Mer receptor tyrosine kinase mediates both tethering and phagocytosis of apoptotic cells

Billions of inflammatory leukocytes die and are phagocytically cleared each day. This regular renewal facilitates the normal termination of inflammatory responses, suppressing pro-inflammatory mediators and inducing their anti-inflammatory counterparts. Here we investigate the role of the receptor tyrosine kinase (RTK) Mer and its ligands Protein S and Gas6 in the initial recognition and capture of apoptotic cells (ACs) by macrophages. We demonstrate extremely rapid binding kinetics of both ligands to phosphatidylserine (PtdSer)-displaying ACs, and show that ACs can be co-opsonized with multiple PtdSer opsonins. We further show that macrophage phagocytosis of ACs opsonized with Mer ligands can occur independently of a requirement for αV integrins. Finally, we demonstrate a novel role for Mer in the tethering of ACs to the macrophage surface, and show that Mer-mediated tethering and subsequent AC engulfment can be distinguished by their requirement for Mer kinase activity. Our results identify Mer as a receptor uniquely capable of both tethering ACs to the macrophage surface and driving their subsequent internalization.

N-Acetylcysteine inhibits platelet-monocyte conjugation in patients with type 2 diabetes with depleted intraplatelet glutathione: a randomised controlled trial

AIMS/HYPOTHESIS

The aim of this study was to determine whether oral dosing with N-acetylcysteine (NAC) increases intraplatelet levels of the antioxidant, glutathione (GSH), and reduces platelet-monocyte conjugation in blood from patients with type 2 diabetes.

METHODS

In this placebo-controlled randomised crossover study, the effect of oral NAC dosing on platelet-monocyte conjugation and intraplatelet GSH was investigated in patients with type 2 diabetes (eligibility criteria: men or post-menopausal women with well-controlled diabetes (HbA(1c) < 10%), not on aspirin or statins). Patients (n = 14; age range 43-79 years, HbA(1c) = 6.9 ± 0.9% [52.3 ± 10.3 mmol/mol]) visited the Highland Clinical Research Facility, Inverness, UK on day 0 and day 7 for each arm of the study. Blood was sampled before and 2 h after oral administration of placebo or NAC (1,200 mg) on day 0 and day 7. Patients received placebo or NAC capsules for once-daily dosing on the intervening days. The order of administration of NAC and placebo was allocated by a central office and all patients and research staff involved in the study were blinded to the allocation until after the study was complete and the data fully analysed. The primary outcome for the study was platelet-monocyte conjugation.

RESULTS

Oral NAC reduced platelet-monocyte conjugation (from 53.1 ± 4.5% to 42.5 ± 3.9%) at 2 h after administration and the effect was maintained after 7 days of dosing. Intraplatelet GSH was raised in individuals with depleted GSH and there was a negative correlation between baseline intraplatelet GSH and platelet-monocyte conjugation. There were no adverse events.

CONCLUSIONS/INTERPRETATION

The NAC-induced normalisation of intraplatelet GSH, coupled with a reduction in platelet-monocyte conjugation, suggests that NAC might help to reduce atherothrombotic risk in type 2 diabetes.

FUNDING

Chief Scientist Office (CZB/4/622), Scottish Funding Council, Highlands & Islands Enterprise and European Regional Development Fund.

TRIAL REGISTRATION

isrctn.org ISRCTN89304265.

Functional and clinical consequences of Fc receptor polymorphic and copy number variants

Receptors for immunoglobulins (Fc receptors) play a central role during an immune response, as they mediate the specific recognition of antigens of almost infinite diversity by leucocytes, thereby linking the humoral and cellular components of immunity. Indeed, engagement of Fc receptors by immunoglobulins initiates a range of immunoregulatory processes that might also play a role in disease pathogenesis. In the circulation, five main types of immunoglobulins (Ig) exist - namely IgG, IgA, IgE, IgM and IgD and receptors with the ability to recognize and bind to IgG (Fc gamma receptor family), IgE (Fc epsilon RI and CD23), IgA (CD89; Fc alpha/microR) and IgM (Fc alpha/microR) have been identified and characterized. However, it is astonishing that nearly all the known human Fc receptors display extensive genetic variation with clear implications for their function, thus representing a substantial genetic risk factor for the pathogenesis of a range of chronic inflammatory disorders.

