Endothelial cells differentially express functional CXC-chemokine receptor-4 (CXCR-4/fusin) under the control of autocrine activity and exogenous cytokines

The chemokine receptor CCR8 mediates human endothelial cell chemotaxis induced by I-309 and Kaposi sarcoma herpesvirus-encoded vMIP-I and by lipoprotein(a)-stimulated endothelial cell conditioned medium

The CC chemokine receptor 8 (CCR8) is expressed on monocytes and type 2 T lymphocytes. CCR8 is the sole receptor for the human CC chemokine I-309, as well as for viral monocyte inflammatory protein-I (vMIP-I), a human chemokine homologue induced in human cells by the Kaposi sarcoma-related human herpesvirus-8. Recently it was found that I-309 messenger RNA and protein are expressed by human umbilical vein endothelial cells (HUVECs) and that the secretion of endothelial I-309 is stimulated by apolipoprotein(a). I-309, vMIP-I, and the conditioned medium from apolipoprotein(a)-stimulated HUVECs induce endothelial chemotaxis. A polyclonal anti-CCR8 antibody and a newly developed murine monoclonal antibody against CCR8 inhibited this activity. The G-protein inhibitor pertussis toxin also inhibited endothelial chemotaxis, providing further evidence for a chemokine receptor-mediated effect. Endothelial cells contain CCR8 mRNA as shown by RNA blot analysis as well by direct sequence analysis. Immunohistochemical studies identified CCR8 and I-309 on the endothelium of human atherosclerotic plaques and in endothelial-derived spindle cells of Kaposi sarcoma. These results indicate that CCR8 is an endothelial receptor that may modulate endothelial function.

Analysis of human lung endothelial cells for susceptibility to HIV type 1 infection, coreceptor expression, and cytotoxicity of gp120 protein

The lung represents a potential target during HIV infection, and the onset of AIDS is associated with severe pulmonary complications in many patients. T-lymphocytes and alveolar macrophages form the majority of HIV-infected cells in the lung. However, other cell types in the lung could participate in HIV-mediated lung pathology and their role has not been investigated. The aims of this study were to determine if human lung microvascular endothelial cells (HLMEC) express HIV receptor and coreceptors, and if HIV can directly infect HLMEC. Specifically, we wished to determine if these cells constitute a viral reservoir in the lung, and if HIV-1 envelope proteins induce cytotoxic effects on HLMEC. Our results showed that by flow cytometry, HLMEC failed to express any CXCR4 or CCR5 on their surface. In contrast, RT-PCR revealed the presence of CXCR4 and CCR5 mRNA, but not CD4 in HLMEC. Two dual-tropic HIV-1 isolates failed to infect HLMEC in vitro, as determined by (1) p24 antigen capture ELISA, (2) reverse transcriptase assay, RT-PCR, and (3) DNA PCR. However, a recombinant HIV-1 gp120 preparation induced apoptotic cell death of HLMEC. These data support the hypothesis that no productive HIV-1 infection of HLMEC occurs in vitro. This suggests that in vivo, HLMEC may not be a major reservoir of HIV in the lung and the primary route for HIV invasion of the lung. Thus, while other mechanisms must play a role in HIV invasion and subsequent dissemination in the lung, lung endothelial cells do represent potential targets for the lethal effects of HIV viral proteins.

Cell cycle-dependent expression of CXC chemokine receptor 3 by endothelial cells mediates angiostatic activity

Endothelial cell receptors for the angiostatic chemokines IFN-gamma-inducible protein of 10 kDa (IP-10) and monokine induced by IFN-gamma (Mig) have not yet been identified, and the mechanisms responsible for the effects of these chemokines on angiogenesis are still unclear. IP-10 and Mig share a common functional receptor on activated T lymphocytes, named CXC chemokine receptor 3 (CXCR3). Using in situ hybridization and immunohistochemistry, we show that CXCR3 is expressed by a small percentage of microvascular endothelial cells in several human normal and pathological tissues. Primary cultures of human microvascular endothelial cells (HMVECs) likewise express CXCR3, although this expression is limited to the S/G2-M phase of their cell cycle. Both IP-10 and Mig, as well as the IFN-gamma-inducible T-cell alpha chemoattractant (I-TAC), which all share high-affinity binding for CXCR3, block HMVEC proliferation in vitro, an effect that can be inhibited by an anti-CXCR3 antibody. These data provide definitive evidence of CXCR3 expression by HMVEC and open new avenues for therapeutic interventions in all conditions in which an angiostatic effect may be beneficial.

I-309 binds to and activates endothelial cell functions and acts as an angiogenic molecule in vivo

Several chemokines have been shown to act as angiogenic molecules or to modulate the activity of growth factors such as fibroblast growth factor 2 (FGF-2) and vascular endothelial growth factor (VEGF). The detection of the CC chemokine receptor (CCR) 8 message in human umbilical vein endothelial cells (HUVECs) by reverse transcription– polymerase chain reaction (RT-PCR) and RNase protection assay (RPA), prompted us to investigate the potential role exerted by the CC chemokine I-309, a known ligand of such receptor, in both in vitro and in vivo angiogenesis assays. We show here that I-309 binds to endothelial cells, stimulates chemotaxis and invasion of these cells, and enhances HUVEC differentiation into capillary-like structures in an in vitro Matrigel assay. Furthermore, I-309 is an inducer of angiogenesis in vivo in both the rabbit cornea and the chick chorioallantoic membrane assay (CAM).

