Combined dysfunctions of immune cells predict nosocomial infection in critically ill patients.

BACKGROUND: Nosocomial infection occurs commonly in intensive care units (ICUs). Although critical illness is associated with immune activation, the prevalence of nosocomial infections suggests concomitant immune suppression. This study examined the temporal occurrence of immune dysfunction across three immune cell types, and their relationship with the development of nosocomial infection. METHODS: A prospective observational cohort study was undertaken in a teaching hospital general ICU. Critically ill patients were recruited and underwent serial examination of immune status, namely percentage regulatory T-cells (Tregs), monocyte deactivation (by expression) and neutrophil dysfunction (by CD88 expression). The occurrence of nosocomial infection was determined using pre-defined, objective criteria. RESULTS: Ninety-six patients were recruited, of whom 95 had data available for analysis. Relative to healthy controls, percentage Tregs were elevated 6-10 days after admission, while monocyte HLA-DR and neutrophil CD88 showed broader depression across time points measured. Thirty-three patients (35%) developed nosocomial infection, and patients developing nosocomial infection showed significantly greater immune dysfunction by the measures used. Tregs and neutrophil dysfunction remained significantly predictive of infection in a Cox hazards model correcting for time effects and clinical confounders {hazard ratio (HR) 2.4 [95% confidence interval (CI) 1.1-5.4] and 6.9 (95% CI 1.6-30), respectively, P=0.001}. Cumulative immune dysfunction resulted in a progressive risk of infection, rising from no cases in patients with no dysfunction to 75% of patients with dysfunction of all three cell types (P=0.0004). CONCLUSIONS: Dysfunctions of T-cells, monocytes, and neutrophils predict acquisition of nosocomial infection, and combine additively to stratify risk of nosocomial infection in the critically ill.

Regulatory role for macrophage migration inhibitory factor in acute respiratory distress syndrome.

Migration inhibitory factor (MIF) is known to exert significant pro-inflammatory effects and has the potential to override the anti-inflammatory action of glucocorticoids. In this study we have identified significant quantities of MIF in the alveolar airspaces of patients with acute respiratory distress syndrome (ARDS). We show in alveolar cells from patients with ARDS that MIF augments pro-inflammatory cytokine secretion (TNF alpha and IL-8), anti-MIF significantly attenuates TNF alpha and IL-8 secretion and MIF overrides, in a concentration-related fashion, the anti-inflammatory effects of glucocorticoids. These findings suggest that MIF may act as a mediator sustaining the pulmonary inflammatory response in ARDS and that an anti-MIF strategy may represent a novel therapeutic approach in inflammatory diseases such as ARDS.

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.

ECM overrides DNA damage-induced cell cycle arrest and apoptosis in small-cell lung cancer cells through beta1 integrin-dependent activation of PI3-kinase.

The emergence of resistance to chemotherapy remains a principle problem in the treatment of small-cell lung cancer (SCLC). We demonstrate that extracellular matrix (ECM) activates phosphatidyl inositol 3-kinase (PI3-kinase) signaling in SCLC cells and prevents etoposide-induced caspase-3 activation and subsequent apoptosis in a beta1 integrin/PI3-kinase-dependent manner. Crucially we show that etoposide and radiation induce G2/M cell cycle arrest in SCLC cells prior to apoptosis and that ECM prevents this by overriding the upregulation of p21(Cip1/WAF1) and p27(Kip1) and the downregulation of cyclins E, A and B. These effects are abrogated by pharmacological and genetic inhibition of PI3-kinase signaling. Importantly we show that chemoprotection is not mediated by altered SCLC cell proliferation or DNA repair. Thus, ECM via beta1 integrin-mediated PI3-kinase activation overrides treatment-induced cell cycle arrest and apoptosis, allowing SCLC cells to survive with persistent DNA damage, providing a model to account for the emergence of acquired drug resistance.

Phagocytosis of aged human neutrophils by macrophages is mediated by a novel "charge-sensitive" recognition mechanism.

