Regulation of phosphatidylserine exposure and phagocytosis of apoptotic T lymphocytes

In lymphocytes, an asymmetric distribution of phospholipids across the plasma membrane is maintained by an ATP-dependent translocase which specifically transports aminophospholipids from the outer to the inner leaflet of the bilayer. During apoptosis, this enzyme is down-regulated and a lipid flipsite, termed the scramblase, is activated. Together, these events lead to the appearance of phosphatidylserine (PS) on the cell surface. In DO11.10 T lymphocyte hybridoma cells undergoing apoptosis, the kinetics of PS externalization are paralleled by the development of PS-sensitive phagocytosis by macrophages. This parallel is also observed when PS externalization is effected directly by application of a Ca2+ ionophore, suggesting that PS externalization is not only necessary, but sufficient, to generate a recognition signal. The broad spectrum aspartate-directed cysteine protease (caspase) inhibitor zVAD-fmk blocks externalization of PS and terminal cell lysis after induction of apoptosis by anti-CD3 antibody, but is ineffective when apoptosis is induced in the same cells by treatment with glucocorticoid. These results suggest that apoptosis induced by glucocorticoid does not require the same zVAD-sensitive caspase steps which are required for Fas/FasL-dependent death induced by anti-CD3 antibody, and that the action of these proteases is also not required for PS externalization. Extracellular Ca2+ is required to complete the later stages of apoptosis in DO11.10 cells, and its removal restores normal transport of PS, suggesting that down-regulation of the aminophospholipid translocase and up-regulation of the scramblase are not effected by irreversible protease cleavage.     

A homogeneous time-resolved fluorescence resonance energy transfer assay for phosphatidylserine exposure on apoptotic cells

A simple, “mix-and-measure” microplate assay for phosphatidylserine (PtdSer) exposure on the surface of apoptotic cells is described. The assay exploits the fact that annexin V, a protein with high affinity and specificity for PtdSer, forms trimers and higher order oligomers on binding to membranes containing PtdSer. The transition from soluble monomer to cell-bound oligomer is detected using time-resolved fluorescence resonance energy transfer from europium chelate-labeled annexin V to Cy5-labeled annexin V. PtdSer detection is achieved by a single addition of a reagent mix containing labeled annexins and calcium ions directly to cell cultures in a 96-well plate, followed by a brief incubation before fluorescence measurement. The assay can be used to quantify PtdSer exposure on both suspension cells and adherent cells in situ. This method is simpler and faster than existing annexin V binding assays based on flow cytometry or microscopy, and it yields precise data with Z’ values of 0.6-0.7.     

Mechanisms of pre‐apoptotic calreticulin exposure in immunogenic cell death

Dying tumour cells can elicit a potent anticancer immune response by exposing the calreticulin (CRT)/ERp57 complex on the cell surface before the cells manifest any signs of apoptosis. Here, we enumerate elements of the pathway that mediates pre‐apoptotic CRT/ERp57 exposure in response to several immunogenic anticancer agents. Early activation of the endoplasmic reticulum (ER)‐sessile kinase PERK leads to phosphorylation of the translation initiation factor eIF2α, followed by partial activation of caspase‐8 (but not caspase‐3), caspase‐8‐mediated cleavage of the ER protein BAP31 and conformational activation of Bax and Bak. Finally, a pool of CRT that has transited the Golgi apparatus is secreted by SNARE‐dependent exocytosis. Knock‐in mutation of eIF2α (to make it non‐phosphorylatable) or BAP31 (to render it uncleavable), depletion of PERK, caspase‐8, BAP31, Bax, Bak or SNAREs abolished CRT/ERp57 exposure induced by anthracyclines, oxaliplatin and ultraviolet C light. Depletion of PERK, caspase‐8 or SNAREs had no effect on cell death induced by anthracyclines, yet abolished the immunogenicity of cell death, which could be restored by absorbing recombinant CRT to the cell surface.     

