Molecular and cellular mechanisms of T lymphocyte apoptosis

Cochinchina momordica seed extract induces apoptosis and cell cycle arrest in human gastric cancer cells via PARP and p53 signal pathways

Cochinchina momordica seed is the dried ripe seed of Momordica cochinchinensis (Lour.) Spreng, which is a kind of fruit and consumed for dietary as well as medicinal uses. In this study, using the human SGC7901 and MKN-28 gastric cancer cell lines, we explored the anticancer activity of the extract from cochinchina momordica seed (ECMS). ECMS inhibited significantly the survival rates of SGC7901 and MKN-28 cells in concentration- and time-dependent manners by MTT assay. The typical apoptotic morphological changes were observed by Hoechst 33258 dye assay after SGC7901 and MKN-28 cells were treated with ECMS for 48 h. Flow cytometry analysis revealed that ECMS-treatment blocked the cells at the S phase of cell cycle. Furthermore, the protein expression levels of poly (ADP-ribose) polymerase (PARP) and Bcl-2 were downregulated notably by ECMS-treatment, whereas those of Fas/Fas-associated death domain, p53, and Bax were upregulated in SGC7901 cells. ECMS dramatically enhanced the enzymatic activities of caspase-3 and caspase-9 whilst slightly increased caspase-8 activity. Taken together, this study demonstrated that ECMS exerted cytotoxic activities via PARP and p53 signal pathways in the human gastric cancer cells.

Tespa1 is involved in late thymocyte development through the regulation of TCR-mediated signaling

Signaling via the T cell antigen receptor (TCR) during the CD4(+)CD8(+) double-positive developmental stage determines thymocyte selection and lineage commitment. Here we describe a previously uncharacterized T cell-expressed protein, Tespa1, with critical functions during the positive selection of thymocytes. Tespa1(-/-) mice had fewer mature thymic CD4(+) and CD8(+) T cells, which reflected impaired thymocyte development. Tespa1 associated with the TCR signaling components PLC-γ1 and Grb2, and Tespa1 deficiency resulted in attenuated TCR signaling, as reflected by defective activation of the Erk-AP-1 and Ca(2+)-NFAT pathways. Our findings demonstrate that Tespa1 is a component of the TCR signalosome and is essential for T cell selection and maturation through the regulation of TCR signaling during T cell development.

Expression of apoptosis regulatory markers in the skin of advanced hepatitis-C virus liver patients

BACKGROUND

Hepatitis-C virus (HCV) infection is considered a major worldwide public health problem with a global prevalence. Maintenance of skin homeostasis requires a delicate balance between proliferation, differentiation, and apoptosis. Meanwhile, it is unclear if there is an altered keratinocyte proliferation/apoptosis balance in advanced liver disease with HCV infection.

AIM

This work aimed to evaluate the epidermal thickness and changes in the expression of apoptosis regulatory markers as well as apoptotic index in skin samples of advanced HCV liver patients compared to normal controls.

MATERIALS AND METHODS

Twenty biopsies were taken from apparently normal skin of advanced HCV liver disease patients, as well as five healthy control subjects. These specimens were used for histometric epidermal measurement, immunohistochemical staining of apoptosis regulatory proteins (Bax, Fas, p53, Caspase-3, Bcl-2, Bcl-xL) as well as the TUNEL technique for detection of apoptotic cells.

RESULTS

The mean epidermal thickness was significantly lower than the control group (P=0.000). There were significant overexpression of pro-apoptotic markers (Bax, Fas, P53, and Caspase-3) in patients (P=0.03, 0.03, 0.003, 0.003 respectively), with increased apoptotic index in HCV liver patients (P=0.002) when compared to normal controls. On the other hand, no statistically significant difference were encountered in the expression of antiapoptotic markers (Bcl-2, Bcl-xL) in HCV patients when compared to normal controls (P=0.5, 0.9, respectively).

CONCLUSION

These findings suggest that an alteration in the proliferation/apoptosis balance is present in the skin of HCV liver patients.

Microarray analysis of spaceflown murine thymus tissue reveals changes in gene expression regulating stress and glucocorticoid receptors

The detrimental effects of spaceflight and simulated microgravity on the immune system have been extensively documented. We report here microarray gene expression analysis, in concert with quantitative RT-PCR, in young adult C57BL/6NTac mice at 8 weeks of age after exposure to spaceflight aboard the space shuttle (STS-118) for a period of 13 days. Upon conclusion of the mission, thymus lobes were extracted from space flown mice (FLT) as well as age- and sex-matched ground control mice similarly housed in animal enclosure modules (AEM). mRNA was extracted and an automated array analysis for gene expression was performed. Examination of the microarray data revealed 970 individual probes that had a 1.5-fold or greater change. When these data were averaged (n = 4), we identified 12 genes that were significantly up- or down-regulated by at least 1.5-fold after spaceflight (P < or = 0.05). The genes that significantly differed from the AEM controls and that were also confirmed via QRT-PCR were as follows: Rbm3 (up-regulated) and Hsph110, Hsp90aa1, Cxcl10, Stip1, Fkbp4 (down-regulated). QRT-PCR confirmed the microarray results and demonstrated additional gene expression alteration in other T cell related genes, including: Ctla-4, IFN-alpha2a (up-regulated) and CD44 (down-regulated). Together, these data demonstrate that spaceflight induces significant changes in the thymic mRNA expression of genes that regulate stress, glucocorticoid receptor metabolism, and T cell signaling activity. These data explain, in part, the reported systemic compromise of the immune system after exposure to the microgravity of space.

Familial NK cell deficiency associated with impaired IL-2- and IL-15-dependent survival of lymphocytes

We previously reported the clinical phenotype of two siblings with a novel inherited developmental and immunodeficiency syndrome consisting of severe intrauterine growth retardation and the impaired development of specific lymphoid lineages, including transient CD8 alphabeta T lymphopenia and a persistent lack of blood NK cells. We describe here the elucidation of a plausible underlying pathogenic mechanism, with a cellular phenotype of impaired survival of both fresh and herpesvirus saimiri-transformed T cells, in the surviving child. Clearly, NK cells could not be studied. However, peripheral blood T lymphocytes displayed excessive apoptosis ex vivo. Moreover, the survival rates of CD4 and CD8 alphabeta T cell blasts generated in vitro, and herpesvirus saimiri-transformed T cells cultured in vitro, were low, but not nil, following treatment with IL-2 and IL-15. In contrast, Fas-mediated activation-induced cell death was not enhanced, indicating a selective excess of cytokine deprivation-mediated apoptosis. In keeping with the known roles of IL-2 and IL-15 in the development of NK and CD8 T cells in the mouse model, these data suggest that an impaired, but not abolished, survival response to IL-2 and IL-15 accounts for the persistent lack of NK cells and the transient CD8 alphabeta T lymphopenia documented in vivo. Impaired cytokine-mediated lymphocyte survival is likely to be the pathogenic mechanism underlying this novel form of inherited and selective NK deficiency in humans.

Further differentiation of murine double-positive thymocytes is inhibited in adenosine deaminase-deficient murine fetal thymic organ culture

Murine fetal thymic organ culture (FTOC) was used to investigate the mechanism by which a lack of adenosine deaminase (ADA) leads to a failure of T cell production in the thymus. We previously showed that T cell development was inhibited beginning at the CD4(-)CD8(-)CD25(+)CD44(low) stage in ADA-deficient FTOC initiated at day 15 of gestation when essentially all thymocytes are CD4(-)CD8(-). In the present study, we asked whether thymocytes at later stages of differentiation would also be sensitive to ADA inhibition by initiating FTOC when substantial numbers of CD4(+)CD8(+) thymocytes were already present. dATP was highly elevated in ADA-deficient cultures, and the recovery of alphabeta TCR(+) thymocytes was inhibited by 94%, indicating that the later stages of thymocyte differentiation are also dependent upon ADA. ADA-deficient cultures were partially rescued by the pan-caspase inhibitor carbobenzoxy-Val-Ala-Asp-fluoromethyl ketone or by the use of apoptotic protease-activating factor-1-deficient mice. Rescue was even more dramatic, with 60- to >200-fold increases in the numbers of CD4(+)CD8(+) cells, when FTOC were performed with an inhibitor of adenosine kinase, the major thymic deoxyadenosine phosphorylating enzyme, or with bcl-2 transgenic mice. dATP levels were normalized by treatment with either carbobenzoxy-Val-Ala-Asp-fluoromethyl ketone or an adenosine kinase inhibitor, but not in cultures with fetal thymuses from bcl-2 transgenic mice. These data suggest that ADA deficiency leads to the induction of mitochondria-dependent apoptosis as a consequence of the accumulation of dATP derived from thymocytes failing the positive/negative selection checkpoint.

