MDM2 is a novel E3 ligase for HIV-1 Vif

The human immunodeficiency virus type 1 (HIV-1) Vif plays a crucial role in the viral life cycle by antagonizing a host restriction factor APOBEC3G (A3G). Vif interacts with A3G and induces its polyubiquitination and subsequent degradation via the formation of active ubiquitin ligase (E3) complex with Cullin5-ElonginB/C. Although Vif itself is also ubiquitinated and degraded rapidly in infected cells, precise roles and mechanisms of Vif ubiquitination are largely unknown. Here we report that MDM2, known as an E3 ligase for p53, is a novel E3 ligase for Vif and induces polyubiquitination and degradation of Vif. We also show the mechanisms by which MDM2 only targets Vif, but not A3G that binds to Vif. MDM2 reduces cellular Vif levels and reversely increases A3G levels, because the interaction between MDM2 and Vif precludes A3G from binding to Vif. Furthermore, we demonstrate that MDM2 negatively regulates HIV-1 replication in non-permissive target cells through Vif degradation. These data suggest that MDM2 is a regulator of HIV-1 replication and might be a novel therapeutic target for anti-HIV-1 drug.

Prevalence and molecular phylogenetic characterization of Trypanosoma (Megatrypanum) minasense in the peripheral blood of small neotropical primates after a quarantine period

Neotropical primates of the Cebidae and Callitrichidae, in their natural habitats, are frequently infected with a variety of trypanosomes including Trypanosoma cruzi, which causes a serious zoonosis, Chagas' disease. The state of trypanosome infection after a 30-day quarantine period was assessed in 85 squirrel monkeys (Saimiri sciureus) and 15 red-handed tamarins (Saguinus midas), that were wild-caught and exported to Japan as companion animals or laboratory animals, for biomedical research, respectively. In addition to many microfilariae of Mansonella (Tetrapetalonema) mariae at a prevalence of 25.9%, and Dipetalonema caudispina at a prevalence of 3.5%, a few trypomastigotes of Trypanosoma (Megatrypanum) minasense were detected in Giemsa-stained thin films of blood from 20 squirrel monkeys at a prevalence of 23.5%. Although few T. minasense trypomastigotes were found in Giemsa-stained blood films from tamarins, a buffy-coat examination detected trypanosomes in 12 red-handed tamarins (80.0%), and PCR amplification of a highly variable region of the small subunit ribosomal RNA genes (SSU rDNA) for Trypanosoma spp. detected the infection in 14 of the 15 tamarins (93.3%). Nucleotide sequences of the amplicons were identical for trypanosomes from tamarins and squirrel monkeys, indicating a high prevalence but low parasitemia of T. minasense in imported Neotropical nonhuman primates. Based on the SSU rDNA and 5.8S rDNA, the molecular phylogenetic characterization of T. minasense indicated that T. minasense is closely related to trypanosomes with Trypanosoma theileri-like morphology and is distinct from Trypanosoma (Tejeraia) rangeli, as well as from T. cruzi. Using some blood samples from these monkeys, amplification and subsequent sequencing of the glycosomal glyceraldehyde-3-phosphate dehydrogenase (gGAPDH) gene fragments detected 4 trypanosome genotypes, including 2 types of T. cruzi clade, 1 type of T. rangeli clade, and 1 T. rangeli-related type, but failed to indicate its phylogenetic position based on the gGAPDH gene. Furthermore, species ordinarily classified in the Megatrypanum by morphological criteria do not form a clade in any molecular phylogenetic trees based on rDNA or gGAPDH genes.

