The CXC-chemokine, H174: expression in the central nervous system

H174 is a new member of the CXC-chemokine family. A cDNA probe containing the entire H174 coding region recognized a predominant inducible transcript of approximately 1.5 kb expressed in interferon (IFN) activated astrocytoma and monocytic cell lines. H174 message can be induced following IFN-alpha, IFN-beta, or IFN-gamma stimulation. H174 message was also detected in IFN treated cultures of primary human astrocytes, but was absent in unstimulated astrocytes. H174, like IP10 and Mig, lacks the ELR sequence associated with the neutrophil specificity characteristic of most CXC-chemokines. Preliminary experiments suggest H174, IP10 and Mig are independently regulated. Recombinant H174 is a weak chemoattractant for monocyte-like cells. H174 can also stimulate calcium flux responses. The data support the classification of H174 as a member of a subfamily of interferon-gamma inducible non-ELR CXC-chemokines. Brain tissues were obtained at autopsy from one patient with AIDS dementia, one patient with multiple sclerosis, and two normal control patients. H174 and Mig were detected by RT-PCR in brain tissue cDNA derived from the patients with pathological conditions associated with activated astrocytes but not in cDNA from control specimens.

Mitogen-activated protein kinase phosphorylates and regulates the HIV-1 Vif protein

The human immunodeficiency virus type 1 (HIV-1) Vif protein plays a critical role in virus replication and infectivity. Here we show that Vif is phosphorylated and regulated by p44/42 mitogen-activated protein kinase (MAPK). Vif phosphorylation by MAPK was demonstrated in vitro as well as in vivo and was shown to occur on serine and threonine residues. Two-dimensional tryptic phosphopeptide mapping indicated that Vif is phosphorylated by MAPK on the same sites in vitro and in vivo. Radioactive peptide sequencing identified two phosphorylation sites, Thr96 and Ser165. These phosphorylation sites do not correspond to the known optimum consensus sequences for phosphorylation by MAPK (PX(S/T)P) nor to the minimum consensus sequence ((S/T)P), indicating that MAPK can phosphorylate proteins at sites other than those containing the PX(S/T)P or (S/T)P motifs. Synthetic Vif peptides corresponding to the local sequences of the phosphorylation sites were not phosphorylated by MAPK, suggesting that recognition of these sites by MAPK is likely to require structural determinants outside the phosphorylation site. Mutations of the Thr96 site, which is conserved among Vif sequences from HIV-1, HIV-2, and SIV, resulted in significant loss of Vif activity and inhibition of HIV-1 replication. These results suggest that MAPK plays a direct role in regulating HIV-1 replication and infectivity by phosphorylating Vif and identify a novel mechanism for activation of HIV-1 replication by mitogens and other extracellular stimuli.

Neuronal apoptosis induced by HIV-1 Tat protein and TNF-alpha: potentiation of neurotoxicity mediated by oxidative stress and implications for HIV-1 dementia

Apoptosis of neurons and non-neuronal cells has been demonstrated in the brain of AIDS patients with dementia. Previous studies suggest that the apoptotic stimuli are likely to be soluble factors. Several candidates for the soluble factors that lead to neuronal apoptosis in HIV-1 infection have been proposed, including the HIV-1 Tat protein and TNF-alpha. The mechanisms that lead to neuronal apoptosis in the brain of AIDS patients in vivo, may involve the combined effects of more than one pro-apoptotic factor. In this study, we examine whether exposure of primary human neurons to the combination of HIV-1 Tat and TNF-alpha can potentiate the induction of neuronal apoptosis compared with exposure to either factor alone. TNF-alpha was shown to potentiate the induction of neuronal apoptosis by HIV-1 Tat via a mechanism that involves increased oxidative stress. Antioxidants inhibited, but did not completely abolish the induction of neuronal apoptosis by Tat, suggesting that other mechanisms are also likely to be involved. These findings suggest that soluble HIV-1 Tat and TNF-alpha may play a role in neuronal apoptosis induced by HIV-1 infection of the CNS, particularly when present in combination. Our findings further suggest that one mechanism whereby combinations of pro-apoptotic factors may potentiate the induction of neuronal apoptosis in the brain of AIDS patients is by increasing oxidative stress. Understanding the role of oxidative stress and other mechanisms that lead to apoptosis in HIV-1 infection of the CNS may advance the development of new therapeutic strategies to prevent neuronal cell death and improve neurologic function in AIDS patients.