I-309 binds to and activates endothelial cell functions and acts as an angiogenic molecule in vivo

Several chemokines have been shown to act as angiogenic molecules or to modulate the activity of growth factors such as fibroblast growth factor 2 (FGF-2) and vascular endothelial growth factor (VEGF). The detection of the CC chemokine receptor (CCR) 8 message in human umbilical vein endothelial cells (HUVECs) by reverse transcription– polymerase chain reaction (RT-PCR) and RNase protection assay (RPA), prompted us to investigate the potential role exerted by the CC chemokine I-309, a known ligand of such receptor, in both in vitro and in vivo angiogenesis assays. We show here that I-309 binds to endothelial cells, stimulates chemotaxis and invasion of these cells, and enhances HUVEC differentiation into capillary-like structures in an in vitro Matrigel assay. Furthermore, I-309 is an inducer of angiogenesis in vivo in both the rabbit cornea and the chick chorioallantoic membrane assay (CAM).

The CXC chemokine receptor 2, CXCR2, is the putative receptor for ELR+ CXC chemokine-induced angiogenic activity

We have previously shown that members of the ELR(+) CXC chemokine family, including IL-8; growth-related oncogenes alpha, beta, and gamma; granulocyte chemotactic protein 2; and epithelial neutrophil-activating protein-78, can mediate angiogenesis in the absence of preceding inflammation. To date, the receptor on endothelial cells responsible for chemotaxis and neovascularization mediated by these ELR(+) CXC chemokines has not been determined. Because all ELR(+) CXC chemokines bind to CXC chemokine receptor 2 (CXCR2), we hypothesized that CXCR2 is the putative receptor for ELR(+) CXC chemokine-mediated angiogenesis. To test this postulate, we first determined whether cultured human microvascular endothelial cells expressed CXCR2. CXCR2 was detected in human microvascular endothelial cells at the protein level by both Western blot analysis and immunohistochemistry using polyclonal Abs specific for human CXCR2. To determine whether CXCR2 played a functional role in angiogenesis, we determined whether this receptor was involved in endothelial cell chemotaxis. We found that microvascular endothelial cell chemotaxis in response to ELR(+) CXC chemokines was inhibited by anti-CXCR2 Abs. In addition, endothelial cell chemotaxis in response to ELR(+) CXC chemokines was sensitive to pertussis toxin, suggesting a role for G protein-linked receptor mechanisms in this biological response. The importance of CXCR2 in mediating ELR(+) CXC chemokine-induced angiogenesis in vivo was also demonstrated by the lack of angiogenic activity induced by ELR(+) CXC chemokines in the presence of neutralizing Abs to CXCR2 in the rat corneal micropocket assay, or in the corneas of CXCR2(-/-) mice. We thus conclude that CXCR2 is the receptor responsible for ELR(+) CXC chemokine-mediated angiogenesis.

Characterization of binding sites for chemokines MCP-1 and MIP-1alpha on human brain microvessels

The presence of binding sites for the beta chemokines monocyte chemoattractant protein-1 (MCP-1) and macrophage inflammatory protein-1alpha (MIP-1alpha) has recently been identified on human brain microvessels. We extend these findings in this report to reveal that such sites exemplify characteristics of the recognized major receptors for MCP-1 and MIP-1alpha: CCR2, and CCR1 and CCR5, respectively. Specifically, labeled MCP-1 binding to isolated brain microvessels was inhibited by unlabeled MCP-1 and MCP-3, the latter another CCR2 ligand, but not by MIP-1alpha. Inhibition of labeled MIP-1alpha binding was achieved with unlabeled MIP-1alpha and RANTES, the latter a beta chemokine that binds to both CCR1 and CCR5, but not by MCP-1. Labeled MIP-1alpha binding was also antagonized by unlabeled MCP-3, which is also recognized by CCR1, and MIP-1beta, which is a ligand for CCR5. Labeled MCP-1 and MIP-1alpha were further observed to be internalized within the endothelial cells of brain microvessels, following their binding to the microvascular surface at 37 degrees C. Additionally, exposure of microvessels to unlabeled MCP-1 or MIP-1alpha was accompanied by the initial loss and subsequent recovery of surface binding sites for these chemokines, which occurred on a time scale consistent with ligand-induced endocytosis and recycling. These collective features bear striking similarity to those that characterize interactions of MCP-1 and MIP-1alpha with their receptors on leukocytes and underscore the concept of cognate chemokine receptors on brain microvascular endothelium.

