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Cholera Toxin Inhibits Lethal Hit Stage of Natural Killer Cell‐Mediated Cytotoxicity

Cytolytic process which was affected by cholera toxin (CT) resulting in the loss of natural killer (NK) cell activity was analyzed. Conjugate formation assay, membrane phospholipid methylation assay and serine esterase (granzyme A) release assay were used to determine the stage of the CT‐induced inhibition of NK cell‐mediated cytotoxicity. A human NK cell line YT cell‐mediated cytotoxicity was completely abolished by CT pretreatment or addition of CT to the assay system. The conjugate formation assay revealed that the binding between YT cells and target cells was not affected by CT. The defined triggering stage which is coupled with membrane phospholipid methylation was not affected by CT treatment, either. On the other hand, the lethal hit stage which is represented by serine esterase (SE) release was completely inhibited by CT treatment of YT cells. Therefore, CT inhibits the stage after binding and triggering—i.e., lethal hit stage of NK cell‐mediated cytotoxicity. The results also suggest that there exists a CT‐sensitive negative cytotoxic signal transduction pathway as well as usual positive signal transduction pathway and these pathways might cross talk each other in the NK cell cytotoxic process.