Autonomic innervation of immune organs and neuroimmune modulation

1. Increasing evidence indicates the occurrence of functional interconnections between immune and nervous systems, although data available on the mechanisms of this bi-directional cross-talking are frequently incomplete and not always focussed on their relevance for neuroimmune modulation. 2. Primary (bone marrow and thymus) and secondary (spleen and lymph nodes) lymphoid organs are supplied with an autonomic (mainly sympathetic) efferent innervation and with an afferent sensory innervation. Anatomical studies have revealed origin, pattern of distribution and targets of nerve fibre populations supplying lymphoid organs. 3. Classic (catecholamines and acetylcholine) and peptide transmitters of neural and non-neural origin are released in the lymphoid microenvironment and contribute to neuroimmune modulation. Neuropeptide Y, substance P, calcitonin gene-related peptide, and vasoactive intestinal peptide represent the neuropeptides most involved in neuroimmune modulation. 4. Immune cells and immune organs express specific receptors for (neuro)transmitters. These receptors have been shown to respond in vivo and/or in vitro to the neural substances and their manipulation can alter immune responses. Changes in immune function can also influence the distribution of nerves and the expression of neural receptors in lymphoid organs. 5. Data on different populations of nerve fibres supplying immune organs and their role in providing a link between nervous and immune systems are reviewed. Anatomical connections between nervous and immune systems represent the structural support of the complex network of immune responses. A detailed knowledge of interactions between nervous and immune systems may represent an important basis for the development of strategies for treating pathologies in which altered neuroimmune cross-talking may be involved.

Mitotic catastrophe constitutes a special case of apoptosis whose suppression entails aneuploidy

A conflict in cell cycle progression or DNA damage can lead to mitotic catastrophe when the DNA structure checkpoints are inactivated, for instance when the checkpoint kinase Chk2 is inhibited. Here we show that in such conditions, cells die during the metaphase of the cell cycle, as a result of caspase activation and subsequent mitochondrial damage. Molecular ordering of these phenomena reveals that mitotic catastrophe occurs in a p53-independent manner and involves a primary activation of caspase-2, upstream of cytochrome c release, followed by caspase-3 activation and chromatin condensation. Suppression of caspase-2 by RNA interference or pseudosubstrate inhibitors as well as blockade of the mitochondrial membrane permeabilization prevent the mitotic catastrophe and allow cells to further proceed the cell cycle beyond the metaphase, leading to asymmetric cell division. Heterokarya generated by the fusion of nonsynchronized cells can be driven to divide into three or more daughter cells when Chk2 and caspases are simultaneously inhibited. Such multipolar divisions, resulting from suppressed mitotic catastrophe, lead to the asymmetric distribution of cytoplasm (anisocytosis), DNA (anisokaryosis) and chromosomes (aneuploidy). Similarly, in a model of DNA damage-induced mitotic catastrophe, suppression of apoptosis leads to the generation of aneuploid cells. Our findings delineate a molecular pathway through which DNA damage, failure to arrest the cell cycle and inhibition of apoptosis can favor the occurrence of cytogenetic abnormalities that are likely to participate in oncogenesis.

Hematopoietic stem cell graft manipulation as a mechanism of immunotherapy

Hematopoietic stem cell transplants (SCT) are used in the treatment of neoplastic diseases, in addition to congenital, autoimmune, and inflammatory disorders. Both autologous and allogeneic SCT are used, depending on donor availability and the type of disease being treated, resulting in different morbidity and outcomes. In both types of SCT, immune regulation via graft manipulation is being studied, although with highly different targeted outcomes. In general, autologous SCT have lower treatment-related morbidity and mortality, but a higher incidence of tumor relapse, and graft manipulation targets immune augmentation and/or the reduction of immune tolerance. In contrast, allogeneic SCT have a higher incidence of treatment-related morbidity and mortality and a significantly longer time of disease progression, and the targeted outcomes or graft manipulation focus on a reduction in graft versus host disease (GVHD). One source of the increased relapse rate and shorter overall survival (OS) following high dose chemotherapy (HDT) and autologous SCT is the immune tolerance that limits host response, both innate and antigen (Ag) specific, against the tumor. The immune tolerance that is observed is due in part to the tumor burden and prior cytotoxic therapy. Therefore, graft manipulation, as an adjuvant therapeutic approach in autologous SCT, is primarily focused on non-specific or specific immune augmentation using cytokines and vaccines. Recently, manipulation of the infused product as a form of cellular therapy has begun to also focus on approaches to reduce immune tolerance found in transplant patients, both prior to and following HDT and SCT. To this end, graft manipulation to reduce the presence of Fas Ligand (FasL)-expressing cells or interleukin (IL)10 and tumor growth factor (TGF)beta production has been proposed. In contrast to autologous transplantation, graft manipulation during allogeneic transplantation is used extensively. This includes limiting the infusion of T cells within the product or as a donor leukocyte infusion (DLI), resulting in a reduction in GVHD and the induction of long-term survivors. Indeed, allogeneic SCT provide the only curative therapy for patients with chronic myelogenous leukemia (CML), refractory acute leukemia, and myelodysplasia. The curative potential of allogeneic SCT is reduced, however, by the development of GVHD, a potentially lethal T-cell-mediated immune response targeting host tissues [Int. Arch. Allergy Immunol. 102 (1993) 309, J. Exp. Med. 183 (1996) 589]. The morbidity and mortality associated with GVHD limit this technology, resulting focus on those patients who have no alternative therapeutic options or who have advanced disease. Thus, allogeneic SCT provide one of the few statistically supported demonstrations of therapeutic efficacy by T cells (comparison of allogeneic to autologous transplantation). In contrast to autologous transplantation, control of GVHD following allogeneic SCT focuses on immune suppression and the induction of tolerance. Here too, graft manipulation is appropriate, and there are numerous studies of T-cell depletion to reduce GVHD, with or without the isolation and infusion of T cells as DLI. Additional strategies are examining the isolation and infusion of T cells with graft versus leukemia (GVL) activity to reduce GVHD and/or the infusion of genetically manipulated and/or selected cellular populations (monocytes or dendritic cells (DC)) to induce tolerance. Therefore, depending upon the type of transplant, the goals associated with graft manipulation can be radically different. In this review, we emphasize using graft manipulation to regulate immune tolerance and anergy in association with SCT. Although this paper focuses on hematopoietic SCT, it should be noted that these strategies are relevant to conditions other than neoplastic and congenital diseases, including solid organ transplants, and autoimmune and inflammatory diseases.