Molecular cloning and characterization of the common marmoset huntingtin gene

We report here for the first time the isolation and identification of the common marmoset (Callithrix jacchus) huntingtin (Htt) gene, whose ortholog in humans is known to be related to Huntington's disease (HD). A 9396 nucleotide complementary DNA (cDNA) carrying the putative full-length open reading frame of the marmoset Htt gene was identified, and highly conserved nucleotide and amino acid sequences among primates were observed. Based on this data and using tools evaluated for the detection of the marmoset Htt gene, we have demonstrated gene silencing against the expression of endogenous Htt gene in immortalized common marmoset mononuclear cells by means of RNA interference (RNAi). Taken together, the data presented here may assist us in realizing a non-human primate HD model with the common marmoset.

Interaction between Hck and HIV-1 Nef negatively regulates cell surface expression of M-CSF receptor

Nef is a multifunctional pathogenetic protein of HIV-1, the interaction of which with Hck, a Src tyrosine kinase highly expressed in macrophages, has been shown to be responsible for the development of AIDS. However, how the Nef-Hck interaction leads to the functional aberration of macrophages is poorly understood. We recently showed that Nef markedly inhibited the activity of macrophage colony-stimulating factor (M-CSF), a primary cytokine for macrophages. Here, we show that the inhibitory effect of Nef is due to the Hck-dependent down-regulation of the cell surface expression of M-CSF receptor Fms. In the presence of Hck, Nef induced the accumulation of an immature under-N-glycosylated Fms at the Golgi, thereby down-regulating Fms. The activation of Hck by the direct interaction with Nef was indispensable for the down-regulation. Unexpectedly, the accumulation of the active Hck at the Golgi where Nef prelocalized was likely to be another critical determinant of the function of Nef, because the expression of the constitutive-active forms of Hck alone did not fully down-regulate Fms. These results suggest that Nef perturbs the intracellular maturation and the trafficking of nascent Fms, through a unique mechanism that required both the activation of Hck and the aberrant spatial regulation of the active Hck.

Efficient regulation of viral replication by siRNA in a non-human primate surrogate model for hepatitis C

RNA interference (RNAi) represents a new technology which could offer potential applications for the therapeutics of human diseases. RNAi-mediated therapy has recently been shown to be effective toward infectious diseases in in vitro and rodent models, however, it remains unclear whether RNAi therapy with systemic application could be effective in primates. In this study, we examined if RNAi therapy could be effective toward infectious diseases by using a non-human primate surrogate model for hepatitis C. Administration into marmosets of cationic liposome-encapsulated siRNA (CL-siRNA) for GB virus B (GBV-B), which is most closely related to hepatitis C virus, repressed GBV-B replication in a dose-dependent manner. Especially, 5 mg/kg of the CL-siRNA completely inhibited the viral replication. Since the serum interferons (IFNs) were induced by CL-siRNA in vivo, inhibition of viral regulation by anti-GBV-B CL-siRNA may include an antiviral effect of IFN. However, contribution of induced IFN may be partial, since the control CL-siRNA which induced a stronger IFN response than GBV-B CL-siRNA could only delay the viral replication. Our results suggest the feasibility of systemic administration of CL-siRNA as an antiviral strategy.

GBV-B as a pleiotropic virus: distribution of GBV-B in extrahepatic tissues in vivo

GB virus B (GBV-B) infection of New World monkeys is considered to be a useful surrogate model for hepatitis C virus (HCV) infection. GBV-B replicates in the liver and induces acute resolving hepatitis but little is known whether the other organs could be permissive for the virus. We investigated the viral tropism of GBV-B in tamarins in the acute stage of viral infection and found that the viral genomic RNA could be detected in a variety of tissues. Notably, a GBV-B-infected tamarin with marked acute viremia scarcely showed a sign of hepatitis, due to preferential infection in lymphoid tissues such as lymph nodes and spleen. These results indicate that GBV-B as well as HCV is a pleiotropic virus in vivo.

