Feline immunodeficiency virus Gag is a nuclear shuttling protein

Lentiviral genomic RNAs are encapsidated by the viral Gag protein during virion assembly. The intracellular location of the initial Gag-RNA interaction is unknown. We previously observed feline immunodeficiency virus (FIV) Gag accumulating at the nuclear envelope during live-cell imaging, which suggested that trafficking of human immunodeficiency virus type 1 (HIV-1) and FIV Gag may differ. Here we analyzed the nucleocytoplasmic transport properties of both Gag proteins. We discovered that inhibition of the CRM1 nuclear export pathway with leptomycin B causes FIV Gag but not HIV-1 Gag to accumulate in the nucleus. Virtually all FIV Gag rapidly became intranuclear when the CRM1 export pathway was blocked, implying that most if not all FIV Gag normally undergoes nuclear cycling. In FIV-infected feline cells, some intranuclear Gag was detected in the steady state without leptomycin B treatment. When expressed individually, the FIV matrix (MA), capsid (CA), and nucleocapsid-p2 (NC-p2) domains were not capable of mediating leptomycin B-sensitive nuclear export of a fluorescent protein. In contrast, CA-NC-p2 did mediate nuclear export, with MA being dispensable. We conclude that HIV-1 and FIV Gag differ strikingly in a key intracellular trafficking property. FIV Gag is a nuclear shuttling protein that utilizes the CRM1 nuclear export pathway, while HIV-1 Gag is excluded from the nucleus. These findings expand the spectrum of lentiviral Gag behaviors and raise the possibility that FIV genome encapsidation may initiate in the nucleus.     

Oral immunization with recombinant listeria monocytogenes controls virus load after vaginal challenge with feline immunodeficiency virus

Recombinant Listeria monocytogenes has many attractive characteristics as a vaccine vector against human immunodeficiency virus (HIV). Wild-type and attenuated Listeria strains expressing HIV Gag have been shown to induce long-lived mucosal and systemic T-cell responses in mice. Using the feline immunodeficiency virus (FIV) model of HIV we evaluated recombinant L. monocytogenes in a challenge system. Five cats were immunized with recombinant L. monocytogenes that expresses the FIV Gag and delivers an FIV Env-expressing DNA vaccine (LMgag/pND14-Lc-env). Control cats were either sham immunized or immunized with wild-type L. monocytogenes (LM-wt). At 1 year after vaginal challenge, provirus could not be detected in any of the nine tissues evaluated from cats immunized with the recombinant bacteria but was detected in at least one tissue in 8 of 10 control animals. Virus was isolated from bone marrow of four of five LMgag/pND14-Lc-env-immunized cats by use of a stringent coculture system but required CD8(+) T-cell depletion, indicating CD8(+) T-cell suppression of virus replication. Control animals had an inverted CD4:CD8 ratio in mesenteric lymph node and were depleted of both CD4(+) and CD8(+) intestinal epithelial T cells, while LMgag/pND14-Lc-env-immunized animals showed no such abnormalities. Vaginal FIV-specific immunoglobulin A was present at high titer in three LMgag/pND14-Lc-env-immunized cats before challenge and in all five at 1 year postchallenge. This study demonstrates that recombinant L. monocytogenes conferred some control of viral load after vaginal challenge with FIV.     

Vif is largely absent from human immunodeficiency virus type 1 mature virions and associates mainly with viral particles containing unprocessed gag

