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

Human Immunodeficiency Virus Type 1 Cellular Entry and Exit in the T Lymphocytic and Monocytic Compartments: Mechanisms and Target Opportunities During Viral Disease

During the course of human immunodeficiency virus type 1 infection, a number of cell types throughout the body are infected, with the majority of cells representing CD4+ T cells and cells of the monocyte-macrophage lineage. Both types of cells express, to varying levels, the primary receptor molecule, CD4, as well as one or both of the coreceptors, CXCR4 and CCR5. Viral tropism is determined by both the coreceptor utilized for entry and the cell type infected. Although a single virus may have the capacity to infect both a CD4+ T cell and a cell of the monocyte-macrophage lineage, the mechanisms involved in both the entry of the virus into the cell and the viral egress from the cell during budding and viral release differ depending on the cell type. These host-virus interactions and processes can result in the differential targeting of different cell types by selected viral quasispecies and the overall amount of infectious virus released into the extracellular environment or by direct cell-to-cell spread of viral infectivity. This review covers the major steps of virus entry and egress with emphasis on the parts of the replication process that lead to differences in how the virus enters, replicates, and buds from different cellular compartments, such as CD4+ T cells and cells of the monocyte-macrophage lineage.

Bioinformatic analysis of HIV-1 entry and pathogenesis

The evolution of human immunodeficiency virus type 1 (HIV-1) with respect to co-receptor utilization has been shown to be relevant to HIV-1 pathogenesis and disease. The CCR5-utilizing (R5) virus has been shown to be important in the very early stages of transmission and highly prevalent during asymptomatic infection and chronic disease. In addition, the R5 virus has been proposed to be involved in neuroinvasion and central nervous system (CNS) disease. In contrast, the CXCR4-utilizing (X4) virus is more prevalent during the course of disease progression and concurrent with the loss of CD4(+) T cells. The dual-tropic virus is able to utilize both co-receptors (CXCR4 and CCR5) and has been thought to represent an intermediate transitional virus that possesses properties of both X4 and R5 viruses that can be encountered at many stages of disease. The use of computational tools and bioinformatic approaches in the prediction of HIV-1 co-receptor usage has been growing in importance with respect to understanding HIV-1 pathogenesis and disease, developing diagnostic tools, and improving the efficacy of therapeutic strategies focused on blocking viral entry. Current strategies have enhanced the sensitivity, specificity, and reproducibility relative to the prediction of co-receptor use; however, these technologies need to be improved with respect to their efficient and accurate use across the HIV-1 subtypes. The most effective approach may center on the combined use of different algorithms involving sequences within and outside of the env-V3 loop. This review focuses on the HIV-1 entry process and on co-receptor utilization, including bioinformatic tools utilized in the prediction of co-receptor usage. It also provides novel preliminary analyses for enabling identification of linkages between amino acids in V3 with other components of the HIV-1 genome and demonstrates that these linkages are different between X4 and R5 viruses.

Envelope variable region 4 is the first target of neutralizing antibodies in early simian immunodeficiency virus mac251 infection of rhesus monkeys

A major goal of AIDS vaccine development is to design vaccination strategies that can elicit broad and potent protective antibodies. The initial viral targets of neutralizing antibodies (NAbs) early after human or simian immunodeficiency virus (HIV/SIV) infection are not known. The identification of early NAb epitopes that induce protective immunity or retard the progression of disease is important for AIDS vaccine development. The aim of this study was to determine the Env residues targeted by early SIV NAbs and to assess the influence of prior vaccination on neutralizing antibody kinetics and specificity during early infection. We previously described stereotypic env sequence variations in SIVmac251-infected rhesus monkeys that resulted in viral escape from NAbs. Here, we defined the early viral targets of neutralization and determined whether the ability of serum antibody from infected monkeys to neutralize SIV was altered in the setting of prior vaccination. To localize the viral determinants recognized by early NAbs, a panel of mutant pseudoviruses was assessed in a TZM-bl reporter gene neutralization assay to define the precise changes that eliminate recognition by SIV Env-specific NAbs in 16 rhesus monkeys. Changing R420 to G or R424 to Q in V4 of Env resulted in the loss of recognition by NAbs in vaccinated monkeys. In contrast, mutations in the V1 region of Env did not alter the NAb profile. These findings indicate that early NAbs are directed toward SIVmac251 Env V4 but not the V1 region, and that this env vaccination regimen did not alter the kinetics or the breadth of NAbs during early infection.