Divalent cation-dependent and -independent augmentation of macrophage phagocytosis of apoptotic neutrophils by CD44 antibody

Phagocytosis of apoptotic neutrophils by macrophages is required for resolution of an inflammatory response. Removal of intact apoptotic neutrophils prevents the release of cytotoxic granules that would otherwise cause tissue damage and may lead to development of fibrosis. Importantly, macrophage phagocytosis of apoptotic neutrophils fails to induce release of proinflammatory mediators, consistent with a 'safe' pathway for disposal of potentially harmful inflammatory cells. One pathway for increasing phagocytosis of apoptotic cells to allow matching of tissue phagocyte capacity to apoptotic cell load in vitro is via antibody-mediated cross-linking of CD44, providing a mechanism for limiting tissue damage during resolution of inflammation. In this study, we have defined divalent cation-dependent and -independent actions of the CD44 antibody. For the divalent cation-independent CD44 antibody effect, we provide evidence that 'enabled' CD32 on the apoptotic neutrophil binds to intact CD44 antibody on the macrophage surface. One implication is that macrophages can phagocytose apoptotic neutrophils that are 'tethered' to the macrophage surface in a manner that is independent of defined apoptotic mechanisms. These data also provide an explanation for the greater efficacy of intact CD44 antibody when compared with F(ab')2 fragments.

Getting to grips with the granulocyte: manipulation of granulocyte behaviour and apoptosis by protein transduction methods

Human granulocytes clearly play an important role in host defence against invading organisms, however they have also been implicated in the pathogenesis and progression of many chronic inflammatory diseases. In addition, these cells have been paramount in gaining a better understanding of many key-signalling pathways regulating fundamental biological processes. Since granulocytes are terminally differentiated and undergo relatively rapid constitutive apoptosis it has been difficult to manipulate intracellular events by transfection or transduction procedures. It has been shown in recent years that the HIV-TAT protein transduction system can be successfully used in granulocytes to manipulate key signalling mechanisms regulating functional responsiveness and survival. In this paper, we review recent literature highlighting important developments using this system in isolated human granulocytes and in inflammatory process in vivo where these cells play a prominent role.

Regulation of granulocyte apoptosis by NF-κB

Granulocyte apoptosis is a crucial part of the successful resolution of inflammation. In vitro results show that activation of NF-κB (nuclear factor κB) in granulocytes is a survival mechanism. NF-κB inhibitors increase the rate of constitutive apoptosis in neutrophils and eosinophils and cause these cells to respond to the pro-apoptotic effects of TNF-α (tumour necrosis factor-α). Results from both in vivo and in vitro experiments suggest that there are at least two important waves of NF-κB activation in inflammatory loci, which increase the expression of COX-2 (cyclooxygenase-2), itself an NF-κB controlled gene. The first wave causes the production of inflammatory mediators such as PGE2 (prostaglandin E2), allowing the establishment of inflammation. The second wave causes the synthesis of PGD2 and its metabolites that induce granulocyte apoptosis by inhibiting NF-κB activation. These metabolites may therefore be important physiological mediators controlling the resolution of inflammation. Although NF-κB is an important target for anti-inflammatory therapy, the timing of inhibition in vivo may be crucial, to ensure that production of PGD2 and its sequential metabolites can occur.

Granulocyte apoptosis: who would work with a ‘real’ inflammatory cell?

The neutrophil granulocyte is a key factor in cellular innate defence mechanisms against infection or tissue damage. Granulocyte apoptosis is now acknowledged to have a critical role in progression of inflammatory responses. Granulocytes are preprogrammed to die with important physiological mechanisms for non-inflammatory clearance. Shutdown of secretory capacity represents an important aspect of the programme of biochemical events that accompany neutrophil apoptosis together with surface molecular changes that serve to identify apoptotic cells as targets for phagocytic removal. Defining the underlying regulatory mechanisms together with the changes in patterns of gene/protein expression associated with granulocyte death remains a challenge. Use of novel strategies for inducing cell death will allow biochemical approaches to dissect the underlying pathways. Although study of granulocyte cell death has especial difficulties when compared with other cell types, there are clearly potential benefits for new therapeutic approaches to treat inflammatory diseases.

