Bacteroides fragilis is the anaerobe most commonly isolated from clinical cases of intra-abdominal
sepsis. In a rodent model of this disease process,
intraperitoneal injection of the capsular
polysaccharide complex (
CPC) from B. fragilis provokes
abscess formation, while subcutaneous administration of this complex confers protection against B. fragilis-induced
intra-abdominal abscesses. The
CPC consists of two discrete
polysaccharides,
polysaccharides A and B (PS A and PS B), each possessing oppositely charged structural groups critical to the ability of these
carbohydrates to induce the formation of
abscesses. Other
bacterial polysaccharides that possess oppositely charged groups (such as the group
antigen or capsular
polysaccharide from Streptococcus pneumoniae type 1 strains) also exhibited potent
abscess-inducing capabilities. We report here that positively and negatively charged groups on
polysaccharides are also essential for inducing protection against
abscess formation. Vaccination of rats with B. fragilis PS A, PS B, or the S. pneumoniae type 1
capsule protected against
intra-abdominal abscesses subsequent to intraperitoneal challenge with each of these
polysaccharides. Chemical conversion of the free amino or carboxyl groups on PS A to uncharged N-acetyl or hydroxymethyl groups, respectively, abrogated the ability of this
polymer to confer protection against
polysaccharide-mediated
abscess formation. Adoptive transfer of splenic T cells from
polysaccharide-vaccinated rats to naive animals demonstrated that T cells mediated this protective activity. T cells transferred from animals vaccinated with a
polysaccharide repeating unit (Salmonella typhi Vi
antigen) that normally contains one carboxyl group but was chemically converted to a
polymer that possesses both free amino and carboxyl groups (accomplished by de-N-acetylating the Vi
antigen) protected naive T-cell recipients against
polysaccharide-induced
abscesses. These results demonstrate that a distinct structural motif associated with the B. fragilis
polysaccharides is necessary for induction of protective immunity against
abscess formation associated with intra-abdominal
sepsis. However, protection is not
antigen specific in a traditional sense. Rather, the protective ability of these structurally dissimilar
polysaccharides is conferred by, and perhaps specific for, a motif of oppositely charged groups.