Phagocyte recognition of cells undergoing apoptosis

Reduction in mitochondrial potential constitutes an early irreversible step of programmed lymphocyte death in vivo

In a number of experimental systems in which lymphocyte depletion was induced by apoptosis-inducing manipulations, no apoptotic morphology and ladder-type DNA fragmentation were detected among freshly isolated peripheral lymphocytes ex vivo. Here we report that one alteration that can be detected among splenocytes stimulated with lymphocyte-depleting doses of dexamethasone (DEX) in vivo is a reduced uptake of 3,3'dihexyloxacarbocyanine iodide (DiOC6[3]), a fluorochrome which incorporates into cells dependent upon their mitochondrial transmembrane potential (delta psi m). In contrast, ex vivo isolated splenocytes still lacked established signs of programmed cell death (PCD):DNA degradation into high or low molecular weight fragments, ultrastructural changes of chromatin arrangement and endoplasmatic reticulum, loss in viability, or accumulation of intracellular peroxides. Moreover, no changes in cell membrane potential could be detected. A reduced delta psi m has been observed in response to different agents inducing lymphoid cell depletion in vivo (superantigen and glucocorticoids [GC]), in mature T and B lymphocytes, as well as their precursors. DEX treatment in vivo, followed by cytofluorometric purification of viable delta psi mlow splenic T cells ex vivo, revealed that this fraction of cells is irreversibly committed to undergoing DNA fragmentation. Immediately after purification neither delta psi mlow, nor delta psi mhigh cells, exhibit detectable DNA fragmentation. However, after short-term culture (37 degrees C, 1 h) delta psi mlow cells show endonucleolysis, followed by cytolysis several hours later. Incubation of delta psi mlow cells in the presence of excess amount of the GC receptor antagonist RU38486 (which displaces DEX from the GC receptor), cytokines that inhibit DEX-induced cell death, or cycloheximide fails to prevent cytolysis. The antioxidant, N-acetylcysteine, as well as linomide, an agent that effectively inhibits DEX or superantigen-induced lymphocyte depletion in vivo, also stabilize the DiOC6(3) uptake. In contrast, the endonuclease inhibitor, aurintricarboxylic acid acts at later stages of apoptosis and only retards the transition from the viable delta psi mlow to the nonviable fraction. Altogether, these data suggest a sequence of PCD-associated events in which a reduction in delta psi m constitutes an obligate irreversible step of ongoing lymphocyte death, preceding other alterations of cellular physiology, and thus allowing for the ex vivo assessment of PCD.

Response to Leydig cell apoptosis in the absence of testicular macrophages

Removal of apoptotic cells from the tissues appears to be a major function of resident tissue macrophages. In order to investigate further the role of testicular macrophages after massive Leydig cell death, adult rats were injected intra-testicularly with liposome-entrapped dichloromethylene diphosphonate (Cl2MDP-lp, right testis) to deplete testicular macrophages, and with NaCl (left testis) as control. Ten days later, the animals were injected intraperitoneally with ethylene dimethane sulphonate (EDS) to induce Leydig cell apoptosis. In macrophage-containing testes there was a 2-fold increase in the number of macrophages on days 1-3 after EDS treatment and Leydig cells were completely eliminated from the interstitium by the second day after treatment. The main differences in the response to Leydig cell death in macrophage-depleted testes were: (1) an early rise in the concentration of small mononuclear, lymphocyte-like cells, (2) a greater influx of circulating monocytes, (3) the existence of variable inflammatory infiltrates on days 3-4, and (4) the disappearance of infiltrating monocytes by day 10. These results suggest that resident macrophages prevent the inflammatory reaction elicited by massive Leydig cell death.

CD36 gene transfer confers capacity for phagocytosis of cells undergoing apoptosis

Phagocyte recognition and ingestion of intact cells undergoing apoptosis are key events in this generally important program of cell death. Insufficient phagocyte capacity for apoptotic cells can result in failure to clear dying cells before membrane integrity is lost, resulting in leakage of noxious cell contents and severe tissue damage. However, no means has been available to increase phagocytic clearance of apoptotic cells. We now report that transfection of the macrophage adhesion molecule CD36 into human Bowes melanoma cells specifically conferred greatly increased capacity to ingest apoptotic neutrophils, lymphocytes, and fibroblasts, comparable to that exhibited by macrophages. Furthermore, when CD36 was transfected into another cell type with limited capacity to take up apoptotic bodies, the monkey COS-7 cell, similar effects were observed. Therefore, CD36 gene transfer can confer "professional" capacity to ingest apoptotic cells upon "amateur" phagocytes.

Lipocortin 1 in apoptosis: mammary regression

BACKGROUND

Enigmatically, degradation of debris generated in programmed cell death (apoptosis) elicits little inflammation. Having previously detected the upregulation of lipocortin 1 (LC1), a 35-kDa protein with anti-inflammatory and immuno-suppressive properties, at sites of non-inflammatory phagocytosis in the central nervous system (J Neurosci Res 36:491-500, 1993), we sought to determine if LC1 was involved in apoptosis.

METHODS

LC1 immunoreactivity in mammary glands of adult rats was quantified in situ using video microdensitometry before and during postlactational regression.

