Intestinal epithelial cells synthesize glucocorticoids and regulate T cell activation.

Glucocorticoids (GCs) are important steroid hormones with widespread activities in metabolism, development, and immune regulation. The adrenal glands are the major source of GCs and release these hormones in response to psychological and immunological stress. However, there is increasing evidence that GCs may also be synthesized by nonadrenal tissues. Here, we report that the intestinal mucosa expresses steroidogenic enzymes and releases the GC corticosterone in response to T cell activation. T cell activation causes an increase in the intestinal expression of the steroidogenic enzymes required for GC synthesis. In situ hybridization analysis revealed that these enzymes are confined to the crypt region of the intestinal epithelial layer. Surprisingly, in situ-produced GCs exhibit both an inhibitory and a costimulatory role on intestinal T cell activation. In the absence of intestinal GCs in vivo, activation by anti-CD3 injection resulted in reduced CD69 expression and interferon-gamma production by intestinal T cells, whereas activation by viral infection led to increased T cell activation. We conclude that the intestinal mucosa is a potent source of immunoregulatory GCs.

Sensitizing antigen-specific CD8+ T cells for accelerated suicide causes immune incompetence.

Death receptor-mediated activation-induced apoptosis of antigen-specific T cells is a major mechanism of peripheral tolerance induction and immune homeostasis. Failure to undergo activation-induced cell death (AICD) is an important underlying cause of many autoimmune diseases. Thus, enhancing the T cell's own suicide mechanism may provide an efficient therapy for the treatment of autoimmune diseases. Bisindolylmaleimide VIII (Bis VIII), a PKC inhibitor, can sensitize T cells for death receptor-induced apoptosis and thus can inhibit the development of T cell-mediated autoimmune disease in vivo. In this study, we have analyzed the functional consequences of accelerated suicide for a protective CD8+ T cell-mediated immune response. Our data indicate that CD8+ T cells are sensitized by Bis VIII to AICD, both in vitro and in vivo. The sensitizing effect of Bis VIII appears to be mediated by specific downmodulation of the antiapoptotic molecule cellular FLICE-like inhibitory protein (cFLIP(L)). Importantly, Bis VIII administration during an acute lymphocytic choriomeningitis virus (LCMV) infection causes the depletion of virus-specific CD8+ T cells and subsequently impaired cytotoxicity and virus clearance. We conclude that resistance to death receptor-induced apoptosis is crucial for the efficient induction of a protective immune response, and that Bis VIII-based immunotherapies have to be applied under well-controlled conditions to avoid the induction of immune incompetence and the inability to respond to pathogen infection.

IL-18-independent cytotoxic T lymphocyte activation and IFN-gamma production during experimental acute graft-versus-host disease.

Acute graft-versus-host disease (GvHD) is a serious complication after allogeneic bone marrow transplantation. Donor-derived T cells infiltrate recipient target organs and cause severe tissue damage, often leading to death of the affected patient. Tissue destruction is a direct result of donor CD8+ T cell activation and cell-mediated cytotoxicity. IL-18 is a novel pro-inflammatory cytokine with potent T(h)1 immune response-promoting and cytotoxic T lymphocyte (CTL)-inducing activity. IL-18 is strongly induced in experimental mouse models and human patients with acute GvHD. However, the precise role of IL-18 in the development of acute GvHD is still unknown. In this study, we have used IL-18-binding protein, a soluble IL-18 decoy receptor, to specifically neutralize IL-18 in vivo and in vitro. Our results demonstrate that IL-18 is induced during GvHD. However, its effect in the induction of GvHD appears to be redundant, since neutralization of IL-18 does not alter any disease parameter analyzed. Our study further shows that IFN-gamma production and CTL induction upon activation by T cell mitogens or by alloantigen does not involve IL-18-mediated amplification, in contrast to lipopolysaccharide-induced IFN-gamma production. We conclude that IL-18 expression correlates with the course of GvHD; however, its effect is dispensable for IFN-gamma and CTL induction for the initiation phase of this disease, most likely due to direct, IL-18-independent, CTL activation.