A trans-receptor mechanism for infection of CD4-negative cells by human immunodeficiency virus type 1

Astrocytes are HIV reservoirs in the brain: A cell type with poor HIV infectivity and replication but efficient cell-to-cell viral transfer

The major barrier to eradicating Human immunodeficiency virus-1 (HIV) infection is the generation of tissue-associated quiescent long-lasting viral reservoirs refractory to therapy. Upon interruption of anti-retroviral therapy (ART), HIV replication can be reactivated. Within the brain, microglia/macrophages and a small population of astrocytes are infected with HIV. However, the role of astrocytes as a potential viral reservoir is becoming more recognized because of the improved detection and quantification of HIV viral reservoirs. In this report, we examined the infectivity of human primary astrocytes in vivo and in vitro, and their capacity to maintain HIV infection, become latently infected, be reactivated, and transfer new HIV virions into neighboring cells. Analysis of human brain tissue sections obtained from HIV-infected individuals under effective and prolonged ART indicates that a small population of astrocytes has integrated HIV-DNA. In vitro experiments using HIV-infected human primary astrocyte cultures confirmed a low percentage of astrocytes had integrated HIV-DNA, with poor to undetectable replication. Even in the absence of ART, long-term culture results in latency that could be transiently reactivated with histone deacetylase inhibitor, tumor necrosis factor-alpha (TNF-α), or methamphetamine. Reactivation resulted in poor viral production but efficient cell-to-cell viral transfer into cells that support high viral replication. Together, our data provide a new understanding of astrocytes' role as viral reservoirs within the central nervous system (CNS).

Evolution of Host Target Cell Specificity During HIV-1 Infection

BACKGROUND

Many details of HIV-1 molecular virology have been translated into lifesaving antiviral drugs. Yet, we have an incomplete understanding of the cells in which HIV-1 replicates in untreated individuals and persists in during antiretroviral therapy.

METHODS

In this review we discuss how viral entry phenotypes have been characterized and the insights they have revealed about the target cells supporting HIV-1 replication. In addition, we will examine whether some HIV-1 variants have the ability to enter cells lacking CD4 (such as astrocytes) and the role that trans-infection plays in HIV-1 replication.

RESULTS

HIV-1 entry into a target cell is determined by whether the viral receptor (CD4) and the coreceptor (CCR5 or CXCR4) are expressed on that cell. Sustained HIV-1 replication in a cell type can produce viral lineages that are tuned to the CD4 density and coreceptor expressed on those cells; a fact that allows us to use Env protein entry phenotypes to infer information about the cells in which a viral lineage has been replicating and adapting.

CONCLUSION

We now recognize that HIV-1 variants can be divided into three classes representing the primary target cells of HIV-1; R5 T cell-tropic variants that are adapted to entering memory CD4+ T cells, X4 T cell-tropic variants that are adapted to entering naïve CD4+ T cells and Mtropic variants that are adapted to entering macrophages and possibly other cells that express low levels of CD4. While much progress has been made, the relative contribution that infection of different cell subsets makes to viral pathogenesis and persistence is still being unraveled.

Antiviral Activity of HIV gp120-Targeting Bispecific T Cell Engager Antibody Constructs

Today's gold standard in HIV therapy is combined antiretroviral therapy (cART). It requires strict adherence by patients and lifelong medication, which can lower the viral load below detection limits and prevent HIV-associated immunodeficiency but cannot cure patients. The bispecific T cell-engaging (BiTE) antibody technology has demonstrated long-term relapse-free outcomes in patients with relapsed and refractory acute lymphocytic leukemia. Here, we generated BiTE antibody constructs that target the HIV-1 envelope protein gp120 (HIV gp120) using either the scFv B12 or VRC01, the first two extracellular domains (1 + 2) of human CD4 alone or joined to the single chain variable fragment (scFv) of the antibody 17b fused to an anti-human CD3ε scFv. These engineered human BiTE antibody constructs showed engagement of T cells for redirected lysis of HIV gp120-transfected CHO cells. Furthermore, they substantially inhibited HIV-1 replication in peripheral blood mononuclear cells (PBMCs) as well as in macrophages cocultured with autologous CD8⁺ T cells, the most potent being the human CD4(1 + 2) BiTE [termed CD(1 + 2) h BiTE] antibody construct and the CD4(1 + 2)L17b BiTE antibody construct. The CD4(1 + 2) h BiTE antibody construct promoted HIV infection of human CD4^-/CD8⁺ T cells. In contrast, the neutralizing B12 and the VRC01 BiTE antibody constructs, as well as the CD4(1 + 2)L17b BiTE antibody construct, did not. Thus, BiTE antibody constructs targeting HIV gp120 are very promising for constraining HIV and warrant further development as novel antiviral therapy with curative potential.IMPORTANCE HIV is a chronic infection well controlled with the current cART. However, we lack a cure for HIV, and the HIV pandemic goes on. Here, we showed in vitro and ex vivo that a BiTE antibody construct targeting HIV gp120 resulted in substantially reduced HIV replication. In addition, these BiTE antibody constructs display efficient killing of gp120-expressing cells and inhibited replication in ex vivo HIV-infected PBMCs or macrophages. We believe that BiTE antibody constructs recognizing HIV gp120 could be a very valuable strategy for a cure of HIV in combination with cART and compounds which reverse latency.

Enigma of HIV-1 latent infection in astrocytes: an in-vitro study using protein kinase C agonist as a latency reversing agent

Purging HIV-1 to cure the infection in patients undergoing suppressive antiretroviral therapy requires targeting all possible viral reservoirs. Other than the memory CD4(+) T cells, several other HIV-1 reservoirs have been identified. HIV-1 infection in the brain as a reservoir is well documented, but not fully characterized. There, microglia, perivascular macrophages, and astrocytes can be infected by HIV-1. HIV-1 infection in astrocytes has been described as a nonproductive and primarily a latent infection. Using primary human astrocytes, we investigated latent HIV-1 infection and tested phorbol 12-myristate 13-acetate (PMA), a protein kinase C agonist, as an HIV-1-latency- reversing agent in infected astrocytes. Chloroquine (CQ) was used to facilitate initial HIV-1 escape from endosomes in astrocytes. CQ significantly increased HIV-1 infection. But treatment with PMA or viral Tat protein was similar to untreated HIV-1-infected astrocytes. Long-term follow-up of VSV-envelope-pseudotyped HIV-1 infected astrocytes showed persistent infection for 110 days, indicating the active state of the virus.

