Coreceptor competition for association with CD4 may change the susceptibility of human cells to infection with T-tropic and macrophagetropic isolates of human immunodeficiency virus type 1

Lipids in infectious diseases - The case of AIDS and tuberculosis

Lipids play a central role in many infectious diseases. AIDS (Acquired Immune Deficiency Syndrome) and tuberculosis are two of the deadliest infectious diseases to have struck mankind. The pathogens responsible for these diseases, Human Immunodeficiency Virus-1 and Mycobacterium tuberculosis, rely on lipids and on lipid membrane properties to gain access to their host cells, to persist in them and ultimately to egress from their hosts. In this Review, we discuss the life cycles of these pathogens and the roles played by lipids and membranes. We then give an overview of therapies that target lipid metabolism, modulate host membrane properties or implement lipid-based drug delivery systems. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.

Membrane organization of virus and target cell plays a role in HIV entry

The initial steps of the Human Immunodeficiency Virus (HIV) replication cycle play a crucial role that arbitrates viral tropism and infection efficiency. Before the release of its genome into the host cell cytoplasm, viruses operate a complex sequence of events that take place at the plasma membrane of the target cell. The first step is the binding of the HIV protein envelope (Env) to the cellular receptor CD4. This triggers conformational changes of the gp120 viral protein that allow its interaction with a co-receptor that can be either CCR5 or CXCR4, defining the tropism of the virus entering the cell. This sequential interaction finally drives the fusion of the viral and host cell membrane or to the endocytosis of the viruses. Here, we discuss how the membrane composition and organization of both the virus and the target cell can affect these steps and thus influence the capability of the viruses to infect cells.

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An overview of lipid role in HIV infection is proposed.

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We discuss the influence of lipid composition on HIV early steps of infection.

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We discuss the role of membrane organization an dynamics in HIV entry.

CCR5/CD4/CXCR4 oligomerization prevents HIV-1 gp120IIIB binding to the cell surface

CCR5 and CXCR4, the respective cell surface coreceptors of R5 and X4 HIV-1 strains, both form heterodimers with CD4, the principal HIV-1 receptor. Using several resonance energy transfer techniques, we determined that CD4, CXCR4, and CCR5 formed heterotrimers, and that CCR5 coexpression altered the conformation of both CXCR4/CXCR4 homodimers and CD4/CXCR4 heterodimers. As a result, binding of the HIV-1 envelope protein gp120IIIB to the CD4/CXCR4/CCR5 heterooligomer was negligible, and the gp120-induced cytoskeletal rearrangements necessary for HIV-1 entry were prevented. CCR5 reduced HIV-1 envelope-induced CD4/CXCR4-mediated cell-cell fusion. In nucleofected Jurkat CD4 cells and primary human CD4(+) T cells, CCR5 expression led to a reduction in X4 HIV-1 infectivity. These findings can help to understand why X4 HIV-1 strains infection affect T-cell types differently during AIDS development and indicate that receptor oligomerization might be a target for previously unidentified therapeutic approaches for AIDS intervention.

Stochastic model of in-vivo X4 emergence during HIV infection: implications for the CCR5 inhibitor maraviroc

The emergence of X4 tropic viral strains throughout the course of HIV infection is associated with poorer prognostic outcomes and faster progressions to AIDS than for patients in whom R5 viral strains predominate. Here we investigate a stochastic model to account for the emergence of X4 virus via mutational intermediates of lower fitness that exhibit dual/mixed (D/M) tropism, and employ the model to investigate whether the administration of CCR5 blockers in-vivo is likely to promote a shift towards X4 tropism. We show that the proposed stochastic model can account for X4 emergence with a median time of approximately 4 years post-infection as a result of: 1.) random stochastic mutations in the V3 region of env during the reverse transcription step of infection; 2.) increasing numbers of CXCR4-expressing activated naive CD4+ T cells with declining total CD4+ T cell counts, thereby providing increased numbers of activated target cells for productive infection by X4 virus. Our model indicates that administration of the CCR5 blocker maraviroc does not promote a shift towards X4 tropism, assuming sufficient efficacy of background therapy (BT). However our modelling also indicates that administration of maraviroc as a monotherapy or with BT of suboptimal efficacy can promote emergence of X4 tropic virus, resulting in accelerated progression to AIDS. Taken together, our results demonstrate that maraviroc is safe and effective if co-administered with sufficiently potent BT, but that suboptimal BT may promote X4 emergence and accelerated progression to AIDS. These results underscore the clinical importance for careful selection of BT when CCR5 blockers are administered in-vivo.

Mechanisms regulating chemokine receptor activity

Co-ordinated movement and controlled positioning of leucocytes is key to the development, maintenance and proper functioning of the immune system. Chemokines and their receptors play an essential role in these events by mediating directed cell migration, often referred to as chemotaxis. The chemotactic property of these molecules is also thought to contribute to an array of pathologies where inappropriate recruitment of specific chemokine receptor-expressing leucocytes is observed, including cancer and inflammatory diseases. As a result, chemokine receptors have become major targets for therapeutic intervention, and during the past 15 years much research has been devoted to understanding the regulation of their biological activity. From these studies, processes which govern the availability of functional chemokine receptors at the cell surface have emerged as playing a central role. In this review, we summarize and discuss current knowledge on the molecular mechanisms contributing to the regulation of chemokine receptor surface expression, from gene transcription and protein degradation to post-translational modifications, multimerization, intracellular transport and cross-talk.

New insights into the mechanisms whereby low molecular weight CCR5 ligands inhibit HIV-1 infection

CC chemokine receptor 5 (CCR5) is a G-protein-coupled receptor for the chemokines CCL3, -4, and -5 and a coreceptor for entry of R5-tropic strains of human immunodeficiency virus type 1 (HIV-1) into CD4(+) T-cells. We investigated the mechanisms whereby nonpeptidic, low molecular weight CCR5 ligands block HIV-1 entry and infection. Displacement binding assays and dissociation kinetics demonstrated that two of these molecules, i.e. TAK779 and maraviroc (MVC), inhibit CCL3 and the HIV-1 envelope glycoprotein gp120 binding to CCR5 by a noncompetitive and allosteric mechanism, supporting the view that they bind to regions of CCR5 distinct from the gp120- and CCL3-binding sites. We observed that TAK779 and MVC are full and weak inverse agonists for CCR5, respectively, indicating that they stabilize distinct CCR5 conformations with impaired abilities to activate G-proteins. Dissociation of [(125)I]CCL3 from CCR5 was accelerated by TAK779, to a lesser extent by MVC, and by GTP analogs, suggesting that inverse agonism contributes to allosteric inhibition of the chemokine binding to CCR5. TAK779 and MVC also promote dissociation of [(35)S]gp120 from CCR5 with an efficiency that correlates with their ability to act as inverse agonists. Displacement experiments revealed that affinities of MVC and TAK779 for the [(35)S]gp120-binding receptors are in the same range (IC(50) ∼6.4 versus 22 nm), although we found that MVC is 100-fold more potent than TAK779 for inhibiting HIV infection. This suggests that allosteric CCR5 inhibitors not only act by blocking gp120 binding but also alter distinct steps of CCR5 usage in the course of HIV infection.

