A block to human immunodeficiency virus type 1 assembly in murine cells

Retroviral hijacking of host transport pathways for genome nuclear export

Recent advances in the study of virus-cell interactions have improved our understanding of how viruses that replicate their genomes in the nucleus (e.g., retroviruses, hepadnaviruses, herpesviruses, and a subset of RNA viruses) hijack cellular pathways to export these genomes to the cytoplasm where they access virion egress pathways. These findings shed light on novel aspects of viral life cycles relevant to the development of new antiviral strategies and can yield new tractable, virus-based tools for exposing additional secrets of the cell. The goal of this review is to summarize defined and emerging modes of virus-host interactions that drive the transit of viral genomes out of the nucleus across the nuclear envelope barrier, with an emphasis on retroviruses that are most extensively studied. In this context, we prioritize discussion of recent progress in understanding the trafficking and function of the human immunodeficiency virus type 1 Rev protein, exemplifying a relatively refined example of stepwise, cooperativity-driven viral subversion of multi-subunit host transport receptor complexes.

Exploiting a rodent cell block for intrinsic resistance to HIV-1 gene expression in human T cells

IMPORTANCE

Unlike humans, mice are unable to support HIV-1 infection. This is due, in part, to a constellation of defined minor, species-specific differences in conserved host proteins needed for viral gene expression. Here, we used precision CRISPR/Cas9 gene editing to engineer a "mousified" version of one such host protein, cyclin T1 (CCNT1), in human T cells. CCNT1 is essential for efficient HIV-1 transcription, making it an intriguing target for gene-based inactivation of virus replication. We show that isogenic cell lines engineered to encode CCNT1 bearing a single mouse-informed amino acid change (tyrosine in place of cysteine at position 261) exhibit potent, durable, and broad-spectrum resistance to HIV-1 and other pathogenic lentiviruses, and with no discernible impact on host cell biology. These results provide proof of concept for targeting CCNT1 in the context of one or more functional HIV-1 cure strategies.

Zebrafish-based platform for emerging bio-contaminants and virus inactivation research

Emerging bio-contaminants such as viruses have affected health and environment settings of every country. Viruses are the minuscule entities resulting in severe contagious diseases like SARS, MERS, Ebola, and avian influenza. Recent epidemic like the SARS-CoV-2, the virus has undergone mutations strengthen them and allowing to escape from the remedies. Comprehensive knowledge of viruses is essential for the development of targeted therapeutic and vaccination treatments. Animal models mimicking human biology like non-human primates, rats, mice, and rabbits offer competitive advantage to assess risk of viral infections, chemical toxins, nanoparticles, and microbes. However, their economic maintenance has always been an issue. Furthermore, the redundancy of experimental results due to aforementioned aspects is also in examine. Hence, exploration for the alternative animal models is crucial for risk assessments. The current review examines zebrafish traits and explores the possibilities to monitor emerging bio-contaminants. Additionally, a comprehensive picture of the bio contaminant and virus particle invasion and abatement mechanisms in zebrafish and human cells is presented. Moreover, a zebrafish model to investigate the emerging viruses such as coronaviridae and poxviridae has been suggested.

Rhabdo-immunodeficiency virus, a murine model of acute HIV-1 infection

Numerous challenges have impeded HIV-1 vaccine development. Among these is the lack of a convenient small animal model in which to study antibody elicitation and efficacy. We describe a chimeric Rhabdo-Immunodeficiency virus (RhIV) murine model that recapitulates key features of HIV-1 entry, tropism and antibody sensitivity. RhIVs are based on vesicular stomatitis viruses (VSV), but viral entry is mediated by HIV-1 Env proteins from diverse HIV-1 strains. RhIV infection of transgenic mice expressing human CD4 and CCR5, exclusively on mouse CD4+ cells, at levels mimicking those on human CD4+ T-cells, resulted in acute, resolving viremia and CD4+ T-cell depletion. RhIV infection elicited protective immunity, and antibodies to HIV-1 Env that were primarily non-neutralizing and had modest protective efficacy following passive transfer. The RhIV model enables the convenient in vivo study of HIV-1 Env-receptor interactions, antiviral activity of antibodies and humoral responses against HIV-1 Env, in a genetically manipulatable host.

HIV-1 Vif's Capacity To Manipulate the Cell Cycle Is Species Specific

Cells derived from mice and other rodents exhibit profound blocks to HIV-1 virion production, reflecting species-specific incompatibilities between viral Tat and Rev proteins and essential host factors cyclin T1 (CCNT1) and exportin-1 (XPO1, also known as CRM1), respectively. To determine if mouse cell blocks other than CCNT1 and XPO1 affect HIV's postintegration stages, we studied HIV-1NL4-3 gene expression in mouse NIH 3T3 cells modified to constitutively express HIV-1-compatible versions of CCNT1 and XPO1 (3T3.CX cells). 3T3.CX cells supported both Rev-independent and Rev-dependent viral gene expression and produced relatively robust levels of virus particles, confirming that CCNT1 and XPO1 represent the predominant blocks to these stages. Unexpectedly, however, 3T3.CX cells were remarkably resistant to virus-induced cytopathic effects observed in human cell lines, which we mapped to the viral protein Vif and its apparent species-specific capacity to induce G2/M cell cycle arrest. Vif was able to mediate rapid degradation of human APOBEC3G and the PPP2R5D regulatory B56 subunit of the PP2A phosphatase holoenzyme in mouse cells, thus demonstrating that VifNL4-3's modulation of the cell cycle can be functionally uncoupled from some of its other defined roles in CUL5-dependent protein degradation. Vif was also unable to induce G2/M cell cycle arrest in other nonhuman cell types, including cells derived from nonhuman primates, leading us to propose that one or more human-specific cofactors underpin Vif's ability to modulate the cell cycle.IMPORTANCE Cells derived from mice and other rodents exhibit profound blocks to HIV-1 replication, thus hindering the development of a low-cost small-animal model for studying HIV/AIDS. Here, we engineered otherwise-nonpermissive mouse cells to express HIV-1-compatible versions of two species-specific host dependency factors, cyclin T1 (CCNT1) and exportin-1 (XPO1) (3T3.CX cells). We show that 3T3.CX cells rescue HIV-1 particle production but, unexpectedly, are completely resistant to virus-induced cytopathic effects. We mapped these effects to the viral accessory protein Vif, which induces a prolonged G2/M cell cycle arrest followed by apoptosis in human cells. Combined, our results indicate that one or more additional human-specific cofactors govern HIV-1's capacity to modulate the cell cycle, with potential relevance to viral pathogenesis in people and existing animal models.

Progress in HIV-1 antibody research using humanized mice

PURPOSE OF REVIEW

Recent discoveries of highly potent broadly HIV-1 neutralizing antibodies provide new opportunities to successfully prevent, treat, and potentially cure HIV-1 infection. To test their activity in vivo, humanized mice have been shown to be a powerful model and were used to investigate antibody-mediated prevention and therapy approaches. In this review, we will summarize recent findings in humanized mice that have informed on the potential use of broadly neutralizing antibodies targeting HIV-1 in humans.

RECENT FINDINGS

Humanized mouse models have been used to demonstrate the antiviral efficacy of HIV-1 neutralizing antibodies in vivo. It has been shown that a combination of antibodies can suppress viremia below the limit of detection and targets the HIV-1 reservoir. Moreover, passively administered antibodies and vector-mediated antibody production protect humanized mice from HIV-1 infection. Finally, immunization studies in knock-in/transgenic mice carrying human antibody gene segments have informed on potential vaccination strategies to induce broad and potent HIV-1 neutralizing antibodies.