Vif is a human immunodeficiency virus type 1 (HIV-1) protein that is essential for the production of infectious virus. Most of Vif synthesized during HIV infection localizes within cells, and the extent of Vif packaging into virions and its function there remain controversial. Here we show that a small but detectable amount of Vif remains associated with purified virions even after their treatment with the protease subtilisin. However, treatment of these virions with 1% Triton X-100 revealed that most of the virion-associated Vif segregated with detergent-resistant virus particles consisting of unprocessed Gag, indicating that detergent-soluble, mature virions contain very little Vif. To investigate the control of Vif packaging in immature virus particles, we tested its association with Gag-containing virus-like particles (VLPs) in a Vif and Gag coexpression system in human cells. Only a small proportion of Vif molecules synthesized in this system became packaged into VLPs, and the VLP-associated Vif was protected from exogenous protease and detergent treatment, indicating that it is stably incorporated into immature virion-like cores. About 10-fold more Vpr than Vif was packaged into VLPs but most of the VLP-associated Vpr was removed by treatment with detergent. Mutagenesis of the C-terminal sequences in Gag previously shown to be responsible for interaction with Vif did not reduce the extent of Vif packaging into Gag VLPs. Surprisingly, short deletions in the capsid domain (CA) of Gag (amino acid residues 284 to 304 and 350 to 362) increased Vif packaging over 10-fold. The 350 to 363 deletion introduced into CA in HIV provirus also increased Vif incorporation into purified virions. Our results show that Vif can be packaged at low levels into aberrant virus particles or immature virions and that Vif is not present significantly in mature virions. Overall, these results indicate that the Vif content in virions is tightly regulated and also argue against a function of virion-associated Vif.     

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.     

The Vif protein of human and simian immunodeficiency viruses is packaged into virions and associates with viral core structures.

The vif gene of human and simian immunodeficiency viruses (HIV and SIV) encodes a late gene product that is essential for viral infectivity in natural target cells. Virions produced in the absence of Vif are abnormal in their ultrastructural morphology and are severely impaired in the ability to complete proviral DNA synthesis upon entry into new target cells. Because previous studies failed to detect Vif protein in virus particles, Vif is believed to influence virus infectivity indirectly, by affecting virion assembly, release, and/or maturation. In this report, we reexamined the possibility that Vif is a virion-associated protein. Utilizing high-titer Vif-specific antibodies, a sensitive immunoblot technique, and highly concentrated virus preparations, we detected a 23-kDa Vif-reactive protein in wild-type HIV type 1 (HIV-1) and a 27-kDa Vif-reactive protein in wild-type SIVSM virions. Neither protein was present in virions derived from vif-deficient HIV-1 and SIVSM proviral constructs. Vif protein content was similar among different strains of HIV-1 and was independent of the cell type (permissive or nonpermissive) used to produce the virus. To determine the subvirion localization of Vif, HIV-1 virions were treated with proteinase K or Triton X-100 to remove virion surface proteins and the viral membrane, respectively, purified through sucrose, and analyzed by immunoblot analysis. Vif protein content was not affected by the removal of external surface proteins or by the removal of the viral membrane and submembrane p17Gag matrix protein. Instead, Vif colocalized with viral core structures which sedimented at a density of 1.25 g/ml on linear sucrose gradients (enveloped HIV-1 particles sediment at a density of 1.17 g/ml). Finally, the amount of Vif protein packaged into virions was estimated to be on the order of 1 molecule of Vif for every 20 to 30 molecules of p24Gag, or between 60 and 100 molecules of Vif per particle. These results indicate that Vif represents an integral component of HIV and SIV particles and raise the possibility that it plays a direct role in early replication events.     

Role of the feline immunodeficiency virus L-domain in the presence or absence of Gag processing: involvement of ubiquitin and Nedd4-2s ligase in viral egress

RNA-enveloped viruses bud from infected cells by exploiting the multivesicular body (MVB) pathway. In this context, ubiquitination of structural viral proteins and their direct interaction with cellular factors involved in the MVB biogenesis through short proline rich regions, named late domains (L-domains), are crucial mechanisms. Here we report that, in contrast with the human immunodeficiency virus (HIV), the feline immunodeficiency virus (FIV), a non-primate lentivirus, is strictly dependent for its budding on a "PSAP"-type L-domain, mapping in the carboxy-terminal region of Gag, irrespective of a functional viral protease. Moreover, we provide evidence that FIV egress is related to Gag ubiquitination, that is, linked to the presence of an active L-domain. Finally, although FIV Gag does not contain a PPxY motif, we show that the Nedd4-2s ubiquitin ligase enhances FIV Gag ubiquitination and it is capable to rescue viral mutants lacking a functional L-domain. In conclusion, our data bring to light peculiar aspects of FIV egress, but we also demonstrate that a non-primate lentivirus shares with HIV-1 a novel mechanism of connection to the cellular budding machinery.     

