The N‐terminal D1 domain of Treponema pallidum flagellin binding to TLR5 is required but not sufficient in activation of TLR5

Abstract

Syphilis is a chronic bacterial infection caused by Treponema pallidum (T pallidum) and the pathogenesis that T pallidum infection induces immunopathological damages in skin and other tissues remains unclear. We have previously reported that recombinant flagellins of T pallidum can elicit IL‐6 and IL‐8 transcriptions via TLR5 pathway. To identify the domains which induced the pro‐inflammatory activity and the importance of the interactions between TLR5 and domains, homology‐based modelling and comparative structural analyses revealed that Tpflagellins can combine with TLR5 directly. Deletion mutations showed that the ND1 domain binding to TLR5 is required but not sufficient in TLR5 activation. Moreover, site‐directed mutagenesis analysis indicated that the arginine residue (Tpflagellins R89) of the ND1 domain and its adjacent residues (Tpflagellins L93 and E113) constitute a hot spot that elicits IL‐6, IL‐8 transcriptions and TLR5 activation, and affects the binding of Tpflagellins to TLR5. Taken together, these results give insight into the pathogenesis of T pallidum and may contribute to the future design of Tpflagellins‐based therapeutics and syphilis vaccine.

Escape of TLR5 Recognition by Leptospira spp.: A Rationale for Atypical Endoflagella

“…We also found in FlaB the consensus in the D0 domain involved in the flagellin/TLR5 complex stabilization ( Figures 6A,C ). We compared the leptospiral FlaB sequences in these 3 consensus binding TLR5 regions with other spirochetes, Borrelia burgdorferi and Treponema spp., the latter known to signal via TLR5 when FlaB are expressed as recombinant proteins ( 42 ) and also with bacteria known to dodge the TLR5 response such as Helicobacter pylori ( 43 ) and Bartonella bacilliformis ( 10 ), presenting variations in those consensus sequences of their flagellins ( Supplementary Figure 4A ). In addition, we also found this FlaB region to be 100% conserved in a panel of major species of Leptospira circulating all over the world, including potential human pathogens, such as L. borgpeterseni, L. kirschneri , L. noguchii , L. weilii , L. santarosai , as well as L. licerasiae , belonging to another clade of species of lower virulence ( 2 ) ( Supplementary Figure 4B ).…”

“…However, our attempts to express recombinant FlaB monomers have failed. We cannot exclude a caveat in our cloning strategy but this failure was quite surprising considering that T. denticola and T. pallidum FlaB were expressed as stable recombinant proteins that were able to signal through TLR5 in THP1 monocytes or in human keratocytes, respectively (42). One hypothesis could be that the FlaBs that encompass a different shape than FliC would need to be stabilized by polymerization into the complex filament structure.…”

The Role of Flagellin B in Vibrio anguillarum-Induced Intestinal Immunity and Functional Domain Identification

“…Many studies have proved that TLR5 recognises flagellins via the extracellular domain, which leads to the expression of a variety of genes. However, due to variation in the sequences and domains of flagellins, flagellins from diverse bacterial species use different TLR5 recognition mechanisms ( 34 ). For activation, the receptor must undergo a ligand-induced conformational change ( 27 ).…”

Bacterial Flagellar Filament: A Supramolecular Multifunctional Nanostructure

“…The TLR5 consensus target sequence is composed of residues 88–98 (LQRIRELAVQA) and located in a conserved site within D1 domain of FliC of β- and γ-proteobacteria. In addition, the C-terminal domain D0 is necessary for TLR5 signaling, as demonstrated by functional studies, as well as the N-terminal D1 motifs (82–101, 110–118) and the C-terminal D1 motif (412–438) that are important for specific interaction of flagellin with TLR5 [ 210 ]. In contrast, some important human pathogens such as H. pylori , C. jejuni and Bartonella bacilliformis species have developed the capacity to produce flagellins with a modified motif (DTVKVKAT, DTIKTKAT or DDIQKSMV, respectively) that cannot be recognized by TLR5 [ 211 ].…”