Interferon gamma-treated human macrophages display enhanced cytolysis and generation of reactive oxygen metabolites but reduced ingestion upon Fc receptor triggering

Macrophage Polarization Modulates FcγR- and CD13-Mediated Phagocytosis and Reactive Oxygen Species Production, Independently of Receptor Membrane Expression

In response to microenvironmental cues, macrophages undergo a profound phenotypic transformation acquiring distinct activation phenotypes ranging from pro-inflammatory (M1) to anti-inflammatory (M2). To study how activation phenotype influences phagocytosis and production of reactive oxygen species (ROS) mediated by receptors for IgG antibodies (Fcγ receptors) and by CD13, human monocyte-derived macrophages were polarized to distinct phenotypes using IFN-γ (Mϕ-IFN-γ), IL-4 (Mϕ-IL-4), or IL-10 (Mϕ-IL-10). Phenotypically, Mϕ-IFN-γ were characterized as CD14⁺CD80⁺CD86⁺ cells, Mϕ-IL-4 as CD209^highCD206⁺CD11b⁺CD14^low, and Mϕ-IL-10 as CD16⁺CD163⁺ cells. Compared to non-polarized macrophages, FcγRI expression increased in Mϕ-IFN-γ and Mϕ-IL-10 and FcγRIII expression increased in Mϕ-IL-10. None of the polarizing cytokines modified FcγRII or CD13 expression. Functionally, we found that cytokine-mediated activation significantly and distinctively affected FcγR- and CD13-mediated phagocytosis and ROS generation. Compared to non-polarized macrophages, FcγRI-, FcγRII-, and CD13-mediated phagocytosis was significantly increased in Mϕ-IL-10 and decreased in Mϕ-IFN-γ, although both cytokines significantly upregulated FcγRI expression. IL-10 also increased phagocytosis of Escherichia coli, showing that the effect of IL-10 on macrophage phagocytosis is not specific for a particular receptor. Interestingly, Mϕ-IL-4, which showed poor FcγR- and CD13-mediated phagocytosis, showed very high phagocytosis of E. coli and zymosan. Coupled with phagocytosis, macrophages produce ROS that contribute to microbial killing. As expected, Mϕ-IFN-γ showed significant production of ROS after FcγRI-, FcγRII-, or CD13-mediated phagocytosis. Unexpectedly, we found that Mϕ-IL-10 can also produce ROS after simultaneous stimulation through several phagocytic receptors, as coaggregation of FcγRI/FcγRII/CD13 induced a belated but significant ROS production. Together, these results demonstrate that activation of macrophages by each cytokine distinctly modulates expression of phagocytic receptors, FcγR- and CD13-mediated phagocytosis, and ROS production.

Interferon gamma induces actin polymerization, Rac1 activation and down regulates phagocytosis in human monocytic cells

IFNγ is a potent activator and IL-10 a powerful inhibitor of macrophage functions. However, neither all cellular functions are enhanced by IFNγ nor IL-10 inhibits all cellular responses. Thus, FcγRs-mediated phagocytosis in monocyte-derived macrophages (MDM) increases after IL-10 treatment, and decreases after treatment with IFNγ, although both IL-10 and IFNγ up regulate FcγRI expression. In this work we investigated the effect of IFNγ and IL-10 on phagocytic signaling by FcγRs in MDM. Treatment with IFNγ diminished phagocytosis of IgG-opsonized SRBC (IgG-SRBC) while treatment with IL-10 increased it. These opposite effects cannot be attributed to changes in FcγR expression induced by each cytokine. Early biochemical responses mediated by FcγRs were distinctly affected by cytokine treatment. Syk phosphorylation and the rise in [Ca(2+)](i) were higher after IL-10 treatment, whereas IFNγ treatment also increased Syk phosphorylation but had no effect on the rise in [Ca(2+)](i). IFNγ treatment led to increased basal levels of F-actin and this effect correlated with the decrease in phagocytosis of both IgG-SRBC and non-opsonized Escherichia coli. IL-10 did not alter F-actin basal levels, and enhanced the phagocytosis of E. coli and IgG-SRBC. The level of F-actin reached after IFNγ treatment was not further increased after stimulation with IgG-SRBC or CCL5, whereas MDM treated with IL-10 showed a slightly higher response than control cells to CCL5. IFNγ increased Rac1-GTP levels. Inhibition of PI3K with LY294002 prevented IFNγ-mediated actin polymerization. Our data suggest that IFNγ induces a higher basal level of F-actin and activation of Rac1, affecting the response to stimuli that induce cytoskeleton rearrangement such as phagocytic or chemotactic stimuli.

