A molecular clone of simian-human immunodeficiency virus (DeltavpuSHIV(KU-1bMC33)) with a truncated, non-membrane-bound vpu results in rapid CD4(+) T cell loss and neuro-AIDS in pig-tailed macaques

Novel Compound Inhibitors of HIV-1NL4-3 Vpu

HIV-1 Vpu targets the host cell proteins CD4 and BST-2/Tetherin for degradation, ultimately resulting in enhanced virus spread and host immune evasion. The discovery and characterization of small molecules that antagonize Vpu would further elucidate the contribution of Vpu to pathogenesis and lay the foundation for the study of a new class of novel HIV-1 therapeutics. To identify novel compounds that block Vpu activity, we have developed a cell-based ‘gain of function’ assay that produces a positive signal in response to Vpu inhibition. To develop this assay, we took advantage of the viral glycoprotein, GaLV Env. In the presence of Vpu, GaLV Env is not incorporated into viral particles, resulting in non-infectious virions. Vpu inhibition restores infectious particle production. Using this assay, a high throughput screen of >650,000 compounds was performed to identify inhibitors that block the biological activity of Vpu. From this screen, we identified several positive hits but focused on two compounds from one structural family, SRI-41897 and SRI-42371. We developed independent counter-screens for off target interactions of the compounds and found no off target interactions. Additionally, these compounds block Vpu-mediated modulation of CD4, BST-2/Tetherin and antibody dependent cell-mediated toxicity (ADCC). Unfortunately, both SRI-41897 and SRI-42371 were shown to be specific to the N-terminal region of NL4-3 Vpu and did not function against other, more clinically relevant, strains of Vpu; however, this assay may be slightly modified to include more significant Vpu strains in the future.

Simian-Human immunodeficiency viruses expressing chimeric subtype B/C Vpu proteins demonstrate the importance of the amino terminal and transmembrane domains in the rate of CD4(+) T cell loss in macaques

Previously, we reported that simian-human immunodeficiency viruses expressing either the lab adapted subtype B (SHIV(KU-1bMC33)) or subtype C (SHIV(SCVpu)) Vpu proteins of human immunodeficiency virus type 1 (HIV-1) had different rates of CD4(+) T cell loss following inoculation into macaques. In this study, we have generated SHIVs that express either the subtype B or subtype C N-terminal (NTD) and transmembrane (TMD) domains and the opposing cytoplasmic domain (SHIV(VpuBC), SHIV(VpuCB)). In culture systems, SHIV(VpuBC) replicated faster than SHIV(VpuCB) while both proteins exhibited similar ability to down-modulate CD4 surface expression. Following inoculation into macaques, SHIV(VpuBC) resulted in rapid CD4(+) T cell loss similar to the parental SHIV(KU-1bMC33), while the rate of CD4(+) T cell loss in those inoculated with SHIV(VpuCB) was intermediate of SHIV(SCVpu) and SHIV(KU-1bMC33). These results emphasize the importance of the Vpu NTD/TMD region in the rate of CD4(+) T cell loss in the pathogenic X4 SHIV/macaque model.

Vif substitution enables persistent infection of pig-tailed macaques by human immunodeficiency virus type 1

Among Old World monkeys, pig-tailed macaques (Pt) are uniquely susceptible to human immunodeficiency virus type 1 (HIV-1), although the infection does not persist. We demonstrate that the susceptibility of Pt T cells to HIV-1 infection is due to the absence of postentry inhibition by a TRIM5 isoform. Notably, substitution of the viral infectivity factor protein, Vif, with that from pathogenic SIVmne enabled replication of HIV-1 in Pt T cells in vitro. When inoculated into juvenile pig-tailed macaques, the Pt-tropic HIV-1 persistently replicated for more than 1.5 to 2 years, producing low but measurable plasma viral loads and persistent proviral DNA in peripheral blood mononuclear cells. It also elicited strong antibody responses. However, there was no decline in CD4(+) T cells or evidence of disease. Surprisingly, the Pt-tropic HIV-1 was rapidly controlled when inoculated into newborn Pt macaques, although it transiently rebounded after 6 months. We identified two notable differences between the Pt-tropic HIV-1 and SIVmne. First, SIV Vif does not associate with Pt-tropic HIV-1 viral particles. Second, while Pt-tropic HIV-1 degrades both Pt APOBEC3G and APOBEC3F, it prevents their inclusion in virions to a lesser extent than pathogenic SIVmne. Thus, while SIV Vif is necessary for persistent infection by Pt-tropic HIV-1, improved expression and inhibition of APOBEC3 proteins may be required for robust viral replication in vivo. Additional adaptation of the virus may also be necessary to enhance viral replication. Nevertheless, our data suggest the potential for the pig-tailed macaque to be developed as an animal model of HIV-1 infection and disease.

Membrane raft association of the Vpu protein of human immunodeficiency virus type 1 correlates with enhanced virus release

The Vpu protein of human immunodeficiency virus type 1 (HIV-1) is known to enhance virion release from certain cell types. To accomplish this function, Vpu interacts with the restriction factor known as bone marrow stromal cell antigen 2 (BST-2)/tetherin. In this study, we analyzed whether the Vpu protein is associated with microdomains known as lipid or membrane rafts. Our results indicate that Vpu partially partitions into detergent-resistant membrane (DRM) fractions when expressed alone or in the context of simian-human immunodeficiency virus (SHIV) infection. The ability to be partitioned into rafts was observed with both subtype B and C Vpu proteins. The use of cholesterol lowering lovastatin/M-β-cyclodextrin and co-patching experiments confirmed that Vpu can be detected in cholesterol rich regions of membranes. Finally, we present data showing that raft association-defective transmembrane mutants of Vpu have impaired enhanced virus release function, but still maintain the ability to down-regulate CD4.

