Interferon-gamma depresses binding of ligand by C3b and C3bi receptors on cultured human monocytes, an effect reversed by fibronectin

RhoG is required for both FcγR- and CR3-mediated phagocytosis

Phagocytosis is a highly ordered process orchestrated by signalling through Rho GTPases to locally organise the actin cytoskeleton and drive particle uptake. Specific Rho family members that regulate phagocytosis are not known, as the majority of studies have relied on the use of dominant-negative mutants and/or toxins, which can inactivate multiple Rho GTPases. To identify the relevant GTPases for phagocytosis through the Fcγ receptor (FcγR) and complement receptor 3 (CR3), we depleted 20 Rho proteins individually in an RNA interference (RNAi) screen. We find that distinct GTPase subsets are required for actin polymerisation and uptake by macrophages: FcγR-dependent engulfment requires Cdc42 and Rac2 (but not Rac1), whereas CR3 requires RhoA. Surprisingly, RhoG is required for particle uptake through both FcγR and CR3. RhoG has been previously linked to Rac and Cdc42 signalling in different model systems, but not to RhoA. Interestingly, we find that RhoG is also recruited and activated at phagocytic cups downstream of FcγR and CR3, irrespective of their distinct actin structures and mechanisms of internalisation. Thus, the functional links between RhoG and RhoA downstream of CR3-dependent phagocytosis are new and unexpected. Our data suggest a broad role for RhoG in consolidating signals from multiple receptors during phagocytosis.

The role of beta2 integrins and lipopolysaccharide-binding protein in the phagocytosis of dead Neisseria meningitidis

Phagocytosis of microbial pathogens is essential for the host immune response to infection. Our previous work has shown that lipooligosaccharide (LOS) expression on the surface of Neisseria meningitidis (Nm) is essential for phagocytosis, but the receptor involved remained unclear. In this study, we show that human CR3 (CD11b/CD18) and CR4 (CD11c/CD18) are phagocytic receptors for Nm as illustrated by the capacity of CR3- and CR4-transfected Chinese hamster ovary (CHO) cells to facilitate Nm uptake. A CR3-signalling mutant failed to internalize Nm, showing that the ability of CR3 to signal is essential for phagocytosis. Internalization of Nm by CR3-transfected CHO cells could be inhibited by the presence of CR3-specific antibodies. Furthermore, dendritic cells from leukocyte adhesion deficiency-1 patients, who have diminished expression of beta2 integrins, showed markedly reduced phagocytosis of Nm. The CR3-mediated phagocytosis required the presence of lipopolysaccharide-binding protein (LBP). Furthermore, the expression of LOS by Nm was essential for LBP binding and phagocytosis via CR3. These results reveal a critical role of CR3 and LBP in the phagocytosis of Nm and provide important insights into the initial interaction meningococci have with the immune system.

Does the Atopic Phenotype Prevent Development of Active Tuberculosis Infection?

This study investigated the relationship between the atopic phenotype and the development of active tuberculosis. A total of 82 human immunodeficiency virus-negative males with active pulmonary tuberculosis and 88 healthy controls were enrolled into this prospective study. Serum immunoglobulin E (IgE) levels were measured and skin prick tests performed before initiation of treatment. Skin prick tests were positive in 34.1% of the tuberculosis patients and 39.8% of the controls. Allergic respiratory symptoms were significantly less frequent in skin prick test positive tuberculosis patients (21.4%) compared with skin prick test positive controls (62.9%). Median IgE levels in atopic tuberculosis patients were significantly higher than in atopic controls. The low rate of atopic respiratory complaints seen in the tuberculosis patients, despite having similar allergic skin prick test sensitivities to the controls, could be attributed to a weak T-helper (Th) 2 immune reaction and its effects on Th1–Th2 interaction.

Th1?Th2 Interaction: Is More Complex than a See-saw?

Infections, such as tuberculosis, measles and diphtheria, may decrease the risk of developing atopic allergic disorders. Patients with pulmonary TB having similar allergic skin prick test sensitivity as healthy controls but unable to show vigorous atopic phenotype in their healthy period might be a sign for Th1-Th2 immune interaction.

Association between airway hyperreactivity and bronchial macrophage dysfunction in individuals with mild asthma

Little is known about the functional capabilities of bronchial macrophages (BMs) and their relationship to airway disease such as asthma. We hypothesize that BMs from asthmatics may be modulated in their function compared with similar cells from healthy individuals. BMs obtained by induced sputum from mild asthmatics (n = 20) and healthy individuals (n = 20) were analyzed using flow cytometry for CD16, CD64, CD11b, CD14, and human leukocyte antigen-DR expression, phagocytosis of IgG opsonized yeast, and oxidant production. Asthma status was assessed by lung function [percent predicted forced vital capacity and forced expiratory volume in 1 s (FEV(1))], percent sputum eosinophils, and nonspecific airway responsiveness [provocative concentration that produces a 20% fall in FEV(1) (PC(20,FEV1))]. Asthmatics with >5% airway eosinophils (AEo+) had decreased BM CD64 expression and phagocytosis compared with asthmatics with <5% eosinophils (AEo-). Among asthmatics, a significant correlation was found between CD64 expression and BM phagocytosis (R = 0.7, P < 0.009). Phagocytosis was also correlated with PC(20,FEV1) (R = 0.6, P < 0.007), lung function (%predicted FEV(1), R = 0.7, P < 0.002) and percent eosinophils (R = -0.6, P < 0.01). In conclusion, BM from asthmatics are functionally modulated, possibly by Th2 cytokines involved in asthma pathology.

Characterization of extracellular nucleotide-induced Mac-1 (alphaM beta2 integrin) surface expression on peripheral blood leukocytes

Extracellular nucleotides, released during vascular injury, stimulate hematopoietic cells resulting in various physiological responses. We have determined that nucleotides can stimulate the expression of Mac-1 on peripheral blood leukocytes. ATP stimulated the expression of Mac-1 in a time- and dose-dependent manner with maximum expression occurring in 5 min at 10 microM ATP. This increase in surface expression was observed in monocytes and granulocytes was dose-dependent and was comparable in extent to the increase induced by the chemotactic peptide, formyl-Met-Leu-Phe. Other nucleotides including 2-MeSADP, ADP, UTP, and 2MeSATP had similar effect. Nucleotide-mediated stimulation of Mac-1 expression in granulocytes was completely inhibited by Ro-31-8220, a specific inhibitor of protein kinase C, while variable inhibition was observed in monocytes. These results demonstrate the stimulation of peripheral blood leukocytes by nucleotides causing an increased surface expression of Mac-1 which may be mediated by the activation of protein kinase C.

