Induction of tolerance in macrophages by cholera toxin B chain

Model systems of human type 1 diabetes have revealed an important role of c ellular immune reactions involving macrophages and T cells in the destructi on of autologous insulin-producing pancreatic beta cells. Recently, the cho lera toxin B chain (CTB) was found to suppress T cell-dependent autoimmune diseases including autoimmune diabetes of nonobese diabetic mice. Therefore , we tested the hypothesis that CTB exerts much of its immunomodulatory act ivity by targeting macrophages. These studies are reviewed here. Cells of t he human monocyte line Mono Mac 6 were exposed to CTB and subsequently test ed for proinflammatory immunoreactivity in response to challenge with endot oxin (LPS from Escherichia coli, 10 ng/ml for 5 h). Incubation of monocytes with CTB (10 mu g/ml) suppressed a later proinflammatory response to LPS a s demonstrated by suppression of TNF alpha release from 6.7 +/- 0.7 ng/ml i n cultures without CTB preexposure to 1.8 +/- 1.1 ng/ml in CTB-pretreated c ells (p < 0.001). In contrast, the release of IL-10 remained inducible afte r CTB pretreatment. RT-PCR analysis showed that the suppression of TNF alph a production occurred at the level of mRNA formation. Control experiments e xcluded a role of possible contamination of CTB by endotoxin or the intact cholera toxin. Tolerance induction was maximal after 5 h of CTB exposure an d persisted for 24 h. The suppressive effect of CTB was dose-dependent and no mote recognizable at less than or equal to 1 mu g/ml. Incubation with IL -10- and TGF beta-neutralizing antibodies during CTB pretreatment prevented tolerization of macrophages. IFN gamma (1,200 U/ml) was found to antagoniz e actions of CTB. In contrast to desensitization by low doses of LPS, toler ance induction by CTB occurred 'silently', i.e. in the absence of a measura ble proinflammatory response. In view of the potent instructive role of the innate immune system on T cell responses these findings are important in u nderstanding how CTB prevents the development of autoimmune diabetes and im proves tolerance to islet autoantigens. Copyright (C) 2000 S. Karger AG, Ba sel.