BST-2 mediated restriction of simian-human immunodeficiency virus

Pathogenic simian-human immunodeficiency viruses (SHIV) contain HIV-1 Vpu and SIV Nef, both shown to counteract BST-2 (HM1.24; CD317; tetherin) inhibition of virus release in a species-specific manner. We show that human and pig-tailed BST-2 (ptBST-2) restrict SHIV. We found that sequential "humanization" of the transmembrane domain (TMD) of the pig-tailed BST-2 (ptBST-2) protein resulted in a fluctuation in sensitivity to HIV-1 Vpu. Our results also show that the length of the TMD in human and ptBST-2 proteins is important for BST-2 restriction and susceptibility to Vpu. Taken together, our results emphasize the importance of tertiary structure in BST-2 antagonism and suggests that the HIV-1 Vpu transmembrane domain may have additional functions in vivo unrelated to BST-2 antagonism.

Identification of amino acids within the second alpha helical domain of the human immunodeficiency virus type 1 Vpu that are critical for preventing CD4 cell surface expression

Human immunodeficiency virus type 1 (HIV-1) encodes for a Vpu protein, which interacts with CD4 resulting in its degradation. In this study, we examined the role of the 10 amino acids within the predicted second alpha-helical domain of the subtype B Vpu cytoplasmic tail in CD4 down-modulation using a VpuEGFP reporter system. Our findings indicate that the invariant leucine at position 63 and, to a lesser extent, the valine at position 68 were required for CD4 down-modulation. Mutation of analogous L63 in Vpu proteins subtypes A2, B(YU-2), C, D, and H also abolished CD4 down-modulation from the cell surface. Co-immunoprecipitation analysis revealed that L63A and V68A mutants were capable of binding CD4 and still retained the ability to interact with h-beta-TrCP1. Taken together, these results indicate that amino acid substitutions in the second alpha-helical domain that retain the predicted structure and binding to h-beta-TrCP1 can influence Vpu-mediated CD4 degradation.

[Accessory proteins of HIV and innate anti-retroviral factors]

The HIV genome encodes several accessory proteins (Vif, Vpr, Vpx, Vpu, and Nef) unique to primate lentiviruses, in addition to the structural (Gag, Pol, and Env) and regulatory (Tat and Rev) proteins. Early studies showed that deletion of accessory proteins has a small or no effect on virus replication in cell cultures. However, recent studies have clearly demonstrated that these proteins are essential for efficient viral replication, dissemination, pathogenicity, and disease progression. Here, we summarize the current knowledge of HIV accessory proteins and their cellular targets, and discuss the functional roles of these biologically unique and important viral proteins for virus replication in vitro and in vivo.

Mutations in the highly conserved SLQYLA motif of Vif in a simian-human immunodeficiency virus result in a less pathogenic virus and are associated with G-to-A mutations in the viral genome

The simian-human immunodeficiency virus (SHIV)/macaque model for human immunodeficiency virus type 1 has become a useful tool to assess the role of accessory genes in lentiviral pathogenesis. In this study, we introduced two amino acid changes in the highly conserved SLQYLA domain (to AAQYLA) of the SIV Vif protein. The resulting virus, SHIV(VifAAQYLA), was used to infect three macaques, which were followed for over six months. Plasma viral loads and circulating CD4(+) T cell levels were assessed during the course of infection. The three macaques inoculated with SHIV(VifAAQYLA) did not develop significant CD4(+) T cell loss over the course of their infection, had plasma viral RNA loads that were over 100-fold lower than macaques inoculated with parental SHIV(KU-1bMC33), and developed no histological lesions in lymphoid tissues. DNA and RT-PCR analysis revealed that only a select number of tissues were infected with this virus. Sequence analysis indicates that the site-directed changes were stable during the first three weeks after inoculation but thereafter the S147A amino acid substitution changed to a threonine in two of three macaques. The L148A substitution remained stable in the vif amplified from the PBMC of all three macaques. Sequence analysis of vif, vpu, env and nef genes revealed G-to-A mutations in the genes amplified from macaques inoculated with SHIV(VifAAQYLA), which were higher than in a macaque inoculated with parental SHIV(KU-1bMC33). We found that the majority (>85%) of the G-to-A mutations were in the context of 5'-TC (minus strand) and not 5'-CC, suggestive that one or more of the rhesus APOBEC3 proteins may be responsible for the observed mutational patterns. The data also suggest that rhesus APOBEC3G probably accounted for a minority of the mutations since its GG-to-AG mutational pattern was infrequently detected. Finally, macaques inoculated with SHIV(VifAAQYLA) developed immunoprecipitating antibody responses against the virus. The results from this study provide the first in vivo evidence of the importance of the SLQYLA domain in viral pathogenesis and show that targeted mutations in vif can lead to a persistent infection with G-to-A changes accumulating in the viral genome.

Requirements of the membrane proximal tyrosine and dileucine-based sorting signals for efficient transport of the subtype C Vpu protein to the plasma membrane and in virus release

Previously, we showed that the Vpu protein from HIV-1 subtype C is more efficiently transported to the cell surface than the well studied subtype B Vpu (Pacyniak et al., 2005) and that a SHIV expressing the subtype C Vpu exhibited a decreased rate of CD4+ T cell loss following inoculation in macaques (Hill et al., 2008). In this study, we examined the role of overlapping tyrosine-based (YXXPhi) and dileucine-based ([D/E]XXXL[L/I]) motifs in the membrane proximal region of the subtype C Vpu (EYRKLL) in Vpu intracellular transport, CD4 surface expression and virus release from the cell surface. We constructed three site-directed mutants of the subtype C vpu and fused these genes to the gene for enhanced green fluorescent protein (EGFP). The first mutation made altered the tyrosine (EARKLL; VpuSCEGFPY35A), the second altered the dileucine motif (EYRKLG; VpuSCEGFPL39G), and the third contained both amino acid substitutions (EARKLG; VpuSCEGFPYL35,39AG) in this region of the Vpu protein. The VpuSCEGFPY35A protein was transported to the cell surface similar to the unmodified VpuSCEGFP1 while VpuSCEGFPL39G was expressed at the cell surface at significantly reduced levels. The VpuSCEGFPYL35,39AG was found to have an intermediate level of cell surface expression. All three mutant Vpu proteins were analyzed for the ability to prevent cell surface expression of CD4. We found that both single mutants did not significantly effect CD4 surface expression while the double mutant (VpuSCEGFPYL35,39AG) was significantly less efficient at preventing cell surface CD4 expression. Chimeric simian human immunodeficiency viruses were constructed with these mutations in vpu (SHIVSCVpuY35A, SHIVSCVpuL39G and SHIVSCVpuYL35,39AG). Our results indicate that SHIVSCVpuL39G replicated much more efficiently and was much more cytopathic than SHIVSCVpu. In contrast, SHIVSCVpuY35A and SHIVSCVpuYL35,39AG replicated less efficiently when compared to the parental SHIVSCVpu. Taken together, these results show for the first time that the membrane proximal tyrosine-based sorting motif in the cytoplasmic domain of Vpu is essential for efficient virus release. These results also indicate that the dileucine-based sorting motif affects the intracellular trafficking of subtype C Vpu proteins, virus replication, and release.

