Cytoplasmic utilization of human immunodeficiency virus type 1 genomic RNA is not dependent on a nuclear interaction with gag

HIV-1 RNA genome packaging: it's G-rated

A member of the Retroviridae, human immunodeficiency virus type 1 (HIV-1), uses the RNA genome packaged into nascent virions to transfer genetic information to its progeny. The genome packaging step is a highly regulated and extremely efficient process as a vast majority of virus particles contain two copies of full-length unspliced HIV-1 RNA that form a dimer. Thus, during virus assembly HIV-1 can identify and selectively encapsidate HIV-1 unspliced RNA from an abundant pool of cellular RNAs and various spliced HIV-1 RNAs. Several "G" features facilitate the packaging of a dimeric RNA genome. The viral polyprotein Gag orchestrates virus assembly and mediates RNA genome packaging. During this process, Gag preferentially binds unpaired guanosines within the highly structured 5' untranslated region (UTR) of HIV-1 RNA. In addition, the HIV-1 unspliced RNA provides a scaffold that promotes Gag:Gag interactions and virus assembly, thereby ensuring its packaging. Intriguingly, recent studies have shown that the use of different guanosines at the junction of U3 and R as transcription start sites results in HIV-1 unspliced RNA species with 99.9% identical sequences but dramatically distinct 5' UTR conformations. Consequently, one species of unspliced RNA is preferentially packaged over other nearly identical RNAs. These studies reveal how conformations affect the functions of HIV-1 RNA elements and the complex regulation of HIV-1 replication. In this review, we summarize cis- and trans-acting elements critical for HIV-1 RNA packaging, locations of Gag:RNA interactions that mediate genome encapsidation, and the effects of transcription start sites on the structure and packaging of HIV-1 RNA.

HIV-1 Gag co-localizes with euchromatin histone marks at the nuclear periphery

IMPORTANCE

The traditional view of retrovirus assembly posits that packaging of gRNA by HIV-1 Gag occurs in the cytoplasm or at the plasma membrane. However, our previous studies showing that HIV-1 Gag enters the nucleus and binds to USvRNA at transcription sites suggest that gRNA selection may occur in the nucleus. In the present study, we observed that HIV-1 Gag trafficked to the nucleus and co-localized with USvRNA within 8 hours of expression. In infected T cells (J-Lat 10.6) reactivated from latency and in a HeLa cell line stably expressing an inducible Rev-dependent HIV-1 construct, we found that Gag preferentially localized with euchromatin histone marks associated with enhancer and promoter regions near the nuclear periphery, which is the favored site HIV-1 integration. These observations support the innovative hypothesis that HIV-1 Gag associates with euchromatin-associated histones to localize to active transcription sites, promoting capture of newly synthesized gRNA for packaging.

Influence of HIV-1 Genomic RNA on the Formation of Gag Biomolecular Condensates

Biomolecular condensates (BMCs) play an important role in the replication of a growing number of viruses, but many important mechanistic details remain to be elucidated. Previously, we demonstrated that the pan-retroviral nucleocapsid (NC) and HIV-1 pr55Gag (Gag) proteins phase separate into condensates, and that HIV-1 protease (PR)-mediated maturation of Gag and Gag-Pol precursor proteins yields self-assembling BMCs that have HIV-1 core architecture. Using biochemical and imaging techniques, we aimed to further characterize the phase separation of HIV-1 Gag by determining which of its intrinsically disordered regions (IDRs) influence the formation of BMCs, and how the HIV-1 viral genomic RNA (gRNA) could influence BMC abundance and size. We found that mutations in the Gag matrix (MA) domain or the NC zinc finger motifs altered condensate number and size in a salt-dependent manner. Gag BMCs were also bimodally influenced by the gRNA, with a condensate-promoting regime at lower protein concentrations and a gel dissolution at higher protein concentrations. Interestingly, incubation of Gag with CD4+ T cell nuclear lysates led to the formation of larger BMCs compared to much smaller ones observed in the presence of cytoplasmic lysates. These findings suggest that the composition and properties of Gag-containing BMCs may be altered by differential association of host factors in nuclear and cytosolic compartments during virus assembly. This study significantly advances our understanding of HIV-1 Gag BMC formation and provides a foundation for future therapeutic targeting of virion assembly.

