Viral entry through CXCR4 is a pathogenic factor and therapeutic target in human immunodeficiency virus type 1 disease

Gaining momentum: stem cell therapies for HIV cure

PURPOSE OF REVIEW

Durable HIV-1 remission has been reported in a person who received allogeneic stem cell transplants (SCTs) involving CCR5 Δ32/Δ32 donor cells. Much of the reduction in HIV-1 burden following allogeneic SCT with or without donor cells inherently resistant to HIV-1 infection is likely due to cytotoxic graft-versus-host effects on residual recipient immune cells. Nonetheless, there has been growing momentum to develop and implement stem cell therapies that lead to durable long-term antiretroviral therapy (ART)-free remission without the need for SCT.

RECENT FINDINGS

Most current research leverages gene editing techniques to modify hematopoietic stem cells which differentiate into immune cells capable of harboring HIV-1. Approaches include targeting genes that encode HIV-1 co-receptors using Zinc Finger Nucleases (ZFN) or CRISPR-Cas-9 to render a pool of adult or progenitor cells resistant to de-novo infection. Other strategies involve harnessing multipotent mesenchymal stromal cells to foster immune environments that can more efficiently recognize and target HIV-1 while promoting tissue homeostasis.

SUMMARY

Many of these strategies are currently in a state of infancy or adolescence; nonetheless, promising preclinical and first-in-human studies have been performed, providing further rationale to focus resources on stem cell therapies.

Spermine and spermidine bind CXCR4 and inhibit CXCR4- but not CCR5-tropic HIV-1 infection

Semen is an important vector for sexual HIV-1 transmission. Although CXCR4-tropic (X4) HIV-1 may be present in semen, almost exclusively CCR5-tropic (R5) HIV-1 causes systemic infection after sexual intercourse. To identify factors that may limit sexual X4-HIV-1 transmission, we generated a seminal fluid-derived compound library and screened it for antiviral agents. We identified four adjacent fractions that blocked X4-HIV-1 but not R5-HIV-1 and found that they all contained spermine and spermidine, abundant polyamines in semen. We showed that spermine, which is present in semen at concentrations up to 14 mM, binds CXCR4 and selectively inhibits cell-free and cell-associated X4-HIV-1 infection of cell lines and primary target cells at micromolar concentrations. Our findings suggest that seminal spermine restricts sexual X4-HIV-1 transmission.

Short Communication: Extremely Severe CD4 Lymphopenia During HIV-1 Primary Infection

We report the case of a patient with a very profound CD4 T cell lymphopenia <20 cells/mm³ in the context of a primary HIV-1 infection, associated with both delayed HIV-specific antibody and CD8 T cell responses. A long-term immune reconstitution was observed after immediate initiation of antiretroviral therapy.

Chandipura virus changes cellular miRNome in human microglial cells

Chandipura virus (CHPV) is a neurotropic virus, known to cause encephalitis in humans. The microRNAs (miRNA/miR) play an important role in the pathogenesis of viral infection. The present study is focused on the role of miRNAs during CHPV (strain 1653514) infection in human microglial cells. The deep sequencing of CHPV-infected human microglial cells identified a total of 12 differentially expressed miRNA (DEMs). To elucidate the role of DEMs, the target gene prediction, Gene Ontology term (GO Term), pathway enrichment analysis, and miRNA-messenger RNA (mRNA) interaction network analysis was performed. The GO terms and pathway enrichment analysis provided 146 enriched genes; which were involved in interferon response, cytokine and chemokine signaling. Further, the WGCNA (weighted gene coexpression network analysis) of the enriched genes were discretely categorized into three modules (blue, brown, and turquoise). The hub genes in the blue module may correlate to CHPV induced neuroinflammation. Altogether, the miRNA-mRNA interaction network and WGCNA study revealed the following pairs, hsa-miR-542-3p and FAF1, hsa-miR-92a-1-5p and MYD88, and hsa-miR-3187-3p and TNFRSF21, which may contribute to neuroinflammation during CHPV infection in human microglial cells.

Productive infection of human neural progenitor cells by R5 tropic HIV-1: opiate co-exposure heightens infectivity and functional vulnerability

OBJECTIVE

HIV type-1 (HIV-1) causes a spectrum of central nervous system (CNS) complications; many are worsened by opiate co-exposure. Human neural progenitor cells (hNPCs) give rise to all CNS neurons and macroglia. We tested the hypothesis that hNPC maturation and fate are altered by HIV and opiates, contributing to HIV-1-related neuropathology. Reports of hNPC infection remain controversial. We rigorously examined this question, testing whether hNPCs propogated infection, and whether HIV affected hNPCs absent their infection.

DESIGN AND METHODS

Primary hNPCs were characterized over multiple passages. Following R5 HIV-1BaL exposure, p24, Nef, and tat assays monitored infection; a serial dilution approach tested infection transfer to naive hNPCs. Bromodeoxyuridine uptake, population doubling time, and immunostaining assessed proliferation and differentiation. Morphine co-exposure assessed opiate interactions. Supernatant from HIV-1BaL-infected PBMCs (HIVsup), HIV-1BaL, and ultraviolet light-inactivated HIVsup were compared to test effects of inflammatory milieu versus virus or infection per se.

RESULTS

The hNPCs (CD4/CD8/Iba/CXC3CL1/CD11b) were infectable and could transfer infection to naive hNPCs. Infection was partly blocked by maraviroc, implicating CCR5. HIVsup reduced hNPC proliferation and caused premature differentiation into neurons/astroglia. Effects on proliferation were due to soluble factors/viral proteins, not infection per se. Morphine co-exposure exacerbated certain functional consequences of HIVsup, and sustained the infection of hNPCs.

CONCLUSION

hNPCs can be infected and propagate virus in vitro. hNPCs or their progeny may represent an underappreciated viral reservoir. Factors from infected cells alter hNPC proliferation and neural cell maturation, which likely compromises CNS structure and function. Morphine-HIV interactions may worsen dysfunction and sustain infection.

Macrophage Densities Correlated with CXC Chemokine Receptor 4 Expression and Related with Poor Survival in Anaplastic Thyroid Cancer

BACKGROUND

Tumor associated macrophages (TAMs) and CXC chemokine receptor 4 (CXCR4) have emerged as potential biomarkers in various human cancers. The aims of this study were to investigate the clinical characteristics of anaplastic thyroid cancer (ATC) patients according to the TAM numbers in the tumor tissue, and to evaluate the associations between CXCR4 expressions and macrophage densities in ATC tumor microenvironment.

METHODS

Total 14 ATC samples from thyroid tissue microarray were used. Immunohistochemical staining was performed using anti-CD163 and anti-CXCR4 antibodies. According to the immunoreactivity of CD163, all subjects were divided into two groups: low-CD163 (n=8) and high-CD163 (n=6) groups.

RESULTS

The mean diagnostic age was 65±7 years and the median tumor size was 4.3 cm, ranging 2.5 to 15 cm. Clinicopathological characteristics were not significantly different between low-CD163 and high-CD163 groups, while age of diagnosis was younger in high-CD163 group than that of low-CD163 group with marginal significance (56.9±5.5 years vs. 67.5±6.8 years, P=0.09). However, overall survival was significantly reduced in high-CD163 group (5.5 months [range, 1 to 10]) compared with low-CD163 groups (8.8 months [range, 6 to 121); log-rank test, P=0.0443). Moreover, high-CD163 group showed strong CXCR4 expressions in both cancer and stromal compartments, while low-CD163 group showed relatively weak, stromal-dominant CXCR4 expressions. Additionally, CD163 and CXCR4 expressions showed a strong positive correlation (γ²=0.432, P=0.013).

CONCLUSION

Increased number of TAMs showed poor overall survival in ATC, suggesting TAMs are potentially a prognostic biomarker for ATC. CXCR4 expression was significantly correlated with CD163-positive TAM densities, which suggest the possible role of CXCR4 in TAM recruitments.

HIV coreceptor CXCR4 antagonists

PURPOSE OF REVIEW

Increases in HIV resistance towards approved antiviral drugs have made it necessary to develop new and potent antiviral drugs with preferably a novel mode of action.

