Hepatic sinusoidal endothelium heterogeneity with respect to the recognition of apoptotic cells

The Scavenger Function of Liver Sinusoidal Endothelial Cells in Health and Disease

The aim of this review is to give an outline of the blood clearance function of the liver sinusoidal endothelial cells (LSECs) in health and disease. Lining the hundreds of millions of hepatic sinusoids in the human liver the LSECs are perfectly located to survey the constituents of the blood. These cells are equipped with high-affinity receptors and an intracellular vesicle transport apparatus, enabling a remarkably efficient machinery for removal of large molecules and nanoparticles from the blood, thus contributing importantly to maintain blood and tissue homeostasis. We describe here central aspects of LSEC signature receptors that enable the cells to recognize and internalize blood-borne waste macromolecules at great speed and high capacity. Notably, this blood clearance system is a silent process, in the sense that it usually neither requires or elicits cell activation or immune responses. Most of our knowledge about LSECs arises from studies in animals, of which mouse and rat make up the great majority, and some species differences relevant for extrapolating from animal models to human are discussed. In the last part of the review, we discuss comparative aspects of the LSEC scavenger functions and specialized scavenger endothelial cells (SECs) in other vascular beds and in different vertebrate classes. In conclusion, the activity of LSECs and other SECs prevent exposure of a great number of waste products to the immune system, and molecules with noxious biological activities are effectively “silenced” by the rapid clearance in LSECs. An undesired consequence of this avid scavenging system is unwanted uptake of nanomedicines and biologics in the cells. As the development of this new generation of therapeutics evolves, there will be a sharp increase in the need to understand the clearance function of LSECs in health and disease. There is still a significant knowledge gap in how the LSEC clearance function is affected in liver disease.

Voices from the dead: The complex vocabulary and intricate grammar of dead cells

Of the roughly one million cells per second dying throughout the body, the vast majority dies by apoptosis, the predominant form of regulated cell death in higher organisms. Long regarded as mere waste, apoptotic cells are now recognized as playing a prominent and active role in homeostatic maintenance, especially resolution of inflammation, and in the sculpting of tissues during development. The activities associated with apoptotic cells are continually expanding, with more recent studies demonstrating their ability to modulate such vital functions as proliferation, survival, differentiation, metabolism, migration, and angiogenesis. In each case, the role of apoptotic cells is active, exerting their effects via new activities acquired during the apoptotic program. Moreover, the capacity to recognize and respond to apoptotic cells is not limited to professional phagocytes. Most, if not all, cells receive and integrate an array of signals from cells dying in their vicinity. These signals comprise a form of biochemical communication. As reviewed in this chapter, this communication is remarkably sophisticated; each of its three critical steps-encoding, transmission, and decoding of the apoptotic cell's "message"-is endowed with exquisite robustness. Together, the abundance and intricacy of the variables at each step comprise the vocabulary and grammar of the language by which dead cells achieve their post-mortem voice. The combinatorial complexity of the resulting communication network permits dying cells, through the signals they emit and the responses those signals elicit, to partake of an expanded role in homeostasis, acting as both sentinels of environmental change and agents of adaptation.

In vitro and in vivo clearance of Rose Bengal Acetate-PhotoDynamic Therapy-induced autophagic and apoptotic cells