SUMMARY

Humanized mouse models are of great value for HIV-1 research. They represent a highly versatile in vivo system to investigate novel approaches for HIV-1 prevention and therapy and expedite the critical translation from basic findings to clinical application.

HIV-1 susceptibility of transgenic rat-derived primary macrophage/T cells and a T cell line that express human receptors, CyclinT1 and CRM1 genes

We developed transgenic (Tg) rats that express human CD4, CCR5, CXCR4, CyclinT1, and CRM1 genes. Tg rat macrophages were efficiently infected with HIV-1 and supported production of infectious progeny virus. By contrast, both rat primary CD4+ T cells and established T cell lines expressing human CD4, CCR5, CyclinT1, and CRM1 genes were infected inefficiently, but this was ameliorated by inhibition of cyclophilin A. The infectivity of rat T cell-derived virus was lower than that of human T cell-derived virus.

The Life-Cycle of the HIV-1 Gag-RNA Complex

Human immunodeficiency virus type 1 (HIV-1) replication is a highly regulated process requiring the recruitment of viral and cellular components to the plasma membrane for assembly into infectious particles. This review highlights the recent process of understanding the selection of the genomic RNA (gRNA) by the viral Pr55^Gag precursor polyprotein, and the processes leading to its incorporation into viral particles.

Coordination of Genomic RNA Packaging with Viral Assembly in HIV-1

The tremendous progress made in unraveling the complexities of human immunodeficiency virus (HIV) replication has resulted in a library of drugs to target key aspects of the replication cycle of the virus. Yet, despite this accumulated wealth of knowledge, we still have much to learn about certain viral processes. One of these is virus assembly, where the viral genome and proteins come together to form infectious progeny. Here we review this topic from the perspective of how the route to production of an infectious virion is orchestrated by the viral genome, and we compare and contrast aspects of the assembly mechanisms employed by HIV-1 with those of other RNA viruses.

Improving combination antiretroviral therapy by targeting HIV-1 gene transcription

INTRODUCTION

Combination Antiretroviral Therapy (cART) has not allowed the cure of HIV. The main obstacle to HIV eradication is the existence of quiescent reservoirs. Several other limitations of cART have been described, such as strict life-long treatment and high costs, restricting it to Western countries, as well as the development of multidrug resistance. Given these limitations and the impetus to find a cure, the development of new treatments is necessary. Areas covered: In this review, we discuss the current status of several efficient molecules able to suppress HIV gene transcription, including NF-kB and Tat inhibitors. We also assess the potential of new proteins belonging to the intriguing DING family, which have been reported to have potential anti-HIV-1 activity by inhibiting HIV gene transcription. Expert opinion: Targeting HIV-1 gene transcription is an alternative approach, which could overcome cART-related issues, such as the emergence of multidrug resistance. Improving cART will rely on the identification and characterization of new actors inhibiting HIV-1 transcription. Combining such efforts with the use of new technologies, the development of new models for preclinical studies, and improvement in drug delivery will considerably reduce drug toxicity and thus increase patient adherence.

HIV-1 and M-PMV RNA Nuclear Export Elements Program Viral Genomes for Distinct Cytoplasmic Trafficking Behaviors

Retroviruses encode cis-acting RNA nuclear export elements that override nuclear retention of intron-containing viral mRNAs including the full-length, unspliced genomic RNAs (gRNAs) packaged into assembling virions. The HIV-1 Rev-response element (RRE) recruits the cellular nuclear export receptor CRM1 (also known as exportin-1/XPO1) using the viral protein Rev, while simple retroviruses encode constitutive transport elements (CTEs) that directly recruit components of the NXF1(Tap)/NXT1(p15) mRNA nuclear export machinery. How gRNA nuclear export is linked to trafficking machineries in the cytoplasm upstream of virus particle assembly is unknown. Here we used long-term (>24 h), multicolor live cell imaging to directly visualize HIV-1 gRNA nuclear export, translation, cytoplasmic trafficking, and virus particle production in single cells. We show that the HIV-1 RRE regulates unique, en masse, Rev- and CRM1-dependent "burst-like" transitions of mRNAs from the nucleus to flood the cytoplasm in a non-localized fashion. By contrast, the CTE derived from Mason-Pfizer monkey virus (M-PMV) links gRNAs to microtubules in the cytoplasm, driving them to cluster markedly to the centrosome that forms the pericentriolar core of the microtubule-organizing center (MTOC). Adding each export element to selected heterologous mRNAs was sufficient to confer each distinct export behavior, as was directing Rev/CRM1 or NXF1/NXT1 transport modules to mRNAs using a site-specific RNA tethering strategy. Moreover, multiple CTEs per transcript enhanced MTOC targeting, suggesting that a cooperative mechanism links NXF1/NXT1 to microtubules. Combined, these results reveal striking, unexpected features of retroviral gRNA nucleocytoplasmic transport and demonstrate roles for mRNA export elements that extend beyond nuclear pores to impact gRNA distribution in the cytoplasm.

RNA helicase MOV10 functions as a co-factor of HIV-1 Rev to facilitate Rev/RRE-dependent nuclear export of viral mRNAs

Human immunodeficiency virus type 1 (HIV-1) exploits multiple host factors during its replication. The REV/RRE-dependent nuclear export of unspliced/partially spliced viral transcripts needs the assistance of host proteins. Recent studies have shown that MOV10 overexpression inhibited HIV-1 replication at various steps. However, the endogenous MOV10 was required in certain step(s) of HIV-1 replication. In this report, we found that MOV10 potently enhances the nuclear export of viral mRNAs and subsequently increases the expression of Gag protein and other late products through affecting the Rev/RRE axis. The co-immunoprecipitation analysis indicated that MOV10 interacts with Rev in an RNA-independent manner. The DEAG-box of MOV10 was required for the enhancement of Rev/RRE-dependent nuclear export and the DEAG-box mutant showed a dominant-negative activity. Our data propose that HIV-1 utilizes the anti-viral factor MOV10 to function as a co-factor of Rev and demonstrate the complicated effects of MOV10 on HIV-1 life cycle.

Postentry restriction of Mason-Pfizer monkey virus in mouse cells

Mason-Pfizer monkey virus (M-PMV) is a prototypical betaretrovirus responsible for simian AIDS (SAIDS) in rhesus macaques. It has been shown previously that mouse cells are resistant to infection by HIV-1 and other primate lentiviruses. However, the susceptibility of mouse cells to primate retroviruses such as M-PMV remains unexplored. In the present study, using single-round green fluorescent protein (GFP) reporter viruses, we showed that various mouse cell lines are unable to support the early stages of M-PMV replication. The block to infection occurs postentry and is independent of the viral envelope. Using quantitative real-time PCR, we showed that the block to infection occurs after reverse transcription but before formation of circular DNA or proviral DNA. Finally, we showed that the M-PMV block in mouse cells is not attributable to the previously characterized mouse restriction factor Fv1. Overall, these findings suggest that mouse cells exhibit a previously uncharacterized block to M-PMV infection.

IMPORTANCE

Here we document a novel postentry restriction to M-PMV infection in mouse cells. The block occurs after reverse transcription but before the formation of circular or proviral DNA and is independent of the previous characterized mouse restriction factor Fv1.