Chemokine receptors in HIV-1 infection of the central nervous system

Several members of the chemokine receptor are used as coreceptors for HIV-1 infection in the central nervous system (CNS). CCR5 and CCR3 are coreceptors together with CD4 for HIV-1 infection of microglia, the major target for HIV-1 infection in the CNS. Microglia express CXCR4, but their infection by HIV-1 viruses that use only CXCR4 as a coreceptor is relatively inefficient. CXCR4 is also expressed in subpopulations of neurons that are resistant to HIV-1 infection. Additional orphan chemokine receptors that can mediate HIV-1 or SIV entry are expressed in the brain or neurally-derived cell lines, but their role in CNS infection has not been defined. The pattern of chemokine receptor expression in the brain is likely to determine the tropism of HIV-1 for particular CNS target cells and to impact inflammatory and degenerative mechanisms associated with CNS infection.

Localization of HIV-1 co-receptors CCR5 and CXCR4 in the brain of children with AIDS

The chemokine receptors CCR5 and CXCR4 are co-receptors together with CD4 for human immunodeficiency virus (HIV)-1 entry into target cells. Macrophage-tropic HIV-1 viruses use CCR5 as a co-receptor, whereas T-cell-line tropic viruses use CXCR4. HIV-1 infects the brain and causes a progressive encephalopathy in 20 to 30% of infected children and adults. Most of the HIV-1-infected cells in the brain are macrophages and microglia. We examined expression of CCR5 and CXCR4 in brain tissue from 20 pediatric acquired immune deficiency syndrome (AIDS) patients in relation to neuropathological consequences of HIV-1 infection. The overall frequency of CCR5-positive perivascular mononuclear cells and macrophages was increased in the brains of children with severe HIV-1 encephalitis (HIVE) compared with children with mild HIVE or non-AIDS controls, whereas the frequency of CXCR4-positive perivascular cells did not correlate with disease severity. CCR5- and CXCR4-positive macrophages and microglia were detected in inflammatory lesions in the brain of children with severe HIVE. In addition, CXCR4 was detected in a subpopulation of neurons in autopsy brain tissue and primary human brain cultures. Similar findings were demonstrated in the brain of adult AIDS patients and controls. These findings suggest that CCR5-positive mononuclear cells, macrophages, and microglia contribute to disease progression in the central nervous system of children and adults with AIDS by serving as targets for virus replication.

Human immunodeficiency virus type 1 Vif protein binds to the Pr55Gag precursor

The Vif protein of human immunodeficiency virus type 1 is required for productive replication in peripheral blood lymphocytes. Previous reports suggest that vif-deleted viruses are limited in replication because of a defect in the late steps of the virus life cycle. One of the remaining questions is to determine whether the functional role of Vif involves a specific interaction with virus core proteins. In this study, we demonstrate a direct interaction between Vif and the Pr55Gag precursor in vitro as well as in infected cells. No interaction is observed between Vif and the mature capsid protein. The Pr55Gag-Vif interaction is detected (i) in the glutathione S-transferase system, with in vitro-translated proteins demonstrating a critical role of the NC p7 domain of the Gag precursor; (ii) with proteins expressed in infected cells; and (iii) by coimmunoprecipitation experiments. Deletion of the C-terminal 22 amino acids of Vif abolishes its interaction with the Pr55Gag precursor. Furthermore, point mutations in the C-terminal domain of Vif which have been previously shown to abolish virus infectivity and binding to cell membranes dramatically decrease the Gag-Vif interaction. These results suggest that the interaction between Vif and the pr55Gag precursor is a critical determinant of Vif function.

Ultrastructure of HIV-1 genomic RNA

The HIV-1 RNA genome is a dimer which consists of two identical strands of RNA linked near their 5' ends by a dimer linkage structure (DLS). We have structurally characterized full-length HIV-1 genomic RNA isolated from HIV-1 virions by electron microscopy. As in other retroviruses, the HIV-1 RNA genome contains a central dimer linkage structure and additional loop structures within each monomer subunit. In contrast to the DLS of other retroviruses, the DLS region of HIV-1 contains a loop of 323 +/- 44 nucleotides. The free 5' ends of the two RNA strands were not visualized, suggesting that the 5' end regions are involved in interstrand complementary base pairing. Computer modeling identified a single stable structure that was consistent with the electron microscopy data. In this model, the two RNA strands are linked at their 5' ends by two contact points derived from "kissing-loop" interactions between r-u5 and SL1 stem-loops and their counterparts on the second strand. These interactions may contribute to the formation of stable HIV-1 RNA dimers in vivo.