Distinct modes of macrophage recognition for apoptotic and necrotic cells are not specified exclusively by phosphatidylserine exposure

The distinction between physiological (apoptotic) and pathological (necrotic) cell deaths reflects mechanistic differences in cellular disintegration and is of functional significance with respect to the outcomes that are triggered by the cell corpses. Mechanistically, apoptotic cells die via an active and ordered pathway; necrotic deaths, conversely, are chaotic and passive. Macrophages and other phagocytic cells recognize and engulf these dead cells. This clearance is believed to reveal an innate immunity, associated with inflammation in cases of pathological but not physiological cell deaths. Using objective and quantitative measures to assess these processes, we find that macrophages bind and engulf native apoptotic and necrotic cells to similar extents and with similar kinetics. However, recognition of these two classes of dying cells occurs via distinct and noncompeting mechanisms. Phosphatidylserine, which is externalized on both apoptotic and necrotic cells, is not a specific ligand for the recognition of either one. The distinct modes of recognition for these different corpses are linked to opposing responses from engulfing macrophages. Necrotic cells, when recognized, enhance proinflammatory responses of activated macrophages, although they are not sufficient to trigger macrophage activation. In marked contrast, apoptotic cells profoundly inhibit phlogistic macrophage responses; this represents a cell-associated, dominant-acting anti-inflammatory signaling activity acquired posttranslationally during the process of physiological cell death.     

Amphiphile-induced phosphatidylserine exposure in human erythrocytes

Nonionic and anionic water-soluble amphiphiles were shown to increase strongly the binding of fluorescein isothiocyanate-conjugated annexin V (FITC-annexin V) in human erythrocytes pretreated with the aminophospholipid translocase (APLT) inhibitor n-ethylmaleimide (NEM). At high sublytic amphiphile-concentrations the binding of FITC-annexin V, monitored in a flow cytometer, was time -and temperature-dependent and occurred heterogeneously in the cell population, with 43-81% of cells being stained above background following incubation for 60 minutes at 37°C. The increased FITC-annexin V binding apparently indicates an increased flop rate of phosphatidylserine (PS) to the outer membrane leaflet. When the NEM-pretreatment was omitted, the FITC-annexin V binding was markedly, but not completely, reduced. In erythrocytes incubated with a zwitter-ionic amphiphile, a small increase in FITC-annexin V binding was detected, while cationic amphiphiles did not induce an increased FITC-annexin V binding. The potency of amphiphiles to induce PS exposure was not related to the type of shape alteration or vesiculation induced. Our results indicate a significant role of the charge status of a membrane intercalated amphiphile for its capability to induce PS exposure.     

New phosphatidylserine receptors: clearance of apoptotic cells and more

Phagocytes recognize apoptotic cells using cell surface receptors, and subsequently engulf these cells. In a recent issue of Nature, two papers reported the identification of novel phagocytic receptors that directly interact with apoptotic cell surface phosphatidylserine (PS). The studies provide new insights into the apoptotic cell clearance process and implicate PS receptors in additional signaling events.     

The presence of oxidized phosphatidylserine on Fas-mediated apoptotic cell surface

A growing body of evidence suggests that phosphatidylserine (PS) oxidation is linked with its transmembrane migration from the inner to the outer leaflet of the plasma membrane during apoptosis. However, there is no direct evidence for the presence of oxidized PS (PSox) on the surface of cells undergoing apoptosis. The present study was performed to detect PSox externalized to the cell surface after Fas engagement in Jurkat cells. Treatment of Jurkat cells with anti-Fas antibody induced caspase-3 activation, chromatin condensation, PS externalization, generation of reactive oxygen species, intracellular glutathione depletion, disruption of mitochondrial transmembrane potential and release of cytochrome c from mitochondria. To determine externalized PS and phosphatidylethanolamine (PE), Jurkat cells were treated with anti-Fas antibody and then labeled with membrane-impermeable fluorescamine, a probe for visualizing lipids that contain primary amino groups. Their total lipids were extracted and subjected to two-dimensional high-performance thin-layer chromatography (HPTLC). The HPTLC plate was sprayed with N,N,N',N'-tetramethyl-p-phenylenediamine dihydrochloride to detect phospholipid hydroperoxides. PSox was present in small amounts within but not on the surface of normal cells. Treatment with anti-Fas antibody increased PSox within the cells and caused PSox to appear on the cell surface. In contrast, PE on the surface of Fas-ligated cells was not oxidized. Thus, the present study demonstrates for the first time the presence of PSox both within and on the surface of apoptotic cells.     