Cell-to-cell contact facilitates HIV transmission from lymphocytes to astrocytes via CXCR4

OBJECTIVES

HIV reservoir in the brain represents a major barrier for curing HIV infection. As the most abundant, long-lived cell type, astrocytes play a critical role in maintaining the reservoir; however, the mechanism of infection remains unknown. Here, we determine how viral transmission occurs from HIV-infected lymphocytes to astrocytes by cell-to-cell contact.

DESIGN AND METHODS

Human astrocytes were exposed to HIV-infected lymphocytes and monitored by live-imaging, confocal microscopy, transmission and three-dimensional electron microscopy. A panel of receptor antagonists was used to determine the mechanism of viral entry.

RESULTS

We found that cell-to-cell contact resulted in efficient transmission of X4 or X4R5-using viruses from T lymphocytes to astrocytes. In co-cultures of astrocytes with HIV-infected lymphocytes, the interaction occurred through a dynamic process of attachment and detachment of the two cell types. Infected lymphocytes invaginated into astrocytes or the contacts occurred via filopodial extensions from either cell type, leading to the formation of virological synapses. In the synapses, budding of immature or incomplete HIV particles from lymphocytes occurred directly onto the membranes of astrocytes. This cell-to-cell transmission could be almost completely blocked by anti-CXCR4 antibody and its antagonist, but only partially inhibited by anti-CD4, ICAM1 antibodies.

CONCLUSION

Cell-to-cell transmission was mediated by a unique mechanism by which immature viral particles initiated a fusion process in a CXCR4-dependent, CD4-independent manner. These observations have important implications for developing approaches to prevent formation of HIV reservoirs in the brain.

Lack of the trans-receptor mechanism of HIV-1 infection: CD4- and coreceptor-independent incorporation of HIV-1-resistant cells into syncytia induced by HIV-1

Human immunodeficiency virus type 1 (HIV-1) infects cells through an interaction of HIV-1 envelope protein with CD4 and an appropriate coreceptor on target cells. This interaction often leads to cell fusion, and formation of syncytia. HIV-1-resistant cells expressing either CD4 or a coreceptor are often surrounding HIV-1-susceptible cells, expressing both CD4 and a compatible coreceptor, in vivo. It is therefore worthwhile to investigate whether these HIV-1-resistant cells could cooperate in HIV-1 infection or cell fusion leading to their incorporation into syncytia. When CD4-positive, coreceptor-negative cells were co-cultured with CD4-negative, coreceptor-positive cells and exposed to HIV-1, HIV-1 infection was not established, indicating that CD4 and the coreceptor expressed on different cell surfaces could not cooperate in HIV-1 entry. However, when HIV-1-resistant cells expressing CD4 or a coreceptor or lacking both were mixed with HIV-1-susceptible cells and inoculated with HIV-1, all these HIV-1-resistant cells were similarly incorporated into syncytia induced by HIV-1, indicating a CD4- and coreceptor-independent incorporation of HIV-1-resistant cells into syncytia. This incorporation was impaired by the transfection of these cells with siRNAs for adhesion molecules. Our study demonstrates that HIV-1-resistant cells can be incorporated into syncytia induced by HIV-1 and this incorporation may partially be mediated through adhesion molecules.

Marrow cell genetic phenotype change induced by human lung cancer cells

Microvesicles have been shown to mediate varieties of intercellular communication. Work in murine species has shown that lung-derived microvesicles can deliver mRNA, transcription factors, and microRNA to marrow cells and alter their phenotype. The present studies evaluated the capacity of excised human lung cancer cells to change the genetic phenotype of human marrow cells. We present the first studies on microvesicle production by excised cancers from human lung and the capacity of these microvesicles to alter the genetic phenotype of normal human marrow cells. We studied 12 cancers involving the lung and assessed nine lung-specific mRNA species (aquaporin, surfactant families, and clara cell-specific protein) in marrow cells exposed to tissue in co-culture, cultured in conditioned media, or exposed to isolated lung cancer-derived microvesicles. We assessed two or seven days of co-culture and marrow which was unseparated, separated by ficoll density gradient centrifugation or ammonium chloride lysis. Under these varying conditions, each cancer derived from lung mediated marrow expression of between one and seven lung-specific genes. Microvesicles were identified in the pellet of ultracentrifuged conditioned media and shown to enter marrow cells and induce lung-specific mRNA expression in marrow. A lung melanoma and a sarcoma also induced lung-specific mRNA in marrow cells. These data indicate that lung cancer cells may alter the genetic phenotype of normal cells and suggest that such perturbations might play a role in tumor progression, tumor recurrence, or metastases. They also suggest that the tissue environment may alter cancer cell gene expression.

Inefficient fusion due to a lack of attachment receptor/co-receptor restricts productive human immunodeficiency virus type 1 infection in human hepatoma Huh7.5 cells

Since the widespread use of the highly active antiretroviral therapy, the incidence of liver disease has increased to become a leading cause of death among human immunodeficiency virus type 1 (HIV-1)-infected individuals. It can be proposed that the ability of HIV-1 to infect hepatocytes could influence liver diseases. Although the presence of HIV-1 was identified in hepatocytes from HIV-1 seropositive patients, the susceptibility of hepatocytes to HIV-1 infection in vitro remains controversial. We present evidence here that human hepatoma cells are not productively infected with CD4-dependent HIV-1 strains because of inefficient fusion related to an absence of cell surface CD4 and CXCR4. However, these cells display an increased susceptibility to infection with a CD4-independent viral isolate through an interaction with galactosyl ceramide, an alternate receptor for HIV-1. This study provides further understanding of the susceptibility of human hepatocytes to HIV-1 infection. However, in vivo investigations are recommended to consolidate these data.