Evidence for involvement of peptidoglycan in the triggering of an oxidative burst by Listeria monocytogenes in phagocytes

We have shown previously that in listeric encephalitis of cattle and rats, nitrotyrosine was produced in microabscesses, implying that both superoxide anion (O(2) (-)) and nitric oxide (NO) are present and react with each other. Evidence of local synthesis of NO by macrophages was provided, but the source of O(2) (-) remained unknown. Here we have examined whether phagocytes exposed to viable and heat-killed Listeria monocytogenes (LMDelta) produce O(2) (-) and, if so, whether this results from direct interaction of phagocytes with the bacterial surface of L. monocytogenes or whether prior opsonization is required. Using lucigenin-enhanced chemiluminescence (LCL) for the measurement of O(2) (-), we show that LMDelta induces an oxidative burst in human neutrophils, monocytes and monocyte-derived macrophages (Mphi). Viability is not required, and opsonization by antibodies and/or complement does not enhance the LCL signal. As Toll-like receptors (TLR) were shown recently to mediate an oxidative burst, TLR agonists representative for pathogen-associated molecular patterns (PAMPs) were tested for their ability to elicit an oxidative burst. These included lipoteichoic acid (LTA), bacterial peptidoglycan (PGN), recombinant flagellin, CpG-containing DNA and double-stranded RNA. Only PGN and flagellin consistently elicited an LCL signal resembling that induced by LMDelta with regard to the kinetics and cell spectrum stimulated. However, flagellin was unlikely to be responsible for the LMDelta-mediated burst, as a flagellin-deficient mutant showed no decrease in LCL. We therefore assume that in LMDelta, core PGN acts as a PAMP and directly induces an oxidative burst in all phagocyte populations. We conclude that in cerebral lesions superoxide anion is generated locally by phagocytes recognizing bacterial PGN.

Generation and functional characterisation of canine bone marrow-derived macrophages

A culture of bone marrow cells from the femurs of canine pups at high concentrations of fetal calf serum under non-adherent conditions allowed the proliferation and differentiation of mononuclear phagocyte lineage cells, as evidenced by morphology and CD14 expression. Cells from other lineages progressively diminished in numbers. Cells collected between 12 and 19 days of culture expressed an array of macrophage activities including ingestion of opsonised erythrocytes, generation of superoxide, up-regulation of procoagulant activity and synthesis of tumour necrosis factor (TNF) upon appropriate stimulation. TNF production was enhanced when the cultures were simultaneously stimulated with canine recombinant, or supernatant-derived, interferon-gamma. In contrast, low levels of inducible nitric oxide (NO) synthase were expressed by only a minority of stimulated macrophages, and nitrite could not be detected in the medium. Therefore, canine macrophages generated by this novel culture system resemble human macrophages in their inefficient and restricted generation of NO upon appropriate stimulation.

Inducible nitric oxide synthase of macrophages. Present knowledge and evidence for species-specific regulation

An important mechanism by which macrophages (M phi) halt the growth of and eliminate a broad array of intracellular pathogens is the production of nitric oxide (NO). NO generation is catalyzed by inducible nitric oxide synthase (iNOS) converting arginine into citrulline and NO. In murine M phi, iNOS activity is regulated largely at the transcriptional level. LPS and IFN-gamma induce iNOS, IL-4 and TGF-beta down-regulate LPS or IFN-gamma induced iNOS. In human M phi, iNOS cannot be induced by conventional activating regimes in vitro. We studied iNOS induction in ruminant monocytes and M phi from various sources (bone marrow, alveolar lavage, peripheral blood) and found that there is a species-specific and differentiation stage-dependent pattern of iNOS regulation in vitro. Notably, cattle M phi and monocytes respond to distinct signals by iNOS expression. Goat monocytes and M phi resemble human, pig and rabbit M phi in that upon treatment with conventional activating stimuli, they express less iNOS than unstimulated murine or bovine M phi and fail to generate detectable amounts of nitrite and nitrate.