This study focuses on the clearance of Rose Bengal Acetate (RBAc)-PhotoDynamic Therapy (PDT)-generated apoptotic and autophagic HeLa cells by murine and human macrophages. Indeed, phagocytosis of dead cells drives the therapeutic efficacy of PDT through both efficient removal of dead/dying cells and macrophages response evoked during engulfment and, up to now, clearance of dying photosensitized cells has been less investigated than PDT mechanisms of cell death induction. RBAc-PDT ensures a long onset of cytotoxicity and a time-related cell death of HeLa cells by signals originating from or converging on almost all intracellular organelles. On this basis, to clarify whether the efficacious cell death commitment is followed by an efficient clearance mechanism, we primarily focused on the analysis of 'eat me' signals exposure and 'find me' signals release, and then investigated the migration, recognition, engulfment and response of murine Raw 264.7 and human blood isolated macrophages. Dead cells secreted 'find me' signals, i.e. fractalkine and Heat Shock Protein 70 (HSP 70), to recruit macrophages and promote their fast phagocytosis. Macrophages phagocytosed apoptotic and autophagic PDT-treated cells more efficiently than the respective positive controls, i.e. puromycin-induced apoptotic and Earle's balanced salt solution-starved autophagic cells. Phagocytosis depends on the glycans exposed on dead cells. The macrophages internalization of photokilled cells elicits the production of Interleukin-10, Transforming Growth Factor-β and Tumour Necrosis Factor-α by macrophages. TNFα production, along with HSP70 release and plasma membrane translocation on dead cells, suggest an immunogenic impact of RBAc-PDT. In fact, macrophages, activated fibroblasts and endothelial cells colonized the inoculum site of photosensitized cells in rat calf muscles, endorsing the hypothesis of immunogenic elicitation of RBAc-PDT.

Heterogeneity in apoptotic responses of microvascular endothelial cells to oxidative stress

Oxidative stress contributes to disease and can alter endothelial cell (EC) function. EC from different vascular beds are heterogeneous in structure and function, thus we assessed the apoptotic responses of EC from lung and heart to oxidative stress. Since protein kinase Cδ (PKCδ) is activated by oxidative stress and is an important modulator of apoptosis, experiments assessed the level of apoptosis in fixed lung and heart sections of PKCδ wild-type (PKCδ(+/+)) and null (PKCδ(-/-)) mice housed under normoxia (21% O(2)) or hyperoxia (~95% O(2)). We noted a significantly greater number of TUNEL-positive cells in lungs of hyperoxic PKCδ(+/+) mice, compared to matched hearts or normoxic organs. We found that 33% of apoptotic cells identified in hyperoxic lungs of PKCδ(+/+) mice were EC, compared to 7% EC in hyperoxic hearts. We further noted that EC apoptosis was significantly reduced in lungs of PKCδ(-/-) hyperoxic mice, compared to lungs of PKCδ(+/+) hyperoxic mice. In vitro, both hyperoxia and H(2)O(2) promoted apoptosis in EC isolated from microvasculature of lung (LMVEC), but not from the heart (HMVEC). H(2)O(2) treatment significantly increased p38 activity in LMVEC, but not in HMVEC. Inhibition of p38 attenuated H(2)O(2)-induced LMVEC apoptosis. Baseline expression of total PKCδ protein, as well as the caspase-mediated, catalytically active PKCδ cleavage fragment, was higher in LMVEC, compared to HMVEC. PKCδ inhibition significantly attenuated H(2)O(2)-induced LMVEC p38 activation. Conversely, overexpression of wild-type PKCδ or the catalytically active PKCδ cleavage product greatly increased H(2)O(2)-induced HMVEC caspase and p38 activation. We propose that enhanced susceptibility of lung EC to oxidant-induced apoptosis is due to increased PKCδ→p38 signaling, and we describe a PKCδ-centric pathway which dictates the differential response of EC from distinct vascular beds to oxidative stress.

Phagocytosis of dying cells: influence of smoking and static magnetic fields

It is becoming evident that failure in the removal of dying cells causes and/or promotes the onset of chronic diseases. Impairment of phagocytosis of apoptotic cells can be due not only to genetic or molecular malfunctioning but also to external/environmental factors. Two of these environmental factors have been recently reported to down regulate the clearance of apoptotic cells: cigarette smoke and static magnetic fields. Cigarette smoke contains highly reactive carbonyls that modify proteins which directly/indirectly affects cellular function. Human macrophages interacting with carbonyl or cigarette smoke modified extracellular matrix (ECM) proteins dramatically down regulated their ability to phagocytose apoptotic neutrophils. It was postulated that changes in the ECM environment as a result of cigarette smoke affect the ability of macrophages to remove apoptotic cells. This decreased phagocytic activity was as a result of sequestration of receptors involved in the uptake of apoptotic cells towards that of recognition of carbonyl adducts on the modified ECM proteins leading to increased macrophage adhesion. Downregulation of the phagocytosis of apoptotic cells was also described when performed in presence of static magnetic fields (SMFs) of moderate intensity. SMFs have been reported to perturb distribution of membrane proteins and glycoproteins, receptors, cytoskeleton and trans-membrane fluxes of different ions, especially calcium [Ca(2+)]i, that in turn, interfere with many different physiological activities, including phagocytosis. The effects of cigarette smoke and SMF on the phagocytosis of dying cells will be here discussed.

