Identification of a neutralizing domain in the external envelope glycoprotein of simian immunodeficiency virus

Electrical and synaptic integration of glioma into neural circuits

High-grade gliomas are lethal brain cancers whose progression is robustly regulated by neuronal activity. Activity-regulated release of growth factors promotes glioma growth, but this alone is insufficient to explain the effect that neuronal activity exerts on glioma progression. Here we show that neuron and glioma interactions include electrochemical communication through bona fide AMPA receptor-dependent neuron-glioma synapses. Neuronal activity also evokes non-synaptic activity-dependent potassium currents that are amplified by gap junction-mediated tumour interconnections, forming an electrically coupled network. Depolarization of glioma membranes assessed by in vivo optogenetics promotes proliferation, whereas pharmacologically or genetically blocking electrochemical signalling inhibits the growth of glioma xenografts and extends mouse survival. Emphasizing the positive feedback mechanisms by which gliomas increase neuronal excitability and thus activity-regulated glioma growth, human intraoperative electrocorticography demonstrates increased cortical excitability in the glioma-infiltrated brain. Together, these findings indicate that synaptic and electrical integration into neural circuits promotes glioma progression.

HIV-1 envelope subregion length variation during disease progression

The V3 loop of the HIV-1 Env protein is the primary determinant of viral coreceptor usage, whereas the V1V2 loop region is thought to influence coreceptor binding and participate in shielding of neutralization-sensitive regions of the Env glycoprotein gp120 from antibody responses. The functional properties and antigenicity of V1V2 are influenced by changes in amino acid sequence, sequence length and patterns of N-linked glycosylation. However, how these polymorphisms relate to HIV pathogenesis is not fully understood. We examined 5185 HIV-1 gp120 nucleotide sequence fragments and clinical data from 154 individuals (152 were infected with HIV-1 Subtype B). Sequences were aligned, translated, manually edited and separated into V1V2, C2, V3, C3, V4, C4 and V5 subregions. V1-V5 and subregion lengths were calculated, and potential N-linked glycosylation sites (PNLGS) counted. Loop lengths and PNLGS were examined as a function of time since infection, CD4 count, viral load, and calendar year in cross-sectional and longitudinal analyses. V1V2 length and PNLGS increased significantly through chronic infection before declining in late-stage infection. In cross-sectional analyses, V1V2 length also increased by calendar year between 1984 and 2004 in subjects with early and mid-stage illness. Our observations suggest that there is little selection for loop length at the time of transmission; following infection, HIV-1 adapts to host immune responses through increased V1V2 length and/or addition of carbohydrate moieties at N-linked glycosylation sites. V1V2 shortening during early and late-stage infection may reflect ineffective host immunity. Transmission from donors with chronic illness may have caused the modest increase in V1V2 length observed during the course of the pandemic.

The HIV envelope gene (env) encodes viral surface proteins (Env) that are vital to the basic processes used by the virus to infect and cause disease in humans. Adaptations in env determine which cells the virus can infect, and permit the virus to avoid elimination by the immune system. Env is one of the most variable genes known, and it can change dramatically over time in a single individual. However, Env-host cell interactions are complex and incompletely understood, and changes in this viral protein during infection have not yet been systematically described. We examined a large number of env sequences from 154 individuals at various stages of HIV infection but who had never received antiretroviral treatment. We found that the env V1V2 region lengthens during chronic infection and becomes more heavily glycosylated. However, these changes partially reverse during late-stage illness, possibly in response to a weakening host immune system. V1V2 lengths are also increasing over time in the epidemic at large, possibly related to the epidemiology of HIV transmission within the subtype B epidemic. These results provide fundamental insights into the biology of HIV.

Structure-antigenicity of the V3 region of SIVmac envelope glycoprotein

The objective of this study was to analyze the immunogenicity and antigenicity of the V3 domain (Cys313-Cys346) of the external envelope glycoprotein gp125 of SIVmac251. The corresponding peptide was synthesized and characterized as linear and cyclic peptides. Our results showed that this region, as for HIV-1, contained an immunodominant epitope. The antigenicity was similar for the linear and cyclic peptides when tested against a panel of 15 sera from SIV infected macaques. Similarly, both peptide structures presented similar immunogenicity as shown by the characterization of the anti-peptide antibodies produced in rabbits against the cyclic and linear forms. But, unexpectedly, the antibodies produced against linear peptides recognized with a relatively higher intensity the native envelope gp140 than those produced against the cyclic structure. Furthermore, we showed that these antibodies recognized better the deglycosylated form of the glycoprotein. But, in contrast to the neutralizing activity obtained with anti-V3 peptides from HIV-1, no antiviral activity was obtained with antibodies generated against linear or cyclic SIVmac V3 peptides.

Induction of anti-simian immunodeficiency virus cellular and humoral immune responses in rhesus macaques by peptide immunogens: correlation of CTL activity and reduction of cell-associated but not plasma virus load following challenge

Lipopeptides which carry the N-terminal moiety tripalmitoyl-S-glyceryl-cysteinyl-seryl-seryl (P(3)CSS) have been shown to have effective adjuvant and transmembrane carrier properties. To test the ability of these constructs to immunize against simian immunodeficiency virus [(SIV)(mac)] infection, rhesus macaques, prescreened for expression of the Mamu-A*01 MHC class I molecule, were immunized at regular intervals with lipopeptides corresponding to known SIV(mac) CTL epitopes alone or in combination with multiple antigenic peptides corresponding to neutralizing epitopes. Both humoral and CTL responses were elicited and the monkeys, along with non-immunized control animals, were challenged intravenously with 20 MID(50) of the homologous, uncloned SIV(mac251-32H) grown in rhesus monkey PBMC. Although none of the monkeys were protected from infection, most demonstrated an anamnestic CTL response with epitope-specific CTL precursor frequencies reaching as high as 1 in 20 total PBMC as measured by limiting dilution CTL assay or 25% of all CD8(+) T-cells using tetrameric MHC-I/peptide complexes. A significant inverse correlation between the levels of CTLp and the number of infected cells in circulation was observed. However, no such correlation with the plasma viral load (RNA copies/ml) was evident.

