From parasites to allergy: a second receptor for IgE

Human allergic responses are triggered when mast cells bind IgE antibodies. IgE production is also a regular feature of helminth infection and parasites can be killed by inflammatory cells such as macrophages, eosinophils and platelets sensitized by reaginic antibody. The IgE receptor they use has been identified only recently. Here André Capron and his colleagues discuss how it differs from the IgE receptor present on mast cells.

Major role of BAX in apoptosis during retinal development and in establishment of a functional postnatal retina

Apoptosis plays a major role in the development of the central nervous system. Previous studies of apoptosis induction during retinal development are difficult to interpret, however, because they explored different mouse strains, different developmental periods, and used different assays. Here, we first established a comprehensive sequential pattern of cell death during the whole development of the C57BL/6J mouse retina, from E10.5 to postnatal day (P) 21 by using the terminal deoxynucleotidyl transferase (TdT) -mediated deoxyuridine triphosphate (dUTP)-biotinylated nick end labeling (TUNEL) assay. We confirmed the existence of three previously described apoptotic peaks and identified another, later peak at P15, in both the outer nuclear layer, in which the photoreceptors differentiate, and the ganglion cell layer. Comparison of wild-type C57BL/6 mice, gld mice, defective in the death ligand fasL, and bax-/- mice, defective in the pro-apoptotic BAX protein, revealed a minor role for FAS ligand but a crucial role for BAX in both apoptosis and normal retinal development. The lack of BAX resulted in thicker than normal inner neuroblastic and ganglion cell layers in adults, with larger numbers of cells and an impaired electroretinogram response related to a decreased number of responsive cells. Our findings indicate that cell death during normal retinal development is important for the modeling of a functional vision organ and showed that the pro-apoptotic BAX protein plays a crucial role in this process.

A novel mechanism for HIV1-mediated bystander CD4+ T-cell death: neighboring dying cells drive the capacity of HIV1 to kill noncycling primary CD4+ T cells

CD4+ T-cell death is a crucial feature of AIDS pathogenesis, but the mechanisms involved remain unclear. Here, we present in vitro findings that identify a novel process of HIV1 mediated killing of bystander CD4+ T cells, which does not require productive infection of these cells but depends on the presence of neighboring dying cells. X4-tropic HIV1 strains, which use CD4 and CXCR4 as receptors for cell entry, caused death of unstimulated noncycling primary CD4+ T cells only if the viruses were produced by dying, productively infected T cells, but not by living, chronically infected T cells or by living HIV1-transfected HeLa cells. Inducing cell death in HIV1-transfected HeLa cells was sufficient to obtain viruses that caused CD4+ T-cell death. The addition of supernatants from dying control cells, including primary T cells, allowed viruses produced by living HIV1-transfected cells to cause CD4+ T-cell death. CD4+ T-cell killing required HIV1 fusion and/or entry into these cells, but neither HIV1 envelope-mediated CD4 or CXCR4 signaling nor the presence of the HIV1 Nef protein in the viral particles. Supernatants from dying control cells contained CD95 ligand (CD95L), and antibody-mediated neutralization of CD95L prevented these supernatants from complementing HIV1 in inducing CD4+ T-cell death. Our in vitro findings suggest that the very extent of cell death induced in vivo during HIV1 infection by either virus cytopathic effects or immune activation may by itself provide an amplification loop in AIDS pathogenesis. More generally, they provide a paradigm for pathogen-mediated killing processes in which the extent of cell death occurring in the microenvironment might drive the capacity of the pathogen to induce further cell death.

The relevance of apoptosis to AIDS pathogenesis

Several recent experimental findings support the hypothesis that apoptosis induced by human immune deficiency virus (HIV) is important in the pathogenesis of acquired immune deficiency syndrome (AIDS). Thus, one potential therapeutic strategy against AIDS may be to block the HIV-mediated apoptosis signal transduction pathway. Induction of apoptosis by HIV infection may prove a useful paradigm for the pathogenesis of a wide range of diseases that involve cell depletion and tissue atrophy.

Caspase-dependent and -independent T-cell death pathways in pathogenic simian immunodeficiency virus infection: relationship to disease progression

Studies of human immunodeficiency virus (HIV) and nonhuman primate models of pathogenic and nonpathogenic simian immunodeficiency virus (SIV) infections have suggested that enhanced ex vivo CD4 T-cell death is a feature of pathogenic infection in vivo. However, the relative contributions of the extrinsic and intrinsic pathways to programmed T-cell death in SIV infection have not been studied. We report here that the spontaneous death rate of CD4+ T cells from pathogenic SIVmac251-infected rhesus macaques ex vivo is correlated with CD4 T-cell depletion and plasma viral load in vivo. CD4+ T cells from SIVmac251-infected macaques showed upregulation of the death ligand (CD95L) and of the proapoptotic proteins Bim and Bak, but not of Bax. Both CD4+ and CD8+ T cells from SIVmac251-infected macaques underwent caspase-dependent death following CD95 ligation. The spontaneous death of CD4+ and CD8+ T cells was not prevented by a decoy CD95 receptor or by a broad-spectrum caspase inhibitor (zVAD-fmk), suggesting that this form of cell death is independent of CD95/CD95L interaction and caspase activation. IL-2 and IL-15 prevented the spontaneous death of CD4+ and CD8+ T cells, whereas IL-10 prevented only CD8 T-cell death and IL-7 had no effect on T-cell death. Our results indicate that caspase-dependent and caspase-independent pathways are involved in the death of T cells in pathogenic SIVmac251-infected primates.