The Vif and Gag proteins of human immunodeficiency virus type 1 colocalize in infected human T cells.

The Vif protein of human immunodeficiency virus type 1 (HIV-1) and other lentiviruses is required for efficient replication in primary cells and certain immortalized cell lines in vitro and, in all likelihood, for the establishment of pathogenic infections in vivo. Current hypotheses concerning Vif's mechanism of action posit that it operates in virus-expressing cells during virion assembly, budding, or maturation such that released virions are modified in a manner that enables them to undergo productive infection in subsequent viral challenges. To gain further insight into the mechanism of action of lentivirus Vif proteins, we have performed a variety of in situ localization and biochemical fractionation studies using cells in which Vif is essential for efficient replication. Double-label immunofluorescence analyses of cells productively infected with HIV-1 or feline immunodeficiency virus revealed dramatic patterns of colocalization between Vif and the virally encoded Gag proteins. Subcellular fractionations of human T cells expressing HIV-1 Vif performed in the absence of any detergent demonstrated that greater than 90% of Vif is associated with cellular membranes. Additional purification using a continuous density gradient indicated that the majority of the membrane-bound Vif copurifies with the plasma membrane. Taken together, these observations suggest that lentivirus Vif and Gag proteins colocalize at the plasma membrane as virion assembly and budding take place. As a result, Vif is able to exert its modulatory effect(s) on these late steps of the virus life cycle.     

Cytoskeleton association and virion incorporation of the human immunodeficiency virus type 1 Vif protein.

The human immunodeficiency virus type 1 (HIV-1) Vif protein has an important role in the regulation of virus infectivity. This function of Vif is cell type specific, and virions produced in the absence of Vif in restrictive cells have greatly reduced infectivity. We show here that the intracellular localization of Vif is dependent on the presence of the intermediate filament vimentin. Fractionation of acutely infected T cells or transiently transfected HeLa cells demonstrates the existence of a soluble and a cytoskeletal form and to a lesser extent the presence of a detergent-extractable form of Vif. Confocal microscopy suggests that in HeLa cells, Vif is predominantly present in the cytoplasm and closely colocalizes with the intermediate filament vimentin. Treatment of cells with drugs affecting the structure of vimentin filaments affect the localization of Vif accordingly, indicating a close association of Vif with this cytoskeletal component. The association of Vif with vimentin can cause the collapse of the intermediate filament network into a perinuclear aggregate. In contrast, analysis of Vif in vimentin-negative cells reveals significant staining of the nucleus and the nuclear membrane in addition to diffuse cytoplasmic staining. In addition to the association of Vif with intermediate filaments, analyses of virion preparations demonstrate that Vif is incorporated into virus particles. In sucrose density gradients, Vif cosediments with capsid proteins even after detergent treatment of virus preparations, suggesting that Vif is associated with the inner core of HIV particles. We propose a model in which Vif has a crucial function as a virion component either by regulating virus maturation or following virus entry into a host cell possibly involving an interaction with the cellular cytoskeletal network.     

Molecular characterization of feline immunodeficiency virus budding

Infection of domestic cats with feline immunodeficiency virus (FIV) is an important model system for studying human immunodeficiency virus type 1 (HIV-1) infection due to numerous similarities in pathogenesis induced by these two lentiviruses. However, many molecular aspects of FIV replication remain poorly understood. It is well established that retroviruses use short peptide motifs in Gag, known as late domains, to usurp cellular endosomal sorting machinery and promote virus release from infected cells. For example, the Pro-Thr/Ser-Ala-Pro [P(T/S)AP] motif of HIV-1 Gag interacts directly with Tsg101, a component of the endosomal sorting complex required for transport I (ESCRT-I). A Tyr-Pro-Asp-Leu (YPDL) motif in equine infectious anemia virus (EIAV), and a related sequence in HIV-1, bind the endosomal sorting factor Alix. In this study we sought to identify and characterize FIV late domain(s) and elucidate cellular machinery involved in FIV release. We determined that mutagenesis of a PSAP motif in FIV Gag, small interfering RNA-mediated knockdown of Tsg101 expression, and overexpression of a P(T/S)AP-binding fragment of Tsg101 (TSG-5′) each inhibited FIV release. We also observed direct binding of FIV Gag peptides to Tsg101. In contrast, mutagenesis of a potential Alix-binding motif in FIV Gag did not affect FIV release. Similarly, expression of the HIV-1/EIAV Gag-binding domain of Alix (Alix-V) did not disrupt FIV budding, and FIV Gag peptides showed no affinity for Alix-V. Our data demonstrate that FIV relies predominantly on a Tsg101-binding PSAP motif in the C terminus of Gag to promote virus release in HeLa cells, and this budding mechanism is highly conserved in feline cells.     

