Infection of macrophages with lymphotropic human immunodeficiency virus type 1 can be arrested after viral DNA synthesis

Human peritoneal macrophages from ascitic fluid can be infected by a broad range of HIV-1 isolates

Macrophages are major HIV target cells. They support both productive and latent HIV-1 infection. Susceptibility of primary macrophages to HIV depends on the anatomical location and activation state of the cells. We demonstrate that peritoneal macrophages (PMs) are abundant in ascitic fluid of patients with liver cirrhosis and are susceptible to HIV-1 infection. PMs expressed CD68, a differentiation marker, exhibited phagocytic activity, and survived in culture for 2 months without additional growth factors. Freshly isolated PMs were susceptible to HIV-1 R5 strains but not to X4-T-cell line-adapted strains. Interestingly, after 7 days in culture, PMs acquired susceptibility to X4-T-cell line-adapted strains. HIV entry inhibitors, TAK779 and AMD3100, blocked HIV infection of PMs, indicating that infection by R5 and X4 strains was mediated by CCR5 and CXCR4, respectively. Although PMs did not express detectable cell surface levels of CXCR4 and CCR5, they did express mRNAs of these HIV coreceptors and responded to stimulation by their natural ligands, SDF-1alpha and RANTES. PMs were susceptible to HIV-1 X4, R5, and X4R5 primary isolates. PMs after 7 days in culture produced greater amounts of X4 and X4R5 HIV than freshly isolated PMs. The day-7 PMs were more susceptible to R5 infection in a single-cycle infection assay, but there was no increase in viral production in a multiple-round infection assay. The level of CXCR4 mRNA and production of CC-chemokines (MIP-1alpha, MIP-1beta, and RANTES) increased significantly during 7 days in culture. Our results indicate that PMs are susceptible to receptor-mediated infection by a broad range of HIV strains. These primary macrophages could provide a valuable system for investigating the role of primary macrophages in HIV pathogenesis.

Maturation of the viral core enhances the fusion of HIV-1 particles with primary human T cells and monocyte-derived macrophages

HIV-1 infection requires fusion of viral and cellular membranes in a reaction catalyzed by the viral envelope proteins gp120 and gp41. We recently reported that efficient HIV-1 particle fusion with target cells is linked to maturation of the viral core by an activity of the gp41 cytoplasmic domain. Here, we show that maturation enhances the fusion of a variety of recombinant viruses bearing primary and laboratory-adapted Env proteins with primary human CD4+ T cells. Overall, HIV-1 fusion was more dependent on maturation for viruses bearing X4-tropic envelope proteins than for R5-tropic viruses. Fusion of HIV-1 with monocyte-derived macrophages was also dependent on particle maturation. We conclude that the ability to couple fusion to particle maturation is a common feature of HIV-1 Env proteins and may play an important role during HIV-1 replication in vivo.

Macrophages exposed to lymphotropic and monocytotropic HIV induce similar CTL responses despite differences in productive infection

Macrophages are accessory cells that are vulnerable to infection by HIV-1. HTLV-IIIB, a lymphotropic strain of HIV, infects macrophages poorly resulting in either no or low levels of virus expression compared to high levels of productive infection after exposure of macrophages to the monocytotropic HIV strain Ada-M. Whether this results in an impaired ability of HTLV-IIIB-exposed macrophages to initiate protective cytotoxic T lymphocyte (CTL) immune responses against these strains is not well defined. We investigated the ability of monocyte-derived macrophages (MDM) exposed to lymphotropic and monocytotropic HIV strains to initiate primary CTL responses in vitro. MDM exposed to HTLV-IIIB induced a specific primary CTL response that was comparable to MDM exposed to the monocytotropic strain Ada-M despite marked differences in productive HIV infection in MDM between the two strains. CTL generated in this model were MHC-restricted, strain-specific, and CD8+. These data demonstrate that high levels of productive HIV infection in accessory cells are not a prerequisite for the generation of a primary CTL response, suggesting a novel immunologic interaction between MDM and lymphotropic HIV strains.

The macrophage response to HIV-1: Intracellular control of X4 virus replication accompanied by activation of chemokine and cytokine synthesis

During human immunodeficiency virus (HIV)-1 infection, T lymphocytes and macrophages play dual roles. They are the primary targets for virus replication, but they are also primary effector cells in acquired and innate immunity, respectively. The authors are now investigating how these roles come together in the response of human monocyte-derived macrophages (MDM) to certain HIV-1. The authors and others have previously shown that MDM permit entry of some X4 virus strains, but control viral replication intracellularly. In the present study, viral DNA synthesis, entry into the nucleus, and transcription to RNA were all observed in X4 virus-infected MDM. MDM arrested HIV-1 replication prior to expression of mature capsid antigen p24 and production of cell-free infectious viral particles. Cell-associated transmissible HIV-1 was detected by cocultivation of infected MDM and susceptible T lymphocytes. A second protective response of MDM to specific R5 as well as X4 HIV-1 was identified in rapid and extensive secretion of tumor necrosis factor-alpha, macrophage inflammatory protein-1alpha, and RANTES. These findings support the view that MDM act aggressively to control HIV-1 replication: X4 strains by severely limiting the progeny virus production and R5 strains by producing beta-chemokines competent to block virus entry into target cells. Optimizing these innate immune responses offers another means to control HIV-1 infection in the human host.