The removal of neutrophils and their histotoxic contents from the inflamed site is a prerequisite for resolution of tissue injury, and a point at which factors critical to the pathogenesis of chronic inflammation may act. Engulfment of intact, senescent neutrophils by macrophages represents an important neutrophil disposal process. In this study the mechanism by which human monocyte-derived macrophages (M phi) recognized and ingested human neutrophils that had been aged in culture was studied using an in vitro phagocytic assay. Inhibition of M phi receptors for Ig Fc and the opsonic complement fragments C3b and iC3b with MAbs to M phi FcR, CR1, CR3, and CR4 had no effect on recognition, and the pattern of inhibition observed when polyanions were included in the medium at 1 mg/ml was different from that reported for the M phi receptor for protein advanced glycosylation end products (AGE), indicating a recognition mechanism different from those proposed for M phi phagocytosis of senescent erythrocytes. Furthermore, although aging neutrophils undergo programmed cell death (or apoptosis), which is directly related to recognition by M phi, the pattern of inhibition observed with monosaccharides was different from that reported to inhibit the binding of apoptotic mouse thymocytes to isologous M phi. By contrast, evidence was obtained for a novel recognition mechanism inhibitable by cationic sugars and amino acids in a charge-dependent fashion, and directly modulated by pH but not affected by inhibitors of the mannose-6-phosphate, sheep erythrocyte, mannosyl-fucosyl, asialoglycoprotein, and scavenger receptors of the macrophage. These observations suggest that hydrogen ions and charged molecules may modulate M phi uptake of senescent neutrophils at inflamed sites, and that recognition itself may involve charged structures on the cells.

Thrombospondin cooperates with CD36 and the vitronectin receptor in macrophage recognition of neutrophils undergoing apoptosis.

We have investigated the cell surface recognition mechanisms used by human monocyte-derived macrophages (M phi) in phagocytosis of intact aging human neutrophils (PMNs) undergoing apoptosis. This study shows that the adhesive protein thrombospondin (TSP) was present in the interaction, both associated with the M phi surface and in solution at a mean concentration of 0.59 micrograms/ml. The interaction was inhibited by treatment of M phi (but not aged PMN) with cycloheximide, but could be "rescued" by replenishment with exogenous TSP. Under control conditions, M phi recognition of aged PMNs was specifically potentiated by purified platelet TSP at 5 micrograms/ml, present either in the interaction or if preincubated with either cell type, suggesting that TSP might act as a "molecular bridge" between the two cell types. In support, both aged PMN and M phi were found to adhere to TSP, and phagocytosis of aged PMN was specifically inhibited by (a) excess soluble TSP; (b) antibodies to TSP that also inhibit TSP-mediated adhesion to aged PMN; and (c) down-regulation of M phi receptors for TSP by plating M phi on TSP-coated surfaces. Furthermore, inhibition with mAbs/Arg-Gly-Asp-Ser peptide of the candidate M phi receptors for TSP, CD36, and alpha v beta 3 exerted synergistic effects on both M phi recognition of aged PMN and M phi adhesion to TSP, indicating that "two point" adhesion of TSP to these M phi structures is involved in phagocytosis of aged PMN. Our findings indicate newly defined roles for TSP and CD36 in phagocytic clearance of senescent neutrophils, which may limit inflammatory tissue injury and promote resolution.

Transient elevations of cytosolic free calcium retard subsequent apoptosis in neutrophils in vitro.

Elevation of cytosolic calcium ([Ca2+]i) has been reported to induce apoptosis in a number of cell types. However, in the neutrophil, which undergoes apoptosis constitutively during aging in vitro, activation by inflammatory mediators elevates [Ca2+]i and prolongs lifespan via inhibition of apoptosis. To examine this paradox, we investigated the effects of modulation of [Ca2+]i upon apoptosis of neutrophils in vitro. Calcium ionophores (A23187, ionomycin) retarded apoptosis in neutrophil populations after 20 h (P < 0.001). Conversely, intracellular Ca(2+)-chelation, using bis-(o-aminophenoxy)-N,N,N'N'-tetraacetic acid (BAPTA) acetoxymethyl ester (AM) promoted apoptosis (P < 0.02). W-7 (an inhibitor of calmodulin) also promoted apoptosis (P < 0.05). Measurements of [Ca2+]i, using fura-2, showed (a) increased apoptosis in neutrophil populations was not associated with elevated [Ca2+]i, (b) neutrophils cultured with ionophore at concentrations inhibiting apoptosis exhibited transient (< 1 h) elevations of [Ca2+]i, to levels previously reported with receptor-mediated stimuli, and (c) BAPTA was able to prevent the elevation of [Ca2+]i and the inhibition of apoptosis produced by ionophore. Modulation of apoptosis occurred without alterations in intracellular pH. Thus, in the neutrophil, unlike lymphoid cells, elevation of [Ca2+]i exerts an inhibitory effect upon apoptosis. Furthermore, these data suggest that transient elevation of [Ca2+]i elicits signaling events leading to prolonged inhibition of apoptosis.