HIV-1 Gag colocalizes with euchromatin histone marks at the nuclear periphery

The retroviral Gag protein of human immunodeficiency virus type 1 (HIV-1) plays a central role in the selection of unspliced viral genomic RNA for packaging into new virions. Previously, we demonstrated that full-length HIV-1 Gag undergoes nuclear trafficking where it associates with unspliced viral RNA (vRNA) at transcription sites. To further explore the kinetics of HIV-1 Gag nuclear localization, we used biochemical and imaging techniques to examine the timing of HIV-1 entry into the nucleus. We also aimed to determine more precisely Gag's subnuclear distribution to test the hypothesis that Gag would be associated with euchromatin, the transcriptionally active region of the nucleus. We observed that HIV-1 Gag localized to the nucleus shortly after its synthesis in the cytoplasm, suggesting that nuclear trafficking was not strictly concentration-dependent. Furthermore, we found that HIV-1 Gag preferentially localized to the transcriptionally active euchromatin fraction compared to the heterochromatin-rich region in a latently-infected CD4+ T cell line (J-Lat 10.6) treated with latency-reversal agents. Interestingly, HIV-1 Gag was more closely associated with transcriptionally-active histone markers near the nuclear periphery, where the HIV-1 provirus was previously shown to integrate. Although the precise function of Gag's association with histones in transcriptionally-active chromatin remains uncertain, together with previous reports, this finding is consistent with a potential role for euchromatin-associated Gag molecules to select newly transcribed unspliced vRNA during the initial stage of virion assembly.

Importance

The traditional view of retroviral assembly posits that HIV-1 Gag selection of unspliced vRNA begins in the cytoplasm. However, our previous studies demonstrated that HIV-1 Gag enters the nucleus and binds to unspliced HIV-1 RNA at transcription sites, suggesting that genomic RNA selection may occur in the nucleus. In the present study, we observed nuclear entry of HIV-1 Gag and co-localization with unspliced viral RNA within 8 hours post-expression. In CD4+ T cells (J-Lat 10.6) treated with latency reversal agents, as well as a HeLa cell line stably expressing an inducible Rev-dependent provirus, we found that HIV-1 Gag preferentially localized with histone marks associated with enhancer and promoter regions of transcriptionally active euchromatin near the nuclear periphery, which favors HIV-1 proviral integration sites. These observations support the hypothesis that HIV-1 Gag hijacks euchromatin-associated histones to localize to active transcription sites, promoting capture of newly synthesized genomic RNA for packaging.

Influence of HIV-1 genomic RNA on the formation of Gag biomolecular condensates

Biomolecular condensates (BMCs) play an important role in the replication of a growing number of viruses, but many important mechanistic details remain to be elucidated. Previously, we demonstrated that pan-retroviral nucleocapsid (NC) and the HIV-1 pr55 ^Gag (Gag) proteins phase separate into condensates, and that HIV-1 protease (PR)-mediated maturation of Gag and Gag-Pol precursor proteins yield self-assembling BMCs having HIV-1 core architecture. Using biochemical and imaging techniques, we aimed to further characterize the phase separation of HIV-1 Gag by determining which of its intrinsically disordered regions (IDRs) influence the formation of BMCs and how the HIV-1 viral genomic RNA (gRNA) could influence BMC abundance and size. We found that mutations in the Gag matrix (MA) domain or the NC zinc finger motifs altered condensate number and size in a salt-dependent manner. Gag BMCs were also bimodally influenced by the gRNA, with a condensate-promoting regime at lower protein concentrations and a gel dissolution at higher protein concentrations. Interestingly, incubation of Gag with CD4 ⁺ T cell nuclear lysates led to the formation of larger BMCs as compared to much smaller ones observed in the presence of cytoplasmic lysates. These findings suggests that the composition and properties of Gag-containing BMCs may be altered by differential association of host factors in nuclear and cytosolic compartments during virus assembly. This study significantly advances our understanding of HIV-1 Gag BMC formation and provides a foundation for future therapeutic targeting of virion assembly.