Characterization of neutralization epitopes of simian immunodeficiency virus (SIV) recognized by rhesus monoclonal antibodies derived from monkeys infected with an attenuated SIV strain

A major limitation in the simian immunodeficiency virus (SIV) system has been the lack of reagents with which to identify the antigenic determinants that are responsible for eliciting neutralizing antibody responses in macaques infected with attenuated SIV. Most of our information on SIV neutralization determinants has come from studies with murine monoclonal antibodies (MAbs) produced in response to purified or recombinant SIV envelope proteins or intact SIV-infected cells for relatively short periods of time. While these studies provide some basic information on the potential immunogenic determinants of SIV envelope proteins, it is unclear whether these murine MAbs identify epitopes relevant to antibody responses elicited in monkeys during infection with either wild-type or attenuated SIV strains. To accomplish maximum biological relevance, we developed a reliable method for the production of rhesus monoclonal antibodies. In the present study, we report on the production and characterization of a unique panel of monoclonal antibodies derived from four individual monkeys inoculated with SIV/17E-CL as an attenuated virus strain at a time when protective immunity from pathogenic challenge was evident. Results from these studies identified at least nine binding domains on the surface envelope glycoprotein; these included linear determinants in the V1, V2, cysteine loop (analogous to the V3 loop in human immunodeficiency virus type 1), and C5 regions, as well as conformational epitopes represented by antibodies that bind the C-terminal half of gp120 and those sensitive to defined mutations in the V4 region. More importantly, three groups of antibodies that recognize closely related, conformational epitopes exhibited potent neutralizing activity against the vaccine strain. Identification of the epitopes recognized by these neutralizing antibodies will provide insight into the antigenic determinants responsible for eliciting neutralizing antibodies in vivo that can be used in the design of effective vaccine strategies.

Naturally occurring V1-env region variants mediate simian immunodeficiency virus SIVmac escape from high-titer neutralizing antibodies induced by a protective subunit vaccine

Macaques which developed high-titer neutralizing antibodies (htNAb) after immunization with a virion-derived oligomeric envelope glycoprotein subunit vaccine were protected against a homologous simian immunodeficiency virus SIVmac challenge. Here we demonstrate that the htNAb could be overcome by V1-env region variants isolated ex vivo from an SIVmac-infected macaque. The results further suggest that the development of V1-env region neutralization escape mutants is also necessary for survival of the virus in infected macaques. The immunological capacity of a single variable region to induce neutralizing antibodies in vaccinated and infected macaques initiate new ideas for a successful vaccine strategy.

Characterization and epitope mapping of neutralizing monoclonal antibodies produced by immunization with oligomeric simian immunodeficiency virus envelope protein

In an attempt to generate broadly cross-reactive, neutralizing monoclonal antibodies (MAbs) to simian immunodeficiency virus (SIV), we compared two immunization protocols using different preparations of oligomeric SIV envelope (Env) glycoproteins. In the first protocol, mice were immunized with soluble gp140 (sgp140) from CP-MAC, a laboratory-adapted variant of SIVmacBK28. Hybridomas were screened by enzyme-linked immunosorbent assay, and a panel of 65 MAbs that recognized epitopes throughout the Env protein was generated. In general, these MAbs detected Env by Western blotting, were at least weakly positive in fluorescence-activated cell sorting (FACS) analysis of Env-expressing cells, and preferentially recognized monomeric Env protein. A subset of these antibodies directed toward the V1/V2 loop, the V3 loop, or nonlinear epitopes were capable of neutralizing CP-MAC, a closely related isolate (SIVmac1A11), and/or two more divergent strains (SIVsmDeltaB670 CL3 and SIVsm543-3E). In the second protocol, mice were immunized with unfixed CP-MAC-infected cells and MAbs were screened for the ability to inhibit cell-cell fusion. In contrast to MAbs generated against sgp140, the seven MAbs produced using this protocol did not react with Env by Western blotting and were strongly positive by FACS analysis, and several reacted preferentially with oligomeric Env. All seven MAbs potently neutralized SIVmac1A11, and several neutralized SIVsmDeltaB670 CL3 and/or SIVsm543-3E. MAbs that inhibited gp120 binding to CD4, CCR5, or both were identified in both groups. MAbs to the V3 loop and one MAb reactive with the V1/V2 loop interfered with CCR5 binding, indicating that these regions of Env play similar roles for SIV and human immunodeficiency virus. Remarkably, several of the MAbs generated against infected cells blocked CCR5 binding in a V3-independent manner, suggesting that they may recognize a region analogous to the conserved coreceptor binding site in gp120. Finally, all neutralizing MAbs blocked infection through the alternate coreceptor STRL33 much more efficiently than infection through CCR5, a finding that has important implications for SIV neutralization assays using CCR5-negative human T-cell lines.

Characterization of neutralizing sites in the second variable and fourth variable region in gp125 and a conserved region in gp36 of human immunodeficiency virus type 2

Several determinants of human immunodeficiency virus (HIV) have been suggested to harbor sites important for neutralization. The third variable region (V3) of the envelope glycoprotein (gp) is an important neutralizing determinant for both serotypes of HIV. The localization of additional neutralizing regions is an urgent task because the virus appears to mutate to phenotypes that escape neutralizing antibodies. Therefore, we have focused on the possibility of finding other immunodominant regions in the envelope glycoproteins of human immunodeficiency virus type 2 (HIV-2). By immunization of guinea pigs with peptides corresponding to different selected regions of gp125 and gp36, we have found three antigenic determinants located in the V2 and V4 regions of the envelope protein gp125, and one region in the glycoprotein gp36, which are important for human antibody binding and also as targets for neutralization. The peptide representing the V2 region had the most pronounced capacity to induce neutralizing anti-HIV-2 antibodies in guinea pigs. Neutralizing activity was also detected in an antipeptide guinea pig sera representing a linear site in gp36, amino acids 644-658. A substitution set of peptides representing the conserved antigenic site in the central part of gp36 was used to identify the role of individual amino acids important for human antibody binding.

