Escherichia coli enterotoxin (LT) and the homologous cholera toxin (CT) are A-B toxins that cause travelers' diarrhea and cholera, respectively. So far, experimental live and killed vaccines against these diseases have been developed using only the nontoxic B portion of these toxins. The enzymatically active A subunit has not been used because it is responsible for the toxicity and it is reported to induce a negligible titer of toxin neutralizing antibodies. We used site-directed mutagenesis to inactivate the ADP-ribosyltransferase activity of the A subunit and obtained nontoxic derivatives of LT that elicited a good titer of neutralizing antibodies recognizing the A subunit. These LT mutants and equivalent mutants of CT may be used to improve live and killed vaccines against cholera and enterotoxinogenic E. coli.

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