Regulation of expression of the human monocyte chemotactic protein-1 receptor (hCCR2) by cytokines

Homocysteine induces monocyte chemoattractant protein-1 expression by activating NF-kappaB in THP-1 macrophages

Homocysteinemia is an independent risk factor for cardiovascular disorders. The recruitment of monocytes is an important event in atherogenesis. Monocyte chemoattractant protein-1 (MCP-1) is a potent chemokine that stimulates monocyte migration into the intima of arterial walls. The objective of the present study was to investigate the effect of homocysteine on MCP-1 expression in macrophages and the underlying mechanism of such effect. Human monocytic cell (THP-1)-derived macrophages were incubated with homocysteine. By nuclease protection assay and ELISA, homocysteine (0.05-0.2 mM) was shown to significantly enhance the expression of MCP-1 mRNA (up to 2.6-fold) and protein (up to 4.8-fold) in these cells. Homocysteine-induced MCP-1 expression resulted in increased monocyte chemotaxis. The increase in MCP-1 expression was associated with activation of nuclear factor (NF)-kappaB due to increased phosphorylation of the inhibitory protein (IkappaB-alpha) as well as reduced expression of IkappaB-alpha mRNA in homocysteine-treated cells. In conclusion, our results demonstrate that homocysteine, at pathological concentration, stimulates MCP-1 expression in THP-1 macrophages via NF-kappaB activation.

Differential expression of chemokine receptors on peripheral blood, synovial fluid, and synovial tissue monocytes/macrophages in rheumatoid arthritis

OBJECTIVE

Since it is likely that monocytes utilize chemokines to migrate to the rheumatoid arthritis (RA) joint, we investigated the expression of C-C chemokine receptors (CCR) 1-6 and C-X-C receptor 3 (CXCR3) in the peripheral blood (PB), synovial fluid (SF), and synovial tissue of patients with RA as well as in the PB of normal subjects.

METHODS

We compared chemokine receptor expression on CD14+ monocytes from normal PB, RA PB, and RA SF using 2-color flow cytometry. Correlations with patient clinical data were determined. Chemokine and receptor expression were investigated in RA synovial tissue by immunohistochemistry and 2-color immunofluorescence to identify CD68+ macrophages.

RESULTS

Most normal PB monocytes expressed CCR1 (87%) and CCR2 (84%), but not CCRs 3, 4, 5, or 6 or CXCR3. RA PB monocytes expressed CCR1 (56%) and CCR2 (76%), with significantly more expressing CCR3 (18%), CCR4 (38%), and CCR5 (17%) compared with normal PB monocytes. Significantly fewer SF monocytes from RA patients expressed CCR1 (17%), CCR2 (24%), and CCR4 (6%) while significantly more expressed CCR3 (35%) and CCR5 (47%) compared with RA and normal PB monocytes; CCR6 and CXCR3 were rarely detected. Clinically, the erythrocyte sedimentation rate was inversely correlated with the expression of CCR1 and CCR4 by RA PB, and CCR5 expression by RA SF was correlated with the SF white blood cell count. CCR1-, CCR2-, and CCR5-immunoreactive cells were found in RA synovial tissue and colocalized with CD68+ macrophages. RA synovial tissue RANTES (regulated upon activation, normally T cell expressed and secreted chemokine)- and monocyte chemoattractant protein 1-immunoreactive cells colocalized with CCR1 and CCR2, respectively, on serial sections. Macrophage inflammatory protein 1alpha (MIP-1alpha) was principally restricted to vascular endothelium, and MIP-1beta+ macrophages were found throughout the sections.

CONCLUSION

Monocytes mainly express CCR1 and CCR2 in normal and RA PB, CCR3 and CCR5 in RA PB and RA SF, and CCR4 in RA PB. The differential expression of chemokine receptors suggests that certain receptors aid in monocyte recruitment from the circulation while others are important in monocyte retention in the joint.

Interleukin-8 and its receptor CXCR2 in atherosclerosis

The participation of inflammatory cells in atherosclerosis is a well-known process that involves numerous molecules including chemotactic cytokines (chemokines) for their entry into the vessel wall. Although the C-C chemokine monocyte chemoattractant protein-1 and its receptor, CCR2, have been implicated in atherosclerosis, the role of the classic C-X-C chemokine, interleukin-8 (KC/growth-related oncogene alpha in mice) and its receptor CXCR2 has not been studied in the pathogenesis of atherosclerosis. Our research has shown that CXCR2 is strongly expressed on macrophages (Mphi) in atherosclerotic lesion. This CXCR2 expression is proatherogenic in that CXCR2 deficiency significantly reduces the progression of advanced atherosclerosis in mice. Although the mechanism still needs to be worked out, it appears that CXCR2 expression on lesion Mphi is essential for these cells to be retained in the lesion.