Vpr.A3A chimera inhibits HIV replication

Several APOBEC3 proteins (A3F and A3G), that are cytidine deaminases restrict human immunodeficiency virus (HIV) replication in the absence of the viral infectivity factor (Vif) protein. However, Vif leads to their degradation and counteracts their effects. Another member, A3A, restricts some retrotransposons and another virus but not HIV. We reasoned that this failure was due to the lack of appropriate targeting. Thus, we fused A3A to another viral protein, Vpr, which binds p6 in Gag and is incorporated into viral cores. Indeed, the Vpr.A3A chimera but not A3A was found abundantly in the viral core. It also restricted potently the replication of HIV and simian immunodeficiency virus in the presence and absence of Vif. Because we identified a high frequency of G to A mutations in viral cDNAs, this antiviral activity was mediated by DNA editing. Interestingly, our fusion protein did not restrict murine leukemia virus, which does not incorporate Vpr. Thus, by targeting appropriately a potent single domain cytidine deaminase, we rendered HIV and simian immunodeficiency virus restriction resistant to Vif.     

Identification of a Cullin5-ElonginB-ElonginC E3 complex in degradation of feline immunodeficiency virus Vif-mediated feline APOBEC3 proteins

Various feline APOBEC3 (fA3) proteins exhibit broad antiviral activities against a wide range of viruses, such as feline immunodeficiency virus (FIV), feline foamy virus (FFV), and feline leukemia virus (FeLV), as well as those of other species. This activity can be counteracted by the FIV Vif protein, but the mechanism by which FIV Vif suppresses fA3s is unknown. In the present study, we demonstrated that FIV Vif could act via a proteasome-dependent pathway to overcome fA3s. FIV Vif interacted with feline cellular proteins Cullin5 (Cul5), ElonginB, and ElonginC to form an E3 complex to induce degradation of fA3s. Both the dominant-negative Cul5 mutant and a C-terminal hydrophilic replacement ElonginC mutant potently disrupted the FIV Vif activity against fA3s. Furthermore, we identified a BC-box motif in FIV Vif that was essential for the recruitment of E3 ubiquitin ligase and also required for FIV Vif-mediated degradation of fA3s. Moreover, despite the lack of either a Cul5-box or a HCCH zinc-binding motif, FIV Vif specifically selected Cul5. Therefore, FIV Vif may interact with Cul5 via a novel mechanism. These finding imply that SOCS proteins may possess distinct mechanisms to bind Cul5 during formation of the Elongin-Cullin-SOCS box complex.     

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.     

Accessory Genes Confer a High Replication Rate to Virulent Feline Immunodeficiency Virus

