Opsonized Sheep Erythrocytes Research Articles

Volatile anesthetics affect macrophage phagocytosis.

BackgroundPerioperative infections, particularly surgical site infections pose significant morbidity and mortality. Phagocytosis is a critical step for microbial eradication. We examined the effect of commonly used anesthetics on macrophage phagocytosis and its mechanism.MethodsThe effect of anesthetics (isoflurane, sevoflurane, propofol) on macrophage phagocytosis was tested using RAW264.7 mouse cells, mouse peritoneal macrophages, and THP-1 human cells. Either opsonized sheep erythrocytes or fluorescent labeled Escherichia coli were used as phagocytic objects. The activation of Rap1, a critical protein in phagocytosis was assessed using the active Rap1 pull-down and detection kit. To examine anesthetic binding site(s) on Rap1, photolabeling experiments were performed using azi-isoflurane and azi-sevoflurane. The alanine scanning mutagenesis of Rap1 was performed to assess the role of anesthetic binding site in Rap1 activation and phagocytosis.ResultsMacrophage phagocytosis was significantly attenuated by the exposure of isoflurane (50% reduction by 1% isoflurane) and sevoflurane (50% reduction by 1.5% sevoflurane), but not by propofol. Photolabeling experiments showed that sevoflurane directly bound to Rap1. Mutagenesis analysis demonstrated that the sevoflurane binding site affected Rap1 activation and macrophage phagocytosis.ConclusionsWe showed that isoflurane and sevoflurane attenuated macrophage phagocytosis, but propofol did not. Our study showed for the first time that sevoflurane served as a novel small GTPase Rap1 inhibitor. The finding will further enrich our understanding of yet-to-be determined mechanism of volatile anesthetics and their off-target effects. The sevoflurane binding site was located outside the known Rap1 functional sites, indicating the discovery of a new functional site on Rap1 and this site would serve as a pocket for the development of novel Rap1 inhibitors.

Screening New Drugs for Immunotoxic Potential: II. Assessment of the Effects of Selective and Nonselective COX-2 Inhibitors on Complement Activation, Superoxide Anion Production and Leukocyte Chemotaxis and Migration Through Endothelial Cells

Results from earlier experiments in our laboratories revealed that both selective and nonselective inhibitors of cyclooxygenase-2 possess little potential for decreasing in vitro phagocytosis by rat macrophages or canine neutrophils and no potential for decreasing in vivo phagocytosis by the intact murine immune system. We now report the results of studies to assess in vitro and ex vivo effects of the drugs on 1) canine complement activation, 2) generation of superoxide anion and hydrogen peroxide (oxidative burst) by canine neutrophils, and 3) leukocytic chemotaxis and transmigration through endothelial cell monolayers. In vitro concentrations of naproxen sodium, SC-236, SC-245, and SC-791 ranging from 0.1 to 10 μM were tested for their abilities to inhibit canine complement-mediated hemolysis of opsonized sheep erythrocytes and to block phorbol myristate acetate-induced oxidative burst in canine neutrophils. Both models responded to known inhibitory agents, leupeptin in the complement activation test and staurosporine in the superoxide anion assay. In contrast, tested nonsteroidal anti-inflammatory drugs produced only trivial changes in complement activation and superoxide anion production. Experiments on plasma and neutrophils isolated from dogs administered an experimental selective COX-2 inhibitor during a 28-day toxicology study revealed no evidence of drug-associated changes in complement activation or formation of superoxide anion. SC-791 reduced chemotaxis of canine leukocytes toward zymosan-activated dog plasma, but not toward leukotriene B4. None of the other drugs tested significantly affected leukocytic chemotaxis. Ibuprofen, SC-245 and SC-791 but not SC-236, reduced transmigration of canine leukocytes through endothelial cell monolayers. Based on the results of these experiments and our earlier studies we have concluded that, although high (suprapharmacologic) concentrations of the drugs may induce in vitro evidence of apparent immunomodulation of the innate immune system, the findings are unlikely to represent a significant human health risk.

In vitro antiphagocytic effect of basil oil on mouse macrophages

The effect of basil oil on the phagocytic capability and respiratory burst response of mouse peritoneal macrophages was studied. The oil inhibited the phagocytosis of opsonized sheep erythrocytes. This inhibition was both dose- and time-dependent and reverted 24 h after removing the extract from the culture medium. On the other hand, the oil showed no effect on concanavalin A and lipopolysaccharide-induced proliferation.

