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

Co-receptor signaling in the pathogenesis of neuroHIV

The HIV co-receptors, CCR5 and CXCR4, are necessary for HIV entry into target cells, interacting with the HIV envelope protein, gp120, to initiate several signaling cascades thought to be important to the entry process. Co-receptor signaling may also promote the development of neuroHIV by contributing to both persistent neuroinflammation and indirect neurotoxicity. But despite the critical importance of CXCR4 and CCR5 signaling to HIV pathogenesis, there is only one therapeutic (the CCR5 inhibitor Maraviroc) that targets these receptors. Moreover, our understanding of co-receptor signaling in the specific context of neuroHIV is relatively poor. Research into co-receptor signaling has largely stalled in the past decade, possibly owing to the complexity of the signaling cascades and functions mediated by these receptors. Examining the many signaling pathways triggered by co-receptor activation has been challenging due to the lack of specific molecular tools targeting many of the proteins involved in these pathways and the wide array of model systems used across these experiments. Studies examining the impact of co-receptor signaling on HIV neuropathogenesis often show activation of multiple overlapping pathways by similar stimuli, leading to contradictory data on the effects of co-receptor activation. To address this, we will broadly review HIV infection and neuropathogenesis, examine different co-receptor mediated signaling pathways and functions, then discuss the HIV mediated signaling and the differences between activation induced by HIV and cognate ligands. We will assess the specific effects of co-receptor activation on neuropathogenesis, focusing on neuroinflammation. We will also explore how the use of substances of abuse, which are highly prevalent in people living with HIV, can exacerbate the neuropathogenic effects of co-receptor signaling. Finally, we will discuss the current state of therapeutics targeting co-receptors, highlighting challenges the field has faced and areas in which research into co-receptor signaling would yield the most therapeutic benefit in the context of HIV infection. This discussion will provide a comprehensive overview of what is known and what remains to be explored in regard to co-receptor signaling and HIV infection, and will emphasize the potential value of HIV co-receptors as a target for future therapeutic development.

Cell Proteins Interacting with the Human Immunodeficiency Virus in Immunoblotting can be Detected by R5- or X4- Tropic Human Immunodeficiency Virus Particles

Introduction:

The present study reported a new immunoblot assay, with revelation by R5- or X4-whole free human immunodeficiency virus (HIV) particles or recombinant gp160.

Materials and Methods:

The assay was optimized to identify cell proteins interacting with HIV. Whole cell lysates were prepared from peripheral blood lymphocytes (PBLs), dendritic cells (DC), monocyte-derived macrophage (MDM), and Henrietta Lacks (Hela, wild-type or transfected with DC-specific intracellular adhesion molecule-3-Grabbing Non-Integrin, HeLa) and Human endometrial cells (HEC-1A) lines; HIV particles used were the R5-tropic HIV-1_JRCSF and the X4-tropic HIV-1_NDK.

Results:

Experiments with PBL lysates and both viruses demonstrated different bands, including a unique band at 105–117 kDa in addition to nonspecific bands. The 105–117 kDa band migrated at the same level of that observed in controls using total PBL lysate and anti-CD4 mAb for detection and thus likely corresponds to the cluster difference (CD) 4 complex. Blots using lysates of DCs, MDM, HeLa cell line, and HEC-1A cell line allowed identifying several bands that positions were similar to that seen by recombinant gp160 or whole R5- or X4-HIV particles.

Conclusion:

Blot of whole lysates of various HIV target cells is recognized by free HIV particles and allows identifying a wide range of HIV-interacting cell proteins. Such optimized assay could be useful to recognize new cellular HIV attachment proteins.

Immune Control of HIV

The human immunodeficiency virus (HIV) infection of the immune cells expressing the cluster of differentiation 4 cell surface glycoprotein (CD4⁺ cells) causes progressive decline of the immune system and leads to the acquired immunodeficiency syndrome (AIDS). The ongoing global HIV/AIDS pandemic has already claimed over 35 million lives. Even after 37 years into the epidemic, neither a cure is available for the 37 million people living with HIV (PLHIV) nor is a vaccine discovered to avert the millions of new HIV infections that continue to occur each year. If left untreated, HIV infection typically progresses to AIDS and, ultimately, causes death in a majority of PLHIV. The recommended combination antiretroviral therapy (cART) suppresses virus replication and viremia, prevents or delays progression to AIDS, reduces transmission rates, and lowers HIV-associated mortality and morbidity. However, because cART does not eliminate HIV, and an enduring pool of infected resting memory CD4⁺ T cells (latent HIV reservoir) is established early on, any interruption to cART leads to a relapse of viremia and disease progression. Hence, strict adherence to a life-long cART regimen is mandatory for managing HIV infection in PLHIV. The HIV-1-specific cytotoxic T cells expressing the CD8 glycoprotein (CD8⁺ CTL) limit the virus replication in vivo by recognizing the viral antigens presented by human leukocyte antigen (HLA) class I molecules on the infected cell surface and killing those cells. Nevertheless, CTLs fail to durably control HIV-1 replication and disease progression in the absence of cART. Intriguingly, <1% of cART-naive HIV-infected individuals called elite controllers/HIV controllers (HCs) exhibit the core features that define a HIV-1 "functional cure" outcome in the absence of cART: durable viral suppression to below the limit of detection, long-term non-progression to AIDS, and absence of viral transmission. Robust HIV-1-specific CTL responses and prevalence of protective HLA alleles associated with enduring HIV-1 control have been linked to the HC phenotype. An understanding of the molecular mechanisms underlying the CTL-mediated suppression of HIV-1 replication and disease progression in HCs carrying specific protective HLA alleles may yield promising insights towards advancing the research on HIV cure and prophylactic HIV vaccine.

Transmission, Evolution, and Endogenization: Lessons Learned from Recent Retroviral Invasions

Viruses of the subfamily Orthoretrovirinae are defined by the ability to reverse transcribe an RNA genome into DNA that integrates into the host cell genome during the intracellular virus life cycle. Exogenous retroviruses (XRVs) are horizontally transmitted between host individuals, with disease outcome depending on interactions between the retrovirus and the host organism. When retroviruses infect germ line cells of the host, they may become endogenous retroviruses (ERVs), which are permanent elements in the host germ line that are subject to vertical transmission. These ERVs sometimes remain infectious and can themselves give rise to XRVs. This review integrates recent developments in the phylogenetic classification of retroviruses and the identification of retroviral receptors to elucidate the origins and evolution of XRVs and ERVs. We consider whether ERVs may recurrently pressure XRVs to shift receptor usage to sidestep ERV interference. We discuss how related retroviruses undergo alternative fates in different host lineages after endogenization, with koala retrovirus (KoRV) receiving notable interest as a recent invader of its host germ line. KoRV is heritable but also infectious, which provides insights into the early stages of germ line invasions as well as XRV generation from ERVs. The relationship of KoRV to primate and other retroviruses is placed in the context of host biogeography and the potential role of bats and rodents as vectors for interspecies viral transmission. Combining studies of extant XRVs and "fossil" endogenous retroviruses in koalas and other Australasian species has broadened our understanding of the evolution of retroviruses and host-retrovirus interactions.

Hyaluronic Acid Conjugated Magnetic Prussian Blue@Quantum Dot Nanoparticles for Cancer Theranostics

A multifunctional nanotheranostic agent was developed by conjugating both hyaluronic acid and bovine serum albumin coated CuInS₂-ZnS quantum dots onto the surface of magnetic Prussian blue nanoparticles. The obtained nanoagent could serve as an efficient contrast agent to simultaneously enhance near infrared (NIR) fluorescence and magnetic resonance (MR) imaging greatly. The coexistence of magnetic core and CD44 ligand hyaluronic acid was found to largely improve the specific uptake of the nanoagent by CD44 overexpressed HeLa cells upon applying an external magnetic field. Both NIR fluorescence and MR imaging in vivo proved high accumulation of the nanoagent at tumor site due to its excellent CD44 receptor/magnetic dual targeting capability. After intravenous injection of the nanoagent and treatment of external magnetic field, the tumor in nude mice was efficiently ablated upon NIR laser irradiation and the tumor growth inhibition was more than 89.95%. Such nanotheranostic agent is of crucial importance for accurately identifying the size and location of the tumor before therapy, monitoring the photothermal treatment procedure in real-time during therapy, assessing the effectiveness after therapy.

