Importance of membrane fusion mediated by human immunodeficiency virus envelope glycoproteins for lysis of primary CD4-positive T cells

Bystander CD4+ T lymphocytes survive in HIV-infected human lymphoid tissue

HIV infection is associated with depletion of CD4(+) T cells. The mechanisms of this phenomenon remain to be understood. In particular, it remains controversial whether and to what extent uninfected ("bystander") CD4(+) T cells die in HIV-infected individuals. We address this question using a system of human lymphoid tissue ex vivo. Tissue blocks were inoculated with HIV-1. After productive infection was established, they were treated with the reverse transcriptase inhibitor nevirapine to protect from infection those CD4(+) T cells that had not yet been infected. These CD4(+) T cells residing in HIV-infected tissue are by definition bystanders. Our results demonstrate that after nevirapine application the number of bystander CD4(+) T cells is conserved. Thus, in the context of HIV-infected human lymphoid tissue, productive HIV infection kills infected cells but is not sufficient to cause the death of a significant number of uninfected CD4(+) T cells.

Cell-surface-expressed HIV-1 envelope induces the death of CD4 T cells during GP41-mediated hemifusion-like events

Cells expressing the HIV-1 envelope glycoprotein complex (gp120/gp41, Env) induce the death of target cells either after cell-to-cell fusion or after cell-to-cell contact in a fusion-independent fashion. Here, we demonstrate that Env-induced death of single cells (including primary CD4 T cells) required gp120 and gp41 function. The gp41 peptide C34, which blocked syncytium formation, completely inhibited the death of single target cells by specifically acting on gp41 function. Moreover, Env-induced single cell death was exclusively observed in CD4 cells and was associated with specific gp41-mediated transfer of lipids from the membrane of Env-expressing cells to the target cell but not with detectable cytoplasm mixing (complete fusion). We conclude that after gp120 function, gp41 mediates close cell-to-cell contacts, thereby triggering cell death in single uninfected cells in the absence of detectable cell-to-cell fusion.

Cell killing by HIV-1 protease

The human immunodeficiency virus protease (HIV-1 PR) was expressed both in the yeast Saccharomyces cerevisiae and in mammalian cells. Inducible expression of HIV-1 PR arrested yeast growth, which was followed by cell lysis. The lytic phenotype included loss of plasma membrane integrity and cell wall breakage leading to the release of cell content to the medium. Given that neither poliovirus 2A protease nor 2BC protein, both being highly toxic for S. cerevisiae, were able to produce similar effects, it seems that this lytic phenotype is specific of HIV-1 PR. Drastic alterations in membrane permeability preceded the lysis in yeast expressing HIV-1 PR. Cell killing and lysis provoked by HIV-1 PR were also observed in mammalian cells. Thus, COS7 cells expressing the protease showed increased plasma membrane permeability and underwent lysis by necrosis with no signs of apoptosis. Strikingly, the morphological alterations induced by HIV-1 PR in yeast and mammalian cells were similar in many aspects. To our knowledge, this is the first report of a viral protein with such an activity. These findings contribute to the present knowledge on HIV-1-induced cytopathogenesis.

Increased mucosal transmission but not enhanced pathogenicity of the CCR5-tropic, simian AIDS-inducing simian/human immunodeficiency virus SHIV(SF162P3) maps to envelope gp120

Through rapid serial transfer in vivo, the chimeric CCR5-tropic simian/human immunodeficiency virus SHIV(SF162) evolved from a virus that is nonpathogenic and poorly transmissible across the vaginal mucosa to a variant that still maintains CCR5 usage but which is now pathogenic and establishes intravaginal infection efficiently. To determine whether envelope glycoprotein gp120 is responsible for increased pathogenesis and transmissibility of the variant SHIV(SF162P3), we cloned and sequenced the dominant envelope gene (encoding P3 gp120) and characterized its functions in vitro. Chimeric SHIV(SF162) virus expressing P3 gp120 of the pathogenic variant, designated SHIV(SF162PC), was also constructed and assessed for its pathogenicity and mucosal transmissibility in vivo. We found that, compared to wild-type SHIV(SF162) gp120, P3 gp120 conferred in vitro neutralization resistance and increased entry efficiency of the virus but was compromised in its fusion-inducing capacity. In vivo, SHIV(SF162PC) infected two of two and two of three rhesus macaques by the intravenous and intravaginal routes, respectively. Nevertheless, although peak viremia reached 10(6) to 10(7) RNA copies per ml of plasma in some infected animals and was associated with depletion of gut-associated CD4(+) lymphocytes, none of the animals maintained a viral set point that would be predictive of progression to disease. Together, the data from this study suggest a lack of correlation between entry efficiency and cytopathic properties of envelope glycoproteins with viral pathogenicity. Furthermore, whereas env gp120 contains the determinant for enhanced mucosal transmissibility of SHIV(SF162P3), the determinant(s) of its increased virulence may require additional sequence changes in env gp41 and/or maps to other viral genes.