Human immunodeficiency virus type 1 Vpu protein interacts with CD74 and modulates major histocompatibility complex class II presentation

The human immunodeficiency virus type 1 (HIV-1) Vpu accessory protein is a transmembrane protein that down regulates CD4 expression and promotes the release of new virions. We screened a human leukocyte-specific yeast two-hybrid expression library to discover novel Vpu-interacting cellular proteins. The major histocompatibility complex class II (MHC II) invariant chain, also called Ii or CD74, was found to be one such protein. We show direct binding of Vpu and CD74 by using a yeast two-hybrid assay and coimmunoprecipitation from HIV-1-infected cells. The cytoplasmic region of Vpu was found to interact with the 30-amino-acid cytoplasmic tail of CD74. Human monocytic U937 cells infected with wild-type or Vpu-defective HIV-1 and transfected cells showed that Vpu down modulated the surface expression of mature MHC II molecules. The reduction in cell surface mature MHC II molecules correlated with decreased antigen presentation to T cells in culture. Thus, the Vpu protein also contributes to viral persistence by attenuating immune responses during HIV infection. This report further exemplifies the rich diversity and redundancy shown by HIV in immune evasion.

Modulation of the severe CD4+ T-cell loss caused by a pathogenic simian-human immunodeficiency virus by replacement of the subtype B vpu with the vpu from a subtype C HIV-1 clinical isolate

Previously, we showed that the Vpu protein from subtype C human immunodeficiency virus type 1 (HIV-1) was efficiently targeted to the cell surface, suggesting that this protein has biological properties that differ from the well-studied subtype B Vpu protein. In this study, we have further analyzed the biological properties of the subtype C Vpu protein. Flow cytometric analysis revealed that the subtype B Vpu (strain HXB2) was more efficient at down-regulating CD4 surface expression than the Vpu proteins from four subtype C clinical isolates. We constructed a simian-human immunodeficiency virus virus, designated as SHIV(SCVpu), in which the subtype B vpu gene from the pathogenic SHIV(KU-1bMC33) was substituted with the vpu from a clinical isolate of subtype C HIV-1 (strain C.96.BW16B01). Cell culture studies revealed that SHIV(SCVpu) replicated with slightly reduced kinetics when compared with the parental SHIV(KU-1bMC33) and that the viral Env and Gag precursor proteins were synthesized and processed similarly compared to the parental SHIV(KU-1bMC33). To determine if substitution of the subtype C Vpu protein affected the pathogenesis of the virus, three pig-tailed macaques were inoculated with SHIV(SCVpu) and circulating CD4+ T-cell levels and viral loads were monitored for up to 44 weeks. Our results show that SHIV(SCVpu) caused a more gradual decline in the rate of CD4+ T cells in pig-tailed macaques compared to those inoculated with parental subtype B SHIV(KU-1bMC33). These results show for the first time that different Vpu proteins of HIV-1 can influence the rate at which CD4+ T-cell loss occurs in the SHIV/pig-tailed macaque model.

High frequency of defective vpu compared with tat and rev genes in brain from patients with HIV type 1-associated dementia

Human immunodeficiency virus type 1 (HIV) infection of the central nervous system frequently causes HIV-associated dementia (HAD) and other neurological disorders. The role of HIV regulatory and accessory proteins in the pathogenesis of these disorders is unclear. Here we analyzed sequences of tat, rev, and vpu genes in 55 subgenomic clones previously shown to encode functional env genes from brain and lymphoid tissues of four AIDS patients with HAD. Phylogenetic analysis showed distinct compartmentalization of tat, rev, and vpu genes in brain versus lymphoid tissues. Nine of 19 vpu sequences from brain of two patients had premature stop codons at positions between amino acids 2 and 30, compared with 0 of 8 from lymphoid tissues. Tat sequences from brain (n = 8 of 8) but not lymphoid (n = 0 of 6) tissue from one patient had a 35 amino acid truncation at the C-terminus. Rev sequences from the brain of one patient (n = 6 of 8) had a 5 amino acid truncation. These results demonstrate a high frequency of defective vpu compared with tat and rev genes in brain from HAD patients, and identify sequence variants of these regulatory/accessory genes that may influence the pathogenesis of HIV-associated neurological disease.