Acute excitotoxic injury induces expression of monocyte chemoattractant protein-1 and its receptor, CCR2, in neonatal rat brain

Chemokines are a family of structurally related cytokines that activate and recruit leukocytes into areas of inflammation. The "CC" chemokine, monocyte chemoattractant protein (MCP)-1 may regulate the microglia/monocyte response to acute brain injury. Recent studies have documented increased expression of MCP-1 in diverse acute and chronic experimental brain injury models; in contrast, there is little information regarding expression of the MCP-1 receptor, CCR2, in the brain. In the neonatal rat brain, acute excitotoxic injury elicits a rapid and intense microglial response. To determine if MCP-1 could be a regulator of this response, we evaluated the impact of excitotoxic injury on MCP-1 and CCR2 expression in the neonatal rat brain. We used a reproducible model of focal excitotoxic brain injury elicited by intrahippocampal injection of NMDA (10 nmol) in 7-day-old rats, to examine injury-induced alterations in MCP-1 and CCR2 expression. RT-PCR assays demonstrated rapid stimulation of both MCP-1 and CCR2 mRNA expression. MCP-1 protein content, measured by ELISA in tissue extracts, increased >30-fold in lesioned tissue 8-12 h after lesioning. CCR2 protein was also detectable in tissue extracts. Double-immunofluorescent labeling enabled localization of CCR2 both to activated microglia/monocytes in the corpus callosum adjacent to the lesioned hippocampus and subsequently in microglia/monocytes infiltrating the pyramidal cell layer of the lesioned hippocampus. These results demonstrate that in the neonatal brain, acute excitotoxic injury stimulates expression of both MCP-1 and its receptor, CCR2, and suggests that MCP-1 regulates the microglial/monocyte response to acute brain injury.

Oxidized LDL reduces monocyte CCR2 expression through pathways involving peroxisome proliferator-activated receptor gamma

The CCR2-mediated recruitment of monocytes into the vessel wall plays an important role in all stages of atherosclerosis. In recent studies, we have shown that lipoproteins can modulate CCR2 expression and have identified native LDL as a positive regulator. In contrast, oxidized LDL (OxLDL), which is mainly formed in the aortic intima, reduces CCR2 expression, promotes monocyte retention, and may cause pathological accumulation of monocytes in the vessel wall. We now provide evidence that OxLDL reduces monocyte CCR2 expression by activating intracellular signaling pathways that may involve peroxisome proliferator-activated receptor gamma (PPARgamma). Receptor-mediated uptake of the lipoprotein particle was required and allows for delivery of the exogenous ligand to the nuclear receptor. The suppression of CCR2 expression by OxLDL was mediated by lipid components of OxLDL, such as the oxidized linoleic acid metabolites 9-HODE and 13-HODE, known activators of PPARgamma. Modified apoB had no such effect. Consistent with a participation of the PPARgamma signaling pathway, BRL49653 reduced CCR2 expression in freshly isolated human monocytes ex vivo and in circulating mouse monocytes in vivo. These results implicate PPARgamma in the inhibition of CCR2 gene expression by oxidized lipids, which may help retain monocytes at sites of inflammation, such as the atherosclerotic lesion.

Monocyte chemoattractant protein-1 is upregulated in rats with volume-overload congestive heart failure

BACKGROUND

Chemokines are potent proinflammatory and immune modulators. Increased expression of chemokines, eg, monocyte chemoattractant protein-1 (MCP-1), has recently been described in clinical and experimental heart failure. The present report is aimed at exploring the expression, localization, and binding site regulation of MCP-1, a member of the C-C chemokine family, in a rat model of volume-overload congestive heart failure (CHF).

METHODS AND RESULTS

An aortocaval fistula was surgically created between the abdominal aorta and inferior vena cava. Rats with CHF were further subdivided into compensated and decompensated subgroups. Northern blot analysis and real-time quantitative polymerase chain reaction demonstrated upregulation of MCP-1 mRNA expression correlating with the severity of CHF (288+/-22, 502+/-62, and 826+/-138 copies/ng total RNA for sham, compensated, and decompensated animals, respectively; n=5, P:<0.05). MCP-1 protein was localized by immunohistochemistry in cardiomyocytes, vascular endothelium and smooth muscle cells, infiltrating leukocytes, and interstitial fibroblasts, and its intensity increased with severity of CHF. In addition, rats with CHF displayed a significant decrease of (125)I-labeled MCP-1 binding sites to myocardium-derived membranes (384.3+/-57.0, 181.3+/-8.8, and 123.3+/-14.1 fmol/mg protein for sham, compensated, and decompensated animals, respectively).

CONCLUSIONS

Volume-overload CHF in rats is associated with alterations in the expression, immunohistochemical localization, and receptor binding of the MCP-1 chemokine in the myocardium. These changes were more pronounced in rats with decompensated CHF. The data suggest that activation of the MCP-1 system may contribute to the progressive cardiac decompensation and development of CHF in rats with aortocaval fistula.