Detection of SRV/D shedding in body fluids of cynomolgus macaques and comparison of partial gp70 sequences in SRV/D-T isolates

We previously reported the isolation of a novel subtype of SRV/D-Tsukuba (SRV/D-T) from two cynomolgus monkeys (Macaca facicularis) in the breeding colony of Tsukuba Primate Research Center (TPRC). We surveyed for SRV/D infection in the TPRC cynomolgus colony using SRV/D-specific PCR primer sets designed based on the entire gag region sequence. The only SRV/D subtype detected in the colony was SRV/D-T with a positive infection rate of 22.4% (n = 49). It has been reported that the mode of transmission of SRV/D is via contact with virus shed in the body fluids. In this report, to investigate the infection route of SRV/D-T in monkeys at TPRC, we performed virus isolation and PCR for detection of the SRV/D genome from peripheral blood mononuclear cells (PBMCs), plasma, saliva, urine, and feces. Virus isolation and PCR detection were positive in plasma, saliva, urine, and fecal samples from all monkeys on which virus was isolated from PBMCs. This suggests that the spread of SRV/D-T infection in TPRC is via contact with virus shed in saliva, urine, and/or feces. Also, comparison of sequences of gp70 on multiple SRV/D-T isolates revealed that there was little intra- and inter-monkey variation, suggesting that SRV/D-T is fairly stable.

Ubiquitination of APOBEC3 proteins by the Vif–Cullin5–ElonginB–ElonginC complex

APOBEC3 proteins are antiviral host factors for a wide variety of retroviruses. HIV-1 Vif overcomes the antiviral activity of APOBEC3G by ubiquitinating the protein. In this study, we examined the ability of Vif to antagonize other family members of APOBEC3 proteins, together with its mechanism. Using HIV infectivity, virion incorporation, immunoprecipitation, and in vitro ubiquitin conjugation assays, we show that the ability of Vif to inhibit antiviral activity of APOBEC3 proteins positively correlates with its ability to bind and ubiquitinate these proteins by a Vif-Cullin5-ElonginB-ElonginC (Vif-BC-Cul5) complex. These results suggest that Vif exhibits its anti-APOBEC3 activity by the ubiquitin ligase activity of the Vif-BC-Cul5 complex.

Expression of HIV-1 accessory protein Vif is controlled uniquely to be low and optimal by proteasome degradation

While the Vif protein of human immunodeficiency virus type 1 (HIV-1) is essential for viral replication in non-permissive cells, it is rapidly degraded intracellularly. We have previously suggested that the rapid turn-over of Vif is biologically meaningful to prevent detrimental effects of this protein at high expression levels. We now studied the mechanism of Vif degradation by examining the blocking effect of protease inhibitors in pulse/chase experiments and by monitoring the extent of Vif ubiquitination. The rapid turn-over of Vif could be blocked by proteasome inhibitors, and Vif was highly ubiquitinated. Cytoskeletal Vif was found to be more stable than soluble cytosolic Vif. These degradation characteristics of Vif were cell type-independent and observed in both non-permissive and permissive cells. Characterization of a series of vif deletion mutants showed that amino acids predicted to be important for formation of beta-strand structures (amino acid nos. 63-70 and 86-89) were critical for maintaining a normal expression level of Vif and for viral infectivity. Finally, we performed comparative stability analysis of the four HIV-1 accessory proteins. Vif was unique in its short half-life and in the magnitude of the degradation. Taken together, we conclude that the proteasome degradation of HIV-1 Vif is a virologically important process and crucial for the function of Vif.

Detection of 14 alleles derived from the MHC class I A locus in cynomolgus monkeys