Characterization and visualization of [125I] stromal cell-derived factor-1alpha binding to CXCR4 receptors in rat brain and human neuroblastoma cells

Stromal cell-Derived Factor-1 (SDF-1alpha), binds to the seven-transmembrane G protein-coupled CXCR4 receptor and modulates cell migration, differentiation, and proliferation. CXCR4 has been reported to be expressed in various tissues including brain. Moreover, CXCR4 has recently been shown to be one of the coreceptors for HIV-1 infection which could be implicated in HIV encephalitis. In the present study, the binding properties and autoradiographic distribution of [125I]SDF-1alpha binding to CXCR4 were characterized in the adult rat brain. SDF-1alpha binding and CXCR4 coupling system were also studied in human neuroblastoma cell line SK-N-SH. The binding of [125I]SDF-1alpha on rat brain sections was specific, time-dependent and reversible. The highest densities of CXCR4 were detected in the choroid plexus of the lateral and the dorsal third ventricle. Lower densities of [125I]SDF-1alpha binding sites were observed in various brain regions including cerebral cortex, anterior olfactory nuclei, hippocampal formation, thalamic nuclei, blood vessels and pituitary gland. In the choroid plexus, the IC(50) and K(d) of [125I]SDF-1alpha binding were respectively 0.6 nM and 0. 36 nM. Similar IC(50) values were obtained in other brain structures. A CXCR4 antagonist, bicyclam, competed with SDF-1alpha binding (30% inhibition at 10(-6) M). In SK-N-SH cells, [125I]SDF-1alpha bound to CXCR4 with a K(d) of 5.0 nM and a maximal binding capacity of 460 fmol/mg of protein. SDF-1alpha induced a rapid and transient intracellular calcium increase in SK-N-SH cells. These findings suggest that CXCR4 is highly expressed in some brain structures and have a regulatory role in the nervous system. The significance of this expression in the brain parenchyma and more specifically in the choroid plexus remains to be clarified in the normal as well as in the infected brain.

Differential expression and responsiveness of chemokine receptors (CXCR1-3) by human microvascular endothelial cells and umbilical vein endothelial cells

The basis for the angiogenic effects of CXC chemokines such as interleukin 8 (IL-8) and for angiostatic chemokines such as interferon-inducible protein 10 (IP-10) has been difficult to assess. We recently reported, based on an RNase protection assay, that human umbilical vein endothelial cells (HUVECs) did not express detectable mRNA for the IL-8 receptors CXCR1 and CXCR2. This raised the possibility of heterogeneity of receptor expression by different endothelial cell (ECs) types. Since systemic angiogenesis induced by IL-8 would more likely involve microvessel ECs, we investigated CXC receptor expression on human microvascular dermal endothelial cells (HMECs). By confocal microscopy and immunofluorescence we observed that HMECs consistently expressed high levels of CXCR1 and CXCR4 (mean fluorescence intensity of 261+/-22.1 and 306.2+/-19, respectively) and intermediate levels of CXCR3 and CXCR2 (173.9+/-30. 2 and 156+/-30.9, respectively). In contrast, only a small proportion of HUVEC preparations expressed low levels of CXCR1, -2, and -3 (66+/-19.9; 49+/-15, and 81.4+/-17.9, respectively). However, both HMECs and HUVECs expressed equal levels of CXCR4. As expected, HMECs had more potent chemotactic responses to IL-8 than HUVECs, and this was correlated with the levels of IL-8 receptors on the ECs. Antibodies to CXCR1 and CXCR2 each had inhibitory effects on chemotaxis of HMECs to IL-8, indicating that both IL-8 receptors contributed to the migratory response of these cells toward IL-8. Assessment of the functional capacity of CXCR3 unexpectedly revealed that HMECs migrated in response to relatively higher concentrations (100-500 ng/ml) of each of the 'angiostatic' chemokines IP-10, ITAC, and MIG. Despite this, the 'angiostatic' chemokines inhibited the chemotactic response of HMECs to IL-8. IL-8 and SDF-1alpha but not IP-10 induced calcium mobilization in adherent ECs, suggesting that signaling events associated with calcium mobilization are separable from those required for chemotaxis. Taken together, our data indicated that functional differences among EC types is dependent on the level of the expression of CXC chemokine receptors. Whether this heterogeneity in receptor expression by ECs reflects distinct differentiation pathways remains to be established.

Downregulation of CXCR4 gene expression in primary human endothelial cells following infection with E1(-)E4(+) adenovirus gene transfer vectors

Infection of human endothelial cells with first-generation E1(-)E4(+) adenovirus (Ad) vectors leads to prolonged cell survival and changes in the cell phenotype to a more quiescent stage. Based on the concept that the CXCR4, the receptor for the endothelial chemoattractant stromal-derived factor-&alpha (SDF-alpha), is constitutively expressed by quiescent, resting endothelial cells, the present study analyzes the effect of Ad vector infection on CXCR4 expression and SDF-alpha responses of human umbilical vein endothelial cells (HUVEC). CXCR4 transcripts were markedly downregulated in E1(-)E4(+) Ad-infected cells 48 h following infection, but not in uninfected control cells or when the cells were infected with an E1(-)E4(-) Ad vector. Analysis of surface CXCR4 expression by flow cytometry demonstrated marked reduction of the CXCR4 receptor on cells infected with E1(-)E4(+) Ad compared to uninfected control cells or E1(-)E4(-) Ad-infected cells. Infection of other cell types which express CXCR4, such as dendritic cells and myeloma cells, did not exhibit CXCR4 receptor downregulation following infection with E1(-)E4(+) Ad. Consistent with the observed downregulation of CXCR4 mRNA and surface protein, infection of the endothelial cells with an E1(-)E4(+) Ad rendered the cells unresponsive to the chemoattractant SDF-alpha compared to naive or E1(-)E4(-) Ad-infected cells. Together, the data suggest that first-generation Ad vectors, likely the E4 region, modify the ability of endothelial cells to respond to at least one important chemoattractant.