Mechanisms determining phenotypic heterogeneity of hepatocytes

This review summarizes results of biochemical and immunohistochemical studies indicating the existence of functional heterogeneity of hepatocytes depending on their localization in the hepatic acinus; this determines characteristic features of metabolism of carbohydrates, lipids, and xenobiotics. The physiological significance of hepatocyte heterogeneity is discussed. According to the proposed model of intercellular communication, the metabolic specialization of hepatocytes is determined by secretory activity of hepatic resident macrophages (Kupffer cells) localized mainly in the periportal zone of the liver acinus. Macrophages participate in secretion of a wide spectrum of intercellular mediators (cytokines, prostaglandins, growth factors) and also in metabolism of numerous blood metabolites and biologically active substances (hormones, lipoproteins, etc.). In the sinusoid and in the space of Disse (also known as perisinusoidal space) they form a concentration gradient of regulatory factors and metabolites inducing the phenotypic differences between hepatocytes.

Phenotypic heterogeneity of the endothelium: II. Representative vascular beds

Endothelial cells, which form the inner cellular lining of blood vessels and lymphatics, display remarkable heterogeneity in structure and function. This is the second of a 2-part review on the phenotypic heterogeneity of blood vessel endothelial cells. The first part discusses the scope, the underlying mechanisms, and the diagnostic and therapeutic implications of phenotypic heterogeneity. Here, these principles are applied to an understanding of organ-specific phenotypes in representative vascular beds including arteries and veins, heart, lung, liver, and kidney. The goal is to underscore the importance of site-specific properties of the endothelium in mediating homeostasis and focal vascular pathology, while at the same time emphasizing the value of approaching the endothelium as an integrated system.

LOX-1 scavenger receptor mediates calcium-dependent recognition of phosphatidylserine and apoptotic cells

The LOX-1 (lectin-like oxidized low-density lipoprotein receptor-1) scavenger receptor regulates vascular responses to oxidized-low-density-lipoprotein particles implicated in atherosclerotic plaque formation. LOX-1 is closely related to C-type lectins, but the mechanism of ligand recognition is not known. Here we show that human LOX-1 recognizes a key cellular phospholipid, PS (phosphatidylserine), in a Ca2+-dependent manner, both in vitro and in cultured cells. A recombinant, folded and glycosylated LOX-1 molecule binds PS, but not other phospholipids. LOX-1 recognition of PS was maximal in the presence of millimolar Ca2+ levels. Mg2+ was unable to substitute for Ca2+ in LOX-1 binding to PS, indicating a Ca2+-specific requirement for bivalent cations. LOX-1-mediated recognition of PS-containing apoptotic bodies was dependent on Ca2+ and was decreased to background levels by bivalent-cation chelation, LOX-1-blocking antibodies or PS-containing liposomes. The LOX-1 membrane protein is thus a Ca2+-dependent phospholipid receptor, revealing novel recognition of phospholipids by mammalian lectins.

