Fig. 1: Key types of antigenic determinants (epitopes) of bacterial polysaccharides.
In homopolymeric polysaccharides, identical monosaccharide residues repeat as a polymer. In heteropolymeric (or complex) PS, the repeat unit (RU, in square brackets) consists of diverse monosaccharide residues and may include branching side chains. The minimal required unit for antigenicity in linear epitopes consists of about 6–7 contiguous monosaccharides121. Linear epitopes may include the terminal monosaccharide residues of glycan chains. Conformational epitopes are formed by residues that are in close spatial proximity but dispersed across their primary sequence122. These require a sequence of residues sufficiently long to build or mimic the spatial conformations necessary for interaction with the antibody. As an example, the homopolymeric conformational epitope of the Neisseria meningitidis group B capsular PS antigen contains ten residues, although only the inner six residues interact with its cognate antibody123,124. Glycan fragments that exceed the length of the identified antigenic epitope are usually used as immunogens in glycoconjugate vaccines125.