Phagocytosis of opsonized apoptotic cells: roles for 'old-fashioned' receptors for antibody and complement

Efficient phagocytic clearance of apoptotic cells is crucial in many biological processes. A bewildering array of phagocyte receptors have been implicated in apoptotic cell clearance, but there is little convincing evidence that they act directly as apoptotic cell receptors. Alternatively, apoptotic cells may become opsonized, whereby naturally occurring soluble factors (opsonins) bind to the cell surface and initiate phagocytosis. Evidence is accumulating that antibodies and complement proteins opsonize apoptotic cells, leading to phagocytosis mediated by well-defined 'old-fashioned' receptors for immunoglobulin-Fc and complement. In this review we summarize the evidence that opsonization is necessary for high capacity clearance of apoptotic cells, which would render putative direct apoptotic cell receptors redundant.

Regulation of Granulocyte Apoptosis by Hemopoietic Growth Factors, Cytokines and Drugs: Potential Relevance to Allergic Inflammation

It has become apparent that the resolution of inflammation depends on the removal of unwanted inflammatory cells, a process governed by physiological apoptosis and non-inflammatory clearance of apoptotic cells. Granulocytes are central to many of the pathophysiological consequences of uncontrolled inflammatory reactions. Hemopoietic factors and cytokines play a critical role in regulating the longevity of these cells in vitro and in vivo. Here we review the progress that has been made in the understanding of granulocyte apoptosis and the implications for immunotherapy and pharmacological strategies in the treatment of allergic inflammatory diseases for therapeutic gain.

Glucocorticoid-mediated regulation of granulocyte apoptosis and macrophage phagocytosis of apoptotic cells: implications for the resolution of inflammation

Glucocorticoids represent one of the most effective clinical treatments for a range of inflammatory conditions, including severe acute inflammation. Although glucocorticoids are known to affect processes involved in the initiation of inflammation, the influence of glucocorticoids on the mechanisms by which acute inflammation normally resolves have received less attention. Apoptosis of granulocytes present at inflamed sites leads to their rapid recognition and internalisation by macrophages, a process which may be important for resolution of inflammation. However, if clearance of either eosinophils or neutrophils is impaired, these cells rapidly undergo secondary necrosis leading to release of pro-inflammatory mediators from the phagocyte, potentially prolonging inflammatory responses. Physiologically relevant concentrations of glucocorticoids accelerate eosinophil apoptosis whilst delaying neutrophil apoptosis during in vitro culture. Here we discuss key pathways regulating the granulocyte apoptotic programme and summarise the effects of glucocorticoids on monocyte differentiation and the consequent changes to apoptotic cell clearance capacity. Definition of the mechanisms underlying resolution of inflammatory responses following glucocorticoid treatment may unveil new targets for modulation of inflammatory disease, allowing co-ordinated augmentation of granulocyte apoptosis together with increased macrophage capacity for clearance of apoptotic cells.

Cyclic AMP regulation of neutrophil apoptosis occurs via a novel protein kinase A-independent signaling pathway

The second messenger molecule cyclic AMP dramatically modulates the apoptotic program in a wide variety of cells, accelerating apoptosis in some and delaying the rate of apoptosis in others. Human neutrophil apoptosis, a process that regulates the fate and numbers of these potentially histotoxic cells in inflammatory sites, is profoundly delayed by the cell-permeable analog of cyclic AMP, dibutyryl-cAMP. We have investigated the mechanisms underlying cyclic AMP-mediated delay of neutrophil apoptosis, and we show that cyclic AMP inhibits loss of mitochondrial potential occurring during constitutive neutrophil apoptosis. Furthermore, we demonstrate that cyclic AMP also suppresses caspase activation in these inflammatory cells. Despite increasing protein kinase A activity, this kinase is unlikely to mediate the effect of cyclic AMP on apoptosis because blockade of protein kinase A activation did not influence the survival effects of cyclic AMP. Further investigation of the signaling mechanism demonstrated that the delay of apoptosis is independent of phosphoinositide 3-kinase and MAPK activation. Our results suggest cyclic AMP delays neutrophil apoptosis via a novel, reversible, and transcriptionally independent mechanism. We show that proteasome activity in the neutrophil is vitally involved in this process, and we suggest that a balance of pro-apoptotic and anti-apoptotic proteins plays a key role in the powerful ability of cyclic AMP to delay neutrophil death.