Platelet factor 4 (CXCL4) facilitates human macrophage infection with HIV-1 and potentiates virus replication

Platelet factor 4 (CXCL4), a member of the CXC chemokine subfamily released in high amounts by activated platelets, has been identified as a monocyte survival factor that induces monocyte differentiation into macrophages. Although CXCL4 has been shown to have biological effects unique to chemokines, nothing is known about the role of CXCL4-derived human macrophages or CXCL4 in human immunodeficiency virus (HIV) disease. In this study, CXCL4-derived macrophages are compared with macrophage-colony stimulating factor (M-CSF)-derived macrophages for their ability to support HIV-1 replication. We show that CXCL4-derived macrophages can be infected with macrophage-tropic HIV-1 that uses either CC-chemokine receptor 5 (CCR5) or CXC-chemokine receptor 4 (CXCR4) as a co-receptor for viral entry. We also find that M-CSF and the chemokines, monocyte chemoattractant protein 1 (MCP-1; CCL2) and macrophage-inflammatory-protein-1-alpha (MIP-1alpha; CCL3) are produced upon R5- and X4-tropic HIV-1 replication in both M-CSF- and CXCL4-derived human macrophages. In addition, CXCL4 added to M-CSF-derived macrophages after virus adsorption and maintained throughout the infection enhances HIV-1 replication. We thus propose a novel role for CXCL4 in HIV disease.

Pairwise comparison of isogenic HIV-1 viruses: R5 phenotype replicates more efficiently than X4 phenotype in primary CD4+ T cells expressing physiological levels of CXCR4

CCR5-using (R5) HIV-1 strains are present during the whole course of the infection in all subjects, whereas CXCR4-using (X4) HIV-1 strains appear only in the late stages of the infection in some subjects. In this study, we tested the hypothesis that this phenomenon might be the result of a replicative advantage of R5 over X4 strains. We compared the infectivity of an R5 and an X4 strain that differ only in their env gene in peripheral blood mononuclear cells. CD4 T cells in culture, where the CXCR4 ligand SDF-1 is absent, overexpress CXCR4 at their surface. Therefore, a cell line producing the chemokine SDF-1, that binds to and induces the internalization of CXCR4, was established by transfer of the SDF-1 gene. We cocultured peripheral blood mononuclear cells with this SDF-1-producing cell line to obtain SDF-1 concentrations that maintained the CD4 T cell surface CXCR4 densities observed in vivo. Under these conditions, the R5 strain appeared to replicate more efficiently than the X4 strain. Thus, in vitro, when CD4 T cells express physiological levels of CXCR4 coreceptors, R5 virions are more fit for replication than X4 virions and in vivo that limited surface expression of CXCR4 on cell targets could contribute to the preponderance of R5 viruses.

Accelerated immunodeficiency by anti-CCR5 treatment in HIV infection

In 50% of progressing HIV-1 patients, CXCR4-tropic (X4) virus emerges late in infection, often overtaking CCR5-tropic (R5) virus as the dominant viral strain. This "phenotypic switch" is strongly associated with rapidly declining CD4(+) T cell counts and AIDS onset, yet its causes remain unknown. Here, we analyze a mathematical model for the mechanism of X4 emergence in late-stage HIV infection and use this analysis to evaluate the utility of a promising new class of antiretroviral drugs -- CCR5 inhibitors -- in dual R5, X4 infection. The model shows that the R5-to-X4 switch occurs as CD4(+) T cell activation levels increase above a threshold and as CD4(+) T cell counts decrease below a threshold during late-stage HIV infection. Importantly, the model also shows that highly active antiretroviral therapy (HAART) can inhibit X4 emergence but that monotherapy with CCR5 blockers can accelerate X4 onset and immunodeficiency if X4 infection of memory CD4(+) T cells occurs at a high rate. Fortunately, when CXCR4 blockers or HAART are used in conjunction with CCR5 blockers, this risk of accelerated immunodeficiency is eliminated. The results suggest that CCR5 blockers will be more effective when used in combination with CXCR4 blockers and caution against CCR5 blockers in the absence of an effective HAART regimen or during HAART failure.

Moesin is required for HIV-1-induced CD4-CXCR4 interaction, F-actin redistribution, membrane fusion and viral infection in lymphocytes

The human immunodeficiency virus 1 (HIV-1) envelope regulates the initial attachment of viral particles to target cells through its association with CD4 and either CXCR4 or CCR5. Although F-actin is required for CD4 and CXCR4 redistribution, little is known about the molecular mechanisms underlying this fundamental process in HIV infection. Using CD4(+) CXCR4(+) permissive human leukemic CEM T cells and primary lymphocytes, we have investigated whether HIV-1 Env might promote viral entry and infection by activating ERM (ezrin-radixin-moesin) proteins to regulate F-actin reorganization and CD4/CXCR4 co-clustering. The interaction of the X4-tropic protein HIV-1 gp120 with CD4 augments ezrin and moesin phosphorylation in human permissive T cells, thereby regulating ezrin-moesin activation. Moreover, the association and clustering of CD4-CXCR4 induced by HIV-1 gp120 requires moesin-mediated anchoring of actin in the plasma membrane. Suppression of moesin expression with dominant-negative N-moesin or specific moesin silencing impedes reorganization of F-actin and HIV-1 entry and infection mediated by the HIV-1 envelope protein complex. Therefore, we propose that activated moesin promotes F-actin redistribution and CD4-CXCR4 clustering and is also required for efficient X4-tropic HIV-1 infection in permissive lymphocytes.