Loss of phospholipid asymmetry and surface exposure of phosphatidylserine is required for phagocytosis of apoptotic cells by macrophages and fibroblasts

Removal of apoptotic cells during tissue remodeling or resolution of inflammation is critical to the restoration of normal tissue structure and function. During apoptosis, early surface changes occur, which trigger recognition and removal by macrophages and other phagocytes. Loss of phospholipid asymmetry results in exposure of phosphatidylserine (PS), one of the surface markers recognized by macrophages. However, a number of receptors have been reported to mediate macrophage recognition of apoptotic cells, not all of which bind to phosphatidylserine. We therefore examined the role of membrane phospholipid symmetrization and PS externalization in uptake of apoptotic cells by mouse macrophages and human HT-1080 fibrosarcoma cells by exposing them to cells that had undergone apoptosis without loss of phospholipid asymmetry. Neither mouse macrophages nor HT-1080 cells recognized or engulfed apoptotic targets that failed to express PS, in comparison to PS-expressing apoptotic cells. If, however, their outer leaflets were repleted with the l-, but not the d-, stereoisomer of sn-1,2-PS by liposome transfer, engulfment by both phagocytes was restored. These observations directly demonstrate that loss of phospholipid asymmetry and PS expression is required for phagocyte engulfment of apoptotic cells and imply a critical, if not obligatory, role for PS recognition in the uptake process.     

Staphylococcal superantigen-like protein 10 binds to phosphatidylserine and apoptotic cells

Staphylococcal superantigen-like proteins (SSLs) are a family of exoproteins that have structural similarities to staphylococcal superantigens. Although SSLs do not have superantigenic activity, some of them have been reported to bind to host immune related molecules and they have been implicated in immune evasion by S. aureus. In this study, we showed that SSL10 is capable of binding to phospholipids. SSL10 bound to phosphatidylserine (PS) containing liposome, but not to phosphatidylcholine liposome. SSL10, but not SSL7, bound to PS containing liposome, suggesting that SSL10 specifically binds to PS. Analysis of PS binding ability among recombinant truncated SSL10 fragments revealed that the β-barrel in the N-terminal oligonucleotide/oligosaccharide-binding (OB)-fold domain contributes to PS binding capacity. Fluorescein isothiocyanate labeled OB-fold of SSL10 stained hydrogen peroxide treated Jurkat cells. Annexin V is widely utilized for detection of apoptosis. Unlike annexin V, the OB-fold domain of SSL10 also bound to apoptotic cells in the presence of EDTA, suggesting that the OB-fold of SSL10 recognizes PS and apoptotic cells in a Ca(2+) independent manner. These findings suggest SSL10 and its derived peptides may be a novel detection tool for apoptotic cells.     

The role of phosphatidylserine in recognition of apoptotic cells by phagocytes

Exposure of phosphatidylserine on the outer leaflet of the plasma membrane is a surface change common to many apoptotic cells. Normally restricted to the inner leaflet, phosphatidylserine appears as a result of decreased aminophospholipid translocase activity and activation of a calcium-dependent scramblase. Phosphatidylserine exposure has several potential biological consequences, one of which is recognition and removal of the apoptotic cell by phagocytes. It is still not clear which receptors mediate PS recognition on apoptotic cells; however, several interesting candidates have been proposed. These include the Class B scavenger and thrombospondin receptor CD36, an oxLDL receptor (CD68), CD14, annexins, beta2 glycoprotein I, gas-6 and a novel activity expressed on macrophages stimulated with digestible particles such as beta-glucan. Whether PS is the sole ligand recognized by phagocytes or whether it associated with other molecules to form a complex ligand remains to be determined.     