Quantitative analysis of the formation of nucleoprotein complexes between HIV-1 Gag protein and genomic RNA using transmission electron microscopy

In HIV, the polyprotein precursor Gag orchestrates the formation of the viral capsid. In the current view of this viral assembly, Gag forms low-order oligomers that bind to the viral genomic RNA triggering the formation of high-ordered ribonucleoprotein complexes. However, this assembly model was established using biochemical or imaging methods that do not describe the cellular location hosting Gag-gRNA complex nor distinguish gRNA packaging in single particles. Here, we studied the intracellular localization of these complexes by electron microscopy and monitored the distances between the two partners by morphometric analysis of gold beads specifically labeling Gag and gRNA. We found that formation of these viral clusters occurred shortly after the nuclear export of the gRNA. During their transport to the plasma membrane, the distance between Gag and gRNA decreases together with an increase of gRNA packaging. Point mutations in the zinc finger patterns of the nucleocapsid domain of Gag caused an increase in the distance between Gag and gRNA as well as a sharp decrease of gRNA packaged into virions. Finally, we show that removal of stem loop 1 of the 5'-untranslated region does not interfere with gRNA packaging, whereas combined with the removal of stem loop 3 is sufficient to decrease but not abolish Gag-gRNA cluster formation and gRNA packaging. In conclusion, this morphometric analysis of Gag-gRNA cluster formation sheds new light on HIV-1 assembly that can be used to describe at nanoscale resolution other viral assembly steps involving RNA or protein-protein interactions.

The HIV 5' Gag Region Displays a Specific Nucleotide Bias Regulating Viral Splicing and Infectivity

Alternative splicing and the expression of intron-containing mRNAs is one hallmark of HIV gene expression. To facilitate the otherwise hampered nuclear export of non-fully processed mRNAs, HIV encodes the Rev protein, which recognizes its intronic response element and fuels the HIV RNAs into the CRM-1-dependent nuclear protein export pathway. Both alternative splicing and Rev-dependency are regulated by the primary HIV RNA sequence. Here, we show that these processes are extremely sensitive to sequence alterations in the 5’coding region of the HIV genomic RNA. Increasing the GC content by insertion of either GFP or silent mutations activates a cryptic splice donor site in gag, entirely deregulates the viral splicing pattern, and lowers infectivity. Interestingly, an adaptation of the inserted GFP sequence toward an HIV-like nucleotide bias reversed these phenotypes completely. Of note, the adaptation yielded completely different primary sequences although encoding the same amino acids. Thus, the phenotypes solely depend on the nucleotide composition of the two GFP versions. This is a strong indication of an HIV-specific mRNP code in the 5′ gag region wherein the primary RNA sequence bias creates motifs for RNA-binding proteins and controls the fate of the HIV-RNA in terms of viral gene expression and infectivity.

CRNKL1 Is a Highly Selective Regulator of Intron-Retaining HIV-1 and Cellular mRNAs