Entry coreceptor use and fusion inhibitor T20 sensitivity of dual-tropic R5X4 HIV-1 in primary macrophage infection

Macrophages are important targets for HIV-1, and R5X4 strains play a central role in pathogenesis, especially in late-stage patients who may receive the fusion inhibitor T20 (enfuvirtide). Sensitivity to T20 varies markedly among HIV-1 strains and is influenced by viral and cellular factors that affect Env/CD4/coreceptor interactions. We addressed the relation between T20 inhibition and the pathway by which R5X4 HIV-1 infects primary macrophages, which express both coreceptors. In U87/CD4/coreceptor cells, T20 sensitivity for entry through CCR5 and CXCR4 was correlated. In macrophages, the proportion of total entry mediated by each coreceptor differed among isolates. Neither pathway was uniformly more or less sensitive to T20, however, nor did the proportion of entry mediated by each coreceptor predict T20 sensitivity. T20 sensitivity for macrophage infection overall correlated modestly with that for entry through CCR5 but not through CXCR4; however, unlike U87 cells, sensitivity of entry through CCR5 and CXCR4 was not correlated. These results suggest that strain-specific factors influence R5X4 T20 sensitivity regardless of the coreceptor used, an absence of systematic differences in efficiency by which R5X4 strains use the 2 coreceptors, and that efficiency and kinetics of interactions with CCR5 are central determinants of macrophage entry even when both pathways are utilized.

A haplotype of the human CXCR1 gene protective against rapid disease progression in HIV-1+ patients

Chemokines and their receptors are key factors in the onset and progression of AIDS. Among them, accumulating evidence strongly indicates the involvement of IL-8 and its receptors, CXCR1 and CXCR2, in AIDS-related conditions. Through extensive investigation of genetic variations of the human CXCR1-CXCR2 locus, we identified a haplotype of the CXCR1 gene (CXCR1-Ha) carrying two nonsynonymous single nucleotide polymorphisms, CXCR1_300 (Met to Arg) in the N terminus extracellular domain and CXCR1_142 (Arg to Cys) in the C terminus intracellular domain. Transfection experiments with CXCR1 cDNAs corresponding to the CXCR1-Ha and the alternative CXCR1-HA haplotype showed reduced expression of CD4 and CXCR4 in CXCR1-Ha cells in human osteosarcoma cells as well as in Jurkat and CEM human T lymphocytes. Furthermore, the efficiency of X4-tropic HIV-1(NL4-3) infection was significantly lower in CXCR1-Ha cells than in CXCR1-HA cells. The results were further confirmed by a series of experiments using six HIV-1 clinical isolates from AIDS patients. A genetic association study was performed by using an HIV-1(+) patient cohort consisting of two subpopulations of AIDS with extreme phenotypes of rapid and slow progression of the disease. The frequency of the CXCR1-Ha allele is markedly less frequent in patients with rapid disease onset than those with slow progression (P = 0.0003). These results provide strong evidence of a protective role of the CXCR1-Ha allele on disease progression in AIDS, probably acting through modulation of CD4 and CXCR4 expression.

CD4 and CCR5 constitutively interact at the plasma membrane of living cells: a confocal fluorescence resonance energy transfer-based approach

Human immunodeficiency virus entry into target cells requires sequential interactions of the viral glycoprotein envelope gp120 with CD4 and chemokine receptors CCR5 or CXCR4. CD4 interaction with the chemokine receptor is suggested to play a critical role in this process but to what extent such a mechanism takes place at the surface of target cells remains elusive. To address this issue, we used a confocal microspectrofluorimetric approach to monitor fluorescence resonance energy transfer at the cell plasma membrane between enhanced blue and green fluorescent proteins fused to CD4 and CCR5 receptors. We developed an efficient fluorescence resonance energy transfer analysis from experiments carried out on individual cells, revealing that receptors constitutively interact at the plasma membrane. Binding of R5-tropic HIV gp120 stabilizes these associations thus highlighting that ternary complexes between CD4, gp120, and CCR5 occur before the fusion process starts. Furthermore, the ability of CD4 truncated mutants and CCR5 ligands to prevent association of CD4 with CCR5 reveals that this interaction notably engages extracellular parts of receptors. Finally, we provide evidence that this interaction takes place outside raft domains of the plasma membrane.

Preferential targeting of CD4-CCR5 complexes with bifunctional inhibitors: a novel approach to block HIV-1 infection

Two receptors, CD4 and one of several chemokine receptors, are required for cellular HIV-1 infection, with CCR5 being the main coreceptor for macrophage-tropic strains. We have designed bifunctional fusion proteins, consisting of RANTES/CCL5 and a single-chain Fv Ab fragment against CD4 to simultaneously block CD4 and CCR5. The fusion proteins bind to both receptors, compete with RANTES/CCL5 binding, and induce down-modulation of CCR5 approximately 10 times more efficiently on CD4+ compared with CD8+ T cells. Moreover, after short incubation and subsequent washout, a significant down-modulation of CCR5 was only seen with the fusion proteins and only on CD4+ cells, but not with unmodified RANTES or on CD4- cells, indicating a preferential targeting of CCR5 on CD4+ T cells. The fusion proteins block M-tropic HIV infection more efficiently than RANTES/CCL5 and CD4 Abs alone or in combination. To our knowledge this is the first report of simultaneous blockade of an HIV-1 receptor and coreceptor with bifunctional inhibitors.

CXCR4 and CCR5 regulation and expression patterns on T- and monocyte-macrophage cell lineages: implications for susceptibility to infection by HIV-1

Chemokine receptor expression may vary dramatically among cell subsets. Therefore, the stage of differentiation and the lineage of CD4 cells may profoundly affect their susceptibility to infection by human immunodeficiency virus type 1 (HIV-1). However, the mechanisms of coreceptor competition for association with HIV-1 glycoproteins remain unknown. Here, we propose mathematical models that address the interdependence of the concentrations of CD4 and CCR5 for efficient infection by M-tropic HIV-1 as well as additional complications originated by coreceptor competition caused by posttranslational modifications that positively or negatively affect the coreceptor ability to form complexes with CD4 and/or HIV-1 envelope. Furthermore, since CCR5 and CXCR4 expression on human leukocytes designate these cells as HIV-1 potential targets, the expression of the major HIV-1 coreceptors are also dynamically modeled/quantified as function of the stage of cell differentiation. Results show that although coreceptor competition degree has limited influence on R5 strain infectivity, the infectivity of CXCR4-using isolates strongly depends on the CD4 expression, according to the coreceptor competition model proposed in Lee et al. [J. Virol. 74(11) (2000) 5016]. Understanding the role of in vivo alterations in CD4, CCR5 and CXCR4 densities on HIV-1 cell entry may help the development of optimal control strategies for AIDS pathogenesis.

Acquired T-cell sensitivity to TRAIL mediated killing during HIV infection is regulated by CXCR4-gp120 interactions

BACKGROUND

Sensitivity towards apoptosis induced by ligation of the tumor necrosis factor family of death receptors is controlled in part by death receptor expression. Whereas cellular activation enhances Fas receptor expression and induces Fas sensitivity, such cellular activation neither alters TRAIL receptor expression nor induces TRAIL sensitivity. Cells infected by HIV acquire sensitivity to TRAIL induced death, although the mechanisms by which this is achieved are undefined.