Susceptibility of lysosomes to rupture is a determinant for plasma membrane disruption in tumor necrosis factor alpha-induced cell death

Since a release of intracellular contents can induce local inflammatory responses, mechanisms that lead to loss of plasma membrane integrity in cell death are important to know. We showed previously that deficiency of the plasma membrane Ca2+ ATPase 4 (PMCA4) in L929 cells impaired tumor necrosis factor alpha (TNF-alpha)-induced enlargement of lysosomes and reduced cell death. The lysosomal changes can be determined by measuring the total volume of intracellular acidic compartments per cell (VAC), and we show here that inhibition of the increase in VAC due to PMCA4 deficiency not only reduced cell death but also converted TNF-alpha-induced cell death from a process involving disruption of the plasma membrane to a cell demise with a nearly intact plasma membrane. The importance of the size of lysosomes in determining plasma membrane integrity during cell death was supported by the observations that chemical inhibitors that reduce VAC also reduced the plasma membrane disruption induced by TNF-alpha in wild-type L929 cells, while increases in VAC due to genetic mutation, senescence, cell culture conditions, and chemical inhibitors all changed the morphology of cell death from one with an originally nearly intact plasma membrane to one with membrane disruption in a number of different cells. Moreover, the ATP depletion-mediated change from apoptosis to necrosis is also associated with the increases of VAC. The increase in lysosomal size may due to intracellular self-digestion of dying cells. Big lysosomes are easy to rupture, and the release of hydrolytic enzymes from ruptured lysosomes can cause plasma membrane disruption.

Differentially expressed genes in HIV-1 tat-expressing CD4(+) T-cell line

Several studies have indicated that human immunodeficiency virus type-1 (HIV-1) transactivating Tat protein is essential for proviral DNA transcription and virus replication. In addition, it is actively released from acutely HIV-1-infected cells and interacts either with the same virus-infected and virus producing cell, or with bystander uninfected cells, influencing the expression of several genes and related cellular functions. The main goal of this paper was to determine the Tat-related expression of basic cellular genes in a permanently tat transfected CD4+ cell line, to identify the cellular genes influenced by the presence of endogenous-exogenous Tat protein. For this purpose, we analyzed, by a cDNA-membrane-array assay, cellular mRNAs expressed in serum-free cultures of lymphoblastoid CD4(+) Jurkat cells, stably transfected with a plasmid constitutively expressing tat gene, in comparison with Jurkat cells transfected with the backbone plasmid only, and parental Jurkat cells. The expression of mRNAs in permanently tat-transfected Jurkat cells showed significant differences in 24 out of 1176 analyzed genes in comparison with parental or backbone plasmid transfected cells. Most of the genes overexpressed in permanently tat-transfected Jurkat cells, belong to transcription factors, or to receptors, adaptors, and mediators of signal transduction pathways, and to factors involved in response to oxidative stress, suggesting a complex regulation of CD4(+) T-lymphoid cell survival and proliferation by HIV-1 Tat protein.

Cellular specificity related to monoglyceride-induced cell death

We have recently observed that monoglycerides (MGs), a family of lipids consisting of a single fatty acid moiety attached to a glycerol backbone, induce rapid dose-dependent apoptosis in murine thymocytes. In this work, we evaluated the sensitivity of various normal and malignant immune and non-immune cells to MGs. We demonstrate that the propensity to MG-induced death displayed by both T and B lymphocytes is clearly modulated according to their differentiation and activation status. For instance, the earliest T and B cell precursors are refractory to MG-mediated cell death. In the T-cell lineage, immature thymocytes are the most susceptible to MG treatment, while B cells from peripheral lymphoid organs appear more sensitive than B-cell subsets from the bone marrow. On the other hand, both activated T and B cells are more resistant to MG exposure than their non-activated counterparts. In addition, other hematopoietic lineages such as natural killer cells, macrophages, and erythroid cells are quite resistant to MG-induced death. Furthermore, using various immortalized cell lines from different tissues, we found that lymphomas and thymomas are the most sensitive among all lineages tested, while epithelial cells and fibroblasts are unaffected by MG treatment. Finally, MG-induced death was shown to be independent of Fas/Fas ligand (FasL) interactions. Altogether, our findings indicate that there is a cellular specificity related to MG-mediated cell death biased towards T and B lymphocytes. This suggests that MGs could potentially be used in the treatment of specific lymphoid disorders by bypassing the requirement for the Fas/FasL system.

Mitogen-activated protein kinase/extracellular signal-regulated kinase signaling in activated T cells abrogates TRAIL-induced apoptosis upstream of the mitochondrial amplification loop and caspase-8

Fas ligand and TNF-related apoptosis-inducing ligand (TRAIL) induce apoptosis in many different cell types. Jurkat T cells die rapidly by apoptosis after treatment with either ligand. We have previously shown that mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) can act as a negative regulator of apoptosis mediated by the Fas receptor. In this study we examined whether MAPK/ERK can also act as a negative regulator of apoptosis induced by TRAIL. Activated Jurkat T cells were efficiently protected from TRAIL-induced apoptosis. The protection was shown to be MAPK/ERK dependent and independent of protein synthesis. MAPK/ERK suppressed TRAIL-induced apoptosis upstream of the mitochondrial amplification loop because mitochondrial depolarization and release of cytochrome c were inhibited. Furthermore, caspase-8-mediated relocalization and activation of Bid, a proapoptotic member of the Bcl family, was also inhibited by the MAPK/ERK signaling. The protection occurred at the level of the apoptotic initiator caspase-8, as the cleavage of caspase-8 was inhibited but the assembly of the death-inducing signaling complex was unaffected. Both TRAIL and Fas ligand have been suggested to regulate the clonal size and persistence of different T cell populations. Our previous results indicate that MAPK/ERK protects recently activated T cells from Fas receptor-mediated apoptosis during the initial phase of an immune response before the activation-induced cell death takes place. The results of this study show clearly that MAPK/ERK also participates in the inhibition of TRAIL-induced apoptosis after T cell activation.

Adenosine kinase inhibition promotes survival of fetal adenosine deaminase-deficient thymocytes by blocking dATP accumulation

Thymocyte development past the CD4(-)CD8(-) stage is markedly inhibited in adenosine deaminase-deficient (ADA-deficient) murine fetal thymic organ cultures (FTOCs) due to the accumulation of ADA substrates derived from thymocytes failing developmental checkpoints. Such cultures can be rescued by overexpression of Bcl-2, suggesting that apoptosis is an important component of the mechanism by which ADA deficiency impairs thymocyte development. Consistent with this conclusion, ADA-deficient FTOCs were partially rescued by a rearranged T cell receptor beta transgene that permits virtually all thymocytes to pass the beta-selection checkpoint. ADA-deficient cultures were also rescued by the adenosine kinase inhibitor 5'-amino-5'-deoxyadenosine (5'A5'dAdo), indicating that the metabolite responsible for the inhibition of thymocyte development is not adenosine or deoxyadenosine, but a phosphorylated derivative of an ADA substrate. Correction of ADA-deficient FTOCs by 5'A5'dAdo correlated with reduced accumulation of dATP, implicating this compound as the toxic metabolite. In ADA-inhibited FTOCs rescued with a Bcl-2 transgene, however, dATP levels were superelevated, suggesting that cells failing positive and negative selection continued to contribute to the accumulation of ADA substrates. Our data are consistent with dATP-induced mitochondrial cytochrome c release followed by apoptosis as the mechanism by which ADA deficiency leads to reduced thymic T cell production.

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.

Quantitation of mitochondrial alterations associated with apoptosis

Mitochondria undergo two major changes during early apoptosis. On the one hand, the outer mitochondrial membrane becomes permeable to proteins, resulting in the release of soluble intermembrane proteins (SIMPs) from the mitochondrion. On the other hand, the inner mitochondrial membrane transmembrane potential (DeltaPsi(m)) is reduced. These changes occur in most, if not all, models of cell death and can be taken advantage of to detect apoptosis at an early stage. Here, we delineate methods for the detection of alterations in the DeltaPsi(m), based on the incubation of cells with cationic lipophilic fluorochromes, the uptake of which is driven by the DeltaPsi(m). Certain DeltaPsi(m)-sensitive dyes can be combined with other fluorochromes to detect simultaneously cellular viability, plasma membrane exposure of phosphatidylserine residues, or the mitochondrial production of reactive oxygen species (ROS). In addition, we describe an immunofluorescence method for the detection of two functionally important proteins translocating from mitochondria, namely, the caspase co-activator cytochrome c and the caspase-independent death effector apoptosis inducing factor (AIF).