Regulation of cell migration in atherosclerosis

In response to vascular injury, monocytes and smooth muscle cells migrate to the intimal space, resulting in the formation of atherosclerotic and restenotic lesions. Several different growth factors and cytokines have been identified as mediators of cellular migration in the development of neointimal lesions. The principle mediator of monocyte adhesion and recruitment to the injured vascular wall is monocyte chemotactic protein-1. Moreover, recent studies have demonstrated that the atherogenic properties of angiotensin II are due to its ability to induce monocyte chemotactic protein-1 and promote monocyte migration to the vascular wall. Ligand-induced activation of receptor tyrosine kinases are the principle mechanism for smooth muscle cell migration following vascular injury. The signaling pathways mediating receptor-tyrosine kinase-induced migration of smooth muscle cells are also discussed.

Mice with a selective deletion of the CC chemokine receptors 5 or 2 are protected from dextran sodium sulfate-mediated colitis: lack of CC chemokine receptor 5 expression results in a NK1.1+ lymphocyte-associated Th2-type immune response in the intestine

The chemokine receptors CCR2 and CCR5 and their respective ligands regulate leukocyte chemotaxis and activation. To determine the role of these chemokine receptors in the regulation of the intestinal immune response, we induced colitis in CCR2- and CCR5-deficient mice by continuous oral administration of dextran sodium sulfate (DSS). Both CCR2- and CCR5-deficient mice were susceptible to DSS-induced intestinal inflammation. The lack of CCR2 or CCR5 did not reduce the DSS-induced migration of macrophages into the colonic lamina propria. However, both CCR5-deficient mice and, to a lesser degree, CCR2-deficient mice were protected from DSS-induced intestinal adhesions and mucosal ulcerations. CCR5-deficient mice were characterized by a greater relative infiltration of CD4+ and NK1.1+ lymphocyte in the colonic lamina propria when compared to wild-type and CCR2-deficient mice. In CCR5-deficient mice, mucosal mRNA expression of IL-4, IL-5, and IL-10 was increased, whereas that of IFN-gamma was decreased, corresponding to a Th2 pattern of T cell activation. In CCR2-deficient mice, the infiltration of Th2-type T cells in the lamina propria was absent, but increased levels of IL-10 and decreased levels of IFN-gamma may have down regulated mucosal inflammation. Our data indicate that CCR5 may be critical for the promotion of intestinal Th1-type immune responses in mice.

Induction of functional IL-8 receptors by IL-4 and IL-13 in human monocytes

IL-8 and related Glu-Leu-Arg (ELR+) CXC chemokines are potent chemoattractants for neutrophils but not for monocytes. IL-13 and IL-4 strongly increased CXCR1 and CXCR2 chemokine receptor expression in human monocytes, macrophages, and dendritic cells. The effect was receptor- and cell type-selective, in that CCRs were not increased and no augmentation was seen in neutrophils. The effect was rapid, starting at 4 h, and concentration dependent (EC50 = 6.2 and 8.3 ng/ml for CXCR1 and CXCR2, respectively) and caused by new transcriptional activity. IL-13/IL-4-treated monocytes showed increased CXCR1 and CXCR2 membrane expression. IL-8 and related ELR+ chemokines were potent and effective chemotactic agents for IL-13/IL-4-treated monocytes, but not for untreated mononuclear phagocytes, with activity comparable to that of reference monocyte attractants, such as MCP-1. In the same cells, IL-8 also caused superoxide release. Macrophages and dendritic cells present in biopsies from Omenn's syndrome and atopic dermatitis patients, two Th2 skewed pathologies, expressed IL-8 receptors by immunohistochemistry. These results show that IL-13 and IL-4 convert IL-8 and related ELR+ chemokines, prototypic neutrophil attractants, into monocyte chemotactic agonists, by up-regulating receptor expression. Therefore, IL-8 and related chemokines may contribute to the accumulation and positioning of mononuclear phagocytes in Th2-dominated responses.

Localisation of mRNA for JE/MCP-1 and its receptor CCR2 in atherosclerotic lesions of the ApoE knockout mouse

MCP-1 has potent chemotactic activity for monocytes and is strongly implicated in the pathogenesis of atherosclerosis. In the present study, we have used in situ hybridisation to examine the gene expression of JE, the murine homologue of MCP-1, and its receptor, CCR2, during the development of atherosclerotic lesions in the ApoE knockout mouse. Interestingly, the earliest expression of JE detected during lesion development was found to be localised in mesenchymal cells in the adventitia and not in the intima. Macrophages were subsequently found to accumulate in these affected regions of the adventitia and these cells were found to express high levels of JE. At this stage, early macrophage-rich lesions with high expression of JE were also seen in the intima, but expression of mRNA for the receptor for JE (CCR2) was only found on adventitial macrophages and not in the intima. This sequence of events suggests that adventitial inflammation may be an important early event in lesion development and responsible for the subsequent accumulation of macrophages in the intima possibly by recruitment from the adventitia as well as via the vessel lumen.

