4 - Capsular Polysaccharides

Abstract

Capsular polysaccharides (CPS) are acidic polysaccharides made up of repeating oligosaccharide units, which may be either linear or branched. The primary structures can be formulated by the respective repeating unit and the joining linkages. The CPSs are sometimes substituted with O-formyl- or O-acetyl-groups (in ester linkage) or with pyruvate (in ketosidic linkage). Amino acids are found linked either to the hydroxyl groups of sugar rings (esters) or to the carboxyl group of hexuronic acids (amides). In general, the CPSs are distinctly longer, up to about 100 kDa per chain, than the polysaccharide moieties (O-PS) of the cell wall (outer membrane) O-antigens of Gram-negative bacteria (LPS, up to about 10–20kDa). In some Gram-negative bacteria, the LPS seems to occur in two physically distinct forms with the same primary structure. The lack of an outer membrane, and hence the lack of a periplasmic space, simplifies the surface presentation of CPS. In general, bacterial CPS induces an immune response that results in the production of protective anticapsular antibodies in the exposed host. With whole bacteria or when coupled to a carrier protein as immunogen, CPS induces a T-cell-dependent immune response that results in the production of IgG antibodies. These can be boosted by subsequent injections of the immunogen. In contrast, pure soluble CPS induces a T-cell-independent response that results in IgM antibodies that cannot be boosted. CPSs may also exert activities that are not primarily involved in immunological processes. The CPS of Bacteroides fragilis, which forms a strong complex of two distinct polysaccharides, induces intestinal abscesses.