Autoimmune gld mutation uncouples suicide and cytokine/proliferation pathways in activated, mature T cells

Immune Tolerance Regulation Is Critical to Immune Homeostasis

The body's immune response plays a critical role in defending against external or foreign antigens while also preserving tolerance to self-antigens. This equilibrium, referred to as immune homeostasis, is paramount for overall health. The regulatory mechanisms governing the maintenance of this delicate immune balance are notably complex. It is currently accepted that immune tolerance is a dynamic outcome regulated by multiple factors, including central and peripheral mechanisms. Its induction or elimination plays a significant role in autoimmune diseases, organ transplantation, and cancer therapy, markedly impacting various major diseases in modern clinical practice. Overall, our current understanding of immune tolerance is still very limited. In this review article, we summarized the main mechanisms that have been known to mediate immune tolerance so far, including endogenous immune tolerance, adaptive immune tolerance, other immune tolerance mechanisms, and the homeostasis of immune tolerance, identified the key factors that regulate immune tolerance, and provided new clues for immune system recovery in many autoimmune diseases, organ transplantation, and tumor therapy.

A novel trivalent non-Fc anti-CD3 Collabody preferentially induces Th1 cell apoptosis in vitro and long-lasting remission in recent-onset diabetic NOD mice

Specific anti-CD3 treatment is deemed to be a promising therapy for allograft rejection and type 1 diabetes (T1D). Fc receptor (FcR) reduced-binding antibodies, by avoiding adverse effects of Fc and FcR interaction, have good therapeutic potential. We generated a trivalent anti-mouse-CD3 Collabody, h145CSA, by using a triplex-forming collagen-like peptide (Gly-Pro-Pro)10 to drive the trimerization of the Fab fragments. Exposure to h145CSA, but not its bivalent counterparts 145-2C11 and h145chIgGAA (FcR reduced-binding format), upregulates FasL expression on Th1 cells and causes Th1 cell apoptosis. Administration of h145CSA invokes minimal mitogenic effects in mice. The ability of multiple dosing of h145CSA to induce splenic CD4+ T-cell depletion is comparable to bivalent antibodies but is characterized by more rapid CD4+ T-cell recovery kinetics. h145CSA is more potent than h145chIgGAA in inducing long-lasting remission in recent-onset diabetic NOD mice. Its therapeutic effect is accompanied by a significantly lower percentage of CD4+IFNγ+ T cells and a higher Treg/Th1 ratio in pancreatic and mesenteric lymph nodes. The results of our study demonstrate that trivalent non-Fc anti-CD3 Collabody has the potential to be used in the treatment of T1D.

miR-155 indicates the fate of CD4+ T cells

MicroRNAs (miRNAs) are a class of short noncoding RNAs that regulate the translation of target messenger RNA (mRNA) and consequently participate in a variety of biological processes at the posttranscriptional level. miR-155, encoded within a region known as the B cell integration cluster (BIC), plays multifunctional roles in shaping lymphocytes ranging from biological development to adaptive immunity. It has been revealed that miR-155 plays a key role in fine-tuning the regulation of lymphocyte subsets, including dendritic cells (DCs), macrophages, B cells, and CD8⁺ and CD4⁺ T cells. Antigen-specific CD4⁺ T lymphocytes are critical for host defense against pathogens and prevention of damage resulting from excessive inflammation. Over the past years, various studies have shown that miR-155 plays a critical role in CD4⁺ T cells function. Therefore, we summarize multiple target genes of miR-155 that regulate aspects of CD4⁺ T cells immunity, particularly CD4⁺ T cells differentiation, in this review. In addition, we also focus on the role of miR-155 in the regulation of immunological diseases, suggesting it as a potential disease biomarker and therapeutic target.

Bcl-2 knockdown accelerates T cell receptor-triggered activation-induced cell death in jurkat T cells

Cell death and survival are tightly controlled through the highly coordinated activation/inhibition of diverse signal transduction pathways to insure normal development and physiology. Imbalance between cell death and survival often leads to autoimmune diseases and cancer. Death receptors sense extracellular signals to induce caspase-mediated apoptosis. Acting upstream of CED-3 family proteases, such as caspase-3, Bcl-2 prevents apoptosis. Using short hairpin RNAs (shRNAs), we suppressed Bcl-2 expression in Jurkat T cells, and this increased TCR-triggered AICD and enhanced TNFR gene expression. Also, knockdown of Bcl-2 in Jurkat T cells suppressed the gene expression of FLIP, TNF receptor-associated factors 3 (TRAF3) and TRAF4. Furthermore, suppressed Bcl-2 expression increased caspase-3 and diminished nuclear factor kappa B (NF-κB) translocation.

Increased protection from vaccinia virus infection in mice genetically prone to lymphoproliferative disorders

Mutations in the genes that encode Fas or Fas ligand (FasL) can result in poor restraints on lymphocyte activation and in increased susceptibility to autoimmune disorders. Because these mutations portend a continuously activated immune state, we hypothesized that they might in some cases confer resistance to infection. To examine this possibility, the immune response to, morbidity caused by, and clearance of vaccinia virus (VACV) Western Reserve was examined in 5- to 7-week-old Fas mutant (lpr) mice, before an overt lymphoproliferative disorder was observable. On day 6 after VACV infection, C57BL/6-lpr (B6-lpr) mice had decreased morbidity, decreased viral titers, and an increased percentage and number of CD4(+) and CD8(+) T cells. As early as day 2 after infection, B6-lpr mice had decreased liver and spleen viral titers and increased numbers of and increased gamma interferon (IFN-γ) production by several different effector cell populations. Depletion of individual effector cell subsets did not inhibit the resistance of B6-lpr mice. Uninfected B6-lpr mice also had increased numbers of NK cells, γδ(+) T cells, and CD44(+) CD4(+) and CD44(+) CD8(+) T cells compared to uninfected B6 mice. Antibody to IFN-γ resulted in increased virus load in both B6 and B6-lpr mice and eliminated the differences in viral titers between them. These results suggest that IFN-γ produced by multiple activated leukocyte populations in Fas-deficient hosts enhances resistance to some viral infections.

Apoptotic signal transduction and T cell tolerance

The healthy immune system makes use of a variety of surveillance mechanisms at different stages of lymphoid development to prevent the occurrence and expansion of potentially harmful autoreactive T cell clones. Disruption of these mechanisms may lead to inappropriate activation of T cells and the development of autoimmune and lymphoproliferative diseases [such as multiple sclerosis, rheumatoid arthritis, lupus erythematosus, diabetes and autoimmune lymphoproliferative syndrome (ALPS)]. Clonal deletion of T cells with high affinities for self-peptide-MHC via programmed cell death (apoptosis) is an essential mechanism leading to self-tolerance. Referred to as negative selection, central tolerance in the thymus serves as the first checkpoint for the developing T cell repertoire and involves the apoptotic elimination of potentially autoreactive T cells clones bearing high affinity T cell receptors (TCR) that recognize autoantigens presented by thymic epithelial cells. Autoreactive T cells that escape negative selection are held in check in the periphery by either functional inactivation ("anergy") or extrathymic clonal deletion, both of which are dependent on the strength and frequency of the TCR signal and the costimulatory context, or by regulatory T cells. This review provides an overview of the different molecular executioners of cell death programs that are vital to intrathymic or extrathymic clonal deletion of T cells. Further, the potential involvement of various apoptotic signaling paradigms are discussed with respect to the genesis and pathophysiology of autoimmune disease.

