Role of the HTLV-III/LAV envelope in syncytium formation and cytopathicity

HIV-1 pathogenesis

Despite considerable advances in HIV science in the past 20 years, the reason why HIV-1 infection is pathogenic is still debated and the goal of eradicating HIV-1 infection remains elusive. A deeper understanding of the interplay between HIV-1 and its host and why simian immunodeficiency virus (SIV) is nonpathogenic in some natural hosts may provide a few answers.

Cytolysis by CCR5-using human immunodeficiency virus type 1 envelope glycoproteins is dependent on membrane fusion and can be inhibited by high levels of CD4 expression

T-tropic (X4) and dualtropic (R5X4) human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins kill primary and immortalized CD4(+) CXCR4(+) T cells by mechanisms involving membrane fusion. However, because much of HIV-1 infection in vivo is mediated by M-tropic (R5) viruses whose envelope glycoproteins use CCR5 as a coreceptor, we tested a panel of R5 and R5X4 envelope glycoproteins for their ability to lyse CCR5(+) target cells. As is the case for CXCR4(+) target cells, HIV-1 envelope glycoproteins expressed by single-round HIV-1 vectors killed transduced CD4(+) CCR5(+) cells in a membrane fusion-dependent manner. Furthermore, a CD4-independent R5 HIV-1 envelope glycoprotein was able to kill CD4-negative target cells expressing CCR5, demonstrating that CD4 is not intrinsically required for the induction of death. Interestingly, high levels of CD4 expression protected cells from lysis and syncytium formation mediated by the HIV-1 envelope glycoproteins. Immunoprecipitation experiments showed that high levels of CD4 coexpression inhibited proteolytic processing of the HIV-1 envelope glycoprotein precursor gp160. This inhibition could be overcome by decreasing the CD4 binding ability of gp120. Studies were also undertaken to investigate the ability of virion-bound HIV-1 envelope glycoproteins to kill primary CD4(+) T cells. However, neither X4 nor R5X4 envelope glycoproteins on noninfectious virions caused death in primary CD4(+) T cells. These results demonstrate that the interaction of CCR5 with R5 HIV-1 envelope glycoproteins capable of inducing membrane fusion leads to cell lysis; overexpression of CD4 can inhibit cell killing by limiting envelope glycoprotein processing.

Mitochondria in HIV-1-induced apoptosis

It is now well admitted that HIV infection leading to AIDS is associated with an abnormal susceptibility of T cells to undergo apoptosis. Recent progress in research into programmed cell death has resulted in the identification of the principal pathways involved in this process. Thus the "extrinsic" as well as the "intrinsic" pathways converge to the mitochondria considered as the main sensor of programmed cell death. This review summarizes our knowledge of the influence of mitochondrial control on T cell death during HIV and SIV infections.

Apoptosis as an HIV strategy to escape immune attack

Viruses have evolved numerous mechanisms to evade the host immune system and one of the strategies developed by HIV is to activate apoptotic programmes that destroy immune effectors. Not only does the HIV genome encode pro-apoptotic proteins, which kill both infected and uninfected lymphocytes through either members of the tumour-necrosis factor family or the mitochondrial pathway, but it also creates a state of chronic immune activation that is responsible for the exacerbation of physiological mechanisms of clonal deletion. This review discusses the molecular mechanisms by which HIV manipulates the apoptotic machinery to its advantage, assesses the functional consequences of this process and evaluates how new therapeutics might counteract this strategy.

A region of the C-terminal tail of the gp41 envelope glycoprotein of human immunodeficiency virus type 1 contains a neutralizing epitope: evidence for its exposure on the surface of the virion

The approximately 150 amino acid C-terminal tail of the gp41 transmembrane glycoprotein of human immunodeficiency virus type 1 (HIV-1) is generally thought to be located inside the virion. However, we show here that both monoclonal IgG and polyclonal epitope-purified IgG specific for the (746)ERDRD(750) epitope that lies within the C-terminal tail neutralized infectious virus. IgG was mapped to the C-terminal tail by its failure to neutralize tail-deleted virus, and by sequencing of antibody-escape mutants. The fact that antibody does not cross lipid membranes, and infectious virus is by definition intact, suggested that ERDRD was exposed on the surface of the virion. This was confirmed by reacting virus and IgG, separating virus and unbound IgG by centrifugation, and showing that virus was neutralized to essentially the same extent as virus that had been in constant contact with antibody. Epitope exposure on virions was independent of temperature and therefore constitutive. Monoclonal antibodies specific to epitopes PDRPEG and IEEE, upstream of ERDRD, also bound to virions, suggesting that they too were located externally. Protease digestion destroyed the ERDRD and PDRPEG epitopes, consistent with their proposed external location. Altogether these data are consistent with part of the C-terminal tail of gp41 being exposed on the outside of the virion. Possible models of the structure of the gp41 tail, taking these observations into account, are discussed.

Rational site-directed mutations of the LLP-1 and LLP-2 lentivirus lytic peptide domains in the intracytoplasmic tail of human immunodeficiency virus type 1 gp41 indicate common functions in cell-cell fusion but distinct roles in virion envelope incorporation

Two highly conserved cationic amphipathic alpha-helical motifs, designated lentivirus lytic peptides 1 and 2 (LLP-1 and LLP-2), have been characterized in the carboxyl terminus of the transmembrane (TM) envelope glycoprotein (Env) of lentiviruses. Although various properties have been attributed to these domains, their structural and functional significance is not clearly understood. To determine the specific contributions of the Env LLP domains to Env expression, processing, and incorporation and to viral replication and syncytium induction, site-directed LLP mutants of a primary dualtropic infectious human immunodeficiency virus type 1 (HIV-1) isolate (ME46) were examined. Substitutions were made for highly conserved arginine residues in either the LLP-1 or LLP-2 domain (MX1 or MX2, respectively) or in both domains (MX4). The HIV-1 mutants with altered LLP domains demonstrated distinct phenotypes. The LLP-1 mutants (MX1 and MX4) were replication defective and showed an average of 85% decrease in infectivity, which was associated with an evident decrease in gp41 incorporation into virions without a significant decrease in Env expression or processing in transfected 293T cells. In contrast, MX2 virus was replication competent and incorporated a full complement of Env into its virions, indicating a differential role for the LLP-1 domain in Env incorporation. Interestingly, the replication-competent MX2 virus was impaired in its ability to induce syncytia in T-cell lines. This defect in cell-cell fusion did not correlate with apparent defects in the levels of cell surface Env expression, oligomerization, or conformation. The lack of syncytium formation, however, correlated with a decrease of about 90% in MX2 Env fusogenicity compared to that of wild-type Env in quantitative luciferase-based cell-cell fusion assays. The LLP-1 mutant MX1 and MX4 Envs also exhibited an average of 80% decrease in fusogenicity. Altogether, these results demonstrate for the first time that the highly conserved LLP domains perform critical but distinct functions in Env incorporation and fusogenicity.

