Nonphlogistic clearance of late apoptotic neutrophils by macrophages: efficient phagocytosis independent of beta 2 integrins

Neutrophils undergo constitutive death by apoptosis, leading to safe nonphlogistic phagocytosis and clearance by macrophages. Recent work has shown that before secondary necrosis, neutrophils exhibiting classical features of apoptosis can progress to a morphologically defined late apoptotic state. However, whether such neutrophils could be safely cleared was unknown. We now report that human late apoptotic neutrophils could be purified from cultured neutrophil populations undergoing constitutive death and were subsequently ingested by human monocyte-derived macrophages by serum-independent mechanisms that did not trigger the release of IL-8 or TNF-alpha. Such ingestion was specifically inhibited by Abs to thrombospondin-1 and the alpha(v)beta(3) vitronectin receptor. Murine bone marrow-derived macrophage phagocytosis of late and early apoptotic neutrophils occurred by similar mechanisms, proceeding with the same efficiency as that observed for wild-type controls when macrophages from [alpha(m)](-/-) or [beta(2)](-/-) mice were used. We conclude that specific nonphlogistic, beta(2) integrin-independent mechanisms involving thrombospondin-1 and alpha(v)beta(3) allow macrophages to ingest late apoptotic neutrophils without eliciting inflammatory cytokine secretion.     

Modulation of phagocytosis of apoptotic neutrophils by supernatant from dexamethasone-treated macrophages and annexin-derived peptide Ac(2-26)

Phagocytic clearance of apoptotic leukocytes plays an important role in the resolution of inflammation. The glucocorticoid-inducible protein annexin 1 and annexin 1-derived peptides show potent anti-inflammatory responses in acute and chronic inflammation. In this study, we report that the annexin 1-derived peptide (Ac(2-26)) significantly stimulates nonphlogistic phagocytosis of apoptotic polymorphonuclear leukocytes (PMNs) by human monocyte-derived macrophages (Mphi). Peptide Ac(2-26)-stimulated phagocytosis is accompanied by rearrangement of the Mphi actin cytoskeleton. To investigate the potential role of endogenous annexin on clearance of apoptotic cells, Mphi were cultured for 5 days in the presence of dexamethasone. Supernatants collected from dexamethasone-treated Mphi significantly enhanced the ability of naive Mphi to engulf apoptotic PMNs. This effect was blocked by an annexin blocking Ab, by immunodepletion of the supernatants, and by the formyl peptide receptor/lipoxin receptor antagonist Boc1. In addition, we show that bone marrow-derived Mphi from annexin 1-null mice present a 40% decreased phagocytosis of apoptotic PMNs compared with cells taken from littermate controls. In conclusion, these results emphasize the pivotal role of annexin 1 as mediator for clearance of apoptotic cells and expand its potential therapeutic role in controlling inflammatory diseases.     

Glucocorticoid augmentation of macrophage capacity for phagocytosis of apoptotic cells is associated with reduced p130Cas expression, loss of paxillin/pyk2 phosphorylation, and high levels of active Rac.

Phagocytic clearance of apoptotic granulocytes has a pivotal role in determining an inflammatory outcome, resolution or progression to a chronic state associated with development of fibrotic repair mechanisms, and/or autoimmune responses. In this study, we describe reprogramming of monocyte to macrophage differentiation by glucocorticoids, resulting in a marked augmentation of their capacity for phagocytosis of apoptotic neutrophils. This monocyte/macrophage phenotype was characterized by decreased phosphorylation, and therefore recruitment of paxillin and pyk2 to focal contacts and a down-regulation of p130Cas, a key adaptor molecule in integrin adhesion signaling. Glucocorticoid-treated cells also displayed higher levels of active Rac and cytoskeletal activity, which were mirrored by increases in phagocytic capability for apoptotic neutrophils. We propose that changes in the capacity for reorganization of cytoskeletal elements induced by glucocorticoids are essential for efficient phagocytic uptake of apoptotic cells.     