On the origin, evolution, and nature of programmed cell death: a timeline of four billion years

Programmed cell death is a genetically regulated process of cell suicide that is central to the development, homeostasis and integrity of multicellular organisms. Conversely, the dysregulation of mechanisms controlling cell suicide plays a role in the pathogenesis of a wide range of diseases. While great progress has been achieved in the unveiling of the molecular mechanisms of programmed cell death, a new level of complexity, with important therapeutic implications, has begun to emerge, suggesting (i) that several different self-destruction pathways may exist and operate in parallel in our cells, and (ii) that molecular effectors of cell suicide may also perform other functions unrelated to cell death induction and crucial to cell survival. In this review, I will argue that this new level of complexity, implying that there may be no such thing as a 'bona fide' genetic death program in our cells, might be better understood when considered in an evolutionary context. And a new view of the regulated cell suicide pathways emerges when one attempts to ask the question of when and how they may have become selected during evolution, at the level of ancestral single-celled organisms.

On the evolution of programmed cell death: apoptosis of the unicellular eukaryote Leishmania major involves cysteine proteinase activation and mitochondrion permeabilization

Leishmania major is a protozoan parasite from one of the most ancient phylogenic branches of unicellular eukaryotes, and containing only one giant mitochondrion. Here we report that staurosporine, that induces apoptosis in all mammalian nucleated cells, also induces in L. major a death process with several cytoplasmic and nuclear features of apoptosis, including cell shrinkage, phosphatidyl serine exposure, maintenance of plasma membrane integrity, mitochondrial transmembrane potential (DeltaPsim) loss and cytochrome c release, nuclear chromatin condensation and fragmentation, and DNA degradation. Nuclear apoptosis-like features were prevented by cysteine proteinase inhibitors, and cell free assays using dying L. major cytoplasmic extracts indicated that the cysteine proteinases involved (i) also induced nuclear apoptosis-like features in isolated mammalian nuclei, and (ii) shared at least two nuclear substrates, but no cleavage site preference, with human effector caspases. Finally, isolated L. major mitochondria released cytochrome c and cysteine proteinases with nuclear pro-apoptotic activity when incubated with human recombinant Bax, even (although much less efficiently) when Bax was deleted of its transmembrane domain required for insertion in mitochondrial outermembranes, implying that L. major mitochondrion may express proteins able to interact with Bax. The recruitment of cysteine proteinases and mitochondria to the cell death machinery may be of very ancient evolutionary origin. Alternately, host/parasite interactions may have exerted selective pressures on the cell death phenotype of kinetoplastid parasites, resulting in the more recent emergence of an apoptotic machinery through a process of convergent evolution.

Possible mechanism of toxicity of zidovudine by induction of apoptosis of CD4+ and CD8+ T-cells in vivo

Some HIV-infected patients have a discordant response to highly active antiretroviral therapy with a low virus load and an incomplete restoration of CD4+ T-cell counts. Zidovudine may limit CD4+ restoration by a hematotoxic mechanism. Apoptosis and T-cell counts were assessed in two patients before and after they switched from zidovudine to stavudine. Whereas CD4+ T-cell apoptosis fell from 52% and 66% before the zidovudine switch to 7% and 12%, respectively, after the switch, the patients' CD4+ counts rose gradually to +183 and +150 cells, respectively. It was therefore hypothesized that zidovudine directly induced apoptosis. Zidovudine withdrawal could be tested before immunological interventions such as interleukin-2 therapy are considered.

Role of CD95-activated caspase-1 processing of IL-1beta in TCR-mediated proliferation of HIV-infected CD4(+) T cells

CD95 plays a critical role in the homeostasis of the immune system, and has been reported to participate in T cell death during HIV infection. Here we report that the response to CD3-TCR stimulation of CD4(+) T cells from HIV-infected individuals and CD4(+) T cells from healthy donors incubated in vitro with HIV-1(Lai) depends on the manner the CD3-TCR complex is engaged. While stimulation by anti-CD3 antibodies in solution induced CD4 T cell apoptosis both in the absence or presence of anti-CD95 antibodies, stimulation by immobilized anti-CD3 antibodies rendered CD4(+) T cells resistant to CD95-mediated death and led to increased CD4 T cell proliferation in response to CD95 ligation. CD95 ligation of CD4(+) T cells led to the activation of caspases, while costimulation induced by anti-CD3 and anti-CD95 mAb prevented the full processing of caspase-3 and caspase-8. Proliferation of CD4(+) T cells induced by CD3-TCR and CD95 costimulation was decreased by treatments with a caspase-1 inhibitor or with neutralizing antibodies to IL-1ss, indicating a requirement for caspase-1-mediated IL-1beta processing and secretion. Our findings suggest a novel mechanism whereby in addition to its role in inducing T cell apoptosis, CD95 signaling during HIV infection may also provide a costimulatory signal leading to an enhancement of CD4 T cell proliferation in response to CD3-TCR complex engagement.