Exposure to perflubron is associated with decreased Syk phosphorylation in human neutrophils.

Liquid ventilation with perflubron is associated with reduced neutrophil recruitment into the lung during acute injury. Perflubron also reduces chemotactic responses, the respiratory burst, and cytokine production in neutrophils and in alveolar macrophages in vitro. In the current studies, the effect of perflubron on neutrophil chemotaxis to formyl-Met-Leu-Phe (fMLP) and phagocytosis of opsonized sheep erythrocytes (EA) correlated with decreased phosphorylation of Syk, an important intracellular second messenger in pathways regulating neutrophil functional responses. Brief (5 min) exposure of neutrophils to perflubron resulted in a dose-dependent reduction in chemotaxis to fMLP and reduced phagocytosis of EA but no apparent morphological changes as seen by electron microscopy. Concurrently, there was a reduction in both total cytosolic tyrosine phosphorylation and Syk phosphorylation. Binding studies indicated that this effect was neither a result of impaired ligand-receptor affinity nor a change in the number of fMLP receptors available on the neutrophil surface. These results suggest that perflubron nonspecifically affects cellular activation as measured by tyrosine phosphorylation perhaps by interfering with transmembrane signal transduction.

Immunomodulatory Activities of Melia azedarach L. Leaf Extracts on Human Monocytes

ABSTRACT Leaf extract from Melia azedarach L. (Meliaceae) inhibited phagocytosis of opsonized sheep erythrocytes and the respiratory burst triggered by the post-receptor stimuli, phorbol 12-myris-tate 13-acetate in human monocytes.

Immunomodulatory activities of Cedrela lilloi and Trichilia elegans aqueous leaf extracts

The effects of Cedrela lilloi and Trichilia elegans (Meliaceae) aqueous leaf extracts on several parameters of the mouse immune system were studied. Both extracts showed a strong anticomplementary activity and inhibited the phagocytosis of opsonized sheep erythrocytes and the activation of the oxidative metabolism by opsonized zymosan on peritoneal macrophages. The in vitro proliferation of spleen T-lymphocytes was also impaired. Furthermore, treatment of mice with the extracts diminished the delayed-type hypersensitivity response to sheep erythrocytes. These results suggest that both extracts exert a marked immunomodulatory effect on the mouse immune system.

Different fates of phagocytosed particles after delivery into macrophage lysosomes.

Phagocytosis in macrophages is often studied using inert polymer microspheres. An implicit assumption in these studies is that such particles contain little or no specific information in their structure that affects their intracellular fate. We tested that assumption by examining macrophage phagosomes containing different kinds of particles and found that although all particles progressed directly to lysosomes, their subsequent fates varied. Within 15 min of phagocytosis, >90% of phagosomes containing opsonized sheep erythrocytes, poly-e-caprolactone microspheres, polystyrene microspheres (PS), or polyethylene glycol-conjugated PS merged with the lysosomal compartment. After that point, however, the characteristics of phagolysosomes changed in several ways that indicated differing degrees of continued interaction with the lysosomal compartment. Sheep erythrocyte phagolysosomes merged together and degraded their contents quickly, poly-e-caprolactone phagolysosomes showed intermediate levels of interaction, and PS phagolysosomes became isolated within the cytoplasm. PS were relatively inaccessible to an endocytic tracer, Texas red dextran, added after phagocytosis. Moreover, immunofluorescent staining for the lysosomal protease cathepsin L decreased in PS phagolysosomes to 23% by 4 h after phagocytosis, indicating degradation of the enzyme without replacement. Finally, PS surface labeled with fluorescein-labeled albumin showed a markedly reduced rate of protein degradation in phagolysosomes, when compared to rates measured for proteins in or on other particles. Thus, particle chemistry affected both the degree of postlysosomal interactions with other organelles and, consequently, the intracellular half-life of particle-associated proteins. Such properties may affect the ability of particles to deliver macromolecules into the major histocompatibility complex class I and II antigen presentation pathways.

Steroid sex hormones and macrophage function: regulation of chemiluminescence and phagocytosis.