Toward the development of a virus-cell-based assay for the discovery of novel compounds against human immunodeficiency virus type 1

The emergence of human immunodeficiency virus type 1 (HIV-1) strains resistant to highly active antiretroviral therapy necessitates continued drug discovery for the treatment of HIV-1 infection. Most current drug discovery strategies focus upon a single aspect of HIV-1 replication. A virus-cell-based assay, which can be adapted to high-throughput screening, would allow the screening of multiple targets simultaneously. HIV-1-based vector systems mimic the HIV-1 life cycle without yielding replication-competent virus, making them potentially important tools for the development of safe, wide-ranging, rapid, and cost-effective assays amenable to high-throughput screening. Since replication of vector virus is typically restricted to a single cycle, a crucial question is whether such an assay provides the needed sensitivity to detect potential HIV-1 inhibitors. With a stable, inducible vector virus-producing cell line, the inhibitory effects of four reverse transcriptase inhibitors (zidovudine, stavudine, lamivudine, and didanosine) and one protease inhibitor (indinavir) were assessed. It was found that HIV-1 vector virus titer was inhibited in a single cycle of replication up to 300-fold without affecting cell viability, indicating that the assay provides the necessary sensitivity for identifying antiviral molecules. Thus, it seems likely that HIV-1-derived vector systems can be utilized in a novel fashion to facilitate the development of a safe, efficient method for screening compound libraries for anti-HIV-1 activity.

Automatic quantitation of HIV-1 mediated cell-to-cell fusion with a digital image analysis system (DIAS): application for rapid screening of HIV-1 fusion inhibitors

Human immunodeficiency viruses type 1 (HIV-1) mediated cell-to-cell fusion plays an important role in HIV-1 spread from infected cells to uninfected cells and in HIV-1 cytopathogenesis. In the present study, we developed a convenient cell fusion assay using a computer-controlled digital image analysis system (DIAS) for automatic quantitation. Compared with a manual quantitative method, DIAS automatic method is less laborious, and more rapid. Furthermore, it is more objective and less dependent on the researchers' experience. This method has great potential to be developed further as a high-throughput screening assay for rapid screening of HIV-1 fusion inhibitors, for evaluating the activity of HIV-1 entry inhibitors and for studying the mechanism of action of anti-HIV-1 agents.

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.

Sequential involvement of Cdk1, mTOR and p53 in apoptosis induced by the HIV-1 envelope

Syncytia arising from the fusion of cells expressing the HIV-1-encoded Env gene with cells expressing the CD4/CXCR4 complex undergo apoptosis following the nuclear translocation of mammalian target of rapamycin (mTOR), mTOR-mediated phosphorylation of p53 on Ser15 (p53(S15)), p53-dependent upregulation of Bax and activation of the mitochondrial death pathway. p53(S15) phosphorylation is only detected in syncytia in which nuclear fusion (karyogamy) has occurred. Karyogamy is secondary to a transient upregulation of cyclin B and a mitotic prophase-like dismantling of the nuclear envelope. Inhibition of cyclin-dependent kinase-1 (Cdk1) prevents karyogamy, mTOR activation, p53(S15) phosphorylation and apoptosis. Neutralization of p53 fails to prevent karyogamy, yet suppresses apoptosis. Peripheral blood mononuclear cells from HIV-1-infected patients exhibit an increase in cyclin B and mTOR expression, correlating with p53(S15) phosphorylation and viral load. Cdk1 inhibition prevents the death of syncytia elicited by HIV-1 infection of primary CD4 lymphoblasts. Thus, HIV-1 elicits a pro-apoptotic signal transduction pathway relying on the sequential action of cyclin B-Cdk1, mTOR and p53.

"Tissue" transglutaminase in AIDS

Apoptosis or programmed cell death (PCD) is an active process of cellular self-destruction, essential for embryonic development and maintenance of homeostasis of multicellular organisms. Programmed cell death induction can serve as a defence mechanism of the host against intracellular microbes. Virus infections trigger host cell apoptosis, which can either limit virus production or contribute directly to viral pathogenesis. Several independent laboratories have identified "tissue" transglutaminase (tTG) as a potentially important player of the cell death program(s). This gene is specifically expressed in cells dying during mammalian development as well as in those undergoing apoptosis in various patho-physiological and experimental settings [Eur. J. Cell Biol. 56 (1991) 170; Piacentini, M., Davies, P.J.A., Fesus, L., 1994. Tissue transglutaminase in cells undergoing apoptosis. In: Tomei, L.D., Cope, F.O. (Eds.), Apoptosis II: The molecular basis of apoptosis in disease. Cold Spring Harbor Lab. Press, pp. 143-165.]. This chapter reviews recent studies concerning the expression and the possible role of "tissue" transglutaminase (tTG) in apoptotic cells; particular emphasis is given to its expression in the cell death pathways associated with HIV infection in the immune system. We propose here that the induction of the tTG gene in cells of the immune system, as well as the detection of the isodipeptide epsilon(gamma-glutamyl)lysine in plasma, are useful markers of apoptosis and might make it possible to monitor disease progression in HIV-infected individuals.

Restoration of wild-type infectivity to human immunodeficiency virus type 1 strains lacking nef by intravirion reverse transcription

Human immunodeficiency virus type 1 (HIV-1) Nef protein exerts several effects, both on infected cells and as a virion protein, which work together to enhance viral replication. One of these activities is the ability to enhance infectivity and the formation of proviral DNA. The mechanism of this enhancement remains incompletely understood. We show that virions with nef deleted can be restored to wild-type infectivity by stimulating intravirion reverse transcription. Particle composition and measures of reverse transcriptase activity remain the same for Nef(+) and Nef(-) virions both before and after natural endogenous reverse transcription (NERT) treatment. The effect of NERT treatment on virions pseudotyped with murine leukemia virus envelope protein was similar to that on particles pseudotyped with HIV-1 envelope protein. However, virions pseudotyped with vesicular stomatitis virus G envelope protein showed no influence of Nef on NERT enhancement of infectivity. These observations suggest that Nef may function at a level prior to reverse transcription. Since NERT treatment results in partial disassembly of the viral core, we speculate that Nef may function at the level of core particle disassembly.

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.

Structural and functional analysis of interhelical interactions in the human immunodeficiency virus type 1 gp41 envelope glycoprotein by alanine-scanning mutagenesis

Membrane fusion by human immunodeficiency virus type 1 (HIV-1) is promoted by the refolding of the viral envelope glycoprotein into a fusion-active conformation. The structure of the gp41 ectodomain core in its fusion-active state is a trimer of hairpins in which three antiparallel carboxyl-terminal helices pack into hydrophobic grooves on the surface of an amino-terminal trimeric coiled coil. In an effort to identify amino acid residues in these grooves that are critical for gp41 activation, we have used alanine-scanning mutagenesis to investigate the importance of individual side chains in determining the biophysical properties of the gp41 core and the membrane fusion activity of the gp120-gp41 complex. Alanine substitutions at Leu-556, Leu-565, Val-570, Gly-572, and Arg-579 positions severely impaired membrane fusion activity in envelope glycoproteins that were for the most part normally expressed. Whereas alanine mutations at Leu-565 and Val-570 destabilized the trimer-of-hairpins structure, mutations at Gly-572 and Arg-579 led to the formation of a stable gp41 core. Our results suggest that the Leu-565 and Val-570 residues are important determinants of conserved packing interactions between the amino- and carboxyl-terminal helices of gp41. We propose that the high degree of sequence conservation at Gly-572 and Arg-579 may result from selective pressures imposed by prefusogenic conformations of the HIV-1 envelope glycoprotein. Further analysis of the gp41 activation process may elucidate targets for antiviral intervention.