Programmed cell death in mature erythrocytes: a model for investigating death effector pathways operating in the absence of mitochondria

Human mature erythrocytes have been considered as unable to undergo programmed cell death (PCD), due to their lack of mitochondria, nucleus and other organelles, and to the finding that they survive two conditions that induce PCD in vitro in all human nucleated cells, treatment with staurosporine and serum deprivation. Here we report that mature erythrocytes can undergo a rapid self-destruction process sharing several features with apoptosis, including cell shrinkage, plasma membrane microvesiculation, phosphatidylserine externalization, and leading to erythrocyte disintegration, or, in the presence of macrophages, to macrophage ingestion of dying erythrocytes. This regulated form of PCD was induced by Ca(2+) influx, and prevented by cysteine protease inhibitors that allowed erythrocyte survival in vitro and in vivo. The cysteine proteinases involved seem not to be caspases, since (i) proforms of caspase 3, while present in erythrocytes, were not activated during erythrocyte death; (ii) cytochrome c, a critical component of the apoptosome, was lacking; and (iii) cell-free assays did not detect activated effectors of nuclear apoptosis in dying erythrocytes. Our findings provide the first identification that a death program can operate in the absence of mitochondria. They indicate that mature erythrocytes share with all other mammalian cell types the capacity to self-destruct in response to environmental signals, and imply that erythrocyte survival may be modulated by therapeutic intervention.

On the evolutionary conservation of the cell death pathway: mitochondrial release of an apoptosis-inducing factor during Dictyostelium discoideum cell death

Mitochondria play a pivotal role in apoptosis in multicellular organisms by releasing apoptogenic factors such as cytochrome c that activate the caspases effector pathway, and apoptosis-inducing factor (AIF) that is involved in a caspase-independent cell death pathway. Here we report that cell death in the single-celled organism Dictyostelium discoideum involves early disruption of mitochondrial transmembrane potential (DeltaPsim) that precedes the induction of several apoptosis-like features, including exposure of the phosphatidyl residues at the external surface of the plasma membrane, an intense vacuolization, a fragmentation of DNA into large fragments, an autophagy, and the release of apoptotic corpses that are engulfed by neighboring cells. We have cloned a Dictyostelium homolog of mammalian AIF that is localized into mitochondria and is translocated from the mitochondria to the cytoplasm and the nucleus after the onset of cell death. Cytoplasmic extracts from dying Dictyostelium cells trigger the breakdown of isolated mammalian and Dictyostelium nuclei in a cell-free system, and this process is inhibited by a polyclonal antibody specific for Dictyostelium discoideum apoptosis-inducing factor (DdAIF), suggesting that DdAIF is involved in DNA degradation during Dictyostelium cell death. Our findings indicate that the cell death pathway in Dictyostelium involves mitochondria and an AIF homolog, suggesting the evolutionary conservation of at least part of the cell death pathway in unicellular and multicellular organisms.

Gamma-interferon induces apoptosis of the B lymphoma WEHI-279 cell line through a CD95/CD95L-independent mechanism

Gamma-interferon (IFN-gamma) a cytokine produced by CD4+ T helper type 1 cells, CD8+ T cells and natural killer (NK) cells, plays a central role in the development of humoral and cell-mediated immunity. IFN-gamma participates in the maturation and differentiation of B cells, but it has been previously reported that IFN-gamma may inhibit the early stages of B cell activation. We report that the inhibition of the B lymphoma cell WEHI-279-proliferation induced by IFN-gamma, involves the induction of typical features of apoptosis (nuclear chromatin condensation and fragmentation, cell shrinkage, phosphatidyl-serine (PS) exposure and mitochondrial membrane potential (delta psim) loss). IFN-gamma-mediated B cell apoptosis was decreased by the addition of the T helper type 2 cytokine, IL-4. WEHI-279 cells express CD95 and undergo apoptosis after treatment with either an agonistic anti-CD95 Ab or with a soluble recombinant CD95L. However, incubation with CD95-Fc or TRAIL-R1-Fc fusion proteins, did not prevent IFN-gamma-mediated apoptosis, suggesting that IFN-gamma-mediated apoptosis occurs independently of CD95/CD95L and TRAIL-R/TRAIL interactions. IFN-gamma-mediated apoptosis is associated with caspase-3 activation that can be prevented by the addition of the broad caspase inhibitor zVAD-fmk. These data indicate that IFN-gamma may play a major role in the regulation of B cell apoptosis, and suggest the involvement of an alternative pathway which is independent of the death receptors.

Inhibition of multicellular development switches cell death of Dictyostelium discoideum towards mammalian-like unicellular apoptosis

The multicellular development of the single celled eukaryote Dictyostelium discoideum is induced by starvation and consists of initial aggregation of the isolated amoebae, followed by their differentiation into viable spores and dead stalk cells. These stalk cells retain their structural integrity inside a stalk tube that support the spores in the fruiting body. Terminal differentiation into stalk cells has been shown to share several features with programmed cell death (Cornillon et al. (1994), J. Cell Sci. 107, 2691-2704). Here we report that, in the absence of aggregation and differentiation, D. discoideum can undergo another form of programmed cell death that closely resembles apoptosis of most mammalian cells, involves loss of mitochondrial transmembrane potential, phosphatidylserine surface exposure, and engulfment of dying cells by neighboring D. discoideum cells. This death has been studied by various techniques (light microscopy and scanning or transmission electron microscopy, flow cytometry, DNA electrophoresis), in two different conditions inhibiting D. discoideum multicellular development. The first one, corresponding to an induced unicellular cell death, was obtained by starving the cells in a "conditioned" cell-free buffer, prepared by previous starvation of another D. discoideum cell population in potassium phosphate buffer (pH 6.8). The second one, corresponding to death of D. discoideum after axenic growth in suspension, was obtained by keeping stationary cells in their culture medium. In both cases of these unicellular-specific cell deaths, microscopy revealed morphological features known as hallmarks of apoptosis for higher eukaryotic cells and apoptosis was further corroborated by flow cytometry. The occurrence in D. discoideum of programmed cell death with two different phenotypes, depending on its multicellular or unicellular status, is further discussed.

