For efficient entry into target cells, primary macrophage-tropic and laboratory-adapted human immunodeficiency viruses type 1 (HIV-1) require particular chemokine receptors, CCR-5 and CXCR-4, respectively, as well as the primary receptor CD4 (refs 1-6). Here we show that a complex of gp120, the exterior envelope glycoprotein, of macrophage-tropic primary HIV-1 and soluble CD4 interacts specifically with CCR-5 and inhibits the binding of the natural CCR-5 ligands, macrophage inflammatory protein (MIP)-1alpha and MIP-1beta (refs 7, 8). The apparent affinity of the interaction between gp120 and CCR-5 was dramatically lower in the absence of soluble CD4. Additionally, in the absence of gp120, an interaction between a two-domain CD4 fragment and CCR-5 was observed. A gp120 fragment retaining the CD4-binding site and overlapping epitopes was able to interact with CCR-5 only if the V3 loop, which can specify HIV-1 tropism and chemokine receptor choice, was also present on the molecule. Neutralizing antibodies directed against either CD4-induced or V3 epitopes on gp120 blocked the interaction of gp12O-CD4 complexes with CCR-5. These results suggest that HIV-1 attachment to CD4 creates a high-affinity binding site for CCR-5, leading to membrane fusion and virus entry.

Lymphocytes that are responsible for regional (tissue-specific) immunity home from the blood to the intestines, inflamed skin or other sites through a multistep process involving recognition of vascular endothelial cells and extravasation. Chemoattractant cytokine molecules known as chemokines regulate this lymphocyte traffic, in part by triggering arrest (stopping) of lymphocytes rolling on endothelium. Here we show that many systemic memory T cells in blood carry the chemokine receptor CCR4 and therefore respond to its ligands, the chemokines TARC and MDC. These cells include essentially all skin-homing cells expressing the cutaneous lymphocyte antigen and a subset of other systemic memory lymphocytes; however, intestinal (alpha4beta7+) memory and naive T cells respond poorly. Immunohistochemistry reveals anti-TARC reactivity of venules and infiltration of many CCR4+ lymphocytes in chronically inflamed skin, but not in the gastrointestinal lamina propria. Moreover, TARC induces integrin-dependent adhesion of skin (but not intestinal) memory T cells to the cell-adhesion molecule ICAM-1, and causes their rapid arrest under physiological flow. Our results suggest that CCR4 and TARC are important in the recognition of skin vasculature by circulating T cells and in directing lymphocytes that are involved in systemic as opposed to intestinal immunity to their target tissues.

Chemokine receptors serve as coreceptors for HIV entry into CD4+ cells. Their expression is thought to determine the tropism of viral strains for different cell types, and also to influence susceptibility to infection and rates of disease progression. Of the chemokine receptors, CCR5 is the most important for viral transmission, since CCR5 is the principal receptor for primary, macrophage-tropic viruses, and individuals homozygous for a defective CCR5 allele (Δ32/ Δ32) are highly resistant to infection with HIV-1. In this study, CCR5-specific mAbs were generated using transfectants expressing high levels of CCR5. The specificity of these mAbs was confirmed using a broad panel of chemokine receptor transfectants, and by their non-reactivity with T cells from Δ32/Δ32 individuals. CCR5 showed a distinct pattern of expression, being abundant on long-term activated, IL-2–stimulated T cells, on a subset of effector/memory T cells in blood, and on tissue macrophages. A comparison of normal and CCR5 Δ32 heterozygotes revealed markedly reduced expression of CCR5 on T cells from the heterozygotes. There was considerable individual to individual variability in the expression of CCR5 on blood T cells, that related to factors other than CCR5 genotype. Low expression of CCR5 correlated with the reduced infectability of T cells with macrophage-tropic HIV-1, in vitro. Anti-CCR5 mAbs inhibited the infection of PBMC by macrophage-tropic HIV-1 in vitro, but did not inhibit infection by T cell–tropic virus. Anti-CCR5 mAbs were poor inhibitors of chemokine binding, indicating that HIV-1 and ligands bind to separate, but overlapping regions of CCR5. These results illustrate many of the important biological features of CCR5, and demonstrate the feasibility of blocking macrophage-tropic HIV-1 entry into cells with an anti-CCR5 reagent.

