Understanding the bacterial polysaccharide antigenicity ofStreptococcus agalactiaeversusStreptococcus 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. S treptococcus agalactiae [Group B Streptococcus (GBS)] andStreptococcus pneumoniae (Pn) are responsible for the majority of life-threatening cases of septicemia, meningitis, and pneumonia in neonates (1, 2). Gram-positive bacteria, such as GBS and Pn, are classified into serotypes according to the unique carbohydrate sequence of the bacterial surface capsular polysaccharide (CPS) and protein antigens. Serotypes vary in antigenicity, immunogenicity, virulence, and geographical distribution (3). Quantification of the structural and dynamic properties responsible for the affinity and specificity of antigenic oligosaccharide-antibody (Ab) interactions is a crucial step in furthering the understanding of the immune response to bacterial and fungal pathogens. In GBS, the CPS is a high-molecularweight polymer composed of varying sequences of ␤-D-, and sometimes L-rhamnopyranose. The glyceryl side chain of the Neu5Ac residues may also be Oacetylated (4). In all GBS strains identified to date, the Neu5Ac residues occur in the terminal position on the side-chain branches of the polymeric repeat unit of the CPS. They play an important role in defining the antigenicity and immunogenicity of the CPS (5). Variations within the CPS sequence result in the nine kno...

Fourier transform mass spectrometry to monitor hyaluronan–protein interactions: use of hydrogen/deuterium amide exchange

“…29 A model for HA 8 was generated by setting the glycosidic (φ,ψ) angles for each linkage (Supplemental Figure 1, see Supplementary material) to the most representative values from the MD data (Supplemental Table 1, see Supplementary material). For ligand docking, a fragment-based docking protocol previously described 30 was modified by employing the rigid whole-molecule docking approach. This modification was adopted because of the large numbers of degrees of freedom present in highly flexible molecules, and the associated potential energy convergence problems.…”

Thermodynamics and Density of Binding of a Panel of Antibodies to High-Molecular-Weight Capsular Polysaccharides

“…Such enthalpy-entropy compensation has been reported for many antibody-carbohydrate interactions (8,12). The serotype 14 PS is an uncharged polymer and is highly conformationally mobile (20). Despite the conformational flexibility of the serotype 14 PS, a conformational epitope consisting of at least four repeating units has been proposed for the serotype 14 PS (21, 45).…”

“…Therefore, the spacing of the binding of the Fabs along the PS calculated from these studies, about 3 nm per bound Fab at saturation, is relatively consistent with published estimates of the size of an antibody binding site (19) and with estimates of the dimensions of an Fab fragment (30,31,35,42), suggesting that anti-PS antibodies of moderately high affinities are capable of binding as densely along the PS chain as permitted by steric interactions between the IgG molecules and may therefore recognize structural or linear epitopes rather than conformational epitopes. Recognition of immunodominant and functionally important conformational epitopes by polyclonal antibodies and MAbs has been proposed for the group B streptococcal type III PS (7,46), the serotype 14 PS (20,21,45), and the Neisseria meningitidis serogroup B PS (6,11,23,24). In contrast, there is evidence to suggest that conformational epitopes may not be present or may not play an important role in generating functional immune responses for serotype 3, 6A, 18C, 19F, or 23F PS (22) or pneumococcal serogroup 9 capsular PS (32,33).…”

The Future of Synthetic Carbohydrate Vaccines: Immunological Studies on Streptococcus pneumoniae Type 14

“…Native Pn14PS is structurally related to and has cross-reactivity with GBSIIIPS [81]. The branched structures of Pn14PS and GBSIIIPS differ only in the absence (in Pn14PS) or presence (in GBSIIIPS) of the ( 23)-linked sialic acid N-acetylneuraminic acid (Neu5Ac) in their side chains: [82]. We reported that type-specific Pn14PS antibodies which recognize the branched structure of Pn14PS have a low affinity for the native GBSIIIPS and do not promote opsonophagocytosis of GBSIII, however desialylation of GBSIIIPS, however, resulted in dramatically higher affinity of anti-Pn14PS antibodies in mice when GBSIIIP was treated by nurimindase (desialylation) [80].…”