Variable constraints on the principal immunodominant domain of the transmembrane glycoprotein of human immunodeficiency virus type 1

A Novel Point-of-Care BioNanoSensor for Rapid HIV Detection and Treatment Monitoring

We report here a new diagnostic approach to the direct detection of HIV in blood or other body fluids that is rapid, sensitive and potentially applicable in a point-of-care setting. The approach follows on the development of a novel BioNanoSensor (BNS) device that utilizes piezoelectric technology to detect the presence of the HIV surface glycoprotein gp120 in a nanoscale format. The detection range of the BNS device for the biomarker gp120 displayed a low-end sensitivity of 6.5×104 HIV viral particles/ml, while using a small fluid sample (5 µl) and with a reaction time of less then 30 seconds. Performance of this device indicated that the BNS has utility for direct detection of HIV particles prior to, and independent from, antibody formation. Accordingly, this device holds utility to monitor the status of HIV infection both early after exposure to virus as well as during chronic HIV infection. The BNS parameters of small sample volume, compact device size, and detection sensitivity indicate that the BNS is potentially useful in the point-of-care and/or home setting for monitoring decisions regarding HIV treatment on a real-time basis.

Escape from human immunodeficiency virus type 1 (HIV-1) entry inhibitors

The human immunodeficiency virus (HIV) enters cells through a series of molecular interactions between the HIV envelope protein and cellular receptors, thus providing many opportunities to block infection. Entry inhibitors are currently being used in the clinic, and many more are under development. Unfortunately, as is the case for other classes of antiretroviral drugs that target later steps in the viral life cycle, HIV can become resistant to entry inhibitors. In contrast to inhibitors that block viral enzymes in intracellular compartments, entry inhibitors interfere with the function of the highly variable envelope glycoprotein as it continuously adapts to changing immune pressure and available target cells in the extracellular environment. Consequently, pathways and mechanisms of resistance for entry inhibitors are varied and often involve mutations across the envelope gene. This review provides a broad overview of entry inhibitor resistance mechanisms that inform our understanding of HIV entry and the design of new inhibitors and vaccines.

Selected amino acid mutations in HIV-1 B subtype gp41 are associated with specific gp120v₃ signatures in the regulation of co-receptor usage

Background

The third variable loop (V3) of the HIV-1 gp120 surface protein is a major determinant of cellular co-receptor binding. However, HIV-1 can also modulate its tropism through other regions in gp120, such as V1, V2 and C4 regions, as well as in the gp41 protein. Moreover, specific changes in gp41 are likely to be responsible for of damage in gp120-CCR5 interactions, resulting in potential resistance to CCR5 inhibitors.

In order to genetically characterize the two envelope viral proteins in terms of co-receptor usage, we have analyzed 526 full-length env sequences derived from HIV-1 subtype-B infected individuals, from our and public (Los Alamos) databases. The co-receptor usage was predicted by the analysis of V3 sequences using Geno2Pheno (G2P) algorithm. The binomial correlation phi coefficient was used to assess covariation among gp120_V3 and gp41 mutations; subsequently the average linkage hierarchical agglomerative clustering was performed.

Results

According to G2P false positive rate (FPR) values, among 526 env-sequences analyzed, we further characterized 196 sequences: 105 with FPR <5% and 91 with FPR >70%, for X4-using and R5-using viruses, respectively.

Beyond the classical signatures at 11/25 V3 positions (S11S and E25D, R5-tropic viruses; S11KR and E25KRQ, X4-tropic viruses), other specific V3 and gp41 mutations were found statistically associated with the co-receptor usage. Almost all of these specific gp41 positions are exposed on the surface of the glycoprotein. By the covariation analysis, we found several statistically significant associations between V3 and gp41 mutations, especially in the context of CXCR4 viruses. The topology of the dendrogram showed the existence of a cluster associated with R5-usage involving E25D_V3, S11S_V3, T22A_V3, S129DQ_gp41 and A96N_gp41 signatures (bootstrap = 0.88). Conversely, a large cluster was found associated with X4-usage involving T8I_V3, S11KR_V3, F20IVY_V3, G24EKR_V3, E25KR_V3, Q32KR_V3, A30T_gp41, A189S_gp41, N195K_gp41 and L210P_gp41 mutations (bootstrap = 0.84).

