Functions of HIV envelope glycans

Glycosylation affects both the three-dimensional structure and antibody binding properties of the HIV-1IIIB GP120 peptide RP135

We have prepared glycosylated analogues of the principal neutralizing determinant of gp120 and studied their conformations by NMR and circular dichroism spectroscopies. The 24-residue peptide from the HIV-1IIIB isolate (residues 308-331) designated RP135, which contains the immunodominant tip of the V3 loop, was glycosylated with both N- and O-linked sugars. The structures of two glycopeptides, one with an N-linked beta-glucosamine (RP135NG) and the other with two O-linked alpha-galactosamine units (RP135digal), were studied by NMR and circular dichroism spectroscopies. Molecular dynamics calculations based on the NMR data obtained in water solutions were performed to explore the conformational substates sampled by the glycopeptides. The data showed that covalently linking a carbohydrate to the peptide has a major effect on the local conformation and imparts additional minor changes at more distant sites of partially defined secondary structure. In particular, the transient beta-type turn comprised of the -Gly-Pro-Gly-Arg- segment at the "tip" of the V3 loop is more highly populated in RP135digal that in the native peptide and N-linked analogue. Binding data for the glycopeptides with 0.5beta, a monoclonal antibody mapped to the RP135 sequence, revealed a significant enhancement in binding for RP135digal as compared with the native peptide, whereas binding was reduced for the N-linked glycopeptide. These data show that glycosylation of V3 loop peptides can affect their conformations as well as their interactions with antibodies. The design of more ordered and biologically relevant conformations of immunogenic regions from gp120 may aid in the design of more effective immunogens for HIV-1 vaccine development.

HIV-1 envelope protein, gp120, has no effects on inositol phosphate production and metabolism in the Jurkat T-cell line either in the presence or absence of receptor stimulation

We have used HPLC techniques to investigate the effects of gp120 upon inositol phosphate turnover in Jurkat E6-1 CD4+ T-cells, to pursue previous reports that this viral coat protein: (a) inhibits receptor-activated inositol phosphate release; (b) stimulates basal inositol phosphate release; (c) inhibits inositol polyphosphate 5-phosphatase. Treatment of cells with up to 10 microg/ml gp120 from between 10 min and 24 h was without effect upon inositol phosphate turnover in both basal cells, and in C305 and OKT3 stimulated cells. This is the first report that biologically competent gp120 does not affect any aspect of inositol phosphate turnover in either basal or receptor-activated lymphocytes.

Delineation of regions important for heteromeric association of hepatitis C virus E1 and E2

Hepatitis C virus (HCV) is the major causative agent of blood-borne non-A non-B hepatitis. The persistence of HCV infection is believed to reflect escape from the host immunosurveillance system by mutations in hypervariable region 1 (HVR1) of the envelope protein 2 (E2). Two envelope proteins of HCV, E1 and E2, have been reported to form a heteromeric complex but the exact organization of the viral envelope proteins remains uncertain. We examined the interaction of E1 and E2 by far- Western blotting using the bacterial recombinant proteins and also by pull-down assay using mammalian expressed proteins. The major E1-interacting site of E2 was mapped within the N-terminal part of E2 (NCD1) (aa 415 to 500 of the polyprotein). Both HVR1 and HVR2, located at the N-terminal part of E2, were dispensable for the interaction. Although several discontinuous regions within NCD1 seemed to contribute to the strong binding to E1, the highly conserved amino acid sequences flanking HVR2 had the most significant effect. The amino acid residues "WHY" from 489 to 491 of E2 played an especially crucial role since the constructs with the internal deletion or substitution of the residues showed severely impaired E1-binding. The N-terminal part of E1 is important for the E2-binding as determined by far-Western blotting using the mammalian- and bacterial-expressed E2 proteins as probes. The mammalian-expressed, glycosylated forms of the E1 and E2 proteins exhibited E1-E2 binding activities similar to those of the bacterial-expressed, nonglycosylated forms in pull-down assays, suggesting that glycosylation is not prerequisite for the heteromeric complex formation of E1 and E2.