Life of psi: how full-length HIV-1 RNAs become packaged genomes in the viral particles

As a member of the retrovirus family, HIV-1 packages its RNA genome into particles and replicates through a DNA intermediate that integrates into the host cellular genome. The multiple genes encoded by HIV-1 are expressed from the same promoter and their expression is regulated by splicing and ribosomal frameshift. The full-length HIV-1 RNA plays a central role in viral replication as it serves as the genome in the progeny virus and is used as the template for Gag and GagPol translation. In this review, we summarize findings that contribute to our current understanding of how full-length RNA is expressed and transported, cis- and trans-acting elements important for RNA packaging, the locations and timing of RNA:RNA and RNA:Gag interactions, and the processes required for this RNA to be packaged into viral particles.

Molecular events accompanying rous sarcoma virus rescue from rodent cells and the role of viral gene complementation

Transformation of rodent cells with avian Rous sarcoma virus (RSV) opened new ways to studying virus integration and expression in nonpermissive cells. We were interested in (i) the molecular changes accompanying fusion of RSV-transformed mammalian cells with avian cells leading to virus rescue and (ii) enhancement of this process by retroviral gene products. The RSV-transformed hamster RSCh cell line was characterized as producing only a marginal amount of env mRNA, no envelope glycoprotein, and a small amount of unprocessed Gag protein. Egress of viral unspliced genomic RNA from the nucleus was hampered, and its stability decreased. Cell fusion of the chicken DF-1 cell line with RSCh cells led to production of env mRNA, envelope glycoprotein, and processed Gag and virus-like particle formation. Proteosynthesis inhibition in DF-1 cells suppressed steps leading to virus rescue. Furthermore, new aberrantly spliced env mRNA species were found in the RSCh cells. Finally, we demonstrated that virus rescue efficiency can be significantly increased by complementation with the env gene and the highly expressed gag gene and can be increased the most by a helper virus infection. In summary, Env and Gag synthesis is increased after RSV-transformed hamster cell fusion with chicken fibroblasts, and both proteins provided in trans enhance RSV rescue. We conclude that the chicken fibroblast yields some factor(s) needed for RSV replication, particularly Env and Gag synthesis, in nonpermissive rodent cells.

IMPORTANCE

One of the important issues in retrovirus heterotransmission is related to cellular factors that prevent virus replication. Rous sarcoma virus (RSV), a member of the avian sarcoma and leukosis family of retroviruses, is able to infect and transform mammalian cells; however, such transformed cells do not produce infectious virus particles. Using the well-defined model of RSV-transformed rodent cells, we established that the lack of virus replication is due to the absence of chicken factor(s), which can be supplemented by cell fusion. Cell fusion with permissive chicken cells led to an increase in RNA splicing and nuclear export of specific viral mRNAs, as well as synthesis of respective viral proteins and production of virus-like particles. RSV rescue by cell fusion can be potentiated by in trans expression of viral genes in chicken cells. We conclude that rodent cells lack some chicken factor(s) required for proper viral RNA processing and viral protein synthesis.

Structure and dynamics of the gp120 V3 loop that confers noncompetitive resistance in R5 HIV-1(JR-FL) to maraviroc

Maraviroc, an (HIV-1) entry inhibitor, binds to CCR5 and efficiently prevents R5 human immunodeficiency virus type 1 (HIV-1) from using CCR5 as a coreceptor for entry into CD4(+) cells. However, HIV-1 can elude maraviroc by using the drug-bound form of CCR5 as a coreceptor. This property is known as noncompetitive resistance. HIV-1(V3-M5) derived from HIV-1(JR-FLan) is a noncompetitive-resistant virus that contains five mutations (I304V/F312W/T314A/E317D/I318V) in the gp120 V3 loop alone. To obtain genetic and structural insights into maraviroc resistance in HIV-1, we performed here mutagenesis and computer-assisted structural study. A series of site-directed mutagenesis experiments demonstrated that combinations of V3 mutations are required for HIV-1(JR-FLan) to replicate in the presence of 1 µM maraviroc, and that a T199K mutation in the C2 region increases viral fitness in combination with V3 mutations. Molecular dynamic (MD) simulations of the gp120 outer domain V3 loop with or without the five mutations showed that the V3 mutations induced (i) changes in V3 configuration on the gp120 outer domain, (ii) reduction of an anti-parallel β-sheet in the V3 stem region, (iii) reduction in fluctuations of the V3 tip and stem regions, and (iv) a shift of the fluctuation site at the V3 base region. These results suggest that the HIV-1 gp120 V3 mutations that confer maraviroc resistance alter structure and dynamics of the V3 loop on the gp120 outer domain, and enable interactions between gp120 and the drug-bound form of CCR5.

Mice transgenic for CD4-specific human CD4, CCR5 and cyclin T1 expression: a new model for investigating HIV-1 transmission and treatment efficacy

Mice cannot be used to evaluate HIV-1 therapeutics and vaccines because they are not infectible by HIV-1 due to structural differences between several human and mouse proteins required for HIV-1 entry and replication including CD4, CCR5 and cyclin T1. We overcame this limitation by constructing mice with CD4 enhancer/promoter-regulated human CD4, CCR5 and cyclin T1 genes integrated as tightly linked transgenes (hCD4/R5/cT1 mice) promoting their efficient co-transmission and enabling the murine CD4-expressing cells to support HIV-1 entry and Tat-mediated LTR transcription. All of the hCD4/R5/cT1 mice developed disseminated infection of tissues that included the spleen, small intestine, lymph nodes and lungs after intravenous injection with an HIV-1 infectious molecular clone (HIV-IMC) expressing Renilla reniformis luciferase (LucR). Furthermore, localized infection of cervical-vaginal mucosal leukocytes developed after intravaginal inoculation of hCD4/R5/cT1 mice with the LucR-expressing HIV-IMC. hCD4/R5/cT1 mice reproducibly developed in vivo infection after inoculation with LucR-expressing HIV-IMC which could be bioluminescently quantified and visualized with a high sensitivity and specificity which enabled them to be used to evaluate the efficacy of HIV-1 therapeutics. Treatment with highly active anti-retroviral therapy or one dose of VRC01, a broadly neutralizing anti-HIV-1 antibody, almost completed inhibited acute systemic HIV-1 infection of the hCD4/R5/cT1 mice. hCD4/R5/cT1 mice could also be used to evaluate the capacity of therapies delivered by gene therapy to inhibit in vivo HIV infection. VRC01 secreted in vivo by primary B cells transduced with a VRC01-encoding lentivirus transplanted into hCD4/R5/cT1 mice markedly inhibited infection after intravenous challenge with LucR-expressing HIV-IMC. The reproducible infection of CD4/R5/cT1 mice with LucR-expressing HIV-IMC after intravenous or mucosal inoculation combined with the availability of LucR-expressing HIV-IMC expressing transmitted/founder and clade A/E and C Envs will provide researchers with a highly accessible pre-clinical in vivo HIV-1-infection model to study HIV-1 acquisition, treatment, and prevention.

Animal models for HIV/AIDS research

The AIDS pandemic continues to present us with unique scientific and public health challenges. Although the development of effective antiretroviral therapy has been a major triumph, the emergence of drug resistance requires active management of treatment regimens and the continued development of new antiretroviral drugs. Moreover, despite nearly 30 years of intensive investigation, we still lack the basic scientific knowledge necessary to produce a safe and effective vaccine against HIV-1. Animal models offer obvious advantages in the study of HIV/AIDS, allowing for a more invasive investigation of the disease and for preclinical testing of drugs and vaccines. Advances in humanized mouse models, non-human primate immunogenetics and recombinant challenge viruses have greatly increased the number and sophistication of available mouse and simian models. Understanding the advantages and limitations of each of these models is essential for the design of animal studies to guide the development of vaccines and antiretroviral therapies for the prevention and treatment of HIV-1 infection.