Human immunodeficiency virus 1 envelope glycoprotein complex-induced apoptosis involves mammalian target of rapamycin/FKBP12-rapamycin-associated protein-mediated p53 phosphorylation

Syncytia arising from the fusion of cells expressing a lymphotropic human immunodeficiency virus (HIV)-1-encoded envelope glycoprotein complex (Env) gene with cells expressing the CD4/CXCR4 complex undergo apoptosis through a mitochondrion-controlled pathway initiated by the upregulation of Bax. In syncytial apoptosis, phosphorylation of p53 on serine 15 (p53S15) precedes Bax upregulation, the apoptosis-linked conformational change of Bax, the insertion of Bax in mitochondrial membranes, subsequent release of cytochrome c, caspase activation, and apoptosis. p53S15 phosphorylation also occurs in vivo, in HIV-1(+) donors, where it can be detected in preapoptotic and apoptotic syncytia in lymph nodes, as well as in peripheral blood mononuclear cells, correlating with viral load. Syncytium-induced p53S15 phosphorylation is mediated by the upregulation/activation of mammalian target of rapamycin (mTOR), also called FKBP12-rapamycin-associated protein (FRAP), which coimmunoprecipitates with p53. Inhibition of mTOR/FRAP by rapamycin reduces apoptosis in several paradigms of syncytium-dependent death, including in primary CD4(+) lymphoblasts infected by HIV-1. Concomitantly, rapamycin inhibits p53S15 phosphorylation, mitochondrial translocation of Bax, loss of the mitochondrial transmembrane potential, mitochondrial release of cytochrome c, and nuclear chromatin condensation. Transfection with dominant negative p53 has a similar antiapoptotic action as rapamycin, upstream of the Bax upregulation/translocation. In summary, we demonstrate that phosphorylation of p53S15 by mTOR/FRAP plays a critical role in syncytial apoptosis driven by HIV-1 Env.

The Biology and Evolution of HIV

▪ Abstract  This review examines the current state of knowledge about HIV/AIDS in terms of its origins, pathogenesis, genetic variation, and evolutionary biology. The HIV virus damages the host's immune system, resulting in AIDS, which is characterized by immunodeficiency, opportunistic infections, neoplasms, and neurological problems. HIV is a complex retrovirus with a high mutation rate. This mutation rate allows the virus to evade host immune responses, and evidence indicates that selection favors more virulent strains with rapid replication. While a number of controversial theories attempt to explain the origin of HIV/AIDS, phylogenetic evidence suggests a zoonotic transmission of HIV to humans and implicates the chimpanzee (Pan troglodytes troglodytes) as the source of HIV-1 infection and the sooty mangabey as the source of HIV-2 infection in human populations. New therapies provide hope for increased longevity among people living with AIDS, but the biology of HIV presents significant obstacles to finding a cure and/or vaccine. HIV continues to be a threat to the global population because of its fast mutation rate, recombinogenic effect, and its use of human defenses to replicate itself.

Sodium lauryl sulfate abrogates human immunodeficiency virus infectivity by affecting viral attachment

The microbicidal activity of sodium lauryl sulfate (SLS) against human immunodeficiency virus type 1 (HIV-1) was studied in cultured cells. Pretreatment of HIV-1(NL4-3) with SLS decreased, in a concentration-dependent manner, its infectivity when using 1G5 as target cells. In the absence of a viral pretreatment period or when 1G5 cells were pretreated with SLS, the surfactant-induced inactivation of viral infectivity was less pronounced, especially at concentrations between 375 and 550 microM. SLS had no effect on HIV-1 when the virus was adsorbed to 1G5 cells by a 2-h incubation period. SLS almost completely inhibited the fusion process by decreasing the attachment of HIV-1 to target cells. SLS also inhibited the infectivity of HIV-1-based luciferase reporter viruses pseudotyped with the amphotropic murine leukemia virus envelope (which enters cells in a CD4-, CCR5-, and CXCR4-independent manner), indicating that SLS may inactivate other envelope viruses. In contrast, no effect was seen with vesicular stomatitis virus envelope glycoprotein G (which enters cells through receptor-mediated endocytosis) pretreated with up to 700 microM SLS. SLS also decreased, in a dose-dependent manner, the HIV-1-dependent syncytium formation between 1G5 and J1.1 cells after a 24-h incubation. The reduction of luciferase activity was more pronounced when J1.1 cells (which express HIV-1 proteins on their surface) were pretreated with SLS rather than 1G5 cells. Taken together, our results suggest that SLS could represent a candidate of choice for use in vaginal microbicides to prevent the sexual transmission of HIV and possibly other pathogens causing sexually transmitted diseases.

CD4(+) and CD8(+) T cell death during human immunodeficiency virus infection in vitro

We have evaluated the death of CD4(+) and CD8(+) T cells during in vitro human immunodeficiency virus (HIV) infection of peripheral blood mononuclear cells (PBMC) and tonsilar tissue. Acute infections with several X4 and R5 HIV isolates induced a decrease in cell viability that was higher in infections with X4 viruses and correlated with an increased rate of CD4(+) T-cell death. In CD4(+) T cells, the primary X4 isolate AOM induced higher levels of death than the laboratory X4 isolates IIIB and NL4-3 or the R5 isolates BaL and MDM. An effect on CD8(+) T-cell viability was exclusively observed in infections by X4 viruses, including the NL4-3 strain, in both PBMC and tonsilar tissue. This effect was dependent on the env gene of the infecting isolate and required productive HIV replication in CD4(+) but not in CD8(+) T cells. Our results suggest that X4 and R5 HIV isolates depleted CD4(+) T cells to a different extent and that CD8(+) T-cell viability may also be affected by mechanisms other than those acting in CD4(+) T cells.

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.

Syncytium formation amplifies apoptotic signals: a new view on apoptosis in HIV infection in vitro

Infection of CD4+ cells with HIV in vitro causes extensive cytopathology. The mechanism that underlies this process is unclear and conflicting data exist regarding whether cytotoxicity is due to necrosis or apoptosis. It was previously reported and is shown here that the coculture of HIV glycoprotein-expressing cells with CD4+ cells results in apoptosis within several hours. This study demonstrates that apoptosis did not occur in single cells and was mediated neither by CD4 nor by coreceptor signaling, indicating that apoptosis was not induced by intra- or intercellular glycoprotein-receptor interaction. Detection of apoptosis required cell-to-cell fusion and undetectable levels of apoptotic cell death were substantially amplified upon syncytium formation. Similar results were obtained with syncytium-forming cultures of measles virus glycoprotein-expressing cells. These findings indicate that the apoptotic cell death observed in cultures of HIV and other syncytium-forming viruses is primarily due to amplification of background apoptosis in the wake of cell-to-cell fusion.

Increased neutralization sensitivity of CD4-independent human immunodeficiency virus variants

Naturally occurring human immunodeficiency virus (HIV-1) variants require the presence of CD4 and specific chemokine receptors to enter a cell. In the laboratory, HIV-1 variants that are capable of bypassing CD4 and utilizing only the CCR5 chemokine receptor for virus entry have been generated. Here we report that these CD4-independent viruses are significantly more sensitive to neutralization by soluble CD4 and a variety of antibodies. The same amino acid changes in the HIV-1 gp120 envelope glycoprotein determined CD4 independence and neutralization sensitivity. The CD4-independent envelope glycoproteins exhibited higher affinity for antibodies against CD4-induced gp120 epitopes but not other neutralizing ligands. The CD4-independent envelope glycoproteins did not exhibit increased lability relative to the wild-type envelope glycoproteins. The utilization of two receptors apparently allows HIV-1 to maintain a more neutralization-resistant state prior to engaging CD4 on the target cell, explaining the rarity of CD4 independence in wild-type HIV-1.

