A dual-tropic primary HIV-1 isolate that uses fusin and the beta-chemokine receptors CKR-5, CKR-3, and CKR-2b as fusion cofactors

A molecularly cloned, pathogenic, neutralization-resistant simian immunodeficiency virus, SIVsmE543-3

An infectious molecular clone of simian immunodeficiency virus SIVsm was derived from a biological isolate obtained late in disease from an immunodeficient rhesus macaque (E543) with SIV-induced encephalitis. The molecularly cloned virus, SIVsmE543-3, replicated well in macaque peripheral blood mononuclear cells and monocyte-derived macrophages and resisted neutralization by heterologous sera which broadly neutralized genetically diverse SIV variants in vitro. SIVsmE543-3 was infectious and induced AIDS when inoculated intravenously into pig-tailed macaques (Macaca nemestrina). Two of four infected macaques developed no measurable SIV-specific antibody and succumbed to a wasting syndrome and SIV-induced meningoencephalitis by 14 and 33 weeks postinfection. The other two macaques developed antibodies reactive in Western blot and virus neutralization assays. One macaque was sacrificed at 1 year postinoculation, and the survivor has evidence of immunodeficiency, characterized by persistently low CD4 lymphocyte subsets in the peripheral blood. Plasma samples from these latter animals neutralized SIVsmE543-3 but with much lower efficiency than neutralization of other related SIV strains, confirming the difficulty by which this molecularly cloned virus is neutralized in vitro. SIVsmE543-3 will provide a valuable reagent for studying SIV-induced encephalitis, mapping determinants of neutralization, and determining the in vivo significance of resistance to neutralization in vitro.

Length polymorphism within the second variable region of the human immunodeficiency virus type 1 envelope glycoprotein affects accessibility of the receptor binding site

Sequential mutations were introduced into the V2 region of human immunodeficiency virus (HIV) type 1 HXB2, affecting the length, charge, and number of potential glycosylation sites. The insertions had no effect on cytopathicity or on the ability of virus to replicate in peripheral blood mononuclear cells and established T-cell lines. However, deletion of amino acids 186 to 188, encoding a conserved glycosylation site, resulted in a nonviable virus, suggesting a minimal length requirement of 40 amino acids for a functional V2 loop. However, all amino acid insertions affected the sensitivity of the variants to neutralization by soluble CD4 and monoclonal antibodies specific for epitopes in the V3 and CD4 binding site regions. Furthermore, these mutant viruses showed resistance to neutralization by HIV-positive human sera. Soluble gp120 mutant glycoproteins showed increased affinities for soluble CD4 and monoclonal antibodies specific for a number of epitopes overlapping the CD4 binding site, confirming that length increases in V2 affect exposure of the CD4 binding site. In summary, these data demonstrate that differences in V2 length modulate immunoreactivity of the envelope glycoprotein and support an association between the V2 and CD4 binding site regions.

Escape of human immunodeficiency virus from immune control

Cytotoxic T lymphocytes (CTL) play a crucial role in the attempt to control infection with human immunodeficiency virus (HIV). Variation in epitopes recognized by CTL is common and frequently offers potential escape routes for mutant virus. Proof of escape, however, requires demonstration of increased frequency of virus particles or provirus that carry the escape sequence. There are now several recorded examples of virus variants that escape from CTL and are then selected. Most dramatic are those in which the CTL response has been dominated by CTL recognizing a single epitope that has suddenly changed, resulting in escape to fixation. This has been seen both early and late in the infection, leaving no doubt that escape occurs. Such escape is likely to be favored when the antiviral CTL response is oligoclonal and focused on a small number of immunodominant epitopes. The heterogeneous CTL response seen in many HIV-infected patients may result from successive waves of virus escape followed by new CTL responses specific for subdominant epitopes. Mutant virus can escape by several different routes, including failure of the mutated peptide to bind to the presenting HLA molecule and altered interactions with T cell receptors (TCR), including antagonism.

Homeostasis of chemokines, interferon production and lymphocyte subsets: implications for AIDS pathogenesis

Certain individuals with elevated levels of macrophage inflammatory protein (MIP)1 alpha, MIP1 beta and RANTES expression appear to be resistant to human immunodeficiency virus (HIV) infection. In this work, we demonstrate that chemokines production by peripheral blood mononuclear cells (PBMCs) are homeostatic parameters varying from one individual to another, and we define optimized experimental conditions to reproducibly assess these parameters. We also studied alpha- and gamma-interferons (IFN alpha and IFN gamma, respectively) which have been implicated in the pathogenesis of acquired immunodeficiency syndrome (AIDS). The kinetics of production of all these cytokines by fresh PBMCs were determined upon stimulation with phytohemagglutinin (PHA), staphylococcus enterotoxin b (SEB) and purified protein derivative (PPD). RANTES and MIP1 alpha are produced early in response to activation, followed by MIP1 beta, (alpha-interferon, gamma-interferon, alpha IFN, gamma-IFN alpha and IFN alpha and gamma. These results suggest that using our methodology, chemokines levels can be reliably determined, permitting the performance of accurate genetic studies using PBMCs from various cohorts (siblings or AIDS related cohorts).

