Shiga toxin 1 from Escherichia coli blocks activation and proliferation ofbovine lymphocyte subpopulations in vitro

Shiga toxin-producing Escherichia coli (STEC) is widespread in the cattle p opulation, but the clinical significance of Shiga toxins (Stx's) for the bo vine species remains obscure. Since Stx's exert immunomodulating effects in other species, we examined the effect of purified Stx1 on a bovine B lymph oma cell line (BL-3) and peripheral blood mononuclear cells (PBMC) isolated from adult bovine blood by viability assays and flow cytometry analysis, S tx1 markedly induced apoptosis in stimulated BL-3 cells. The susceptibility of this B-cell-derived cell line was induced only by either lipopolysaccha ride (LPS) or pokeweed mitogen, while cultures stimulated with T-cell mitog ens were unaffected by the toxin, In contrast, Stx1 did not induce cellular death-neither apoptosis nor necrosis-in primary cultures of PBMC but hinde red the mitogen-induced increase in metabolic activity, The influence of St x1 on single PBMC subpopulations varied with the type of mitogenic stimulus applied. Stimulation with phytohemagglutinin P particularly induced the pr oliferation of bovine CD8-expressing (BoCD8(+)) cells, and this proliferati ve response was blocked by Stx1, On the other hand, Stx1 reduced the portio n of viable B cells in the presence of LPS, Modulation of activation marker expression (BoCD25 and BoCD71) by Stx1 indicated that the toxin hindered t he proliferation of cells by blocking their activation. In conclusion, we a ssume that Stx1 contributes to the pathogenesis of STEC associated diarrhea in calves by suppressing the mucosa-associated immune response. The useful ness of cattle as a model in which to study Stx-induced immunomodulation is discussed.