Induction of rapid and extensive beta-chemokine synthesis in macrophages by human immunodeficiency virus type 1 and gp120, independently of their coreceptor phenotype

Human immunodeficiency virus type 1 (HIV-1) interacts with its target cells through CD4 and a coreceptor, generally CCR5 or CXCR4. Macrophages display CD4, CCR5, and CXCR4 that are competent for binding and entry of virus. Virus binding also induces several responses by lymphocytes and macrophages that can be dissociated from productive infection. We investigated the responses of macrophages to exposure to a series of HIV-1 species, R5 species that productively infect and X4 species that do not infect macrophages. We chose to monitor production of several physiologically relevant factors within hours of treatment to resolve virally induced effects that may be unlinked to HIV-1 production. Our novel findings indicate that independently of their coreceptor phenotype and independently of virus replication, exposure to certain R5 and X4 HIV-1 species induced secretion of high levels of macrophage inflammatory protein 1alpha (MIP-1alpha), MIP-1beta, RANTES, and tumor necrosis factor alpha. However two of the six R5 species tested, despite efficient infection, were unable to induce rapid chemokine production. The acute effects of virus on macrophages could be mimicked by exposure to purified R5 or the X4 HIV-1 envelope glycoprotein gp120. Depletion of intracellular Ca(2+) or inhibition of protein synthesis blocked the chemokine induction, implicating Ca(2+)-mediated signal transduction and new protein synthesis in the response. The group of viruses able to induce this chemokine response was not consistent with coreceptor usage. We conclude that human macrophages respond rapidly to R5 and X4 envelope binding by production of high levels of physiologically active proteins that are implicated in HIV-1 pathogenesis.

Characterization of a late entry event in the replication cycle of human immunodeficiency virus type 2

Certain human cell lines and primary macrophage cultures are restricted to infection by some primary isolates of human immunodeficiency virus type 2 (HIV-2), although early steps of the viral life cycle such as fusion at the plasma membrane and reverse transcription are fully supported. The late postintegration events, transcription, translation, assembly, budding, and maturation into infectious virions are functional in restrictive cells. Apart from primary macrophages, the restrictive cell types are actively dividing, and nuclear import of preintegration complexes (PICs) is not required for infection. We therefore postulate that the PICs are trapped in a cellular compartment, preventing subsequent steps in the replication cycle that lead to integration of the provirus. To test this we showed that HIV-2 particles pseudotyped with vesicular stomatitis virus envelope G protein, which delivers HIV into an endocytic compartment, could overcome the block to infection. We suggest that delivery of the viral core into an appropriate cellular compartment is a critical step during the entry process of HIV.

Analysis of cellular factors influencing the replication of human immunodeficiency virus type I in human macrophages derived from blood of different healthy donors

We analyzed parameters influencing HIV-1 infectibility of cells of the monocyte/macrophage lineage (MO/MAC) isolated from different healthy donors. The proportion of in vitro-infected cells and replication kinetics in different donor MAC ranged from 0.03 to 99% p24 antigen-positive MAC and from undetectable RT activity up to 5 x 10(6) cpm/ml/90 min, respectively. As a quantitative measurement for HIV-1 susceptibility of donor MO/MAC, we determined TCID(50) values of defined virus stocks which varied up to 3000-fold depending on the donor MAC used for titration. As host factors which may influence the viral infection we determined the expression of virus receptors CD4, CCR5, CXCR4, and CCR3 as well as the secretion of the natural ligands of CCR5, which altogether showed no correlation with HIV-1 infectibility of the cells. Moreover, other MO-derived secretory factors which might affect viral infection of these cells could be excluded. Furthermore, expression of maturation-related antigens CD14, CD16, HLA-DR, and MAX.1/CPM was determined. Analysis of the reverse transcription process revealed that restricted HIV-1 infection was reflected by highly reduced or even undetectable full-length HIV-1 DNA formation, although early and intermediate transcripts appeared, suggesting that viral replication is blocked after entry at the level of early reverse transcription.

Spleen necrosis virus-derived C-type retroviral vectors for gene transfer to quiescent cells

Gene therapy applications of retroviral vectors derived from C-type retroviruses have been limited to introducing genes into dividing target cells. Here, we report genetically engineered C-type retroviral vectors derived from spleen necrosis virus (SNV), which are capable of infecting nondividing cells. This has been achieved by introducing a nuclear localization signal (NLS) sequence into the matrix protein (MA) of SNV by site-directed mutagenesis. This increased the efficiency of infecting nondividing cells and was sufficient to endow the virus with the capability to efficiently infect growth-arrested human T lymphocytes and quiescent primary monocyte-derived macrophages. We demonstrate that this vector actively penetrates the nucleus of a target cell, and has potential use as a gene therapy vector to transfer genes into nondividing cells.