Production and characterization of SIV envelope-specific rhesus monoclonal antibodies from a macaque asymptomatically infected with a live SIV vaccine

Five rhesus monoclonal antibodies (RhMAbs) were produced by rhesus EBV transformation of peripheral blood B cells from a rhesus macaque that had been asymptomatically infected with an attenuated, macrophage-tropic SIV strain, 17E-Cl. These MAbs recognized conformation-dependent epitopes on SIV gp120 and could not be mapped using synthetic peptides. All five RhMAbs were able to neutralize the vaccine strain and a heterologous isolate, SIV/DeltaB670. The RhMAbs did not cross-react with HIV-2; by contrast, four human MAbs derived from an HIV-2-infected person were broadly cross-reactive with both SIV and HIV-2 gp120s. Cross-competition analysis indicated that the five RhMAbs could be placed in two groups recognizing two nonoverlapping epitopes; while the HMAbs were placed in two additional competition groups. Binding of the three group I RhMAbs (1.7F, 3.11B, and 1.10A) as well as HMAb 17A was shown to be sensitive to specific amino acid alterations in V4 occurring in natural env variants. The results of this study demonstrate that RhEBV transformation provides a means to probe rhesus antibody responses to SIV infection at the monoclonal level. RhMAbs will allow structural and functional studies of envelope glycoprotein determinants that elicit protective immune responses against SIV.

Hypervariable epitope constructs representing variability in envelope glycoprotein of SIV induce a broad humoral immune response in rabbits and rhesus macaques

Using synthetic peptides, we developed an approach to account for protein epitope variability. We have prepared, in a single synthesis, a cocktail of peptides we have designated a hypervariable epitope construct (HEC), which collectively represents much of the in vivo variability seen in an epitope. Eight HECs representing the in vivo variability seen throughout the envelope glycoprotein of the simian immunodeficiency virus (SIV) were designed and synthesized. The constructs were collectively conjugated to KLH (HEC-KLH) or recombinant gp130 (HEC-rgp130) and used to immunize rabbits and rhesus macaques, respectively. Using sera collected from rabbits immunized with HEC-KLH, we demonstrated that individual components of the immunogen were recognized as antigen in ELISAs, and that the induced antibodies cross-reacted with several strains of SIV as well as with a strain of HIV-2. Following immunization of macaques with HEC-rgp130 antiviral antibodies were induced. These antibodies were still present 9.5 months after the last boost and were also capable of recognizing several different strains of SIV, including SIVmac239, SIVmac251, and SIVsmH3, as well as a strain of HIV-2 (HIV-2ROD). In addition, the antibodies were also capable of neutralizing SIV viral infectivity in vitro. Peripheral blood lymphocytes (PBLs) from immunized macaques proliferated in response to whole proteins and virus. Finally, sera from monkeys immunized with SIV, rgp130, and HIV-2 as well as sera from HIV-2-positive humans recognized HECs in ELISAs, demonstrating the relevance of these epitopes in vivo. This approach can be used as an effective method for generating a strong, broadly cross-reactive humoral response against HIV and can serve as an important component of combination vaccines against HIV and AIDS.

Antigenicity of linear and cyclic peptides mimicking the disulfide loops in HIV-2 envelope glycoprotein: synthesis, reoxidation and purification

The external envelope glycoprotein (gp125) of human immunodeficiency virus type 2 (HIV-2) contains 22 cysteine residues. The positions of the 11 disulfide bridges in HIV-2 gp125 were determined by analogy with the experimental position of the disulfide bonds found in the gp120 of HIV-1. Peptides expected to mimic all 11 disulfide-bonded domains containing from 13 to 47 amino acids were synthesized by the solid-phase method according to 9-fluorenylmethoxycarbonyl strategy, except for peptide 5, which was assembled according to t-butoxycarbonyl (Boc) strategy. Analysis of all the crude peptides showed that the expected peptides were obtained with good yields, between 75% and 85%. Peptides were purified further by high-performance liquid chromatography (HPLC) on an Aquapore RPC30 C8 column. Peptide homogeneity was more than 90%. For each peptide, linear peptides (L) were SH-iodoacetamidated, whereas cyclization of peptides (C) was performed by air oxidation. Oxidation kinetics was followed with the Ellman test and HPLC. Cyclic peptides were purified by HPLC and characterized by fast atom bombardment mass spectrometry. This analysis showed that a small quantity (<10%) of dimeric peptides (2 and 8) and cyclic peptides containing oxidized methionine or tryptophan residues (4, 9 and 10) were formed. To assess the relevance of conformation for the antigenicity of disulfide-bonded loops of HIV-2 gp125, the antigenicity of linear and cyclic peptides was tested against a set of 76 HIV-2 positive human sera by enzyme-linked immunosorbent assay. Peptides 2, 4 and 9, mimicking the V1, V2 and V3 regions of the external envelope glycoprotein (gp 125) of HIV-2, were the most highly reactive with HIV-2 positive human sera tested at the dilution of 1:50. Cyclic peptides generally were recognized more than linear peptides, as shown by their greater inhibition (2 to 10 times more) of antigen-antibody complexes. Structure-antigenicity of peptide V3, the most reactive peptide (75% of the HIV-2 positive sera tested), was analyzed further. Cyclic peptide 9C had a higher affinity for anti-gp125 antibodies than linear peptide 9L. In addition, circular dichroism showed that linear and cyclic peptides 9 had a similar structure, but when analyzed in aqueous solution or in trifluoroethanol (TFE), the structural difference shown with antibodies was not confirmed. No significant difference was observed between the antigenicity of linear and cyclic peptides 1, 8 and 11, mimicking the C1, C2 and C4 regions of HIV-1 gp125. These peptides were weakly reactive with HIV-2 positive sera. This result agrees with the low immunogenicity of conserved regions.

Simian immunodeficiency virus (SIV) envelope-specific Fabs with high-level homologous neutralizing activity: recovery from a long-term-nonprogressor SIV-infected macaque

An antibody phage display library was constructed from RNA extracted from lymph node cells of a simian immunodeficiency virus (SIV)-infected long-term-nonprogressor macaque. Seven gp120-reactive Fabs were obtained by selection of the library against SIV monomeric gp120. Although each of the Fabs was unique in sequence, there were two distinct groups based on epitope recognition, neutralizing activity in vitro, and molecular analysis. Group 1 Fabs did not neutralize SIV and bound to a linear epitope in the V3 loop of the SIV envelope. In contrast, two of the group 2 Fabs neutralized homologous, neutralization-sensitive SIVsm isolates with high efficiency but failed to neutralize heterologous SIVmac isolates. Based on competition enzyme-linked immunosorbent assays with mouse monoclonal antibodies of known specificity, these Fabs reacted with a conformational epitope that includes domains V3 and V4 of the SIV envelope. These neutralizing and nonneutralizing Fabs provide valuable standardized and renewable reagents for studying the role of antibody in preventing or modifying SIV infection in vivo.