SDF-1 activity on microvascular endothelial cells: consequences on angiogenesis in in vitro and in vivo models

The chemokine stromal cell-derived factor-1 (SDF-1) has been shown to be involved in cell migration. As the receptor CXCR-4 is expressed on endothelial cells and upregulated by angiogenic factors, we were prompted to study the effect of SDF-1 on angiogenesis in endothelial cells from microvasculature. This study demonstrates that SDF-1 induces an angiogenic effect in vitro, primarily in a tridimensional fibrin gel. The increase in capillary tube formation was evident after a 10-day incubation with SDF-1. This was associated with a mild increase in VEGF production by microvascular endothelial cells (ELISA and rt-PCR) and a potent chemotactic effect. SDF-1 also induced an in vivo angiogenic activity as shown in the model of the rabbit corneal pocket. However, the angiogenesis was located in an area rich in inflammatory cells. The results of our study suggest that these data underline the potential role of SDF-1 in angiogenesis as the microvascular endothelial cells were greatly involved in this process.

HIV-1 gp120- and gp160-induced apoptosis in cultured endothelial cells is mediated by caspases

The immune dysfunction and cell destruction that occur in the human immunodeficiency virus (HIV)-infected host appear to result from the direct cytopathic effects of viral infection and the effects of viral proteins on uninfected bystander cells. Recently, the α-chemokine receptor CXCR4 has been reported to mediate apoptosis in neuronal cells and in CD4+ and CD8+ T cells after its binding to HIV-1 envelope proteins. In the current study, it was observed that human umbilical vein endothelial cells (HUVEC) undergo apoptosis after their treatment with the HIV-1 envelope proteins gp120/160. Anti-CXCR4 monoclonal antibody decreased HIV-1 gp120/160-induced apoptosis, suggesting that the CXCR4 chemokine receptor mediates the apoptotic effects of these HIV envelope glycoproteins. Further studies revealed that caspases play an important role in this process because the pretreatment of cells with a general caspase enzyme inhibitor decreased the extent of HUVEC apoptosis induced by gp120/160. In addition, it was found that caspase-3 was activated on HIV-1 gp120/160 treatment of these cells. It was also observed that gp120/160 treatment slightly increased the expression of the pro-apoptotic molecule Bax. These results suggest that HIV-1 envelope glycoproteins can disrupt endothelial integrity through the interaction with CXCR4, thereby facilitating virus transit out of the bloodstream and contributing to the vascular injury syndromes seen in acquired immunodeficiency syndrome.

HIV-1 gp120- and gp160-induced apoptosis in cultured endothelial cells is mediated by caspases

The immune dysfunction and cell destruction that occur in the human immunodeficiency virus (HIV)-infected host appear to result from the direct cytopathic effects of viral infection and the effects of viral proteins on uninfected bystander cells. Recently, the α-chemokine receptor CXCR4 has been reported to mediate apoptosis in neuronal cells and in CD4+ and CD8+ T cells after its binding to HIV-1 envelope proteins. In the current study, it was observed that human umbilical vein endothelial cells (HUVEC) undergo apoptosis after their treatment with the HIV-1 envelope proteins gp120/160. Anti-CXCR4 monoclonal antibody decreased HIV-1 gp120/160-induced apoptosis, suggesting that the CXCR4 chemokine receptor mediates the apoptotic effects of these HIV envelope glycoproteins. Further studies revealed that caspases play an important role in this process because the pretreatment of cells with a general caspase enzyme inhibitor decreased the extent of HUVEC apoptosis induced by gp120/160. In addition, it was found that caspase-3 was activated on HIV-1 gp120/160 treatment of these cells. It was also observed that gp120/160 treatment slightly increased the expression of the pro-apoptotic molecule Bax. These results suggest that HIV-1 envelope glycoproteins can disrupt endothelial integrity through the interaction with CXCR4, thereby facilitating virus transit out of the bloodstream and contributing to the vascular injury syndromes seen in acquired immunodeficiency syndrome.

Chemokine signaling in inflammation

The events that lead to an inflammatory response are characterized by recognition of the site of injury by inflammatory cells, specific recruitment of subpopulations of leukocytes into tissue, removal of the offending agent and "debridement" of the injured cells/tissue, and repair of the site of injury with attempts to reestablish normal parenchymal, stromal, and extracellular matrix relationship. The molecular regulation of this complex physiologic process involves the interaction between cell surface, extracellular matrix, and soluble mediators, such as chemokines. Chemokine activities are mediated through G-protein coupled receptors. This is the largest known family of cell-surface receptors, which mediate transmission of stimuli as diverse as hormones, peptides, glycopeptides, and chemokines. In this review, we will focus on the signaling pathways involved in the production and function of chemokines as they relate to the inflammatory response.

