HIV-1 macrophage tropism is determined at multiple levels of the viral replication cycle

Detailed Characterization of Early HIV-1 Replication Dynamics in Primary Human Macrophages

Macrophages are natural target cells of human immunodeficiency virus type 1 (HIV-1). Viral replication appears to be delayed in these cells compared to lymphocytes; however, little is known about the kinetics of early post-entry events. Time-of-addition experiments using several HIV-1 inhibitors and the detection of reverse transcriptase (RT) products with droplet digital PCR (ddPCR) revealed that early replication was delayed in primary human monocyte-derived macrophages of several donors and peaked late after infection. Direct imaging of reverse-transcription and pre-integration complexes (RTC/PIC) by click-labeling of newly synthesized DNA further confirmed our findings and showed a concomitant shift to the nuclear stage over time. Altering the entry pathway enhanced infectivity but did not affect kinetics of viral replication. The addition of viral protein X (Vpx) enhanced productive infection and accelerated completion of reverse transcription and nuclear entry. We propose that sterile alpha motif (SAM) and histidine/aspartate (HD) domain-containing protein 1 (SAMHD1) activity lowering deoxyribonucleotide triphosphate (dNTP) pools is the principal factor delaying early HIV-1 replication in macrophages.

Guanylate Binding Protein (GBP) 5 Is an Interferon-Inducible Inhibitor of HIV-1 Infectivity

Guanylate binding proteins (GBPs) are an interferon (IFN)-inducible subfamily of guanosine triphosphatases (GTPases) with well-established activity against intracellular bacteria and parasites. Here we show that GBP5 potently restricts HIV-1 and other retroviruses. GBP5 is expressed in the primary target cells of HIV-1, where it impairs viral infectivity by interfering with the processing and virion incorporation of the viral envelope glycoprotein (Env). GBP5 levels in macrophages determine and inversely correlate with infectious HIV-1 yield over several orders of magnitude, which may explain the high donor variability in macrophage susceptibility to HIV. Antiviral activity requires Golgi localization of GBP5, but not its GTPase activity. Start codon mutations in the accessory vpu gene from macrophage-tropic HIV-1 strains conferred partial resistance to GBP5 inhibition by increasing Env expression. Our results identify GBP5 as an antiviral effector of the IFN response and may explain the increased frequency of defective vpu genes in primary HIV-1 strains.

Generation of transmitted/founder HIV-1 infectious molecular clones and characterization of their replication capacity in CD4 T lymphocytes and monocyte-derived macrophages

Genome sequences of transmitted/founder (T/F) HIV-1 have been inferred by analyzing single genome amplicons of acute infection plasma viral RNA in the context of a mathematical model of random virus evolution; however, few of these T/F sequences have been molecularly cloned and biologically characterized. Here, we describe the derivation and biological analysis of ten infectious molecular clones, each representing a T/F genome responsible for productive HIV-1 clade B clinical infection. Each of the T/F viruses primarily utilized the CCR5 coreceptor for entry and replicated efficiently in primary human CD4(+) T lymphocytes. This result supports the conclusion that single genome amplification-derived sequences from acute infection allow for the inference of T/F viral genomes that are consistently replication competent. Studies with monocyte-derived macrophages (MDM) demonstrated various levels of replication among the T/F viruses. Although all T/F viruses replicated in MDM, the overall replication efficiency was significantly lower compared to prototypic "highly macrophage-tropic" virus strains. This phenotype was transferable by expressing the env genes in an isogenic proviral DNA backbone, indicating that T/F virus macrophage tropism mapped to Env. Furthermore, significantly higher concentrations of soluble CD4 were required to inhibit T/F virus infection compared to prototypic macrophage-tropic virus strains. Our findings suggest that the acquisition of clinical HIV-1 subtype B infection occurs by mucosal exposure to virus that is not highly macrophage tropic and that the generation and initial biological characterization of 10 clade B T/F infectious molecular clones provides new opportunities to probe virus-host interactions involved in HIV-1 transmission.

Polymorphism in HIV-1 dependency factor PDE8A affects mRNA level and HIV-1 replication in primary macrophages

Four genome-wide RNAi screens have recently identified hundreds of HIV-1 dependency factors (HDFs). Previously, we reported a large variation in the ability of HIV-1 to replicate in monocyte-derived macrophages (MDM) derived from >400 healthy seronegative blood donors. Here we determined whether SNPs in genes encoding newly identified HDFs were associated with this variation in HIV-1 replication. We found a significant association between the minor allele of SNP rs2304418 in phosphodiesterase 8A (PDE8A) and lower HIV-1 replication (p=2.4×10(-6)). The minor allele of SNP rs2304418 was also significantly associated with lower PDE8A mRNA levels in MDM (p=8.3×10(-5)). In accordance with this, overexpression of PDE8A in HEK293T cells resulted in increased HIV-1 replication, while subsequent knock-down of PDE8A decreased replication. This study links host genetic variation in a newly identified HDF to variation in HIV-1 replication in a relevant primary target cell for HIV-1 and may provide new leads for treatment of this infection.