Autologous neutralizing antibodies to the transmitted/founder viruses emerge late after simian immunodeficiency virus SIVmac251 infection of rhesus monkeys

While the simian immunodeficiency virus (SIV)-infected rhesus monkey is an important animal model for human immunodeficiency virus type 1 (HIV-1) infection of humans, much remains to be learned about the evolution of the humoral immune response in this model. In HIV-1 infection, autologous neutralizing antibodies emerge 2 to 3 months after infection. However, the ontogeny of the SIV-specific neutralizing antibody response in mucosally infected animals has not been defined. We characterized the kinetics of the autologous neutralizing antibody response to the transmitted/founder SIVmac251 using a pseudovirion-based TZM-bl cell assay and monitored env sequence evolution using single-genome amplification in four rhesus animals that were infected via intrarectal inoculations. We show that the SIVmac251 founder viruses induced neutralizing antibodies at 5 to 8 months after infection. Despite their slow emergence and low titers, these neutralizing antibodies selected for escape mutants that harbored substitutions and deletions in variable region 1 (V1), V2, and V4 of Env. The neutralizing antibody response was initially focused on V4 at 5 to 8 months after infection and then targeted V1/V2 and V4 by 16 months. These findings reveal a striking delay in the development of neutralizing antibodies in SIVmac-infected animals, thus raising questions concerning the suitability of SIVmac251 as a challenge strain to screen AIDS vaccines that elicit neutralizing antibodies as a means to prevent virus acquisition. They also illustrate the capacity of the SIVmac quasispecies to modify antigenic determinants in response to very modest titers of neutralizing antibodies.

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.

Importance of the V1/V2 loop region of simian-human immunodeficiency virus envelope glycoprotein gp120 in determining the strain specificity of the neutralizing antibody response

Plasma samples from individuals infected with human immunodeficiency virus type 1 (HIV-1) are known to be highly strain specific in their ability to neutralize HIV-1 infectivity. Such plasma samples exhibit significant neutralizing activity against autologous HIV-1 isolates but typically exhibit little or no activity against heterologous strains, although some cross-neutralizing activity can develop late in infection. Monkeys infected with the simian-human immunodeficiency virus (SHIV) clone DH12 generated antibodies that neutralized SHIV DH12, but not SHIV KB9. Conversely, antibodies from monkeys infected with the SHIV clone KB9 neutralized SHIV KB9, but not SHIV DH12. To investigate the role of the variable loops of the HIV-1 envelope glycoprotein gp120 in determining this strain specificity, variable loops 1 and 2 (V1/V2), V3, or V4 were exchanged individually or in combination between SHIV DH12 and SHIV KB9. Despite the fact that both parental viruses exhibited significant infectivity and good replication in the cell lines examined, 3 of the 10 variable-loop chimeras exhibited such poor infectivity that they could not be used further for neutralization assays. These results indicate that a variable loop that is functional in the context of one particular envelope background will not necessarily function within another. The remaining seven replication-competent chimeras allowed unambiguous assignment of the sequences principally responsible for the strain specificity of the neutralizing activity present in SHIV-positive plasma. Exchange of the V1/V2 loop sequences conferred a dominant loss of sensitivity to neutralization by autologous plasma and a gain of sensitivity to neutralization by heterologous plasma. Substitution of V3 or V4 had little or no effect on the sensitivity to neutralization. These data demonstrate that the V1/V2 region of HIV-1 gp120 is principally responsible for the strain specificity of the neutralizing antibody response in monkeys infected with these prototypic SHIVs.