Female sex hormones modulate a variety of humoral and cell-mediated immunologic functions. In this study, the effects of estrogen, progesterone, and testosterone on the chemiluminescence (CL) response and phagocytic ability of male rat peritoneal macrophages (M luminal diameter) were examined. In M luminal diameter pretreated with 10(-2) ng/ml of 17 beta-estradiol (E2) for 20 hours, the CL generated in response to phorbol myristate acetate (PMA), 1,2-dioctanoyl-rac-glycerol (C8:0), or opsonized zymosan (OZ) was significantly increased by 135%, 140%, and 136% of control values, respectively. In addition, M luminal diameter treated with 10(-5) ng/ml or 10 ng/ml of E2 exhibited a significantly greater PMA- or OZ-stimulated CL response than did untreated controls. At 10(-2) ng/ml, progesterone enhanced and testosterone reduced the CL response, but these changes were not statistically significant. In time course studies, the PMA-stimulated CL response of M luminal diameter treated with 10(-2) ng/ml of E2 or progesterone for 5 h was significantly less than that of the untreated group. In the presence of endotoxin (12 pg/ml), the CL response in M luminal diameter treated with E2 or testosterone was significantly depressed as compared to untreated controls. Phagocytosis of opsonized sheep erythrocytes also was significantly enhanced (140% to 190% of control) when M luminal diameter were pretreated with 10(-12) M to 10(-8) M of either E2 or progesterone. These findings suggest that, at physiological concentrations, E2 is capable of modulating both CL generation and phagocytic uptake by M luminal diameter in a manner not shared by other steroid hormones.

Immunomodulatory activities of extracellular metallothionein. II. Effects on macrophage functions.

Metallothionein (MT) is a thiol-rich protein that is rapidly induced by exposure to heavy metal cations. We have previously demonstrated that exogenous MT stimulates murine splenocytes to proliferate, but inhibits humoral responses to antigen. These observations suggest that metallothionein released from cells has a complex role in heavy metal-mediated immune dysfunction. Here we examine one possible mechanism by which MT mediates suppression of humoral immunity. Exposure of macrophages to 20 microM MT did not affect their ability to engulf opsonized sheep erythrocytes, but in the presence of 20 microM MT, peritoneal macrophages were stimulated to produce increased levels of oxygen radicals. These results correlated with observations that while macrophage phagocytosis of opsonized Candida albicans was unaltered by the presence of exogenous MT, killing of the engulfed yeast cells was dramatically enhanced by 20 microM MT. Amounts of free cadmium and zinc equimolar to that added as Zn,Cd-MT had no effect on candidacidal activity. MT was also found to significantly decrease lymphocyte proliferation mediated by macrophage activity. Biotinylated MT (MT-b) bound specifically to the plasma membranes of these macrophages, suggesting that membrane-associated molecules of the macrophage may transduce a signal mediated by MT binding. These results demonstrate that macrophages are a sensitive target for MT-mediated immunomodulation and that some of the consequences of the MT interaction with macrophages may be alterations in the capacity to produce an effective immune response and increased extracellular exposure to damaging free radicals.

In vitro antiphagocytic effect of Melia azedarach leaf extracts on mouse peritoneal exudate cells

The effect of Melia azedarach L. (Meliaceae) leaf extract on the phagocytic capability and respiratory burst of mouse peritoneal exudate cells was studied. The extract inhibited the phagocytosis of opsonized sheep erythrocytes. This inhibition was both dose- and time-dependent and reverted 48 h after removing the extract from the culture medium. Furthermore, chemiluminescence in treated cells was also impaired using either receptor (opsonized zymosan) or postreceptor (PMA) stimuli.

Interaction of Staphylococcus aureus with human neutrophils and the down-regulation of TNF receptors.

We have shown previously that pre-exposure of neutrophils to TNF significantly enhanced their killing of opsonized Staphylococcus aureus. We now demonstrate that the ability of TNF to enhance the bactericidal activity is dependent on preincubation time; enhancement was still evident when TNF and bacteria were added simultaneously to neutrophils but if TNF addition was delayed by 5 min, no enhancement was seen. Evidence is presented that suggests that this could be related to a down-regulation of TNF receptors by the bacteria, but in addition, the release of TNF receptor fragments may contribute to the inhibition observed. Scatchard analyses demonstrated a decrease from approximately 3000 TNF receptor (receptor binding) sites per cell to 450 following treatment with S. aureus, but essentially no change in receptor affinity. Using mAb directed against the type A (75 kDa) receptor (utr-1) and the type B (55 kDa) receptor (htr-9), it was found that the expression of both receptors was decreased following treatment with the bacteria. The time course of loss of these receptors showed that the surface expression of both molecules was markedly decreased by 5 min which correlated with the loss in ability of TNF to enhance the bactericidal activity. In contrast to changes seen in the binding of TNF, similarly treated neutrophils showed essentially no change in the binding of radiolabeled tripeptide FMLP and, if anything, an increase in the expression of the CD11b Ag (CR3 receptor). When another phagocytic stimulus was used, opsonized fungi (Torulopsis glabrata), a similar depression of TNF binding was also found, but opsonized sheep erythrocytes had no effect on the TNF binding, suggesting that the effects on the TNF receptor cannot be explained simply on the basis of particle phagocytosis.