Early dysregulation of cripto-1 and immunomodulatory genes in the cerebral cortex in a macaque model of neuroAIDS

Human immunodeficiency virus type 1 (HIV-1) and related primate lentiviruses are known to enter the central nervous system (CNS) during the primary phase of infection. Neuroinvasion by simian immunodeficiency virus and simian human immunodeficiency virus (SHIV) is characterized by transient meningitis and astrocytosis. In this report, we used targeted cytokine cDNA arrays to analyze cortical brain tissue from four pig-tailed macaques inoculated for 2 weeks with pathogenic SHIV(50OLNV) and a normal age-matched pig-tailed macaque. Our results revealed that eight genes were significantly upregulated in all four macaques. These included: leukocyte interferon inducible peptide, corticotrophin releasing factor receptor 1, interleukin 6, CDW40 antigen, cysteine-rich fibroblast growth factor, neurotrophin 3, ciliary neurotrophin factor receptor and cripto-1. The upregulation of three of these genes was confirmed by reverse transcriptase PCR (RT-PCR). Since cripto-1 had not been previously identified within specific cell types within the primate central nervous system, we performed immunohistochemical studies, which revealed the presence of cripto-1 in neurons. RT-PCR studies demonstrated that cripto-1 mRNA was widely expressed in the CNS. These results indicate that immunomodulatory genes are upregulated during the primary phase of infection of the central nervous system. Cripto-1, which acts as a survival factor in tumor cells and may be neuroprotective, is expressed in neurons within the CNS and is upregulated during viral invasion.

APOBEC3G expression is restricted to neurons in the brains of pigtailed macaques

The Vif protein of human immunodeficiency virus-1 (HIV-1) has been shown to interact with members of the APOBEC family of cytidine deaminases, particularly APOBEC3G/F. In this study, we isolated RNA from 12 regions of the brain from two pigtailed macaques that were exsanguinated and perfused with saline. Our results indicate that APOBEC3G was detected in all regions of the brain analyzed. Immunoblot analysis using lysates prepared from these same regions of the brain and a monoclonal antibody to APOBEC3G confirmed the RT-PCR findings. To determine which cell types express APOBEC3G, immunohistochemical studies were performed using this monoclonal antibody on whole brain sections. Our results clearly show that the pyramidal neurons within the gray matter of cerebral and cerebellar cortices express APOBEC3G. However, APOBEC3G expression in the pyramidal neurons appeared to be nuclear or associated with nuclei. In contrast to our findings in the cerebral cortex, immunohistochemical analysis of the spleen and kidney tissues revealed that APOBEC3G expression in the cells of these tissues was predominantly cytoplasmic. We further investigated the expression of APOBEC3G in astrocytes. Immunohistochemical staining of serial sections was performed using antibodies to glial fibrillary acidic protein (GFAP) and APOBEC3G. As expected, the cortical and cerebellar white matter showed extensive immunostaining of astrocytes with the antibody against GFAP but a lack of reactivity to the antibody to APOBEC3G. Additionally, Immunoblot analysis of lysates prepared from primary human fetal astrocytes revealed a lack of APOBEC3G expression. Taken together, these results indicate that APOBEC3G expression is restricted to neurons in the brain and that astrocytes and microglia probably do not express this protein or express it at levels undetectable by immunohistochemistry. These finding have implications for the brain as a potential reservoir for Vif-defective viruses.

A single amino acid substitution within the transmembrane domain of the human immunodeficiency virus type 1 Vpu protein renders simian–human immunodeficiency virus (SHIVKU-1bMC33) susceptible to rimantadine

Previous studies from our laboratory have shown that the transmembrane domain (TM) of the Vpu protein of human immunodeficiency virus type 1 (HIV-1) contributes to the pathogenesis of SHIV(KU-1bMC33) in macaques and that the TM domain of Vpu could be replaced with the M2 protein viroporin from influenza A virus. Recently, we showed that the replacement of the TM domain of Vpu with that of the M2 protein of influenza A virus resulted in a virus (SHIV(M2)) that was sensitive to rimantadine [Hout, D.R., Gomez, M.L., Pacyniak, E., Gomez, L.M., Inbody, S.H., Mulcahy, E.R., Culley, N., Pinson, D.M., Powers, M.F., Wong, S.W., Stephens, E.B., 2006. Substitution of the transmembrane domain of Vpu in simian human immunodeficiency virus (SHIV(KU-1bMC33)) with that of M2 of influenza A results in a virus that is sensitive to inhibitors of the M2 ion channel and is pathogenic for pig-tailed macaques. Virology 344, 541-558]. Based on previous studies of the M2 protein which have shown that the His-X-X-X-Trp motif within the M2 is essential to the function of the M2 proton channel, we have constructed a novel SHIV in which the alanine at position 19 of the TM domain was replaced with a histidine residue resulting in the motif His-Ile-Leu-Val-Trp. The SHIV(VpuA19H) replicated with similar kinetics as the parental SHIV(KU-1bMC33) and pulse-chase analysis revealed that the processing of viral proteins was similar to SHIV(KU-1bMC33). This SHIV(VpuA19H) virus was found to be more sensitive to the M2 ion channel blocker rimantadine than SHIV(M2). Electron microscopic examination of SHIV(VpuA19H)-infected cells treated with rimantadine revealed an accumulation of viral particles at the cell surface and within intracellular vesicles, which was similar to that previously observed to SHIV(M2)-infected cells treated with rimantadine. These data indicate that the Vpu protein of HIV-1 can be converted into a rimantadine-sensitive ion channel with the alteration of one amino acid and provide additional evidence that drugs targeting the Vpu TM/ion channel can be effective anti-HIV-1 drugs.

Substitution of the transmembrane domain of Vpu in simian-human immunodeficiency virus (SHIVKU1bMC33) with that of M2 of influenza A results in a virus that is sensitive to inhibitors of the M2 ion channel and is pathogenic for pig-tailed macaques