Genome-wide association study identifies single nucleotide polymorphism in DYRK1A associated with replication of HIV-1 in monocyte-derived macrophages

BACKGROUND

HIV-1 infected macrophages play an important role in rendering resting T cells permissive for infection, in spreading HIV-1 to T cells, and in the pathogenesis of AIDS dementia. During highly active anti-retroviral treatment (HAART), macrophages keep producing virus because tissue penetration of antiretrovirals is suboptimal and the efficacy of some is reduced. Thus, to cure HIV-1 infection with antiretrovirals we will also need to efficiently inhibit viral replication in macrophages. The majority of the current drugs block the action of viral enzymes, whereas there is an abundance of yet unidentified host factors that could be targeted. We here present results from a genome-wide association study identifying novel genetic polymorphisms that affect in vitro HIV-1 replication in macrophages.

METHODOLOGY/PRINCIPAL FINDINGS

Monocyte-derived macrophages from 393 blood donors were infected with HIV-1 and viral replication was determined using Gag p24 antigen levels. Genomic DNA from individuals with macrophages that had relatively low (n = 96) or high (n = 96) p24 production was used for SNP genotyping with the Illumina 610 Quad beadchip. A total of 494,656 SNPs that passed quality control were tested for association with HIV-1 replication in macrophages, using linear regression. We found a strong association between in vitro HIV-1 replication in monocyte-derived macrophages and SNP rs12483205 in DYRK1A (p = 2.16 × 10(-5)). While the association was not genome-wide significant (p<1 × 10(-7)), we could replicate this association using monocyte-derived macrophages from an independent group of 31 individuals (p = 0.0034). Combined analysis of the initial and replication cohort increased the strength of the association (p = 4.84 × 10(-6)). In addition, we found this SNP to be associated with HIV-1 disease progression in vivo in two independent cohort studies (p = 0.035 and p = 0.0048).

CONCLUSIONS/SIGNIFICANCE

These findings suggest that the kinase DYRK1A is involved in the replication of HIV-1, in vitro in macrophages as well as in vivo.

HIV-1 and the macrophage

Macrophages and CD4+ T cells are natural target cells for HIV-1, and both cell types contribute to the establishment of the viral reservoir that is responsible for continuous residual virus replication during antiretroviral therapy and viral load rebound upon treatment interruption. Scientific findings that support a critical role for the infected monocyte/macrophage in HIV-1-associated diseases, such as neurological disorders and cardiovascular disease, are accumulating. To prevent or treat these HIV-1-related diseases, we need to halt HIV-1 replication in the macrophage reservoir. This article describes our current knowledge of how monocytes and certain macrophage subsets are able to restrict HIV-1 infection, in addition to what makes macrophages respond less well to current antiretroviral drugs as compared with CD4+ T cells. These insights will help to find novel approaches that can be used to meet this challenge.

Donor variation in in vitro HIV-1 susceptibility of monocyte-derived macrophages

Primary human cells from different donors vary in their susceptibility to in vitro infection with HIV-1. In order to perform genetic analysis to identify host factors that affect HIV-1 susceptibility, it is important that a clear phenotype is defined. Here, we report a standardized method to study variation for in vitro HIV-1 infection in monocyte-derived macrophages (MDM) from large numbers of individuals. With this assay, HIV-1 susceptibility of MDM from 489 different donors shows more than 3 log variation and a good correlation with the 32 base pair deletion in the CCR5 co-receptor (ccr5 Delta 32 genotype) of the donors. However, in 7 of 12 donors completely resistant to infection with CCR5-using HIV-1, this was not explained by the ccr5 Delta 32 genotype, showing evidence that other host factors are likely to influence HIV-1 replication in MDM. Infections with VSV-G pseudotyped HIV-1 indeed confirmed the existence of post-entry level restrictions in MDM.