Identification of the V1 region as a linear neutralizing epitope of the simian immunodeficiency virus SIVmac envelope glycoprotein

The sequence variability of viral structure polypeptides has been associated with immune escape mechanisms. The V1 region of simian immunodeficiency virus (SIV) is a highly variable region of the SIVmac env gene. Here, we describe the V1 region as a linear neutralizing epitope. V1 region-specific neutralizing antibodies (NAb) were first demonstrated in a rabbit infected with a recombinant vaccinia virus carrying the env gene of human immunodeficiency virus type 2 strain ben (HIV-2ben). Since we detected in this animal V1 region-specific NAb that were able to neutralize not only human immunodeficiency virus type 2 but also SIVmac32H, we investigated whether a similar immune response is evoked in macaques (Macaca mulatta) either infected with SIVmac or immunized with the external glycoprotein (gp130) of the same virus. Distinctly lower NAb titers were found in the SIVmac-infected animals than in the gp130-immunized macaques. Since the NAb titers in both groups were high enough for competition experiments, we used five overlapping peptides encompassing the whole V1 region for a detailed identification of the epitope. In each of the 12 macaques investigated, we detected a high level of NAb reacting with at least one peptide located in the central part of the V1 region. The relatively high degree of divergence, especially within the central part of the V1 region, which characterized the evolution of the retroviral sequences from the original inoculum in the infected macaques suggests the development of escape mutants. Furthermore, 3 of 12 animals developed NAb directed against the amino-terminal end of the V1 region epitope. Sequence analysis, however, revealed relatively low levels of genetic drift and genetic variability within this part of the V1 region. The induction of V1 env-specific NAb not only in gp130-immunized macaques but also in SIVmac-infected animals in combination with the increased genetic variability of this region in vivo indicates a marked biological significance of this epitope for the virus.

Neutralization sensitivity of cell culture-passaged simian immunodeficiency virus

CEMx174- and C8166-45-based cell lines which contain a secreted alkaline phosphatase (SEAP) reporter gene under the control of a tat-responsive promoter derived from either SIVmac239 or HIV-1(NL4-3) were constructed. Basal levels of SEAP activity from these cell lines were low but were greatly stimulated upon transfection of tat expression plasmids. Infection of these cell lines with simian immunodeficiency virus (SIV) or human immunodeficiency virus type 1 (HIV-1) resulted in a dramatic increase in SEAP production within 48 to 72 h that directly correlated with the amount of infecting virus. When combined with chemiluminescent measurement of SEAP activity in the cell-free supernatant, these cells formed the basis of a rapid, sensitive, and quantitative assay for SIV and HIV infectivity and neutralization. Eight of eight primary isolates of HIV-1 that were tested induced readily measurable SEAP activity in this system. While serum neutralization of cloned SIVmac239 was difficult to detect with other assays, neutralization of SIVmac239 was readily detected at low titers with this new assay system. The neutralization sensitivities of two stocks of SIVmac251 with different cell culture passage histories were tested by using sera from SIV-infected monkeys. The primary stock of SIVmac251 had been passaged only twice through primary cultures of rhesus monkey peripheral blood mononuclear cells, while the laboratory-adapted stock had been extensively passaged through the MT4 immortalized T-cell line. The primary stock of SIVmac251 was much more resistant to neutralization by a battery of polyclonal sera from SIV-infected monkeys than was the laboratory-adapted virus. Thus, SIVmac appears to be similar to HIV-1 in that extensive laboratory passage through T-cell lines resulted in a virus that is much more sensitive to serum neutralization.

Specific N-linked and O-linked glycosylation modifications in the envelope V1 domain of simian immunodeficiency virus variants that evolve in the host alter recognition by neutralizing antibodies

During progression to AIDS in simian immunodeficiency virus (SIV) Mne-infected macaques, viral variants are selected that encode sequences with serine and threonine changes in variable region 1 (V1) of the surface component of the viral envelope protein (Env-SU). Because these serine and threonine amino acid changes are characteristic of sites for O-linked and N-linked glycosylation, we examined whether they were targets for modification by carbohydrates. For this purpose, we used several biochemical methods for analyzing the Env-SU protein encoded by chimeras of SIVMneCL8 and envelope sequences cloned from an SIVMneCL8-infected Macaca nemestrina during clinical latency and just after the onset of AIDS. The addition of an N-linked glycan was demonstrated by changes in the electrophoretic mobility of Env-SU, and this was verified by specific glycanase digestions and a detailed analysis of the molecular mass of partially purified Env-SU by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). Molecular mass calculations by MALDI-TOF MS also demonstrated an increased mass, from 102.3 to 103.5 kDa, associated with serine and threonine residues predicted to be O-linked glycosylation sites. Together, these data provide the first direct evidence that the carbohydrate profile of Env-SU is distinct in SIV variants that evolve during infection of the host. Moreover, our studies show that these changes in glycosylation in V1 were directly associated with changes in antigenicity. Specifically, serine and threonine changes in V1 allowed the virus to escape neutralization by macaque sera that contained antibodies that could neutralize the parental virus, SIVMneCL8. The escape from antibody recognition appeared to be influenced by either O-linked or N-linked carbohydrate additions in V1. Moreover, when glycine residues were engineered at the positions where serine and threonine changes evolve in V1 of SIVMneCL8, there was no change in antigenicity compared to SIVMneCL8. This suggests that the amino acids in V1 are not part of the linear epitope recognized by neutralizing antibody. More likely, V1-associated carbohydrates mask the major neutralizing epitope of SIV. These experiments indicate that the selection of novel glycosylation sites in the V1 region of envelope during the course of disease is driven by humoral immune responses.

