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

Inhibiting antibodies targeting receptor-binding pockets in proteins is a major focus in the development of vaccines and in antibody-based therapeutic strategies. Here, by using a common mannose-specific fimbrial adhesin of Escherichia coli, FimH, we demonstrate that locking the adhesin in a low-binding conformation induces the production of binding pocket-specific, adhesion-inhibiting antibodies. A di-sulfide bridge was introduced into the conformationally dynamic FimH lectin domain, away from the mannose-binding pocket but rendering it defective with regard to mannose binding. Unlike the native, functionally active lectin domain, the functionally defective domain was potent in inducing inhibitory monoclonal antibodies that blocked FimH-mediated bacterial adhesion to epithelial cells and urinary bladder infection in mice. Inhibition of adhesion involved direct competition between the antibodies and mannose for the binding pocket. Binding pocket-specific inhibitory antibodies also were abundant in polyclonal immune serum raised against the functionally defective lectin domain. The monoclonal antibodies elicited against the binding-defective protein bound to the high-affinity conformation of the adhesin more avidly than to the low-affinity form. However, both soluble mannose and blood plasma more strongly inhibited antibody recognition of the high-affinity FimH conformation than the low-affinity form. We propose that in the functionally active conformation the binding-pocket epitopes are shielded from targeted antibody development by ligand masking and that strong immunogenicity of the binding pocket is unblocked when the adhesive domain is in the nonbinding conformation.