HIV-1 coreceptor preference is distinct from target cell tropism: a dual-parameter nomenclature to define viral phenotypes

HIV-1 infection of cells is mediated by engagement between viral envelope glycoproteins (Env) and a receptor complex comprising CD4 and one of two chemokine receptors, CCR5 and CXCR4, expressed on the surface of target cells. Most CD4+-transformed T cell lines express only CXCR4, but primary lymphocytes and macrophages, the main cellular targets for infection in vivo, express both coreceptors. Cell- and viral strain-specific utilization of these coreceptor pathways, rather than coreceptor expression per se, regulates lymphocyte and macrophage entry and tropism. Virus use of coreceptor[s] (R5, X4, or R5 and X4) and its target cell tropism (lymphocytes, macrophages, and/or transformed T cell lines) are related but distinct characteristics of Envs. A comprehensive classification schema of HIV-1 Env phenotypes that addresses both tropism and coreceptor use is proposed. Defining Env phenotype based on both parameters is important in the development of entry inhibitors and vaccines, for understanding changes in Env that evolve over time in vivo, and for discerning differences among viral species that underlie aspects of pathogenesis and transmission. Recognizing how tropism is related to, yet differs from, coreceptor selectivity is critical for understanding the mechanisms by which these viral characteristics impact pathogenesis.

HIV-1 fitness and macrophages

HIV-1 comprises a collection of closely related, but not identical, viruses or quasispecies. Fitness represents a selective advantage for propagation among populations of organisms competing in a particular environment and is an important characteristic of viruses because of a link between fitness and pathogenesis. Environmental differences based on the type of cell that is targeted for infection or the cell type that produces virus, impact fitness. CD4-expressing cells of lymphocyte or macrophage lineage are the principal host cells for HIV-1, although the milieu in lymphocytes is distinct from the macrophage environment from the perspective of cell half-life and activation, signal transduction and expression of coreceptors, and bioavailability of antiretroviral drugs. Multiple viral determinants, including entry via envelope glycoproteins, replication by reverse transcriptase, and virion maturation by protease activity, contribute to fitness in different cells and provide targets for current antiretroviral therapies. This review focuses on fitness of HIV-1 in macrophages and examines the impact of protease inhibitors on fitness of quasispecies and an unexplained discordance between fitness and pathogenesis.

Abrogation of postentry restriction of HIV-1-based lentiviral vector transduction in simian cells

HIV-1 replication in simian cells is restricted at an early postentry step because of the presence of an inhibitory cellular factor. This block reduces the usefulness of HIV-1-based lentiviral vectors in primate animal models. Here, we demonstrate that substitution of the cyclophilin A (CyPA) binding region in the capsid of an HIV-1-based lentiviral vector (LV) with that of the macrophage tropic HIV-1 Ba-L resulted in a vector that was resistant to the inhibitory effect and efficiently transduced simian cells. Notably, the chimeric gag LV efficiently transduced primary simian hematopoietic progenitor cells, a critical cellular target in gene therapy. The alterations in the CyPA binding region did not affect CyPA incorporation; however, transduction by the gag chimeric LV seemed to be relatively insensitive to cyclosporin A, indicating that it does not require CyPA for early postentry steps. In dual infection experiments, the gag chimeric LV failed to remove the block to transduction of the WT LV, suggesting that the gag chimeric LV did not saturate the inhibitory simian cellular factor. These data suggest that the CyPA binding region of capsid contains a viral determinant involved in the postentry restriction of HIV-1-based lentiviral vectors. Overall, the findings demonstrate that the host range of HIV-1-based LV can be altered by modifications in the packaging construct.

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.

Early pathogenesis of transmucosal feline immunodeficiency virus infection

To identify the early target cells and tissues in transmucosal feline immunodeficiency virus (FIV) infection, cats were exposed to a clade C FIV isolate via the oral-nasal or vaginal mucosa and multiple tissues were examined by virus isolation coculture (VI), DNA PCR, catalyzed tyramide signal-amplified in situ hybridization (TSA-ISH), and immunohistochemistry between days 1 and 12 postinoculation (p.i.). FIV RNA was detected in tonsil and oral or vaginal mucosa as early as 1 day p.i. by TSA-ISH and in retropharyngeal, tracheobronchial, or external iliac lymph nodes and sometimes in spleen or blood mononuclear cells by day 2, indicating that regional and distant spread of virus-infected cells occurred rapidly after mucosal exposure. By day 8, viral RNA, DNA, and culturable virus were uniformly detected in regional and distant tissues, connoting systemic infection. TSA-ISH proved more sensitive than DNA PCR in detecting early FIV-infected cells. In mucosal tissues, the earliest demonstrable FIV-bearing cells were either within or subjacent to the mucosal epithelium or were in germinal centers of regional lymph nodes. The FIV(+) cells were of either of two morphological types, large stellate or small round. Those FIV RNA(+) cells which could be colabeled for a phenotype marker, were labeled for either dendritic-cell-associated protein p55 or T-lymphocyte receptor antigen CD3. These studies indicate that FIV crosses mucous membranes within hours after exposure and rapidly traffics via dendritic and T cells to systemic lymphoid tissues, a pathway similar to that thought to occur in the initial phase of infection by the human and simian immunodeficiency viruses.