Production and characterization of monoclonal antibodies to simian immunodeficiency virus envelope glycoproteins

Twelve monoclonal antibodies (MAbs), TB1 to TB12, were produced against a soluble vaccinia recombinant envelope glycoprotein (gp140) from simian immunodeficiency virus SIVmac251. These MAbs recognized SIV gp140 with a relatively high affinity (K0.5 from 6.7 x 10(-8) to 4 x 10(-9) M). All the MAbs except TB9, TB11, and TB12 cross-reacted with HIV-2 envelope glycoproteins, but none of the 12 MAbs recognized those from HIV-1. Using a panel of 87 overlapping synthetic peptides containing 20 amino acid residues, with an overlap of 10 amino acids and spanning the entire primary sequence of gp140, 3 linear epitopes were identified. The first mapped with a neutralizing MAb, TB12, which recognized a linear sequence around amino acids 28-31 within the N-terminal end of the external envelope glycoprotein. The two other new nonneutralizing MAbs recognized linear epitopes around amino acid sequence 380-381 by MAbs TB1, TB2, and TB3, and at the transmembrane glycoprotein amino acids 581-600 by MAb TB6. Seven of the 12 MAbs, TB4, TB5, TB7-9, TB10, and TB11, failed to bind the linear synthetic peptides in ELISA. Moreover, among these seven MAbs only MAbs TB4, TB5, TB9, and TB10 failed to recognize SIV envelope glycoproteins in Western blot (WB) or ELISA after reduction of disulfide bridges by dithiothreitol (DTT), suggesting that they are directed against conformational or discontinuous epitopes. It is of interest to note that MAb TB10 can block the binding of gp140 to the CD4 receptor when the MAb is previously incubated with gp140. Consistent with this result, MAb TB10 cannot bind to gp140 that has been previously complexed with the CD4 receptor. All these results suggest that MAb TB10 recognizes a conformational or discontinuous epitope overlapping or close to the CD4-binding site. These properties are probably implicated in the neutralizing activity observed with this MAb.

Location, exposure, and conservation of neutralizing and nonneutralizing epitopes on human immunodeficiency virus type 2 SU glycoprotein

Eleven rat monoclonal antibodies (MAbs) that recognize the SU glycoprotein of human immunodeficiency virus type 2 (HIV-2) ROD were produced and characterized. Binding sites for eight of these MAbs were mapped to epitopes within the Cl, V1/V2, C2, and V3 envelope regions. The three other MAbs defined at least two conformation-dependent, strain-specific epitopes outside Vl/V2, V3, and the CD4-binding site. The MAbs were used to probe the tertiary structure of oligomeric envelope glycoprotein expressed on the surfaces of infected cells. Epitopes at the apices of V2 and V3 were exposed on the native molecule, whereas other epitopes on V1/V2, Cl, and C2 were hidden. The MAbs defined three neutralization targets on exposed domains: two linear epitopes in the V2 and the V3 loops and one conformational epitope outside V1, V2, and V3.

Neutralising epitopes of simian immunodeficiency virus envelope glycoprotein

Monoclonal and polyclonal antibodies with weak SIV neutralising activity bind to the V2 and V4 regions of gp120 or bind to the amino acids DWNND in gp41. Antibodies with the most potent neutralising activity recognise conformation-dependent epitopes involving the V3 and V4 regions of gp120. Monoclonal antibodies that map to the V3 region of SIVmac failed to neutralise. However, one antibody to SIV AGM neutralised but only in the presence of soluble CD4.

Neutralizing monoclonal antibody against a external envelope glycoprotein (gp110) of SIVmac251

Three monoclonal antibodies (M318T, M56S and M815) against an external envelope glycoprotein (gp110) of simian immunodeficiency virus (SIV) mac251 were obtained by immunizing BALB/c mice with recombinant gp110 (rgp110). All three monoclonal antibodies reacted with the surface of cells infected with SIVmac251 but not with that of uninfected counterparts. The binding activity of these monoclonal antibodies against native gp110 was confirmed by means of Western blotting. One of them, M318T neutralized SIVmac251 infection both by cell-free and cell-associated viruses. M318T cross-reacted with human immunodeficiency virus type 2 strains (HIV-2 GH1 and ROD isolates) and SIVmac239 isolates. However, the antibody did not cross-neutralize these viral strains. Epitope mapping revealed that the neutralizing epitope recognized by M318T was localized at 8 residues between amino acids 178 and 185 (KRDKTKEY) in gp110, corresponding to the V2 region of human immunodeficiency virus type 1 (HIV-1). Because some antibodies against the V2 region of HIV-1 reportedly neutralize virus infection by interfering with CD4-gp120 interaction, we tested the activity of M318T against the binding of CD4-gp110. However, M318T did not inhibit CD4-gp110 interaction, suggesting the involvement of another unknown mechanism of M318T-mediated neutralization. In analogy with the V2 region of HIV-1, the V2 region of SIV contains a type specific neutralizing epitope recognized by M318T. Although some amino acid sequence in the epitope was conserved for the isolates of SIV and HIV-2 and there was cross-reactivity of the antibody against the strains, neutralization by M318T was associated with a single amino acid (182 T) in the epitope.

Identification and characterization of monoclonal antibodies specific for polymorphic antigenic determinants within the V2 region of the human immunodeficiency virus type 1 envelope glycoprotein

We have identified six monoclonal antibodies (MAbs) mapping to both linear and conformation-dependent epitopes within the V2 region of the human immunodeficiency virus type 1 clone HXB10. Three of the MAbs (12b, 66c, and 66a) were able to neutralize the molecular clones HXB10 and HXB2, with titers in the range of 9.5 to 20.0 micrograms/ml. MAbs mapping to the crown of the V2 loop (12b, 60b, and 74) bound poorly to cell surface-expressed oligomeric gp120, suggesting an explanation for the poor or negligible neutralizing activity of MAbs to this region. In contrast, MAbs 12b and 60b demonstrated good reactivity with recombinant gp120 in an enzyme-linked immunosorbent assay format, suggesting differential epitope exposure between the recombinant and native forms of gp120. Cross-competition analysis of these MAbs and additional V1V2 MAbs for gp120 binding enabled us to assign the MAbs to six groups (A to F). Selection of neutralization escape mutants with MAbs 10/76b and 11/68b, belonging to nonoverlapping competition groups, identified amino acid changes at residues 165 (I to T) and 185 (D to N), respectively. Interestingly, these escape variants remained sensitive to neutralization by the nonselecting V2 MAbs. All MAbs demonstrated good recognition of IIIB viral gp120 yet failed to neutralize nonclonal stocks of IIIB. In addition, MAbs 12b and 62c bound MN and RF viral gp120, respectively, yet failed to neutralize the respective isolates. Cloning and expression of a library of gp120 and V1V2 fragments from IIIB-, MN-, and RF-infected H9 cultures identified a number of polymorphic sites, resulting in antigenic variation and subsequent loss of V2 MAb recognition. In contrast, the V3 region from the clones of the same isolates showed no amino acid changes, suggesting that the V2 region is polymorphic in long-term-passaged laboratory isolates and may account for the reduced antibody recognition observed.