A basic understanding of the major histocompatibility complex (MHC) class I, which, together with T-cell receptors, is a key player in antigen recognition by cytotoxic T lymphocytes, is necessary to study the cellular immune response to intracellular pathogens. The MHC has hardly been reported in cynomolgus monkeys ( Macaca facicularis), although cynomolgus monkeys have been frequently used as the surrogate animal model. We attempted to determine the nucleotide sequences of the MHC class I A locus of cynomolgus monkeys ( Mafa-A) and eventually 34 independent sequences of Mafa-A were obtained from 29 cynomolgus monkeys. These 34 sequences were classified into 14 Mafa-A alleles according to the results of phylogenetic analyses using the neighbor-joining method. One to three Mafa-A alleles were obtained from a single animal. We also tried to establish a multiplex PCR-SSP method for convenient typing of Mafa-A alleles. cDNA from a family of cynomolgus monkeys, which is composed of four sirs and four dams, were examined by multiplex PCR-SSP. The result of multiplex PCR-SSP showed that an individual cynomolgus monkey had two or three Mafa-A alleles, suggesting that the A locus of cynomolgus monkeys might be duplicated.

Codon optimization of the HIV-1 vpu and vif genes stabilizes their mRNA and allows for highly efficient Rev-independent expression

Two HIV-1 accessory proteins, Vpu and Vif, are notoriously difficult to express autonomously in the absence of the viral Tat and Rev proteins. We examined whether the codon bias observed in the vpu and vif genes relative to highly expressed human genes contributes to the Rev dependence and low expression level outside the context of the viral genome. The entire vpu gene as well as the 5' half of the vif gene were codon optimized and the resulting open reading frames (ORFs) (vphu and hvif, respectively) were cloned in autonomous expression vectors under the transcriptional control of the CMV promoter. Codon optimization efficiently removed the expression block observed in the native genes and allowed high levels of Rev- and Tat-independent expression of Vpu and Vif. Most of the higher protein levels detected are accounted for by enhanced steady-state levels of the mRNA encoding the optimized species. Nuclear run-on experiments show for the first time that codon optimization has no effect on the rate of transcriptional initiation or elongation of the vphu mRNA. Instead, optimization of the vpu gene was found to stabilize the vphu mRNA in the nucleus and enhance its export to the cytoplasm. This was achieved by allowing the optimized mRNA to use a new CRM I-independent nuclear export pathway. This work provides a better understanding of the molecular mechanisms underlying the process of codon optimization and introduces novel tools to study the biological functions of the Vpu and Vif proteins independently of other viral proteins.

High level expression of human immunodeficiency virus type-1 Vif inhibits viral infectivity by modulating proteolytic processing of the Gag precursor at the p2/nucleocapsid processing site

The human immunodeficiency virus type-1 Vif protein has a crucial role in regulating viral infectivity. However, we found that newly synthesized Vif is rapidly degraded by cellular proteases. We tested the dose dependence of Vif in non-permissive H9 cells and found that Vif, when expressed at low levels, increased virus infectivity in a dose-dependent manner. Surprisingly, however, the range of Vif required for optimal virus infectivity was narrow, and further increases in Vif severely reduced viral infectivity. Inhibition of viral infectivity at higher levels of Vif was cell type-independent and was associated with an accumulation of Gag-processing intermediates. Vif did not act as a general protease inhibitor but selectively inhibited Gag processing at the capsid and nucleocapsid (NC) boundary. Identification of Vif variants that were efficiently packaged but were unable to modulate Gag processing suggests that Vif packaging was necessary but insufficient for the production of 33- and 34-kDa processing intermediates. Interestingly, these processing intermediates, like Vif, associated with viral nucleoprotein complexes more rigidly than mature capsid and NC. We conclude that virus-associated Vif inhibits processing of a subset of Gag precursor molecules at the p2/NC primary cleavage site. Modulation of processing of a small subset of Gag molecules by physiological levels of Vif may be important for virus maturation. However, the accumulation of such processing intermediates at high levels of Vif is inhibitory. Thus, rapid intracellular degradation of Vif may have evolved as a mechanism to prevent such inhibitory effects of Vif.