Definition of the stage of host cell genetic restriction of replication of human immunodeficiency virus type 1 in monocytes and monocyte-derived macrophages by using twins

Using identical (ID) twins, we have previously demonstrated that host cell genes exert a significant impact on productive human immunodeficiency virus (HIV) infection of monocytes and macrophages (J. Chang et al., J. Virol. 70:7792-7803, 1996). Therefore, the stage in the replication cycle at which these host genetic influences act was investigated in a study using 8 pairs of ID twins and 10 pairs of sex- and age-matched unrelated donors (URDs). In the first phase of the study, blood monocytes and monocyte-derived macrophages (MDM) of ID twins and URDs were infected with 15 HIV type 1 strains. Four well-characterized primary isolates and HIV-BaL were then examined in more detail. The host cell genetic effect in MDM was exerted predominantly prior to complete reverse transcription, as the HIV DNA level and p24 antigen levels were concordant (r = 0.91, P = 0.0001) and similar between the pairs of ID twin pairs (r = 0.96, P = 0.0001) but discordant between URD pairs (r = 0.11, P = 0.3) in both phases of the study. To further examine genetic influence on viral entry, we examined the proportion of CCR5 membrane expression on MDM. As expected, there was wide variability in proportion of MDM expressing CCR5 among URDs (r = 0. 58, P = 0.2); however, this variability was significantly reduced between ID twin pairs (r = 0.81, P = 0.01). Differences in viral entry did not necessarily correlate with CCR5 expression, and only very low levels of CCR5 expression restricted HIV entry and production. In summary, the host cell genetic effect on HIV replication in macrophages appears to be exerted predominantly pre-reverse transcription. Although CCR5 was necessary for infection, other unidentified host genes are likely to limit productive infection.

HIV-1 envelope determinants for cell tropism and chemokine receptor use

Isolates of human immunodeficiency virus type-1 (HIV-1) display marked differences in their ability to replicate in macrophages and transformed T-cell lines in vitro, a property that has important implications for disease pathogenesis. The restriction in replication between these two CD4-positive cell types is largely at the level of viral entry and is regulated by the viral envelope (env) gene. The envelope protein (Env) is responsible for fusion of the viral and host membranes, and a particular region of Env called the V3-loop has been implicated in regulating viral tropism. However, other regions of Env, such as the V1- and V2-loops, have been shown to modulate the effects of the V3-loop. The discovery that Env initially binds the CD4 molecule on the target cell surface and then makes subsequent interactions with one of several members of the chemokine receptor family has greatly enhanced the molecular understanding of HIV-1 entry. The differential use of chemokine receptors by different viral isolates and their expression in different cell types largely explains viral tropism. The same regions in Env responsible for virus tropism have also been shown to play an important role in mediating chemokine receptor use. The recent crystallization of HIV-1 Env in complex with CD4 illuminates the architecture of the components involved in mediating fusion between the viral and host membranes. The spatial relationship between variable structures of Env previously implicated in tropism and chemokine receptor use and conserved Env structures potentially involved in chemokine receptor binding are discussed.

Exposure to bacterial products renders macrophages highly susceptible to T-tropic HIV-1

Microbial coinfections variably influence HIV-1 infection through immune activation or direct interaction of microorganisms with HIV-1 or its target cells. In this study, we investigated whether exposure of macrophages to bacterial products impacts the susceptibility of these cells to HIV-1 of different cellular tropisms. We demonstrate that () macrophages exposed to bacterial cell wall components such as lipopolysaccharide (LPS) (Gram-negative rods), lipoteichoic acid (Gram-positive cocci), and lipoarabinomannan (Mycobacteria) become highly susceptible to T cell (T)-tropic HIV-1 (which otherwise poorly replicate in macrophages) and variably susceptible to macrophage (M)-tropic HIV-1; () LPS-stimulated macrophages secrete a number of soluble factors (i.e., chemokines, interferon, and proinflammatory cytokines) that variably affect HIV infection of macrophages, depending on the virus phenotype in question; and () LPS-stimulated macrophages express CCR5 (a major coreceptor for M-tropic HIV-1) at lower levels and CXCR4 (a major coreceptor for T-tropic HIV-1) at higher levels compared with unstimulated macrophages. We hypothesize that a more favorable environment for T-tropic HIV-1 and a less favorable or even unfavorable environment for M-tropic HIV-1 secondary to exposure of macrophages to those bacterial products may accerelate a transition from M- to T-tropic viral phenotype, which is indicative of disease progression.

Viral interference in HIV-1 infected cells

The study of viral interference in HIV-1 infected cells has revealed several different means whereby infected cells resist superinfection. The most familiar of these, down-modulation of cellular receptors for virus, can be accomplished through the independent action of at least three HIV-1 proteins. Both the principal viral receptor CD4 and the chemokine receptors which serve as co-receptors are subject to down-modulation as a consequence of infection. Elucidation of the specificity of co-receptor utilisation by HIV-1 strains is an exciting, ongoing task which has opened new avenues to the understanding of viral replication and pathogenesis. Novel routes to resistance to superinfection have been discovered during HIV-1 infection and their investigation may reveal new pathways to control HIV-1 and the loss of immunological function with AIDS. Copyright 1998 John Wiley & Sons, Ltd.

