The activation state of human intrahepatic lymphocytes.

The immune tolerance induced by the liver as an allograft is difficult to reconcile with the evidence that the liver selectively accumulates activated T cells from the circulation. However, much of this information is based on murine liver lymphocytes that were isolated using enzymatic digestion. In the present study we made use of a novel resource, the lymphocytes isolated during the perfusion of living donor liver lobe prior to transplantation. These healthy human liver lymphocytes displayed surface markers indicating a high degree of activation of natural killer cells, CD56(+) T cells, CD4(+) T cells and CD8(+) T cells. These properties were independent of enzymatic treatment or the details of cell isolation. We conclude that the healthy human liver is a site of intense immunological activity.

Cytotoxic T-cell response following mouse liver transplantation is independent of the initial site of T-cell priming.

BACKGROUND: Liver transplantation in the mouse results in systemic induction of tolerance. The underlying mechanisms may also account for the persistence of chronic liver infections. It has therefore been hypothesized that antigen (Ag) presentation within the liver by nonprofessional antigen-presenting cells (APC) leads to incomplete T-cell activation, ultimately resulting in tolerance induction. We tested this hypothesis in an orthotopic mouse liver transplantation model. METHODS: Mouse liver transplantation was used to manipulate antigen presentation in major histocompatibility complex (MHC)-disparate donor and recipient strains. The effect of restricted Ag presentation was studied using CD8+ T-cell receptor transgenic OT-I cells. Transgenic OT-I cells were activated by injection of their cognate peptide antigen SIINFEKL, which could be presented by the MHC class I of only one of the mouse strains. Depending on the strain combination, Ag presentation was restricted to either the transplanted liver itself, the recipient (excluding the transplanted liver), or systemically throughout the recipient. Extrahepatic Ag presentation by passenger leukocytes was eliminated by using donors of chimeric bone marrow. RESULTS: OT-I cells encountering antigen only in the transplanted liver were activated, underwent extensive proliferation, and developed effector functions, based on IFN-gamma production and in vivo cytotoxicity assays. This T-cell activation and differentiation within the liver was comparable to animals with systemic Ag presentation and to animals with absent hepatic-parenchymal Ag presentation. CONCLUSIONS: The restricted presentation of antigen in the liver showed no immunosuppressive effect on activation of CD8+ T cells. In contrast, the liver may be an excellent priming site for naive CD8+ T cells.

Cleavage of E2F-1-regulating proteins and activation of E2F-1 during CD95-induced death of thymocytes.

The CD95 death receptor activates caspases that cleave a variety of intracellular substrates, including cell cycle control proteins. However, the significance of this cleavage for the induction of apoptosis is unclear. In this study, CD95-induced cleavage of the G1/S checkpoint regulator proteins, retinoblastoma protein (pRb) and murine-double-minute-2 (mdm-2), was associated with an increased protein concentration of a key transcription factor, E2F-1, which is regulated by both of them. Furthermore, DNA-binding activity to E2F sites is increased. In thymocytes, CD95-induced apoptosis was associated with increased E2F-1 DNA-binding activity, while thymocytes that lacked E2F-1 were less susceptible to CD95-induced apoptosis. We conclude that the G1/S checkpoint is an important target of CD95 signalling. CD95-activated caspases cleave regulator proteins to increase E2F-1 activity, and inappropriate activation of E2F-1 is part of the mechanism of CD95-induced apoptosis.

Antigen presentation by liver cells controls intrahepatic T cell trapping, whereas bone marrow-derived cells preferentially promote intrahepatic T cell apoptosis.

Systemic activation and proliferation of CD8(+) T cells result in T cell accumulation in the liver, associated with T cell apoptosis and liver injury. However, the role of Ag and APC in such accumulation is not clear. Bone marrow chimeras were constructed to allow Ag presentation in all tissues or alternatively to restrict presentation to either bone marrow-derived or non-bone marrow-derived cells. OVA-specific CD8(+) T cells were introduced by adoptive transfer and then activated using peptide, which resulted in clonal expansion followed by deletion. Ag presentation by liver non-bone marrow-derived cells was responsible for most of the accumulation of activated CD8(+) T cells. In contrast, Ag presentation by bone marrow-derived cells resulted in less accumulation of T cells in the liver, but a higher frequency of apoptotic cells within the intrahepatic T cell population. In unmodified TCR-transgenic mice, Ag-induced T cell deletion and intrahepatic accumulation of CD8(+) T cells result in hepatocyte damage, with the release of aminotransaminases. Our experiments show that such liver injury may occur in the absence of Ag presentation by the hepatocytes themselves, arguing for an indirect mechanism of liver damage.

