Enhanced and accelerated lymphoproliferation in Fas-null mice

Functional Fas Expression in Human Thymic Epithelial Cells

Fas, a cell surface receptor, can induce apoptosis after cross-linking with its ligand. We report that Fas antigen is constitutively expressed in medullary epithelial cells of the human thymus. Expression is decreased in cultured thymic epithelial cells (TEC), similarly to HLA-DR antigen. TEC are resistant to anti-Fas–induced apoptosis after 4 days of primary culture, and this resistance is reversed by concomitant addition of cycloheximide. Cycloheximide also downregulated the expression of Fas-associated phosphatase-1, which has been found to inhibit Fas-induced apoptosis. This phosphatase could be involved in the resistance to Fas-induced apoptosis observed on day 4 of TEC culture. When TEC were subcultured after 10 to 13 days of primary culture, exposure to interleukin-1-β, tumor necrosis factor-, and interferon-γ, alone or together, reinduced Fas mRNA and protein expression. In coculture with activated thymocytes, TEC also upregulated Fas protein expression. Cytokine-activated TEC became sensitive to apoptosis induced by an agonistic anti-Fas antibody. This apoptosis was inhibited by Z-VAD-fmk but not by Z-DEVD-fmk and DEVDase activity was slightly increased in Fas-stimulated TEC, suggesting that DEVDase activity is not sufficient to induce TEC apoptosis. Taken together, these data show that the Fas receptor is expressed in medullary epithelial cells of the human thymus and is able to induce apoptosis.

Functional Fas Expression in Human Thymic Epithelial Cells

Fas, a cell surface receptor, can induce apoptosis after cross-linking with its ligand. We report that Fas antigen is constitutively expressed in medullary epithelial cells of the human thymus. Expression is decreased in cultured thymic epithelial cells (TEC), similarly to HLA-DR antigen. TEC are resistant to anti-Fas–induced apoptosis after 4 days of primary culture, and this resistance is reversed by concomitant addition of cycloheximide. Cycloheximide also downregulated the expression of Fas-associated phosphatase-1, which has been found to inhibit Fas-induced apoptosis. This phosphatase could be involved in the resistance to Fas-induced apoptosis observed on day 4 of TEC culture. When TEC were subcultured after 10 to 13 days of primary culture, exposure to interleukin-1-β, tumor necrosis factor-, and interferon-γ, alone or together, reinduced Fas mRNA and protein expression. In coculture with activated thymocytes, TEC also upregulated Fas protein expression. Cytokine-activated TEC became sensitive to apoptosis induced by an agonistic anti-Fas antibody. This apoptosis was inhibited by Z-VAD-fmk but not by Z-DEVD-fmk and DEVDase activity was slightly increased in Fas-stimulated TEC, suggesting that DEVDase activity is not sufficient to induce TEC apoptosis. Taken together, these data show that the Fas receptor is expressed in medullary epithelial cells of the human thymus and is able to induce apoptosis.

Evidence of Alternative or Concomitant Use of Perforin- and Fas-Dependent Pathways in a T Cell-Mediated Negative Regulation of Ig Production

To study the possible involvement of perforin (Pfp)- and/or Fas-dependent cytotoxicity pathways in a T cell-mediated negative regulation of Ig production, we used the T cell-induced Ig-allotype suppression model. T splenocytes from Igha/a mice, when neonatally transferred into histocompatible Igha/b F1 or Ighb/b congenic hosts, are intrinsically able to totally, specifically, and chronically suppress the production of IgG2a of the Ighb haplotype (IgG2ab). It has not been established whether the suppression effectors, which are anti-IgG2ab MHC class I-restricted CD8+ T cells, cytolyse IgG2ab+ B targets or whether they only silence Ig production. In this study, using T cells from Igha/a Pfp+/+ or Pfpo/o mice, the latter obtained by crossbreeding, and B cells from Ighb/b Fas+/+ or Faslpr/lpr (lymphoproliferation) mice in appropriate adoptive transfer models, we demonstrated that: 1) under blockage of the Pfp-mediated pathway, Igha/a T cells were still able to induce suppression against wild-type IgG2ab+ B cells, 2) IgG2ab+ B cells with impaired Fas expression were also subjected to suppression by WT Igha/a T splenocytes, and 3) the suppression establishment was totally inhibited when both Pfp- and Fas-dependent mechanisms were simultaneously blocked, i.e., when Igha/a Pfpo/o T cells were used to induce suppression against Ighb/b Faslpr/lpr B cells. These results provide the first demonstration of the existence of alternative or simultaneous use of the major cytotoxic mechanisms in a T cell-mediated down-regulation of an Ig production.

A role for perforin in downregulating T-cell responses during chronic viral infection

Cytotoxic T cells secrete perforin to kill virus-infected cells. In this study we show that perforin also plays a role in immune regulation. Perforin-deficient (perf -/-) mice chronically infected with lymphocytic choriomeningitis virus (LCMV) contained greater numbers of antiviral T cells compared to persistently infected +/+ mice. The enhanced expansion was seen in both CD4 and CD8 T cells, but the most striking difference was in the numbers of LCMV-specific CD8 T cells present in infected perf -/- mice. Persistent LCMV infection of +/+ mice results in both deletion and anergy of antigen-specific CD8 T cells, and our results show that this peripheral "exhaustion" of activated CD8 T cells occurred less efficiently in perf -/- mice. This excessive accumulation of activated CD8 T cells resulted in immune-mediated damage in persistently infected perf -/- mice; approximately 50% of these mice died within 2 to 4 weeks, and mortality was fully reversed by in vivo depletion of CD8 T cells. This finding highlights an interesting dichotomy between the role of perforin in viral clearance and immunopathology; perforin-deficient CD8 T cells were unable to clear the LCMV infection but were capable of causing immune-mediated damage. Finally, this study shows that perforin also plays a role in regulating T-cell-mediated autoimmunity. Mice that were deficient in both perforin and Fas exhibited a striking acceleration of the spontaneous lymphoproliferative disease seen in Fas-deficient (lpr) mice. Taken together, these results show that the perforin-mediated pathway is involved in downregulating T-cell responses during chronic viral infection and autoimmunity and that perforin and Fas act independently as negative regulators of activated T cells.

