Role of HIV-2 envelope in Lv2-mediated restriction

Capsid-dependent lentiviral restrictions

Host antiviral proteins inhibit primate lentiviruses and other retroviruses by targeting many features of the viral life cycle. The lentiviral capsid protein and the assembled viral core are known to be inhibited through multiple, directly acting antiviral proteins. Several phenotypes, including those known as Lv1 through Lv5, have been described as cell type-specific blocks to infection against some but not all primate lentiviruses. Here we review important features of known capsid-targeting blocks to infection together with several blocks to infection for which the genes responsible for the inhibition still remain to be identified. We outline the features of these blocks as well as how current methodologies are now well suited to find these antiviral genes and solve these long-standing mysteries in the HIV and retrovirology fields.

The innate immune factor RPRD2/REAF and its role in the Lv2 restriction of HIV

Intracellular innate immunity involves co-evolved antiviral restriction factors that specifically inhibit infecting viruses. Studying these restrictions has increased our understanding of viral replication, host-pathogen interactions, and pathogenesis, and represent potential targets for novel antiviral therapies. Lentiviral restriction 2 (Lv2) was identified as an unmapped early-phase restriction of HIV-2 and later shown to also restrict HIV-1 and simian immunodeficiency virus. The viral determinants of Lv2 susceptibility have been mapped to the envelope and capsid proteins in both HIV-1 and HIV-2, and also viral protein R (Vpr) in HIV-1, and appears dependent on cellular entry mechanism. A genome-wide screen identified several likely contributing host factors including members of the polymerase-associated factor 1 (PAF1) and human silencing hub (HUSH) complexes, and the newly characterized regulation of nuclear pre-mRNA domain containing 2 (RPRD2). Subsequently, RPRD2 (or RNA-associated early-stage antiviral factor) has been shown to be upregulated upon T cell activation, is highly expressed in myeloid cells, binds viral reverse transcripts, and potently restricts HIV-1 infection. RPRD2 is also bound by HIV-1 Vpr and targeted for degradation by the proteasome upon reverse transcription, suggesting RPRD2 impedes reverse transcription and Vpr targeting overcomes this block. RPRD2 is mainly localized to the nucleus and binds RNA, DNA, and DNA:RNA hybrids. More recently, RPRD2 has been shown to negatively regulate genome-wide transcription and interact with the HUSH and PAF1 complexes which repress HIV transcription and are implicated in maintenance of HIV latency. In this review, we examine Lv2 restriction and the antiviral role of RPRD2 and consider potential mechanism(s) of action.

HIV-1 Accessory Protein Vpr Interacts with REAF/RPRD2 To Mitigate Its Antiviral Activity

The human immunodeficiency virus type 1 (HIV-1) accessory protein Vpr enhances viral replication in both macrophages and, to a lesser extent, cycling T cells. Virion-packaged Vpr is released in target cells shortly after entry, suggesting it is required in the early phase of infection. Previously, we described REAF (RNA-associated early-stage antiviral factor; RPRD2), a constitutively expressed protein that potently restricts HIV replication at or during reverse transcription. Here, we show that a virus without an intact vpr gene is more highly restricted by REAF and, using delivery by virus-like particles (VLPs), that Vpr alone is sufficient for REAF degradation in primary macrophages. REAF is more highly expressed in macrophages than in cycling T cells, and we detected, by coimmunoprecipitation assay, an interaction between Vpr protein and endogenous REAF. Vpr acts quickly during the early phase of replication and induces the degradation of REAF within 30 min of viral entry. Using Vpr F34I and Q65R viral mutants, we show that nuclear localization and interaction with cullin 4A-DBB1 (DCAF1) E3 ubiquitin ligase are required for REAF degradation by Vpr. In response to infection, cells upregulate REAF levels. This response is curtailed in the presence of Vpr. These findings support the hypothesis that Vpr induces the degradation of a factor, REAF, that impedes HIV infection in macrophages.IMPORTANCE For at least 30 years, it has been known that HIV-1 Vpr, a protein carried in the virion, is important for efficient infection of primary macrophages. Vpr is also a determinant of the pathogenic effects of HIV-1 in vivo A number of cellular proteins that interact with Vpr have been identified. So far, it has not been possible to associate these proteins with altered viral replication in macrophages or to explain why Vpr is carried in the virus particle. Here, we show that Vpr mitigates the antiviral effects of REAF, a protein highly expressed in primary macrophages and one that inhibits virus replication during reverse transcription. REAF is degraded by Vpr within 30 min of virus entry in a manner dependent on the nuclear localization of Vpr and its interaction with the cell's protein degradation machinery.