Complementation of diverse HIV-1 Env defects through cooperative subunit interactions: a general property of the functional trimer

BACKGROUND

The HIV-1 Env glycoprotein mediates virus entry by catalyzing direct fusion between the virion membrane and the target cell plasma membrane. Env is composed of two subunits: gp120, which binds to CD4 and the coreceptor, and gp41, which is triggered upon coreceptor binding to promote the membrane fusion reaction. Env on the surface of infected cells is a trimer consisting of three gp120/gp41 homo-dimeric protomers. An emerging question concerns cooperative interactions between the protomers in the trimer, and possible implications for Env function.

RESULTS

We extended studies on cooperative subunit interactions within the HIV-1 Env trimer, using analysis of functional complementation between coexpressed inactive variants harboring different functional deficiencies. In assays of Env-mediated cell fusion, complementation was observed between variants with a wide range of defects in both the gp120 and gp41 subunits. The former included gp120 subunits mutated in the CD4 binding site or incapable of coreceptor interaction due either to mismatched specificity or V3 loop mutation. Defective gp41 variants included point mutations at different residues within the fusion peptide or heptad repeat regions, as well as constructs with modifications or deletions of the membrane proximal tryptophan-rich region or the transmembrane domain. Complementation required the defective variants to be coexpressed in the same cell. The observed complementation activities were highly dependent on the assay system. The most robust activities were obtained with a vaccinia virus-based expression and reporter gene activation assay for cell fusion. In an alternative system involving Env expression from integrated provirus, complementation was detected in cell fusion assays, but not in virus particle entry assays.

CONCLUSION

Our results indicate that Env function does not require every subunit in the trimer to be competent for all essential activities. Through cross-talk between subunits, the functional determinants on one defective protomer can cooperatively interact to trigger the functional determinants on an adjacent protomer(s) harboring a different defect, leading to fusion. Cooperative subunit interaction is a general feature of the Env trimer, based on complementation activities observed for a highly diverse range of functional defects.

The paradoxical dynamism of marrow stem cells: considerations of stem cells, niches, and microvesicles

Marrow stem cell regulation represents a complex and flexible system. It has been assumed that the system was intrinsically hierarchical in nature, but recent data has indicated that at the progenitor/stem cell level the system may represent a continuum with reversible alterations in phenotype occurring as the stem cells transit cell cycle. Short and long-term engraftment, in vivo and in vitro differentiation, gene expression, and progenitor numbers have all been found to vary reversibly with cell cycle. In essence, the stem cells appear to show variable potential, probably based on transcription factor access, as they proceed through cell cycle. Another critical component of the stem cell regulation is the microenvironment, so-called niches. We propose that there are not just several unique niche cells, but a wide variety of niche cells which continually change phenotype to appropriately interact with the continuum of stem cell phenotypes. A third component of the regulatory system is microvesicle transfer of genetic information between cells. We have shown that marrow cells can express the genetic phenotype of pulmonary epithelial cells after microvesicle transfer from lung to marrow cells. Similar transfers of tissue specific mRNA occur between liver, brain, and heart to marrow cells. Thus, there would appear to be a continuous genetic modulation of cells through microvesicle transfer between cells. We propose that there is an interactive triangulated Venn diagram with continuously changing stem cells interacting with continuously changing areas of influence, both being modulated by transfer of genetic information by microvesicles.

Successful application of hyperbranched multidisplacement genomic amplification to detect HIV-1 sequences in single neurons removed from autopsy brain sections by laser capture microdissection

To confirm studies suggesting that HIV-1 infects neurons and to determine whether CD8(+) T lymphocytes traffic to HIV-1-infected neurons, we used laser capture microdissection to remove hippocampal neurons with and without perineuronal CD8(+) T cells from AIDS patients with HIV-1 encephalitis (HIVE) or without HIVE and from normal controls. We used hyperbranched multidisplacement amplification for whole gene amplification (MDA-WGA) plus two rounds of PCR to amplify housekeeping sequences (HK(+)) and, in HK(+) samples, to amplify HIV-1 gag, nef, and pol sequences. Sample size and, in single neurons, MDA-WGA correlated with housekeeping gene amplification (P < 0.05), whereas patient group and postmortem interval did not (P > 0.05). Neuronal viral sequences correlated with HIVE (43% vs. 13% and 0 in non-HIVE and controls, respectively) and, in HIVE cases, with perineuronal CD8(+) T lymphocytes (70% in CD8(+) samples vs. 37% of CD8(-) samples). Our results suggest that MDA-WGA is a useful technique when analyzing DNA from single cells from autopsy brains, supporting prior studies that show that neurons may contain HIV-1 neuronal sequences in vivo. The association between neuronal infection and perineuronal CD8(+) T cells supports our hypothesis that these cells specifically traffic to infected neurons but raises the possibility that CD8(+) T cells, if infected, could transmit virus to neurons.

In vitro derived dendritic cells trans-infect CD4 T cells primarily with surface-bound HIV-1 virions

In the prevailing model of HIV-1 trans-infection, dendritic cells (DCs) capture and internalize intact virions and transfer these virions to interacting T cells at the virological synapse. Here, we show that HIV-1 virions transmitted in trans from in vitro derived DCs to T cells principally originate from the surface of DCs. Selective neutralization of surface-bound virions abrogated trans-infection by monocyte-derived DCs and CD34-derived Langerhans cells. Under conditions mimicking antigen recognition by the interacting T cells, most transferred virions still derived from the cell surface, although a few were transferred from an internal compartment. Our findings suggest that attachment inhibitors could neutralize trans-infection of T cells by DCs in vivo.

Dendritic cells (DCs) patrol peripheral mucosal sites, capturing and processing potential pathogens into antigenic peptides for presentation to T cells of lymphoid organs, and thereby initiating an immune response. HIV-1 had been proposed to use DCs as “Trojan horses,” hiding inside the DCs and surviving the degradation pathway to gain access to the lymph nodes and spread to the T cells. Our study challenges this “Trojan horse” model by showing that only HIV-1 virions bound to the surface of DCs, and not internalized virions, are transmitted to T cells. Even when T cells specifically recognized the antigen presented by DCs, the infection of T cells was principally mediated by virions remaining at the surface of the DCs. Interestingly, in this context of antigen-specific recognition, which increases the trafficking toward the immunological synapse of DC internal vesicles, where HIV-1 virions seem to hide, a few internal virions could infect T cells. Our findings suggest that in vivo transmission to T cells of HIV-1 virions captured by DCs should be more sensitive to neutralization than previously expected.