CCR5 is a chemokine receptor expressed by T cells and macrophages, which also functions as the principal coreceptor for macrophage (M)-tropic strains of HIV-1. To understand the molecular basis of the binding of chemokines and HIV-1 to CCR5, we developed a number of mAbs that inhibit the various interactions of CCR5, and mapped the binding sites of these mAbs using a panel of CCR5/CCR2b chimeras. One mAb termed 2D7 completely blocked the binding and chemotaxis of the three natural chemokine ligands of CCR5, RANTES (regulated on activation normal T cell expressed and secreted), macrophage inflammatory protein (MIP)-1α, and MIP-1β, to CCR5 transfectants. This mAb was a genuine antagonist of CCR5, since it failed to stimulate an increase in intracellular calcium concentration in the CCR5 transfectants, but blocked calcium responses elicited by RANTES, MIP-1α, or MIP-1β. This mAb inhibited most of the RANTES and MIP-1α chemotactic responses of activated T cells, but not of monocytes, suggesting differential usage of chemokine receptors by these two cell types. The 2D7 binding site mapped to the second extracellular loop of CCR5, whereas a group of mAbs that failed to block chemokine binding all mapped to the NH2-terminal region of CCR5. Efficient inhibition of an M-tropic HIV-1–derived envelope glycoprotein gp120 binding to CCR5 could be achieved with mAbs recognizing either the second extracellular loop or the NH2-terminal region, although the former showed superior inhibition. Additionally, 2D7 efficiently blocked the infectivity of several M-tropic and dual-tropic HIV-1 strains in vitro. These results suggest a complicated pattern of HIV-1 gp120 binding to different regions of CCR5, but a relatively simple pattern for chemokine binding. We conclude that the second extracellular loop of CCR5 is an ideal target site for the development of inhibitors of either chemokine or HIV-1 binding to CCR5.

CCR4, a chemokine receptor for macrophage-derived chemokine (MDC) and thymus and activation-regulated chemokine (TARC), has been implicated as a preferential marker for Th2 lymphocytes. Following in vitro polarization protocols, most Th2 lymphocytes express CCR4 and respond to its ligands TARC and MDC, whereas Th1 lymphocytes express CXC chemokine receptor 3 and CCR5 (but not CCR4). We show in this study that CCR4 is a major receptor for MDC and TARC on T lymphocytes, as anti-CCR4 mAbs significantly inhibit the migration of these cells to MDC and TARC. CCR4 is also highly expressed in most single-positive CD4+ thymocytes and on a major fraction of blood nonintestinal (α4β7−) memory CD4 lymphocytes, including almost all skin memory CD4+ cells expressing the cutaneous lymphocyte Ag (CLA), but weakly or not expressed in other subsets in thymus and blood. Interestingly, major fractions of circulating CCR4+ memory CD4 lymphocytes coexpress the Th1-associated receptors CXC chemokine receptor 3 and CCR5, suggesting a potential problem in using these markers for Th1 vs Th2 lymphocyte cells. Moreover, although production of Th2 cytokines in blood T cells is associated with CCR4+ CD4 lymphocytes, significant numbers of freshly isolated circulating CCR4+ memory CD4 lymphocytes (including both CLA+ and CLA− fractions) readily express the Th1 cytokine IFN-γ after short-term stimulation. Our results are consistent with a role for CCR4 as a major trafficking receptor for systemic memory T cells, and indicate that the patterns and regulation of chemokine receptor expression in vivo are more complex than indicated by current in vitro models of Th1 vs Th2 cell generation.

