title: Understanding the bacterial polysaccharide antigenicity of Streptococcus agalactiae versus Streptococcus pneumoniae.
abstract: Bacterial surface capsular polysaccharides (CPS) that are similar in carbohydrate sequence may differ markedly in immunogenicity and antigenicity. The structural origin of these phenomena is poorly understood. Such a case is presented by the Gram-positive bacteria Streptococcus agalactiae (Group B Streptococcus; GBS) type III (GBSIII) and Streptococcus pneumoniae (Pn) type 14 (Pn14), which share closely related CPS sequences. Nevertheless, antibodies (Abs) against GBSIII rarely cross-react with the CPS from Pn14. To establish the origin for the variation in CPS antigenicity, models for the immune complexes of CPS fragments from GBSIII and Pn14, with the variable fragment (Fv) of a GBS-specific mAb (mAb 1B1), are presented. The complexes are generated through a combination of comparative Ab modeling and automated ligand docking, followed by explicitly solvated 10-ns molecular dynamics simulations. The relationship between carbohydrate sequence and antigenicity is further quantified through the computation of interaction energies using the Molecular Mechanics-Generalized Born Surface Area (MM-GBSA) method, augmented by conformational entropy estimates. Despite the electrostatic differences between Pn14 and GBSIII CPS, analysis indicates that entropic penalties are primarily responsible for the loss of affinity of the highly flexible Pn14 CPS for mAb 1B1. The similarity of the solution conformation of the relatively rigid GBSIII CPS with that in the immune complex characterizes the previously undescribed 3D structure of the conformational epitope. The analysis provides a comprehensive interpretation for a large body of biochemical and immunological data related to Ab recognition of bacterial polysaccharides and should be applicable to other Ab-carbohydrate interactions.
authors.affiliation: Complex Carbohydrate Research Center, University of Georgia, 315 Riverbend Road, Athens, GA 30602, USA.
authors.forename: Renuka
authors.initials: R
authors.lastname: Kadirvelraj
authors.forename: Jorge
authors.initials: J
authors.lastname: Gonzalez-Outeiriño
authors.forename: B Lachele
authors.initials: BL
authors.lastname: Foley
authors.forename: Meredith L
authors.initials: ML
authors.lastname: Beckham
authors.forename: Harold J
authors.initials: HJ
authors.lastname: Jennings
authors.forename: Simon
authors.initials: S
authors.lastname: Foote
authors.forename: Michael G
authors.initials: MG
authors.lastname: Ford
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Woods
mesh_terms: D000595:Amino Acid Sequence; D000939:Epitopes / Q000737:chemistry; D008954:Models, Biological; D008968:Molecular Conformation; D008969:Molecular Sequence Data; D011134:Polysaccharides / Q000737:chemistry*; D011135:Polysaccharides, Bacterial / Q000737:chemistry*; D017386:Sequence Homology, Amino Acid; D012984:Software; D055672:Static Electricity; D013292:Streptococcus agalactiae / Q000737:chemistry*; D013296:Streptococcus pneumoniae / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D000939:Epitopes; D011134:Polysaccharides; D011135:Polysaccharides, Bacterial