Role of 14-3-3epsilon, c-Myc/Max, and Akt phosphorylation in HIV-1 gp 120-induced mesangial cell proliferation

Focal glomerulosclerosis (FGS) is the predominant glomerular lesion in patients with human immunodeficiency virus (HIV)-associated nephropathy. Initial mesangial cell hyperplasia and subsequent hypoplasia are common features of FGS. In the present study we evaluated the effect of HIV-1 glycoprotein (gp) 120 on human mesangial cell (HMC) growth. HIV-1 gp 120 stimulated HMC proliferation at lower concentrations, whereas it suppressed cell proliferation at higher concentrations. In parallel to the modulation of cell growth, gp 120 at low concentrations resulted in an increase in the expression of c-Myc, Max, and 14-3-3epsilon proteins and phosphorylation of ATP-dependent tyrosine kinases (Akt) at Ser(473). However, the expression of these proteins decreased with increasing concentrations of gp 120. Furthermore, gp 120 also exhibited a dose-dependent inhibition of Akt phosphorylation at Ser-473 without any significant alteration of Akt expression. Little or no effects of gp 120 were observed on the expression of extracellular signal-regulated kinase (ERK), phospho-ERK, Bcl-2, and Bax proteins. At a higher concentration, gp 120 not only promoted HMC apoptosis but also enhanced expression of Fas and FasL. These results suggest that HIV-1 gp 120 induces alterations in conflicting survival signaling pathways that contribute to the potential dual effects of gp 120 in promoting or inhibiting HMC proliferation.

Resistance to apoptosis in HIV-infected CD4+ T lymphocytes is mediated by macrophages: role for Nef and immune activation in viral persistence

Apoptosis or programmed cell death may play a critical role in AIDS pathogenesis through depletion of both CD4(+) and CD8(+) T lymphocytes. Using a reporter virus, a recombinant HIV infectious clone expressing the green fluorescent protein (GFP), apoptosis was measured in productively infected CD4(+) T lymphocytes, in the presence and absence of autologous macrophages. The presence of macrophages in the culture increased the frequency of nonapoptotic GFP-positive productively infected CD4(+) T lymphocytes. The appearance of nonapoptotic productively infected CD4(+) T lymphocytes in the culture required intercellular contacts between macrophages and PBLs and the expression of the HIV Nef protein. The presence of macrophages did not reduce apoptosis when CD4(+) T lymphocytes were infected with a GFP-tagged virus deleted for the nef gene. TNF-alpha (TNF) expressed on the surface of macrophages prevented apoptosis in nef-expressing, productively infected CD4(+) T lymphocytes. Similarly, following TNF stimulation, apoptosis was diminished in Jurkat T cells transfected with a nef-expressing plasmid. TNF stimulation of nef-expressing Jurkat T cells resulted in NF-kappaB hyperactivation, which has been shown to deliver anti-apoptotic signals. Our results indicate that intercellular contacts with macrophages increase the rate of productively infected nonapoptotic CD4(+) T lymphocytes. The survival of productively infected CD4(+) T lymphocytes requires Nef expression as well as activation by TNF expressed on the surface of macrophages and might participate in the formation and maintenance of viral reservoirs in HIV-infected persons.

Apoptosis of syncytia induced by the HIV-1-envelope glycoprotein complex: influence of cell shape and size

Cells stably transfected with a lymphotropic HIV-1 Env gene form syncytia when cocultured with CD4(+)CXCR4(+) cells. Heterokaryons then spontaneously undergo apoptosis, while manifesting signs of mitochondrial membrane pemeabilization as well as nuclear chromatin condensation. Modulation of cellular geometry was achieved by growing syncytia on self-assembled monolayers of terminally substituted alkanethiolates designed to control the adhesive properties of the substrates. Spreading of syncytia, induced by culturing them on small circular adhesive islets (diameter 5 microm), placed at a distance that cells can bridge (10 microm), inhibited spontaneous and staurosporin-induced signs of apoptosis, both at the mitochondrial and at the nuclear levels, and allowed for the generation of larger syncytia. Transient cell spreading conferred a memory of apoptosis inhibition which was conserved upon adoption of a conventional cell shape. Limiting syncytium size by culturing them on square-shaped planar adhesive islands of defined size (400 to 2500 microm(2)), separated by nonadhesive regions, enhanced the rate of apoptotic cell death, as indicated by an accelerated permeabilization of the outer mitochondrial membrane, loss of the mitochondrial inner transmembrane potential, and an increased frequency of nuclear apoptosis. In conclusion, external constraints on syncytial size and shape strongly modulate their propensity to undergo apoptosis.

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

In established T-cell lines, the membrane-fusing capacity of the human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins mediates cytopathic effects, both syncytium formation and single-cell lysis. Furthermore, changes in the HIV-1 envelope glycoproteins are responsible for the increased CD4(+) T-cell-depleting ability observed in infected monkeys upon in vivo passage of simian-human immunodeficiency virus (SHIV) chimeras. In this study, a panel of SHIV envelope glycoproteins and their mutant counterparts defective in membrane-fusing capacity were expressed in primary human CD4(+) T cells. Compared with controls, all of the functional HIV-1 envelope glycoproteins induced cell death in primary CD4(+) T-cell cultures, whereas the membrane fusion-defective mutants did not. Death occurred almost exclusively in envelope glycoprotein-expressing cells and not in bystander cells. Under standard culture conditions, most dying cells underwent lysis as single cells. When the cells were cultured at high density to promote syncytium formation, the envelope glycoproteins of the passaged, pathogenic SHIVs induced more syncytia than those of the respective parental SHIV. These results demonstrate that the HIV-1 envelope glycoproteins induce the death of primary CD4(+) T lymphocytes by membrane fusion-dependent processes.

Apoptosis control in syncytia induced by the HIV type 1-envelope glycoprotein complex: role of mitochondria and caspases

Syncytia arising from the fusion of cells expressing a lymphotropic HIV type 1-encoded envelope glycoprotein complex (Env) with cells expressing the CD4/CXC chemokine receptor 4 complex spontaneously undergo cell death. Here we show that this process is accompanied by caspase activation and signs of mitochondrial membrane permeabilization (MMP), including the release of intermembrane proteins such as cytochrome c (Cyt-c) and apoptosis-inducing factor (AIF) from mitochondria. In Env-induced syncytia, caspase inhibition did not suppress AIF- and Cyt-c translocation, yet it prevented all signs of nuclear apoptosis. Translocation of Bax to mitochondria led to MMP, which was inhibited by microinjected Bcl-2 protein or bcl-2 transfection. Bcl-2 also prevented the subsequent nuclear chromatin condensation and DNA fragmentation. The release of AIF occurred before that of Cyt-c and before caspase activation. Microinjection of AIF into syncytia sufficed to trigger rapid, caspase-independent Cyt-c release. Neutralization of endogenous AIF by injection of an antibody prevented all signs of spontaneous apoptosis occurring in syncytia, including the Cyt-c release and nuclear apoptosis. In contrast, Cyt-c neutralization only prevented nuclear apoptosis, and did not affect AIF release. Our results establish that the following molecular sequence governs apoptosis of Env-induced syncytia: Bax-mediated/Bcl-2-inhibited MMP --> AIF release --> Cyt-c release --> caspase activation --> nuclear apoptosis.