The natural history of B‐chronic lymphocytic leukemia (CLL) is not entirely explained by intrinsic defects of the neoplastic cell, but is also favored by microenvironmental signals. As CLL cells retain the capacity to respond to CD40 ligand (CD40L) and as CD4+ T cells are always present in involved tissues, we asked whether malignant CLL cells might produce T cell‐attracting chemokines. We studied the chemokine expression of CD19+/CD5+ malignant B cells from peripheral blood (PB), lymph nodes (LN) or bone marrow (BM) of 32 patients and found a major difference. LN‐ and BM‐, but not PB‐derived cells, expressed a readily detectable reverse transcription‐PCR band for CCL22 and one for CCL17 of variable intensity. CD40 ligation of PB cells induced the mRNA expression of both CCL22 and CCL17. CCL22 was also released in the culture supernatants. These supernatants induced the migration of activated CD4+, CD40L+ T cells expressing the CCL22 receptor, CCR4. T cell migration was abrogated by anti‐CCL22 antibodies. Immunohistochemistry and cytofluorography studies revealed that a proportion of CD4+ T cells in CLL LN and BM expressed CD40L. Our data demonstrate that malignant CLL cells chemo‐attract CD4+ T cells that in turn induce a strong chemokine production by the leukemic clone, suggesting a vicious circle, leading to the progressive accumulation of the neoplastic cells.

Human immunodeficiency virus type 1 (HIV-1) requires the presence of specific chemokine receptors in addition to CD4 to enter target cells. The chemokine receptor CCR5 is used by the macrophage-tropic strains of HIV-1 that predominate during the asymptomatic stages of infection. Here we identify a small tyrosine-rich region of CCR5 proximal to the N-terminal cysteine that is critical for entry of macrophage-tropic and dual-tropic variants of HIV-1. HIV-1 infection of cells expressing CCR5 mutants with changes in this region was substantially reduced compared with the infection of cells bearing wild-type CCR5. Simian immunodeficiency virus (SIVmac239) entry was also ablated on a subset of these mutants but enhanced on others. These differences in virus entry were correlated with the relative ability of soluble, monomeric HIV-1 and SIVmac239 gp120 glycoproteins to bind the CCR5 mutants. These results identify a region of CCR5 that is necessary for the physical association of the gp120 envelope glycoprotein with CCR5 and for HIV-1 infection.

The mechanisms by which chemokines bind and signal through their receptors are complex and poorly understood. In the present study, we sought to dissect these processes and to map important functional domains of the two CXC chemokine (interleukin-8) receptors, CXCR1 (formally IL-8RA) and CXCR2 (formally IL-8RB), using blocking monoclonal antibodies (mAbs) to the receptors and a series of chimeras between CXCR1 and CXCR2. A panel of specific mAbs against CXCR1 or CXCR2, generated by immunizing mice with transfectants expressing either receptor, were shown to effectively block IL-8-and/or growth-related oncogene ␣ (GRO␣) -mediated ligand binding, chemotaxis, elastase release, and VCAM-1 binding in CXCR1 and CXCR2 transfectants and/or human neutrophils. Of particular interest was an anti-CXCR1 mAb, 7D9, that inhibited chemotaxis, elastase release, and VCAM-1 binding but had no detectable effects on ligand binding. The epitopes of these blocking mAbs were mapped by using a series of CXCR1/2 chimera transfectants and synthetic peptides. Most of the anti-CXCR1 antibodies, except 7D9, mapped to the amino acid sequence WDFDDL (CXCR1 residues 10 -15), and all the anti-CXCR2 antibodies mapped to the amino acid sequence FEDFW (CXCR2 residues 6 -10). The epitope of mAb 7D9 mainly involved a region within the first 45 residues of CXCR1, and it appeared to be conformation-sensitive. These results support a model in which the binding and signaling of IL-8 with its receptor occur in at least two discrete steps involving distinct domains of the receptor. This model is consistent with the notion that discrete conformational changes of the receptor secondary to ligand binding are required to trigger various biological responses. Moreover, the ligand binding and chemotaxis properties of each CXCR1/2 chimeric receptor to IL-8 and GRO␣ were determined. It was found that each is distinct in its ability to confer ligand binding and chemotactic response to IL-8 and GRO␣, and two conclusions could be made. 1) The N-terminal segment of CXCR1 is a dominant determinant of receptor subtype selectivity, consistent with previous studies using rabbit/human CXCR1/2 chimeras; and 2) the specificity determinant for GRO␣ binding in CXCR2 involves sequences in the N terminus, distal to the first 15 residues, as well as other parts of the receptor.