Type 1 Fimbrial Adhesin FimH Elicits an Immune Response That Enhances Cell Adhesion of Escherichia coli

Abstract

Escherichia coli causes about 90% of urinary tract infections (UTI), and more than 95% of all UTI-causing E. coli express type 1 fimbriae. The fimbrial tip-positioned adhesive protein FimH utilizes a shear forceenhanced, so-called catch-bond mechanism of interaction with its receptor, mannose, where the lectin domain of FimH shifts from a low-to a high-affinity conformation upon separation from the anchoring pilin domain. Here, we show that immunization with the lectin domain induces antibodies that exclusively or predominantly recognize only the high-affinity conformation. In the lectin domain, we identified four high-affinity-specific epitopes, all positioned away from the mannose-binding pocket, which are recognized by 20 separate clones of monoclonal antibody. None of the monoclonal or polyclonal antibodies against the lectin domain inhibited the adhesive function. On the contrary, the antibodies enhanced FimH-mediated binding to mannosylated ligands and increased by severalfold bacterial adhesion to urothelial cells. Furthermore, by natural conversion from the high-to the low-affinity state, FimH adhesin was able to shed the antibodies bound to it. When whole fimbriae were used, the antifimbrial immune serum that contained a significant amount of antibodies against the lectin domain of FimH was also able to enhance FimH-mediated binding. Thus, bacterial adhesins (or other surface antigens) with the ability to switch between alternative conformations have the potential to induce a conformation-specific immune response that has a function-enhancing rather than -inhibiting impact on the protein. These observations have implications for the development of adhesin-specific vaccines and may serve as a paradigm for antibody-mediated enhancement of pathogen binding.

Conformational inactivation induces immunogenicity of the receptor-binding pocket of a bacterial adhesin

“…As shown in Fig. 2A (20). The level of mannose-binding inhibition exerted by individual anti-LD mut antibodies did not depend on their antigen-binding affinity, as shown in Fig.…”

“…Although it would be interesting to determine the epitope recognized by mAb255, so far our mutant library screening has not revealed the epitope recognized by mAb255. Another FimH-specific functional property of mannose is its ability to induce FimH in the low-affinity conformation to switch (in a reversible way) to the high-affinity conformation allosterically, as demonstrated by the ability of mAb21 (the active state-specific antibody) to recognize FimH K12 in the presence but not in the absence of soluble mannose (20). As shown in Fig.…”

“…There was no overlap between the putative mAb475 epitope and the mAb21 epitope comprising residues 26, 29, and 153-157 located at the bottom of the LD, away from the binding pocket (20); mutations in the mAb21 epitope had no inhibiting effect on mannose binding (Table S1). …”

“…S3B). In contrast, different LD nat -elicited mAbs that recognize four separate classes of LD nat epitopes (20) bound fimbrial FimH equally well but failed to block bacterial adhesion (S3 A and B). Binding of inhibitory antibodies to fimbrial FimH was abolished in the presence of mannose (Fig.…”

“…We believe our findings provide insight into strategies to elicit neutralizing and functional antibodies against a broad spectrum of receptor-binding proteins. purified LD do not inhibit but instead actually enhance bacterial adhesion by stabilizing fimbrial FimH in the active conformation (20), raising questions about the utility of active LD or FimH as the optimal vaccine candidate.…”