Arsenic-interferon-alpha-triggered apoptosis in HTLV-I transformed cells is associated with tax down-regulation and reversal of NF-kappa B activation

Human T-cell lymphotropic virus type I (HTLV-I)-associated adult T-cell leukemia/lymphoma (ATL) is a malignancy of mature activated T cells resistant to conventional chemotherapy. The viral transactivator protein Tax plays a critical role in HTLV-I-induced transformation and apoptosis resistance by inducing I kappa B-alpha degradation, resulting in the activation of the NF-kappa Bpathway. In these HTLV-I-transformed cells, arsenic trioxide (As) and interferon (IFN)-alpha synergize to induce cell cycle arrest and apoptosis. We demonstrate that cell death induction is only partly dependent upon caspase activation and is not associated with modulation of bcl-2, bax, or p53 expression. However, combined As and IFN induce the degradation of Tax, associated with an up-regulation of I kappa B-alpha resulting in a sharp decrease in RelA DNA binding nuclear factor (NF)-kappa B complexes because of the cytoplasmic retention of RelA. Taken the role of Tax in HTLV-I-induced transformation, its down-regulation probably accounts for cell death induction through inactivation of the NF-kappa B pathway. Such specific targeting of the viral oncoprotein by As-IFN treatment, reminiscent of As targeting of promyelocytic leukemia/retinoic acid receptor-alpha in acute promyelocytic leukemia, provides strong rational for combined As-IFN therapy in ATL patients. (Blood. 2000;96:2849-2855)

Early changes in peripheral blood T cells during primary infection of rhesus macaques with a pathogenic SIV

Primary infection of rhesus macaques with pathogenic strains of simian immunodeficiency virus (SIV) leads to rapid and dynamic changes in both viral load and T cell counts in the peripheral blood. We have performed a sequential analysis of peripheral blood CD4 and CD8 T cells in five macaques during the 8 weeks following SIVmac251 infection. We observed a transient lymphopenia of both CD4 and CD8 T cells during the first 2 weeks, followed by a rebound. The primary phase of infection was associated with changes in the T cells expressing CD25, CD69, or HLA-DR and with a priming of the peripheral blood CD4 and CD8 T cells for a process of apoptosis in vitro that was enhanced by CD95 (Fas) ligation, and was detected in two macaques as early as 7 days after infection. Despite the small numbers of animals studied, the importance of the early transient CD4 and CD8 T lymphopenia was positively correlated with the viral load. No correlation was found, however, between the level of activation markers expressed or of priming for apoptosis in peripheral blood T cells and the viral load. Our findings suggest the possibility that the early activation and priming for apoptosis of CD4 and CD8 T cells may involve indirect, host-related, mechanisms, or alternatively, that the T cells that remain in the peripheral blood during primary infection do not adequately reflect the viral-mediated changes in T cell activation and death that may occur in the lymphoid organs throughout the body.

Ineffective cellular immune response associated with T-cell apoptosis in susceptible Mycobacterium bovis BCG-infected mice

It has previously been reported that inhibition of delayed-type hypersensitivity-mediating functions of T cells during mycobacterial infection in mice is haplotype dependent. In the present study, we show that Mycobacterium bovis BCG infection induced, in susceptible C57BL/6 and BALB/c mice but not in resistant C3H/HeJ and DBA/2 mice, an important splenomegaly. An in vitro defect in T-cell proliferation in response to T-cell receptor (TCR) stimulation with mitogens or anti-CD3 antibodies was associated with enhanced levels of CD4(+) and CD8(+) T-cell apoptosis in susceptible but not in resistant mice 2 weeks after infection. Further investigations of C57BL/6 and C3H/HeJ mice revealed that in vivo splenomegaly was associated with destruction of the lymphoid tissue architecture, liver cellular infiltrates, and increased numbers of apoptotic cells in both spleen and liver tissue sections. Infection of C57BL/6 mice but not of C3H/HeJ mice induced massive production of tumor necrosis factor alpha (TNF-alpha) in serum, as well as an increase in Fas and Fas ligand (FasL) expression in T cells. In vitro addition of neutralizing anti-TNF-alpha antibodies led to a significant reduction in CD3-induced T-cell apoptosis of both CD4(+) and CD8(+) T cells of C57BL/6 mice, while the blockade of Fas-FasL interactions reduced apoptosis only in CD4(+) but not in CD8(+) T cells. Together, these results suggest that TNF-alpha and Fas-FasL interactions play a role in the activation-induced cell death (AICD) process associated with a defect in T-cell proliferation of the susceptible C57BL/6 mice. T-cell death by apoptosis may represent one of the important components of the ineffective immune response against mycobacterium-induced immunopathology in susceptible hosts.

Phosphorylation of Bcl-2 and mitochondrial changes are associated with apoptosis of lymphoblastoid cells induced by normal immunoglobulin G

Normal human immunoglobulin G induces apoptosis in human lymphoblastoid cells which involves antibody-mediated Fas ligation and the activation of caspases. Here, we show that Bcl-2 is phosphorylated on serine upon treatment of CEM T cells with normal IgG and that the overexpression of Bcl-2 in stable transfectants of CEM T cells prevents IgG-induced cell death. Treatment of CEM cells with normal IgG also results in a reduction in mitochondrial transmembrane potential and in the release of cytochrome c (Cyt c) into cytosol. The findings are concordant with earlier observations that apoptosis induced by IgG is associated with the activation of caspases. Our results demonstrate that Bcl-2 controls apoptosis induced by normal IgG and support a central role for Bcl-2 and mitochondria in antibody-mediated selection of lymphocyte repertoires.