Reduced Fitness of HIV-1 Resistant to Cxcr4 Antagonists

HIV-1 strains with a syncytium-inducing phenotype that use CXCR4 (X4 strains) have been associated with faster disease progression and AIDS. Antiviral agents designed to block CXCR4 may prevent the emergence of X4 HIV strains but resistant strains that maintain the X4 phenotype can be raised by sequential passage in cell cultures. We have demonstrated that a laboratory adapted strain (NL4-3) and a cloned clinical isolate (CI-1) of HIV-1 cultured in the presence of the CXCR4 antagonist, AMD3100, became resistant to the compound without a change in co-receptor use. These strains became resistant through divergence with respect to the wild-type virus. Conversely, a clinical isolate made resistant to AMD3100 switched co-receptor use from X4 to R5 through a change in diversity from the original virus population. When dual infection competition/heteroduplex tracking assays were performed, all AMD3100-resistant strains, regardless of co-receptor use showed a significantly diminished fitness compared with the wild-type virus. Single virus infections, at a similar multiplicity of infection, also indicated that the wild-type strains possess better replicative ability than their corresponding resistant strains. Thus, viral resistance development to a CXCR4 antagonist such as AMD3100 is associated with reduced viral fitness.

Addition of a single gp120 glycan confers increased binding to dendritic cell-specific ICAM-3-grabbing nonintegrin and neutralization escape to human immunodeficiency virus type 1

The potential role of dendritic cell-specific ICAM-3-grabbing nonintegrin (DC-SIGN) binding in human immunodeficiency virus transmission across the mucosal barrier was investigated by assessing the ability of simian-human immunodeficiency chimeric viruses (SHIVs) showing varying degrees of mucosal transmissibility to bind the DC-SIGN expressed on the surface of transfected cells. We found that gp120 of the highly transmissible, pathogenic CCR5-tropic SHIV(SF162P3) bound human and rhesus DC-SIGN with an efficiency threefold or greater than that of gp120 of the nonpathogenic, poorly transmissible parental SHIV(SF162), and this increase in binding to the DC-SIGN of the SHIV(SF162P3) envelope gp120 translated into an enhancement of T-cell infection in trans. The presence of an additional glycan at the N-terminal base of the V2 loop of SHIV(SF162P3) gp120 compared to that of the parental virus was shown to be responsible for the increase in binding to DC-SIGN. Interestingly, this glycan also conferred escape from autologous neutralization, raising the possibility that the modification occurred as a result of immune selection. Our data suggest that more-efficient binding of envelope gp120 to DC-SIGN could be relevant to the enhanced mucosal transmissibility of SHIV(SF162P3) compared to that of parental SHIV(SF162).

Stimulation of enveloped virus infection by beta-amyloid fibrils

Alzheimer's disease is characterized by deposition of beta-amyloid peptide (Abeta) into plaques in the brain, leading to neuronal toxicity and dementia. Human immunodeficiency virus type 1 (HIV-1) infection of the central nervous system can also cause a dementia, and amyloid deposition in the central nervous system is significantly higher in HIV-1-infected individuals compared with uninfected controls. Here we report that Abeta fibrils stimulated, by 5-20-fold, infection of target cells expressing CD4 and an appropriate coreceptor by multiple HIV-1 isolates but did not permit infection of cells lacking these receptors. Abeta enhanced infection at the stage of virus attachment or entry into the cell. Abeta fibrils also stimulated infection by amphotrophic Moloney leukemia virus, herpes simplex virus, and viruses pseudotyped with the envelope glycoprotein of vesicular stomatitis virus. Other synthetic fibril-forming peptides similarly enhanced viral infection and may be useful in gene delivery applications utilizing retroviral vectors. These data suggest that Abeta deposition may increase the vulnerability of the central nervous system to enveloped viral infection and that amyloidogenic peptides could be useful in enhancing gene transfer by enveloped viral vectors.