The Vpu protein of human immunodeficiency virus type 1 has been shown to shunt the CD4 receptor molecule to the proteasome for degradation and to enhance virus release from infected cells. The exact mechanism by which the Vpu protein enhances virus release is currently unknown but some investigators have shown that this function is associated with the transmembrane domain and potential ion channel properties. In this study, we determined if the transmembrane domain of Vpu could be functionally substituted with that of the prototypical viroporin, the M2 protein of influenza A virus. We constructed chimeric vpu gene in which the transmembrane domain of Vpu was replaced with that of the M2 protein of influenza. This chimeric vpu gene was substituted for the vpu gene in the genome of a pathogenic simian human immunodeficiency virus, SHIVKU-1bMC33. The resulting virus, SHIVM2, synthesized a Vpu protein that had a slightly different Mr compared to the parental SHIVKU-1bMC33, reflecting the different sizes of the two Vpu proteins. The SHIVM2 was shown to replicate with slightly reduced kinetics when compared to the parental SHIVKU-1bMC33 but electron microscopy revealed that the site of maturation was similar to the parental virus SHIVKU1bMC33. We show that the replication and spread of SHIVM2 could be blocked with the antiviral drug rimantadine, which is known to target the M2 ion channel. Our results indicate a dose dependent inhibition of SHIVM2 with 100 microM rimantadine resulting in a >95% decrease in p27 released into the culture medium. Rimantadine did not affect the replication of the parental SHIVKU-1bMC33. Examination of SHIVM2-infected cells treated with 50 microM rimantadine revealed numerous viral particles associated with the cell plasma membrane and within intracytoplasmic vesicles, which is similar to HIV-1 mutants lacking a functional vpu. To determine if SHIVM2 was as pathogenic as the parental SHIVKU-1bMC33 virus, two pig-tailed macaques were inoculated and followed for up to 8 months. Both pig-tailed macaques developed severe CD4+ T cell loss within 1 month of inoculation, high viral loads, and histological lesions consistent with lymphoid depletion similar to the parental SHIVKU-1bMC33. Taken together, these results indicate for the first time that the TM domain of the Vpu protein can be functionally substituted with the TM of M2 of influenza A virus, and shows that compounds that target the TM domain of Vpu protein of HIV-1 could serve as novel anti-HIV-1 drugs.

Scrambling of the amino acids within the transmembrane domain of Vpu results in a simian-human immunodeficiency virus (SHIVTM) that is less pathogenic for pig-tailed macaques

Previous studies have shown that the transmembrane (TM) domain of the subtype B Vpu enhances virion release from cells and some studies have shown that this domain may form an oligomeric structure with properties of an ion channel. To date, no studies have been performed to assess the role of this domain in virus pathogenesis in a macaque model of disease. Using a pathogenic molecular clone of simian human immunodeficiency virus (SHIVKU-1bMC33), we have generated a novel virus in which the transmembrane domain of the Vpu protein was scrambled but maintained hydrophobic in nature (SHIVTM), which presumably would disrupt any ion channel TM properties of this protein. Vectors expressing the Vpu as a fusion protein with the enhanced green fluorescent protein (VpuTMEGFP) indicate that it was transported to the same intracellular compartment as the unmodified Vpu protein but did not down-regulate cell surface expression of CD4. To assess the pathogenicity of SHIVTM, three pig-tailed macaques were inoculated with the SHIVTM and monitored for 6-8 months for CD4+ T cell levels, viral loads and the stability of the sequence of the vpu gene. Our results indicated that unlike the parental SHIVKU-1bMC33, inoculation of macaques with SHIVTM did not cause a severe CD4+ T cell loss over the course of their infections. Sequence analysis of the vpu gene analyzed from sequential PBMC samples derived from macaques revealed that the scrambled TM was stable during the course of infection. At necropsy, examination of tissues revealed low viral loads and none of the pathology commonly observed in lymphoid and non-lymphoid tissues following inoculation with the pathogenic parental SHIVKU-1bMC33 virus. Thus, these results show for the first time that the TM domain of Vpu contributes to the pathogenicity of SHIVKU-1bMC33 in pig-tailed macaques.

Fusion of the upstream vpu sequences to the env of simian human immunodeficiency virus (SHIV(KU-1bMC33)) results in the synthesis of two envelope precursor proteins, increased numbers of virus particles associated with the cell surface and is pathogenic for pig-tailed macaques

Previous studies have shown that the gene coding for the Vpu protein of the human immunodeficiency virus type 1 (HIV-1) is 5' to the env gene, is in a different reading frame, and overlaps the env by 90 nucleotides. In this study, we examined the processing of the Env protein as well as the maturation and infectivity of a virus (SHIV(Vpenv)) in which a single nucleotide was removed at the vpu-env junction, fusing the first 162 bases of vpu to the env ORF. Pulse-chase analysis revealed that SHIV(Vpenv)-infected cells gave rise to two precursor glycoprotein species (gp160 and gp175). Immune precipitation results also revealed that an anti-Vpu serum could immune precipitate the gp175 precursor, suggesting that the amino-terminal Vpu sequence was fused to the Env protein. Growth curves revealed that the SHIV(Vpenv)-inoculated cultures released approximately three times more p27 into the culture medium than parental SHIV(KU-1bMC33). Electron microscopy revealed that while both viruses matured at the cell plasma membrane, significantly higher quantities of virus particles were cell associated on SHIV(Vpenv)-infected cells compared to cultures inoculated with parental SHIV(KU-1bMC33). Furthermore, virus was observed maturing into intracellular vesicles of SHIV(Vpenv)-infected cells. To assess the pathogenicity of SHIV(Vpenv), three pig-tailed macaques were inoculated with the SHIV(Vpenv) and monitored for 6 months for CD4(+) T cell levels, viral loads, and the stability of the deletion at the vpu-env junction. Our results indicated that SHIV(Vpenv) caused a severe CD4(+) T cell loss in all three macaques within weeks of inoculation. Sequence analysis of the vpu gene analyzed from sequential PBMC samples derived from macaques revealed that this mutation was stable during the period of rapid CD4(+) T cell loss. Sequence analysis showed that with increasing time of infection, the one base pair deletion was repaired in all three macaques inoculated with SHIV(Vpenv) with the reversion occurring at 10 weeks in macaque CT1G and at 12 weeks in macaque CP3R and CT1R. These results indicate that fusion of the first 54 amino acids of Vpu to Env results in intracellular maturation of virus, and accumulation of virus within intracellular vesicles as well as on the cell plasma membrane. Our results indicate that while fusion of the vpu gene to env results in a virus that is still pathogenic for pig-tailed macaques, there is a selective pressure to maintain the vpu and env genes in separate reading frames.