Membrane-bound Fas (Apo-1/CD95) ligand on leukemic cells: A mechanism of tumor immune escape in leukemia patients

There is evidence from bone marrow transplantation that T cells may be involved in the immunologic control of leukemia. But many patients relapse despite a potent graft-versus-leukemia effect mediated by allogeneic T cells. The expression of the FasL protein has been suggested as a mechanism of tumor immune escape. We, therefore, evaluated the capacity of leukemic cells from patients with acute or chronic myelogenous leukemia to escape the allogeneic or autologous immune response by bearing the FasL molecule. Although almost all leukemic cells express the 37-kD form of FasL, only 54% of acute myeloblastic leukemia and 27% of chronic myeloid leukemia (CML) cells bore a FasL with killing properties, as assessed by the ability of leukemic cells to cause the apoptosis of a Fas-sensitive target cell line or autologous activated T cells in 3 tested leukemic cases. Experiments with a recombinant Fas-Fc molecule confirmed the role of Fas/FasL in leukemic-mediated cell death. Only CML leukemic cells from certain individuals contained the 26-kD truncated form of FasL. Thus, myeloid leukemic cells from some, but not all patients can set up a mechanism of immune escape involving the Fas/FasL pathway. This leukemic escape may have implications for patients eligible for adoptive cellular immunotherapy.

PML induces a novel caspase-independent death process

PML nuclear bodies (NBs) are nuclear matrix-associated structures altered by viruses and oncogenes. We show here that PML overexpression induces rapid cell death, independent of de novo transcription and cell cycling. PML death involves cytoplasmic features of apoptosis in the absence of caspase-3 activation, and caspase inhibitors such as zVAD accelerate PML death. zVAD also accelerates interferon (IFN)-induced death, suggesting that PML contributes to IFN-induced apoptosis. The death effector BAX and the cdk inhibitor p27KIP1 are novel NB-associated proteins recruited by PML to these nuclear domains, whereas the acute promyelocytic leukaemia (APL) PML/RAR alpha oncoprotein delocalizes them. Arsenic enhances targeting of PML, BAX and p27KIP1 to NBs and synergizes with PML and IFN to induce cell death. Thus, cell death susceptibility correlates with NB recruitment of NB proteins. These findings reveal a novel cell death pathway that neither requires nor induces caspase-3 activation, and suggest that NBs participate in the control of cell survival.

The immunomodulating glycoprotein extract from Klebsiella pneumoniae RU 41740 exerts a suppressive effect on human monocyte death by apoptosis

Programmed cell death or apoptosis is a physiological cell suicide process that can be suppressed by survival factors. Monocytes undergo rapid apoptosis in culture, unless signalled by cytokines or the bacterial lipopolysaccharide LPS. We have investigated the effect on monocyte apoptosis of the immunostimulating agent RU 41740 (Biostim), a glycoprotein extract from the Klebsiella pneumoniae K2O1 strain that is used for the prevention of recurrent infections. RU 41740, as LPS, strongly enhanced monocyte survival in vitro, an effect related to apoptosis suppression. RU 41740 at concentration ranging from 1 ng/ml to 10 microg/ml prevented apoptosis induced both by survival factor deprival and by gamma-irradiation. Our observation suggests that enhancement of monocyte survival may represent a component of the reported immunostimulating effect of this compound.

Cellular and molecular mechanisms of senescent erythrocyte phagocytosis by macrophages. A review

Human red blood cells (RBCs) have a life-span of 120 days in circulation, after which they are phagocytized by resident macrophages. Extensive studies have been undertaken by many investigators in order to elucidate the cellular and molecular mechanisms of the erythrophagocytosis. The critical questions addressed by physiologists, clinicians and biochemists are: 'which of the many traumatic blemishes that appear on the erythrocyte surface as it winds its way through the circulation is the primary signal for clearance of the effete RBC from the circulation?', or 'What is the critical signal that it, and it alone, will activate the resident macrophage to adhere to and engulf it?'. Numerous, and often conflicting, hypotheses have been proposed. Each investigator focusing on but one of the many modifications that afflict the cell surface of the ageing erythrocyte, viz changes in either or both the carbohydrate or peptidic moieties of glycoproteins; abolishment of the pre-existing asymmetry in the lipid bilayer with the exposure of phosphatidylserine residues; or alterations in spectrin, to mention but a few. Many of these investigators also have invoked an intermediary role for auto-immune antibodies that recognise the change(s) on the erythrocyte surface and thereby serve as opsonins as a prelude to the erythrophagocytosis. The objective of the present review is to evaluate the data in support of the various hypotheses, and to submit some of our own recent observations involving the use of flow cytometric procedures that: i) provide evidence that the cell surface sialic acid serves as a determinant of the life-span; ii) characterise the senescent erythrocyte population that is specifically captured and phagocytized by macrophages (utilising the rapid and sensitive procedure we developed for quantification of in vitro erythrophagocytosis); and finally iii) provide evidence for the existence of an alternative pathway that is independent of immunoglobulins.