The Bcl-2 family in autoimmune and degenerative disorders

Members of the Bcl-2 family are essential regulators of programmed cell death and thus play a major role in the development and function of many tissues. The balance between pro-survival and pro-apoptotic members of the family decides whether a cell will live or die. This mechanism allows organisms to get rid of cells that are no longer needed or have become dangerous. Deregulation of apoptosis is a major contributing factor in the development of many diseases. A deeper understanding of how the Bcl-2 family proteins orchestrate death in normal and pathologic conditions is thus relevant not only for disease etiology, but also to try to prevent these various disorders. Experiments with transgenic and gene-ablated mice have helped elucidate the function of the different members of the Bcl-2 family and their physiological roles. The present review highlights the role of Bcl-2 family members in autoimmune and degenerative disorders, with a particular focus on the mouse models that have been used to study their function.

The many roles of FAS receptor signaling in the immune system

FAS belongs to the subgroup of the tumor necrosis factor receptor (TNF-R) family that contains an intracellular "death domain" and triggers apoptosis. Its physiological ligand FASL is a member of the TNF cytokine family. Studies with mutant mice and cells from human patients have shown that FAS plays critical roles in the immune system, including the killing of pathogen-infected cells and the death of obsolete and potentially dangerous lymphocytes. Fas thereby functions as a guardian against autoimmunity and tumor development. FAS triggers apoptosis through FADD-mediated recruitment and activation of caspase-8. In certain cells such as hepatocytes, albeit not lymphocytes, FAS-induced apoptosis requires amplification through proteolytic activation of the proapoptotic BCL-2 family member BID. Curiously, several components of the FAS signaling machinery have been implicated in nonapoptotic processes, including cellular activation, differentiation, and proliferation. This review describes current understanding of Fas-induced apoptosis signaling and proposes experimental strategies for future advances.

c-FLIP protects mature T lymphocytes from TCR-mediated killing

Although c-FLIP has been identified as an important player in the extrinsic (death receptor-induced) apoptosis pathway, its endogenous function in mature T lymphocytes remains undefined. c-FLIP may inhibit or promote T cell death as previous data demonstrate that the c-FLIP(L) isoform can promote or inhibit caspase 8 activation while the c-FLIP(S) isoform promotes or inhibits T cell death when overexpressed. Although the c-FLIP(R) isoform inhibits cell death in cell lines, its function in T cells remains unknown. To investigate the function of c-FLIP in mature T cells, we have generated several genetic mouse models with c-FLIP or its individual isoforms deleted in mature T cells. Surprisingly, we found that c-FLIP protects mature T cells not only from apoptosis induced by the death receptors Fas and TNFR but also from TCR-mediated and spontaneous apoptosis. Thus, c-FLIP plays an essential role in protecting mature T cells from a death signal induced through the TCR itself and is required for naive T cell survival. Our results demonstrate that c-FLIP functions beyond the extrinsic death pathway.

Cell cycle regulation by FasL and Apo2L/TRAIL in human T-cell blasts. Implications for autoimmune lymphoproliferative syndromes

The Fas-FasL pathway plays an important role in the homeostasis of mature lymphocytes, with defects causing autoimmune lymphoproliferative syndromes (ALPS). Human T-cell blasts are not sensitive to FasL or Apo2L/TRAIL-induced apoptosis unless they get reactivated, but either of those ligands inhibits their growth in the absence of cell death induction due to a cell cycle arrest in S-G2/M. In the present work, we have studied the mechanism(s) by which FasL or Apo2L/TRAIL regulate T-cell blast cell cycle in healthy donors and in two types of ALPS patients. Our data indicate that in human CD8+ T-cell blasts, Fas ligation, and especially Apo2L/TRAIL induce the p53-dependent decrease in cyclin-B1 levels. However, the induction of the negative cell cycle regulator p21WAF1 by FasL or Apo2L/TRAIL in either CD4+ or CD8+ T-cell blasts seems to be the main regulatory mechanism. This mechanism is dependent on caspase activation and on H2O2 generation. The increase in p21 levels by FasL or Apo2L/TRAIL is concomitant with p53 increases only in CD8+ T-cell blasts, with p21 levels maintained high for longer times than p53 levels. In CD4+ T-cell blasts p21 levels are controlled through a transient and p53-independent mechanism. The present results suggest that the etiology of ALP syndromes could be related not only to defects in apoptosis induction, but also in cell cycle regulation.

Diminished myelin-specific T cell activation associated with increase in CTLA4 and Fas molecules in multiple sclerosis patients treated with IFN-beta

Multiple sclerosis (MS) is a chronic inflammatory disease of the white matter of the central nervous system (CNS) characterized by focal areas of demyelination. Interferon-beta (IFN-beta) provides an effective treatment that lessens the frequency and severity of exacerbations in relapsing-remitting multiple sclerosis (RRMS), but the mechanisms by which IFN-beta is efficient remain uncertain. The data presented here demonstrate that IFN-beta impairs the proliferative response to myelin basic protein (MBP) and myelin, as well as increasing the expression of the CTLA4 intracellular molecule. Moreover, this treatment increases the expression of surface Fas molecules and of the soluble form of these molecules. Our hypothesis is that the increase in Fas and CTLA4 molecules in MS patients may lead to lymphocyte apoptosis, which suggests possible mechanisms underlying the therapeutic response to IFN-beta.

B cells in glomerulonephritis: focus on lupus nephritis

The production of pathogenic antibody has been traditionally viewed as the principle contribution of B cells to the pathogenesis of immune-mediated glomerulonephritis. However, it is increasingly appreciated that B cells play a much broader role in such diseases, functioning as antigen-presenting cells, regulators of T cells, dendritic cells, and macrophages and orchestrators of local lymphatic expansion. In this review, we provide an overview of basic B cell biology and consider the evidence implicating B cells in one of the archetypal immune-mediated glomerulonephritides, lupus nephritis.