Fibroblast growth factor modulates HIV coreceptor CXCR4 expression by neural cells. HNRC Group

Recent studies suggest that the chemokine receptor CXCR4 may be involved in mediating the neurodegenerative process in the brains of patients with acquired immunodeficiency disease (AIDS). In this context, we hypothesize that neurotrophic factors, such as fibroblast growth factor (FGF), might protect against human immunodeficiency virus (HIV)-mediated neurotoxicity via regulating the expression of CXCR4 in neural cells. For this purpose, levels of CXCR4 were determined in neuronal and glial cell lines after FGF1 and 2 treatment. In addition, levels of CXCR4 immunoreactivity were associated with levels of FGF1 immunoreactivity in the brains of HIV-positive patients. These studies showed that neuronal CXCR4 levels decreased in a dose-dependent manner after exposure to FGF. Conversely, glial CXCR4 was increased in a dose-dependent manner after FGF2 treatment. These effects were dependent on the FGF receptor tyrosine kinase signaling pathway, because FGF-induced effects on CXCR4 were blocked by the tyrosine kinase inhibitor, 5'-deoxy-5'methylthioadenosine, or by anti-FGF receptor antibody. Stromal cell-derived factor-1, the ligand for CXCR4, and HIV gp120 neurotoxicity was attenuated by FGF1 in a dose-dependent manner in vitro, further supporting physiological relevance. In the brains of AIDS patients, the levels of neural CXCR4 immunoreactivity were inversely associated with FGF levels. Taken together, these results support the possibility that the neuroactive effects of FGF in HIV encephalitis might be mediated through regulation of the expression of CXCR4.

CXC chemokine receptor 4 expression and stromal cell-derived factor-1alpha-induced chemotaxis in CD4+ T lymphocytes are regulated by interleukin-4 and interleukin-10

We report that interleukin (IL)-4 and IL-10 can significantly up- or down-regulate CXC chemokine receptor 4 (CXCR4) expression on CD4+ T lymphocytes, respectively. Stromal cell-derived factor-1alpha (SDF-1alpha)-induced CD4+ T-lymphocyte chemotaxis was also correspondingly regulated by IL-4 and IL-10. IL-4 and IL-10 up- or down-regulated CXCR4 mRNA expression in CD4+ T lymphocytes, respectively, as detected by real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR). Scatchard analysis revealed a type of CXCR4 with affinity (Kd approximately 6.3 nM), and approximately 70,000 SDF-1alpha-binding sites per cell, among freshly isolated CD4+ T lymphocytes, and two types of CXCR4 with different affinities (Kd1 approximately 4.4 nM and Kd2 approximately 14.6 nM), and a total of approximately 130,000 SDF-1alpha-binding sites per cell, among IL-4-stimulated CD4+ T lymphocytes. The regulation of CXCR4 expression in CD4+ T lymphocytes by IL-4 and IL-10 could be blocked by a selective inhibitor of protein kinase (staurosporine) or by a selective inhibitor of cAMP- and cGMP-dependent protein kinase (H-8), indicating that these cytokines regulate CXCR4 on CD4+ T lymphocytes via both cAMP and cGMP signalling pathways. The fact that cyclosporin A or ionomycin were able to independently change the CXCR4 expression and block the effects of IL-4 and IL-10 on CXCR4 expression implied that the capacity of IL-4 and IL-10 to regulate CXCR4 on CD4+ T lymphocytes is not linked to calcium-mobilization stimulation. These results indicate that the effects of IL-4 and IL-10 on the CXCR4-SDF-1 receptor-ligand pair may be of particular importance in the cytokine/chemokine environment concerning the inflammatory processes and in the progression of human immunodeficiency virus (HIV) infection.

CXCR4 on human endothelial cells can serve as both a mediator of biological responses and as a receptor for HIV-2

It has been shown that deletion of the chemokine receptor, CXCR4, causes disordered angiogenesis in mouse models. In the present studies, we examined the distribution and trafficking of CXCR4 in human endothelial cells, tested their responses to the CXCR4 ligand, SDF-1, and asked whether endothelial cell CXCR4 can serve as a cell surface receptor for the binding of viruses. The results show that CXCR4 is present on endothelial cells from coronary arteries, iliac arteries and umbilical veins (HUVEC), but expression was heterogeneous, with some cells expressing CXCR4 on their surface, while others did not. Addition of SDF-1 caused a rapid decrease in CXCR4 surface expression. It also caused CXCR4-mediated activation of MAPK, release of PGI(2), endothelial migration, and the formation of capillary-like structures by endothelial cells in culture. Co-culture of HUVEC with lymphoid cells that were chronically infected with a CD4-independent/CXCR4-tropic variant of HIV-2 resulted in the formation of multinucleated syncytia. Formation of the syncytia was inhibited by each of several different CXCR4 antibodies. Thus, our findings indicate: (1) that CXCR4 is widely expressed on human endothelial cells; (2) the CXCR4 ligand, SDF-1, can evoke a wide variety of responses from human endothelial cells; and (3) CXCR4 on endothelial cells can serve as a receptor for isolates of HIV that can utilize chemokine receptors in the absence of CD4.

Expression of VEGFR-2 and AC133 by circulating human CD34+ cells identifies a population of functional endothelial precursors