The vpu protein of human immunodeficiency virus type 1 plays a protective role against virus-induced apoptosis in primary CD4(+) T lymphocytes

Previous data revealed that primary cultures of peripheral blood mononuclear cells (PBMCs) were killed by apoptosis at higher rates after infection with two CRF01_AE primary isolates of human immunodeficiency virus type 1 (HIV-1) than after infection with five other CRF01_AE primary isolates, five subtype B primary isolates, and two subtype B laboratory strains. Here, we show evidence that mutations at the vpu gene which were exclusively identified only in the two CRF01_AE isolates mentioned above are involved in their abilities to induce massive apoptosis in primary CD4(+) T lymphocytes. The rates of virus production by these two isolates in the culture media of infected PBMCs were lower (the same as those of the other CRF01_AE isolates) than those of the subtype B isolates. To confirm the correlation between the higher apoptosis-inducing abilities and the mutations at the vpu gene, infectious molecular clone pNL4-3-based vpu mutants were constructed and examined for their apoptosis induction levels. The apoptosis induction levels after introduction of the vpu mutations were greatly increased in primary CD4(+) T lymphocytes. In contrast, the apoptosis induction abilities of these vpu mutants were lower in human T-cell line MT-4. Thus, the Vpu protein of HIV-1 could play a protective role against virus-induced apoptosis in primary CD4(+) T lymphocytes.

The primary phase of infection by pathogenic simian-human immunodeficiency virus results in disruption of the blood-brain barrier

Using the simian-human immunodeficiency virus (SHIV), we have investigated whether the blood-brain barrier (BBB) is compromised during the early stages of infection. Five macaques were inoculated with pathogenic SHIV(50OLNV) for 2 weeks at which time macaques were anesthetized, perfused with saline, and sacrificed. The brains were removed and examined for the disruption of the blood-brain barrier by immunohistochemical staining for the plasma protein fibrinogen in the neural parenchyma. Our results indicate a disruption of the BBB in the five of five macaques inoculated with SHIV(50OLNV) for 2 weeks. Zonula occludens 1 (ZO-1), which is a marker for the tight junctions formed by brain vascular endothelial cells, was largely absent in areas that showed fibrinogen deposition in all five macaques. To determine if the BBB integrity correlated with the initial stages of infection, the brains from two macaques were analyzed that had progressed to end-stage disease following inoculation with pathogenic SHIV(50OLNV) but developed no neuropathology and from two macaques that were inoculated with a gene-deleted, nonpathogenic virus (novpuSHIV(KU-1bMC33)) for over 1 year. Our results indicate that unlike the macaques sacrificed during the acute phase of infection, immunohistochemical staining for fibrinogen in the neural parenchyma was negative and ZO-1 staining was readily detected in the endothelial cells of the blood vessels. The results of this study indicate that the transient loss of BBB integrity is a function of the high level of virus replication that occurs during the acute phase of infection and provides important information on the early stages of lentivirus neuroinvasion.

The HIV-1 Vpu protein: a multifunctional enhancer of viral particle release

HIV accessory genes are expressed throughout the viral life cycle and regulate wide-ranging aspects of virus replication including viral infectivity (Vif and Nef), viral gene expression (Vpr) and progeny virion production (Vpu). While in many cases the molecular basis of accessory protein function is not fully understood, a consensus is emerging that these viral products are generally devoid of enzymatic activity and instead act as multifunctional adapters, subverting normal cellular processes to serve the needs of the virus. This review focuses on presenting our current knowledge of the HIV-1-specific Vpu protein and its essential role in regulating viral particle release, viral load and expression of the CD4 receptor.

The presence of the casein kinase II phosphorylation sites of Vpu enhances the CD4(+) T cell loss caused by the simian-human immunodeficiency virus SHIV(KU-lbMC33) in pig-tailed macaques

The simian-human immunodeficiency virus (SHIV)/ macaque model for human immunodeficiency virus type 1 has become a useful tool to assess the role of Vpu in lentivirus pathogenesis. In this report, we have mutated the two phosphorylated serine residues of the HIV-1 Vpu to glycine residues and have reconstructed a SHIV expressing this nonphosphorylated Vpu (SHIV(S52,56G)). Expression studies revealed that this protein was localized to the same intracellular compartment as wild-type Vpu. To determine if this virus was pathogenic, four pig-tailed macaques were inoculated with SHIV(S52,56G) and virus burdens and circulating CD4(+) T cells monitored up to 1 year. Our results indicate that SHIV(S52,56G) caused rapid loss in the circulating CD4(+) T cells within 3 weeks of inoculation in one macaque (CC8X), while the other three macaques developed no or gradual numbers of CD4(+) T cells and a wasting syndrome. Histological examination of tissues revealed that macaque CC8X had lesions in lymphoid tissues (spleen, lymph nodes, and thymus) that were typical for macaques inoculated with pathogenic parental SHIV(KU-1bMC33) and had no lesions within the CNS. To rule out that macaque CC8X had selected for a virus in which there was reversion of the glycine residues at positions 52 and 56 to serine residues and/or compensating mutations occurred in other genes associated with CD4 down-regulation, sequence analysis was performed on amplified vpu sequences isolated from PBMC and from several lymphoid tissues at necropsy. Sequence analysis revealed a reversion of the glycine residues back to serine residues in this macaque. The other macaques maintained low virus burdens, with one macaque (P003) developing a wasting syndrome between months 9 and 11. Histological examination of tissues from this macaque revealed a thymus with severe atrophy that was similar to that of a previously reported macaque inoculated with a SHIV lacking vpu (Virology 293, 2002, 252). Sequence analysis revealed no reversion of the glycine residues in the vpu sequences isolated from this macaque. These results contrast with those from four macaques inoculated with the parental pathogenic SHIV(KU-1bMC33), all of which developed severe CD4(+) T cell loss within 1 month after inoculation. Taken together, these results indicate that casein kinase II phosphorylation sites of Vpu contributes to the pathogenicity of the SHIV(KU-1bMC33) and suggest that the SHIV(KU-1bMC33)/pig-tailed macaque model will be useful in analyzing amino acids/domains of Vpu that contribute to the pathogenesis of HIV-1.