Macrophage phagocytosis of aging neutrophils in inflammation. Programmed cell death in the neutrophil leads to its recognition by macrophages.

Mechanisms governing the normal resolution processes of inflammation are poorly understood, yet their elucidation may lead to a greater understanding of the pathogenesis of chronic inflammation. The removal of neutrophils and their potentially histotoxic contents is one prerequisite of resolution. Engulfment by macrophages is an important disposal route, and changes in the senescent neutrophil that are associated with their recognition by macrophages are the subject of this investigation. Over 24 h in culture an increasing proportion of human neutrophils from peripheral blood or acutely inflamed joints underwent morphological changes characteristic of programmed cell death or apoptosis. Time-related chromatin cleavage in an internucleosomal pattern indicative of the endogenous endonuclease activation associated with programmed cell death was also demonstrated. A close correlation was observed between the increasing properties of apoptosis in neutrophils and the degree of macrophage recognition of the aging neutrophil population, and a direct relationship between these parameters was confirmed within aged neutrophil populations separated by counterflow centrifugation into fractions with varying proportions of apoptosis. Macrophages from acutely inflamed joints preferentially ingested apoptotic neutrophils and histological evidence was presented for occurrence of the process in situ. Programmed cell death is a phenomenon of widespread biological importance and has not previously been described in a cell of the myeloid line. Because it leads to recognition of intact senescent neutrophils that have not necessarily disgorged their granule contents, these processes may represent a mechanism for the removal of neutrophils during inflammation that also serves to limit the degree of tissue injury.

Neutrophil-mediated injury to endothelial cells. Enhancement by endotoxin and essential role of neutrophil elastase.

The neutrophil has been implicated as an important mediator of vascular injury, especially after endotoxemia. This study examines neutrophil-mediated injury to human microvascular endothelial cells in vitro. We found that neutrophils stimulated by formyl-methionyl-leucyl-phenylalanine (FMLP), the complement fragment C5a, or lipopolysaccharide (LPS) (1-1,000 ng/ml) alone produced minimal endothelial injury over a 4-h assay. In contrast, neutrophils incubated with endothelial cells in the presence of low concentrations of LPS (1-10 ng/ml) could then be stimulated by FMLP or C5a to produce marked endothelial injury. Injury was maximal at concentrations of 100 ng/ml LPS and 10(-7) M FMLP. Pretreatment of neutrophils with LPS resulted in a similar degree of injury, suggesting that LPS effects were largely on the neutrophil. Endothelial cell injury produced by LPS-exposed, FMLP-stimulated neutrophils had a time course similar to that induced by the addition of purified human neutrophil elastase, and different from that induced by hydrogen peroxide (H2O2). Further, neutrophil-mediated injury was not inhibited by scavengers of a variety of oxygen radical species, and occurred with neutrophils from a patient with chronic granulomatous disease, which produced no H2O2. In contrast, the specific serine elastase inhibitor methoxy-succinyl-alanyl-alanyl-prolyl-valyl-chloromethyl ketone inhibited 63% of the neutrophil-mediated injury and 64% of the neutrophil elastase-induced injury. However, neutrophil-mediated injury was not inhibited significantly by 50% serum, 50% plasma, or purified alpha 1 proteinase inhibitor. These results suggest that, in this system, chemotactic factor-stimulated human neutrophil injury of microvascular endothelial cells is enhanced by small amounts of LPS and may be mediated in large part by the action of neutrophil elastase.

Human circulating eosinophils secrete macrophage migration inhibitory factor (MIF). Potential role in asthma.