Annexin-1 and peptide derivatives are released by apoptotic cells and stimulate phagocytosis of apoptotic neutrophils by macrophages

The resolution of inflammation is a dynamically regulated process that may be subverted in many pathological conditions. Macrophage (Mphi) phagocytic clearance of apoptotic leukocytes plays an important role in the resolution of inflammation as this process prevents the exposure of tissues at the inflammatory site to the noxious contents of lytic cells. It is increasingly appreciated that endogenously produced mediators, such as lipoxins, act as potent regulators (nanomolar range) of the phagocytic clearance of apoptotic cells. In this study, we have investigated the intriguing possibility that apoptotic cells release signals that promote their clearance by phagocytes. We report that conditioned medium from apoptotic human polymorphonuclear neutrophils (PMN), Jurkat T lymphocytes, and human mesangial cells promote phagocytosis of apoptotic PMN by Mphi and THP-1 cells differentiated to a Mphi-like phenotype. This prophagocytic activity appears to be dose dependent, sensitive to the caspase inhibitor zVAD-fmk, and is associated with actin rearrangement and release of TGF-beta1, but not IL-8. The prophagocytic effect can be blocked by the formyl peptide receptor antagonist Boc2, suggesting that the prophagocytic factor(s) may interact with the lipoxin A(4) receptor, FPRL-1. Using nanoelectrospray liquid chromatography mass spectrometry and immunodepletion and immunoneutralization studies, we have ascertained that annexin-1 and peptide derivatives are putative prophagocytic factors released by apoptotic cells that promote phagocytosis of apoptotic PMN by M[phi] and differentiated THP-1 cells. These data highlight the role of annexin-1 and peptide derivatives in promoting the resolution of inflammation and expand on the therapeutic anti-inflammatory potential of annexin-1.     

Novel role of ICAM3 and LFA-1 in the clearance of apoptotic neutrophils by human macrophages

Apoptotic cells express eat-me signals which are recognized by several receptors mainly on professional phagocytes of the mononuclear phagocyte system. This “engulfment synapse” can define a safe and effective clearance of apoptotic cells in order to maintain tissue homeostasis in the entire body. We show that the expression of four genes related to apoptotic cell clearance is strongly up-regulated in human macrophages 30 min after administration of apoptotic neutrophils. Out of these the significant role of the up-regulated intercellular adhesion molecule 3 (ICAM3) in phagocytosis of apoptotic neutrophils could be demonstrated in macrophages by gene silencing as well as treatment with blocking antibodies. Blocking ICAM3 on the surface of apoptotic neutrophils also resulted in their decreased uptake which confirmed its role as an eat-me signal expressed by apoptotic cells. In macrophages but not in neutrophils silencing and blocking integrin alphaL and beta2 components of lymphocyte function-associated antigen 1 (LFA-1), which can strongly bind ICAM3, resulted in a decreased phagocytosis of apoptotic cells indicating its possible role to recognize ICAM3 on the surface of apoptotic neutrophils. Finally, we report that engulfing portals formed in macrophages during phagocytosis are characterized by accumulation of ICAM3, integrin alphaL and beta2 which show co-localization on the surface of phagocytes. Furthermore, their simultaneous knock-down in macrophages resulted in a marked deficiency in phagocytosis and a slight decrease in the anti-inflammatory effect of apoptotic neutrophils. We propose that ICAM3 and LFA-1 act as recognition receptors in the phagocytosis portals of macrophages for engulfment of apoptotic neutrophils.     

CD14-dependent clearance of apoptotic cells by human macrophages: the role of phosphatidylserine

Apoptotic-cell clearance is dependent on several macrophage surface molecules, including CD14. Phosphatidylserine (PS) becomes externalised during apoptosis and participates in the clearance process through its ability to bind to a novel receptor, PS-R. CD14 has the proven ability to bind phospholipids and may function as an alternative receptor for the externalised PS of apoptotic cells. Here we demonstrate that CD14 does not function preferentially as a PS receptor in apoptotic-cell clearance. Compared with phosphatidylcholine and phosphatidylethanolamine, PS was the least active phospholipid binding to human monocyte-derived macrophages and showed no specificity for soluble or membrane-anchored CD14. Significantly, PS-containing liposomes failed to inhibit CD14-dependent uptake of apoptotic cells by macrophages. PS exposure was, however, found to be insufficient for either CD14-dependent or CD14-independent apoptotic-cell uptake by phagocytes. The additional features that enable apoptotic-cell clearance are derived from mechanisms that can be divorced temporally from those responsible for the morphological features of apoptosis.     