Engagement of the Lewis X Antigen (CD15) Results in Monocyte Activation

We previously reported that monocyte adhesion to tumor necrosis factor-α (TNF-α)–treated endothelial cells increased expression of tissue factor and CD36 on monocytes. Using immunological cross-linking to mimic receptor engagement by natural ligands, we now show that CD15 (Lewis X), a monocyte counter-receptor for endothelial selectins may participate in this response. We used cytokine production as a readout for monocyte activation and found that CD15 cross-linking induced TNF-α release from peripheral blood monocytes and cells from the monocytic cell line MM6. Quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) showed an increase in steady-state TNF-α mRNA after 3 to 4 hours of cross-linking. CD15 cross-linking also concomitantly increased interleukin-1β (IL-1β) mRNA, while no apparent change was observed in the levels of β-actin mRNA, indicating specificity. To examine transcriptional regulation of cytokine genes by CD15 engagement, a CAT plasmid reporter construct containing IL-1β promoter/enhancer sequences was introduced into MM6. Subsequent cross-linking of CD15 increased CAT activity. CD15 engagement by monoclonal antibody also attenuated IL-1β transcript degradation, demonstrating that signaling via CD15 also had posttranscriptional effects. Nuclear extracts of anti-CD15 cross-linked cells demonstrated enhanced levels of the transcriptional factor activator protein-1, minimally changed nuclear factor-κB, and did not affect SV40 promoter specific protein-1. We conclude that engagement of CD15 on monocytes results in monocyte activation. In addition to its well-recognized adhesive role, CD15 may function as an important signaling molecule capable of initiating proinflammatory events in monocytes that come into contact with activated endothelium.

Engagement of the Lewis X Antigen (CD15) Results in Monocyte Activation

We previously reported that monocyte adhesion to tumor necrosis factor-α (TNF-α)–treated endothelial cells increased expression of tissue factor and CD36 on monocytes. Using immunological cross-linking to mimic receptor engagement by natural ligands, we now show that CD15 (Lewis X), a monocyte counter-receptor for endothelial selectins may participate in this response. We used cytokine production as a readout for monocyte activation and found that CD15 cross-linking induced TNF-α release from peripheral blood monocytes and cells from the monocytic cell line MM6. Quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) showed an increase in steady-state TNF-α mRNA after 3 to 4 hours of cross-linking. CD15 cross-linking also concomitantly increased interleukin-1β (IL-1β) mRNA, while no apparent change was observed in the levels of β-actin mRNA, indicating specificity. To examine transcriptional regulation of cytokine genes by CD15 engagement, a CAT plasmid reporter construct containing IL-1β promoter/enhancer sequences was introduced into MM6. Subsequent cross-linking of CD15 increased CAT activity. CD15 engagement by monoclonal antibody also attenuated IL-1β transcript degradation, demonstrating that signaling via CD15 also had posttranscriptional effects. Nuclear extracts of anti-CD15 cross-linked cells demonstrated enhanced levels of the transcriptional factor activator protein-1, minimally changed nuclear factor-κB, and did not affect SV40 promoter specific protein-1. We conclude that engagement of CD15 on monocytes results in monocyte activation. In addition to its well-recognized adhesive role, CD15 may function as an important signaling molecule capable of initiating proinflammatory events in monocytes that come into contact with activated endothelium.

Growth hormone activation of human monocytes for superoxide production but not tumor necrosis factor production, cell adherence, or action against Mycobacterium tuberculosis

We have previously demonstrated that growth hormone (GH) is a human macrophage-activating factor which primes monocytes for enhanced production of H2O2 in vitro. This report extends our observations to other monocyte functions relevant to infection. We find that GH also primes monocytes for O2- production, to a degree similar to the effect of gamma interferon. Neither macrophage-activating factor alone stimulates monocytes to release bioactive tumor necrosis factor. However, GH, unlike gamma interferon, does not synergize with endotoxin for enhanced tumor necrosis factor production. In further contrast, GH does not alter monocyte adherence or morphology, while phagocytosis and killing of Mycobacterium tuberculosis by GH-treated monocytes are also unaffected. Therefore, despite the multiplicity of the effects of GH on the immune system in vivo, its effects on human monocytes in vitro appear to be limited to priming for the release of reactive oxygen intermediates.

Upregulation of mouse CD14 expression in Kupffer cells by lipopolysaccharide

Western blot analysis showed that a monoclonal antibody against recombinant mouse CD14 (mCD14), designated rmC5-3, specifically reacted with mouse macrophage cell line J774, but not myeloma cell line NS1. Fluorographic and immunocytochemical analysis demonstrated specific binding of rmC5-3 with mouse resident macrophages, inflammatory monocytes and neutrophils, and macrophage cell lines. Immunohistochemical staining using rmC5-3 showed that CD14-positive Kupffer cells (KC) were small in number in the liver in nonstimulated mice. The number of stained KC, which were rich in the midzonal and periportal regions, gradually increased with time after intraperitoneal injection of lipopolysaccharide (LPS), peaked 6 h after injection, and returned to normal by 20 h after injection. Staining intensity over time was proportional to the number of KC. A slight increase in mCD14 expression was observed in peritoneal macrophages 2 h after LPS administration in vivo using flow cytometric analysis. mCD14 mRNA became detectable at 1 h after the intraperitoneal injection of LPS (20 micrograms/mice), and the level dramatically increased with time, peaking at 3 h, and sharply dropped at 6 h. The resident peritoneal macrophages demonstrated a constitutively high mCD14 mRNA expression, which slightly increased 2 h after LPS (100 ng/ml) stimulation in vitro. The level of mCD14 expression in macrophages did not increase after intraperitoneal injection of LPS (20 micrograms/mice).