Levels of soluble FasL and FasL gene expression during the development of graft-versus-host disease in DLT-treated patients

Three patients with different clinical symptoms of graft-versus-host disease (GVHD) who had received donor lymphocyte transfusion (DLT) for the treatment of relapsed leukaemia after an allogeneic bone marrow transplantation (BMT) from HLA-matched sibling donors were analysed for the presence of soluble FasL (sFasL) in the sera and for the expression of the Fas ligand (FasL) gene in the peripheral blood mononuclear cells (PBMNC). Two patients who demonstrated liver damage with increased levels of serum bilirubin showed significantly increased levels of serum sFasL. The increase in the sFasL level was observed prior to the increase in the bilirubin during the clinical courses of both patients. The high dose of methyl predonisolone administered to one of these patients greatly reduced the levels of sFasL in the serum. The bilirubin levels were also reduced thereafter. The third patient (without liver damage) did not show any increase in the serum sFasL level. The expression of the FasL gene in the PBMNC of these three patients was examined. All three patients showed increased levels of the FasL gene expression during their clinical courses. However, only one patient showed a parallel alteration of FasL gene expression with sFasL in the serum. These cases provide evidence that the Fas/FasL system is closely associated with human GVHD, especially in the development of liver GVHD.

Timing and Casting for Actors of Thymic Negative Selection

We have recently proposed a new model for the differentiation pathway of αβ TCR thymocytes, with the CD4 and CD8 coreceptors undergoing an unexpectedly complex series of expression changes. Taking into account this new insight, we reinvestigated the timing of thymic negative selection. We found that, although endogenous superantigen-driven thymic negative selection could occur at different steps during double-positive/single-positive cell transition, this event was never observed among CD4lowCD8low TCRint CD69+ thymocytes, i.e., within the first subset to be generated upon TCR-mediated activation of immature double-positive cells. We confirm a role for CD40/CD40L interaction, and the absence of involvement of CD28 costimulation, in thymic deletion in vivo. Surprisingly, we found that thymic negative selection was impaired in the absence of Fas, but not FasL, molecule expression. Finally, we show involvement in opposing directions for p59fyn and SHP-1 molecules in signaling for thymic negative selection.

Effector and predisposing genes in murine lupus

This Publication is No. 11789-IMM from the Department of Immunology, The Scripps Research Institute, 10550 North Torrey Pines Road/IMM3, La Jolla, CA 92037. The work of the author reported herein was supported, in part, by NIH grants AR39555, AR31203, and AG15061.

Blocked negative selection of developing T cells in mice expressing the baculovirus p35 caspase inhibitor

Clonal deletion in the thymus by apoptosis is involved in purging the immune system of self-reactive T lymphocytes (negative selection). Cysteine proteases (caspases) belonging to the CPP32 family are activated during this process. We have produced transgenic mice expressing baculovirus p35, a broad-range caspase inhibitor. Thymocytes from p35 transgenic mice were resistant in vitro to several apoptosis-inducing agents; this resistance correlated with the inhibition of CPP32-like activity. Negative selection in vivo of thymocytes triggered by two exogenous antigens, staphylococcal enterotoxin B superantigen and an antigenic peptide in the F5 T-cell receptor transgenic model, was specifically inhibited in p35 transgenic mice. Our results provide direct evidence for caspase involvement in negative selection during thymocyte development.

Apoptosis in B-chronic lymphocytic leukaemia

B-chronic lymphocytic leukaemia (B-CLL) is characterised by the progressive accumulation of monoclonal B cells, which may be the result of several factors leading to extended B-CLL cell lifespan, increased proliferative capacity and diminished cell death. Here we review the implications of several signals mediated by receptors, such as surface IgM, CD6 and CD40, for the B-CLL cell survival, together with data on gene modulation in relation to the apoptosis process in B-CLL cells. We also describe some features of the Fas/FasL system in B-CLL that hypothetically might contribute to the accumulation of leukaemic cells and the progression of the disease, by downregulating the apoptotic response or avoiding the autologous immune response.

Involvement of Fas/Fas ligand system-mediated apoptosis in the development of concanavalin A-induced hepatitis

Concanavalin A (Con A)-induced hepatitis is an experimental hepatitis model in which hepatic injury is caused by the action of cytokines produced by T cells. Using IFN-gamma-deficient mice, we previously demonstrated that IFN-gamma plays a central role in Con A-induced hepatitis. Here, we show that development of the disease is completely suppressed in gld/gld mice, in which Fas ligand is defective. In contrast, suppression of the disease in Ipr/Ipr mice was incomplete, since a small amount of the fas mRNA was produced in these mice. The data indicate that activation of the Fas/Fas ligand system is a necessary step in the development of Con A-induced hepatitis. Furthermore, we found that not only fas but also caspase-1 expression was reduced in IFN-gamma-deficient mice. Since caspase-1 is an integral component of Fas signal transduction, these observations suggest that IFN-gamma-induced activation of both fas and caspase-1 expression causes enhancement of hepatocyte apoptosis resulting in the development of hepatitis.

Pathological findings in human autoimmune lymphoproliferative syndrome

The defects in lymphocyte apoptosis that underlie the autoimmune lymphoproliferative syndrome (ALPS) are usually attributable to inherited mutations of the CD95 (Fas) gene. In this report, we present the histopathological and immunophenotypic features seen in the lymph nodes (n = 16), peripheral blood (n = 10), bone marrow (n = 2), spleen (n = 3), and liver (n = 2) from 10 patients with ALPS. Lymph nodes showed marked paracortical hyperplasia. Interfollicular areas were expanded and populated by T cell receptor-alphabeta CD3+ CD4-CD8- (double-negative, DN) T cells that were negative for CD45RO. CD45RA+ T cells were increased in all cases studied. The paracortical infiltrate was a result of both reduced apoptosis and increased proliferation, as measured by in situ detection of DNA fragmentation and staining with MIB-1, respectively. The paracortical proliferation may be extensive enough to suggest a diagnosis of malignant lymphoma. Many of the paracortical lymphocytes expressed markers associated with cytotoxicity, such as perforin, TIA-1, and CD57. CD25 was negative. In addition, most lymph nodes exhibited florid follicular hyperplasia, often with focal progressive transformation of germinal centers; in some cases, follicular involution was seen. A polyclonal plasmacytosis also was present. The spleens were markedly enlarged, more than 10 times normal size. There was expansion of both white pulp and red pulp, with increased DN T cells. DN T cells also were observed in liver biopsies exhibiting portal triaditis. In the peripheral blood, the T cells showed increased expression of HLA-DR and CD57 but not CD25. CD45RA+ T cells were increased in the four cases studied. Polyclonal B cell lymphocytosis with expansion of CD5+ B cells was a characteristic finding. Taken together, the histopathological and immunophenotypic findings, particularly in lymph nodes and peripheral blood, are sufficiently distinctive to suggest a diagnosis of ALPS. Of note, two affected family members of one proband developed lymphoma (T-cell-rich B-cell lymphoma and nodular lymphocyte predominance Hodgkin's disease, respectively).