RESULTS

LC1 is present in the mammary ducts but is absent from the alveoli during lactation. One day after weaning, however, LC1 is detected in the lactiferous cells and, as apoptosis proceeds over the ensuing 4 days, total LC1 in the gland increases > 10-fold over resting levels. LC1 remains high in both the apoptotic cells and epithelial phagocytes through day 10, but the total LC1 per gland drops as the apoptotic cells are cleared.

CONCLUSIONS

Published experiments have shown that LC1 specifically binds Ca++ and phosphatidylserine, and that these affinities are modulated by tyrosine phosphorylation and cross-linking with transglutaminase. Thus, LC1 appears to be a candidate for several putative activities in apoptosis (e.g., phagocyte recognition via phosphatidylserine binding and/or buffering intracellular Ca++) in addition to its anti-inflammatory role.

Phenotypic and functional changes in peripheral blood monocytes during progression of human immunodeficiency virus infection. Effects of soluble immune complexes, cytokines, subcellular particulates from apoptotic cells, and HIV-1-encoded proteins on monocytes phagocytic function, oxidative burst, transendothelial migration, and cell surface phenotype

We postulated that changes in the cell surface display of molecules that facilitate cell-cell and cell-matrix adhesions may reflect the changing immunosurveillance capacity of blood monocytes during progression of human immunodeficiency virus (HIV) infections. In Centers for Disease Control (CDC) stage A patients, whose monocytes' ability to phagocytose bacteria and generate reactive oxygen intermediates is often increased, the frequency of monocytes expressing CD49d, HLA-DP, HLA-DQ, and an activation epitope of CD11a/CD18 was increased and monocyte transendothelial migration was unimpaired. In CDC stage B/C patients, whose monocytes' ability to phagocytose bacteria and migrate across confluent endothelial monolayers was diminished, surface expression of CD49e and CD62L and the percentage of monocytes expressing CD18, CD11a, CD29, CD49e, CD54, CD58, CD31, and HLA-I were significantly decreased. Incubating normal donor monocytes with immune complexes in vitro reproduced the phenotypic and functional abnormalities seen in stage B/C patients. By contrast, in vitro stimulation with subcellular particulates released by apoptotic lymphocytes reproduced changes seen in stage A patients' monocytes. Although circulating monocytes appear to be activated at all stages, these data suggest that the high levels of circulating immune complexes, found predominantly in the later stages of HIV infection, may be particularly instrumental in reducing the monocyte's capacity to maintain surveillance against infection.

Apoptosis and functional Fas antigen in rheumatoid arthritis synoviocytes

OBJECTIVE

To determine whether apoptosis occurs in rheumatoid arthritis (RA) synoviocytes, and if this phenomenon is dependent on the Fas/Apo-1 pathway.

METHODS

Apoptotic change in vivo was examined in RA synovial cells by several standard methods. The ability of cells to undergo Fas-induced apoptosis was determined in vitro.

RESULTS

Typical apoptotic change was demonstrated in RA synovial cells by each method. Anti-Fas antibody induced apoptotic synovial cell death in vitro.

CONCLUSION

This is the first reported study to demonstrate apoptosis in RA synovial cells. The findings indicate that rheumatoid synoviocytes undergo Fas-mediated apoptosis.

Impaired alveolar macrophage function in smoke inhalation injury

The high incidence of both bacterial pneumonia and the adult respiratory distress syndrome (ARDS) associated with smoke inhalation injury (SII) may result, at least in part, from smoke-induced injury to the alveolar macrophage (AM). Specifically, we hypothesized that AM antimicrobial function, ability to phagocytose apoptotic PMNs, and capacity to prevent apoptosis in PMNs are impaired by smoke. To test these hypotheses, AMs were harvested by bronchoalveolar lavage from sheep before and after the animal was exposed to cotton smoke. The two populations of AMs were incubated with Pseudomonas aeruginosa (PSA) in vitro. Normal AMs (NAMs) phagocytosed a mean of 99 +/- 11% of the PSA placed in their wells, whereas smoke-exposed AMs (SAMs) ingested only 60 +/- 8%. NAMs killed 80 +/- 8% of PSA ingested, whereas SAMs killed only 56 +/- 16% (P < 0.05). When sheep PMNs, allowed to undergo apoptosis, were incubated with the two AM populations, 66 +/- 3% of the NAMs and 40 +/- 6% of the SAMs demonstrated phagocytosis of these apoptotic PMNs (P < 0.05). Fresh sheep PMNs were incubated in unconditioned media, NAM and SAM-conditioned media, and followed over 48 hr for the development of apoptosis and maintenance of viability. The NAM-conditioned media markedly prevented apoptosis and augmented PMN survival relative to the unconditioned and SAM-conditioned media (P < 0.05). The poor antimicrobial function known to be characteristic of apoptotic PMNs, together with the directly impaired antimicrobial function of AMs, may contribute to the infectious complications of SII. If the PMNs recruited to the lung in SII are not properly supported by the AMs following smoke injury, large numbers may undergo apoptosis. If not properly disposed of by these SAMs, the apoptotic PMNs could eventually lyse, releasing tissue toxins, resulting in escalation of lung injury and leading to ARDS.