HIV Fusion in Dendritic Cells Occurs Mainly at the Surface and Is Limited by Low CD4 Levels

HIV-1 poorly infects monocyte-derived dendritic cells (MDDCs). This is in large part due to SAMHD1, which restricts viral reverse transcription. Pseudotyping HIV-1 with vesicular stomatitis virus G protein (VSV-G) strongly enhances infection, suggesting that earlier steps of viral replication, including fusion, are also inefficient in MDDCs. The site of HIV-1 fusion remains controversial and may depend on the cell type, with reports indicating that it occurs at the plasma membrane or, conversely, in an endocytic compartment. Here, we examined the pathways of HIV-1 entry in MDDCs. Using a combination of temperature shift and fusion inhibitors, we show that HIV-1 fusion mainly occurs at the cell surface. We then asked whether surface levels or intracellular localization of CD4 modulates HIV-1 entry. Increasing CD4 levels strongly enhanced fusion and infection with various HIV-1 isolates, including reference and transmitted/founder strains, but not with BaL, which uses low CD4 levels for entry. Overexpressing coreceptors did not facilitate viral infection. To further study the localization of fusion events, we generated CD4 mutants carrying heterologous cytoplasmic tails (LAMP1 or Toll-like receptor 7 [TLR7]) to redirect the molecule to intracellular compartments. The intracellular CD4 mutants did not facilitate HIV-1 fusion and replication in MDDCs. Fusion of an HIV-2 isolate with MDDCs was also enhanced by increasing surface CD4 levels. Our results demonstrate that MDDCs are inefficiently infected by various HIV-1 and HIV-2 strains, in part because of low CD4 levels. In these cells, viral fusion occurs mainly at the surface, and probably not after internalization.IMPORTANCE Dendritic cells (DCs) are professional antigen-presenting cells inducing innate and adaptive immune responses. DCs express the HIV receptor CD4 and are potential target cells for HIV. There is debate about the sensitivity of DCs to productive HIV-1 and HIV-2 infection. The fusion step of the viral replication cycle is inefficient in DCs, and the underlying mechanisms are poorly characterized. We show that increasing the levels of CD4 at the plasma membrane allows more HIV fusion and productive infection in DCs. We further demonstrate that HIV fusion occurs mainly at the cell surface and not in an intracellular compartment. Our results help us understand why DCs are poorly sensitive to HIV infection.

RNA-Associated Early-Stage Antiviral Factor Is a Major Component of Lv2 Restriction

Human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) replication in human cells is restricted at early postentry steps by host inhibitory factors. We previously described and characterized an early-phase restriction of HIV-1 and -2 replication in human cell lines, primary macrophages, and peripheral blood mononuclear cells. The restriction was termed lentiviral restriction 2 (Lv2). The viral determinants of Lv2 susceptibility mapped to the HIV-2 envelope (Env) and capsid (CA). We subsequently reported a whole-genome small interfering RNA screening for factors involved in HIV that identified RNA-associated early-stage antiviral factor (REAF). Using HIV-2 chimeras of susceptible and nonsusceptible viruses, we show here that REAF is a major component of the previously described Lv2 restriction. Further studies of the viral CA demonstrate that the CA mutation I73V (previously called I207V), a potent determinant for HIV-2, is a weak determinant of susceptibility for HIV-1. More potent CA determinants for HIV-1 REAF restriction were identified at P38A, N74D, G89V, and G94D. These results firmly establish that in HIV-1, CA is a strong determinant of susceptibility to Lv2/REAF. Similar to HIV-2, HIV-1 Env can rescue sensitive CAs from restriction. We conclude that REAF is a major component of the previously described Lv2 restriction.IMPORTANCE Measures taken by the host cell to combat infection drive the evolution of pathogens to counteract or sidestep them. The study of such virus-host conflicts can point to possible weaknesses in the arsenal of viruses and may lead to the rational design of antiviral agents. Here we describe our discovery that the host restriction factor REAF fulfills the same criteria previously used to describe lentiviral restriction (Lv2). We show that, like the HIV-2 CA, the CA of HIV-1 is a strong determinant of Lv2/REAF susceptibility. We illustrate how HIV counteracts Lv2/REAF by using an envelope with alternative routes of entry into cells.

HIV-2 infects resting CD4+ T cells but not monocyte-derived dendritic cells

Background

Human Immunodeficiency Virus-type 2 (HIV-2) encodes Vpx that degrades SAMHD1, a cellular restriction factor active in non-dividing cells. HIV-2 replicates in lymphocytes but the susceptibility of monocyte-derived dendritic cells (MDDCs) to in vitro infection remains partly characterized.