Macrophage migration inhibitory factor (MIF) is a potent proinflammatory mediator that has been shown to potentiate lethal endotoxemia and to play a potentially important regulatory role in human acute respiratory distress syndrome (ARDS). We have investigated whether eosinophils are an important source of MIF and whether MIF may be involved in the pathophysiology of asthma. Unstimulated human circulating eosinophils were found to contain preformed MIF. Stimulation of human eosinophils with phorbol myristate acetate in vitro yielded significant release of MIF protein. For example, eosinophils stimulated with phorbol myristate acetate (100 nM, 8 h, 37 degreesC) released 1,539+/-435 pg/10(6) cells of MIF, whereas unstimulated cells released barely detectable levels (< 142 pg/10(6) cells, mean+/-SEM, n = 8). This stimulated release was shown to be (a) concentration- and time-dependent, (b) partially blocked by the protein synthesis inhibitor cycloheximide, and (c) significantly inhibited by the protein kinase C inhibitor Ro-31,8220. In addition, we show that the physiological stimuli C5a and IL-5 also cause significant MIF release. Furthermore, bronchoalveolar lavage fluid obtained from asthmatic patients contains significantly elevated levels of MIF as compared to nonatopic normal volunteers (asthmatic, 797.5+/-92 pg/ml; controls, 274+/-91 pg/ml). These results highlight the potential importance of MIF in asthma and other eosinophil-dependent inflammatory disorders.

The neutrophil.

In 'beneficial inflammation', which is the major component of our innate immune system, it is possible to predict an 'ideal' sequence of cellular events: neutrophil migration would be rapid; time of contact with endothelial cells minimized; matrix degradation localized, with specific turn-on and turn-off of degradation mechanisms; neutrophil secretion and disintegration would be kept to a minimum during bacterial killing; and finally, rapid cessation of neutrophil migration and rapid removal of intact senescent cells would occur. Any doubts that the cellular events of the early stages of acute inflammation normally involve highly sophisticated cellular interactions, presumably designed to minimize tissue perturbation, should be dispelled by two elegant recent studies of neutrophil-endothelial interaction. Clearly, defects in the control of these processes could tip the balance towards cell injury or excessive matrix degradation and initiate amplification mechanisms leading to persistent inflammation and disease. The further identification of molecular mechanisms of these events should permit specific intervention in neutrophil-mediated disease. However, it is important to remember, firstly, that the neutrophil is just a part of the highly redundant inflammatory process and the removal of one 'strand' does not mean that the whole 'web' breaks down, and secondly, that impairment of neutrophil mechanisms may critically impair our anti-bacterial defences. Therefore, continued attempts should be made to define how cells and mediators interact in concert, to determine the fine specificity of molecular mechanisms and, in parallel, to identify 'time windows' in diseases, during which these mechanisms are more critical to the processes damaging the host than they are essential to its defences.

Inhibition of neuropeptide-stimulated tyrosine phosphorylation and tyrosine kinase activity stimulates apoptosis in small cell lung cancer cells.

Small cell lung cancer (SCLC) cell growth is sustained by multiple autocrine and paracrine growth loops involving neuropeptides. The bombesin family of peptides are autocrine growth factors in H345 SCLC cells and provide a paradigm for the study of growth factors and mitogenic signaling in SCLC cells. We show that bombesin (and other neuropeptides) stimulates protein tyrosine phosphorylation (particularly focal adhesion kinase) and protein tyrosine kinase (PTK) activity in intact SCLC cells. Furthermore, the broad spectrum neuropeptide receptor antagonist [D-Arg, D = Phe, D-Trp, Leu11]substance P inhibits all neuropeptide-mediated signals (including PTK activation), SCLC cell growth in vivo and in vitro, and also increases the natural rate of apoptosis seen in growing SCLC cell lines. Hence the effect of selective PTK inhibition on SCLC cell growth and apoptosis was examined. We show that selective inhibition of PTK activity, with genistein and (3,4,5-tri-hydroxyphenyl)-methylene(-propanedinitrile) tyrphostin-25 inhibits basal and neuropeptide-stimulated SCLC cell growth. Genistein and tyrphostin-25 also stimulate apoptosis in SCLC cells. Inhibition of proliferation in these cells is intimately linke to apoptosis, because these changes occurred without any effect on SCLC cell cycle kinetics, suggesting that apoptosis occurs independently of the cell cycle and that failure to progress through the cell cycle results in apoptosis. Because tyrphostin-25 fails to influence p53 or Bcl-2 expression in these cells, this mode of programmed cell death appears to be via a p53- and Bcl-2-independent mechanism. These results provide evidence that tyrosine phosphorylation is a mitogenic signal in SCLC cells and suggest that regulation of the level of protein tyrosine phosphorylation represents a critical determinant of whether SCLC cells survive and proliferate or die by apoptosis. Thus PTK inhibition may provide a novel therapeutic option in SCLC that has become resistant to conventional chemotherapeutic agents.

