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Conformation and Cross-Protection in Group B Streptococcus Serotype III and Streptococcus pneumoniae Serotype 14: A Molecular Modeling Study.

ABSTRACT: Although the branched capsular polysaccharides of Streptococcus agalactiae serotype III (GBSIII PS) and Streptococcus pneumoniae serotype 14 (Pn14 PS) differ only in the addition of a terminal sialic acid on the GBSIII PS side chains, these very similar polysaccharides are immunogenically distinct. Our simulations of GBSIII PS, Pn14 PS and the unbranched backbone polysaccharide provide a conformational rationale for the different antigenic epitopes identified for these PS. We find that side chains stabilize the proximal ? dGlc(1?6) ? dGlcNAc backbone linkage, restricting rotation and creating a well-defined conformational epitope at the branch point. This agrees with the glycotope structure recognized by an anti-GBSIII PS functional monoclonal antibody. We find the same dominant solution conformation for GBSIII and Pn14 PS: aside from the branch point, the backbone is very flexible with a "zig-zag" conformational habit, rather than the helix previously proposed for GBSIII PS. This suggests a common strategy for bacterial evasion of the host immune system: a flexible backbone that is less perceptible to the immune system, combined with conformationally-defined branch points presenting human-mimic epitopes. This work demonstrates how small structural features such as side chains can alter the conformation of a polysaccharide by restricting rotation around backbone linkages.

SUBMITTER: Kuttel MM

PROVIDER: S-EPMC6469160 | biostudies-literature | 2019 Feb

REPOSITORIES: biostudies-literature

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Conformation and Cross-Protection in Group B Streptococcus Serotype III and Streptococcus pneumoniae Serotype 14: A Molecular Modeling Study.

Kuttel Michelle M MM Ravenscroft Neil N

Pharmaceuticals (Basel, Switzerland) 20190213 1

Although the branched capsular polysaccharides of Streptococcus agalactiae serotype III (GBSIII PS) and Streptococcus pneumoniae serotype 14 (Pn14 PS) differ only in the addition of a terminal sialic acid on the GBSIII PS side chains, these very similar polysaccharides are immunogenically distinct. Our simulations of GBSIII PS, Pn14 PS and the unbranched backbone polysaccharide provide a conformational rationale for the different antigenic epitopes identified for these PS. We find ...[more]

PMID: 30781826

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Project description:Streptococcus pneumoniae (the pneumococcus) is a human pathogen of global importance, classified into serotypes based on the type of capsular polysaccharide produced. Serotyping of pneumococci is essential for disease surveillance and vaccine impact measurement. However, the accuracy of serotyping methods can be affected by previously undiscovered variants. Previous studies have identified variants of serotype 14, a highly invasive serotype included in all licensed vaccine formulations. However, the potential of these variants to influence serotyping accuracy and evade vaccine-induced protection has not been investigated. In this study, we screened 1,386 nasopharyngeal swabs from children hospitalized with acute respiratory infection in Papua New Guinea for pneumococci. Swabs containing pneumococci (n = 1,226) were serotyped by microarray to identify pneumococci with a divergent serotype 14 capsule locus. Three serotype 14 variants ('14-like') were isolated and characterized further. The serotyping results of these isolates using molecular methods varied depending on the method, with 3/3 typing as nontypeable (PneumoCaT), 3/3 typing as serotype 14 (seroBA), and 2/3 typing as serotype 14 (SeroCall and quantitative PCR). All three isolates were nontypeable by phenotypic methods (Quellung and latex agglutination), indicating the absence of capsule. Illumina and nanopore sequencing were employed to examine their capsule loci and revealed unique mutations. Lastly, when incubated with sera from vaccinated individuals, the 14-like isolates evaded serotype-specific opsonophagocytic killing. Our study highlights the need for phenotypic testing to validate serotyping data derived from molecular methods. The convergent evolution of capsule loss underscores the importance of studying pneumococcal population biology to monitor the emergence of pneumococci capable of vaccine escape, globally. IMPORTANCE Pneumococcus is a pathogen of major public health importance. Current vaccines have limited valency, targeting a subset (up to 20) of the more than 100 capsule types (serotypes). Precise serotyping methods are therefore essential to avoid mistyping, which can reduce the accuracy of data used to inform decisions around vaccine introduction and/or maintenance of national vaccination programs. In this study, we examine a variant of serotype 14 (14-like), a virulent serotype present in all currently licensed vaccine formulations. Although these 14-like pneumococci no longer produce a serotype 14 capsule, widely used molecular methods can mistype them as serotype 14. Importantly, we show that 14-like pneumococci can evade opsonophagocytic killing mediated by vaccination. Despite the high accuracy of molecular methods for serotyping, our study reemphasizes their limitations. This is particularly relevant in situations where nonvaccine type pneumococci (e.g., the 14-likes in this study) could potentially be misidentified as a vaccine type (e.g., serotype 14).

Dataset Information

Conformation and Cross-Protection in Group B Streptococcus Serotype III and Streptococcus pneumoniae Serotype 14: A Molecular Modeling Study.

Publications

Conformation and Cross-Protection in Group B Streptococcus Serotype III and <i>Streptococcus pneumoniae</i> Serotype 14: A Molecular Modeling Study.

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