Cutting Edge: CXCR4 Is a Functional Coreceptor for Infection of Human Macrophages by CXCR4-Dependent Primary HIV-1 Isolates

The identification of HIV-1 coreceptors has provided a molecular basis for the tropism of different HIV-1 strains. CXC chemokine receptor-4 (CXCR4) mediates the entry of both primary and T cell line-adapted (TCLA) syncytia-inducing strains. Although macrophages (Mφ) express CXCR4, this coreceptor is assumed to be nonfunctional for HIV-1 infection. We addressed this apparent paradox by infecting human monocyte-derived Mφ with primary and TCLA isolates that were rigorously characterized for coreceptor usage and by adding the natural CXCR4 ligand, stem cell differentiation factor-1, to specifically block CXCR4-mediated entry. Our results show that primary HIV-1 isolates that selectively use CXCR4 productively infected both normal and C-C chemokine receptor-5-null Mφ. By contrast, Mφ supported the entry of CXCR4-dependent TCLA strains with variable efficiency but were not productively infected. Thus, the tropism of HIV isolates results from complex virus/host cell interactions both at the entry and postentry levels.

Human immunodeficiency virus type 1 T-lymphotropic strains enter macrophages via a CD4- and CXCR4-mediated pathway: replication is restricted at a postentry level

The human immunodeficiency virus type 1 (HIV-1) laboratory strains adapted to T-cell lines, as well as most syncytium-inducing primary isolates, replicate poorly in macrophages, which, beside CD4(+) T lymphocytes, are major targets of HIV-1. In the present work, we used a semiquantitative PCR-based technique to study viral entry into cells, kinetics of reverse transcription, and translocation of the viral DNA into the nucleus of macrophages infected with different HIV-1 strains. Our results demonstrate that T-lymphotropic strains efficiently enter macrophages. Entry was inhibited by a monoclonal antibody against CD4 and by stromal cell-derived factor 1alpha, a natural ligand of CXCR4, suggesting that both CD4 and CXCR4 act as receptors on macrophages for HIV-1 T-lymphotropic strains. Analysis of the kinetics of reverse transcription and nuclear import revealed that the most pronounced differences between T-lymphotropic and macrophagetropic strains occurred at the level of nuclear translocation of viral DNA, although a delay in reverse transcription was also observed. These results suggest that postentry steps are critical for restricted replication of T-lymphotropic HIV-1 strains in macrophages.

Analysis of the phenotype and phagocytic activity of monocytes/macrophages from cattle infected with the bovine leukaemia virus

The bovine leukaemia virus (BLV) is a retrovirus that infects mainly B lymphocytes of cattle, but proviral DNA can also be isolated from monocytes/macrophages. This study investigated the effect of BLV infection on surface antigens on freshly isolated peripheral blood monocytes and cultured monocyte-derived macrophages, with and without lipopolysaccharide (LPS) stimulation. The effect of BLV infection on phagocytic activity of CD14+ monocytes was also assessed. The percentage of monocytes expressing the surface antigens CD11b, CD32 (FcgammaRII), MHC class II and the surface antigen recognised by mAb DH59B were increased in BLV-positive cattle. In contrast, expression intensity of all markers was low in samples from BLV-positive cattle. CD14+ monocytes from BLV-positive cattle showed less Fcgamma-receptor-mediated phagocytosis compared to monocytes from BLV-negative cattle. After 7 days in culture, there was evidence for shedding/downregulation of surface antigens on monocyte-derived macrophages, in particular on cells from BLV-positive cattle. LPS stimulation decreased the percentage of cells expressing the measured markers in monocyte-derived macrophages taken from BLV-negative cattle, but not in cultures derived from BLV-positive cattle. The results provide further evidence for an altered function of monocytes and macrophages in BLV-infected cattle.

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

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

Abortive infection in HeLaCD4 cells by a primary HIV type 1 isolate: implications for differential host cell tropism

The emergence of T cell-tropic, syncytium-inducing (T-tropic/SI) HIV-1 variants from the background of macrophage-tropic, non-syncytium-inducing (M-tropic/NSI) strains is associated with disease progression in infected individuals. HIV89.6 is a primary isolate with a transitional phenotype: like M-tropic strains it replicates in primary macrophages and lymphocytes but not in most transformed cells, yet it is also syncytium inducing. We have shown that HIV89.6 can utilize both the M-tropic and T-tropic cofactors CCR-5 and CXCR-4, respectively, in conjunction with CD4 for fusion and entry into otherwise nonpermissive nonhuman cells. To better understand the nature of restricted HIV89.6 infection of transformed cells, we analyzed its interaction with CD4-expressing transformed human HeLaCD4-LTR/beta-Gal cells, which contain the beta-galactosidase gene linked to the HIV-1 LTR. Here we show that HIV89.6 enters these cells and undergoes reverse transcription and integration. Furthermore, HIV89.6 induces LTR-driven beta-galactosidase expression, indicating Tat-dependent trans-activation, in a similar number of cells as the permissive T-tropic/SI isolate HIV(HXB). Acute infection with HIV89.6, however, produces markedly lower levels of p24 antigen and infectious virus per trans-activation-positive cell than HIV(HXB). In contrast, transfection results in high levels of expression for both viruses but HIV89.6 still fails to establish spreading infection. HIV89.6 is also blocked after entry in two other nonpermissive cell lines, SUP-T1 and U937. HIV89.6 arrest in HeLaCD4-LTR/beta-Gal cells at a stage subsequent to entry, reverse transcription, integration, and Tat expression is a novel level at which HIV-1 strain- and cell-specific restrictions define host cell tropism. These studies emphasize that complex patterns of tropism are determined by the interplay of permissive or restricted virus-cell interactions at multiple steps in the replication cycle.