The presence of a constitutively active phosphoinositide 3-kinase in small cell lung cancer cells mediates anchorage-independent proliferation via a protein kinase B and p70s6k-dependent pathway.

Small cell lung cancer (SCLC) is characterized by early and widespread metastases. Anchorage-independent growth is pivotal to the ability of tumor cells to survive and metastasize in vivo and, under in vitro conditions, allows transformed cells to form colonies in semisolid medium. Here, we report that of five SCLC cell lines tested, all exhibited high basal constitutive phosphoinositide 3-kinase (PI 3-kinase) activity, which results in high basal protein kinase B (PKB) and ribosomal p70 S6 kinase activity (p70s6k). Inhibition of PI 3-kinase activity markedly inhibited SCLC cell proliferation in liquid culture as a result of stimulating apoptosis and promoting cell cycle delay in G1. Furthermore, PI 3-kinase inhibition reduced basal SCLC cell colony formation in agarose semisolid medium that could not be overcome by the addition of neuropeptide growth factors. Thus, constitutive PI 3-kinase activity in SCLC cells plays an important role in promoting the growth and anchorage independence of SCLC. This is not due to activating ras mutations or increased basal src or focal adhesion kinase activity. These data represent the first description of constitutively activated PI 3-kinase/PKB in any human cancer. Constitutive activation of these integrin-dependent signaling events provides a molecular explanation for the anchorage-independent growth of SCLC cells and may account for the nonadherent phenotype and highly metastatic nature of this aggressive cancer. Up-regulation of the PI 3-kinase/PKB pathway may, therefore, represent a novel target for therapeutic intervention in SCLC.

Nitric oxide: a key regulator of myeloid inflammatory cell apoptosis.

Apoptosis of inflammatory cells is a critical event in the resolution of inflammation, as failure to undergo this form of cell death leads to increased tissue damage and exacerbation of the inflammatory response. Many factors are able to influence the rate of apoptosis in neutrophils, eosinophils, monocytes and macrophages. Among these is the signalling molecule nitric oxide (NO), which possesses both anti- and proapoptotic properties, depending on the concentration and flux of NO, and also the source from which NO is derived. This review summarises the differential effects of NO on inflammatory cell apoptosis and outlines potential mechanisms that have been proposed to explain such actions.

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.

Inhibition of apoptosis and prolongation of neutrophil functional longevity by inflammatory mediators.

Neutrophil apoptosis leads to macrophage ingestion of intact senescent neutrophils. This may represent a neutrophil removal mechanism that is important both in the control of inflammatory tissue injury and for the normal resolution processes of inflammation. Because apoptosis is likely to be a key control process in cell and tissue homeostasis, a number of inflammatory mediators were tested for their ability to modulate the rate of apoptosis in populations of neutrophils aging in culture. Endotoxic lipopolysaccharide, human recombinant complement factor 5a, and human recombinant granulocyte-macrophage colony-stimulating factor all markedly inhibited the rate of neutrophil apoptosis in a concentration-dependent fashion, without inducing necrosis (as assessed by trypan blue exclusion). This inhibitory effect on the rate of neutrophil apoptosis was shown by morphological criteria and confirmed by gel electrophoresis of extracted DNA. Inhibition of apoptosis of aging neutrophil populations was associated with prolongation of the functional life span of the population as assessed by the ability of neutrophils to spread on glass surfaces, to polarize in response to deliberate stimulation with N-formyl-Met-Leu-Phe (fMLP), and to release the granule enzyme marker myeloperoxidase on fMLP stimulation. These observations show that inflammatory mediators prolong the functional life span of neutrophils through modulation of apoptosis. Further elucidation of these mechanisms will lead to a better understanding of the processes controlling neutrophil residence and function in inflamed tissues and may provide further insights into the molecular mechanisms of apoptosis, which is of widespread importance in tissue biology.