Receptor for advanced glycation end products binds to phosphatidylserine and assists in the clearance of apoptotic cells

Clearance of apoptotic cells is necessary for tissue development, homeostasis and resolution of inflammation. The uptake of apoptotic cells is initiated by an 'eat-me' signal, such as phosphatidylserine, on the cell surface and phagocytes recognize the signal by using specific receptors. In this study, we show that the soluble form of the receptor for advanced glycation end products (RAGE) binds to phosphatidylserine as well as to the apoptotic thymocytes. RAGE-deficient (Rage(-/-)) alveolar macrophages showed impaired phagocytosis of apoptotic thymocytes and defective clearance of apoptotic neutrophils in Rage(-/-) mice. Our results indicate that RAGE functions as a phosphatidylserine receptor and assists in the clearance of apoptotic cells.     

A new insight into phagocytosis of apoptotic cells: proteolytic enzymes divert the recognition and clearance of polymorphonuclear leukocytes by macrophages

The recognition of phosphatidylserine (PS) on the surface of any apoptotic cell is considered to be a key event for its clearance. We challenge this concept by showing that pretreatment of neutrophils with either host or bacterial protease affects their uptake by human monocyte-derived macrophages without having an effect on cell-surface PS presentation. Specifically, whereas preincubation of apoptotic neutrophils with cathepsin G or thrombin significantly inhibited their uptake, gingipains R or clostripain enhanced phagocytosis by macrophages. Moreover, bacterial proteinases sensitized healthy neutrophils for uptake by macrophages, whereas endogenous proteinases were unable to elicit this effect. This stimulation was apparently owing to the combined effect of proteolytic cleavage of an antiphagocytic signal (CD31) and the generation of a novel 'eat-me' signal on the neutrophil surface. These results argue that neutrophil recognition and phagocytosis by macrophages is mediated by a protein ligand whose proteolytic modification could affect the local inflammatory process.     

Uptake of lactoferrin by mononuclear phagocytes inhibits their ability to form hydroxyl radical and protects them from membrane autoperoxidation.

Human mononuclear phagocytes do not contain the iron-binding protein lactoferrin that we have previously demonstrated inhibits the potential for human neutrophils to generate hydroxyl radical in the presence of an exogenous iron catalyst of the Haber-Weiss reaction. Previous work by other investigators has suggested that mononuclear phagocytes (monocytes and monocyte-derived macrophages (MDM] have the capacity to bind exogenous lactoferrin via lactoferrin-specific membrane surface receptors. Accordingly, we examined the possibility that uptake of iron-free (apo) lactoferrin by human mononuclear phagocytes could play a role in limiting the potential for generation of hydroxyl radical during the monocyte/MDM respiratory burst. When monocytes or MDM were incubated in the presence of apo-lactoferrin, cell-associated lactoferrin increased in proportion to the concentration of lactoferrin provided. Similar results were obtained with iron-loaded (diferric) milk lactoferrin. Consistent with the in vivo importance of these findings, we found that lactoferrin was intimately associated with human alveolar macrophages obtained by bronchoalveolar lavage. The fucose polymer fucoidan inhibited lactoferrin uptake whereas exogenous transferrin or MDM exposure to IFN-gamma was without effect. Scatchard binding analysis confirmed the presence of a lactoferrin-specific receptor with a calculated kDa of 3.56 x 10(-6) M and 3.4 x 10(7) binding sites per cell. Subcellular fractionation studies indicated that twofold more of the lactoferrin which became cell-associated over the 1-h incubation time could be found in the cytoplasmic fraction compared to the plasma membrane-containing fraction, consistent with previous evidence by others for internalization of lactoferrin by mononuclear phagocytes. When lactoferrin-loaded monocytes/MDM were incubated in lactoferrin-free media, evidence for release of lactoferrin was obtained by SDS-PAGE and immunoblot analysis, suggesting the presence of a recyclable pool of cell-associated lactoferrin. To assess the impact of lactoferrin loading on monocyte/MDM hydroxyl radical formation, lactoferrin-loaded phagocytes were stimulated with PMA in the presence of catalytic iron. Hydroxyl radical generation by lactoferrin-loaded cells was decreased to about 50% of control cells. Similarly, monocytes that had been lactoferrin-loaded demonstrated a 28% decrease in autooxidation of their membrane when stimulated in the presence of catalytic iron. These data suggest that lactoferrin binding may play an important role in maintaining optimal mononuclear phagocyte function and protecting adjacent tissue from untoward phagocyte-associated hydroxyl radical generation.     