Proteolysis of fodrin (non-erythroid spectrin) during apoptosis

Several recent studies have implicated proteases as important triggers of apoptosis. Thus far, substrates that are cleaved during apoptosis have been elusive. In this report we demonstrate that cleavage of alpha-fodrin (non-erythroid spectrin) accompanies apoptosis, induced by activation via the CD3/T cell receptor complex in a murine T cell hybridoma, ligation of the Fas (CD95) molecule on a human T cell lymphoma line and other Fas-expressing cells, or treatment of cells with staurosporine, dexamethasone, or synthetic ceramide. Furthermore, inhibition of activation-induced apoptosis by pretreatment of T hybridoma cells with antisense oligonucleotides directed against c-myc also inhibited fodrin proteolysis, confirming that this cleavage process is tightly coupled to apoptosis. Fodrin cleavage during apoptosis may have implications for the membrane blebbing seen during this process.

A macrophage receptor for oxidized low density lipoprotein distinct from the receptor for acetyl low density lipoprotein: partial purification and role in recognition of oxidatively damaged cells

The binding and uptake of oxidatively modified low density lipoprotein (OxLDL) by mouse peritoneal macrophages occurs, in part, via the well characterized acetyl LDL receptor. However, several lines of evidence indicate that as much as 30-70% of the uptake can occur via a distinct receptor that recognizes OxLDL with a higher affinity than it recognizes acetyl LDL. We describe the partial purification and characterization of a 94- to 97-kDa plasma membrane protein from mouse peritoneal macrophages that specifically binds OxLDL. This receptor is shown to be distinct from the acetyl LDL receptor as well as from two other macrophage proteins that also bind OxLDL--the Fc gamma RII receptor and CD36. We suggest that this OxLDL-binding membrane protein participates in uptake of OxLDL by murine macrophages and also represents a receptor responsible for macrophage binding and phagocytosis of oxidatively damaged cells.

Upregulation of surface markers on dying thymocytes

Using terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) to detect cells undergoing early apoptosis, we have defined the surface markers expressed on CD4+CD8+ thymocytes undergoing spontaneous or steroid-induced apoptosis in tissue culture. Some surface markers, e.g., CD4, CD8, and heat stable antigen, are downregulated on apoptotic thymocytes. Surprisingly, however, other markers are upregulated; this applies to T cell receptor beta/CD3, CD69, and CD25 expression. Upregulation of these markers is restricted to a discrete subset of apoptotic cells.

Apoptosis in B lymphocytes: the WEHI-231 perspective

In this review we summarize recent work on the molecular basis of apoptosis in the murine B cell lymphoma WEHI-231. WEHI-231 cells undergo apoptosis in response to antigen receptor cross-linking with anti-Ig reagents. Death is specifically triggered via surface IgM (sIgM); cross-linking sIgD, Ia or FcR has no effect. Apoptosis is preceded by growth arrest in the G0/G1 phase of the cell cycle and may not occur in all currently available WEHI-231 sublines. The continuous passage of WEHI-231 cells in different laboratories has yielded variants that differ greatly in their response to anti-Ig treatment because apoptotic cells tend to be negatively selected in culture. Resistant and susceptible variants undergo growth arrest in response to anti-Ig but only susceptible cells go on to die by apoptosis. Cells resistant to anti-Ig have intact apoptotic machinery as indicated by their susceptibility to dexamethasone, irradiation and other treatments. However, anti-Ig-resistant cells are also resistant to apoptosis induced by the immunosuppressants cyclosporin A, FK-506 and rapamycin. We discuss the experimental evidence indicating that the apoptotic machinery in WEHI-231 cells is pre-activated but under constant negative regulation by short-lived protein inhibitors. Inhibition is removed by a mediator released in response to anti-Ig treatment in susceptible sublines. The mediator of death is the sphingosine derivative, ceramide, presumably produced by membrane sphingomyelinases activated by anti-Ig. A hypothetical model on how ceramide kills WEHI-231 is presented.

Morphostasis and immunity

Scientists have traditionally been resistant to fundamental changes in perspective. New ideas are rejected if they challenge essential, accepted paradigms. Here I present a concept that, I believe, represents a paradigm shift in the way self/non-self discrimination is perceived. Traditional opinion has it that lymphocytes carry out this discrimination. I propose an alternative view. Self/non-self discrimination is driven by mechanisms closely related to those that lead to cell sorting in disaggregated embryos. Lymphocytes are only used to classify cells according to their mode of death (apoptosis or necrosis). The hypothesis outlines the process of morphostasis (tissue homeostasis). It fills in much detail about the gradual evolution of the mammalian immune system. Earlier versions of this hypothesis have been reflexly rejected by numerous journals. Until recently, I too was unsure of the validity of the core concept. Recent publications have dispelled this doubt from my mind. A paradigm shift is due.