B-cell continuous epitopes of the SIVmac-251 envelope protein in experimentally infected macaques

The humoral immune response of 34 macaques experimentally infected with SIVmac-251 was studied using a combination of an epitope library and synthetic peptides. The course of the immune response was checked for up to 9 months postinfection with a panel of clones expressing SIV fragments. A systematic study was performed with synthetic peptides covering the whole transmembrane (TM) and external (SU) envelope proteins. Seven major immunodominant epitopes were characterized. Four are localized in the SU protein: one in the V1 region (111-130), one in the Cys loop of the V3 region (311-330) and two in the C-terminal end (501-520 and 511-530). Three are localized in the TM protein: one in the extracellular domain (601-619), one in the anchor domain (731-750) and one in the intracytoplasmic domain (861-881). Among these epitopes, only one, 601-619, was found to be reactive with all sera and can be defined as the principal immunodominant epitope.

Characterization of monoclonal antibodies to human immunodeficiency virus type 2 envelope glycoproteins

Twelve murine monoclonal antibodies (MAbs) to human immunodeficiency virus type 2 (isolate ROD) envelope glycoproteins have been generated and characterized. Nine MAbs were specific to the external gp125 and three reacted with the transmembrane gp36. A large majority of MAbs displayed a significant affinity for the native gp140 precursor and were shown to bind to viral antigens on the surface of fixed HIV-2-infected cells. In Western blot analysis, the 12 MAbs showed varying profiles of cross-reactivity, but none of the MAbs cross-reacted with the HIV-1LAI envelope. Six MAbs reacted exclusively with the homologous HIV-2ROD isolate whereas only two MAbs displayed cross-reactivity with HIV-2ROD, HIV-2EHO, and SIVmac251. The four other MAbs cross-reacted with either HIV-2EHO or SIVmac251. Results of competitive binding assays indicated that the three anti-gp36 MAbs shared the same competition group, whereas at least eight competition groups were defined with the nine anti-gp125 MAbs. The epitopes of the three anti-gp36 and four anti-gp125 MAbs have been delineated using synthetic peptides or by immunological screening of an SIVmac251 peptide library expressed in yeast. The anti-gp36 MAbs are directed against the same domain of the transmembrane gp36 corresponding to the major antigenic determinant of HIV-2 and HIV-1. The four anti-gp125 MAbs recognize four distinct epitopes localized in the V2, V3, and C1 domains. None of the 12 MAbs displayed neutralizing activity against HIV-2ROD, including the 2 MAbs directed against the V2 and V3 domains.(ABSTRACT TRUNCATED AT 250 WORDS)

Vaccine-induced neutralizing antibodies directed in part to the simian immunodeficiency virus (SIV) V2 domain were unable to protect rhesus monkeys from SIV experimental challenge

The potential of the simian immunodeficiency virus (SIV) variable 2 (V2) domain as an effective region to boost SIV-neutralizing antibodies and to protect against live SIV challenge was tested in rhesus macaques. In this study, two rhesus macaques were primed with vaccinia virus recombinants expressing the surface glycoprotein gp140 of SIVmac and were given booster injections with the SIVmac V2 domain presented by a highly immunogenic carrier, the hepatitis B surface antigen (HBsAg). The two vaccinated macaques exhibited SIV-neutralizing antibodies after primer injections that were enhanced by the V2/HBsAg injections. Part of these SIV-neutralizing antibodies were directed specifically to the V2 region, as shown by neutralization-blocking experiments. However, despite having consistent SIV-neutralizing antibody titers, animals were not protected against homologous challenge with BK28, the molecular clone of SIVmac251. No SIV envelope-specific cellular cytotoxic response was detected throughout the immunization protocol, suggesting that neutralizing antibodies directed to SIV envelope gp140 and especially to the V2 domain were unable on their own to protect against SIV challenge. Furthermore, the vaccinees seemed to have higher viral loads than control animals after challenge, raising the question of whether neutralizing antibodies induced by vaccination and directed to the SIV envelope selected viral escape mutants, as shown previously in SIV-infected macaques. This mechanism is certainly worthy of intensive investigation and raises some concern for SIV envelope-targeted immunization.

Effects of natural sequence variation on recognition by monoclonal antibodies neutralize simian immunodeficiency virus infectivity

The determinants of immune recognition by five monoclonal antibodies (KK5, KK9, KK17, Senv7.1, and Senv101.1) that neutralize simian immunodeficiency virus infectivity were analyzed. These five neutralizing monoclonal antibodies were generated to native SIVmac251 envelope glycoprotein expressed by a vaccinia virus recombinant vector. All five recognize conformational or discontinuous epitopes and require native antigen for optimal recognition. These monoclonal antibodies also recognize SIVmac239 gp120, but they do not recognize gp120 of two natural variants of SIVmac239, 1-12 and 8-22, which evolved during the course of persistent infection in vivo (D.P.W. Burns and R.C. Desrosiers, J. Virol. 65:1843-1854, 1991). Recombinant viruses which were constructed by exchanging variable regions between SIVmac239 and variant 1-12 were used to define domains important for recognition. Radioimmunoprecipitation analysis demonstrated that sequence changes in variable regions 4 and 5 (V4/V5) were primarily responsible for the loss of recognition of the 1-12 variant. Site-specific mutants were used to define precise changes that eliminate recognition by these neutralizing antibodies. Changing N-409 to D, deletion of KPKE, and deletion of KEQH in V4 each resulted in loss of recognition by all five monoclonal antibodies. SIVs with these natural sequence changes are still replication competent and viable. Changing A-417 to T or A/N-417/418 to TK in V4 or Q-477 to K in V5 did not alter recognition detectably. These results define specific, naturally occurring sequence changes in V4 of SIVmac that result in loss of recognition by one class of SIVmac neutralizing antibodies.

