# Bacterial Flagellar Filament: A Supramolecular Multifunctional Nanostructure
## 6 Filament-Host Interaction
### Host Immune Response to Flagella
FLS2 is a homodimer and upon flg22 binding associates with another receptor kinase, BAK1, through their ectodomains, triggering a signaling cascade that leads to stomatal closure [216]. Binding of flg22 also induces FLS2 internalization and degradation, serving as a negative regulation mechanism. FLS2 exhibits different ligand specificities among plant species. In tomato plants, for example, a shorter version of the flagellin epitope with 15 amino acids is enough to elicit response, but not in _Arabidopsis_ and _Nicotiana benthamiana_[216]. Intracellularly, the presence of flagellin is detected through NOD-like receptors (NLR) in mice (Figure 7). Two of these, NAIP5 and NAIP6, specifically recognize the C-terminal part of the flagellin D0 domain. Upon flagellin binding, NAIP5 associates with NLRC4 to form an inflammasome that activates caspase-1. Caspase-1, in turn, induces the inflammatory response by cleaving and activating interleukins IL-18 and IL-18, and triggers the lytic form of cell death called pyroptosis. This mechanism, however, was not registered in humans, in which flagellin was not able to activate the only human NAIP molecule. The structure of the flagellin-NAIP5-NLRC4 inflammasome complex shows that NAIP5 alone binds flagellin, making numerous contacts with both helices of the D0 domain [217]. Six different regions of NAIP5 (N-terminal helix, BIR1, HD1, HD2, ID, and LRR) form a flagellin-binding pocket and confer ligand specificity. Based on two-hybrid assays, flagellins from five pathogenic bacteria (_L. pneumoniaeilia_, _S. typhimurium_, _Y. enterocolitica_, _Photorlabus luminescens_ and _P. aeruginosa_) showed a positive result while those from five others including EPEC, EHEC, _Shigella flexneri_, _Chromobacterium violaceum_ and _Burkholderia thailandensis_ were unable to interact with NAIP5. Additionally, those flagellins that interacted with NAIP5 also induced inflammasome activation in bone marrow-derived macrophages. These studies confirmed that the ability of flagellins to bind NAIP5 correlates with their ability to induce an immune response, and that NLRC4 acts as an adaptor through which inflammasome activation signals generated from different NAIP receptors are transduced to caspase-1 [202]. It was recently revealed that FliC is capable of inducing potent IgG1 anti-FliC responses in the absence of TLR5, the inflammasome, and MyD88. Based on this, a third pathway that promotes anti-FliC antibodies was proposed, independent of TLR5 and inflammasome, and this requires the presence of all four domains of FliC. [PERSON] et al. described different FliC deletions that had reduced immunogenicity, which indicated that the D2/D3 domains may be the most critical for MyD88-independent IgG1 anti-flagellin responses [218]. Mice immunized with FliC and different truncated versions made antibodies that showed reactivity to the D0/D1 and D2/D3 domains, indicating that both components of the molecule are antigenic. The D3 domain of FliC influences immunogenicity independently of the known innate recognition sites in the D0/D1 domains to augment antibody production, suggesting that full-length FliC and the preservation of all four domains would be critical for optimal immunogenicity and primary and secondary antibody responses in flagellin-based vaccines [203]. The nature of the third pathway is poorly understood and requires further investigation.