The critical role of protein kinase C-theta in Fas/Fas ligand-mediated apoptosis

A functional immune system not only requires rapid expansion of antigenic specific T cells, but also requires efficient deletion of clonally expanded T cells to avoid accumulation of T cells. Fas/Fas ligand (FasL)-mediated apoptosis plays a critical role in the deletion of activated peripheral T cells, which is clearly demonstrated by superantigen-induced expansion and subsequent deletion of T cells. In this study, we show that in the absence of protein kinase C-theta (PKC-theta), superantigen (staphylococcal enterotoxin B)-induced deletion of Vbeta8(+) CD4(+) T cells was defective in PKC-theta(-/-) mice. In response to staphylococcal enterotoxin B challenge, up-regulation of FasL, but not Fas, was significantly reduced in PKC-theta(-/-) mice. PKC-theta is thus required for maximum up-regulation of FasL in vivo. We further show that stimulation of FasL expression depends on PKC-theta-mediated activation of NF-AT pathway. In addition, PKC-theta(-/-) T cells displayed resistance to Fas-mediated apoptosis as well as activation-induced cell death (AICD). In the absence of PKC-theta, Fas-induced activation of apoptotic molecules such as caspase-8, caspase-3, and Bid was not efficient. However, AICD as well as Fas-mediated apoptosis of PKC-theta(-/-) T cells were restored in the presence of high concentration of IL-2, a critical factor required for potentiating T cells for AICD. PKC-theta is thus required for promoting FasL expression and for potentiating Fas-mediated apoptosis.

Cloning and Functional Characterization of the Murine Caspase-3 Gene Promoter

Several studies have shown that the levels of caspase-3 are upregulated under different conditions of apoptosis. Previously, we have shown that activation of T cells through the TCR leads to the upregulation of caspase-3 levels. These findings highlight the importance of regulating the expression of caspase-3 in order to prevent premature cell death. To better understand the regulation of the caspase-3 gene, a portion of the 5'- untranslated region was cloned, sequenced, and characterized. The segment of the 5'-flanking region of the caspase-3 gene was also cloned upstream of a luciferase reporter gene, demonstrating that this fragment contains promoter activity. Higher luciferase expression was found with several of the promoter deletion constructs in Jurkat T cells but not the mouse Neuro-2A neuroblastoma cell line, suggesting the presence of a T-cell-specific regulated region. The importance of these sequences is further supported by the genomic organization of the human and mouse caspase-3 promoter regions. These findings demonstrated that the -2245/+14 region of the caspase-3 promoter shows constitutive levels of expression, and that several regions of the promoter play a role in basal regulation. Finally, some of the conserved transcription factor binding sites identified between the human and mouse promoters appear to play an important role in lymphoid cells.

T-lymphocyte death during shutdown of an immune response

The immune system serves to protect organisms from infectious pathogens. During infection, lymphocytes and cells of the innate immune system, expressing receptors that recognize foreign antigens, proliferate and differentiate to develop effector functions that help to kill the pathogens. Effector functions, such as cellular or antibody mediated cytotoxicity, and inflammatory cytokines can be harmful to the host. To limit damage to healthy tissue, mechanisms have evolved to shut down immune responses, including cell inactivation and cell death. Here we discuss recent studies demonstrating that the death of antigen-activated T lymphocytes during termination of an immune response is initiated by the BH3-only Bcl-2 family member Bim and also requires its multi-BH domain pro-apoptotic relatives Bax and Bak.

Selective up-regulation of caspase-3 gene expression following TCR engagement

Activation-induced cell death (AICD) in T lymphocytes depends on the expression of Fas-ligand, which triggers the apoptotic process after binding to its receptor Fas. This leads to the activation of cysteine proteases of the caspase family and especially of caspase-3, a critical effector protein during AICD. We have previously observed the up-regulation of caspase-3 expression in effector but not memory T cells stimulated in vivo. In this study, we further characterized the regulation of caspase expression following T cell receptor (TCR) signaling and demonstrate that a three-fold increase in caspase-3 mRNA levels was observed by semi-quantitative and real-time RT-PCR analysis. Caspase-3 expression was selectively increased among five different caspases following TCR stimulation, as assessed by RNase protection assay. Real-time RT-PCR analysis demonstrated that a three-fold up-regulation in caspase-3 mRNA levels was observed following TCR triggering, whereas caspase-8 mRNA levels remained unchanged. The increase in caspase-3 mRNA levels occurred before cleavage and activation of caspase-3 and in the absence of apoptosis. TCR-mediated induction in caspase-3 expression was not dependent on STAT1 activation, since following stimulation of KOX-14 cells the transcription factor was not phosphorylated. Together, these results show that TCR activation triggers the selective increase in caspase-3 mRNA levels, independently of caspase activity and the induction of apoptosis.

Activation-induced cell death in T cells

A properly functioning immune system is dependent on programmed cell death at virtually every stage of lymphocyte development and activity. This review addresses the phenomenon of activation-induced cell death (AICD) in T lymphocytes, in which activation through the T-cell receptor results in apoptosis. AICD can occur in a cell-autonomous manner and is influenced by the nature of the initial T-cell activation events. It plays essential roles in both central and peripheral deletion events involved in tolerance and homeostasis, although it is likely that different forms of AICD proceed via different mechanisms. For example, while AICD in peripheral T cells is often caused by the induction of expression of the death ligand, Fas ligand (CD95 ligand, FasL), it does not appear to be involved in AICD in thymocytes. This and other mechanisms of AICD are discussed. One emerging model that may complement other forms of AICD involves the inducible expression of FasL by nonlymphoid tissues in response to activated T lymphocytes. Induction of nonlymphoid FasL in this manner may serve as a sensing mechanism for immune cell infiltration, which contributes to peripheral deletion.

Responding naive T cells differ in their sensitivity to Fas engagement: early death of many T cells is compensated by costimulation of surviving T cells

Engagement of Fas (CD95) induces death of activated T cells but can also potentiate T-cell response to CD3 ligation. Yet, the effects of Fas-mediated signals on activation of naive T cells have remained controversial. We followed naive T cells responding under Fas ligation. Ligation of Fas simultaneously with activation by antigen-bearing dendritic cells promoted early death in half of the responding naive murine CD4 T cells. Surprisingly, it simultaneously accelerated cell division and interferon-gamma (IFN-gamma) production among surviving T cells. These cells developed quickly an activation-associated phenotype (CD44(hi), CD62L(lo)), responded vigorously to antigen rechallenge, were partially resistant to subsequent induction of cell death via Fas, and were long-lived in vivo. Compared with cells becoming apoptotic, the surviving cells expressed lower levels of Fas and higher levels of T-cell receptor (TCR), CD4, and interleukin-2 receptor (IL-2R). Their survival was associated with expression of antiapoptotic cellular FLICE-inhibitory protein (c-FLIP), Bcl-X(L), and Bcl-2. Thus, at the time of T-cell activation there is a subtle balance in the effects of Fas ligation that differs on a cell-to-cell basis. Factors that predict cell survival include expression levels of Fas, TCR, CD4, and IL-2R. Early death of some cells and a pronounced response of the surviving cells suggest that Fas ligation can both up- and down-regulate a primary T-cell response.