The mechanism and significance of deletion of parasite-specific CD4(+) T cells in malaria infection

It is thought that both helper and effector functions of CD4(+) T cells contribute to protective immunity to blood stage malaria infection. However, malaria infection does not induce long-term immunity and its mechanisms are not defined. In this study, we show that protective parasite-specific CD4(+) T cells were depleted after infection with both lethal and nonlethal species of rodent PLASMODIUM: It is further shown that the depletion is confined to parasite-specific T cells because (a) ovalbumin (OVA)-specific CD4(+) T cells are not depleted after either malaria infection or direct OVA antigen challenge, and (b) the depletion of parasite-specific T cells during infection does not kill bystander OVA-specific T cells. A significant consequence of the depletion of malaria parasite-specific CD4(+) T cells is impaired immunity, demonstrated in mice that were less able to control parasitemia after depletion of transferred parasite-specific T cells. Using tumor necrosis factor (TNF)-RI knockout- and Fas-deficient mice, we demonstrate that the depletion of parasite-specific CD4(+) T cells is not via TNF or Fas pathways. However, in vivo administration of anti-interferon (IFN)-gamma antibody blocks depletion, suggesting that IFN-gamma is involved in the process. Taken together, these data suggest that long-term immunity to malaria infection may be affected by an IFN-gamma-mediated depletion of parasite-specific CD4(+) T cells during infection. This study provides further insight into the nature of immunity to malaria and may have a significant impact on approaches taken to develop a malaria vaccine.

Derivatives of monoglycerides as apoptotic agents in T-cells

Recently, lipids have received considerable attention for their potential to induce apoptosis when added exogenously to cells. In this study, we directly demonstrate that murine T-cells undergo rapid apoptosis following treatment with various forms of monoglycerides, which are a family of naturally occurring lipids consisting of a single fatty acid moiety attached to a glycerol backbone. The potency of these lipids varied depending on their chemical structure, whereas glycerol backbone or corresponding fatty acids alone were ineffective. Moreover, monoglyceride-mediated apoptosis was suppressed either by Bcl-2 overexpression, treatment with a broad inhibitor of caspases, or RNA and protein synthesis inhibitors. In addition, treatment of cells with derivatives of monoglycerides induced a calcium flux, which could be inhibited by both extracellular (EGTA) or intracellular (EGTA-AM) calcium chelators. To our knowledge, this is the first report demonstrating a role for derivatives of monoglycerides as inducers of apoptosis in mammalian cells.

Modulatory effects of octreotide on anti-CD3 and dexamethasone-induced apoptosis of murine thymocytes

In an attempt to elucidate the effects of somatostatin on two crucial processes that regulated T-cell differentiation and selection in thymus in this study, we investigated in vivo and in vitro the effects of octreotide (SMS 201-995) on dynamics of apoptosis, induced by dexamethasone (DEX) or by anti-CD3 monoclonal antibodies (mAb). The data were estimated by analysis of absolute cellularity, DNA fragmentation and maturational stage of thymocytes, detecting the CD4 and/or CD8 and T cell receptor (TCR) expression on thymocytes. The results, obtained by estimation of subdiploid peak of DNA and ladder DNA formation, have shown that SMS given in vivo, may potentiate the early phase of DEX-induced nuclear fragmentation (at 24 h), accelerating simultaneously the elimination of thymic cells with double positive (DP) CD4high CD8high phenotype (expressed both as percentage and absolute number). On the contrary, SMS, given both in vivo and in vitro, down-regulated the late process (at 72 h) of nuclear fragmentation, induced by anti-CD3 mAb, minimizing simultaneously the elimination of DP cells (expressed both as percentage and absolute number). In anti-CD3-treated cultures of thymocytes, SMS retarded also the elimination of immature thymocytes, expressing the TRC alpha/betalow or intermediate phenotype. The data emphasize that octreotide might have important regulatory effect on processes of thymic differentiation and maturation, which are crucial for T cell selection, induction of tolerance and prevention of autoimmune diseases.

Regeneration and tolerance factor is expressed during T-lymphocyte activation and plays a role in apoptosis

Regeneration and tolerance factor (RTF) is a protein cloned from the thymus and expressed on B lymphocytes in normal pregnancy, B lymphocytic leukemia lines, and T and B lymphocytes in individuals with HIV infection. Findings, using the Jurkat T-cell model, revealed that RTF is upregulated after activation and anti-RTF antibody-induced apoptosis. In this article anti-RTF antibody-induced apoptosis of both unstimulated and activated T lymphocytes. RTF expression was examined in human PBMC or purified T lymphocytes after their in vitro activation. Kinetic studies indicated maximal RTF cell surface expression on activated T lymphocytes occurred between expression of the early activation antigen CD69 and the IL-2alpha receptor (CD25) by multiparameter flow cytometry. RTF receptor expression correlated with Fas (CD95) and CD25 receptor expression (r2 = 0.6 and 0.5, respectively). RTF surface expression was dependent on the stimuli used to activate T lymphocytes. T lymphocytes obtained maximal RTF expression when activated through the TCR signal complex using anti-CD3epsilon antibody alone when compared with T lymphocytes activated with costimulation provided by anti-CD28 antibody alone or with anti-CD28 and anti-CD3epsilon antibody. RTF is expressed under conditions of both activation and anergy. The RTFs increased concentration on the surface of anergic T cells may protect these cells from apoptosis because increased RTF concentrations inhibited anti-RTF induced apoptosis. These data further characterize the expression of RTF on activated T lymphocytes and the role of anti-RTF antibody in T-lymphocyte apoptosis.

Fas-FasL interaction modulates nitric oxide production in Trypanosoma cruzi-infected mice

During acute Trypanosoma cruzi infection in mice, many leucocytes undergo apoptosis. Although apoptosis has been ascribed to increased levels of nitric oxide (NO) and Fas-FasL interaction, the importance of this phenomenon in modulating the host response against T. cruzi is unknown. Herein, the role of NO- and Fas-FasL-induced apoptosis in modulating the immune response to T. cruzi was evaluated using mice deficient in Fas expression (MRL/MpJ-Fas lpr) and inducible nitric oxide synthase (iNOS) knockout mice (iNOS-/-). The results showed that besides decreasing apoptosis induction after infection, impairment of the Fas-FasL interaction resulted in decreased NO production, as a consequence of enhanced T helper 2 (Th2) cytokine production. Differently, blockage of NO-induced apoptosis resulted in uncontrolled cytokine production, rather than a biased Th2 cytokine pattern. Together, these results suggested that Fas and FasL-induced apoptosis could be implied in modulation of the immune response against T. cruzi by interfering with cytokine and NO production during the acute phase of the infection.

The ALG-2/AIP-complex, a modulator at the interface between cell proliferation and cell death? A hypothesis

During the development of an organism cell proliferation, differentiation and cell death are tightly balanced, and are controlled by a number of different regulators. Alterations in this balance are often observed in a variety of human diseases. The role of Ca(2+) as one of the key regulators of the cell is discussed with respect to two recently discovered proteins, ALG-2 and AIP, of which the former is a Ca(2+)-binding protein, and the latter is substrate to various kinases. The two proteins interact with each other in a Ca(2+)-dependent manner, and the role of the complex ALG-2/AIP as a possible modulator at the interface between cell proliferation and cell death is discussed.