A Designed "Nested" Dimer of Cyanovirin-N Increases Antiviral Activity

Cyanovirin-N (CV-N) is an antiviral lectin with potent activity against enveloped viruses, including HIV. The mechanism of action involves high affinity binding to mannose-rich glycans that decorate the surface of enveloped viruses. In the case of HIV, antiviral activity of CV-N is postulated to require multivalent interactions with envelope protein gp120, achieved through a pseudo-repeat of sequence that adopts two near-identical glycan-binding sites, and possibly involves a 3D-domain-swapped dimeric form of CV-N. Here, we present a covalent dimer of CV-N that increases the number of active glycan-binding sites, and we characterize its ability to recognize four glycans in solution. A CV-N variant was designed in which two native repeats were separated by the “nested” covalent insertion of two additional repeats of CV-N, resulting in four possible glycan-binding sites. The resulting Nested CV-N folds into a wild-type-like structure as assessed by circular dichroism and NMR spectroscopy, and displays high thermal stability with a T_m of 59 °C, identical to WT. All four glycan-binding domains encompassed by the sequence are functional as demonstrated by isothermal titration calorimetry, which revealed two sets of binding events to dimannose with dissociation constants K_d of 25 μM and 900 μM, assigned to domains B and B’ and domains A and A’ respectively. Nested CV-N displays a slight increase in activity when compared to WT CV-N in both an anti-HIV cellular assay and a fusion assay. This construct conserves the original binding specifityies of domain A and B, thus indicating correct fold of the two CV-N repeats. Thus, rational design can be used to increase multivalency in antiviral lectins in a controlled manner.

Genetic, transcriptomic, and epigenetic studies of HIV-associated neurocognitive disorder

The Human Genome Project, coupled with rapidly evolving high-throughput technologies, has opened the possibility of identifying heretofore unknown biological processes underlying human disease. Because of the opaque nature of HIV-associated neurocognitive disorder (HAND) neuropathogenesis, the utility of such methods has gained notice among NeuroAIDS researchers. Furthermore, the merging of genetics with other research areas has also allowed for application of relatively nascent fields, such as neuroimaging genomics, and pharmacogenetics, to the context of HAND. In this review, we detail the development of genetic, transcriptomic, and epigenetic studies of HAND, beginning with early candidate gene association studies and culminating in current "omics" approaches that incorporate methods from systems biology to interpret data from multiple levels of biological functioning. Challenges with this line of investigation are discussed, including the difficulty of defining a valid phenotype for HAND. We propose that leveraging known associations between biology and pathology across multiple levels will lead to a more reliable and valid phenotype. We also discuss the difficulties of interpreting the massive and multitiered mountains of data produced by current high-throughput omics assays and explore the utility of systems biology approaches in this regard.

Genetic diversity of the highly variable V1 region interferes with Human Immunodeficiency Virus type 1 envelope functionality

Background

The HIV envelope (Env) promotes viral entry in the host cell. During this process, Env undergoes several conformational changes to ensure its function. At the same time, the gp120 component of Env is the protein of the virus presenting the largest genetic diversity. Understanding how the virus maintains the balance between the competing requirements for maintenance of functionality and antigenic variation of this protein is central for the comprehension of its strategies of evolution and can highlight vulnerable aspects of its replication cycle. We focused on the variable domains V1 and V2 of the HIV-1 gp120 that are involved in conformational changes and are critical for viral escape from antibody neutralization.

Results

Despite the extensive sequence diversity found in the epidemic for these regions and their location on the external face of the protein, we observed that replacing V1V2 of one primary isolate with that of another severely interferes with Env functionality in more than half of the cases studied. Similar results were obtained for intra- and intersubtype chimeras. These observations are indicative of an interference of genetic diversity in these regions with Env functionality. Therefore, despite the extensive sequence diversity that characterizes these regions in the epidemic, our results show that functional constraints seem to limit their genetic variation. Defects in the V1V2 chimeras were not relieved by the insertion of the V3 region from the same isolate, suggesting that the decrease in functionality is not due to perturbation of potential coevolution networks between V1V2 and V3. Within the V1V2 domain, the sequence of the hypervariable loop of the V1 domain seems to be crucial for the functionality of the protein.

Conclusions

Besides the well-documented role of V1V2 in the interplay with the immune response, this work shows that V1 is also involved in the selection of functional envelopes. By documenting a compromise between the opposing forces of sequence diversification and retention of functionality, these observations improve our understanding of the evolutionary trajectories of the HIV-1 envelope gene.

Effects of replicative fitness on competing HIV strains

We develop an n-strain model to show the effects of replicative fitness of competing viral strains exerting selective density-dependant infective pressure on each other. A two strain model is used to illustrate the results. A perturbation technique and numerical simulations were used to establish the existence and stability of steady states. More than one infected steady states governed by the replicative fitness resulted from the model exhibiting either strain replacement or co-infection. We found that the presence of two or more HIV strains could result in a disease-free state that, in general, is not globally stable.

Engineered lentiviral vectors pseudotyped with a CD4 receptor and a fusogenic protein can target cells expressing HIV-1 envelope proteins

Lentiviral vectors (LVs) derived from human immunodeficiency virus type 1 (HIV-1) are promising vehicles for gene delivery because they not only efficiently transduce both dividing and non-dividing cells, but also maintain long-term transgene expression. Development of an LV system capable of transducing cells in a cell type-specific manner can be beneficial for certain applications that rely on targeted gene delivery. Previously it was shown that an inverse fusion strategy that incorporated an HIV-1 receptor (CD4) and its co-receptor (CXCR4 or CCR5) onto vector surfaces could confer to LVs the ability to selectively deliver genes to HIV-1 envelope-expressing cells. To build upon this work, we aim to improve its relatively low transduction efficiency and circumvent its inability to target multiple tropisms of HIV-1 by a single vector. We investigated a method to create LVs co-enveloped with the HIV-1 cellular receptor CD4 and a fusogenic protein derived from the Sindbis virus glycoprotein and tested its efficiency to selectively deliver genes into cells expressing HIV-1 envelope proteins. The engineered LV system yields a higher level of transduction efficiency and a broader tropism towards cells displaying the HIV-1 envelope protein (Env) than the previously developed system. Furthermore, we demonstrated in vitro that this engineered LV can preferentially deliver suicide gene therapy to HIV-1 envelope-expressing cells. We conclude that it is potentially feasible to target LVs towards HIV-1-infected cells by functional co-incorporation of the CD4 and fusogenic proteins, and provide preliminary evidence for further investigation on a potential alternative treatment for eradicating HIV-1-infected cells that produce drug-resistant viruses after highly active antiretroviral therapy (HAART).

Pillars article: HIV-1 entry cofactor: functional cDNA cloning of a seven-transmembrane, G protein-coupled receptor. Science. 1996. 272: 872-877

A cofactor for HIV-1 (human immunodeficiency virus-type 1) fusion and entry was identified with the use of a novel functional complementary DNA (cDNA) cloning strategy. This protein, designated “fusin,” is a putative G protein–coupled receptor with seven transmembrane segments. Recombinant fusin enabled CD4-expressing nonhuman cell types to support HIV-1 Env-mediated cell fusion and HIV-1 infection. Antibodies to fusin blocked cell fusion and infection with normal CD4-positive human target cells. Fusin messenger RNA levels correlated with HIV-1 permissiveness in diverse human cell types. Fusin acted preferentially for T cell line–tropic isolates, in comparison to its activity with macrophage-tropic HIV-1 isolates.

