Induction of tolerance in macrophages by cholera toxin B chain

Model systems of human type 1 diabetes have revealed an important role of cellular immune reactions involving macrophages and T cells in the destruction of autologous insulin-producing pancreatic beta cells. Recently, the cholera 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 activity by targeting macrophages. These studies are reviewed here. Cells of the human monocyte line Mono Mac 6 were exposed to CTB and subsequently tested for proinflammatory immunoreactivity in response to challenge with endotoxin (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 as demonstrated by suppression of TNF alpha release from 6.7 +/- 0.7 ng/ml in cultures without CTB preexposure to 1.8 +/- 1.1 ng/ml in CTB-pretreated cells (p < 0.001). In contrast, the release of IL-10 remained inducible after CTB pretreatment. RT-PCR analysis showed that the suppression of TNF alpha production occurred at the level of mRNA formation. Control experiments excluded a role of possible contamination of CTB by endotoxin or the intact cholera toxin. Tolerance induction was maximal after 5 h of CTB exposure and 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 antagonize actions of CTB. In contrast to desensitization by low doses of LPS, tolerance induction by CTB occurred 'silently', i.e. in the absence of a measurable proinflammatory response. In view of the potent instructive role of the innate immune system on T cell responses these findings are important in understanding how CTB prevents the development of autoimmune diabetes and improves tolerance to islet autoantigens. Copyright (C) 2000 S. Karger AG, Basel.