Infectivity and neutralization of simian immunodeficiency virus with FLAG epitope insertion in gp120 variable loops

A FLAG epitope tag was substituted within variable loop 1 (V1), 2 (V2), or 4 (V4) of the gp120 envelope glycoprotein of simian immunodeficiency virus strain 239 (SIV239) to evaluate the extent to which each variable loop may serve as a target for antibody-mediated neutralization. Two sites within each variable loop of SIV239 were chosen for individual epitope tag insertions. FLAG epitope substitutions were also made in the V1, V2, and V4 loops of a neutralization-sensitive derivative of SIV239, SIV316. Of the 10 FLAG-tagged recombinant viruses analyzed, three (SIV239FV1b, SIV239FV2b, and SIV239FV4a) replicated with kinetics similar to those of the parental strain, SIV239, in both CEMx174 cells and the immortalized rhesus monkey T-cell line 221. The SIV316FV1b and SIV316FV4a FLAG variants replicated with a substantial lag, and the five remaining recombinants did not replicate detectably. Both gp160 and gp120 from replication-competent FLAG variants could be immunoprecipitated from transfected 293T cells by the anti-gp120 rhesus monoclonal antibody (RhMAb) 3.11H, the anti-FLAG MAb M2, and CD4-immunoglobulin, whereas only unprocessed gp160 was detected in 293T cells transfected with replication-defective variants. Furthermore, gp120 was detectably incorporated only into virions that were infectious. SIV239FV1b was sensitive to neutralization by MAb M2, with a 50% inhibitory concentration of 1 mug/ml. Neither SIV239FV2b nor SIV239FV4a was sensitive to M2 neutralization. The ability of the M2 antibody to neutralize SIV239FV1b infectivity was associated with an increased ability of the M2 antibody to detect native, oligomeric SIV239FV1b envelope protein on the surfaces of cells relative to that for the other SIV FLAG variants. Furthermore, SIV239FV1b was globally more sensitive to antibody-mediated neutralization than was parental SIV239 when these strains were screened with a panel of anti-SIV MAbs of various specificities. These results indicate that the V1 loop can serve as an effective target for neutralization on SIV239FV1b. However, antibody-mediated neutralization of this variant, similar to that of other SIV239 variants that have been studied previously, was associated with a global increase in neutralization sensitivity. These results suggest that the variable loops on the neutralization-resistant SIV239 strain are difficult for antibodies to access effectively and that mutations that allow neutralization have global effects on the trimeric envelope glycoprotein structure and accessibility.

Humoral Immunity and the Evolution of HIV-2

Human immunodeficiency virus type 2 (HIV-2) evolved from the zoonotic transmission of simian immunodeficiency virus (SIV) that naturally infects sooty mangabeys found in West Africa. Using sera from HIV-2-infected humans, we discovered that an hypervariable region (the V4 loop) of HIV-2 induces antibody responses only weakly reactive against itself but strongly reactive against analogous sequences from the V4 loop of strains of SIV. Available sequence data indicates that all strains of HIV-2 have large deletions in the V4 region that truncate an immunodominant neutralizing B cell epitope among strains of SIV. Infection of a macaque with a sequenced clone of HIV-2 similarly elicited antibodies that poorly recognized the V4 loop of HIV-2 but readily bound to analogous SIV sequences. Our data are consistent with a scenario whereby a disparate antibody response directed against the V4 loop may have influenced the selective expansion and survival of HIV-2 in humans.

Convergent evolution of SIV env after independent inoculation of rhesus macaques with infectious proviral DNA

The env gene of three simian immunodeficiency virus (SIV) variants developed convergent mutations during disease progression in six rhesus macaques. The monkeys had been inoculated with supercoiled plasmids encoding infectious proviruses of SIVmac239 (a pathogenic, wild-type strain), SIVdelta3 (the live attenuated vaccine strain derived from SIVmac239), or SIVdelta3+ (a pathogenic progeny virus that had evolved from SIVdelta3). All six monkeys developed immunodeficiency and progressed to fatal disease. Although many divergent mutations arose in env among the different hosts, three regions consistently mutated in all monkeys studied; these similar mutations developed independently even though the animals had received only a single infectious molecular clone rather than standard viral inocula that contain viral quasispecies. Together, these data indicate that the env genes of SIVmac239, SIVdelta3, and SIVdelta3+, in the context of different proviral backbones, evolve similarly in different hosts during disease progression.