Epithelial cells as phagocytes: apoptotic epithelial cells are engulfed by mammary alveolar epithelial cells and repress inflammatory mediator release

Clearance of apoptotic cells is critical to tissue homeostasis and resolution of inflammatory lesions. Macrophages are known to remove dying cells and release anti-inflammatory mediators in response; however, many cells traditionally thought of as poor phagocytes can mediate this function as well. In the lactating mammary gland following weaning, alveolar epithelial cell death is massive, yet the gland involutes rapidly, attaining its prepregnancy state in a matter of days. We found histologic evidence of apoptotic cell phagocytosis by viable mammary epithelial cells (MEC) in the involuting mouse mammary gland. Cultured MEC were able to engulf apoptotic cells in vitro, utilizing many of the same receptors used by macrophages, including the phosphatidylserine receptor (PSR), CD36, the vitronectin receptor alpha(v)beta3, and CD91. In addition, MEC, like macrophages, produced TGFbeta in response to stimulation of the PSR by apoptotic cells or the anti-PSR ab 217G8E9, and downregulated endotoxin-stimulated proinflammatory cytokine production. These data support the hypothesis that amateur phagocytes play a significant role in apoptotic cell clearance and its regulation of inflammation.

Enhanced expression of the mannose receptor by endothelial cells of the liver and spleen microvascular beds in the macrophage-deficient PU.1 null mouse

Mice null for the haematopoietic lineage-specific transcription factor PU.1 lack mature Mphi and are compromised in their ability to clear cellular debris from the blood circulation. We investigated the possibility that non-professional phagocytes may partially compensate for the lack of Mphi in clearance functions. In the absence of Kupffer cells (resident liver Mphi) in the PU.1 null mice, electron microscopy revealed ingested debris in sinusoidal endothelial cells and hepatocytes although debris was also seen free in blood vessels. To investigate whether an increased clearance function of non-professional phagocytes might be linked to expression of Mphi-associated phagocytic and pinocytic receptors by other cells in PU.1 null mouse, we examined expression of several candidate proteins by immunocytochemistry and Western blotting. We found mannose receptor (MR) comparably expressed in PU.1 null and PU.1+ mice liver and spleen whereas class A scavenger receptor was substantially reduced and complement receptor 3 was absent in PU.1 null animals. By morphometric analysis, liver and spleen sinusoidal endothelial cells were seen to express significantly more MR in the PU.1 null mouse. This study provides the first evidence of apparently compensatory alterations in the microvasculature of the Mphi-deficient PU.1 null mouse.

Bioeffects of moderate-intensity static magnetic fields on cell cultures

The interaction of static magnetic fields (SMFs) with living organisms is a rapidly growing field of investigation. However, despite the increasing number of studies on the effects of the interaction of SMFs with living organisms, many gaps in our knowledge still remain. One reason why it is extremely important to deeply understand the true mode of action of MFs on living organisms, is the need to protect human health in consideration of the probable future introduction of new technologies such as magnetically levitated trains and the therapeutical use of MFs (e.g. magnetic resonance imaging, MRI, coupling of MF exposure with chemotherapy). The lack of knowledge of the morphological modifications brought about by exposure to moderate-intensity SMFs prompted us to investigate the bioeffects of 6mT SMFs on different cell types, by means of light and electron microscopy, confocal laser scanning microscopy and immuno- or cytochemistry. In the present article we report our own and other data from the literature on the morphological studies of the bioeffects of moderate-intensity SMFs. We focus on morphological modifications related to cell shape, cell surface, cytoskeleton, and plasma membrane expression of molecules and carbohydrate residues. The effects of exposure to moderate-intensity SMF for 24 or 48 h, on apoptosis, on apoptotic related gene products, on macrophagic differentiation and on phagocytosis of apoptotic cells in primary cell cultures (transformed or stabilized cell lines) will be also discussed. Moderate-intensity (6mT) SMFs induced modifications of cell shape, cell surface and cytoskeleton, progressively achieved during the entire period of exposure. In general, at the end of the exposure period, the cells had a less flat shape due to partial detachment from the culture dishes or a more round-elongated shape (in relation to adhesion growth or in suspension growth respectively) with many irregular lamellar microvilli, while the morphology of the organelles remained unmodified. In parallel with cell shape changes, the microfilaments and microtubules, as well as the quantity and distribution of surface ConA-FITC and Ricinus Comm.-FITC labelling sites, were modified in a time-dependent manner. Apoptosis was influenced in a cell type-dependent manner: for some cells spontaneous apoptosis decreased while, for others, it increased to about 20% after 24h of continuous exposure. The induction of apoptosis was likely due to the increment of [Ca(2+)]i during exposure. Cell proliferation was only slightly affected. Indeed, in addition to the cell type, the time of exposure was also an important factor in the intensity of the effects produced. Both apoptotic rate and cell and surface shape were influenced by exposure to SMFs when simultaneously administered with apoptogenic drugs. Apoptotic cells were cleared by an efficient and fast process of phagocytosis mediated by specific epitopes, externalized during the formation of the apoptotic cells, on the dead cells and by specific receptors on the phagocytes (both "professional" and "nonprofessional"). The recognition of apoptotic lymphocytes as well as of control cells exposed for at least 24h to 6mT SMF by liver sinusoidal cells was influenced by the cell surface modifications which both apoptotic or normal exposed cells underwent during the induction of apoptosis or SMF exposure. The degree of macrophagic differentiation of human pro-monocytic U937 cells induced by phorbol ester was decreased by exposure to 6mT SMFs, with a consequent fall in cell adhesion and increased polarization of pseudopodia and cytoplasmic protrusions. Differentiation alone, or in combination with exposure to SMFs, affects distribution and quantity of cell surface carbohydrate residues, surface expression of markers of macrophage differentiation, and phagocytic capability. The increasing amount of data reporting on the bioeffects of SMFs is leading researchers to an understanding of how important it is to fully understand the mode of action of MFs on living organisms. Indeed, even if the perturbations of biological systems by SMFs are sublethal at shorter times of exposure, these perturbations could, especially at longer times of exposure, evolve into a progressive accumulation of modifications, whose ultimate effects still need to be clarified.

