# Bacterial Flagellar Filament: A Supramolecular Multifunctional Nanostructure
## 6 Filament-Host Interaction
### Host Immune Response to Flagella
jejuni_ and _Bartonella bacilliformis_ species have developed the capacity to produce flagellins with a modified motif (DTVKVKAT, DTIKTAT or DDIQKSMV, respectively) that cannot be recognized by TLR5 [211]. Their unique flagellin sequences contain amino acid substitutions in the TLR5 recognition site that permit TLR5 evasion, as well as compensatory mutations that preserve bacterial motility. Interestingly, cell signaling assays and structure-based analysis demonstrated that the replacement of the motif of the best-characterized flagellin FliC from _S. Typhimurium_ (QRVRELAV) by the corresponding motif from _H. pylori_ abolishes the interaction with TLR5. Two of the scarce flagellated commensal gut bacteria genera, _Eubacterium_ and _Roseburia_, contain flagellin proteins with conserved residues (L87, Q88, R89, E92, L93, and Q96) that are found in pro-inflammatory flagellins of the \(\beta\)- and \(\gamma\)-proteobacteria, and which are critical for TLR5 signaling and flagellin polymerization [111]. When two human intestinal epithelial cell lines (T84 and HT-29) were exposed to flagellins isolated from some _Roseburia inulinurans_ and _Eubacterium rectale_, they exhibited increased secretion of pro-inflammatory interleukin-8 (IL-8) [111]. Hence, flagellin-specific immune responses should modulate the microbiome's production of flagellin to help to maintain mucosal barrier integrity and gut homeostasis [119]. The structure of the complex between zebrafish TLR5 and FliC from \(S\). Dublin showed that the contact area is extensive and involves several critical sites [212]. Two regions of FliC were essential for primary interaction with TLR5 and the establishment of TLR5-FliC 1:1 heterodimers: the C-terminal \(\alpha\)-helix of FliC D1 domain, which is in close contact with leucine-rich repeats 1-6 (LRR1-6), and the motif comprised of residues 89-96 of FliC D1 domain that binds to LRR7-10. Dimerization of heterodimers through LRR12-13 of TLR5 results in a functional 2:2 complex. The TLR5-activating main motif (89-96) is buried deep within the filament in the flagellum core (not exposed in the intact flagella) and only flagellin monomers are able to induce TLR5 signaling, while filaments do not. Furthermore, proper folding and three-dimensional structure are critical for TLR5 recognition of flagellin and must be taken into account when designing flagellin-based adjuvants for vaccines. Bacterial flagellin can also be recognized by the innate immune system in plants, through a pattern recognition receptor (PRR) which is analogous to TLR5 from animals (Figure 7). By screening _Arabidopsis_ mutants, a receptor that binds a highly conserved 22-residue peptide derived from the flagellin N-terminus (flg22) was identified [213]. This receptor, Flagellin-sensitive 2 (FLS2), contains 28 copies of LRR in its ectodomain (ECD)  
Figure 7: Flagellin recognition by the host. Extracellular detection—TLR5 receptor recognizes region on the D1 domain of flagellin and upon binding, TLR5 dimerizes starting a signaling cascade in the cytoplasm that leads to the alteration of gene expression; Intracellular detection (detected in mice only)—Nai9 5 and 6 (NOD-like receptors) recognize C terminus of the flagellin. They recrute another NOD-like receptor NLRP4, which oligomerizes and activates caspase I forming a structure called inflammasomeome. Inflammasome activates molecules such as interleukins and gasdermin D leading to pyrotosis; Detection in plants—Plant receptor FLS2 recognizes N-terminal fragment of flagellin and upon binding FLS2 dimerizes with BAKI, protein kinase, starting a downstream signaling cascade that leads to the stomatal closure. connected via a single transmembrane region to a cytoplasmic Ser/Thr kinase domain that is absent in animal TLRs [214]. FLS2 mutants are insensitive to flg22 treatment, indicating that FLS2 is essential and specific for flagellin-induced immunity. The crystal structure of the FLS2 ectodomain and the flg22 ligand revealed that LRRs 9-15 of FLS2 contribute to flg22 responsiveness [215].