HIV-1-specific cell-mediated immunity is enhanced by co-inoculation of TCA3 expression plasmid with DNA vaccine

We developed a candidate DNA vaccine designated pCMV160IIIB with pcREV (pCMV160IIIB/REV) that encodes gp160 of human immunodeficiency virus (HIV)-1IIIB and Rev driven by the cytomegalovirus (CMV) promotor. This vaccine induced both HIV-1-specific antibodies and cytotoxic T lymphocyte (CTL) activity. In the present study, we inoculated the TCA3 expression plasmid into mouse skeletal muscle with pCMV160IIIB/REV to determine whether this cytokine expression plasmid was able to modify the immune response. Results of a delayed-type hypersensitivity (DTH) assay using footpad swelling as well as those of a CTL assay clearly demonstrated that cell-mediated immunity (CMI) elicited by co-inoculation of pCMV160IIIB/REV with the TCA3 expression plasmid was markedly enhanced compared with that obtained using pCMV160IIIB/REV alone. When TCA3 expression plasmid was inoculated with anti-TCA3 antibody, enhancement of the DTH response was suppressed below the level of that obtained with pCMV160IIIB/REV alone. The titre of HIV-1-specific IgG2a was slightly high when pCMV160IIIB/REV was co-inoculated with this plasmid, suggesting that T-helper 1 (Th1) response was predominant in TCA3-inoculated mice. Infiltration of mononuclear cells was seen in the muscles at sites where TCA3 expression plasmid had been inoculated. Our present data suggest that TCA3 expression plasmid has potent adjuvant activity that results in an augmented CMI response.

The Role of the Envelope Glycoprotein in the Depletion of T Helper Cells in Human Immunodeficiency Virus Infection

Infection with the human immunodeficiency virus (HIV) causes gradual depletion of CD4+ T helper lymphocytes and destruction of the lymphoid tissue, which ultimately leads to a fatal defect of the cellular immune system. Paramount to the understanding of the pathogenesis of HIV infection is to elucidate the mechanism which underlies the loss of T helper cells. Various ideas have been proposed in order to explain this issue. Several hypotheses have focused on the role of the envelope glycoprotein in this process. This review summarizes the data obtained and concepts proposed regarding the involvement of the HIV glycoprotein in the pathology of CD4+ T cell depletion.

Synthetic peptide from the V3 loop consensus motif with a potent anti-HIV activity inhibits ristocetin-mediated vWF-GPIb interaction

The V3 loop consensus motif. Arg-Gly-Pro-Gly-Arg-Ala-Phe-Val-Thr-Ile (HIV-1 IIIB), inhibits an interaction of HIV with CD4-positive lymphocytes. Recently, both proline-rich peptides and peptides containing proline-glycine loops (beta-turns) form a complex with ristocetin dimers. These peptides interact with ristocetin-loaded platelet membrane glycoprotein (GP) Ib and act as inhibitors of von Willebrand factor (vWF)-GPIb interaction by preventing the subsequent formation of ristocetin dimer bridges. The Pro-Gly sequence is also present in the V3 loop consensus motif, Arg-Gly-Pro-Gly-Arg-Ala-Phe-Val-Thr-Ile (HIV-1 IIIB). In this report, we have evaluated the effect of the HIV-1 IIIB peptide on vWF binding to GPIb. This peptide only inhibited vWF binding to GPIb as well as platelet aggregation in the presence of ristocetin while it had no effect on botrocetin-mediated vWF interaction with platelets. The peptide inhibited a binding of anti-vWF monoclonal antibody (RG-46) to immobilized vWF. Furthermore, ristocetin inhibited the binding of HIV-1 IIIB peptide to immobilized CXC-chemokine receptor-4 (CXCR-4) peptide. These results indicate that ristocetin may prevent HIV infection and would be useful a tool to understand the mechanism of HIV tissue tropism and infection.