Selective transmission of R5 HIV-1 variants: where is the gatekeeper?

To enter target cells HIV-1 uses CD4 and a coreceptor. In vivo the coreceptor function is provided either by CCR5 (for R5) or CXCR4 (for X4 HIV-1). Although both R5 and X4 HIV-1 variants are present in body fluids (semen, blood, cervicovaginal and rectal secretions), R5 HIV-1 appears to transmit infection and dominates early stages of HIV disease. Moreover, recent sequence analysis of virus in acute infection shows that, in the majority of cases of transmission, infection is initiated by a single virus. Therefore, the existence of a "gatekeeper" that selects R5 over X4 HIV-1 and that operates among R5 HIV-1 variants has been suggested. In the present review we consider various routes of HIV-transmission and discuss potential gatekeeping mechanisms associated with each of these routes. Although many mechanisms have been identified none of them explains the almost perfect selection of R5 over X4 in HIV-1 transmission. We suggest that instead of one strong gatekeeper there are multiple functional gatekeepers and that their superimposition is sufficient to protect against X4 HIV-1 infection and potentially select among R5 HIV-1 variants. In conclusion, we propose that the principle of multiple barriers is more general and not restricted to protection against X4 HIV-1 but rather can be applied to other phenomena when one factor has a selective advantage over the other(s). In the case of gatekeepers for HIV-1 transmission, the task is to identify them and to decipher their molecular mechanisms. Knowledge of the gatekeepers' localization and function may enable us to enhance existing barriers against R5 transmission and to erect the new ones against all HIV-1 variants.

Pharmacotherapy of HIV-1 Infection: Focus on CCR5 Antagonist Maraviroc

Sustained inhibition of HIV-1, the goal of antiretroviral therapy, is often impeded by the emergence of viral drug resistance. For patients infected with HIV-1 resistant to conventional drugs from the viral reverse transcriptase and protease inhibitor classes, the recently approved entry and integration inhibitors effectively suppress HIV-1 and offer additional therapeutic options. Entry inhibitors are particularly attractive because, unlike conventional antiretrovirals, they target HIV-1 extracellularly, thereby sparing cells from both viral- and drug-induced toxicities. The fusion inhibitor enfuvirtide and the CCR5 antagonist maraviroc are the first entry inhibitors licensed for patients with drug-resistant HIV-1, with maraviroc restricted to those infected with CCR5-tropic HIV-1 (R5 HIV-1) only. Vicriviroc (another CCR5 antagonist) is in Phase III clinical trials, whereas the CCR5 antibodies PRO 140 and HGS 004 are in early stages of clinical development. Potent antiviral synergy between maraviroc and CCR5 antibodies, coupled with distinct patterns of resistance, suggest their combinations might be particularly effective in patients. In addition, given that oral administration of maraviroc achieves high drug levels in cervicovaginal fluid, combinations of maraviroc and other CCR5 inhibitors could be effective in preventing HIV-1 transmission. Moreover, since CCR5 antagonists prevent rejection of transplanted organs, maraviroc could both suppress HIV-1 and prolong organ survival for the growing number of HIV-1 patients with kidney or liver failure necessitating organ transplantation. Thus, maraviroc offers an important treatment option for patients with drug-resistant R5 HIV-1, who presently account for >50% of drug-resistance cases.

Functional and genetic analysis of coreceptor usage by dualtropic HIV-1 subtype C isolates

It is widely documented that a complete switch from the predominant CCR5 (R5) to CXCR4 (X4) phenotype is less common for HIV-1 subtype C (HIV-1C) compared to other major subtypes. We investigated whether dualtropic HIV-1C isolates represented dualtropic, mixed R5 and X4 clones or both. Thirty of 35 functional HIV-1 env clones generated by bulk PCR amplification from peripheral blood mononuclear cells (PBMCs) infected with seven dualtropic HIV-1C isolates utilized CXCR4 exclusively. Five of 35 clones displayed dualtropism. Endpoint dilution of one isolate did not yield a substantial proportion of R5-monotropic env clones. Sequence-based predictive algorithms showed that env sequences from PBMCs, CXCR4 or CCR5-expressing cell lines were indistinguishable and all possessed X4/dualtropic characteristics. We describe HIV-1C CXCR4-tropic env sequence features. Our results suggest a dramatic loss of CCR5 monotropism as dualtropism emerges in HIV-1C which has important implications for the use of coreceptor antagonists in therapeutic strategies for this subtype.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

Virus entry via the alternative coreceptors CCR3 and FPRL1 differs by human immunodeficiency virus type 1 subtype

Human immunodeficiency virus type 1 (HIV-1) infects target cells by binding to CD4 and a chemokine receptor, most commonly CCR5. CXCR4 is a frequent alternative coreceptor (CoR) in subtype B and D HIV-1 infection, but the importance of many other alternative CoRs remains elusive. We have analyzed HIV-1 envelope (Env) proteins from 66 individuals infected with the major subtypes of HIV-1 to determine if virus entry into highly permissive NP-2 cell lines expressing most known alternative CoRs differed by HIV-1 subtype. We also performed linear regression analysis to determine if virus entry via the major CoR CCR5 correlated with use of any alternative CoR and if this correlation differed by subtype. Virus pseudotyped with subtype B Env showed robust entry via CCR3 that was highly correlated with CCR5 entry efficiency. By contrast, viruses pseudotyped with subtype A and C Env proteins were able to use the recently described alternative CoR FPRL1 more efficiently than CCR3, and use of FPRL1 was correlated with CCR5 entry. Subtype D Env was unable to use either CCR3 or FPRL1 efficiently, a unique pattern of alternative CoR use. These results suggest that each subtype of circulating HIV-1 may be subject to somewhat different selective pressures for Env-mediated entry into target cells and suggest that CCR3 may be used as a surrogate CoR by subtype B while FPRL1 may be used as a surrogate CoR by subtypes A and C. These data may provide insight into development of resistance to CCR5-targeted entry inhibitors and alternative entry pathways for each HIV-1 subtype.

HIV-1 Transmission, Replication Fitness and Disease Progression

Upon transmission, human immunodeficiency virus type 1 (HIV-1) establishes infection of the lymphatic reservoir, leading to profound depletion of the memory CD4(+) T cell population, despite the induction of the adaptive immune response. The rapid evolution and association of viral variants having distinct characteristics with different stages of infection, the level of viral burden, and rate of disease progression suggest a role for viral variants in this process. Here, we review the literature on HIV-1 variants and disease and discuss the importance of viral fitness for transmission and disease.

HIV-1 Transmission, Replication Fitness and Disease Progression

Upon transmission, human immunodeficiency virus type 1 (HIV-1) establishes infection of the lymphatic reservoir, leading to profound depletion of the memory CD4+ T cell population despite the induction of the adaptive immune response. The rapid evolution and association of viral variants having distinct characteristics during different stages of infection, the level of viral burden, and rate of disease progression suggest a role for viral variants in this process. Here, we review the literature on HIV-1 variants and disease and discuss the importance of viral fitness for transmission and disease.

Vaccinia-based reporter gene cell-fusion assays to quantitate functional interactions of HIV envelope glycoprotein with receptors

This unit describes quantitation of functional interactions between HIV envelope glucoprotein and target cell receptors, using assay of cell fusion-dependent reporter gene activation. The method is particularly useful since it isolates the fusion reaction from the rest of the HIV replication cycle, and obviates the need for infectious HIV particles. Reporter Gene Cell Fusion Assays to Quantitate Functional Interactions of HIV Envelope Glycoprotein with Receptors.