Conclusions

Our results show that gp120_V3 and several specific amino acid changes in gp41 are associated together with CXCR4 and/or CCR5 usage. These findings implement previous observations that determinants of tropism may reside outside the V3-loop, even in the gp41. Further studies will be needed to confirm the degree to which these gp41 mutations contribute directly to co-receptor use.

Ldlr-/- mice display decreased susceptibility to Western-type diet-induced obesity due to increased thermogenesis

The low-density lipoprotein receptor (Ldlr) is a key molecule involved with lipid clearance. The Ldlr(-/-) mouse has been used extensively as a model for studying atherosclerosis. This study sought to characterize the energy balance phenotype of Ldlr(-/-) mice with respect to weight gain, body composition, energy expenditure (EE), glucose homeostasis, and leptin sensitivity. Adult Ldlr(-/-) mice and Ldlr(+/+) controls on a C57Bl/6J background were fed either a chow or a high-fat, high-sucrose Western-type diet (WTD) for eight wk. Physiological studies of food intake, EE, activity, insulin sensitivity, and leptin responsiveness were performed. The effect of these diet interventions on circulating leptin and on leptin gene expression was also examined. On the chow diet, Ldlr(-/-) mice had lower EE and higher activity levels relative to controls. On the WTD, Ldlr(-/-) mice gained less weight relative to Ldlr(+/+) mice, specifically gaining less fat mass. Increased thermogenesis in Ldlr(-/-) mice fed the WTD was detected. Additionally, leptin responsiveness was blunted in chow-fed Ldlr(-/-) mice, suggesting a novel role for the Ldlr pathway that extends to leptin's regulation of energy balance. In addition to its known role in lipid transport, these results demonstrate the importance of the Ldlr in energy homeostasis and suggest a direct physiological link between altered lipid transport and energy balance.

The disulfide loop of gp41 is critical to the furin recognition site of HIV gp160

The importance of the HIV gp41 conserved disulfide loop to envelope function has been examined by mutational and functional analyses. Based on a luciferase-reporter entry assay, mutants gp41-CC/AA (C598A/C604A) and gp41-Delta (deletion of residues 596-606) result in a nonfunctional envelope protein. Western blot analysis shows both mutants to be properly expressed but not processed to form gp120 and gp41, which explains their nonfunctionality. The presence of mutant gp160 on the cell surface, as well as their ability to bind to sCD4, suggests that the mutations have disrupted processing at the furin recognition site encoded within the gp120 conserved domain 5, without resulting in an overall misfolding of the protein. With respect to the furin recognition site, the mutations are sequentially distant, which implies that the gp41 disulfide loop is interacting with gp120 C5 in gp160. In addition, we have modeled the gp120-gp41 interaction in unprocessed precursor gp160 using structural data available for gp120 and gp41 domains in isolation, supplemented by mutagenesis data. We suggest that the mutations have altered the interaction between gp120 C5 and the gp41 disulfide loop, resulting in decreased accessibility of the furin recognition site and implying that the interaction between the gp120 C5 and gp41 loop is a conformational requirement for gp160 processing. The sensitivity of this interaction could be exploited in future antivirals designed to disrupt HIV pathogenesis by disrupting gp160 processing.