The HIV-1 Rev protein is a nuclear export factor for unspliced and incompletely spliced HIV-1 RNAs. Without Rev, these intron-retaining RNAs are trapped in the nucleus. A genome-wide screen identified nine proteins of the spliceosome, which all enhanced expression from the HIV-1 unspliced RNA after CRISPR/Cas knockdown. Depletion of DHX38, WDR70, and four proteins of the Prp19-associated complex (ISY1, BUD31, XAB2, and CRNKL1) resulted in a more than 20-fold enhancement of unspliced HIV-1 RNA levels in the cytoplasm. Targeting of CRNKL1, DHX38, and BUD31 affected nuclear export efficiencies of the HIV-1 unspliced RNA to a much larger extent than splicing. Transcriptomic analyses further revealed that CRNKL1 also suppresses cytoplasmic levels of a subset of cellular mRNAs, including some with selectively retained introns. Thus, CRNKL1-dependent nuclear retention is a novel cellular mechanism for the regulation of cytoplasmic levels of intron-retaining HIV-1 mRNAs, which HIV-1 may have harnessed to direct its complex splicing pattern.IMPORTANCE To regulate its complex splicing pattern, HIV-1 uses the adaptor protein Rev to shuttle unspliced or partially spliced mRNA from the nucleus to the cytoplasm. In the absence of Rev, these RNAs are retained in the nucleus, but it is unclear why. Here we identify cellular proteins whose depletion enhances cytoplasmic levels of the HIV-1 unspliced RNA. Depletion of one of them, CRNKL1, also increases cytoplasmic levels of a subset of intron-retaining cellular mRNA, suggesting that CRNKL1-dependent nuclear retention may be a basic cellular mechanism exploited by HIV-1.

HIV-1 Gag Forms Ribonucleoprotein Complexes with Unspliced Viral RNA at Transcription Sites

The ability of the retroviral Gag protein of Rous sarcoma virus (RSV) to transiently traffic through the nucleus is well-established and has been implicated in genomic RNA (gRNA) packaging Although other retroviral Gag proteins (human immunodeficiency virus type 1, HIV-1; feline immunodeficiency virus, FIV; Mason-Pfizer monkey virus, MPMV; mouse mammary tumor virus, MMTV; murine leukemia virus, MLV; and prototype foamy virus, PFV) have also been observed in the nucleus, little is known about what, if any, role nuclear trafficking plays in those viruses. In the case of HIV-1, the Gag protein interacts in nucleoli with the regulatory protein Rev, which facilitates nuclear export of gRNA. Based on the knowledge that RSV Gag forms viral ribonucleoprotein (RNPs) complexes with unspliced viral RNA (USvRNA) in the nucleus, we hypothesized that the interaction of HIV-1 Gag with Rev could be mediated through vRNA to form HIV-1 RNPs. Using inducible HIV-1 proviral constructs, we visualized HIV-1 Gag and USvRNA in discrete foci in the nuclei of HeLa cells by confocal microscopy. Two-dimensional co-localization and RNA-immunoprecipitation of fractionated cells revealed that interaction of nuclear HIV-1 Gag with USvRNA was specific. Interestingly, treatment of cells with transcription inhibitors reduced the number of HIV-1 Gag and USvRNA nuclear foci, yet resulted in an increase in the degree of Gag co-localization with USvRNA, suggesting that Gag accumulates on newly synthesized viral transcripts. Three-dimensional imaging analysis revealed that HIV-1 Gag localized to the perichromatin space and associated with USvRNA and Rev in a tripartite RNP complex. To examine a more biologically relevant cell, latently infected CD4+ T cells were treated with prostratin to stimulate NF-κB mediated transcription, demonstrating striking localization of full-length Gag at HIV-1 transcriptional burst site, which was labelled with USvRNA-specific riboprobes. In addition, smaller HIV-1 RNPs were observed in the nuclei of these cells. These data suggest that HIV-1 Gag binds to unspliced viral transcripts produced at the proviral integration site, forming vRNPs in the nucleus.

Advantages and Limitations of Integrated Flagellin Adjuvants for HIV-Based Nanoparticle B-Cell Vaccines