Molecular mechanisms of erythrophagocytosis. Characterization of the senescent erythrocytes that are phagocytized by macrophages

We have recently developed a flow cytometric assay for the quantitation of erythrophagocytosis, using PKH 26-labeled erythrocytes as the target cells. Using this assay we have shown that there is extensive phagocytosis of desialylated erythrocytes. Furthermore, we have demonstrated that it is the densest population of erythrocytes obtained on a self-forming gradient of Percoll that shows the greatest susceptibility to phagocytosis. We designate this population of erythrocytes as fraction X; it is even denser than the fraction 5 found previously. This population of erythrocytes corresponds to zone X previously seen in the dot-plot of the flow cytometric analyses of human erythrocytes. Further scrutiny of this fraction indicates that a) it shows the greatest reactivity with annexin V, which is specific for the detection of phosphatidylserine (PS) exposed on the outer leaflet of the erythrocyte membrane, b) it is the most susceptible to erythrophagocytosis by resident murine peritoneal macrophages, and c) this erythrophagocytosis of PKH 26-labeled erythrocytes can be inhibited by annexin V and by liposomes containing PS. Scanning electron microscopy of fraction X shows two populations of erythrocytes: (A) spheroechinocytes with filipodes and (B) echinocytes without filipods. After a 2-h period of phagocytosis, the cells remaining in fraction X show a decrease in population A, commensurate with a decrease in reactivity with FITC-labeled annexin V from 65.5 to 24%.

Mycobacterium bovis Bacillus Calmette Guérin infection prevents apoptosis of resting human monocytes

Apoptosis plays an essential role in the development and homeostasis of multicellular organisms. Some infectious agents interfere with this programmed cell death to their own benefit. Here, we show that infection of resting human monocytes with Mycobacterium bovis Bacillus Calmette Guérin (BCG) increases monocyte viability by preventing them from undergoing apoptosis. Heat-killed BCG also prevented apoptosis, indicating that replication of BCG is not required to prevent cell death. Analysis of BCG-infected monocytes revealed an up-regulation of the A1 mRNA, whereas the bcl-2 mRNA was not up-regulated. Interestingly, preinfection with BCG renders the cells resistant to interleukin (IL)-10-induced apoptosis which may be one of the mechanisms mycobacteria use to modulate immune responses. BCG infection was also accompanied by an impairment of the capacity of monocytes to secrete IL-10 and by an induction of the capacity to secrete tumor necrosis factor-alpha, two cytokines known to induce and prevent human monocyte apoptosis, respectively. Since it has been reported that apoptosis is involved in killing of intracellular mycobacteria, the prevention of apoptosis may represent a strategy for mycobacterial survival in the infected host.

A role for T-helper type-1 and type-2 cytokines in the regulation of human monocyte apoptosis

T-helper type-1 (Th1) and type-2 (Th2) cytokines, respectively, favor T-cell-mediated immunity and defense against intracellular pathogens or antibody-mediated immunity and defense against extracellular pathogens. Here we report that type-1 and type-2 cytokines also exert a regulatory effect on human monocyte survival. Interleukin-12 (IL-12) enhanced survival in long-term (10 days) cultures of adherent monocytes, whereas IL-10 induced death by apoptosis. In short-term cultures (2 days), the Th2 cytokines, IL-10 and IL-4, enhanced apoptosis; however, the Th1 cytokines, IL-12 and IL-2 only showed a reducing effect on monocyte apoptosis in culture conditions that decreased monocyte adhesion leading to increased levels of spontaneous apoptosis; finally, the Th1 cytokine, interferon-gamma (IFN-gamma), acted in a dose-dependent fashion: At high concentrations, IFN-gamma enhanced apoptosis, which is an effect related to IL-10 secretion and reduced by antibodies to IL-10. Th1 cytokines reduced monocyte apoptosis induced by several stimuli: IL-2 reduced apoptosis induced by either IL-10 or high concentrations of IFN-gamma, IL-12 reduced apoptosis induced by either the ligation of the Fas (CD95) molecule or gamma-irradiation, and IFN-gamma (at low doses that did not trigger apoptosis) reduced apoptosis induced by gamma-irradiation. These findings suggest that the regulatory role of type-1 and type-2 cytokines on the development of immune responses and inflammatory reactions also involves the regulation of monocyte death by apoptosis.

Interleukin-10-mediated T cell apoptosis during the T helper type 2 cytokine response in murine Schistosoma mansoni parasite infection

The pathogenesis of infection with the helminth parasite Schistosoma (S) mansoni in mice has been reported to involve a T helper (Th)1 to Th2 cytokine switch, associated with a pathogenic granulomatous response to parasite eggs and to a global defect in Th1-cell effector functions. Here we report that the Th2 cytokine response, which begins 6 weeks after infection, at the time of parasite egg laying (i) does not occur in the context of a genuine Th1 to Th2 cytokine switch, but is associated with a persistent capacity of Th1 (or Th0) cells to secrete IL-2 and IFN-gamma in response to T cell receptor (TCR) stimulation; (ii) is associated, in vitro, with spontaneous death by apoptosis of a significant fraction of the CD4 and CD8 T cells, which is greatly enhanced by TCR stimulation; and (iii) is associated, in vivo, with numerous and large clusters of apoptotic cells in the spleen and in the inflammatory infiltrates surrounding the parasite egg deposits in the liver. The in vitro addition of antibodies to the Th2 cytokine IL-10 had both a preventive effect on TCR-induced T cell apoptosis and an enhancing effect on TCR-induced T cell secretion of Th1 cytokines. Taken together, these findings suggest that the downregulation of Th1-cell-mediated effector functions in S. mansoni-infected mice may not be related to a lack of Th1 cell production, but to a process of IL-10-mediated and activation-induced premature T cell death, that include Th1 (or Th0) cells. Further identification of mechanisms involved in the regulation of T cell apoptosis has implications for the understanding of the pathogenesis of immunosuppression associated with chronic infectious diseases.