FIV as a Model for HIV: An Overview

Animal models for human immunodeficiency virus (HIV) infection play a key role in understanding the pathogenesis of AIDS and the development of therapeutic agents and vaccines. As the only lentivirus that causes an immunodeficiency resembling that of HIV infection, in its natural host, feline immunodeficiency virus (FIV) has been a unique and powerful model for AIDS research. FIV was first described in 1987 by Niels Pedersen and co-workers as the causative agent for a fatal immunodeficiency syndrome observed in cats housed in a cattery in Petaluma, California. Since this landmark observation, multiple studies have shown that natural and experimental infection of cats with biological isolates of FIV produces an AIDS syndrome very similar in pathogenesis to that observed for human AIDS. FIV infection induces an acute viremia associated with Tcell alterations including depressed CD4 :CD8 T-cell ratios and CD4 T-cell depletion, peripheral lymphadenopathy, and neutropenia. In later stages of FIV infection, the host suffers from chronic persistent infections that are typically self-limiting in an immunocompetent host, as well as opportunistic infections, chronic diarrhea and wasting, blood dyscracias, significant CD4 T-cell depletion, neurologic disorders, and B-cell lymphomas. Importantly, chronic FIV infection induces a progressive lymphoid and CD4 T-cell depletion in the infected cat. The primary mode of natural FIV transmission appears to be blood-borne facilitated by fighting and biting. However, experimental infection through transmucosal routes (rectal and vaginal mucosa and perinatal) have been well documented for specific FIV isolates. Accordingly, FIV disease pathogenesis exhibits striking similarities to that described for HIV-1 infection.

Tetraspanins CD9 and CD81 modulate HIV-1-induced membrane fusion

Protein organization on the membrane of target cells may modulate HIV-1 transmission. Since the tetraspanin CD81 is associated to CD4, the receptor of HIV-1 envelope protein (Env; gp120/gp41), we have explored the possibility that this molecule may modulate the initial steps of HIV-1 infection. On the other hand, CD81 belongs to the tetraspanin family, which has been described as organizers of protein microdomains on the plasma membrane. Therefore, the role of CD81 and other related tetraspanin, CD9, on the cell-to-cell fusion process mediated by HIV-1 was studied. We found that anti-tetraspanin Abs enhanced the syncytia formation induced by HIV-1 envelope proteins and viral entry in human T lymphoblasts. In addition, anti-CD81 Abs triggered its clustering in patches, where CD4 and CXCR4 were included. Moreover, the knocking down of CD81 and CD9 expression resulted in an increase in syncytia formation and viral entry. Accordingly, overexpression of CD81 and CD9 rendered cells less susceptible to Env-mediated syncytia formation. These data indicate that CD9 and CD81 have an important role in membrane fusion induced by HIV-1 envelope.

Structural characterization of the feline-immunodeficiency-virus envelope glycoprotein 36 ectodomain for the development of new antivirals

In the fight against the human HIV, new targets are being explored, such as the proteins involved in the process of fusion of the virus with the host cell. Recently, the first generation of fusion inhibitors (enfuvirtide), targeting gp41 (virus envelope glycoprotein 41), has become commercially available. However, this promising class of drugs has to be improved in respect of their efficacy and bioavailability. Considering the strong homologies between HIV and FIV (feline immunodeficiency virus), as well as the highly conserved structure of the transmembrane envelope protein among species, FIV represents a relevant model of pre-screening studies for HIV. Taking into account (i) sequence homologies between the ectodomain of HIV gp41 and FIV gp36 (envelope glycoprotein 36), (ii) structural data available for gp41 and (iii) the fact that synthetic peptides derived from gp36 are effective inhibitors of FIV infection, we designed several peptides derived from gp36 sequence. We checked that these peptides had the same structural features as the corresponding peptides from gp41 HIV by CD, analytical ultracentrifugation and 1H-2H (hydrogen-deuterium) exchange combined with MS. By combining this latter technique with surface-plasmon-resonance assays, we identified the amino acid residues of the C-terminal region of the ectodomain of gp36 that are critical for interaction with the N-terminal region. This gave clues for therapy and vaccines against FIV, thus providing helpful data for treatments against HIV.