Expression of the long form of human FLIP by retroviral gene transfer of hemopoietic stem cells exacerbates experimental autoimmune encephalomyelitis

Subsidence of inflammation and clinical recovery in experimental autoimmune encephalomyelitis (EAE) is postulated to involve apoptosis of inflammatory cells. To test this concept, we examined the effects of overexpressing the long form of human FLICE-inhibitory protein, a potent inhibitor of death receptor-mediated apoptosis, in myelin oligodendrocyte glycoprotein-induced EAE in DBA/1 mice. We found that overexpression of the long form of human FLICE-inhibitory protein by retroviral gene transfer of hemopoietic stem cells led to a clinically more severe EAE in these mice compared with control mice receiving the retroviral vector alone. The exacerbated disease was evident by an enhanced and prolonged inflammatory reaction in the CNS of these animals compared with control mice. The acute phase of EAE was characterized by a massive infiltration of macrophages and granulocytes and a simultaneous increase in TNF-alpha production in the CNS. In the chronic phase of the disease, there was a prolonged inflammatory response in the form of persistent CD4(+) T and B cells in the CNS and a peripheral Th1 cytokine bias caused by elevated levels of IFN-gamma and reduced levels of IL-4 in the spleen. Our findings demonstrate that death receptor-mediated apoptosis can be important in the pathogenesis of EAE and further emphasize the need for effective apoptotic elimination of inflammatory cells to achieve disease remission.

NFATC2 transcription factor regulates cell cycle progression during lymphocyte activation: evidence of its involvement in the control of cyclin gene expression

Upon antigen stimulation, lymphocytes enter in cell cycle and proliferate, and most of the activated T cells die by apoptosis. Many of the proteins that regulate lymphocyte activation are Under the control of transcription factors belonging to the NFAT family. As previously demonstrated, NFATC2-/- mice consistently showed a marked increase in lymphocyte proliferation. Here, we evaluate the role of NFATC2 in regulating lymphocyte proliferation and its involvement in the control of cell cycle progression during lymphocyte activation. NFATC2-/- lymphocytes, including CD4+ T cells and B cells, hyperproliferated upon stimulation when compared with NFATC2+/+ cells. Analysis of cell death demonstrated that NFATC2-/- lymphocytes displayed an increased rate of apoptosis after antigen stimulation in addition to the hyperproliferation. Cell cycle analysis after antigen stimulation showed that NFATC2-/- cultures contained more cycling cells when compared with NFATC2+/+ cultures, which is related to a shortening in time of cell division upon activation. Furthermore, hyperproliferation of NFATC2-/- lymphocytes is correlated to an overexpression of cyclins A2, B1, E, and F. Taken together, our results suggest that the NFATC2 transcription factor plays an important role in the control of cell cycle during lymphocyte activation and may act as an inhibitor of cell proliferation in normal cells.

Role of the passive apoptotic pathway in graft-versus-host disease

Donor T cells have been shown to undergo apoptosis during graft-vs-host disease (GVHD). Although active apoptosis mediated through Fas/Fas ligand interactions has been implicated in GVHD, little is known about the role of the passive apoptotic pathway. To examine this question, we compared the ability of normal donor T cells and T cells overexpressing the antiapoptotic protein, Bcl-x(L), to mediate alloreactive responses in vitro and lethal GVHD in vivo. In standard MLCs, T cells that overexpressed Bcl-x(L) had significantly higher proliferative responses but no difference in cytokine phenotype. Overexpression of Bcl-x(L) prolonged survival of both resting and alloactivated CD4(+) and CD8(+) T cells as assessed by quantitative flow cytometry, accounting for the higher proliferative responses. Analysis of engraftment in murine transplantation experiments demonstrated an increase in donor T cell chimerism in animals transplanted with Bcl-x(L) T cells, suggesting that overexpression of Bcl-x(L) prolonged T cell survival in vivo as well. Notably, transplantation of Bcl-x(L) T cells into nonirradiated F(1) recipients also significantly exacerbated GVHD as assessed by mortality and pathological damage in the gastrointestinal tract. However, when mice were irradiated no difference in GVHD mortality was observed between animals transplanted with wild-type and Bcl-x(L) T cells. These data demonstrate that the passive apoptotic pathway plays a role in the homeostatic survival of transplanted donor T cells. Moreover, the susceptibility of donor T cells to undergo passive apoptosis is a significant factor in determining GVHD severity under noninflammatory but not inflammatory conditions.

Lymphocyte-mediated cytotoxicity

Virtually all of the measurable cell-mediated cytotoxicity delivered by cytotoxic T lymphocytes and natural killer cells comes from either the granule exocytosis pathway or the Fas pathway. The granule exocytosis pathway utilizes perforin to traffic the granzymes to appropriate locations in target cells, where they cleave critical substrates that initiate DNA fragmentation and apoptosis; granzymes A and B induce death via alternate, nonoverlapping pathways. The Fas/FasL system is responsible for activation-induced cell death but also plays an important role in lymphocyte-mediated killing under certain circumstances. The interplay between these two cytotoxic systems provides opportunities for therapeutic interventions to control autoimmune diseases and graft vs. host disease, but oversuppression of these pathways may also lead to increased viral susceptibility and/or decreased tumor cell killing.

Sp1 is the major fasl gene activator in abnormal CD4(-)CD8(-)B220(+) T cells of lpr and gld mice

The abnormal CD4(-)CD8(-)TCRalpha beta(+)B220(+) double-negative (DN) T cells that accumulate in lpr and gld mice are refractory to TCR cross-linking and IL-2 stimulation, yet they have an activated phenotype and express a high level of fasl mRNA. Specific binding sites for Sp1, NFAT, Egr, and NF-kappaB have been identified in the promoter region of the fasl gene. To determine the critical factor for fasl gene activation, fasl promoter reporter and mutant constructs were transiently transfected into the abnormal DN T cells. The data demonstrate that the Sp1 binding site is the major response element that regulates fasl promoter activity. Moreover, the abnormal DN T cells contain in their nuclei a high level of Sp1, a low level of NFAT and NF-kappaB, and a very low level of Egr. Ectopic expression of Egr-3 but not Sp1 protein in the abnormal DN T cells enhanced fasl promoter activity, suggesting that the Egr but not Sp1 was limiting for fasl gene activation. Comparison between the abnormal DN T cells and the Sertoli TM4 cells showed a strong correlation between Sp1 expression and fasl mRNA level and FasL function. Our study has identified Sp1 as the major transcription factor responsible for fasl gene activation in the abnormal DN T cells that are defective in signal transduction through TCR and IL-2R, thereby, implicating a novel regulatory pathway for fasl gene activation during the physiological development and elimination of the abnormal DN T cells.