Expression and Function of Fas on Cells Damaged by γ-Irradiation in B6 and B6/lpr Mice

Fas (CD95) is a cell surface protein that mediates apoptosis. lpr is a mutation of the Fas gene caused by a retroviral insertion resulting in premature termination of transcription and aberrant splicing of Fas mRNA. Mice homozygous for the lpr gene develop lymphoproliferation and produce autoantibodies closely resembling those of human systemic lupus erythematosus. While lpr mice have been reported to express low levels of normally spliced Fas mRNA, it is unknown whether they express functional Fas protein. Here we show that splenocytes from lpr mice that have been damaged by γ-irradiation expressed Fas protein. Fas was up-regulated on irradiated B6 cells and could be detected on B6/lpr cells undergoing apoptosis following in vitro culture. Detection of Fas on live lpr cells was demonstrable when apoptosis was blocked by zinc. In a short term chimera system, Fas was shown to play a role, in vivo, in the disposition of radiation-injured cells from both normal and lpr mice. The addition of anti-Fas Ab to in vitro cultures resulted in an increase in apoptosis in both B6 and B6/lpr cells. Detection of intact Fas message and low levels of Fas protein in lpr mice has led to the consideration of lpr as a leaky mutation. This study demonstrates that lpr mice can produce functional Fas protein. This system is also appropriate for identifying the in vivo role of Fas/FasL in apoptosis following other cell manipulations.

Efficient peripheral clonal elimination of B lymphocytes in MRL/lpr mice bearing autoantibody transgenes

Peripheral B cell tolerance was studied in mice of the autoimmune-prone, Fas-deficient MRL/ lpr.H-2(d) genetic background by introducing a transgene that directs expression of membrane-bound H-2Kb antigen to liver and kidney (MT-Kb) and a second transgene encoding antibody reactive with this antigen (3-83mu delta, anti-Kk,b). Control immunoglobulin transgenic (Ig-Tg) MRL/lpr.H-2(d) mice lacking the Kb antigen had large numbers of splenic and lymph node B cells bearing the transgene-encoded specificity, whereas B cells of the double transgenic (Dbl-Tg) MRL/lpr.H-2(d) mice were deleted as efficiently as in Dbl-Tg mice of a nonautoimmune B10.D2 genetic background. In spite of the severely restricted peripheral B cell repertoire of the Ig-Tg MRL/lpr.H-2(d) mice, and notwithstanding deletion of the autospecific B cell population in the Dbl-Tg MRL/lpr.H-2(d) mice, both types of mice developed lymphoproliferation and exhibited elevated levels of IgG anti-chromatin autoantibodies. Interestingly, Dbl-Tg MRL/lpr.H-2(d) mice had a shorter lifespan than Ig-Tg MRL/lpr.H-2(d) mice, apparently as an indirect result of their relative B cell lymphopenia. These data suggest that in MRL/lpr mice peripheral B cell tolerance is not globally defective, but that certain B cells with receptors specific for nuclear antigens are regulated differently than are cells reactive to membrane autoantigens.

TNF-related ligands and their receptors

Multicellular organisms have the challenging task of coordinating the activities of many distinct cell types. This coordination is accomplished largely by cell-associated and soluble signalling molecules that act locally or distantly to alter target-cell physiology. The tumour necrosis factor family of cytokines are type II transmembrane proteins that are important regulators of homeostasis and have been implicated as mediators of disease. These molecules serve as ligands for a family of cell-surface receptors termed the tumour necrosis factor/nerve growth factor (TNF/NGF) receptor family. The receptors are type I transmembrane proteins capable of mediating a wide range of responses in vitro and in vivo. Signal transduction is mediated by several newly discovered cytoplasmic proteins that couple these receptors to downstream signalling events. The elucidation and use of spontaneously occurring mutants in TNF-related ligands and receptors in addition to gene-targeting experiments have begun to clarify the diverse biological effects mediated by this superfamily of cytokines.

Two Signaling Pathways Can Increase Fas Expression in Human Thymocytes

Fas, a cell surface receptor, can induce apoptosis after cross-linking with its ligand. Fewer than 3% of human thymocytes strongly express Fas. We report that Fas antigen expression can be upregulated by two signaling pathways in vitro, one mediated by anti-CD3 and the other by interleukin-7 + interferon-γ. The two signaling pathways differed in several respects. (1) Fas expression increased in all thymic subsets after cytokine activation, but only in the CD4 lineage after anti-CD3 activation. (2) Fas upregulation was inhibited by cyclosporin A (a calcineurin inhibitor) in anti-CD3–activated but not in cytokine-activated thymocytes. (3) Cycloheximide (a metabolic inhibitor) inhibited Fas upregulation in cytokine-activated thymocytes but not in anti-CD3–activated thymocytes. (4) Cytokine-activated thymocytes were more susceptible than anti-CD3–activated thymocytes to Fas-induced apoptosis, a difference mainly accounted for by CD4+ cells. The nature of the stimulus might thus influence the susceptibility of human thymocytes to Fas-induced apoptosis.

© 1998 by The American Society of Hematology.

Two Signaling Pathways Can Increase Fas Expression in Human Thymocytes

Fas, a cell surface receptor, can induce apoptosis after cross-linking with its ligand. Fewer than 3% of human thymocytes strongly express Fas. We report that Fas antigen expression can be upregulated by two signaling pathways in vitro, one mediated by anti-CD3 and the other by interleukin-7 + interferon-γ. The two signaling pathways differed in several respects. (1) Fas expression increased in all thymic subsets after cytokine activation, but only in the CD4 lineage after anti-CD3 activation. (2) Fas upregulation was inhibited by cyclosporin A (a calcineurin inhibitor) in anti-CD3–activated but not in cytokine-activated thymocytes. (3) Cycloheximide (a metabolic inhibitor) inhibited Fas upregulation in cytokine-activated thymocytes but not in anti-CD3–activated thymocytes. (4) Cytokine-activated thymocytes were more susceptible than anti-CD3–activated thymocytes to Fas-induced apoptosis, a difference mainly accounted for by CD4+ cells. The nature of the stimulus might thus influence the susceptibility of human thymocytes to Fas-induced apoptosis.

© 1998 by The American Society of Hematology.