Role of the liver in peripheral tolerance: induction through oral antigen feeding

Using a murine liver transplant model, we studied the liver's role in peripheral tolerance. Livers from BALB/c mice fed with ovalbumin (OVA) at either a low or high dose were transplanted into syngeneic recipients. Non-fed recipients were controls. Orthotopic liver transplantation (OLTx) was followed by OVA immunization and delayed-type hypersensitivity (DTH) challenge. The ex vivo adoptive transfer effect of liver nonparenchymal cells (NPCs) or spleen cells (SCs) from OVA-fed mice was examined. In vitro proliferative assays and cytokine profiles were conducted on NPCs and SCs from transplant recipients. Livers from all OVA-fed mice after 10 days transferred tolerance to OVA-naïve mice. The time course of adoptive transfer of liver NPCs from high-dose OVA-fed mice transferred OVA tolerance within 24 h; low-dose OVA-fed mice required > or = 4 days to transfer tolerance. The in vitro proliferative response of the NPCs to OVA revealed a decreased response in both dosage groups over the control group. Our results suggest that the liver plays an important role in inducing peripheral tolerance in a mucosal tolerance model, especially feeding high-dose OVA.

Liver sinusoidal endothelial cells are insufficient to activate T cells

Liver sinusoidal endothelial cells (LSEC) have been reported to express MHC class II, CD80, CD86, and CD11c and effectively stimulate naive T cells. Because dendritic cells (DC) are known to possess these characteristics, we sought to directly compare the phenotype and function of murine LSEC and DC. Nonparenchymal cells from C57BL/6 mice were obtained by collagenase digestion of the liver followed by density gradient centrifugation. From the enriched nonparenchymal cell fraction, LSEC (CD45(-)) were then isolated to 99% purity using immunomagnetic beads. Flow cytometric analysis of LSEC demonstrated high expression of CD31, von Willebrand factor, and FcgammaRs. However, unlike DC, LSEC had low or absent expression of MHC class II, CD86, and CD11c. LSEC demonstrated a high capacity for Ag uptake in vitro and in vivo. Although acetylated low-density lipoprotein uptake has been purported to be a specific function of LSEC, we found DC captured acetylated low-density lipoprotein to a similar extent in vivo. Consistent with their phenotype, LSEC were poor stimulators of allogeneic T cells. Furthermore, in the absence of exogenous costimulation, LSEC induced negligible proliferation of CD4(+) or CD8(+) TCR-transgenic T cells. Thus, contrary to previous reports, our data indicate that LSEC alone are insufficient to activate naive T cells.