Astrocyte apoptosis induced by HIV-1 transactivation of the c-kit protooncogene

HIV-1 infection of the central nervous system (CNS) frequently causes dementia and other neurological disorders. The mechanisms of CNS injury in HIV-1 infection are poorly understood. Apoptosis of neurons and astrocytes is induced by HIV-1 infection in vitro and in brain tissue from AIDS patients, but the apoptotic stimuli have not been identified. We report herein that HIV-1 infection of primary brain cultures induces the receptor tyrosine kinase protooncogene c-kit and that high levels of c-Kit expression are associated with astrocyte apoptosis. Overexpression of c-Kit in an astrocyte-derived cell line in the absence of HIV-1 induces rapid apoptotic death. The apoptotic mechanism requires the c-Kit tyrosine kinase domain. The mechanism of c-kit induction by HIV-1 involves transactivation of the c-kit promoter by the HIV-1 Nef protein. These studies demonstrate that c-Kit can induce astrocyte apoptosis and suggest that this mechanism may play a role in CNS injury caused by HIV-1 infection. We propose that c-Kit can serve dual functions as a growth factor receptor or apoptosis inducer.

CCR3 and CCR5 are co-receptors for HIV-1 infection of microglia

Several members of the chemokine receptor family are used together with CD4 for HIV-1 entry into target cells. T cell line-tropic (T-tropic) HIV-1 viruses use the chemokine receptor CXCR4 as a co-receptor, whereas macrophage-tropic (M-tropic) primary viruses use CCR5 (refs 2-6). Individuals with defective CCR5 alleles exhibit resistance to HIV-1 infection, suggesting that CCR5 has an important role in vivo in HIV-1 replication. A subset of primary viruses can use CCR3 as well as CCR5 as a co-receptor, but the in vivo contribution of CCR3 to HIV-1 infection and pathogenesis is unknown. HIV-1 infects the central nervous system (CNS) and causes the dementia associated with AIDS. Here we report that the major target cells for HIV-1 infection in the CNS, the microglia, express both CCR3 and CCR5. The CCR3 ligand, eotaxin, and an anti-CCR3 antibody inhibited HIV-1 infection of microglia, as did MIP-1beta, which is a CCR5 ligand. Our results suggest that both CCR3 and CCR5 promote efficient infection of the CNS by HIV-1.

Role of Vif in human immunodeficiency virus type 1 reverse transcription

The Vif protein of human immunodeficiency virus type 1 (HIV-1) is important for virion infectivity. Previous studies have shown that vif mutant HIV-1 virions are defective in their ability to synthesize proviral DNA in vivo. Here, we examine the role of Vif in viral DNA synthesis in the endogenous reverse transcriptase (RT) reaction, an in vitro assay in which virions synthesize viral DNA by using endogenous viral RNA as a template. vif mutant virions showed a significant reduction in endogenous RT activity despite similar levels of exogenous RT activity. Analysis of the viral DNA products on agarose gels demonstrated that this reflects reduced synthesis of short minus- and plus-strand DNA products in addition to those of full genomic length. Quantitative PCR analysis of endogenous reverse transcription provided further evidence for reduced formation of both initial and completed reverse transcripts. Vif had no effect on genomic RNA dimerization or the stability of the RNA dimer linkage. These results suggest that Vif is important for an early event after virus entry but preceding or during the early stages of viral DNA synthesis. This may be due to an intrinsic effect on reverse transcription or a preceding postentry event(s), such as virion uncoating or disassembly of the virion core. Drugs targeted to Vif function may provide a new therapeutic approach to inhibiting HIV-1 reverse transcription.

