The P2X7 receptor is a key modulator of the PI3K/GSK3ß/VEGF signaling network: evidence in experimental neuroblastoma.

Neuroblastoma (NB) is an aggressive pediatric tumor, responsible for 15% of cancer-related deaths in childhood, lacking an effective treatment in its advanced stages. The P2X7 receptor for extracellular ATP was associated to NB cell proliferation and recently emerged as a promoter of tumor engraftment, growth and vascularization. In an effort to identify new therapeutic options for neuroblastoma, we studied the role of P2X7 receptor in NB biology. We first analyzed the effect of P2X7 activation or down-modulation of the main biochemical ways involved in NB progression: the PI3K/Akt/GSK3ß/MYCN and the HIF1α/VEGF pathways. In ACN human NB cells, P2X7 stimulation enhanced PI3K/Akt, while decreasing GSK3ß activity. In the same model, P2X7 silencing or antagonist administration reduced the activity of PI3K/Akt and increased that of GSK3ß, leading to a decrease in cellular glycogen stores. Similarly, P2X7 downmodulation caused a reduction in HIF1α levels and vascular endothelial growth factor (VEGF) secretion. Systemic administration of two different P2X7 antagonists (AZ10606120 or A740003) in nude/nude mice reduced ACN-derived tumor growth. An even stronger effect of P2X7 blockade was obtained in a syngeneic immune-competent neuroblastoma model: Neuro2A cells injected in AlbinoJ mice. Together with tumor regression, treatment with P2X7 antagonists caused downmodulation of the Akt/HIF1α axis, leading to reduced VEGF content and decreased vessel formation. Interestingly, in both experimental models, P2X7 antagonists strongly reduced the expression of the probably best-accepted oncogene in NB: MYCN. Finally, we associated P2X7 overexpression with poor prognosis in advanced-stage NB patients. Taken together, our data suggest that P2X7 receptor is an upstream regulator of the main signaling pathways involved in NB growth, metabolic activity and angiogenesis, and a promising therapeutic target for neuroblastoma treatment.

Induction of macrophage glutamine: fructose-6-phosphate amidotransferase expression by hypoxia and by picolinic acid.

We studied the expression of glutamine: fructose-6-phosphate amidotransferase (GFAT), the rate limiting enzyme in the hexosamine biosynthetic pathway controlling protein glycosylation. We obtained the first evidence that the GFAT mRNA and protein are constitutively expressed in murine mononuclear phagocytes (Mf) and inducible by picolinic acid (PA), a catabolite of tryptophan, hypoxia and desferrioxamine (DFX). These stimuli share the property to transactivate gene expression through the Hypoxia Responsive Element (HRE). The promoter of GFAT contains the consensus sequence of HRE in position 74/-65 (GFAT-HRE), and we studied the role of HRE on the activation of the promoter utilizing appropriate expression vectors. We found that GFAT-HRE is essential for the response to hypoxia, PA or DFX and that Hypoxia Inducible Factor-1alpha (HIF-1alpha) can augment this response. Finally, we demonstrate that iron chelation is part of the mechanism by which PA and DFX activate GFAT expression. Our results provide the first indication that hypoxia, PA or DFX induce the transcription of GFAT gene in murine Mf cell lines and that the HRE of the promoter is essential for this response.

Newborn liver gene transfer by an HIV-2-based lentiviral vector.

Newborn gene therapy, because it can prevent the damage caused by the onset of a disease, deserves specific attention. To evaluate gene transfer in tissues of newborn mice, we used a human immunodeficiency virus (HIV)-2 based lentiviral vector pseudotyped with vesicular stomatitis virus G glycoprotein expressing the green fluorescent protein reporter gene under the control of the cytomegalovirus promoter. We found that very low doses of HIV-2 could infect and be expressed in newborn mice. Under these conditions, the virus was preferentially expressed in the liver and hepatocytes were the predominant target. The treatment was not toxic, the infected liver cells proliferated and the transduced gene was stably expressed. Adult mice could be infected by HIV-2, but the vector was detected in the liver only utilizing the sensitive method of polymerase chain reaction coupled with Southern blot. Direct comparison between newborn and adult recipients demonstrated a much greater efficiency of liver transduction in the newborn mouse. These results indicate that the combination of early intervention and low multiplicity of infection may be a strategy for preferentially and efficiently targeting newborn liver for gene therapy applications.

Picolinic acid- or desferrioxamine-inducible autocrine activation of macrophages engineered to produce IFNgamma: an approach for gene therapy.