Granulocyte macrophage colony-stimulating factor contributes to enhanced monocyte survival in chronic atopic dermatitis

Evidence suggesting that prolonged effector cell survival may contribute to perpetuation of inflammation prompted us to ask whether monocyte macrophages, the predominate inflammatory cell in the lesion of chronic atopic dermatitis (AD), exhibit enhanced survival in AD. Cultures of peripheral blood monocytes from patients with chronic AD, psoriasis, and from normal (NL) donors were examined for morphologic features and DNA fragmentation characteristic of cells undergoing the process of apoptosis (programmed cell death). Cultures of AD monocytes exhibited a significantly lower incidence of apoptosis than did cultures of NL monocytes (45 vs 68%, P < 0.01), or psoriatic monocytes (45 vs 80%, P < 0.01). Furthermore, AD monocytes were unresponsive to both IL-1, an inhibitor of apoptosis, and IL-4, an enhancer of apoptosis, in comparison to cultured NL monocytes. Of note, GM-CSF in a concentration-dependent fashion, decreased the incidence of apoptosis in NL monocyte cultures and rendered them unresponsive to these cytokines. These findings suggested that GM-CSF may enhance monocyte survival in AD. In support of this hypothesis, AD monocyte cultures produced fivefold more GM-CSF than did cultures of NL monocytes or psoriatic monocytes (P < 0.05). Additionally, there was a significantly greater number of GM-CSF mRNA expressing cells detected by in situ hybridization in biopsies of lesions of chronic AD than in acute AD or NL skin (P < 0.05). Finally, NL monocytes incubated with supernatants obtained from monocytes of AD patients exhibited significant inhibition of apoptosis, an effect that could be ablated by a neutralizing antibody to GM-CSF. Taken together, these data strongly suggest that increased production of GM-CSF by cells from patients with AD inhibits monocyte apoptosis and may contribute to the chronicity of this inflammatory disease.

Mechanisms for nitric oxide-induced cell death: involvement of apoptosis

Recent observations suggest that apoptosis, the natural, active cell death process is also triggered in several pathological conditions including ischemic brain insult, and neuro-degenerative and autoimmune diseases. We have investigated the mechanisms involved in the development of apoptosis in neuronal and pancreatic cells and in macrophages, which were exposed to either chemical donors of nitric oxide or to inducers of the nitric oxide synthase. In this overview, we summarize current evidence for the involvement of apoptosis in the cytotoxicity of nitric oxide and discuss possible mechanisms that may lead to cell death.

Programmed cell death in development

Although cell death has long been recognized to be a significant element in the process of embryonic morphogenesis, its relationships to differentiation and its mechanisms are only now becoming apparent. This new appreciation has come about not only through advances in the understanding of cell death in parallel immunological and pathological situations, but also through progress in developmental genetics which has revealed the roles played by death in the cell lineages of invertebrate embryos. In this review, we discuss programmed cell death as it is understood in developmental situations, and its relationship to apoptosis. We describe the morphological and biochemical features of apoptosis, and some methods for its detection in tissues. The occurrence of programmed cell death during invertebrate development is reviewed, as well as selected examples in vertebrate development. In particular, we discuss cell death in the early vertebrate embryo, in limb development, and in the nervous system.

The effects of glucocorticoids on human eosinophils

It is readily apparent that eosinophils are important targets of steroid effects. Disruption of the cytokine network by glucocorticoids can indirectly diminish eosinophil generation, survival, and function. In addition, glucocorticoids appear to have direct effects on the survival, and perhaps on the function, of these cells. Rapid advances in our knowledge of the biochemistry of signal transduction, as well as the interaction between cell surface receptors and RNA transcription machinery, will allow detailed analysis of the mechanisms by which the function and life cycle of eosinophils are influenced by glucocorticoids.

The immunological potential of apoptotic debris produced by tumor cells and during HIV infection

Apoptosis is a major cause of cell death in health and disease. In contrast to necrosis, apoptosis does not induce an inflammatory response and the cellular debris produced by apoptosis has been assumed to be biologically inert. This review challenges this assumption by suggesting that apoptotic debris (especially in the context of growing tumors or during HIV infection) may have immunological activities, mainly immunosuppressive but perhaps also immunostimulatory. In many cases, the surface of apoptotic cells differs from normal cells in that phosphatidylserine (PS) is aberrantly exposed on the external face of the cell membrane. Liposomes composed of PS may down-modulate macrophage anti-leishmanial activities, suppress macrophage TNF production, suppress lymphocyte proliferation, and increase macrophage proliferation. "Membrane shedding" has been described in certain malignancies where apoptosis may be occurring, and the shed tumor membrane vesicles have been shown to reduce MHC class II expression on macrophages and decrease lymphocyte responsiveness, perhaps because of their ganglioside content. Finally, the apoptotic debris from HIV-infected cells may bear on its surface viral proteins which contain immunosuppressive peptide sequences. This debris may also use viral envelope proteins to fuse into macrophages and thereby avoid phagocytosis and lysosomal destruction. These considerations suggest that the flux of apoptosing cells and debris through the immune system that occurs during tumor growth and HIV infection should not be assumed to be immunologically neutral. In particular, HIV-related apoptosis may have immunosuppressive effects in addition to the numerical depletion of lymphocytes.