Apoptosis induced by HIV-1 infection of the central nervous system

Apoptosis plays a role in AIDS pathogenesis in the immune system, but its role in HIV-1-induced neurological disease is unknown. In this study, we examine apoptosis induced by HIV-1 infection of the central nervous system (CNS) in an in vitro model and in brain tissue from AIDS patients. HIV-1 infection of primary brain cultures induced apoptosis in neurons and astrocytes in vitro as determined by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) and propidium iodide staining and by electron microscopy. Apoptosis was not significantly induced until 1-2 wk after the time of peak virus production, suggesting induction by soluble factors rather than by direct viral infection. Apoptosis of neurons and astrocytes was also detected in brain tissue from 10/11 AIDS patients, including 5/5 patients with HIV-1 dementia and 4/5 nondemented patients. In addition, endothelial cell apoptosis was frequently detected in the brain of AIDS patients and was confirmed by electron microscopy. Most of the apoptotic cells were not localized adjacent to HIV-1-infected cells, providing further evidence for induction by soluble factors. In six non-AIDS control patients with normal brain, apoptotic cells were absent or limited to rare astrocytes. However, TUNEL-positive neurons and astrocytes were frequently detected in seven patients with Alzheimer's disease or abundant senile plaques. These studies suggest that apoptosis is a mechanism of CNS injury in AIDS which is likely to be induced by soluble factors. The apoptosis of endothelial cells in the CNS raises the possibility that some of these factors may be blood-derived.

Phosphorylation of Vif and its role in HIV-1 replication

Vif is a 23-kDa protein encoded by human immunodeficiency virus, type 1 (HIV-1) which is important for virion infectivity. Here, we describe the phosphorylation of HIV-1 Vif and its role in HIV-1 replication. In vivo studies demonstrated that Vif is highly phosphorylated on serine and threonine residues. To identify phosphorylation sites and characterize the Vif kinase(s), Vif was expressed in Escherichia coli and purified for use as a substrate in in vitro kinase assays. The purified Vif protein was phosphorylated in vitro on serine and threonine residues by a kinase(s) present in both cytosol and membrane fractions. Phosphorylation of Vif was stimulated by phorbol 12-myristate 13-acetate and inhibited by staurosporine and hypericin, a drug with potent anti-HIV activity. The Vif kinase(s) was resistant to inhibitors of protein kinase C, cAMP-dependent kinase, and cGMP-dependent kinase, suggesting that it is distinct from these enzymes. To identify the phosphorylation sites, 32P-labeled Vif was digested by V8 protease and the peptides were resolved by reverse-phase high performance liquid chromatography. Radioactive peptide sequencing identified three phosphorylation sites within the C terminus, Ser144, Thr155, and Thr188. Two-dimensional tryptic phosphopeptide mapping indicated that these sites are also phosphorylated in vivo. Both Ser144 and Thr188 are contained in the recognition motifs (R/KXXS*/T* and R/KXXXS*/T*) used by serine/threonine protein kinases such as cGMP-dependent kinase and PKC. Ser144 is present in the motif SLQXLA, which is the most highly conserved sequence among all lentivirus Vif proteins. Mutation of Ser144 to alanine resulted in loss of Vif activity and >90% inhibition of HIV-1 replication. These studies suggest that phosphorylation of Vif by a serine/threonine protein kinase(s) plays an important role in regulating HIV-1 replication and infectivity.

Biological activity of human immunodeficiency virus type 1 Vif requires membrane targeting by C-terminal basic domains

Human immunodeficiency virus type 1 (HIV-1) encodes a Vif protein which is important for virus replication and infectivity. Vif is a cytoplasmic protein which exists in both membrane-associated and soluble forms. The membrane-associated form is an extrinsic membrane protein which is tightly associated with the cytoplasmic side of membranes. We have analyzed the mechanism of membrane targeting of Vif and its role in HIV-1 replication. Mutagenesis studies demonstrate that C-terminal basic domains are required for membrane association. Vif mutations which disrupt membrane association also inhibit HIV-1 replication, indicating that membrane localization of Vif is likely to be required for its biological activity in vivo. Membrane binding of Vif is almost completely abolished by trypsin treatment of membranes. These results demonstrate that membrane localization of Vif requires C-terminal basic domains and interaction with a membrane-associated protein(s). This interaction may serve to direct Vif to a specific cellular site, since immunofluorescence staining and plasma membrane fractionation studies show that Vif is localized predominantly to an internal cytoplasmic compartment rather than to the plasma membrane. The mechanism of membrane targeting of Vif is different in some respects from that of other extrinsic membrane proteins, such as Ras, Src, and MARCKS, which utilize a basic domain together with a lipid modification for membrane targeting. Membrane targeting of Vif is likely to play an important role in HIV-1 replication and thus may be a therapeutic target.