Silent cleanup of very early apoptotic cells by macrophages

Apoptotic cells are phagocytosed as soon as they appear in vivo. In this study, we first determined precisely at what stage apoptotic cells are phagocytosed by macrophages, and then examined the subsequent cytokine production. Phagocytosis was confirmed by flow cytometry and confocal laser microscopy, whereas the subsequent response was examined by ELISA and RT-PCR for quantitative and semiquantitative measurement of the protein and mRNA levels of cytokines, respectively. Even the cell populations containing very early apoptotic cells, such as IL-2-dependent CTLL-2 cells cultured in the absence of IL-2 for 4 h and a murine leukemic cell line, P388 cells, treated with etoposide for 5 h, were phagocytosed by macrophages. Although the cell populations containing the very early apoptotic cells used in this study were FITC-Annexin V-negative and did not show a decrease in cell size as compared with untreated cells, they showed a very small increase in phosphatidylserine on the cell surface, as detected with Cy3-Annexin V, and a decrease in mitochondrial membrane potential, indicating that the cell populations had already started the apoptotic process. Phagocytosis of such populations containing very early apoptotic cells was inhibited by phospho-L-serine much more significantly than Arg-Gly-Asp-Ser. In addition, macrophages hardly produced either proinflammatory or anti-inflammatory cytokines after phagocytosis, thus being an almost null response. These results are contrary to the generally accepted concept that the phagocytosis of apoptotic cells leads to the production of anti-inflammatory cytokines, suggesting instead that cells starting to undergo apoptosis are quickly phagocytosed by macrophages without any inflammation in vivo.

The role of the macrophage in apoptosis: hunter, gatherer, and regulator

Clearance of cellular corpses is a critical feature of apoptosis in vivo during development, tissue homeostasis, and resolution of inflammation. As the professional phagocytes of the body, macrophages play a key role in this process. By recognizing emerging signals using several different receptors, macrophages engulf apoptotic cells swiftly and efficiently. In addition, the binding of apoptotic cells profoundly down-regulates the ability of the macrophage to produce inflammatory mediators by inducing the release of antiinflammatory mediators. Finally, macrophages may actually induce cell death in specific cells during embryogenesis. Abnormalities of apoptotic cell clearance may contribute to the pathogenesis of chronic inflammatory diseases, including those of autoimmune etiology. It is also possible that certain malignant tumor cells co-opt the mechanisms for apoptotic cell clearance to avoid immune surveillance by subverting macrophage and dendritic cell responses.

Phagocytosis of apoptotic cells by liver: a morphological study

The present review deals with the morphological features of the removal of apoptotic cells by liver. The engulfment of cells undergoing apoptosis can be considered a specialized form of phagocytosis, playing a major role in the general tissue homeostasis in physiological and pathological conditions. In fact, defects of phagocytosis of apoptotic cells might have deleterious consequences for neighboring healthy cells, i.e., pathogenesis of inflammatory disease or dysregulation of the immune system. Phagocytosis of apoptotic cells by liver is a complex phenomenon, involving multiple molecular mechanisms of recognition (i.e., lectin-like receptors and receptors for externalized phosphatydilserine) of both parenchymal (hepatocytes) and nonparenchymal (Kupffer and endothelial cells) liver cells, often operating in cooperation. The data discussed in the present review are drawn from studies of phagocytosis of apoptotic cells in the liver, carried out with in vivo and in situ adhesion experiments as well as in vitro assays. Our results indicate that the three main liver cell types (hepatocytes, Kupffer, and endothelial cells) are able to recognize and internalize apoptotic cells by means of specific receptors (galactose and mannose-specific receptor; receptor for phosphatydilserine) and by cytoskeletal reorganization that favors the engulfment of the apoptotic cells. The "flags" for the identification of apoptotic cells by the liver are modifications of the surface of dead cells, i.e., sugar residues and phosphatydilserine exposition. Vitronectin receptor is not involved in such a recognition. The adhesions between modified cell surfaces of apoptotic cells and phagocytes generate cytoplasmatic signaling pathways that drive apoptotic cells to their final fate within the phagocytes (i.e., lysosomal digestion).