OBJECTIVE

To define the mechanism by which cells from HIV infected patients acquire sensitivity to TRAIL mediated killing.

DESIGN

In vitro assessment of TRAIL receptor expression and TRAIL sensitivity.

METHODS

Treatment of Jurkat T cells, peripheral blood lymphocytes from HIV negative donors, or human osteogenic seroma (HOS) cells expressing CD4, CXCR4 or CCR5 with T tropic gp120, M tropic gp120, or agonistic antibodies against CD4, CXCR4 or CCR5. TRAIL receptors were measured by flow cytometry or reverse transcription-PCR and TRAIL sensitivity was assessed by incubation with recombinant TRAIL followed by Annexin V fluorescein isothiocyanate/Propidium Iodide (PI) staining.

RESULTS

Treatment of uninfected Jurkat T cells, as well as primary T cells with gp120 results in the upregulation of TRAIL death receptor expression and acquired sensitivity to TRAIL mediated cell death. The increase in TRAIL death receptor expression and acquisition of TRAIL sensitivity requires the chemokine coreceptor CXCR4 but not CCR5 or the CD4 receptor.

CONCLUSIONS

These results indicate that chemokine receptor interactions regulate TRAIL receptor expression and provide an explanation for the acquired T cell sensitivity to TRAIL mediated killing death during HIV infection.

Immunomodulatory effects of the HIV-1 gp120 protein on antigen presenting cells: implications for AIDS pathogenesis

Antigen presenting cell (APC) function is central to the development of an effective anti-viral immune response. Among APC, monocytes, macrophages and dendritic cells (DC) form the principal non-T cell compartment involved in in vivo HIV infection, and these cells play important and well-established roles in multiple aspects of viral pathogenesis. HIV infection may result in APC defects, which could ultimately contribute to the loss of CD4+ T cell responses observed early in HIV infection, when the CD4+ T cell number is still within the normal range. Extensive in vitro studies have demonstrated that the envelope glycoproteins of HIV-1 exert profound influences on various cell populations of the immune system, including hematopoietic progenitors, T and B lymphocytes, monocytes/ macrophages and DC, as well as on neuronal cells. The demonstration of the presence of envelope proteins both free in the circulation and bound to the surface of CD4+ cells suggests that gp120 interactions with non-infected cells can influence cellular functions in vivo, thus contributing to the immunopathogenesis of AIDS. This paper provides an overview of the present knowledge on gp120 binding, signal transduction triggering and interference with macrophage and DC functions and it highlights the importance of this interaction in the pathogenesis of AIDS.

Productive infection of primary murine astrocytes, lymphocytes, and macrophages by human immunodeficiency virus type 1 in culture

A mouse model of human immunodeficiency virus type 1 (HIV-1) infection would be extremely valuable for evaluation of therapies and vaccines; however, multiple blocks to productive infection of NIH 3T3 and other mouse cell lines have been reported. The authors investigated the replication of HIV-1 in primary mouse astrocytes, lymphocytes, and macrophages in culture by infection with intact HIV-1 pseudotyped with the vesicular stomatitis virus G envelope glycoprotein (VSV-G) or with the envelope glycoprotein of amphotropic murine leukemia virus. Astrocytes, lymphocytes, and macrophages were susceptible to productive infection as variously assayed by detection of p24 and Tat proteins, viral protease-mediated processing of Gag, appropriately spliced viral RNA, and infectious progeny virus. As expected, NIH 3T3 cells were not susceptible to productive infection by VSV/NL4. Susceptibility mapped neither to the Fv locus nor to a possible polymorphism in cyclin T1. This study indicates that there are no intrinsic intracellular barriers to HIV-1 replication in primary mouse cells when virus entry is efficient.

Protecting from R5-tropic HIV: individual and combined effectiveness of a hammerhead ribozyme and a single-chain Fv antibody that targets CCR5

The CCR5 chemokine receptor is important for most clinical strains of HIV to establish infection. Individuals with naturally occurring polymorphisms in the CCR5 gene who have reduced or absent CCR5 are apparently otherwise healthy, but are resistant to HIV infection. With the goal of reducing CCR5 and protecting CCR5+ cells from R5-tropic HIV, we used Tag-deleted SV40-derived vectors to deliver several anti-CCR5 transgenes: 2C7, a single-chain Fv (SFv) antibody; VCKA1, a hammerhead ribozyme; and two natural CCR5 ligands, MIP-1alpha and MIP-1beta, modified to direct these chemokines, and hence their receptor to the endoplasmic reticulum. These transgenes were delivered using recombinant, Tag-deleted SV40-derived vectors to human CCR5+ cell lines and primary cells: monocyte-derived macrophages and brain microglia. All transgenes except MIP-1alpha decreased CCR5, as assayed by immunostaining, Northern blotting, and cytofluorimetry (FACS). Individually, all transgenes except MIP-1alpha protected from low challenge doses of HIV. At higher dose HIV challenges, protection provided by all transgenes diminished, the SFv and the ribozyme being most potent. Vectors carrying these two transgenes were used sequentially to deliver combination anti-CCR5 genetic therapy. This approach gave approximately additive reduction in CCR5, as measured by FACS and protected from higher dose HIV challenges. Reducing cell membrane CCR5 using anti-CCR5 transgenes, alone or in combinations, may therefore provide a degree of protection from R5-tropic strains of HIV.

Macrophages and HIV-1: dangerous liaisons

HIV-1, like the other lentiviruses, has evolved the ability to infect nondividing cells including macrophages. HIV-1 replication in monocytes/macrophages entails peculiar features and differs in many respects from that in CD4 T lymphocytes. HIV-1 exhibits different tropism for CD4 T cells and macrophages. The virus can enter macrophages via several routes. Mitosis is not required for nuclear import of viral DNA or for its integration into the host cell genome. Specific cellular factors are required for HIV-1 transcription in macrophages. The assembly and budding of viral particles in macrophages take place in late endosomal compartments. Viral particles can use the exosome pathway to exit cells. Given their functions in host defence against pathogens and the regulation of the immune response plus their permissivity to HIV-1 infection, monocytes/macrophages exert a dual role in HIV infection. They contribute to the establishment and persistence of HIV-1 infection, and may activate surrounding T cells favouring their infection. Furthermore, monocytes/macrophages act as a Trojan horse to transmit HIV-1 to the central nervous system. They also exhibit antiviral activity and express many molecules that inhibit HIV-1 replication. Activated microglia and macrophages may also exert a neurotrophic and neuroprotective effect on infected brain regulating glutamate metabolism or by secretion of neurotrophins. This review will discuss specific aspects of viral replication in monocytes/macrophages and the role of their interactions with the cellular environment in HIV-1 infection swinging between protection and pathogenesis.