Macrophage (Mphi)-based vectors are highly mobile cellular shuttles designed to deliver therapeutic genes within the tissues. We engineered a mouse Mphi cell line to express the murine interferon-gamma (IFNgamma) under the control of an inducible promoter containing the hypoxia-responsive element, which can be triggered by hypoxia and other stimuli. We show that this Mphi vector can be induced to produce IFNgamma under normoxic conditions by stimulation with picolinic acid (PA), a catabolite of tryptophan, or desferrioxamine (DFX), an iron-chelating drug. The Mphi vector responds to IFNgamma with the induction of IRF-1 and of other IFNgamma-inducible genes, the expression of Ia antigens and induction of phagocytic activity. Inducible nitric oxygen synthase gene expression, nitric oxide production, as well as TNFalpha secretion were enhanced by PA or DFX as the sole stimuli. None of the above responses could be triggered individually by PA or DFX in control, normal Mphi, indicating that the Mphi vector overcame the need for costimulatory molecules derived from the immune system for its full activation. Furthermore, we demonstrate that extracellular iron can downregulate such response, thereby identifying an additional tool for the fine tuning of the Mphi vector response to stimulation.

Regulation of taurine transport in murine macrophages.

We studied the regulation of taurine transport in ANA1 murine macrophage cell line. Taurine uptake was upregulated by hypertonicity and downregulated by bacterial lypopolysaccharide (LPS) and other stimuli leading to macrophage activation. However combined stimulation with LPS plus hypertonic shock evoked an increase of taurine uptake that was even higher than with hypertonic shock alone. Taurine transport was not modified by LPS in GG2EE macrophages derived from C3H/Hej mouse strain, which harbour a mutated Toll-like receptor 4 (TLR4) and thus are not activated by LPS. The extracellular signal-regulated kinase (ERK) inhibitor PD98059 abrogates the effect of both LPS and hyperosmotic shock on ANA1 taurine uptake, while the p38 inhibitor SB203580 reduces the taurine uptake in control conditions and impairs only the response to hypertonicity. These results suggest that the effect of LPS on taurine transport depends on ERK pathway and can be influenced by environmental conditions.

Generation of high-titer retroviral vector-producing macrophages as vehicles for in vivo gene transfer.

The goal of this project was to develop a novel gene transfer system based on macrophages (Mphi) as shuttles of recombinant retroviral vectors carrying therapeutic or marker genes. The murine Mphi cell line WGL5 was used as a source of Mphi for this study. We generated retrovirus-producing Mphi by transducing the WGL5 cells with a replication-defective retroviral vector carrying the enhanced green fluorescent protein (EGFP) reporter gene and the Moloney murine leukemia virus (MoMLV) as helper virus. We demonstrated stable integration of the recombinant retrovirus in the Mphi genome, efficient recombinant retrovirus production, and EGFP gene delivery to different cell lines in vitro. To evaluate Mphi-mediated EGFP gene transfer in vivo, allogeneic mice were injected s.c. with the retrovirus-producing WGL5 Mphi, that gave rise to solid tumor masses at the injection site, highly infiltrated with host leukocytes. We observed EGFP fluorescence in tumor-infiltrating CD4(+) and CD8(+) host T lymphocytes, providing direct evidence of the ability of engineered Mphi to mediate EGFP gene delivery to host cells in vivo. Moreover, we showed that retrovirus-producing Mphi could home to different organs in vivo following i.v. injection into mice. These data demonstrate that Mphi can be engineered as cellular vehicles for recombinant retroviruses carrying heterologous genes and suggest potential applications of this novel vector system for gene therapy.

Engineering of macrophages to produce IFN-gamma in response to hypoxia.

Activation of murine macrophages (Mphi) requires the collaboration of signals derived from the immune system and the environment. In this study, we engineered a murine Mphi cell line to become activated in response to an environmental signal, hypoxia, as the sole stimulus. Hypoxia is a condition of low oxygen tension, occurring in several pathological tissues, which acts in synergy with IFN-gamma to induce full Mphi activation. We transfected the ANA-1 murine Mphi cell line with a construct containing the IFN-gamma gene controlled by a synthetic promoter inducible by hypoxia (HRE3x-Tk), and we characterized the cellular and molecular biology of the engineered Mphi under normoxia or hypoxia. Engineered Mphi in normoxia expressed basal levels of IFN-gamma mRNA and protein that were strongly augmented by shifting the cells to hypoxia. Furthermore, they responded to the synthesized IFN-gamma with induction of IFN-responsive factor-1 and 2'-5'-oligoadenylate synthase expression. Under normoxic conditions, the engineered Mphi had a significant constitutive level of Ia Ags and Fc receptors. Hypoxia induced further augmentation of Ia and Fc expression. Finally, hypoxia induced inducible NO synthase expression, and subsequent reoxygenation led to the production of NO. In conclusion, the engineered Mphi, which produce IFN-gamma in an inducible manner, express new biochemical and functional properties in response to low oxygen environment as the sole stimulus, thereby circumventing the need for costimulation by other immune system-derived signals.