Results

Here, we investigated HIV-2 replication in primary CD4+ T lymphocytes, both activated and non-activated, as well as in MDDCs. We focused on the requirement of Vpx for productive HIV-2 infection, using the reference HIV-2 ROD strain, the proviral clone GL-AN, as well as two primary HIV-2 isolates. All HIV-2 strains tested replicated in activated CD4+ T cells. Unstimulated CD4+ T cells were not productively infected by HIV-2, but viral replication was triggered upon lymphocyte activation in a Vpx-dependent manner. In contrast, MDDCs were poorly infected when exposed to HIV-2. HIV-2 particles did not potently fuse with MDDCs and did not lead to efficient viral DNA synthesis, even in the presence of Vpx. Moreover, the HIV-2 strains tested were not efficiently sensed by MDDCs, as evidenced by a lack of MxA induction upon viral exposure. Virion pseudotyping with VSV-G rescued fusion, productive infection and HIV-2 sensing by MDDCs.

Conclusion

Vpx allows the non-productive infection of resting CD4+ T cells, but does not confer HIV-2 with the ability to efficiently infect MDDCs. In these cells, an entry defect prevents viral fusion and reverse transcription independently of SAMHD1. We propose that HIV-2, like HIV-1, does not productively infect MDDCs, possibly to avoid triggering an immune response mediated by these cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-014-0131-7) contains supplementary material, which is available to authorized users.

A variant macaque-tropic human immunodeficiency virus type 1 is resistant to alpha interferon-induced restriction in pig-tailed macaque CD4+ T cells

Human immunodeficiency virus type 1 (HIV-1) antagonizes innate restriction factors in order to infect and persistently replicate in a host. In a previous study, we demonstrated that HIV-1 NL4-3 with a simian immunodeficiency virus mne (SIVmne) vif gene substitution (HSIV-vif-NL4-3) could infect and replicate in pig-tailed macaques (PTM), indicating that APOBEC3 proteins are primary barriers to transmission. Because viral replication was persistent but low, we hypothesized that HSIV-vif-NL4-3 may be suppressed by type I interferons (IFN-I), which are known to upregulate the expression of innate restriction factors. Here, we demonstrate that IFN-α more potently suppresses HSIV-vif-NL4-3 in PTM CD4(+) T cells than it does pathogenic SIVmne027. Importantly, we identify a variant (HSIV-vif-Yu2) that is resistant to IFN-α, indicating that the IFN-α-induced barrier can be overcome by HSIV-vif chimeras in PTM CD4(+) T cells. Interestingly, HSIV-vif-Yu2 and HSIV-vif-NL4-3 are similarly restricted by PTM BST2/Tetherin, and neither virus downregulates it from the surface of infected PTM CD4(+) T cells. Resistance to IFN-α-induced restriction appears to be conferred by a determinant in HSIV-vif-Yu2 that includes env su. Finally, we show that the Yu-2 env su allele may overcome an IFN-α-induced barrier to entry. Together, our data demonstrate that the prototype macaque-tropic HIV-1 clones based on NL4-3 may not sufficiently antagonize innate restriction in PTM cells. However, variants with resistance to IFN-α-induced restriction factors in PTM CD4(+) T cells may enhance viral replication by overcoming a barrier early in the viral replication cycle.

A whole genome screen for HIV restriction factors

Background

Upon cellular entry retroviruses must avoid innate restriction factors produced by the host cell. For human immunodeficiency virus (HIV) human restriction factors, APOBEC3 (apolipoprotein-B-mRNA-editing-enzyme), p21 and tetherin are well characterised.

Results

To identify intrinsic resistance factors to HIV-1 replication we screened 19,121 human genes and identified 114 factors with significant inhibition of infection. Those with a known function are involved in a broad spectrum of cellular processes including receptor signalling, vesicle trafficking, transcription, apoptosis, cross-nuclear membrane transport, meiosis, DNA damage repair, ubiquitination and RNA processing. We focused on the PAF1 complex which has been previously implicated in gene transcription, cell cycle control and mRNA surveillance. Knockdown of all members of the PAF1 family of proteins enhanced HIV-1 reverse transcription and integration of provirus. Over-expression of PAF1 in host cells renders them refractory to HIV-1. Simian Immunodeficiency Viruses and HIV-2 are also restricted in PAF1 expressing cells. PAF1 is expressed in primary monocytes, macrophages and T-lymphocytes and we demonstrate strong activity in MonoMac1, a monocyte cell line.

Conclusions

We propose that the PAF1c establishes an anti-viral state to prevent infection by incoming retroviruses. This previously unrecognised mechanism of restriction could have implications for invasion of cells by any pathogen.