Mice with an increased glucocorticoid receptor gene dosage show enhanced resistance to stress and endotoxic shock

Targeted mutagenesis of the glucocorticoid receptor has revealed an essential function for survival and the regulation of multiple physiological processes. To investigate the effects of an increased gene dosage of the receptor, we have generated transgenic mice carrying two additional copies of the glucocorticoid receptor gene by using a yeast artificial chromosome. Interestingly, overexpression of the glucocorticoid receptor alters the basal regulation of the hypothalamo-pituitary-adrenal axis, resulting in reduced expression of corticotropin-releasing hormone and adrenocorticotrope hormone and a fourfold reduction in the level of circulating glucocorticoids. In addition, primary thymocytes obtained from transgenic mice show an enhanced sensitivity to glucocorticoid-induced apoptosis. Finally, analysis of these mice under challenge conditions revealed that expression of the glucocorticoid receptor above wild-type levels leads to a weaker response to restraint stress and a strongly increased resistance to lipopolysaccharide-induced endotoxic shock. These results underscore the importance of tight regulation of glucocorticoid receptor expression for the control of physiological and pathological processes. Furthermore, they may explain differences in the susceptibility of humans to inflammatory diseases and stress, depending on individual prenatal and postnatal experiences known to influence the expression of the glucocorticoid receptor.

Regeneration and tolerance factor's potential role in T-cell activation and apoptosis

Regeneration and tolerance factor (RTF) is a novel membrane protein that has a diverse expression pattern and immunoregulatory properties. RTF is expressed in vivo on the surface of individuals with B cell chronic lymphocytic leukemia and on activated T lymphocytes of HIV infected individuals as determined by their coexpression with CD38 and HLA-DR. The unique expression patterns of this protein in vivo lead us to investigate its expression in vitro. The activation of human PBMCs through the TCR, using anti-CD3 antibody and PMA, upregulated cell surface expression of RTF from 2. 3% to 91.2% (mean channel fluorescence [MCF] increased threefold). The activation of Jurkat T cells through the TCR upregulated surface expression of RTF from 8.3% (MCF-1.3) to 58.7% (MCF-13.1). The Jurkat T-cell line was used as a model system to explore RTF's role in cellular activation. Using the Jurkat T-cell model, we found anti-RTF antibody induces apoptosis. The addition of anti-RTF antibody increased annexin V binding by threefold compared with the IgG1 kappa isotype control antibody (p < 0.00002) and activated caspase 3. These data indicate that RTF is expressed during T-cell activation and may be associated with apoptosis.

Dose-dependent apoptosis induced by low concentrations of methylmercury in murine splenic Fas+ T cell subsets

Methylmercury chloride (MeHgCl) is known to induce cellular and humoral immunodeficiencies in mice. In this study, the involvement of lymphoid subset disorders due to low concentrations of methylmercury (0.001-1.0 microM) has been examined. Cytofluorometric analysis of splenic cells exposed in vitro to low concentrations of MeHgCl for 48 h revealed two distinct populations: the first expressed a typical lymphoid forward light scatter (FSC)/side light scatter (SSC) pattern (R1 region), and the second was characterized by a lower FSC and a higher SSC (R2 region). A dose-dependent shift of cells from R1 region toward R2 region was observed in splenic cells treated with MeHgCl. The apoptotic state of cells in the R2 region was confirmed by the TdT-mediated dUTP nick end labeling (TUNEL) assay. Analysis of DNA content in splenic lymphoid cells showed that low concentrations of MeHgCl increased both hypoploid cells and cells in G0-G1/S phase, both in the R1 and R2 regions. However, the numbers of cells in G0-G1/S and G2/M phases were decreased, but hypoploid cells increased in both regions due to exposure to 1 microM MeHgCl. MeHgCl-induced apoptosis disappeared when splenic cells were pretreated with anti-Fas antibodies, indicating that Fas expressing cells were the target cells for MeHgCl-mediated apoptosis. Furthermore, T cells from the V beta 8 family were found to be more sensitive to apoptosis induced by low concentrations of MeHgCl. Taken together, these results suggest that MeHgCl at low concentrations mediates the development of apoptosis in peripheral T cell via the Fas/FasL pathway.

Increased apoptosis of peripheral blood T cells following allogeneic hematopoietic cell transplantation

Lymphopenia and immune deficiency are significant problems following allogeneic hematopoietic cell transplantation (HCT). It is largely assumed that delayed immune reconstruction is due to a profound decrease in thymus-dependent lymphopoiesis, especially in older patients, but apoptosis is also known to play a significant role in lymphocyte homeostasis. Peripheral T cells from patients who received HCT were studied for evidence of increased cell death. Spontaneous apoptosis was measured in CD3+ T cells following a 24-hour incubation using 7-amino-actinomycin D in conjunction with the dual staining of cell surface antigens. Apoptosis was significantly greater among CD3+ T cells taken from patients 19-23 days after transplantation (30.4% ± 12.5%,P < .05), and 1 year after transplantation (9.7% ± 2.8%, P < .05) compared with healthy controls (4.0% ± 1.5%). Increased apoptosis occurred preferentially in HLA (human leukocyte antigen)-DR positive cells and in both CD3+/CD4+ and CD3+/CD8+ T-cell subsets, while CD56+/CD3− natural killer cells were relatively resistant to apoptosis. The extent of CD4+T-cell apoptosis was greater in patients with grade II-IV acute graft-versus-host disease (GVHD) (33.9% ± 11.3%) compared with grade 0-I GVHD (14.6 ± 6.5%, P < .05). T-cell apoptosis was also greater in patients who received transplantations from HLA-mismatched donors (39.5% ± 10.4%,P < .05) or HLA-matched unrelated donors (32.1% ± 11.4%, P < .05) compared with patients who received transplantations from HLA-identical siblings (19.6% ± 6.7%). The intensity of apoptosis among CD4+ T cells was significantly correlated with a lower CD4+ T-cell count. Together, these observations suggest that activation of T cells in vivo, presumably by alloantigens, predisposes the cells to spontaneous apoptosis, and this phenomenon is associated with lymphopenia. Activation-induced T-cell apoptosis may contribute to delayed immune reconstitution following HCT.

Increased apoptosis of peripheral blood T cells following allogeneic hematopoietic cell transplantation

Lymphopenia and immune deficiency are significant problems following allogeneic hematopoietic cell transplantation (HCT). It is largely assumed that delayed immune reconstruction is due to a profound decrease in thymus-dependent lymphopoiesis, especially in older patients, but apoptosis is also known to play a significant role in lymphocyte homeostasis. Peripheral T cells from patients who received HCT were studied for evidence of increased cell death. Spontaneous apoptosis was measured in CD3+ T cells following a 24-hour incubation using 7-amino-actinomycin D in conjunction with the dual staining of cell surface antigens. Apoptosis was significantly greater among CD3+ T cells taken from patients 19-23 days after transplantation (30.4% ± 12.5%,P &lt; .05), and 1 year after transplantation (9.7% ± 2.8%, P &lt; .05) compared with healthy controls (4.0% ± 1.5%). Increased apoptosis occurred preferentially in HLA (human leukocyte antigen)-DR positive cells and in both CD3+/CD4+ and CD3+/CD8+ T-cell subsets, while CD56+/CD3− natural killer cells were relatively resistant to apoptosis. The extent of CD4+T-cell apoptosis was greater in patients with grade II-IV acute graft-versus-host disease (GVHD) (33.9% ± 11.3%) compared with grade 0-I GVHD (14.6 ± 6.5%, P &lt; .05). T-cell apoptosis was also greater in patients who received transplantations from HLA-mismatched donors (39.5% ± 10.4%,P &lt; .05) or HLA-matched unrelated donors (32.1% ± 11.4%, P &lt; .05) compared with patients who received transplantations from HLA-identical siblings (19.6% ± 6.7%). The intensity of apoptosis among CD4+ T cells was significantly correlated with a lower CD4+ T-cell count. Together, these observations suggest that activation of T cells in vivo, presumably by alloantigens, predisposes the cells to spontaneous apoptosis, and this phenomenon is associated with lymphopenia. Activation-induced T-cell apoptosis may contribute to delayed immune reconstitution following HCT.