Multifactorial nature of noncytolytic CD8+ T cell-mediated suppression of HIV replication: beta-chemokine-dependent and -independent effects

Chemokines were originally characterized by their ability to direct migration and induce activation of selected leukocyte populations. The beta-chemokines MIP-1 alpha, MIP-beta, and RANTES have been implicated in the suppression of viral replication by CD8+ T cells from HIV-infected individuals. The present study was undertaken to evaluate the effect of beta-chemokines on HIV replication in cocultures of dendritic cells (DCs) and CD4+ T cells, and an in vitro model of the lymphoid microenvironment. In the acute infection system, where DCs from uninfected individuals are pulsed with HIV and cocultured with autologous CD4+ T cells, no inhibition of replication of monocytotropic or T cell tropic viral isolates by MIP-1 alpha, MIP-1 beta, and RANTES, alone or in combination, was observed. In contrast, in an endogenous infection system, where the DCs and CD4+ T cells were obtained from HIV-infected subjects, addition of recombinant beta-chemokines suppressed HIV replication. However, neutralizing antibodies to beta-chemokines did not affect the suppressive activity of CD8+ T cells from HIV-infected donors in either system, suggesting that CD8+ T cell-mediated suppression is not due exclusively to beta-chemokines. Furthermore, no significant differences in secretion of MIP-1 alpha, MIP-1 beta, and RANTES by purified CD8+ T cells were noted in uninfected versus HIV-infected donors, regardless of the stage of disease. These results indicate that HIV suppression by CD8+ T cells derived from HIV-infected donors is a multifactorial phenomenon and not limited to the action of MIP-1 alpha, MIP-1 beta, and RANTES.

Human chemokines: an update

Interleukin 8, the first chemokine to be characterized, was discovered nearly ten years ago. Today, more than 30 human chemokines are known. They are often upregulated in inflammation and act mainly on leukocytes inducing migration and release responses. The present review deals largely with the new developments of the last three years. Several structural studies have shown that most chemokines form dimers. The dimers, however, dissociate upon dilution, and the monomers constitute the biologically active form. Chemokine activities are mediated by seven-transmembrane-domain, G protein coupled receptors, five of which were discovered in the past three years. The primary receptor-binding domain of all chemokines is near the NH2 terminus, and antagonists can be obtained by truncation or substitutions in this region. Major progress has been made in the understanding of chemokine actions on T lymphocytes that respond to several CC chemokines but also to IP10 and Mig, two CXC chemokines that selectively attract T cells via a novel receptor. Effects of chemokines on angiogenesis and tumor growth have been reported, but the data are still contradictory and the mechanisms unknown. Of considerable interest is the recent discovery that some chemokines function as HIV-suppressive factors by interacting with chemokine receptors which, together with CD4, were recognized as the binding sites for HIV-1.

Host genes and HIV: the role of the chemokine receptor gene CCR5 and its allele

Since the late 1970s, 8.4 million people worldwide, including 1.7 million children, have died of AIDS, and an estimated 22 million people are infected with human immunodeficiency virus (HIV)(1). During 1995 and 1996, major clinical and laboratory discoveries regarding HIV pathogenesis provided new hope for the prevention and treatment of HIV infection. One major discovery was that members of the chemokine receptor family serve as cofactors for HIV entry into cells. We describe the role of allelic polymorphism in the gene coding for the CCR5 chemokine receptor with regard to susceptibility to and disease course of HIV infection. We also examine the effect of this discovery on medical and public health practices.

Sensitivity to inhibition by beta-chemokines correlates with biological phenotypes of primary HIV-1 isolates

Primary HIV-1 isolates were evaluated for their sensitivity to inhibition by beta-chemokines RANTES (regulated upon activation, normal T-cell expressed and secreted), macrophage inflammatory protein 1 alpha (MIP-1 alpha), and MIP-1 beta. Virus isolates of both nonsyncytium-inducing (NSI) and syncytium-inducing (SI) biological phenotypes recovered from patients at various stages of HIV-1 infection were assessed, and the results indicated that only the isolates with the NSI phenotype were substantially inhibited by the beta-chemokines. More important to note, these data demonstrate that resistance to inhibition by beta-chemokines RANTES, MIP-1 alpha, and MIP-1 beta is not restricted to T cell line-adapted SI isolates but is also a consistent property among primary SI isolates. Analysis of isolates obtained sequentially from infected individuals in whom viruses shifted from NSI to SI phenotype during clinical progression exhibited a parallel loss of sensitivity to beta-chemokines. Loss of virus sensitivity to inhibition by beta-chemokines RANTES, MIP-1 alpha, and MIP-1 beta was furthermore associated with changes in the third variable (V3) region amino acid residues previously described to correlate with a shift of virus phenotype from NSI to SI. Of interest, an intermediate V3 genotype correlated with a partial inhibition by the beta-chemokines. In addition, we also identified viruses sensitive to RANTES, MIP-1 alpha, and MIP-1 beta of NSI phenotype that were isolated from individuals with AIDS manifestations, indicating that loss of sensitivity to beta-chemokine inhibition and shift in viral phenotype are not necessarily prerequisites for the pathogenesis of HIV-1 infection.