Monitoring early fusion dynamics of human immunodeficiency virus type 1 at single-molecule resolution

The fusion of human immunodeficiency virus type 1 (HIV-1) to host cells is a dynamic process governed by the interaction between glycoproteins on the viral envelope and the major receptor, CD4, and coreceptor on the surface of the cell. How these receptors organize at the virion-cell interface to promote a fusion-competent site is not well understood. Using single-molecule force spectroscopy, we map the tensile strengths, lifetimes, and energy barriers of individual intermolecular bonds between CCR5-tropic HIV-1 gp120 and its receptors CD4 and CCR5 or CXCR4 as a function of the interaction time with the cell. According to the Bell model, at short times of contact between cell and virion, the gp120-CD4 bond is able to withstand forces up to 35 pN and has an initial lifetime of 0.27 s and an intermolecular length of interaction of 0.34 nm. The initial bond also has an energy barrier of 6.7 k(B)T (where k(B) is Boltzmann's constant and T is absolute temperature). However, within 0.3 s, individual gp120-CD4 bonds undergo rapid destabilization accompanied by a shortened lifetime and a lowered tensile strength. This destabilization is significantly enhanced by the coreceptor CCR5, not by CXCR4 or fusion inhibitors, which suggests that it is directly related to a conformational change in the gp120-CD4 bond. These measurements highlight the instability and low tensile strength of gp120-receptor bonds, uncover a synergistic role for CCR5 in the progression of the gp120-CD4 bond, and suggest that the cell-virus adhesion complex is functionally arranged about a long-lived gp120-coreceptor bond.

CCR5Delta32 59537-G/A promoter polymorphism is associated with low translational efficiency and the loss of CCR5Delta32 protective effects

We have recently demonstrated that the CCR5Delta32 protein interacts with CCR5 and CXCR4 and down-modulates their cell surface expression. We have also reported the absence of detectable expression of the truncated CCR5Delta32 protein in four out of six human immunodeficiency virus-infected (HIV(+)) CCR5(-/-) individuals. To explain the defect in protein expression in these samples, we cloned and sequenced the promoter regions of the six HIV(+) individuals. We have identified several polymorphisms in the CCR5Delta32 promoter region, but these polymorphisms were not associated with significant differences in mRNA levels. Coupled in vitro transcription/translation and polyribosome analysis demonstrated a strong association between a variant genotype designated CCR5Delta32 59537-A/A and a low translation efficiency. Protein analysis indicated that the peripheral blood mononuclear cells from two of the HIV(+) CCR5(-/-) individuals carrying the CCR5Delta32 59537-A/A variant expressed trace amounts of CCR5Delta32 protein compared to the individuals carrying the CCR5Delta32 59537-G/G genotype. The results imply that the absence of CCR5Delta32 protein in two HIV(+) individuals is due to a genetic defect in the translation of the protein. Together, these results highlight the importance of the CCR5Delta32 protein as an HIV suppressive factor and provide further insight into the mechanism of the protective effect of the CCR5Delta32 mutation.

CCR5Delta32 protein expression and stability are critical for resistance to human immunodeficiency virus type 1 in vivo

Human immunodeficiency virus type 1 (HIV-1) infection of individuals carrying the two alleles of the CCR5Delta32 mutation (CCR5(-/-)) has rarely been reported, but how the virus overcomes the CCR5Delta32 protective effect in these cases has not been delineated. We have investigated this in 6 infected (HIV(+)) and 25 HIV(-) CCR5(-/-) individuals. CD4(+) T lymphocytes isolated from HIV(-) CCR5(-/-) peripheral blood mononuclear cells (PBMCs) showed lower levels of CXCR4 expression that correlated with lower X4 Env-mediated fusion. Endogenous CCR5Delta32 protein was detected in all HIV(-) CCR5(-/-) PBMC samples (n = 25) but not in four of six unrelated HIV(+) CCR5(-/-) PBMC samples. Low levels were detected in another two HIV(+) CCR5(-/-) PBMC samples. The expression of adenovirus 5 (Ad5)-encoded CCR5Delta32 protein restored the protective effect in PBMCs from three HIV(+) CCR5(-/-) individuals but failed to restore the protective effect in PBMCs isolated from another three HIV(+) CCR5(-/-) individuals. In the latter samples, pulse-chase analyses demonstrated the disappearance of endogenous Ad5-encoded CCR5Delta32 protein and the accumulation of Ad5-encoded CCR5 during the chase periods. PBMCs isolated from CCR5(-/-) individuals showed resistance to primary X4 but were readily infected by a lab-adapted X4 strain. Low levels of Ad5-encoded CCR5Delta32 protein conferred resistance to primary X4 but not to lab-adapted X4 virus. These data provide strong support for the hypothesis that the CCR5Delta32 protein actively confers resistance to HIV-1 in vivo and suggest that the loss or reduction of CCR5Delta32 protein expression may account for HIV-1 infection of CCR5(-/-) individuals. The results also suggest that other cellular or virally induced factors may be involved in the stability of CCR5Delta32 protein.

Distinct requirements for signal peptidase processing and function in the stable signal peptide subunit of the Junín virus envelope glycoprotein

The arenavirus envelope glycoprotein (GP-C) retains a cleaved and stable signal peptide (SSP) as an essential subunit of the mature complex. This 58-amino-acid residue peptide serves as a signal sequence and is additionally required to enable transit of the assembled GP-C complex to the Golgi, and for pH-dependent membrane fusion activity. We have investigated the C-terminal region of the Junín virus SSP to study the role of the cellular signal peptidase (SPase) in generating SSP. Site-directed mutagenesis at the cleavage site (positions -1 and -3) reveals a pattern of side-chain preferences consistent with those of SPase. Although position -2 is degenerate for SPase cleavage, this residue in the arenavirus SSP is invariably a cysteine. In the Junín virus, this cysteine is not involved in disulfide bonding. We show that replacement with alanine or serine is tolerated for SPase cleavage but prevents the mutant SSP from associating with GP-C and enabling transport to the cell surface. Conversely, an arginine mutation at position -1 that prevents SPase cleavage is fully compatible with GP-C-mediated membrane fusion activity when the mutant SSP is provided in trans. These results point to distinct roles of SSP sequences in SPase cleavage and GP-C biogenesis. Further studies of the unique structural organization of the GP-C complex will be important in identifying novel opportunities for antiviral intervention against arenaviral hemorrhagic disease.

Inhibition of human immunodeficiency virus envelope glycoprotein- mediated single cell lysis by low-molecular-weight antagonists of viral entry

The coexpression of human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins and receptors leads to the lysis of single cells by a process that is dependent upon membrane fusion. This cell lysis was inhibited by low-molecular-weight compounds that interfere with receptor binding or with receptor-induced conformational transitions in the envelope glycoproteins. A peptide, T20, potently inhibited cell-cell fusion but had no effect on single cell lysis mediated by the HIV-1 envelope glycoproteins. Thus, critical events in the lysis of single cells by the HIV-1 envelope glycoproteins occur in intracellular compartments accessible only to small inhibitory compounds.

