Isolation of diphtheria toxin-sensitive mouse cells from a toxin-resistant population transfected with monkey DNA

Diphtheria toxin (DTX)-sensitive mouse cells were isolated from a toxin-resistant thymidine kinase (TK)-negative L-M(TK-) mouse cell population that was transfected with DNA from highly toxin-sensitive monkey Vero cells. Sensitivity to DTX was screened by using a replica plate assay. The purified toxin-sensitive mouse cells were characterized with respect to their ability to bind, internalize, and translocate DTX into the cytosol. In contrast to the L-M(TK-) cells, these DTX-sensitive mouse cells were able to bind and internalize radioiodinated toxin into intracellular vesicles at 37 degrees C. Specific binding of radioiodinated toxin to their cell surface (at 4 degrees C) could not be demonstrated. However, the following evidence for functional receptors capable of binding DTX was obtained: (i) when the toxin-sensitive mouse cells were first allowed to bind DTX at 4 degrees C, followed by washing the cells and shifting the temperature to 37 degrees C (allowing cell surface-bound toxin to enter the cells), the cells were killed; (ii) when cells with surface-bound DTX were exposed briefly to an acidic medium (allowing the toxin to penetrate the plasma membrane directly), protein synthesis was inhibited; and (iii) when cells were incubated with DTX in the presence of the CRM 197, a nontoxic form of DTX with binding properties similar to native DTX, the cytotoxic effect of DTX was markedly decreased. The results demonstrate that the toxin-sensitive mouse cells are killed by a mechanism similar to that observed in naturally occurring toxin-sensitive cell lines. The data further suggest that the transfected mouse cells express functional receptors for DTX.