Specific blockade by CD54 and MHC II of CD40-mediated signaling for B cell proliferation and survival.

Regulation of B lymphocyte proliferation is critical to maintenance of self-tolerance, and intercellular interactions are likely to signal such regulation. Here, we show that coligation of either the adhesion molecule ICAM-1/CD54 or MHC II with CD40 inhibited cell cycle progression and promoted apoptosis of mouse splenic B cells. This resulted from specific blockade of NF-kappa B induction, which normally inhibits apoptosis. LPS- or B cell receptor (BCR)-induced proliferation was not inhibited by these treatments, and mAb-induced association of CD40 with other B cell surface molecules did not have these effects. Addition of BCR or IL-4 signals did not overcome the effect of ICAM-1 or MHC II on CD40-induced proliferation. FasL expression was not detected in B cell populations. These results show that MHC II and ICAM-1 specifically modulate CD40-mediated signaling, so inhibiting proliferation and preventing inhibition of apoptosis.

Involvement of CD1 in peripheral deletion of T lymphocytes is independent of NK T cells.

During peripheral T cell deletion, lymphocytes accumulate in nonlymphoid organs including the liver, a tissue that expresses the nonclassical, MHC-like molecule, CD1. Injection of anti-CD3 Ab results in T cell activation, which in normal mice is followed by peripheral T cell deletion. However, in CD1-deficient mice, the deletion of the activated T cells from the lymph nodes was impaired. This defect in peripheral T cell deletion was accompanied by attenuated accumulation of CD8(+) T cells in the liver. In tetra-parental bone marrow chimeras, expression of CD1 on the T cells themselves was not required for T cell deletion, suggesting a role for CD1 on other cells with which the T cells interact. We tested whether this role was dependent on the Ag receptor-invariant, CD1-reactive subset of NK T cells using two other mutant mouse lines that lack most NK T cells, due to deletion of the genes encoding either beta(2)-microglobulin or the TCR element J alpha 281. However, these mice had no abnormality of peripheral T cell deletion. These findings indicate a novel role for CD1 in T cell deletion, and show that CD1 functions in this process through mechanisms that does not involve the major, TCR-invariant set of NK T cells.

Differential regulation of cell cycle-related proteins by CD95 engagement in thymocytes and T cell leukemic cell line, Jurkat.

CD95 engagement results in apoptosis in thymocytes and in the Jurkat human leukemic T cell line. Biochemical analyses in CD95-engaged thymocytes and Jurkat cells revealed dysregulation of the G1/S cell cycle control point. Cyclin E was upregulated upon CD95 engagement, suggesting G1-to-S progression, but there was no upregulation of cyclin A. Instead, cyclin E was degraded by caspases. In addition, c-myc that normally acts on S-phase progression through the activation of cdc25A appeared to be involved in the inhibition of S-phase progression after CD95 ligation. This implies that G1-->S progression and apoptosis are intimately linked in cells undergoing CD95 ligation. Furthermore, our data suggest that CD95-induced apoptosis occurs at the G1/S phase transition. We therefore suggest that CD95 engagement not only triggers death signals but also affects the G1/S checkpoint.

Functional flexibility in T cells: independent regulation of CD4+ T cell proliferation and effector function in vivo.

Proliferation and differentiation of CD4+ T cells are often correlated, but it is not clear whether they are mechanistically linked. When antigen-specific T cells are present at high frequency in vivo, they all respond to antigenic peptide stimulation by expressing activation markers, but only a subset begins to proliferate. However, noncycling cells may synthesize the effector cytokine IFNgamma even though their cell cycle is blocked in G1. These data show that proliferation and effector function are not rigidly linked in T cells. Instead, CD4+ T cells have the flexibility to engage in or bypass clonal expansion based on the integration of multiple signals, including the frequency of other responding T cells.

Induction of murine hepatocyte death by membrane-bound CD95 (Fas/APO-1)-ligand: characterization of an in vitro system.