Membrane damage and interleukin-1 production in murine macrophages exposed to listeriolysin O.

To obtain some insight into the interaction between listeriolysin O (LLO) and the macrophage membrane, we examined the effect of purified Listeria monocytogenes hemolysin on the viability and functions of mouse peritoneal exudate macrophages. The study showed that purified LLO impaired a variety of functions of the macrophages. First, it suppressed the luminol-dependent chemiluminescence response of macrophages. Second, it suppressed the phagocytic ingestion of opsonized sheep erythrocytes and latex beads. Third, exposure of macrophages to LLO resulted in an increase in dead cells, as determined by the trypan blue dye exclusion method. An interesting observation of this study is that the LLO-induced production of interleukin-1 from macrophages could not be blocked by preincubation with cholesterol, while the membrane-damaging ability could be blocked by cholesterol. The dissociation of the blocking effects of cholesterol suggests that the interleukin-1-inducing ability of LLO may be distinct from its membrane-damaging ability.

Glutamine and macrophage function

The effects of glutamine concentration on the phagocytosis of an opsonized antigen, the synthesis of RNA, and the production of interleukin-1 (IL-1) by macrophages were investigated in vitro. A minimum of 0.125 mmol/L glutamine was required for a significant increase in phagocytosis of opsonized sheep erythrocytes, compared with that recorded for macrophages cultured in the absence of glutamine. The synthesis of 3H-RNA by macrophages also required 0.125 mmol/L glutamine in the culture medium before it was significantly increased above the levels of control cultures. A minimum of 0.03 mmol/L glutamine was required for the induction of significant levels of IL-1 by lipopolysaccharide (LPS)-stimulated macrophages. Therefore, recent findings suggesting that decreases in plasma glutamine resulting from major burn injury, sepsis, trauma, and surgery may be partly responsible for the associated impairment of immune function now have a basis in both phagocytosis and in modulation of the synthesis of IL-1 (the first cytokine of the interleukin cascade that leads to specific immunity) by macrophages, in addition to the previously established dependency of lymphocytes on external sources of glutamine for their replication.

Phagocytic Functions of Pulmonary Alveolar Macrophages in Genetically Selected Lean and Obese Swine and the Effects of Exogenous Linolenic Acid upon Cell Function

Alveolar macrophages from genetically selected obese and lean swine were compared for in vitro phagocytic capabilities, using Fc(γ)- and C3-mediated phagocytosis. Cells from obese pigs were significantly more effective at Fc(γ)-mediated phagocytosis than those from lean pigs, both for percentage of total cells phagocytosing ( P<0.044) and for the average number of opsonized sheep erythrocytes (SRBC) ingested per phagocyte ( P<0.045). A seasonal interaction was noted for average number of SRBC ingested per phagocyte: the relative difference in macrophage responses between obese and lean groups became significantly more pronounced during winter and spring months ( P<0.080). Macrophages from obese pigs also exhibited higher phagocytic activities at C3-mediated phagocytosis than did cells from lean pigs, but these differences were significant only for average number of SRBC ingested per phagocyte ( P<0.080). Exogenous linolenic acid was added to selected cultures undergoing Fc(γ)-mediated phagocytosis. Addition of the fatty acid frequently caused enhanced phagocytosis. Macrophages from obese pigs were stimulated by fatty acid treatment more frequently than cells from lean pigs ( P<0.05). Relatively greater enhancement was also seen in cells from obese pigs, when compared with those from lean swine ( P<0.025). These results suggest that genetically transferred factors are of primary importance in alveolar macrophage phagocytic responses and that linolenic acid can induce increased phagocytic activity by porcine alveolar macrophages in vitro.

Production of a factor (CIF) from normal fibroblast cells inhibiting tumor necrosis factor/cachectin production.