A novel, glycan-dependent epitope in the V2 domain of human immunodeficiency virus type 1 gp120 is recognized by a highly potent, neutralizing chimpanzee monoclonal antibody

An anti-gp120 monoclonal antibody (MAb), C108G (gamma 1, kappa), was isolated from a chimpanzee that had been infected with strain IIIB of human immunodeficiency virus type 1 (HIV-1IIIB) and subsequently immunized with the recombinant glycoprotein rgp160MN. This MAb is specific for the IIIB strain of HIV-1 and related clones and exhibits very potent neutralization of these viruses; e.g., 100% neutralization of approximately 8 x 10(3) infectious units of HXB2 was achieved with 125 ng of C108G per ml. Commensurate with this potent neutralizing activity, the apparent affinity of C108G for rgp160LAI was very high, i.e., approximately 3 x 10(10) liters/mol. The C108G epitope was not destroyed by reduction of gp120 disulfide bonds but was profoundly disrupted by removal of N-linked sugars from gp120. Despite the importance of a glycan(s) in forming the C108G epitope, specific binding of C108G to synthetic peptides overlapping in amino acids 162 to 169 of the V2 region was detected, albeit with an affinity approximately 2,000-fold lower than that of C108G's binding to glycosylated envelope protein. This epitope mapping correlated with results of competition assays using MAbs of known epitope specificities. To our knowledge, this is the first description of an anti-V2 MAb raised in response to HIV-1 infection. Its potent neutralizing activity and epitope specificity indicate that the V2 domain of gp120 may be an effective target of the protective immune response and, therefore, potentially an important component of HIV vaccines.

Hypervariable epitope constructs as a means of accounting for epitope variability

Epitope variability is one of the greatest obstacles to development of synthetic peptide vaccines. Based on a recently described hypervariable epitope (aa 414-434) on the envelope glycoprotein (gp130) to simian immunodeficiency virus (SIVmac142), we have developed a novel approach to account for epitope variability. We have prepared, in a single synthesis, a cocktail of peptides, designated a hypervariable epitope construct (HEC), which collectively represent all the in vivo variability seen in an epitope. The HEC represents permutations of amino acid substitutions found in the epitope and has been able to induce antibodies with enhanced binding to native SIV and broad immunoreactivity to related epitope analogues.

Specificity of antipeptide antibodies produced against V2 and V3 regions of the external envelope of human immunodeficiency virus type 2

The V2 region of simian immunodeficiency virus (SIV) and V3 region of human immunodeficiency virus type 1 (HIV-1) have been reported to be neutralization epitopes. We analysed the corresponding regions in HIV-2. Synthetic peptides modeling the V2 (aa 149-168) and V3 (CV3: aa 298-315 and NV3: aa 306-324) regions of the HIV-2 external envelope glycoprotein were coupled to KLH and used as immunogens in rabbits. We characterized the resulting antiV2 and antiV3 antibodies for their ability to recognize native and deglycosylated HIV-2 envelope glycoprotein, to block gp-CD4 interaction and to inhibit syncytium formation in vitro. The three synthetic peptides induced antibodies able to recognize specifically the native HIV-2 envelope glycoprotein with a significant avidity (K0.5 between 6 x 10(-7) and 8 x 10(-9) M). Interestingly, the reactivity of antibodies produced against the V2 peptide, which contains two potential sites of N-glycosylation, was higher against the fully deglycosylated than glycosylated HIV-2 external envelope glycoprotein (gp105). The antipeptide antibodies were used to investigate the topography of these regions in the preformed gp-CD4 complex in indirect immunofluorescence assays. The V2 and V3 regions in the complex remained accessible to their respective antibodies. Moreover, preincubation of gp105 with anti V2 or anti V3 antibodies did not prevent gp-CD4 interaction. Thus the V2 and V3 regions are not directly involved in the gp105 binding site for the CD4 receptor. Finally, in contrast with results obtained with antibodies produced against the V3 region of HIV-1 gp120 and monoclonal antibodies produced against the V3 of SIV, antibodies produced against V2 and V3 of HIV-2 were unable to inhibit syncytium formation induced by HIV-2 in vitro.

Passive immunization of cynomolgus macaques with immune sera or a pool of neutralizing monoclonal antibodies failed to protect against challenge with SIVmac251

In the first of two passive transfer experiments, three groups of four macaques were injected intraperitoneally with a normal serum pool, an immune serum pool (pool 1) collected 132-172 weeks postinfection with the 11/88 pool of SIVmac251, or with a pool of four neutralizing monoclonal antibodies (KK9, 17, 54, and 56) raised against gp120 of the 11/88 pool. Sera were given at a dose of 13 ml/kg whereas the MAb pool was given at 30 ml/kg. In a second experiment, a further four macaques were injected with an immune serum pool (pool 2) collected 12 weeks postinfection with simian-grown SIVmac251 at a dose of 19 ml/kg. Animals in both experiments were challenged with SIVmac251 grown in simian peripheral blood lymphocytes. Despite high levels of circulating antibodies in the serum of animals that received either the immune serum pools or the MAbs, all macaques became infected following challenge. The results described are in contrast to a previous report in which passive transfer of sera from animals infected with SIVsm successfully protected against challenge with the homologous virus grown in human PBMCs. Challenge with SIVmac251 grown in simian PBMCs may be the reason for these conflicting results. Nevertheless, the results suggest that in this model the presence of circulating neutralizing antibodies alone does not necessarily confer protection against challenge with SIVmac251 grown in simian cells.

Characterization of mutants of human immunodeficiency virus type 1 that have escaped neutralization by a monoclonal antibody to the gp120 V2 loop

The biologically cloned human immunodeficiency virus type 1 (HIV-1) RF isolate is sensitive to neutralization by the murine monoclonal antibody (MAb) G3-4 to a conformationally sensitive epitope in the V2 loop of HIV-1 gp120. To assess how variation in the V2 amino acid sequence affects neutralization by this MAb, we cultured RF in the presence of G3-4 to select neutralization escape mutants. Three such mutants resistant to G3-4 neutralization were generated from three independent experiments. Solubilized gp120 from each of these escape mutants had a reduced affinity for G3-4 and also for two other V2 MAbs that were able to bind the wild-type RF gp120. PCR sequencing of the entire gp120 of the wild-type RF virus and the escape mutants showed that amino acid substitutions had occurred only at two positions, Y177H and L179P, both in V2. Experimental introduction of the Y177H substitution into the RF V2 loop in the context of the NL4-3 molecular clone re-created the G3-4-resistant phenotype. The L179P mutant was not viable. Thus, our findings confirm that the HIV-1 V2 loop contains the conformationally sensitive neutralization epitope recognized by G3-4 and that a single amino acid substitution within this region can result in escape variants that arise from immune selection pressure.