Induction of nitric oxide synthase in bovine mononuclear phagocytes is differentiation stage-dependent

Bovine monocytes and monocyte-derived macrophages (MDM) activated by various means were assessed for induction of inducible nitric oxide synthase (iNOS), using the Griess assay, Northern blotting and reverse transcription/polymerase chain reaction (RT-PCR). Interferon-gamma (IFN-gamma) induced little, if any, iNOS expression and NO production in MDM, although these cells responded to IFN-gamma in other regards. In contrasts, MDM produced copious amounts of NO when stimulated with LPS or Salmonella dublin, and this was paralleled by high steady state levels of iNOS mRNA. Heat-killed Listeria monocytogenes induced more iNOS mRNA and nitrite than IFN-gamma, but much less than L. mono-cytogenes and IFN-gamma combined. Monocytes differed from M phi with respect to iNOS induction and nitrite production in several regards: (i) LPS and S. dublin induced only low levels of iNOS mRNA and nitrite in monocytes, although cells responded to these stimuli in various other ways: (ii) IFN-gamma alone induced in monocytes iNOS mRNA generation and NO formation, although to a low and variable degree; (iii) upon maximal stimulation (e.g. by L. monocytogenes and IFN-gamma combined), monocytes produced much less nitrite than MDM, and mRNA levels were lower. Regulation of macrophage iNOS varies considerably between species. We provide the first evidence in any species that the steady state levels of iNOS mRNA and NO generation in monocytes and macrophages activated by various means depend on the stage of mononuclear phagocyte differentiation.

Dexamethasone and methylprednisolone affect rat peritoneal phagocyte chemiluminescence after administration in vivo

Production of reactive oxygen compounds by peritoneal monocytes/macrophages was studied in rats exposed to dexamethasone or methylprednisolone in the drinking water. Luminol-amplified chemiluminescence was measured in preparations of peritoneal leukocytes activated ex vivo by serum opsonized zymosan, N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP) or phorbol 12-myristate 13-acetate (PMA). After dexamethasone administration for 1 day (approximately 0.13 mg/kg per 24 h) a significant reduction in chemiluminescence was found in cells stimulated with serum opsonized zymosan, while responses to fMLP and PMA stimulation were significantly reduced after 2 days. The maximal inhibition obtained after 5-8 days of dexamethasone administration (plasma levels < 5 nM) was 92.0 +/- 1.2%, 87.6 +/- 0.2% and 84.5 +/- 3.1% in cells stimulated with serum opsonized zymosan, fMLP and PMA, respectively. Administration of dexamethasone or methylprednisolone for 48 h gave a dose-dependent reduction of chemiluminescence. ED50 values of dexamethasone were estimated at 0.06-0.15 mg/kg for the different stimulators (plasma concentrations 5-10 nM). Estimated ED50 values for methylprednisolone were 35-36 mg/kg. Since the percentage of mononuclear phagocytes in the peritoneal cell population did not change significantly with dose or time of dexamethasone exposure, this study indicates that glucocorticoids have a depressive effect on the monocyte/macrophage 'respiratory burst' in vivo. The results are consistent with the hypothesis that these effects are mediated by glucocorticoid receptors. Although the pathway activated by serum opsonized zymosan was more rapidly inhibited than the fMLP- and PMA-activated pathways, the responses induced by the different stimulators were similarly affected, suggesting a modulation of common components in the activation pathways, possibly protein kinase C or the NADPH-oxidase complex, after administration of low pharmacological doses of glucocorticoids in vivo.

Regulation of microglial function by interferons

Cultured neonatal rat microglia were pretreated with varying doses of either purified interferon (IFN) alpha/beta or recombinant IFN gamma for 24 or 48 h and the following functional parameters examined; superoxide anion production, interleukin-1 secretion and chemotaxis. IFN gamma produced a marked increase in superoxide anion levels when PMA was used to initiate superoxide anion production but had no effect in OPZ-stimulated microglia. Treatment with IFN alpha/beta potentiated superoxide anion production in both PMA and OPZ-stimulated cells. Interleukin-1 activity was decreased by treatment with IFN gamma for 24 h while IFN alpha/beta increased IL-1 activity at 48 h. Removal of serum from the treatment media prevented the action of IFN alpha/beta on IL-1 production. Treatment with IFN alpha/beta or gamma decreased chemotaxis of microglia in response to zymosan activated serum. The data indicate that IFN gamma and alpha/beta can regulate microglial function and that this regulation may differ from that seen for other monocytically derived macrophages.