Inhibition of NF-kappaB in T cells blocks lymphoproliferation and partially rescues autoimmune disease in gld/gld mice

The Fas ligand (FasL)/Fas pathway is crucial for the maintenance of homeostasis of the peripheral immune system. Its importance is illustrated by the spontaneous mouse mutants gld andlpr which lack functional FasL and Fas receptor, respectively. These animals develop lymphadenopathy, splenomegaly, increased serum Ig and autoantibodies, leading to an autoimmune syndromeand premature death. The Rel/NF-kappaB family of transcription factors plays an important role in peripheral lymphocyte proliferation and survival. In this report, we studied the consequences of T cell-specific inhibition of NF-kappaB on the development of the gld phenotype. Transgenic gld/gld mice expressing a non-degradable form of IkappaBalpha under the control of T cell-specific regulatory elements show dramatically reduced lymphadenopathy, splenomegaly, and an almost complete elimination of Thy-1(+)B220(+)CD4(-)CD8(-) abnormal T cells, correlating with reduced proliferative responses and increased apoptosis of peripheral T cells upon TCR triggering. Interestingly, the B cell abnormalities that are characteristic of gld/gld mice, such as the production of autoantibodies, high levels of serum Ig, and the development of glomerulonephritis, are partially corrected. These results suggest that the T cell-specific inhibition of NF-kappaB opens apoptotic pathways distinct from FasL/Fas which, along with a diminished proliferative response, blocks splenomegaly and lymphadenopathy and partially rescues autoimmune disease in gld/gld mice.

Identification of Fas-L-expressing apoptotic T lymphocytes in normal human peripheral blood : in vivo suicide

Fas-L molecules expressed by in vitro stimulated T cells may be critically involved in suicidal activation-induced cell death (AICD) of such cells through engagement of their Fas receptors. A similar suicide of T cells was postulated to occur even in vivo, to eliminate dangerous activated lymphocytes; however, the demonstration of suicidal AICD of T cells in healthy humans in vivo is still lacking. We therefore investigated the possible occurrence of Fas-L-linked suicidal apoptosis of T cells in normal human peripheral blood. For this purpose, we took advantage of immunoelectron microscopy, which allows simultaneous visualization of the morphological apoptotic cellular changes together with surface expression of Fas-L molecules. Very few T lymphocytes were observed showing the ultrastructural features of apoptotic lymphocytes; these occasional apoptotic T cells, together with the majority of the normal T cell population, expressed the Fas molecule on the plasma membrane, as expected. Interestingly, the apoptotic cells were also Fas-L-positive, whereas normal T cells were Fas-L-negative. Such Fas-L-associated T cell suicide operating in vivo in healthy individuals is presumably able to suppress immune responses and prevent autoreactivity, thus maintaining the homeostasis of human blood.

Activation-induced T cell death occurs at G1A phase of the cell cycle

Peripheral negative selection of cycling T cells after TCR engagement and deletion of activated T cells after an immune response occur by an apoptotic process termed activation-induced cell death (AICD). The cross-linking of TCR-CD3 complex with anti-CD3 monoclonal antibody led to significant apoptotic cell death in peripheral blood T cells. To further define cell cycle restriction points for triggering AICD in T cells, we evaluated the association between cell cycle progression and death signal transduction. Simultaneous DNA / RNA quantification analysis revealed that T cells entering G1A phase of the cell cycle may acquire sensitivity to AICD. The activation of caspase-3 was induced when T cells entered G1A phase. Up-regulation of cyclin-dependent kinases (Cdk4 and Cdk6) and cyclin D3 was initiated in TCR-stimulated T cells entering G1A phase and expression of these markers steadily increased as T cells progressed from G1A into G1B phase. Interestingly, caspase-3 inhibitors could inhibit the up-regulation of these G1 cell cycle regulators and induce G0 / G1A arrest as well as the inhibition of AICD. On the basis of these results, AICD signals are most likely transduced into TCR-stimulated T cells entering G1A phase. T cells that fail to progress from G1A into G1B phase undergo AICD.

Alloantigen-driven T cell death mediated by Fas ligand and tumor necrosis factor-alpha is not essential for the induction of allograft acceptance

BACKGROUND

Fas ligand (FasL)-Fas and tumor necrosis factor alpha (TNFalpha)-tumor necrosis factor receptor (TNFR) interactions regulate immune responses and contribute to self-tolerance by mediating antigen-driven T cell apoptosis. It is not known whether FasL and TNFalpha, expressed by the recipient's lymphoid or nonlymphoid cells, are essential for the apoptosis of alloreactive T lymphocytes and the induction of allograft acceptance.

METHODS

We compared the survival of fully allogeneic vascularized cardiac allografts between wild-type (wt) and FasL-mutant (gld) recipient mice. In addition, we studied cardiac allograft survival in gld mice injected with TNFalpha-neutralizing antibody. Allograft acceptance (graft survival >100 days) was induced by treating the recipients with CTLA4Ig, a recombinant fusion protein that blocks B7-CD28 T cell costimulation. In vivo alloantigen-driven apoptosis of mature CD4+ and CD8+ T lymphocytes was analyzed in mice repeatedly stimulated with allogeneic splenocytes.

RESULTS

We found that CTLA4Ig induces 100% long-term acceptance of cardiac allografts in wt and gld mice. Similarly, CTLA4Ig induced 100% allograft acceptance in gld recipients injected with TNFalpha-neutralizing antibody. In vivo alloantigen-driven apoptosis of mature CD4+ and CD8+ T cells was significantly reduced in gld mice and in wt mice treated with anti-TNFalpha antibody. However, neutralizing TNFalpha activity in gld mice failed to abrogate alloantigen-driven T cell apoptosis.

CONCLUSIONS

These data indicate that: (1) FasL and TNFalpha expression are not obligatory for the induction of long-term allograft acceptance by CTLA4Ig and (2) FasL- and TNFalpha-independent death pathways contribute to alloantigen-driven T cell apoptosis.

Rapid B cell apoptosis induced by antigen receptor ligation does not require Fas (CD95/APO-1), the adaptor protein FADD/MORT1 or CrmA-sensitive caspases but is defective in both MRL-+/+ and MRL-lpr/lpr mice

Antigen receptor ligation-induced apoptosis is thought to play a role in self-tolerance by deleting autoreactive lymphocytes. Antigen receptor ligation-induced apoptosis of mature T cells and T cell lines requires autocrine or paracrine activation of Fas (CD95/APO-1). Whether B cell antigen receptor (BCR)-mediated apoptosis requires Fas or related molecules is unclear. Here we demonstrate that expression of either CrmA, the cowpox virus serpin, or an inhibitor of the adapter protein FADD/MORT1 blocks Fas-mediated apoptosis but has no effect on BCR ligation-induced apoptosis of the B cell line WEHI-231. In contrast, expression of Bcl-2 blocks BCR-mediated but not Fas-induced apoptosis in WEHI-231 cells. These results indicate that BCR ligation activates an apoptotic signaling pathway distinct from Fas-mediated apoptosis in WEHI-231 cells, and that BCR-mediated apoptosis of WEHI-231 cells does not require Fas or related molecules such as DR3, DR4 and DR5, as all of these death receptors require FADD/MORT1 and/or CrmA-sensitive caspases for induction of apoptosis. Moreover, extensive BCR ligation induces death of mature B cells from C57BL/6-lpr/lpr mice as efficiently as those from C57BL/6 mice, indicating that Fas is not essential for BCR-mediated apoptosis of mature B cells. In contrast, BCR ligation-induced apoptosis is reduced in mature B cells from MRL mice and this is not affected by the lpr mutation. Since MRL-lpr/lpr mice but not C57BL/6-lpr/lpr mice develop severe autoimmune disease, defects in BCR-mediated apoptosis in the MRL background, together with lpr mutation, may contribute to the development of severe autoimmune disease in MRL-lpr/lpr mice by allowing survival of self-reactive B cells.