Immunological and pathological consequences of mutations in both Fas and Fas ligand

The lpr mutation in mice results in premature termination of transcription of the gene encoding the apoptosis-signaling receptor Fas. As a result, lpr mice develop severe lymphoproliferation and lupus-like autoantibodies. Growing evidence suggests that the lpr mutation is "leaky" and that low levels of Fas are expressed in lpr mice. To determine if Fas expressed in lpr mice is contributing to apoptosis we generated a novel strain of mice (B6/lprgld) which is homozygous for both the lpr mutation and the gld mutation which encodes nonfunctional Fas ligand (FasL) protein. If low levels of Fas in B6/lpr mice contribute to apoptosis and lessen the severity of disease, the B6/lprgld mice, which also lack functional FasL, would be expected to develop a more severe form of disease than B6/lpr mice. Our results revealed no significant increase in either lymphoproliferation or autoimmunity in B6/lprgld mice compared to B6/lpr or B6/gld mice. Additionally, no increase in surface expression of Fas was detected by flow cytometry on B6/lprgld lymphocytes compared to B6/lpr lymphocytes. However, histological examination of B6/lprgld liver revealed a marked increase in lymphocytic infiltration, compared to livers of B6/lpr and B6/gld mice. Our results suggest that, while low levels of Fas in lpr mice may not be contributing to amelioration of lymphoproliferation or autoimmunity, they may be partially protecting the liver from abnormalities which arise in the absence of Fas-mediated apoptosis.

A role for Fas in negative selection of thymocytes in vivo

To seek information on the role of Fas in negative selection, we examined subsets of thymocytes from normal neonatal mice versus Fas-deficient lpr/lpr mice injected with graded doses of antigen. In normal mice, injection of 1-100 microg of staphylococcal enterotoxin B (SEB) induced clonal elimination of SEB-reactive Vbeta8+ cells at the level of the semi-mature population of HSAhi CD4+ 8- cells found in the thymic medulla; deletion of CD4+ 8+ cells was minimal. SEB injection also caused marked elimination of Vbeta8+ HSAhi CD4+ 8- thymocytes in lpr/lpr mice. Paradoxically, however, elimination of these cells in lpr/lpr mice was induced by low-to-moderate doses of SEB ( Collapse

Key Words Collapse

MESH Headings

• Amino Acid Sequence
• Animals
• Antibodies, Monoclonal/immunology
• Antibodies, Monoclonal/metabolism
• CD4-Positive T-Lymphocytes/immunology
• CD8-Positive T-Lymphocytes/immunology
• Clone Cells/immunology
• Enterotoxins/immunology
• Flow Cytometry
• Immune Tolerance/immunology
• Mice
• Mice, Inbred Strains
• Mice, Transgenic
• Molecular Sequence Data
• Ovalbumin/immunology
• Peptide Fragments/immunology
• Receptors, Antigen, T-Cell/genetics
• Thymus Gland/physiology
• fas Receptor/physiology

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Grants

• P01 AG001743 NIA NIH HHS
• R37 CA038355 NCI NIH HHS
• CA25803 NCI NIH HHS
• CA38355 NCI NIH HHS
• AI21487 NIAID NIH HHS

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Affiliation(s)

H Kishimoto Department of Immunology, IMM4, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, USA
C D Surh
J Sprent

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Ultraviolet Light-Induced Immune Tolerance Is Mediated via the Fas/Fas-Ligand System

Hapten sensitization through UV-exposed skin induces tolerance that is mediated via the induction of hapten-specific T suppressor cells. However, the detailed mechanisms underlying tolerance induction remain unclear to date. We show here that the apoptosis-related surface Ag Fas (APO-1, CD95) and its ligand, Fas ligand (FasL) are critically involved, since Fas-deficient lpr mice and FasL-deficient gld mice do not develop UV-induced tolerance. Adoptive transfer experiments revealed that the mediation of tolerance does not require the expression of Fas or FasL by the T suppressor cells but does require the expression of both molecules by the cells of mice receiving the T suppressor cells. To identify the mechanisms involved, the effect of suppressor cells on Ag-presenting dendritic cells (DC) was studied. Coincubation of hapten-pulsed DC with T cells that were obtained from UV-tolerized mice resulted in an enhanced death rate of DC, and this cell death was dependent upon Fas expression. The addition of IL-12, which recently was found to break established tolerance in vivo, prevented DC death. Moreover, IL-12 did not only rescue DC from T suppressor cell-induced death but also from apoptosis induced by rFasL, suggesting that IL-12 may interfere with the Fas/FasL system. Together, these data indicate a crucial role for the Fas/FasL system in UV-induced tolerance, and suggest that UV-induced T suppressor cells may act by inducing the cell death of APCs via the Fas pathway. The ability of IL-12 to break established tolerance may be due to the prevention of DC death induced by T suppressor cells.

In vitro thymocyte maturation is associated with reduced cellular susceptibility to Fas-mediated apoptosis

We have developed a novel system in which the susceptibility of murine thymocytes to Fas-mediated apoptosis can be modulated. Thymocyte susceptibility to Fas decreases under in vitro culture conditions that promote aspects of thymocyte maturation. The hyporesponsive state is specific for the Fas pathway, since cellular susceptibility to other apoptotic stimuli is not reduced. Hyporesponsiveness is not associated with alterations in the thymocyte subset distribution, decreased expression of full-length Fas protein, or alterations in FADD, Bcl-2, or Bcl-XL expression. Hyporesponsiveness is overcome by increasing the strength of the Fas cross-linking stimulus, leading us to propose that reduced thymocyte susceptibility to apoptosis results from altered Fas signaling. The block in Fas signaling resides proximal to ceramide generation, since Fas-hyporesponsive thymocytes are susceptible to ceramide-induced apoptosis. Further characterization of Fas signaling in these in vitro cultured thymocytes may facilitate the identification of factors regulating the susceptibility of wild-type lymphocytes to Fas.

Transgenic Expression of Fas in T Cells Blocks Lymphoproliferation But Not Autoimmune Disease in MRL-lpr Mice

Fas is a member of the TNF receptor family. Binding of Fas ligand to Fas induces apoptosis in Fas-bearing cells. Fas is expressed in various cells, including thymocytes, peripheral T cells, and activated B cells. The mouse lpr mutation is a loss of function mutation of Fas. MRL-lpr/lpr mice develop lymphadenopathy and splenomegaly, and produce multiple autoantibodies, which results in autoimmune disease. In this report, we describe the establishment of a line of Fas transgenic MRL-lpr mice in which mouse Fas cDNA was expressed using the T cell-specific murine lck promoter. The transgenic mice expressed functional Fas in thymocytes and peripheral T cells, but not in B cells. The transgenic mice did not accumulate abnormal T cells (Thy-1+ B220+), but still accumulated B cells (Thy-1− B220+); they produced a large quantity of Igs (IgG1 and IgG2a), including anti-DNA Abs, and developed glomerulonephritis. These results suggest that autoreactive or activated B cells must be killed through Fas expressed in the B cells by the Fas ligand expressed in activated T cells.