Recombinant HIV envelope expressed in an alpha-glucosidase I-deficient CHO cell line and its parental cell line in the presence of 1-deoxynojirimycin is functional

alpha-Glucosidase inhibitors-e.g., 1-deoxynojirimycin (DNM)-interfere with HIV infectivity in CD4+ cell cultures but have proven unsuccessful in clinical trials. In vitro, several HIV Env properties, including the cleavage of the Env precursor gp 160, the immunoreactivity of the third variable domain (V3) of Env, the binding to the CD4 receptor, and the induction of the membrane fusion between the virus and the host cell, have been reported to be altered by such inhibitors. We have studied these properties for Env expressed via a recombinant vaccinia virus in two Chinese hamster ovary cell lines, an alpha-glucosidase I-deficient cell line and its parental cell line, treated with DNM under conditions that have been reported to alter Env properties. The glycosylation of Env, but not the quantity produced, varied in accordance with the experimental conditions. However, irrespective of these conditions, Env cleavage, V3 immunoreactivity, CD4 binding, membrane expression, and ability to induce syncytium formation were similar. Thus, neither the alpha-glucosidase I deficiency nor DNM treatment had a significant effect on the properties of Env produced here. Cellular mechanisms that may allow the normal expression of Env are discussed and may offer an explanation for the many discrepant results obtained to date on the effects of DNM on HIV Env.

Effect of various glycosidase treatments on the resistance of the HIV-1 envelope to degradation

Using a CD4-binding assay to assess the conformation of the human immunodeficiency virus envelope glycoprotein (CHO+ Env), we studied the effect of treatment with various glycosidases on the stability of Env in denaturing environments and in biological media: cleavage from Env of either high-mannose-type glycans (HMT- Env) by endoglycosidase H or sialic acid residues (Sial- Env) by sialidase did not alter Env stability whereas its complete deglycosylation (CHO- Env) by N-glycanase had a large effect. The influence of glycan removal on Env sensitivity to proteases was also studied. Thrombin cleavage within V3 was affected by N-glycanase treatment; both HMT- Env and CHO- Env displayed an increased sensitivity to other endoproteases. Thus, partial deglycosylation increases Env sensitivity to proteases but only its total deglycosylation alters its stability.

Differential regulation of the antibody responses to Gag and Env proteins of human immunodeficiency virus type 1

We have studied the antibody responses to Env and Gag antigens of human immunodeficiency virus type 1 (HIV-1) in several cohorts of HIV-1-infected individuals: long-term nonprogressors, progressors to disease, acute seroconvertors, and recipients of HIV-1 protease inhibitors. We conclude that the antibody responses to Env and Gag antigens are differentially regulated and that changes in the plasma viral load in the measurable range (500 to 10(8) RNA copies per ml) do not directly affect the antibody responses to these HIV-1 proteins. We provide quantitative estimates of HIV-1-specific immunoglobulin G concentrations in plasma, which can be in excess of 1 mg/ml for both anti-gp120 and anti-p24 once the immune response to HIV-1 has stabilized after seroconversion. We discuss the apparent paradox that the absence of anti-Gag antibodies (which have, at best, limited antiviral activity) is indicative of disease progression, while the retention of anti-Env antibodies (which do have antiviral activity) is of limited (or no) prognostic value. We show that the disappearance of anti-Gag antibodies during disease progression is highly unlikely to be due to immune complexing; instead, we believe that it reflects the loss of T-cell help that is more necessary for the anti-Gag than the anti-Env response.

Synthesis of alpha-D-Glcp-(1-->2)-alpha-D-Glcp-(1-->3) -alpha-D-Glcp-O-(CH2)8 COOCH3 for use in the assay of alpha-glucosidase I activity

The chemical synthesis of alpha-D-Glcp-(1-->2)-alpha-D-Glcp p-(1-->3) -alpha-D-Glcp-O-(CH2)8 COOCH3 (9), a substrate specific for alpha-glucosidase I, is reported. This enzyme removes the terminal alpha-D-Glcp unit to produce alpha-D-Glcp-(1-->3)-alpha-D-Glcp-O-(CH2)8 COOCH3 (10). This is the first synthetic substrate described for glucosidase I that allows kinetic evaluation of substrates and inhibitors of this enzyme. Tetramethylrhodamine was coupled to 9 through an ethylenediamine linker to produce a brilliant red derivative. Addition of this fluorescent dye did not affect enzyme binding to the substrate, as determined by a comparison of the Km value (1.3 mM). The fluorescent label allows visual detection of 2-3 pmol of product by TLC.