Feline immunodeficiency virus (FIV) is a lentivirus that causes AIDS in domestic cats, similar to human immunodeficiency virus (HIV)/AIDS in humans. The FIV accessory protein Vif abrogates the inhibition of infection by cat APOBEC3 restriction factors. FIV also encodes a multifunctional OrfA accessory protein that has characteristics similar to HIV Tat, Vpu, Vpr, and Nef. To examine the role of vif and orfA accessory genes in FIV replication and pathogenicity, we generated chimeras between two FIV molecular clones with divergent disease potentials: a highly pathogenic isolate that replicates rapidly in vitro and is associated with significant immunopathology in vivo, FIV-C36 (referred to here as high-virulence FIV [HV-FIV]), and a less-pathogenic strain, FIV-PPR (referred to here as low-virulence FIV [LV-FIV]). Using PCR-driven overlap extension, we produced viruses in which vif, orfA, or both genes from virulent HV-FIV replaced equivalent genes in LV-FIV. The generation of these chimeras is more straightforward in FIV than in primate lentiviruses, since FIV accessory gene open reading frames have very little overlap with other genes. All three chimeric viruses exhibited increased replication kinetics in vitro compared to the replication kinetics of LV-FIV. Chimeras containing HV-Vif or Vif/OrfA had replication rates equivalent to those of the virulent HV-FIV parental virus. Furthermore, small interfering RNA knockdown of feline APOBEC3 genes resulted in equalization of replication rates between LV-FIV and LV-FIV encoding HV-FIV Vif. These findings demonstrate that Vif-APOBEC interactions play a key role in controlling the replication and pathogenicity of this immunodeficiency-inducing virus in its native host species and that accessory genes act as mediators of lentiviral strain-specific virulence.     

Human immunodeficiency virus type 1 Vif- mutant particles from restrictive cells: role of Vif in correct particle assembly and infectivity.

Disruption of the vif gene of human immunodeficiency virus (HIV) type 1 affects virus infectivity to various degrees, depending on the T-cell line used. We have concentrated our studies on true phenotypic Vif- mutant particles produced from CEMx174 or H9 cells. In a single round of infection, Vif- virus is approximately 25 (from CEMx174 cells) to 100 (from H9 cells) times less infectious than wild-type virus produced from these cells or than the Vif- mutant produced from HeLa cells. Vif- virions recovered from restrictive cells, but not from permissive cells, are abnormal both in terms of morphology and viral protein content. Notably, they contain much reduced quantities of envelope proteins and altered quantities of Gag and Pol proteins. Although wild-type and Vif- virions from restrictive cells contain similar quantities of viral RNA, no viral DNA synthesis was detectable after acute infection of target cells with phenotypically Vif- virions. To examine the possible role of Vif in viral entry, attempts were made to rescue the Vif- defect in H9 cells by pseudotyping Vif+ and Vif- HIV particles with amphotropic murine leukemia virus envelope. Vif- particles produced in the presence of HIV envelope could not be propagated when pseudotyped. In contrast, when only the murine leukemia virus envelope was present, significant propagation of Vif- HIV particles could be detected. These results demonstrate that Vif is required for proper assembly of the viral particle and for efficient HIV Env-mediated infection of target cells.     

Expression of an immunogenic region of HIV by a filamentous bacteriophage vector.

Vectors derived from the Escherichia coli filamentous phage, fd-tet, expressing parts of the human immunodeficiency virus (HIV) gag genes were constructed and analyzed. The immunoreactive domain of HIV Gag antigens was produced in the form of a fusion protein, with a phage minor coat protein, called protein III, playing an important role in phage infectivity. A micropanning procedure, utilizing the strong affinity of biotinylated antibody to streptavidin, was applied for the selection of clones. A simple preparation procedure consisting of polyethyleneglycol precipitation of the recombinant phage from the E. coli supernatant allowed us to detect HIV antigens by enzyme-linked immunosorbent assay (ELISA). Cloned FUSE-gag, as isolated using anti-Gag RL4.72.1 monoclonal antibody (mAb), contained a nucleotide sequence coding for 91 amino acids of HIV Gag p24. It specifically reacted with the mAb in the ELISA. Construction of the mAb-selectable phages permitted localization of epitopes for mAb. Infectivity of the phage clone was specifically neutralized by the anti-HIV mAb. Immunoelectroblotting analysis of recombinant phages revealed the presence of an about 65-kDa band reacting with anti-HIV mAb. This Mr corresponded to the size of the fused form of the FUSE 1 protein III. Human sera from HIV-infected and uninfected individuals reacted with recombinant protein III, as well as the original form of protein III.     