Cellular entry via an actin and clathrin-dependent route is required for Lv2 restriction of HIV-2

Lv2 is a human factor that restricts infection of some HIV-2 viruses after entry into particular target cells. HIV-2 MCR is highly susceptible to Lv2 whereas HIV-2 MCN is not. The block is after reverse transcription but prior to nuclear entry. The viral determinants for this restriction have been mapped to the HIV-2 envelope and the capsid genes. Our model of Lv2 restriction suggests that the route taken into a cell is important in determining whether a productive infection occurs. Here we characterised the infectious routes used by MCN and MCR using chemical compounds and molecular techniques to distinguish between potential pathways. Our results suggest that susceptible MCR can enter restrictive HeLa(CD4) cells via two pathways; a clathrin/AP2 mediated endocytic route that is sensitive to Lv2 restriction and an alternative, non-clathrin mediated route, which results in more efficient infection.

A novel envelope mediated post entry restriction of murine leukaemia virus in human cells is Ref1/TRIM5α independent

BACKGROUND

'Intrinsic' resistance to retroviral infection was first recognised with the Friend virus susceptibility gene (Fv1), which determines susceptibility to murine leukaemia virus (MLV) infection in different murine species. Similarly, the tripartite motif (TRIM) family of proteins determine lentiviral restriction in a primate host-species specific manner. For example rhesus TRIM5α (rhTRIM5α) can potently restrict HIV-1 infection while human TRIM5α (huTRIM5α) only has a mild effect on SIVmac and HIV-1 infectivity (Lv1). Human TRIM5α is able to restrict MLV-N virus replication, but is ineffective against MLV-B or MLV-NB virus infection. Lv2 restriction of some HIV-2 viruses is seen in human cells. Like Lv1, Lv2 is a post-entry restriction factor, whose viral determinants have been mapped to the viral capsid (CA). Unlike Lv1, however, Lv2 is determined by envelope (Env) in addition to CA. Here we present evidence of a novel Env determined post entry restriction to infection in human cells of pseudotyped MLV-B and MLV-NB cores.

RESULTS

We generated retroviral vectors pseudotyped with various gamma and lentiviral Envs on MLV-B and -NB CAs containing a green fluorescent protein (GFP) reporter. Flow cytometry was used to determine transduction efficiencies in NP2/CD4/CXCR4 (glioma cell line stably transduced with the HIV receptors) and HeLa/CD4 cell lines. The HeLa/CD4 cell line restricted both MLV CAs in an Env dependent manner, compared to NP2/CD4/CXCR4 cells. Quantitative polymerase chain reaction (QT-PCR) analysis of reverse transcription (RT) transcripts demonstrates that this restriction occurs at a post entry and RT level. siRNA knockdown of huTRIM5α ruled out a direct role for this cellular component in mediating this restriction. We describe a previously unobserved Env determined restriction of MLV-B and MLV-NB CAs in HeLa/CD4 cells when pseudotyped with HIV-2 and RD114 Envs, but not gibbon ape leukaemia virus (GALV), HIV-1 or Amphotrophic (Ampho) Envs.

CONCLUSIONS

Our data further demonstrate the variability of Env and CA mediated susceptibility to post entry host cell restriction. We discuss the relevance of these findings in light of the growing evidence supporting the complexities involved in innate host immunity to retroviral infection.

High natural permissivity of primary rabbit cells for HIV-1, with a virion infectivity defect in macrophages as the final replication barrier

An immunocompetent, permissive, small-animal model would be valuable for the study of human immunodeficiency virus type 1 (HIV-1) pathogenesis and for the testing of drug and vaccine candidates. However, the development of such a model has been hampered by the inability of primary rodent cells to efficiently support several steps of the HIV-1 replication cycle. Although transgenesis of the HIV receptor complex and human cyclin T1 have been beneficial, additional late-phase blocks prevent robust replication of HIV-1 in rodents and limit the range of in vivo applications. In this study, we explored the HIV-1 susceptibility of rabbit primary T cells and macrophages. Envelope-specific and coreceptor-dependent entry of HIV-1 was achieved by expressing human CD4 and CCR5. A block of HIV-1 DNA synthesis, likely mediated by TRIM5, was overcome by limited changes to the HIV-1 gag gene. Unlike with mice and rats, primary cells from rabbits supported the functions of the regulatory viral proteins Tat and Rev, Gag processing, and the release of HIV-1 particles at levels comparable to those in human cells. While HIV-1 produced by rabbit T cells was highly infectious, a macrophage-specific infectivity defect became manifest by a complex pattern of mutations in the viral genome, only part of which were deamination dependent. These results demonstrate a considerable natural HIV-1 permissivity of the rabbit species and suggest that receptor complex transgenesis combined with modifications in gag and possibly vif of HIV-1 to evade species-specific restriction factors might render lagomorphs fully permissive to infection by this pathogenic human lentivirus.