Generation and functional characterization of ovine bone marrow-derived macrophages

A method for the culturing and propagation of ovine bone marrow-derived macrophages (BMM) in vitro is described. Bone marrow cells from sterna of freshly slaughtered sheep were cultured in hydrophobic (teflon foil) bags in the presence of high serum concentrations (20% autologous serum and 20% fetal calf serum). During an 18 day culture period in the absence of added conditioned medium, and without medium change, a strong enrichment of mononuclear phagocytes was achieved. Whereas the number of macrophages increased four to fivefold during this time, granulocytes, lymphoid cells, stem cells and undifferentiated progenitor cells were reduced to less than 3% of their numbers at Day 0. This resulted in BMM populations of 94 +/- 3% purity. These cells had morphological and histochemical characteristics of differentiated macrophages, and they performed functions similar to those of non-activated, unprimed human monocyte-derived macrophages. Thus, they avidly ingested erythrocytes coated with IgG of heterologous or homologous origin. They expressed a modest level of procoagulant activity, but upon triggering with lipopolysaccharide (LPS), a marked increase in cell-associated procoagulant activity was observed. LPS triggering promoted the secretion of interleukin-1, as evidenced by measurement of murine thymocyte costimulatory activity, and transforming growth factor-beta. Using the mouse L929 cell cytotoxicity assay as an indication of tumor necrosis factor (TNF) activity, no TNF activity was detected in the same supernatants, a result possibly due to species restriction. BMM generated low levels of O2- upon triggering with phorbol 12-myristate 13-acetate (PMA). On the other hand, no O2- production was observed upon stimulation with zymosan opsonized with ovine or human serum. Using luminol-enhanced chemiluminescence (CL) as a more sensitive indicator of an oxidative burst, both PMA or zymosan were able to trigger CL, but the response was subject to partial inhibition by sodium azide, an inhibitor of myeloperoxidase. This points to non-macrophage cells contributing also to the CL response, and is consistent with the view that unprimed BMM elicit a low oxidative burst upon triggering with strong inducers of a burst. Our functional characterization now allows us to apply priming and activation protocols and to relate their effect to functional alterations.

LPS-induced, but not interferon-gamma-induced procoagulant activity of suspended human macrophages is followed by a refractory state of low procoagulant expression

Monocyte-derived macrophages cultured under a variety of conditions were assessed for expression of procoagulant activity (PCA) upon induction by various triggers, using a semiautomated turbidimetric recalcification time assay in a kinetic ELISA reader. Macrophages cultured in a nonadherent (teflon) culture system and seeded in microtiter plates responded with PCA expression to lipopolysaccharide (LPS), to toxic shock-syndrom toxin-1 (TSST-1) and to surface-bound IgG, but not to surface-bound albumin, nor to interferon-gamma (IFN-gamma). In contrast, macrophages stimulated in teflon containers by IFN-gamma showed a strong PCA response peaking around 24 hr after stimulation, but they failed to secrete tumour necrosis factor (TNF). Suspended IFN-gamma-stimulated cells showed a similar response upon 2nd stimulation by LPS or IgG after adherence to microtiter plates as did nonprimed counterparts. In contrast, cells primed in suspension, then cultured in adherence secreted dramatically enhanced amounts of TNF when compared with nonprimed cells. Macrophages stimulated in suspension with LPS showed a PCA response of similar magnitude, which was accompanied by TNF secretion. PCA of both IFN-gamma-primed and LPS-exposed suspension culture cells was largely due to the surface expression of tissue factor, and to a lesser extent of a prothrombinase-like activity, as evidenced by PCA testing with factor-X-deficient plasma. The kinetics of LPS-induced PCA differed from IFN-gamma-induced PCA, in that PCA peaked at 6 hr and fell to insignificant levels after 24 hr. When transferred to microtiter plates at this time, they could be restimulated neither with LPS, nor with surface-adherent IgG nor with IFN-gamma. Evidence was obtained that the failure to express PCA was due to a refractory state of the cells rather than to the generation of cell-bound or secreted inhibitors of coagulation. The loss of PCA expression could be prevented by pre-exposure to IFN-gamma. Thus, PCA expression may be dissociated from other functional and/or activation parameters (e.g. TNF secretion). For the first time, a state in which cells are completely unresponsive to PCA induction has been identified. Should lower LPS concentrations also be found to induce such a refractory state, our results may be of pathophysiological significance.