Envelope sequence variation, neutralizing antibodies, and primate lentivirus persistence

Studies in ungulate lentivirus systems clearly indicate that neutralization escape variants emerge over time in chronically infected animals. Studies in the EIAV system, in particular, have provided strong evidence that the humoral branch of the immune system is at least one selective force acting on an array of viral variants. In previous studies with the ungulate lentiviruses, molecularly cloned virus was never used, and plaque-purified virus was only sometimes used; the genetic determinants responsible for antigenic variation and immune selection were not determined. While molecular clones are available for HIV-1, immune selection studies have been hampered in this system by the fact that HIV-1 is infectious only for chimpanzees, which do not develop disease and are available in only limited numbers. Experiments on immune selection in humans are generally complicated by lack of knowledge on the time of infection and the genetic make-up of the infecting virus. Our studies on SIV immune selection summarized in this review provide definitive evidence that neutralization-resistant variants emerge in an individual during persistent infection by primate lentiviruses. By cloning viral envelope genes from rhesus monkeys over time and obtaining sequential serum samples from them, we have been able to study not only the evolution of envelope sequences but also the emergence of neutralization-resistant variants. Reciprocal neutralization studies were performed using parental and variant specific sera, and immune selection was demonstrated using molecularly cloned virus of defined sequence. During the course of persistent infection with SIV and HIV, there is clear selective pressure for change in discrete variable regions of envelope. The host neutralizing antibody response appears to be at least one of the selective forces driving sequence change in envelope since one result of the sequence variation is the emergence of neutralization escape mutants. This indicates that neutralizing antibodies do serve to limit HIV and SIV replication during the lengthy asymptomatic stage of infection. The coincidence of neutralization domains of HIV and/or SIV with variable regions V1, V2, V3, V4, V5, and V6 suggests a direct relationship between neutralization domains and the emergence of sequence variants. However, different selective forces may be responsible all or in part for driving sequence changes in some variable domains (summarized in Table 2). For example, alterations in cell and/or tissue tropism may be responsible at least in part for driving change in V3 and the cytotoxic T-lymphocyte response may be responsible for driving change in the signal peptide (V0; Henderson et al. 1992; Wei and Cresswell 1992).(ABSTRACT TRUNCATED AT 400 WORDS)

Probing the structure of the V2 domain of human immunodeficiency virus type 1 surface glycoprotein gp120 with a panel of eight monoclonal antibodies: human immune response to the V1 and V2 domains

We have analyzed a panel of eight murine monoclonal antibodies (MAbs) that depend on the V2 domain for binding to human immunodeficiency virus type 1 (HIV-1) gp120. Each MAb is sensitive to amino acid changes within V2, and some are affected by substitutions elsewhere. With one exception, the MAbs were not reactive with peptides from the V2 region, or only poorly so. Hence their ability to bind recombinant strain IIIB gp120 depended on the preservation of native structure. Three MAbs cross-reacted with strain RF gp120, but only one cross-reacted with MN gp120, and none bound SF-2 gp120. Four MAbs neutralized HIV-1 IIIB with various potencies, and the one able to bind MN gp120 neutralized that virus. Peptide serology indicated that antibodies cross-reactive with the HxB2 V1 and V2 regions are rarely present in HIV-1-positive sera, but the relatively conserved segment between the V1 and V2 loops was recognized by antibodies in a significant fraction of sera. Antibodies able to block the binding of V2 MAbs to IIIB or MN gp120 rarely exist in sera from HIV-1-infected humans; more common in these sera are antibodies that enhance the binding of V2 MAbs to gp120. This enhancement effect of HIV-1-positive sera can be mimicked by several human MAbs to different discontinuous gp120 epitopes. Soluble CD4 enhanced binding of one V2 MAb to oligomeric gp120 but not to monomeric gp120, perhaps by inducing conformational changes in the oligomer.

International collaboration comparing neutralization and binding assays for monoclonal antibodies to simian immunodeficiency virus

Thirteen laboratories characterized a coded panel of 10 MAbs to SIVmac251 envelope protein in a collaboration organized by the National Institute of Allergy and Infectious Diseases (NIAID). The MAbs were examined against SIV isolates in neutralization and radioimmune precipitation, immunoblot, enzyme-linked immunosorbent, and radioimmune assays. Although laboratories employed diverse neutralization assays that varied in sensitivity there was agreement on the relative ability of the MAbs to neutralize SIVmac251. Additionally, even though the quantity of any single MAb required to neutralize SIVmac251 varied between laboratories, there was agreement on the rank-order strength fo the five neutralizing MAbs. Based on the data from this study, the MAbs were classified according to their neutralization potential as high efficiency (MAb concentration, < 5 micrograms/ml), low efficiency (MAb concentration, 5-100 micrograms/ml), or nonneutralizing (MAb concentration, > 100 micrograms/ml). The MAbs could be assigned to four serological groups based on ability to cross-neutralize and bind different SIV isolates. The distinction between groups I, II, and III were based on the limited neutralization data obtained with the sooty mangabey isolate.

An epitope on the surface envelope glycoprotein (gp130) of simian immunodeficiency virus (SIVmac) involved in viral neutralization and T cell activation

SIVmac infection of macaques is an important animal model for HIV infection and AIDS; this model is being utilized for development of antiviral therapies and vaccines. In the present article, we sought to identify neutralization epitopes of SIVmac envelope surface glycoprotein (gp130). Algorithms were used to predict antigenicity of specific regions. Four regions from the primary amino acid sequence of the viral surface glycoprotein were selected. A synthetic peptide representing one of these regions (414-434) induced virus-neutralizing antibodies in mice; in addition, this peptide induced T cell-proliferative responses in macaques. To address the in vivo relevance of these observations, we demonstrated that experimentally infected macaques produce antibodies to the neutralization epitope. In addition, rhesus macaques protected against infection by an inactivated SIV vaccine develop antibodies that bind to peptide 414-434. These observations demonstrate that the region that includes the sequence 414-434 in the fourth variable domain (V4) of SIVmac gp130 contains both a linear neutralization epitope and a T cell epitope.