Fas/Fas Ligand Signaling During Gestational T Cell Development

Most thymocytes express high levels of Fas Ag (Apo-1/CD95); however, the role of Fas/Fas ligand-mediated apoptosis in thymocyte development remains unclear. During gestational development of thymocytes in C57BL/6(B6) +/+ mice, the highest levels of Fas ligand mRNA and Fas ligand protein expression were detected at gestational day (GD) 15, and there was a ninefold decrease in Fas ligand mRNA expression between GD 15 and 17 accompanied by a sixfold increase in Fas mRNA. Apoptotic thymocytes were first detected in the medulla at GD 15, and increasing numbers of cortical clusters and scattered, single apoptotic cells were present on GD 16 and 17. Thus, early apoptosis correlated with high expression of Fas ligand. High levels of Fas ligand mRNA were maintained throughout gestational development in thymocytes of Fas-deficient B6-lpr/lpr mice, but cortical clusters and scattered apoptotic cells were decreased relative to B6 +/+ mice before GD 17. Kinetic analysis of fetal thymic organ cultures treated with anti-Fas Ab demonstrated that thymocytes become sensitive to Fas-mediated apoptosis during the transition from the CD4−CD8− to the CD4+CD8+ phenotype. More mature CD4+CD8+ thymocytes and CD4+ and CD8+ thymocytes became resistant to Fas-mediated apoptosis after GD 17, despite high expression of Fas. However, low avidity engagement of the TCR on Fas-sensitive CD4+CD8+ thymocytes before GD 17 induced resistance to Fas-mediated apoptosis. The present results indicate that Fas plays a critical role in mediating apoptosis during early gestational thymocyte development and that thymocytes that receive a survival signal through TCR/CD3 become resistant to Fas-mediated apoptosis.

A role for FADD in T cell activation and development

FADD is a cytoplasmic adapter molecule that links the family of death receptors to the activation of caspases during apoptosis. We have produced transgenic mice expressing a dominantly interfering mutant of FADD, lacking the caspase-dimerizing death effector domain, as well as mice overexpressing the poxvirus serpin, CrmA, an inhibitor of caspases downstream of FADD. While thymocytes from either line of mice were completely protected from CD95-dependent cytotoxicity, neither transgene afforded protection from apoptosis induced during thymocyte selection and neither led to the lymphoproliferative disorders associated with deficiencies in CD95. However, in FADD dominant negative (FADDdd) mice, early thymocyte development was retarded and peripheral lymphocyte pools were devoid of normal populations of T cells. We show that thymocytes and peripheral T cells from FADDdd display signaling anomalies, implying that FADD plays a previously uncharacterized role in T cell development and activation.

Tolerant B lymphocytes acquire resistance to Fas-mediated apoptosis after treatment with interleukin 4 but not after treatment with specific antigen unless a surface immunoglobulin threshold is exceeded

Susceptibility to Fas-mediated apoptosis in nontolerant B cells is regulated in a receptor-specific fashion. To explore the regulation of Fas killing in tolerant, autoreactive B cells, mice doubly transgenic for hen egg lysozyme (HEL)-specific B cell receptors and soluble HEL were examined. Engagement of CD40 led to enhanced Fas expression and acquisition of sensitivity to Fas-mediated apoptosis in tolerant B cells, similar to that observed in nontolerant, receptor transgenic B cells. Engagement of surface immunoglobulin by specific (HEL) antigen failed to induce Fas resistance in tolerant B cells, in contrast to its effect on nontolerant B cells; however, cross-linking of biotinylated HEL with streptavidin induced similar levels of Fas resistance in tolerant and nontolerant B cells, which approximated the degree of Fas resistance produced by anti-Ig. Unlike surface Ig (sIg) engagement, physiological engagement of IL-4 receptors produced similar levels of Fas resistance in tolerant and nontolerant B cells. Thus, tolerant B cells differ from nontolerant B cells in the diminished capacity of surface immunoglobulin engagement to produce Fas resistance; however, tolerant B cells can be induced to become resistant to Fas-mediated apoptosis by IL-4 or by higher order cross-linking of sIg receptors.

Soluble FasR ligand-binding domain: high-yield production of active fusion and non-fusion recombinant proteins using the baculovirus/insect cell system

We used the recombinant baculovirus/insect cell system to express two soluble forms of the mouse Fas receptor (mFasR) extracellular domain (ECD): a monomer comprising the entire ligand-binding portion of mFasR followed by a carboxy-terminal hexa-histidine extension aiding purification by immobilized metal affinity chromatography and an immunoadhesin in which the same 148 residues were fused to the Fc portion of a truncated human IgG1 immunoglobulin heavy chain. Both constructs harboured a 24 base pairs insertion placed upstream of the initiating ATG [Peakman, Charles, Sydenham, Gewert, Page, and Makoff (1992) Nucleic Acids Res. 20, 6111-6112]. Despite its hexa-histidine extension, the monovalent recombinant protein from crude culture media failed to bind immobilized Ni2+ unless proteins were first precipitated twice by ammonium sulphate. The overall procedure then yielded approximately 10mg/l of protein which could be purified to near homogeneity using two additional chromatographic steps. The glycosylated polypeptide migrated as a band of Mr=(21-31) x 10(3) in SDS/PAGE and was monomeric in physiological buffers. Under non-reducing conditions, denaturation in 6 M guanidinium chloride was reversible after slow removal of the denaturing agent. The mFasR immunoadhesin was secreted (approximately 5-10 mg/l) as a disulphide-linked homodimer, and endowed with ligand-binding activity since it could bind FasL on the surface of D11S, FasL-expressing cells. When tested for their ability to inhibit FasR-dependent cell lysis, the soluble dimeric immunoadhesin markedly inhibited FasL-mediated cytotoxicity (IC50 approximately 30 nM), and was approximately 6 times as effective as its monomeric counterpart.

Involvement of the Fas (CD95) system in peripheral cell death and lymphoid organ development

Fas-mediated apoptosis is a form of cell death that operates through a Fas-Fas ligand (FasL) interaction. In this study we investigated the role of the Fas system during development of normal and Fas-mutated lymphocytes. Irradiated RAG2-/-recipients were reconstituted with bone marrow cells from B6 and lpr mice (Fas defective) or from B6 and gld mice (FasL defective), and analyzed for long-term development. The results showed a primary role of the Fas system in peripheral cell death and thymic colonization. In the periphery, the interaction in vivo between Fas+ and Fas-T cell populations indicated that cellular homeostasis was defective. Indeed, we observed a FasL-mediated cytotoxic effect on normal-derived T cells, explaining the dominance of lpr T cells in the mixed chimeras. The Fas mutation affected neither cell activation nor cell proliferation, as the effector (Fas-) and target (Fas+) cells behaved similarly with regard to activation marker expression and cell cycle status. However, Fas-T cells failed to seed the periphery and the thymus in the long term. We suggest that this could be due to the fact that FasL is involved in the structural organization of the lymphoid compartment.

