Pneumococcal Capsule Polysaccharides

Most bacterial capsules are polysaccharides with a specific structure. In many respects, the capsular polysaccharide can be considered an identifying feature of the organism. The pneumococci are divided into types (or groups) that are defined by a particular immunological precipitin reaction of the capsular polysaccharide with an antibody preparation obtained by inoculation of the polysaccharide into an animal host such as a rabbit. The reaction is usually highly specific and depends on the structure of the polysaccharide. S. pneumoniae has been found to have over 80 different immunological types and, therefore, 80 different polysaccharide structures. 

Not all of the pneumococcal capsular polysaccharides have had their structures completely determined. Because of their antigenicity and use in producing vaccines, there has been great interest in this field of carbohydrate chemistry. We will consider only the structures of six types of 5. pneumoniae capsular polysaccharides that have had their structures definitively determined. 

Type 2 polysaccharide has an unusual structure with a repeating tetrasaccharide unit containing a sequence of three a- and -linked L-rhamnopyranose residues linked to position 3, and a single a-D-glucopyranose linked at position 4. The polysaccharide has a branched disaccharide unit of a-D-glucopyranose uronic acid-(l 6)-a-D-glucopyranose linked 1 2 to the second L-rhanmose residue 

Type 3 polysaccharide is one of the first pneumococcal capsule polysaccharides to have its structure determined [184]. It has a simpler structure than type 2 polysaccharide, with only two monosaccharide residues, D-glucopyranose and d-glucopyranose uronic acid, with alternating p-1 - 3 and p-1 - 4 glycosidic linkages in a repeating disaccharide of the following structure [184]  

Type 8 polysaccharide consists of a repeating tetrasaccharide with P-D-glu-copyranosyl uronic acid linked 1 — 4 to p-D-glucopyranose and with a-D-glu-copyranosyl linked 1 4 to a-D-galactopyranose. The polysaccharide contains 

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Prevnar is a glycoconjugate heptavalent pneumococcal vaccine for young children made from the polysaccharide capsule of the pneumococcus (20). Although older vaccines made from pneumococcus capsular polysaccharide are available for adults, young children cannot make antibodies to the capsule. By coupling the pneumococcal capsular polysaccharide with a protein carrier, a vaccine was created that will trigger an infant s immune system to produce antibodies. 

Pneumococcal— These polysaccharides control the immunological type specificity of the pneumococci by their presence in the bacterial capsule. Their constituents are D-glucose, D-glucuronic add, aldobionic adds and amino sugars. The proportions of each vary from one type to another and certain constituents may be missing. 

Capsules of pneumococci consist largely of a polysaccharide slime, but they contain in addition some protein and small amounts of other substances. The type-specificity and virulence of the pneumococci are due to their capsular polysaccharides (see Chapter VIII). The specificity of reaction for different pneumococcal types is due to variation in the capsular polysaccharide molecules. The antigenic polysaccharides are isolable from the bacteria but are more often prepared from the culture broth into which they are liberated by bacterial autolysis. Though there are more than 70 known types of pneumococci, little is known of the structures of their capsular polysaccharides save that from type III pneumococcus. This is shown to be a linear molecule of D-glucose and D-glucuronic acid units in equal amounts. They are linked alternately so that the molecule may be regarded as a chain of aldobiouronic acid units 131) ... 

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