The intracytoplasmic domain of gp41 mediates polarized budding of human immunodeficiency virus type 1 in MDCK cells

Human immunodeficiency virus type 1 (HIV-1) has been shown to exhibit a specific basolateral release in polarized epithelial cells. Previous investigators have used vaccinia virus recombinants expressing HIV proteins to demonstrate that virus release is nonpolarized in the absence of viral envelope glycoproteins. In this study, we developed a transient expression system which allows the use of Madin-Darby canine kidney polarized epithelial cells directly grown on semipermeable membranes. This procedure allowed us to investigate polarized HIV viral budding following introduction of proviral DNA constructs. Expression of env gene products in trans demonstrated the ability to polarize env-negative viruses in a dose-dependent manner. The targeting signal for polarized virus release was shown to be present in the envelope gp41 transmembrane protein and absent from the gp120 portion of env. At least part of this signal is within the gp41 intracytoplasmic domain. Mutants of the p17gag matrix protein were shown to be nonpolarized only when unable to interact with the envelope glycoproteins. Together, these data are consistent with a model of polarized virus budding in which capsid proteins, lacking a targeting signal, are targeted for specific basolateral release via an interaction of p17 with the envelope glycoprotein containing the polarization signal in its intracytoplasmic domain.

Role of vif during packing of the core of HIV-1

The viral infectivity factor gene vif of human immunodeficiency virus type 1 (HIV-1) has been shown to enhance the cell-free infectivity of HIV-1 virus particles. Previous studies have demonstrated that vif increases viral infectivity at the time of virus production, most likely by affecting viral protein processing, virus assembly, or virus maturation. The effect of vif on the assembly and maturation of HIV-1 propagated in CEM, Jurkat, and SupT1 cells was examined by electron microscopy and goniometer analysis. CEM and Jurkat cells are nonpermissive and partially permissive for the replication of vif--defective viruses, respectively, while SupT1 cells are completely permissive. In CEM and Jurkat cultures, the morphology of immature vif+ and vif- virions was similar but immature virus particles were observed at a slightly higher frequency in cultures infected with the vif- virus. At later stages of virus maturation, however, nonhomogeneous packing of the core was detected in the majority of vif- virus particles produced in CEM and Jurkat cells. In the absence of vif, the cone-shaped virus core contained dense material in its broad end but, in contrast to vif+ virions, the material inside its narrow end appeared transparent. The narrow part of the vif- virus core was surrounded by a shell and was attached to the viral envelope by a core-envelope link structure. Vif- virus particles with a lateral body of core material adjacent to the viral envelope were also observed more frequently in CEM and Jurkat cultures. In contrast, in SupT1 cultures the morphology of mature vif+ and vif- virus particles was similar. These results suggest that vif is associated with an effect during the final stages of packing of the viral nucleoprotein core. This effect may be important for the infectivity of HIV-1 virus particles.

Inhibition of energy metabolism alters the processing of amyloid precursor protein and induces a potentially amyloidogenic derivative

The cellular mechanisms which lead to the generation and pathological deposition of beta amyloid in Alzheimer's disease are unknown. In this report we describe the proteolytic processing of the amyloid precursor protein (APP) to an 11.5-kDa COOH-terminal derivative which contains the full-length beta amyloid sequence. This processing step normally occurs at low levels in parallel with APP maturation in the secretory pathway. Inhibition of oxidative energy metabolism by sodium azide or the mitochondrial uncoupler carbonyl cyanide m-chlorophenylhydrazone increased the proteolysis of APP to the 11.5-kDa derivative by about 80-fold with accumulation of this APP derivative in the Golgi complex. Agents which inhibit protein transport in the secretory pathway, including monensin and brefeldin A, also increased the production of the 11.5-kDa derivative. Inhibition of APP maturation demonstrated that the 11.5-kDa derivative could be produced by proteolysis of immature APP. These results demonstrate that APP processing to potentially amyloidogenic COOH-terminal derivatives occurs in either the endoplasmic reticulum or Golgi complex and can be modulated by the state of cellular energy metabolism. Deficits in oxidative energy metabolism have recently been found in the cerebral cortex of patients with Alzheimer's disease. These findings raise the possibility that energy-related metabolic stress may lead to altered metabolism of APP and contribute to amyloidosis in Alzheimer's disease.