CD8+ T-cell-derived soluble factor(s), but not beta-chemokines RANTES, MIP-1 alpha, and MIP-1 beta, suppress HIV-1 replication in monocyte/macrophages

It has been demonstrated that CD8+ T cells produce a soluble factor(s) that suppresses human immuno-deficiency virus (HIV) replication in CD4+ T cells. The role of soluble factors in the suppression of HIV replication in monocyte/macrophages (M/M) has not been fully delineated. To investigate whether a CD8+ T-cell-derived soluble factor(s) can also suppress HIV infection in the M/M system, primary macrophages were infected with the macrophage tropic HIV-1 strain Ba-L. CD8+ T-cell-depleted peripheral blood mononuclear cells were also infected with HIV-1 IIIB or Ba-L. HIV expression from the chronically infected macrophage cell line U1 was also determined in the presence of CD8+ T-cell supernatants or beta-chemokines. We demonstrate that: (i) CD8+ T-cell supernatants did, but beta-chemokines did not, suppress HIV replication in the M/M system; (ii) antibodies to regulated on activation normal T-cell expressed and Secreted (RANTES), macrophage inflammatory protein 1 alpha (MIP-1 alpha) and MIP-1 beta did not, whereas antibodies to interleukin 10, interleukin 13, interferon alpha, or interferon gamma modestly reduced anti-HIV activity of the CD8+ T-cell supernatants; and (iii) the CD8+ T-cell supernatants did, but beta-chemokines did not, suppress HIV-1 IIIB replication in peripheral blood mononuclear cells as well as HIV expression in U1 cells. These results suggest that HIV-suppressor activity of CD8+ T cells is a multifactorial phenomenon, and that RANTES, MIP-1 alpha, and MIP-1 beta do not account for the entire scope of CD8+ T-cell-derived HIV-suppressor factors.

Host factors and the pathogenesis of HIV-induced disease

The level of human immunodeficiency virus (HIV) replication in patients reflects a balance between stimulatory and inhibitory host factors (particularly endogenous cytokines). New information concerning the cellular co-receptors for HIV and the cellular tropism of different strains of virus will advance our understanding of HIV-induced pathogenesis and suggests new therapeutic and preventive strategies.

Molecular cloning and characterization of a murine pre-B-cell growth-stimulating factor/stromal cell-derived factor 1 receptor, a murine homolog of the human immunodeficiency virus 1 entry coreceptor fusin

Pre-B-cell growth-stimulating factor/ stromal cell-derived factor 1 (PBSF/SDF-1) is a member of the CXC group of chemokines that is initially identified as a bone marrow stromal cell-derived factor and as a pre-B-cell stimulatory factor. Although most chemokines are thought to be inducible inflammatory mediators, PBSF/SDF-1 is essential for perinatal viability,. B lymphopoiesis, bone marrow myelopoiesis, and cardiac ventricular septal formation, and it has chemotactic activities on resting lymphocytes and monocytes. In this paper, we have isolated a cDNA that encodes a seven transmembrane-spanning-domain receptor, designated pre-B-cell-derived chemokine receptor (PB-CKR) from a murine pre-B-cell clone, DW34. The deduced amino acid sequence has 90% identity with that of a HUMSTSR/fusin, a human immunodeficiency virus 1 (HIV-1) entry coreceptor. However, the second extracellular region has lower identity (67%) compared with HUMSTSR/fusin. PB-CKR is expressed during embryo genesis and in many organs and T cells of adult mice. Murine PBSF/SDF-1 induced an increase in intracellular free Ca2+ in DW34 cells and PB-CKR-transfected Chinese hamster ovary (CHO) cells, suggesting that PB-CKR is a functional receptor for murine PBSF/SDF-1. Murine PBSF/ SDF-1 also induced Ca2+ influx in fusin-transfected CHO cells. On the other hand, considering previous results that HIV-1 does not enter murine T cells that expressed human CD4, PB-CKR may not support HIV-1 infection. Thus, PB-CKR will be an important tool for functional mapping of HIV-1 entry coreceptor fusin and for understanding the function of PBSF/SDF-1 further.

Chemokines: molecular double agents

Chemokines are best known as signalling molecules that attract leukocytes to sites of inflammation, but recent results show they are also important regulators of hematopoiesis during development.

