Antibodies Damage the Resilience of Fimbriae, Causing Them To Be Stiff and Tangled

Abstract

As adhesion fimbriae are a major virulence factor for many pathogenic Gram-negative bacteria, they are also potential targets for antibodies. Fimbriae are commonly required for initiating the colonization that leads to disease, and their success as adhesion organelles lies in their ability to both initiate and sustain bacterial attachment to epithelial cells. The ability of fimbriae to unwind and rewind their helical filaments presumably reduces their detachment from tissue surfaces with the shear forces that accompany significant fluid flow. Therefore, the disruption of functional fimbriae by inhibiting this resilience should have high potential for use as a vaccine to prevent disease. In this study, we show that two characteristic biomechanical features of fimbrial resilience, namely, the extension force and the extension length, are significantly altered by the binding of antibodies to fimbriae. The fimbriae that were studied are normally expressed on enterotoxigenic Escherichia coli, which are a major cause of diarrheal disease. This alteration in biomechanical properties was observed with bivalent polyclonal antifimbrial antibodies that recognize major pilin subunits but not with the Fab fragments of these antibodies. Thus, we propose that the mechanism by which bound antibodies disrupt the uncoiling of natural fimbria under force is by clamping together layers of the helical filament, thereby increasing their stiffness and reducing their resilience during fluid flow. In addition, we propose that antibodies tangle fimbriae via bivalent binding, i.e., by binding to two individual fimbriae and linking them together. Use of antibodies to disrupt physical properties of fimbriae may be generally applicable to the large number of Gram-negative bacteria that rely on these surface-adhesion molecules as an essential virulence factor. IMPORTANCE Our study shows that the resiliency of colonization factor antigen I (CFA/I) and coli surface antigen 2 (CS2) fimbriae, which are current targets for vaccine development, can be compromised significantly in the presence of antifimbrial antibodies. It is unclear how the humoral immune system specifically interrupts infection after the attachment of enterotoxigenic Escherichia coli (ETEC) to the epithelial surface. Our study indicates that immunoglobulins, in addition to their welldocumented role in adaptive immunity, can mechanically damage the resilience of fimbriae of surface-attached ETEC, thereby revealing a new mode of action. Our data suggest a mechanism whereby antibodies coat adherent and free-floating bacteria to impede fimbrial resilience. Further elucidation of this possible mechanism is likely to inform the development and refinement of preventive vaccines against ETEC diarrhea.KEYWORDS pili, IgG, vaccine, CFA/I, CS2, optical tweezers

Antibodies induced by enterotoxigenic Escherichia coli (ETEC) adhesin major structural subunit and minor tip adhesin subunit equivalently inhibit bacteria adherence in vitro

“…Antibodies against a fimbrial major structural subunit, unless this major subunit is also the adhesive subunit which is directly involved in bacterial adherence, do not serve as a direct blockage to prevent the recognition between a fimbria and host receptors. A recent study, however, demonstrated that anti-fimbria antibodies, particularly antibodies to ETEC major structural subunits can physically damage fimbria adherence function [23]. Anti-fimbria or anti-major subunit antibodies recognize adjacent major subunits and lock extension and contraction movement of coil-structured fimbriae.…”

“…Anti-fimbria or anti-major subunit antibodies recognize adjacent major subunits and lock extension and contraction movement of coil-structured fimbriae. That makes fimbria “stiff and tangled”, resulting in reduction or prevention of ETEC fimbria adherence to host cells [23]. Since the molecular sizes of IgG and more particularly IgA antibodies to CFA major subunits are much larger compared to a typical ETEC tip subunit (15 to 50 kDa), anti-major subunit antibodies recognized the major structural subunits adjacent to the tip subunit can physically prevent a tip from adhering to host receptors (S2 Fig).…”

Therapeutic Approaches Targeting the Assembly and Function of Chaperone-Usher Pili

“…Bivalent polyclonal antibodies have also been used to diminish the compliance of CFA/I and coli surface antigen 2 (CS2) pili, which play essential roles in ETEC pathogenesis. These antibodies, which recognize major pilin subunits, decrease pilus resilience during fluid flow by clamping together layers of the helical fiber or two individual pili, thereby increasing their stiffness and entangling them (110,111). The salivary peptide histatin-5 was also found to bind to and stiffen CFA/I pili, inhibiting ETEC colonization in the gastrointestinal tract (112).…”

Molecular Strategy for Blocking Isopeptide Bond Formation in Nascent Pilin Proteins

“…Bacterial infections are initiated by the attachment of bacteria to their host cells, which is mediated by specialized filamentous structures known as pili (1)(2)(3). As these adhesins are recognized virulence factors, pili have emerged as a target for the development of new vaccines and anti-adhesives (4)(5)(6)(7)(8)(9). In Gram-positive bacteria, pili are assembled as a single polypeptide composed of up to hundreds of pilin protein repeats placed in tandem (10)(11)(12).…”