Surface molecules involved in the adherence of recombinant interferon-gamma (rIFN-gamma)-stimulated human monocytes to vascular endothelial cells

During an inflammatory reaction, an increased number of circulating monocytes adhere to the endothelial cells (EC) of the vessel wall. This process is mediated by molecules located on the surface of monocytes and EC. Locally released inflammatory mediators can modulate monocyte-EC interaction. In an earlier study we reported that stimulation of monolayers of cultured venous EC with rIFN-gamma enhanced their adhesiveness for monocytes. The aim of the present study was to investigate the effect of rIFN-gamma on peripheral blood monocytes with regard to the expression of surface molecules and the binding to non-stimulated or cytokine-stimulated EC. Flow cytometric analysis demonstrated that monocytes stimulated with 500 U/ml rIFN-gamma for 24 h showed increased expression of CR3 (CD11b/CD18), p150,95 (CD11c/CD18) and Fc gamma RI (CD64); the expression of LFA-1 (CD11a/CD18), L-selectin, CD14 and VLA-4 (CD49d/CD29) did not change or was slightly reduced. Upon stimulation with rIFN-gamma monocytes showed an enhanced binding to both non-stimulated or rIFN-gamma-stimulated EC. This was even more pronounced when EC had been stimulated with rIL-1 alpha for 24 h. The increased binding of rIFN-gamma-stimulated monocytes to rIL-1 alpha-stimulated EC was further analysed. Studies with MoAbs against adhesion molecules on monocytes revealed that the binding of rIFN-gamma-stimulated monocytes, but not that of non-stimulated monocytes, to rIL-1 alpha-stimulated EC was inhibited by about 30-60% with MoAbs against CD11a, CD11b, CD18, L-selectin or CD14. MoAbs against CD11c or CD49d had little or no effect. From these results, we conclude that exposure of monocytes to rIFN-gamma enhances their adhesiveness to cytokine-stimulated EC by a mechanism which involves CD11a/CD18, CD11b/CD18, CD14 and L-selectin on monocytes.

Fc receptors expression and function in mononuclear phagocytes from AIDS patients: modulation by IFN-gamma

Fc-receptor (FcR)-mediated phagocytosis and FcR (FcRI, FcRII and FcRIII) membrane expression was studied on freshly separated and cultured monocytes (Mo) from 20 AIDS patients and 20 healthy controls. Both Mo and Mo-derived macrophages from AIDS patients presented a significant defect in their capacity to ingest IgG-coated erythrocytes (EA) compared to control cells. This functional defect did not depend on a decline in the number of FcR+ cells or on a decrease in the expression of FcR on their surface. In fact, the percentages of phagocytes reacting with anti-FcRI MoAb (32.2) or anti-FcRII MoAb (IV.3) were similar for controls and AIDS patients, while the percentage of FcRIII-positive Mo (MoAb 3G8) was higher in the AIDS population than in controls, though this difference was not seen on cultured Mo. The level of FcRI expression, evaluated as mean fluorescence intensity (MFI), was higher on freshly separated Mo from AIDS patients than from controls but this difference disappeared also with differentiation of Mo to Mo-derived macrophages in vitro. Parallel analysis of FcRII and FcRIII on phagocytes revealed no differences in the MFI between the AIDS and control groups. Some observations suggested that this functional defect might be secondary to phagocyte priming by circulating IFN-gamma: (1) in vitro stimulation of Mo with hrIFN-gamma, which increased FcRI expression, actually reduced phagocytosis of IgG-coated particles; and (2) IFN-gamma concentrations were increased in AIDS patients' plasma. In spite of these findings, no significant correlation was found between plasma IFN-gamma concentrations and FcR-mediated ingestion in AIDS patients, making the hypothesis uncertain. Even if the basis for the impaired FcR-mediated phagocytosis in AIDS patients remains unclear, this functional defect may have a role in the immunopathogenesis of AIDS, constituting a component cause of the immunodeficiency.

Phagocytosis of Pseudomonas aeruginosa by macrophages: receptor-ligand interactions

Phagocytic cells play a critically important role in host defense against infection with Pseudomonas aeruginosa. Recent observations on the receptors and ligands that mediate ingestion of this bacterium by phagocytic cells and the factors that modulate phagocytosis have provided the theoretical underpinning for novel therapeutic strategies.

Enhancement of macrophage candidacidal activity by interferon-gamma. Increased phagocytosis, killing, and calcium signal mediated by a decreased number of mannose receptors

In contrast to its macrophage-activating capacity, IFN-gamma downregulates expression of the macrophage mannose receptor (MMR), which mediates uptake of Candida and other microorganisms. We found that IFN-gamma induced a concentration-dependent increase in the capacity of human monocyte-derived macrophages to ingest and kill both opsonized and unopsonized Candida albicans and to release superoxide anion upon stimulation with Candida. Mannan or mannosylated albumin inhibited this activated uptake of unopsonized Candida, but glucan did not. Addition of mAb to complement receptor (CR) 3 did not inhibit ingestion; macrophages that lacked CR3 (leukocyte adhesion defect) showed normal upregulation of ingestion by IFN-gamma. The increased candidacidal activity of IFN-gamma-activated macrophages was associated with reduced expression of MMR by a mean of 79% and decreased pinocytic uptake of 125I-mannosylated BSA by 73%; K(uptake) of pinocytosis was not changed. Exposure of resident macrophages to unopsonized Candida did not elicit a transient increase in intracellular free Ca2+ ([Ca2+]i); macrophages activated by IFN-gamma expressed a brisk increase in [Ca2+]i on exposure to Candida. These data suggest that macrophage activation by IFN-gamma can enhance resistance to C. albicans infection in spite of downregulation of the MMR, perhaps through enhanced coupling of the MMR to microbicidal functions.

