design better adjuvants, increasing the number of vaccines available for infectious diseases. |