Phagocytosis of apoptotic cells by macrophages induces novel signaling events leading to cytokine-independent survival and inhibition of proliferation: activation of Akt and inhibition of extracellular signal-regulated kinases 1 and 2

Recent evidence indicates that phagocytic clearance of apoptotic cells, initially thought to be a silent event, can modulate macrophage (M phi) function. We show in this work that phagocytic uptake of apoptotic cells or bodies, in the absence of serum or soluble survival factors, inhibits apoptosis and maintains viability of primary cultures of murine peritoneal and bone marrow M phi with a potency approaching that of serum-supplemented medium. Apoptotic uptake also profoundly inhibits the proliferation of bone marrow M phi stimulated to proliferate by M-CSF. While inhibition of proliferation is an unusual property for survival factors, the combination of increased survival and decreased proliferation may aid the M phi in its role as a scavenger during resolution of inflammation. The ability of apoptotic cells to promote survival and inhibit proliferation appears to be the result of simultaneous activation of Akt and inhibition of the mitogen-activated protein kinases extracellular signal-regulated kinase (ERK)1 and ERK2 (ERK1/2). While several activators of the innate immune system, or danger signals, also inhibit apoptosis and proliferation, danger signals and necrotic cells differ from apoptotic cells in that they activate, rather than inhibit, ERK1/2. These signaling differences may underlie the opposing tendencies of apoptotic cells and danger signals in promoting tolerance vs immunity.     

Friendly fire against neutrophils: proteolytic enzymes confuse the recognition of apoptotic cells by macrophages

Physiologically the only acceptable fate for almost all damaged or unwanted cells is their apoptotic death, followed by engulfment of the corpses by healthy neighbors or professional phagocytes. Efficient clearance of cells that have succumbed to apoptosis is crucial for normal tissue homeostasis, and for the modulation of immune responses. The disposal of apoptotic cells is finely regulated by a highly redundant system of receptors, bridging molecules and 'eat me' signals. The complexity of the system is reflected by the term: 'engulfment synapse', used to describe the interaction between a phagocytic cell and its target. In healthy humans, dying neutrophils are the most abundant and important targets for such recognition and engulfment. In inflammation the scope and importance of this complicated task is further increased. Paradoxically, despite growing evidence highlighting the priority of neutrophils clearance, the recognition of these cells by phagocytes is not as well understood as the recognition of other apoptotic cell types. New findings indicate that the interaction of phosphatidylserine (PS) on apoptotic neutrophils with its receptor on macrophages is not as critical for the specific clearance of neutrophil corpses it was previously believed. In this review we focus on recent findings regarding alternative, PS-independent "eat me" signals expressed on neutrophils during cell death and activation. Based on our own research, we emphasize the clearance of dying neutrophils, especially at the focus of bacterial infection; and the associated inflammatory reaction, which occurs in a highly proteolytic milieu containing both host and bacteria-derived proteinases. In these environments, eat-me signals expressed by neutrophils are drastically modified; arguing against the phospholipid-based detection of apoptotic cells, but supporting the importance of proteinaceous ligand(s) for the recognition of neutrophils by macrophages. In this context we discuss the effect of the gingipain R (Rgp) proteinases from Porphyromonas gingivalis on neutrophils interactions with macrophages. Since the recognition of apoptotic neutrophils is an important fundamental process, serving multiple functions in the regulation of immunity and homeostasis, we hypothesize that many pathogenic bacteria may have developed similar strategies to confuse macrophage-neutrophil interaction as a common pathogenic strategy.     