The great advantage of virus-like particle (VLP) nano-vaccines is their structural identity to wild-type viruses, ensuring that antigen-specific B-cells encounter viral proteins in their natural conformation. "Wild-type" viral nanoparticles can be further genetically or biochemically functionalized with biomolecules (antigens and adjuvants). Flagellin is a potent inducer of innate immunity and it has demonstrated adjuvant effectiveness due to its affinity for toll-like receptor 5 (TLR5). In contrast to most TLR ligands, flagellin is a protein and can induce an immune response against itself. To avoid side-effects, we incorporated a less inflammatory and less immunogenic form of flagellin as an adjuvant into HIV-based nanoparticle B-cell-targeting vaccines that display either the HIV-1 envelope protein (Env) or a model antigen, hen egg lysozyme (HEL). While flagellin significantly enhanced HEL-specific IgG responses, anti-Env antibody responses were suppressed. We demonstrated that flagellin did not activate B-cells directly in vitro, but might compete for CD4+ T-cell help in vivo. Therefore, we hypothesize that in the context of VLP-based B-cell nano-vaccines, flagellin serves as an antigen itself and may outcompete a less immunogenic antigen with its antibody response. In contrast, in combination with a strong immunogen, the adjuvant activity of flagellin may dominate over its immunogenicity.

NONO Detects the Nuclear HIV Capsid to Promote cGAS-Mediated Innate Immune Activation

Detection of viruses by innate immune sensors induces protective antiviral immunity. The viral DNA sensor cyclic GMP-AMP synthase (cGAS) is necessary for detection of HIV by human dendritic cells and macrophages. However, synthesis of HIV DNA during infection is not sufficient for immune activation. The capsid protein, which associates with viral DNA, has a pivotal role in enabling cGAS-mediated immune activation. We now find that NONO is an essential sensor of the HIV capsid in the nucleus. NONO protein directly binds capsid with higher affinity for weakly pathogenic HIV-2 than highly pathogenic HIV-1. Upon infection, NONO is essential for cGAS activation by HIV and cGAS association with HIV DNA in the nucleus. NONO recognizes a conserved region in HIV capsid with limited tolerance for escape mutations. Detection of nuclear viral capsid by NONO to promote DNA sensing by cGAS reveals an innate strategy to achieve distinction of viruses from self in the nucleus.

Visualization of RAS/MAPK Signaling In Situ by the Proximity Ligation Assay (PLA)

RAS/MAPK signaling responds to diverse extracellular cues and regulates a wide array of cellular processes. Given its biological importance, abnormalities in RAS/MAPK signaling cascade have been intimately implicated in numerous human diseases, including cancer. Herein, we describe a novel methodology to study activation of this pivotal signaling pathway. The Proximity Ligation Assay (PLA) is employed to monitor kinase-substrate interactions between MEK1 and HSF1, or MEK1 and ERK1 in situ.

Orchestrating the Selection and Packaging of Genomic RNA by Retroviruses: An Ensemble of Viral and Host Factors

Infectious retrovirus particles contain two copies of unspliced viral RNA that serve as the viral genome. Unspliced retroviral RNA is transcribed in the nucleus by the host RNA polymerase II and has three potential fates: (1) it can be spliced into subgenomic messenger RNAs (mRNAs) for the translation of viral proteins; or it can remain unspliced to serve as either (2) the mRNA for the translation of Gag and Gag-Pol; or (3) the genomic RNA (gRNA) that is packaged into virions. The Gag structural protein recognizes and binds the unspliced viral RNA to select it as a genome, which is selected in preference to spliced viral RNAs and cellular RNAs. In this review, we summarize the current state of understanding about how retroviral packaging is orchestrated within the cell and explore potential new mechanisms based on recent discoveries in the field. We discuss the cis-acting elements in the unspliced viral RNA and the properties of the Gag protein that are required for their interaction. In addition, we discuss the role of host factors in influencing the fate of the newly transcribed viral RNA, current models for how retroviruses distinguish unspliced viral mRNA from viral genomic RNA, and the possible subcellular sites of genomic RNA dimerization and selection by Gag. Although this review centers primarily on the wealth of data available for the alpharetrovirus Rous sarcoma virus, in which a discrete RNA packaging sequence has been identified, we have also summarized the cis- and trans-acting factors as well as the mechanisms governing gRNA packaging of other retroviruses for comparison.

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.