New high-performance liquid chromatographic method for the detection of picolinic acid in biological fluids.

A HPLC method is described to quantify picolinic acid in milk, blood serum and tissue culture supernatant. The method requires very little sample preparation because acid precipitation allows total recovery of picolinic acid. High specificity and sensitivity were obtained using ion-pair chromatography on a C18 reversed-phase column with tetrabutylammonium hydrogen sulfate as ion pairing reagent. We describe the conditions for the automated testing of multiple samples and for the detection of L-tryptophan and L-kynurenine together with picolinic acid. This system will be utilized to elucidate the relationship between picolinic acid production and human disease. Furthermore, we provide the first evidence of picolinic acid in human blood serum.

The tryptophan catabolite picolinic acid selectively induces the chemokines macrophage inflammatory protein-1 alpha and -1 beta in macrophages.

We previously found that the tryptophan catabolite picolinic acid (PA) is a costimulus for the activation of macrophage effector functions. In this study, we have investigated the ability of PA to modulate the expression of chemokines in macrophages. We demonstrate that PA is a potent activator of the inflammatory chemokines MIP (macrophage inflammatory protein)-1 alpha and MIP-1 beta (MIPs) mRNA expression in mouse macrophages in a dose- and time-dependent fashion and through a de novo protein synthesis-dependent process. The induction by PA occurred within 3 h of treatment and reached a peak in 12 h. The stimulatory effects of PA were selective for MIPs because other chemokines, including monocyte chemoattractant protein-1, RANTES, IFN-gamma-inducible protein-10, MIP-2, and macrophage-derived chemokine, were not induced under the same experimental conditions and were not an epiphenomenon of macrophage activation because IFN-gamma did not affect MIPs expression. Induction of both MIP-1 alpha and MIP-1 beta by PA was associated with transcriptional activation and mRNA stabilization, suggesting a dual molecular mechanism of control. Iron chelation could be involved in MIPs induction by PA because iron sulfate inhibited the process and the iron-chelating agent, desferrioxamine, induced MIPs expression. We propose the existence of a new pathway leading to inflammation initiated by tryptophan catabolism that can communicate with the immune system through the production of PA, followed by secretion of chemokines by macrophages. These results establish the importance of PA as an activator of macrophage proinflammatory functions, providing the first evidence that this molecule can be biologically active without the need for a costimulatory agent.

Flavopiridol, a protein kinase inhibitor, down-regulates hypoxic induction of vascular endothelial growth factor expression in human monocytes.

We have investigated the effects of flavopiridol, a novel protein kinase inhibitor that is selective for cyclin-dependent kinases, on hypoxia-induced vascular endothelial growth factor (VEGF) expression in human monocytes. We found that hypoxia induces a time-dependent increase of VEGF mRNA expression and protein levels in human monocytes. Flavopiridol showed a minimal effect on the constitutive levels of VEGF mRNA but completely blocked hypoxia-induced VEGF mRNA and protein expression. The inhibitory effects of flavopiridol on VEGF mRNA induction also occurred in the presence of cycloheximide. The transcriptional activation of either a VEGF promoter-luciferase construct or a hypoxia-inducible factor 1 reporter plasmid was not affected by addition of flavopiridol in transient transfection experiments. In contrast, actinomycin D experiments demonstrated that flavopiridol dramatically decreased VEGF mRNA stability. These data provide the first evidence that flavopiridol can affect gene expression by altering mRNA stability. We propose that flavopiridol may interfere with one or more signaling events, leading to hypoxia-induced, protein kinase-modulated, RNA protein binding activity. An important clinical implication of our results is that flavopiridol, presently under investigation in clinical trials, might have antiangiogenic as well as direct antiproliferative effects.

An electrogenic amino acid transporter in the apical membrane of cultured human bronchial epithelial cells.