Cellular cytotoxicity mediated by isotype-switch variants of a monoclonal antibody to human neuroblastoma

The biological property of an antibody is determined by its antigen binding characteristics and its isotype-related effector functions. We have established monoclonal antibodies of different isotypes by stepwise selection and cloning of the hybridoma CE7. The original CE7 secretes an IgG1/kappa (CE7 gamma 1) antibody that recognises a 185 kD cell surface glycoprotein expressed on all human sympatho-adrenomedullary cells. Isotype-switch variants were isolated in the following sequence: from the original CE7 gamma 1, CE7 gamma 2b variants were isolated, and from a CE7 gamma 2b variant CE7 gamma 2a variants were isolated. The antibodies of three different isotype variant cell lines possess identical antigen binding characteristics, but display distinct effector functions as demonstrated by antibody dependent cell-mediated cytotoxicity (ADCC). ADCC was performed with the neuroblastoma line IMR-32 as the target cells, and different FcR gamma positive cells were either freshly isolated from human peripheral blood leukocytes or cultured for 6-10 days and tested as potential effector cells. Tumour lysis mediated by monocyte-derived macrophages depended on the presence of CE7 gamma 2a antibodies; antibodies from the CE7 hybridomas of gamma 2b and gamma 1 isotypes were virtually inactive in ADCC assay. Pre-exposure of macrophages to rIFN-gamma enhanced their ADCC activity, a result that is compatible with the notion that the high affinity Fc IgG receptor (FcR gamma I/CD64) is involved in the triggering of ADCC in macrophages. In contrast to macrophages, mononuclear cells, nonadherent cells and monocytes displayed considerable non-specific lytic activity, which was little influenced by the presence of antibody regardless of the isotype added.

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.

Evidence that exposure to fibrinogen or to antibodies directed against Mac-1 (CD11b/CD18; CR3) modulates human monocyte effector functions

We have recently shown that the treatment of fibrinogen-coated monocytes (MO) with anti-fibrinogen as well as the exposure of MO to surface-bound fibrinogen (Fg) or to albumin haptenized with the Fg C-gamma-terminal pentadecapeptide, induces on oxidative burst. Using chemiluminescence (CL) for indicating an oxidative burst, and the ingestion of IgG-coated erythrocytes as a test of phagocytosis, we have now studied the impact of Fg on MO effector functions. MO that had been either pretreated with Fg and washed, or that were exposed to surface-adsorbed Fg, exhibited impaired Fc receptor-mediated phagocytosis. A similar impairment was observed when MO were pretreated with activating agents such as phorbol myristate acetate, n-formyl-methionyl-leucyl-phenylalanine (fMLP) or the calcium ionophore A23187. Moreover, following exposure to Fg or IgG, MO exhibited a reduced oxidative burst upon stimulation with a variety of agents. Similarly, MO pretreated or coincubated with anti-Mac-1 exhibited a reduced oxidative burst upon stimulation. Our results raise the possibility that inflammatory mononuclear phagocytes experience a functional modulation upon encountering fibrin by interacting with specific receptors for fibrin(ogen). This type of modulation is analogous to effects induced by the triggering of Fc gamma receptors. MO showed a decreased oxidative burst when either pretreated or coincubated with anti-Mac-1 antibodies, whereas antibodies directed against other MO surface constituents had no, or a weak effect only. This is compatible with the suggestion that Mac-1 acts as a fibrin (ogen) receptor.

Effect of IgG for intravenous use on Fc receptor-mediated phagocytosis by human monocytes

Polyspecific IgG given intravenously at high doses (IVIG) is used for immunomodulatory therapy in autoimmune diseases such as idiopathic thrombocytopenic purpura and myasthenia gravis. It is assumed that the clinical effect is brought about in part by a modulation of mononuclear phagocyte function, in particular by an inhibition of Fc receptor (FcR) mediated phagocytosis. In the present study, the effect of IVIG on FcR-mediated phagocytosis by monocytes was analysed in vitro. Since monocytes exposed to minute amounts of surface-bound IgG displayed impaired phagocytosis of IgG-coated erythrocytes (EA), the effect of IVIG was studied with mononuclear cells suspended in teflon bags in medium containing 10% autologous serum and IVIG (2-10 mg/ml). Monocytes pre-exposed to IVIG and then washed, displayed impaired ingestion of EA when compared with control cells cultured in 10% autologous serum only. The decrease in phagocytosis was observed with sheep erythrocytes treated with either rabbit IgG or bovine IgG1 and with anti-D-treated human erythrocytes. This suggests that phagocytosis via both FcR type I (FcRI) and type II (FcRII) was decreased. The impairment of phagocytosis was dependent on the presence of intact IgG and was mediated by IVIG from nulliparous donors and from multigravidae to the same extent, suggesting that alloantibodies contained in IVIG have a minor role in modulating FcR-mediated phagocytosis by monocytes. A flow cytometric analysis using anti-FcRI, FcRII and FcRII monoclonal antibodies showed that IVIG treatment upregulated FcRI expression but did not significantly alter the expression of FcRII and FcRIII.