PI3-K/AKT regulation of NF-kappaB signaling events in suppression of TNF-induced apoptosis

We found that in MCF-7 breast carcinoma cells, PI3K and Akt suppressed a dose-dependent induction of apoptosis by tumor necrosis factor alpha (TNF). PI3K and Akt stimulated NF-kappaB activation in a dose-dependent manner, suggesting a common link between these two pathways. TNF has been shown to activate both an apoptotic cascade, as well as a cell survival signal through NF-kappaB. PI3K and AKT cell survival signaling were correlated with increased TNF-stimulated NF-kappaB activity in MCF-7 cells. We demonstrate that while both TNFR1 and NIK are partially involved in Akt-induced NF-kappaB stimulation, a dominant negative IkappaBalpha completely blocked Akt-NF-kappaB cross-talk. PI3K-Akt signaling activated NF-kappaB through both TNFR signaling-dependent and -independent mechanisms, potentially representing a mechanism by which Akt functions to suppress apoptosis in cancer.

Control of apoptotic DNA degradation

Apoptotic DNA degradation has been thought to be a cell-autonomous process. Recent evidence suggests that heterophagic recognition and engulfment of dying cells by non-apoptotic cells may be critical for the activation and/or action of apoptogenic DNases.

Cross-talk between phosphatidylinositol 3-kinase and sphingomyelinase pathways as a mechanism for cell survival/death decisions

Peptide hormones act to regulate apoptosis through activation of multiple pro- and anti-apoptotic signaling cascades of which lipid signaling events represent an important facet of the cellular rheostat that determines survival and death decisions. Activation of sphingomyelinase, which generates ceramide, is an intermediate in cellular stress responses and induction of apoptosis in many systems. Conversely, phosphatidylinositol 3-kinase (PI3K) is a critical signaling molecule involved in regulating cell survival and proliferation pathways. In the present study, we investigate cross-talk between the PI3K and sphingomyelinase pathways as a mechanism for regulation of cell survival/death decisions. We show that phorbol ester, insulin-like growth factor 1, and a constitutively active PI3K suppress both tumor necrosis factor-induced apoptosis and ceramide generation. Conversely, inhibition of the PI3K pathway with expression of a kinase-dead PI3K both prevented survival signaling and enhanced tumor necrosis factor-induced ceramide generation. The ability of exogenous sphingomyelinase to induce ceramide generation was partially suppressed by expression of constitutively active PI3K and enhanced by inhibition of PI3K suggesting that cross-talk between PI3K and ceramide generation within cells is regulated subsequent to activation of sphingomyelinase.

Specific inhibition of glucocorticoid-induced thymocyte apoptosis by substance P

Glucocorticoids (GC) are strong inducers of thymocyte apoptosis. In the present study we looked into the possibility that the neuropeptide substance P (SP) might serve as an antagonist to GC-induced apoptosis. Indeed, SP inhibited hydrocortisone (HC)-induced apoptosis of CD4+CD8+ thymocytes in mice, both in vivo and in vitro. It also inhibited HC-induced apoptosis in the T cell hybridoma line 2B4.11, which is sensitive to GC. The inhibitory effect was complete if SP was given with HC or within 1 h after it; partial inhibitory effect could be seen at 2 h and no effect at 3 h. The presence of the SP antagonist nullified SP effect. The effect was specific to both components of the system (i.e., HC as apoptosis inducer and SP as its inhibitor), as judged from comparison to three other apoptosis-inducing means (irradiation, thymic epithelial cells, or retinoic acid), and to two other neuropeptides (somatostatin and vasoactive intestinal peptide). SP/HC antagonism was further demonstrated in two relevant molecular events: 1) HC augmented GC receptor production in our cell system and this was inhibited by SP; and 2) HC reduced the expression of the transcription factor NF-kappaB, SP increased it and when both were present, SP effect dominated. On the other hand, the level of IkappaB (NF-kappaB inhibitory molecule) was decreased by SP, preserved at a relatively high level with HC, and when both SP and HC were present, SP effect dominated. The intensity of SP effect, both in vivo and in vitro, its specificity, its inhibition by SP antagonist, as well as the previously documented presence of SP and its receptor in the thymus suggest that SP might be a physiological antagonist of the potent thymocyte apoptosis induced by GC.

STAT1 affects lymphocyte survival and proliferation partially independent of its role downstream of IFN-gamma

Lymphocytes derived from mice deficient in STAT1 showed reduced apoptosis and enhanced proliferation in vitro. To understand the involvement of STAT1 in the observed reduction in apoptosis, we examined the levels of caspase and bcl-2 family genes that are involved in cell survival and/or apoptosis. The levels of caspase 1 and 11, two enzymes involved in both cytokine protein processing and induction of apoptosis, were reduced in STAT1-/- cells compared with wild-type. However, the levels of bcl-2 genes were comparable in both mice. STAT1-/- cells also displayed an enhanced proliferation following TCR stimulation. This hyperproliferation could not be ascribed completely to the loss of IFN-gamma-mediated antiproliferation. First, similar phenotypes were also observed in fibroblasts and pre-B cells derived from STAT1-/- mice, which do not produce IFN-gamma. Second, comparisons with cells lacking the gene for IFN-gamma or with cells treated with neutralizing Abs to IFN-gamma only partially mimicked the STAT1-/- phenotype. Interestingly, the kinetics of degradation of p27kip1, a CDK inhibitor, following TCR ligation were faster, and, concomitantly, the up-regulation of CDK2 kinase activity and protein levels were increased in stimulated T cells of STAT1-/- mice relative to those of wild-type mice. Furthermore, STAT1-/- animals were more susceptible to carcinogen-induced thymic tumors, a possible consequence of altered T cell growth and/or survival. These results demonstrate an essential role for STAT1 for lymphocyte survival and proliferation that is only partially dependent on IFN-gamma signaling.

Impaired Fas response and autoimmunity in Pten+/- mice

Inactivating mutations in the PTEN tumor suppressor gene, encoding a phosphatase, occur in three related human autosomal dominant disorders characterized by tumor susceptibility. Here it is shown that Pten heterozygous (Pten+/-) mutants develop a lethal polyclonal autoimmune disorder with features reminiscent of those observed in Fas-deficient mutants. Fas-mediated apoptosis was impaired in Pten+/- mice, and T lymphocytes from these mice show reduced activation-induced cell death and increased proliferation upon activation. Phosphatidylinositol (PI) 3-kinase inhibitors restored Fas responsiveness in Pten+/- cells. These results indicate that Pten is an essential mediator of the Fas response and a repressor of autoimmunity and thus implicate the PI 3-kinase/Akt pathway in Fas-mediated apoptosis.

Constitutive impaired TCR/CD3-mediated activation of T cells in IDDM patients co-exist with normal co-stimulation pathways

IDDM is a T cell-mediated autoimmune disease which is paradoxically associated with T cell functional deficiencies. The proliferative response of PBMC under CD3-, Vbeta2-, Vbeta8- and Vbeta7-stimulation was investigated in IDDM and NIDDM patients, non-diabetic first-degree relatives and control subjects. Despite normal surface expression of the TCR/CD3 complex, the TCR/CD3-mediated proliferation of PBMC from IDDM patients was significantly impaired compared to control subjects (P<0.05). This defect was specific for the autoimmune disease, constitutive and not linked to the class II MHC genotype, to metabolic disturbances or to presence of specific autoantibodies. Inefficient activation of T cells was not related to a lower capacity of CD28 to transduce co-stimulative signals because proliferative responses under CD2/CD28 stimulations were similar in IDDM and control groups. The IL-2/IL-2 receptor system was functional because unstimulated PBMC proliferated in response to increasing amounts of IL-2. Nevertheless, despite normal expression of CD25, addition of IL-2 did not normalize the proliferative defect linked to IDDM. In conclusion, excluding a faulty co-stimulation pathway, these results are in favour of a constitutive defect in the CD3/TCR transduction machinery, increasing sensitivity to apoptosis or anergy in T cells from IDDM patients.

