Shiga toxins (Stxs) produced by enterohaemorrhagic Escherichia coli or Shigella dysenteriae damage human endothelial cells predominantly in cooperation with pro-inflammatory cytokines, such as TNF-α. However, in this study, in vitro IFN-γ pre-treatment resulted in human lung microvascular endothelial cells becoming over 10 000-fold less sensitive to Stxs. In contrast, in their basal condition, they were extremely sensitive to Stxs. Interestingly, TNF-α addition to IFN-γ reverted the Stx-resistant phenotype, which corresponded with its well-established enhancing effect on Stx toxicity. Toxin binding to the cell was barely affected by IFN-γ. Also, the toxin uptake in the Stx-resistant phenotype was more than 100-fold greater than that of normal cells, when compared at Stx concentrations resulting in equivalent degrees of cell damage. Protein synthesis was inhibited by nearly 90% in the Stx-resistant phenotype after 24 h toxin exposure. This indicated that the intracellular toxin was active as an N-glycosidase, while cells were still over 60% viable, suggesting a possible unknown cytotoxic function of Stx. In conclusion, this study shows a unique effect of IFN-γ in the suppression of the toxicity of Stxs in a human microvascular endothelial cell model and the involvement of a novel mechanism in this suppression.
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