Do inflammatory cells participate in mammary gland involution?

The processes by which the involuting mammary gland clears residual milk and milk fat, as well as apoptotic cells, have gone largely unstudied in the modern literature. Here we review the evidence for and against the involvement of professional phagocytes of hematopoietic lineage in this process. Additionally we present evidence that mammary epithelial cells themselves are capable of phagocytosis and may be responsible for the majority of apoptotic cell and residual milk clearance during murine involution. In this scheme these cells regulate their cytokine production in response to apoptotic cells in a manner similar to other cells, including macrophages. The ensuing model describes a process of involution that actively suppresses an inflammatory response in the gland, allowing for effective tissue remodeling and damage prevention.

Biology and significance of T-cell apoptosis in the liver

The liver has emerged as an organ with distinct immunological properties. In this review, we summarize evidence that shows that the liver can remove apoptotic, or non-apoptotic but activated, CD8+ T cells from the circulation and induce apoptosis in these activated T cells by either active or passive mechanisms. Hepatitis viruses, particularly hepatitis C virus, often establish persistent infection. We review evidence that suggests that these viruses exploit intrahepatic tolerance mechanisms to protect themselves from immune attack.

Apoptosis: the quiet death silences the immune system

Apoptosis plays an essential role in maintaining cellular homeostasis during development, differentiation, and pathophysiological processes. In the immune system, recent investigations reveal that during the course of T-cell development in the thymus, negative selection of autoreactive immature T-cells is a typical apoptotic process. In addition, apoptosis is also involved in cytotoxic killing of target cells and the regulation of lymphocyte homeostasis during immune responses. Interestingly, recent evidence has suggested that cells dying by apoptosis are actively involved in immunosuppression in various circumstances. We have shown that apoptotic cells could inhibit the expression of CD69 during T-cell activation. Furthermore, apoptotic cells phagocytosed by macrophages and/or dendritic cells are immunosuppressive, a process likely mediated by the production of transforming growth factor-beta1. Since apoptosis is a common mechanism by which excessive cells in many tissues and organs are eliminated in various pathophysiological processes, we believe that further investigation into the mechanisms by which apoptotic cells affect the immune system will not only lead to a better understanding of the significance of apoptosis during immune responses, but will also provide novel strategies for the management of autoimmune diseases and transplantation.

Apoptosis in diseases of the liver

Apoptosis, or programmed cell death, and the elimination of apoptotic cells are crucial factors in the maintenance of liver health Apoptosis allows hepatocytes to die without provoking a potentially harmful inflammatory response In contrast to necrosis, apoptosis is tightly controlled and regulated via several mechanisms, including Fas/Fas ligand interactions, the effects of cytokines such as tumor necrosis factor alpha (TNF-alpha) and transforming growth factor beta (TGF-beta), and the influence of pro- and antiapoptotic mitochondria-associated proteins of the B-cell lymphoma-2 (Bcl-2) family. Efficient elimination of apoptotic cells in the liver relies on Kupffer cells and endothelial cells and is thought to be regulated by the expression of certain cell surface receptors. Liver disease is often associated with enhanced hepatocyte apoptosis, which is the case in viral and autoimmune hepatitis, cholestatic diseases, and metabolic disorders. Disruption of apoptosis is responsible for other diseases, for example, hepatocellular carcinoma. Use and abuse of certain drugs, especially alcohol, chemotherapeutic agents, and acetaminophen, have been associated with increased apoptosis and liver damage. Apoptosis also plays a role in transplantation-associated liver damage, both in ischemia/reperfusion injury and graft rejection. The role of apoptosis in various liver diseases and the mechanisms by which apoptosis occurs in the liver may provide insight into these diseases and suggest possible treatments.