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.

Thalidomide analogue CC-3052 reduces HIV+ neutrophil apoptosis in vitro

Thalidomide has significant immunomodulatory properties and has been used successfully in the treatment of oral ulcers and wasting in HIV patients. However, its use is limited by its poor bioavailability due to low solubility and short half life in solution, and teratogenic and neurotoxic side-effects. Recently, water-soluble analogues of thalidomide with significantly greater immunomodulatory activity and reduced side-effects have become available. We examined the effect of thalidomide and one analogue, CC-3052, on neutrophil apoptosis following culture for 20 h in vitro. Apoptosis was assessed by reduced CD16 expression and Annexin V binding using flow cytometry. Thalidomide or CC-3052 alone had no effect on neutrophil apoptosis when used at physiological levels. However, when used together with prostaglandin E2 (10-7 M), a potent adenylate cyclase activator, CC-3052 but not thalidomide (both 10-5 M) reduced apoptosis in neutrophils from normal and HIV+ donors. The reduced apoptosis could not be attributed to the ability of CC-3052 to reduce tumour necrosis factor-alpha (TNF-alpha) production, but may be due to its PDE4 inhibitor properties, as it increased [cAMP]i, and mimicked the effect of increasing [cAMP]i using dibutryl cAMP, a membrane-permeable analogue of cAMP. The results suggest a role for thalidomide analogue CC-3052 in reducing persistent activation of the TNF-alpha system in HIV without markedly impairing neutrophil viability.

Soluble E-selectin acts in synergy with platelet-activating factor to activate neutrophil beta 2-integrins. Role of tyrosine kinases and Ca2+ mobilization

Selectins play a critical role in neutrophil recruitment to sites of inflammation, in tethering and rolling of neutrophils on vascular endothelium, as well as triggering beta(2)-integrin-mediated adhesion. We have previously demonstrated potential pro-inflammatory effects of soluble E-selectin upon neutrophil effector functions, using a soluble recombinant molecule (E-zz), which increased beta(2)-integrin-mediated adhesion, decreased beta(2)-integrin-dependent migration, and triggered reactive oxygen species generation and release. In this study, we have examined the intracellular signals following neutrophil activation by soluble E-selectin. We show that exposure of neutrophils to E-selectin and platelet-activating factor (PAF) in combination induced a synergistic effect upon beta(2)-integrin-mediated adhesion. Although soluble E-selectin did not induce Ca(2+) mobilization in neutrophils by itself, elevation of intracellular Ca(2+) was specifically prolonged in response to PAF but not leukotriene B(4) or N-formyl-Met-Leu-Phe. The prolonged Ca(2+) mobilization observed in the presence of E-selectin was dependent on Ca(2+) influx from intracellular stores rather than influx of extracellular Ca(2+) through SKF 96365-sensitive channels. The specific alteration of Ca(2+) mobilization reported here appears not to have a role in the synergistic effects of E-selectin and PAF upon neutrophil O(2) release but may be involved in augmentation of beta(2)-integrin-mediated adhesion.

Molecular characterization of the surface of apoptotic neutrophils: implications for functional downregulation and recognition by phagocytes

We have used a panel of monoclonal antibodies and lectins to examine the profile of surface molecule expression on human neutrophils that have undergone spontaneous apoptosis during in vitro culture. Neutrophil apoptosis was found to be accompanied by down-regulation of the immunoglobulin superfamily members PECAM-1 (CD31), ICAM-3 (CD50), CD66acde, and CD66b and the integrin-associated proteins CD63 and urokinase plasminogen activator receptor (CD87) that may alter the potential for adhesive interactions. Cellular interactions may be further influenced by the reduction of the expression of surface carbohydrate moieties, including sialic acid. Reduced expression of FcgammaRII (CD32), complement receptor type 1 (CD35) and receptors for pro-inflammatory mediators C5a (CD88) and TNFalpha (CD120b) associated with apoptosis might limit neutrophil responsiveness to stimuli that trigger degranulation responses. Although many of the receptors we have examined are expressed at reduced levels on apoptotic neutrophils, we found that there was differential loss of certain receptors (e.g. CD16, CD15 and CD120b) and increased expression of aminopeptidase-N (CD13). Together with our previous data showing that expression of certain molecules e.g. LFA-3 (CD58) is not altered during neutrophil apoptosis, these data are suggestive of specific changes in receptor mobilisation and shedding associated with apoptosis. Although reduced expression of CD63 (azurophilic granules) and CR1 (specific granules) indicates that granule mobilisation does not accompany apoptosis, a monoclonal antibody (BOB78), that recognises a 90 kDa antigen localised in intracellular granules, defines a subpopulation of apoptotic neutrophils that exhibit nuclear degradation yet retain intact plasma membranes. BOB78 positive neutrophils were found to bind biotinylated thrombospondin, suggesting that this mAb defines surface molecular changes associated with exposure of thrombospondin binding moieties.