Murine embryonic fibroblast cells produce a factor designated cytotoxin-inhibiting factor (CIF) which inhibits tumor necrosis factor (TNF) and interleukin 1 production as well as tumoricidal activity by lipopolysaccharide-activated macrophages. This study determines the physiologic conditions of CIF production in serum-free medium. CIF production was largely dependent upon the presence of lipopolysaccharide. A quantitative correlation between fibroblast cell number, lipopolysaccharide concentration, and incubation time was established. Evidence is presented that CIF inhibited the production or release of TNF. CIF did not destroy TNF after production and release nor did it sequester secreted TNF. The supernatant fluids which inhibited TNF production did not suppress the capability of resting macrophages to phagocytize opsonized sheep erythrocytes, suggesting that only functions expressed in the activated state are inhibited.

Reversal of Virus-Induced Alveolar Macrophage Bactericidal Dysfunction by Cyclooxygenase Inhibition In Vitro

Virus infection of alveolar macrophages (AM) both in vivo and in vitro has been associated with a decreased ability of these cells to kill bacteria, together with enhanced production of metabolites of arachidonic acid. These metabolites, especially PGE2, may be inhibitory to some phagocyte functions. Primary cultures of bovine AM obtained by bronchoalveolar lavage of normal cattle were infected in vitro with parainfluenza-3 (PI3 virus) virus. Killing of Staphylococcus epidermidis by AM was determined on days 1-4 post-infection (p.i.) PI3 virus-infected AM killed significantly fewer bacteria on day 4 p.i. compared to uninfected controls (12.1 +/- 1.3% infected vs. 52.7 +/- 7.2% controls, P less than or equal to 0.05). Bacterial killing by virus-infected AM, but not control AM, was significantly enhanced on day 4 p.i. by addition of cyclooxygenase inhibitors 1 hr prior to bactericidal assay (28.0 +/- 4.5% indomethacin, 36.0 +/- 4.1% mefenamic acid, 38.6 +/- 7.3% piroxicam, 37.0 +/- 6.4% NDGA, 44.9 +/- 7.7% ETYA, P less than or equal to 0.05). Phagocytosis of opsonized sheep erythrocytes and superoxide generation by virus-infected AM were not significantly increased by cyclooxygenase inhibition. Phagosome-lysosome fusion was severely impaired in virus-infected AM. Pretreatment of virus-infected AM with indomethacin significantly enhanced the percentage of cell expressing fusion activity. This data suggests that in vitro bactericidal dysfunction associated with virus infection of AM is partially the result of enhanced production of prostaglandins or thromboxane by AM and/or an abnormal response to normal levels of endogenously produced cyclooxygenase metabolites. The data further indicate the presence of cyclooxygenase sensitive (phagosome-lysosome fusion) and insensitive (phagocytic) components of virus-induced bactericidal dysfunction in AM.

Characterization of Murine Macrophage Fc Receptor-Dependent Phagocytosis and Antibody-Dependent Cellular Cytotoxicity during in vitro Culture with Interferons-Gamma, Alpha/Beta and/or Fetal Bovine Serum

Resident and oil-elicited inflammatory peritoneal macrophages (PMΦ) from competent C3HeB/FeJ and genetically deficient C3H/HeJ mice were characterized for their Fe receptor (FcR)-dependent binding, phagocytic and ADCC functions during in vitro differentiation under the influence of mouse recombinant interferon-γ (rIFN-γ), interferon-α/β (IFN-α/β) fetal bovine serum (FBS), or in serum-free medium. Freshly cultured resident PMΦ from C3HeB/FeJ mice had low levels of FcR-mediated phagocytosis in response to mouse monoclonal IgGγ2a, IgGγ2b or IgGγ1, opsonized sheep erythrocytes as compared to oil-elicited inflammatory PMΦ from the same strain. Resident PMΦ were uniformly upregulated in their FcR-dependent phagocytosis after 24-48h in vitro culture with FBS to levels approximating that of freshly cultured inflammatory PMΦ which were also further upregulated after 24h in vitro culture with FBS. Both resident and inflammatory PMΦ were upregulated largely by an autostimulatory process in that they increased their FcR-mediated phagocytosis in serum-free RPMI-1640 medium without the addition of rIFN-γ or IFN-α/β, although FBS further augmented FcR upregulation. A synergistic effect of FBS and rIFN-γ was required for total reconstitution of FcR-mediated phagocytosis of FcR-incompetent C3H/HeJ inflammatory PMΦ in that FBS or rIFN-γ alone only partially reconstituted FcR function, whereas in combination full reconstitution occurred. Thus, macrophages from competent C3HeB/FeJ mice were upregulated in their FcR-mediated functions largely by an autostimulatory process, presumably dependent on endogenous of IFN-β, whereas, genetically-deficient C3H/HeJ macrophages required exogenous rIFN-γ in combination with fetal bovine serum for synergistic reconstitution of FcR functions. The uniform upregulation of FcR-dependent effector functions in vitro appears to provide an efficient system for enhanced immune function during differentiation which may be applicable to in vivo situations.

