Characterization of Shiga-Like Toxin I Mitogenic Activity on Porcine Lymphocytes

The Shiga-like toxins, a family of cytotoxins produced by certain pathogenic strains of Escherichia coli, have been associated with outbreaks of diarrheal disease, hemorrhagic colitis, and hemolytic uremic syndrome. The mechanism by which SLT\u27s function has always been ascribed to their cytotoxic activity. We tested SLT-I for its ability to activate porcine lymphocytes in vitro. SLT-I was optimally mitogenic at 0.05 HeLa cytotoxic units (-0.1 ng/ml) of protein enriched toxin preparation, a protein concentration 10,000 fold lower than the optimal mitogenic concentration of ConA (1 µg/ml), a conventional T cell mitogen. SLT-I mitogenic activity increased with dose to optimal stimulation, then mitogenesis declined. Only supernatants of cultures carrying the toxin prophage, not of the non-toxic parent strain, caused mitogenesis. Monoclonal antibody recognizing the B subunit of the toxin blocked mitogenesis; and sugars found on the ceramide to which SLT-I binds on vero cells (as a cytotoxin) did not block mitogenic activity. Antibodies to porcine CD4, CD8, and class II MHC (DR-like) molecules effectively inhibited SLT-I induced stimulation. SLT-I had cytotoxic activity against macrophages at concentrations ten times greater than the mitogenic optimum. Our studies suggest that SLT-I may be a superantigen. We believe that SLT-I acts systemically by inducing a loss of normal immune regulatory control and is important in the control bf host immune response in the course of enteric infection with SLT-I producing E.coli