Identification of shared populations of human immunodeficiency virus type 1 infecting microglia and tissue macrophages outside the central nervous system

Infection of microglia and other cells of the macrophage/monocyte lineage in the central nervous system (CNS) by human immunodeficiency virus type I (HIV-1) underlies the development of giant cell encephalitis (GCE). It is currently unknown whether GCE depends on the emergence of virus populations specifically adapted to replicate in cells of the monocyte/macrophage lineage and whether this also leads to the specific targeting of macrophages in other nonlymphoid tissues. Autopsy samples from lymph node, brain (frontal region), lung, and full-thickness colon sections were obtained from nine study subjects with GCE and from nine without. The two groups showed no significant differences in CD4 counts, disease progression, or treatment history before death. Genetic relatedness between variants recovered from lymph node and nonlymphoid tissues was assessed by sequence comparison of V3 and p17(gag) regions using a newly developed method that scores the sample composition at successive nodes in a neighbor-joining tree. The association index enabled objective, numerical comparisons on the degree of tissue compartmentalization to be made. High proviral loads and p24 antigen expression in the brain were confined to the nine individuals with GCE. GCE was also associated with significantly higher proviral loads in colon samples (median of the GCE(+) group: 1,010 copies/10(6) cells; median of GCE(-) group, 10/10(6) cells; P = 0.006). In contrast, there were no significant differences in proviral load between the GCE(+) and GCE(-) groups in lymph node or lung samples, where HIV infection was manifested predominantly by infiltrates of lymphoid cells. V3 sequences from brain samples of individuals with GCE showed the greatest compartmentalization from those of lymph node, although samples from other tissues, particularly the colon, frequently contained variants phylogenetically related to those found in brain. The existence of shared, distinct populations of HIV specifically distributed in cells of the monocyte/macrophage lineage was further indicated by immunocytochemical detection of CD68(+), multinucleated giant cells expressing p24 antigen in samples of lung and colon in two individuals with GCE. This study provides the basis for future investigation of possible phenotypic similarities that underline the shared distributions of HIV variants infecting microglia and tissue macrophages outside the CNS.

Macrophage tropism of human immunodeficiency virus type 1 isolates from brain and lymphoid tissues predicts neurotropism independent of coreceptor specificity

The viral determinants that underlie human immunodeficiency virus type 1 (HIV-1) neurotropism are unknown, due in part to limited studies on viruses isolated from brain. Previous studies suggest that brain-derived viruses are macrophage tropic (M-tropic) and principally use CCR5 for virus entry. To better understand HIV-1 neurotropism, we isolated primary viruses from autopsy brain, cerebral spinal fluid, blood, spleen, and lymph node samples from AIDS patients with dementia and HIV-1 encephalitis. Isolates were characterized to determine coreceptor usage and replication capacity in peripheral blood mononuclear cells (PBMC), monocyte-derived macrophages (MDM), and microglia. Env V1/V2 and V3 heteroduplex tracking assay and sequence analyses were performed to characterize distinct variants in viral quasispecies. Viruses isolated from brain, which consisted of variants that were distinct from those in lymphoid tissues, used CCR5 (R5), CXCR4 (X4), or both coreceptors (R5X4). Minor usage of CCR2b, CCR3, CCR8, and Apj was also observed. Primary brain and lymphoid isolates that replicated to high levels in MDM showed a similar capacity to replicate in microglia. Six of 11 R5 isolates that replicated efficiently in PBMC could not replicate in MDM or microglia due to a block in virus entry. CD4 overexpression in microglia transduced with retroviral vectors had no effect on the restricted replication of these virus strains. Furthermore, infection of transfected cells expressing different amounts of CD4 or CCR5 with M-tropic and non-M-tropic R5 isolates revealed a similar dependence on CD4 and CCR5 levels for entry, suggesting that the entry block was not due to low levels of either receptor. Studies using TAK-779 and AMD3100 showed that two highly M-tropic isolates entered microglia primarily via CXCR4. These results suggest that HIV-1 tropism for macrophages and microglia is restricted at the entry level by a mechanism independent of coreceptor specificity. These findings provide evidence that M-tropism rather than CCR5 usage predicts HIV-1 neurotropism.

Evidence for a post-entry barrier to R5 HIV-1 infection of CD4 memory T cells

BACKGROUND

HIV-1 strains R5 and X4 can infect CD4 memory T cells in vivo. Anti-CD3/28 stimulation induces beta-chemokines and CCR5 down-regulation and renders these cells resistant to R5 HIV-1 infection. Here we describe an additional cellular mechanism that blocks productive R5 HIV-1 infection of CD4 memory T cells.