Cutting Edge: Lymphocyte Inhibitor of TRAIL (TNF-Related Apoptosis-Inducing Ligand): A New Receptor Protecting Lymphocytes from the Death Ligand TRAIL

Apoptosis can be triggered by the engagement of cell surface receptors by their ligands. A growing number of receptors belonging to the TNF receptor family have been identified that contain a conserved cytoplasmic death domain. These include Fas, TNF-R1, lymphocyte-associated receptor of death (LARD), DR4, and TNF-related apoptosis-inducing ligand receptor inducer of cell killing-2 (TRICK2). The latter two are receptors for the cytotoxic ligand TNF-related apoptosis-inducing ligand (TRAIL), and one of the paradoxes raised by the cloning of these molecules was why do most cells not die upon contact with the widely expressed TRAIL molecule? This is a particular problem for lymphocytes that express DR4 and TRICK2 and are in constant circulation through TRAIL-expressing tissues. We have cloned LIT (lymphocyte inhibitor of TRAIL), which lacks a death domain. LIT is expressed predominantly on PBL, where it can competitively inhibit TRAIL-induced apoptosis through DR4/TRICK2, and may function to modulate lymphocyte sensitivity to TRAIL.

TNF Receptor-Deficient Mice Reveal Striking Differences Between Several Models of Thymocyte Negative Selection

Central tolerance depends upon Ag-mediated cell death in developing thymocytes. However, the mechanism of induced death is poorly understood. Among the known death-inducing proteins, TNF was previously found to be constitutively expressed in the thymus. The role of TNF in thymocyte negative selection was therefore investigated using TNF receptor (TNFR)-deficient mice containing a TCR transgene. TNFR-deficient mice displayed aberrant negative selection in two models: an in vitro system in which APC are cultured with thymocytes, and a popular in vivo system in which mice are treated with anti-CD3 Abs. In contrast, TNFR-deficient mice displayed normal thymocyte deletion in two Ag-induced in vivo models of negative selection. Current models of negative selection and the role of TNFR family members in this process are discussed in light of these results.

Biochemical and molecular mechanisms regulating apoptosis

In eukaryotes, the regulation of tissue cell numbers is a critical homeostatic objective that is achieved through tight control of apoptosis, mitosis and differentiation. While much is known about the genetic regulation of cell growth and differentiation, the molecular basis of apoptosis is less well understood. Genes involved in both cell proliferation and apoptosis reflect the role of some stimuli in both of these processes, the cell response depending on the overall cellular milieu. Recent research has given fascinating insights into the complex genetic and molecular mechanisms regulating apoptosis. A picture is emerging of the initiation in certain cells, after an apoptotic trigger, of sequential gene expression and specific signal transduction cascades that guide cells along the cell death pathway. Changes in gene expression precede the better known biochemical and morphological changes of apoptosis. It seems possible that, as a result of increased understanding of the cellular events preceding cell death, apoptosis may become more amenable to manipulation by appropriate drug- and gene-based therapies.

Why do defects in the Fas-Fas ligand system cause autoimmunity?

We have previously isolated genes that encode Fas and Fas ligand, a receptor-ligand pair that mediates an apoptotic signal. We also have demonstrated that lpr and gld mice, well-known animal models of autoimmune disease are loss-of-function mutants of the Fas and Fas ligand genes, respectively. Patients with autoimmune lymphoproliferative disorders have been found to bear mutations of the Fas gene. These findings indicate that the Fas-Fas ligand system plays an important role in the maintenance of self-tolerance among both humans and mice. During T-cell development, mouse T cells initially express Fas in the thymus and maintain their expression thereafter. Peripheral B cells usually express Fas at much lower levels than do T cells, but various stimuli enhance Fas expression on B cells. In contrast, among the lymphocyte subsets, only activated T cells and natural killer cells express readily detectable levels of Fas ligand. Reactivation of previously activated T cells through T-cell receptors induces apoptosis. This phenomenon (activation-induced cell death) is mediated by means of the Fas-Fas ligand interaction. We recently discovered that peripheral naive T cells in mice are susceptible to Fas ligand but not to agonistic anti-Fas antibodies. To our surprise, engagement of T-cell receptors on naive T cells was shown to induce Fas ligand resistance. On the basis of these findings and other reports, we discuss how the breakdown of self-tolerance occurs as the result of defects in the Fas-Fas ligand system.

The role of the bcl-2/ced-9 gene family in cancer and general implications of defects in cell death control for tumourigenesis and resistance to chemotherapy

Cell production within an organ is determined by the rate of immigration, proliferation, differentiation, emigration and death of cells. Abnormalities in any one of these processes will disturb normal control of cell production, thereby eliciting hyperplasia can be an early event in neoplasia. Cell death, apoptosis, is a physiological process responsible for removing unwanted cells. It is used in multi-cellular organisms for tissue remodelling during embryogenesis, regulation of cell turnover and as a defence strategy against invading pathogens. In this review article we describe the role of the bcl-2/ced-9 gene family in cancer and discuss the general implications of defects in the apoptosis program for tumourigenesis and resistance of cancer cells to chemotherapy in light of current knowledge of the molecular mechanisms of cell death.