Mapping of independent V3 envelope determinants of human immunodeficiency virus type 1 macrophage tropism and syncytium formation in lymphocytes

The V3 region of the human immunodeficiency virus type 1 (HIV-1) envelope protein is known to have a major influence on macrophage tropism as well as the ability to cause syncytium formation or fusion in CD4-positive lymphocyte cultures. Using infectious molecular HIV-1 clones, a series of mutant clones was created which allowed detailed mapping of V3 amino acid positions involved in these properties. In these experiments the non-syncytium-inducing phenotype in T cells did not always correlate with macrophage tropism. Macrophage tropism appeared to depend on the presence of certain combinations of amino acids at five specific positions within and just outside of the V3 loop itself, whereas syncytium formation in lymphocytes was influenced by substitution of particular residues at two to four positions within V3. In most cases, different V3 amino acid positions were found to independently influence macrophage tropism and syncytium formation in T cells and position 13 was the only V3 location which appeared to simultaneously influence both macrophage tropism and syncytium formation in lymphocytes.

Molecular cloning and chromosomal mapping of a novel human gene, ChemR1, expressed in T lymphocytes and polymorphonuclear cells and encoding a putative chemokine receptor

We describe the cloning of a human gene, named ChemR1, encoding a new putative chemokine receptor sharing 48% identity with CC-chemokine receptor (CCR)4 and 44% identity with CCR1. It displays four extracellular cysteines that are conserved among all other chemokine receptors. ChemR1 transcripts were detected by Northern blotting in the T lymphoblastic cell lines Jurkat and MOLT-4, but not in the pre-B lymphoblastic cell line JM-1. ChemR1 receptor transcripts were also detected by reverse transcription and polymerase chain reaction analysis in unstimulated CD4+ and CD8+ T cells and polymorphonuclear cells prepared from peripheral blood. The chromosomal localization was performed by radiation hybrid mapping and testing of a panel of yeast artificial chromosome clones. This allowed the assignment of the ChemR1 receptor gene to the p21.3-24 region of human chromosome 3, in close proximity with the functionally characterized CCR. Future work is required to identify the ligand(s) of this new chemokine receptor and to define its role in the recruitment of white blood cell populations.

Primary, syncytium-inducing human immunodeficiency virus type 1 isolates are dual-tropic and most can use either Lestr or CCR5 as coreceptors for virus entry

A panel of primary syncytium-inducing (SI) human immunodeficiency virus type 1 isolates that infected several CD4+ T-cell lines, including MT-2 and C8166, were tested for infection of blood-derived macrophages. Infectivity titers for C8166 cells and macrophages demonstrated that primary SI strains infected macrophages much more efficiently than T-cell line-adapted HIV-1 strains such as LAI and RF. These primary SI strains were therefore dual-tropic. Nine biological clones of two SI strains, prepared by limiting dilution, had macrophage/C8166 infectivity ratios similar to those of their parental viruses, indicating that the dual-tropic phenotype was not due to a mixture of non-SI/macrophage-tropic and SI/T-cell tropic viruses. We tested whether the primary SI strains used either Lestr (fusin) or CCR5 as coreceptors. Infection of cat CCC/CD4 cells transiently expressing Lestr supported infection by T-cell line-adapted strains including LAI, whereas CCC/CD4 cells expressing CCR5 were sensitive to primary non-SI strains as well as to the molecularly cloned strains SF-162 and JR-CSF. Several primary SI strains, as well as the molecularly cloned dual-tropic viruses 89.6 and GUN-1, infected both Lestr+ and CCR5+ CCC/CD4 cells. Thus, these viruses can choose between Lestr and CCR5 for entry into cells. Interestingly, some dual-tropic primary SI strains that infected Lestr+ cells failed to infect CCR5+ cells, suggesting that these viruses may use an alternative coreceptor for infection of macrophages. Alternatively, CCR5 may be processed or presented differently on cat cells so that entry of some primary SI strains but not others is affected.

CD4-independent infection by HIV-2 is mediated by fusin/CXCR4

Several members of the chemokine receptor family have been shown to function in association with CD4 to permit HIV-1 entry and infection. However, the mechanism by which these molecules serve as CD4-associated cofactors is unclear. In the present report, we show that one member of this family, termed Fusin/ CXCR4, is able to function as an alternative receptor for some isolates of HIV-2 in the absence of CD4. This conclusion is supported by the finding that (1) CD4-independent infection by these viruses is inhibited by an anti-Fusin monoclonal antibody, (2) Fusin expression renders human and nonhuman CD4-negative cell lines sensitive to HIV-2-induced syncytium induction and/or infection, and (3) Fusin is selectively down-regulated from the cell surface following HIV-2 infection. The finding that one chemokine receptor can function as a primary viral receptor strongly suggests that the HIV envelope glycoprotein contains a binding site for these proteins and that differences in the affinity and/or the availability of this site can extend the host range of these viruses to include a number of CD4-negative cell types.