Restricted entry of R5 HIV Type 1 strains into eosinophilic cells

A cell culture system previously developed by our laboratory demonstrated that T cell-tropic (CXCR4-using) but not macrophage-tropic (CCR5-using) HIV-1 strains productively infected eosinophilic cells. In the current study, an improved model was used to determine the level of this viral restriction by assessing viral entry and coreceptor usage. The model was improved by using AML14.3D10 cells that were engineered to express CCR3 in addition to the major HIV-1 coreceptors, CD4, CXCR4, and CCR5, thus making them more like primary eosinophils. A polymerase chain reaction (PCR) assay was used to detect viral entry. In the PCR assay, primers specific for early reverse transcription products were used to amplify minus strand viral DNA from HIV-1-infected AML14.3D10-CCR3 eosinophilic cells. Coreceptor blocking experiments, using the CXCR4 antagonist AMD3100, were performed to determine coreceptor usage by the CXCR4-using (X4) strain known to productively infect the cells. Virus production was measured by p24 immunoassay. As expected, viral DNA was detected in AML14.3D10-CCR3 cells infected with X4 HIV-1, and cell viability was decreased during maximal viral production. Conversely, viral DNA was not detected in eosinophilic cells exposed to a CCR5-using (R5) HIV-1 strain that is also capable of using CCR3, indicating that R5 HIV-1 is unable to enter eosinophilic cells despite the presence of the appropriate coreceptors. Infection of AML14.3D10-CCR3 cells by HTLV-III(B) was completely inhibited by AMD3100, indicating that X4 HIV-1 enters the AML14.3D10-CCR3 cell line by using the CXCR4 coreceptor exclusively. Since X4 strains predominate during the late stages of HIV-1 infection in many patients, when eosinophil numbers also tend to increase, the ability of these HIV-1 strains to infect eosinophilic cells has important implications for the involvement of eosinophils in the pathogenesis of AIDS.

Constitutive association of cell surface CCR5 and CXCR4 in the presence of CD4

Chemokine receptors CCR5 and CXCR4 are the major coreceptors of HIV-1 infection and also play fundamental roles in leukocyte trafficking, metastasis, angiogenesis, and embyogenesis. Here, we show that transfection of CCR5 into CXCR4 and CD4 expressing 3T3 cells enhances the cell surface level of CXCR4. In CCR5 high expressing cells, cell surface level of CXCR4 was incompletely modulated in the presence of the CXCR4 ligand CXCL12/SDF-1alpha. CCR5 was resistant to ligand-dependent modulation with the CCR5 ligand CCL5/RANTES. Confocal laser microscopy revealed that CCR5 was colocalized with CXCR4 on the cell surface. In CD4 expressing CCR5 and CXCR4 double positive NIH 3T3 cells, immunoprecipitation followed by Western blot analysis revealed that CCR5 was associated with CXCR4 and CD4. CXCR4 and CCR5 were not co-immunoprecipitated in cells expressing CCR5 and CXCR4 but without CD4 expression. Compared to NIH 3T3CD4 cells expressing CXCR4, the entry of an HIV-1 X4 isolate (HCF) into NIH 3T3CD4 expressing both CXCR4 and CCR5 was reduced. Our data indicate that chemokine receptors interact with each other, which may modulate chemokine-chemokine receptor interactions and HIV-1 coreceptor functions.

Effect of a CCR5 inhibitor on viral loads in macaques dual-infected with R5 and X4 primate immunodeficiency viruses

Human immunodeficiency virus type 1 (HIV-1) fusion with its target cells is initiated by sequential interactions between its envelope glycoprotein, CD4, and a co-receptor, usually CCR5 or CXCR4. Small molecules that bind to CCR5 and prevent its use by R5 HIV-1 strains are now being developed clinically as antiviral drugs. To test whether a block to CCR5 promotes the replication of viruses that enter cells via CXCR4 and are associated with accelerated disease progression, we administered a small molecule CCR5 inhibitor, CMPD 167, to three macaques dual-infected with both R5 (SIVmac251) and X4 (SHIV-89.6P) viruses. CMPD 167 caused a rapid and substantial (on average, 50-fold) suppression of R5 virus replication in each animal. In two of the animals, but not in the third, a rapid, transient, 8- to 15-fold increase in the amount of plasma X4 virus occurred. In neither animal was the increase in X4 viral load sustained throughout therapy, however. These observations may have relevance for the development of CCR5 inhibitors for treatment of HIV-1 infection of humans.

The CCR5 and CXCR4 coreceptors--central to understanding the transmission and pathogenesis of human immunodeficiency virus type 1 infection

In this review, we will discuss what is known, what is suspected, and what still remains obscure about the central role played by coreceptor expression and usage in the transmission and pathogenic consequences of human immunodeficiency virus type 1 (HIV-1) infection. An emphasis will be on the HIV-1 phenotypic variants that are defined by their usage of the CCR5 or CXCR4 coreceptors, and how the different cellular tropism of these variants influences how and where HIV-1 replicates in vivo. We will also review what might happen when coreceptor antagonists are used clinically to treat HIV-1 infection.

Inhibition of HIV-1 infection by a CCR5-binding cyclophilin from Toxoplasma gondii

The activation of murine dendritic cells by Toxoplasma gondii has recently been shown to depend on a parasite protein that signals through the chemokine receptor CCR5. Here we demonstrate that this molecule, cyclophilin-18 (C-18), is an inhibitor of HIV-1 cell fusion and infection with cell-free virus. T gondii C-18 efficiently blocked syncytium formation between human T cells and effector cells expressing R5 but not X4 envelopes. Neither human nor Plasmodium falciparum cyclophilins possess such inhibitory activity. Importantly, C-18 protected peripheral blood leukocytes from infection with multiple HIV-1 R5 primary isolates from several clades. C-18 bound directly to human CCR5, and this interaction was partially competed by the beta-chemokine macrophage inflammatory protein 1 beta (MIP-1 beta) and by HIV-1 R5 gp120. In contrast to several other antagonists of HIV coreceptor function, C-18 mediated inhibition did not induce beta-chemokines or cause CCR5 downmodulation, suggesting direct blocking of envelope binding to the receptor. These data support the further development of C-18 derivatives as HIV-1 inhibitors for preventing HIV-1 transmission and for postexposure prophylaxis.