GLUT-1-independent infection of the glioblastoma/astroglioma U87 cells by the human T cell leukemia virus type 1

The human glucose transporter protein 1 (GLUT-1) functions as a receptor for human T cell leukemia virus (HTLV). GLUT-1 is a twelve-transmembrane cell surface receptor with six extracellular (ECL) and seven intracellular domains. To analyze HTLV-1 cytotropism, we utilized polyclonal antibodies to a synthetic peptide corresponding to the large extracellular domain of GLUT-1. The antibodies caused significant blocking of envelope (Env)-mediated fusion and pseudotyped virus infection of HeLa cells but had no significant effect on infection of U87 cells. This differential effect correlated with the detection of high-level surface expression of GLUT-1 on HeLa cells and very weak staining of U87 cells. To investigate this in terms of viral cytotropism, we cloned GLUT-1 cDNA from U87 cells and isolated two different versions of cDNA clones: the wild-type sequence (encoding 492 residues) and a mutant cDNA with a 5-base pair deletion (GLUT-1Delta5) between nucleotides 1329 and 1333. The deletion, also detected in genomic DNA, resulted in a frame-shift and premature termination producing a truncated protein of 463 residues. Transfection of the wild-type GLUT-1 but not GLUT-1Delta5 cDNA into CHO cells resulted in efficient surface expression of the human GLUT-1. Co-expression of GLUT-1 with GLUT-1Delta5 produces a trans-inhibition by GLUT-1Delta5 of GLUT-1-mediated HTLV-1 envelope (Env)-mediated fusion. Co-immunoprecipitation experiments demonstrated physical interaction of the wild-type and mutant proteins. Northern blot and RT-PCR analyses demonstrated lower GLUT-1 RNA expression in U87 cells. We propose two mechanisms to account for the impaired cell surface expression of GLUT-1 on U87 cells: low GLUT-1 RNA expression and the formation of GLUT-1/GLUT-1Delta5 heterodimers that are retained intracellularly. Significant RNAi-mediated reduction of endogenous GLUT-1 expression impaired HTLV-1 Env-mediated fusion with HeLa cells but not with U87 cells. We propose a GLUT-1-independent mechanism of HTLV-1 infection of U87 cells. The results may have important implications for HTLV-1 neurotropism and pathogenesis.

Infection of CD4+ T lymphocytes by the human T cell leukemia virus type 1 is mediated by the glucose transporter GLUT-1: Evidence using antibodies specific to the receptor's large extracellular domain

To analyze HTLV-1 cytotropism, we developed a highly sensitive vaccinia virus-based assay measuring activation of a reporter gene upon fusion of two distinct cell populations. We used this system in a functional cDNA screening to isolate and confirm that the glucose transporter protein 1 (GLUT-1) is a receptor for HTLV-1. GLUT-1 is a ubiquitously expressed plasma membrane glycoprotein with 12 transmembrane domains and 6 extracellular loops (ECL). We demonstrate for the first time that peptide antibodies (GLUT-IgY) raised in chicken to the large extracellular loop (ECL1) detect GLUT-1 at the cell surface and inhibit envelope (Env)-mediated fusion and infection. Efficient GLUT-IgY staining was detected with peripheral blood CD4(+) lymphocytes purified by positive selection. Further, GLUT-IgY caused efficient inhibition of Env-mediated fusion and infection of CD4(+) T and significantly lower inhibition of CD8(+) T lymphocytes. The specificity of GLUT-IgY antibodies to GLUT-1 was demonstrated by ECL1 peptide competition studies. Grafting ECL1 of GLUT-1 onto the receptor-negative GLUT-3 conferred significant receptor activity. In contrast, grafting ECL1 of GLUT-3 onto GLUT-1 resulted in a significant loss of the receptor activity. The ECL1-mediated receptor activity was efficiently blocked with four different human monoclonal antibody (HMab) to HTLV-1 Env. The ECL1-derived peptide blocked HTLV-1 Env-mediated fusion with several nonhuman mammalian cell lines. The results demonstrate the utilization of cell surface GLUT-1 in HTLV-1 infection of CD4(+) T lymphocytes and implicate a critical role for the ECL1 region in viral tropism.

Furin cleavage of the SARS coronavirus spike glycoprotein enhances cell-cell fusion but does not affect virion entry

The fusogenic potential of Class I viral envelope glycoproteins is activated by proteloytic cleavage of the precursor glycoprotein to generate the mature receptor-binding and transmembrane fusion subunits. Although the coronavirus (CoV) S glycoproteins share membership in this class of envelope glycoproteins, cleavage to generate the respective S1 and S2 subunits appears absent in a subset of CoV species, including that responsible for the severe acute respiratory syndrome (SARS). To determine whether proteolytic cleavage of the S glycoprotein might be important for the newly emerged SARS-CoV, we introduced a furin recognition site at single basic residues within the putative S1–S2 junctional region. We show that furin cleavage at the modified R667 position generates discrete S1 and S2 subunits and potentiates membrane fusion activity. This effect on the cell–cell fusion activity by the S glycoprotein is not, however, reflected in the infectivity of pseudotyped lentiviruses bearing the cleaved glycoprotein. The lack of effect of furin cleavage on virion infectivity mirrors that observed in the normally cleaved S glycoprotein of the murine coronavirus and highlights an additional level of complexity in coronavirus entry.

R5 and X4 HIV envelopes induce distinct gene expression profiles in primary peripheral blood mononuclear cells

HIV envelope binds to and signals through its primary cellular receptor, CD4, and through a coreceptor, either CC chemokine receptor 5 (CCR5) or CXC chemokine receptor 4 (CXCR4). Here, we evaluate the response of peripheral blood mononuclear cells to a panel of genetically diverse R5 and X4 envelope proteins. Modulation of gene expression was evaluated by using oligonucleotide microarrays. Activation of transcription factors was evaluated by using an array of oligonucleotides encoding transcription factor binding sites. Responses were strongly influenced by coreceptor specificity. Treatment of cells from CCR5delta32 homozygous donors with glycoprotein (gp)120 derived from an R5 virus demonstrated that the majority of responses elicited by R5 envelopes required engagement of CCR5. R5 envelopes, to a greater extent than X4 envelopes, induced the expression of genes belonging to mitogen-activated protein kinase signal transduction pathways and genes regulating the cell cycle. A number of genes induced by R5, but not X4, envelopes were also up-regulated in the resting CD4+ T cell population of HIV-infected individuals. These results suggest that R5 envelope facilitates replication of HIV in the pool of resting CD4+ T cells. Additionally, signaling by R5 gp120 may facilitate the transmission of R5 viruses by inducing a permissive environment for HIV replication.

Human immunodeficiency virus type 1 coreceptor switching: V1/V2 gain-of-fitness mutations compensate for V3 loss-of-fitness mutations

Human immunodeficiency virus type 1 (HIV-1) entry into target cells is mediated by the virus envelope binding to CD4 and the conformationally altered envelope subsequently binding to one of two chemokine receptors. HIV-1 envelope glycoprotein (gp120) has five variable loops, of which three (V1/V2 and V3) influence the binding of either CCR5 or CXCR4, the two primary coreceptors for virus entry. Minimal sequence changes in V3 are sufficient for changing coreceptor use from CCR5 to CXCR4 in some HIV-1 isolates, but more commonly additional mutations in V1/V2 are observed during coreceptor switching. We have modeled coreceptor switching by introducing most possible combinations of mutations in the variable loops that distinguish a previously identified group of CCR5- and CXCR4-using viruses. We found that V3 mutations entail high risk, ranging from major loss of entry fitness to lethality. Mutations in or near V1/V2 were able to compensate for the deleterious V3 mutations and may need to precede V3 mutations to permit virus survival. V1/V2 mutations in the absence of V3 mutations often increased the capacity of virus to utilize CCR5 but were unable to confer CXCR4 use. V3 mutations were thus necessary but not sufficient for coreceptor switching, and V1/V2 mutations were necessary for virus survival. HIV-1 envelope sequence evolution from CCR5 to CXCR4 use is constrained by relatively frequent lethal mutations, deep fitness valleys, and requirements to make the right amino acid substitution in the right place at the right time.

CD46 on glial cells can function as a receptor for viral glycoprotein-mediated cell-cell fusion

Membrane cofactor protein (CD46) is a regulator of complement activation that also serves as the entry receptor for human herpes virus 6 (HHV-6) and measles virus (MV) into human cells. While it is clear that oligodendrocytes and astrocytes are cell types commonly infected by these viruses, it is unclear whether oligodendrocytes express CD46, or which are the cellular mechanisms underlying the infection. We show that adult oligodendrocytes, as well as astrocytes and microglial cells, express CD46 on the cellular surface. Moreover, we employed a quantitative fusion assay to demonstrate that HHV-6A infection of T lymphocytes enables cell-cell fusion of these cells to astrocytes or to oligodendroglial cells. This fusion is mediated by the interaction between viral glycoproteins expressed on the membrane of the infected cells and CD46 on the glial targets, and is also observed using cells expressing recombinant MV glycoproteins. These data suggest a mechanism that involves cell-cell fusion by which certain viruses could spread the infection from the periphery to the cells in the nervous system.