HIV-1 nucleocapsid protein localizes efficiently to the nucleus and nucleolus

The HIV-1 nucleocapsid (NC) is an essential viral protein containing two highly conserved retroviral-type zinc finger (ZF) motifs, which functions in multiple stages of the HIV-1 life cycle. Although a number of functions for NC either in its mature form or as a domain of Gag have been revealed, little is known about the intracellular localization of NC and, moreover, its role in Gag protein trafficking. Here, we have investigated various forms of HIV-1 NC protein for its cellular localization and found that the NC has a strong nuclear and nucleolar localization activity. The linker region, composed of a stretch of basic amino acids between the two ZF motifs, was necessary and sufficient for the activity.

Divergence of primary cognate B- and T-cell proliferative responses to subcutaneous and intravenous immunization with virus-like particles

A major advantage of virus-like particle (VLP) vaccines against HIV is their structural identity to wild-type viruses, ensuring that antigen-specific B-cells encounter the envelope protein in its natural conformation. For the induction of affinity-matured antibodies, the B-cells must also obtain help from T-cells that are restricted by linear epitopes. Using B- and T-cell transgenic mouse models, we compared the efficacy of modified HIV-VLPs delivered by subcutaneous and intravenous immunization to stimulate primary B- and T-cell proliferative responses in different lymphoid organs. VLPs containing an influenza virus hemagglutinin epitope within the HIV-Gag protein induced comparable primary cognate T-cell proliferative responses in the draining lymph node and the spleen, irrespective of the delivery route. In contrast, after subcutaneous immunization with HIV-Gag VLPs containing hen egg lysozyme (HEL) on their surface, the proliferative response of transgenic HEL-specific B-cells was restricted to the draining lymph nodes, while intravenous VLP immunization primarily induced a B-cell proliferative response in the spleen. In vitro co-culture experiments further revealed that the presentation of VLP-associated surface antigens by dendritic cells to cognate B-cells is inefficient. This is consistent with a direct triggering of the B-cell proliferative response by the VLPs and suggests that HIV VLPs may indeed be suitable to directly promote the expansion of B-cells specific for conformational epitopes that are unique to functionally-active Env spikes on the virion. Further investigations are warranted to explore potential differences in the quality and protective potency of HIV-specific antibody responses induced by the two routes.

Cytoplasmic HIV-RNA in monocytes determines microglial activation and neuronal cell death in HIV-associated neurodegeneration

Despite highly active antiretroviral therapy, HIV-associated neurocognitive disorders (HAND) are still highly prevalent. Direct neurotoxicity of microglia activated by HIV-infected monocytes independent from viral replication may account for this observation. To investigate underlying molecular and viral determinants, human monocytoid cells (U937) transduced with HIV-particles were co-cultured with primary human microglia or astrocytes. Using genetically-engineered HIV-particles key steps of infection were examined. Levels of pro-inflammatory/neurotoxic cytokines were investigated in co-culture supernatants by flow cytometry. Neurotoxicity mediated by the supernatants was analysed using primary cortical rat neurons. To corroborate our findings, cytokine profiles in cerebrospinal fluid (CSF) of neuropsychologically asymptomatic HIV positive (HIV(+)) patients (n=45) were correlated with neurofilament H (NfH) as surrogate of neuronal/axonal degeneration. In contrast to direct exposure of HIV to microglia, only the presence of HIV-transduced monocytoid cells strongly activated human microglia as evidenced by enhanced secretion of CXCL10, CCL5, CCL2, and IL-6 (1.3-7.1-fold; p<0.01) leading to two-fold increased neurotoxicity (p<0.001). In direct comparison, astrocyte activation by HIV-transduced monocytoid cells was limited. Using different mutant HIV-particles we show that the presence of cytoplasmic HIV-RNA in monocytoid cells is the viral determinant for this unique microglial activation pattern and subsequent neuronal cell death; reverse transcription and expression of viral genes were not essential. In CSF of presymptomatic HIV(+) patients, CXCL10, CCL5 and IL-6 were correlated with NfH as surrogate marker of neurodegeneration as well as CSF-pleocytosis. In conclusion, cytosolic viral RNA in monocytes is mandatory for subsequent microglial activation and neurotoxicity; activated astrocytes may augment neuroinflammation. In addition, neuroinflammation and neurodegeneration occur even in preclinical HIV(+) patients and are associated with cytokines regulated in vitro. Our data may aid in the development of biomarkers and glia-directed therapeutic approaches of HAND.