Cell context-dependent involvement of ATR in early stages of retroviral replication

Retroviral DNA integration leaves behind a single-strand DNA discontinuity at each virus:host DNA junction. It has long been proposed that cellular proteins detect and repair the integrated DNA and that failure to do so might lead to apoptotic cell death, but their identity remains unknown. PIKK family members ATM, DNA-PKcs and ATR have all been proposed to be important for HIV-1 replication, but these findings turned out to be very controversial. In order to clarify their role in retroviral replication, we analyzed the effect of pharmacological inhibitors and of a dominant-negative version of ATR on the replication of retroviruses in cell lines relevant to HIV-1 infection. Our data show that ATR and probably other PIKKs as well are involved in retroviral replication in some but not all cell lines and that ATR increases the frequency of retroviral transduction by a mechanism other than the enhancement of infected cell survival.

Recent advancements for the evaluation of anti-viral activities of natural products

Significant progress has been achieved for the development of novel anti-viral drugs in the recent years. Large numbers of these newly developed drugs belong to three groups of compounds, nucleoside analogues, thymidine kinase-dependent nucleotide analogues and specific viral enzyme inhibitors. It has been found that the natural products, like plant extract, plant-derived compounds (phytochemicals) and so on, as well as traditional medicines, like Ayurvedic, traditional Chinese medicine (TCM), Chakma medicines and so on, are the potential sources for potential and novel anti-viral drugs based on different in vitro and in vivo approaches. In this chapter some of these important approaches utilised in the drug discovery process of potential candidate(s) for anti-viral agents are being discussed. The key conclusion is that natural products are one of the most important sources of novel anti-viral agents.

Modulation of HIV-1 infectivity and cyclophilin A-dependence by Gag sequence and target cell type

Background

HIV-1 Gag proteins are essential for virion assembly and viral replication in newly infected cells. Gag proteins are also strong determinants of viral infectivity; immune escape mutations in the Gag capsid (CA) protein can markedly reduce viral fitness, and interactions of CA with host proteins such as cyclophilin A (CypA) and TRIM5α can have important effects on viral infectivity. Little information, however, is available concerning the extent that different primary Gag proteins affect HIV-1 replication in different cell types, or the impact on viral replication of differences in the expression by target cells of proteins that interact with CA. To address these questions, we compared the infectivity of recombinant HIV-1 viruses expressing Gag-protease sequences from primary isolates in different target cells in the presence or absence of agents that disrupt cyclophilin A – CA interactions and correlated these results with the viral genotype and the expression of cyclophilin A and TRIM5α by the target cells.

Results

Viral infectivity was governed by the nature of the Gag proteins in a target cell-specific fashion. The treatment of target cells with agents that disrupt CypA-CA interactions often produced biphasic dose-response curves in which viral infectivity first increased and subsequently decreased as a function of the dose used. The extent that treatment of target cells with high-dose CypA inhibitors impaired viral infectivity was dependent on several factors, including the viral genotype, the nature of the target cell, and the extent that treatment with low-dose CypA inhibitors increased viral infectivity. Neither the presence of polymorphisms in the CA CypA-binding loop, the level of expression of CypA, or the level of TRIM5α expression could, alone, explain the differences in the shape of the dose-response curves observed or the extent that high-dose CypA inhibitors reduced viral infectivity.

Conclusion

Multiple interactions between host-cell factors and Gag can strongly affect HIV-1 infectivity, and these vary according to target cell type and the origin of the Gag sequence. Two of the cellular activities involved appear to be modulated in opposite directions by CypA-CA interactions, and Gag sequences determine the intrinsic sensitivity of a given virus to each of these cellular activities.

Human cyclin T1 expression ameliorates a T-cell-specific transcriptional limitation for HIV in transgenic rats, but is not sufficient for a spreading infection of prototypic R5 HIV-1 strains ex vivo

Background

Cells derived from native rodents have limits at distinct steps of HIV replication. Rat primary CD4 T-cells, but not macrophages, display a profound transcriptional deficit that is ameliorated by transient trans-complementation with the human Tat-interacting protein Cyclin T1 (hCycT1).

Results

Here, we generated transgenic rats that selectively express hCycT1 in CD4 T-cells and macrophages. hCycT1 expression in rat T-cells boosted early HIV gene expression to levels approaching those in infected primary human T-cells. hCycT1 expression was necessary, but not sufficient, to enhance HIV transcription in T-cells from individual transgenic animals, indicating that endogenous cellular factors are critical co-regulators of HIV gene expression in rats. T-cells from hCD4/hCCR5/hCycT1-transgenic rats did not support productive infection of prototypic wild-type R5 HIV-1 strains ex vivo, suggesting one or more significant limitation in the late phase of the replication cycle in this primary rodent cell type. Remarkably, we identify a replication-competent HIV-1 GFP reporter strain (R7/3 YU-2 Env) that displays characteristics of a spreading, primarily cell-to-cell-mediated infection in primary T-cells from hCD4/hCCR5-transgenic rats. Moreover, the replication of this recombinant HIV-1 strain was significantly enhanced by hCycT1 transgenesis. The viral determinants of this so far unique replicative ability are currently unknown.