Age-related increase of peripheral CD4+ CD8+ double-positive T lymphocytes in cynomolgus monkeys: longitudinal study in relation to thymic involution

The age-related increase of peripheral CD4+ CD8+ double-positive (DP) T cells in cynomolgus monkeys has been reported previously. Because the percentage of DP T cells in cynomolgus monkeys increases abruptly in parallel with the thymic involution occurring at around 11 years of age, it was suggested that thymic involution was associated with this increase. Therefore, a longitudinal study was carried out over 5 years to clarify the exact time when DP T lymphocytes start to increase in relation to the thymic involution. Twelve cynomolgus monkeys at 6 years of age were classified into three groups, based on their percentage of DP T cells, as follows: DP-High (>5% DP T cells); DP-Middle (1-5% DP T cells); and DP-Low (<1% DP T cells). In the DP-High group, the percentage of DP T cells showed an abrupt increase, of >10%, in monkeys at 7 years of age, and the prevalence of this subset correlated with a distinctive increase in the percentage of memory T cells (CD4+ CD29(high), CD8+ CD28-), indicating an association with the maturation of immune function, including thymic involution. To assess the thymic function, the coding joint of T-cell receptor excision circles (cjTREC) levels in sorted T cells were analysed by polymerase chain reaction (PCR)-enzyme-linked immunosorbent assay (ELISA). The cjTREC in the T cells of the DP-High group (4362 +/- 3139 copies/105 T cells) was significantly lower than that (22 722 +/- 4928 copies/105 T cells) of the DP-Low group. Moreover, the mean copy number of cjTREC in naive T cells was also significantly different between the DP-High and the DP-Low group (0.457 +/- 0.181 and 1.141 +/- 0.107, respectively). These findings suggest that thymic involution has an influence on the age-related increase of DP T cells in cynomolgus monkeys.

Susceptibility of HVS-immortalized lymphocytic HSC-F cells to various strains and mutants of HIV/SIV

Susceptibility of HSC-F, a cynomolgus macaque cell line immortalized by Herpesvirus saimiri, to infection with various primate immunodeficiency viruses were monitored. While NL432 clone of human immunodeficiency virus type 1 (HIV-1) did not grow at all in HSC-F cells, GH123 and GL-AN clones of HIV-2, and MA239 clone of simian immunodeficiency virus isolated from macaque monkeys (SIVMAC) did grow in these cells. In addition, NM-3 clone of a chimeric simian and human immunodeficiency virus (SHIV) grew fairly well in HSC-F cells. Mutational analyses of accessory genes of GL-AN were successfully performed in the HSC-F cells. These results have thus demonstrated the importance of this cell line for molecular biological studies on HIV/SIV.

Naturally occurring amino acid substitutions in the HIV-2 ROD envelope glycoprotein regulate its ability to augment viral particle release

The envelope glycoprotein of HIV-2 ROD10 has the intriguing ability to enhance the rate of viral particle release from infected cells. However, not all HIV-2 envelope glycoproteins are active in this regard. Indeed, we have previously noted that, despite a high degree of identity with that of ROD10, the envelope protein of the ROD14 isolate was unable to enhance virus production. In this study, site-directed mutagenesis was employed to reveal that a single naturally occurring alanine-to-threonine substitution at position 598, located in the extracellular part of the TM subunit, fully accounted for the lack of activity of the ROD14 Env in HeLa and 12D7 cells. A second mutation at position 422, substituting a lysine residue in ROD10 for an arginine in ROD14, was additionally required for efficient virus release from infected H9 cells, suggesting cell-type-specific requirements for this activity. Interestingly, the ROD14 Env protein exhibited a trans-dominant negative effect on particle release by ROD10 Env, suggesting that the viral release activity of the HIV-2 ROD envelope protein may be regulated by its ability to assemble into functional oligomeric structures.