Blockade of HIV-1 infection of New World monkey cells occurs primarily at the stage of virus entry

HIV-1 naturally infects chimpanzees and humans, but does not infect Old World monkeys because of replication blocks that occur after virus entry into the cell. To understand the species-specific restrictions operating on HIV-1 infection, the ability of HIV-1 to infect the cells of New World monkeys was examined. Primary cells derived from common marmosets and squirrel monkeys support every phase of HIV-1 replication with the exception of virus entry. Efficient HIV-1 entry typically requires binding of the viral envelope glycoproteins and host cell receptors, CD4 and either CCR5 or CXCR4 chemokine receptors. HIV-1 did not detectably bind or utilize squirrel monkey CD4 for entry, and marmoset CD4 was also very inefficient compared with human CD4. A marmoset CD4 variant, in which residues 48 and 59 were altered to the amino acids found in human CD4, supported HIV-1 entry efficiently. The CXCR4 molecules of both marmosets and squirrel monkeys supported HIV-1 infection, but the CCR5 proteins of both species were only marginally functional. These results demonstrate that the CD4 and CCR5 proteins of New World monkeys represent the major restriction against HIV-1 replication in these primates. Directed adaptation of the HIV-1 envelope glycoproteins to common marmoset receptors might allow the development of New World monkey models of HIV-1 infection.

Evidence for a cytopathogenicity determinant in HIV-1 Vpr

HIV-1 is cytopathic for CD4(+) T lymphocytes in vitro and this property of HIV-1 is generally considered to account for some of its in vivo cytopathogenicity. Thus, the extent of lymphocyte depletion correlates with the level of viremia whereas low levels of viral replication are typically associated with stable lymphocyte levels and asymptomatic infection such as is observed in non-progressors. Here, we describe a non-progressor who did not fit this general pattern in that CD4(+) T lymphocyte homeostasis was maintained in the face of high-level viral replication. Biological viral isolates from this patient replicated in primary lymphocytes without inducing cytopathicity. Because this phenotype is reminiscent of Vpr-deleted viruses, we examined the contribution of the Vpr gene to the viral phenotype. Vpr alleles derived from this patient contained both premature stop codons and an unusual Q3R polymorphism. Insertion of patient-derived Vpr alleles or a Q3R substitution into a cytopathic HIV-1 clone resulted in a marked impairment of cytopathicity without affecting viral replication efficiency. The effect of Vpr on cytopathicity was unrelated to reported activities of Vpr including virion association, interaction with uracil DNA glycosylase, G(2) arrest, or enhancement of macrophage infection but correlated with the ability of Vpr to induce host cell apoptosis. This study suggests the presence of a determinant of in vivo cytopathogenicity within HIV-1 Vpr and further indicates that viral replication can be uncoupled from cytopathicity in vitro and in vivo.

Human retroviruses after 20 years: a perspective from the past and prospects for their future control

Among viruses the human retroviruses may be of special interest to immunologists, because they target cells of the immune system, particularly mature CD4+ T cells, impair their function and cause them to grow abnormally (human T-cell leukemia virus, HTLV) or to die (human immunodeficiency virus, HIV). Human retroviruses cause disease ranging from neurological disorders and leukemias (HTLV-1) to AIDS (acquired immunodeficiency virus) (HIV) and promote development of several types of malignancies (HIV). They share many common features, but their contrasts are greater, especially the far greater replication and variation of HIV associated with its greater genomic complexity. Both have evolved striking redundancy for mechanisms which promote their survival. Thus, HTLV has redundant mechanisms for promoting growth of provirus containing T cells needed for virus continuity, because it is chiefly through its cellular DNA provirus that HTLV replicates and not through production of virions. Conversely, HIV has redundancy in its mechanisms for promoting virion replication and escape from the host immune system. It is via these redundant mechanisms that they produce disease: leukemias from mechanisms promoting T-cell proliferation (HTLV-1) and AIDS from mechanisms promoting virus replication and T-cell death (HIV). The practical challenges for the future are clear. For HTLV-1, education and control of breastfeeding. For HIV, the formidable tasks now ahead in part demand new kinds of talent, talents that will foster greater insights into the development of therapy for the developing countries, new forms of less toxic therapies for all infected persons, a continued and expanded commitment to education, and a persistent 'never say die' commitment to the development of a truly preventive vaccine with all the scientific and nonscientific challenges that these objectives face.