RECENT FINDINGS

Entry inhibitors constitute a new class of drugs to treat infection by HIV-1. The first member of this class, enfuvirtide, previously known as T-20 and targeting gp41, has now been licensed for therapeutic use. Several other classes of entry inhibitors are in various stages of preclinical or clinical development.

SUMMARY

In this review we focus on the chemokine receptor inhibitors targeting CXCR4, which is one of the main HIV coreceptors, besides CCR5, for viral entry.

Biological signature characteristics of primary isolates from human immunodeficiency virus type 1 group O in ex vivo human tonsil histocultures

Human immunodeficiency virus type 1 (HIV-1) group M viruses have achieved a global distribution, while HIV-1 group O viruses are endemic only in particular regions of Africa. Here, we evaluated biological characteristics of group O and group M viruses in ex vivo models of HIV-1 infection. The replicative capacity and ability to induce CD4 T-cell depletion of eight group O and seven group M primary isolates were monitored in cultures of human peripheral blood mononuclear cells and tonsil explants. Comparative and longitudinal infection studies revealed HIV-1 group-specific activity patterns: CCR5-using (R5) viruses from group M varied considerably in their replicative capacity but showed similar levels of cytopathicity. In contrast, R5 isolates from group O were relatively uniform in their replicative fitness but displayed a high and unprecedented variability in their potential to deplete CD4 T cells. Two R5 group O isolates were identified that cause massive depletion of CD4 T cells, to an extent comparable to CXCR4-using viruses and not documented for any R5 isolate from group M. Intergroup comparisons found a five- to eightfold lower replicative fitness of isolates from group O than for isolates from group M yet a similar overall intrinsic pathogenicity in tonsil cultures. This study establishes biological ex vivo characteristics of HIV-1 group O primary isolates. The current findings challenge the belief that a grossly reduced replicative fitness or inherently impaired cytopathicity of viruses from this group underlies their low global prevalence.

CXCR4 expression determines functional activity of bone marrow-derived mononuclear cells for therapeutic neovascularization in acute ischemia

OBJECTIVE

Bone marrow-derived mononuclear cells (BMCs) improve the functional recovery after ischemia. However, BMCs comprise a heterogeneous mixture of cells, and it is not known which cell types are responsible for the induction of neovascularization after cell therapy. Because cell recruitment is critically dependent on the expression of the SDF-1-receptor CXCR4, we examined whether the expression of CXCR4 may identify a therapeutically active population of BMCs.

METHODS AND RESULTS

Human CXCR4(+) and CXCR4(-) BMCs were sorted by magnetic beads. CXCR4(+) BMCs showed a significantly higher invasion capacity under basal conditions and after SDF-1 stimulation. Hematopoietic or mesenchymal colony-forming capacity did not differ between CXCR4(+) and CXCR4(-) BMCs. Injection of CXCR4(+) BMCs in mice after induction of hindlimb ischemia significantly improved the recovery of perfusion compared to injection of CXCR4(-) BMCs. Likewise, capillary density was significantly increased in CXCR4(+) BMC-treated mice. Because part of the beneficial effects of cell therapy were attributed to the release of paracrine effectors, we analyzed BMC supernatants for secreted factors. Importantly, supernatants of CXCR4(+) BMCs were enriched in the proangiogenic cytokines HGF and PDGF-BB.

CONCLUSIONS

CXCR4(+) BMCs exhibit an increased therapeutic potential for blood flow recovery after acute ischemia. Mechanistically, their higher migratory capacity and their increased release of paracrine factors may contribute to enhanced tissue repair.

Selective expression of human immunodeficiency virus Nef in specific immune cell populations of transgenic mice is associated with distinct AIDS-like phenotypes

We previously reported that CD4C/human immunodeficiency virus (HIV)(Nef) transgenic (Tg) mice, expressing Nef in CD4(+) T cells and cells of the macrophage/dendritic cell (DC) lineage, develop a severe AIDS-like disease, characterized by depletion of CD4(+) T cells, as well as lung, heart, and kidney diseases. In order to determine the contribution of distinct populations of hematopoietic cells to the development of this AIDS-like disease, five additional Tg strains expressing Nef through restricted cell-specific regulatory elements were generated. These Tg strains express Nef in CD4(+) T cells, DCs, and macrophages (CD4E/HIV(Nef)); in CD4(+) T cells and DCs (mCD4/HIV(Nef) and CD4F/HIV(Nef)); in macrophages and DCs (CD68/HIV(Nef)); or mainly in DCs (CD11c/HIV(Nef)). None of these Tg strains developed significant lung and kidney diseases, suggesting the existence of as-yet-unidentified Nef-expressing cell subset(s) that are responsible for inducing organ disease in CD4C/HIV(Nef) Tg mice. Mice from all five strains developed persistent oral carriage of Candida albicans, suggesting an impaired immune function. Only strains expressing Nef in CD4(+) T cells showed CD4(+) T-cell depletion, activation, and apoptosis. These results demonstrate that expression of Nef in CD4(+) T cells is the primary determinant of their depletion. Therefore, the pattern of Nef expression in specific cell population(s) largely determines the nature of the resulting pathological changes.

Exceptional molecular and coreceptor-requirement properties of molecular clones isolated from an Human Immunodeficiency Virus Type-1 subtype C infection

BACKGROUND

The pathogenic significance of coreceptor switch in the viral infection of HIV-1 is not completely understood. This situation is more complex in subtype C infection where coreceptor switch is either absent or extremely rare. To gain insights into the mechanisms that underlie coreceptor requirement of subtype C, we screened several primary viral isolates and identified a clinical sample that demonstrated a potential to grow on standard T-cell lines with no detectable CCR5 expression. The subject was diagnosed with HIV-1 associated dementia in the absence of opportunistic infections of the brain. To isolate molecular clones from this virus, we devised a novel strategy based on anchor primers that target a sequence in the reverse transcriptase, highly conserved among diverse subtypes of HIV-1.

RESULTS

Using this strategy, we isolated 8 full-length molecular clones from the donor. Two of the eight molecular clones, 03In94_D17 and 03In94_D24, (D17 and D24) generated replication-competent viruses. Phylogenetic analysis of the full-length viral sequences revealed that both clones were non-recombinant subtype C viruses. They contain intact open reading frames in all the viral proteins. Both the viral clones are endowed with several unique molecular and biological properties. The viral promoter of the clones is characterized by the presence of four NF-kB binding elements, a feature rarely seen in the subtype C HIV-1 LTR. Interestingly, we identified the coexistence of two different forms of Rev, a truncated form common to subtype C and a full-length form less common for this subtype, in both proviral and plasma virus compartments. An exceptional property of the viruses, atypical of subtype C, is their ability to use a wide range of coreceptors including CCR5, CXCR4, and several others tested. Sequence analysis of Env of D17 and D24 clones identified differences within the variable loops providing important clues for the expanded coreceptor use. The V1, V2 and V4 loops in both of the molecular clones are longer due to the insertion of several amino acid residues that generated potential N-linked glycosylation sites.

CONCLUSION

The exceptional biological and molecular properties of these clones make them invaluable tools to understand the unique pathogenic characteristics of subtype C.

