Relationship between alowtoxicity ofthemutantA subunit ofenterotoxigenic Escherichia coli enterotoxin andits strong adjuvant action

SUMMARY Intheworkdescribed hereitwas determined ifandhowunnicking intheA subunit ofEscherichia coli enterotoxin atArgl92 ornearby residues affected biological activities ofthetoxin. Themutant toxin was constructed to lackthenicksiteintheA subunit bydeleting thetripeptide Argl92-Thrl93-Ilel94, whichisessential fortoxicity. Themutanttoxin didnotexhibit agmatine ADP-ribosyltransferase activity inthepresenceor absence oftheADP-ribosylation factor and hadless diarrhoeal activity andlowerinduction ofcyclic AMP thandidLT.Themutanttoxin exhibited a muchstronger adjuvant action on antibody responsestomeasles virus, keyhole limpt haemocyanin, bovine immunoglobulin andovalbumin compared withLT.Thealtered toxicity of themutanttoxin mightbeclosely related tothepotentadjuvant action on antibody responsesto antigens. Therelationship between twoactivities isdiscussed.