Intrapulmonary Versus Nasal Transduction of Murine Airways With GP64-pseudotyped Viral Vectors

Persistent viral vector-mediated transgene expression in the airways requires delivery to cells with progenitor capacity and avoidance of immune responses. Previously, we observed that GP64-pseudotyped feline immunodeficiency virus (FIV)-mediated gene transfer was more efficient in the nasal airways than the large airways of the murine lung. We hypothesized that in vivo gene transfer was limited by immunological and physiological barriers in the murine intrapulmonary airways. Here, we systematically investigate multiple potential barriers to lentiviral gene transfer in the airways of mice. We show that GP64-FIV vector transduced primary cultures of well-differentiated murine nasal epithelia with greater efficiency than primary cultures of murine tracheal epithelia. We further demonstrate that neutrophils, type I interferon (IFN) responses, as well as T and B lymphocytes are not the major factors limiting the transduction of murine conducting airways. In addition, we observed better transduction of GP64-pseudotyped vesicular stomatitis virus (VSV) in the nasal epithelia compared with the intrapulmonary airways in mice. VSVG glycoprotein pseudotyped VSV transduced intrapulmonary epithelia with similar efficiency as nasal epithelia. Our results suggest that the differential transduction efficiency of nasal versus intrapulmonary airways by FIV vector is not a result of immunological barriers or surface area, but rather differential expression of cellular factors specific for FIV vector transduction.

Human CRM1 augments production of infectious human and feline immunodeficiency viruses from murine cells

Productive replication of human immunodeficiency virus type 1 (HIV-1) occurs efficiently only in humans. The posttranscriptional stages of the HIV-1 life cycle proceed poorly in mouse cells, with a resulting defect in viral assembly and release. Previous work has shown that the presence of human chromosome 2 increases HIV-1 production in mouse cells. Recent studies have shown that human chromosome region maintenance 1 (hCRM1) stimulates Gag release from rodent cells. Here we report that expressions of hCRM1 in murine cells resulted in marked increases in the production of infectious HIV-1 and feline immunodeficiency virus (FIV). HIV-1 production was also increased by hSRp40, and a combination of hCRM1 and hSRp40 resulted in a more-than-additive effect on HIV-1 release. In contrast, the overexpression of mouse CRM1 (mCRM1) minimally affected HIV-1 and FIV production and did not antagonize hCRM1. In the presence of hCRM1 there were large increases in the amounts of released capsid, which paralleled the increases in the infectious titers. Consistent with this finding, the ratios of unspliced to spliced HIV-1 mRNAs in mouse cells expressing hCRM1 and SRp40 became similar to those of human cells. Furthermore, imaging of intron-containing FIV RNA showed that hCRM1 increased RNA export to the cytoplasm.By testing chimeras between mCRM1 and hCRM1 and comparing those sequences to feline CRM1, we mapped the functional domain to HEAT (Huntingtin, elongation factor 3, protein phosphatase 2A, and the yeast kinase TOR1) repeats 4A to 9A and a triple point mutant in repeat 9A, which showed a loss of function. Structural analysis suggested that this region of hCRM1 may serve as a binding site for viral or cellular factors to facilitate lentiviral RNA nuclear export.

Animal models in virus research: their utility and limitations

Viral diseases are important threats to public health worldwide. With the number of emerging viral diseases increasing the last decades, there is a growing need for appropriate animal models for virus studies. The relevance of animal models can be limited in terms of mimicking human pathophysiology. In this review, we discuss the utility of animal models for studies of influenza A viruses, HIV and SARS-CoV in light of viral emergence, assessment of infection and transmission risks, and regulatory decision making. We address their relevance and limitations. The susceptibility, immune responses, pathogenesis, and pharmacokinetics may differ between the various animal models. These complexities may thwart translating results from animal experiments to the humans. Within these constraints, animal models are very informative for studying virus immunopathology and transmission modes and for translation of virus research into clinical benefit. Insight in the limitations of the various models may facilitate further improvements of the models.

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.

Production and use of HIV-1 luciferase reporter viruses

This unit describes the production and use of HIV-1 luciferase reporter viruses. These viruses are used to rapidly and accurately quantify HIV-1 in cell culture. Protocols are also presented for screening and characterization of small molecule inhibitors of HIV-1 entry into cells.

Productive replication and evolution of HIV-1 in ferret cells

A rodent or other small animal model for HIV-1 has not been forthcoming, with the principal obstacles being species-specific restriction mechanisms and deficits in HIV-1 dependency factors. Some Carnivorans may harbor comparatively fewer impediments. For example, in contrast to mice, the domestic cat genome encodes essential nonreceptor HIV-1 dependency factors. All Feliformia species and at least one Caniformia species also lack a major lentiviral restriction mechanism (TRIM5α/TRIMCyp proteins). Here we investigated cells from two species in another carnivore family, the Mustelidae, for permissiveness to the HIV-1 life cycle. Mustela putorius furo (domesticated ferret) primary cells and cell lines did not restrict HIV-1, feline immunodeficiency virus (FIV), equine infectious anemia virus (EIAV), or N-tropic murine leukemia virus (MLV) postentry and supported late HIV-1 life cycle steps comparably to human cells. The ferret TRIM5α gene exon 8, which encodes the B30.2 domain, was found to be pseudogenized. Strikingly, ferret (but not mink) cells engineered to express human HIV-1 entry receptors supported productive spreading replication, amplification, and serial passage of wild-type HIV-1. Nevertheless, produced virions had relatively reduced infectivity and the virus accrued G→A hypermutations, consistent with APOBEC3 protein pressure. Ferret cell-passaged HIV-1 also evolved amino acid changes in the capsid cyclophilin A binding loop. We conclude that the genome of this carnivore can provide essential nonreceptor HIV-1 dependency factors and that ferret APOBEC3 proteins with activity against HIV-1 are likely. Even so, unlike in cat cells, HIV-1 can replicate in ferret cells without vif substitution. The virus evolves in this novel nonprimate cell adaptive landscape. We suggest that further characterization of HIV-1 adaptation in ferret cells and delineation of Mustelidae restriction factor repertoires are warranted, with a view to the potential for an HIV-1 animal model.

TRIM5α and Species Tropism of HIV/SIV

Human immunodeficiency virus type 1 (HIV-1) infects humans and chimpanzees but not old world monkeys (OWMs) such as the rhesus monkey (Rh) and cynomolgus monkey (CM). HIV-1 efficiently enters cells of OWMs but encounters a block before reverse transcription. This narrow host range is attributed to a barrier in the host cell. In 2004, the screening of a Rh cDNA library identified tripartite motif 5α (TRIM5α) as a cellular antiviral factor. TRIM5α is one of splicing variants produced by TRIM5 gene and TRIM5 proteins are members of the TRIM family containing RING, B-box 2, and coiled-coil domains. The RING domain is frequently found in E3 ubiquitin ligase and TRIM5α is degraded via the ubiquitin–proteasome-dependent pathway. Among TRIM5 splicing variants, TRIM5α alone has an additional C-terminal PRYSPRY (B30.2) domain. Previous studies have shown that sequence variation in variable regions of the PRYSPRY domain among different monkey species affects species-specific retrovirus infection, while amino acid sequence differences in the viral capsid protein determine viral sensitivity to restriction. TRIM5α recognizes the multimerized capsid proteins (viral core) of an incoming virus by its PRYSPRY domain and is thus believed to control retroviral infection. There are significant intraspecies variations in the Rh-TRIM5 gene. It has also been reported that some Rh and CM individuals have retrotransposed cyclophilin A open reading frame in the TRIM5 gene, which produces TRIM5–cyclophilin A fusion protein (TRIMCyp). TRIMCyp, which was originally identified as an anti-HIV-1 factor of New World owl monkeys, is an interesting example of the gain of a new function by retrotransposition. As different TRIM5 genotypes of Rh showed different levels of simian immunodeficiency virus replication in vivo, the TRIM5 genotyping is thought to be important in acquired immunodeficiency syndrome monkey models.