Lipid antioxidant, etoposide, inhibits phosphatidylserine externalization and macrophage clearance of apoptotic cells by preventing phosphatidylserine oxidation

Apoptosis is associated with the externalization of phosphatidylserine (PS) in the plasma membrane and subsequent recognition of PS by specific macrophage receptors. Selective oxidation of PS precedes its externalization/recognition and is essential for the PS-dependent engulfment of apoptotic cells. Because etoposide is a potent and selective lipid antioxidant that does not block thiol oxidation, we hypothesized that it may affect PS externalization/recognition without affecting other features of the apoptotic program. We demonstrate herein that etoposide induced apoptosis in HL-60 cells without the concomitant peroxidation of PS and other phospholipids. HL-60 cells also failed to externalize PS in response to etoposide treatment. In contrast, oxidant (H2O2)-induced apoptosis was accompanied by PS externalization and oxidation of different phospholipids, including PS. Etoposide potentiated H2O2-induced apoptosis but completely blocked H2O2-induced PS oxidation. Etoposide also inhibited PS externalization as well as phagocytosis of apoptotic cells by J774A.1 macrophages. Integration of exogenous PS or a mixture of PS with oxidized PS in etoposide-treated HL-60 cells reconstituted the recognition of these cells by macrophages. The current data demonstrate that lipid antioxidants, capable of preventing PS peroxidation, can block PS externalization and phagocytosis of apoptotic cells by macrophages and hence dissociate PS-dependent signaling from the final common pathway for apoptosis.     

Stimulation of erythrocyte phosphatidylserine exposure by lead ions

Pb⁺ intoxication causes anemia that is partially due to a decreased life span of circulating erythrocytes. As shown recently, a Ca²⁺-sensitive erythrocyte scramblase is activated by osmotic shock, oxidative stress, and/or energy depletion, leading to exposure of phosphatidylserine at the erythrocyte surface. Because macrophages are equipped with phosphatidylserine receptors, they bind, engulf, and degrade phosphatidylserine-exposing cells. The present experiments were performed to explore whether Pb⁺ ions trigger phosphatidylserine exposure of erythrocytes. The phosphatidylserine exposure was estimated on the basis of annexin binding as determined using fluorescence-activated cell sorting (FACS) analysis. Exposure to Pb⁺ ions [0.1 µM Pb(NO₃)₂] significantly increased annexin binding. This effect was paralleled by erythrocyte shrinkage, which was apparent on the basis of the decrease in forward scatter in FACS analysis. The effect of Pb⁺ ions on cell volume was virtually abolished, and the effect of Pb⁺ ions on annexin binding was blunted after increase of extracellular K⁺ concentration. Moreover, both effects of Pb⁺ ions were partially prevented in the presence of clotrimazole (10 µM), an inhibitor of the Ca²⁺-sensitive K⁺ channels in the erythrocyte cell membrane. Whole cell patch-clamp experiments disclosed a significant activation of a K⁺-selective conductance after Pb⁺ ion exposure, an effect requiring higher (10 µM) concentrations, however. In conclusion, Pb⁺ ions activate erythrocyte K⁺ channels, leading to erythrocyte shrinkage, and also activate the erythrocyte scramblase, leading to phosphatidylserine exposure. The effect could well contribute to the reported decreased life span of circulating erythrocytes during Pb⁺ intoxication. cell volume; annexin; apoptosis; Gardos channel; calcium     

Stimulation of erythrocyte phosphatidylserine exposure by paclitaxel.

Side effects of cytostatic treatment include development of anemia resulting from either decreased generation or accelerated clearance of circulating erythrocytes. Recent experiments revealed a novel kind of stress-induced erythrocyte death, i.e. eryptosis, which is characterized by enhanced cytosolic Ca(2+) levels, increased ceramide formation and exposure of phosphatidylserine at the cell surface. The present study explored whether cytostatic treatment with paclitaxel (Taxol) triggers eryptosis. Blood was drawn from cancer patients before and after infusion of 175 mg/m2 Taxol. The treatment significantly decreased the hematocrit and significantly increased the percentage of annexin-V-binding erythrocytes in vivo (by 37%). In vitro incubation of human erythrocytes with 10 microM paclitaxel again significantly increased annexin-V-binding (by 129%) and augmented the increase of annexin-V-binding following cellular stress. The enhanced phosphatidylserine exposure was not dependent on caspase-activity but paralleled by erythrocyte shrinkage, increase of cytosolic Ca(2+) activity, ceramide formation and activation of calpain. Phosphatidylserine exposure was similarly induced by docetaxel but not by carboplatin or doxorubicin. Moreover, eryptosis was triggered by the Ca(2+) ionophore ionomycin (10 microM). In mice, ionomycin-treated eryptotic erythrocytes were rapidly cleared from circulating blood and sequestrated into the spleen. In conclusion, our data strongly suggest that paclitaxel-induced anemia is at least partially due to induction of eryptosis.     

