Bacterial lipopolysaccharide rapidly inhibits expression of C-C chemokine receptors in human monocytes

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.

Selective Inhibition of Expression of the Chemokine Receptor CCR2 in Human Monocytes by IFN-γ

IFN-γ is a potent activator of mononuclear phagocyte function and promotes the development of Th1 responses. Moreover, it induces and modulates chemokine production in a variety of cell types, including mononuclear phagocytes. In the present study, we examined the effect of IFN-γ on the expression of CC chemokine receptors in human monocytes. IFN-γ selectively and rapidly inhibited expression of the monocyte chemotactic protein (MCP) receptor CCR2 with an ED50 of ∼50 U/ml. The effect was rapid (detectable after 1 h) and reversible. Other chemokine receptors (CCR1, CCR3, CCR4, and CCR5) were not substantially affected, and CXCR4 was reduced. IFN-γ acted in concert with LPS, TNF-α, and IL-1β in inhibiting CCR2 expression. IFN-γ-treated monocytes showed a shorter half-life of CCR2 mRNA compared with untreated cells, whereas the rate of nuclear transcription was unaffected. The inhibition of CCR2 mRNA expression by IFN-γ was associated with a lower number of surface receptors and lower chemotactic responsiveness. Thus, IFN-γ, an inducer of MCP-1 and MCP-3 in mononuclear phagocytes, selectively inhibits expression of the MCP receptor CCR2 in monocytes. These results are consistent with an emerging paradigm of divergent regulation by several agents of chemokine production and receptor expression in monocytes. The inhibition of MCP-1R expression may serve as a means of retaining mononuclear phagocytes at sites of inflammation and as a feedback mechanism in the regulation of recruitment from the blood.

A novel lipopolysaccharide inducible C-C chemokine receptor related gene in murine macrophages

To identify genes induced in activated macrophages, we screened a cDNA library prepared from the lipopolysaccharide (LPS)-treated cell line, RAW264, using the suppression subtractive hybridization technique. One of the clones isolated was dramatically induced by LPS in macrophages. The predicted protein sequence of this gene contains the domain unique to seven transmembrane receptors, and shows similarity with mouse C-C chemokine receptor 5 (CCR5). Therefore, we designated it LPS inducible C-C chemokine receptor related gene (L-CCR). Northern blot analysis revealed that L-CCR was specifically expressed in differentiated macrophages after LPS stimulation. These results show that L-CCR is a novel C-C chemokine receptor related gene induced by LPS in macrophages and may play an important role in inflammatory responses.

Role of the beta-chemokine receptors CCR3 and CCR5 in human immunodeficiency virus type 1 infection of monocytes and microglia

Human immunodeficiency virus type 1 (HIV-1) infection in mononuclear phagocyte lineage cells (monocytes, macrophages, and microglia) is a critical component in the pathogenesis of viral infection. Viral replication in macrophages serves as a reservoir, a site of dissemination, and an instigator for neurological sequelae during HIV-1 disease. Recent studies demonstrated that chemokine receptors are necessary coreceptors for HIV-1 entry which determine viral tropism for different cell types. To investigate the relative contribution of the beta-chemokine receptors CCR3 and CCR5 to viral infection of mononuclear phagocytes we utilized a panel of macrophage-tropic HIV-1 strains (from blood and brain tissue) to infect highly purified populations of monocytes and microglia. Antibodies to CD4 (OKT4A) abrogated HIV-1 infection. The beta chemokines and antibodies to CCR3 failed to affect viral infection of both macrophage cell types. Antibodies to CCR5 (3A9) prevented monocyte infection but only slowed HIV replication in microglia. Thus, CCR5, not CCR3, is an essential receptor for HIV-1 infection of monocytes. Microglia express both CCR5 and CCR3, but antibodies to them fail to inhibit viral entry, suggesting the presence of other chemokine receptors for infection of these cells. These studies demonstrate the importance of mononuclear phagocyte heterogeneity in establishing HIV-1 infection and persistence.

Mature dendritic cells respond to SDF-1, but not to several beta-chemokines

Immature dendritic cells (DCs) are highly motile, but after differentiation they stop migration. Chemokines are chemotactic cytokines that direct leukocyte trafficking, therefore we looked for the expression and function of chemokine receptors in immature and mature DCs. As a model, we used the human DCs that develop from CD14+ peripheral blood monocytes cultured with GM-CSF and IL-4. After 6-7 days in culture, these cells have the characteristics of immature DCs, but can be induced to mature further by inflammatory stimuli or by monocyte conditioned medium (MCM). Immature DCs express mRNA for CXCR4, CCR3 and CCR5. The receptors are expressed on the cell surface, as assessed with monoclonal antibodies, and are functional (with the exception of CCR3) as assessed by CA++ mobilization in response to specific chemokines. Further differentiation and maturation of DC in MCM causes a downregulation of expression and function of the beta-chemokine receptors, while CXCR4 still remains, and signals a calcium flux on mature DCs. We argue that the downregulation of beta-chemokine receptors during maturation helps to stop DC movement after T cells have been identified in lymphoid organs or at sites of delayed-type hypersensitivity.

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.

Interleukin 10 increases CCR5 expression and HIV infection in human monocytes

The immunosuppressive and antiinflammatory cytokine interleukin (IL) 10 selectively upregulates the expression of the CC chemokine receptors CCR5, 2, and 1 in human monocytes by prolonging their mRNA half-life. IL-10-stimulated monocytes display an increased number of cell surface receptors for, and better chemotactic responsiveness to, relevant agonists than do control cells. In addition, IL-10-stimulated monocytes are more efficiently infected by HIV BaL. This effect was associated to the enhancement of viral entry through CCR5. These data add support to an emerging paradigm in which pro- and antiinflammatory molecules exert reciprocal and opposing influence on chemokine agonist production and receptor expression.