Local amplification of glucocorticoids by 11 beta-hydroxysteroid dehydrogenase type 1 promotes macrophage phagocytosis of apoptotic leukocytes

Glucocorticoids promote macrophage phagocytosis of leukocytes undergoing apoptosis. Prereceptor metabolism of glucocorticoids by 11beta-hydroxysteroid dehydrogenases (11beta-HSDs) modulates cellular steroid action. 11beta-HSD type 1 amplifies intracellular levels of active glucocorticoids in mice by reactivating corticosterone from inert 11-dehydrocorticosterone in cells expressing the enzyme. In this study we describe the rapid (within 3 h) induction of 11beta-HSD activity in cells elicited in the peritoneum by a single thioglycolate injection in mice. Levels remained high in peritoneal cells until resolution. In vitro experiments on mouse macrophages demonstrated that treatment with inert 11-dehydrocorticosterone for 24 h increased phagocytosis of apoptotic neutrophils to the same extent as corticosterone. This effect was dependent upon 11beta-HSD1, as 11beta-HSD1 mRNA, but not 11beta-HSD2 mRNA, was expressed in these cells; 11-dehydrocorticosterone was ineffective in promoting phagocytosis by Hsd11b1(-/-) macrophages, and carbenoxolone, an 11beta-HSD inhibitor, prevented the increase in phagocytosis elicited in wild-type macrophages by 11-dehydrocorticosterone. Importantly, as experimental peritonitis progressed, clearance of apoptotic neutrophils was delayed in Hsd11b1(-/-) mice. These data point to an early role for 11beta-HSD1 in promoting the rapid clearance of apoptotic cells during the resolution of inflammation and indicate a novel target for therapy.     

Vascular endothelial growth factor enhances macrophage clearance of apoptotic cells

Efficient clearance of apoptotic cells from the lung by alveolar macrophages is important for the maintenance of tissue structure and function. Lung tissue from humans with emphysema contains increased numbers of apoptotic cells and decreased levels of vascular endothelial growth factor (VEGF). Mice treated with VEGF receptor inhibitors have increased numbers of apoptotic cells and develop emphysema. We hypothesized that VEGF regulates apoptotic cell clearance by alveolar macrophages (AM) via its interaction with VEGF receptor 1 (VEGF R1). Our data show that the uptake of apoptotic cells by murine AMs and human monocyte-derived macrophages is inhibited by depletion of VEGF and that VEGF activates Rac1. Antibody blockade or pharmacological inhibition of VEGF R1 activity also decreased apoptotic cell uptake ex vivo. Conversely, overexpression of VEGF significantly enhanced apoptotic cell uptake by AMs in vivo. These results indicate that VEGF serves a positive regulatory role via its interaction with VEGF R1 to activate Rac1 and enhance AM apoptotic cell clearance.     

Lipopolysaccharide induces rapid production of IL-10 by monocytes in the presence of apoptotic neutrophils

There is growing evidence that apoptotic neutrophils have an active role to play in the regulation and resolution of inflammation following phagocytosis by macrophages and dendritic cells. However, their influence on activated blood monocytes, freshly recruited to sites of inflammation, has not been defined. In this work, we examined the effect of apoptotic neutrophils on cytokine production by LPS-activated monocytes. Monocytes stimulated with LPS in the presence of apoptotic neutrophils for 18 h elicited an immunosuppressive cytokine response, with enhanced IL-10 and TGF-beta production and only minimal TNF-alpha and IL-1beta cytokine production. Time-kinetic studies demonstrated that IL-10 production was markedly accelerated in the presence of apoptotic neutrophils, whereas there was a sustained reduction in the production of TNF-alpha and IL-1beta. This suppression of proinflammatory production was not reversible by depletion of IL-10 or TGF-beta or by addition of exogenous IFN-gamma. It was demonstrated, using Transwell experiments, that monocyte-apoptotic cell contact was required for induction of the immunosuppressive monocyte response. The response of monocytes contrasted with that of human monocyte-derived macrophages in which there was a reduction in IL-10 production. We conclude from these data that interaction between activated monocytes and apoptotic neutrophils creates a unique response, which changes an activated monocyte from being a promoter of the inflammatory cascade into a cell primed to deactivate itself and other cells.     

Dynamics of cytochemically distinct subpopulations of macrophages in elicited rat peritoneal exudates

