Structural basis of the abscess-modulating polysaccharide A2 fromBacteroides fragilis

Abstract

Zwitterionic capsular polysaccharides from pathogenic bacteria have peculiar immunological properties. They are capable of eliciting T-cell proliferation and modulating the course of abscess formation. To understand the molecular basis of this characteristic immune response, we are conducting detailed structure-function studies on these polysaccharides. We have identified, purified, and characterized an abscess-modulating polysaccharide, PS A2, from the clinical strain Bacteroides fragilis 638R. Here, we report the elucidation of both the chemical and three-dimensional structures of PS A2 by NMR spectroscopy, chemical methods, gas chromatography-mass spectrometry, and restrained molecular dynamics calculations. PS A2 consists of a pentasaccharide repeating unit containing mannoheptose, N-acetylmannosamine, 3-acetamido-3,6-dideoxyglucose, 2-amino-4-acetamido-2,4,6-trideoxygalactose, fucose, and 3-hydroxybutanoic acid. PS A2 is zwitterionic and carries one cationic free amine and one anionic carboxylate in each repeating unit. It forms an extended right-handed helix with two repeating units per turn and a pitch of 20 Å. Positive and negative charges are exposed on the outer surface of the polymer in a regularly spaced pattern, which renders them easily accessible to other molecules. The helix is characterized by repeated large grooves whose lateral boundaries are occupied by the charges. The three-dimensional structure of PS A2 explicitly suggests mechanisms of interaction between zwitterionic polysaccharides and proteins.

Zwitterionic Polysaccharides Stimulate T Cells with No Preferential Vβ Usage and Promote Anergy, Resulting in Protection against Experimental Abscess Formation

“…This mechanism would be especially sensitive to the size of the bridging polymer. Any Zps molecule with a size Ͼ17 kDa would therefore be able to crosslink several TCRs, but also possibly anchor MHC class II molecules (42), thus explaining why these latter molecules are as essential as the TCR to yield a productive response by the T cell. Given the polyclonal nature of the T cell response to Zps, this argues in favor of a limited specificity for polysaccharides as long as they are zwitterionic.…”

Glycosaminoglycans are a potential cause of rheumatoid arthritis

“…If a self-reactive cell encounters its self-antigen after it matures, the cell still is generally inactivated because of the added requirement of a costimulatory signal. Because GAGs can bind to many types of cell surface receptors, as well as cytokines and other soluble protein messengers (8)(9)(10)(11)(12), and also because polysaccharides are capable of cross-linking multiple receptors on a single cell or multiple cells (35), GAGs could act as ''superantigens'' and provide the necessary signals to promote the expansion of GAG-binding cells. Furthermore, irregular amounts of highly acidic and multimolecule-binding GAGs could change the microenvironment and dynamics of the immune system.…”

Bacteroides: the Good, the Bad, and the Nitty-Gritty

“…The zwitterionic motif is necessary for the activities of this group of molecules, including promoting the formation of abscesses (282). The structural basis of the abscess-modulating activity of the polysaccharide has been extensively studied; one model suggests that grooves in the polysaccharide may serve as "docking sites" for ␣-helices of specific molecules (e.g., immunomodulating molecules such as major histocompatibility/antigen molecular complexes) and thus trigger specific T-cell responses which then lead to abscess formation (289).…”

“…Two distinct high-molecular-weight polysaccharides (PS-A and PS-B) that are coexpressed were described (179,283), and the structures of these two polysaccharides were elucidated (14). PS-A is made up of repeating tetrasaccharide units, and PS-B is made up of repeating hexasaccharide units (283,289). Other strains of B. fragilis were subsequently analyzed, and all possessed a complex capsular polysaccharide composed of at least two different polysaccharides; these polysaccharides were antigenically diverse, although some crossreactivity with the prototype capsular polysaccharide was seen (180).…”