METHODS

Blood-derived CD4 memory T cells and CD4 T-cell clones were infected with primary R5 and X4 HIV-1 strains. Virus replication was correlated with CCR5 expression and beta-chemokine production. Virus entry and infectivity were measured by PCR for early and late products of HIV reverse transcription respectively.

RESULTS

R5 strains were up to 1000-fold less infectious than X4 viruses for CD4 memory T cells. This resistance was independent of CCR5 levels and of the Delta-32 mutation and the CCR2-V64I/CCR5-59653T linked mutations. Blocking endogenous beta-chemokines relieved minimally this restriction. At the single cell level, CD4 memory cells were either permissive or non-permissive for R5 HIV-1 infection. R5 HIV titre was up to 10-fold lower than X4 virus titre even in a permissive clone. However, R5 viruses replicated as efficiently as X4 viruses in the permissive clone when neutralizing anti-beta chemokine antibodies were added. Non-permissive cells blocked a post-entry step of the virus life-cycle and expressed early but not late HIV transcripts. Neutralizing anti-beta chemokine antibodies promoted R5 virus replication marginally in the non-permissive clone.

CONCLUSION

Some blood memory CD4 T cells retard R5 HIV-1 replication via endogenous beta-chemokines whereas others block productive R5 HIV-1 infection by an additional mechanism that interferes with a post-entry step of the virus life cycle. These natural barriers might contribute to lower pathogenicity of R5 HIV-1 strains for CD4 memory T cells than X4 viruses that emerge late in disease.

Diminished human immunodeficiency virus type 1 reverse transcription and nuclear transport in primary macrophages arrested in early G(1) phase of the cell cycle

Previously, we and others have demonstrated that the process of reverse transcription of human immunodeficiency virus type 1 (HIV-1) is disturbed in nondividing macrophages and quiescent T lymphocytes. Here we analyzed which phase of the cell cycle in macrophages is crucial for early steps in the HIV-1 replication cycle. HIV-1 Ba-L-inoculated macrophages arrested early in the G(1) phase by n-butyrate contained incomplete products of reverse transcription. In gamma-irradiated macrophages, reverse transcription was successfully completed but proviral integration could not be detected. In these cells, nuclear import was disturbed as reflected by the absence of two-long-terminal-repeat circles. In macrophages arrested late in G(1) phase by aphidicolin or 5, 6-dichloro-1-beta-D-ribofuranosyl-benzimidazole (DRB), reverse transcription was unaffected. Proviral integration occurred efficiently in DRB-treated macrophages, whereas integrated proviral DNA could not be detected after aphidicolin treatment. Arrest at G(2) phase of the cell cycle by nocodazole did not affect reverse transcription or proviral integration. Treatment of macrophages with hydroxyurea (HU), which reduces the intracellular deoxynucleoside triphosphate (dNTP) pool by blocking the de novo synthesis of dNTP, resulted in a dose-dependent inhibition of HIV-1 reverse transcription. This could partially be restored by the addition of nucleoside precursors. Addition of nucleoside precursors enhanced both reverse transcription and cell proliferation. However, the disturbed reverse transcription observed in the nonproliferating and n-butyrate-treated macrophages could not be restored by addition of nucleoside precursors. Similar to observations in quiescent T lymphocytes, incomplete proviral DNA species were arrested in the cytoplasm of the macrophages. Our results indicate that also in primary macrophages the intracellular nucleotide pools and other cellular factors that coincide with late G(1) phase of the cell cycle may contribute to efficient reverse transcription and nuclear localization.

AIDS dementia complex

Human immunodeficiency virus (HIV) infection is often complicated by the development of AIDS dementia complex (ADC). This article examines the typical and atypical presentations of ADC, along with aspects of the prevalence and natural history of the disorder. Salient aspects of the neuropathology, neurovirology, neuroimmunology, and pathogenesis are also considered. The intricacies of management of ADC, especially in the era of highly active antiretroviral therapy, are fully evaluated. Finally, this information is synthesized into an approach to the diagnosis of ADC in a particular patient.