A retroviral DNA vaccine vector

A versatile DNA vaccine (pdIV3) was constructed by replacing the integrase, vif, vpx, and vpr genes of a pathogenic simian immunodeficiency virus (SIV) molecular clone with a linker containing unique cloning sites. The 5' long terminal repeat (LTR) is truncated and transcription is controlled by a cytomegalovirus (CMV) promoter. The construct expresses Gag and Env in vitro and noninfectious virus particles are produced from transfected cells. The ability of pdIV3 to promote cellular and humoral immune responses, along with the flexibility of the linker design to allow insertion of immunostimulatory genes in future constructs, makes this a useful base vector for immunization against primate lentiviruses. We present the construction of a retroviral plasmid designed to serve as a template for the development of safe and effective vaccines against primate immunodeficiency retroviruses. This vaccine component should facilitate the simultaneous induction of cellular and humoral immune responses that protect primates against infection with SIV and human immunodeficiency virus (HIV) and the development of acquired immune deficiency syndrome (AIDS). This plasmid could induce the appropriate immune response required to attack both cell-free and cell-associated viruses. The lack of infectivity, the inability to integrate, and the SIV origin make this construct a safe alternative to attenuated vaccines based on HIV. In addition, we intend to develop this construct as an immunotherapeutic approach to lower the viremia in AIDS patients.

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.

The kinetics of specific immune responses in rhesus monkeys inoculated with live recombinant BCG expressing SIV Gag, Pol, Env, and Nef proteins

Development of an effective preventive or therapeutic vaccine against HIV-1 is an important goal in the fight against AIDS. Effective virus clearance and inhibition of spread to target organs depends principally on the cellular immune response. Therefore, a vaccine against HIV-1 should elicit virus-specific cytotoxic lymphocyte (CTL) responses to eliminate the virus during the cell-associated stages of its life cycle. The vaccine should also be capable of inducing immunity at the mucosal surfaces, the primary route of transmission. Recombinant Bacille Calmette-Guérin (BCG) expressing viral proteins offers an excellent candidate vaccine in view of its safety and ability to persist intracellularly, resulting in the induction of long-lasting immunity and stimulation of the cellular immune response. BCG can be administered orally to induce HIV-specific immunity at the mucosal surfaces. The immunogenicity of four recombinant BCG constructs expressing simian immunodeficiency virus (SIV) Gag, Pol, Env, and Nef proteins was tested in rhesus macaques. A single simultaneous inoculation of all four recombinants elicited SIV-specific IgA and IgG antibody, and cellular immune responses, including CTL and helper T cell proliferation. Our results demonstrate that BCG recombinant vectors can induce concomitant humoral and cellular immune responses to the major proteins of SIV.

Immunogenicity of a vaccine preparation representing the variable regions of the HIV type 1 envelope glycoprotein

Variability of the major antigenic sites of the envelope glycoprotein of HIV-1 constitutes a major problem in the formulation of effective vaccines. We have prepared a synthetic peptide vaccine that represents the major hypervariable epitopes (V1 through V5) of the clade B HIV-1 envelope glycoprotein (gp120). We refer to this preparation as variable epitope immunogen or VEI vaccine. This construct takes into consideration the type and frequency of amino acid substitutions found at each epitope during the evolution of the virus in individual patients and in the target population. Immunization of mice, rabbits, and rhesus macaques with the VEI vaccine resulted in the induction of long-lasting, high-titered HIV-1 antibodies, including antibodies that neutralize primary isolates. We also documented lymphocyte proliferative responses to the VEI vaccine, its individual components, analogs, and subtype-specific peptides representing the major hypervariable regions of HIV-1 gp120. Delayed-type hypersensitivity responses to these antigens were also demonstrated in mice. Our results show that this vaccine is highly immunogenic and safe in animals. Our data suggest that this formulation could become an important component of combination vaccine approaches against HIV-1 and other antigenically variable pathogens.