Specific recognition of the HIV-1 genomic RNA by the Gag precursor

During assembly of HIV-1 particles in infected cells, the viral Pr55(Gag) protein (or Gag precursor) must select the viral genomic RNA (gRNA) from a variety of cellular and viral spliced RNAs. However, there is no consensus on how Pr55(Gag) achieves this selection. Here, by using RNA binding and footprinting assays, we demonstrate that the primary Pr55(Gag) binding site on the gRNA consists of the internal loop and the lower part of stem-loop 1 (SL1), the upper part of which initiates gRNA dimerization. A double regulation ensures specific binding of Pr55(Gag) to the gRNA despite the fact that SL1 is also present in spliced viral RNAs. The region upstream of SL1, which is present in all HIV-1 RNAs, prevents binding to SL1, but this negative effect is counteracted by sequences downstream of SL4, which are unique to the gRNA.

Novel vaccine regimen elicits strong airway immune responses and control of respiratory syncytial virus in nonhuman primates

Induction of long-lasting immunity against viral respiratory tract infections remains an elusive goal. Using a nonhuman primate model of human respiratory syncytial virus (hRSV) infection, we compared mucosal and systemic immune responses induced by different DNA delivery approaches to a novel parenteral DNA prime-tonsillar adenoviral vector booster immunization regimen. Intramuscular (i.m.) electroporation (EP) of a DNA vaccine encoding the fusion protein of hRSV induced stronger systemic immune responses than intradermal EP, tattoo immunization, and conventional i.m. DNA injection. A single EP i.m., followed by two atraumatic tonsillar immunizations with the adenoviral vector, elicited strong systemic immune responses, an unique persistent CD4(+) and CD8(+) T cell response in the lower respiratory tract and protection from intranasal hRSV challenge. Thus, parenteral DNA priming followed by booster immunization targeted to a mucosal inductive site constitutes an effective vaccine regimen for eliciting protective immune responses at mucosal effector sites.

IMPORTANCE

The human respiratory syncytial virus (hRSV) is the most common cause of severe respiratory tract disease in infancy and leads to substantial morbidity and morality in the elderly. In this study, we compared the immunogenicity and efficacy of several gene-based immunization protocols in rhesus macaques. Thereby, we found that the combination of an initially parenterally delivered DNA vaccine with a subsequent atraumatic tonsillar adenoviral vector immunization results in a strong systemic immune response accompanied by an exceptional high T-cell response in the mucosa. Strikingly, these animals were protected against a RSV challenge infection controlling the viral replication indicated by a 1,000-fold-lower viral load in the lower respiratory tract. Since mucosal cellular responses of this strength had not been described in earlier RSV vaccine studies, this heterologous DNA prime-tonsillar boost vaccine strategy is very promising and should be pursued for further preclinical and clinical testing.

Nuclear trafficking of retroviral RNAs and Gag proteins during late steps of replication

Retroviruses exploit nuclear trafficking machinery at several distinct stages in their replication cycles. In this review, we will focus primarily on nucleocytoplasmic trafficking events that occur after the completion of reverse transcription and proviral integration. First, we will discuss nuclear export of unspliced viral RNA transcripts, which serves two essential roles: as the mRNA template for the translation of viral structural proteins and as the genome for encapsidation into virions. These full-length viral RNAs must overcome the cell's quality control measures to leave the nucleus by co-opting host factors or encoding viral proteins to mediate nuclear export of unspliced viral RNAs. Next, we will summarize the most recent findings on the mechanisms of Gag nuclear trafficking and discuss potential roles for nuclear localization of Gag proteins in retrovirus replication.