Fas ligand-induced apoptosis

The immune response is regulated not only by cell proliferation and differentiation, but also by programmed cell death, or apoptosis. In response to various stimuli, death factors bind to their respective receptors and activate the apoptotic death program in target cells. A cascade of specific proteases termed caspases mediates the apoptotic process. The activated caspases cleave various cellular components, a process that leads to morphological changes of the cells and nuclei, as well as to degradation of the chromosomal DNA. Loss-of-function mutations in the signaling molecules involved in apoptosis cause hyper-proliferation of cells in mouse and human. In contrast, exaggeration of this death cascade causes the destruction of various tissues.

Molecular and cellular mechanisms regulating T and B cell apoptosis through Fas/FasL interaction

Fas (CD95) and Fas ligand (FasL) are a receptor/ligand pair critically involved in lymphocyte homeostasis and peripheral tolerance such that genetic defect in either Fas or FasL results in an autoimmune lymphoproliferative syndrome. Fas is a type I transmembrane protein and a member of the tumor necrosis factor receptor (TNFR) family whereas FasL is a type II transmembrane protein and a member of TNF family. Binding of Fas by FasL induces apoptosis of the Fas-expressing cells. In the past few years, Fas/FasL interaction has been connected to a series of important phenomena previously viewed as independent immune processes. The activation-induced T cell death (AICD) and the FasL-mediated cytotoxicity by activated T cells are two critical mechanisms that can account for most of these phenomena. It is in the context of the two mechanisms that we discuss in this review the molecular and cellular events that occur during T/T and T/B interactions that account for the down-regulation of the immune response. We have also discussed recent advances in the areas of FasL gene regulation, lymphokine regulation of AICD, and regulation of B cell susceptibility to FasL. Investigation in these areas should help elucidate the role of Fas/FasL in the complex network of regulatory mechanisms that control immune response and autoimmunity.

The dual functions of fas ligand in the regulation of peripheral CD8+ and CD4+ T cells

Although Fas ligand (FasL) is well characterized for its capacity to deliver a death signal through its receptor Fas, recent work demonstrates that FasL also can receive signals facilitating antigen (Ag)-specific proliferation of CD8(+) T cells. The fact that the gld mutation differentially influences the proliferative capacity of CD8(+) and CD4(+) T cells presented the intriguing possibility that a single molecule may play opposing roles in these two subpopulations. The present study focuses on how these positive and negative regulatory roles are balanced. We show that naive CD4(+) T cells are responsive to FasL-mediated costimulation on encounter with Ag when Fas-mediated death is prevented. Thus, the machinery responsible for transducing the FasL positive reverse signal operates in both CD4(+) and CD8(+) T cells. Instead, differential control of FasL expression distinguishes the role of FasL in these two T cell subpopulations. FasL costimulation occurs immediately on T cell receptor ligation and correlates with the up-regulation of FasL expression on CD8(+) and naive CD4(+) T cells, both of which are sensitive to the FasL costimulatory signal. Conversely, FasL-initiated death occurs late in an immune response when high levels of FasL expression are maintained on CD4(+) T cells that are sensitive to Fas-mediated death, but not on CD8(+) T cells that are relatively insensitive to this signal. This careful orchestration of FasL expression during times of susceptibility to costimulation and conversely, to death, endows FasL with the capacity to both positively and negatively regulate the peripheral T cell compartment.

Vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide inhibit antigen-induced apoptosis of mature T lymphocytes by inhibiting Fas ligand expression

Apoptosis in T and B lymphocytes is a major element controlling the immune response. The Ag-induced cell death (AICD) in T cells is a main mechanism for maintaining peripheral tolerance and for limiting an ongoing immune response. AICD is initiated by Ag re-engagement of the TCR and is mediated through Fas/Fas ligand (FasL) interactions. Vasoactive intestinal peptide (VIP) and the structurally related pituitary adenylate cyclase-activating polypeptide (PACAP) are two multifunctional neuropeptides present in the lymphoid microenvironment that act primarily as anti-inflammatory agents. In the present study we investigated whether VIP and PACAP affect AICD in mature peripheral T cells and T cell hybridomas. VIP and PACAP reduce in a dose-dependent manner anti-CD3-induced apoptosis in Con A/IL-2-preactivated peripheral T cells and the murine T hybridomas 2B4.11 and A1.1. A functional study demonstrates that the inhibition of AICD is achieved through the inhibition of activation-induced FasL expression at protein and mRNA levels. VIP/PACAP-mediated inhibition of both AICD and FasL expression is mediated through the specific receptors VPAC1 and VPAC2. Of obvious biological significance is the fact that VIP and PACAP prevent Ag-induced clonal deletion of CD4+ T cells, but not that of CD8+ T cells. By affecting FasL expression, VIP and PACAP may play a physiological role in both the generation of memory T cells and the inhibition of FasL-mediated T cell cytotoxicity.

FasL promoter activation by IL-2 through SP1 and NFAT but not Egr-2 and Egr-3

Recently activated peripheral T cells treated with IL-2 for 4 days expressed Fas ligand (FasL)-mediated cytotoxicity. These IL-2-treated T cells had high nuclear expression of SP1 and NFAT, but lacked the Egr-2 and Egr-3 that could be induced by anti-CD3 stimulation and had been implicated in FasL gene activation. A minimal promoter region that responded to IL-2 was identified by transient transfection assays using deletion mutants. The data suggests that the GGGCGGAAA site present in the 5' end of the minimal FasL promoter is critical to IL-2-induced FasL gene activation. The GGGCGGAAA sequence contains an overlapping site used by two transcription factor families, one (GGGCGG) for the SP1 family and the other (GGAAA) for the NFAT family. FasL promoter activity was partially but statistically significantly reduced with constructs mutated at either site. More activity was lost with a construct mutated at both sites. In contrast, mutation at the Egr site had no effect on IL-2-induced FasL promoter activity. Our study identified a new FasL promoter site responding to IL-2-induced SP1 and NFAT factors. Furthermore, the nuclei of IL-2-treated cells express SP1 and NFAT, but not Egr-2 and Egr-3, for FasL gene activation.