Qualitative and quantitative changes in rat peritoneal macrophage (M phi) subpopulations, differing in their ultrastructural peroxidatic staining characteristics were followed over the course of a thioglycollate (TG) broth-induced inflammatory response. In addition, selected functional features of the normal steady-state and 4-day TG-induced populations of M phi were compared. The steady-state population consisted primarily of M phi with peroxidatic staining limited to the nuclear envelope (NE) and rough endoplasmic reticulum (RER); such cells are called resident M phi. Within hours of TG injection, there was an influx of monocyte-derived exudate M phi, the number of which reached a maximum, by 24 hr. During the next 24 hr, the proportion of exudate M phi decreased with a concomitant increase in peroxidatic activity (PA)-negative M phi. These two cell types continued to predominate for the next 48 hr during which there was a gradual increase in resident M phi and so-called "exudate-resident" M phi, the latter of which exhibits both exudate and resident PA patterns. Thus, the 4-day TG-induced population consisted of four cytochemically distinct M phi subpopulations: approximately 50% PA-negative M phi, approximately 25% exudate M phi, approximately 15% resident M phi, and approximately 10% exudate-resident M phi. Differences in Fc receptors and complement receptors 1 and 3 were noted between the two populations in the presence of progenitors that give rise to colonies of M phi in liquid culture in response to murine-derived colony-stimulating factor 1. The implications of these results in regard to the origin(s) of M phi diversity are discussed.     

Galectin-3 functions as an opsonin and enhances the macrophage clearance of apoptotic neutrophils

Galectin-3, a β-galactoside binding, endogenous lectin,takes part in various inflammatory events and is produced insubstantial amounts at inflammatory foci. We investigated whetherextracellular galectin-3 could participate in the phagocyticclearance of apoptotic neutrophils by macrophages, a processof crucial importance for termination of acute inflammation.Using human leukocytes, we show that exogenously added galectin-3increased the uptake of apoptotic neutrophils by monocyte-derivedmacrophages (MDM). Both the proportion of MDM that engulfedapoptotic prey and the number of apoptotic neutrophils thateach MDM engulfed were enhanced in the presence of galectin-3.The effect was lactose-inhibitable and required galectin-3 affinityfor N-acetyllactosamine, a saccharide typically found on cellsurface glycoproteins, since a mutant lacking this activitywas without effect. The enhanced uptake relied on the presenceof galectin-3 during the cellular interaction and was paralleledby lectin binding to apoptotic cells as well as MDM in a lactose-dependentmanner. These findings suggest that galectin-3 functions asa bridging molecule between phagocyte and apoptotic prey, actingas an opsonin. The process of clearance, whereby apoptotic neutrophilsare removed by macrophages, is crucial for the resolution ofacute inflammation and our data imply that the increased levelsof galectin-3 often found at inflammatory sites could potentlyaffect this process.     

Neutrophils Are Not Less Sensitive Than Other Blood Leukocytes to the Genomic Effects of Glucocorticoids

BackgroundNeutrophils are generally considered less responsive to glucocorticoids compared to other inflammatory cells. The reported increase in human neutrophil survival mediated by these drugs partly supports this assertion. However, it was recently shown that dexamethasone exerts potent anti-inflammatory effects in equine peripheral blood neutrophils. Few comparative studies of glucocorticoid effects in neutrophils and other leukocytes have been reported and a relative insensitivity of neutrophils to these drugs could not be ruled out.ObjectiveWe assessed glucocorticoid-responsiveness in equine and human peripheral blood neutrophils and neutrophil-depleted leukocytes.MethodsBlood neutrophils and neutrophil-depleted leukocytes were isolated from 6 healthy horses and 4 human healthy subjects. Cells were incubated for 5 h with or without LPS (100 ng/mL) alone or combined with hydrocortisone, prednisolone or dexamethasone (10⁻⁸ M and 10⁻⁶ M). IL-1β, TNF-α, IL-8, glutamine synthetase and GR-α mRNA expression was quantified by qPCR. Equine neutrophils were also incubated for 20 h with or without the three glucocorticoids and cell survival was assessed by flow cytometry and light microscopy on cytospin preparations.ResultsWe found that glucocorticoids down-regulated LPS-induced pro-inflammatory mRNA expression in both cell populations and species. These drugs also significantly increased glutamine synthetase gene expression in both equine cell populations. The magnitude of glucocorticoid response between cell populations was generally similar in both species. We also showed that dexamethasone had a comparable inhibitory effect on pro-inflammatory gene expression in both human and equine neutrophils. As reported in other species, glucocorticoids significantly increase the survival in equine neutrophils.ConclusionsGlucocorticoids exert genomic effects of similar magnitude on neutrophils and on other blood leukocytes. We speculate that the poor response to glucocorticoids observed in some chronic neutrophilic diseases such as severe asthma or COPD is not explained by a relative lack of inhibition of these drugs on pro-inflammatory cytokines expression in neutrophils.     