Regulation of phosphatidylserine exposure in red blood cells

The exposure of phosphatidylserine (PS) on the outer membrane leaflet of red blood cells (RBCs) serves as a signal for eryptosis, a mechanism for the RBC clearance from blood circulation. The process of PS exposure was investigated as function of the intracellular Ca(2+) content and the activation of PKCα in human and sheep RBCs. Cells were treated with lysophosphatidic acid (LPA), 4-bromo-A23187, or phorbol-12 myristate-13 acetate (PMA) and analysed by flow cytometry, single cell fluorescence video imaging, or confocal microscopy. For human RBCs, no clear correlation existed between the number of cells with an elevated Ca(2+) content and PS exposure. Results are explained by three different mechanisms responsible for the PS exposure in human RBCs: (i) Ca(2+)-stimulated scramblase activation (and flippase inhibition) by LPA, 4-bromo-A23187, and PMA; (ii) PKC activation by LPA and PMA; and (iii) enhanced lipid flop caused by LPA. In sheep RBCs, only the latter mechanism occurs suggesting absence of scramblase activity.     

ABC1 promotes engulfment of apoptotic cells and transbilayer redistribution of phosphatidylserine

ATP-binding-cassette transporter 1 (ABC1) has been implicated in processes related to membrane-lipid turnover. Here, using in vivo loss-of-function and in vitro gain-of-function models, we show that ABC1 promotes Ca2+-induced exposure of phosphatidylserine at the membrane, as determined by a prothrombinase assay, membrane microvesiculation and measurement of transbilayer redistribution of spin-labelled phospholipids. That ABC1 promotes engulfment of dead cells is shown by the impaired ability of ABC1-deficient macrophages to engulf apoptotic preys and by the acquisition of phagocytic behaviour by ABC1 transfectants. Release of membrane phospholipids and cholesterol to apo-AI, the protein core of the cholesterol-shuttling high-density lipoprotein (HDL) particle, is also ABC1-dependent. We propose that both the efficiency of apoptotic-cell engulfment and the efflux of cellular lipids depend on ABC1-induced perturbation of membrane phosphatidylserine turnover. Transient local exposure of anionic phospholipids in the outer membrane leaflet may be sufficient to alter the general properties of the membrane and thus influence discrete physiological functions.     

CD300b regulates the phagocytosis of apoptotic cells via phosphatidylserine recognition