Embryonic stem cell-derived microvesicles reprogram hematopoietic progenitors: evidence for horizontal transfer of mRNA and protein delivery

Membrane-derived vesicles (MV) are released from the surface of activated eucaryotic cells and exert pleiotropic effects on surrounding cells. Since the maintenance of pluripotency and undifferentiated propagation of embryonic stem (ES) cells in vitro requires tight cell to cell contacts and effective intercellular signaling, we hypothesize that MV derived from ES cells (ES-MV) express stem cell-specific molecules that may also support self-renewal and expansion of adult stem cells. To address this hypothesis, we employed expansion of hematopoietic progenitor cells (HPC) as a model. We found that ES-MV (10 microg/ml) isolated from murine ES cells (ES-D3) in serum-free cultures significantly (i) enhanced survival and improved expansion of murine HPC, (ii) upregulated the expression of early pluripotent (Oct-4, Nanog and Rex-1) and early hematopoietic stem cells (Scl, HoxB4 and GATA 2) markers in these cells, and (iii) induced phosphorylation of MAPK p42/44 and serine-threonine kinase AKT. Furthermore, molecular analysis revealed that ES-MV express Wnt-3 protein and are selectively highly enriched in mRNA for several pluripotent transcription factors as compared to parental ES cells. More important, this mRNA could be delivered by ES-MV to target cells and translated into the corresponding proteins. The biological effects of ES-MV were inhibited after heat inactivation or pretreatment with RNAse, indicating a major involvement of protein and mRNA components of ES-MV in the observed phenomena. We postulate that ES-MV may efficiently expand HPC by stimulating them with ES-MV expressed ligands (e.g., Wnt-3) as well as increase their pluripotency after horizontal transfer of ES-derived mRNA.

Brain CD8+ and cytotoxic T lymphocytes are associated with, and may be specific for, human immunodeficiency virus type 1 encephalitis in patients with acquired immunodeficiency syndrome

CD8+ T cells infiltrate brains with human immunodeficiency virus type-1 (HIV-1) encephalitis (HIVE) and related animal models; their perineuronal localization suggests cytotoxic T cell (CTL)-mediated neuronal killing. Because CTLs have not been identified in acquired immunodeficiency syndrome (AIDS) brains, the authors identified their cytotoxic granules in autopsy AIDS brains with HIVE and without HIVE (HIVnE) plus controls (7 to 13 cases/group) and determined gene expression profiles of CTL-associated genes in a separate series of cases. CD3+ and CD8+ T cells were significantly increased (P < .01) in perivascular spaces and inflammatory nodules in HIVE but were rare or absent in brain parenchyma in HIVnE and control brains. Eight HIVE brains contained granzyme B+ T cells and five contained perforin+ T cells. Their T-cell origin was confirmed by colocalization of CD8 and granzyme B in the same cell and the absence of CD56+ natural killer cells. The CTLs directly contacted with neurons, as the authors showed previously for CD3+ and CD8+ T cells. CTLs were rare or absent in HIV nonencephalitis (HIVnE) and controls. Granzyme B and H precursor gene expression was up-regulated and interleukin (IL)-12A precursor, a maturation factor for natural killer cells and CTLs, was down-regulated in HIVE versus HIVnE brain. This study demonstrates, for the first time, CTLs in HIVE and shows that parenchymal T cells and CTLs are sensitive biomarkers for HIVE. Consequently, CD8+ T cells and CTLs could mediate brain injury in HIVE and may represent an important biomarker for productive brain infection by HIV-1.

Therapeutic role of CD8+ T cells in HIV-1 infection: targets and suppressors of viral replication

CD8+ T cells are pivotal in controlling viral replication in HIV-1-infected subjects. However, in chronic infection, HIV-1-specific CD8+ T cells fail to adequately control infection, presenting incomplete maturation and more severe functional impairment with advanced disease. Accumulating evidence has shown that CD8+ T cells can also be productively infected by HIV-1. Whether HIV-1 infection of CD8+ T lymphocytes impacts on their antiviral activity remains to be determined. This review explores the potential mechanisms of HIV-1 infection of CD8+ T cells, its likely contribution to the immunopathogenesis of HIV-1 infection and potential therapeutic interventions.

Evolution of cell recognition by viruses: a source of biological novelty with medical implications

The picture beginning to form from genome analyses of viruses, unicellular organisms, and multicellular organisms is that viruses have shared functional modules with cells. A process of coevolution has probably involved exchanges of genetic information between cells and viruses for long evolutionary periods. From this point of view present-day viruses show flexibility in receptor usage and a capacity to alter through mutation their receptor recognition specificity. It is possible that for the complex DNA viruses, due to a likely limited tolerance to generalized high mutation rates, modifications in receptor specificity will be less frequent than for RNA viruses, albeit with similar biological consequences once they occur. It is found that different receptors, or allelic forms of one receptor, may be used with different efficiency and receptor affinities are probably modified by mutation and selection. Receptor abundance and its affinity for a virus may modulate not only the efficiency of infection, but also the capacity of the virus to diffuse toward other sites of the organism. The chapter concludes that receptors may be shared by different, unrelated viruses and that one virus may use several receptors and may expand its receptor specificity in ways that, at present, are largely unpredictable.

