Semisynthetic Glycoconjugate Vaccine Candidates against Escherichia coli O25B Induce Functional IgG Antibodies in Mice

Abstract

Extraintestinal pathogenic Escherichia coli (ExPEC) is a major health concern due to emerging antibiotic resistance. Along with O1A, O2, and O6A, E. coli O25B is a major serotype within the ExPEC group, which expresses a unique O -antigen. Clinical studies with a glycoconjugate vaccine of the above-mentioned O -types revealed O25B as the least immunogenic component, inducing relatively weak IgG titers. To evaluate the immunological properties of semisynthetic glycoconjugate vaccine candidates against E. coli O25B, we here report the chemical synthesis of an initial set of five O25B glycan antigens differing in length, from one to three repeat units, and frameshifts of the repeat unit. The oligosaccharide antigens were conjugated to the carrier protein CRM 197 . The resulting semisynthetic glycoconjugates induced functional IgG antibodies in mice with opsonophagocytic activity against E. coli O25B. Three of the oligosaccharide–CRM 197 conjugates elicited functional IgGs in the same order of magnitude as a conventional CRM 197 glycoconjugate prepared with native O25B O -antigen and therefore represent promising vaccine candidates for further investigation. Binding studies with two monoclonal antibodies (mAbs) revealed nanomolar anti-O25B IgG responses with nanomolar K D values and with varying binding epitopes. The immunogenicity and mAb binding data now allow for the rational design of additional synthetic antigens for future preclinical studies, with expected further improvements in the functional antibody responses. Moreover, acetylation of a rhamnose residue was shown to be likely dispensable for immunogenicity, as a deacylated antigen was able to elicit strong functional IgG responses. Our findings strongly support the feasibility of a semisynthetic glycoconjugate vaccine against E. coli O25B.

Development in the Concept of Bacterial Polysaccharide Repeating Unit-Based Antibacterial Conjugate Vaccines

“…For example, while we were in the process of preparing this Review, Pereira and coworkers reported the synthesis and evaluation of some new glycoconjugate vaccine candidates for Escherichia coli O25B. 295 Therefore, it is challenging to cover the conjugate vaccines against all bacterial pathogens, and we chose to review only some common bacteria and their vaccine development. Furthermore, even among the few bacterial pathogens selected, it is impossible to include all the research related to glycoconjugate vaccine development.…”

“…Finally, we want to point out that the topic of this Review is very broadthe research area has been developing rapidly in the past few decades and is still growing. For example, while we were in the process of preparing this Review, Pereira and co-workers reported the synthesis and evaluation of some new glycoconjugate vaccine candidates for Escherichia coli O25B . Therefore, it is challenging to cover the conjugate vaccines against all bacterial pathogens, and we chose to review only some common bacteria and their vaccine development.…”

Glycovaccine Design: Optimization of Model and Antitubercular Carrier Glycosylation via Disuccinimidyl Homobifunctional Linker

“…Among the different options, the introduction of an amino group is one of the most convenient since it allows further derivatization with bifunctional or bivalent linkers to form active esters that are able to react with the ε-amino group of lysine residues on the carrier protein surface. The use of disuccinimidyl bivalent linkers (such as disuccinimidyl glutarate (DSG) or disuccinimidyl adipate (DSA)) that are able to react with amino-functionalized oligosaccharides to form reactive esters falls into that category [ 4 ], and several examples showing the application of this synthetic strategy to the development/production of glycovaccines can be found in the literature [ 5 , 6 , 7 , 8 , 9 , 10 , 11 ].…”

“…Most of the protocols reported in the literature mention purification approaches based on liquid–liquid extraction and/or precipitation using different organic solvents, such as ethyl acetate or chloroform [ 5 , 6 , 7 ], or even water [ 13 ]. The effectiveness of these purification approaches has not been proven since, in most of these experimental works, glycan attachment to the carrier is monitored by SDS-PAGE and/or MALDI-TOF-MS analysis [ 6 , 7 , 9 , 11 ]. The use of these analytical methods only provides the average glycan loading, and no information regarding any side reactions or alterations occurring during conjugation, such as the conjugation with residual linker, can be obtained.…”

Progress toward the development of an effective vaccine for Extraintestinal pathogenicE. coli(ExPEC): The application of the multiple-protein subunits vaccine in different murine models

“…However, the ineffectiveness of heat-killed inactivated bacterial vaccines in preventing uncomplicated UTIs and significant adverse effects of bacterial-lysate vaccines limited their prevention function [44]. Several attempts have been made to develop O-specific polysaccharide (O-antigen) conjugate vaccines [45][46][47][48]. For instance, ExPEC4V (bioconjugate O1A, O2, O6A, and O25B) has elicited functional antibody responses and is currently in phase 2 randomized controlled trial [49][50][51][52][53].…”