Upregulation of the apoptosis-associated protein Grb3-3 in HIV-1-infected human CD4(+) lymphocytes

The mechanism(s) by which HIV-1 infection contributes to depletion of CD4(+) T cell is not well understood. In this report, we investigated whether a recently identified isoform of growth factor receptor bound protein (Grb2), named Grb3-3, a signaling molecule that is associated with the MAP kinase pathway and with apoptosis could be involved. We find that Grb3-3 is markedly up-regulated following HIV-1 infection of CD4(+) peripheral blood mononuclear cells undergoing apoptosis. Although IL-2 deprived CD4(+) cells also undergo apoptosis to a similar extent, Grb3-3 upregulation is not detected under these experimental conditions. Transient overexpression of Grb3-3 in Jurkat T-cells also causes apoptosis. Upon staurosporine stimulation, Grb3-3 predisposes Sup-T1 cell to apoptosis. Finally, analysis of the HIV-1 genes responsible for Grb3-3 expression demonstrates that Tat and Nef can independently induces its expression, suggesting these two earliest viral gene products of HIV-1 may share some common pathway(s) in up-regulating Grb3-3 expression.

Destruction of primary CD4(+) T cells by cell-cell interaction in human immunodeficiency virus type 1 infection in vitro

Infection of CD4(+) T lymphocytes with human immunodeficiency virus (HIV) in vitro is accompanied by extensive cytopathicity. The mechanism of cell death is unclear, but may be related to expression of the viral envelope glycoprotein. Here, it is demonstrated that T cell destruction in primary T cells occurs upon contact of infected with uninfected lymphocytes. Cell death was due to the interaction of the envelope glycoprotein with CD4 and subsequent fusion of the cells. Agents that interfered with cell-to-cell fusion such as a monoclonal antibody to CD4 and the peptide T20 prevented T cell death and depletion. In contrast, single-cell lysis due to expression and intracellular processing of the envelope glycoprotein was insignificant. These results suggest that cell-to-cell fusion and concomitant rapid cell death promote the depletion of T cells in HIV-infected individuals.

Envelope glycoprotein determinants of increased fusogenicity in a pathogenic simian-human immunodeficiency virus (SHIV-KB9) passaged in vivo

Changes in the envelope glycoprotein ectodomains of a nonpathogenic simian-human immunodeficiency virus (SHIV-89.6) that was serially passaged in vivo have been shown to be responsible for the increased pathogenicity of the resulting virus, SHIV-KB9 (G. B. Karlsson, et al., J. Exp. Med. 188:1159-1171, 1998). The 12 amino acid changes in the envelope glycoprotein ectodomains resulted in increased chemokine receptor-binding and syncytium-forming abilities. Here we identify the envelope glycoprotein determinants of these properties. A single amino acid change in the gp120 third variable (V3) loop was both necessary and sufficient for the observed increase in the binding of the SHIV-KB9 gp120 glycoprotein to the CCR5 chemokine receptor. The increased syncytium-forming ability of SHIV-KB9 involved, in addition to the V3 loop change, changes in the second conserved (C2) region of gp120 (residue 225) and in the gp41 ectodomain (residues 564 and 567). The C2 and gp41 ectodomain changes influenced syncytium formation in a cooperative manner. Changes in the V1/V2 gp120 variable loops exerted a negative effect on syncytium formation and chemokine receptor binding, supporting a previously described role of these changes in immune evasion. The definition of the passage-associated changes that determine the efficiency of chemokine receptor binding and membrane fusogenicity will allow evaluation of the contribution of these properties to in vivo CD4-positive lymphocyte depletion.

Caspase-dependent apoptosis of cells expressing the chemokine receptor CXCR4 is induced by cell membrane-associated human immunodeficiency virus type 1 envelope glycoprotein (gp120)

Human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins interact with CD4 and chemokine receptors on T cells to deliver signals that trigger either activation, anergy, or apoptosis. However, the molecular mechanisms driving these responses remain poorly understood. In this study we demonstrate that apoptosis is induced upon HIV-1 envelope binding to the chemokine receptor CXCR4. Cells expressing a mutant form of CXCR4 with a C-terminal deletion were also sensitive to HIV-1 envelope-mediated apoptosis, indicating that the cytoplasmic tail of CXCR4 is not required to induce the apoptotic pathway. The specificity of this process was analyzed using several inhibitors of gp120-CD4-CXCR4 interaction. Monoclonal antibodies directed against the gp120-binding site on CD4 (ST4) and against CXCR4 (MAB173) prevented the apoptotic signal in a dose-dependent manner. The cell death program was also inhibited by SDF-1alpha, the natural ligand of CXCR4, and by suramin, a G protein inhibitor that binds with a high affinity to the V3 loop of HIV-1 gp120 envelope protein. These results highlight the role played by gp120-binding on CXCR4 to trigger programmed cell death. Next, we investigated the intracellular signal involved in gp120-induced apoptosis. This cell death program was insensitive to pertussis toxin and did not involve activation of the stress- and apoptosis-related MAP kinases p38(MAPK) and SAPK/JNK but was inhibited by a broad spectrum caspase inhibitor (z-VAD.fmk) and a relatively selective inhibitor of caspase 3 (z-DEVD.fmk). Altogether, our results demonstrate that HIV induces a caspase-dependent apoptotic signaling pathway through CXCR4.

Fusion of uninfected T-cells occurs with immature HIV-1 protease-mutant, but not morphologically similar protease inhibitor derived particles

Protease inhibitors are widely used in the treatment of human immunodeficiency virus type 1 (HIV-1)-infected individuals and show a drastic effect on the reduction of virus load. We previously reported that doughnut-shaped, protease-defective gp120-containing HIV-1 particles from an L-2 cell clone, carrying a provirus with mutations at the pol (protease), env (gp41) and nef genes, rapidly and more effectively induces virus particle-mediated syncytia formation of uninfected T-cells, than a parental wild-type laboratory strain of HIV-1 (LAI). In this study, we examined the possibility of whether enhanced syncytia formation is mediated by morphologically similar doughnut-shaped particles obtained after treatment of LAI-infected cells with the protease inhibitors L-689, 502, DMP-323, RO-31-8959, and KNI-272. Utilizing such protease inhibitor-induced particles and a clone of MOLT-4 cells, we could not detect any enhancement of syncytia formation, over that seen with wild-type LAI particles. This result should alleviate concerns of patients on highly active antiretroviral therapy (HAART), that protease inhibitors might accelerate progression of the disease through enhanced production of defective, 'immature'-appearing particles.

Could Nef and Vpr proteins contribute to disease progression by promoting depletion of bystander cells and prolonged survival of HIV-infected cells?