Evaluation of live feline immunodeficiency virus vaccines with modified antigenic properties

Live-attenuated viruses have typically been generated from pathogenic viruses by genetic modifications that modified their replicative capacity. The present study investigated whether modification of the antigenic properties of live-attenuated viruses might improve upon the protection that such vaccines afford against lentivirus infection. In a previous study, random amino acid substitutions were introduced into the transmembrane envelope glycoprotein of the feline immunodeficiency virus (FIV), within a highly conserved domain (principal immunodominant domain) bearing immunodominant B-cell epitopes. Amongst a wide set of mutants, mutations that modified antibody specificity without abolishing infectivity ex vivo were selected. In the present study, two such mutants, TN14 and TN92, were evaluated for their replicative capacities and pathogenic properties in vivo in comparison with the parental virus, FIV 34TF10. No significant differences in viral load were observed between mutant and parental viruses. After 1 year of infection, all animals were subjected to a heterologous intraclade superinfection with a primary strain of FIV. Whilst both parental and modified viruses protected cats from high viral loads after superinfection, the TN92 virus afforded a higher degree of protection (P=0·0079). Such improvement in protection might correlate with a decrease in the immunogenicity of a B-cell epitope potentially involved in antibody enhancement of infection.

Differentiation of a passive vaccine and the humoral immune response toward infection: analysis of phage displayed peptides

Antibody-genes undergo molecular events that produce unique binding-sites that recognize specific epitopes, thus, leading to B-cell clonal variation. As a result, different binding-site structures (paratope internal images) are produced even when two distinct B-cells bind one and the same epitope. Paratope structural variation can be exploited to enable one to evaluate antibody-diversity in a single polyclonal serum sample. This is accomplished through the selection of antibody-specific peptides isolated from combinatorial phage displayed peptide libraries. As an example, we demonstrate the analysis of macaque sera containing passively administered antibodies, given as a therapeutic vaccine and antibodies actively produced by the virus-infected monkeys.

Biochemical and HIV-1 coreceptor properties of K26R, a new CCR5 Variant in China's Sichuan population

Despite multiple exposures to HIV-1, some individuals remain uninfected. This resistance to HIV infection has been associated with homozygosity for a 32-basepair deletion in the CCR5 receptor gene. This variant is frequent in caucasians but extremely rare in Asians and Africans. Identifying variations in the CCR5 gene that affect susceptibility to HIV infection in non-caucasians is therefore of great interest. In this report, we identify 5 CCR5 coding region variants in a Chinese population. The K26R mutation is an undescribed gene variant, whereas 228delK was already found in caucasians and G106R, C178R, and R223Q were previously described in Asian populations and functionally analyzed. As the function of K26R was still unknown, we focused our work on studying its chemokine receptor activity and HIV coreceptor properties compared with wild-type CCR5 and G106R, an already analyzed mutant taken as another control. We observed that K26R displayed alteration in MIP1-beta/CCL4 and RANTES/CCL5 ligand binding and exhibited a slightly decrease for HIV coreceptor properties.

Alanine scanning mutants of the HIV gp41 loop

Based on mutagenesis and structural studies of human immunodeficiency virus (HIV) envelope proteins, the loop region of gp41 is thought to directly interact with gp120. The importance of the HIV gp41 loop region to envelope function has been systematically examined by alanine scanning of all gp41 loop residues and the subsequent characterization of the mutagenic effects on viral entry, envelope expression, envelope processing, and gp120 association with gp41. With respect to the wild-type gp41, mutational effects on viral entry fall into four classes as follows: 1) little or no effect (G594A, S599A, G600A, K601A, N611A, S615A, N616A, and L619A); 2) significantly reduced entry (I595A, L602A, I603A, V608A, and K617A); 3) abolished entry (L593A, W596A, G597A, T606A, W610A, W614A, S618A, and I622A); and 4) enhanced entry (T605A, P609A, S613A, E620A, and Q621A). The reduced functionality of many mutants was apparently due to either disruption of envelope processing (L593A and T606A), viral incorporation of the envelope (W610A, W614A, and I662A), or increased dissociation of gp120 (W596A, G597A, and S618A). The extreme sensitivity of the gp120-gp41 interaction to alanine substitutions (e.g. the G597A and S618A mutants are relatively conservative substitutions) suggests that this association is an attractive and novel target for future drug discovery efforts.