Subtype-specific conformational differences within the V3 region of subtype B and subtype C human immunodeficiency virus type 1 Env proteins

The V3 region of the human immunodeficiency virus type 1 gp120 Env protein is a key domain in Env due to its role in interacting with the coreceptors CCR5 and CXCR4. We examined potential subtype-specific V3 region differences by comparing patterns of amino acid variability and probing for subtype-specific structures using 11 anti-V3 monoclonal antibodies (V3 MAbs). Differences between the subtypes in patterns of variability were most evident in the stem and turn regions of V3 (positions 9 to 24), with the two subtypes being very similar in the base region. The characteristics of the binding of V3 MAbs to Env proteins of the subtype B virus JR-FL and the subtype C virus BR025 suggested three patterns, as each group of MAbs recognized a specific conformation- or sequence-based epitope. Viruses pseudotyped with Env from JR-FL and BR025 were resistant to neutralization by the V3 MAbs, although the replacement of the Env V3 region of the SF162 virus with the JR-FL V3 created a pseudotyped virus that was hypersensitive to neutralization. A single mutation in V3 (H13R) made this chimeric Env selectively resistant to one group of V3 MAbs, consistent with the mAb binding properties. We hypothesize that there are intrinsic differences in V3 conformation between subtype B and subtype C that are localized to the stem and turn regions and that these differences have two important biological consequences: first, subtype B and subtype C V3 regions can have subtype-specific epitopes that will inherently limit antibody cross-reactivity, and second, V3 conformational differences may potentiate the frequent evolution of R5- into X4-tropic variants of subtype B but limit subtype C virus from using the same mechanism to evolve X4-tropic variants as efficiently.

The role of N-glycosylation sites on the CXCR4 receptor for CXCL-12 binding and signaling and X4 HIV-1 viral infectivity

The chemokine receptor CXCR4 functions as one of the HIV-1 coreceptors and can be considered as an attractive target for the development of novel anti-HIV drugs. Here, we investigated the effect of its two known N-glycosylation sites g1 (NYT) and g2 (NVS) on the antiviral potential of several classes of entry inhibitors. The lack of g1 clearly affected the binding of the amino-terminal directed 2B11 mAb, but not the 12G5 mAb. No dramatic effects on CXCL-12 binding and CXCL-12-induced intracellular calcium responses were observed. Importantly, the anti-HIV-1 activity and antagonistic activity of the prototype compound of CXCR4 inhibitors, AMD3100, were not affected by the presence or absence of the CXCR4 N-glycans. Since CXCR4 N-glycans play a less important role in viral entry compared to the N-glycans on the HIV envelope, cells expressing CXCR4 N-glycosylation mutants might be no relevant alternative to allow HIV-1 escape from antivirals.

HIV co-receptor inhibitors as novel class of anti-HIV drugs

Entry inhibitors constitute a new class of drugs to treat infection by human immunodeficiency virus type 1 (HIV-1). The first member of this class, enfuvirtide, previously known as T-20 and targeting gp41, has now been licensed for therapeutic use. Several other entry inhibitors are in various stages of pre-clinical or clinical development. In this review we focus on the chemokine receptor inhibitors targeting CCR5 and CXCR4 that are the main HIV co-receptors for viral entry.

Simian immunodeficiency virus variants that differ in pathogenicity differ in fitness under rapid cell turnover conditions

Simian immunodeficiency virus (SIV) has been shown to progress through a number of changes that lead to the emergence of pathogenic viral variants in macaques initially infected with a mildly cytopathic variant, SIVMneCL8. One of these late-stage isolates, SIVMne170, replicates to high levels in vivo and causes a rapid disease course when reintroduced into naïve macaques, resulting in a viral set point up to 3,000-fold higher than the set point of the parental virus, SIVMneCL8. However, in cell culture both viruses replicate with similar kinetics. One major difference between in vivo and in vitro cultures is the life span of the infected cells. Here, we manipulated the life span of infected cells in vitro, and we show that the fitness of SIVMne170 in cultures with a limited cell life span dramatically increased compared to its fitness in cultures with a nonlimited life span of cells. The increase in fitness was at least partially due to the fact that the rapid turnover system eliminates the negative influence of the cytopathic effects associated with replication of SIVMne170. Because the relative fitness of SIVMneCL8 and SIVMne170 observed in the rapid turnover system more accurately reflects their fitness in vivo, the system represents an improved approach to comparing relative fitness of viruses.

The ins and outs of HIV replication

The life cycle of HIV-1 involves a series of steps necessary for the successful infection of human target cells. First the RNA genome enters the cytoplasm after the fusion of the viral membrane and that of the target cell. The RNA genome is then converted to DNA form through the process of reverse transcription. The DNA genome is then integrated into the host cell DNA. Next, viral proteins and more copies of the viral genome are produced. These components assemble to form new virions that are then able to propagate. The cellular proteins involved in HIV-1 entry have been known for more than a decade now and the study of the cellular and viral components involved in HIV-1 entry has led to the development of many therapeutic strategies and drugs designed to block viral replication. Recently, there have been significant advances in the understanding of HIV-1 assembly as a consequence of the identification of the cellular factors that mediate this process. This review will provide a basic outline of the current understanding of HIV-1 entry and exit.

Enhanced replication of R5 HIV-1 over X4 HIV-1 in CD4(+)CCR5(+)CXCR4(+) T cells

To enter human cells, HIV-1 usually uses CD4 and 1 of 2 coreceptors: CCR5 and CXCR4. Interestingly, even though CCR5 is expressed on far fewer T cells than is CXCR4, many patients in early- and late-stage HIV disease maintain high levels of CCR5-tropic (R5) viruses. We hypothesized that such high R5 viral loads may be sustained because, relative to CXCR4-tropic (X4) HIV-1 infection, R5 HIV-1 infection of permissive CD4(+)CCR5(+)CXCR4(+) T cells results in the production of significantly more infectious virus particles per target cell. To investigate this possibility, we compared the levels of virus production per target cell after isogenic R5 and X4 HIV-1 infection of 2 in vitro primary human lymphocyte culture systems: T-cell receptor-stimulated blood-derived CD4(+) T cells and tonsil histoculture (which requires no exogenous stimulation for ex vivo infection). We provide evidence that R5 HIV-1 does indeed compensate for a small target cell population by producing, on average, 5 to 10 times more infectious virus per CCR5(+) target cell than X4 HIV-1. This replicative advantage may contribute to the predominance of R5 HIV-1 in vivo.

Enhanced Replicative Capacity and Pathogenicity of HIV-1 Isolated From Individuals Infected With Drug-Resistant Virus and Declining CD4+ T-Cell Counts

Virologic failure on continuous antiretroviral therapy (ART) is associated with variable changes in CD4 T-cell counts: peripheral CD4 T-cell counts decrease in conjunction with a resurgence of plasma virus (nonresponders) or remain stable or continue to increase despite ongoing virus replication (discordant responders). This study found that HIV-1 isolated from nonresponders had significantly greater replicative capacity in activated peripheral blood mononuclear cells (PBMCs) as well as an enhanced ability to induce apoptosis in both HIV-1-infected and HIV-1-uninfected CD4 T cells compared with virus isolated from discordant responders. Enhanced replicative capacity in PBMCs of virus isolated from nonresponders was inhibited by AMD3100, a CXCR4 antagonist. Virus quasispecies isolated from PBMCs from nonresponders used both CC chemokine receptor 5 (CCR5) and CX chemokine receptor 4 (CXCR4) for entry, in contrast to virus isolated from PBMCs from discordant responders, which predominantly used CCR5. In contrast, virus isolated from plasma from both groups predominantly used CCR5. In summary, although drug resistance may lead to impaired viral fitness, the capacity of virus quasispecies from PBMCs to use CXCR4 may have significant consequences on viral replicative capacity and potentially on clinical outcome.

HIV chemokine receptor inhibitors as novel anti-HIV drugs

The chemokine receptors CXCR4 and CCR5 are the main coreceptors used by the T-cell-tropic (CXCR4-using, X4) and macrophage-tropic (CCR5-using, R5) HIV-1 strains, respectively, for entering their CD4+ target cells. In this review, we focus on the function of these chemokine receptors in HIV infection and their role as novel targets for viral inhibition. Besides some modified chemokines with antiviral activity, several low-molecular weight CCR5 and CXCR4 antagonistic compounds have been described with potent antiviral activity. The best CXCR4 antagonists described are the bicyclam derivatives, which consistently block X4 but also R5/X4 viral replication in PBMCs. We believe that chemokine receptor antagonists will become important new antiviral drugs to combat AIDS. Both CXCR4 and CCR5 chemokine receptor inhibitors will be needed in combination and even in combinations of antiviral drugs that also target other aspects of the HIV replication cycle to obtain optimum antiviral therapeutic effects.