Neutralization of human immunodeficiency virus type 1 by sCD4-17b, a single-chain chimeric protein, based on sequential interaction of gp120 with CD4 and coreceptor

We designed a novel single-chain chimeric protein, designated sCD4-17b, for neutralization of human immunodeficiency virus type 1 (HIV-1). The recombinant protein contains domains 1 and 2 of soluble CD4 (sCD4), connected via a flexible polypeptide linker to a single-chain variable region construct of 17b, a human monoclonal antibody that targets a conserved CD4-induced epitope on gp120 overlapping the coreceptor binding region. We hypothesized that the sCD4 moiety would bind gp120 and expose the 17b epitope; the 17b moiety would then bind, thereby blocking coreceptor interaction and neutralizing infection. The sCD4-17b protein, expressed by a recombinant vaccinia virus, potently neutralized a prototypic R5 clade B primary isolate, with a 50% inhibitory concentration of 3.2 nM (0.16 microg/ml) and >95% neutralization at 32 nM (1.6 microg/ml). The individual components (sCD4 and 17b, singly or in combination) had minimal effects at these concentrations, demonstrating that the activity of sCD4-17b reflected the ability of a single chimeric molecule to bind gp120 simultaneously via two independent moieties. sCD4-17b was highly potent compared to the previously characterized broadly cross-reactive neutralizing monoclonal antibodies IgGb12, 2G12, and 2F5. Multiple primary isolates were neutralized, including two previously described as antibody resistant. Neutralization occurred for both R5 and X4 strains and was not restricted to clade B. However, several primary isolates were insensitive over the concentration range tested, despite the known presence of binding sites for both CD4 and 17b. sCD4-17b has potential utility for passive immunization against HIV-1 in several contexts, including maternal transmission, postexposure prophylaxis, and sexual transmission (topical microbicide).

Platelet- and megakaryocyte-derived microparticles transfer CXCR4 receptor to CXCR4-null cells and make them susceptible to infection by X4-HIV

OBJECTIVE

Under some circumstances the HIV virus may infect cells that do not express receptors essential to HIV-entry. We hypothesized that platelet- and megakaryocyte-derived microparticles (MP) could play a role in such infections. MP are circular membrane fragments shed from the surface of eukaryotic cells. After adhesion to target cells, MP may transfer membrane-associated proteins to these cells. We found that peripheral blood platelet- (PMP) and megakaryocyte-derived MP (MegaMP) that highly express CXCR4 may transfer this receptor from the surface of platelets or megakaryocytes to the surface of CXCR4-null cells.

DESIGN

Since this mechanism could potentially allow CD4+/CXCR4-null cells to become infected by T-tropic HIV, we incubated several human CD4+/CXCR4-null cells such as normal erythroblasts, glioblastomas U87, MAGI and hematopoietic cell lines UT-7, HEL and TF-1 with PMP or MegaMP. We found that these cells became CXCR4+. We next exposed these cells to X4-HIV (IIIB) and evaluated their susceptibility to infection by PCR, ELISA, and morphological analysis.

RESULTS

We observed in all instances that after CD4+/CXCR4-null cell lines 'acquired' CXCR4 from PMP or MegaMP, they could became infected by X4 HIV.

CONCLUSIONS

We postulate that both PMP and MegaMP may play a novel and important role in spreading HIV-1 infection by transferring the CXCR4 co-receptor to CD4+/CXCR4-null cells.

Methamphetamine enhances cell-associated feline immunodeficiency virus replication in astrocytes

Human immunodeficiency virus (HIV) infection among substance abusers is on the rise worldwide. Psychostimulants, and in particular methamphetamine (METH), have detrimental effects on the immune system as well as causing a progressive neurodegeneration, similar to HIV infection. Many Lentivirinae, including feline immunodeficiency virus (FIV), penetrate into the central nervous system early in the course of infection with astrocytes serving as a reservoir of chronic brain infection. We demonstrate that the FIV-Maryland isolate infects feline primary and cell line (G355-5)-cultured astrocytes only under cell-associated conditions. Infected astrocytes yielded a new astrocytotropic isolate, capable of cell-free infection (termed FIV-MD-A). This isolate contained four amino acid substitutions in the envelope polyprotein resulting in a change in net charge as compared to FIV-MD. Infection for both isolates was dependent upon a functional astrocyte CXCR4 receptor. Methamphetamine increased significantly FIV replication in feline astrocytes for cell-associated infection only, with no effect on peripheral blood mononuclear cells or astrocytes infected with FIV-MD-A. This viral replication was related to proviral copy number, suggesting the effect of METH is at the viral entry or integration into host genome levels, but not at the translational level. Thus, lentiviral infection of the brain in the presence of the psychostimulant METH may result in enhanced astrocyte viral replication, producing a more rapid and increased brain viral load.

Inter-retroviral fusion mediated by human immunodeficiency virus or murine leukemia virus glycoproteins: independence of cellular membranes and membrane vesicles

We have recently demonstrated for the first time that inter-retroviral membrane fusion, i.e., membrane fusion between individual retroviral particle populations with incorporated HIV-1 Env and cellular receptors, respectively, can occur (Sparacio et al. 2000, Virology 271: 248-252). We have extended these analyses here and confirmed that fusion between particles can occur in the extracellular medium independent of any cellular membranes and that luciferase transduction, mediated by the fused structures, is independent of significant potential contribution by contaminating membrane vesicles. We have additionally analyzed whether membrane fusion between HIV-like particles can be mediated by amphotropic murine leukemia virus (MuLV) glycoprotein and its respective cellular receptor, PiT-2. We demonstrate that PiT-2 can be incorporated into HIV-like particles and can fuse with MuLV-Env-carrying particles. This occurs only in the situation in which the incorporated MuLV-Env protein has been activated to fusion activity by HIV protease-mediated removal of the C-terminal R-peptide and is completely inhibited when the respective particles are generated in the presence of the HIV protease inhibitor, Saquinavir.

Detection of HIV-1 gene sequences in hippocampal neurons isolated from postmortem AIDS brains by laser capture microdissection

We employed laser capture microdissection to remove individual pyramidal neurons from the CA1, CA3, and CA4 regions of formalin-fixed, paraffin-embedded hippocampus from 8 AIDS brains and 2 HIV-1-seronegative normal brains. We amplified HIV-1 gag and nef gene sequences using separate, double round PCR reactions for each of the primer sets. In all 3 hippocampal regions, amplification efficiency was best with sequence length between 284 and 324 bp; HIV-1 nef gene sequences were more common than HIV-1 gag sequences; and rank order for percent positive amplification was CA3 > CA4 > CA1 samples. These results are the first to detect HIV-1 gene sequences in microdissected human tissue. They indicate that brain neurons in vivo contain HIV-1 DNA sequences consistent with latent infection by this virus, and suggest that neurons display a selective vulnerability for HIV infection. Neuronal HIV infection could contribute to neuronal injury and death or act as a potential viral reservoir if reactivated.