New CCR5 variants associated with reduced HIV coreceptor function in southeast Asia

BACKGROUND

Despite multiple exposure to HIV-1, some individuals remain uninfected. This resistance has been associated with homozygosity for a 32 base pair deletion in the gene for the CCR5 receptor. This variant occurs frequently in Caucasians but is extremely rare in Asians or Africans.

OBJECTIVE

To identify variations in CCR5 receptor gene that affect susceptibility to HIV infection in non-Caucasians.

METHODS

CCR5 coding region polymorphisms were screened in three groups of Vietnamese subjects: 47 HIV-1 infected intravascular drug users, 50 highly HIV-1-exposed but seronegative intravascular drug users and 37 HIV-1-unexposed seronegative individuals. DNA was analysed by denaturing high performance liquid chromatography; this was followed by examination of the biochemical and HIV coreceptor properties of the coding regions.

RESULTS

Five CCR5 coding region variants were identified in this Vietnamese population. The S185R, I254T and C269F mutations have not been previously described; G106R and R223Q have already been found in other Asian populations, but the functional properties of G106R is not known. These variants differed in biochemical and HIV coreceptor properties. S185R and I254T variants had receptor and coreceptor activities comparable to that of the wild type, whereas C269F and G106R behaved differently. This latter pair are poorly expressed at the cell surface, weakly bind macrophage inflammatory protein 1beta (CCL4) and RANTES (CCL5), and display reduced HIV-1 coreceptor efficiency.

CONCLUSIONS

Among the five CCR5 variants found in this Vietnamese population, G106R and C269F displayed significant modifications of their receptor and coreceptor properties, which may contribute to susceptibility to HIV-1 infection and/or disease progression within this population.

Generation and characterization of single-chain antibody fragments specific against transmembrane envelope glycoprotein gp46 of maedi-visna virus

A single-chain antibody fragments (scFv) was developed directed against transmembrane envelope glycoprotein gp46 of the virus maedi-visna, by the application of the antibody phage display library. To get specific scFv binders, the library was panned against the biotinylated peptide of 20 amino acids corresponding to the principal immunodominant domain of gp46 protein. The number of positively binding scFvs was evaluated by scFv-phage ELISA, BstN1 fingerprinting and DNA sequencing. The scFvs were expressed in soluble form and purified by immobilized metal affinity chromatography (IMAC) with a yield of 2-2.5 mg/l. Two scFvs have shown to recognize gp46 and gp150 proteins in Western blot analysis. The scFvs also recognized the virus in infected cells as shown by immunofluorescence assay. The affinity of the obtained antibody fragments to gp46 peptide was measured by surface plasmon resonance, and the resulting K(A) was in the 10(6)-10(7)lmol(-1) range. The application of characterized scFvs for expression as intrabodies in intracellular immunization against virus maedi-visna infection and for the diagnosis of this virus is discussed.

Envelope variants from women recently infected with clade A human immunodeficiency virus type 1 confer distinct phenotypes that are discerned by competition and neutralization experiments

Women infected with clade A human immunodeficiency virus type 1 harbor a virus population that is genetically diverse in the envelope gene, a fact that contrasts with the homogeneous virus population identified in newly infected men. It is not known whether viral genetic diversity at this early stage of infection is manifested as phenotypic diversity. This is a significant question because phenotypic diversity in the viral population that establishes infection in women may have important implications for pathogenesis and therapeutic intervention. Thus, in this study we compared the biological properties of three pairs of chimeric viruses that contained envelope genes representative of variant groups in each of three infected women-Q23, Q45, and Q47. Envelope chimeras were evaluated for replication in stimulated and resting peripheral blood mononuclear cells alone and in competition, for coreceptor use, and for neutralization sensitivity. All viruses utilized CCR5 exclusively and had a non-syncytium-inducing phenotype on MT-2 cells and in primary culture. There were no significant differences in replication parameters between paired variants in individual cultures. However, in competition experiments, one chimera of each variant pair always dominated. The dominant virus from Q23 and Q47, but not from Q45, infected a significantly higher number of CCR5- and CD4-expressing GHOST cells than the weaker chimeras. Significantly, chimeric viruses from Q47 and Q45 showed markedly different neutralization sensitivity to antibodies to CCR5 and gp120, respectively. These data indicate that distinct envelope genotypes identified in clade A-infected women near seroconversion confer unique phenotypes that affect viral fitness and that may be due, in part, to different requirements for relative configuration of CD4 and CCR5 on infected cells.