A growing body of literature suggests that the HIV accessory proteins Nef and Vpr could be involved in depletion of CD4(+) and non-CD4(+) cells and tissue atrophy, and in delaying the death of HIV-infected cells. Cell depletion is likely to be predominantly a bystander effect because the number of cells dying far outnumbers HIV-infected cells and is not confined to CD4(+) cells. The myristylated N-terminal region of Nef has severe membrane disordering properties, and when present in the extracellular medium causes rapid lysis in vitro of a wide range of CD4(+) and non-CD4(+) cells, suggesting a role for extracellular Nef in the depletion of bystander cells. A direct role for HIV-1 Nef in cytopathicity is supported by studies in HIV-infected Hu Liv/Thy SCID mice, in transgenic mice expressing nef gene alone, and in rhesus macaques infected with SIV/HIV chimeric virus containing HIV-1 nef. The N-terminal region of Nef has been directly implicated in development of simian AIDS. Extracellular Vpr and C-terminal fragments of Vpr cause membrane permeabilization and apoptosis of a wide range of CD4(+) and non-CD4(+) cells, and could also contribute to depletion of bystander cells. A direct in vivo role for Vpr in thymocyte depletion, thymic atrophy, and nephropathy is suggested in studies with vpr transgenic mice. Intracellular Nef and Vpr could help HIV-infected cells evade cell death by inhibiting apoptosis of infected cells and by avoiding virus-specific CTL response. Nef and Vpr are potential targets for therapeutic intervention and vaccine development, and strategies that prevent the death of bystander cells while promoting the early death of HIV-infected cells could arrest or retard progression to AIDS.

The long cytoplasmic tail of gp41 is required in a cell type-dependent manner for HIV-1 envelope glycoprotein incorporation into virions

Lentiviruses, including HIV-1, have transmembrane envelope (Env) glycoproteins with cytoplasmic tails that are quite long compared with those of other retroviruses. However, mainly because of the lack of biochemical studies performed in cell types that are targets for HIV-1 infection, no clear consensus exists regarding the function of the long lentiviral Env cytoplasmic tail in virus replication. In this report, we characterize the biological and biochemical properties of an HIV-1 mutant lacking the gp41 cytoplasmic tail. We find that the gp41 cytoplasmic tail is necessary for the efficient establishment of a productive, spreading infection in the majority of T cell lines tested, peripheral blood mononuclear cells, and monocyte-derived macrophages. Biochemical studies using a high-level, transient HIV-1 expression system based on pseudotyping with the vesicular stomatitis virus glycoprotein demonstrate that in HeLa and MT-4 cells, mutant Env incorporation into virions is reduced only 3-fold relative to wild type. In contrast, gp120 levels in virions produced from a number of other T cell lines and primary macrophages are reduced more than 10-fold by the gp41 truncation. The Env incorporation defect imposed by the cytoplasmic tail truncation is not the result of increased shedding of gp120 from virions or reduced cell-surface Env expression. These results demonstrate that in the majority of T cell lines, and in primary cell types that serve as natural targets for HIV-1 infection in vivo, the gp41 cytoplasmic tail is essential for efficient Env incorporation into virions.

Unique morphological alterations of the HTLV-I transformed C8166 cells by infection with HIV-1

C8166 cells express T lymphocyte markers, a monocyte-specific esterase, taxpolypeptide of HTLV-I. In spite of this transactivator, their HIV-1 yield is low. Their culture conditions were modified, and infected cells were immobilized on a poly-L-lysine sheet under semisolid overlays to study their phenotypic alterations and HIV-1 production by microscopy and electron microscopy. Another lymphoid cultures (MT-4, CEM, CEM-ss, AdCEM) similarly treated were infected with either HIV-1/RF or IIIB. Specificity of HIV-1 was compared to the effects of vesicular stomatitis virus (VSV). Unlike other cultures, HIV-1/RF infected C8166 cells in Eagle s MEM exhibited surface projections resembling hairy leukemia cells, which was followed by balloon degeneration and apoptosis. Immobilized HIV-1 infected cultures formed flat syncytia with several interdigitating dendritic projections. Syncytia shrunk with condensed nuclear material and axon-like filaments characteristic for infected macrophages. VSV induced enlargement and necrotic lysis of all cell types. Early postinfection with HIV-1, electron microscopy revealed irreversible membrane fusion above cell nuclei, and transient fusion between filaments. Transient presence of coated vesicles containing intact HIV-1 particles, Birbeck granule-like structures of Langerhans cells, fibrillar-lamellar structures resembling hairy leukemia or Sézary cells were detected. Late postinfection, high proportion of HIV-1 bud from polarized cytoplasm was empty particle, while that bud and entrapped in cytoplasmic vacuoles contained two or multiple cores in a fused envelope. The effect of early gene products of HIV-1 on HTLV-I and C8166 cells might elicit their latent potentials for monocyte or interdigitating dendritic cells, while in the later phase HTLV-I products might alter HIV-1 virion assembly.

Sterilization of HIV with irradiation: relevance to infected bone allografts

BACKGROUND

Bone allograft banks commonly sterilize frozen bone by irradiation. The dose-response relationship for HIV is calculated and the dose required to inactivate the bioburden of virus that may be present in allograft bone is determined.

METHODS

A virus titre experiment is performed using irradiated frozen HIV. The virus is maintained on dry ice (approximately -70 degrees C) and is exposed to a cobalt 60 source with 0-40 kGy irradiation at 5 kGy intervals. Lymphocyte cell cultures are exposed to serial dilutions of the irradiated virus. The virus titre is quantified by cytological changes of HIV infection and p24 immunofluorescence.

RESULTS

There is a linear relationship between the virus titre and the radiation dose delivered. The inactivation rate of irradiated virus was 0.1134 log10 tissue culture infective doses 50/mL per kGy (95% confidence intervals, 0.1248-0.1020). The irradiation dose required to inactivate the HIV bioburden in allograft bone is 35 kGy. The irradiation dose required to achieve a sterility assurance level of 10(-6) is 89 kGy. This dose exceeds current recommendations for sterilizing medical products and the current practice of many bone banks.

CONCLUSIONS

It is concluded that gamma irradiation should be disregarded as a significant virus inactivation method for bone allografts.

Reversion of a human immunodeficiency virus type 1 matrix mutation affecting Gag membrane binding, endogenous reverse transcriptase activity, and virus infectivity

We previously characterized mutations in the human immunodeficiency virus type 1 matrix (MA) protein that displayed reduced infectivity in single-round assays, defects in the stable synthesis of viral DNA in infected cells, and impaired endogenous reverse transcriptase activity. The mutants, which contained substitutions in a highly conserved Leu at MA amino acid 20, also increased binding of Gag to membrane. To elucidate further the role of MA in the virus replication cycle, we have characterized a viral revertant of an amino acid 20 mutant (20LK). The revertant virus, which replicates with essentially wild-type kinetics in H9 cells, contains second-site compensatory changes at MA amino acids 73 (E-->K) and 82 (A-->T), while retaining the original 20LK mutation. Single-cycle infectivity assays, performed with luciferase-expressing viruses, show that the 20LK/73EK/82AT triple mutant displays markedly improved infectivity relative to the original 20LK mutant. The stable synthesis of viral DNA in infected cells is also significantly increased compared with that of 20LK DNA. Furthermore, activity of revertant virions in endogenous reverse transcriptase assays is restored to near-wild-type-levels. Interestingly, although 20LK/73EK/82AT reverses the defects in replication kinetics, postentry events, and endogenous reverse transcriptase activity induced by the 20LK mutation, the reversion does not affect the 20LK-imposed increase in Gag membrane binding. Mutants containing single and double amino acid substitutions were constructed, and their growth kinetics were examined. Only virus containing all three changes (20LK/73EK/82AT) grew with significantly accelerated kinetics; 73EK, 73EK/82AT, and 20LK/82AT mutants displayed pronounced defects in virus particle production. Viral core-like complexes were isolated by sucrose density gradient centrifugation of detergent-treated virions. Intriguingly, the protein composition of wild-type and mutant detergent-resistant complexes differed markedly. In wild-type and 20LK complexes, MA was removed following detergent solubilization of the viral membrane. In contrast, in revertant preparations, the majority of MA cosedimented with the detergent-resistant complex. These results suggest that the 20LK/73EK/82AT mutations induced a significant alteration in MA-MA or MA-core interactions.