Protein-tyrosine phosphorylation regulates apoptosis in human eosinophils and neutrophils

Early signaling events that control the process of programmed cell death are largely unknown. Tyrosine phosphorylation plays a major role in transmembrane signal transduction through most cell surface receptors. Granulocyte/macrophage colony-stimulating factor (GM-CSF), a cytokine released by activated T cells, has been shown to increase tyrosine phosphorylation in several cells and to inhibit granulocyte cell death in vitro. In this study, we demonstrate that the effect of GM-CSF on granulocyte cell death can be blocked by the tyrosine kinase inhibitor genistein, suggesting that increases in tyrosine phosphorylation are essential to inhibit cell death. To analyze the role of tyrosine phosphorylation for the regulation of granulocyte cell death more precisely, we increased levels of tyrosine phosphorylation using the protein-tyrosine phosphatase inhibitor phenylarsine oxide (PAO). Similar to GM-CSF, treatment of the cells with PAO was followed by high increases in tyrosine phosphorylation and inhibition of programmed cell death in human eosinophils and neutrophils. Strikingly, at low concentrations of the inhibitor and low induction of tyrosine phosphorylation, acceleration of apoptosis was observed. Genistein and herbimycin A reversed the effects of PAO on tyrosine phosphorylation and granulocyte apoptosis. These results suggest that programmed eosinophil and neutrophil death is regulated by early events of signal transduction pathways such as tyrosine phosphorylation.

T-cell apoptosis detected in situ during positive and negative selection in the thymus

Because of positive and negative selection to molecules of the major histocompatibility complex (MHC), only a small proportion of the massive numbers of T cells generated in the thymus are selected for export. Immature thymocytes have a rapid turnover, and it has long been assumed that most thymocytes die in situ, presumably from apoptosis. This has yet to be proved, however, and conventional staining techniques have shown only minimal evidence of cell death in the normal thymus. Using a method for detecting cells with DNA strand breaks, we now present direct evidence for apoptosis in the normal thymus. In sections of thymus from adult mice, apoptotic cells are scattered throughout the cortex and are engulfed locally by F4/80+ macrophages. Apoptosis in the thymic cortex is not reduced in MHC-deficient mice, which suggests that T-cell death is primarily a reflection of lack of positive selection rather than negative selection. Direct evidence for apoptosis due to negative selection was obtained by crossing a V beta 5 transgenic line to I-E+ and I-E- mice: I-E+ mice are known to eliminate V beta 5+ T cells in the thymus whereas I-E- mice do not. In marked contrast to I-E- mice, the medulla of I-E+ V beta 5 transgenic mice contains dense aggregates of apoptotic cells; these cells are engulfed by a distinct population of F4/80- MAC-3+ macrophages. Negative selection of V beta 5+ cells is thus restricted to the medulla.

Recognition of apoptotic cells by human macrophages: inhibition by a monocyte/macrophage-specific monoclonal antibody

Cells undergoing death by apoptosis are rapidly engulfed by phagocytes in vivo, a highly efficient process which prevents leakage of potentially dangerous intracellular contents from dying cells to neighboring tissue. We have tested a panel of monoclonal antibodies (mAb) specifying a range of human monocyte/macrophage surface antigens for their capacity to inhibit the in vitro recognition of apoptotic cells by human peripheral blood monocyte-derived macrophages. The results identify the antigen defined by the 61D3 mAb, a widely-used marker of monocyte/macrophage lineage cells, as an important mediator of apoptotic cell recognition. In our system, apoptotic, but not viable, cells were recognized by the cultured macrophages and 61D3 was found to inhibit the recognition of all apoptotic cell types tested, including Ca2+ ionophore-treated or growth factor-depleted B and T lymphocyte lines, tonsillar germinal center B cells, irradiated peripheral blood lymphocytes and senescing neutrophils. Furthermore, the apoptotic cell recognition pathway specified by 61D3 could be distinguished from that involving the macrophage alpha v beta 3 vitronectin receptor which has been shown previously to play an important role in the recognition of apoptotic cells. These results provide further evidence that the mechanisms underlying rapid clearance of apoptotic cells involve multiple phagocyte receptors.

The specific recognition by macrophages of CD8+,CD45RO+ T cells undergoing apoptosis: a mechanism for T cell clearance during resolution of viral infections

During viral infections, CD8+,CD45RO+ T populations expand. These primed cells express abundant levels of cytoplasmic granules that contain perforin and TIA-1. Recent work has suggested that the majority of this CD8+ population downregulates Bcl-2 protein expression and is destined to undergo apoptosis. In this study we have investigated the elimination of these apoptotic CD8+ T cells by both human monocyte-derived and murine bone marrow macrophages. We have found that these phagocytes recognize and ingest both apoptotic CD8+ and CD4+ T cells using an alpha v beta 3 (vitronectin receptor)/CD36/thrombospondin recognition system, with the same receptors being used in the recognition of apoptotic neutrophils. These data provide new evidence for a mechanism that enables the clearance of greatly increased populations of CD8+ effector cells which are found during viral infections. This enables cellular homeostasis to occur in the host upon resolution of viral diseases in vivo.