Antigen-specific and nonspecific deletion of immature cortical thymocytes caused by antigen injection

Analysis of antigen-induced negative selection of thymocytes in T cell receptor (TCR)-transgenic mice is complicated by the presence of an antigen-responsive peripheral T cell compartment. Our experiments address the question of whether and how peripheral T cell activation can affect immature thymocytes. Following three daily injections of peptide antigen into mice expressing a peptide-specific transgenic TCR and deficient for TAP1, we and others have found profound deletion of the CD4+CD8+ (DP) thymocyte subset. However, our work shows that even though mature CD8+ T cells are inefficiently selected in TAP1-deficient mice, there was a striking degree of peripheral expansion and activation of CD8+ peripheral T cells. Furthermore, when cells from TCR-transgenic mice were adoptively transferred, we found that deletion of nontransgenic DP thymocytes occurred in Thy-1-congenic and even more efficiently in TAP1-deficient recipients after repeated peptide injection resulting in peripheral T cell activation. In the adoptive transfer experiments the degree of deletion of immature bystander thymocytes was decreased upon blocking of TNF. These data show that deletion of DP thymocytes can result from excessive peripheral T cell activation and identify TNF as an important effector molecule for this process. When steps are taken to avoid peripheral T cell activation, peptide antigen can induce TCR-mediated thymocyte deletion, presumably in the thymus cortex, since injection of TAP1-deficient TCR-transgenic mice resulted in deletion of immature DP thymocytes prior to detectable peripheral T cell expansion and activation. This effect was not blocked by inhibiting tumor necrosis factor activity. In addition, DP depletion was seen in the absence of peripheral T cell activation when antibody-mediated depletion of CD8+ T cells was performed. Our work clearly shows that two mechanisms for deletion of DP thymocytes exist: deletion induced by antigen presentation in the thymus and deletion as a consequence of repeated stimulation of mature peripheral T cells.

Correction of Fas (CD95) deficiency by haploidentical bone marrow transplantation

Recently, an inherited syndrome characterized by nonmalignant lymphoproliferation with autoimmune manifestations, caused by mutations of the Fas (CD95) receptor gene has been described. Because of disease severity, i.e. unremitting lymphoproliferation in a child with complete Fas deficiency, a haploidentical bone marrow transplantation (BMT) was performed despite the known resistance of Fas-deficient lpr mice to bone marrow transplantation. Marrow graft was rejected early; however, a second attempt using bone marrow from the mother led to engraftment and to control of lymphoproliferation and of autoimmune thrombocytopenia up to the last follow-up at 24 months after BMT. This single case shows that resistance to bone marrow engraftment caused by survival of Fas-deficient cells can be overcome.

Characterization of Fas (Apo-1, CD95)-Fas ligand interaction

The death-inducing receptor Fas is activated when cross-linked by the type II membrane protein Fas ligand (FasL). When human soluble FasL (sFasL, containing the extracellular portion) was expressed in human embryo kidney 293 cells, the three N-linked glycans of each FasL monomer were found to be essential for efficient secretion. Based on the structure of the closely related lymphotoxin alpha-tumor necrosis factor receptor I complex, a molecular model of the FasL homotrimer bound to three Fas molecules was generated using knowledge-based protein modeling methods. Point mutations of amino acid residues predicted to affect the receptor-ligand interaction were introduced at three sites. The F275L mutant, mimicking the loss of function murine gld mutation, exhibited a high propensity for aggregation and was unable to bind to Fas. Mutants P206R, P206D, and P206F displayed reduced cytotoxicity toward Fas-positive cells with a concomitant decrease in the binding affinity for the recombinant Fas-immunoglobulin Fc fusion proteins. Although the cytotoxic activity of mutant Y218D was unaltered, mutant Y218R was inactive, correlating with the prediction that Tyr-218 of FasL interacts with a cluster of three basic amino acid side chains of Fas. Interestingly, mutant Y218F could induce apoptosis in murine, but not human cells.

Rheumatic diseases in an MRL strain of mice with a deficit in the functional Fas ligand

OBJECTIVE

To characterize Fas antigen expression on the cell surface, and to determine the effect of this expression in rheumatic diseases using a newly established gld-congenic MRL strain of mice (MRL/gld), which is defective in its functional Fas ligand (Fas-L).

METHODS

Flow cytometric analyses of lymphoid cells and macrophages were performed using anti-Fas and other cell surface markers. Histopathologic manifestations were examined using immunochemistry and light and electron microscopy. Serum levels of IgG and anti-DNA antibodies were measured by single radial immunodiffusion and enzyme-linked immunosorbent assay, respectively.

RESULTS

MRL/gld mice developed systemic lymphadenopathy with an accumulation of Thy1.2+, B220+ and CD4-, CD8- T cells, which both express the Fas antigen. Splenic B cells positive for surface IgM and/or surface IgD, and resident peritoneal macrophages exhibited up-regulated expression of the Fas antigen, at much higher levels than those observed in MRL/MpJ-+/+ (MRL/+) mice. Forms of rheumatic disease were observed in these mice, although not in C3H/HeJ-gld/gld mice. These forms included diffuse glomerulonephritis, granulomatous arteritis, and arthritis, and were associated with the infiltration of mononuclear cells expressing the Fas antigen. Serum levels of IgG and anti-DNA antibodies were significantly increased in MRL/gld mice compared with MRL/+ mice.

CONCLUSION

Rheumatic disease was generated by the gld gene in mice with an MRL background, as it is by the lpr gene, which is a Fas deletion mutant, associated with autoimmune traits. Rheumatic disease in this MRL strain was initiated by an incapacity for Fas/Fas-L-induced apoptosis, resulting in the development of autoimmunity and allowing for a persistent immune response in the affected lesions.

Resistance of cultured peripheral T cells towards activation-induced cell death involves a lack of recruitment of FLICE (MACH/caspase 8) to the CD95 death-inducing signaling complex

Phytohemagglutinin-activated peripheral CD95+ T cells (day 1 T cells) are resistant to CD95-mediated apoptosis. After prolonged interleukin-2 treatment, these T cells become CD95-mediated apoptosis-sensitive (day 6 T cells). To elucidate the molecular mechanism of apoptosis resistance, day 1 and day 6 T cells were tested for formation of the CD95 death-inducing signaling complex (DISC). DISC-associated active Fas-associated DD protein (FADD)-like interleukin-1 beta-converting enzyme-like protease (FLICE) also referred to as MACH/caspase 8 was only found in apoptosis-sensitive day 6 T cells. Further-analysis of mRNA and protein expression levels of apoptosis-signaling molecules FADD, receptor interacting protein, hematopoietic cell protein tyrosine phosphatase, Fas-associated phosphatase-1, FLICE, bel-2, bcl-xL, and, bax-alpha showed that only the expression level of bcl-xL correlated with T cell resistance to CD95-mediated apoptosis (day 1 T cells: bcl-xhiL; day 6 T cells: bcl-XloL). In T cells activated in vitro, up-regulation of bcl-xL, has previously been correlated with general apoptosis resistance. However, the experiments presented suggest that resistance to CD95-mediated apoptosis in T cells can also be regulated at the level of recruitment of FLICE to the DISC.