Fas-mediated apoptosis of CD4+ and CD8+ T cells from human immunodeficiency virus-infected persons: differential in vitro preventive effect of cytokines and protease antagonists

Human immunodeficiency syndrome (HIV) infection leads to a progressive loss of T-cell-mediated immunity associated with T-cell apoptosis. We report here that CD4+ and CD8+ T cells from HIV-1-infected persons are sensitive to Fas (CD95/APO-1)-mediated death induced either by an agonistic anti-Fas antibody or by the physiologic soluble Fas ligand, although showing no sensitivity to tumor necrosis factor alpha-induced death. CD4+ and CD8+ T-cell apoptosis induced by Fas ligation was enhanced by inhibitors of protein synthesis and was prevented either by a soluble Fas receptor decoy or an antagonistic anti-Fas antibody. Fas-mediated apoptosis could also be prevented in a CD4+ or CD8+ T-cell-type manner (1) by several protease antagonists, suggesting the involvement of the interleukin-1beta (IL-1beta)-converting enzyme (ICE)-related cysteine protease in CD4+ T-cell death and of both a CPP32-related cysteine protease and a calpain protease in CD8+ T-cell death; and (2) by three cytokines, IL-2, IL-12, and IL-10, that exerted their effects through a mechanism that required de novo protein synthesis. Finally, T-cell receptor (TCR)-induced apoptosis of CD4+ T cells from HIV-infected persons involved a Fas-mediated death process, whereas TCR stimulation of CD8+ T cells led to a different Fas-independent death process. These findings suggest that Fas-mediated T-cell death is involved in acquired immunodeficiency syndrome (AIDS) pathogenesis and that modulation of Fas-mediated signaling may represent a target for new therapeutic strategies aimed at the prevention of CD4+ T-cell death in AIDS.

A combinatorial peptide library around variation of the human immunodeficiency virus (HIV-1) V3 domain leads to distinct T helper cell responses

The hypervariable domain of the HIV gp120, the V3 loop domain, represents a target for neutralizing antibodies and for HIV vaccine strategies. In this study, we have investigated in murine species the potential cross-reactivity of immune responses elicited by immunization either with individual V3 peptides, derived from distinct HIV sequences (BRU, RF, SF2, MN and ELI sequences), or with a V3 combinatorial peptide library. We observed that individual V3 peptides are immunogenic but elicit a specific B- and T-cell immune response that is mainly restricted to the sequence of the immunizing peptide. In particular, T-cell responses that depend on T-cell receptor recognition of peptides bound to the molecules encoded by the major histocompatibility complex were significantly influenced by small differences in the peptide amino acid sequence. The combinatorial V3 peptide library, previously described as B- and T-cell immunogens, induced a more broadly reactive immune response, specially when T-cell cytokine secretion was used as a readout for restimulation of T-cells with individual V3 peptides. These data suggest that amino acid variations in the sequence of an antigenic peptide could lead to the induction of different transducing signals in the primed T-cell population and to the activation of T-cells with distinct cytokine secretion properties. These observations may have implications in the understanding of antigenic variability and in the design of vaccine strategies.

T helper type 1/T helper type 2 cytokines and T cell death: preventive effect of interleukin 12 on activation-induced and CD95 (FAS/APO-1)-mediated apoptosis of CD4+ T cells from human immunodeficiency virus-infected persons

Human immunodeficiency virus (HIV) infection leads to a progressive loss of CD4+ T helper (Th) type 1 cell-mediated immunity that is associated with defective in vitro CD4+ T cell proliferation and abnormal T cell death by apoptosis in response to T cell receptor (TCR) stimulation. Quantification of interleukin (IL)-2, interferon gamma, IL-4, IL-5, and IL-10 secretion by immunoassays, and of interferon gamma, IL-4 and IL-10 messenger RNA expression by competitive reverse transcriptase polymerase chain reaction after in vitro stimulation of the TCR revealed a similar Th1 cytokine profile in T cells from HIV-infected persons and from controls. These data indicated that the loss of CD4+ Th1 cell function in HIV-infected persons is not related to a Th1 to Th2 cytokine switch as previously proposed, but to a process of activation-induced death of CD4+ Th1 cells. Despite the absence of elevated levels of Th2 cytokines, apoptosis of CD4+ T cells, but not of CD8+ T cells, was prevented in vitro by antibodies to IL-10 or IL-4, two Th2 cytokines that downregulate Th1 cell responses, or by the addition of recombinant IL-12, a cytokine that upregulates Th1 functions. TCR-induced apoptosis of T cell hybridomas and preactivated T cells has been shown to involve the CD95 (Fas/Apo-1) molecule. CD4+ and CD8+ T cells from HIV-infected persons expressed high levels of the CD95 molecule, and, in contrast to T cells from controls, were highly sensitive to antibody-mediated CD95 ligation, which induced apoptosis in a percentage of T cells similar to that induced by TCR stimulation. As TCR-induced apoptosis, CD95-mediated apoptosis of CD4+ T cells, but not of CD8+ T cells, was prevented by the addition of recombinant IL-12. Together, these findings suggest that apoptosis of CD4+ T cells from HIV-infected persons involves an abnormal sensitivity to CD95 ligation, and to TCR stimulation in the presence of normal levels of Th2 cytokines. The preventive effect of IL-12 on both mechanisms has potential implications for the design of immunotherapy strategies aimed at the upregulation of CD4+ Th1 cell functions in AIDS.