Down-modulation of the CXCR4 co-receptor by intracellular expression of a single chain variable fragment (SFv) inhibits HIV-1 entry into primary human brain microvascular endothelial cells and post-mitotic neurons

Our laboratories previously demonstrated that expression of a single chain variable antibody fragment (SFv), anti-CXCR4 SFv, in human lymphoid cells suppresses surface display of the chemokine co-receptor CXCR4 and inhibits infectious entry of human immunodeficiency virus type I (HIV-1). We now sought to extend these results to two types of central nervous system (CNS) cells, primary isolated human brain microvascular endothelial cells (MVECs), and post-mitotic differentiated human neurons, both of which normally express significant levels of CXCR4. The anti-CXCR4 SFv expression construct was delivered using an HIV-1-based vector, and control cells received LacZ-expressing viral particles. Upon intracellular expression of the anti-CXCR4 SFv, immunostaining revealed a marked reduction in surface display of CXCR4 on both cell types. Consequently, post-mitotic neurons expressing the anti-CXCR4 SFv were significantly protected from HIV-1 infection, as measured by HIV-1 p24 antigen production, and partial protection was observed in human brain MVECs. The ability to selectively down-modulate the surface expression of CXCR4 in CNS cells may allow for the development of clinical molecular therapy strategies against HIV-1-related neurodegenerative disorders and neuroinvasion.

Human immunodeficiency virus type 1 genetic diversity in the nervous system: Evolutionary epiphenomenon or disease determinant?

Over the past decade there has been a revolution in the understanding and care of human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS)-associated disease. Much of this progress stems from a broader recognition of the importance of differences in viral types, including receptor preference(s), replication properties, and reservoirs, as contributing factors to immunosuppresion and disease progression. In contrast, there is limited conceptualizatin of viral diversity and turnover in the brain and circulation in relation to neurocognitive impairments. Herein, the authors review current concepts regarding viral molecular diversity and phenotypes together with features of HIV-1 neuroinvasion, neurotropism, neurovirulence and neurosusceptiblity. Viral genetic and antigenic diversity is reduced within the brain compared to blood or other systemic organs within individuals. Conversely, viral molecular heterogeneity is greater in patients with HIV-associated dementia compared to nondemented patients, depending on the viral gene examined. Individual viral proteins exert multiple neuropathogenic effects, although the neurological consequences of different viral polymorphisms remain uncertain. Nonetheless, host genetic polymorphisms clearly influence neurological disease outcomes and likely dictate both acquired and innate immune responses, which in turn shape viral evolution within the host. Emerging issues include widespread antiretroviral therapy resistance and increasing awareness of viral superinfections together with viral recombination, all of which are likely to impact on both HIV genetic variation and neuropathogenesis. With the persisting prevalence of HIV-induced neurocognitive disabilities, despite marked improvements in managing immunosuppression, it remains imperative to fully define and understand the mechanisms by which viral dynamics and diversity contribute to neurological disease, permitting the development of new therapeutic strategies.

Functional correlation of P-glycoprotein expression and genotype with expression of the human immunodeficiency virus type 1 coreceptor CXCR4

The aim of this study was to investigate the relationship between lymphocyte P-glycoprotein (P-gp) expression and genotype in vivo and the expression of lymphocyte receptors critical in the life cycle of human immunodeficiency virus type 1 (HIV-1), i.e., CD4, CCR5, and CXCR4. Using flow cytometry to quantify each membrane receptor/transporter, we demonstrate a highly significant correlation between P-gp protein expression and the expression of CXCR4 (rho = 0.874; P < 0.0001). Furthermore, confocal microscopy showed colocalized expression of CXCR4 and P-gp in the lymphocyte membrane. This significant relationship was also apparent at the mRNA level by use of reverse transcription-PCR (rho = 0.61; P < 0.005) and was present in both phytohemagglutinin-stimulated and unstimulated peripheral blood mononuclear cells. Genotypic analysis of the C3435T single-nucleotide polymorphism of P-gp confirmed significantly higher levels of P-gp in C (range, 2.45 to 11.00 relative fluorescence units [RFU])- than in T (range, 0.25 to 5.00 RFU)-homozygous individuals (P = 0.0088; 95% confidence interval [95% CI], 0.7 to 6.3 RFU). An equivalent association between CXCR4 levels and C (range, 12.7 to 44.1 RFU) versus T (range, 3 to 18.9 RFU) genotype was also demonstrated (P = 0.0019; 95% CI, 5.4 to 23.7). Functionally, although these correlates had no impact on HIV-1 production from either X4- or R5-tropic virus, expression correlated significantly with the activity of the HIV-1 protease inhibitor (PI) saquinavir for both P-gp (rho = 0.75; P = 0.0019) and CXCR4 (rho = 0.71; P = 0.0041). This study defines an association between P-gp (expression and genotype) and CXCR4 that may have implications for the selection of viral tropism and the access of drugs to protease for specific tropic types. The interplay between these two proteins may also influence the viral genotypes which escape effective chemotherapy and which therefore have the opportunity to evolve resistance to PIs.

Safety, Pharmacokinetics, and Antiviral Activity of AMD3100, a Selective CXCR4 Receptor Inhibitor, in HIV-1 Infection

AMD3100 is a CXCR4 receptor inhibitor with anti-HIV-1 activity in vitro. We tested the safety, pharmacokinetics, and antiviral effect of AMD3100 administered for 10 days by continuous intravenous infusion in an open-label dose escalation study from 2.5 to 160 microg/kg/h. Forty HIV-infected patients with an HIV RNA level >5000 copies/mL on stable antiretroviral (ARV) regimens or off therapy were enrolled. Syncytium-inducing (SI) phenotype in an MT-2 cell assay was required in higher dose cohorts. Most subjects were black (55%), male (98%), and off ARV therapy. HIV phenotype was SI (30%), non-SI (45%), or not tested (25%). One patient (5 microg/kg/h) had serious and possibly drug-related thrombocytopenia. Two patients (40 and 160 microg/kg/h) had unexpected, although not serious, premature ventricular contractions. Most patients in the 80- and 160-microg/kg/h cohorts had paresthesias. Steady-state blood concentration and area under the concentration-time curve were dose proportional across all dose levels; the median terminal elimination half-life was 8.6 hours (range: 8.1-11.1 hours). Leukocytosis was observed in all patients, with an estimated maximum effect of 3.4 times baseline (95% confidence interval: 2.9-3.9). Only 1 patient, the patient whose virus was confirmed to use purely CXCR4 and who also received the highest dose (160 microg/kg/h), had a significant 0.9-log10 copies/mL HIV RNA drop at day 11. Overall, however, the average change in viral load across all patients was +0.03 log10 HIV RNA. Given these results, AMD3100 is not being further developed for ARV therapy, but development continues for stem cell mobilization.

R5 human immunodeficiency virus type 1 (HIV-1) replicates more efficiently in primary CD4+ T-cell cultures than X4 HIV-1

In this report, we present evidence that R5 human immunodeficiency virus type 1 (HIV-1) replicates more efficiently in primary CD4+ T cells than X4 HIV-1. By comparing CD3/CD28-costimulated CD4+ T-cell cultures infected by several X4 and R5 HIV-1 strains, we determined that R5-infected CD4+ T cells produce more virus over time than X4-infected CD4+ T cells. In the first comparison, we found that more cells were infected by the X4-tropic strain LAI than by the R5-tropic strain JR-CSF and yet that higher levels of viral production were detected in the R5-infected cultures. The differential viral production was partially due to the severe cytopathic effects of the X4 virus. We also compared cultures infected with the isogenic HIV-1 strains NL4-3 (X4) and 49.5 (R5). We found that fewer cells were infected by the R5 strain, and yet similar levels of viral production were detected in both infected cultures. Cell death played less of a role in the differential viral production of these strains, as the cell viability remained comparable in both X4- and R5-infected cultures over time. The final comparison involved the primary R5-tropic isolate KP1 and the primary dual-tropic isolate KP2. Although both strains infected similar numbers of cells and induced comparable levels of cytopathicity, viral production was considerably higher in the R5-infected culture. In summary, these data demonstrate that R5 HIV-1 has an increased capacity to replicate in costimulated CD4+ T cells compared to X4 HIV-1.