Inhibition of beta-amyloid production by activation of protein kinase C

The cellular factors regulating the generation of beta-amyloid from the amyloid precursor protein (APP) are unknown. Activation of protein kinase C (PKC) by phorbol ester treatment inhibited the generation of the 4-kDa beta-amyloid peptide in transfected COS cells, a human glioma cell line, and human cortical astrocytes. An analogue of diacylglycerol, the endogenous cellular activator of PKC, also inhibited the generation of beta-amyloid. Activation of PKC increased the level of secreted APP in transfected COS cells but did not significantly affect the level of secreted APP in primary human astrocytes or in the glioma cell line. Cell-associated APP and the secreted APP derivative, but not beta-amyloid, were phosphorylated on serine residues. Activation of PKC did not increase the level of APP phosphorylation, suggesting that PKC modulates the proteolytic cleavage of APP indirectly by phosphorylation of other substrates. These results indicate that PKC activation inhibits beta-amyloid production by altering APP processing and suggest that beta-amyloid production can be regulated by the phospholipase C-diacylglycerol signal transduction pathway.

Effects of CD4 synthetic peptides on HIV type I envelope glycoprotein function

Benzylated derivatives of a peptide (CD4(81-92)) representing the CDR3-like region of CD4 were previously found to inhibit gp120 binding, HIV-1 infectivity, and syncytium formation. These results have been interpreted to indicate a role for the corresponding CD4 region in these processes. The peptide (TbYICbEbVEDQKAcEE) is the prototype of a series of similar CD4(81-92) derivatives. We report that this peptide noncompetitively inhibits binding to CD4 of both gp120 and a mAb (MAX.16H5), both of which recognize the CDR2-like region of CD4. The binding of an antibody (Leu 3a) that is directed against a different area of the D1 domain of CD4 was also inhibited. The peptide derivative inhibited both HIV-1- and HTLV-1-mediated syncytium formation in the same concentration range. Nonbenzylated cyclic and linear peptides representing the CDR3-like region of CD4 (CD4(84-101)) had only minor effects on gp120 binding which were not sequence specific. The results of this study suggest that the effects of benzylated CD4(81-92) derivatives on HIV-1 binding or fusion should not be used to reach conclusions about the function of the corresponding CD4 region.

Effects of CD4 synthetic peptides on HIV type I envelope glycoprotein function

Benzylated derivatives of a peptide (CD4(81-92)) representing the CDR3-like region of CD4 were previously found to inhibit gp120 binding, HIV-1 infectivity, and syncytium formation. These results have been interpreted to indicate a role for the corresponding CD4 region in these processes. The peptide (TbYICbEbVEDQKAcEE) is the prototype of a series of similar CD4(81-92) derivatives. We report that this peptide noncompetitively inhibits binding to CD4 of both gp120 and a mAb (MAX.16H5), both of which recognize the CDR2-like region of CD4. The binding of an antibody (Leu 3a) that is directed against a different area of the D1 domain of CD4 was also inhibited. The peptide derivative inhibited both HIV-1- and HTLV-1-mediated syncytium formation in the same concentration range. Nonbenzylated cyclic and linear peptides representing the CDR3-like region of CD4 (CD4(84-101)) had only minor effects on gp120 binding which were not sequence specific. The results of this study suggest that the effects of benzylated CD4(81-92) derivatives on HIV-1 binding or fusion should not be used to reach conclusions about the function of the corresponding CD4 region.

Allelic variation in the effects of the nef gene on replication of human immunodeficiency virus type 1

The effects of the viral gene nef on human immunodeficiency virus type 1 (HIV-1) replication in culture were investigated using nef alleles of the HIV-1 IIIB and ELI strains. The results demonstrate significant allelic variation in the effect of nef on virus replication in both an established human CD4+ T-cell line and primary human lymphocytes. In the context of the HXB2 virus, the ELI nef allele but not the IIIB nef allele permits initiation of efficient low-multiplicity infection in primary peripheral blood mononuclear cells, including unfractionated peripheral blood lymphocytes, T cells, and monocyte/macrophages. Within the same genetic context, the IIIB nef allele slightly retards replication of the virus in a T-cell line, whereas the ELI nef allele accelerates replication of the virus. Sequences in the IIIB and ELI genomes outside of nef also moderate the effects of nef on HIV-1 replication. nef did not appear to determine the host-cell preference of the virus. These studies may help to reconcile apparently conflicting reports on the role of nef in HIV-1 replication and suggest that HIV-1 nef may play an important role in viral pathogenesis.