Human immunodeficiency virus type 1 coreceptors participate in postentry stages in the virus replication cycle and function in simian immunodeficiency virus infection

Primate lentiviruses use chemokine coreceptors in addition to the CD4 receptor to initiate virus infection. Simian immunodeficiency virus (SIV) productively infects human cells expressing CD4 and the human allele of the chemokine coreceptor CCR-5 as efficiently as it infects macaque cells expressing human CD4, suggesting that SIV can function with either a simian or a human coreceptor in conjunction with human CD4. In the same macaque cells expressing human CD4, the replication of human immunodeficiency virus type 1 (HIV-1) is blocked at several stages of infection; some isolates are restricted prior to reverse transcription, while others, including some macrophage-tropic and primary isolates, are restricted at a step after reverse transcription but prior to migration of the preintegration complex to the nucleus. Both blocks in HIV-1 replication can be relieved by either expression of the appropriate human coreceptor (CCR-5 or CXCR-4) or expression of SIV gene products in cis with the HIV-1 envelope as a chimera between SIV and HIV-1 (SHIV). Thus, a virus with a SIV core and HIV-1 envelope can efficiently infect macaque cells expressing human CD4, presumably by interacting with the simian coreceptor, whereas a virus with an HIV-1 core and an HIV-1 envelope requires expression of the human allele of the coreceptor for productive infection of these cells. These studies suggest that there are interactions among the coreceptor, the viral envelope, and another viral gene product that govern postentry steps of virus replication. These data are consistent with the hypothesis that such interactions may be required for translocation of the virus core to the nucleus. Moreover, the differential abilities of SIV and HIV-1 to function in these processes with heterologous primate coreceptors may have implications for cross-species transmission.

The presence of host-derived HLA-DR1 on human immunodeficiency virus type 1 increases viral infectivity

Human immunodeficiency virus type 1 (HIV-1) incorporates several host cell components when budding out of the infected cell. One of the most abundant host-derived molecules acquired by HIV-1 is the HLA-DR determinant of the major histocompatibility complex class II (MHC-II) molecules. The fact that CD4 is the natural ligand of MHC-II prompted us to determine if such virally embedded cellular components can affect the biology of the virus. Herein, we report for the first time that the incorporation of cellular HLA-DR1 within HIV-1 enhances its infectivity. This observation was made possible with virions bearing or not bearing on their surfaces host-derived HLA-DR1 glycoproteins. Such virus stocks were prepared by a transient-expression system based on transfection of 293T cells with a recombinant luciferase-encoding HIV-1 molecular clone along with plasmids encoding the alpha and beta chains of HLA-DR1. Cell-free virions recovered from transfected cells were shown to have efficiently incorporated host-derived HLA-DR1 glycoproteins. Infectivity was increased by a factor of 1.6 to 2.3 for virions bearing on their surfaces host-derived HLA-DR1. The observed enhancement of HIV-1 infectivity was independent of the virus stocks used and was seen in several T-lymphoid cell lines, in a premonocytoid cell line, and in primary peripheral blood mononuclear cells. Finally, we determined that the presence of virion-bound cellular HLA-DR1 is associated with faster kinetics of virus infection. Taken together, these results suggest that HLA-DR-1-bearing HIV-1 particles had a greater infectivity per picogram of viral p24 protein than HLA-DR1-free virions.

Cell type-specific fusion cofactors determine human immunodeficiency virus type 1 tropism for T-cell lines versus primary macrophages