Infection of human astrocytoma cells with simian-human immunodeficiency virus results in up-regulation of gene expression and altered growth properties

Recent reports of human immunodeficiency virus-1 (HIV-1) infection of astrocytes suggest a role for astrocytes in HIV encephalitis. In this study, we infected a human astrocytoma cell line with a pathogenic simian HIV (SHIV(50OLNV)) and examined growth patterns and immunomodulatory genes. Approximately 1% of uninfected cells in culture expressed glial fibrillary acid protein (GFAP) whereas 40% of the cells expressed GFAP at 7 days post-inoculation along altered growth patterns. Using targeted cytokine cDNA arrays, we found that SHIV(50OLNV) infection resulted in the up-regulation of several genes including metalloproteinase bone morphogenic protein 1 and chemokines monocyte chemoattractant protein 1 and stromal cell derived factor 1alpha. These data suggest that astrocytic activation, altered morphology and up-regulation of immunomodulatory genes in response to SHIV infection may participate in initiation of inflammation and trafficking of infected monocytes/macrophages into the central nervous system, potentiating the development of HIV encephalitis.

Pathogenic and nef-interrupted simian-human immunodeficiency viruses traffic to the macaque CNS and cause astrocytosis early after inoculation

Several studies have shown that deletion of the nef gene of simian immunodeficiency virus (SIV) and simian-human immunodeficiency virus (SHIV) results in attenuated viruses. However, studies have not critically examined trafficking of attenuated viruses to the central nervous system (CNS) at early stages after inoculation. In this study, we investigated the colocalization of pathogenic and vpu-negative, nef-interrupted SHIVs at early stages following inoculation. The first virus, designated SHIV(50OLNV), was isolated from the lymph node of a pig-tailed macaque which developed severe CD4+ T cell loss and neurological disease. The second virus was a molecularly cloned virus in which the vpu gene was deleted and the gene for the enhanced green fluorescent protein from the jellyfish Aequoria victora had been inserted in-frame within the nef gene of the pathogenic SHIV(KU-1bMC33) (designated SHIV(KU-1bEGFP)). Three pig-tailed macaques were inoculated intravenously with equivalent amounts of two viruses, two macaques were inoculated with SHIV(KU-1bEGFP), and two macaques were inoculated with SHIV(50OLNV). The peripheral blood mononuclear cells (PBMCs) were isolated from bleeds obtained 3, 7, 10, and 14 days postinoculation and monitored for syncytia-inducing virus and for fluorescent cells. Virus was detected in the PBMCs as early as 3 days postinoculation and was present throughout the course of this short-term study. At 14 days postinoculation, the macaques were sacrificed and examined for virus in lymphoid tissues and different regions of the CNS following necropsy. Our results revealed the presence of both viruses in lymphoid and CNS tissues, although SHIV(50OLNV) was present to a much greater extent. Histological examination revealed that one macaque displayed signs of meningitis and all three macaques developed massive cortical astrocyte activation as demonstrated by immunostaining for glial fibrillary acidic protein, but only limited microglial activation. In the two macaques inoculated with SHIV(50OLNV), astrocyte activation similar to that in the macaques inoculated with both viruses was observed while no astrocyte activation was observed in macaques inoculated with SHIV(KU-1bEGFP). Thus, this study demonstrates that SHIVs with an intact nef(SHIV(50OLNV)) as well as those lacking a vpu gene and with a nonfunctional nef gene (SHIV(KU-1bEGFP)) are capable of invading the CNS and that pathogenic SHIVs are capable of causing reactive astrocytosis early after inoculation.

Innate differences between simian-human immunodeficiency virus (SHIV)(KU-2)-infected rhesus and pig-tailed macaques in development of neurological disease

Neurological disease associated with HIV infection results from either primary replication of the virus or a combination of virus infection and replication of opportunistic pathogens in the CNS. Recent studies indicate that the primary infection is mediated mainly by viruses that utilize CCR5 as the coreceptor; it is not known whether the syndrome can be mediated by viruses that use the CXCR4 coreceptor. The macaque model of the disease using simian immunodeficiency virus (SIV) has confirmed that CCR5-using viruses such as SIV(mac)251 can cause primary disease in the CNS. In this report we have examined the role of simian-human immunodeficiency virus (SHIV)(KU-2), a CXCR4 virus which replicates productively in rhesus macrophages, in causing CNS disease. A survey of archival brain tissues from SHIV(KU-2)-infected rhesus and pig-tailed macaques that succumbed to AIDS showed productive viral replication in the CNS of 10 of 14 rhesus animals. Eight of these 10 had additional infections with opportunistic pathogens. In contrast, 21 of 22 pig-tailed macaques had no evidence of productive viral infection in the brain. In an earlier study we had shown that inoculation of SHIV-infected rhesus macaques with eggs of Schistosoma mansoni, a potent inducer of IL-4, resulted in enhanced replication of the virus in tissue macrophages. In the present study, we compared the replication of the virus in macrophages from normal rhesus and pig-tailed macaques and determined further whether exogenous IL-4 could cause enhancement of virus replication in these cells. These studies showed that the virus replicated productively in rhesus macrophages, and this was enhanced significantly after recombinant macaque IL-4 was added to the medium. IL-4 also caused enhancement of virus production in macrophages isolated from virus-infected animals. In contrast, the virus replicated only minimally in pig-tailed macaque macrophages and supplemental IL-4 had negligible effects. The data thus suggested that failure of pig-tailed macaques to develop encephalitis was due to the innate resistance of macrophages from this species of macaque to support replication of SHIV(KU-2). The ability of the virus to replicate in the brains of rhesus macaques was dependent on coinfection in the brain with opportunistic pathogens which presumably induced both macrophages and IL-4 in the CNS microenvironment. A supportive role for IL-4 in the CNS disease was suggested by the presence of IL-4 RNA in the encephalitic brains of rhesus macaques and reduced levels of this cytokine in the brains from pig-tailed macaques.