KSHV-encoded CC chemokine vMIP-III is a CCR4 agonist, stimulates angiogenesis, and selectively chemoattracts TH2 cells

Kaposi's sarcoma-associated herpesvirus (KSHV) encodes 3 genes that are homologous to cellular chemokines. vMIP-III, the product of open reading frame K4.1, is the most distantly related to human chemokines and has yet to be characterized. We have examined the interaction of vMIP-III with chemokine receptors, its expression in KS lesions, and its in ovo angiogenic properties. We show expression of vMIP-III in KS lesions and demonstrate the stimulation of angiogenesis by this chemokine, like vMIP-I and vMIP-II, in the chick chorioallantoic membrane assay. vMIP-III does not block human immunodeficiency virus entry through the coreceptors CCR3, CCR5, or CXCR4. However, vMIP-III is an agonist for the cellular chemokine receptor CCR4. CCR4 is expressed by TH2-type T cells. Consistent with this, vMIP-III preferentially chemoattracts this cell type. Because of these biologic properties and because it is expressed in KS lesions, vMIP-III may play an important role in the pathobiology of KS.

Experimental gliosarcoma induces chemokine receptor expression in rat brain

Macrophage/microglial infiltration is a characteristic feature of brain tumors. The functional role(s) of these cells is complex and could include both trophic and suppressive effects on tumors. Information has recently emerged about the molecular signals that regulate the accumulation and function of monocytes in pathological disorders. Recent data indicate that the chemokine, monocyte chemoattractant protein-1 (MCP-1), a potent monocyte activating and chemotactic factor, is a primary regulator of the macrophage response in brain tumors. We hypothesized that if MCP-1 regulates macrophage/microglial infiltration, then expression of the specific MCP-1 receptor, CCR2, will be induced in peritumoral tissue and/or within brain tumors. Identification of a specific receptor that is preferentially expressed in brain tumors could be important both in terms of tumor biology and as a potential therapeutic target. We used an established experimental gliosarcoma model, induced by intracranial transplantation of cultured 9L cells into adult rat brain, to test this hypothesis. RT-PCR analysis showed high levels of both MCP-1 and CCR2 mRNA and Western blot analysis demonstrated increased CCR2 protein in tumor extracts. Immunocytochemistry showed CCR2 immunoreactive microglia in peritumoral tissue and, unexpectedly, that intrinsic tumor cells, rather than monocytes, were the predominant source of CCR2. These results demonstrate that CCR2 expression is markedly upregulated in this brain tumor model.

Delayed clearance of zymosan-induced granuloma and depressed phagocytosis of macrophages with concomitant up-regulated kinase activities of Src-family in a human monocyte chemoattractant protein-1 transgenic mouse

A human monocyte chemoattractant protein-1 (hMCP-1) transgenic mouse (Tgm) line which constitutively produces a large amount of hMCP-1 (7-13 ng/ml in the serum) was established. Although expression of the transgene was detected in various tissues, an accumulation of macrophages (Mphi) was seen in only lymphoid organs which might be attributed to the high concentration of hMCP-1 in these organs. A reduced phagocytosis by peritoneal Mphi in vivo and a delayed clearance of granulomas in the liver following zymosan administration were observed in these Tgm. However, peritoneal exudate cells (PEC) from Tgm exhibited normal in vitro phagocytic activity and nitric oxide (NO) production upon stimulation with IFN-gamma as compared with those from non-Tgm. In addition, high activities of src-family protein tyrosine kinases (PTK), Fgr and Hck, were also noted in the peritoneal resident cells from Tgm, whereas the level of mitogen-activated protein kinase (MAPK) activity was almost the same as that of non-Tgm. It was suggested that the low functional activities of Tgm Mphi seen in vivo were attributed to down-regulation of the unique transducing system of hMCP-1 signals under the influence of a high concentration of the hMCP-1. It seemed that the depressed functions were recovered when the peritoneal cells were released ex vivo from such a high hMCP-1 environment.

Granulocyte macrophage colony-stimulating factor and interleukin-3 regulate chemokine and chemokine receptor expression in bone marrow macrophages

The beta-chemokine macrophage inflammatory protein-1 alpha (MIP-1alpha) and its associated receptors are involved in the regulation of pro-inflammatory and haemopoietic processes. This study was designed to investigate regulation of expression MIP-1alpha and its receptors by other haemopoietic cytokines. Murine bone marrow macrophages (BMM) were treated with or without GM-CSF or IL-3 and expression of MIP-1alpha, other chemokines and their receptors examined by Northern blotting. Receptor levels were also examined using Scatchard analysis and functional tests. Treatment of BMM with GM-CSF revealed a striking increase in MIP-1alpha mRNA levels, relative to untreated cells with a corresponding increase in MIP-1alpha protein. A similar increase in mRNA levels was found when BMM were treated with IL-3. An increase in the expression of three other beta-chemokines namely MIP-1beta, MCP-1 and MCP-3, was also found following treatment with GM-CSF or IL-3. We have additionally examined the expression of the known beta-chemokine receptors in BMM and observed an increase in CCR1 mRNA levels following treatment with GM-CSF and IL-3, but no change was seen in the level of CCR5 expression. The increase in CCR1 expression was reflected in an increase in the number of cell surface receptors for MIP-1alpha on the GM-CSF treated BMM and in an enhanced response of the GM-CSF treated BMM to CCR1 ligands. These data suggest that GM-CSF and IL-3 may be involved in mechanisms regulating expression levels of MIP-1alpha and its receptors.