Homeostatic regulation of CD8+ T cells by perforin

To prevent uncontrolled expansion, the massive proliferation of T cells during an acute immune response has to be followed by controlled deletion. Here we show that similar to Fas, perforin is not only an important effector molecule of cytotoxic T lymphocytes (CTL) but also involved in down-regulating peripheral T cells. Mice deficient for both the CTL effector molecule perforin and the apoptosis-inducing Fas ligand spontaneously develop infiltration of highly activated CD8(+) T cells in kidney and liver and die between 5 and 12 weeks of age. Injection of staphylococcal enterotoxin B (SEB) into perforin-deficient mice results in dramatically increased selective expansion and prolonged persistence of CD8(+), but not CD4(+), SEB-reactive T cells. Also, secondary immunization of TCR transgenic perforin-deficient mice with the lymphocytic choriomeningitis virus glycoprotein-derived epitope peptide leads to an increased proliferation of transgenic CD8(+) T cells, that is not explained by failure to deplete professional antigen-presenting cells. These results point to a novel mechanism of T cell homeostasis in which the acquisition of perforin-dependent cytotoxic activity regulates the expansion and persistence of CD8(+) effector T cells in vivo.

THP-1 monocytic leukemia cells express Fas ligand constitutively and kill Fas-positive Jurkat cells

Normal human monocytes express Fas and are susceptible to Fas ligand (FasL)-induced apoptosis. Because the myeloid leukemia cell lines HL-60 and THP-1 can be differentiated into functional monocytes and macrophages, we studied their expression of Fas and Fas ligand (FasL) to determine whether there were differentiation-associated changes in these proteins. THP-1, HL-60 and HCW-2, both before and after treatment with PMA, expressed high levels of Fas ligand (FasL), but did not express Fas. The FasL expressed by THP-1 cells was functional as measured by their ability to kill Jurkat T-cells by apoptosis. The THP-1 Fas gene appears to be silent, because bacterial lipopolysaccharide (LPS) induced Fas expression in fully differentiated THP-1 cells. Our results suggest that FasL expression by leukemia cells may account in part for the pathophysiology of myeloid leukemia, and that PMA-differentiated THP-1 cells, while possessing many of the functional properties of normal macrophages, are abnormal with respect to a major apoptotic pathway.

Inherited human Caspase 10 mutations underlie defective lymphocyte and dendritic cell apoptosis in autoimmune lymphoproliferative syndrome type II

Caspases are cysteine proteases that mediate programmed cell death in phylogenetically diverse multicellular organisms. We report here two kindreds with autoimmune lymphoproliferative syndrome (ALPS) type II, characterized by abnormal lymphocyte and dendritic cell homeostasis and immune regulatory defects, that harbor independent missense mutations in Caspase 10. These encode amino acid substitutions that decrease caspase activity and interfere with death receptor-induced apoptosis, particularly that stimulated by Fas ligand and TRAIL. These results provide evidence that inherited nonlethal caspase abnormalities cause pleiotropic apoptosis defects underlying autoimmunity in ALPS type II.

Mature T lymphocyte apoptosis--immune regulation in a dynamic and unpredictable antigenic environment

Apoptosis of mature T lymphocytes preserves peripheral homeostasis and tolerance by countering the profound changes in the number and types of T cells stimulated by diverse antigens. T cell apoptosis occurs in at least two major forms: antigen-driven and lymphokine withdrawal. These forms of death are controlled in response to local levels of IL-2 and antigen in a feedback mechanism termed propriocidal regulation. Active antigen-driven death is mediated by the expression of death cytokines such as FasL and TNF. These death cytokines engage specific receptors that assemble caspase-activating protein complexes. These signaling complexes tightly regulate cell death but are vulnerable to inherited defects. Passive lymphokine withdrawal death may result from the cytoplasmic activation of caspases that is regulated by mitochondria and the Bcl-2 protein. The human disease, Autoimmune Lymphoproliferative Syndrome (ALPS) is due to dominant-interfering mutations in the Fas/APO-1/CD95 receptor and other components of the death pathway. The study of ALPS patients reveals the necessity of apoptosis for preventing autoimmunity and allows the genetic investigation of apoptosis in humans. Immunological, cellular, and molecular evidence indicates that throughout the life of a T cell, apoptosis may be evoked in excessive, harmful, or useless clonotypes to preserve a healthy and balanced immune system.

Galectin-1, an alternative signal for T cell death, is increased in activated macrophages

Galectin-1 belongs to an evolutionarily conserved family of animal beta-galactoside-binding proteins, which exert their functions by crosslinking the oligosaccharides of specific glycoconjugate ligands. During the past decade, attempts to identify the functional role of galectin-1 suggested participation in the regulation of the immune response. Only in the last few years has the molecular mechanism involved in these properties been clearly elucidated, revealing a critical role for galectin-1 as an alternative signal in the generation of T cell death. In the present study we will discuss the latest advances in galectin research in the context of the regulation of the immune response, not only at the central level but also at the periphery. Moreover, we will review the purification, biochemical properties and functional significance of a novel galectin-1-like protein from activated rat macrophages, whose expression is differentially regulated according to the activation state of the cells. The novel role of a carbohydrate-binding protein in the regulation of apoptosis is providing a breakthrough in galectin research and extending the interface between immunology, glycobiology and clinical medicine.

Role of mitogen-activated protein kinases in activation-induced apoptosis of T cells

A member of the mitogen-activated protein (MAP) kinase family, Jun N-terminal kinase (JNK), has been implicated in regulating apoptosis in various cell types. We have investigated the requirement for another type of MAP kinase, extracellular signal-regulated protein kinase (ERK) in activation-induced cell death (AICD) of T cells. AICD is the process by which recently activated T cells undergo apoptosis when restimulated through the T-cell antigen receptor. Here we show that both JNK and ERK are activated rapidly upon T-cell receptor (TCR) ligation prior to the onset of AICD. A chemical inhibitor of ERK activation, PD 098059, inhibits ERK activation and apoptosis, while JNK activation is not inhibited. This suggests that JNK activation is not sufficient for apoptosis. TCR cross-linking induces expression of the apoptosis-inducing factor, Fas ligand (FasL), and its expression correlates with ERK activation. In addition, apoptosis induced by direct ligation of the Fas receptor by anti-Fas antibody is not associated with ERK activation and is not inhibited by PD 098059. These data suggest that ERK activation is an early event during T-cell apoptosis induced by antigen-receptor ligation, and is not involved in apoptosis per se but in the expression of FasL. MAP kinase family members may be similarly involved in inducing apoptosis signals in other cell types.