Work in this laboratory previously demonstrated that the tropism of different human immunodeficiency type 1 isolates for infection of human CD4+ continuous cell lines (e.g., T-cell lines and HeLa-CD4 transformants) versus primary macrophages is associated with parallel intrinsic fusogenic specificities of the corresponding envelope glycoproteins (Envs). For T-cell line-tropic isolates, it is well established that the target cell must also contain a human-specific fusion cofactor(s) whose identity is unknown. In this study, we tested the hypothesis that the Env fusion specificities underlying T-cell line versus macrophage tropism are determined by distinct cell type-specific fusion cofactors. We applied a recombinant vaccinia virus-based reporter gene assay for Env-CD4-mediated cell fusion; the LAV and Ba-L Envs served as prototypes for T-cell line-tropic and macrophage-tropic isolates, respectively. We examined CD4+ promyeloctic and monocytic cell lines that are infectible by T-cell line-tropic isolates and become susceptible to macrophage-tropic strains only after treatment with differentiating agents. We observed parallel changes in fusion specificity: untreated cells supported fusion by the LAV but not the Ba-L Env, whereas cells treated with differentiating agents acquired fusion competence for Ba-L. These results suggest that in untreated cells, the block to infection by macrophage-tropic isolates is at the level of membrane fusion; furthermore, the differential regulation of fusion permissiveness for the two classes of Envs is consistent with the existence of distinct fusion cofactors. To test this notion directly, we conducted experiments with transient cell hybrids formed between CD4-expressing nonhuman cells (murine NIH 3T3) and different human cell types. Hybrids formed with HeLa cells supported fusion by the LAV Env but not by the Ba-L Env, whereas hybrids formed with primary macrophages showed the opposite specificity; hybrids formed between HeLa cells and macrophages supported fusion by both Envs. These results suggest the existence of cell type-specific fusion cofactors selective for each type of Env, rather than fusion inhibitors for discordant Env-cell combinations. Finally, analyses based on recombinant protein expression and antibody blocking did not support the proposals by others that the CD44 or CD26 antigens are involved directly in the entry of macrophage-tropic isolates.

Interference to human immunodeficiency virus type 1 infection in the absence of downmodulation of the principal virus receptor, CD4

It is thought that interference during human immunodeficiency virus type 1 (HIV-1) infection is established by downmodulation of the principal virus receptor, CD4. Here we present evidence to the contrary. At various times after primary infection, we superinfected T cells in vitro by exposure to a genetically distinct viral clone or to a virus carrying the chloramphenicol acetyltransferase gene. Replication of each virus strain was determined by restriction enzyme analysis of total cellular DNA, by PCR amplification of viral DNA, or by assay of cell extracts for chloramphenicol acetyltransferase activity. We found that efficient viral interference is established within 24 h of infection at a multiplicity of infection of 1. At that time, expression of viral structural proteins was low and infected cells displayed undiminished levels of surface CD4 and were fully susceptible to virus binding and fusion. Superinfection by either cell-free HIV-1 or cocultivation was blocked. Cells resistant to superinfection by HIV-1 remained susceptible to Moloney murine leukemia and vaccinia viruses. No interference was observed 4 h after primary infection or in cells infected with either UV-inactivated HIV-1 or a mutant virus defective in virus-cell fusion activity, indicating that binding of primary virus to CD4 is insufficient to prevent superinfection. The minimum viral requirements for this interference are that HIV-1 must be able to enter cells and synthesize viral DNA; Tat-mediated transcription is dispensable. Our results support the existence of a novel pathway to interference to HIV-1 infection, which we term postentry interference, which blocks superinfection during intracellular phases of the virus life cycle.

HIV-1 protein expression from synthetic circles of DNA mimicking the extrachromosomal forms of viral DNA

We have constructed circular forms of human immunodeficiency virus type 1 viral DNA in vitro that closely resemble the single and double long terminal repeat circular forms of unintegrated viral DNA formed in the nuclei of infected cells. We have analyzed viral protein expression after transient transfection of these circular DNAs into HeLa cells and compared it with expression from a transfected linearized plasmid containing an integrated provirus. Both circular forms are expressed, as judged by the appearance of extracellular p24, and expression is trans-activated by human immunodeficiency virus type 1 Tat. Viral p24 production, however, is approximately an order of magnitude lower than that obtained with transfected integrated viral DNA. Similar data were obtained when a luciferase reporter gene was substituted for the coding regions of the viral DNA. Positional effects of the transcriptional initiation and termination signals in the long terminal repeat appear to account for some of the low expression levels. These data suggest that unintegrated circular viral DNAs are transcriptionally active, although at low levels, and may contribute to overall viral replication in infected people under some conditions.

Fusogenic selectivity of the envelope glycoprotein is a major determinant of human immunodeficiency virus type 1 tropism for CD4+ T-cell lines vs. primary macrophages

We investigated the relationship between the fusion selectivity of the envelope glycoprotein (env) and the tropism of different human immunodeficiency virus type 1 (HIV-1) isolates for CD4+ human T-cell lines vs. primary macrophages. Recombinant vaccinia viruses were prepared encoding the envs from several well-characterized HIV-1 isolates with distinct cytotropisms. Cells expressing the recombinant envs were mixed with various CD4+ partner cell types; cell fusion was monitored by a quantitative reporter gene assay and by syncytia formation. With CD4+ continuous cell lines as partners (T-cell lines, HeLa cells expressing recombinant CD4), efficient fusion occurred with the envs from T-cell line-tropic isolates (IIIB, LAV, SF2, and RF) but not with the envs from macrophage-tropic isolates (JR-FL, SF162, ADA, and Ba-L). The opposite selectivity pattern was observed with primary macrophages as cell partners; stronger fusion occurred with the envs from the macrophage-tropic than from the T-cell line-tropic isolates. All the envs showed fusion activity with peripheral blood mononuclear cells as partners, consistent with the ability of this cell population to support replication of all the corresponding HIV-1 isolates. These fusion selectivities were maintained irrespective of the cell type used to express env, thereby excluding a role for differential host cell modification. We conclude that the intrinsic fusion selectivity of env plays a major role in the tropism of a HIV-1 isolate for infection of CD4+ T-cell lines vs. primary macrophages, presumably by determining the selectivity of virus entry and cell fusion.