Chemokines and atherosclerosis

Chemokines or chemotactic cytokines represent an expanding family of structurally related small molecular weight proteins, recognised as being responsible for leukocyte trafficking and activation. Soon after the discovery of this class of cytokines, about a decade ago, monocyte chemoattractant protein-1 (MCP-1) was found to be highly expressed in human atherosclerotic lesions and postulated to be central in monocyte recruitment into the arterial wall and developing lesions. In this review, we will discuss our present knowledge about MCP-1 and its receptor CCR2 and their role in atherogenesis. Although less well established, other chemokines such as RANTES, MIP-1alpha and MIP-1beta have also been implicated in atherosclerotic lesion formation as are a number of more recently discovered chemokines like MCP-4, ELC and PARC. The role of these chemokines in the progression of atherosclerosis will be discussed as well as the emerging role of IL-8, mostly know for its effects on neutrophils. Particular attention will be given not only to the involvement of chemokines in the inflammatory recruitment of monocytes/macrophages, but also to their role in the related local immune responses and vascular remodelling which occur during the formation of unstable atherosclerotic plaques.

Role of the first extracellular loop in the functional activation of CCR2. The first extracellular loop contains distinct domains necessary for both agonist binding and transmembrane signaling

The physiological cellular responses to monocyte chemoattractant protein-1 (MCP-1), a potent chemotactic and activating factor for mononuclear leukocytes, are mediated by specific binding to CCR2. The aim of this investigation is to identify receptor microdomains that are involved in high affinity agonist binding and receptor activation. The results from our functional studies in which we utilized neutralizing antisera against CCR2 are consistent with a multidomain binding model, previously proposed by others. The first extracellular loop was of particular interest, because in addition to a ligand-binding domain it contained also information for receptor activation, crucial for transmembrane signaling. Replacement of the first extracellular loop of CCR2 with the corresponding region of CCR1 decreased the MCP-1 binding affinity about 10-fold and prevented transmembrane signaling. A more detailed analysis by site-directed mutagenesis revealed that this receptor segment contains two distinct microdomains. The amino acid residues Asn(104) and Glu(105) are essential for high affinity agonist binding but are not involved in receptor activation. In contrast, the charged amino acid residue His(100) does not contribute to ligand binding but is vital for receptor activation and initiation of transmembrane signaling. We hypothesize that the interaction of agonist with this residue initiates the conformational switch that allows the formation of the functional CCR2-G protein complex.

Up-Regulation of CCR2 Chemokine Receptor Expression and Increased Susceptibility to the Multitropic HIV Strain 89.6 in Monocytes Exposed to Glucocorticoid Hormones

Glucocorticoid hormones (GC) are potent antiinflammatory agents widely used in the treatment of diverse human diseases. The present study was aimed at assessing the effect of GC on chemokine receptor expression in human monocytes. Dexamethasone (Dex) up-regulated mRNA expression of the monocyte chemotactic protein (MCP-1, CCL2) chemokine receptor CCR2. The effect was selective in that other chemokine receptors were not substantially affected. Stimulation by Dex was observed after 4 h of exposure at concentrations of 10−7 to 10−5 M. Steroids devoid of GC activity were inactive, and the GC receptor antagonist, RU486, inhibited stimulation. Dex did not affect the rate of nuclear transcription, but augmented the CCR2 mRNA half-life. Augmentation of CCR2 expression by Dex was associated with increased chemotaxis. Finally, Dex treatment induced productive replication of the HIV strain 89.6, which utilizes CCR2 as entry coreceptor, in freshly isolated monocytes. Together with previous findings, these results indicate that at least certain pro- and antiinflammatory molecules have reciprocal and divergent effects on expression of a major monocyte chemoattractant, MCP-1, and of its receptor (CCR2). Augmentation of monocyte CCR2 expression may underlie unexplained in vivo effects of GC as well as some of their actions on HIV infection.

Expression of CCR2 by endothelial cells : implications for MCP-1 mediated wound injury repair and In vivo inflammatory activation of endothelium