HIV coreceptors: role of structure, posttranslational modifications, and internalization in viral-cell fusion and as targets for entry inhibitors

The human immunodeficiency virus (HIV) envelope glycoprotein forms trimers on the virion surface, with each monomer consisting of two subunits, gp120 and gp41. The gp120 envelope component binds to CD4 on target cells and undergoes conformational changes that allow gp120 to interact with certain G-protein-coupled receptors (GPCRs) on the same target membranes. The GPCRs that function as HIV coreceptors were found to be chemokine receptors. The primary coreceptors are CCR5 and CXCR4, but several other chemokine receptors were identified as "minor coreceptors", indicating their ability support entry of some HIV strains in tissue cultures. Formation of the tri-molecular complexes stabilizes virus binding and triggers a series of conformational changes in gp41 that facilitate membrane fusion and viral cell entry. Concerted efforts are underway to decipher the specific interactions between gp120/CD4, gp120/coreceptors, and their contributions to the subsequent membrane fusion process. It is hoped that some of the transient conformational intermediates in gp120 and gp41 would serve as targets for entry inhibitors. In addition, the CD4 and coreceptors are primary targets for several classes of inhibitors currently under testing. Our review summarizes the current knowledge on the interactions of HIV gp120 with its receptor and coreceptors, and the important properties of the chemokine receptors and their regulation in primary target cells. We also summarize the classes of coreceptor inhibitors under development.

Potential role for CD63 in CCR5-mediated human immunodeficiency virus type 1 infection of macrophages

Macrophages and CD4(+) lymphocytes are the principal target cells for human immunodeficiency virus type 1 (HIV-1) infection, but the molecular details of infection may differ between these cell types. During studies to identify cellular molecules that could be involved in macrophage infection, we observed inhibition of HIV-1 infection of macrophages by monoclonal antibody (MAb) to the tetraspan transmembrane glycoprotein CD63. Pretreatment of primary macrophages with anti-CD63 MAb, but not MAbs to other macrophage cell surface tetraspanins (CD9, CD81, and CD82), was shown to inhibit infection by several R5 and dualtropic strains, but not by X4 isolates. The block to productive infection was postfusion, as assessed by macrophage cell-cell fusion assays, but was prior to reverse transcription, as determined by quantitative PCR assay for new viral DNA formation. The inhibitory effects of anti-CD63 in primary macrophages could not be explained by changes in the levels of CD4, CCR5, or beta-chemokines. Infections of peripheral blood lymphocytes and certain cell lines were unaffected by treatment with anti-CD63, suggesting that the role of CD63 in HIV-1 infection may be specific for macrophages.

Follicular dendritic cell-mediated up-regulation of CXCR4 expression on CD4 T cells and HIV pathogenesis

Follicular dendritic cells (FDCs) represent a major reservoir of HIV, and active infection occurs surrounding these cells, suggesting that this microenvironment is highly conducive to virus transmission. Because CD4 T cells around FDCs in germinal centers express the HIV coreceptor, CXCR4, whereas CD4 lymphocytes in many other sites do not, it prompted the hypothesis that FDCs may increase CXCR4 expression on CD4 T cells, thereby facilitating infection. To test this, HIV receptor/coreceptor expression was determined on CD4 T cells cultured with or without FDCs, and its consequence to infection was assessed by measuring virus binding and entry. FDCs had little effect on CCR5 or CD4 expression but increased CXCR4 expression on CD4 T cells. FDC-mediated up-regulation of CXCR4 on CD4 T cells occurred by 24 h and was sustained for at least 96 h in vitro, and FDC-CD4 T cell contact was necessary. Importantly, increased CXCR4 expression directly correlated with increased binding and entry of HIV-1 X4 isolates. Furthermore, CD4(+)CD57(+) germinal center T cells expressed high levels of CXCR4 and supported enhanced entry of X4 HIV compared with other CD4 T cells from the same tissue. Thus, in addition to serving as a reservoir of infectious virus, FDCs render surrounding germinal center T cells highly susceptible to infection with X4 isolates of HIV-1.

Altering expression levels of human immunodeficiency virus type 1 gp120-gp41 affects efficiency but not kinetics of cell-cell fusion

Human immunodeficiency virus (HIV) entry into a host cell requires the fusion of virus and cellular membranes that is driven by interaction of the viral envelope glycoproteins gp120 and gp41 (gp120/gp41) with CD4 and a coreceptor, typically either CXCR4 or CCR5. The stoichiometry of gp120/gp41:CD4:CCR5 necessary to initiate membrane fusion is not known. To allow an examination of early events in gp120/gp41-driven membrane fusion, we developed a novel real-time cell-cell fusion assay. Using this assay to study fusion kinetics, we found that altering the cell surface density of gp120/gp41 affected the maximal extent of fusion without dramatically altering fusion kinetics. Collectively, these observations are consistent with the view that gp120/gp41-driven membrane fusion requires the formation of a threshold number of fusion-active intercellular gp120/gp41:CD4:CCR5 complexes. Furthermore, the probability of reaching this threshold is governed, in part, by the surface density of gp120/gp41.

Expression of CD4 controls the susceptibility of THP-1 cells to infection by R5 and X4 HIV type 1 isolates

The monocytic THP-1 cell line has been used to study HIV-monocyte/macrophage interactions and the relationship between differentiation, virus production, and virus latency. Undifferentiated THP-1 cells are susceptible to infection by T-tropic human immunodeficiency virus type 1 (HIV-1) isolates that use the coreceptor CXCR4 (X4 strains). Treatment with phorbol 12-myristate 13-acetate (PMA) induces differentiation of THP-1 cells into adherent macrophage-like cells, which are susceptible to M-tropic, CCR5-dependent isolates (R5 strains). The aim of this study was to determine whether variabilities observed in the susceptibility of THP-1 cells to HIV-1 infection may be related to the differential expression of CD4, CCR5, and CXCR4. Both propagation and PMA treatment of THP-1 cells resulted in a marked decrease in CD4-positive cells, whereas the expression of CCR5 and CXCR4 was not reduced during propagation. Both coreceptors were also relatively "resistant" to PMA-induced downregulation when compared with the low percentage of CD4-positive cells in differentiated cultures. In undifferentiated THP-1 cells, low CD4 expression significantly reduced the susceptibility of the cells to infection with the R5 HIV-1(BaL) isolate, whereas a PMA-induced decrease in CD4 expression reduced permissiveness of the cells to the X4 HIV-1(IIIB) isolate. Thus, cell surface CD4 plays a primary role in determining how efficiently THP-1 cells can be infected with the X4 and the R5 isolates.

CCR5 and CXCR4 expression correlated with X4 and R5 HIV-1 infection yet not sustained replication in Th1 and Th2 cells

OBJECTIVE

To investigate the infectivity of T-helper (Th)1 and Th2 cells (derived from ccr5 wild-type and homozygous ccr5 Delta 32) to R5 and X4 HIV-1.