Nucleotide and amino acid mutations in human immunodeficiency virus corresponding to CD4+ decline

In a meta-analysis, gene sequences of the HIV-1 V3 and surrounding envelope region from studies examining longitudinally derived blood and plasma human immunodeficiency virus forms were analyzed for changes over disease course. CD4+ counts were used as a marker of disease progression; 58 subjects, followed an average of 56 months, were included. Genetic diversification was found early in disease progression. In mid-progression (CD4+ counts dropping from 488 to 329/mm3) diversification did not increase while loop charge dramatically increased. This is consistent with a charged form that dominates and induces disease progression at this critical time. Although the overall mean increase in loop charge was significant, this increase and the transition to amino acids known to change tropism occurred in only half of the subjects who progressed. Those with rapidly progressing disease (within 2 years post-infection) began with a loop charge similar to the end stage of normal progressors. DNA from blood-cell-derived sequences differed from concurrently obtained plasma counterparts by one nucleotide out of 238, but this difference was not reflected in differences in glycosylation patterns, loop charge, or tropism-conferring amino acids. Plasma-derived forms were poorer predictors of future viral forms than were cell-derived sequences.

A single amino acid change and truncated TM are sufficient for simian immunodeficiency virus to enter cells using CCR5 in a CD4-independent pathway

Entry of HIV and SIV into susceptible cells is mediated by CD4 and chemokine receptors, which act as coreceptors. To study cell entry of SIV, we constructed a cell line, xKLuSIV, derived from non-susceptible human K562 cells, that express the firefly luciferase reporter gene under control of a minimal SIV long terminal repeat (LTR). Using these susceptible cells, we studied the entry of a well-characterized molecularly cloned macrophage-tropic SIV. xKLuSIV cells that express rhesus macaque CD4 and/or the rhesus chemokine receptor CCR5 are susceptible to infection with the macrophage-tropic, neurovirulent strain SIV/17E-Fr, but only xKLuSIV cells expressing both CCR5 and CD4 were susceptible to infection by the macrophage-tropic, non-neurovirulent strain SIV/17E-Cl. CCR5-dependent, CD4-independent infection by SIV/17E-Fr was abrogated by pre-incubation of the cells with AOP-RANTES, a ligand for CCR5. In addition to viral entry occurring by a CD4-independent mechanism, neutralization of SIV/17E-Fr with rhesus mAbs from 3 different neutralization groups blocked entry into x KLuSIV cells by both CD4-dependent and -independent mechanisms. Triggering the env glycoprotein of SIV-17 EFr with soluble CD4 had no significant effect in infectivity, but triggering of the same glycoprotein of SIV/17E-Cl allowed it to enter cells in a CD4-independent fashion. Using mutant molecular clones, we studied the determinants for CD4 independence, all of which are confined to the env gene. We report here that truncation of the TM at amino acid 764 and changing a single amino acid (R751G) in the SIV envelope transmembrane protein (TM) conferred the observed CD4-independent phenotype. Our data suggest that the envelope from the neurovirulent SIV/17E-Fr interacts with CCR5 in a CD4-independent manner, and changes in the TM protein of this virus are important components that contribute to neurovirulence in SIV.

Functional analysis of naturally occurring mutations in the open reading frame of CCR5 in HIV-infected Chinese patients and healthy controls

We studied polymorphism of the HIV coreceptor CC chemokine receptor (CCR) 5 in 1099 Chinese adults residing in Hong Kong, including 785 HIV-negative healthy donors and 314 HIV-positive patients. Ten mutants in the CCR5 open reading frame were identified, 7 of which were nonsynonymous. The frequencies of these alleles did not show a significant difference between HIV patients and healthy controls. G106R, Delta32, R223Q, 299(FS), and S336I were cloned from prevalent mutant genes, and their effects on HIV infection were analyzed by a series of in vitro experiments to determine their transcription levels, expression levels, conformational changes, and HIV coreceptor function. R223Q is the most prevalent CCR5 mutant in ethnic Chinese, with a frequency of 0.046, which does not affect HIV infection in vitro, however. The S336I mutant also does not affect its transcription, expression, or HIV coreceptor function. Similar to 299(FS), the mutant G106R located in the third transmembrane domain results in diminished HIV coreceptor function in vitro through conformation changes in ECL2.

Changes in the human immunodeficiency virus V3 region that correspond with disease progression: a meta-analysis

In order to determine the changes in the human immunodeficiency virus type-1 (HIV-1) envelope that corresponds with disease progression, a meta-analysis of viral forms was performed using HIV-1 sequences obtained from GenBank. Studies were selected that included longitudinally derived V3 envelope region sequences from multiple time points along with CD4 values as a marker of disease progression. Studies with a total of 58 subjects, 327 time points, and 380,000 total amino acid residues were included in this meta-analysis. Changes at specific amino acid sites over the course of disease progression stages were analyzed. The most common specific changes were found at amino acid sites 324D to N, 306S/G to R, and 360N to R. Other sites had changes from one amino acid type to another including the appearance of a basic form at 327, a charged form at 319, and 320D/E changing to basic or neutral. The timing of these changes was contrasted to CD4 decline with changes at 324 and 327 appearing before and 306, 320, and 319 appearing after the initiation of CD4 decline.

Requirements for CEACAMs and cholesterol during murine coronavirus cell entry

Previous reports have documented that cholesterol supplementations increase cytopathic effects in tissue culture and also intensify in vivo pathogenicities during infection by the enveloped coronavirus murine hepatitis virus (MHV). To move toward a mechanistic understanding of these phenomena, we used growth media enriched with methyl-beta-cyclodextrin or cholesterol to reduce or elevate cellular membrane sterols, respectively. Cholesterol depletions reduced plaque development 2- to 20-fold, depending on the infecting MHV strain, while supplementations increased susceptibility 2- to 10-fold. These various cholesterol levels had no effect on the binding of viral spike (S) proteins to cellular carcinoembryonic antigen-related cell adhesion molecule (CEACAM) receptors, rather they correlated directly with S-protein-mediated membrane fusion activities. We considered whether cholesterol was indirectly involved in membrane fusion by condensing CEACAMs into "lipid raft" membrane microdomains, thereby creating opportunities for simultaneous binding of multiple S proteins that subsequently cooperate in the receptor-triggered membrane fusion process. However, the vast majority of CEACAMs were solubilized by cold Triton X-100 (TX-100), indicating their absence from lipid rafts. Furthermore, engineered CEACAMs appended to glycosylphosphatidylinositol anchors partitioned with TX-100-resistant lipid rafts, but cells bearing these raft-associated CEACAMs were not hypersensitive to MHV infection. These findings argued against the importance of cholesterol-dependent CEACAM localizations into membrane microdomains for MHV entry, instead suggesting that cholesterol had a more direct role. Indeed, we found that cholesterol was required even for those rare S-mediated fusions taking place in the absence of CEACAMs. We conclude that cholesterol is an essential membrane fusion cofactor that can act with or without CEACAMs to promote MHV entry.