Inhibition of HIV-1 fusion with small interfering RNAs targeting the chemokine coreceptor CXCR4

RNA interference (RNAi) is an evolutionarily conserved process by which plants and animals protect their genomes utilizing small, double-stranded RNAs to degrade target RNAs in a sequence-specific manner. Post-transcriptional gene silencing by these moieties can lead to degradation of both cellular and viral RNAs. It has recently been shown that double-stranded, small interfering RNAs (siRNAs) of 21-25 nucleotides can be transfected into relevant cells to target specific RNAs. This approach was utilized to inhibit human immunodeficiency virus type I (HIV-1) infection in human cells. siRNAs with homology to a motif in the mRNA that encodes for the HIV-1 chemokine coreceptor CXCR4 was utilized. Complementary studies via immunofluorescence microscopy and fluorescence-activated cell sorting demonstrated downregulation of CXCR4 from the surface of cells transfected with the specific siRNAs. As well, siRNAs without sequence homology to CXCR4 were used as controls and demonstrated no downregulation of CXCR4. siRNAs targeted to another chemokine coreceptor, APJ, showed specificity for downregulation of APJ but had no effects on CXCR4. Transfections with siRNAs targeting CXCR4 mRNA were shown to inhibit HIV-1 envelope fusion, which is relatively resistant to most viral inhibitors targeting chemokine coreceptors. The specificity of this effect was demonstrated by the inhibition of fusion by CXCR4-tropic and dual-tropic (CXCR4 and CCR5) envelope glycoproteins from HIV-1 on CXCR4+ indicator cells, but the lack of effects by siRNAs targeting CXCR4 mRNA on dual-tropic HIV-1 envelopes in CCR5+ indicator cells utilizing these fusion assays. Interestingly, siRNAs targeting CXCR4 selectively inhibited CXCR4-tropic cell-free virus infection of human cells but at only modest levels as compared to cell:cell fusion. siRNA may be a potential molecular therapeutic approach to alter a cellular cofactor critical for infection of human cells by relevant strains of HIV-1. The targeting of a cellular cofactor, rather than the HIV-1-specific mRNAs or genomic RNA, holds promise as the rapid mutational ability of the HIV-1 genome may obviate the potential clinical use of RNAi directly against this virus.

In vivo evolution of X4 human immunodeficiency virus type 1 variants in the natural course of infection coincides with decreasing sensitivity to CXCR4 antagonists

CXCR4-using (X4) human immunodeficiency virus type 1 (HIV-1) variants evolve from CCR5-restricted (R5) HIV-1 variants. Early after their first appearance in vivo, X4 HIV-1 variants additionally use CCR5. The ability to use CCR5 in addition to CXCR4 is generally lost late in infection. Here we studied whether this evolution of the coreceptor repertoire is also reflected in a changing sensitivity of X4 variants to CXCR4 antagonists such as peptide T22 and the synthetic compound AMD3100. We observed differences in the concentrations of CXCR4 antagonists needed to suppress replication of X4 HIV variants from different patients. In general, late X4 HIV variants were less sensitive to AMD3100 than were early R5X4 HIV variants. The differences between early R5X4 HIV variants and late X4 variants were less pronounced for T22-mediated inhibition. These results suggest an ongoing evolution of X4 virus variants toward more efficient usage of the cellular entry complex.

Establishment of a novel CCR5 and CXCR4 expressing CD4+ cell line which is highly sensitive to HIV and suitable for high-throughput evaluation of CCR5 and CXCR4 antagonists

BACKGROUND

CCR5 and CXCR4 are the two main coreceptors essential for HIV entry. Therefore, these chemokine receptors have become important targets in the search for anti-HIV agents. Here, we describe the establishment of a novel CD4+ cell line, U87.CD4.CCR5.CXCR4, stably expressing both CCR5 and CXCR4 at the cell surface.

RESULTS

In these cells, intracellular calcium signalling through both receptors can be measured in a single experiment upon the sequential addition of CXCR4- and CCR5-directed chemokines. The U87.CD4.CCR5.CXCR4 cell line reliably supported HIV-1 infection of diverse laboratory-adapted strains and primary isolates with varying coreceptor usage (R5, X4 and R5/X4) and allows to investigate the antiviral efficacy of combined CCR5 and CXCR4 blockade. The antiviral effects recorded in these cells with the CCR5 antagonist SCH-C and the CXCR4 antagonist AMD3100 were similar to those noted in the single CCR5- or CXCR4-transfected U87.CD4 cells. Furthermore, the combination of both inhibitors blocked the infection of all evaluated HIV-1 strains and isolates.

CONCLUSIONS

Thus, the U87.CD4.CCR5.CXCR4 cell line should be useful in the evaluation of CCR5 and CXCR4 antagonists with therapeutic potential and combinations thereof.

Cell surface receptors for gammaretroviruses

Evidence obtained during the last few years has greatly extended our understanding of the cell surface receptors that mediate infections of retroviruses and has provided many surprising insights. In contrast to other cell surface components such as lectins or proteoglycans that influence infections indirectly by enhancing virus adsorption onto specific cells, the true receptors induce conformational changes in the viral envelope glycoproteins that are essential for infection. One surprise is that all of the cell surface receptors for gamma-retroviruses are proteins that have multiple transmembrane (TM) sequences, compatible with their identification in known instances as transporters for important solutes. In striking contrast, almost all other animal viruses use receptors that exclusively have single TM sequences, with the sole proven exception we know of being the coreceptors used by lentiviruses. This evidence strongly suggests that virus genera have been prevented because of their previous evolutionary adaptations from switching their specificities between single-TM and multi-TM receptors. This evidence also implies that gamma-retroviruses formed by divergent evolution from a common origin millions of years ago and that individual viruses have occasionally jumped between species (zoonoses) while retaining their commitment to using the orthologous receptor of the new host. Another surprise is that many gamma-retroviruses use not just one receptor but pairs of closely related receptors as alternatives. This appears to have enhanced viral survival by severely limiting the likelihood of host escape mutations. All of the receptors used by gamma-retroviruses contain hypervariable regions that are often heavily glycosylated and that control the viral host range properties, consistent with the idea that these sequences are battlegrounds of virus-host coevolution. However, in contrast to previous assumptions, we propose that gamma-retroviruses have become adapted to recognize conserved sites that are important for the receptor's natural function and that the hypervariable sequences have been elaborated by the hosts as defense bulwarks that surround the conserved viral attachment sites. Previously, it was believed that binding to receptors directly triggers a series of conformational changes in the viral envelope glycoproteins that culminate in fusion of the viral and cellular membranes. However, new evidence suggests that gamma-retroviral association with receptors triggers an obligatory interaction or cross-talk between envelope glycoproteins on the viral surface. If this intermediate step is prevented, infection fails. Conversely, in several circumstances this cross-talk can be induced in the absence of a cell surface receptor for the virus, in which case infection can proceed efficiently. This new evidence strongly implies that the role of cell surface receptors in infections of gamma-retroviruses (and perhaps of other enveloped animal viruses) is more complex and interesting than was previously imagined. Recently, another gammaretroviral receptor with multiple transmembrane sequences was cloned. See Prassolov, Y., Zhang, D., Ivanov, D., Lohler, J., Ross, S.R., and Stocking, C. Sodium-dependent myo-inositol transporter 1 is a receptor for Mus cervicolor M813 murine leukemia virus.

Mutations at the CXCR4 interaction sites for AMD3100 influence anti-CXCR4 antibody binding and HIV-1 entry

The interaction of the CXCR4 antagonist AMD3100 with its target is greatly influenced by specific aspartate residues in the receptor protein, including Asp(171) and Asp(262). We have now found that aspartate-to-asparagine substitutions at these positions differentially affect the binding of four different anti-CXCR4 monoclonal antibodies as well as the infectivity of diverse human immunodeficiency virus type 1 (HIV-1) strains and clinical isolates. Mutation of Asp(262) strongly decreased the coreceptor efficiency of CXCR4 for wild-type but not for AMD3100-resistant HIV-1 NL4.3. Thus, resistance of HIV-1 NL4.3 to AMD3100 is associated with a decreased dependence of the viral gp120 on Asp(262) of CXCR4, pointing to a different mode of interaction of wild-type versus AMD3100-resistant virus with CXCR4.