Human immunodeficiency virus type 1 Env with an intersubunit disulfide bond engages coreceptors but requires bond reduction after engagement to induce fusion

A mutant human immunodeficiency virus (HIV) envelope protein (Env) with an engineered disulfide bond between the gp120 and gp41 subunits (SOS-Env) was expressed on cell surfaces. With the disulfide bond intact, these cells did not fuse to target cells expressing CD4 and CCR5, but the fusion process did advance to an intermediate state: cleaving the disulfide bond with a reducing agent after but not before binding to target cells allowed fusion to occur. Through the use of an antibody directed against CCR5, it was found that at the intermediate stage, SOS-Env had associated with coreceptors. Reducing the disulfide bond after this intermediate had been reached resulted in hemifusion at low temperature and fusion at physiological temperature. The addition of C34 or N36, peptides that prevent six-helix bundle formation, at the hemifused state blocked the fusion that would have resulted after raising the temperature. Thus, Env has not yet folded into six-helix bundles after hemifusion has been achieved. Because SOS-Env binds CCR5, it is suggested that the conformational changes in wild-type Env that result from this binding cause disengagement of gp120 from gp41 in the region of the engineered bond. It is proposed that this disengagement is the event that directly frees gp41 to undergo the conformational changes that lead to fusion. The intermediate state achieved prior to reduction of the disulfide bond was stable. The capture of this configuration of Env could yield a suitable antigen for vaccine development, and it may also be a target for pharmacological intervention against HIV-1 entry.

Fcgamma receptor-mediated suppression of human immunodeficiency virus type 1 replication in primary human macrophages

Permissiveness of monocytes and macrophages to human immunodeficiency virus (HIV) infection is modulated by various stimuli. In this study we demonstrate that stimulation of primary monocytes and monocyte-derived macrophages (MDM) through the receptors for the Fc portion of immunoglobulin G (IgG) (FcgammaR) inhibits HIV type 1 (HIV-1) replication. Viral p24 production was decreased by 1.5 to 3 log units in MDM infected with both R5 and X4 HIV-1 strains upon stimulation by immobilized IgG but not upon stimulation by soluble IgG or by F(ab')(2) IgG fragments. Although MDM activation by immobilized IgG induced high levels of macrophage-derived chemokine secretion as well as a sustained down-regulation of CD4 and a transient decrease in CCR5 expression, these factors did not appear to play a major role in the suppression of HIV-1 replication. Single-cycle infection of FcgammaR-stimulated MDM with HIV-1 virions pseudotyped with either HIV-1 R5 or vesicular stomatitis virus G envelopes was inhibited, suggesting a postentry restriction of viral replication. PCR analyses of HIV-1 DNA intermediate replication forms suggested that reverse transcription is not affected by stimulation with immobilized human IgG, at least during the first replication cycle. The accumulation of PCR products corresponding to nuclear unintegrated two-long-terminal-repeat circles and the relative decrease of integrated HIV-1 DNA signals suggest an inhibition of proviral integration. Our data, showing that FcgammaR-mediated activation of MDM is a potent mechanism of HIV-1 suppression, raise the possibility that FcgammaR cross-linking by immune complexes may contribute to the control of viral replication in macrophages.