Differential expression of chemokine receptors and chemotactic responsiveness of type 1 T helper cells (Th1s) and Th2s

T helper cells type 1 (Th1s) that produce interferon-gamma predominantly mediate cellular immune responses and are involved in the development of chronic inflammatory conditions, whereas Th2s which produce large amounts of IL-4 and IL-5 upregulate IgE production and are prominent in the pathogenesis of allergic diseases. The precise factors determining whether Th1- or Th2-mediated immune responses preferentially occur at a peripheral site of antigen exposure are largely unknown. Chemokines, a superfamily of polypeptide mediators, are a key component of the leukocyte recruitment process. Here we report that among four CXC (CXCR1-4) and five CC (CCR1-5) chemokine receptors analyzed, CXCR3 and CCR5 are preferentially expressed in human Th1s. In contrast, Th2s preferentially express CCR4 and, to a lesser extent, CCR3. In agreement with the differential chemokine receptor expression, Th1s and Th2s selectively migrate in response to the corresponding chemokines. The differential expression of chemokine receptors may dictate, to a large extent, the migration and tissue homing of Th1s and Th2s. It may also determine different susceptibility of Th1s and Th2s to human immunodeficiency virus strains using different fusion coreceptors.

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.

CXCR-4 is expressed by primary macrophages and supports CCR5-independent infection by dual-tropic but not T-tropic isolates of human immunodeficiency virus type 1

Primary macrophages are infected by macrophage (M)-tropic but not T-cell line (T)-tropic human immunodeficiency virus type 1 (HIV-1) strains, and CCR5 and CXCR-4 are the principal cofactors utilized for CD4-mediated entry by M-tropic and T-tropic isolates, respectively. Macrophages from individuals homozygous for an inactivating mutation of CCR5 are resistant to prototype M-tropic strains that depend on CCR5 but are permissive for a dual-tropic isolate, 89.6, that can use both CCR5 and CXCR-4, as well as CCR2b, CCR3, and CCR8. Here we show that 89.6 entry into CCR5-deficient macrophages is blocked by an anti-CXCR-4 antibody and by the CXCR-4-specific chemokine SDF but not by the ligands to CCR2b or CCR3. Reverse transcription-PCR demonstrated expression of CXCR-4 but not CCR3 or CCR8 in macrophages, while CCR2b was variable. Macrophage surface expression of CXCR-4 was confirmed by immunofluorescence staining and flow cytometry. Thus, CXCR-4 is expressed by primary macrophages and functions as a cofactor for entry by dual-tropic but not T-tropic HIV-1 isolates, and macrophage resistance to T-tropic strains does not result from a lack of the T-tropic entry cofactor CXCR-4. Since CXCR-4 on macrophages can be used by some but not other isolates, these results indicate that HIV-1 strains differ in how they utilize chemokine receptors as cofactors for entry and that the ability of a chemokine receptor to mediate HIV-1 entry differs, depending on the cell type in which it is expressed.

Regulation of the receptor specificity and function of the chemokine RANTES (regulated on activation, normal T cell expressed and secreted) by dipeptidyl peptidase IV (CD26)-mediated cleavage

CD26 is a leukocyte activation marker that possesses dipeptidyl peptidase IV activity but whose natural substrates and immunological functions have not been clearly defined. Several chemo-kines, including RANTES (regulated on activation, normal T cell expressed and secreted), have now been shown to be substrates for recombinant soluble human CD26. The truncated RANTES(3-68) lacked the ability of native RANTES(1-68) to increase the cytosolic calcium concentration in human monocytes, but still induced this response in macrophages activated with macrophage colony-stimulating factor. Analysis of chemokine receptor messenger RNAs and patterns of desensitization of chemokine responses showed that the differential activity of the truncated molecule results from an altered receptor specificity. RANTES(3-68) showed a reduced activity, relative to that of RANTES(1-68), with cells expressing the recombinant CCR1 chemokine receptor, but retained the ability to stimulate CCR5 receptors and to inhibit the cytopathic effects of HIV-1. Our results indicate that CD26-mediated processing together with cell activation-induced changes in receptor expression provides an integrated mechanism for differential cell recruitment and for the regulation of target cell specificity of RANTES, and possibly other chemokines.

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.

The interplay between primary and secondary cytokines. Cytokines involved in the regulation of monocyte recruitment

Cytokines and chemokines play significant roles in the recruitment of leucocytes from the circulation and are therefore crucial determinants of inflammatory reactions and immunity. As knowledge of the actions of these agents and their regulatory mechanisms increases, so does the opportunity to devise pharmacological means of intervening in the activities of these molecules. It is now believed, for instance, that the actions of the classical pro-inflammatory cytokine interleukin (IL)-1 are inhibited by the expression and presentation of a 'decoy' receptor, which acts as a 'molecular trap' for IL-1. There is evidence to suggest that IL-6, formerly considered a purely anti-inflammatory cytokine, also has proinflammatory properties. Furthermore, regulation of the chemokine system, which can be achieved by regulating agonist production or altering the expression of chemokine receptors, has recently been demonstrated. These and other new findings, which may prove to have considerable relevance as regards current or future strategies for the treatment of inflammation, infection, neoplasia etc., are presented and discussed in this article.