Conclusion

Thus, hCycT1 expression is beneficial to de novo HIV infection in a transgenic rat model, but additional genetic manipulations of the host or virus are required to achieve full permissivity.

Independent genesis of chimeric TRIM5-cyclophilin proteins in two primate species

The host range of retroviruses is influenced by antiviral proteins such as TRIM5, a restriction factor that recognizes and inactivates incoming retroviral capsids. Remarkably, in Owl monkeys (omk), a cyclophilin A (CypA) cDNA has been transposed into the TRIM5 locus, resulting in the expression of a TRIM5-CypA fusion protein (TRIMCyp) that restricts retroviral infection based on the retroviral capsid-binding specificity of CypA. Here, we report that the seemingly improbable genesis of TRIMCyp has, in fact, occurred twice, and pigtailed macaques (pgt) express an independently generated TRIMCyp protein. The omkTRIMCyp and pgtTRIMCyp proteins restrict infection by several lentiviruses, but their specificities are distinguishable. Surprisingly, pgtTRIMCyp cannot bind to or restrict HIV-1 capsids as a consequence of a point mutation close to the Cyp:capsid-binding interface that was acquired during or after transposition of pgtCypA. However, the same mutation confers on pgtTRIMCyp the ability to restrict FIV in the presence of cyclosporin A, a drug that normally abolishes the interaction between pgtTRIMCyp or omkTRIMCyp and lentiviral capsids. Overall, an intuitively unlikely evolutionary event has, in fact, occurred at least twice in primates and represents a striking example of convergent evolution in divergent species.

Primary T-cells from human CD4/CCR5-transgenic rats support all early steps of HIV-1 replication including integration, but display impaired viral gene expression

Background

In vivo studies on HIV-1 pathogenesis and testing of antiviral strategies have been hampered by the lack of an immunocompetent small animal model that is highly susceptible to HIV-1 infection. Since native rodents are non-permissive, we developed transgenic rats that selectively express the HIV-1 receptor complex, hCD4 and hCCR5, on relevant target cells. These animals display a transient low-level plasma viremia after HIV-1_YU-2 infection, demonstrating HIV-1 susceptibility in vivo. However, unlike macrophages, primary CD4 T-cells from double-transgenic animals fail to support viral spread ex vivo. To identify quantitative limitations or absolute blocks in this rodent species, we quantitatively assessed the efficiency of key steps in the early phase of the viral replication cycle in a side-by-side comparison in infected cell lines and primary T-cells from hCD4/hCCR5-transgenic rats and human donors.

Results

Levels of virus entry, HIV-1 cDNA synthesis, nuclear import, and integration into the host genome were shown to be remarkably similar in cell lines and, where technically accessible, in primary T-cells from both species. In contrast, a profound impairment at the level of early HIV gene expression was disclosed at the single-cell level in primary rat T-cells and most other rat-derived cells. Macrophages were a notable exception, possibly reflecting the unique transcriptional milieu in this evolutionarily conserved target cell of all lentiviruses. Importantly, transient trans-complementation by ex vivo nucleofection with the Tat-interacting protein Cyclin T1 of human origin markedly elevated HIV gene expression in primary rat T-cells.

Conclusion

This is the first study that has quantitatively determined the efficiency of consecutive steps in the HIV-1 replication cycle in infected primary HIV target cells from a candidate transgenic small animal and compared it to human cells. Unlike cells derived from mice or rabbits, rat cells complete all of the early steps in the HIV-1 replication cycle, including provirus integration in vivo, with high efficiency. A deficiency in gene expression was disclosed at the single cell level and could be counteracted by the human pTEFb transcription complex factor Cyclin T1. Collectively, these results provide the basis for the advancement of this transgenic rat model through strategies aimed at boosting HIV-1 gene expression in primary rat CD4 T-cells, including human Cyclin T1 transgenesis.