Automated quantitation of cell-mediated HIV type 1 infection of human syncytiotrophoblast cells by fluorescence in situ hybridization and laser scanning cytometry

Infection of human placental syncytiotrophoblast cells with HIV requires direct contact with infected leukocytes. In vitro investigations into mechanisms regulating placental HIV transmission and into the development of therapeutic interventions have been hampered by difficulties inherent in quantitating HIV levels in cocultures of infected lymphocytes and adherent multinucleated syncytiotrophoblast cells. Here, we have used fluorescence in situ hybridization (FISH) for the direct detection of HIV-1 RNA within syncytiotrophoblast cells combined with laser scanning cytometry (LSC) to quantitate HIV levels exclusively in the syncytiotrophoblast cells. HIV-1-infected lymphocytic MOLT-4 cells were cocultured with primary human syncytiotrophoblast cells. Lymphocytic cells were identified with an anti-vimentin antibody and Cy5. HIV RNA was localized by in situ hybridization, using a digoxigenin-labeled riboprobe detected by Oregon Green, and nuclei were stained with 7-aminoactinomycin D. The three-color cocultures were analyzed by LSC to remove unwanted cell populations and quantitate HIV expression levels. The total HIV RNA level (green fluorescence integral) in each colony was normalized for cell size by dividing by the total DNA content (red fluorescence integral). The nuclear-normalized fluorescence integral was 2.3 times higher in infected cocultures than in uninfected cultures. When cocultures were incubated with 10 microM AZT, the green/red fluorescence integral value was significantly lower than that of cocultures incubated in the absence of AZT, corresponding to a 78% reduction in fluorescence. Laser scanning cytometry can be used to quantitate cell-mediated HIV infection in syncytiotrophoblast cells and should allow drug assessment studies and studies aimed at understanding the mechanism of virus entry into trophoblast cells to be carried out.

DC-SIGNR, a DC-SIGN homologue expressed in endothelial cells, binds to human and simian immunodeficiency viruses and activates infection in trans

DC-SIGN, a C-type lectin expressed on the surface of dendritic cells (DCs), efficiently binds and transmits HIVs and simian immunodeficiency viruses to susceptible cells in trans. A DC-SIGN homologue, termed DC-SIGNR, has recently been described. Herein we show that DC-SIGNR, like DC-SIGN, can bind to multiple strains of HIV-1, HIV-2, and simian immunodeficiency virus and transmit these viruses to both T cell lines and human peripheral blood mononuclear cells. Binding of virus to DC-SIGNR was dependent on carbohydrate recognition. Immunostaining with a DC-SIGNR-specific antiserum showed that DC-SIGNR was expressed on sinusoidal endothelial cells in the liver and on endothelial cells in lymph node sinuses and placental villi. The presence of this efficient virus attachment factor on multiple endothelial cell types indicates that DC-SIGNR could play a role in the vertical transmission of primate lentiviruses, in the enabling of HIV to traverse the capillary endothelium in some organs, and in the presentation of virus to CD4-positive cells in multiple locations including lymph nodes.

HIV type 1 molecular clones able to use the Bonzo/STRL-33 coreceptor for virus entry

We describe the cloning of env genes from the mother-infant HIV-1 isolate pair P6-v3 and M6-v3. These viruses are unusual in that they can use the coreceptor Bonzo/STRL33 as well as CCR5 and, in the case of M6, CXCR4, to enter transfected cell lines in vitro. The phenotype of the parental isolates is generally reflected by the properties of the cloned env genes, when these are used in an Env-complementation assay of virus entry. Chimeric viruses were also made that contain the env genes of P6-v3 and M6-v3 inserted into the background of the infectious molecular clone, HIV-1 NL4-3. Some of the chimeric viruses derived from HIV1 P6-v3 were able to use Bonzo for entry into transfected cell lines, albeit to a lesser extent than they could use CCR5. There are some indications that one of these chimeric viruses, P6-v3-22-1, can use a coreceptor other than CCR5, perhaps Bonzo, to enter mitogen-stimulated PBMC, although only weakly. However, formal proof that this virus can use Bonzo in primary cells has not been obtained. The P6-v3-22-1 chimeric virus was unable to infect CD4-negative, placental cell lines, in the presence or absence of soluble CD4. Env sequence analysis revealed several differences among viruses with different tropisms, most notably a four amino acid deletion in the central region of the V3 loop that distinguishes the R5 virus P6-v3-25-4 from the R5, Bonzo virus P6-v3-22-1.

Donor- and ligand-dependent differences in C-C chemokine receptor 5 reexpression

N-terminal modifications of the chemokine RANTES bind to C-C chemokine receptor 5 (CCR5) and block human immunodeficiency virus type 1 (HIV-1) infection with greater efficacy than native RANTES. Modified RANTES compounds induce rapid CCR5 internalization and much slower receptor reexpression than native RANTES, suggesting that receptor sequestration is one mode of anti-HIV activity. The rates of CCR5 internalization and reexpression were compared using the potent n-nonanoyl (NNY)-RANTES derivative and CD4(+) T cells derived from donors with different CCR5 gene polymorphisms. NNY-RANTES caused even more rapid receptor internalization and slower reexpression than aminooxypentane (AOP)-RANTES. Polymorphisms in the promoter and coding regions of CCR5 significantly affected the receptor reexpression rate after exposure of cells to NNY-RANTES. These observations may be relevant for understanding the protective effects of different CCR5 genotypes against HIV-1 disease progression.