B Cell Apoptosis Triggered by Antigen Receptor Ligation Proceeds Via a Novel Caspase-Dependent Pathway

In contrast to positive signaling leading to proliferation, the mechanisms involved in negative signaling culminating in apoptosis after B cell Ag receptor (BCR) ligation have received little study. We find that apoptosis induced by BCR cross-linking on EBV-negative mature and immature human B cell lines involves the following sequential, required events: a cyclosporin A-inhibitable, likely calcineurin-mediated step; and activation of caspase-2, -3, and -9. Caspase-2 is activated early and plays a major role in the apoptotic pathway, while caspase-9 is activated later in the apoptotic pathway and most likely functions to amplify the apoptotic signal. Caspase-8 and -1, which are activated by ligation of the CD95 and TNF-R1 death receptors, are not involved. Apoptosis induced by BCR ligation thus proceeds via a previously unreported intracellular signaling pathway.

Primed T Cells Are More Resistant to Fas-Mediated Activation-Induced Cell Death than Naive T Cells

Memory T cells respond in several functionally different ways from naive T cells and thus function as efficient effector cells. In this study we showed that primed T cells were more resistant to Fas-mediated activation-induced cell death (AICD) than naive T cells using OVA-specific TCR transgenic DO10 mice and Fas-deficient DO10 lpr/lpr mice. We found that apoptosis was efficiently induced in activated naive T cells at 48 and 72 h after Ag restimulation (OVA peptide; 0.3 and 3 μM), whereas apoptosis was not significantly increased in activated primed T cells at 24–72 h after Ag restimulation. We further showed that the resistance to AICD in primed T cells was due to the decreased sensitivity to apoptosis induced by Fas-mediated signals, but TCR-mediated signaling equally activated both naive and primed T cells to induce Fas and Fas ligand expressions. Furthermore, we demonstrated that primed T cells expressed higher levels of Fas-associated death domain-like IL-1β-converting enzyme inhibitory protein (FLIP), an inhibitor of Fas-mediated apoptosis, at 24–48 h after Ag restimulation than naive T cells. In addition, Bcl-2 expression was equally observed between activated naive and primed T cells after Ag restimulation. Thus, these results indicate that naive T cells are sensitive to Fas-mediated AICD and are easily deleted by Ag restimulation, while primed/memory T cells express higher levels of FLIP after Ag restimulation, are resistant to Fas-mediated AICD, and thus function as efficient effector cells for a longer period.

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.

Plasma Membrane Potential in Thymocyte Apoptosis

Apoptosis is accompanied by major changes in ion compartmentalization and transmembrane potentials. Thymocyte apoptosis is characterized by an early dissipation of the mitochondrial transmembrane potential, with transient mitochondrial swelling and a subsequent loss of plasma membrane potential (ΔΨp) related to the loss of cytosolic K+, cellular shrinkage, and DNA fragmentation. Thus, a gross perturbation of ΔΨp occurs at the postmitochondrial stage of apoptosis. Unexpectedly, we found that blockade of plasma membrane K+ channels by tetrapentylammonium (TPA), which leads to a ΔΨp collapse, can prevent the thymocyte apoptosis induced by exposure to the glucocorticoid receptor agonist dexamethasone, the topoisomerase inhibitor etoposide, γ-irradiation, or ceramide. The TPA-mediated protective effect extends to all features of apoptosis, including dissipation of the mitochondrial transmembrane potential, loss of cytosolic K+, phosphatidylserine exposure on the cell surface, chromatin condensation, as well as caspase and endonuclease activation. In strict contrast, TPA is an ineffective inhibitor when cell death is induced by the potassium ionophore valinomycin, the specific mitochondrial benzodiazepine ligand PK11195, or by primary caspase activation by Fas/CD95 cross-linking. These results underline the importance of K+ channels for the regulation of some but not all pathways leading to thymocyte apoptosis.

Apoptotic human lymphocytes have diminished CD4 and CD8 receptor expression

We used quantitative multiparameter flow cytometric assays to simultaneously detect viable, apoptotic, and necrotic human peripheral blood mononuclear cells (PBMC) and immunophenotyped lymphocyte subsets within the PBMC. Apoptosis was induced by a spectrum of treatments, including camptothecin, cisplatin, dexamethasone, hyperthermia, staurosporine, and etoposide in anti-CD3 mAb-stimulated cells and by cyclohexamide in both quiescent and stimulated cells; apoptosis in the latter was augmented by anti-fas mAb. We found that CD4(+) and CD8(+) cells were significantly underrepresented in the apoptotic PBMC and that the percentage of CD4(+) and CD8(+) PBMC each markedly decreased as apoptosis increased. This suggested that surface expression of these receptors was lessened on apoptotic CD4(+) and CD8(+) cells. This was directly confirmed by observation of sorted CD4(+) PBMC. This analysis of a wide variety of apoptotic stimuli demonstrates that diminished CD4 and CD8 surface receptor expression is a common feature of human T lymphocyte apoptosis.

The cyclin-dependent kinase Cdk2 regulates thymocyte apoptosis

Aberrant activation of cell cycle molecules has been postulated to play a role in apoptosis ("catastrophic cell cycle"). Here we show that in noncycling developing thymocytes, the cyclin- dependent kinase Cdk2 is activated in response to all specific and nonspecific apoptotic stimuli tested, including peptide-specific thymocyte apoptosis. Cdk2 was found to function upstream of the tumor suppressor p53, transactivation of the death promoter Bax, alterations of mitochondrial permeability, Bcl-2, caspase activation, and caspase-dependent proteolytic cleavage of the retinoblastoma protein. Inhibition of Cdk2 completely protected thymocytes from apoptosis, mitochondrial changes, and caspase activation. These data provide the first evidence that Cdk2 activity is crucial for the induction of thymocyte apoptosis.

Interleukin-7: physiological roles and mechanisms of action

Interleukin-7 (IL-7), a product of stromal cells, provides critical signals to lymphoid cells at early stages in their development. Two types of cellular responses to IL-7 have been identified in lymphoid progenitors: (1) a trophic effect and (2) an effect supporting V(D)J recombination. The IL-7 receptor is comprised of two chains, IL-7R alpha and gamma(c). Following receptor crosslinking, rapid activation of several classes of kinases occurs, including members of the Janus and Src families and PI3-kinase. A number of transcription factors are subsequently activated including STATs, c-myc, NFAT and AP-1. However, it remains to be determined which, if any, previously identified pathway leads to the trophic or V(D)J endpoints. The trophic response to IL-7 involves protecting lymphoid progenitors from a death process that resembles apoptosis. This protection is partly mediated by IL-7 induction of Bcl-2, however other IL-7-induced events are probably also involved in the trophic response. The V(D)J response to IL-7 is partly mediated through increased production of Rag proteins (which cleave the target locus) and partly by increasing the accessibility of a target locus to cleavage through chromatin remodeling.

Triggering HLA-DR molecules on human peripheral monocytes induces their death

Although engagement of MHC class II molecules on human monocytes triggers various cellular events, their possible role in monocyte death is yet unknown. We demonstrate that ligation of MHC class II on primary monocytes induces a rapid cell death that has all the characteristics of monocyte apoptosis, does not require de novo protein synthesis, is independent from both Fas and TNF-alpha systems, and is not rescued by ligation of CD40. However, cell-cell interactions that involve the beta2-integrin CD18 seem to be critical for the execution of this monocyte death. Priming monocytes with IFN-gamma enhances significantly their HLA-DR-mediated death whereas LPS treatment effectively reverses this death process. Thus, our results describe the MHC class II molecules, in particular HLA-DR, as mediators of monocyte death and suggest that this novel pathway of monocyte death might have an important role in controlling the outcome of inflammatory process and the regulation of monocyte hemostasis.