Release of WBC-derived IL-1 receptor antagonist into supernatants of RBCs: influence of storage time and filtration

BACKGROUND

Transfusion-associated immunodepression may be related to the transfer of immunoinhibitory cytokines with blood components.

STUDY DESIGN AND METHODS

After evidence of increasing concentrations of IL-1 receptor antagonist (IL-1RA) but not of IL-10 was obtained in supernatants of stored RBC units that were WBC-reduced by centrifugation (C-RBCs) in a pilot study, IL-1RA concentrations were determined weekly in supernatants of C-RBCs and in units that underwent prestorage WBC reduction by in-line filtration (F-RBCs) over a 49-day storage. For assessing total IL-1RA content, complete cell lysis by repeated freezing and thawing was done. The results were related to the changes in WBC count during storage. The dependency of IL-1RA content on preparation procedures was assessed.

RESULTS

The prestorage IL-1RA concentration in C-RBCs (859 +/- 218 pg/mL) was significantly higher than in F-RBC (75 +/- 13 pg/mL). Whereas no changes were seen in F-RBCs during storage, IL-1RA levels in C-RBC supernatants drastically increased to levels about 50 times those in normal plasma (16,327 +/- 2,686 pg/mL on Day 49). Follow-up analysis revealed stringent correlation between IL-1RA release into supernatants and the current loss of WBCs (r = 0.79, n = 42; p<0.001). The total IL-1RA content did not change during storage and was directly dependent on prestorage WBC count. Preparation procedures altered the IL-1RA content only by WBC reduction.

CONCLUSION

The immunosuppressive cytokine IL-1RA is transmitted by RBCs in relation to WBC content and storage time.

CD14-dependent clearance of apoptotic cells: relevance to the immune system

Until very recently, the function of CD14 was thought to be limited to innate immune responses to bacterial and other microbial structures. It is now known that macrophage CD14 serves an additional unexpected function, namely as a receptor involved in the recognition and phagocytosis of cells undergoing apoptosis. In stark contrast to its role in eliciting pro-inflammatory responses following binding of microbial ligands, macrophage CD14 mediates clearance of apoptotic cells without inciting inflammation. Increasing interest in the profoundly important final stage of apoptosis - the engulfment process - together with significant advances in knowledge of the immunological consequences of apoptotic-cell clearance and of the means by which signal transduction may be achieved following CD14-ligand binding have begun to produce a clearer picture of the role of CD14 in the immune system.

Exposure of N-acetylglucosamine decreases early in dexamethasone-induced apoptosis in thymocytes, demonstrated by flow cytometry using wheat germ agglutinin and pokeweed mitogen

In the present paper we describe changes in the exposure of oligosaccharides containing N-acetylglucosamine (Glc-NAc) during apoptosis of mouse thymocytes. The structures containing this sugar were probed with fluorescein isothiocyanate-labelled lectins, wheat germ agglutinin and pokeweed mitogen in flow cytometric assays. Both lectins bind to structures containing Glc-NAc. The present report describes experiments in which two different dual-staining techniques were used to simultaneously identify apoptotic cells and measure their lectin exposure. In these experiments, we observed an early and substantial decrease in the exposure of Glc-NAc-containing structures associated with the onset of apoptosis, before or simultaneously with phosphatidylserine exposure. This was followed by an increase in the exposure of Glc-NAc-containing structures after longer incubation times, when a large proportion of cells was demonstrated to have fragmented DNA. These results provide evidence for major changes in the structure of plasma membrane oligosaccharides during apoptosis. The initial decrease may be a by-product of the hydrolysis of glycosphingolipids to yield ceramide for apoptotic signalling or a deliberate process related to the removal of cell adhesion signalling structures, associated with the separation of the apoptotic cell from its neighbours. The later increase in Glc-NAc-containing structures may be the result of the incorporation of internal membranes into the plasma membrane or a deliberate production of prophagocytic signals by a still-functioning Golgi apparatus.