Understanding the basis of CD4(+) T-cell depletion in macaques infected by a simian-human immunodeficiency virus

The efficacy of candidate AIDS vaccines to mediate protection against viral infection and pathogenesis is evaluated, at a preclinical stage, in animal models. One model that is favored because the infecting virus is closely related to HIV-1 and because of the rapidity of pathogenic outcomes is the infection of Old World monkeys by simian-human immunodeficiency virus (SHIV) chimerae. We investigated the basis for the depletion of CD4(+) T lymphocytes in a SHIV-macaque model. Molecularly cloned SHIVs, SHIV-89.6 and SHIV-KB9, differ in the ability to cause CD4(+) T-cell loss at a given level of virus replication in monkeys. The envelope glycoproteins of the pathogenic SHIV-KB9 mediate membrane-fusion in cultured T lymphocytes more efficiently than the envelope glycoproteins of the non-pathogenic SHIV-89.6. The minimal envelope glycoprotein region that specifies this increase in membrane-fusing capacity was sufficient to convert SHIV-89.6 into a virus that causes profound CD4(+) T-cell depletion in monkeys. Conversely, two single amino acid changes that decrease the membrane-fusing ability of the SHIV-KB9 envelope glycoproteins also attenuated the CD4(+) T-cell destruction that accompanied a given level of virus replication in SHIV-infected monkeys. Thus, the ability of the HIV-1 envelope glycoproteins to fuse membranes, which has been implicated in the induction of viral cytopathic effects in vitro, contributes to the capacity of the pathogenic SHIV to deplete CD4(+) T lymphocytes in vivo.

Human immunodeficiency virus type 1 neutralization measured by flow cytometric quantitation of single-round infection of primary human T cells

There is currently intensive research on the design of novel human immunodeficiency virus type 1 (HIV-1) vaccine immunogens that can elicit potent neutralizing antibodies. A prerequisite for comparing and optimizing these strategies is the ability to precisely measure neutralizing antibody responses. To this end, we sought to develop an assay that directly quantifies single-round HIV-1 infection of peripheral blood mononuclear cells (PBMC). Initial experiments demonstrated that essentially all productively infected PBMC could be identified by flow cytometric detection of intracellular p24 antigen (p24-Ag). After infection of PBMC with HIV-1, p24(+) lymphocytes could be distinguished beginning 1 day postinfection, and the majority of CD8(-) T cells were p24-Ag positive by 3 to 4 days postinfection. To directly quantify first-round infection, we included a protease inhibitor in PBMC cultures. The resulting 2-day assay was highly sensitive and specific for the detection of HIV-1-infected PBMC. Serial dilutions of virus stocks demonstrated that the number of target cells infected was directly related to the amount of infectious virus input into the assay. In neutralization assays, the flow cytometric enumeration of first-round infection of PBMC provided quantitative data on the number of target cells infected and on the inactivation of infectious virus due to reaction with antibody. We also used this single-round assay to compare the percentage of cells expressing p24-Ag to the number of copies of HIV-1 gag per 100 PBMC. The precision and reproducibility of this assay will facilitate the measurement of HIV-1 neutralization, particularly incrementally improved neutralizing antibody responses generated by new candidate vaccines.

Inhibition of HIV-1 replication by novel lentiviral vectors expressing transdominant Rev and HIV-1 env antisense

Retroviral vectors expressing transdominant negative mutants of Rev (TdRev) inhibit HIV-1 replication by preventing the nuclear export of unspliced viral transcripts, thus inhibiting the synthesis of Gag-Pol, Env and reducing the levels of genomic RNA available for packaging. Due to these effective mechanisms of inhibition, production of HIV-1-based lentiviral vectors expressing TdRev has been difficult. Here we describe HIV-based vectors in which expression of TdRev is negatively regulated by Rev expression. In these vectors, we maintained the wild-type HIV-1 Tat/Rev exons and intron configuration and its mode of splicing regulation. The second Rev exon was mutated to encode TdRev. Inhibition of TdRev expression by Rev during vector production yields high titer vector preparations. A second vector containing an additional anti-HIV gene (env-antisense) was constructed by flipping a 1.2-kb env fragment contained within the Tat/TdRev intron. SupT1 cells and primary CD4+ lymphocytes transduced with these vectors inhibit HIV-1 replication and show a preferential advantage for survival. Although these vectors are poorly mobilized to secondary target cells by wild-type HIV-1, they reduce the infectivity of the wild-type virions escaping inhibition.