Upregulation of the macrophage scavenger receptor in response to different forms of injury in the CNS

The monoclonal antibody 2F8 was used to localize the macrophage scavenger receptor by immunohistochemistry. In control adult mice, macrophage scavenger receptor expression in the brain was restricted to stromal and epiplexus macrophages of the choroid plexus, meningeal macrophages and to perivascular sites. Microglia did not express the receptor. In the developing mouse brain, macrophage scavenger receptor expression was high on meningeal macrophages and detectable on immature microglia in the supraventricular corpus callosum, cingulum, cavum septum and the periaqueductal area. In the aged mouse brain, the pattern of macrophage scavenger receptor expression was no different from that in the young adult brain. Macrophage scavenger receptor expression on resident microglia and recruited macrophages was detected 24 h after an intrahippocampal injection of either lipopolysaccharide or kainic acid. Macrophage scavenger receptor expression was also detected in microglia 3 days after optic nerve crush both in the nerve segment distal to the crush site and in the superior colliculus. These studies indicate a potential role for the macrophage scavenger receptor in the CNS in the clearance of debris during acute neuronal degeneration.

Activation of the alternative pathway of complement by apoptotic Jurkat cells

Jurkat T cells die of apoptosis upon exposure to anti-Fas mAb. Here we show that although the alternative complement pathway generally does not attack homologous cells, anti-Fas-induced apoptotic Jurkat T cells were attacked antibody-independently by the alternative pathway of human complement and opsonized with iC3b, which is a ligand of the complement receptor type 3 (CR3) of phagocytes. These results suggest that apoptotic cells become the targets of the homologous alternative complement pathway, which facilitates the clearance of apoptotic cells by phagocytes.

In vitro inhibition of natural-killer-mediated lysis by chromatin fragments

A qualitative impairment of natural killer (NK) function and the presence of circulating DNA have been independently reported in clinical situations such as cancer and lupus. The existence of receptors for chromatin fragments at the leukocyte membrane raised the question of the relation between the presence of chromatin fragments in the extracellular medium and the impairment of NK function. The present study shows that plasmas from patients with metastatic cancer and with pathological DNA concentrations inhibited significantly the NK activity of normal lymphocytes as compared to cancer plasmas with DNA concentrations in the normal range. In vitro, it was demonstrated that chromatin fragments inhibited the NK-mediated cytotoxicity in a dose-dependent manner. Inhibitory concentrations of nucleosomes (2.5-10 micrograms/ml) were lower than those of DNA and histones alone (100 micrograms/ml). Inhibitory effects of nucleosomes, DNA and histones differed also according to the effector population used: nucleosomes were effective whatever the CD56+ cell enrichment of the effector population, while DNA inhibition needed T cells, and histone inhibition probably resulted from a subtoxic effect, prevented by the presence of adherent cells. Finally we found that nucleosomes could inhibit the NK function only when they were present in the extracellular medium. Taken together, these data suggest that the persistence of nucleosomal DNA at sites of cell death or in the blood might be responsible, at least partly, for the NK activity impairment observed in pathological circumstances characterized by a high rate of cell death phenomena such as cancer.

Isolation of a nitric oxide inhibitor from mammary tumor cells and its characterization as phosphatidyl serine

Macrophages from mice bearing large D1-DMBA-3 mammary tumors have a decreased capacity to kill tumor targets. This effect is due to an impaired ability to produce nitric oxide (NO) in response to lipopolysaccharide (LPS) stimulation. Here we report that the DA-3 tumor cell line, derived from the in vivo adenocarcinoma D1-DMBA-3, produces a factor that inhibits both NO production/release and cytotoxicity of LPS-activated peritoneal exudate macrophages (PEM). However, other complex macrophage functions such as phagocytosis, superoxide production, mitochondrial dehydrogenase activity, and synthesis of proteins were not reduced by this factor. The NO inhibitor has been found to be lipid in nature. Lipid extracts from DA-3 cell culture supernatants were purified by repeated silica gel column chromatography. The active molecule was unambiguously characterized as phosphatidyl serine (PS) by fast atom bombardment tandem mass spectrometry. Preliminary results indicate a lack of induced NO synthase (iNOS) activity in the lysates of LPS-activated PEM pretreated with PS. The ubiquity of PS in the inner leaflet of biological membranes and its NO inhibitory property, suggest that this phospholipid may be one of the long elusive molecules responsible for regulating physiological levels of NO in the host and hence preventing cellular dysfunction and/or tissue damage. Furthermore, the possible overexpression and shedding of PS by DA-3 tumor cells may represent a novel mechanism to impair macrophage cytotoxicity, a host function that contributes to the protection against developing neoplasms.

Granulocyte apoptosis and the control of inflammation

We have described a novel pathway available for the clearance of extravasated granulocytes from inflamed tissues whereby aging granulocytes undergo apoptosis, a process which leads to their phagocytosis by inflammatory macrophages. By contrast with necrosis, which may also be seen at inflamed sites, apoptosis represents a granulocyte fate which by a number of mechanisms would tend to limit inflammatory tissue injury and promote resolution rather than progression of inflammation: (i) apoptosis is responsible for macrophage recognition of senescent neutrophils with intact cell membranes which exclude vital dyes and retain their potentially histotoxic granule contents; (ii) the apoptotic neutrophil loses its ability to secrete granule enzymes on deliberate external stimulation; (iii) the macrophage possesses a huge phagocytic capacity for apoptotic neutrophils which it rapidly ingests and degrades without disgorging neutrophil contents; and (iv) the macrophage utilizes a novel phagocytic recognition mechanism which fails to trigger the release of pro-inflammatory macrophage mediators during the phagocytosis of apoptotic neutrophils. Preliminary characterization of the recognition mechanism implicates the integrin alpha v beta 3 (vitronectin receptor) and CD36 (thrombospondin receptor) on the macrophage surface. Macrophage phagocytosis of apoptotic neutrophils is greatly influenced by the microenvironmental pH and by the presence of cationic molecules. Moreover, it can be specifically modulated by external cytokines and intracellular second messenger systems. By controlling the functional longevity of neutrophil and eosinophil granulocytes and their subsequent removal by macrophages, granulocyte apoptosis, with its potential for modulation by external mediators, is likely to play a key dynamic role in the control of the 'tissue load' of granulocytes at inflamed sites.(ABSTRACT TRUNCATED AT 250 WORDS)