Patterns of chemokine receptor fusion cofactor utilization by human immunodeficiency virus type 1 variants from the lungs and blood

Human immunodeficiency virus type 1 (HIV-1) infection is highly compartmentalized, with distinct viral genotypes being found in the lungs, brain, and other organs compared with blood. CCR5 and CXCR4 are the principal HIV-1 coreceptors, and a number of other molecules support entry in vitro but their roles in vivo are uncertain. To address the relationship between tissue compartmentalization and the selective use of entry coreceptors, we generated functional env clones from primary isolates derived from the lungs and blood of three infected individuals and analyzed their use of the principal, secondary, orphan, and virus-encoded coreceptors for fusion. All Env proteins from lung viruses used CCR5 but not CXCR4, while those from blood viruses used CCR5 or CXCR4 or both. The orphan receptor APJ was widely used for fusion by Env proteins from both blood and lung viruses, but none used the cytomegalovirus-encoded receptor US28. Fusion mediated by the secondary coreceptors CCR2b, CCR3, CCR8, and CX3CR1 and orphan receptors GPR1, GPR15, and STRL33 was variable and heterogeneous, with relatively broad utilization by env clones from isolates of one subject but limited use by env clones from the other two subjects. However, there was no clear distinction between blood and lung viruses in secondary or orphan coreceptor fusion patterns. In contrast to fusion, none of the secondary or orphan receptors enabled efficient productive infection. These results confirm, at the level of cofactor utilization, previous observations that HIV-1 populations in the lungs and blood are biologically distinct and demonstrate diversity within lung-derived as well as blood-derived quasispecies. However, the heterogeneity in coreceptor utilization among clones from each isolate and the lack of clear distinction between lung- and blood-derived Env proteins argue against selective coreceptor utilization as a major determinant of compartmentalization.

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.

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.

Continued utilization of CCR5 coreceptor by a newly derived T-cell line-adapted isolate of human immunodeficiency virus type 1

The differential use of CC chemokine receptor 5 (CCR5) and CXC chemokine receptor 4 (CXCR4) may be intimately involved in the transmission and progression of human immunodeficiency virus infection. Changes in coreceptor utilization have also been noted upon adaptation of primary isolates (PI) to growth in established T-cell lines. All of the T-cell line-adapted (TCLA) viruses studied to date utilize CXCR4 but not CCR5. This observation had been suggested as an explanation for the sensitivity of TCLA, but not PI, viruses to neutralization by recombinant gp120 antisera and V3-directed monoclonal antibodies, but recent studies have shown coreceptor utilization to be independent of neutralization sensitivity. Here we describe a newly isolated TCLA virus that is sensitive to neutralization but continues to utilize both CXCR4 and CCR5 for infection. This finding further divorces coreceptor specificity from neutralization sensitivity and from certain changes in cell tropism. That the TCLA virus can continue to utilize CCR5 despite the changes that occur upon adaptation and in the apparent absence of CCR5 expression in the FDA/H9 T-cell line suggests that the interaction between envelope protein and coreceptor may be mediated by multiple weak interactions along a diffuse surface.

Patterns of CCR5, CXCR4, and CCR3 usage by envelope glycoproteins from human immunodeficiency virus type 1 primary isolates

Coreceptor usage by Envs from diverse primary human immunodeficiency virus type 1 isolates was analyzed by a vaccinia virus-based expression and assay system. Usage of recombinant CCR5 and CXCR4 correlated closely with fusogenicity toward macrophages and T-cell lines expressing endogenous coreceptors. Surprisingly, recombinant CCR3 was utilized by most primary and T-cell-line-adapted Envs. Endogenous CXCR4 in macrophages was functional as a coreceptor.

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.

Viral phenotype in mother-to-child HIV-1 transmission and disease progression of vertically acquired HIV-1 infection

Human immunodeficiency virus-1 (HIV-1) primary isolates differ in replicative capacity on peripheral blood mononuclear cells, tropism for primary monocyte-derived macrophages (MDM) and T-cell lines and syncytium-inducing (SI) capability on MT-2 cells in vitro. To assess the role of viral phenotype in mother-to-child HIV-1 transmission and the progression of vertically acquired HIV-1 infection, we studied 57 HIV-1-infected women at the time of delivery and 24 HIV-1-infected infants. Eight mothers transmitted the infection to their children. Primary isolates, obtained from 7 and 33 transmitting and non-transmitting mothers, respectively, differed in replicative capacity and SI activity, and no significant differences between the two groups were found regarding these viral properties. However, all primary isolates from transmitting mothers, but about half of those from non-transmitting mothers, were able to infect and replicate in MDM, regardless of their replicative capacity and/or SI activity; moreover, the monocyto-macrophage tropism of the maternal isolate correlated with an increased risk of transmission. Viral isolates from HIV-1-infected children were typed before 2 months of age; all but four showed a tropism for MDM, further supporting the notion that monocyto-macrophage tropic variants are selectively transmitted from mother to child and/or selectively replicated upon transmission. Clinical follow-up disclosed that 7/11 infants with a rapid/high replicating virus but none of the 17 with a slow replicating virus developed severe symptoms of disease and/or severe immunodepression by 1 year of age. By means of competitive RNA-polymerase chain reaction (PCR), a relationship was found between viral phenotype and dynamics of HIV-1 replication early in life in children who experienced different patterns of disease progression.