HIV-susceptible transgenic rats allow rapid preclinical testing of antiviral compounds targeting virus entry or reverse transcription

The current testing of anti-HIV drugs is hampered by the lack of a small animal that is readily available and easy to handle; can be infected systemically with HIV type 1 (HIV-1); harbors the major HIV-1 target cells in a physiological frequency, organ distribution, and activation state; and is established as a pharmacological model. Here, we explored the potential of outbred Sprague-Dawley rats that transgenically express the HIV-1 receptor complex on CD4 T cells and macrophages as a model for the preclinical evaluation of inhibitors targeting virus entry or reverse transcription. The concentrations of the peptidic fusion inhibitor enfuvirtide or the nonnucleoside reverse transcriptase inhibitor efavirenz required to inhibit HIV-1 infection of cultured primary CD4 T cells and macrophages from human CD4 and CCR5-transgenic rats differed by no more than 3-fold from those required for human reference cultures. Prophylactic treatment of double-transgenic rats with a weight-adapted pediatric dosing regimen for either enfuvirtide (s.c., twice-daily) or efavirenz (oral, once-daily) achieved a 92.5% or 98.8% reduction, respectively, of the HIV-1 cDNA load in the spleen 4 days after i.v. HIV-1 challenge. Notably, a once-daily dosing regimen for enfuvirtide resulted in a approximately 5-fold weaker inhibition of infection, unmasking the unfavorable pharmacokinetic characteristics of the synthetic peptide in the context of an efficacy trial. This work provides proof of principle that HIV-susceptible transgenic rats can allow a rapid and predictive preclinical evaluation of the inhibitory potency and of the pharmacokinetic properties of antiviral compounds targeting early steps in the HIV replication cycle.

Factors that influence VSV-G pseudotyping and transduction efficiency of lentiviral vectors—in vitro andin vivo implications

Pseudotyping viral vectors with vesicular stomatitis virus glycoprotein (VSV-G) enables the transduction of an extensive range of cell types from different species. We have discovered two important parameters of the VSV-G-pseudotyping phenomenon that relate directly to the transduction potential of lentiviral vectors: (1) the glycosylation status of VSV-G, and (2) the quantity of glycoprotein associated with virions. We measured production-cell and virion-associated quantities of two isoform variants of VSV-G, which differ in their glycosylation status, VSV-G1 and VSV-G2, and assessed the impact of this difference on the efficiency of mammalian cell transduction by lentiviral vectors. The glycosylation of VSV-G at N336 allowed greater maximal expression of VSV-G in HEK293T cells, thus facilitating vector pseudotyping. The transduction of primate cell lines was substantially affected (up to 50-fold) by the degree of VSV-G1 or VSV-G2 incorporation, whereas other cell lines, such as D17 (canine), were less sensitive to virion-associated VSV-G1/2 quantities. These data indicate that the minimum required concentration of virion-associated VSV-G differs substantially between cell species/types. The implications of these data with regard to VSV-G-pseudotyped vector production, titration, and use in host-cell restriction studies, are discussed.

Human TRIM5alpha mediated restriction of different HIV-1 subtypes and Lv2 sensitive and insensitive HIV-2 variants

In order to characterize the antiviral activity of human TRIM5α in more detail human derived indicator cell lines over expressing wild type human TRIM5α were generated and challenged with HIV-1 and HIV-2 viruses pseudotyped with HIV envelope proteins in comparison to VSV-G pseudotyped particles. HIV envelope protein pseudotyped particles (HIV-1[NL4.3], HIV-1[BaL]) showed a similar restriction to infection (12 fold inhibition) compared to VSV-G pseudotyped viruses after challenging TZM-huTRIM5α cells. For HIV-2 a stronger restriction to infection was observed when the homologous envelope protein Env42S was pseudotyped onto these particles compared to VSV-G pseudotyped HIV-2 particles (8.6 fold inhibition versus 3.4 fold inhibition). It has been shown that HIV-2 is restricted by the restriction factor Lv2, acting on capsid like TRIM5α. A mutation of amino acid 73 (I73V) of HIV-2 capsid renders this virus Lv2-insensitive. Lv2-insensitive VSV-G pseudotyped HIV-2/I73V particles showed a similar restriction to infection as did HIV-2[VSV-G] particles (4 fold inhibition). HIV-2 envelope protein (Env42S)-pseudotyped HIV-2/I73V particles revealed a 9.3 fold increase in infection in TZM cells but remained restricted in TZM-huTRIM5α cells (80.6 fold inhibition) clearly indicating that at least two restriction factors, TRIM5α and Lv2, act on incoming HIV-2 particles. Further challenge experiments using primary isolates from different HIV-1 subtypes and from HIV-1 group O showed that wild type human TRIM5α restricted infection independent of coreceptor use of the infecting particle but to variable degrees (between 1.2 and 19.6 fold restriction).