Endothelial cell proliferation and migration may play a central role in angiogenesis, wound healing, and atherosclerosis. Although CXC chemokines can act on endothelial cells by influencing proliferation, an involvement of CC chemokines and endothelial expression of chemokine receptors remains to be elucidated. Reverse transcription-polymerase chain reaction, RNase protection, Western blot, and flow cytometric analysis showed that human umbilical vein endothelial cells express mRNA and surface protein of the monocyte chemotactic protein-1 (MCP-1) receptor CCR2, which was upregulated by inflammatory cytokines. MCP-1 induced migration of endothelial cells in a transwell assay, which was inhibited by the 9-76 MCP-1 receptor antagonist. Increased secretion of MCP-1 or interleukin-8, but not RANTES, on endothelial injury suggested a functional role of CCR2 in wound repair as measured by ELISA. After mechanical injury to endothelial monolayers, which spontaneously closed within 24 hours, wound repair was delayed by the 9-76 antagonist and by a blocking monoclonal antibody to MCP-1, but not to interleukin-8, and was improved by exogenous MCP-1. This was confirmed by quantification of cell migration into the wound area, whereas proliferation and viability were unaltered by MCP-1 or its analogue. Notably, immunohistochemistry of inflamed tissue revealed CCR2 staining on arterial, venous, and venular endothelium affected by cellular infiltration. This is the first demonstration of endothelial CCR2 expression ex vivo, inferring its involvement in inflammatory conditions. Thus endothelial cells express functional CCR2 that may have important implications for endothelial wound repair and inflammatory reactions.

Downregulation by tumor necrosis factor-alpha of monocyte CCR2 expression and monocyte chemotactic protein-1-induced transendothelial migration is antagonized by oxidized low-density lipoprotein: a potential mechanism of monocyte retention in atherosclerotic lesions

The subintimal infiltration with monocytes is crucially involved in the development of complex atherosclerotic plaques. Monocyte chemotactic protein-1 (MCP-1) and its receptor CCR2 are important for monocyte extravasation and formation of atherosclerotic lesions. However, mechanisms of monocyte persistence in atherosclerotic plaques remain to be elucidated. Flow cytometric analysis revealed that monocytoid Mono Mac 6 cells that had transmigrated endothelium towards a MCP-1 gradient expressed higher levels of CCR2 than the non-migratory fraction, while input cells were intermediate, suggesting that high CCR2 levels are essential for transendothelial chemotaxis. Pretreatment of Mono Mac 6 cells or isolated human blood monocytes with the inflammatory cytokine tumor necrosis factor (TNF)-alpha dose- and time-dependently reduced MCP-1-induced transendothelial chemotaxis, which was inhibited by the CCR2 receptor antagonist 9-76 analog. This was paralleled by a decrease in CCR2 surface protein and mRNA expression. as assessed by flow cytometry and reverse transcription-polymerase chain reaction, inferring that inhibition of monocyte transmigration was due to downregulation of CCR2 to levels insufficient for chemotaxis. In contrast, treatment of monocytes with oxidized low-density protein (oxLDL) containing oxidized lipids, such as cholesteryl linoleate 13-hydroxide. but not with LDL, increased CCR2 protein and mRNA expression. Notably, oxLDL counteracted the TNF-alpha-mediated downregulation of CCR2 and CCR2-dependent transendothelial chemotaxis. Macrophage-colony-stimulating factor hardly affected CCR2 expression and function, suggesting that differentiation was not responsible for effects on CCR2. In conclusion, TNF-alpha impairs MCP-1-induced transendothelial migration of monocytes by downregulating CCR2 which appears critical for migration. Exposure to oxLDL antagonized the effects of TNF-alpha, and may thus contribute to monocyte retention and perpetuation of a chronic inflammatory reaction in unstable atherosclerotic lesions.

Distinct scavenger receptor expression and function in the human CD14(+)/CD16(+) monocyte subset

The CD14(+)/CD16(+) subset of human blood monocytes, which expresses low levels of the lipopolysaccharide receptor CD14 and high levels of the Fc receptor CD16 and exhibits features of mature tissue macrophages, is expanded in certain inflammatory conditions and may be relevant in atherosclerosis. Scavenger receptors (ScR) are important for lipid accumulation into macrophage-derived foam cells in atherogenesis and for the clearance of pathogens. Hence, we compared the function and expression of ScR in CD33(low) CD16(+) and CD33(high) CD14(++) monocyte subsets. Double immunofluorescence analysis of isolated monocytes revealed that the CD33(low) subset showed lower specific, ScR-mediated binding of DiI-labeled modified low-density lipoproteins (LDL) than CD33(high) cells. Differences in modified LDL binding between subsets were accompanied by changes in mRNA expression. RT-PCR in sorted cells indicated lower ScR class A type I/II (ScR-AI/II) mRNA levels in CD14(+)/CD16(+) than in CD14(++) cells, whereas CD36 transcripts were unaltered. This was paralleled by findings in mostly CD16(+) monocyte-derived macrophages showing a marked reduction in ScR-mediated binding of acetylated LDL, but not in the binding of oxidized LDL, and lower expression of ScR-AI/II mRNA, but not CD36 transcripts, after exposure to tumor necrosis factor-alpha for 48 h in vitro. Thus the subset of CD14(+)/CD16(+) monocytes shows distinct ScR function and expression, possibly reflecting a preactivation by cytokines with a predilection for specific inflammatory or vascular conditions, e.g., atherogenesis.

