The pathogenic bacterium Shigella is a leading cause of diarrheal disease and mortality, disproportionately affecting young children in low-income countries. The increasing prevalence of antibiotic resistance in Shigella necessitates an effective vaccine, for which the bacterial lipopolysaccharide O-antigen is the primary target. S. flexneri serotype 6 has been proposed as a multivalent vaccine component to ensure broad protection against Shigella. We have previously explored the conformations of S. flexneri O-antigens from serogroups Y, 2, 3, and 5 that share a common saccharide backbone (serotype Y). Here we consider serogroup 6, which is of particular interest because of an altered backbone repeat unit with non-stoichiometric O-acetylation, the antigenic and immunogenic importance of which have yet to be established. Our simulations show significant conformational changes in serogroup 6 relative to the serotype Y backbone. We further find that O-acetylation has little effect on conformation and hence may not be essential for the antigenicity of serotype 6. This is corroborated by an in vivo study in mice, using Generalized Modules for Membrane Antigens (GMMA) as O-antigen delivery systems, that shows that O-acetylation does not have an impact on the immune response elicited by the S. flexneri serotype 6 O-antigen.

No abstract

Streptococcus pneumoniae is an encapsulated gram-negative bacterium and a significant human pathogen. The capsular polysaccharide (CPS) is essential for virulence and a target antigen for vaccines. Although widespread introduction of pneumococcal conjugate vaccines (PCVs) has significantly reduced disease, the prevalence of non-vaccine serotypes has increased. On the basis of the CPS, S. pneumoniae serogroup 10 comprises four main serotypes 10A, 10B, 10C, and 10F; as well as the recently identified 10D. As it is the most prevalent, serotype 10A CPS has been included as a vaccine antigen in the next generation PCVs. Here we use molecular modeling to provide conformational rationales for the complex cross-reactivity reported between serotypes 10A, 10B, 10C, and 10F anti-sera. Although the highly mobile phosphodiester linkages produce very flexible CPS, shorter segments are conformationally defined, with exposed β-D-galactofuranose (β DGalf) side chains that are potential antibody binding sites. We identify four distinct conformational epitopes for the immunodominant β DGalf that assist in rationalizing the complex asymmetric cross-reactivity relationships. In particular, we find that strongly cross-reactive serotypes share common epitopes. Further, we show that human intelectin-1 has the potential to bind the exposed exocyclic 1,2-diol of the terminal β DGalf in each serotype; the relative accessibility of three- or six-linked β DGalf may play a role in the strength of the innate immune response and hence serotype disease prevalence. In conclusion, our modeling study and relevant serological studies support the inclusion of serotype 10A in a vaccine to best protect against serogroup 10 disease.

Pasteurella multocida, an encapsulated gram-negative bacterium, is a significant veterinary pathogen. P. multocida is classified into five serogroups (A, B, D, E, and F) based on the bacterial capsular polysaccharide (CPS), which is important for virulence. Serogroups B and E are the primary causative agents of bovine hemorrhagic septicemia that is associated with significant yearly losses of livestock worldwide, primarily in low- and middle-income countries. P. multocida disease is currently managed by whole-cell vaccination, albeit with limited efficacy. CPS is an attractive antigen target for an improved vaccine: CPS-based vaccines have proven highly effective against human bacterial diseases and could provide longer-term protection against P. multocida. The recently elucidated CPS repeat units of serogroups B and E both comprise a ManNAcA/GlcNAc disaccharide backbone with Fruf side chain, but differ in their glycosidic linkages, and a glycine side chain in serogroup B. Interestingly, the Haemophilus influenzae types e and d CPS have the same backbone residues. Here comparative modeling of P. multocida serogroups B and E and H. influenzae types e and d CPS identifies a significant impact of small structural differences on both the chain conformation and the exposed potential antibody-binding epitopes. Further, Fruf and/or glycine side chains shield the immunogenic amino-sugar CPS backbone—a possible common strategy for immune evasion in both P. multocida and H. influenzae. As the lack of common epitopes suggests limited potential for cross-reactivity, a bivalent CPS-based vaccine may be necessary to provide adequate protection against P. multocida types B and E.