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.
In wild-type BALB/c mice, i.p. administration of acetaminophen (APAP; 750 mg/kg) induced intrahepatic IFN-gamma mRNA expression and a marked increase in serum transaminase levels, leading to acute lethality of approximately 45%. Histopathological examination showed centrilobular hepatic necrosis with leukocyte infiltration and a large number of apoptotic hepatocytes 10 and 24 h after APAP challenge. mRNA expression of intercellular adhesion molecule 1, vascular cell adhesion molecule 1, interleukin (IL) 1alpha, IL-1beta, IL-6, tumor necrosis factor alpha, monocyte chemoattractant protein 1, macrophage inflammatory protein (MIP) 1alpha, MIP-2, KC, IP-10, Mig, Fas, and inducible nitric oxide synthase was enhanced in the liver of wild-type mice injected with APAP. To clarify the role of IFN-gamma in this process, IFN-gamma-deficient mice were treated in the same manner. All IFN-gamma-deficient mice survived with reduced serum transaminase elevation and attenuated hepatic necrosis, leukocyte infiltration, and hepatocyte apoptosis. The gene expression of all molecules was significantly attenuated in IFN-gamma-deficient mice. Administration of an anti-IFN-gamma neutralizing antibody even 2 or 8 h after APAP challenge to wild-type mice alleviated APAP-induced liver injury, and all mice survived. Thus, IFN-gamma is responsible for APAP-induced liver injury by mediating leukocyte infiltration, hepatocyte apoptosis, and NO production as well as cytokine and chemokine production. Moreover, immunoneutralization of IFN-gamma may be therapeutically effective for developing APAP-induced liver injury.
IL-12 and IL-18 synergistically enhance IFN-γ mRNA transcription by activating STAT4 and AP-1, respectively. However, it is still unknown how STAT4/AP-1 elicit IFN-γ promoter activation. Using an IL-12/IL-18-responsive T cell clone, we investigated the mechanisms underlying synergistic enhancement of IFN-γ mRNA expression induced by these two cytokines. Synergy was observed in a reporter gene assay using an IFN-γ promoter fragment that binds AP-1, but not STAT4. An increase in c-Jun, a component of AP-1, in the nuclear compartment was elicited by stimulation with either IL-12 or IL-18, but accumulation of serine-phosphorylated c-Jun was induced only by IL-18 capable of activating c-Jun N-terminal kinase. The binding of AP-1 to the relevant promoter sequence depended on the presence of STAT4. STAT4 bound with c-Jun, and a phosphorylated c-Jun-STAT4 complex most efficiently interacted with the AP-1-relevant promoter sequence. Enhanced cobinding of STAT4 and c-Jun to the AP-1 sequence was also observed when activated lymph node T cells were exposed to IL-12 plus IL-18. These results show that STAT4 up-regulates AP-1-mediated IFN-γ promoter activation without directly binding to the promoter sequence, providing a mechanistic explanation for IL-12/IL-18-induced synergistic enhancement of IFN-γ gene expression.
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.
While CD28 functions as the major T cell costimulatory receptor, a number of other T cell molecules have also been described to induce T cell costimulation. Here, we investigated the mechanisms by which costimulatory molecules other than CD28 contribute to T cell activation. Non-CD28 costimulatory molecules such as CD5, CD9, CD2, and CD44 were present in the detergent-insoluble glycolipid-enriched (DIG) fraction/raft of the T cell surface, which is rich in TCR signaling molecules and generates a TCR signal upon recruitment of the TCR complex. Compared with CD3 ligation, coligation of CD3 and CD5 as an example of DIG-resident costimulatory molecules led to an enhanced association of CD3 and DIG. Such a DIG redistribution markedly up-regulated TCR signaling as observed by ZAP-70/LAT activation and Ca2+ influx. Disruption of DIG structure using an agent capable of altering cholesterol organization potently diminished Ca2+ mobilization induced by the coligation of CD3 and CD5. This was associated with the inhibition of the redistribution of DIG although the association of CD3 and CD5 was not affected. Thus, the DIG-resident costimulatory molecules exert their costimulatory effects by contributing to an enhanced association of TCR/CD3 and DIG.