Enhancing the proteolytic maturation of human immunodeficiency virus type 1 envelope glycoproteins

In virus-infected cells, the envelope glycoprotein (Env) precursor, gp160, of human immunodeficiency virus type 1 is cleaved by cellular proteases into a fusion-competent gp120-gp41 heterodimer in which the two subunits are noncovalently associated. However, cleavage can be inefficient when recombinant Env is expressed at high levels, either as a full-length gp160 or as a soluble gp140 truncated immediately N-terminal to the transmembrane domain. We have explored several methods for obtaining fully cleaved Env for use as a vaccine antigen. We tested whether purified Env could be enzymatically digested with purified protease in vitro. Plasmin efficiently cleaved the Env precursor but also cut at a second site in gp120, most probably the V3 loop. In contrast, a soluble form of furin was specific for the gp120-gp41 cleavage site but cleaved inefficiently. Coexpression of Env with the full-length or soluble form of furin enhanced Env cleavage but also reduced Env expression. When the Env cleavage site (REKR) was mutated in order to see if its use by cellular proteases could be enhanced, several mutants were found to be processed more efficiently than the wild-type protein. The optimal cleavage site sequences were RRRRRR, RRRRKR, and RRRKKR. These mutations did not significantly alter the capacity of the Env protein to mediate fusion, so they have not radically perturbed Env structure. Furthermore, unlike that of wild-type Env, expression of the cleavage site mutants was not significantly reduced by furin coexpression. Coexpression of Env cleavage site mutants and furin is therefore a useful method for obtaining high-level expression of processed Env.

HIV-1 infection of primary effusion lymphoma cell line triggers Kaposi's sarcoma-associated herpesvirus (KSHV) reactivation

Kaposi's sarcoma-associated herpesvirus (KSHV) is a gammaherpesvirus etiologically linked to primary effusion lymphoma (PEL), to a subset of multicentric Castleman's disease and to Kaposi's sarcoma (KS), the most common neoplasm associated with AIDS. Among KSHV-infected individuals, the risk of KS is much higher in those with human immunodeficiency-1 (HIV-1) infection than among those with other types of immunosuppression, suggesting a direct action of HIV-1 on KSHV replication. We show in our report that BC-3 cells, a chronically KSHV-infected B-cell line of a PEL origin, are permissive to HIV-1, offering a new tool for studying the interactions between these 2 viruses. In these cells, HIV-1 infection leads to reactivation of latent KSHV genomes, as demonstrated by the expression of KSHV lytic viral mRNAs. Although recombinant HIV-1 gp120 fails to enhance herpesvirus expression, transient transfection of the HIV-1 trans-activator Tat suffices to reactivate latent KSHV. By showing that HIV-1 infection directly reactivates latent KSHV, our results suggest a direct role of HIV-1 in the onset of KS in coinfected individuals.

Functional analysis of the disulfide-bonded loop/chain reversal region of human immunodeficiency virus type 1 gp41 reveals a critical role in gp120-gp41 association

Human immunodeficiency virus type 1 (HIV-1) entry into cells is mediated by the surface-exposed envelope protein (SU) gp120, which binds to cellular CD4 and chemokine receptors, triggering the membrane fusion activity of the transmembrane (TM) protein gp41. The core of gp41 comprises an N-terminal triple-stranded coiled coil and an antiparallel C-terminal helical segment which is packed against the exterior of the coiled coil and is thought to correspond to a fusion-activated conformation. The available gp41 crystal structures lack the conserved disulfide-bonded loop region which, in human T-lymphotropic virus type 1 (HTLV-1) and murine leukemia virus TM proteins, mediates a chain reversal, connecting the antiparallel N- and C-terminal regions. Mutations in the HTLV-1 TM protein gp21 disulfide-bonded loop/chain reversal region adversely affected fusion activity without abolishing SU-TM association (A. L. Maerz, R. J. Center, B. E. Kemp, B. Kobe, and P. Poumbourios, J. Virol. 74:6614-6621, 2000). We now report that in contrast to our findings with HTLV-1, conservative substitutions in the HIV-1 gp41 disulfide-bonded loop/chain reversal region abolished association with gp120. While the mutations affecting gp120-gp41 association also affected cell-cell fusion activity, HIV-1 glycoprotein maturation appeared normal. The mutant glycoproteins were processed, expressed at the cell surface, and efficiently immunoprecipitated by conformation-dependent monoclonal antibodies. The gp120 association site includes aromatic and hydrophobic residues on either side of the gp41 disulfide-bonded loop and a basic residue within the loop. The HIV-1 gp41 disulfide-bonded loop/chain reversal region is a critical gp120 contact site; therefore, it is also likely to play a central role in fusion activation by linking CD4 plus chemokine receptor-induced conformational changes in gp120 to gp41 fusogenicity. These gp120 contact residues are present in diverse primate lentiviruses, suggesting conservation of function.