Emerging data suggest that a subset of circulating human CD34+ cells have phenotypic features of endothelial cells. Whether these cells are sloughed mature endothelial cells or functional circulating endothelial precursors (CEPs) is not known. Using monoclonal antibodies (MoAbs) to the extracellular domain of the human vascular endothelial receptor-2 (VEGFR-2), we have shown that 1.2 ± 0.3% of CD34+ cells isolated from fetal liver (FL), 2 ± 0.5% from mobilized peripheral blood, and 1.4 ± 0.5% from cord blood were VEGFR-2+. In addition, most CD34+VEGFR-2+ cells express hematopoietic stem cell marker AC133. Because mature endothelial cells do not express AC133, coexpression of VEGFR-2 and AC133 on CD34+ cells phenotypically identifies a unique population of CEPs. CD34+VEGFR-2+ cells express endothelial-specific markers, including VE-cadherin and E-selectin. Also, virtually all CD34+VEGFR-2+ cells express the chemokine receptor CXCR4 and migrate in response to stromal-derived factor (SDF)-1 or VEGF. To quantitate the plating efficiency of CD34+ cells that give rise to endothelial colonies, CD34+ cells derived from FL were incubated with VEGF and fibroblast growth factor (FGF)-2. Subsequent isolation and plating of nonadherent FL-derived VEGFR-2+ cells with VEGF and FGF-2 resulted in differentiation of AC133+VEGFR-2+ cells into adherent AC133−VEGFR-2+Ac-LDL+(acetylated low-density lipoprotein) colonies (plating efficiency of 3%). In an in vivo human model, we have found that the neo-intima formed on the surface of left ventricular assist devices is colonized with AC133+VEGFR-2+ cells. These data suggest that circulating CD34+ cells expressing VEGFR-2 and AC133 constitute a phenotypically and functionally distinct population of circulating endothelial cells that may play a role in neo-angiogenesis.

Chemokine receptors and their role in vascular biology

Chemokines play an important role in the process of leukocyte recruitment and activation at sites of inflammation. Until recently, the actions of chemokines and the expression of their receptors have only been described on different leukocyte populations. However, increasing evidence has suggested that non-haematopoietic cell types are capable of binding and responding to a number of chemokines. The functional expression of certain chemokine receptors has recently been described on vascular endothelial and smooth muscle cells. These findings provide new insight into the activities of chemokines and indicate that these molecules have a more widespread cellular target than first envisaged. Studies carried out to date indicate that chemokines and their respective receptors play an important role in the regulation of angiogenesis and angiostasis. They may also be involved in developmental and pathological processes such as organ vascularization, embryogenesis and arteriosclerosis.

Essential role for the p55 tumor necrosis factor receptor in regulating hematopoiesis at a stem cell level

Hematopoietic stem cell (HSC) self-renewal is a complicated process, and its regulatory mechanisms are poorly understood. Previous studies have identified tumor necrosis factor (TNF)-alpha as a pleiotropic cytokine, which, among other actions, prevents various hematopoietic progenitor cells from proliferating and differentiating in vitro. However, its role in regulating long-term repopulating HSCs in vivo has not been investigated. In this study, mice deficient for the p55 or the p75 subunit of the TNF receptor were analyzed in a variety of hematopoietic progenitor and stem cell assays. In older p55(-/-) mice (>6 mo), we identified significant differences in their hematopoietic system compared with age-matched p75(-/-) or wild-type counterparts. Increased marrow cellularity and increased numbers of myeloid and erythroid colony-forming progenitor cells (CFCs), paralleled by elevated peripheral blood cell counts, were found in p55-deficient mice. In contrast to the increased myeloid compartment, pre-B CFCs were deficient in older p55(-/-) mice. In addition, a fourfold decrease in the number of HSCs could be demonstrated in a competitive repopulating assay. Secondary transplantations of marrow cells from primary recipients of p55(-/-) marrow revealed impaired self-renewal ability of p55-deficient HSCs. These data show that, in vivo, signaling through the p55 subunit of the TNF receptor is essential for regulating hematopoiesis at the stem cell level.

Stromal-cell derived factor is expressed by dendritic cells and endothelium in human skin

Stromal-cell derived factor or SDF-1 is a CXC chemokine constitutively expressed by stromal bone marrow cell cultures that binds to the G-protein-coupled receptor CXCR4. SDF-1/CXCR4 represents a unique, nonpromiscuous ligand/receptor pair that plays an essential role in prenatal myelo- and lymphopoiesis as well as in cardiovascular and neural development. SDF-1 prevents entry of CXCR4-dependent (X4) HIV viruses in T lymphocytes, by binding and internalizing CXCR4. The expression pattern of SDF-1 protein in normal tissues is not known. Here we describe an analysis of SDF-1 mRNA and protein in normal and inflamed skin by in situ hybridization and immunohistochemistry, using a novel anti-SDF-1 monoclonal antibody. We also describe the expression pattern of CXCR4 receptor by immunohistochemistry. Our results show that SDF-1 protein and mRNA are normally expressed by endothelial cells, pericytes, and either resident or explanted CD1a+ dendritic cells. Epithelial cells of sweat glands but not keratinocytes also express SDF-1. In various inflammatory skin diseases, a large number of mononuclear cells and fibroblasts in close contact with CXCR4-positive lymphocytic infiltrates also express SDF-1. CXCR4 was also detected in many different normal cell types, including endothelial and epithelial cells, which points to a role for SDF-1/CXCR4 cell signaling in vascular and epithelial homeostasis. The demonstration of SDF-1 expression in dendritic and endothelial cells provides new insights into the mechanisms of normal and pathological lymphocyte circulation and makes it possible to envisage a role for locally secreted SDF-1 in the selective incapacity of mucosal dendritic cells to support and propagate infection by X4 HIV isolates.

