Mitogenic activity of purified capsular polysaccharide A from Bacteroides fragilis: Differential stimulatory effect on mouse and rat lymphocytes in vitro

Bacteroides fragilis; a Gram-negative colonic bacterium, induces the format ion of abscesses associated with intra-abdominal sepsis in humans, The sing ular ability of this organism to modulate abscess formation in experimental rodent models resides in the structurally distinct and ionically charged c apsular polysaccharides A (PS A) and B (PS B), The regulation of abscess fo rmation in animals is dependent on T lymphocytes, However, the manner in wh ich PS A interacts with T cells remains unknown. We therefore tested the T cell stimulatory capacity of purified PS A on mouse and rat lymphocytes in cellular proliferation assays and found that the PS A molecule possesses mi togenic characteristics distinguishable from those of the polyclonal B cell activator LPS, the T cell mitogen Con A, and staphylococcal enterotoxin A superantigen. Further, PS A stimulated proliferation of normal mouse and ra t lymphocytes differentially. Mouse B cells responded to PS A in a fashion that did not require exogenous APC function, while rat T lymphocyte respons es to PS A required APC function derived from autologous or xenogenic feede r cells, Cellular depletion experiments showed that the CD4(+) subset of ra t spleen cells was the primary responder cell type to PS A in vitro, The di fferential stimulatory effects of PS A on mouse and rat lymphocytes may ref lect its ability to stimulate different lymphocyte subsets in vivo through the activities of receptor/counter-receptor pairs present on responder lymp hocytes and cognate APC.