Specific inhibition of HIV-1 coreceptor activity by synthetic peptides corresponding to the predicted extracellular loops of CCR5

We used synthetic peptides to the extracellular loops (ECLs) of CCR5 to examine inhibitory effects on HIV infection/fusion with primary leukocytes and cells expressing recombinant CCR5. We show for the first time that peptides derived from the first, second, or third ECL caused dose-dependent inhibition of fusion and infection, although with varying potencies and specificities for envelope glycoproteins (Envs) from different strains. The first and third ECL peptides inhibited Envs from the R5 Ba-L strain and the R5X4 89.6 strain, whereas the second ECL peptide inhibited Ba-L but not 89.6 Env. None of the peptides affected fusion mediated by Env from the X4 LAV strain. Fusion mediated by Envs from several primary HIV-1 isolates was also inhibited by the peptides. These findings suggest that various HIV-1 strains use CCR5 domains in different ways. Experiments involving peptide pretreatment and washing, modulation of the expression levels of Env and CCR5, analysis of CCR5 peptide effects against different coreceptors, and inhibition of radiolabeled glycoprotein (gp) 120 binding to CCR5 suggested that the peptide-blocking activities reflect their interactions with gp120. The CCR5-derived ECL peptides thus provide a useful approach to analyze structure-function relationships involved in HIV-1 Env-coreceptor interactions and may have implications for the design of drugs that inhibit HIV infection.

Neuraminidase treatment of respiratory syncytial virus-infected cells or virions, but not target cells, enhances cell-cell fusion and infection

Respiratory syncytial virus (RSV) infection of HeLa cells induces fusion, but transient expression of the three viral glycoproteins induces fusion poorly, if at all. We found that neuraminidase treatment of RSV-infected cells to remove sialic acid (SA) increases fusion dramatically and that the same treatment of transiently transfected cells expressing the three viral glycoproteins, or even cells expressing the fusion (F) protein alone, results in easily detectable fusion. Neuraminidase treatment of the effector cells, expressing the viral glycoproteins, enhanced fusion while treatment of the target cells did not. Likewise, infectivity was increased by treating virions with neuraminidase, but not by treating target cells. Reduction of charge repulsion by removal of the negatively charged SA is unlikely to explain this effect, since removal of negative charges from either membrane would reduce charge repulsion. Infection with neuraminidase-treated virus remained heparan-sulfate-dependent, indicating that a novel attachment mechanism is not revealed by SA removal. Interestingly, neuraminidase enhancement of RSV infectivity was less pronounced in a virus expressing both the G and the F glycoproteins, compared to virus expressing only the F glycoprotein, possibly suggesting that the G protein sterically hinders access of the neuraminidase to its fusion-enhancing target.

Human and simian immunodeficiency virus capsid proteins are major viral determinants of early, postentry replication blocks in simian cells

The cells of most Old World monkey species exhibit early, postentry restrictions on infection by human immunodeficiency virus type 1 (HIV-1) but not by simian immunodeficiency virus of macaques (SIV(mac)). Conversely, SIV(mac), but not HIV-1, infection is blocked in most New World monkey cells. By using chimeric HIV-1/SIV(mac) viruses capable of a single round of infection, we demonstrated that a major viral determinant of this restriction is the capsid (CA) protein. The efficiency of early events following HIV-1 and SIV(mac) entry is apparently determined by the interaction of the incoming viral CA and species-specific host factors.

Species-specific effects of HIV-1 Nef-mediated MHC-I downmodulation

In vitro studies have revealed that human immunodeficiency virus-1 (HIV-1) Nef functionally interacts with amino acid residues in the cytoplasmic tail of major histocompatibility complex class I (MHC-I) molecules, reducing their expression on the cell surface and protecting them from cytotoxic T lymphocyte (CTL) lysis. To obtain a better understanding of Nef's effects in vivo, it would be helpful to have a mouse model system. However, it is not known whether Nef will affect murine MHC-I proteins. We find that Nef downmodulates human MHC-I HLA-A2 more efficiently than murine MHC-I molecules in HeLa cells and that Nef does not function efficiently in murine endothelial cells. Studies with chimeric molecules indicate that the MHC-I cytoplasmic tail is primarily responsible for species-specific differences. However, there are also effects attributable to the extracellular domain.

Membrane fusion tropism and heterotypic functional activities of the Nipah virus and Hendra virus envelope glycoproteins

Nipah virus (NiV) and Hendra virus (HeV) are novel paramyxoviruses from pigs and horses, respectively, that are responsible for fatal zoonotic infections of humans. The unique genetic and biological characteristics of these emerging agents has led to their classification as the prototypic members of a new genus within the Paramyxovirinae subfamily called HENIPAVIRUS: These viruses are most closely related to members of the genus Morbillivirus and infect cells through a pH-independent membrane fusion event mediated by the actions of their attachment (G) and fusion (F) glycoproteins. Understanding their cell biological features and exploring the functional characteristics of the NiV and HeV glycoproteins will help define important properties of these emerging viruses and may provide new insights into paramyxovirus membrane fusion mechanisms. Using a recombinant vaccinia virus system and a quantitative assay for fusion, we demonstrate NiV glycoprotein function and the same pattern of cellular tropism recently reported for HeV-mediated fusion, suggesting that NiV likely uses the same cellular receptor for infection. Fusion specificity was verified by inhibition with a specific antiserum or peptides derived from the alpha-helical heptads of NiV or HeV F. Like that of HeV, NiV-mediated fusion also requires both F and G. Finally, interactions between the glycoproteins of the paramyxoviruses have not been well defined, but here we show that the NiV and HeV glycoproteins are capable of highly efficient heterotypic functional activity with each other. However, no heterotypic activity was observed with envelope glycoproteins of the morbilliviruses Measles virus and Canine distemper virus.

Human Herpesvirus 6 and Measles Virus Employ Distinct CD46 Domains for Receptor Function

We employed a quantitative cell fusion assay to identify structural domains of CD46 required for its function as a receptor for human herpesvirus 6 (HHV-6). We examined the activities of recombinant variants of CD46, including different isoforms as well as engineered truncations and molecular chimeras with decay-accelerating factor, a related protein in the family of regulators of complement activation (RCA). We observed strong receptor activity for all four CD46 isoforms, which differ in the membrane-proximal extracellular and cytoplasmic domains, indicating that the critical determinants for HHV-6 receptor activity reside outside the C-terminal portion of CD46. Analysis of the short consensus repeat (SCR) regions that comprise most of the extracellular portion of CD46 indicated a strong dependence on SCRs 2 and 3 and no requirement for SCRs 1 or 4. Fusion-inhibition studies with SCR-specific monoclonal antibodies supported the essential role of SCRs 2 and 3 in HHV-6 receptor activity. These findings contrast markedly with fusion mediated by measles virus glycoproteins for which we observed a strict dependence on SCRs 1 and 2, consistent with previous reports. These results expand the emerging notion that CD46 and other members of the RCA family are co-opted in distinct manners by different infectious pathogens.

Restriction of measles virus RNA synthesis by a mouse host cell line: trans-complementation by polymerase components or a human cellular factor(s)

The mouse epithelial MODE-K cell line expressing human CD46 or CD150 cellular receptors was found to be nonpermissive for measles virus (MV) replication. The virus binding and membrane fusion steps were unimpaired, but only very limited amounts of virus protein and RNA synthesized were detected after the infection. In a minigenome chloramphenicol acetyltransferase assay, MODE-K cells were as able as the permissive HeLa cells in supporting MV polymerase activity. The restriction phenotype of MODE-K cells could be alleviated by providing, in trans, either N-P-L or N-P functional protein complexes but not by P-L complexes or individual N, P, and L proteins. Several human x mouse (HeLa x MODE-K) somatic hybrid clones expressing human CD46 were isolated and found to be either nonpermissive or permissive according to their human chromosomal contents. The MV-restricted phenotype exhibited by the MODE-K cell line suggests that a cellular factor(s) can control MV transcription, possibly by stabilizing the incoming virus polymerase templates.