We performed Ussing chamber experiments on cultured human bronchial epithelial cells to look for the presence of electrogenic dibasic amino acid transport. Apical but not basolateral L-arginine (10-1, 000 microM) increased the short-circuit current. Maximal effect and EC50 were approximately 3.5 microA/cm2 and 80 microM, respectively, in cells from normal subjects and cystic fibrosis patients. The involvement of nitric oxide was ruled out because a nitric oxide synthase inhibitor (NG-nitro-L-arginine methyl ester) did not decrease the arginine-dependent current. Apical L-lysine, L-alanine, and L-proline, but not aspartic acid, were also effective in increasing the short-circuit current, with EC50 values ranging from 26 to 971 microM. Experiments performed with radiolabeled arginine demonstrated the presence of an Na+-dependent concentrative transporter on the apical membrane of bronchial cells. This transporter could be important in vivo to maintain a low amino acid concentration in the fluid covering the airway surface.

Functional role for the myeloid differentiation antigen CD14 in the activation of human monocytes by IL-2.

Human monocytes express functional IL-2Rs and are directly activated by IL-2 to exert effector and secretory functions. In this study, we demonstrate that the myeloid differentiation Ag CD14 participates in monocyte activation by IL-2. Engagement of CD14 by specific mAbs resulted in the selective and dose-dependent suppression of IL-2-induced, but not of IFN-gamma-induced, monocyte tumoricidal activity. Furthermore, anti-CD14 mAbs effectively inhibited the secretion of IL-8 and IL-1beta in response to IL-2. Preincubation of monocytes with mAbs directed to selected epitopes on CD14 blocked the binding of IL-2 to the cell surface, providing a possible explanation for the inhibition of IL-2-triggered responses. A critical role for CD14 in IL-2-mediated monocyte activation was further demonstrated by experiments with the human U937 promonocytic cell line. These cells are negative for CD14 and unresponsive to IL-2 despite the expression of the beta and gamma subunits of the IL-2R. U937 cells acquired the capacity to respond to IL-2 following transfection with the human CD14 cDNA (U937/CD14). Stimulation of U937/CD14 cells with IL-2 up-regulated the constitutive levels of IL-8 mRNA, whereas no change in IL-8 mRNA basal expression was observed in control cells transfected with the vector alone (U937/Neo). Accordingly, increased secretion of IL-8 by U937/CD14, but not by U937/Neo cells, was detected following exposure to IL-2. Expression of IL-1beta was also augmented by IL-2 in U937/CD14 cells. These data provide the first evidence that CD14 expression is required for the response of monocytic cells to IL-2.

Functional requirement of the hypoxia-responsive element in the activation of the inducible nitric oxide synthase promoter by the iron chelator desferrioxamine.

We have previously reported that a 19-base pair element of the 5'-flanking region of the inducible nitric oxide synthase (iNOS) gene containing a sequence homology to a hypoxia-responsive enhancer (iNOS-HRE) mediates picolinic acid (PA)- or hypoxia-induced activation of the iNOS promoter in interferon-gamma (IFN-gamma)-treated murine macrophages. The iron chelator desferrioxamine (DFX) induces the activity of the human erythropoietin enhancer in Hep3B cells. We have investigated the influence of DFX on the activation of the iNOS promoter and iNOS gene expression in ANA-1 macrophages. We have found that DFX induced DNA-binding activity to the hypoxia-inducible factor 1 (HIF-1) consensus sequence of the iNOS promoter and activated the iNOS-HRE in murine macrophages. These activities of DFX were associated with a synergistic induction of iNOS mRNA expression and iNOS transcription in IFN-gamma-treated ANA-1 macrophages. Functional analysis of the 5'-flanking region of the iNOS gene demonstrated that IFN-gamma plus DFX activated the full-length iNOS promoter and that the iNOS-HRE was required for DFX-induced iNOS transcriptional activity. We also investigated the role of iron metabolism in the DFX- or PA-dependent induction of HIF-1 activity and iNOS expression. We demonstrate that addition of iron sulfate completely abolished DFX or PA induction of HIF-1 binding and iNOS-HRE activation and abrogated IFN-gamma plus either DFX- or PA-induced iNOS expression. These data establish that DFX is a co-stimulus for the transcriptional activation of the iNOS gene in IFN-gamma-treated macrophages, and they provide evidence that the iNOS-HRE is required for the DFX-dependent activation of the iNOS promoter. Furthermore, our results indicate that the iNOS-HRE is a regulatory element of the iNOS promoter responsive to iron chelation.

The Csk-like proteins Lsk, Hyl, and Matk represent the same Csk homologous kinase (Chk) and are regulated by stem cell factor in the megakaryoblastic cell line MO7e.