Lysophosphatidic Acid and Sphingosine 1-Phosphate Protection of T Cells from Apoptosis in Association with Suppression of Bax

Members of a subfamily of G protein-coupled receptors (GPCRs), encoded by five different endothelial differentiation genes (edgs), specifically mediate effects of lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) on cellular proliferation and differentiation. Mechanisms of suppression of apoptosis by LPA and S1P were studied in the Tsup-1 cultured line of human T lymphoblastoma cells, which express Edg-2 and Edg-4 GPCRs for LPA and Edg-3 and Edg-5 GPCRs for S1P. At 10−10 M to 10−7 M, both LPA and S1P protected Tsup-1 cells from apoptosis induced by Abs to Fas, CD2, and CD3 plus CD28 in combination. Apoptosis elicited by C6 ceramide was inhibited by S1P, but not by LPA, in part because ceramide suppressed expression of Edg-2 and Edg-4 surface receptors for LPA without affecting Edg-3 surface receptors for S1P. At 10−9 M to 10−7 M, LPA and S1P significantly suppressed cellular levels of the apoptosis-promoting protein Bax, without altering the levels of Bcl-xL or Bcl-2 assessed by Western blots and immunoassays. Transfections of pairs of antisense plasmids for Edg-2 plus Edg-4 and Edg-3 plus Edg-5, and hygromycin selection of transfectants with reduced expression of the respective Edg R proteins in Western blots, inhibited both protection from apoptosis and reduction in cellular levels of Bax by LPA and S1P. Thus, LPA and S1P protection from apoptosis is mediated by distinct Edg GPCRs and may involve novel effects on Bax regulatory protein.

The role of c-FLIP in modulation of CD95-induced apoptosis

Upon stimulation, CD95 (APO-1/Fas) recruits the adapter molecule Fas-associated death domain protein (FADD)/MORT1 and caspase-8 (FADD-like interleukin-1beta-converting enzyme (FLICE)/MACH/MCH5) into the death-inducing signaling complex (DISC). Recently, a molecule with sequence homology to caspase-8 was identified, termed cellular FLICE-inhibitory protein (c-FLIP). c-FLIP has been controversially reported to possess apoptosis-promoting and -inhibiting functions. Using c-FLIP-specific monoclonal antibodies, we now show that c-FLIP is expressed in two isoforms, both of which, like FADD and caspase-8, are recruited to the CD95 DISC in a stimulation-dependent fashion. In stably transfected BJAB cells, c-FLIP blocks caspase-8 activation at the DISC and thereby inhibits CD95-mediated apoptosis. During this process, both caspase-8 and c-FLIP undergo cleavage between the p18 and p10 subunits, generating two stable intermediates of 43 kDa that stay bound to the DISC. c-FLIP has been suggested to play a role in protecting activated peripheral T cells from CD95-mediated apoptosis (Irmler, M., Thome, M., Hahne, M., Schneider, P., Hofmann, K., Steiner, V., Bodmer, J. L. , Schroter, M., Burns, K., Mattmann, C., Rimoldi, D., French, L. E., and Tschopp, J. (1997) Nature 388, 190-195). In contrast to this hypothesis, neither caspase-8 nor c-FLIP were cleaved in these cells, ruling out c-FLIP as the main factor regulating DISC activity. Moreover, recruitment of FADD, caspase-8, and c-FLIP to the DISC was strongly reduced in the apoptosis-resistant but readily detectable in the apoptosis-sensitive T cells.

A link between cell cycle and cell death: Bax and Bcl-2 modulate Cdk2 activation during thymocyte apoptosis

Resting thymocytes undergoing apoptosis in response to specific stimuli degrade the cdk inhibitor p27(Kip1) and upregulate Cdk2 kinase activity. Inhibition of Cdk2 kinase activity efficiently blocks cell death via certain apoptosis pathways whereas overexpression of Cdk2 accelerates such cell death, suggesting its involvement in the signal transduction pathways activated by certain apoptotic stimuli. We found that Cdk2 activation during thymocyte apoptosis can be regulated by p53, Bax and Bcl-2. The highly elevated Cdk2 kinase activity in the apoptosing thymocytes is not associated with its canonical cyclins, cyclin E and cyclin A, and requires de novo synthesis of proteins for activation to take place. We therefore propose Cdk2 activation to be a crucial event in distinct pathways of apoptosis and the point at which the cell cycle and cell death pathways interact.

Vav regulates peptide-specific apoptosis in thymocytes

The protooncogene Vav functions as a GDP/GTP exchange factor (GEF) for Rho-like small GTPases involved in cytoskeletal reorganization and cytokine production in T cells. Gene-targeted mice lacking Vav have a severe defect in positive and negative selection of T cell antigen receptor transgenic thymocytes in vivo, and vav-/- thymocytes are completely resistant to peptide-specific and anti-CD3/anti-CD28-mediated apoptosis. Vav acts upstream of mitochondrial pore opening and caspase activation. Biochemically, Vav regulates peptide-specific Ca2+ mobilization and actin polymerization. Peptide-specific cell death was blocked both by cytochalasin D inhibition of actin polymerization and by inhibition of protein kinase C (PKC). Activation of PKC with phorbol ester restored peptide-specific apoptosis in vav-/- thymocytes. Vav was found to bind constitutively to PKC-theta in thymocytes. Our results indicate that peptide-triggered thymocyte apoptosis is mediated via Vav activation, changes in the actin cytoskeleton, and subsequent activation of a PKC isoform.

PML is essential for multiple apoptotic pathways

The PML gene of acute promyelocytic leukaemia (APL) encodes a cell growth and tumour suppressor, however, the mechanisms by which PML suppresses tumorigenesis are poorly understood. We show here that Pml is required for Fas- and caspase-dependent DNA-damage-induced apoptosis. We also found that Pml is essential for induction of programmed cell death by Fas, tumour necrosis factor alpha (TNF), ceramide and type I and II interferons (IFNs). As a result, Pml-/- mice and cells are protected from the lethal effects of ionizing radiation and anti-Fas antibody. Pml is required for caspase 1 and caspase 3 activation upon exposure to these stimuli. The PML-RAR alpha fusion protein of APL renders haemopoietic progenitor cells resistant to Fas-, TNF- and IFN-induced apoptosis with a lack of caspase 3 activation, thus acting as a Pml dominant-negative product. These results demonstrate that Pml is a mediator of multiple apoptotic signals, and implicate inhibition of apoptosis in the pathogenesis of APL.

The central role of the mitochondrial megachannel in apoptosis: evidence obtained with intact cells, isolated mitochondria, and purified protein complexes

The mitochondrial megachannel (also called permeability transition pore) is a polyprotein complex formed in the contact site between the inner and the outer mitochondrial membranes and participates in the regulation of mitochondrial membrane permeability. We have obtained three independent lines of evidence suggesting the implication of the mitochondrial megachannel in apoptosis. First, in intact cells, apoptosis is accompanied by an early dissipation of the mitochondrial transmembrane potential (delta psi m). In several models of apoptosis, specific agents inhibiting the mitochondrial megachannels prevent this delta psi m dissipation and simultaneously abolish the manifestations of caspase- and endonuclease activation, indicating that megachannel opening is a critical event of the apoptotic process. Second, mitochondria are rate-limiting for caspase and nuclease activation in several cell-free systems of apoptosis. Isolated mitochondria release apoptogenic factors capable of activating pro-caspases or endonucleases upon opening of the mitochondrial megachannel in vitro. Third, opening of the purified megachannel reconstituted into liposomes is inhibited by recombinant Bcl-2 or Bcl-XL, two apoptosis-inhibitory proteins which also prevent megachannel opening in cells and isolated mitochondria. This indicates that the megachannel is under the direct regulatory control of anti-apoptotic members of the Bcl-2 family. Altogether, our results suggest that megachannel opening is sufficient and (mostly) necessary for triggering apoptosis.