A cell-free stock of simian-human immunodeficiency virus that causes AIDS in pig-tailed macaques has a limited number of amino acid substitutions in both SIVmac and HIV-1 regions of the genome and has offered cytotropism

We have examined both the sequence changes in the LTR, gag, vif, vpr, vpx, tat, rev, vpu, env, and nef genes and the cell tropism of a cell-free stock of chimeric simian-human immunodeficiency virus (SHIV) isolated from the cerebrospinal fluid of a pig-tailed macaque (PNb) that developed AIDS. This virus (SHIVKU-1) is highly pathogenic when inoculated into other macaques. DNA sequence analysis of PCR-amplified products revealed a total of 5 nucleotide changes in the LTR while vif had 2 consensus amino acid changes. The gag, vif, and vpx had no consensus amino acid substitutions, whereas vpr had 1 consensus substitution. The tat and rev genes of the HXB2 region of SHIVKU-1 had 2 and 1 consensus amino acid changes, respectively. The vpu gene of the HXB2 region of SHIV, which originally had an ACG at the beginning of the gene, reverted to an initiation ATG codon and in addition contained a consensus amino acid substitution at position 69 of this protein. As expected, the majority of the nucleotide substitutions were found in the env and nef genes. Thirteen and 5 amino acid changes were predicted for the corresponding Env and Nef proteins, respectively. In addition, one-third of the env gene clones isolated from the SHIVKU-1 stock had a 5-amino-acid deletion in the V4 region. Using three independent assays, we determined that the changes in the SHIVKU-1 were associated with an increase in the efficiency of replication in macrophages. The strikingly few consensus changes in the virus suggest that conversion of this virus to one capable of causing AIDS in pig-tailed macaques was associated with relatively few changes in the viral envelope and/or accessory genes. These results will provide the basis for the development of a pathogenic, molecular clone of SHIV capable of causing AIDS in pig-tailed macaques.

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

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

Twin studies demonstrate a host cell genetic effect on productive human immunodeficiency virus infection of human monocytes and macrophages in vitro

Biological and genetic variability is a prominent feature of human immunodeficiency virus (HIV) strains, especially in tropism, syncytium formation, and replicative capacity. To determine whether there were variable host cell effects on HIV replication in monocytes, three different strains of low-passage-number monocytotropic blood isolates of HIV and the laboratory-adapted strain Ba-L were inoculated into panels of adherent monocytes drawn from 44 different donors, and peak extracellular HIV p24 antigen titers were compared. The clinical HIV strains showed patterns of either moderate or low-level replication in most donor monocytes (20 to 4,000 pg/ml). However, within this range there was marked variation in peak titers in most donors. HIV type 1 Ba-L replicated in all donor monocytes to much higher levels with less variability (30 to 40 ng/ml). Furthermore, replication of 21 clinical blood-derived strains of HIV in blood monocytes and monocyte-derived macrophages (MDM) from pairs of identical twins and age-matched unrelated donors (URD) of the same sex were compared. In all of the seven pairs of identical twins, the kinetics of replication (measured by extracellular HIV p24 antigen) of panels of four clinical HIV type 1 isolates in monocytes were similar within pairs. However, marked and significant differences in kinetics of HIV production occurred within 10 of the 12 unrelated donor pairs (P = 0.0007). The remaining two URD pairs showed similar kinetic patterns, but only one pair had the same HLA-DR genotype. Similar results were observed with monocytes/MDMs obtained from a second bleed of the same donor. Hence, discordant patterns of HIV replication kinetics between URD monocyte pairs contrasted with concordant patterns in identical twin monocytes. These data strongly suggest a host cell genetic effect on productive viral replication in monocytes and MDMs. So far, no consistent genetic linkage of HIV replication pattern with HLA-DR genotype has been observed.

The inhibition of HIV replication in monocytes by interleukin 10 is linked to inhibition of cell differentiation

The effect of exogenous recombinant interleukin 10 on the replication of low-passage HIV-1 strains in blood-derived monocytes and monocyte-derived-macrophages (MDMs) was examined at various stages of cell maturation after adherence to the plastic substrate. Interleukin 10 inhibited extracellular production of HIV-1 to a greater degree in monocytes infected within 24 hr of adherence than those infected at 5-7 days. Inhibition of viral production as extracellular p24 antigen was most marked when interleukin 10 was preincubated with monocytes for 24-96 hr (optimum, 48 hr), and increased between 2 and 100 ng/ml. Neutralizing antibody to IL-10 reversed the inhibition. Inhibition of HIV production from monocytes and macrophages was maximal at 1 week after a single addition of cytokine, but then HIV production rose to control levels. Interleukin 10 was also found to inhibit reversibly the normal increase in size and maturation of both uninfected and HIV-infected monocytes during 10-15 days of adherence. In addition, cytoplasmic and membrane expression of CD26, a marker of macrophage maturation, was markedly inhibited but the proportion of detaching, apoptotic, or necrotic cells was also not increased. Hence, interleukin 10 reversibly inhibits both monocyte maturation and HIV production from infected monocytes with similar kinetics, suggesting that inhibition of monocyte maturation by IL-10 may have a marked effect of HIV production by these cells.