Specific Interactions between Retrovirus Env and Gag Proteins in Rat Neurons

In this work we have studied the intracellular localization properties of the Gag and Env proteins of Moloney murine leukemia virus (MLV) and human immunodeficiency virus (HIV) in dorsal root ganglion (DRG) neurons of rat. These neurons form thick bundles of axons, which facilitates protein localization studies by immunofluorescence analyses. When such neuron cultures were infected with recombinant Semliki Forest virus particles carrying the gag genes of either retrovirus, the expressed Gag proteins were localized to both the somatic and the axonal regions of the DRG neurons. In contrast, the Env proteins were confined only to the somatic region. When the Gag and Env proteins were coexpressed, the Gag proteins were also excluded from the axons. This effect of the Env proteins was shown to be dependent on the concentration of the Gag proteins in the neuron and also to be specific for homologous pairs of retrovirus proteins. Therefore, the results suggest that there are specific interactions between the Env and the Gag proteins of MLV and HIV in the DRG neurons.     

Human immunodeficiency virus type 1 gag-specific mucosal immunity after oral immunization with papillomavirus pseudoviruses encoding gag

Mucosal surfaces are the primary portals for human immunodeficiency virus (HIV) transmission. Because systemic immunization, in general, does not induce effective mucosal immune responses, a mucosal HIV vaccine is urgently needed. For this study, we developed papillomavirus pseudoviruses that express HIV-1 Gag. The pseudoviruses are synthetic, nonreplicating viruses, yet they can produce antigens for a long time in the immune system. Here we show that oral immunization of mice by the use of papillomavirus pseudoviruses encoding Gag generated mucosal and systemic Gag-specific cytotoxic T lymphocytes that effectively lysed Gag-expressing target cells. Furthermore, the pseudoviruses generated Gag-specific gamma interferon-producing T cells and serum immunoglobulin G (IgG) and mucosal IgA. In contrast, oral immunization with plasmid DNA encoding HIV-1 Gag did not induce specific immune responses. Importantly, oral immunization with the pseudoviruses induced Gag-specific memory cytotoxic T lymphocytes and protected mice against a rectal mucosal challenge with a recombinant vaccinia virus expressing HIV-1 Gag. Thus, papillomavirus pseudoviruses encoding Gag are a promising mucosal vaccine against AIDS.     

The regulation of primate immunodeficiency virus infectivity by Vif is cell species restricted: a role for Vif in determining virus host range and cross-species transmission.

The primate immunodeficiency virus Vif proteins are essential for replication in appropriate cultured cell systems and, presumably, for the establishment of productive infections in vivo. We describe experiments that define patterns of complementation between human and simian immunodeficiency virus (HIV and SIV) Vif proteins and address the determinants that underlie functional specificity. Using human cells as virus producers, it was found that the HIV-1 Vif protein could modulate the infectivity of HIV-1 itself, HIV-2 and SIV isolated from African green monkeys (SIVAGM). In contrast, the Vif proteins of SIVAGM and SIV isolated from Sykes' monkeys (SIVSYK) were inactive for all HIV and SIV substrates in human cells even though, at least for the SIVAGM protein, robust activity could be demonstrated in cognate African green monkey cells. These observations suggest that species-specific interactions between Vif and virus-producing cells, as opposed to between Vif and virus components, may govern the functional consequences of Vif expression in terms of inducing virion infectivity. The finding that the replication of murine leukemia virus could also be stimulated by HIV-1 Vif expression in human cells further supported this notion. We speculate that species restrictions to Vif function may have contributed to primate immunodeficiency virus zoonosis.     

The value of primate models for studying human immunodeficiency virus pathogenesis

Abstract: Research on human immunodeficiency virus (HIV) infection is compromised by the obvious limitation in having for study only virus-infected individuals or those exposed to the virus. Steps involved in transmission or pathogenesis require planned experimentation. The identification of animal models of acquired immunodeficiency syndrome (AIDS) has therefore been helpful for evaluating phases of HIV pathogenesis. Of the seven subgenera of lentiviruses now recognized, two share the characteristics with HIV of a T cell tropism and the associated loss of CD4+ cells in the host associated with disease: the feline immunodeficiency virus (FIV) and the simian immunodeficiency virus (SIV) (Table 1). The other animal lentiviruses grow best in macrophages and their infection generally reflects clinical sequellae of infection of this cell type. This review addresses those features of SIV, HIV, and SHIV infections of non-human primates that illustrate the importance of the animal models of AIDS.