Antibodies from HIV-positive and AIDS patients bind to an HIV envelope multivalent vaccine

A major problem impeding development of an effective HIV vaccine is the rapid antigenic variability that is characteristic of several envelope glycoprotein epitopes. Frequent mutations alter the composition of the most immunogenic regions of the envelope glycoprotein. We have prepared a synthetic immunogen representing the evolution of the major hypervariable epitopes on the envelope glycoprotein (gp120) of HIV-1. Five synthetic constructs, representing each of the HIV-1 gp120 hypervariable epitopes were tested for recognition by antibodies from patients infected with HIV-1 from different geographic regions worldwide. An HIV-1 human plasma panel provided a representation of the antibodies recognizing subtype-specific epitope sequences prevalent at different parts of the world. The vaccine construct was recognized by antibodies from HIV-1-positive individuals infected with subtypes A, B, C, D, E, and F. Antibodies in pooled HIV-1 patient sera from San Francisco also recognized all five constructs. This complex immunogen was recognized by antibodies in sera from individual HIV-1-positive and AIDS patients from Puerto Rico and Canada, with a strong binding to the complete vaccine and the V3 component. Altogether, our results demonstrate that antibodies from seropositive patients infected with different HIV-1 clades recognize and bind to the HIV hypervariable epitope construct vaccine preparation and its individual components.

Hypervariable epitope construct: a synthetic immunogen that overcomes MHC restriction of antigen presentation

Vaccines are not universal in their ability to induce favorable immune responses in all individuals because the major histocompatibility complex (MHC) molecules needed for presentation of vaccine components to T cells are limited in the peptides they recognize and bind. A heterogeneous cocktail of related peptides synthesized simultaneously and representing amino acids 414-434 of the SIV envelope protein was used to induce immune responses stronger than those induced by a single T cell peptide synthesized conventionally and representing the same region of the viral envelope. The heterogeneous peptide mixture called a hypervariable epitope construct (HEC) was capable of overcoming MHC restriction in peptide presentation in four different inbred mouse strains, including a strain that was a poor responder to the AA 414-434 single sequence peptide (SSP). HEC induced proliferation responses 15 times better than those induced by SSP. Antibodies elicited by HEC but not SSP immunization effectively bind viral antigen. The 414-434 HEC and the 414-434 SSP were also tested for their ability to upregulate the expression of MHC class I molecules on the surface of the mutant RMA-S murine cell line. Surface display of MHC molecules was measured by confocal microscopy followed by calculation of fluorescence intensity of images. HECs upregulated expression of MHC molecules 30% more than SSP peptides. Our findings suggest that HEC cocktails could be effective components of subunit vaccines to help overcome the unresponsiveness observed in outbred animals and in humans as a result of MHC-restricted antigen presentation.

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.

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.

HIV/SIV glycoproteins: structure-function relationships

The various functions of human (HIV) and simian (SIV) immunodeficiency virus glycoproteins are similar, so it may be assumed that the overall structure of the folded proteins will be maintained. To preserve structure there must be constraints on sequence variation. The majority of mutations tolerated will be involved in immune escape but changes at some positions are known to have direct effects on glycoprotein expression and function. This allows the virus to change its phenotype and escape immune pressure. These properties will influence the fitness of the virus to infect and replicate in potential hosts. A better understanding of the structure-function relationships of HIV/SIV glycoproteins will assist in the development of vaccines and antivirals. Here, we identify similarities and differences between HIV-1 subtypes and HIV/SIV types that may be relevant to the phenotypes of the various groups. The results are discussed in relation to what is known of domain-function associations for HIV/SIV glycoproteins.

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.

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.

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.

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)