Fas and Fas-ligand expression in seminomatous testes

Sixteen seminomas with surrounding tissue containing normal and precancerous (cis) seminiferous tubules were examined for the expression of Fas (CD95, APO-1) and Fas ligand (FasL) (CD95L). This was done by analyzing frozen specimens using immunohistochemistry with antibodies directed against Fas and FasL. The study showed that varying numbers (mean approx. 20%) of Fas-positive lymphocytes were present among tumor-infiltrating lymphocytes, but very few FasL-positive lymphocytes. Fas was not expressed by normal seminiferous tubules and only occasional Fas-positive epithelial cells were seen in cis tubules. FasL was expressed in 9 out of 10 cases in virtually all normal seminiferous tubules, mainly as a thin layer at the base of the seminiferous epithelium. In precancerous tubules, this layer was discontinuous and less pronounced. Rete testis expressed FasL in 2 out of 2 cases with rete present and Fas in 1 out of 1 case. Invasive tumor cells did not express Fas or FasL. The data are discussed in relation to immune reactions to seminomas and to the concept of the testis being an immunologically privileged area.

Biology of the interleukin-2 receptor

Studies of the biology of the IL-2 receptor have played a major part in establishing several of the fundamental principles that govern our current understanding of immunology. Chief among these is the contribution made by lymphokines to regulation of the interactions among vast numbers of lymphocytes, comprising a number of functionally distinct lineages. These soluble mediators likely act locally, within the context of the microanatomic organization of the primary and secondary lymphoid organs, where, in combination with signals generated by direct membrane-membrane interactions, a wide spectrum of cell fate decisions is influenced. The properties of IL-2 as a T-cell growth factor spawned the view that IL-2 worked in vivo to promote clonal T-cell expansion during immune responses. Over time, this singular view has suffered from increasing appreciation that the biologic effects of IL-2R signals are much more complex than simply mediating T-cell growth: depending on the set of conditions, IL-2R signals may also promote cell survival, effector function, and apoptosis. These sometimes contradictory effects underscore the fact that a diversity of intracellular signaling pathways are potentially activated by IL-2R. Furthermore, cell fate decisions are based on the integration of multiple signals received by a lymphocyte from the environment; IL-2R signals can thus be regarded as one input to this integration process. In part because IL-2 was first identified as a T-cell growth factor, the major focus of investigation in IL-R2 signaling has been on the mechanism of mitogenic effects in cultured cell lines. Three critical events have been identified in the generation of the IL-2R signal for cell cycle progression, including heterodimerization of the cytoplasmic domains of the IL-2R beta and gamma(c) chains, activation of the tyrosine kinase Jak3, and phosphorylation of tyrosine residues on the IL-2R beta chain. These proximal events led to the creation of an activated receptor complex, to which various cytoplasmic signaling molecules are recruited and become substrates for regulatory enzymes (especially tyrosine kinases) that are associated with the receptor. One intriguing outcome of the IL-2R signaling studies performed in cell lines is the apparent functional redundancy of the A and H regions of IL-2R beta, and their corresponding downstream pathways, with respect to the proliferative response. Why should the receptor complex induce cell proliferation through more than one mechanism or pathway? One possibility is that this redundancy is an unusual property of cultured cell lines and that primary lymphocytes require signals from both the A and the H regions of IL-2R beta for optimal proliferative responses in vivo. An alternative possibility is that the A and H regions of IL-2R beta are only redundant with respect to proliferation and that each region plays a unique and essential role in regulating other aspects of lymphocyte physiology. As examples, the A or H region could prove to be important for regulating the sensitivity of lymphocytes to AICD or for promoting the development of NK cells. These issues may be resolved by reconstituting IL-2R beta-/-mice with A-and H-deleted forms of the receptor chain and analyzing the effect on lymphocyte development and function in vivo. In addition to the redundant nature of the A and H regions, there remains a large number of biochemical activities mediated by the IL-2R for which no clear physiological role has been identified. Therefore, the circumstances are ripe for discovering new connections between molecular signaling events activated by the IL-2R and the regulation of immune physiology. Translating biochemical studies of Il-2R function into an understanding of how these signals regulate the immune system has been facilitated by the identification of natural mutations in IL-2R components in humans with immunodeficiency and by the generation of mice with targeted mutations in these gen

Apoptotic regulation of T cells and absence of immune deficiency in virus-infected gamma interferon receptor knockout mice

Acute viral infections often induce a transient period of immune deficiency in which the host's T cells fail to proliferate in response to T-cell mitogens and fail to make an antigen-specific memory recall response. This has been associated with the enhanced sensitivity of these highly activated T cells to undergo apoptosis, or activation-induced cell death (AICD), upon T-cell receptor ligation. Here we show that gamma interferon receptor-deficient (IFN-gamma R-/-) mice mount a T-cell response to lymphocytic choriomeningitis virus (LCMV) infection but fail to undergo the transient immune deficiency. Instead, their T cells were hyperproliferative and relatively, but not completely, resistant to AICD. The immune response returned to homeostasis, but with delayed kinetics, in parallel with delayed clearance of the virus. Wild-type mice receiving high doses of disseminating LCMV Clone 13 are known to undergo clonal exhaustion of their virus-specific cytotoxic T lymphocytes (CTL). To determine whether this process was mediated by AICD associated with IFN-gamma or with Fas-Fas ligand interactions, LCMV-specific precursor CTL frequencies were examined in LCMV Clone 13-infected IFN-gamma R-/- or lpr (Fas-deficient) mice. In both instances, viral persistence was established and CTL precursors were greatly eliminated. This finding indicates that clonal exhaustion of CTL does not require IFN-gamma or Fas, even though both molecules influence AICD and the transient immune deficiency seen in the LCMV infection.

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-α, because none of these cell populations had detectable TNF-α production.

Involvement of APO2 ligand/TRAIL in activation-induced death of Jurkat and human peripheral blood T cells

The interaction of Fas with Fas ligand (FasL) mediates activation-induced cell death (AICD) of T hybridomas and of mature T lymphocytes. The TNF/TNF receptor system also plays a significant role in AICD of mature T cells and in the maintenance of peripheral tolerance. We previously demonstrated that in human Jurkat leukemia cells, AICD is triggered mainly by the rapid release of preformed FasL upon TCR stimulation. In the present work, we show that the cytotoxic cytokine APO2 ligand (APO2L; also known as TRAIL) is constitutively expressed as an intracytoplasmic protein in Jurkat T cells and derived sublines. APO2L is also detected in fresh human peripheral blood mononuclear cells (PBMC) from a significant number of donors, and the amount of both FasL and APO2L substantially increases upon blast generation. A neutralizing anti-APO2L monoclonal antibody (mAb) partially suppresses the cytotoxicity induced by supernatants of phytohemagglutinin (PHA)-prestimulated Jurkat or human PBMC on non-activated Jurkat cells, indicating that APO2L is released by these cells and contributes to AICD. A combination of neutralizing anti-APO2L and anti-Fas mAb blocks around 60 % of the toxicity associated with supernatants from PHA-activated human PBMC. These results show that FasL and APO2L account for the majority of cytotoxic activity released during AICD, and suggest that additional uncharacterized factors may also contribute to this process.