Chemokine receptor CCR2 expression and monocyte chemoattractant protein-1-mediated chemotaxis in human monocytes. A regulatory role for plasma LDL

The subendothelial accumulation of macrophage-derived foam cells is one of the hallmarks of atherosclerosis. The recruitment of monocytes to the intima requires the interaction of locally produced chemokines with specific cell surface receptors, including the receptor (CCR2) for monocyte chemoattractant protein-1 (MCP-1). We have previously reported that monocyte CCR2 gene expression and function are effectively downregulated by proinflammatory cytokines. In this study we identified low density lipoprotein (LDL) as a positive regulator of CCR2 expression. Monocyte CCR2 expression was dramatically increased in hypercholesterolemic patients compared with normocholesterolemic controls. Similarly, incubation of human THP-1 monocytes with LDL induced a rapid increase in CCR2 mRNA and protein. By 24 hours the number of cell surface receptors was doubled, causing a 3-fold increase in the chemotactic response to MCP-1. The increase in CCR2 expression and chemotaxis was promoted by native LDL but not by oxidized LDL. Oxidized LDL rapidly downregulated CCR2 expression, whereas reductively methylated LDL, which does not bind to the LDL receptor, had only modest effects on CCR2 expression. A neutralizing anti-LDL receptor antibody prevented the effect of LDL, suggesting that binding and internalization of LDL were essential for CCR2 upregulation. The induction of CCR2 expression appeared to be mediated by LDL-derived cholesterol, because cells treated with free cholesterol also showed increased CCR2 expression. These data suggest that elevated plasma LDL levels in conditions such as hypercholesterolemia enhance monocyte CCR2 expression and chemotactic response and potentially contribute to increased monocyte recruitment to the vessel wall in chronic inflammation and atherogenesis.

Identification of the lectin-like receptor for oxidized low-density lipoprotein in human macrophages and its potential role as a scavenger receptor

A new receptor for oxidized low-density lipoprotein (LDL), lectin-like oxidized LDL receptor-1 (LOX-1), has recently been cloned from bovine endothelial cells and human lung. A limited tissue-distribution study suggested that the protein was mainly produced by the vascular endothelium. In the present study we demonstrate that LOX-1 is also expressed in macrophages, where it may function as a scavenger receptor. LOX-1 was not detected in undifferentiated THP-1 cells or in freshly isolated human blood monocytes. However, mature human monocyte-derived macrophages and differentiated THP-1 cells showed high levels of LOX-1 transcripts. Consistent with these results, immunofluorescence staining and FACS analysis demonstrated that LOX-1 protein is expressed on the plasma membrane of macrophages. Western-blot analysis of membranes from macrophages (but not those from monocytes) identified a single band, with an apparent molecular mass of about 40 kDa, that displayed oxidized LDL-binding activity. These results suggest that differentiation induces the expression of LOX-1 in macrophages, where it may play a role as a scavenger receptor and/or a receptor for oxidized LDL.

Prostaglandin E2 Induces Resistance to Human Immunodeficiency Virus-1 Infection in Monocyte-Derived Macrophages: Downregulation of CCR5 Expression by Cyclic Adenosine Monophosphate

The chemokine receptor CCR5 can function as a coreceptor for human immunodeficiency virus-1 (HIV-1) entry into CD4+ T cells and macrophages, especially during the early stages of HIV-1 infection. The regulation of CCR5 expression may affect not only leukocyte migration, but also infectivity by HIV-1 and, therefore, acquired immunodeficiency syndrome (AIDS) pathogenesis. We report here that agents which increase intracellular concentrations of cyclic adenosine monophosphate (cAMP) rapidly downregulate CCR5 gene expression, with consequent loss of CCR5 expression and function in monocytes/macrophages. Chemotaxis and intracellular Ca2+mobilization in monocytes pretreated with prostaglandin E2or dibutyryl-cAMP for 24 hours were significantly reduced in response to the CCR5 ligand, MIP-1β. Moreover, HIV-1 entry into monocyte-derived macrophages pretreated with dibutyryl-cAMP or prostaglandin E2 was markedly decreased. Our findings suggest that resistance to HIV-1 can be induced by agents which increase cellular levels of cAMP and that this may suggest additional therapeutic strategies to limit infection by HIV-1.

MCP-1, MCP-2 and MCP-3 expression in multiple sclerosis lesions: an immunohistochemical and in situ hybridization study