In vivo evolution of human immunodeficiency virus type 1 toward increased pathogenicity through CXCR4-mediated killing of uninfected CD4 T cells

The destruction of the immune system by progressive loss of CD4 T cells is the hallmark of AIDS. CCR5-dependent (R5) human immunodeficiency virus type 1 (HIV-1) isolates predominate in the early, asymptomatic stages of HIV-1 infection, while CXCR4-dependent (X4) isolates typically emerge at later stages, frequently coinciding with a rapid decline in CD4 T cells. Lymphocyte killing in vivo primarily occurs through apoptosis, but the importance of apoptosis of HIV-1-infected cells relative to apoptosis of uninfected bystander cells is controversial. Here we show that in human lymphoid tissues ex vivo, apoptosis of uninfected bystander CD4 T cells is a major mechanism of lymphocyte depletion caused by X4 HIV-1 strains but is only a minor mechanism of depletion by R5 strains. Further, X4 HIV-1-induced bystander apoptosis requires the interaction of the viral envelope glycoprotein gp120 with the CXCR4 coreceptor on CD4 T cells. These results emphasize the contribution of bystander apoptosis to HIV-1 cytotoxicity and suggest that in association with a coreceptor switch in HIV disease, T-cell killing evolves from an infection-restricted stage to generalized toxicity that involves a high degree of bystander apoptosis.

Evaluation of SDF-1/CXCR4-induced Ca2+ signaling by fluorometric imaging plate reader (FLIPR) and flow cytometry

BACKGROUND

The chemokine receptors CXCR4 and CCR5 are the main coreceptors for human immunodeficiency virus (HIV) 1 to enter its target cells. The antiviral activity of their natural ligands (stromal cell-derived factor 1 [SDF-1], regulated on activation normal T-cell expressed and secreted, (RANTES) and macrophage inflammatory proteins 1alpha and 1beta, MIP-1alpha and MIP-1beta) and the finding that individuals deficient in CCR5 are relatively resistant to HIV infection led to the concept that chemokine receptor antagonists can play an important role in anti-HIV therapy. AMD3100, the prototype compound of the bicyclams, is one of the most potent and selective CXCR4 antagonists described to date. The search for new chemokine receptor antagonists involves the evaluation of compounds for their ability to block the specific chemokine-induced transient intracellular Ca(2+) flux. We evaluated two cell-based-fluorescent methods with the use of the Fluorometric Imaging Plate Reader (FLIPR) system and a flow cytometric assay to measure the SDF-1-induced intracellular Ca(2+) mobilization via CXCR4. Both assay systems were compared for their sensitivity, advantages, and system-dependent limitations.

METHODS

CXCR4(+) lymphocytic and monocytic cell lines commonly used in laboratories studying acquired immunodeficiency syndrome and freshly isolated peripheral blood mononuclear cells (PBMCs) were loaded with the Ca(2+) indicator Fluo-3. After washing, the cells were preincubated with the CXCR4 antagonist AMD3100. Then the increase in intracellular Ca(2+) concentration after stimulation with SDF-1 was examined by the FLIPR and the flow cytometer, which monitored the change in green fluorescence intensity of Ca(2+)-bound Fluo-3. Surface CXCR4 expression was also determined flow cytometrically.

RESULTS

In all five CXCR4(+) cell lines, SDF-1 elicited a transient increase in intracellular Ca(2+) concentration. For each cell line, the magnitude of response was related to the level of CXCR4 expression: the cells with the highest CXCR4 level showed the strongest Ca(2+) response. AMD3100 effected a dose-dependent inhibition of the SDF-1-induced Ca(2+) flux in all cell lines examined. The FLIPR was more sensitive than the flow cytometer in detecting minor Ca(2+) responses. In freshly isolated PBMCs, the Ca(2+) response was due solely to the stimulation of monocytes and granulocytes, whereas the lymphocyte population, although expressing CXCR4, did not respond at all. This phenomenon could be observed with the flow cytometer and not with the FLIPR.

CONCLUSIONS

The FLIPR system is a most adequate to monitor intracellular Ca(2+) mobilization and to evaluate chemokine receptor antagonists. However, flow cytometry and its multiparameter analysis allow additional characterization of the cells involved in the chemokine receptor-mediated signal transduction.

Susceptibility of diverse primary HIV isolates with varying co-receptor specificity's to CXCR4 antagonistic compounds

The chemokine receptors CCR5 and CXCR4 are an obvious target for HIV therapies. Two compounds, T-22 and AMD-3100, have been shown to inhibit infection of CXCR4-using HIV-1 isolates. The specificity of T-22 and AMD-3100 was further confirmed by their ability to block entry of HIV-1 in GHOST-CXCR4 transfected cells with no effect on viral entry in the GHOST-CCR5 cells. The ability of T-22 to block replication of diverse HIV-1 isolates (group M, subtypes A, B, D, E, and F as well as group O) and HIV-2 primary isolates with varying coreceptor specificities ranging from exclusive CCR5 usage to multiple coreceptor usage was examined in detail. T-22 was found to be highly effective (>90%) at blocking infection of diverse HIV-1 (subtypes A-F, and group O) and HIV-2 isolates that use multiple coreceptors in human PBMCs homozygous for a 32-bp deletion in CCR5 (CCR5-/-), but less effective in CCR5 +/+ PBMCs. Additionally, sequential primary HIV-1 isolates obtained from a longitudinal cohort who had switched from single coreceptor usage to a broad range of multiple receptors could be blocked effectively by both T-22 and AMD-3100 in CCR5-/- PBMCs. Our data suggest that CXCR4 antagonistic compounds are highly effective in blocking the entry of X4-tropic HIV-1, and that these compounds could be a useful additive to current anti-retroviral therapy for clinical management of HIV disease.

Coreceptor phenotype of natural human immunodeficiency virus with nef deleted evolves in vivo, leading to increased virulence

The Sydney Blood Bank Cohort is a group of patients with slowly progressive infection by a human immunodeficiency virus strain containing spontaneous deletions within the nef long terminal repeat region. In 1999, 18 years after the initial infection, one of the members (D36) developed AIDS. In this work, we used an ex vivo human lymphoid cell culture system to analyze two viral isolates obtained from this patient, one prior to the onset of AIDS in 1995 and one after disease progression in 1999. Both D36 isolates were less potent in depleting CD4(+) T cells than a reference dualtropic, nef-bearing viral isolate. However, the 1999 isolate was measurably more cytotoxic to CD4(+) T cells than the 1995 isolate. Interestingly, although both isolates were nearly equally potent in depleting CCR5(+) CD4(+) T cells, the cytotoxic effect of the 1999 isolate toward CCR5(-) CD4(+) T cells was significantly higher. Furthermore, GHOST cell infection assays and blocking experiments with the CXCR4 inhibitor AMD3100 showed that the later D36 1999 isolate could infect both CCR5(+) and CCR5(-) CXCR4(+) cells efficiently, while infection by the 1995 isolate was nearly completely restricted to CCR5(+) cells. Sequence analysis of the V1/V2 and V3 regions of the viral envelope protein gp120 revealed that the more efficient CXCR4 usage of the later isolate might be caused by an additional potential N-glycosylation site in the V1/V2 loop. In conclusion, these data show that an in vivo evolution of the tropism of this nef-deleted strain toward an X4 phenotype was associated with a higher cytopathic potential and progression to AIDS.

Conserved CXCR4 usage and enhanced replicative capacity of HIV-2/287, an isolate highly pathogenic in Macaca nemestrina

OBJECTIVE

To investigate viral properties that contribute to the pathogenic potential of HIV-2 in macaques.