The CD300 receptor family members are a group of molecules that modulate a variety of immune cell processes. We show that mouse CD300b (CLM7/LMIR5), expressed on myeloid cells, recognizes outer membrane-exposed phosphatidylserine (PS) and does not, as previously reported, directly recognize TIM1 or TIM4. CD300b accumulates in phagocytic cups along with F-actin at apoptotic cell contacts, thereby facilitating their engulfment. The CD300b-mediated activation signal is conveyed through CD300b association with the adaptor molecule DAP12, and requires a functional DAP12 ITAM motif. Binding of apoptotic cells promotes the activation of the PI3K-Akt kinase pathway in macrophages, while silencing of CD300b expression diminishes PI3K-Akt kinase activation and impairs efferocytosis. Collectively, our data show that CD300b recognizes PS as a ligand, and regulates the phagocytosis of apoptotic cells via the DAP12 signaling pathway.In both developing and mature multicellular organisms, large numbers of apoptotic cells are continually generated and must be cleared by neighboring cells or ‘professional'' phagocytes.^{1, 2, 3, 4} If not properly cleared, they become necrotic, pro-inflammatory and immunogenic, potentially leading to the development of autoimmune diseases, such as systemic lupus erythematous (SLE).^{5, 6, 7, 8} Therefore, phagocytes possess sensing systems to facilitate the clearance of apoptotic cells.^{1, 2, 3} Once guided to their location by diffusible ‘find me'' signals, phagocytes recognize apoptotic cells through their display of characteristic cell surface molecules (‘eat me'' signals).^{4, 7} The most common signal promoting phagocytosis is the recognition of phosphatidylserine (PS), which when exposed on the outer leaflet of the plasma membrane signals phagocytes to engulf apoptotic cells.² Multiple receptors for PS exist on phagocytic cells, although not necessarily simultaneously; these include stabilins,^{9, 10} T cell Ig mucin (TIM) 1 and TIM4,^{11, 12} BAI1,¹³ MFGE8, which bridges PS to integrin αvβ3,¹⁴ and Protein S and Gas6, which bridge PS to TAM receptors.¹⁵ Recently, we and others demonstrated that the CD300 family members, human and mouse CD300a,^{16, 17} and mouse CD300f,^{18, 19} also bind PS, and their expression regulates apoptotic cell phagocytosis.The CD300 family contains both activating and inhibitory receptor members.²⁰ CD300b has a short intracellular tail and gains activation potential by association with DNAX activating protein of 12 kDa (DAP12) or DAP10 adaptor molecules.²¹ CD300b is predominantly expressed on myeloid cells, including neutrophils, macrophages and mast cells. Antibody cross-linking of human and mouse CD300b has been shown to induce the release of inflammatory cytokines from mast cells.²¹ The ligand for CD300b remains a matter of debate. A recent study found that a soluble form of CD300b, released in response to Toll-like receptor ligation, recognizes unknown ligands on the surface of macrophages, resulting in the release of inflammatory cytokines.²² Others have identified the PS-binding receptors TIM1 and TIM4 as endogenous ligands for CD300b, but not PS itself.²³Here, we show that CD300b binds to PS, and recognizes PS on TIM1 or TIM4 expressing cells rather than TIM1 or TIM4 alone. We found that CD300b promotes PS-dependent apoptotic cell phagocytosis upon ectopic expression in cell lines, without the need for additional PS receptors. In addition, CD300b-mediated phagocytosis requires the association of the adaptor protein DAP12 for effective signaling. Inhibition of CD300b function by either anti-CD300b antibody treatment or siRNA transfection significantly decreases macrophage-dependent phagocytosis of apoptotic cells. Furthermore, CD300b silencing in macrophages severely impairs the apoptotic cell-induced phosphorylation of PI3K, Akt and Syk, but not Erk. Thus, our data show that CD300b is an activating receptor that has an important role in macrophage-mediated clearance of apoptotic cells.     

Cell surface-expressed phosphatidylserine as therapeutic target to enhance phagocytosis of apoptotic cells

Impaired efferocytosis has been shown to be associated with, and even to contribute to progression of, chronic inflammatory diseases such as atherosclerosis. Enhancing efferocytosis has been proposed as strategy to treat diseases involving inflammation. Here we present the strategy to increase ‘eat me'' signals on the surface of apoptotic cells by targeting cell surface-expressed phosphatidylserine (PS) with a variant of annexin A5 (Arg-Gly-Asp–annexin A5, RGD–anxA5) that has gained the function to interact with α_vβ₃ receptors of the phagocyte. We describe design and characterization of RGD–anxA5 and show that introduction of RGD transforms anxA5 from an inhibitor into a stimulator of efferocytosis. RGD–anxA5 enhances engulfment of apoptotic cells by phorbol-12-myristate-13-acetate-stimulated THP-1 (human acute monocytic leukemia cell line) cells in vitro and resident peritoneal mouse macrophages in vivo. In addition, RGD–anxA5 augments secretion of interleukin-10 during efferocytosis in vivo, thereby possibly adding to an anti-inflammatory environment. We conclude that targeting cell surface-expressed PS is an attractive strategy for treatment of inflammatory diseases and that the rationally designed RGD–anxA5 is a promising therapeutic agent.     

