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Zusammenfassung:
The gut microbiome, a complex community of archaea, fungi, viruses, and bacteria in the gastrointestinal tract, plays a crucial role in host health and homeostasis. Consequently, understanding the specifics of host-microbe interactions has great therapeutic potential. Toll-like receptor 5 (TLR5), an innate immune receptor, specifically responds to bacterial flagellin. TLR5 overactivity is linked to pro-inflammatory diseases like IBD, while disrupted activity increases susceptibility to pathogens. I investigated how flagellins from the microbiome are able to modulate TLR5 responses, to promote survival and gut homeostasis. I performed a screen on binding interactions between a truncated TLR5 construct (TLR5 N14 ), which exclusively binds the flagellin D1 domain, and flagellins from common commensal bacteria. Comparing these results to activity data allowed the identification of "silent" flagellins, which bind to TLR5 but elicit a weak immune response. These were widespread throughout the dataset, suggesting a common mechanism of immune evasion. Mutagenic studies of the stimulatory Salmonella typhimurium FliC (StFliC) and silent Roseburia hominis FlaB (RhFlaB) flagellins indicated that TLR5 signalling was modulated by interactions at the highly conserved flagellin D1 and D0 domains. The latter was thought to contain an allosteric binding site, which induces conformational change in TLR5. Through a novel kinetic analysis of D1 domain binding, I identified distinct kinetic binding profiles between TLR5 N14 and StFliC or RhFlaB. StFliC displayed comparatively slower association and dissociation rates, implying it formed longer-lasting complexes despite its weaker overall binding strength. This suggests a temporal mechanism of immune evasion, where a shorter binding period limits signal propagation. RhFlaB binding was solely dependent on a well-defined MAMP, whereas StFliC also interacted at a secondary binding interface, which could explain its more stable complex. The comparison of a novel RhFlaB structure to StFliC revealed distinct surface characteristics, which likely drive their unique binding profiles. My work revealed a new class of host-microbe interactions, elucidated how microbes modulate TLR5 immune responses, and identified structural characteristics that impact binding kinetics. These findings suggest novel avenues for developing immune receptor modulators, emphasizing the significant scientific and medical potential of continued research into host-microbe interactions.