Vpx and Vpr proteins of HIV-2 up-regulate the viral infectivity by a distinct mechanism in lymphocytic cells

Mutants of human immunodeficiency virus type 2 (HIV-2) carrying a frame-shift mutation in vpx, vpr, and in both genes were monitored for their growth potentials in a newly established lymphocytic cell line, HSC-F. Worthy of note, the replication of a vpx single mutant, but not vpr, was severely impaired in these cells, and that of a vpx-vpr double mutant was more damaged. Defective replication sites of the vpx single and vpx-vpr double mutants were demonstrated to be mapped, respectively, to the nuclear import of viral genome, and to both, this process and the virus assembly/release stage. While the mutational effect of vpr was small, the replication efficiency in one cycle of the vpx mutant relative to that of wild-type virus was estimated to be 10%. The growth phenotypes of the vpx, vpr, and vpx-vpr mutant viruses in HSC-F cells were essentially repeated in primary human lymphocytes. In primary human macrophages, whereas the vpx and vpx-vpr mutants did not grow at all, the vpr mutant grew equally as well as the wild-type virus. These results strongly suggested that Vpx is critical for up-regulation of HIV-2 replication in natural target cells by enhancing the genome nuclear import, and that Vpr promotes HIV-2 replication somewhat, at least in lymphocytic cells, at a very late replication phase.

nef gene is required for robust productive infection by simian immunodeficiency virus of T-cell-rich paracortex in lymph nodes

The pathogenesis of AIDS virus infection in a nonhuman primate AIDS model was studied by comparing plasma viral loads, CD4(+) T-cell subpopulations in peripheral blood mononuclear cells, and simian immunodeficiency virus (SIV) infection in lymph nodes for rhesus macaques infected with a pathogenic molecularly cloned SIVmac239 strain and those infected with its nef deletion mutant (Deltanef). In agreement with many reports, whereas SIVmac239 infection induced AIDS and depletion of memory CD4(+) T cells in 2 to 3 years postinfection (p.i.), Deltanef infection did not induce any manifestation associated with AIDS up to 6.5 years p.i. To explore the difference in SIV infection in lymphoid tissues, we biopsied lymph nodes at 2, 8, 72, and 82 weeks p.i. and analyzed them by pathological techniques. Maximal numbers of SIV-infected cells (SIV Gag(+), Env(+), and RNA(+)) were detected at 2 weeks p.i. in both the SIVmac239-infected animals and the Deltanef-infected animals. In the SIVmac239-infected animals, most of the infected cells were localized in the T-cell-rich paracortex, whereas in the Deltanef-infected animals, most were localized in B-cell-rich follicles and in the border region between the paracortex and the follicles. Analyses by double staining of CD68(+) macrophages and SIV Gag(+) cells and by double staining of CD3(+) T cells and SIV Env(+) cells revealed that SIV-infected cells were identified as CD4(+) T cells in either the SIVmac239 or the Deltanef infection. Whereas the many functions of Nef protein were reported from in vitro studies, our finding of SIVmac239 replication in the T-cell-rich paracortex in the lymph nodes supports the reported roles of Nef protein in T-cell activation and enhancement of viral infectivity. Furthermore, the abundance of SIVmac239 infection and the paucity of Deltanef infection in the T-cell-rich paracortex accounted for the differences in viral replication and pathogenicity between SIVmac239 and the Deltanef mutant. Thus, our in vivo study indicated that the nef gene enhances SIV replication by robust productive infection in memory CD4(+) T cells in the T-cell-rich region in lymphoid tissues.

Human immunodeficiency virus type 1 Vif is efficiently packaged into virions during productive but not chronic infection

Packaging of the human immunodeficiency virus type 1 Vif protein into virus particles is mediated through an interaction with viral genomic RNA and results in the association of Vif with the nucleoprotein complex. Despite the specificity of this process, calculations of the amount of Vif packaged have produced vastly different results. Here, we compared the efficiency of packaging of Vif into virions derived from acutely and chronically infected H9 cells. We found that Vif was efficiently packaged into virions from acutely infected cells (60 to 100 copies per virion), while packaging into virions from chronically infected H9 cells was near the limit of detection (four to six copies of Vif per virion). Superinfection by an exogenous Vif-defective virus did not rescue packaging of endogenous Vif expressed in the chronically infected culture. In contrast, exogenous Vif expressed by superinfection of wild-type virus was readily packaged (30 to 40 copies per virion). Biochemical analyses suggest that the differences in the relative packaging efficiencies were not due to gross differences in the steady-state distribution of Vif in chronically or acutely infected cells but are likely due to differences in the relative rates of de novo synthesis of Vif. Despite its low packaging efficiency, endogenously expressed Vif was sufficient to direct the production of viruses with almost wild-type infectivity. The results from our study provide novel insights into the biochemical properties of Vif and offer an explanation for the reported differences regarding Vif packaging.