Oxidative stress inhibits the phagocytosis of apoptotic cells that have externalized phosphatidylserine

The efficient phagocytosis of apoptotic cells by macrophages reduces the potential for an inflammatory response by ensuring that the dying cells are cleared before their intracellular contents are released. Early apoptotic cells are targeted for phagocytosis through the translocation of phosphatidylserine (PS) from the inner to the outer leaflet of the plasma membrane. In this report, we show that the oxidant H(2)O(2) inhibits phagocytosis of apoptotic cells even though the cells express functional PS on their surface. Thus, B lymphoma cells induced to undergo apoptosis by the chemotherapy drug etoposide are efficiently phagocytosed by macrophages in a process that is mediated by PS (inhibitable by PS liposomes). Exposure of the apoptotic cells to H(2)O(2) inhibits phagocytosis even though the cells still express functional PS on their surface. In addition, Jurkat cells and thymocytes induced to undergo apoptosis by H(2)O(2) alone are poorly phagocytosed. Inhibition of phagocytosis by H(2)O(2) cannot be attributed to oxidative inactivation or redistribution of PS on the cell surface. The results indicate that PS externalization is necessary but is not sufficient to target apoptotic cells for phagocytosis. Another phagocytosis recognition factor must therefore exist to facilitate uptake of apoptotic cells, and this factor is sensitive to modification by H(2)O(2).     

Caspase-independent phosphatidylserine exposure during apoptosis of primary T lymphocytes

Exposure of phosphatidylserine (PS) on the outer leaflet of the plasma membrane is a key feature of apoptosis. As the signals underlying these phenomena are unknown, it is generally assumed that PS exposure is a consequence of caspase activation, another hallmark of apoptosis. In this study we investigated the role of caspases in PS externalization during apoptosis of activated PBL triggered by drugs (etoposide, staurosporine), CD95 engagement, or IL-2 withdrawal. Anti-CD95 mAb induces a rapid activation of caspases, followed by PS exposure and mitochondrial transmembrane potential (DeltaPsim) disruption. In contrast, etoposide (ETO), staurosporine (STS), or IL-2 withdrawal triggers concomitant caspase activation, PS exposure, and DeltaPsim disruption. Such kinetics suggest that PS exposure could be independent of caspase activation. As expected, in activated PBL treated by anti-CD95 mAb, the pan-caspase inhibitor Cbz-Val-Ala-Asp(OMe)-fluoromethylketone and the caspase-8 inhibitor Cbz-Leu-Glu-Thr-Asp(OMe)-fluoromethylketone, but not the caspase-9 inhibitor Cbz-Leu-Glu-His-Asp(OMe)-fluoromethylketone, inhibit PS externalization and DeltaPsim disruption. Surprisingly, during apoptosis induced by ETO, STS, or IL-2 withdrawal, none of those caspase inhibitors prevents PS externalization or DeltaPsim disruption, whereas they all inhibit DNA fragmentation as well as the morphological features of nuclear apoptosis. In Jurkat and H9 T cell lines, as opposed to activated PBL, PS exposure is inhibited by Cbz-Val-Ala-Asp(OMe)-fluoromethylketone during apoptosis induced by CD95 engagement, ETO, or STS. Thus, caspase-independent PS exposure occurs in primary T cells during apoptosis induced by stimuli that do not trigger death receptors.     

Carbohydrate chains and phosphatidylserine successively work as signals for apoptotic cell removal

At an early stage of apoptosis, Jurkat cells transiently become susceptible to binding and phagocytosis by macrophages through the polylactosamine-type carbohydrate chains of CD43 [J. Biol. Chem. 279 (2004) 5967]. Susceptibility of apoptotic Jurkat cells to macrophage recognition was studied over an extended time range of 0-24 h including a later stage. Jurkat cells incubated with appropriate concentrations of apoptosis-inducing agents etoposide or anti-Fas antibody became susceptible to macrophage-binding at 2 h, and the susceptibility fell to the control level at 4 or 6 h. However, it increased again at later hours (6-24 h). Flow cytometric analyses of CD43 and phosphatidylserine (PS) on the apoptotic cells indicated that CD43 began to degrade at around 4 h, and PS is externalized significantly at 4 or 6 h. The macrophage-binding at 2 h was prevented by glycosidase treatment of Jurkat cells, but not by annexin V. Conversely, the later binding at 12 or 18 h was not prevented by glycosidase treatment, but was done so by annexin V. These results suggest that Jurkat cells become susceptible to phagocytic removal at an early stage of apoptosis by the carbohydrate-mediated mechanism, and at a later stage by the PS-mediated mechanism.