Presence of Intact vpu and nef genes in nonpathogenic SHIV is essential for acquisition of pathogenicity of this virus by serial passage in macaques

Use of the macaque model of human immunodeficiency virus (HIV) pathogenesis has shown that the accessory genes nef and vpu are important in the pathogenicity of simian immunodeficiency virus (SIV) and simian-human immunodeficiency virus (SHIV). We examined the ability of two nonpathogenic SHIVs, SHIV(PPC) and DeltavpuDeltanefSHIV(PPC), to gain pathogenicity by rapid serial passage in macaques. In this study, each virus was passaged by blood intravenously four times at 4-week intervals in macaques. Animals were monitored for 40 weeks for levels of CD4 T cells and quantitative measures of virus infection. DeltavpuDeltanefSHIV(PPC) maintained a limited phase of productive replication in the four animals, with no loss of CD4(+) T cells, whereas SHIV(PPC) became more pathogenic in later passages, judging by plasma viral load and viral mRNA in lymph nodes, infectious peripheral blood mononuclear cells and CD4(+) T cell loss. The nef, LTR, and env of the SHIV(PPC) viruses underwent numerous mutations, compared to DeltavpuDeltanefSHIV(PPC). This study confirms the seminal role that nef, LTR, and vpu could play in regulation of pathogenesis of HIV infection.

Deletion of the vpu sequences prior to the env in a simian-human immunodeficiency virus results in enhanced Env precursor synthesis but is less pathogenic for pig-tailed macaques

The Vpu protein of human immunodeficiency virus type 1 (HIV-1) has been reported to enhance virion release from infected cells and to down-regulate the expression of CD4 on infected cells. Previous studies have shown that Vpu and the envelope glycoprotein precursor (gp160) are translated from different reading frames of the same bicistronic messenger RNA (mRNA). In order to assess the effect of the Vpu sequences 5' to the Env open reading frame on Env biosynthesis and pathogenesis, we have constructed a deletion mutant of a molecularly cloned chimeric simian--human immunodeficiency virus (SHIV(KU-1bMC33)) in which the entire coding region of vpu upstream of env had been deleted (novpuSHIV(KU-1bMC33)). While both SHIV(KU-1bMC33) and novpuSHIV(KU-1bMC33) synthesized comparable amounts of env mRNA in infected cells, the novpuSHIV(KU-1bMC33)-infected cells synthesized more Env precursor when standardized against the p57 Gag precursor protein. While more Env was synthesized than Gag in novpuSHIV(KU-1bMC33)-infected cells, pulse--chase analysis revealed that p27 Gag protein was released from infected cells with delayed kinetics, a reflection of the lack of a Vpu protein. Inoculation of novpuSHIV(KU-1bMC33) into two pig-tailed macaques resulted in no loss of circulating CD4(+) T cells. However, replicating virus could be detected in the lymphoid tissues (lymph nodes, spleen, thymus) 1 year after inoculation and the thymus of one of the macaques exhibited severe atrophy. The results of these studies indicate that the Vpu coding sequences upstream of Env may attenuate the level of Env precursor biosynthesis but significantly contribute to the pathogenesis of this SHIV in pig-tailed macaques.

A simian human immunodeficiency virus with a nonfunctional Vpu (deltavpuSHIV(KU-1bMC33)) isolated from a macaque with neuroAIDS has selected for mutations in env and nef that contributed to its pathogenic phenotype

Previous studies have shown that passage of nonpathogenic SHIV-4 through a series of macaques results in the selection of variants of the virus that are capable of causing rapid subtotal loss of CD4(+) T cells and AIDS within 6-8 months following inoculation into pig-tailed macaques. Using a pathogenic variant of SHIV-4 known as SHIV(KU-1bMC33), we reported that a mutant of this virus with the majority of the vpu deleted was still capable of causing profound CD4(+) T cell loss and neuroAIDS in pig-tailed macaques (McCormick-Davis et al., 2000, Virology 272, 112-116). In this study, we have analyzed the tissue-specific changes in the env and nef in one macaque that developed neuroAIDS (macaque 50 O) and in three macaques that developed only a moderate or no significant loss of CD4(+) T cells and no neurological disease (macaques 50 Y, 20220, 20228) following inoculation with DeltavpuSHIV(KU-1bMC33). Sequence analysis of the gp120 region of env isolated from lymphoid tissues (lymph node and spleen) of macaques 50 Y, 20220, and 20228 revealed no consensus amino acid substitutions. In contrast, analysis of the gp120 sequences isolated from lymphoid and CNS tissues (parietal cortex, basal ganglia, and pons) of macaque 50 O revealed numerous amino acid substitutions. The significance of the amino acid substitutions in gp120 was supported by neutralization assays which showed that the virus isolated from the lymph node of macaque 50 O was neutralization resistant compared to the parental SHIV(KU-1bMC33). Analysis of changes in the nef gene from macaque 50 O revealed in-frame deletions in Nef that ranged from 4 to 13 amino acids in length, whereas the nef genes isolated from the other three macaques revealed no deletions or consensus amino acid substitutions. Inoculation of the virus isolated from the lymph node of the macaque which developed neuroAIDS, SHIV(50OLNV), into four pig-tailed macaques resulted in a severe loss of the circulating CD4(+) T cells within 2 weeks postinoculation, which was maintained for up to 20 weeks postinoculation, confirming that this virus had indeed become more pathogenic in pig-tailed macaques. Taken together, these observations suggest that DeltavpuSHIV(KU-1bMC33) has a low pathogenic phenotype in macaques but that individual pig-tailed macaques can select for additional mutations within the Env and Nef which can compensate for the lack of an intact Vpu and ultimately increase its pathogenicity.