Packaging cell lines for simian foamy virus type 1 vectors

Foamy viruses are nonpathogenic retroviruses that offer several unique opportunities for gene transfer in various cell types from different species. We have previously demonstrated the utility of simian foamy virus type 1 (SFV-1) as a vector system by transient expression assay (M. Wu et al., J. Virol. 72:3451-3454, 1998). In this report, we describe the first stable packaging cell lines for foamy virus vectors based on SFV-1. We developed two packaging cell lines in which the helper DNA is placed under the control of either a constitutive cytomegalovirus (CMV) immediate-early gene or inducible tetracycline promoter for expression. Although the constitutive packaging expressing cell line had a higher copy number of packaging DNA, the inducible packaging cell line produced four times more vector particles. This result suggested that the structural gene products in the constitutively expressing packaging cell line were expressed at a level that is not toxic to the cells, and thus vector production was reduced. The SFV-1 vector in the presence of vesicular stomatitis virus envelope protein G (VSV-G) produced an insignificant level of transduction, indicating that foamy viruses could not be pseudotyped with VSV-G to generate high-titer vectors. The availability of stable packaging cell lines represents a step toward the use of an SFV-1 vector delivery system that will allow scaled-up production of vector stocks for gene therapy.

High major histocompatibility complex-unrestricted lysis of simian immunodeficiency virus envelope-expressing cells predisposes macaques to rapid AIDS progression

Before the development of virus-specific immune responses, peripheral blood mononuclear cells (PBMC) from uninfected rhesus monkeys and human beings have the capacity to lyse target cells expressing simian immunodeficiency virus (SIV) or human immunodeficiency virus-1 (HIV) envelope (gp130 and gp120) antigens. Lysis by naive effector cells does not require major histocompatibility complex (MHC)-restricted antigen presentation, is equally effective for allogeneic and xenogeneic targets, and is designated MHC-unrestricted (UR) lysis. UR lysis is not sensitive to EGTA and does not require de novo RNA or protein synthesis. Several kinds of envelope-expressing targets, including cells that poorly express MHC class I antigens, can be lysed. CD4(+) effectors are responsible for most of the lytic activity. High lysis is correlated with high expression of HIV or SIV envelope, specifically, the central one-third of the gp130 molecule, and lysis is completely inhibited by a monoclonal antibody against envelope. Our work extends observations of human lymphocytes expressing HIV gp120 to the SIV/rhesus monkey model for AIDS. Additionally, we address the relevance of UR lysis in vivo. A survey of PBMC from 56 uninfected rhesus monkeys indicates that 59% of the individuals had peak UR lytic activity above 15% specific lysis. Eleven of these monkeys were subsequently infected with SIV. Animals with UR lytic activity above 15% specific lysis were predisposed to more rapid disease progression than animals with low UR lytic activity, suggesting a strong correlation between this form of innate immunity and disease progression to AIDS.

Transient Tat activation of the HIVLAV/Lai-1 LTR by primary HIV-1 phenotypic variants in HeLaT4LTRbeta-gal cells

The rapid/high and slow/low phenotypic variants of primary HIV-1 isolates can be distinguished by their differential co-receptor utilization and their ability to productively infect established cell lines. To reveal possible differences in Tat-mediated transactivation, the potential for primary isolate Tat proteins to transactivate the LTR from the laboratory strain HIVLAV/Lai-1 was examined. Using either cell-mediated or PEG-induced fusion of cells infected with primary HIV-1 isolates and HeLaT4LTRbeta-gal cells, it was clear that the Tat protein encoded by all patient isolates efficiently activated transcription from the HIVLAV/Lai-1 LTR. However, infection of HeLaT4LTRbeta-gal cells by primary HIV-1 isolates was transient, suggesting the development of a postpenetration host control of HIV-1 replication at the level of tat activation, a feature not observed for the laboratory-adapted strain HIVIIIB. Although plasmid vectors based on the HIVLAV/Lai-1 LTR remain useful for the development of susceptible established cell lines for titrating primary HIV-1 isolates, the efficacy of such a system would depend upon the stability/duration of Tat activation.

The regulation of apoptosis by microbial pathogens

In the past few years, there has been remarkable progress unraveling the mechanism and significance of eukaryotic programmed cell death (PCD), or apoptosis. Not surprisingly, it has been discovered that numerous, unrelated microbial pathogens engage or circumvent the host's apoptotic program. In this chapter, we briefly summarize apoptosis, emphasizing those studies which assist the reader in understanding the subsequent discussion on PCD and pathogens. We then examine the relationship between virulent bacteria and apoptosis. This section is organized to reflect both common and diverse mechanisms employed by bacteria to induce PCD. A short discussion of parasites and fungi is followed by a detailed description of the interaction of viral pathogens with the apoptotic machinery. Throughout the review, apoptosis is considered within the broader contexts of pathogenesis, virulence, and host defense. Our goals are to update the reader on this rapidly expanding field and identify topics in the current literature which demand further investigation.

Marked increase in anti-HIV activity, as well as inhibitory activity against HIV entry mediated by CXCR4, linked to enhancement of the binding ability of tachyplesin analogs to CXCR4

T22 ([Tyr5,12, Lys7]-polyphemusin II) is a strong anti-HIV compound. Six analogs of T22 and two natural forms were synthesized. Of them, all downsized peptides (14 residues; TW70, T131, T134, and T140) showed a higher selectivity index than did other, 17- or 18-residue peptides. In particular, T134 and T140 showed both lower cytotoxicity and higher antiviral activity than did T22 against HIV infection of MT-4 cells, an HTLV-I-bearing T cell line. To clarify the inhibitory mode of T22 and its analogs, we used a single-round replication assay (luciferase assay), in which different envelope-bearing pseudotypes were used to infect CXCR4- or CCR5-bearing U87 cells via CD4. All of the analogs inhibited T cell line-tropic strain HXB-2 (X4) and dual-tropic strain 89.6 (R5X4) HIV infections mediated by CXCR4, but had no effect on macrophage-tropic strain ADA (R5) or 89.6 HIV infections mediated by CCR5. The inhibition by T134 (IC50 of 2.70 nM) and T140 (IC50 of 0.432 nM) was also stronger than that by T22 (IC50 of 5.05 nM). The binding of anti-CXCR4 monoclonal antibody 12G5 to lymphoma-derived T cell line Sup-T1 was more efficiently blocked by T134 and T140 than by T22. Taken together, T22 and its analogs T134 and T140 exerted their inhibition by specific binding to CXCR4. The marked increase in the anti-HIV activity of T134 and T140 was ascribed to an enhancement in their ability to bind to CXCR4.