p53-dependent apoptosis suppresses tumor growth and progression in vivo

To determine the contribution of p53 loss to tumor progression, we have induced abnormal proliferation in the brain choroid plexus epithelium of transgenic mice using a SV40 T antigen fragment that perturbs pRB family function but does not affect p53 function. Tumors induced by this mutant develop slowly compared with those induced by wild-type T antigen. Suppressed tumor growth is directly attributable to p53 function, since rapid tumor development occurs when the T antigen fragment is expressed in p53-null mice. In p53-heterozygous mice, stochastic loss of the wild-type p53 allele results in the focal emergence of aggressive tumor nodules characteristic of tumor progression. In each case, aggressive tumor development in the absence of p53 function corresponds to a decrease in the level of apoptosis. These results provide in vivo evidence that p53-dependent apoptosis, occurring in response to oncogenic events, is a critical regulator of tumorigenesis.

Cell-mediated cytotoxic mechanisms

There are two competing, but probably really complementary, models for the mechanism of cell-mediated cytotoxicity. One depends upon contact-mediated transmembrane signaling, and the other on the exocytosis of toxic materials by the killer cell. There is exciting news on both fronts. Transmembrane signaling has been shown to involve the surface molecule Fas/APO-1 on targets and its ligand on cytotoxic T cells. The Fas ligand has been cloned, and is a member of the tumor necrosis factor family. The major cytolytic molecule in the exocytosis pathway is perforin; perforin knock-out mice have been produced, and they display many intriguing abnormalities. It has been a bumper year for cytotoxicologists.

Pharmacological inhibition of programmed lymphocyte death

Programmed cell death (PCD) is a necessary process that helps to regulate the lifespan of lymphocytes and maintain the compartmental balance of lymphoid organs. In addition, PCD is required for the generation and maintenance of self-tolerance. Strategies that inhibit PCD cause profound alterations in the (patho)physiology of the immune system. Here, Guido Kroemer and Carlos Martínez-A. discuss the multiplicity of PCD-inducing pathways, which have been revealed through the use of PCD-inhibitory agents, and analyse the levels at which these agents act.

Flow-cytometric analysis of apoptotic and nonapoptotic T-cell receptor-transgenic thymocytes following in vitro presentation of antigen

The use of flow cytometry to detect apoptotic thymocytes is now well established. We have further developed the technique of Hoechst 33342 vital staining to identify discrete stages of murine thymocyte apoptosis (induced by 37 degrees C culture), in conjunction with propidium iodide (PI), cell scatter profile, and surface marker analysis. The first detectable stage was an increase in Hoechst fluorescence without any change in plasma membrane permeability (measured by PI staining). At this early stage thymocytes had already reduced in size, fragmented their DNA, and for the predominant CD4+ CD8+ double positive population, reduced expression of CD4 and CD8. Subsequent to this stage thymocytes continued to reduce in size and decrease expression of CD4 and CD8, though this was accompanied by an increase in membrane permeability. This technique was applied to an in vitro antigen-specific deletion system, where apoptosis of T cell-receptor-transgenic thymocytes was induced upon presentation of self-antigen. Although self-antigen-induced apoptotic thymocytes showed similar characteristics to those undergoing spontaneous apoptosis, there was a significant population of nonapoptotic CD4+ 8+ thymocytes that also had reduced expression of CD4 and CD8. Therefore, we have been able to show that the reduced expression of CD4 and CD8 is not limited to apoptotic thymocytes.

Apoptosis. Its significance in cancer and cancer therapy

Apoptosis is a distinct mode of cell death that is responsible for deletion of cells in normal tissues; it also occurs in specific pathologic contexts. Morphologically, it involves rapid condensation and budding of the cell, with the formation of membrane-enclosed apoptotic bodies containing well-preserved organelles, which are phagocytosed and digested by nearby resident cells. There is no associated inflammation. A characteristic biochemical feature of the process is double-strand cleavage of nuclear DNA at the linker regions between nucleosomes leading to the production of oligonucleosomal fragments. In many, although not all of the circumstances in which apoptosis occurs, it is suppressed by inhibitors of messenger RNA and protein synthesis. Apoptosis occurs spontaneously in malignant tumors, often markedly retarding their growth, and it is increased in tumors responding to irradiation, cytotoxic chemotherapy, heating and hormone ablation. However, much of the current interest in the process stems from the discovery that it can be regulated by certain proto-oncogenes and the p53 tumor suppressor gene. Thus, c-myc expression has been shown to be involved in the initiation of apoptosis in some situations, and bcl-2 has emerged as a new type of proto-oncogene that inhibits apoptosis, rather than stimulating mitosis. In p53-negative tumor-derived cell lines transfected with wild-type p53, induction of the gene has, in rare cases, been found to cause extensive apoptosis, instead of growth arrest. Finally, the demonstration that antibodies against a cell-surface protein designated APO-1 or Fas can enhance apoptosis in some human lymphoid cell lines may have therapeutic implications.