Cytokines and PGE2 modulate the phagocytic function of the beta-glucan receptor in macrophages

Under serum-free conditions the beta-glucan receptor of mouse macrophages mediates phagocytosis of beta-1,3-D-glucan-coated microbeads (diameter 2 microns). IFN-gamma increases the phagocytic function of the beta-glucan receptor in a dose-dependent manner, giving the plateau level at 100 U/ml. Maximum activity appears 9 h after addition of IFN-gamma to the cells. The effect disappears within 24 h. The effect of IFN-gamma may be a result of augmented receptor synthesis since treatment with cycloheximide reduces the phagocytosis. IL-1 also increases the phagocytic function of the beta-glucan receptor giving a dose-dependent response and with the plateau level reached at 10 U/ml. Maximum activity is found 4 h after addition of IL-1 to macrophages. The effect disappears within 24 h. TNF does not alter the phagocytic function of the beta-glucan receptor, but TNF together with IL-1 prolongs the effect of IL-1. PGE2 reduces the phagocytic function of the beta-glucan receptor. Maximum reduction is achieved with 8 ng/ml. Time-course studies show the lowest phagocytic activity 9 h after addition of PGE2 to the cells.

CD14: cell surface receptor and differentiation marker

In the past, CD14 has been viewed simply as a useful marker molecule for monocytes and macrophages. Now, new findings on its role in binding of LPS-LBP complexes and in signal transduction have engendered renewed interest in the properties of CD14. Here, CD14 function, its expression in different cell types and the regulation of expression, including the generation of soluble CD14, are described, and the diagnostic value of CD14 in various diseases is discussed.

Increased expression of IgG Fc receptor type I on neutrophils and monocytes from HIV-infected subjects

Interferon-gamma (IFN-gamma) induces de novo expression of IgG Fc receptor type I (FcRI) on neutrophils and significantly raises the level of these receptors on monocytes. Since increased concentrations of IFN-gamma have been observed in sera from patients with HIV infection, FcRI expression might also be increased on these subjects' phagocytes. FcRI expression was assessed by indirect immunofluorescence staining of phagocytes in whole blood from 40 healthy controls and 55 HIV+ subjects, 24 belonging to CDC class III and 31 to CDC class IV; 42 were intravenous drug abusers (IVDA) and 13 were homosexual men. Plasma levels of IFN-gamma were measured using a modified immunoradiometric assay. The mean linear fluorescence intensity, used as a relative measure of receptor expression, was significantly higher on unseparated neutrophils from HIV+ subjects in CDC classes III (P < 0.001) and IV (P < 0.0001) than from controls. Similar changes in FcRI expression were observed on monocytes from HIV+ subjects. While no differences were observed between IVDA and homosexual HIV+ patients, there was a significant association between FcRI expression and the patients' CDC stage, those in class IV having the highest FcRI levels. Plasma IFN-gamma concentrations were significantly higher in HIV+ patients than in controls and a positive correlation with the stages of HIV infection was again observed. FcRI expression was also increased on freshly purified neutrophils from five HIV+ patients in CDC class IV but did not increase further after 18 h incubation with IFN-gamma, a treatment that up-regulated FcRI expression on control neutrophils. These data suggest that: (i) FcRI evaluation may be a sensitive marker for the biological activity of IFN-gamma in vivo; (ii) phagocytes from HIV+ subjects are activated in vivo by IFN-gamma, expressing increased levels of FcRI; (iii) these IFN-gamma-activated cells may play a role in the pathogenesis of AIDS.

Monocyte-derived macrophage function in HIV-infected subjects: in vitro modulation by rIFN-gamma and rGM-CSF

We investigated monocyte-derived macrophage function in 25 HIV-positive patients, 19 in the CDC class III and 6 class IV; 17 were intravenous drug abusers (IVDA) and 8 were homosexual men. Macrophages from HIV-positive patients behaved normally in assays of superoxide anion (O2-) production and candidacidal activity. After 3 days' treatment with 200 U/ml recombinant interferon-gamma (rIFN-gamma) or 250 U/ml recombinant granulocyte/macrophage-colony stimulating factor (rGM-CSF), both control and HIV-positive patients' phagocytes expressed the activated state, as indicated by the increased O2- production in response to phagocytable or soluble stimuli; however, these cytokines did not enhance candidacidal activity. Compared to appropriate HIV-negative controls (18 healthy heterosexuals, 4 homosexuals and 4 IVDA), macrophages from 19 of the 25 HIV-positive patients presented a significant defect in their Fc receptor (FcR)-dependent phagocytosis, independently from the CDC stage, AZT therapy, or life style. Treatment of macrophages with rIFN-gamma impaired their capacity to ingest IgG-coated erythrocytes, both in controls and HIV-positive subjects. Treatment of phagocytes with rGM-CSF significantly increased their FcR-dependent phagocytosis in controls, whereas in HIV-positive patients and in HIV-negative homosexuals and IVDA only an upward tendency was observed. Although the mechanism of the impaired FcR-dependent phagocytosis in HIV-positive patients remain to be clarified, our results suggest that this functional defect may be secondary to phagocyte priming by circulating IFN-gamma in vivo. This macrophage alteration may be implicated in the immunodeficiency of HIV-positive patients. However, considering the potential role of FcRs in HIV infection enhancement, the defective FcR function might even be a protective mechanism against FcR-mediated HIV dissemination. In the light of these findings, the immunotherapeutic potential of IFN-gamma and GM-CSF in HIV infection merits further investigation.

Effect of cytokines and lipopolysaccharide on CD14 antigen expression in human monocytes and macrophages