Unfractionated spleen cells taken from tumor-bearing mice 2 weeks after tumor implantation contained tumor-primed T cells which produced cytokines including IL-2 and IFN-gamma when cultured in vitro. With progressive tumor growth this initial lymphokine-producing capacity decreased. Here, we investigated the ability of IL-12 to (i) restore suppressed IFN-gamma production, (ii) cause tumor regression and (ii) induce anti-tumor protective immunity. Addition of rIL-12 to spleen cell cultures from 4- to 10-week-old tumor-bearing mice resulted in a striking enhancement in the production of IFN-gamma compared with cultures of these cells in the absence of rIL-12 or of normal spleen cells in the presence of rIL-12. Five i.p. injections of rIL-12 into mice bearing s.c. tumors induced complete tumor regression. This was found when rIL-12 was given at early (1-2 weeks), intermediate (4-5 weeks) or even late (7 weeks) stages of tumor growth. Furthermore, IL-12-treated mice which rejected the primary tumor exhibited complete resistance to a rechallenge with the same tumor but did not reject a second syngenetic tumor. Immunohistochemical analyses following IL-12 treatment revealed that CD4+ and CD8+ T cells infiltrate the tumor. More importantly, IFN-gamma mRNA expression was observed in fresh tumor masses from tumor-bearing mice receiving IL-12 treatment. The importance of IFN-gamma was further demonstrated by the observation that the systemic administration of anti-IFN-gamma mAb prior to IL-12 treatment completely abrogated the anti-tumor effect of IL-12. Thus, these results indicate that administration of modest levels of rIL-12 to tumor-bearing mice results in tumor regression through mechanisms involving reversal of suppressed IFN-gamma production by anti-tumor T cells and the establishment of a tumor-specific protective immune response.
The chemokine receptors CCR5 and CXCR3 have been implicated as playing a central role in directing a Th1 inflammatory response. Here, we investigated whether a synthetic CCR5 antagonist affects the process of T cell migration to sites of inflammation. Immunization of DBA/1 mice with type II collagen resulted in typical arthritis, which is associated with cellular infiltration. Treatment with a CCR5 antagonist strikingly affected the development of arthritis by reducing both incidence and severity of disease. There was no substantial difference between collagen‐immunized mice with and without antagonist treatment in the induction of anti‐collagen T cell responses and the capacity to produce IL‐12. This endogenous IL‐12 functioned to induce comparable levels of CCR5 in these two immunized groups of T cells. Whereas a massive infiltration of inflammatory cells including CCR5+ T cells occurred in the joints of mice immunized without antagonist, cellular infiltration in the antagonist‐treated group was only marginal. These results indicate that administration of a CCR5 antagonist inhibits the development of arthritis not by affecting the generation of collagen‐sensitized T cells but by interfering with their migration to joint lesions.
CC chemokine receptor (CCR) 5 and CXC chemokine receptor (CXCR)3 are expressed on T helper cell type 1 cells and have been implicated in their migration to sites of inflammation. Our preceding study demonstrated that a nonpeptide synthetic CCR5 antagonist, TAK-779 (N, N-dimethyl-N-[4-[[[2-(4-methylphenyl)-6, 7-dihydro-5H-benzocyclohepten-8-yl]carbon-yl]amino]benzyl]-tetrahydro-2H-pyran4-aminium chloride, inhibits the development of experimentally induced arthritis by modulating the migration of CCR5(+)/CXCR3(+) T cells to joints. The present study investigated the functional properties of TAK-779, including the effect of this antagonist on CXCR3 function. For this purpose, transfectants expressing mouse CCR5 (mCCR5) or mCXCR3 and expressing mCCR4 or mCXCR4 as controls were established by introducing each relevant gene into 2B4 T cells and were subjected to the following assays. First, the ligand binding to chemokine receptors was assayed by incubating transfectants with [(125)I]-labeled relevant ligand or with the unlabeled relevant ligand followed by staining with anti-ligand antibody. Second, chemokine-induced lymphocyte function-associated antigen-1 (LFA-1) activation was assayed by measuring the adhesion of cells to microculture plates coated with purified intercellular adhesion molecule-1. Third, chemokine-stimulated chemotaxis was assayed by observing the cell migration through transwells. In these assays, TAK-779 blocked the ligand binding as well as LFA-1 up-regulating and chemotactic function of mCXCR3 and mCCR5 but did not elicit a biologically significant inhibition of those functions of mCCR4 and mCXCR4. These observations indicate the unique target specificity of TAK-779 and explain why this antagonist efficiently blocks the migration of T cells expressing CCR5 and CXCR3 to sites of inflammation.
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