DESIGN

We compared HIV-2/287, a virus highly pathogenic in Macaca nemestrina, with its non-pathogenic progenitor HIV-2 EHO, for coreceptor usage and ability to infect human and macaque peripheral blood mononuclear cells (PBMC).

METHODS

Coreceptor usage was determined in GHOST cells expressing known coreceptors, and in PBMC with coreceptor-specific inhibitors. Infectivity in PBMC was determined by virus titration and p27 antigen production. Early and late products of reverse transcription were measured by PCR with primers specific for the long terminal repeat (LTR), or the gag region, respectively.

RESULTS

Both viruses preferentially infect HOS-CD4 cells expressing CXCR4. Inhibition by CXCR4-specific peptide TW70 and monoclonal antibody 12G5 indicated that both viruses use predominantly CXCR4 to infect macaque and human PBMC. HIV-2/287 showed greater infectivity than HIV-2 EHO in macaque cells, but the situation was reversed in human cells. Kinetic analysis of reverse transcription products revealed no restriction in reverse transcription following HIV-2 EHO infection of macaque PBMC. However, comparison of the level of newly initiated HIV-2 EHO DNA in macaque and human PBMC indicated that there is an early restriction, prior to the initiation of reverse transcription.

CONCLUSIONS

Results indicate that the adaptation of HIV-2 EHO in M. nemestrina to a highly pathogenic virus HIV-2/287 is not correlated with a shift in or an expansion of coreceptor usage, but with the acquisition of an ability to overcome restrictions for growth in macaque PBMC.

Evidence for common structural determinants of human immunodeficiency virus type 1 coreceptor activity provided through functional analysis of CCR5/CXCR4 chimeric coreceptors

Human immunodeficiency virus type 1 (HIV-1) infection in vivo is dependent upon the interaction of the viral envelope glycoprotein gp120 with CC chemokine receptor 5 (CCR5) or CXC chemokine receptor 4 (CXCR4). To study the determinants of the gp120-coreceptor association, we generated a set of chimeric HIV-1 coreceptors which express all possible combinations of the four extracellular domains of CCR5 and CXCR4. Stable U87 astroglioma cell lines expressing CD4 and individual chimeric coreceptor proteins were tested against a variety of R5, X4, and R5X4 envelope glycoproteins and virus strains for their ability to support HIV-1-mediated cell fusion and infection, respectively. Each of the cell lines promoted fusion with cells expressing an HIV envelope glycoprotein, except for U87.CD4.5455, which presents the first extracellular loop (ECL1) and flanking sequences of CXCR4 in the context of CCR5. However, all of the chimeric coreceptors allowed productive infection by one or more of the viral strains tested. Viral phenotype was a predictive factor for the observed activity of the chimeric molecules; X4 and R5X4 HIV strains utilized a majority of the chimeras, while R5 strains were limited in their ability to infect cells expressing these chimeric molecules. The expression of CCR5 ECL2 within the CXCR4 backbone supported infection by an R5 primary isolate, but no chimeras bearing the N terminus of CCR5 exhibited activity with R5 strains. Remarkably, the introduction of any CXCR4 domain into the CCR5 backbone was sufficient to allow utilization by multiple X4 strains. However, critical determinants within ECL2 and/or ECL3 of CXCR4 were apparent for all X4 viruses upon replacement of these domains in CXCR4 with CCR5 sequences. Unexpectedly, chimeric coreceptor-facilitated entry was blocked in all cases by the presence of the CXCR4-specific inhibitor AMD3100. Our data provide proof that CCR5 contains elements that support usage by X4 viral strains and demonstrate that the gp120 interaction sites of CCR5 and CXCR4 are structurally related.

HIV-1 Vpr enhances viral burden by facilitating infection of tissue macrophages but not nondividing CD4+ T cells

Prior experiments in explants of human lymphoid tissue have demonstrated that human immunodeficiency virus type 1 (HIV-1) productively infects diverse cellular targets including T cells and tissue macrophages. We sought to determine the specific contribution of macrophages and T cells to the overall viral burden within lymphoid tissue. To block infection of macrophages selectively while preserving infection of T cells, we used viruses deficient for viral protein R (Vpr) that exhibit profound replication defects in nondividing cells in vitro. We inoculated tonsil histocultures with matched pairs of congenic viruses that differed only by the presence of a wild-type or truncated vpr gene. Although these viruses exhibited no reduction in the infection or depletion of T cells, the ability of the Vpr-deficient R5 virus to infect tissue macrophages was severely impaired compared with matched wild-type R5 virus. Interestingly, the Vpr-deficient R5 virus also exhibited a 50% reduction in overall virus replication compared with its wild-type counterpart despite the fact that macrophages represent a small fraction of the potential targets of HIV-1 infection in these tissues. Collectively, these data highlight the importance of tissue macrophages in local viral burden and further implicate roles for CC chemokine receptor 5, macrophages, and Vpr in the life cycle and pathogenesis of HIV-1.

HIV-1 actively replicates in naive CD4(+) T cells residing within human lymphoid tissues

Although HIV-1 gene expression is detected in naive, resting T cells in vivo, such cells are resistant to productive infection in vitro. However, we found that the endogenous microenvironment of human lymphoid tissues supports de novo infection and depletion of this population. Cell cycle analysis and DNA labeling experiments established that these cells were definitively quiescent and thus infected de novo. Quantitation of the "burst size" within naive cells further demonstrated that these cells were productively infected and contributed to the local viral burden. These findings demonstrate that lymphoid tissues support active HIV-1 replication in resting, naive T cells. Moreover, these cells are not solely reservoirs of latent virus but are permissive hosts for viral replication that likely targets them for elimination.

Cytopathicity of human immunodeficiency virus type 1 primary isolates depends on coreceptor usage and not patient disease status

It has been hypothesized that human immunodeficiency virus type 1 (HIV-1) evolves toward increased cytopathicity in conjunction with disease progression in infected patients. A viral property known to evolve in some but not all patients is coreceptor utilization, and it has been shown that a switch in coreceptor utilization is sufficient for the development of increased cytopathicity. To test the hypothesis that the evolution of other viral properties also contributes to accelerating cytopathicity in vivo, we used human lymphoid tissue explants to assay the cytopathicity of a panel of primary HIV-1 isolates derived from various stages of disease characterized by the presence or absence of changes in coreceptor preference. We found no evidence of coreceptor-independent increases in cytopathicity in isolates obtained either before coreceptor preference changes or from patients who progressed to AIDS despite an absence of coreceptor evolution. Instead, the cytopathicity of all HIV-1 isolates was determined solely by their coreceptor utilization. These results argue that HIV-1 does not evolve toward increased cytopathicity independently of changes in coreceptor utilization.

Susceptibility of rat-derived cells to replication by human immunodeficiency virus type 1

Progress in developing a small animal model of human immunodeficiency virus type 1 (HIV-1) disease would greatly facilitate studies of transmission, pathogenesis, host immune responses, and antiviral strategies. In this study, we have explored the potential of rats as a susceptible host. In a single replication cycle, rat cell lines Rat2 and Nb2 produced infectious virus at levels 10- to 60-fold lower than those produced by human cells. Rat-derived cells supported substantial levels of early HIV-1 gene expression, which was further enhanced by overexpression of human cyclin T1. Rat cells displayed quantitative, qualitative, and cell-type-specific limitations in the late phase of the HIV-1 replication cycle including relative expression levels of HIV-1 Gag proteins, intracellular Gag processing, and viral egress. Nb2 cells were rendered permissive to HIV-1 R5 viruses by coexpression of human CD4 and CCR5, indicating that the major restriction on HIV-1 replication was at the level of cellular entry. We also found that primary rat lymphocytes, macrophages, and microglia expressed considerable levels of early HIV-1 gene products following infection with pseudotyped HIV-1. Importantly, primary rat macrophages and microglia, but not lymphocytes, also expressed substantial levels of HIV-1 p24 CA and produced infectious virions. Collectively, these results identify the rat as a promising candidate for a transgenic small animal model of HIV-1 infection and highlight pertinent cell-type-specific restrictions that are features of this species.