Regulation of CXCR4 conformation by the small GTPase Rac1: implications for HIV infection

The chemokine receptor CXCR4 is a critical regulator of cell migration and serves as a coreceptor for HIV-1. The chemokine stromal cell derived factor-1, also known as CXCL12, binds to CXCR4 and exerts its biologic functions partly through the small guanosine triphosphate hydrolase (GTPase) Rac1 (ras-related C3 botulinum toxin substrate 1). We show in different cell types, including CD34(+) hematopoietic stem and progenitor cells, that inhibition of Rac1 causes a reversible conformational change in CXCR4, but not in the related receptors CXCR7 or CCR5. Biochemical experiments showed that Rac1 associates with CXCR4. The conformational change of CXCR4 on Rac1 inhibition blocked receptor internalization and impaired CXCL12-induced Gα(i) protein activation. Importantly, we found that the conformation adopted by CXCR4 after Rac1 inhibition prevents HIV-1 infection of both the U87-CD4-CXCR4 cell line and of primary peripheral blood mononuclear cells. In conclusion, our data show that Rac1 activity is required to maintain CXCR4 in the responsive conformation that allows receptor signaling and facilitates HIV-1 infection; this implies that Rac1 positively regulates CXCR4 function and identifies the Rac1-CXCR4 axis as a new target for preventing HIV-1 infection.

Evolution of a species-specific determinant within human CRM1 that regulates the post-transcriptional phases of HIV-1 replication

The human immunodeficiency virus type-1 (HIV-1) Rev protein regulates the nuclear export of intron-containing viral RNAs by recruiting the CRM1 nuclear export receptor. Here, we employed a combination of functional and phylogenetic analyses to identify and characterize a species-specific determinant within human CRM1 (hCRM1) that largely overcomes established defects in murine cells to the post-transcriptional stages of the HIV-1 life cycle. hCRM1 expression in murine cells promotes the cytoplasmic accumulation of intron-containing viral RNAs, resulting in a substantial stimulation of the net production of infectious HIV-1 particles. These stimulatory effects require a novel surface-exposed element within HEAT repeats 9A and 10A, discrete from the binding cleft previously shown to engage Rev's leucine-rich nuclear export signal. Moreover, we show that this element is a unique feature of higher primate CRM1 proteins, and discuss how this sequence has evolved from a non-functional, ancestral sequence.

Latent HIV-1 infection of resting CD4⁺ T cells in the humanized Rag2⁻/⁻ γc⁻/⁻ mouse

Persistent human immunodeficiency virus type 1 (HIV-1) infection of resting CD4⁺ T cells, unaffected by antiretroviral therapy (ART), provides a long-lived reservoir of HIV infection. Therapies that target this viral reservoir are needed to eradicate HIV-1 infection. A small-animal model that recapitulates HIV-1 latency in resting CD4⁺ T cells may accelerate drug discovery and allow the rational design of nonhuman primate (NHP) or human studies. We report that in humanized Rag2⁻/⁻ γ(c)⁻/⁻ (hu-Rag2⁻/⁻ γ(c)⁻/⁻) mice, as in humans, resting CD4⁺ T cell infection (RCI) can be quantitated in pooled samples of circulating cells and tissue reservoirs (e.g., lymph node, spleen, bone marrow) following HIV-1 infection with the CCR5-tropic JR-CSF strain and suppression of viremia by ART. Replication-competent virus was recovered from pooled resting CD4⁺ T cells in 7 of 16 mice, with a median frequency of 8 (range, 2 to 12) infected cells per million T cells, demonstrating that HIV-1 infection can persist despite ART in the resting CD4⁺ T cell reservoir of hu-Rag2⁻/⁻ γ(c)⁻/⁻ mice. This model will allow rapid preliminary assessments of novel eradication approaches and combinatorial strategies that may be challenging to perform in the NHP model or in humans, as well as a rigorous analysis of the effect of these interventions in specific anatomical compartments.

Molecular determinants that regulate plasma membrane association of HIV-1 Gag

Human immunodeficiency virus type 1 assembly is a multistep process that occurs at the plasma membrane (PM). Targeting and binding of Gag to the PM are the first steps in this assembly process and are mediated by the matrix domain of Gag. This review highlights our current knowledge on viral and cellular determinants that affect specific interactions between Gag and the PM. We will discuss potential mechanisms by which the matrix domain might integrate three regulatory components, myristate, phosphatidylinositol-(4,5)-bisphosphate, and RNA, to ensure that human immunodeficiency virus type 1 assembly occurs at the PM.

Humanized mouse models of HIV infection

Because of the limited tropism of HIV, in vivo modeling of this virus has been almost exclusively limited to other lentiviruses, such as simian immunodeficiency virus, that reproduce many important characteristics of HIV infection. However, there are significant genetic and biological differences among lentiviruses and some HIV-specific interventions are not effective against other lentiviruses in nonhuman hosts. For these reasons, much emphasis has recently been placed on developing alternative animal models that support HIV replication and recapitulate key aspects of HIV infection and pathogenesis in humans. Humanized mice, CD34+ hematopoietic progenitor cell transplanted immunodeficient mice, and in particular mice also implanted with human thymus/liver tissue (bone marrow liver thymus mice) that develop a functional human immune system, have been the focus of a great deal of attention as possible models to study virtually all aspects of HIV biology and pathogenesis. Humanized mice are systemically reconstituted with human lymphoid cells, offering rapid, reliable, and reproducible experimental systems for HIV research. Peripheral blood of humanized mice can be readily sampled longitudinally to assess reconstitution with human cells and to monitor HIV replication, permitting the evaluation of multiple parameters of HIV infection such as viral load levels, CD4+ T-cell depletion, immune activation, as well as the effects of therapeutic interventions. Of high relevance to HIV transmission is the extensive characterization and validation of the reconstitution with human lymphoid cells of the female reproductive tract and of the gastrointestinal tract of humanized bone marrow liver thymus mice that renders them susceptible to both vaginal and rectal HIV infection. Other important attributes of all types of humanized mice include: (i) their small size and cost that make them widely accessible; (ii) multiple cohorts of humanized mice can be made from multiple human donors and each cohort has identical human cells, permitting control of intragenetic variables; (iii) continuous de novo production of human immune cells from the transplanted CD34+ cells within each humanized mouse facilitates long-term experiments; (iv) both primary and laboratory HIV isolates can be used for experiments; and (v) in addition to therapeutic interventions, rectal and vaginal HIV prevention approaches can be studied. In summary, humanized mice can have an important role in virtually all aspects of HIV research, including the analysis of HIV replication, the evaluation of HIV restriction factors, the characterization of successful biomedical HIV prevention strategies, the evaluation of new treatment regimens, and the evaluation of novel HIV eradication strategies.

