SummaryThis study investigates the molecular mechanisms underlying the induction of and protection from T cell activation-associated hepatic injury. When BALB/c mice were given a single intravenous injection of concanavalin A (Con A) (>10.3 rag/mouse), they developed acute hepatic injury as assessed by a striking increase in plasma transaminase levels within 24 h. Histopathologically, only the liver was injured while moderate infiltration of T cells and polymorphonuclear cells occurred in the portal areas and around the central veins. The induction of hepatic injury was dependent on the existence as well as the activation of T cells, as untreated BALB/c nu/nu mice or BALB/c mice pretreated with a T cell-specific immunosuppressive drug, FK506, failed to develop disease. Significant increases in the levels of various cytokines in the plasma were detected before an increase in plasma transaminase levels. Within 1 h after Con A injection, tumor necrosis factor (TNF) levels peaked, this being followed by production of two other inflammatory cytokines, interleukin 6 (IL-6) and IL-1. Passive immunization with anti-TNF but not with anti-IL-1 or anti-IL-6 antibody, conferred significant levels of protection. Moreover, administration of rlL-6 before Con A injection resulted in an IL-6 dose-dependent protection. A single administration of a given dose of rlL-6 completely inhibited the release of transaminases, whereas the same regimen induced only 40-50% inhibition of TNF production. More than 80% inhibition of TNF production required four consecutive rlL-6 injections. These results indicate that: (a) TNFs are critical cytokines for inducing T cell activation-associated (Con A-induced) hepatitis; (b) the induction of hepatitis is almost completely controlled by rlL-6; and (c) rlL-6 exerts its protective effect through multiple mechanisms including the reduction of TNF production.

CD4+CD25+ regulatory T cells inhibit organ-specific autoimmune diseases induced by CD4+CD25−T cells and are potent suppressors of T cell activation in vitro. Their mechanism of suppression remains unknown, but most in vitro studies suggest that it is cell contact–dependent and cytokine independent. The role of TGF-β1 in CD4+CD25+ suppressor function remains unclear. While most studies have failed to reverse suppression with anti–transforming growth factor (TGF)-β1 in vitro, one recent study has reported that CD4+CD25+ T cells express cell surface TGF-β1 and that suppression can be completely abrogated by high concentrations of anti–TGF-β suggesting that cell-associated TGF-β1 was the primary effector of CD4+CD25+-mediated suppression. Here, we have reevaluated the role of TGF-β1 in CD4+CD25+-mediated suppression. Neutralization of TGF-β1 with either monoclonal antibody (mAb) or soluble TGF-βRII-Fc did not reverse in vitro suppression mediated by resting or activated CD4+CD25+ T cells. Responder T cells from Smad3−/− or dominant-negative TGF-β type RII transgenic (DNRIITg) mice, that are both unresponsive to TGF-β1–induced growth arrest, were as susceptible to CD4+CD25+-mediated suppression as T cells from wild-type mice. Furthermore, CD4+CD25+ T cells from neonatal TGF-β1−/− mice were as suppressive as CD4+CD25+ from TGF-β1+/+ mice. Collectively, these results demonstrate that CD4+CD25+ suppressor function can occur independently of TGF-β1.

A single intravenous injection of concanavalin A (Con A) induces T-cell activation and an acute hepatitis in mice. This study investigated the role of interferon gamma (IFN-gamma) in the pathogenesis of this hepatitis model. Striking increases in the plasma levels of various cytokines, including tumor necrosis factor (TNF), interleukin-2 (IL-2), and IFN-gamma, were detected before the increase in plasma aminotransferase levels induced by Con A injection. TNF levels peaked within 2 hours, whereas IFN-gamma levels peaked at 6 hours after Con A injection. In contrast to a sharp peak of TNF levels, high IFN-gamma levels were detected for a more prolonged period. Passive immunization with anti-IFN-gamma monoclonal antibody (MAb) conferred a dose-dependent protection against liver injury in this model. This protection was observed when anti-IFN-gamma MAb was administered at least 30 minutes before Con A injection but not when given 1 hour after Con A injection. The protection from Con A-induced hepatitis was also induced by administration of rIL-6 before Con A injection. rIL-6 treatment induced significant albeit incomplete inhibition of IFN-gamma and TNF production, whereas this regimen did not affect IL-2 production. Despite striking protective effects of rIL-6 or anti-IFN-gamma MAb, comparable levels of cellular (both T cell and polymorphonuclear cell) infiltration were detected in liver sections from animals untreated, or treated with either rIL-6 or anti-IFN-gamma MAb. Moreover, electron microscopic examination showed that infiltrating T cells exhibited a blastoid appearance in all groups. These results indicate that IFN-gamma plays a critical role in the development of Con A-induced acute hepatitis and suggest that IL-6 administration can regulate the manifestation of hepatitis through mechanisms including the reduced production of inflammatory cytokines such as IFN-gamma.

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Liver disease, including hepatitis and cirrhosis, repmodel. Striking increases in the plasma levels of various resent a major cause of morbidity and mortality. In cytokines, including tumor necrosis factor (TNF), in-chronic hepatitis, liver parenchymal cells are destroyed terleukin-2 (IL-2), and IFN-g, were detected before the by host lymphoid cells, especially by activated T cells increase in plasma aminotransferase levels induced by responding to viral antigens. 1 The cellular and molecuCon A injection. TNF levels peaked within 2 hours, lar mechanisms underlying the development of hepatowhereas IFN-g levels peaked at 6 hours after Con A injeccellular destruction have not been studied extensively tion. In contrast to a sharp peak of TNF levels, high IFNg levels were detected for a more prolonged period. because of the lack of availability of suitable animal Passive immunization with anti-IFN-g monoclonal anti-models. body (MAb) conferred a dose-dependent protectionRecently, a new hepatitis model was developed in against liver injury in this model. This protection was which liver-specific inflammatory lesions are induced observed when anti-IFN-g MAb was administered at by injection of concanavalin A (Con A). 2 Importantly, least 30 minutes before Con A injection but not when the hepatitis could be induced without pretreatment given 1 hour after Con A injection. The protection from with D-galactosamine, 3 which is an absolute requireCon A-induced hepatitis was also induced by adminis-ment for the induction of hepatitis by injection of endotration of rIL-6 before Con A injection. rIL-6 treatment toxin from gram-negative bacteria such as lipopolysacinduced significant albeit incomplete inhibition of IFNcharide (LPS). 4,5 Furthermore, the hepatic injury g and TNF production, whereas this regimen did not affect IL-2 production. Despite striking protective ef-appears to be a consequence of T-cell activation. 2,6 Our fects of rIL-6 or anti-IFN-g MAb, comparable levels of initial study 6 using this model showed that tumor necellular (both T cell and polymorphonuclear cell) infil-crosis factor (TNF) and interleukin-6 (IL-6) function as tration were detected in liver sections from animals un-positive and negative modulators, respectively, for the treated, or treated with either rIL-6 or anti-IFN-g MAb. development of Con A-induced hepatitis. Despite the Moreover, electron microscopic examination showed massive production of interferon gamma (IFN-g) by that infiltrating T cells exhibited a blastoid appearance Con A-stimulated T cells in vitro, the involvement of in all groups. These results indicate that IFN-g plays a this cytokine in the pathogenesis in Con A-induced critical role in the development of Con A-induced acute hepatitis has not been investigated. It is increasingly hepatitis and suggest that IL-6 administration can reguevident that IFN-g produced by T cells after stimula-