Chemokines are low molecular weight chemotactic cytokines that have been shown to play a central role in the perivascular transmigration and accumulation of specific subsets of leukocytes at sites of tissue damage. Two major families have been defined depending on the positioning of four conserved cysteines. The CXC chemokines predominantly attract neutrophils, whereas the CC chemokines predominantly attract monocytes and other leukocyte cell types. Members of the monocyte chemotactic protein (MCP)-1 family form a major component of the CC family of chemokines and are considered the principal chemokines involved in the recruitment of monocytes/macrophages and activated lymphocytes. In this study we addressed the expression and distribution of MCP-1, -2 and -3 in multiple sclerosis (MS) lesions of differing ages and levels of inflammatory activity using immunohistochemistry and in situ hybridization. In acute and chronic-active MS lesions immunoreactivity for MCP-1, -2 and -3 was prominent throughout the lesion center with reactivity diminishing abruptly at the lesion edge. Hypertrophic astrocytes were strongly reactive and inflammatory cells showed variable reactivity. Outside of the lesion only hypertrophic astrocytes were reactive. The results obtained by in situ hybridization for MCP-1 were in agreement with those obtained by immunostaining. In more chronic lesions immunoreactivity for MCP-1, -2 and -3 was considerably diminished, and in chronic-silent lesions immunoreactivity was restricted to a few scattered reactive astrocytes. Normal control brains showed no immunoreactivity for MCP-1, -2 and -3. Although the cellular distribution of all three members of this family was similar, antibodies to MCP-3 gave prominent staining of the extracellular matrix that was not noted for MCP-1 and -2. These results support the conclusion that members of the MCP family of chemokines are involved in the development of MS lesions in the central nervous system.

Chemokine receptors in human endothelial cells. Functional expression of CXCR4 and its transcriptional regulation by inflammatory cytokines

Chemokines play an important role in the regulation of endothelial cell (EC) function, including proliferation, migration and differentiation during angiogenesis, and re-endothelialization after injury. In this study, reverse transcriptase-polymerase chain reaction was used to reveal expression of various CXC and CC chemokine receptors in human umbilical vein EC. Northern analysis showed that CXCR4 was selectively expressed in vascular EC, but not in smooth muscle cells. Compared with other chemokines, stromal cell-derived factor-1alpha (SDF-1alpha), the known CXCR4 ligand, was an efficacious chemoattractant for EC, causing the migration of approximately 40% input cells with an EC50 of 10-20 nM. Of the chemokines tested, only SDF-1alpha induced a rapid, though variable mobilization of intracellular Ca2+ in EC. Experiments with actinomycin D demonstrated that CXCR4 transcripts were short-lived, indicating a rapid mRNA turnover. Interferon-gamma (IFN-gamma) caused a pronounced down-regulation of CXCR4 mRNA in a concentration- and time-dependent manner. In a striking functional correlation, IFN-gamma treatment also attenuated the chemotactic response of EC to SDF-1alpha. IL-1beta, tumor necrosis factor-alpha, and lipopolysaccharide produced a time course-dependent biphasic effect on CXCR4 transcription. Expression of CXCR4 in EC is significant, more so as it and several CC chemokine receptors have been shown to serve as fusion co-receptors along with CD4 during human immunodeficiency virus infection. Taken together, these findings provide evidence of chemokine receptor expression in EC and offer an explanation for the action of chemokines like SDF-1alpha on the vascular endothelium.

The molecular and cellular biology of the chemokines

Chemokines are the largest mammalian cytokine family so far identified. They have distinct molecular structures and are unique among the cytokines in their chemotactic properties and ability to inhibit bone marrow progenitor-cell growth. A wealth of in vitro and in vivo data show that chemokines act as directional forces in immune and inflammatory responses, working with integrins to target subsets of effector cells to specific tissue sites. The selective action of individual chemokines on leucocyte subsets makes them ideal molecules to sort, as well as direct, leucocyte traffic. In addition to their in vitro and in vivo actions on haemopoietic precursors, chemokines can also modulate angiogenesis and some may activate leucocytes. There is still much to learn about the chemokines and their intracellular pathways, and more ligands and receptors will undoubtedly be discovered. However, chemokines are already known to be central to the co-ordination of leucocyte responses and represent important therapeutic targets in a number of diseases.

Severe reduction in leukocyte adhesion and monocyte extravasation in mice deficient in CC chemokine receptor 2

CC chemokine receptor 2 (CCR2) is a prominent receptor for the monocyte chemoattractant protein (MCP) group of CC chemokines. Mice generated by gene targeting to lack CCR2 exhibit normal leukocyte rolling but have a pronounced defect in MCP-1-induced leukocyte firm adhesion to microvascular endothelium and reduced leukocyte extravasation. Constitutive macrophage trafficking into the peritoneal cavity was not significantly different between CCR2-deficient and wild-type mice. However, after intraperitoneal thioglycollate injection, the number of peritoneal macrophages in CCR2-deficient mice did not rise above basal levels, whereas in wild-type mice the number of macrophages at 36 h was approximately 3.5 times the basal level. The CCR2-deficient mice showed enhanced early accumulation and delayed clearance of neutrophils and eosinophils. However, by 5 days neutrophils and eosinophils in both CCR2-deficient and wild-type mice had returned to near basal levels, indicating that resolution of this inflammatory response can occur in the absence of macrophage influx and CCR2-mediated activation of the resident peritoneal macrophages. After intravenous injection with yeast beta-glucan, wild-type mice formed numerous large, well-defined granulomas throughout the liver parenchyma, whereas CCR2-deficient mice had much fewer and smaller granulomas. These results demonstrate that CCR2 is a major regulator of induced macrophage trafficking in vivo.