Structural comparison of a 15 residue peptide from the V3 loop of HIV-1IIIb and an O-glycosylated analogue

As part of a program to study the effect of glycosylation on the three-dimensional structures of HIV-1IIIB V3 peptide constructs, we have examined the solution structures of a 15 residue peptide (RIQRGPGRAFVTIGK, P18IIIB)- originally mapped as an epitope recognized by CD8+ Dd class I MHC-restricted murine cytotoxic T-lymphocytes (CTL), and an analogue (P18IIIB-g), O-glycosylated with an alpha-galactosamine on Thr-12, using NMR, circular dichroism and molecular modeling methods. Our studies show that the peptides sample mainly random conformations in aqueous solution near 25 degrees C and become more ordered by the addition of trifluoroethanol. Upon decreasing the temperature to 5 degrees C, a reverse turn is formed around the immunodominant tip (G5-R8). Glycosylation on T12 'tightens' the turn slightly as suggested by NOE and CD analysis. In addition, the sugar has a defined conformation with respect to the peptide backbone and influences the local peptide conformation. These data suggest that simple glycosylation may influence the conformational equilibrium of a V3 peptide which contains a domain critical for antibody recognition and virus neutralization. We also show that the ability of cytotoxic T-lymphocytes (CTL) to lyse tumor cells presenting P18IIIB was completely abrogated by threonine glycosylation.

A model for alignment of Env V1 and V2 hypervariable domains from human and simian immunodeficiency viruses

HIV-1 env gene encodes a multifunctional glycoprotein that is involved in virus infectivity, interactions between the virus and the host immune system, and phenotypic characteristics of virus isolates in culture. A number of Env functions map by genetic analysis to V3, one of five hypervariable domains that compose the surface component of Env gp120. V1 and V2 hypervariable domains of Env also contribute to the phenotype of HIV-1, although relationships between V1 and V2 genotypes and biological characteristics of HIV-1 are not well defined. One limitation to genetic analysis of V1 and V2 is the extensive length variation that results from in-frame deletions or duplications of nucleotides and renders alignments difficult among V1 and V2 sequences from different populations of viruses. We developed a model to facilitate rational alignments of V1 and V2 domains independent of their length. The alignment strategy constrains gap placement in V1 and V2 so that glycan modification motifs and potential alpha helices are intact. The alignment model accommodates the spectrum of HIV-1 subtypes, as well as HIV-2 and SIV V1 and V2 sequences. The model will facilitate genetic analysis and interpretation of amino acid changes in the hypervariable domains. For example, charged and uncharged amino acids are conserved in defined positions in each of the V1 and V2 hypervariable domains from a subset of HIV-1 subtype B isolates. Biochemical characteristics of amino acids in V1 and V2 appear unrelated to cytotropic or syncytium-inducing phenotypes of the viruses.

Conformational preferences of a chimeric peptide HIV-1 immunogen from the C4-V3 domains of gp120 envelope protein of HIV-1 CAN0A based on solution NMR: comparison to a related immunogenic peptide from HIV-1 RF

A critical problem to overcome on HIV vaccine design is the variability among HIV strains. One strategy to solve this problem is the construction of multicomponent immunogens reflective of common HIV motifs. Currently, it is not known if these motifs should be based primarily on amino acid sequence or higher-order structure of the viral proteins of a combination of the two. In this paper, we report NMR-derived solution conformations for a sympathetic peptide taken from the C4 and V3 domains of HIV-1 CAN0A gp120 envelope protein. This peptide, designated T1-SP10CAN0(A), is compared to a recently reported C4-V3 peptide. T1-SP10RF(A) from the HIV-1 RF strain [de Lorimier et al. (1994) Biochemistry 33, 2055-2062], in terms of conformational features and immune responses in mice [Haynes et al. (1995) AIDS Res. Hum. Retroviruses 11, 211-221]. The T1 segment of 16 amino acids from the gp120 C4 domain is identical in both peptides and exhibits nascent helical character. The SP10 region, taken from the gp120 V3 loop, differs from that of T1-SP10RF(A) in both sequence and conformations. A reverse turn is observed at the conserved GPGX sequence. The rest of the Sp10 domain is extended with the exception of the last three residues which show evidence for a helical arrangement. Modeling of the turn region of the T1-SP10CAN0(A) peptide shows exposure of a continuous apolar stretch of side chains similar to that reported in the crystal structure of a V3 peptide from HIV-1 MN complexed with a monoclonal antibody [Rini et al. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 6325-6329]. this hydrophobic patch is interrupted by a charged Lys residue in the T1-SP10RF(A) peptide. This observation suggests that the HIV-1 CAN0A and HIV-1 RF C4-V3 peptides can induce widely different anti-HIV antibodies. consistent with immunogenic results.