Antibody neutralization of HIV-1

Neutralizing antibodies are a major component of host defense against viruses, and appear to be particularly important in limiting the spread of cell-free virus. Results from vaccine trials in animal models suggest that these antibodies may contribute to protection against human immunodeficiency virus (HIV) infection. Here, Pascal Poignard and colleagues discuss recent developments in this area, with particular emphasis on the measurement, specificity and mechanism of the antibody response, and its significance for vaccine production.

A cytopathic infectivity assay of human immunodeficiency virus type 1 in human primary macrophages

In addition to CD4+ T lymphocytes, cells of monocyte/macrophage lineage are a major target for human immunodeficiency virus type 1 (HIV-1) infection. In vitro studies of HIV-1 infection in human monocyte-derived macrophages can be undertaken by a reproducible cell-based assay. A macrophage-based infectivity assay was developed based on the semi-quantitative scoring of HIV-1 induced cytopathology in monolayer macrophage cultures. The assay exhibited dilution-dependent linearity with all three primary macrophage-tropic isolates tested. The end-point infectivity titers determined by this assay correlated with the results obtained by detecting viral p24 antigen in the culture supernatant. The applications of the assay in both neutralization and anti-viral protocols yielded identical results with the more time-consuming and costly p24 formats. Since the assay offers a simple and low-cost method of measuring HIV-1 infectivity in human primary macrophages, it can be used quite easily for large-scale screening or evaluation of candidate vaccines and anti-viral agents.

Temporal relationship between elongation of the HIV type 1 glycoprotein 120 V2 domain and the conversion toward a syncytium-inducing phenotype

The second and third variable domains (V2 and V3) of the human immunodeficiency virus type 1 (HIV-1) gp120 envelope molecule have been shown to be determinants of syncytium-inducing (SI) capacity. Previously we have reported evidence that increased length of the V2 domain and duplication or relocation of potential N-linked glycosylation sites in V2 might be used as prognostic markers for evolution toward an SI phenotype. Here, we used a PCR assay that discriminates a 6-nucleotide difference in the length of the V2 domain, with a sensitivity of 1 elongated V2 domain when present in a background of 125 to 625 short V2 domains. Analysis of DNA isolated directly from PBMCs from 11 HIV-1-infected individuals prior to SI phenotype conversion revealed, however, that the usefulness of this PCR for V2 length polymorphism as predictive marker for SI phenotype evolution is limited. The strong association as observed in our previous study between elongation of the V2 domain and an SI phenotype prompted us to expand our first analysis. An extremely significant correlation was observed between V2 length and virus phenotype for samples obtained at about the moment of SI conversion, but not for samples obtained 3 to 35 months after SI phenotype conversion, suggesting that changes in V2 may be only transiently required to allow SI phenotype evolution. This possibly only transient nature of V2 elongation may explain the discrepancy between results by our group and others.

Macrophage-tropic variants initiate human immunodeficiency virus type 1 infection after sexual, parenteral, and vertical transmission

Macrophage-tropic, non-syncytium-inducing, HIV-1 variants predominate in the asymptomatic phase of infection and may be responsible for establishing infection in an individual exposed to the mixture of HIV-1 variants. Here, genotypical and phenotypical characteristics of virus populations, present in sexual, parenteral, or vertical donor-recipient pairs, were studied. Sequence analysis of the V3 domain confirmed the presence of a homogeneous virus population in recently infected individuals. Biological HIV-1 clones were further characterized for syncytium inducing capacity on the MT2 cell line and for macrophage tropism as defined by the appearance of proviral DNA upon inoculation of monocyte-derived macrophages. Both sexual and parenteral transmission cases revealed a selective outgrowth in the recipient of the most macrophage-tropic variant(s) present in the donor. In three out of five vertical transmission cases, more than one highly macrophage-tropic virus variant was present in the child shortly after birth, suggestive of transmission of multiple variants. In three primary infection cases, homogeneous virus populations of macrophage-tropic, non-syncytium-inducing variants were present prior to seroconversion, thus excluding humoral immunity as the selective pressure in favour of macrophage-tropic variants. These observations may have important implications for vaccine development.