CD66-dependent neutrophil activation: a possible mechanism for vascular selectin-mediated regulation of neutrophil adhesion.

We have examined the role of CD66 in the modulation of neutrophil adhesion and effector function. Engagement of neutrophil CD66 with anti-carcinoembryonic antigen (anti-CEA) Ig results in activation-associated phenomena including shape change, activation of beta 2-integrins, and priming of the respiratory burst. Anti-CEA Ig-treated neutrophils underwent transient shape change distinct from that induced by formyl-Met-Leu-Phe (fMLP). fMLP stimulated beta 2-integrin up-regulation and 70% loss of L-selectin, whereas only low-level up-regulation of the beta 2-integrins, without loss of L-selectin, occurred with anti-CEA Ig. Anti-CEA F(ab')2 fragments and whole Ig augmented beta 2-integrin-dependent adhesion. Anti-CEA Ig-induced beta 2-integrin activation was transient, whereas fMLP-induced activation persisted longer. Although they did not cause a significant increase in respiratory burst activity, CEA Ig and F(ab')2 fragments of antibody primed neutrophils so that the subsequent fMLP-induced respiratory burst was significantly increased.

Phagocytosis of apoptotic neutrophils does not induce macrophage release of thromboxane B2.

Senescent human neutrophils undergo programmed cell death (apoptosis), leading to their recognition and phagocytosis by mature macrophages. At inflamed sites in vivo these processes may represent a neutrophil removal mechanism with the potential to limit the histotoxic capacity of these cells. Phagocytosis can provoke marked proinflammatory responses by macrophages. A macrophage proinflammatory response to the ingestion of apoptotic neutrophils would limit the efficacy of this neutrophil removal mechanism as a component of inflammatory resolution. In the present study we examined two macrophage proinflammatory responses; secretion of the granule enzyme N-acetyl-beta-D-glucosaminidase (NAG) and release of the membrane lipid-derived inflammatory mediator thromboxane A2 (TxA2, measured as TxB2). By contrast with the marked release of NAG and TxB2 elicited by phagocytosis of control particles (opsonised zymosan and immunoglobulin G-coated erythrocytes), macrophage ingestion of apoptotic neutrophils resulted in minimal release of NAG and no release of TxB2; indeed, there was a small depression of TxB2 release that was not due to a toxic effect of neutrophil uptake because macrophages ingesting apoptotic neutrophils retained marked TxB2 responses to subsequent stimulation with opsonised zymosan. Furthermore, there was significant TxB2 release in response to macrophage phagocytosis of apoptotic neutrophils that had been coated with opsonic serum, demonstrating that the lack of macrophage response was determined by the mechanism of recognition rather than the properties of the apoptotic particle itself. These observations are consistent with the hypothesis that macrophage clearance of senscent neutrophils undergoing apoptosis is an injury-limiting mechanism that favors resolution rather than persistence of the inflammatory response and are consistent with observations that the waves of apoptotic cell removal seen in embryological removal and thymic involution do not trigger an inflammatory response.

Coupling of neutrophil apoptosis to recognition by macrophages: coordinated acceleration by protein synthesis inhibitors.

Onset of apoptosis in many cell types, including the neutrophil granulocyte, leads to recognition and ingestion by macrophages, a key regulatory step in clearance of inflammatory cells from inflamed sites. These studies examined the requirement for protein synthesis in neutrophil apoptosis and in the recognition of apoptotic neutrophils by monocyte-derived macrophages. Treatment with cycloheximide or actinomycin D produced a time- and concentration-dependent acceleration of apoptosis in populations of neutrophils purified from human peripheral blood. Both compounds caused significant promotion of apoptosis after 8 h (apoptosis was 7.7 +/- 2.9%, mean +/- SEM, in control populations, 57.5 +/- 4.9% in cycloheximide-treated, and 73.4 +/- 5.5% in actinomycin D-treated populations, n = 4, P < 0.001), which was associated with loss of neutrophil functional ability (assessed by shape change on N-formyl-methionyl-leucyl-phenylalanine stimulation) and increased macrophage recognition and ingestion of neutrophil populations with accelerated apoptosis. These results support the existence of survival proteins, which act as intracellular suppressors of programmed cell death. However, protein synthesis was not required for the recognition process because macrophage recognition was increased pari passu with the morphology of apoptosis.