Properties of the surface envelope glycoprotein associated with virulence of simian-human immunodeficiency virus SHIV(SF33A) molecular clones

In vivo adaptation of simian-human immunodeficiency virus (SHIV) clone SHIV(SF33) resulted in the emergence of pathogenic isolate SHIV(SF33A), which caused a rapid and severe CD4(+) T-cell depletion when inoculated into rhesus macaques. Two molecular clones generated by inserting the env V1-to-V5 region amplified from SHIV(SF33A)-infected animals into the parental SHIV(SF33) genome retained a pathogenic phenotype. The gp120 envelope glycoproteins of pathogenic clones SHIV(SF33A2) and SHIV(SF33A5) conferred a threefold increase in viral entry and fusogenicity compared to the parental glycoprotein. Changes in gp120 were also responsible for a higher replication capacity and cytopathicity in primary CD4(+) T-cell cultures. Last, gp120 carried the determinants of SHIV(SF33A) neutralization resistance. Thus, changes in SHIV(SF33A) gp120 produced a set of properties that could account for the pathogenic phenotype observed in vivo. Measurement of antibody binding to SHIV(SF33A) viral particles revealed an increased exposure of the CD4-induced epitope recognized by the 17b monoclonal antibody in a region that was shown to contribute to coreceptor binding. Exposure of this epitope occurred in the absence of CD4 binding, suggesting that the envelope glycoprotein of pathogenic SHIV(SF33A) clones folded in a conformation that was primed for interaction with CXCR4 or for the subsequent step of fusion.

Gene therapy of HIV-1 infection using lentiviral vectors expressing anti-HIV-1 genes

The use of vectors based on primate lentiviruses for gene therapy of human immunodeficiency virus type 1 (HIV-1) infection has many potential advantages over the previous murine retroviral vectors used for delivery of genes that inhibit replication of HIV-1. First, lentiviral vectors have the ability to transduce dividing and nondividing cells that constitute the targets of HIV-1 infection such as resting T cells, dendritic cells, and macrophages. Lentiviral vectors can also transfer genes to hematopoietic stem cells with a superior gene transfer efficiency and without affecting the repopulating capacity of these cells. Second, these vectors could be potentially mobilized in vivo by the wild-type virus to secondary target cells, thus expanding the protection to previously untransduced cells. And finally, lentiviral vector backbones have the ability to block HIV-1 replication by several mechanisms that include sequestration of the regulatory proteins Tat and Rev, competition for packaging into virions, and by inhibition of reverse transcription in heterodimeric virions with possible generation of nonfunctional recombinants between the vector and viral genomes. The inhibitory ability of lentiviral vectors can be further increased by expression of anti-HIV-1 genes. In this case, the lentiviral vector packaging system has to be modified to become resistant to the anti-HIV-1 genes expressed by the vector in order to avoid self-inhibition of the vector packaging system during vector production. This review focuses on the use of lentiviral vectors as the main agents to mediate inhibition of HIV-1 replication and discusses the different genetic intervention strategies for gene therapy of HIV-1 infection.

Unique monoclonal antibody recognizing the third extracellular loop of CXCR4 induces lymphocyte agglutination and enhances human immunodeficiency virus type 1-mediated syncytium formation and productive infection

To increase insight into the structural basis of CXCR4 utilization in human immunodeficiency virus type 1 (HIV-1) infection, a new generation of three monoclonal antibodies (MAbs) was developed in WKA rats. The A80 MAb, which binds an epitope in the third extracellular loop (ECL3) of CXCR4, has unique biologic properties that provide novel insights into CXCR4 function. This agent enhanced syncytium formation in activated human peripheral blood mononuclear cells (PBMC) infected with X4 or R5 and CEM cells infected with X4 HIV-1 strains. Exposure to A80 increased the productive infection of activated CD4(+) T cells and CEM cells with R5 and X4 viruses, respectively. This antibody uniquely induced agglutination of PBMC and CEM cells but did not activate calcium mobilization. Agglutination induced by A80 was inhibited by stromal cell-derived factor 1, T22, and phorbol 12-myristate 13-acetate but was not significantly altered by pretreatment of cells with pertussis toxin, wortmannin, or MAbs to LFA-1, ICAM-1, ICAM-2, and ICAM-3. The binding of the A145 and A120 MAbs was mapped to the N-terminal extracellular domain and a conformational epitope involving ECL1 and ECL2, respectively. Both of these MAbs inhibited HIV-1 infection and lacked the novel properties of A80. These results suggest a new role for CXCR4 in homologous lymphocyte adhesion that is ligand independent and in HIV-1 infection.

