It has been shown that active immunization of rats with the capsular polysaccharide of Bacteroides fragilis protects these animals against abscess development following intraperitoneal challenge with this species. Passive transfer of hyperimmune globulin from immunized animals to nonimmune recipients provided protection against B. fragilis bacteremia in challenged animals, but did not confer protection against abscess development. On the other hand, adoptive transfer of spleen cells from immunized animals to nonimmunized recipients resulted in protection against abscesses following challenge with B. fragilis. These data suggested that a T cell-dependent immune response was involved in protection against abscess development after immunization with B. fragilis capsular antigen. To determine the possible role of cell-mediated immunity prompted by the capsular antigen, inbred congenitally athymic OLA/Rnu rats and their phenotypically normal littermates were actively immunized. Despite the development of high titers of anti-B. fragilis capsular antibody, 100% of actively immunized athymic rats developed abscesses, as did 100% of unimmunized athymic control rats. However, no phenotypically normal littermate control rats that were actively immunized developed abscesses, while 100% of phenotypically normal unimmunized rats developed abscesses. Additional studies showed that adoptive transfer of T cell-enriched spleen cell preparations from Wistar-Lewis rats immunized with the capsular polysaccharide to nonimmune recipients also resulted in protection against B. fragilis-induced abscesses. We conclude that the protection afforded by immunization with B. fragilis capsule against intraabdominal abscesses caused by that organism is T cell-mediated and does not require the presence of serum antibody.
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