Glycosylation and stability of mature HIV envelope glycoprotein conformation under various conditions

The role of the glycans of the mature human immunodeficiency virus (HIV) envelope (gp160) in its stability in various conditions was studied. gp160 conformation was monitored through its subsequent ability to bind [125I]CD4. Treatment of glycosylated (CHO+) gp160 with (i) sodium dodecyl sulfate (SDS) concentrations above 0.01% impaired subsequent CD4 binding while 0.3% SDS abolished it; (ii) beta-mercaptoethanol (MSH) concentrations above 0.01% impaired CD4 binding while 0.03% MSH abolished it; (iii) 2 M guanidine-HCl had no effect; (iv) temperatures between 50 degrees C and 80 degrees C altered CD4 binding while, above 80 degrees C, the binding was abolished; (v) CD4 binding was decreased by 50% by 2 freeze-thaw cycles but was not further affected by subsequent (up to 15) cycles; (vi) gp160 incubation in serum or cell lysate had no effect on CD4 binding. Glycanase treated (CHO-) gp160 binding activity was only 3-fold lower than that of CHO+ gp160. Only 2 M guanidine-HCl and heating at 70 degrees C differentially affected the binding of CHO+ and CHO- gp160, the effects being larger for CHO- gp160. CHO- gp160 binding was impaired after incubation in either serum or cell lysate. Thus, glycans stabilize gp160 conformation in some environments. However, CHO- gp160 appears to be resistant to denaturation as compared to other glycoproteins reported in the literature.

Molecular biology and pathogenesis of animal lentivirus infections

Lentiviruses are a subfamily of retroviruses that are characterized by long incubation periods between infection of the host and the manifestation of clinical disease. Human immunodeficiency virus type 1, the causative agent of AIDS, is the most widely studied lentivirus. However, the lentiviruses that infect sheep, goats, and horses were identified and studied prior to the emergence of human immunodeficiency virus type 1. These and other animal lentiviruses provide important systems in which to investigate the molecular pathogenesis of this family of viruses. This review will focus on two animal lentivirus models: the ovine lentivirus visna virus; and the simian lentivirus, simian immunodeficiency virus. These animal lentiviruses have been used to examine, in particular, the pathogenesis of lentivirus-induced central nervous system disease as models for humans with AIDS as well as other chronic diseases.

Inhibition of human immunodeficiency virus infection of CD4+ cells by CD4-free glycopeptides from monocytic U937 cells

We have previously demonstrated that human immunodeficiency virus (HIV) envelope glycoproteins have specific carbohydrate-binding properties for mannosyl/N-acetylglucosaminyl residues presented at high density on a carrier in vitro. Here, we investigated whether HIV envelope glycoprotein gp120 was able to interact with surface membrane carbohydrates of CD4+ cells by means of such lectin-carbohydrate interactions. CD4-free tryptic glycopeptides, prepared from the membrane of CD4+ monocytic U937 cells and partially purified by ConA-agarose affinity chromatography, could be eluted by mannan but not by methyl-alpha-mannose or methyl-alpha-glucose, which strongly suggests that they displayed oligomannosidic structures. These glycopeptides bound in a mannosyl-specific manner to radiolabeled recombinant gp120. Deglycosylation with N-glycanase which, as expected, strongly diminished binding of the glycopeptides to concanavalin A also abolished their interaction with gp120. In addition, the glycopeptides inhibited HIV infection of both U937 and CD4+ lymphoid CEM cells when preincubated with the virus. These findings indicate that, independently of the binding to CD4, mannosyl structures on CD4+ cells may play a role through lectin-carbohydrate interactions in envelope glycoprotein binding to a putative coreceptor(s) of HIV.

Eukaryotic virus display: engineering the major surface glycoprotein of the Autographa californica nuclear polyhedrosis virus (AcNPV) for the presentation of foreign proteins on the virus surface

We describe the development of the baculovirus Autographa californica nuclear polyhedrosis virus (AcNPV) as a vector for the display of distinct proteins on the viral surface in a manner that is analogous to the established bacterial "phage display" systems. As a model system, the marker gene encoding the 26kDa protein glutathione-S-transferase (GST) was used to construct several fusions with the major baculovirus glycoprotein gp64 gene. Following expression in Spodoptera frugiperda (Sf9) cells, the yield and cellular distribution of each GST-gp64 protein was assessed by Western blot of both cell and supernatant fractions. One fusion, in which GST was inserted between the leader peptide and the nature protein, was found to be efficiently secreted into the cell medium. In the context of expression of the full length gp64, the hybrid GST-gp64 was shown by immunogold labelling to be incorporated onto the virion surface. In addition, the affinity purification of the soluble transmembrane gp64-GST fusion protein resulted in the co-purification of wild type gp64 suggesting that co-oligomerization of the GST-tagged fusion and the wild type molecule was the basis for virion incorporation. The HIV major surface glycoprotein, gp120 was also efficiently displayed in functional form on the viral surface following fusion to the amino terminus of gp64. A general expression vector, pAcSurf-2, was constructed in which multiple cloning sites were positioned in-phase between the gp64 signal sequence and the sequence encoding the mature protein under the control of the polyhedrin promoter.