Pharmacological manipulation of granulocyte apoptosis: potential therapeutic targets

Resolution of inflammation involves the clearance of excess or effete inflammatory cells by a process of physiological programmed cell death (apoptosis) and the subsequent recognition and removal of apoptotic cells by phagocytes. The therapeutic induction of apoptosis for the resolution of chronic inflammation and the general pharmacology of apoptosis have become subjects of increasing interest. In this article, some of the unique and important differences in the control of apoptosis of various inflammatory cells (particularly neutrophil and eosinophil granulocytes) are highlighted. It is suggested that apoptosis can be specifically regulated pharmacologically and could be exploited to develop new drug therapies.

Extracellular matrix proteins protect small cell lung cancer cells against apoptosis: a mechanism for small cell lung cancer growth and drug resistance in vivo

Resistance to chemotherapy is a principal problem in the treatment of small cell lung cancer (SCLC). We show here that SCLC is surrounded by an extensive stroma of extracellular matrix (ECM) at both primary and metastatic sites. Adhesion of SCLC cells to ECM enhances tumorigenicity and confers resistance to chemotherapeutic agents as a result of beta1 integrin-stimulated tyrosine kinase activation suppressing chemotherapy-induced apoptosis. SCLC may create a specialized microenvironment, and the survival of cells bound to ECM could explain the partial responses and local recurrence of SCLC often seen clinically after chemotherapy. Strategies based on blocking beta1 integrin-mediated survival signals may represent a new therapeutic approach to improve the response to chemotherapy in SCLC.

Glucocorticoids promote nonphlogistic phagocytosis of apoptotic leukocytes

Phagocyte recognition, uptake, and nonphlogistic degradation of neutrophils and other leukocytes undergoing apoptosis promote the resolution of inflammation. This study assessed the effects of anti-inflammatory glucocorticoids on this leukocyte clearance mechanism. Pretreatment of "semimature" 5-day human monocyte-derived macrophages (M phi) for 24 h with methylprednisolone, dexamethasone, and hydrocortisone, but not the nonglucocorticoid steroids aldosterone, estradiol, and progesterone, potentiated phagocytosis of apoptotic neutrophils. These effects were specific in that the potentiated phagocytosis of apoptotic neutrophils was completely blocked by the glucocorticoid receptor antagonist RU38486, and glucocorticoids did not promote 5-day M phi ingestion of opsonized erythrocytes. Similar glucocorticoid-mediated potentiation was observed with 5-day M phi uptake of alternative apoptotic "targets" (eosinophils and Jurkat T cells) and in uptake of apoptotic neutrophils by alternative phagocytes (human glomerular mesangial cells and murine M phi elicited into the peritoneum or derived from bone marrow). Importantly, methylprednisolone-mediated enhancement of the uptake of apoptotic neutrophils did not trigger the release of the chemokines IL-8 and monocyte chemoattractant protein-1. Furthermore, longer-term potentiation by methylprednisolone was observed in maturing human monocyte-derived M phi, with greater increases in 5-day M phi uptake of apoptotic cells being observed the earlier glucocorticoids were added during monocyte maturation into M phi. We conclude that potentiation of nonphlogistic clearance of apoptotic leukocytes by phagocytes is a hitherto unrecognized property of glucocorticoids that has potential implications for therapies aimed at promoting the resolution of inflammatory diseases.