Recently, the cDNAs for Lsk, Matk and Hyl, three Csk-related protein tyrosine kinases, have been cloned. We have examined the relationship of Lsk, Matk and Hyl, and found that the gene for each of these proteins is localized to the same region of human chromosome 19. Further, the proteins encoded by Lsk and Matk cDNAs are immunologically similar. These data strongly suggest that Lsk, Hyl and Matk are the same gene product. Previous reports demonstrating expression of Hyl and Matk in hematopoietic lineages led us to investigate the regulation of Lsk expression in response to stem cell factor (SCF) and granulocyte-macrophage colony stimulating factor (GM-CSF) in M07e, a human leukemic cell line. Induction of Lsk/Hyl/Matk protein and mRNA was observed after treatment with SCF but not with GM-CSF. GM-CSF and IL-3, potent mitogens, had no effect on Lsk/Hyl/Matk expression. In contrast, PMA induced Lsk/Hyl/Matk but did not stimulate proliferation. Therefore, induction of Lsk/ Hyl/Matk does not correlate with the capacity to stimulate proliferation. None of the stimuli examined increased Csk protein or mRNA expression. These data demonstrate differential regulation of Csk family members by cytokines and suggest a role for Lsk/ Hyl/Matk in responses mediated by SCF and PMA. Further, our data demonstrate that, as has been seen in blood monocytes, cytokine driven translational control of Lsk/Hyl/ Matk is likely a critical mode of regulation. Lastly, since our studies strongly suggest that the Lsk, Hyl and Matk kinases are related and regulated distinctly from Csk, we and several of the original authors have agreed to rename this kinase the Csk homologous kinase (Chk).

Nuclear factor kB is activated by arachidonic acid but not by eicosapentaenoic acid.

The omega-6 arachidonic acid supplementation of the human promonocytic cell line U937 strongly stimulates the nuclear translocation of the transcription factor NF-kB. Inhibitors of arachidonate oxidative metabolism prevent NF-kB activation, indirectly indicating a role for prostaglandin and leukotriene metabolites in the genesis of this phenomenon. Of note, omega-3 eicosapentaenoic acid does not exert any effect on NF-kB DNA binding. In subsequent experiments, prostaglandin E2 consistently showed the ability to activate NF-kB in U937 promonocytic cells, as well as in J774 macrophages. NF-kB activation by arachidonate, together with the lack of effect by eicosapentaenoic acid, suggests a way to modulate the expression of certain genes by means of a suitable dietary n-6/n-3 fatty acid ratio.

Regulation of inducible nitric oxide synthase expression in IFN-gamma-treated murine macrophages cultured under hypoxic conditions.

We recently reported that a hypoxia-responsive element mediates a novel pathway of transcriptional activation of the inducible nitric oxide synthase (iNOS) promoter in murine macrophages treated with IFN-gamma plus hypoxia (1% O2). In this study, we investigated the expression of NOS activity and the regulation of NOS induction in IFN-gamma treated ANA-1 murine macrophages or thioglycollate-elicited peritoneal macrophages cultured under hypoxic conditions. We found that murine macrophages stimulated with IFN-gamma plus hypoxia, despite a significant accumulation of iNOS mRNA, did not release nitrite into culture supernatant. However, cytosol from macrophages treated with IFN-gamma plus hypoxia contained significant levels of iNOS protein and enzymatic activity. Experiments in which cells were treated with IFN-gamma plus hypoxia and then cultured in normoxic conditions (20% O2) demonstrated that reoxygenation was required to achieve detectable accumulation of nitrite in the culture supernatant. Furthermore, we demonstrated that IL-4 inhibited IFN-gamma plus hypoxia-dependent induction of iNOS mRNA expression, iNOS protein, and enzymatic activity. Experiments in which ANA-1 macrophages were transfected transiently with the full-length iNOS promoter linked to a chloramphenicol acetyltransferase reporter gene demonstrated that IL-4 also down-regulated the IFN-gamma plus hypoxia-induced activation of the iNOS promoter. These data establish that hypoxia is a costimulus with IFN-gamma for the induction of iNOS activity in ANA-1 macrophages as well as in murine peritoneal macrophages, and they provide the first evidence that IL-4 inhibits hypoxia-inducible gene expression. In addition, our results suggest that hypoxia, which occurs in many pathologic conditions, may play an important role in the activation of murine macrophages.

A hypoxia-responsive element mediates a novel pathway of activation of the inducible nitric oxide synthase promoter.