The 52 kD myeloid membrane glycoprotein CD14 represents the receptor for complexes of lipopolysaccharide (LPS) and LPS binding protein (LBP); it is involved in LPS induced tumor necrosis factor-alpha production. Expression of CD14 increases in monocytes differentiating into macrophages, and it is reduced by rIFNg in monocytes in vitro. In the present study CD14 membrane antigen expression was investigated in cultures of human mononuclear leucocytes (PBL), in elutriated, purified monocytes, and in blood monocyte derived Teflon cultured macrophages. Cells were incubated for 15 or 45 h with rIL-1, rIL-2, rIL-3, rIL-5, rIL-6, rTNFa, rGM-CSF, rM-CSF, rTGFb1, rIFNa, lipopolysaccharide (LPS), and, as a control, rIFNg. The monoclonal antibodies Leu-M3 and MEM 18 were used for labelling of CD14 antigen by indirect immunofluorescence and FACS analysis of scatter gated monocytes or macrophages. IFNg concentrations were determined in PBL culture supernatants by ELISA. rIFNa and rIL-2 reduced CD14 in 15 and 45 h PBL cultures, an effect mediated by endogenous IFNg, since it was abolished by simultaneous addition of an anti-IFNg antibody. rIFNa and rIL-2 were ineffective in purified monocytes or macrophages. rIL-4 strongly reduced CD14 in PBL and purified monocytes after 45 h, whereas in macrophages the decrease was weak, although measurable after 15 h. The other cytokines investigated did not change CD14 antigen expression. Cycloheximide alone reduced CD14, but when added in combination with rIFNg the effect on CD14 downregulation was more pronounced. The effect of rIFNg on CD14 in PBL cultures was dose-dependently inhibited by rIL-4 and this inhibition is probably due to an IL-4 mediated blockade of IFNg secretion. LPS at a low dose increased CD14, at a high dose it produced a variable decrease of CD14 in PBL, which was probably due to LPS induced IFNg secretion. LPS strongly enhanced CD14 in 45 h cultures of purified monocytes. The results, showing that CD14 antigen expression is upregulated by LPS and downregulated by rIFNg and rIL-4, suggest that the LPS-LBP receptor is involved in the feedback response of IFNg and IL-4 to LPS stimulation.

Suppression by human recombinant gamma interferon of in vitro macrophage nonopsonic and opsonic phagocytosis and killing

Although gamma interferon (IFN-gamma) exerts profound effects on the state of activation of macrophages, its influence on receptor-mediated phagocytosis and killing of extracellular bacteria is poorly understood. Human monocytes cultured in the presence of human recombinant IFN-gamma exhibited an enhanced capacity to produce superoxide anion. Although these cells bound greater numbers of particles via Fc receptors, their capacity to phagocytose by these receptors or to bind or ingest particles via receptors for C3bi, mannose, or unopsonized Pseudomonas aeruginosa was substantially depressed in a dose-dependent fashion (0.1 to 1,000 U of IFN-gamma per ml). Macrophage phagocytosis of P. aeruginosa and Staphylococcus aureus opsonized with whole serum or with serum deficient in immunoglobulin or complement was also depressed. Macrophages cultured in the presence of IFN-gamma had a diminished capacity to kill both unopsonized and opsonized P. aeruginosa. We conclude that IFN-gamma inhibits macrophage nonopsonic and opsonic receptor-mediated phagocytosis and killing but enhances oxidative radical generation; its production may exacerbate host tissue damage during chronic infection with extracellular pathogens.

Signals arising from antigen-presenting cells

It has been customary to consider that antigen-presenting cells provide, in addition to the presented antigen, a second or co-stimulatory signal that leads to T-cell growth and effector function. The recent literature indicates that this two-signal notion oversimplifies the function of antigen-presenting cells. Instead it is useful to consider four groups of events: the formation of peptide-MHC complexes, the role of soluble cytokines, the action of antigen-presenting cell-T cell molecular couples distinct from the receptor for peptide MHC, and the function of antigen-presenting cells in situ.

Discrimination between intracellular uptake and surface adhesion of bacterial pathogens

Most bacterial pathogens initiate infectious diseases by adhering to host cells. Bacterial adherence to nonphagocytic cells usually leads to extracellular colonization; however, many invasive microorganisms enter host cells after binding to the host cell surface. It is unclear why bacterial adherence can result in these two different fates for the microorganism. Analyses of model systems, such as the uptake of enteropathogenic Yersinia into cultured cells, indicate that the particular mammalian cell receptors bound and the nature of the binding event dictate whether the bacterium remains extracellular or enters host cells.

Enhancement of macrophage superoxide anion production by amphotericin B

Amphotericin B (AmB) appears to have some important immunomodulatory effects, but its mechanism of action has not been explained. We investigated the effects of AmB on activation of human monocyte-derived macrophages. Macrophages cultured in the presence of AmB had an enhanced capacity to produce superoxide anion after stimulation with phorbol myristate acetate. This enhancement was dose dependent within a therapeutic range of AmB levels (0.1 to 3.0 mg/liter). Macrophages cultured in the presence of AmB had enhanced surface expression of Ia antigen; phagocytosis of unopsonized zymosan, opsonized Staphylococcus aureus, or erythrocytes opsonized with C3bi or immunoglobulin G paradoxically appeared to be reduced, but results did not achieve statistical significance. AmB appears to activate macrophages and may do so via direct effects on the plasma membrane.

Binding of Mycobacterium avium-Mycobacterium intracellulare to human leukocytes

We examined nonopsonic binding of Mycobacterium avium-Mycobacterium intracellulare (MAI) by human leukocytes. Macrophages (M phi) avidly bound fluorescently labeled MAI in the absence of serum proteins. Binding appeared to be mediated by a lineage-specific, proteinaceous receptor on M phi, since (i) binding of labeled bacteria could be competitively inhibited by unlabeled MAI, (ii) treatment of M phi with trypsin ablated the ability of M phi to bind MAI, and (iii) the capacity to bind MAI was observed on monocytes, M phi, and stimulated polymorphonuclear cells but not on lymphocytes or unstimulated polymorphonuclear cells. The receptor for MAI appeared mobile in the plane of the membrane, since spreading of M phi on a carpet of immobilized, unlabeled MAI down modulated binding of labeled MAI added in suspension. The receptor required neither calcium nor magnesium for activity and appeared different from other known receptors for intracellular pathogens.