Apoptosis in a unicellular eukaryote (Trypanosoma cruzi): implications for the evolutionary origin and role of programmed cell death in the control of cell proliferation, differentiation and survival

The origin of programmed cell death (PCD) has been linked to the emergence of multicellular organisms. Trypanosoma cruzi, a member of one of the earliest diverging eukaryotes, is a protozoan unicellular parasite that undergoes three major differentiation changes and requires two different hosts. We report that the in vitro differentiation of the proliferating epimastigote stage into the G0/G1 arrested trypomastigote stage is associated with massive epimastigote death that shows the cytoplasmic and nuclear morphological features and DNA fragmentation pattern of apoptosis, the most frequent phenotype of PCD in multicellular organisms. Apoptosis could be accelerated or prevented by modifying culture conditions or cell density, indicating that extracellular signals influenced the epimastigote decision between life and death. Epimastigotes responded to complement-mediated immunological agression by undergoing apoptosis, while undergoing necrosis in response to nonphysiological saponin-mediated damage. PCD may participate into the optimal adaptation of T. cruzi to its different hosts, and the avoidance of a local competition between a G0/G1 arrested stage and its proliferating progenitor. The existence of a regulated cell death programme inducing an apoptotic phenotype in a unicellular eukaryote provides a paradigm for a widespread role for PCD in the control of cell survival, which extends beyond the evolutionary constraints that may be specific to multicellular organisms and raises the question of the origin and nature of the genes involved. Another implication is that PCD induction could represent a target for therapeutic strategies against unicellular pathogens.

YOPRO-1 permits cytofluorometric analysis of programmed cell death (apoptosis) without interfering with cell viability

In the absence of cell permeabilization, the impermeant nuclear dye YOPRO-1 permits accurate analysis of apoptosis using cytofluorometry or fluorescent microscopy. Several immune cell populations were studied including dexamethasone-treated thymocytes, irradiated peripheral blood mononuclear cells and a growth factor-depleted tumor B cell line. Excellent correlation values were found with acridine orange using cytofluorometry and with eosin-hematoxylin using optical microscopy. Under fluorescent microscopy, YOPRO-1-fluorescent cells demonstrate the morphological features of cells undergoing apoptosis such as nuclear shrinkage and fragmentation. An important characteristic of the dye that differs from all other nuclear dyes previously used for the detection of apoptosis is that it does not label living cells. Cell sorting after flow cytofluorometry analysis confirmed that only the apoptotic cell population was labelled with YOPRO-1. Further studies showed that while incubation of living cells with Hoechst 33342 almost completely abrogated the capacity of T cells to proliferate in response to several stimuli, YOPRO-1 had no inhibitory effect. This new simple, rapid and reproducible use of the YOPRO-1 dye should prove useful in the analysis of apoptotic cells as well as for investigations of the functional properties of living cells in a culture containing apoptotic cells.

[Mechanisms of immunodeficiency in human immunodeficiency virus infection]

Human immunodeficiency virus (HIV) infection leads within about 10 years to the progressive collapse of the 2 most complex regulatory networks of the human body: the immune system, and the central nervous system. AIDS results from the loss, in each organ, of a selective cell population: CD4 + T cells in the immune system, leading to immune in competence; neurons in the brain, leading to dementia. Research on AIDS pathogenesis has mainly focused on 2 different approaches: 1. the cytopathic effect of HIV is the main driving force in the collapse of the immune system; 2. the immune system is a major participant in its own destruction, and HIV is the trigger of the immunopathological disturbances that results in disease. These virological and immunological views of AIDS pathogenesis are not mutually exclusive. An integrated approach is required in order to fully understand AIDS, and to design the optimal therapeutic strategies aimed at preventing the development of disease.

From AIDS to parasite infection: pathogen-mediated subversion of programmed cell death as a mechanism for immune dysregulation

Premature cell death can result either from cell injury or degeneration, leading to necrosis, or from the activation of a physiological cell-suicide process, termed programmed cell death or apoptosis, that is regulated by intercellular signalling. This process plays an essential role in the selection of developing lymphocytes, and is also involved in the function of the mature adaptative immune system. A growing number of experimental findings during the last 4 years has provided support to our hypothesis that inappropriate HIV-mediated dysregulation of programmed T-cell death is relevant to AIDS pathogenesis. A series of recent experimental results also supports the general concept that the persistence and pathogenesis of several infectious pathogens, ranging from retroviruses to parasites, may be related to their capacity to dysregulate programmed cell death in various cell populations including lymphocytes. Subversion by pathogens of the physiological control of programmed cell death provides a paradigm for the pathogenesis of a wide range of infectious diseases that involve immune dysregulation and suggests therapeutic potential for the in vivo modulation of cell signalling.

The mixotope: a combinatorial peptide library as a T cell and B cell immunogen

We report a new approach in peptide vaccine strategy based on combinatorial synthesis. A library of 7.5 x 10(5) related peptides, termed mixotope, was derived from the sequence of the third hypervariable domain (V3 loop) of the human immunodeficiency virus (HIV) envelope protein. This preparation induced a strong immune response in all syngeneic and outbred rodents tested. The response directed against the mixotope included antibodies, CD4+ T helper cells (TH1 and TH2) and CD8+ T cells. In rodents immunized with the mixotope, the T cell response directed against individual V3 peptide sequences (BRU, MN, RF, SF2, and ELI) as measured by T cell proliferation and interleukin (IL)-2 production, was found to be major histocompatibility complex haplotype-dependent. However, additional experiments performed in mice indicated that selectivity was less restrictive when using IL-3 secretion to explore T cell activation. This combinatorial antigen could be considered as a series of agretopic motifs framing a multiplicity of closely related epitopes for T cell recognition and able to elicit a T cell and B cell repertoire. This new construct may therefore provide a basis for the design of future vaccine strategies.