Kappa-opioid receptor agonist inhibition of HIV-1 envelope glycoprotein-mediated membrane fusion and CXCR4 expression on CD4(+) lymphocytes

Our previous studies have shown that the suppressive effect of kappa-opioid receptor (KOR) ligand treatment on HIV-1(AT) (a T-tropic strain) expression in acutely infected CD4(+) lymphocytes is time-dependent. This finding implied that the inhibition observed following treatment with KOR agonists such as U50,488 (trans-3,4-dichloro-N-methyl-N[2-(1-pyrolidinyl)cyclohexyl]benzeneaceamide methanesulfonate) occurs at an early step in the viral replication cycle, perhaps as early as viral entry. In the present study, we examined the hypothesis that U50,488 treatment of CD4(+) lymphocytes inhibits HIV-1 envelope (Env) glycoprotein-mediated membrane fusion. We used a vaccinia virus-based assay to measure the effects of U50,488 treatment of CD4(+) lymphocytes on HIV-1 IIIB Env glycoprotein-mediated fusogenic activity, based on the cytoplasmic activation of a reporter gene. The results show that U50,488 inhibited Env-mediated cell fusion in a bell-shaped concentration-response manner with suppression ranging between 31 and 98% at concentrations of 10(-8) and 10(-10)M (N=9 experiments). U50,488 was also found to inhibit cell fusion when monitored in situ with 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-gal) staining. Blockade of the inhibitory activity of U50,488 by the KOR antagonist nor-bialtorphimine (nor-BNI) suggested that U50,488 was acting via a KOR-related mechanism. Using flow cytometry, we demonstrated that the chemokine co-receptor CXCR4, but not CD4, is down-regulated as a consequence of KOR activation, with 44.2+/-3.5% suppression at 10(-10)M U50,488. These findings support the hypothesis that KOR-related activation of CD4(+) lymphocytes inhibits HIV-1 entry via down-regulation of CXCR4.

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

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

Functional expression and membrane fusion tropism of the envelope glycoproteins of Hendra virus

Hendra virus (HeV) is an emerging paramyxovirus first isolated from cases of severe respiratory disease that fatally affected both horses and humans. Understanding the mechanisms of host cell infection and cross-species transmission is an important step in addressing the risk posed by such emerging pathogens. We have initiated studies to characterize the biological properties of the HeV envelope glycoproteins. Recombinant vaccinia viruses encoding the HeV F and G open reading frames were generated and glycoprotein expression was verified by metabolic labeling and detection using specific antisera. Glycoprotein function and cellular tropism were examined with a quantitative assay for HeV-mediated membrane fusion. Fusion specificity was verified through specific inhibition by anti-HeV antiserum and a peptide corresponding to one of the alpha-helical heptad repeats of F. HeV requires both F and G to mediate fusion. Permissive target cells have been identified, including cell lines derived from cat, bat, horse, human, monkey, mouse, and rabbit. Fusion negative cell types have also been identified. Protease treatments of the target cells abolished fusion activity, suggesting that the virus is employing a cell-surface protein as its receptor.

The coreceptor mutation CCR5Delta32 influences the dynamics of HIV epidemics and is selected for by HIV

We explore the impact of a host genetic factor on heterosexual HIV epidemics by using a deterministic mathematical model. A protective allele unequally distributed across populations is exemplified in our models by the 32-bp deletion in the host-cell chemokine receptor CCR5, CCR5Delta32. Individuals homozygous for CCR5Delta32 are protected against HIV infection whereas those heterozygous for CCR5Delta32 have lower pre-AIDS viral loads and delayed progression to AIDS. CCR5Delta32 may limit HIV spread by decreasing the probability of both risk of infection and infectiousness. In this work, we characterize epidemic HIV within three dynamic subpopulations: CCR5/CCR5 (homozygous, wild type), CCR5/CCR5Delta32 (heterozygous), and CCR5Delta32/CCR5Delta32 (homozygous, mutant). Our results indicate that prevalence of HIV/AIDS is greater in populations lacking the CCR5Delta32 alleles (homozygous wild types only) as compared with populations that include people heterozygous or homozygous for CCR5Delta32. Also, we show that HIV can provide selective pressure for CCR5Delta32, increasing the frequency of this allele.

Functional analysis of recombinant respiratory syncytial virus deletion mutants lacking the small hydrophobic and/or attachment glycoprotein gene

Respiratory syncytial virus (RSV) produces three envelope glycoproteins, the attachment glycoprotein (G), the fusion (F) protein, and the small hydrophobic (SH) protein. It had been assumed, by analogy with other paramyxoviruses, that the G and F proteins would be required for the first two steps of viral entry, attachment and fusion. However, following repeated passage in cell culture, a viable mutant RSV that lacked both the G and SH genes was isolated (R. A. Karron, D. A. Buonagurio, A. F. Georgiu, S. S. Whitehead, J. E. Adamus, M. L. Clements-Mann, D. O. Harris, V. B. Randolph, S. A. Udem, B. R. Murphy, and M. S. Sidhu, Proc. Natl. Acad. Sci. USA 94:13,961--13,966, 1997). To explore the roles of the G, F, and SH proteins in virion assembly, function, and cytopathology, we have modified the full-length RSV cDNA and used it to rescue infectious RSV lacking the G and/or SH genes. The three resulting viruses and the parental virus all contain the green fluorescent protein (GFP) gene that serves to identify infected cells. We have used purified, radiolabeled virions to examine virus production and function, in conjunction with GFP to quantify infected cells. We found that the G protein enhances virion binding to target cells but plays no role in penetration after attachment. The G protein also enhances cell-to-cell fusion, presumably via cell-to-cell binding, and enhances virion assembly or release. The presence or absence of the G protein in virions has no obvious effect on the content of F protein or host cell proteins in the virion. In growth curve experiments, the viruses lacking the G protein produced viral titers that were at least 10-fold lower than titers of viruses containing the G protein. This reduction is due in large part to the less efficient release of virions and the lower infectivity of the released virions. In the absence of the G protein, virus expressing both the F and SH proteins displayed somewhat smaller plaques, lower fusion activity, and slower viral entry than the virus expressing the F protein alone, suggesting that the SH protein has a negative effect on virus fusion in cell culture.

Cytokines and HIV-1: interactions and clinical implications

Cytokines play an important role in controlling the homoeostasis of the immune system. Infection with HIV results in dysregulation of the cytokine profile in vivo and in vitro. During the course of HIV-1 infection secretion of T-helper type 1 (Th1) cytokines, such as interleukin (IL)-2, and antiviral interferon (IFN)-gamma, is generally decreased, whereas production of T helper type 2 (Th2) cytokines, IL-4, IL-10, proinflammatory cytokines (IL-1, IL-6, IL-8) and tumour necrosis factor (TNF)-alpha, is increased. Such abnormal cytokine production contributes to the pathogenesis of the disease by impairing cell-mediated immunity. A number of cytokines have been shown to modulate in vitro HIV-1 infection and replication in both CD4 T lymphocytes and cells of macrophage lineage. HIV-inductive cytokines include: TNF-alpha, TNF-beta, IL-1 and IL-6, which stimulate HIV-1 replication in T cells and monocyte-derived macrophages (MDM), IL-2, IL-7 and IL-15, which upregulate HIV-1 in T cells, and macrophage-colony stimulating factor, which stimulates HIV-1 in MDM. HIV-suppressive cytokines include: IFN-alpha, IFN-beta and IL-16, which inhibit HIV-1 replication in T cells and MDM, and IL-10 and IL-13, which inhibit HIV-1 in MDM. Bifunctional cytokines such as IFN-gamma, IL-4 and granulocyte-macrophage colony-stimulating factor have been shown to have both inhibitory and stimulatory effects on HIV-1. The beta-chemokines, macrophage-inflammatory protein (MIP)-1alpha, MIP-1beta and RANTES are important inhibitors of macrophage-tropic strains of HIV-1, whereas the alpha-chemokine stromal-derived factor-1 suppresses infection of T-tropic strains of HIV-1. This review outlines the interactions between cytokines and HIV-1, and presents clinical applications of cytokine therapy combined with highly active antiretroviral therapy or vaccines.