CD4-dependent, antibody-sensitive interactions between HIV-1 and its co-receptor CCR-5

The beta-chemokine receptor CCR-5 is an essential co-factor for fusion of HIV-1 strains of the non-syncytium-inducing (NSI) phenotype with CD4+ T-cells. The primary binding site for human immunodeficiency virus (HIV)-1 is the CD4 molecule, and the interaction is mediated by the viral surface glycoprotein gp120 (refs 6, 7). The mechanism of CCR-5 function during HIV-1 entry has not been defined, but we have shown previously that its beta-chemokine ligands prevent HIV-1 from fusing with the cell. We therefore investigated whether CCR-5 acts as a second binding site for HIV-1 simultaneously with or subsequent to the interaction between gp120 and CD4. We used a competition assay based on gp120 inhibition of the binding of the CCR-5 ligand, macrophage inflammatory protein (MIP)-1beta, to its receptor on activated CD4+ T cells or CCR-5-positive CD4- cells. We conclude that CD4 binding, although not absolutely necessary for the gp120-CCR-5 interaction, greatly increases its efficiency. Neutralizing monoclonal antibodies against several sites on gp120, including the V3 loop and CD4-induced epitopes, inhibited the interaction of gp120 with CCR-5, without affecting gp120-CD4 binding. Interference with HIV-1 binding to one or both of its receptors (CD4 and CCR-5) may be an important mechanism of virus neutralization.

CD4-induced interaction of primary HIV-1 gp120 glycoproteins with the chemokine receptor CCR-5

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.

Possible role of the V3 domain of gp120 in resistance to an amphotericin B derivative (MS8209) blocking human immunodeficiency virus entry

MS8209, an amphotericin B derivative blocking human immunodeficiency virus type 1 (HIV-1) entry after CD4 binding, neutralized the HIV-2 strains EHO and ROD10 but not ROD(CEM). In the V3 domain of gp120, ROD(CEM) differed from ROD10 at two positions (a threonine instead of an isoleucine at position 312 and an arginine instead of a glutamine at position 329), and drug resistance was conferred to HIV-1 by substitution of the ROD(CEM) V3 but not the ROD10 V3. V3 mutations may prevent the interaction of gp120 with MS8209 or modify the mechanism of virus entry, rendering it less accessible to neutralization.

Regions in beta-chemokine receptors CCR5 and CCR2b that determine HIV-1 cofactor specificity

Macrophage-tropic (M-tropic) HIV-1 strains use the beta-chemokine receptor CCR5, but not CCR2b, as a cofactor for membrane fusion and infection, while the dual-tropic strain 89.6 uses both. CCR5/2b chimeras and mutants were used to map regions of CCR5 important for cofactor function and specificity. M-tropic strains required either the amino-terminal domain or the first extracellular loop of CCR5. A CCR2b chimera containing the first 20 N-terminal residues of CCR5 supported M-tropic envelope protein fusion. Amino-terminal truncations of CCR5/CCR2b chimeras indicated that residues 2-5 are important for M-tropic viruses, while 89.6 is dependent on residues 6-9. The identification of multiple functionally important regions in CCR5, coupled with differences in how CCR5 is used by M- and dual-tropic viruses, suggests that interactions between HIV-1 and entry cofactors are conformationally complex.

Herpes simplex virus-1 entry into cells mediated by a novel member of the TNF/NGF receptor family

We identified and cloned a cellular mediator of herpes simplex virus (HSV) entry. Hamster and swine cells resistant to viral entry became susceptible upon expression of a human cDNA encoding this protein, designated HVEM (for herpesvirus entry mediator). HVEM was shown to mediate the entry of several wild-type HSV strains of both serotypes. Anti-HVEM antibodies and a soluble hybrid protein containing the HVEM ectodomain inhibited HVEM-dependent infection but not virus binding to cells. Mutations in the HSV envelope glycoprotein gD significantly reduced HVEM-mediated entry. The contribution of HVEM to HSV entry into human cells was demonstrable in activated T cells. HVEM, the first identified mediator of HSV entry, is a new member of the TNF/NGF receptor family.

A mechanism of restricted human immunodeficiency virus type 1 expression in human glial cells