The distinct leukocyte integrins of mouse spleen dendritic cells as identified with new hamster monoclonal antibodies

Hybridoma fusions with hamster hosts were undertaken to generate mAbs to mouse spleen dendritic cells. Two mAb were obtained and used to uncover the distinct integrins of these APC. One, 2E6, bound a determinant common to all members of the CD11/CD18 family, most likely the shared 90 kD CD18 beta chain. 2E6 immunoprecipitated the characteristic beta 2 integrin heterodimers from lymphocytes (p180, 90; CD11a) and macrophages (p170,90; CD11b), but from dendritic cells, a p150,90 (presumably CD11c) integrin was the predominant species. 2E6 inhibited the binding function of the CD11a and CD11b integrins on B cells and macrophages in appropriate assays, but 2E6 exerted little or no inhibition on the clustering of dendritic cells to T cells early in primary MLR, suggesting a CD11/CD18-independent mechanism for this binding. The second mAb, N418, precipitated a 150, 90 kD heterodimer that shared the 2E6 CD18 epitope. This N418 epitope may be the murine homologue of the previously characterized human CD11c molecule, but the epitope was only detected on dendritic cells. N418 did not react with peritoneal macrophages, anti-Ig-induced spleen B blasts, or bulk lymph node cells. When used to stain sections of spleen, N418 stained dendritic cells in the T-dependent areas, much like anti-class II mAbs that were also generated in these fusions. In addition, N418 revealed nests of dendritic cells that punctuated the rim of marginal zone macrophages between red and white pulp. This localization positioned most dendritic cells at regions where arterial vessels and T cells enter the white pulp. We conclude that the p150, 90 heterodimer is the major beta 2 integrin of spleen dendritic cells, and we speculate that it may function to localize these APC at sites that permit access to the recirculating pool of resting T cells.

The leukocyte cell surface receptor(s) for the iC3b product of complement

CR3 is probably the major adhesion molecule on monocytes and neutrophils. Its function as a phagocytic receptor for iC3b-coated particles has been well characterized. CR3 also has binding affinity for other ligands, including those that compete with iC3b such as fibrinogen, factor X, and beta-glucan, and those that do not such as bacterial LPS. CR3 binding to endothelial cells probably plays an important role in the extravascular migration of monocytes and neutrophils, but the ligand that it recognizes on endothelial cells has not been identified. Structurally CR3 belongs to the integrin family, and it shares a common subunit with p150,95 and LFA-1. The expression of these three membrane antigens appear to be limited to leukocytes, and they are sometimes referred to collectively as the leukocyte integrins. All three antigens have a common binding affinity for bacterial LPS. p150,95 also has affinity for iC3b, but p150,95/iC3b-dependent cellular responses has not been demonstrated. Its status as a complement receptor therefore awaits further experimental support.

Regulated expression of Mg2+ binding epitope on leukocyte integrin alpha subunits

The leukocyte integrins LFA-1, CR3 and p150,95 are a family of heterodimeric receptors that mediate divalent cation-dependent cellular adhesion reactions. In this study we describe a novel antibody-defined epitope present on the leukocyte integrin alpha subunits indicating that the antibody recognizes a structural feature common to all three polypeptides. Antibody recognition further differs from that of previously described anti-leukocyte integrin antibodies in that it is strictly dependent upon the presence of Mg2+. This suggests that the epitope is located within, or in close proximity to, the three conserved cation binding domains and is therefore a measure of Mg2+ bound to the leukocyte integrins and thus reflects functionally active molecules. The epitope can be induced on polymorphonuclear leukocytes, a subset of T cells and on monocytes but is absent or much reduced at low temperature or in the presence of metabolic inhibitors. These observations have considerable implications for the regulation of leukocyte integrin function suggesting that Mg2+ binding to the extracellular domain(s) of the alpha subunits is controlled from within the cell. We suggest that one mechanism by which ligand binding by these molecules can be 'switched' on and off in response to external signals is by regulation of Mg2+ binding to these molecules, converting from the cation-free, 'inactive' to the Mg2+-bound, 'active' form.

Surface Mo1 (CD11b/CD18) glycoprotein is up-modulated by neutrophils recruited to sites of inflammation in vivo

Inasmuch as the recruitment of polymorphonuclear leukocytes (PMNs) to inflammatory foci in vivo involves adhesion-dependent events (e.g., margination, diapedesis, and directed migration), we sought to characterize the relationship between the local accumulation of PMNs in sterile peritonitis and their surface expression of the adhesion-promoting plasma membrane glycoprotein, Mo1 (CD11b/CD18). In an immunofluorescence analysis of PMNs isolated from rats injected intraperitoneally with sterile 1% glycogen solution, we detected a significant enhancement of surface Mo1 expression by exudative peritoneal PMNs. In contrast, no significant rise in Mo1 expression was noted over time by circulating intravascular PMNs (isolated simultaneously). However, these intravascular PMNs had the capacity to increase their surface Mo1 density upon exposure to peritoneal fluid supernatant at 37 degrees C. These results demonstrate that PMNs at sites of inflammation in vivo do up-modulate their surface expression of the adhesion-promoting Mo1 glycoprotein during their recruitment from the circulating, intravascular leukocyte pool.

Macrophages: current views on their differentiation, structure, and function

Macrophages are large mononuclear phagocytes that represent the major differentiated elements of the mononuclear phagocytic system. They arise from distinct progenitors in the bone marrow, and their immediate precursors, the monocytes, emigrate from the vascular compartment into many tissues and organs where they develop into mature macrophages. The latter display diverse morphological and functional characteristics, depending on the environmental stimuli that they receive. This phenotypic heterogeneity is, therefore, the final consequence of a series of down-regulation of some cellular processes and the up-regulation of others. The kinetics of the production of macrophages and their participation in various physiological and pathological phenomena is the subject of this review.

The interaction of ethanol with human monocyte IgG-Fc receptors, characterized by monoclonal antibodies raised against two distinct receptor subpopulations

Human blood monocytes (Mo) in medium containing 10% autologous serum were exposed to ethanol (160 mM) at 37 degrees C for 15 min. After being washed, the cells were incubated with murine monoclonal antibodies (MoAb), one binding to the 40 kDa Fc receptor (FcR) (MoAb IV3) and the other to the 72 kDa FcR (MoAb 32). The incubation was performed with and without excess of human or rabbit IgG. The amount of receptor-bound MoAb was evaluated by fluorescein isothiocyanate (FITC)-labelled anti-mouse IgG. Most control Mo bound MoAb IV3 (90 +/- 8% stained cells) while only 52 +/- 2% Mo were positively stained by MoAb 32. The staining increased in the presence of human IgG in the assay mixtures. Pre-incubation with 160 mM ethanol reduced the MoAb IV3 binding (61 +/- 2% stained cells), but had no significant effect on the binding of MoAb 32. Wash-out experiments indicated normalization of receptor function (MoAb IV3 binding) after 4 h. Treatment of medium or serum with ethanol before incubation of the Mo had no effect. It was concluded that a brief exposure of human Mo to ethanol leads to changes in a subpopulation of IgG FcR. Since these changes appeared when MoAb were used against the receptors, they probably represent a decrease in functional receptors and not changes in affinity.