DESIGN

It remains unclear whether infection of Th1 and Th2 CD4 cells by R5 and X4 viruses mirrors their co-receptor expression profile as no direct quantitation of coreceptor levels and infection has been performed. In addition, it is unknown whether the lack of CCR5 expression affects the degree of Th1/Th2 polarization.

METHODS

Surface expression of CCR5 and CXCR4 was determined by quantitative fluorescence activated cell sorter analysis on in vitro differentiated Th1 and Th2 cells. R5 (Ba-L) and X4 (IIIB) HIV-1 isolates were used for infection studies and the efficiency of viral entry was determined by quantitative real time polymerase chain reaction detection of reverse transcribed proviral DNA.

RESULTS

Cell surface density of CCR5 molecules was eight-fold higher in Th1 versus Th2 subsets (P = 0.005) whereas CXCR4 surface density was four-fold higher in Th2 versus Th1 subsets (P = 0.006). Preferential infection and entry of Th1 cells by R5 HIV-1 was not associated with preferential replication, as eventually the R5-virus replicated to a higher level in Th2 cells in spite of lower initial viral infection/entry. By contrast, Th2 cells preferentially supported X4-virus infection and replication. High beta chemokine secretion by Th1 cells was associated with a lower R5 replication rate.

CONCLUSIONS

Th1 and Th2 cells differ in their infection efficiency for R5 and X4 HIV-1. ccr5 Delta 32-homozygous individuals maintain the ability for Th1/Th2 polarization, i.e., the expression of CCR5 is not required for Th1/Th2 polarization.

Numerical modelling of the perturbation of HIV-1 during combination anti-retroviral therapy

A competitive, chaos-free, implicit, finite-difference method is developed and used for a novel deterministic model for the perturbation of HIV by combination antiretroviral therapy. The compartmental model monitors the interaction between HIV and CD4(+) T cells, its principal target and site of replication in vivo, in the presence of reverse transcription inhibitors and protease inhibitors. The model exhibits two steady states, an uninfected (trivial) steady state (with no virus present) and an endemically infected state (with virus and infected T cells present). Stability and bifurcation analyses together with numerical simulations of the resulting dynamical system are reported.

Regulation of human immunodeficiency virus type 1 infection, beta-chemokine production, and CCR5 expression in CD40L-stimulated macrophages: immune control of viral entry

Mononuclear phagocytes (MP) and T lymphocytes play a pivotal role in the host immune response to human immunodeficiency virus type 1 (HIV-1) infection. Regulation of such immune responses can be mediated, in part, through the interaction of the T-lymphocyte-expressed molecule CD40 ligand (CD40L) with its receptor on MP, CD40. Upregulation of CD40L on CD4+ peripheral blood mononuclear cells during advanced HIV-1 disease has previously been reported. Based on this observation, we studied the influence of CD40L-CD40 interactions on MP effector function and viral regulation in vitro. We monitored productive viral infection, cytokine and beta-chemokine production, and beta-chemokine receptor expression in monocyte-derived macrophages (MDM) after treatment with soluble CD40L. Beginning 1 day after infection and continuing at 3-day intervals, treatment with CD40L inhibited productive HIV-1 infection in MDM in a dose-dependent manner. A concomitant and marked upregulation of beta-chemokines (macrophage inhibitory proteins 1alpha and 1beta and RANTES [regulated upon activation normal T-cell expressed and secreted]) and the proinflammatory cytokine tumor necrosis factor alpha (TNF-alpha) was observed in HIV-1-infected and CD40L-treated MDM relative to either infected or activated MDM alone. The addition of antibodies to RANTES or TNF-alpha led to a partial reversal of the CD40L-mediated inhibition of HIV-1 infection. Surface expression of CD4 and the beta-chemokine receptor CCR5 was reduced on MDM in response to treatment with CD40L. In addition, treatment of CCR5- and CD4-transfected 293T cells with secretory products from CD40L-stimulated MDM prior to infection with a CCR5-tropic HIV-1 reporter virus led to inhibition of viral entry. In conclusion, we demonstrate that CD40L-mediated inhibition of viral entry coincides with a broad range of MDM immune effector responses and the down-modulation of CCR5 and CD4 expression.

Inhibition of R5X4 dualtropic HIV-1 primary isolates by single chemokine co-receptor ligands

The susceptibility of HIV-1 to chemokine-mediated inhibition may be lost as a consequence of the expanded usage of chemokine co-receptors frequently occurring in clade B isolates obtained from individuals with advanced disease. Since chemokine-based immune intervention is under intense investigation, it is crucial to determine its potential effect on primary dualtropic HIV isolates characterized by simultaneous utilization of CCR5 and CXCR4 chemokine co-receptors (R5X4 viruses). In the present study, the CCR5 binding chemokine regulated upon activation normal T cell expressed and secreted (RANTES) strongly inhibited the replication of two of eight primary R5X4 viruses in mitogen-activated primary peripheral blood mononuclear cells (PBMC). The CXCR4 antagonist AMD3100 efficiently suppressed the replication of other two HIV isolates, whereas the remaining four viruses were partially inhibited by treatment with either RANTES or AMD3100. The potency of chemokine-mediated inhibition was influenced by PBMC donor variability, but it was usually independent from the levels of expression of CCR5 or CXCR4. Dual co-receptor usage was maintained by the viruses after two serial passages on U87.CD4 astrocytic cell lines expressing exclusively either CCR5 or CXCR4. The gp120 env variable domains were sequenced before and after passages on U87.CD4 cells. Virus replication into U87.CD4-CXCR4 cells did not result in changes in the V3 region but perturbed the dominant env V4 sequence. Interestingly, double passage onto U87.CD4-CXCR4 cells determined the loss of susceptibility to RANTES inhibition. In conclusion, interference with CCR5 may efficiently inhibit the replication of at least some dualtropic HIV-1 strains, whereas forced CXCR4 usage may result in viral escape from CCR5-dependent inhibitory effects.

Viral exploitation and subversion of the immune system through chemokine mimicry

The chemokine superfamily of leukocyte chemoattractants coordinates development and deployment of the immune system by signaling through a family of G protein-coupled receptors. The importance of this system to antimicrobial host defense has been supported by the discovery of numerous herpesviruses and poxviruses that encode chemokine mimics able to block chemokine action. However, specific herpesviruses and lentiviruses can also exploit the immune system through chemokine mimicry, for example, to facilitate viral dissemination or, as in the case of HIV-1, to directly infect leukocyte target cells. The study of viral mimicry of chemokines and chemokine receptors is providing important new concepts in viral immunopathogenesis, new anti-inflammatory drug leads and new targets and concepts for antiviral drug and vaccine development.