Hepatocytes constitutively express CD95 (also called Fas/APO-1) and are therefore potential targets for CD95-ligand (CD95L)-mediated injury. To study this mechanism of cell death in hepatocytes we developed an in vitro model of liver cell apoptosis using membrane-bound CD95L as the inducing agent. Primary mouse hepatocytes were cocultured with NIH 3T3 fibroblasts, stably transfected with mouse CD95L (F(CD95L+)). Fibroblasts stably transfected with vector only (F(CD95L-)) served as controls. Hepatocytes from mice expressing low levels of CD95 (Fas(lpr) mice) served as controls for effects unrelated to CD95. Morphologic and biochemical studies indicate that CD95 is expressed in cultured mouse hepatocytes. Membrane-bound CD95 from transfected fibroblasts destroyed all cocultured hepatocytes within 24 hours in the absence of protein synthesis inhibitors. Characteristic features of apoptosis were observed in dying hepatocytes and occurred in the following sequence: formation of cytoplasmic blebs and nuclear condensation after 3 hours; nuclear fragmentation and DNA strand breaks after 4 hours. These changes were observed only when normal hepatocytes were cocultured with F(CD95L+) and were not observed with F(CD95L-) or in hepatocytes from Fas(lpr) mice. Anti-CD95 antibody (Jo2) evoked similar changes in hepatocytes, although to a much lesser extent. We conclude that coculture of mouse hepatocytes with F(CD95L+) is a useful in vitro model for CD95-mediated apoptosis induced by CD95L. The high incidence of apoptosis caused by membrane-bound CD95L differs from the much smaller effects induced by the Jo2 antibody. In view of the high sensitivity of hepatocytes towards CD95L we speculate that CD95L-induced liver damage in vivo may be minimized by restricting exposure of hepatocytes to CD95L.

The liver as a site of T-cell apoptosis: graveyard, or killing field?

The liver is a site at which apoptotic CD8+ cells accumulate during the clearance phase of peripheral immune responses. Normal mouse liver contains an unusual mixture of lymphocytes in which natural killer (NK) and NK-T cells are abundant and apoptotic T cells are present, and we interpret these cell populations as, respectively, agents and targets of an intrahepatic T-cell trapping and killing mechanism. In support of this idea, direct perfusion of activated lymphocyte populations through the normal liver results in the selective retention of activated CD8+ T cells. T cells trapped in this manner undergo apoptosis in the liver. This mechanism could explain the importance of the liver in oral tolerance, the phenomenon of tolerance induced by portal vein infusion of antigenic cells, the tolerance to allogeneic liver allografts, and the persistence of some liver pathogens including hepatitis C.

CD95/Fas signaling in T lymphocytes induces the cell cycle control protein p21cip-1/WAF-1, which promotes apoptosis.

Ligation of CD95 on T lymphocytes resulted in the up-regulation of a cell cycle control protein, p21cip-1/WAF-1, an inhibitor of cyclin-dependent kinases. This up-regulation was completely blocked by the cysteine protease inhibitor Z-VAD-fmk (benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone), whereas DEVD-CHO (succinyl-Asp-Glu-Val-Asp-aldehyde), a caspase 3 inhibitor, had no effect. In Faslpr-cg mice, a point mutation in the death domain of CD95 results in failure to recruit FADD (Fas-associated death domain), and in the present study this mutation prevented both CD95-mediated apoptosis and p21cip-1/WAF-1 induction. During apoptotic cell death due to irradiation, p21cip-1/WAF-1 is up-regulated by a p53-dependent pathway that responds to DNA damage. However, CD95-induced up-regulation of p21cip-1/WAF-1 in T cells was p53-independent. T cells deficient in p21cip-1/WAF-1 were less susceptible to CD95-induced apoptosis. We conclude that in T cells, ligation of CD95 and activation of caspases cause the induction of p21cip-1/WAF-1, which acts to promote cell death.

Death and destruction of activated T lymphocytes.

The massive clonal expansion that occurs during an antigen-specific immune response results in the flooding of immune organs with activated T lymphocytes. At the end of a specific response, the vast majority of these activated T cells are cleared from the immune system. The T cells receive signals through specific death receptors that are expressed as a result of activation. Death receptors transmit their apoptotic signals through the activation of caspases. Function of the death receptors is intimately linked to cell-cycle control, and many cell-cycle control proteins are caspase substrates. Among CD8+ T cells, apoptotic death occurs at a specific site, the sinusoids of the liver. The liver appears to contain a mechanism for the trapping and killing of activated T cells, rendering it an immunologically privileged site.

Selective retention of activated CD8+ T cells by the normal liver.

Activation-induced cell death resulting in peripheral deletion of CD8+ T cells is associated with the accumulation of large numbers of apoptotic T cells in the liver. The hypothesis that this accumulation results from the intrahepatic trapping of T cells from the circulating pool predicts that the liver should retain T cells, which subsequently undergo apoptosis. Here we test this prediction. Perfusion of the liver with lymphocyte mixtures showed retention of activated, but neither resting nor apoptosing, T cells. This trapping was selective for CD8+ cells and was mediated primarily by ICAM-1 constitutively expressed on sinusoidal endothelial cells and Kupffer cells. T cells trapped in the liver became apoptotic. The normal liver is therefore a "sink" for activated T cells.

Caspase-3 controls both cytoplasmic and nuclear events associated with Fas-mediated apoptosis in vivo.