Variable-loop-deleted variants of the human immunodeficiency virus type 1 envelope glycoprotein can be stabilized by an intermolecular disulfide bond between the gp120 and gp41 subunits

We have described an oligomeric gp140 envelope glycoprotein from human immunodeficiency virus type 1 that is stabilized by an intermolecular disulfide bond between gp120 and the gp41 ectodomain, termed SOS gp140 (J. M. Binley, R. W. Sanders, B. Clas, N. Schuelke, A. Master, Y. Guo, F. Kajumo, D. J. Anselma, P. J. Maddon, W. C. Olson, and J. P. Moore, J. Virol. 74:627-643, 2000). In this protein, the protease cleavage site between gp120 and gp41 is fully utilized. Here we report the characterization of gp140 variants that have deletions in the first, second, and/or third variable loop (V1, V2, and V3 loops). The SOS disulfide bond formed efficiently in gp140s containing a single loop deletion or a combination deletion of the V1 and V2 loops. However, deletion of all three variable loops prevented formation of the SOS disulfide bond. Some variable-loop-deleted gp140s were not fully processed to their gp120 and gp41 constituents even when the furin protease was cotransfected. The exposure of the gp120-gp41 cleavage site is probably affected in these proteins, even though the disabling change is in a region of gp120 distal from the cleavage site. Antigenic characterization of the variable-loop-deleted SOS gp140 proteins revealed that deletion of the variable loops uncovers cryptic, conserved neutralization epitopes near the coreceptor-binding site on gp120. These modified, disulfide-stabilized glycoproteins might be useful as immunogens.

Sensitivity to a nonpeptidic compound (RPR103611) blocking human immunodeficiency virus type 1 Env-mediated fusion depends on sequence and accessibility of the gp41 loop region

The triterpene RPR103611 is an efficient inhibitor of membrane fusion mediated by the envelope proteins (Env, gp120-gp41) of CXCR4-dependent (X4) human immunodeficiency virus type 1 (HIV-1) strains, such as HIV-1(LAI) (LAI). Other X4 strains, such as HIV-1(NDK) (NDK), and CCR5-dependent (R5) HIV-1 strains, such as HIV-1(ADA) (ADA), were totally resistant to RPR103611. Analysis of chimeric LAI-NDK Env proteins identified a fragment of the NDK gp41 ectodomain determining drug resistance. A single difference at position 91, leucine in LAI and histidine in NDK, apparently accounted for their sensitivity or resistance to RPR103611. We had previously identified a mutation of isoleucine 84 to serine in a drug escape LAI variant. Both I84 and L91 are located in the "loop region" of gp41 separating the proximal and distal helix domains. Nonpolar residues in this region therefore appear to be important for the antiviral activity of RPR103611 and are possibly part of its target. However, another mechanism had to be envisaged to explain the drug resistance of ADA, since its gp41 loop region was almost identical to that of LAI. Fusion mediated by chimeric Env consisting of LAI gp120 and ADA gp41, or the reciprocal construct, was fully blocked by RPR103611. The gp120-gp41 complex of R5 strains is stable, relative to that of X4 strains, and this stability could play a role in their drug resistance. Indeed, when the postbinding steps of ADA infection were performed under mildly acidic conditions (pH 6.5 or 6.0), a treatment expected to favor dissociation of gp120, we achieved almost complete neutralization by RPR103611. The drug resistance of NDK was partially overcome by preincubating virus with soluble CD4, a gp120 ligand inducing conformational changes in the Env complex. The antiviral efficacy of RPR103611 therefore depends on the sequence of the gp41 loop and the stability of the gp120-gp41 complex, which could limit the accessibility of this target.