Immunoregulatory Defects in Inflammatory Bowel Disease

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Project 1: The study of murine models of intestinal inflammation has been a rich source of new insights concerning the pathogenesis and treatment of human Crohn's disease. One such model that has been extensively studied in this laboratory is the model known as TNBS-colitis, a haptene-induced colitis due to an excessive IL-12-driven, Th1 T cell-mediated immune response in SJL/J mice. In the present study, we explored the use of recombinant B chain of cholera toxin (rCT-B) as an agent for the treatment of TNBS-colitis. In initial studies we showed that oral administration of rCT-B at the time of TNBS-colitis induction by intra-rectal TNBS instillation, prevents the development of colitis or, at later time when TNBS-colitis is well established, brings about resolution of the colitis. In further studies we showed that such CT-B administration is accompanied by prevention or reversal of IFN-gamma secretion (the hallmark of a Th1 response) without a concomitant effect on IL-4 secretion. This effect on IFN-gamma secretion is not due to an effect on the secretion of the counter-regulatory cytokines IL-10 or TGF-beta but, instead, is due to marked inhibition of IL-12 secretion. Finally, we showed that rCT-B administration results in increased apoptosis of lamina propria cells, an effect previously shown to be indicative of IL-12 deprivation. From these studies, rCT-B emerges as a powerful inhibitor of Th1 T cell-driven inflammation that can potentially be applied to the treatment of Crohn's disease. Project 2: In further studies of TNBS-colitis we explored the capacity of TGF-beta delivered via a DNA plasmid to inhibit the Th1 T cell-mediated inflammation. In these studies we initially showed that a single intranasal dose of a plasmid encoding active TGF-beta1 (pCMV-TGF-beta1) prevents the development of TNBS-colitis. In addition, such plasmid administration abrogates TNBS-colitis after it has been established, whereas in contrast, i.p. administration of rTGF-beta1 protein does not have this effect. In further studies addressing the mechanism of the effect of the TGF-beta DNA plasmid we showed first that intranasal pCMV-TGF-beta1 administration leads to the expression of TGF-beta1 mRNA in the intestinal lamina propria and spleen for two weeks, as well as the appearance of TGF-beta1 producing T cells and macrophages in these tissues; in addition, it is not associated with the occurrence of tissue fibrosis. We then showed that TGF-beta1-producing cells cause marked suppression of IL-12 and IFN-gamma production and enhancement of IL-10 production; in addition, they inhibit IL-12R beta2 chain expression. Finally, we showed that co-administration of anti-IL-10 at the time of pCMV-TGF-beta1 administration prevents the enhancement of IL-10 production and reverses the suppression of IL-12, but not IFN-gamma secretion; however, anti-IL-10 leads to increased TNF-alpha production, especially in established colitis. Taken together, these studies demonstrate that TGF-beta1 inhibition of a Th1-mediated colitis is due to: 1) suppression of IL-12 secretion by IL-10 induction; and 2) inhibition of IL-12 signaling via down-regulation of IL-12R beta2 chain expression; in addition, TGF-beta1 may also have an inhibitory effect on IFN-gammatranscription.