Changes in human immunodeficiency virus type 1 envelope glycoproteins responsible for the pathogenicity of a multiply passaged simian-human immunodeficiency virus (SHIV-HXBc2)

In vivo passage of a poorly replicating, nonpathogenic simian-human immunodeficiency virus (SHIV-HXBc2) generated an efficiently replicating virus, KU-1, that caused rapid CD4(+) T-lymphocyte depletion and AIDS-like illness in monkeys (S. V. Joag, Z. Li, L. Foresman, E. B. Stephens, L.-J. Zhao, I. Adany, D. M. Pinson, H. M. McClure, and O. Narayan, J. Virol. 70:3189-3197, 1996). The env gene of the KU-1 virus was used to create a molecularly cloned virus, SHIV-HXBc2P 3.2, that differed from a nonpathogenic SHIV-HXBc2 virus in only 12 envelope glycoprotein residues. SHIV-HXBc2P 3.2 replicated efficiently and caused rapid and persistent CD4(+) T-lymphocyte depletion in inoculated rhesus macaques. Compared with the envelope glycoproteins of the parental SHIV-HXBc2, the SHIV-HXBc2P 3.2 envelope glycoproteins supported more efficient infection of rhesus monkey peripheral blood mononuclear cells. Both the parental SHIV-HXBc2 and the pathogenic SHIV-HXBc2P 3.2 used CXCR4 but none of the other seven transmembrane segment receptors tested as a second receptor. Compared with the parental virus, viruses with the SHIV-HXBc2P 3.2 envelope glycoproteins were more resistant to neutralization by soluble CD4 and antibodies. Thus, changes in the envelope glycoproteins account for the ability of the passaged virus to deplete CD4(+) T lymphocytes rapidly and specify increased replicative capacity and resistance to neutralization.

Does HIV cause depletion of CD4+ T cells in vivo by the induction of apoptosis?

The central pathogenic feature of AIDS is the dramatic loss of CD4+ lymphocytes. Despite more than a decade of intense research, the exact mechanism by which HIV causes this is still not understood. A major model for T cell depletion, proposed originally by Ameison and Capron in a report published in 1991, is that HIV sensitizes CD4+ T cells for activation-induced apoptosis. The apoptotic model of T cell depletion is discussed, and experiments that address the questions of whether apoptosis is restricted to infected cells or 'bystander' T cells, and whether T cell apoptosis requires participation of separate HIV-infected haematopoietic cell populations, are reviewed.

Tyrosine phosphorylation induced by C4 peptide constructs from HIV Gp120

Treatment of HUT78 cells with CD4-binding peptide constructs derived from the C4 domain of HIV-1 gp120 results in autophosphorylation of a src-related kinase, p56lck. This leads to p56lck activation and the subsequent phosphorylation of tyrosine residues in several intracellular proteins. The phosphorylation is specific to the C4 peptides as no new phosphorylation occurs when the cells are treated with control peptides or polymers. The induction of tyrosine phosphorylation by the C4 peptide constructs depends on the capability of the peptide to assume a helical conformation because similar peptide constructs that were not able to form helices did not induce cellular tyrosine phosphorylation.

Characterization of antigens expressed in normal baboon trophoblast and cross-reactive with HIV/SIV antibodies

Electron microscopic studies have revealed the presence of endogenous retroviral (ERV) particles in normal primate placental tissues. These particles have ultrastructural similarities to type C retroviral particles and are mainly associated with the trophoblast. In normal human placental tissues, they have antigenic similarity with exogenous retroviruses, such as the human immunodeficiency virus (HIV), and may have a role to play in the regulation of cellular gene expression, syncytiotrophoblast formation or pregnancy-related immunosuppression. In this study, a panel of antibodies (polyclonal and monoclonal antibodies) against viral proteins (anti-HIV and anti-SIV) and endogenous retroviral (ERV) proteins were assessed by immunohistochemistry and immunoblotting, for their cross-reactivity with ERV particles isolated from normal baboon placental tissues. The antibodies (anti-HERV-K RT, anti-ERV3 env, anti-HIV-1 p17, anti-HIV-2 gp120) reacted positively with the syncytiotrophoblast and each antibody recognized one or two proteins of molecular weights (MW) 38, 58 or 64 kDa present in the baboon placental villous tissues and SIV-infected molt-4 Cl8 cells, but not in uninfected cells. The results of this study confirm the specific expression of retroviral cross-reactive antigens in normal baboon placental tissues and suggest placental cellular proteins may have antigenic similarity with those recognized by anti-HIV/SIV antibodies. The role of these retroviral-related proteins expressed at the maternal-fetal interface remain unclear.

HIV-1 envelope determinants for cell tropism and chemokine receptor use

Isolates of human immunodeficiency virus type-1 (HIV-1) display marked differences in their ability to replicate in macrophages and transformed T-cell lines in vitro, a property that has important implications for disease pathogenesis. The restriction in replication between these two CD4-positive cell types is largely at the level of viral entry and is regulated by the viral envelope (env) gene. The envelope protein (Env) is responsible for fusion of the viral and host membranes, and a particular region of Env called the V3-loop has been implicated in regulating viral tropism. However, other regions of Env, such as the V1- and V2-loops, have been shown to modulate the effects of the V3-loop. The discovery that Env initially binds the CD4 molecule on the target cell surface and then makes subsequent interactions with one of several members of the chemokine receptor family has greatly enhanced the molecular understanding of HIV-1 entry. The differential use of chemokine receptors by different viral isolates and their expression in different cell types largely explains viral tropism. The same regions in Env responsible for virus tropism have also been shown to play an important role in mediating chemokine receptor use. The recent crystallization of HIV-1 Env in complex with CD4 illuminates the architecture of the components involved in mediating fusion between the viral and host membranes. The spatial relationship between variable structures of Env previously implicated in tropism and chemokine receptor use and conserved Env structures potentially involved in chemokine receptor binding are discussed.

Inhibition of HIV type 1 infectivity by coexpression of a wild-type and a defective glycoprotein 120

An amino acid substitution (D --> K) in the C3 region of HIV-1 gp120 has previously been shown to inhibit binding of virions to CD4+ cells. We have introduced the same mutation into the HIV-1 isolate LAV-I(BRU), in which the mutation is denoted D373K. Here we show that the D373K envelope protein is processed and incorporated into virus particles, but that D373K virions have no detectable infectivity (below 0.1% relative to wild type). When D373K and the wild-type envelope gene were cotransfected in 293 cells at a 4:1 ratio, the resultant infectivity of the HIV-1 supernatant was reduced more than 100-fold. When the same ratio of plasmids was tested in COS-1 cells the inhibition of HIV-1 was an order of magnitude less than observed in 293 cells. COS-1 and 293 cells differed in that only 293 cells displayed saturation of virus production with respect to the envelope protein. Our data fit a simple model: when virion formation is saturated with envelope protein, expression and incorporation of a defective envelope protein imply a corresponding dilution of wild-type protein on the surface of virions. The cooperative function of wild-type envelope proteins is subsequently compromised, and a trans-dominant inhibition of virus infectivity is observed.

CD4(+) T-lymphocyte depletion in human lymphoid tissue ex vivo is not induced by noninfectious human immunodeficiency virus type 1 virions

We tested infectious human immunodeficiency virus type 1 (HIV-1), noninfectious but conformationally authentic inactivated whole HIV-1 virions, and purified gp120 for the ability to induce depletion of CD4(+) T cells in human lymphoid tissues ex vivo. Infectious CXCR4-tropic HIV-1, but not matched inactivated virions or gp120, mediated CD4(+) T-cell depletion, consistent with mechanisms requiring productive infection.