Bimodal Response to Shiga Toxin 2 Subtypes Results from Relatively Weak Binding to the Target Cell.

There are two major antigenic forms of Shiga toxin (Stx), Stx1 and Stx2, which bind the same receptor and act on the same target but nonetheless differ in potency. Stx1a is more toxic to cultured cells, but Stx2 subtypes are more potent in animal models. To understand this phenomenon in cultured cells, we used a system that combines flow cytometry with a fluorescent reporter to monitor the Stx-induced inhibition of protein synthesis in single cells. We observed that Vero cells intoxicated with Stx1a behave differently than those intoxicated with Stx2 subtypes: cells challenged with Stx1a exhibited a population-wide loss of protein synthesis, while cells exposed to Stx2a or Stx2c exhibited a dose-dependent bimodal response in which one subpopulation of cells was unaffected (i.e., no loss of protein synthesis). Cells challenged with a hybrid toxin containing the catalytic subunit of Stx1a and the cell-binding subunit of Stx2a also exhibited a bimodal response to intoxication, while cells challenged with a hybrid toxin containing the catalytic subunit of Stx2a and the cell-binding subunit of Stx1a exhibited a population-wide loss of protein synthesis. Other experiments further supported a primary role for the subtype of the B subunit in the outcome of host-Stx interactions. Our collective observations indicate that the bimodal response to Stx2 subtypes is due to relatively weak binding between Stx2 and the host cell that reduces the total functional pool of Stx2 in comparison to that of Stx1a. This explains, in part, the molecular basis for the differential cellular toxicity between Stx1a and Stx2 subtypes.