The Fc receptor, FcRI, and other activation molecules on human mononuclear phagocytes after treatment with interferon-gamma

Human monocytes and the myeloid cell lines U937 and HL60 have been tested with monoclonal antibodies (MoAbs) reactive with 22 different cell surface molecules before and after treatment with interferon-gamma (IFN-gamma). An increase in the expression of the high affinity Fc receptor, FcRI, and the receptor for interleukin 2, IL-2R, were the most consistent alterations which were observed. In addition, expression of the gp55 molecule recognized by CD14 MoAbs was decreased on monocytes. Of the MHC Class II molecules, there was little expression by the myeloid cell lines and no enhancement after IFN-gamma treatment. In contrast monocytes expressed all three MHC Class II subloci with DR much greater than DQ and DP. However there was much variation in IFN-gamma-mediated increase in expression of the individual subregions. In monocytes, the alteration in expression of FcRI, IL-2R, gp55 and MHC Class II molecules took place in a co-ordinate fashion and reached a plateau only after 48 h. In U937 cells, activation proceeded more rapidly and was at maximum levels between 12-16 h. This increase in FcRI appears to be a hallmark of IFN-gamma activation for mononuclear phagocytes (Mph) as the other alterations are either not found on all types of Mph (gp55, MHC Class II) or are induced by other cytokines on Mph and on other cells (IL-2R, MHC Class II). Conversely, other cytokines do not induce FcRI on Mph. These results also suggest that the cell membrane phenotypic changes induced in Mph by IFN-gamma may not be extensive and that FcRI must play a specific role in the IFN-gamma-activated Mph.

Properties of isolated red pulp macrophages from mouse spleen

Dense monolayers of large, adherent macrophages were prepared from the red pulp of mouse spleen. These sinus-lining phagocytes resembled liver Kupffer cells in morphology, as well as expression of F4/80 and class II MHC antigens and receptors for IgG. C3-coated red cells attached at low levels to spleen macrophages, but attachment and endocytosis were enhanced on fibronectin-coated surfaces. The ionophore A23187 induced spleen macrophages to synthesize prostaglandin E2, but like Kupffer cells, spleen macrophages did not synthesize leukotrienes and made relatively small amounts of HETE and 12-hydroxyheptadecanoic acid. Resident spleen macrophages did not produce H2O2, but splenic inflammatory cells, induced by infection of animals with Listeria monocytogenes, actively released H2O2. We conclude that the functional properties of resident, sinusoidal-lining macrophages in liver and spleen are similar to one another but distinct from other pools of phagocytes.

Up-regulation of the iC3b receptor (CR3) is neither necessary nor sufficient to promote neutrophil aggregation

The iC3b receptor (CR3) is required for neutrophil adhesive functions, including homotypic aggregation. Because stimuli that enhance neutrophil adhesion also induce up-regulation of surface CR3, it is widely held that these two responses are causally related. We have dissociated CR3 display (immunofluorescence) from CR3 function (aggregation). Neutrophils isolated at 4 degrees C and rewarmed to 37 degrees C up-regulated surface CR3 twofold, but did not aggregate. The kinetics of FMLP-induced CR3 up-regulation were discordant with those of aggregation. In the absence of extracellular divalent cations, CR3 expression increased twofold after exposure to FMLP, but neutrophils did not aggregate. FMLP elicited 3.5-fold more aggregation than the ionophore A23187, yet less than one-half as much CR3 up-regulation. 3 mM sodium salicylate inhibited aggregation 55 +/- 4%, but had no effect on CR3 up-regulation. Conversely, 1 mM tetracaine completely inhibited CR3 up-regulation, while significantly enhancing aggregation. Neutroplasts expressed CR3, but did not up-regulate the receptor; in contrast, FMLP induced CR3-dependent aggregation of neutroplasts. We conclude that, although constitutive surface CR3 is required for neutrophil aggregation, the up-regulation of CR3 is neither necessary nor sufficient to promote cell-cell adhesion.

Complement receptor type 3 (CR3) binds to an Arg-Gly-Asp-containing region of the major surface glycoprotein, gp63, of Leishmania promastigotes

The major surface glycoprotein of Leishmania promastigotes, gp63, was isolated and reconstituted into a lipid membrane immobilized on the surface of 5-micron-diameter silica beads. These beads bound to the macrophage (MO), and the extent of binding correlated with the density of gp63 on the bead. The bead thus facilitated analysis of the binding specificity of a single ligand, gp63, without contribution from other molecules present on the surface of intact promastigotes. Plating of MO onto substrates coated with antibodies directed against several cell surface receptors indicated that the complement receptor CR3 was necessary for binding gp63. CR3 recognizes a portion of C3 that contains the sequence R G D. Since gp63 also contains such a sequence, we tested the ability of a synthetic peptide based on the R G D-containing region of gp63 to inhibit the binding of gp63 beads. The R G D-containing peptide from gp63 inhibited the binding of both gp63 beads and EC3bi to MO. Similarly, peptides previously shown to inhibit the binding of C3bi also inhibited the attachment of gp63 beads. The synthetic peptide from the R G D region of gp63 also reduced the binding of intact promastigotes to MO. These results indicate that gp63 binds directly to CR3.

Activation of monocytes--more than one process. Differential effect of cytokines on monocytes

Monocytes and macrophages respond to a number of exogenous agents by alterations in metabolism and gene expression in a process loosely called 'activation'. The question arises whether these alterations in cellular activity are the pleiotropic effects of one programmed activation process or result from separately programmed activation pathways. We report that certain cytokines (interleukin 1 (IL-1 alpha, IL-1 beta) and interleukin 2 (IL-2] which all activate monocytes, induce the synthesis of thromboplastin (TPL) and (IL-1 beta only) factor VII. Interferon-gamma (IFN-gamma) and tumour necrosis factor alpha (TNF alpha) do not induce monocyte procoagulant activity, although they both activate monocytes in several respects. Our data thus show that different states of activation are induced. Similar observations have been made by others using different systems.