Viral activation from latency during retrodifferentiation of U937 cells exposed to phorbol ester followed by infection with human immunodeficiency virus type 1

To determine the mechanism underlying the human immunodeficiency virus type 1 (HIV-1) latency and its activation in monocyte/macrophage lineage, the human promonocytic cell line U937 was infected with HIV-1 after differentiation with varied doses of phorbol 12-myristate 13-acetate (PMA). Variously differentiated intermediate stages were generated in U937 cells in a dose-dependent manner. When these cells were infected with lymphotropic HIV-1, the kinetics of the production of HIV-1 DNA, the appearance of HIV-1 antigen-positive cells, and viral production in the conditioned media were slower at higher doses of PMA. This different susceptibility to the infection was not due to the rate of HIV-1 adsorption. Viral replication from latency in the differentiated cells was activated in proportion with the retrodifferentiation observed in long-term cultures of the host cells. Thus, our data demonstrate the close correlation between the regulation of HIV-1 replication and the differentiation stage of monocyte/macrophage lineage cells at the time of HIV-1 infection. The retrodifferentiation phenomenon in infected cells seems to be particularly important for understanding the mechanisms for HIV-1 activation from latency.

Replication of macrophage-tropic and T-cell-tropic strains of human immunodeficiency virus type 1 is augmented by macrophage-endothelial cell contact

Macrophages perform a central role in the pathogenesis of human immunodeficiency virus type 1 (HIV-1) infection and have been implicated as the cell type most prominent in the development of central nervous system impairment. In this study, we evaluated the effect of interaction between macrophages and endothelial cells on HIV-1 replication. Upregulation of HIV-1 replication was consistently observed in monocyte-derived macrophages (hereafter called macrophages) cocultured with either umbilical vein endothelial cells or brain microvascular endothelial cells. HIV-1 p24 antigen production of laboratory-adapted strains and patient-derived isolates was increased 2- to 1,000-fold in macrophage-endothelial cocultures, with little or no detectable replication in cultures containing endothelial cells only. The upregulation of HIV-1 in macrophage-endothelial cocultures was observed not only for viruses with the non-syncytium-inducing, macrophage-tropic phenotype but also for viruses previously characterized as syncytium inducing and T-cell tropic. In contrast, cocultures of macrophages with glioblastoma, astrocytoma, cortical neuronal, fibroblast, and placental cells failed to increase HIV-1 replication. Enhancement of HIV-1 replication in macrophage-endothelial cocultures required cell-to-cell contact; conditioned media from endothelial cells or macrophage-endothelial cocultures failed to augment HIV-1 replication in macrophages. Additionally, antibody to leukocyte function-associated antigen (LFA-1), a macrophage-endothelial cell adhesion molecule, inhibited the enhanced HIV-1 replication in macrophage-endothelial cell cocultures. Thus, these data indicate that macrophage-endothelial cell contact enhances HIV-1 replication in macrophages for both macrophage-tropic and previously characterized T-cell-tropic strains and that antibody against LFA-1 can block the necessary cell-to-cell interaction required for the observed upregulation. These findings may have important implications for understanding the ability of HIV-1 to replicate efficiently in tissue macrophages, including those in the brain and at the blood-brain barrier.

V3-independent determinants of macrophage tropism in a primary human immunodeficiency virus type 1 isolate

Human immunodeficiency virus type 1 isolates differ in their ability to productively infect macrophages, and several groups have mapped the genetic basis for macrophage tropism to regions of env that include the third hypervariable region (V3 loop). We recently described a primary isolate (89.6) which is highly macrophage tropic and yet differs from other macrophage-tropic strains studied in that it is cytopathic in T cells. Genetic mapping of macrophage tropism determinants in this virus was done by using chimeras generated with the prototypic non-macrophage-tropic strain HXB2. Replacement of a 2.7-kb env-containing region of HXB with corresponding sequences from 89.6 conferred the macrophage-tropic phenotype, but insertion of the 89.6 V3 loop along with V4/V5 sequences did not. Conversely, placement of HXB sequences that included V3 into 89.6 did not impair this strain's ability to replicate in macrophages. Sequence analysis of V3 shows that 89.6 differs markedly from previously described macrophage-tropic consensus sequences and that it is more similar to highly charged non-macrophage-tropic strains. This suggests either that macrophage tropism is defined by structural determinants resulting from complex interactions among multiple env regions rather than V3 sequence-specific requirements or that there are multiple mechanisms by which different strains may establish productive macrophage infection. In addition, because the HXB V3 loop supports productive macrophage infection in the background of 89.6, phenotypic characterization of V3 sequences should be considered specific to the viral context in which they are placed.