Expression and function of chemokine receptors on human thymocytes: implications for infection by human immunodeficiency virus type 1

The presence or absence of the receptor CD4 and the coreceptors CCR5 and CXCR4 restrict the cell tropism of human immunodeficiency virus type 1 (HIV-1). Despite the importance of thymic infection by HIV-1, conflicting reports regarding the expression of HIV-1 coreceptors on human thymocytes have not been resolved. We assayed the expression and function of the major HIV-1 coreceptors, CCR5 and CXCR4, as well as CCR4 and CCR7 as controls, on human thymocytes. We detected CCR5 on 2.5% of thymocytes, CXCR4 on 53% of the cells, and CCR4 on 16% and CCR7 on 11% of human thymocytes. Moreover, infection by R5 HIV-1 did not significantly induce expression of CCR5. We found that two widely used anti-CCR5 monoclonal antibodies cross-reacted with CCR8, which may account for discrepancies among published reports of CCR5 expression on primary cells. This cross-reactivity could be eliminated by deletion of amino acids 2 through 4 of CCR8. Chemotaxis assays showed that SDF-1, which binds CXCR4; MDC, which binds CCR4; and ELC, which binds CCR7, mediated significant chemotaxis of thymocytes. In contrast, MIP-1beta, whose receptor is CCR5, did not induce significant chemotaxis. Our results indicate that CXCR4, CCR4, CCR7, and their chemokine ligands may be involved in thymocyte migration during development in the thymus. CCR5 and its ligands, however, are likely not involved in these processes. Furthermore, the pattern of CCR5 and CXCR4 expression that we found may explain the greater susceptibility of human thymocytes to infection by HIV-1 isolates capable of using CXCR4 in cell entry compared to those that use only CCR5.

Inhibition of HIV-1 infection by down-regulation of the CXCR4 co-receptor using an intracellular single chain variable fragment against CXCR4

CXCR4 is the major co-receptor used by X4 strains of human immunodeficiency virus type I (HIV-1). In HIV-1-infected patients, the appearance of X4 strains (T cell line-tropic) correlates with disease progression. Since its discovery, the CXCR4 co-receptor has been a major target for different agents which block its function, such as stromal-derived factor 1alpha (SDF-1alpha) and the anti-CXCR4 monoclonal antibody, 12G5. In the present studies, the 12G5 hybridoma was used to construct a single-chain variable antibody fragment (SFv). Murine leukemia virus (MLV) and simian virus 40 (SV(40)) were utilized as delivery vehicles for the anti-CXCR4 SFv. Intracellular expression of the anti-CXCR4 SFv led to down-regulation of this critical co-receptor, as demonstrated by immunostaining. This effect significantly and specifically protected transduced cells from challenge with HIV-1, as measured by HIV-1 p24 antigen expression. Inhibition of HIV-1 replication was specific for X4 HIV-1 strains as demonstrated by MAGI assays. HeLa-CD4/betagal-CCR5 cells expressing the anti-CXCR4 SFv showed significant inhibition of infectivity by the X4 HIV-1 strain NL4-3, but not with the R5 HIV-1 strain Bal. Thus, this anti-HIV-1 molecular therapy has the potential to inhibit HIV-1 replication and virion spread. Targeting CXCR4 by intracellular immunization could be of additional benefit to certain HIV-1-infected patients on highly active antiretroviral therapy (HAART).

Differential coreceptor expression allows for independent evolution of non-syncytium-inducing and syncytium-inducing HIV-1

We demonstrated previously that CD45RA(+) CD4(+) T cells are infected primarily by syncytium-inducing (SI) HIV-1 variants, whereas CD45RO(+) CD4(+) T cells harbor both non-SI (NSI) and SI HIV-1 variants. Here, we studied evolution of tropism for CD45RA(+) and CD45RO(+) CD4(+) cells, coreceptor usage, and molecular phylogeny of coexisting NSI and SI HIV-1 clones that were isolated from four patients in the period spanning SI conversion. NSI variants were CCR5-restricted and could be isolated throughout infection from CD45RO(+) CD4(+) cells. SI variants seemed to evolve in CD45RO(+) CD4(+) cells, but, in time, SI HIV-1 infection of CD45RA(+) CD4(+) cells equaled infection of CD45RO(+) CD4(+) cells. In parallel with this shift, SI HIV-1 variants first used both coreceptors CCR5 and CXCR4, but eventually lost the ability to use CCR5. Phylogenetically, NSI and SI HIV-1 populations diverged over time. We observed a differential expression of HIV-1 coreceptors within CD45RA(+) and CD45RO(+) cells, which allowed us to isolate virus from purified CCR5(+) CXCR4(-) and CCR5(-) CXCR4(+) CD4(+) cells. The CCR5(+) subset was exclusively infected by CCR5-dependent HIV-1 clones, whereas SI clones were preferentially isolated from the CXCR4(+) subset. The differential expression of HIV-1 coreceptors provides distinct cellular niches for NSI and SI HIV-1, contributing to their coexistence and independent evolutionary pathways.

Cytopathicity of human immunodeficiency virus type 2 (HIV-2) in human lymphoid tissue is coreceptor dependent and comparable to that of HIV-1

Epidemiological studies have shown that human immunodeficiency virus type 2 (HIV-2) is markedly less pathogenic than HIV-1 in vivo. Individuals infected with HIV-2 exhibit a remarkably slow rate of disease development, and these clinical properties have been attributed presumptively to an "attenuated" phenotype of HIV-2 itself. Here, we investigated the impact of coreceptor usage on the cytopathicity of HIV-2 and compared its pathogenic potential with that of HIV-1 in a unique human lymphoid histoculture model. We found that HIV-2 strains, as well as closely related simian immunodeficiency viruses (SIV), displayed mildly or highly aggressive cytopathic phenotypes depending on their abilities to use the coreceptor CCR5 or CXCR4, respectively. A side-by-side comparison of primary X4 HIV-1 and HIV-2 strains revealed similar, high degrees of cytopathicity induced by both HIV types. Furthermore, we found that HIV-2 coreceptor specificity for CCR5 and CXCR4 determined the target cell population for T-cell depletion in lymphoid tissue. Finally, utilization of the alternate coreceptors BOB and Bonzo did not significantly increase the cytopathic properties of HIV-2. These findings demonstrate that coreceptor preference is a key regulator of target cell specificity and the cytopathic potential of HIV-2, with indistinguishable rules compared with HIV-1. Moreover, HIV-2 strains are not characterized by an intrinsically lower cytopathicity than HIV-1 strains. Therefore, direct cytopathic potential per se does not explain the unique behavior of HIV-2 in people, highlighting that other unknown factors need to be elucidated as the basis for their lesser virulence in vivo.

Use of inhibitors to evaluate coreceptor usage by simian and simian/human immunodeficiency viruses and human immunodeficiency virus type 2 in primary cells

We have used coreceptor-targeted inhibitors to investigate which coreceptors are used by human immunodeficiency virus type 1 (HIV-1), simian immunodeficiency viruses (SIV), and human immunodeficiency virus type 2 (HIV-2) to enter peripheral blood mononuclear cells (PBMC). The inhibitors are TAK-779, which is specific for CCR5 and CCR2, aminooxypentane-RANTES, which blocks entry via CCR5 and CCR3, and AMD3100, which targets CXCR4. We found that for all the HIV-1 isolates and all but one of the HIV-2 isolates tested, the only relevant coreceptors were CCR5 and CXCR4. However, one HIV-2 isolate replicated in human PBMC even in the presence of TAK-779 and AMD3100, suggesting that it might use an undefined, alternative coreceptor that is expressed in the cells of some individuals. SIV(mac)239 and SIV(mac)251 (from macaques) were also able to use an alternative coreceptor to enter PBMC from some, but not all, human and macaque donors. The replication in human PBMC of SIV(rcm) (from a red-capped mangabey), a virus which uses CCR2 but not CCR5 for entry, was blocked by TAK-779, suggesting that CCR2 is indeed the paramount coreceptor for this virus in primary cells.