Escherichia coli heat-labile enterotoxin (LT) is known to be a potent adjuvant of both the mucosal and systemic immune systems but the mechanism of action leading to adjuvant activity remains incompletely understood. This study investigates the action of LT and LT mutants with impaired enzymatic activity, on the function of dendritic cells. Wild-type LT and LTR72, which retains some ADP ribosyltransferase activity, induced a selective increase in cell surface expression of B7.1, and a selective decrease of CD40 expression on mouse bone marrow derived dendritic cells. LTK63 and LT-B had no obvious effect on the expression of these antigens on similar dendritic cells. LT-treated dendritic cells also showed a profoundly impaired ability to present protein antigen (ovalbumin) to cognate T cells, although this effect was not observed with non-toxic LT mutants. LT and LTR72-treated cells showed a slower rate of receptor-mediated endocytosis as measured by flow cytometric analysis of uptake of fluorescently labelled dextran. Furthermore, confocal microscopy showed changes in the intracellular distribution of endocytosed molecules, and of the class II containing acidic antigen processing compartments. This response of dendritic cells to toxin is likely to play an important role in determining the adjuvant activity of these molecules. © 2002 Elsevier Science Ltd. All rights reserved.
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