Potent, broad-spectrum inhibition of human immunodeficiency virus type 1 by the CCR5 monoclonal antibody PRO 140

CCR5 serves as a requisite fusion coreceptor for clinically relevant strains of human immunodeficiency virus type 1 (HIV-1) and provides a promising target for antiviral therapy. However, no study to date has examined whether monoclonal antibodies, small molecules, or other nonchemokine agents possess broad-spectrum activity against the major genetic subtypes of HIV-1. PRO 140 (PA14) is an anti-CCR5 monoclonal antibody that potently inhibits HIV-1 entry at concentrations that do not affect CCR5's chemokine receptor activity. In this study, PRO 140 was tested against a panel of primary HIV-1 isolates selected for their genotypic and geographic diversity. In quantitative assays of viral infectivity, PRO 140 was compared with RANTES, a natural CCR5 ligand that can inhibit HIV-1 entry by receptor downregulation as well as receptor blockade. Despite their divergent mechanisms of action and binding epitopes on CCR5, low nanomolar concentrations of both PRO 140 and RANTES inhibited infection of primary peripheral blood mononuclear cells (PBMC) by all CCR5-using (R5) viruses tested. This is consistent with there being a highly restricted pattern of CCR5 usage by R5 viruses. In addition, a panel of 25 subtype C South African R5 viruses were broadly inhibited by PRO 140, RANTES, and TAK-779, although approximately 30-fold-higher concentrations of the last compound were required. Interestingly, significant inhibition of a dualtropic subtype C virus was also observed. Whereas PRO 140 potently inhibited HIV-1 replication in both PBMC and primary macrophages, RANTES exhibited limited antiviral activity in macrophage cultures. Thus CCR5-targeting agents such as PRO 140 can demonstrate potent and genetic-subtype-independent anti-HIV-1 activity.

Cooperative subunit interactions within the oligomeric envelope glycoprotein of HIV-1: functional complementation of specific defects in gp120 and gp41

The envelope glycoprotein (Env) of HIV-1 is displayed on the surface of the virion or infected cell as an oligomer of multiple gp120/gp41 complexes. We sought to unravel the relationships between this oligomeric structure and the requirements for sequential interactions with CD4 and coreceptor (CCR5 or CXCR4). We used a quantitative cell fusion assay to examine the effects of coexpressing pairs of Envs, each nonfunctional because of a specific defect in one of the essential properties. We observed efficient fusion activity upon coexpression of two Env variants, one containing a gp41 subunit with a mutated fusion peptide and the other containing a gp120 subunit with a mutated CD4 binding site or a mismatched coreceptor specificity. We also observed fusion upon coexpression of two Env variants with distinct gp120 defects, i.e., a CD4 binding site mutation and the incorrect coreceptor specificity determinants. Coimmunoprecipitation experiments verified the efficient formation of mixed oligomers, suggesting that the observed fusion reflected subunit complementation within the oligomeric complex. These results support a model in which cooperative subunit interactions within the Env oligomer result in concerted conformational changes upon receptor binding, resulting in activation for fusion. The implications of these findings for Env function and virus neutralization are discussed.

Interferon γ and interleukin 6 modulate the susceptibility of macrophages to human immunodeficiency virus type 1 infection

The effect of interferon γ (IFN-γ) and interleukin 6 (IL-6) on infection of macrophages with human immunodeficiency virus type 1 (HIV-1) was investigated. By using a polymerase chain reaction–based viral entry assay and viral infectivity assay, it was demonstrated that IL-6 and IFN-γ augmented susceptibility of monocyte-derived macrophages (MDMs) to infection with T-cell tropic CXCR4-utilizing (X4) HIV-1 strains. Consistent with this finding, IFN-γ and IL-6 augmented fusion of MDMs with T-tropic envelope-expressing cells. The enhanced fusion of cytokine-treated MDMs with T-tropic envelopes was inhibited by the CXCR4 ligand, SDF-1, and by T22 peptide. IFN-γ and IL-6 did not affect expression of surface CXCR4 or SDF-1–induced Ca++ flux in MDMs. In contrast to the effect of IFN-γ on the infection of MDMs with X4 strains, IFN-γ inhibited viral entry and productive infection of MDMs with macrophage-tropic (M-tropic) HIV-1. Consistent with this finding, IFN-γ induced a decrease in fusion with M-tropic envelopes that correlated with a modest reduction in surface CCR5 and CD4 on MDMs. It was further demonstrated that macrophage inflammatory protein (MIP)-1α and MIP-β secreted by cytokine-treated MDMs augmented their fusion with T-tropic–expressing cells and inhibited their fusion with M-tropic envelope-expressing cells. These data indicate that proinflammatory cytokines, which are produced during opportunistic infections or sexually transmitted diseases, may predispose macrophages to infection with X4 strains that, in turn, could accelerate disease progression.

Interferon γ and interleukin 6 modulate the susceptibility of macrophages to human immunodeficiency virus type 1 infection

The effect of interferon γ (IFN-γ) and interleukin 6 (IL-6) on infection of macrophages with human immunodeficiency virus type 1 (HIV-1) was investigated. By using a polymerase chain reaction–based viral entry assay and viral infectivity assay, it was demonstrated that IL-6 and IFN-γ augmented susceptibility of monocyte-derived macrophages (MDMs) to infection with T-cell tropic CXCR4-utilizing (X4) HIV-1 strains. Consistent with this finding, IFN-γ and IL-6 augmented fusion of MDMs with T-tropic envelope-expressing cells. The enhanced fusion of cytokine-treated MDMs with T-tropic envelopes was inhibited by the CXCR4 ligand, SDF-1, and by T22 peptide. IFN-γ and IL-6 did not affect expression of surface CXCR4 or SDF-1–induced Ca++ flux in MDMs. In contrast to the effect of IFN-γ on the infection of MDMs with X4 strains, IFN-γ inhibited viral entry and productive infection of MDMs with macrophage-tropic (M-tropic) HIV-1. Consistent with this finding, IFN-γ induced a decrease in fusion with M-tropic envelopes that correlated with a modest reduction in surface CCR5 and CD4 on MDMs. It was further demonstrated that macrophage inflammatory protein (MIP)-1α and MIP-β secreted by cytokine-treated MDMs augmented their fusion with T-tropic–expressing cells and inhibited their fusion with M-tropic envelope-expressing cells. These data indicate that proinflammatory cytokines, which are produced during opportunistic infections or sexually transmitted diseases, may predispose macrophages to infection with X4 strains that, in turn, could accelerate disease progression.