Expression of functional CXCR4 chemokine receptors on human colonic epithelial cells

In addition to their role as regulators of leukocyte migration and activation, chemokines and their receptors also function in angiogenesis, growth regulation, and HIV-1 pathogenesis--effects that involve the action of chemokines on nonhematopoietic cells. To determine whether chemokine receptors are expressed in human colonic epithelium, HT-29 cells were examined by RT-PCR for the expression of the chemokine receptors for lymphotactin, fractalkine, CCR1-10, and CXCR1-5. The only receptor consistently detected was CXCR4 (fusin/LESTR), although HT-29 cells did not express mRNA for its ligand, stromal cell-derived factor (SDF-1alpha). Flow cytometric analysis with anti-CXCR4 antibody indicated that the CXCR4 protein was expressed on the surface of roughly half of HT-29 cells. CXCR4 was also expressed in colonic epithelial cells in vivo as shown by immunohistochemistry on biopsies from normal and inflamed human colonic mucosa. The mRNA for SDF-1alpha and other CC and CXC chemokines was present in normal colonic biopsies. The CXCR4 receptor in HT-29 cells was functionally coupled, as demonstrated by the elevation in [Ca2+]i, which occurred in response to 25 nM SDF-1alpha and by the SDF-1alpha-induced upregulation of ICAM-1 mRNA. Sodium butyrate downregulated CXCR4 expression and induced differentiation of HT-29 cells, suggesting a role for CXCR4 in maintenance and renewal of the colonic epithelium. This receptor, which also serves as a coreceptor for HIV, may mediate viral infection of colonic epithelial cells.

Effect of extracellular human immunodeficiency virus type 1 glycoprotein 120 on primary human vascular endothelial cell cultures

During the course of an HIV-1 infection, free infectious and noninfectious virus particles, and free HIV-1 proteins, circulate within the host, exposing the host endothelium to these viral factors, even if the endothelium is not infected. This suggests that extracellular HIV-1 proteins could influence endothelial cell function, leading to pathogenesis. In light of this, we have used primary cultured human vascular endothelial cells (HUVECs) to screen for effects of the HIV-1 protein gp120 on endothelial cell function. The results of this study show that short exposure of HUVEC cultures to this protein causes significant levels of cytotoxicity. Further, using several different assays, we have shown that this cytotoxic effect on HUVECs appears to be due to induction of an apoptotic program. The biphasic nature of gp120 titration curves suggests that multiple cellular factors are mediating these gp120-induced effects. Competition studies appear to confirm this by showing that the apoptotic effect is mediated through two cell surface receptors on HUVECs, CCR5 and CXCR4. Alternatively, competition studies examining CD4 receptors suggests that CD4 played no role in gp12O-induced effects on HUVECs.

Embryonic expression and function of the chemokine SDF-1 and its receptor, CXCR4

Directed cell movement is integral to both embryogenesis and hematopoiesis. In the adult, the chemokine family of secreted proteins signals migration of hematopoietic cells through G-coupled chemokine receptors. We detected embryonic expression of chemokine receptor messages by RT-PCR with degenerate primers at embryonic day 7.5 (E7.5) or by RNase protection analyses of E8.5 and E12.5 tissues. In all samples, the message encoding CXCR4 was the predominate chemokine receptor detected, particularly at earlier times (E7.5 and E8.5). Other chemokine receptor messages (CCR1, CCR4, CCR5, CCR2, and CXCR2) were found in E12.5 tissues concordant temporally and spatially with definitive (adult-like) hematopoiesis. Expression of CXCR4 was compared with that of its only known ligand, stromal cell-derived factor-1 (SDF-1), by in situ hybridization. During organogenesis, these genes have dynamic and complementary expression patterns particularly in the developing neuronal, cardiac, vascular, hematopoietic, and craniofacial systems. Defects in the first four of these systems have been reported in CXCR4- and SDF-1-deficient mice. Our studies suggest new potential mechanisms for some of these defects as well as additional roles beyond the scope of the reported abnormalities. Earlier in development, expression of these genes correlates with migration during gastrulation. Migrating cells (mesoderm and definitive endoderm) contain CXCR4 message while embryonic ectoderm cells express SDF-1. Functional SDF-1 signaling in midgastrula cells as well as E12.5 hematopoietic progenitors was demonstrated by migration assays. Migration occurred with an optimum dose similar to that found for adult hematopoietic cells and was dependent on the presence of SDF-1 in a gradient. This work suggests roles for chemokine signaling in multiple embryogenic events.

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.

Key role of chemokines and chemokine receptors in inflammation, immunity, neoplasia, and infectious disease

Leukocyte homing is a complex, multistep process involving the coordinated expression of adhesion molecules and certain chemotactic cytokines, termed chemokines. Although chemokines initially burst into the literature as potent inflammatory mediators, it is now clear that they are involved in a variety of processes including lymphocyte maturation, angiogenesis, and tumor growth. Furthermore, a variety of important pathogens manipulate various chemokine/receptor pathways to infect the host and evade the immune system.