Both caspase-1- and caspase-3-like activities are required for Fas-mediated apoptosis. However, the role of caspase-1 and caspase-3 in mediating Fas-induced cell death is not clear. We assessed the contributions of these caspases to Fas signaling in hepatocyte cell death in vitro. Although wild-type, caspase-1(-/-), and caspase-3(-/-) hepatocytes were killed at a similar rate when cocultured with FasL expressing NIH 3T3 cells, caspase-3(-/-) hepatocytes displayed drastically different morphological changes as well as significantly delayed DNA fragmentation. For both wild-type and caspase-1(-/-) apoptotic hepatocytes, typical apoptotic features such as cytoplasmic blebbing and nuclear fragmentation were seen within 6 hr, but neither event was observed for caspase-3(-/-) hepatocytes. We extended these studies to thymocytes and found that apoptotic caspase-3(-/-) thymocytes exhibited similar "abnormal" morphological changes and delayed DNA fragmentation observed in hepatocytes. Furthermore, the cleavage of various caspase substrates implicated in mediating apoptotic events, including gelsolin, fodrin, laminB, and DFF45/ICAD, was delayed or absent. The altered cleavage of these key substrates is likely responsible for the aberrant apoptosis observed in both hepatocytes and thymocytes deficient in caspase-3.

IL-18 augments perforin-dependent cytotoxicity of liver NK-T cells.

The liver contains abundant cytotoxic cells, including NK-T cells, NK cells, and CTLs. However, the regulation of this cytotoxicity is not fully understood. In this study, we investigated the effect of a recently described cytokine, IL-18, which is present in large quantities in the liver, on the cytotoxicity of intrahepatic lymphocyte subpopulations. This effect of IL-18 was assessed by assaying the in vitro cytotoxicity of purified NK-T, NK, and T cells against a CD95- and perforin-sensitive T cell line, Jurkat. The results show that IL-18 enhances the killing activity of liver NK-T cells by a CD95-independent, perforin-dependent pathway. IL-18 also augments liver NK cell activity, but the exact mechanisms of this killing remain to be elucidated. Finally, the augmentation of the killing activities of liver NK-T and NK cells by IL-18 is not due to soluble TNF-alpha, because none of these cell populations had detectable TNF-alpha production.

TCR ligation on CD8+ T cells creates double-negative cells in vivo.

The lack of CD95 in mice is associated with an accumulation of TCR alphabeta+ CD4- CD8- (double-negative (DN)) cells in the lymph nodes (LNs) and other organs. To test the hypothesis that these DN cells arise from TCR alphabeta+ CD8+ cells after activation via the TCR, we have crossed an MHC class I-restricted TCR transgene (tg) onto the lpr genotype to generate two TCR-transgenic experimental groups, TCRtg+ lpr/+ (CD95-intact) and TCRtg+ lpr/lpr (CD95-deficient). Specific peptide administration resulted in peripheral deletion of TCR alphabeta cells from the LNs of CD95-intact and CD95-deficient mice. On day 3 after peptide administration in the CD95-deficient but not the CD95-intact mice, there was a ninefold increase in the percentage of DN cells in the LN; this increase returned to control levels by day 10. Peripheral deletion was associated with an accumulation of TCR alphabeta+ CD8high cells in the livers of mice of both genotypes by day 3, which returned to control levels by day 10 without an increase in the percentage or total number of DN cells. Our data show that the in vivo stimulation of TCR alphabeta+ CD8+ cells in the absence of CD95 results in an initial accumulation and an eventual loss of DN cells. This identifies a role for CD95 after TCR alphabeta stimulation in the efficient removal of TCR alphabeta+ CD8+ cells after the down-regulation of CD8. CD95 is not essential for this process, because other mechanisms can compensate, but such mechanisms are less efficient in the LN.

Specific resistance of T cells to CD95-induced apoptosis during S phase of the cell cycle.

We have examined the susceptibility of mouse thymocytes and Con A-activated mature T cells to CD95 (Fas; APO-1)-induced apoptosis at different phases of the cell cycle. Signaling through CD95, induced by murine CD95 ligand expressed on fibroblasts, resulted in the preferential apoptosis of T cells in G0-G1 phase of the cell cycle. T cells in S phase were selectively protected. CD95-induced apoptosis was inhibited by exogenous IL-2, which increases the percentage of cells in S phase. The inverse relationship between DNA synthesis and apoptosis in CD95-ligated T cells was not observed during the spontaneous death of T cells in culture or during propriocidal apoptosis due to TCR cross-linking, to which cells in S phase were susceptible. These results show that in T cells there is no distinctively apoptosis-vulnerable phase of the cell cycle; instead, apoptosis can occur at different phases of the cycle depending on the apoptotic stimulus.