Thymocyte Fas Expression Is Dysregulated in Myasthenia Gravis Patients With Anti-Acetylcholine Receptor Antibody

Myasthenia gravis (MG) is a human autoimmune disease mediated by anti-acetylcholine receptor (AChR) antibodies. The thymus is probably the site where the autoimmune response is triggered and maintained. Recent reports have linked various autoimmune disease with defective Fas expression. We thus analyzed Fas expression in thymocytes and peripheral blood lymphocytes (PBL) from MG patients. The proportion of a thymocyte subpopulation with strong Fas expression (Fashi) was markedly enhanced in MG patients with anti-AChR antibodies (P < .0003, compared with controls). In this group of patients, the proportion of CD4+Fashi and CD4+CD8+Fashi thymocytes were significantly increased (P < .002 for both subsets). Fashi thymocytes were enriched in activated cells and showed intermediate CD3 expression. They were preferentially Vβ5.1-expressing cells, previously shown to be enriched in potentially autoreactive cells. The proliferative response of thymocytes from MG patients to peptides from the AChR was abolished after depletion of Fashi cells. Fashi thymocytes were sensitive to an agonistic anti-Fas antibody. In peripheral blood, Fashi lymphocytes proportion was not significantly modified in MG patients whatever their anti-AChR antibody titer, compared with controls. Altogether, these results indicate that Fashi thymocytes, which accumulate in MG patients with anti-AChR antibodies, could be involved in the autoimmune response that targets the AChR.

Essential roles of the Fas ligand in the development of hepatitis

The Fas ligand (FasL) is expressed in activated T cells and induces apoptosis in Fas-bearing cells. A cytotoxic T lymphocyte (CTL) clone specific for hepatitis B surface antigen (HBsAg) causes an acute liver disease in HBsAg transgenic mice. Here we observed that the CTL clone killed hepatocytes expressing HBsAg in a Fas-dependent manner. Administration of the soluble form of Fas into HBsAg transgenic mice prevented the CTL-induced liver disease. In the second model, mice were primed with Propionibacterium acnes. A subsequent challenge with lipopolysaccharide (LPS) killed the mice by inducing liver injury. Neutralization of FasL rescued the mice from LPS-induced mortality, and Fas-null mice were resistant to LPS-induced mortality. These results suggest that FasL has an essential role in the development of hepatitis.

Naturally occurring primary deficiencies of the immune system

Naturally occurring genetic disorders of the immune system provide many models for the study of its development and function. In a way, their analysis complements the information provided by the generation of genetic defects in mice created using homologous recombination techniques. In this review, the recent findings made in three areas are focused upon deficiencies in T cell differentiation and in T lymphocyte activation, and on the control process of peripheral immune response.

The role of the Ikaros gene in lymphocyte development and homeostasis

The Ikaros gene, which encodes a family of hemopoietic-specific zinc finger proteins, is described as a central regulator of lymphocyte differentiation. During fetal development, it is required at the earliest stage of T cell and B cell specification. In the adult, however, lymphoid lineages rely on Ikaros at distinct phases of their development. Its activity is essential for the generation of B cell but not of T cell precursors, although the differentiation of the latter is not normal. A significant increase in CD4 thymocytes and their immediate precursors is detected, and because these cells lack markers that correlate with positive selection, a deregulation in their maturation process is suggested. Furthermore, Ikaros-null thymocytes hyperproliferate in response to T cell receptor (TCR) signaling; within days after their appearance in the thymus, clonally expanding populations are detected. Deregulated TCR-mediated responses and the fast kinetics of tumor development in these mutant thymocytes implicate Ikaros as a central tumor suppressor gene for the T cell lineage. In addition, lack of natural killer cells and selective defects in gamma delta T cells and dendritic antigen-presenting cells point to Ikaros as an essential factor for the establishment of early branchpoints of the T cell pathway. The dominant interference activity of Ikaros isoforms unable to bind DNA and their effects in lymphocyte development suggest that Ikaros works in concert with other factors. The role of Aiolos, a lymphoid-restricted and structurally related gene, in lymphoid differentiation is discussed. A model is proposed that defines Ikaros as the backbone of a complex regulatory protein network that controls cell fate decisions and regulates homeostasis in the hemo-lymphoid system. Changes in this regulatory network may reflect differentiation and proliferation adjustments made in hemo-lymphoid progenitors and precursors as they give rise to the cells of our immune system.

CrmA expression in T lymphocytes of transgenic mice inhibits CD95 (Fas/APO-1)-transduced apoptosis, but does not cause lymphadenopathy or autoimmune disease

The cysteine protease interleukin-1beta converting enzyme (ICE) is implicated as an effector of apoptosis in mammalian cells. Proteolytic activity of ICE can be blocked in vitro by the cytokine response modifier A (crmA), a serpin-like protease inhibitor encoded by cowpox virus. Here we show that CD2 enhancer-driven expression of crmA in T lymphocytes of transgenic mice (CD2-crmA mice) reduces CD95 (Fas/APO-1)-transduced apoptosis in vitro to the level seen in CD95-deficient mutant lpr mice, but does not protect against gamma-radiation or corticosteroid-induced cell death. Unlike lpr mice, CD2-crmA transgenic mice developed neither T cell hyperplasia nor serum autoantibodies. These results provide evidence that the phenotype of lpr mice is not simply due to failure of CD95 to trigger T cell apoptosis mediated by ICE.

Apoptosis and the maintenance of homoeostasis in the immune system

The regulation of cell proliferation and the selection against autoreactive cells in the lymphoid system both occur through the induction of apoptosis. Many of the signals that induce apoptosis in lymphocytes are now well defined. Interactions between Fas and its ligand have emerged as a major mechanism for the deletion of activated peripheral T cells and autoreactive B cells. Although the signal-transduction pathway leading from engagement of Fas to apoptosis is not entirely clear, significant advances have been made recently. There has also been progress in the elucidation of the mechanisms that regulate apoptosis in the immune system.

Fas ligand in human serum

The Fas ligand (FasL), a member of the tumor necrosis factor family, induces apoptosis in Fas-bearing cells. The membrane-bound human FasL was found to be converted to a soluble form (sFasL) by the action of a matrix metalloproteinase-like enzyme. Two neutralizing monoclonal anti-human FasL antibodies were identified, and an enzyme-linked immunosorbent assay (ELISA) for sFasL in human sera was established. Sera from healthy persons did not contain a detectable level of sFasL, whereas those from patients with large granular lymphocytic (LGL) leukemia and natural killer (NK) cell lymphoma did. These malignant cells constitutively expressed FasL, whereas peripheral NK cells from healthy persons expressed FasL only on activation. These results suggested that the systemic tissue damage seen in most patients with LGL leukemia and NK-type lymphoma is due to sFasL produced by these malignant cells. Neutralizing anti-FasL antibodies or matrix metalloproteinase inhibitors may be of use in modulating such tissue damage.