Membrane-fusing capacity of the human immunodeficiency virus envelope proteins determines the efficiency of CD+ T-cell depletion in macaques infected by a simian-human immunodeficiency virus

The mechanism of the progressive loss of CD4+ T lymphocytes, which underlies the development of AIDS in human immunodeficiency virus (HIV-1)-infected individuals, is unknown. Animal models, such as the infection of Old World monkeys by simian-human immunodeficiency virus (SHIV) chimerae, can assist studies of HIV-1 pathogenesis. Serial in vivo passage of the nonpathogenic SHIV-89.6 generated a virus, SHIV-89.6P, that causes rapid depletion of CD4+ T lymphocytes and AIDS-like illness in monkeys. SHIV-KB9, a molecularly cloned virus derived from SHIV-89.6P, also caused CD4+ T-cell decline and AIDS in inoculated monkeys. It has been demonstrated that changes in the envelope glycoproteins of SHIV-89.6 and SHIV-KB9 determine the degree of CD4+ T-cell loss that accompanies a given level of virus replication in the host animals (G. B. Karlsson et. al., J. Exp. Med. 188:1159-1171, 1998). The envelope glycoproteins of the pathogenic SHIV mediated membrane fusion more efficiently than those of the parental, nonpathogenic virus. Here we show that the minimal envelope glycoprotein region that specifies this increase in membrane-fusing capacity is sufficient to convert SHIV-89.6 into a virus that causes profound CD4+ T-lymphocyte depletion in monkeys. We also studied two single amino acid changes that decrease the membrane-fusing ability of the SHIV-KB9 envelope glycoproteins by different mechanisms. Each of these changes attenuated the CD4+ T-cell destruction that accompanied a given level of virus replication in SHIV-infected monkeys. Thus, the ability of the HIV-1 envelope glycoproteins to fuse membranes, which has been implicated in the induction of viral cytopathic effects in vitro, contributes to the capacity of the pathogenic SHIV to deplete CD4+ T lymphocytes in vivo.

Envelope glycoprotein determinants of neutralization resistance in a simian-human immunodeficiency virus (SHIV-HXBc2P 3.2) derived by passage in monkeys

The simian-human immunodeficiency virus SHIV-HXBc2 contains the envelope glycoproteins of a laboratory-adapted, neutralization-sensitive human immunodeficiency virus type 1 variant, HXBc2. Serial in vivo passage of the nonpathogenic SHIV-HXBc2 generated SHIV KU-1, which causes rapid CD4(+) T-cell depletion and an AIDS-like illness in monkeys. A molecularly cloned pathogenic SHIV, SHIV-HXBc2P 3.2, was derived from the SHIV KU-1 isolate and differs from the parental SHIV-HXBc2 by only 12 envelope glycoprotein amino acid residues. Relative to SHIV-HXBc2, SHIV-HXBc2P 3.2 was resistant to neutralization by all of the antibodies tested with the exception of the 2G12 antibody. The sequence changes responsible for neutralization resistance were located in variable regions of the gp120 exterior envelope glycoprotein and in the gp41 transmembrane envelope glycoprotein. The 2G12 antibody, which neutralized SHIV-HXBc2 and SHIV-HXBc2P 3.2 equally, bound the HXBc2 and HXBc2P 3.2 envelope glycoproteins on the cell surface comparably. The ability of the other tested antibodies to achieve saturation was less for the HXBc2P 3.2 envelope glycoproteins than for the HXBc2 envelope glycoproteins, even though the affinity of the antibodies for the two envelope glycoproteins was similar. Thus, a highly neutralization-sensitive SHIV, by modifying both gp120 and gp41 glycoproteins, apparently achieves a neutralization-resistant state by decreasing the saturability of its envelope glycoproteins by antibodies.