Lectin-mediated effects on HIV type 1 infection in vitro

Lectins with specificity for terminal mannose residues and anti-mannan antibodies neutralize HIV-1 infection in vitro. This is assumed to be caused by binding of the agents to the viral glycoproteins. In this study we show that one such agent, the Galanthus nivalis lectin (GNA), also blocks infection at the target cell level. To explore the effect of GNA on HIV infection we used the two HIV-1 isolates LAV and NDK, representing in the first case a prototype virus and in the latter case a highly cytopathic virus, which spreads preferentially via cell-to-cell contact. MT-4 cells were used as target cells and infection was determined from the occurrence of syncytia. Cell-to-cell infection was studied with CEM cells persistently infected with the two virus isolates. GNA, at concentrations in the nanogram per milliliter range, neutralized the HIV-1 isolates LAV, NDK, and MN as well as HIV-2ROD. Pretreatment of cells with the lectin, before addition of virus, or of infected cells, also blocked infection. This effect was more pronounced with HIV-1NDK than with HIV-1LAV. Mannosidase treatment of the target cells abolished the GNA effect on HIV-1NDK infection. It is concluded that GNA inhibits infection of several HIV isolates. It neutralizes infection by binding to the virion but also blocks infection at the target cell level. The latter effect may be different for different virus isolates. Mannosyl residuals at the cell surface are targets for GNA modulation of infection with the cytopathic HIV-1NDK. These do not represent essential virus receptors.

Immunological evidence for interactions between the first, second, and fifth conserved domains of the gp120 surface glycoprotein of human immunodeficiency virus type 1

We have used a combination of genetic and immunological techniques to explore how amino acid substitutions in the second conserved (C2) domain of gp120 from human immunodeficiency virus type 1 (HIV-1) affect the conformation of the protein. It was reported previously (R. L. Willey, E. K. Ross, A. J. Buckler-White, T. S. Theodore, and M. A. Martin. J. Viol. 63:3595-3600, 1989) that an asparagine-glutamine (N/Q) substitution at C2 residue 267 of HIV-1 NL4/3 reduced virus infectivity, but that infectivity was restored by a compensatory amino acid change (serine-glutamine; S/N) at residue 128 in the C1 domain. Here we show that the 267 N/Q substitution causes the abnormal exposure of a segment of C1 spanning residues 80 to 120, which compromises the integrity of the CD4-binding site. The reversion substitution at residue 128 restores the normal conformation of the C1 domain and recreates a high-affinity CD4-binding site. The gp120 structural perturbation caused by changes in C2 extends also to the C5 domain, and we show by immunological analysis that there is a close association between areas of the C1 and C5 domains. This association might be important for forming a complex binding site for gp41 (E. Helseth, U. Olshevsky, C. Furman, and J. Sodroski. J. Virol. 65:2119-2123, 1991). Segments of the C1 and C2 domains are predicted to form amphipathic alpha helices. We suggest that these helices might be packed together in the core of the folded gp120 molecule, that the 267 N/Q substitution disrupts this interdomain association, and that the 128 S/N reversion substitution restores it.

Effect of sialic acid removal on the antibody response to the third variable domain of human immunodeficiency virus type-1 envelope glycoprotein

The gp160 envelope glycoprotein of human immunodeficiency virus type-1 (HIV-1) is an essential component of current vaccine trials. The glycans of gp160, part of which are highly sialylated, have been shown to influence gp160 immunogenicity. Here, using a panel of synthetic V3 peptides, we characterized the anti-V3 antibodies generated in rabbits immunized by desialylated recombinant gp160LAI. Amino acid residues flanking the GPGR tip of V3 were necessary for the recognition by anti-V3 antibodies raised against either the native or desialylated gp160. Both types of antibodies reacted to V3 peptides of MN and SF2 strains and with a North American/European V3 consensus peptide, while anti-desialylated gp160LAI antibodies reacted in addition to the V3 of CDC4, WMJ2 and NY5 strains. Yet, the V3 peptides did not significantly differ in their secondary structure, as determined by circular dichroism. The titer and avidity for V3MN of anti-desialylated gp160LAI antibodies were significantly lower than those of anti-native gp160LAI, which likely accounts for the inability of anti-desialylated gp160LAI sera to neutralize HIV-1MN-induced syncytia. These results indicate that V3 immunogenicity may be influenced by subtle directed changes in the gp160 glycosylation pattern.