Picolinic acid, a catabolite of L-tryptophan, activates the transcription of the inducible nitric oxide synthase gene (iNOS) in IFN-gamma-treated murine macrophages. We performed functional studies on the 5' flanking region of the iNOS gene linked to a CAT reporter gene to identify the cis-acting element(s) responsible for the activation of iNOS transcription by picolinic acid. Transient transfection assays showed that the full-length iNOS promoter in the murine macrophage cell line ANA-1 was activated by the synergistic interaction between IFN-gamma and picolinic acid. Deletion or mutation of the iNOS promoter region from -227 to -209, containing a sequence homology to a hypoxia-responsive enhancer (iNOS-HRE), decreased picolinic acid- but not LPS-induced CAT activity by more than 70%. Functional studies using a tk promoter-CAT reporter gene plasmid demonstrated that the iNOS-HRE was sufficient to confer inducibility by picolinic acid but not by IFN-gamma or LPS. Electrophoretic mobility shift assays confirmed that picolinic acid alone induced a specific binding activity to the iNOS-HRE. Furthermore, we found that the iNOS-HRE activity was inducible by hypoxia and that hypoxia in combination with IFN-gamma activated the iNOS promoter in transient transfection assays and induced iNOS transcription and mRNA expression. These data establish that the iNOS-HRE is a novel regulatory element of the iNOS promoter activity in murine macrophages and provide the first evidence that iNOS is a hypoxia-inducible gene.

Leukemia inhibitory factor induces interleukin-8 and monocyte chemotactic and activating factor in human monocytes: differential regulation by interferon-gamma.

Leukemia inhibitory factor (LIF) is a cytokine released at the site of injuries where there is a recruitment of monocytes and polymorphonuclear cells. We analyzed the effect of LIF on human monocytes, which are a major source of chemotactic factors. We showed that supernatants of monocytes treated with LIF (50 ng/mL) for 18 hours had chemotactic activity for neutrophils and monocytes that was neutralized by anti-interleukin-8 (anti-IL-8) and anti-monocyte chemotactic and activating factor (anti-MCAF) neutralizing antibodies. Northern blot analysis showed induction of IL-8 and MCAF RNA in monocytes treated with LIF. Both IL-8 and MCAF mRNA were induced within 3 hours of stimulation. IL-8 and MCAF mRNAs expression peaked at 6 hours and 18 hours, respectively. Interferon-gamma (IFN-gamma), a potent monocyte activator, inhibited IL-8 induction by LIF. On the contrary, IFN-gamma by itself induced MCAF and did not affect the LIF-induced MCAF. These results indicate that LIF released at the site of injury by inducing IL-8 and MCAF can play an important role in recruiting leukocytes and that IFN-gamma can differentially regulate this recruitment.

IL-4 inhibits IL-2-induced tumoricidal activity and secretory functions of human monocytes. Modulation of IL-2 binding and IL-2 receptor beta gamma chain expression.

Human monocytes express functional IL-2 receptors (IL-2R) and are directly activated by IL-2 to exert effector and secretory functions. In this study, we show that IL-4 selectively suppressed, in a dose-dependent manner, IL-2-induced monocyte tumoricidal activity, without affecting IFN-gamma-dependent cytotoxicity. This effect was specific because a neutralizing anti-IL-4 mAb completely restored IL-2-activated cytolysis. Furthermore, IL-4 effectively blocked the secretion of proinflammatory cytokines by IL-2-stimulated monocytes. Binding studies with biotin-conjugated IL-2 demonstrated that monocyte stimulation with IL-2 increased IL-2 binding to the cell surface, and that treatment with IL-4 inhibited this augmentation, providing a possible explanation for the decreased responsiveness of monocytes to IL-2 in the presence of IL-4. However, IL-4 suppressive effects could not be ascribed to the down-regulation of the individual components of the IL-2R complex. In fact, co-treatment of monocytes with IL-2 and IL-4 increased the expression of IL-2R gamma chain above the levels induced by IL-2 alone, whereas it did not significantly affect the expression of IL-2R beta chain. Thus, the inhibition of IL-2 binding by IL-4 may be due to the recruitment of the gamma chain into the IL-4-IL-4R system, making it unavailable for participation in the formation of IL-2 binding sites. These findings provide the first evidence of the ability of IL-4 to suppress IL-2-mediated activation of human monocytes and suggest that IL-4 may play an important role in vivo as an inhibitory signal that controls the response of monocytes to IL-2.