bcl-2 inhibits apoptosis of neutrophils but not their engulfment by macrophages

Neutrophils, the most common inflammatory leukocytes, have the most limited life span of all blood cells. After they undergo apoptosis, they are recognized and engulfed by macrophages. bcl-2, a proto-oncogene rearranged and deregulated in B cell lymphomas bearing the t(14;18) translocation, is known to inhibit programmed death. bcl-2 expression is localized in early myeloid cells of the bone marrow but is absent in mature neutrophils. Transgenic mice that expressed bcl-2 in mature neutrophils showed that bcl-2 blocked neutrophil apoptosis. Despite this, homeostasis of neutrophil population is essentially unaffected. In fact, macrophage uptake of neutrophils expressing bcl-2 still occurred. This transgenic model indicates that the mechanism that triggers phagocytosis of aging neutrophils operates independently of the process of apoptosis regulated by bcl-2.

Interleukin-2 prevention of apoptosis in human neutrophils

Evidence is presented that interleukin (IL)-2 maintains viability of human polymorphonuclear cells (PMN) in culture by preventing these cells from undergoing programmed cell death (PCD) and induces the synthesis of new RNA and protein. Our laboratory has recently discovered that human PMN constitutively express IL-2 beta receptor and more importantly, PMN are able to respond functionally to IL-2 by enhanced growth inhibitory activity against an opportunistic fungal pathogen, Candida albicans. We now report that IL-2 was able to interfere with the PCD process and reduce the number of apoptotic PMN to < 40% in 72-h culture. Freshly isolated PMN usually underwent a time-dependent aging process and > 80% of PMN cultured in medium alone for 72 h showed morphologic features of PCD as depicted by hematoxylin and eosin staining as well as by electron microscopy. During the PCD process, untreated PMN not only exhibited condensed nuclear structure and decrease in cell size, but also displayed DNA fragmentation. DNA fragmentation in PMN was prevented by IL-2. Prevention of PCD by IL-2 was associated with an increase in new RNA and protein synthesis in PMN, which may reflect cytokine induction, such as tumor necrosis factor, as we have recently shown. Thus, our data expands our current understanding of PMN in that they may be an active component of the immune system, with a longer life-span when activated than expected.

Growth factors as survival factors: regulation of apoptosis

Apoptosis is now widely recognized as a common form of cell death and represents a mechanism of cell clearance in many physiological situations where deletion of cells is required. Peptide growth factors, initially characterised as stimulators of cell proliferation, have now been shown to inhibit death in many cell types. Deprivation of growth factors leads to the induction of apoptosis, i.e. condensation of chromatin and degradation in oligonucleosome-sized fragments, formation of plasma and nuclear membrane blebs and cell fragmentation into apoptotic bodies which can be taken up by neighbouring cells. Here we discuss the mechanism(s) by which growth factors may inhibit apoptosis.

Dicing with death: dissecting the components of the apoptosis machinery

Apoptosis, a mode of cell death commonly observed when death is a desirable or programmed event, has several characteristic structural features. These features appear to be induced by a range of gene products which, together, supervise and participate in the controlled dismantling of the cell. In this article the molecular components of the apoptotic machinery and the proteins implicated in the regulation of this mechanism of cell death are discussed.

Temporal analysis of events associated with programmed cell death (apoptosis) of sympathetic neurons deprived of nerve growth factor

The time course of molecular events that accompany degeneration and death after nerve growth factor (NGF) deprivation and neuroprotection by NGF and other agents was examined in cultures of NGF-dependent neonatal rat sympathetic neurons and compared to death by apoptosis. Within 12 h after onset of NGF deprivation, glucose uptake, protein synthesis, and RNA synthesis fell precipitously followed by a moderate decrease of mitochondrial function. The molecular mechanisms underlying the NGF deprivation-induced decrease of protein synthesis and neuronal death were compared and found to be different, demonstrating that this decrease of protein synthesis is insufficient to cause death subsequently. After these early changes and during the onset of neuronal atrophy, inhibition of protein synthesis ceased to halt neuronal degeneration while readdition of NGF or a cAMP analogue remained neuroprotective for 6 h. This suggests a model in which a putative killer protein reaches lethal levels several hours before the neurons cease to respond to readdition of NGF with survival and become committed to die. Preceding loss of viability by 5 h and concurrent with commitment to die, the neuronal DNA fragmented into oligonucleosomes. The temporal and pharmacological characteristics of DNA fragmentation is consistent with DNA fragmentation being part of the mechanism that commits the neuron to die. The antimitotic and neurotoxin cytosine arabinoside induced DNA fragmentation in the presence of NGF, supporting previous evidence that it mimicked NGF deprivation-induced death closely. Thus trophic factor deprivation-induced death occurs by apoptosis and is an example of programmed cell death.