Characterization of a discontinuous human immunodeficiency virus type 1 gp120 epitope recognized by a broadly reactive neutralizing human monoclonal antibody

While one hypervariable, linear neutralizing determinant on the human immunodeficiency virus type 1 (HIV-1) gp120 envelope glycoprotein has been well characterized, little is known about the conserved, discontinuous gp120 epitopes recognized by neutralizing antibodies in infected individuals. Here, the epitope recognized by a broadly reactive neutralizing monoclonal antibody (F105) derived from an HIV-1-infected patient was characterized by examining the effects of changes in conserved gp120 amino acids on antibody reactivity. The F105 epitope was disrupted by changes in gp120 amino acids 256 and 257, 368 to 370, 421, and 470 to 484, which is consistent with the discontinuous nature of the epitope. Three of these regions are proximal to those previously shown to be important for CD4 binding, which is consistent with the ability of the F105 antibody to block gp120-CD4 interaction. Since F105 recognition was more sensitive to amino acid changes in each of the four identified gp120 regions than was envelope glycoprotein function, replication-competent mutant viruses that escaped neutralization by the F105 antibody were identified. These studies identify a conserved, functional HIV-1 gp120 epitope that is immunogenic in man and may serve as a target for therapeutic or prophylactic intervention.

Effects of changes in gp120-CD4 binding affinity on human immunodeficiency virus type 1 envelope glycoprotein function and soluble CD4 sensitivity

Mutant gp120 glycoproteins exhibiting a range of affinities for CD4 were tested for ability to form syncytia and to complement an env-defective provirus for replication. Surprisingly, gp120 mutants that efficiently induced syncytia and/or complemented virus replication were identified that exhibited marked (up to 50-fold) reductions in CD4-binding ability. Temperature-dependent changes in gp120, which result in a seven- to ninefold increase in affinity for CD4, were shown not to be necessary for subsequent membrane fusion or virus entry events. Mutant glycoproteins demonstrating even relatively small decreases in CD4-binding ability exhibited reduced sensitivity to soluble CD4. The considerable range of CD4-binding affinities tolerated by replication-competent HIV-1 variants has important implications for antiviral strategies directed at the gp120-CD4 interaction.

Changes in the transmembrane region of the human immunodeficiency virus type 1 gp41 envelope glycoprotein affect membrane fusion

The charged amino acids near or within the membrane-spanning region of the human immunodeficiency virus type 1 gp41 envelope glycoprotein were altered. Two mutants were defective for syncytium formation and virus replication even though levels of envelope glycoproteins on the cell or virion surface and CD4 binding were comparable to those of the wild-type proteins. Thus, in addition to anchoring the envelope glycoproteins, sequences proximal to the membrane-spanning gp41 region are important for the membrane fusion process.

Rapid complementation assays measuring replicative potential of human immunodeficiency virus type 1 envelope glycoprotein mutants

Rapid assays which measure the ability of mutant human immunodeficiency virus type 1 envelope glycoproteins to mediate cell-free and/or cell-to-cell transmission of virus are described. By using these assays, envelope glycoprotein mutants with varying degrees of syncytium-forming ability were tested for ability to complement viral replication in trans. As expected, mutants that dramatically affect association of the gp120-gp41 envelope subunits, CD4 binding, or membrane fusion were unable to form syncytia or to support cell-free or cell-to-cell transmission. Surprisingly, some membrane fusion-defective mutants significantly attenuated in syncytium-forming ability were able to complement viral replication. Conversely, mutations in the carboxyl terminus of gp41 transmembrane glycoprotein, although not affecting syncytium-forming ability, significantly attenuated both forms of virus transmission. These results indicate that syncytium formation is not sufficient for cell-to-cell transmission of human immunodeficiency virus type 1. Furthermore, virus transmission appears to be less sensitive to inhibition of membrane fusion than is syncytium formation.