Intravirion processing of the human immunodeficiency virus type 1 Vif protein by the viral protease may be correlated with Vif function

The human immunodeficiency virus type 1 (HIV-1) Vif protein is specifically packaged into virus particles through an interaction with viral genomic RNA in which it associates with the viral nucleoprotein complex. We now demonstrate for the first time that virus-associated Vif is subject to proteolytic processing by the viral protease (Pr). Pr-dependent processing of Vif was observed both in vivo and in vitro. In vivo processing of Vif was cell type independent and evident by the appearance of a 7-kDa processing product, which was restricted to cell-free virus preparations. Processing of Vif required an active viral Pr and was sensitive to Pr inhibitors such as ritonavir. The processing site in Vif was characterized both in vivo and in vitro and mapped to Ala(150). Interestingly, the Vif processing site is located in a domain that is highly conserved among HIV-1, HIV-2, and simian immunodeficiency virus Vif isolates. Mutations at or near the processing site did not affect protein stability or packaging efficiency but had dramatic effects on Vif processing. In general, mutations that markedly increased or decreased the sensitivity of Vif to proteolytic processing severely impaired or completely abolished Vif function. In contrast, mutations at the same site that had little or no effect on processing efficiency also did not influence Vif function. None of the mutants affected the ability of the virus to replicate in permissive cell lines. Our data suggest that mutations in Vif that cause a profound change in the sensitivity to Pr-dependent processing also severely impaired Vif function, suggesting that intravirion processing of Vif is important for the production of infectious viruses.

Different effects of Nef-mediated HLA class I down-regulation on human immunodeficiency virus type 1-specific CD8(+) T-cell cytolytic activity and cytokine production

A previous study using a Nef-defective human immunodeficiency virus type 1 (HIV-1) mutant suggested that Nef-mediated down-regulation of HLA class I on the infected cell surface affects the cytolytic activity of HIV-1-specific cytotoxic T-lymphocyte (CTL) clones for HIV-1-infected primary CD4(+) T cells. We confirmed this effect by using a nef-mutant HIV-1 strain (NL-M20A) that expresses a Nef protein which does not induce down-regulation of HLA class I molecules but is otherwise functional. HIV-1-specific CTL clones were not able to kill primary CD4(+) T cells infected with a Nef-positive HIV-1 strain (NL-432) but efficiently lysed CD4(+) T cells infected with NL-M20A. Interestingly, CTL clones stimulated with NL-432-infected CD4(+) T cells were able to produce cytokines, albeit at a lower level than when stimulated with NL-M20A-infected CD4(+) T cells. This indicates that Nef-mediated HLA class I down-regulation affects CTL cytokine production to a lesser extent than cytolytic activity. Replication of NL-432 was partially suppressed in a coculture of HIV-1-infected CD4(+) T cells and HIV-1-specific CTL clones, while replication of NL-M20A was completely suppressed. These results suggest that HIV-1-specific CD8(+) T cells are able to partially suppress the replication of HIV-1 through production of soluble HIV-1-suppressive factors such as chemokines and gamma interferon. These findings may account for the mechanism whereby HIV-1-specific CD8(+) T cells are able to partially but not completely control HIV-1 replication in vivo.