Quantitation of surface antigens on cultured murine epidermal Langerhans cells: rapid and selective increase in the level of surface MHC products

It was recently discovered that murine epidermal Langerhans cells (LC) changed significantly in function and phenotype when maintained in culture. Notably, accessory cell function for primary immune responses increased while cytologic markers like ATPase, nonspecific esterase, and Birbeck granules were lost. To further analyze LC differentiation, we used flow cytometry and a panel of 22 monoclonal antibodies to quantitate changes in surface antigens at the single-cell level. A striking change was a fivefold increase in the amount of Ia antigens (which are expressed on class II MHC products) during the first day of culture. The increase was evident within 3 h and reached a plateau at 15-24 h. Both I-A and I-E products behaved similarly. The increase in Ia was blocked by 1 microgram/ml cycloheximide. Expression of other surface antigens was then monitored on Ia+ LC by two-color flow cytometry. Low levels of class I (H-2D and H-2K) MHC products were detected on freshly isolated LC, and these antigens also increased severalfold during the first day of culture. Fc receptors (identified with the 2.4G2 mAb) and the F4/80 macrophage antigen decreased, as reported previously. Three antigens that were detected in fresh suspensions were expressed at constant levels in culture. These were the C3bi receptor and the pan leukocyte and interdigitating cell antigens. Several leukocyte antigens that were not found initially on LCs did not appear, including B220 anti-B cell, 33D1 anti-dendritic cell, and CD4, CD5, CD8 T-cell specificities. We conclude that the surface of cultured LCs undergoes selective changes in culture. As a result, the cells are rich in Ia and H-2 and have detectable C3bi receptors, but have little or no LFA-1, Ti, CD4, 5, and 8, 33D1, 2.4G2, F4/80, and B220 antigens.

Modulation of surface CD11/CD18 glycoproteins (Mo1, LFA-1, p150,95) by human mononuclear phagocytes

Mo1, LFA-1, and p150,95 are structurally related glycoproteins of the CD11/CD18 complex that are expressed on the membrane of human leukocytes. In the neutrophil, the surface expression of the CD11/CD18 complex is up-modulated (Mo1 greater than p150,95 much greater than LFA-1) by stimulatory factors that include calcium ionophore A23187, phorbol myristate acetate (PMA), and N-L-formyl-L-leucyl-L-phenylalanine (fMLP). Here, in an immunofluorescence analysis, we have examined CD11/CD18 glycoprotein expression by human monocytes, pulmonary alveolar macrophages (PAM, obtained by bronchoalveolar lavage), and breast milk macrophages (BMM) as compared to neutrophils before and after exposure to A23187 (1 microM), fMLP (0.1 microM), or PMA (0.1 microgram/ml) for 15 min at 37 degrees C. Unstimulated monocytes within unfractionated blood mononuclear cells kept at 4 degrees C (n = 13) expressed all three CD11/CD18 glycoproteins, and exposure to A23187 resulted in significant increases in the surface expression of Mo1 (median of 5.7-fold), LFA-1 (median of 2.1-fold), and p150,95 (median of 7.2-fold). Exposure to fMLP- or PMA-stimulated increases of lesser magnitude. CD11/CD18 expression by PAM (n = 9) was barely detectable and was unaffected by exposure to A23187. In contrast, BMM (n = 11) expressed all three CD11/CD18 glycoproteins (with considerable variability among specimens), but no increase was stimulated by A23187. These results demonstrate that monocytes, like neutrophils, have the capacity to respond to activating factors with an increase in CD11/CD18 glycoprotein expression; macrophage differentiation is accompanied by a loss (PAM) or retention (BMM) of CD11/CD18 expression that is unmodulated in response to activation.

CD16. Developmentally regulated IgG Fc receptors on cultured human monocytes

We have demonstrated that one Fc receptor for IgG (FcR) (CD16) on cultured human monocytes appears to be a developmentally regulated membrane protein. This receptor appears to contain less carbohydrate (if any) than does its counterpart on human neutrophils. Expression of CD16 on cultured monocytes increases with respect to both percentage of positive cells and numbers of sites per cell with length of time in culture. This was in contrast to expression of other types of FcRs that either decreased (CDw32) or did not change (FcRp72). Unlike an FcR that binds monomeric IgG (FcRp72), expression of CD16 on monocytes from most normal individuals was not influenced by IFN-gamma. After 14 d in culture, CD16 appeared to be the predominant FcR on cultured monocytes, and was capable of mediating both ligand attachment and phagocytosis. These findings support the hypothesis that CD16 plays an important role in mediating immunophagocytosis.

The binding of antigen presenting cells to T lymphocytes

In the introduction, we asked how MHC molecules on the surfaces of APC make contact with antigen-specific receptors on the surfaces of T cells. We have reviewed two models in which antigen-specific contacts occur with primary leukocyte populations in vitro. One system involves resting T cells; the other sensitized T lymphoblasts. At the onset of a primary immune response, dendritic cells seem critical for binding and activating both CD4+ and CD8+ subsets. Given the current evidence, we suggest that dendritic cells literally find the right T cell clone, and not vice versa, and that dendritic cells do so by first binding and surveying T cells by an antigen-independent mechanism. In the efferent limb of immunity, other types of APC including B cells and macrophages bind and stimulate freshly sensitized T lymphoblasts. Freshly isolated epidermal Langerhans cells do not cluster T cells by an antigen-independent mechanism but acquire this capacity during epidermal suspension culture. Under the control of GM-CSF, the Langerhans cell becomes a powerful accessory cell for the primary or sensitization limb of T-dependent immune responses like the MLR and primary antibody response. Isolated lymphoid dendritic cells have many features in common with interdigitating cells in lymphoid T areas, and may be related to some of the irregularly-shaped Ia+ cells in certain epithelia and interstitial regions. Contact with dendritic cells may be important in both central and peripheral pathways of T cell sensitization in situ.