Molecular function of the CD4 D1 domain in coreceptor-mediated entry by HIV type 1

The surface molecule CD4 plays a key role in initiating cellular entry by the human immunodeficiency virus type 1 (HIV-1), and it is now recognized as acting synergistically with select chemokine receptors (coreceptors) in the infection process. The present study was undertaken to determine whether the extracellular region of CD4 is sufficient to induce fusion of HIV-1 virions with target cells in the absence of its anchoring function. Using pseudotype reporter viruses to quantitate infection, soluble CD4 (sCD4) was tested for its ability to induce fusion by viruses utilizing CCR5 as their coreceptor. We found that sCD4 was competent to replace membrane-bound CD4 to trigger infection mediated by several HIV-1 envelopes. Furthermore, in a comparison of the envelopes of HIV-1 NL4-3 and a chimera containing the gp120 V3 loop of Ba-L, the V3 region was found to be one factor affecting susceptibility to induction by sCD4. In addition, using truncated and mutant derivatives of sCD4, the amino-terminal D1 domain of CD4 was found to be necessary and sufficient for induction of fusion and to require an intact gp120-binding site for this activity. These results delineate determinants on CD4 and gp120 required for fusion induction in collaboration with a coreceptor, and suggest a mechanism whereby CD4 may contribute to viral infection in trans.

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.

Expression and coreceptor function of APJ for primate immunodeficiency viruses

APJ is a seven transmembrane domain G-protein-coupled receptor that functions as a coreceptor for some primate immunodeficiency virus strains. The in vivo significance of APJ coreceptor function remains to be elucidated, however, due to the lack of an antibody that can be used to assess APJ expression, and because of the absence of an antibody or ligand that can block APJ coreceptor activity. Therefore, we produced a specific monoclonal antibody (MAb 856) to APJ and found that it detected this receptor in FACS, immunofluorescence, and immunohistochemistry studies. MAb 856 also recognized APJ by Western blot, enabling us to determine that APJ is N-glycosylated. Using this antibody, we correlated APJ expression with coreceptor activity and found that APJ had coreceptor function even at low levels of expression. However, we found that APJ could not be detected by FACS analysis on cell lines commonly used to propagate primate lentiviruses, nor was it expressed on human PBMC cultured under a variety of conditions. We also found that some viral envelope proteins could mediate fusion with APJ-positive, CD4-negative cells, provided that CD4 was added in trans. These findings indicate that in some situations APJ use could render primary cell types susceptible to virus infection, although we have not found any evidence that this occurs. Finally, the peptide ligand for APJ, apelin-13, efficiently blocked APJ coreceptor activity.

Expression and coreceptor activity of STRL33/Bonzo on primary peripheral blood lymphocytes

CCR5 and CXCR4 are the major coreceptors that mediate human immunodeficiency virus 1 (HIV-1) infection, while most simian immunodeficiency virus (SIV) isolates use CCR5. A number of alternative coreceptors can also mediate infection of some virus strains in vitro, although little is known about their in vivo relevance. Therefore, we characterized the expression pattern and coreceptor activity of one of these alternative coreceptors, STRL33/Bonzo, using a newly developed monoclonal antibody. In addition to being highly expressed (approximately 1000-7000 STRL33 ABS [antibody binding sites]) on specific subsets of natural killer cells (CD3−/CD16−/low/CD56+ and CD3−/CD16low/CD56−) and CD19+ B lymphocytes (approximately 300-5000 STRL33 ABS), STRL33 was expressed at levels sufficient to support virus infection on freshly isolated, truly naive CD4+/CD45RA+/CD62L+cells (6000-11 000 ABS). STRL33 expression on peripheral blood mononuclear cells (PBMCs) was increased by mitogenic stimulation (OKT3/IL-2 [interleukin-2] had a greater effect than phytohemaglutinin (PHA)/IL-2), but it was dramatically decreased upon Ficoll purification. Infection of CCR5− human peripheral blood lymphocytes (PBLs) showed that 2 different SIV envelope (Env) proteins mediated entry into STRL33+cells. More importantly, the preferential infection of STRL33+ cells in CCR5− PBLs by an R5/X4/STRL33 HIV-1 maternal isolate in the presence of a potent CXCR4 antagonist (AMD3100) suggests that STRL33 can be used as a coreceptor by HIV-1 on primary cells. Rhesus macaque (rh) STRL33 was used less efficiently than human STRL33 by the majority of SIV Env proteins tested despite similar levels of expression, thereby making it less likely that STRL33 is a relevant coreceptor in the rhesus macaque system. In summary, the expression pattern and coreceptor activity of STRL33 suggest its involvement in trafficking of tumor-infiltrating lymphocytes and indicate that STRL33 may be a relevant coreceptor in vivo.

Expression and coreceptor activity of STRL33/Bonzo on primary peripheral blood lymphocytes

CCR5 and CXCR4 are the major coreceptors that mediate human immunodeficiency virus 1 (HIV-1) infection, while most simian immunodeficiency virus (SIV) isolates use CCR5. A number of alternative coreceptors can also mediate infection of some virus strains in vitro, although little is known about their in vivo relevance. Therefore, we characterized the expression pattern and coreceptor activity of one of these alternative coreceptors, STRL33/Bonzo, using a newly developed monoclonal antibody. In addition to being highly expressed (approximately 1000-7000 STRL33 ABS [antibody binding sites]) on specific subsets of natural killer cells (CD3−/CD16−/low/CD56+ and CD3−/CD16low/CD56−) and CD19+ B lymphocytes (approximately 300-5000 STRL33 ABS), STRL33 was expressed at levels sufficient to support virus infection on freshly isolated, truly naive CD4+/CD45RA+/CD62L+cells (6000-11 000 ABS). STRL33 expression on peripheral blood mononuclear cells (PBMCs) was increased by mitogenic stimulation (OKT3/IL-2 [interleukin-2] had a greater effect than phytohemaglutinin (PHA)/IL-2), but it was dramatically decreased upon Ficoll purification. Infection of CCR5− human peripheral blood lymphocytes (PBLs) showed that 2 different SIV envelope (Env) proteins mediated entry into STRL33+cells. More importantly, the preferential infection of STRL33+ cells in CCR5− PBLs by an R5/X4/STRL33 HIV-1 maternal isolate in the presence of a potent CXCR4 antagonist (AMD3100) suggests that STRL33 can be used as a coreceptor by HIV-1 on primary cells. Rhesus macaque (rh) STRL33 was used less efficiently than human STRL33 by the majority of SIV Env proteins tested despite similar levels of expression, thereby making it less likely that STRL33 is a relevant coreceptor in the rhesus macaque system. In summary, the expression pattern and coreceptor activity of STRL33 suggest its involvement in trafficking of tumor-infiltrating lymphocytes and indicate that STRL33 may be a relevant coreceptor in vivo.