The human immunodeficiency virus type 1 accessory protein Vpu induces apoptosis by suppressing the nuclear factor kappaB-dependent expression of antiapoptotic factors

Human immunodeficiency virus (HIV) type 1 Vpu is an integral membrane protein with a unique affinity for betaTrCP (TrCP), a key member of the SkpI-Cullin-F-box E3 ubiquitin ligase complex that is involved in the regulated degradation of cellular proteins, including IkappaB. Remarkably, Vpu is resistant to TrCP-mediated degradation and competitively inhibits TrCP-dependent degradation of IkappaB, resulting in the suppression of nuclear factor (NF)-kappaB activity in Vpu-expressing cells. We now report that Vpu, through its interaction with TrCP, potently contributes to the induction of apoptosis in HIV-infected T cells. Vpu-induced apoptosis is specific and independent of other viral proteins. Mutation of a TrCP-binding motif in Vpu abolishes its apoptogenic property, demonstrating a close correlation between this property of Vpu and its ability to inhibit NF-kappaB activity. The involvement of NF-kappaB in Vpu-induced apoptosis is further supported by the finding that the levels of antiapoptotic factors Bcl-xL, A1/Bfl-1, and TNF receptor-associated factor (TRAF)1, all of which are expressed in an NF-kappaB-dependent manner, are reduced and, at the same time, levels of active caspase-3 are elevated. Thus, Vpu induces apoptosis through activation of the caspase pathway by way of inhibiting the NF-kappaB-dependent expression of antiapoptotic genes.

Cyclophilin A-independent replication of a human immunodeficiency virus type 1 isolate carrying a small portion of the simian immunodeficiency virus SIV(MAC) gag capsid region

Hybrid viruses between human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus strain mac (SIV(MAC)) are invaluable to various fields of HIV-1 research. To date, however, no replication-competent HIV-1 strain containing the gag capsid (CA) region of SIV(MAC) has been reported. To obtain the viable gag gene chimeric virus in an HIV-1 background, seven HIV-1 strains carrying a part of SIV(MAC) CA or a small deletion in the CA region were constructed and examined for their biological and biochemical characteristics. While all the recombinants and mutants were found to express Gag and to produce progeny virions on transfection, only one chimeric virus, which has 18 bp of SIV gag CA sequence in place of the region encoding the HIV-1 CA cyclophilin A (CyPA)-binding loop, was infectious for human cell lines. Although this chimeric virus was unable to grow in monkey lymphocytic cells like wild-type (wt) HIV-1 did, it grew much better than wt virus in the presence of cyclosporin A in a human cell line which supports HIV-1 replication in a CyPA-dependent manner. These results indicate that the transfer of a small portion of the SIV(MAC) CA region to HIV-1 could confer the CyPA-independent replication potential of SIV(MAC) on the virus.

Human immunodeficiency virus type 1 Vif protein is packaged into the nucleoprotein complex through an interaction with viral genomic RNA

The human immunodeficiency virus type 1 (HIV-1) Vif protein plays a critical role in the production of infectious virions. Previous studies have demonstrated the presence of small amounts of Vif in virus particles. However, Vif packaging was assumed to be nonspecific, and its functional significance has been questioned. We now report that packaging of Vif is dependent on the packaging of viral genomic RNA in both permissive and restrictive HIV-1 target cells. Mutations in the nucleocapsid zinc finger domains that abrogate packaging of viral genomic RNA abolished packaging of Vif. Additionally, an RNA packaging-defective virus exhibited significantly reduced packaging of Vif. Finally, deletion of a putative RNA-interacting domain in Vif abolished packaging of Vif into virions. Virion-associated Vif was resistant to detergent extraction and copurified with components of the viral nucleoprotein complex and functional reverse transcription complexes. Thus, Vif is specifically packaged into virions as a component of the viral nucleoprotein complex. Our data suggest that the specific association of Vif with the viral nucleoprotein complex might be functionally significant and could be a critical requirement for infectivity of viruses produced from restrictive host cells.