Production of HIV particles is regulated by altering sub-cellular localization and dynamics of Rev induced by double-strand RNA binding protein

Human immunodeficiency virus (HIV)-1 encoded Rev is essential for export from the nucleus to the cytoplasm, of unspliced and singly spliced transcripts coding for structural and nonstructural viral proteins. This process is spatially and temporally coordinated resulting from the interactions between cellular and viral proteins. Here we examined the effects of the sub-cellular localization and dynamics of Rev on the efficiency of nucleocytoplasmic transport of HIV-1 Gag transcripts and virus particle production. Using confocal microscopy and fluorescence recovery after bleaching (FRAP), we report that NF90ctv, a cellular protein involved in Rev function, alters both the sub-cellular localization and dynamics of Rev in vivo, which drastically affects the accumulation of the viral protein p24. The CRM1–dependent nuclear export of Gag mRNA linked to the Rev Response Element (RRE) is dependent on specific domains of the NF90ctv protein. Taken together, our results demonstrate that the appropriate intracellular localization and dynamics of Rev could regulate Gag assembly and HIV-1 replication.

Endogenous CD317/Tetherin limits replication of HIV-1 and murine leukemia virus in rodent cells and is resistant to antagonists from primate viruses

Human CD317 (BST-2/tetherin) is an intrinsic immunity factor that blocks the release of retroviruses, filoviruses, herpesviruses, and arenaviruses. It is unclear whether CD317 expressed endogenously in rodent cells has the capacity to interfere with the replication of the retroviral rodent pathogen murine leukemia virus (MLV) or, in the context of small-animal model development, contributes to the well-established late-phase restriction of human immunodeficiency virus type 1 (HIV-1). Here, we show that small interfering RNA (siRNA)-mediated knockdown of CD317 relieved a virion release restriction and markedly enhanced the egress of HIV-1, HIV-2, and simian immunodeficiency virus (SIV) in rat cells, including primary macrophages. Moreover, rodent CD317 potently inhibited MLV release, and siRNA-mediated depletion of CD317 in a mouse T-cell line resulted in the accelerated spread of MLV. Several virus-encoded antagonists have recently been reported to overcome the restriction imposed by human or monkey CD317, including HIV-1 Vpu, envelope glycoproteins of HIV-2 and Ebola virus, Kaposi's sarcoma-associated herpesvirus K5, and SIV Nef. In contrast, both rat and mouse CD317 showed a high degree of resistance to these viral antagonists. These data suggest that CD317 is a broadly acting and conserved mediator of innate control of retroviral infection and pathogenesis that restricts the release of retroviruses and lentiviruses in rodents. The high degree of resistance of the rodent CD317 restriction factors to antagonists from primate viruses has implications for HIV-1 small-animal model development and may guide the design of novel antiviral interventions.

Excessive RNA splicing and inhibition of HIV-1 replication induced by modified U1 small nuclear RNAs

HIV-1 RNA undergoes a complex splicing process whereby over 40 different mRNA species are produced by alternative splicing. In addition, approximately half of the RNA transcripts remain unspliced and either are used to encode Gag and Gag-Pol proteins or are packaged into virions as genomic RNA. It has previously been shown that HIV-1 splicing is regulated by cis elements that bind to cellular factors. These factors either enhance or repress definition of exons that are flanked by the HIV-1 3' splice sites. Here we report that expression of modified U1 snRNPs with increased affinity to HIV-1 downstream 5' splice sites and to sequences within the first tat coding exon act to selectively increase splicing at the upstream 3' splice sites in cotransfected 293T cells. This results in a decrease of unspliced viral RNA levels and an approximately 10-fold decrease in virus production. In addition, excessive splicing of viral RNA is concomitant with a striking reduction in the relative amounts of Gag processing intermediates and products. We also show that T cell lines expressing modified U1 snRNAs exhibit reduced HIV-1 replication. Our results suggest that induction of excessive HIV-1 RNA splicing may be a novel strategy to inhibit virus replication in human patients.

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.

Replication competent molecular clones of HIV-1 expressing Renilla luciferase facilitate the analysis of antibody inhibition in PBMC

Effective vaccine development for human immunodeficiency virus type 1 (HIV-1) will require assays that ascertain the capacity of vaccine immunogens to elicit neutralizing antibodies (NAb) to diverse HIV-1 strains. To facilitate NAb assessment in peripheral blood mononuclear cell (PBMC)-based assays, we developed an assay-adaptable platform based on a Renilla luciferase (LucR) expressing HIV-1 proviral backbone. LucR was inserted into pNL4-3 DNA, preserving all viral open reading frames. The proviral genome was engineered to facilitate expression of diverse HIV-1 env sequences, allowing analysis in an isogenic background. The resulting Env-IMC-LucR viruses are infectious, and LucR is stably expressed over multiple replications in PBMC. HIV-1 neutralization, targeting TZM-bl cells, was highly correlative comparing virus (LucR) and cell (firefly luciferase) readouts. In PBMC, NAb activity can be analyzed either within a single or multiple cycles of replication. These results represent advancement toward a standardizable PBMC-based neutralization assay for assessing HIV-1 vaccine immunogen efficacy.

SRp40 and SRp55 promote the translation of unspliced human immunodeficiency virus type 1 RNA

Nuclear RNA processing events, such as 5' cap formation, 3' polyadenylation, and pre-mRNA splicing, mark mRNA for efficient translation. Splicing enhances translation via the deposition of the exon-junction complex and other multifunctional splicing factors, including SR proteins. All retroviruses synthesize their structural and enzymatic proteins from unspliced genomic RNAs (gRNAs) and must therefore exploit unconventional strategies to ensure their effective expression. Here, we report that specific SR proteins, particularly SRp40 and SRp55, promote human immunodeficiency virus type 1 (HIV-1) Gag translation from unspliced (intron-containing) viral RNA. This activity does not correlate with nucleocytoplasmic shuttling capacity and, in the case of SRp40, is dependent on the second RNA recognition motif and the arginine-serine (RS) domain. While SR proteins enhance Gag expression independent of RNA nuclear export pathway choice, altering the nucleotide sequence of the gag-pol coding region by codon optimization abolishes this effect. We therefore propose that SR proteins couple HIV-1 gRNA biogenesis to translational utilization.

A low-molecular-weight entry inhibitor of both CCR5- and CXCR4-tropic strains of human immunodeficiency virus type 1 targets a novel site on gp41

A low-molecular-weight human immunodeficiency virus type 1 (HIV-1) inhibitor, PF-68742 (molecular weight, 573), has been identified in a high-throughput screen for compounds that block HIV-1 envelope glycoprotein (Env)-mediated fusion. The compound is shown to be potent against R5 and X4 isolates in both cell-cell fusion and antiviral assays (50% effective concentrations of approximately 0.1 to 1 muM). Postfusion and HIV-1 pseudotyping control experiments confirm that PF-68742 is an entry inhibitor with Env as the specific target for antiviral action. PF-68742 was not able to block binding of monomeric gp120 to soluble CD4 or the binding of gp120:CD4 complexes to cell-associated CCR5, thus distinguishing PF-68742 from described gp120 antagonists and coreceptor binders. Escape variants of HIV-1(NL4-3) were selected, and all resistant viruses were found to contain a common G514R (HxB2 numbering) mutation in Env, located proximal to the furin cleavage site in the fusion peptide of gp41. When introduced into wild-type NL4-3 gp41, G514R conferred resistance to PF-68742. Resistance via G514R is shown to be associated with enhancement of virion infectivity by PF-68742 that may result from altered properties of inhibitor-bound Env, rather than from a loss of compound binding. Wild-type viruses and those with substitutions in the disulfide loop (DSL) region of gp41 were also examined for PF-68742 sensitivity. Here, complete resistance to PF-68742 was found to occur through changes outside of position 514, including in the gp41 DSL region. The results highlight PF-68742 as a starting point for novel therapies against HIV-1 and provide new insights into models of Env-mediated fusion.