Mapping the domains of CD134 as a functional receptor for feline immunodeficiency virus

The feline homologue of CD134 is the primary binding receptor for feline immunodeficiency virus (FIV), targeting the virus preferentially to activated CD4+ helper T cells. However, strains of FIV differ in utilization of CD134; the prototypic strain PPR requires a minimal determinant in the first cysteine-rich domain (CRD1) of feline CD134 to confer near-optimal receptor function, while strains such as GL8 require additional determinants in the CD134 CRD2. We map this determinant to a loop in CRD2 governing the interaction between the receptor and its ligand; the amino acid substitutions S78N-S79Y-K80E restored full viral receptor activity to the CDR2 of human CD134 in the context of feline CD134, with tyrosine-79 appearing to be the critical residue for restoration of receptor function.

Antiretroviral potential of human tripartite motif-5 and related proteins

TRIM5alpha is a potent inhibitor of infection by diverse retroviruses and is encoded by one of a large family of TRIM genes. We found that several TRIM motifs among a panel of selected human TRIM proteins (TRIM1, 5, 6, 18, 19, 21 22, 34) could inhibit infection when artificially targeted to an incoming HIV-1 capsid. Conversely, when ectopically expressed as authentic full-length proteins, most lacked activity against a panel of retroviruses. The exceptions were TRIM1, TRIM5 and TRIM34 proteins. Weak but specific inhibition of HIV-2/SIV(MAC) and EIAV by TRIM34 was noted, and human TRIM5alpha modestly, but specifically, inhibited an HIV-1 strain carrying a mutation in the cyclophilin binding loop (G89V). Restriction activity observed in ectopic expression assays was sometimes not detectable in corresponding RNAi-based knockdown experiments. However, endogenous owl monkey TRIMCyp potently inhibited an SIV(AGM) strain. Overall, sporadic examples of intrinsic antiretroviral activity exist in this panel of TRIM proteins.

Determinants of human immunodeficiency virus type 1 resistance to membrane-anchored gp41-derived peptides

The expression of a membrane-anchored gp41-derived peptide (M87) has been shown to confer protection from infection through human immunodeficiency virus type 1 (HIV-1) (Hildinger et al., J. Virol. 75:3038-3042, 2001). In an effort to characterize the mechanism of action of this membrane-anchored peptide in comparison to the soluble peptide T-20, we selected resistant variants of HIV-1(NL4-3) and HIV-1(BaL) by serial virus passage using PM1 cells stably expressing peptide M87. Sequence analysis of the resistant isolates showed different patterns of selected point mutations in heptad repeat regions 1 and 2 (HR1 and HR2, respectively) for the two viruses analyzed. For HIV-1(NL4-3) a single amino acid change at position 33 in HR1 (L33S) was selected, whereas for HIV-1(BaL) the majority of the sequences obtained showed two amino acid changes, one in HR1 and one in HR2 (I48V/N126K). In both selections the most important contiguous 3-amino-acid sequence, GIV, within HR1, associated with resistance to soluble T-20, was not changed. Site-directed mutagenesis studies confirmed the importance of the characterized point mutations to confer resistance to M87 as well as to soluble T-20 and T-649. Replication capacity and dual-color competition assays revealed that the double mutation I48V/N126K in HIV-1(BaL) results in a strong reduction of viral fitness, whereas the L33S mutation in HIV-1(NL4-3) did enhance viral fitness compared to the respective parental viruses. However, the selected point mutations did not confer resistance to the more recently described optimized membrane-anchored fusion inhibitor M87o (Egelhofer et al., J. Virol. 78:568-575, 2004), strengthening the importance of this novel antiviral concept for gene therapy approaches.

An envelope-determined, pH-independent endocytic route of viral entry determines the susceptibility of human immunodeficiency virus type 1 (HIV-1) and HIV-2 to Lv2 restriction

We identified a postentry restriction, termed Lv2, which determines the cellular tropism of two related human immunodeficiency virus type 2 (HIV-2) isolates and is dependent on the sequence of the capsid (CA) and envelope (Env) proteins. To explain the reliance on both CA and Env, we proposed that restrictive Envs deliver susceptible capsids to a compartment where Lv2 is active whereas nonrestrictive Envs deliver capsids into a compartment where Lv2 is either absent or less active. To test this model, we used compounds that affect endocytic pathways (ammonium chloride, bafilomycin A1, hypertonic sucrose) or lipid rafts (methyl-beta-cyclodextrin) to treat restrictive cells and show that restricted virus can be rescued from Lv2 if a lipid-raft-dependent, pH-independent endocytic pathway is inhibited. Furthermore, viral entry into HeLa/CD4 cells containing a tailless CD4 receptor, located outside lipid rafts, was fully permissive. Finally, we show that a variety of primary HIV-1 and HIV-2 viruses are susceptible to Lv2. Thus, we show that the route of entry, determined by the viral envelope, can influence cellular tropism by avoiding intracellular blocks to infection.