Protection against Experimental Melioidosis with a Synthetic manno-Heptopyranose Hexasaccharide Glycoconjugate

Abstract

Melioidosis is an emerging infectious disease caused by Burkholderia pseudomallei and is associated with high morbidity and mortality rates in endemic areas. Antibiotic treatment is protracted and not always successful; even with appropriate therapy, up to 40% of individuals presenting with melioidosis in Thailand succumb to infection. In these circumstances, an effective vaccine has the potential to have a dramatic impact on both the scale and the severity of disease. Currently, no vaccines are licensed for human use. A leading vaccine candidate is the capsular polysaccharide consisting of a homopolymer of unbranched 1→3 linked 2-O-acetyl-6-deoxy-β-d-manno-heptopyranose. Here, we present the chemical synthesis of this challenging antigen using a novel modular disaccharide assembly approach. The resulting hexasaccharide was coupled to the nontoxic Hc domain of tetanus toxin as a carrier protein to promote recruitment of T-cell help and provide a scaffold for antigen display. Mice immunized with the glycoconjugate developed IgM and IgG responses capable of recognizing native capsule, and were protected against infection with over 120 × LD50 of B. pseudomallei strain K96243. This is the first report of the chemical synthesis of an immunologically relevant and protective hexasaccharide fragment of the capsular polysaccharide of B. pseudomallei and serves as the rational starting point for the development of an effective licensed vaccine for this emerging infectious disease.

Isotype switching: Mouse IgG3 constant region drives increased affinity for polysaccharide antigens

“…Indeed, a hexamer conjugated to protein is sufficient to confer protection in a mouse model. 28 Dillon et al. demonstrate that an IgG3 anti-CPS antibody (3C5; composed of Vh6 and IgKV19/28 chains) raised against heat killed B. pseudomallei has a strong affinity for purified CPS.…”

Novel multi-component vaccine approaches for Burkholderia pseudomallei

“…In another study, intraperitoneal immunization with chemically synthesized CPS conjugated to tetanus toxoid promoted IgM and IgG serum antibodies. While the CPS-specific antibody titres were considerably less than that induced by native CPS, BALB/c mice were still significantly protected from B. pseudomallei challenge, with 66% of mice surviving until the study end-point at day 35 [47]. In addition to CPS-based conjugates, immunization with the B. pseudomallei O-polysaccharide (OPS II) conjugated to a carrier protein AcrA from Campylobacter jejuni provided similar levels of protection to immunization with whole-cell killed bacteria against a highly lethal challenge [48].…”

“…Importantly, B. pseudomallei appears to express only a single capsule serotype, suggesting that a CPS-based vaccine could confer protection against all B. pseudomallei strains. While the CPS-specific antibody titres were considerably less than that induced by native CPS, BALB/c mice were still significantly protected from B. pseudomallei challenge, with 66% of mice surviving until the study end-point at day 35 [47]. In a recent study, B. pseudomallei CPS was purified and covalently linked to recombinant CRM197, a non-toxic mutant of diphtheria toxin.…”

Glycoconjugate vaccines: some observations on carrier and production methods

“…Another strategy has been to produce polysaccharides synthetically in blocks. The exact number can vary, such as a di-and tetra-saccharides (Budhadev & Mukhopadhyay, 2015), di-and tri-saccharides (Mandal, 2014;Mondal, Liao, Mondal, & Guo, 2015;Si & Misra, 2016), three disaccharides (Scott et al, 2016) and two tri-saccharides (Seeberger, Pereira, & Govindan, 2017). However, it should be noted that it is possible to compare assembly methods and one study revealed that linear assembly may be more effective than block assembly (Dhara & Misra, 2015;Seeberger et al, 2017).…”