In vivo stable transduction of humanized liver tissue in chimeric mice via high-capacity adenovirus-lentivirus hybrid vector

We developed hybrid vectors employing high-capacity adenovirus as a first-stage carrier encoding all the components required for in situ production of a second-stage lentivirus, thereby achieving stable transgene expression in secondary target cells. Such vectors have never previously been tested in normal tissues, because of the scarcity of suitable in vivo systems permissive for second-stage lentivirus assembly. Here we employed a novel murine model in which endogenous liver tissue is extensively reconstituted with engrafted human hepatocytes, and successfully achieved stable transduction by the second-stage lentivirus produced in situ from first-stage adenovirus. This represents the first demonstration of the functionality of adenoviral-lentiviral hybrid vectors in a normal parenchymal organ in vivo.

Retroviral assembly and budding occur through an actin-driven mechanism

The assembly and budding of a new virus is a fundamental step in retroviral replication. Yet, despite substantial progress in the structural and biochemical characterization of retroviral budding, the underlying physical mechanism remains poorly understood, particularly with respect to the mechanism by which the virus overcomes the energy barrier associated with the formation of high membrane curvature during viral budding. Using atomic force, fluorescence, and transmission electron microscopy, we find that both human immunodeficiency virus and Moloney murine leukemia virus remodel the actin cytoskeleton of their host. These actin-filamentous structures assemble simultaneously with or immediately after the beginning of budding, and disappear as soon as the nascent virus is released from the cell membrane. Analysis of sections of cryopreserved virus-infected cells by transmission electron microscopy reveals similar actin filament structures emerging from every nascent virus. Substitution of the nucleocapsid domain implicated in actin binding by a leucine-zipper domain results in the budding of virus-like particles without remodeling of the cell's cytoskeleton. Notably, viruses carrying the modified nucleocapsid domains bud more slowly by an order of magnitude compared to the wild-type. The results of this study show that retroviruses utilize the cell cytoskeleton to expedite their assembly and budding.

HSV type 2 infection increases HIV DNA detection in vaginal tissue of mice expressing human CD4 and CCR5

The goal of this study was to develop an in vivo murine model that can be used to study the influence of HSV-2 on HIV infection. Mice expressing transgenes for human CD4, CCR5, and Cyclin T1 were infected intravaginally with HSV-2 and 3-7 days later infected with HIV. HIV DNA was detected by real-time PCR. The frequency of detection of HIV DNA was significantly higher (65%) in vaginal tissue of HSV-2-infected mice compared to mock-infected mice (35%) when HIV was given 3 days after HSV-2. HSV-2-infected mice also had significantly higher levels of HIV DNA in vaginal tissue. HIV DNA was not detected in vaginal tissue of mice lacking human CD4. Longer periods (5 or 7 days) between infection with HSV-2 and HIV did not increase the frequency of detection or the amount of HIV DNA detected. HIV DNA was also detected in lymph nodes from some of the mice that were infected intravaginally with HSV-2 and HIV. Flow cytometric and mRNA analysis of human CD4 in vaginal tissue suggested that HSV-2 infection increased the number of T cells expressing human CD4 in vaginal tissue. This study provides evidence that HIV infection of cells occurs in the vagina of mice expressing human CD4, CCR5, and Cyclin T1 and that HSV-2 infection increases HIV infection. These findings demonstrate that this model can be used to study the mechanisms responsible for increased susceptibility to HIV in HSV-2-infected persons and for testing preventative treatments.

Matrix mediates the functional link between human immunodeficiency virus type 1 RNA nuclear export elements and the assembly competency of Gag in murine cells

Human immunodeficiency virus type 1 (HIV-1) assembles poorly in murine cells, reflecting inefficient targeting of the Gag structural polyprotein to the plasma membrane. Virus particle production can be restored by replacing the cis-acting Rev response element (RRE) in Gag-Pol mRNAs with multiple copies of the CTE (4xCTE), suggesting a mechanistic link between HIV-1 RNA trafficking and productive Gag assembly. In this report, we demonstrate that Gag molecules generated from RRE-dependent transcripts are intrinsically defective for assembly in murine 3T3 cells. When controlled for the intracellular Gag level, modulations of the Gag matrix (MA) domain that enhance Gag membrane association (e.g., deletion of the MA globular head) substantially improve assembly for Gag derived from RRE- but not 4xCTE-dependent transcripts. Gag mutants carrying a leucine zipper replacement of the nucleocapsid (NC) domain remain largely assembly defective when derived from RRE-dependent transcripts, indicating that the defect does not reflect aberrant NC/RNA-driven Gag multimerization. We further demonstrate that single changes in uncharged amino acids implicated in Gag/MA myristoyl switch regulation, most notably replacing the leucine at position 21 with serine, improve assembly for Gag derived from RRE-dependent transcripts. In sum, we provide genetic evidence to suggest that HIV-1 RNA metabolism specifically modulates the activation of MA-dependent membrane targeting.

Synergistic effect of human CycT1 and CRM1 on HIV-1 propagation in rat T cells and macrophages

BACKGROUND

In vivo studies of 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. Although transgenic rats that express the HIV-1 receptor complex hCD4 and hCCR5 are susceptible to infection, HIV-1 replicates very poorly in these animals. To demonstrate the molecular basis for developing a better rat model for HIV-1 infection, we evaluated the effect of human CyclinT1 (hCycT1) and CRM1 (hCRM1) on Gag p24 production in rat T cells and macrophages using both established cell lines and primary cells prepared from hCycT1/hCRM1 transgenic rats.

RESULTS

Expression of hCycT1 augmented Gag production 20-50 fold in rat T cells, but had little effect in macrophages. Expression of hCRM1 enhanced Gag production 10-15 fold in macrophages, but only marginally in T cells. Expression of both factors synergistically enhanced p24 production to levels approximately 10-40% of those detected in human cells. R5 viruses produced in rat T cells and macrophages were fully infectious.

CONCLUSION

The expression of both hCycT1 and hCRM1 appears to be fundamental to developing a rat model that supports robust propagation of HIV-1.

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.

Replication of equine infectious anemia virus in engineered mouse NIH 3T3 cells

We employed the equine lentivirus equine infectious anemia virus (EIAV) to investigate the cellular restrictions for lentivirus replication in murine NIH 3T3 cells. The results of these studies demonstrate that NIH 3T3 cells expressing the EIAV receptor ELR1 and equine cyclin T1 supported productive replication of EIAV and produced infectious virions at levels similar to those found in a reference permissive equine cell line. The studies presented here demonstrate, for the first time, differential levels of restriction for EIAV and human immunodeficiency virus type 1 (HIV-1) replication in murine cells and suggest that these differences can be exploited to reveal critical virus-cell interactions required for HIV-1 assembly and budding of lentivirus particles.