Transfer of the chemokine receptor CCR5 between cells by membrane-derived microparticles: a mechanism for cellular human immunodeficiency virus 1 infection

The release of microparticles from eukaryotic cells is a well-recognized phenomenon. We demonstrate here that the chemokine receptor CCR5, the principal co-receptor for macrophage-tropic human immunodeficiency virus (HIV)-1, can be released through microparticles from the surface of CCR5+ Chinese hamster ovary cells and peripheral blood mononuclear cells. Microparticles containing CCR5 can transfer the receptor to CCR5- cells and render them CCR5+. The CCR5 transfer to CCR5-deficient peripheral blood mononuclear cells homozygous for a 32-base-pair deletion in the CCR5 gene enabled infection of these cells with macrophage-tropic HIV-1. In monocytes, the transfer of CCR5 could be inhibited by cytochalasin D, and transferred CCR5 could be downmodulated by chemokines. A transfer of CCR5 from peripheral blood mononuclear cells to endothelial cells during transendothelial migration could be demonstrated. Thus, the transfer of CCR5 may lead to infection of tissues without endogenous CCR5 expression. Moreover, the intercellular transfer of membrane proteins by microparticles might have broader consequences for intercellular communication beyond the effects seen for HIV-1.

DC-SIGN, a dendritic cell-specific HIV-1-binding protein that enhances trans-infection of T cells

Dendritic cells (DC) capture microorganisms that enter peripheral mucosal tissues and then migrate to secondary lymphoid organs, where they present these in antigenic form to resting T cells and thus initiate adaptive immune responses. Here, we describe the properties of a DC-specific C-type lectin, DC-SIGN, that is highly expressed on DC present in mucosal tissues and binds to the HIV-1 envelope glycoprotein gp120. DC-SIGN does not function as a receptor for viral entry into DC but instead promotes efficient infection in trans of cells that express CD4 and chemokine receptors. We propose that DC-SIGN efficiently captures HIV-1 in the periphery and facilitates its transport to secondary lymphoid organs rich in T cells, to enhance infection in trans of these target cells.

Anti-human immunodeficiency virus type 1 gene therapy in human central nervous system-based cells: an initial approach against a potential viral reservoir

Studies have demonstrated that human immunodeficiency virus type 1 (HIV-1) infection of central nervous system (CNS)-based cells in vivo results in a series of devastating clinical conditions collectively termed acquired immune deficiency syndrome (AIDS) dementia complex (ADC). Gene therapy for these neurovirological disorders necessitates utilization of a vector system that can mediate in vivo delivery and long-term expression of an antiretroviral transgene in nondividing/postmitotic CNS cellular elements. The present studies focus on the transfer of an anti-HIV-1 gene to primary isolated CNS microvascular endothelial cells (MVECs) and neuronal-based cells, for its effects in protecting these cells from HIV-1 infection. By using an HIV-1-based vector system, it was possible to efficiently transduce and maintain expression of a marker transgene, beta-galactosidase (beta-Gal), in human CNS MVECs, human fetal astrocytes, plus immature and mature (differentiated) NT2 cells. Significant transduction of the marker gene, beta-Gal, in CNS-based cells prompted the utilization of this system with an anti-HIV-1 gene therapeutic construct, RevM10, a trans-dominant negative mutant Rev protein. Initially, it was not possible to generate any HIV-1 vector particles with the RevM10 gene in the transducing construct, because of inhibitory effects on the HIV-1 vector by this gene product. However, the vector could be partially rescued by adding an additional construct that supplied wild-type rev, in trans, during a multiple construct transfection in the packaging 293T cells. Thus, it was possible to significantly improve the titer of RevM10-expressing viral particles generated from these cells. Moreover, this RevM10 vector transduced the neuronal precursor cell line NT2, retinoic acid-differentiated human neurons (hNT) from the precursor cells, and primary isolated human brain MVECs with high efficiency. RevM10 generated from the HIV-1-based vector system potently inhibited replication of diverse HIV-1 strains in human CNS MVECs and neuronal cells. The data generated from these studies represent an initial approach for future development of anti-HIV-1 gene therapy in the CNS.

Sequential CD4-coreceptor interactions in human immunodeficiency virus type 1 Env function: soluble CD4 activates Env for coreceptor-dependent fusion and reveals blocking activities of antibodies against cryptic conserved epitopes on gp120

We devised an experimental system to examine sequential events by which the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) interacts with CD4 and coreceptor to induce membrane fusion. Recombinant soluble CD4 (sCD4) activated fusion between effector cells expressing Env and target cells expressing coreceptor (CCR5 or CXCR4) but lacking CD4. sCD4-activated fusion was dose dependent, occurred comparably with two- and four-domain proteins, and demonstrated Env-coreceptor specificities parallel to those reported in conventional fusion and infectivity systems. Fusion activation occurred upon sCD4 preincubation and washing of the Env-expressing effector cells but not the coreceptor-bearing target cells, thereby demonstrating that sCD4 exerts its effects by acting on Env. These findings provide direct functional evidence for a sequential two-step model of Env-receptor interactions, whereby gp120 binds first to CD4 and becomes activated for subsequent functional interaction with coreceptor, leading to membrane fusion. We used the sCD4-activated system to explore neutralization by the anti-gp120 human monoclonal antibodies 17b and 48d. These antibodies reportedly bind conserved CD4-induced epitopes involved in coreceptor interactions but neutralize HIV-1 infection only weakly. We found that 17b and 48d had minimal effects in the standard cell fusion system using target cells expressing both CD4 and coreceptor but potently blocked sCD4-activated fusion with target cells expressing coreceptor alone. Both antibodies strongly inhibited sCD4-activated fusion by Envs from genetically diverse HIV-1 isolates. Thus, the sCD4-activated system reveals conserved Env-blocking epitopes that are masked in native Env and hence not readily detected by conventional systems.