We characterized in detail the life cycle of human immunodeficiency virus type 1 (HIV-1) in human glioma H4/CD4 cells which stably express transfected CD4 DNA (B. Volsky, K. Sakai, M. Reddy, and D. J. Volsky, Virology 186:303-308, 1992). Infection of cloned H4/CD4 cells with the N1T strain of cell-free HIV-1 (HIV-1/N1T) was rapid and highly productive as measured by the initial expression of viral DNA, RNA, and protein, but all viral products declined to low levels by 14 days after infection. Chronically infected, virus-producing H4/CD4 cells could be obtained by cell cloning, indicating that HIV-1 DNA can integrate and remain expressed in these cells. The HIV-1 produced in H4/CD4 cells was noninfectious to glial cells, but it could be transmitted with low efficiency to CEM cells. Examination of viral protein composition by immunoprecipitation with AIDS serum or anti-gp120 antibody revealed that HIV-1/N1T-infected H4/CD4 cells produced all major viral proteins including gp160, but not gp120. Deglycosylation experiments with three different glycosidases determined that the absence of gp120 was not due to aberrant glycosylation of gp160, indicating a defect in gp160 proteolytic processing. Similar results were obtained in acutely and chronically infected H4/CD4 cells. To determine the generality of this HIV-1 replication phenotype in H4/CD4 cells, nine different viral clones were tested for replication in H4/CD4 cells by transfection. Eight were transiently productive like N1T, but one clone, NL4-3, established a long-lived productive infection in H4/CD4 cells, produced infectious progeny virus, and produced both gp160 and gp120. We conclude that for most HIV-1 strains tested, HIV-1 infection of H4/CD4 is restricted to a single cycle because of the defective processing of gp160, resulting in the absence of gp120 on progeny virus.

The role of a mutant CCR5 allele in HIV-1 transmission and disease progression

A 32-nucleotide deletion (delta 32) within the beta-chemokine receptor 5 (CCR5) gene has been described in subjects who remain uninfected despite extensive exposure to HIV-1. This allele was found to be common in the Caucasian population with a frequency of 0.0808, but was not found in people of African or Asian ancestry. To determine its role in HIV-1 transmission and disease progression, we analyzed the CCRS genotype of 1252 homosexual men enrolled in the Chicago component of the Multicenter AIDS Cohort Study (MACS). No infected participant was found to be homozygous for the delta 32 allele, whereas 3.6% of at-risk but uninfected Caucasian participants were homozygous, showing the highly protective role of this genotype against sexual acquisition of HIV-1. No evidence was found to suggest that heterozygotes were protected against HIV-1 infection, but a limited protective role against disease progression was noted. The delta 32 allele of CCR5 is therefore an important host factor in HIV-1 transmission and pathogenesis.

Genetic restriction of HIV-1 infection and progression to AIDS by a deletion allele of the CKR5 structural gene. Hemophilia Growth and Development Study, Multicenter AIDS Cohort Study, Multicenter Hemophilia Cohort Study, San Francisco City Cohort, ALIVE Study

The chemokine receptor 5 (CKR5) protein serves as a secondary receptor on CD4(+) T lymphocytes for certain strains of human immunodeficiency virus-type 1 (HIV-1). The CKR5 structural gene was mapped to human chromosome 3p21, and a 32-base pair deletion allele (CKR5Delta32) was identified that is present at a frequency of approximately0.10 in the Caucasian population of the United States. An examination of 1955 patients included among six well-characterized acquired immunodeficiency syndrome (AIDS) cohort studies revealed that 17 deletion homozygotes occurred exclusively among 612 exposed HIV-1 antibody-negative individuals (2.8 percent) and not at all in 1343 HIV-1-infected individuals. The frequency of CKR5 deletion heterozygotes was significantly elevated in groups of individuals that had survived HIV-1 infection for more than 10 years, and, in some risk groups, twice as frequent as their occurrence in rapid progressors to AIDS. Survival analysis clearly shows that disease progression is slower in CKR5 deletion heterozygotes than in individuals homozygous for the normal CKR5 gene. The CKR5Delta32 deletion may act as a recessive restriction gene against HIV-1 infection and may exert a dominant phenotype of delaying progression to AIDS among infected individuals.

Cofactors provide the entry keys. HIV-1

The identification of the cofactors required for HIV-1 entry into cells promises to provide new insights into viral transmission and pathogenesis, and opens new avenues for AIDS therapy and prophylaxis.

The CXC chemokine SDF-1 is the ligand for LESTR/fusin and prevents infection by T-cell-line-adapted HIV-1

A putative chemokine receptor that we previously cloned and termed LESTR has recently been shown to function as a co-receptor (termed fusin) for lymphocyte-tropic HIV-1 strains. Cells expressing CD4 became permissive to infection with T-cell-line-adapted HIV-1 strains of the syncytium-inducing phenotype after transfection with LESTR/fusin complementary DNA. We report here the indentification of a human chemokine of the CXC type, stromal cell-derived factor 1 (SDF-1), as the natural ligand for LESTR/fusin, and we propose the term CXCR-4 for this receptor, in keeping with the new chemokine-receptor nomenclature. SDF-1 activates Chinese hamster ovary (CHO) cells transfected with CXCR-4 cDNA as well as blood leukocytes and lymphocytes. In cell lines expressing CXCR-4 and CD4, and in blood lymphocytes, SDF-1 is a powerful inhibitor of infection by lymphocyte-tropic HIV-1 strains, whereas the CC chemokines RANTES, MIP-1 alpha and MIP-1 beta, which were shown previously to prevent infection with primary, monocyte-tropic viruses, are inactive. In combination with CC chemokines, which block the infection with monocyte/macrophage-tropic viruses, SDF-1 could help to decrease virus load and prevent the emergence of the syncytium-inducing viruses which are characteristic of the late stages of AIDS.