Apoptotic cell-derived ICAM-3 promotes both macrophage chemoattraction to and tethering of apoptotic cells

A wide range of molecules acting as apoptotic cell-associated ligands, phagocyte-associated receptors or soluble bridging molecules have been implicated within the complex sequential processes that result in phagocytosis and degradation of apoptotic cells. Intercellular adhesion molecule 3 (ICAM-3, also known as CD50), a human leukocyte-restricted immunoglobulin super-family (IgSF) member, has previously been implicated in apoptotic cell clearance, although its precise role in the clearance process is ill defined. The main objective of this work is to further characterise the function of ICAM-3 in the removal of apoptotic cells. Using a range of novel anti-ICAM-3 monoclonal antibodies (mAbs), including one (MA4) that blocks apoptotic cell clearance by macrophages, alongside apoptotic human leukocytes that are normal or deficient for ICAM-3, we demonstrate that ICAM-3 promotes a domain 1-2-dependent tethering interaction with phagocytes. Furthermore, we demonstrate an apoptosis-associated reduction in ICAM-3 that results from release of ICAM-3 within microparticles that potently attract macrophages to apoptotic cells. Taken together, these data suggest that apoptotic cell-derived microparticles bearing ICAM-3 promote macrophage chemoattraction to sites of leukocyte cell death and that ICAM-3 mediates subsequent cell corpse tethering to macrophages. The defined function of ICAM-3 in these processes and profound defect in chemotaxis noted to ICAM-3-deficient microparticles suggest that ICAM-3 may be an important adhesion molecule involved in chemotaxis to apoptotic human leukocytes.     

Programmed cell clearance: Molecular regulation of the elimination of apoptotic cell corpses and its role in the resolution of inflammation

Programmed cell clearance is a physiological process of elimination of apoptotic cell corpses. Recent studies have disclosed several ligand-receptor interactions that dictate the recognition or non-recognition of cells by macrophages and other phagocytes. The externalization of the anionic phospholipid, phosphatidylserine is effectively recognized by specific receptors on professional phagocytes and facilitates the clearance of apoptotic cells. Macrophage disposal of cells at sites of inflammation is believed to play an important role in the resolution of the inflammatory process, and recent studies have suggested a role for the NADPH oxidase in the process of macrophage elimination of activated neutrophils. The present review will focus on the molecular regulation of programmed cell clearance, and discuss the role of cell elimination in the resolution of inflammation.     

Proteinase 3, the autoantigen in granulomatosis with polyangiitis, associates with calreticulin on apoptotic neutrophils, impairs macrophage phagocytosis, and promotes inflammation

Proteinase 3 (PR3) is the target of anti-neutrophil cytoplasm Abs in granulomatosis with polyangiitis, a form of systemic vasculitis. Upon neutrophil apoptosis, PR3 is coexternalized with phosphatidylserine and impaired macrophage phagocytosis. Calreticulin (CRT), a protein involved in apoptotic cell recognition, was found to be a new PR3 partner coexpressed with PR3 on the neutrophil plasma membrane during apoptosis, but not after degranulation. The association between PR3 and CRT was demonstrated in neutrophils by confocal microscopy and coimmunoprecipitation. Evidence for a direct interaction between PR3 and the globular domain of CRT, but not with its P domain, was provided by surface plasmon resonance spectroscopy. Phagocytosis of apoptotic neutrophils from healthy donors was decreased after blocking lipoprotein receptor-related protein (LRP), a CRT receptor on macrophages. In contrast, neutrophils from patients with granulomatosis with polyangiitis expressing high membrane PR3 levels showed a lower rate of phagocytosis than those from healthy controls not affected by anti-LRP, suggesting that the LRP-CRT pathway was disturbed by PR3-CRT association. Moreover, phagocytosis of apoptotic PR3-expressing cells potentiated proinflammatory cytokine in vitro by human monocyte-derived macrophages and in vivo by resident murine peritoneal macrophages, and diverted the anti-inflammatory response triggered by the phagocytosis of apoptotic cells after LPS challenge in thioglycolate-elicited murine macrophages. Therefore, membrane PR3 expressed on apoptotic neutrophils might amplify inflammation and promote autoimmunity by affecting the anti-inflammatory "reprogramming" of macrophages.