The proteins of lymphocyte- and macrophage-tropic strains of simian immunodeficiency virus are processed differently in macrophages

Since the pathogenesis of SIVmac disease complex is thought to be explained by the tropism of the infecting virus for either CD4+ T-lymphocytes or macrophages or both types of cells, we compared the infection in primary macaque macrophages with molecularly cloned, lymphocyte-tropic SIVmac239 and a cloned, macrophage-tropic chimeric virus (SIVmac239/17E) whose env gene was derived from brain of a macaque (17E) dying from SIV-induced encephalopathy. SIVmac239/17E caused a productive, syncytial cytopathic infection accompanied by accumulation of virus particles within cytoplasmic vesicles of the macrophages. Pulse-chase and immune precipitation studies showed that both the viral glycoprotein precursor (gp160) and the gag precursor (p57) were cleaved into gp120 and p27, respectively, and both were released into the culture medium of infected cells, although most of the p27 remained cell associated. SIVmac239 also infected macrophages, but in comparison to SIVmac239/17E, minimal virus replication occurred. Immunocytostaining revealed that while occasional syncytia were observed in cultures, the majority of the infected cells were not associated with syncytium formation. Ultrastructural studies did not reveal the accumulation of virions within infected macrophages. Pulse-chase studies showed that both gp160 and p57 were produced but were cleaved inefficiently and only minimal amounts of gp120 and p27 were released into the culture medium, even after prolonged incubation times. The processing of proteins of the two viruses was indistinguishable in lymphocytes. Since these two viruses are identical except for changes within the env gene, these results indicate that efficient assembly and release of SIV from blood-derived macrophages is mediated by changes in the envelope glycoprotein.

Efficient synthesis of viral nucleic acids following monocyte infection by HIV-1

The replication of human immunodeficiency virus type 1 in mononuclear phagocytes (blood monocytes, tissue macrophages, and dendritic cells) is an important feature of HIV-1 pathogenesis. Although most primary HIV-1 isolates are able to productively infect monocytes, some reports suggest that rates of viral DNA synthesis and virus replication are reduced in HIV-1-infected monocytes as compared to infected T cells. In this study we compare kinetics of viral DNA synthesis in CD4+ T cells and monocytes following HIV-1 infection. Our results indicate that reverse transcription of viral nucleic acids following infection of monocytes occurs at rates equal to or greater than that observed following infection of T cells. These studies reveal no postentry restrictions to HIV-1 replication following infection in monocytes. Moreover, the results support the notion that both monocytes and CD4+ T cells are equally permissive for virus replication in infected individuals.

Alpha interferon-induced antiretroviral activities: restriction of viral nucleic acid synthesis and progeny virion production in human immunodeficiency virus type 1-infected monocytes

Alpha interferon (IFN-alpha) restricts multiple steps of the human immunodeficiency virus type 1 (HIV-1) life cycle. A well-described effect of IFN-alpha is in the modulation of viral nucleic acid synthesis. We demonstrate that IFN-alpha influences HIV-1 DNA synthesis principally by reducing the production of late products of reverse transcription. The magnitude of IFN-alpha-induced downregulation of HIV-1 DNA and/or progeny virion production was dependent on the IFN-alpha concentration, the duration of cytokine administration, the multiplicity of infection, the viral strain, and the cycles of viral infection. Interestingly, reductions in viral DNAs could not fully account for the observed IFN-alpha-induced abrogation of progeny virion production. These data, by our investigation of both single-cycle and spreading viral infections, support a predominant but not exclusive effect of IFN-alpha on viral DNA synthesis.

Human immunodeficiency virus type 1 variants with increased replicative capacity develop during the asymptomatic stage before disease progression

We examined the replicative properties of a series of sequential isolates and biological clones of human immunodeficiency virus type 1 (HIV-1) obtained from an individual who progressed from seroconversion to AIDS in approximately 5 years. HIV-1 isolated soon after seroconversion replicated slowly and to low levels in cultures of peripheral blood mononuclear cells; however, subsequent isolates obtained during asymptomatic infection showed a marked increase in replication kinetics. This was examined in more detail by using a panel of 35 biological clones of HIV-1 generated from sequential patient peripheral blood mononuclear cell samples. Each clone was evaluated for replication in primary macrophages and CD4+ T lymphocytes and for the ability to induce syncytium formation in MT-2 cell cultures. Consistent with earlier observations, we found that all of the clones isolated just after seroconversion were slowly replicating and non-syncytium inducing (NSI). However, NSI variants with increased replication kinetics in macrophages were identified soon thereafter. These variants preceded the appearance of NSI and syncytium-inducing variants, with rapid replication in both macrophages and CD4+ T lymphocytes. To determine whether changes in the rate of replication could be traced to the early stages of the virus life cycle, PCR assays were used to evaluate entry and reverse transcription of selected biological clones in macrophages and CD4+ T lymphocytes. We found there was no inherent block to entry or reverse transcription for the slowly replicating variants; however, this does not preclude the possibility that small differences in the rate of entry may account for larger differences in the replication kinetics over many cycles. Overall, our results demonstrate that rapidly replicating variants of HIV-1 emerge during the asymptomatic period in a patient who subsequently progressed clinically, suggesting that these variants may play an important role in HIV-1 pathogenesis.