The lymphocyte chemoattractant SDF-1 is a ligand for LESTR/fusin and blocks HIV-1 entry

Chemokines are chemotactic cytokines that activate and direct the migration of leukocytes. There are two subfamilies, the CXC and the CC chemokines. We recently found that the CXC-chemokine stromal cell-derived factor-1 (SDF-1) is a highly efficacious lymphocyte chemoattractant. Chemokines act on responsive leukocyte subsets through G-protein-coupled seven-transmembrane receptors, which are also used by distinct strains of HIV-1 as cofactors for viral entry. Laboratory-adapted and some T-cell-line-tropic (T-tropic) primary viruses use the orphan chemokine receptor LESTR/fusin (also known as fusin), whereas macrophage-tropic primary HIV-1 isolates use CCR-5 and CCR-3 (refs 7-11), which are receptors for known CC chemokines. Testing of potential receptors demonstrated that SDF-1 signalled through, and hence 'adopted', the orphan receptor LESTR, which we therefore designate CXC-chemokine receptor-4 (CXCR-4). SDF-1 induced an increase in intracellular free Ca2+ and chemotaxis in CXCR-4-transfected cells. Because SDF-1 is a biological ligand for the HIV-1 entry cofactor LESTR, we tested whether it inhibited HIV-1. SDF-1 inhibited infection by T-tropic HIV-1 of HeLa-CD4 cells, CXCR-4 transfectants, and peripheral blood mononuclear cells (PBMCs), but did not affect CCR-5-mediated infection by macrophage-tropic (M-tropic) and dual-tropic primary HIV-1.

Resistance to HIV-1 infection in caucasian individuals bearing mutant alleles of the CCR-5 chemokine receptor gene

HIV-1 and related viruses require co-receptors, in addition to CD4, to infect target cells. The chemokine receptor CCR-5 (ref.1) was recently demonstrated to be a co-receptor for macrophage-tropic (M-tropic) HIV-1 strains, and the orphan receptor LESTR (also called fusin) allows infection by strains adapted for growth in transformed T-cell lines (T-tropic strains). Here we show that a mutant allele of CCR-5 is present at a high frequency in caucasian populations (allele frequency, 0.092), but is absent in black populations from Western and Central Africa and Japanese populations. A 32-base-pair deletion within the coding region results in a frame shift, and generates a non-functional receptor that does not support membrane fusion or infection by macrophage- and dual-tropic HIV-1 strains. In a cohort of HIV-1 infected caucasian subjects, no individual homozygous for the mutation was found, and the frequency of heterozygotes was 35% lower than in the general population. White blood cells from an individual homozygous for the null allele were found to be highly resistant to infection by M-tropic HIV-1 viruses, confirming that CCR-5 is the major co-receptor for primary HIV-1 strains. The lower frequency of heterozygotes in seropositive patients may indicate partial resistance.

Homozygous defect in HIV-1 coreceptor accounts for resistance of some multiply-exposed individuals to HIV-1 infection

Rare individuals have been multiply exposed to HIV-1 but remain uninfected. The CD4+ T-cells of two of these individuals, designated EU2 and EU3, are highly resistant in vitro to the entry of primary macrophagetropic virus but are readily infectable with transformed T-cell line adapted viruses. We report here on the genetic basis of this resistance. We found that EU2 and EU3 have a homozygous defect in CKR-5, the gene encoding the recently described coreceptor for primary HIV-1 isolates. These individuals appear to have inherited a defective CKR-5 allele that contains an internal 32 base pair deletion. The encoded protein is severely truncated and cannot be detected at the cell surface. Surprisingly, this defect has no obvious phenotype in the affected individuals. Thus, a CKR-5 allele present in the human population appears to protect homozygous individuals from sexual transmission of HIV-1. Heterozygous individuals are quite common (approximately 20%) in some populations. These findings indicate the importance of CKR-5 in HIV-1 transmission and suggest that targeting the HIV-1-CKR-5 interaction may provide a means of preventing or slowing disease progression.

Chemokine receptors - the new frontier for AIDS research

CD4 is widely known as the HIV receptor, but is insufficient to allow viral infection. Recently, members of the family of chemokine receptors have been identified as the missing co-receptors, which act with CD4 to allow the virus to enter cells. These discoveries open up the possibilities of novel therapeutic strategies to combat HIV infection and AIDS.