Selective secretion of chemoattractants for haemopoietic progenitor cells by bone marrow endothelial cells: a possible role in homing of haemopoietic progenitor cells to bone marrow

To elucidate the mechanisms by which haemopoietic progenitor cells lodge in the bone marrow, we examined the secretion of chemoattractants for haemopoietic progenitor cells by bone marrow and lung endothelial cells. The bone marrow endothelial cells, but not lung endothelial cells, secreted chemoattractants for the haemopoietic progenitor cell line, FDCP-2, and normal haemopoietic progenitor cells. Checkerboard analysis demonstrated that the conditioned medium of the bone marrow endothelial cells had chemotactic activity and random motility-stimulating activity. The bone marrow endothelial cells expressed stromal-cell-derived factor-1 (SDF-1) mRNA and produced SDF-1 protein, whereas the lung endothelial cells did not. Adhesion of FDCP-2 cells to the bone marrow endothelial cells was partially inhibited by anti-SDF-1 antibody. These findings suggest that the chemoattractants for haemopoietic progenitor cells including SDF-1 and random motility-stimulating factor(s) selectively secreted by the bone marrow endothelial cells may contribute to the homing of haemopoietic progenitor cells to bone marrow.

Induction of inflammatory angiogenesis by monocyte chemoattractant protein-1

Almost any growth of tumors is to some extent associated with an inflammatory reaction which may be anti-tumorigenic by acting directly on tumor cells or protumorigenic cells presumably by inducing tumor-associated angiogenesis. In this study, we have analyzed the angiogenesis-inducing capacity of monocyte chemoattractant protein-1 (MCP-1), a key regulatory molecule of monocyte trafficking to sites of inflammation. MCP-1 was found to be potently angiogenic when implanted into rabbit cornea, exerting potency similar to the specific angiogenic vascular endothelial growth factor (VEGF)-A(121). MCP-1-induced angiogenesis in the cornea is associated with prominent recruitment of macrophages, whereas VEGF-A(121)-induced corneal angiogenesis is devoid of inflammatory cell recruitment. Based on these findings, we studied MCP-1 expression and macrophage recruitment in human invasive ductal mammary carcinomas in comparison with the physiological angiogenic processes in bovine ovarian corpus luteum. Macrophage recruitment was always associated with MCP-1 expression. High macrophage counts in mammary tumors corresponded with poor prognosis. In contrast, physiological ovarian angiogenesis was associated with only minimal inflammatory recruitment of macrophages. Our data show that MCP-1 is an indirect inflammation-associated inducer of angiogenesis and demonstrate distinct qualitative differences between tumor angiogenesis in human mammary tumors and physiological angiogenesis in the ovary.

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.

Visualization of chemokine binding sites on human brain microvessels

The chemokines monocyte chemoattractant protein-1 (MCP-1) and macrophage inflammatory protein-1alpha (MIP-1alpha) aid in directing leukocytes to specific locales within the brain and spinal cord during central nervous system inflammation. However, it remains unclear how these chemokines exert their actions across a vascular barrier, raising speculation that interaction with endothelial cells might be required. Therefore, experiments were performed to determine whether binding domains for these chemokines exist along the outer surface of brain microvessels, a feature that could potentially relay chemokine signals from brain to blood. Using a biotinylated chemokine binding assay with confocal microscopy and three-dimensional image reconstruction, spatially resolved binding sites for MCP-1 and MIP-alpha around human brain microvessels were revealed for the first time. Binding of labeled MCP-1 and MIP-1alpha could be inhibited by unlabeled homologous but not heterologous chemokine, and was independent of the presence of heparan sulfate, laminin, or collagen in the subendothelial matrix. This is the first evidence of specific and separate binding domains for MCP-1 and MIP-1alpha on the parenchymal surface of microvessels, and highlights the prospect that specific interactions of chemokines with microvascular elements influence the extent and course of central nervous system inflammation.

Vascular endothelial growth factor and basic fibroblast growth factor induce expression of CXCR4 on human endothelial cells: In vivo neovascularization induced by stromal-derived factor-1alpha

The contribution of chemokines toward angiogenesis is currently a focus of intensive investigation. Certain members of the CXC chemokine family can induce bovine capillary endothelial cell migration in vitro and corneal angiogenesis in vivo, and apparently act via binding to their receptors CXCR1 and CXCR2. We used an RNAse protection assay that permitted the simultaneous detection of mRNA for various CXC chemokine receptors in resting human umbilical vein endothelial cells (HUVECs) and detected low levels of only CXCR4 mRNA. Stimulation of HUVECs with vascular endothelial growth factor (VEGF) or basic fibroblast growth factor (bFGF) up-regulated levels of only CXCR4 mRNA. CXCR4 specifically binds the chemokine stromal-derived factor-1alpha (SDF-1alpha). Competitive binding studies using 125I-labeled SDF-1alpha with Scatchard analysis indicated that VEGF or bFGF induced an average number of approximately 16,600 CXCR4 molecules per endothelial cell, with a Kd = 1.23 x 10(-9) mol/L. These receptors were functional as HUVECs and human aorta endothelial cells (HAECs) migrated toward SDF-1alpha. Although SDF-1alpha-induced chemotaxis was inhibited by the addition of a neutralizing monoclonal CXCR4 antibody, endothelial chemotaxis toward VEGF was not altered; therefore, the angiogenic effect of VEGF is independent of SDF-1alpha. Furthermore, subcutaneous SDF-1alpha injections into mice induced formation of local small blood vessels that was accompanied by leukocytic infiltrates. To test whether these effects were dependent on circulating leukocytes, we successfully obtained SDF-1alpha-induced neovascularization from cross sections of leukocyte-free rat aorta. Taken together, our data indicate that SDF-1alpha acts as a potent chemoattractant for endothelial cells of different origins bearing CXCR4 and is a participant in angiogenesis that is regulated at the receptor level by VEGF and bFGF.