Inability of human immunodeficiency virus type 1 produced in murine cells to selectively incorporate primer formula

Attempts to use the mouse as a model system for studying AIDS are stymied by the multiple blocks to human immunodeficiency virus type 1 (HIV-1) replication that exist in mouse cells at the levels of viral entry, transcription, and Gag assembly and processing. In this report, we describe an additional block in the selective packaging of tRNA(3Lys) into HIV-1 produced in murine cells. HIV-1 and murine leukemia virus (MuLV) use tRNA(3Lys) and tRNA(Pro), respectively, as primers for reverse transcription. Selective packaging of tRNA(3Lys) into HIV-1 produced in human cells is much stronger than that for tRNA(Pro) incorporation into MuLV produced in murine cells, and different packaging mechanisms are used. Thus, both lysyl-tRNA synthetase and GagPol are required for tRNA(3Lys) packaging into HIV-1, but neither prolyl-tRNA synthetase nor GagPol is required for tRNA(Pro) packaging into MuLV. In this report, we show that when HIV-1 is produced in murine cells, the virus switches from an HIV-1-like incorporation of tRNA(3Lys) to an MuLV-like packaging of tRNA(Pro). The primer binding site in viral RNA remains complementary to tRNA(3Lys), resulting in a significant decrease in reverse transcription and infectivity. Reduction in tRNA(3Lys) incorporation occurs even though both murine lysyl-tRNA synthetase and HIV-1 GagPol are packaged into the HIV-1 produced in murine cells. Nevertheless, the murine cell is able to support the select incorporation of tRNA(3Lys) into another retrovirus that uses tRNA(3Lys) as a primer, the mouse mammary tumor virus.

HIV traffics through a specialized, surface-accessible intracellular compartment during trans-infection of T cells by mature dendritic cells

In vitro, dendritic cells (DCs) bind and transfer intact, infectious HIV to CD4 T cells without first becoming infected, a process known as trans-infection. trans-infection is accomplished by recruitment of HIV and its receptors to the site of DC–T cell contact and transfer of virions at a structure known as the infectious synapse. In this study, we used fluorescent microscopy to track individual HIV particles trafficking in DCs during virus uptake and trans-infection. Mature DCs rapidly concentrated HIV into an apparently intracellular compartment that lacked markers characteristic of early endosomes, lysosomes, or antigen-processing vesicles. Live cell microscopy demonstrated that the HIV-containing compartment was rapidly polarized toward the infectious synapse after contact with a T cell; however, the bulk of the concentrated virus remained in the DCs after T cell engagement. Individual virions were observed emerging from the compartment and fusing with the T cell membrane at the infectious synapse. The compartmentalized HIV, although engulfed by the cytoplasm, was fully accessible to HIV envelope-specific inhibitors and other membrane-impermeable probes that were delivered to the cell surface. These results demonstrate that HIV resides in an invaginated domain within DCs that is both contiguous with the plasma membrane and distinct from endocytic vesicles. We conclude that HIV virions are routed through this specialized compartment, which allows individual particles to be delivered to T cells during trans-infection.

Dendritic cells (DCs) patrol mucosal areas of the body, where they engulf invading pathogens and transport them to immune tissues. There the DCs degrade the microbes and present antigenic peptides to T lymphocytes to elicit specific immune responses. HIV-1 has appropriated this feature of the immune system to better establish and maintain infection of its primary target–CD4-positive T cells. DCs efficiently bind and degrade HIV, however a portion of the virus remains intact and can be transmitted into CD4 T cells, a process called trans-infection. DC maturation by various stimuli dramatically increases their capacity to trans-infect. Here we report that mature DCs concentrate infectious HIV into a pocket-like compartment that resides within the cell but remains physically connected to the cell surface. This structure is distinct from the intracellular degradative compartments that are used for microbial processing and presentation. The intact viral particles are retained within this compartment for extended periods, and individual particles can emerge and infect T cells at the cellular interface. We hypothesize that DCs form this compartment to sequester HIV off of the cell surface, however escape of virions from the pocket results in efficient infection of T cells during immune presentation events.

Murine models for HIV vaccination and challenge

HIV-1 only infects humans and chimpanzees. SIV or SHIV are, therefore, used as models for HIV in rhesus, cynomologus and pigtail macaques. Since conducting experiments in primate models does not fully mimic infection or vaccination against HIV-1 and is expensive, there is a great need for small-animal models in which it is possible to study HIV-1 infection, immunity and vaccine efficacy. This review summarizes the available murine models for studying HIV-1 infection with an emphasis on our experience of the HIV-1-infected-cell challenge as a model for evaluating candidate HIV-1 vaccines. In the cell-based challenge model, several important factors that, hopefully, can be related to vaccine efficacy in humans were discovered: the efficiency of combining plasmid DNA representing several of the viral genes originating from multiple clades of HIV-1, the importance of adjuvants activating innate and induced immunity and the enhanced HIV eradication by drug-conjugated antibody.

Mouse T-cells restrict replication of human immunodeficiency virus at the level of integration

Background

The development of an immunocompetent, genetically modified mouse model to study HIV-1 pathogenesis and to test antiviral strategies has been hampered by the fact that cells from native mice do not or only inefficiently support several steps of the HIV-1 replication cycle. Upon HIV-1 infection, mouse T-cell lines fail to express viral proteins, but the underlying replication barrier has thus far not been unambiguously identified. Here, we performed a kinetic and quantitative assessment of consecutive steps in the early phase of the HIV-1 replication cycle in T-cells from mice and humans.

Results

Both T-cell lines and primary T-cells from mice harbor a severe post-entry defect that is independent of potential species-specTR transactivation. Reverse transcription occurred efficiently following VSV-G-mediated entry of virions into mouse T-cells, and abundant levels of 2-LTR circles indicated successful nuclear import of the pre-integration complex. To probe the next step in the retroviral replication cycle, i.e. the integration of HIV-1 into the host cell genome, we established and validated a nested real-time PCR to specifically quantify HIV-1 integrants exploiting highly repetitive mouse B1 elements. Importantly, we demonstrate that the frequency of integrant formation is diminished 18- to > 305-fold in mouse T-cell lines compared to a human counterpart, resulting in a largely abortive infection. Moreover, differences in transgene expression from residual vector integrants, the transcription off which is cyclin T1-independent, provided evidence for an additional, peri-integrational deficit in certain mouse T-cell lines.

Conclusion

In contrast to earlier reports, we find that mouse T-cells efficiently support early replication steps up to and including nuclear import, but restrict HIV-1 at the level of chromosomal integration.

Relief of preintegration inhibition and characterization of additional blocks for HIV replication in primary mouse T cells

Development of a small animal model to study HIV replication and pathogenesis has been hampered by the failure of the virus to replicate in non-primate cells. Most studies aimed at achieving replication in murine cells have been limited to fibroblast cell lines, but generating an appropriate model requires overcoming blocks to viral replication in primary T cells. We have studied HIV-1 replication in CD4(+) T cells from human CD4/CCR5/Cyclin T1 transgenic mice. Expression of hCD4 and hCCR5 in mouse CD4(+) T cells enabled efficient entry of R5 strain HIV-1. In mouse T cells, HIV-1 underwent reverse transcription and nuclear import as efficiently as in human T cells. In contrast, chromosomal integration of HIV-1 proviral DNA was inefficient in activated mouse T cells. This process was greatly enhanced by providing a secondary T cell receptor (TCR) signal after HIV-1 infection, especially between 12 to 24 h post infection. This effect was specific for primary mouse T cells. The pathways involved in HIV replication appear to be PKCtheta-, CARMA1-, and WASp-independent. Treatment with Cyclosporin A (CsA) further relieved the pre-integration block. However, transcription of HIV-1 RNA was still reduced in mouse CD4(+) T cells despite expression of the hCyclin T1 transgene. Additional post-transcriptional defects were observed at the levels of Gag expression, Gag processing, Gag release and virus infectivity. Together, these post-integration defects resulted in a dramatically reduced yield of infectious virus (300-500 fold) after a single cycle of HIV-1 replication. This study implies the existence of host factors, in addition to those already identified, that are critical for HIV-1 replication in mouse cells. This study also highlights the differences between primary T cells and cell lines regarding pre-integration steps in the HIV-1 replication cycle.