The chicken macrophage response to carbohydrate-based irritants: Temporal changes in peritoneal cell populations

Temporal changes in the recruitment and functional activation of Cornell K strain chicken peritoneal macrophages elicited by starch and dextrans were determined. During a 72-hr period following intraperitoneal injection, Sephadex (cross-linked dextran) elicited a gradually increasing number of peritoneal exudate cells (PECs) which contained ≥78% macrophages in the adherent cell population. In contrast, starch tended to elicit higher numbers of PECs early in the stimulation process, with heterophils comprising a major proportion of the adherent cells. Later in the time course, macrophages were the predominant adherent cell type. Comparisons of PEC recruitment using Sephadex vs. low molecular weight dextrans demonstrated that the low-molecular weight dextrans elicited fewer PECs than did Sephadex; unlike Sephadex, however, the low molecular weight dextrans generated a significant peak of heterophil infiltration that was similar, although delayed, to that produced with starch. Sephadex-stimulated peritoneal macrophages exhibited time-dependent functional acquisitions for 1) the phagocytosis of sheep erythrocytes, 2) the engulfment of opsonized sheep erythrocytes, and 3) opsonin-erythrocyte rosetting. Acquisition of phagocytic activity for uncoated vs. opsonized erythrocytes occurred under very different rates during the time course. The results suggest that both the nature of the irritant and the time point of cell harvest can be appropriately selected to provide the particular cell population and functional status desired for subsequent in vitro analyses.

Triggering of interferon γ-primed macrophages by various known complement activators for nonspecific tumor cytotoxicity

Five known complement activators were evaluated for their capacity to directly activate murine macrophages and to trigger activation of lymphokine primed macrophages for nonspecific tumor cytotoxicity. Bacterial lipopolysaccharide (LPS), Lipid A, polyinosinic-polycytidylic acid, cobra venom factor (CVF), and zymosan directly activated macrophages in a dose-dependent fashion at high concentrations. Subactivating concentrations of each of these agents were found to effectively trigger macrophages which were preprimed either by macrophage-activating factor or by murine recombinant interferon γ for enhanced tumoricidal activity. An Fc receptor blockade with opsonized sheep erythrocytes abrogated LPS-mediated direct activation and triggering of interferon γ-primed macrophages, but had no inhibitory effect on direct activation or triggering by CVF for nonspecific tumor cytotoxicity. This study characterizes the capacity of a diverse group of known complement activators to serve as second signal triggers for culmination of the activation process of interferon-primed macrophages for nonspecific tumoricidal activity. These findings suggest that complement activators may directly activate macrophages by stimulation of interferon β production by macrophages for self-priming and, as we have shown, act as self-triggers. The putative role of macrophage-associated complement components in the activation process is discussed.

Effects of leukocyte inhibitory factor (LIF) on neutrophil phagocytosis and bactericidal activity.

Human leukocyte inhibitory factor (LIF) is a lymphokine initially defined by its ability to inhibit the random migration of neutrophils. We have recently demonstrated that LIF also potentiates a number of f-met-leu-phe-mediated functions as well as enhancing one Fc receptor-mediated function (antibody-dependent cellular cytotoxicity). In this paper, we have extended our studies involving the effects of LIF on the neutrophil, specifically its effect on phagocytosis and bactericidal activity. We demonstrate that LIF (2 U/ml) potentiates phagocytosis of opsonized heat-killed Staphylococcus aureus (up to 57.2%) and sheep erythrocytes (124.4%) as well as unopsonized latex particles (59.9%). Phagocytosis of opsonized sheep erythrocytes was inhibited by an anti-neutrophil Fc receptor antibody with control PMN but not using the LIF-treated PMN. LIF (1/2 to 1 U) also potentiates the killing of S. aureus by up to 51.6%. Higher concentrations of LIF (greater than or equal to 4 U) inhibits killing. These effects were shown not to be associated with an increase in Fc receptor availability. It is therefore possible that potentiation of these neutrophil activities by LIF may occur either as a result of increased receptor turnover or, more likely, secondary to an increase in nonspecific neutrophil adherence. These studies further support the concept that LIF may have an important role in vivo in inflammation and immunity.