HIV-derived lentiviral particles promote T-cell independent activation and differentiation of naïve cognate conventional B2-cells in vitro

In animal models, lentiviral particles (LP) were shown to be promising HIV vaccine candidates. Since little is known about the direct impact of LP on antigen-specific B cells, we incorporated Hen Egg Lysozyme (HEL) into LP (HEL-LP) derived from HIV to study their effect on HEL-specific, B cell receptor-transgenic B-cells (HEL(+)B-cells) in vitro. We observed preferential binding of HEL-LP to HEL(+)B-cells and their efficient internalization. HEL-LP were able to effectively cross-link B-cell receptors as indicated by the loss of surface CD62L. In the absence of CD4(+) T-cells, other activation events induced by LP in cognate naïve B-cells included increased expression of activation and co-stimulatory molecules as well as an enhanced proliferative response. Additionally, the B-cell phenotype shifted toward a germinal center pattern with further differentiation into memory and IgG3- and IgA-producing cells. The observed CD4(+) T-cell independent activation and differentiation may be due to LP-induced expression of CD40L by a subset of cognate B-cells. Thus, even in the absence of CD4(+) T-cells LP provide strong direct activation signals to cognate naïve B-cells, which may contribute to the strong humoral immune responses observed after LP immunization.

The HIV-1 Rev protein enhances encapsidation of unspliced and spliced, RRE-containing lentiviral vector RNA

Background

During the RNA encapsidation process of human immunodeficiency virus (HIV) viral genomic, unspliced RNA (gRNA) is preferentially incorporated into assembling virions. However, a certain amount of spliced viral transcripts can also be detected in viral particles. Recently, we observed that nuclear export of HIV and lentiviral vector gRNA by Rev is required for efficient encapsidation. Since singly-spliced HIV transcripts also contain the Rev-response element (RRE), we investigated if the encapsidation efficiency of RRE-containing spliced HIV-vector transcripts is also increased by the viral Rev protein.

Findings

Starting with a lentiviral vector imitating the splicing pattern of HIV, we constructed vectors that express an unspliced transcript either identical in sequence to the singly-spliced or the fully-spliced RNA of the parental construct. After transfection of the different lentiviral vectors cytoplasmic and virion-associated RNA levels and vector titers were determined in the presence and absence of Rev. Rev enhanced the infectious titer of vectors containing an RRE 6 to 37-fold. Furthermore, Rev strongly increased encapsidation efficiencies of all RRE-containing transcripts up to 200-fold. However, a good correlation between encapsidation efficiency and lentiviral vector titer could only be observed for the gRNA. The infectious titer of the vector encoding the fully-spliced RNA without RRE as well as the encapsidation efficiency of all transcripts lacking the RRE was not influenced by Rev. Interestingly, the splicing process itself did not seem to interfere with packaging, since the encapsidation efficiencies of the same RNA expressed either by splicing or as an unspliced transcript did not differ significantly.

Conclusions

Rev-mediated nuclear export enhances the encapsidation efficiency of RRE-containing lentiviral vector RNAs independently of whether they have been spliced or not.

NC-mediated nucleolar localization of retroviral gag proteins

The assembly and release of retrovirus particles from the cell membrane is directed by the Gag polyprotein. The Gag protein of Rous sarcoma virus (RSV) traffics through the nucleus prior to plasma membrane localization. We previously reported that nuclear localization of RSV Gag is linked to efficient packaging of viral genomic RNA, however the intranuclear activities of RSV Gag are not well understood. To gain insight into the properties of the RSV Gag protein within the nucleus, we examined the subnuclear localization and dynamic trafficking of RSV Gag. Restriction of RSV Gag to the nucleus by mutating its nuclear export signal (NES) in the p10 domain or interfering with CRM1-mediated nuclear export of Gag by leptomycin B (LMB) treatment led to the accumulation of Gag in nucleoli and discrete nucleoplasmic foci. Retention of RSV Gag in nucleoli was reduced with cis-expression of the 5' untranslated RU5 region of the viral RNA genome, suggesting the psi (Ψ) packaging signal may alter the subnuclear localization of Gag. Fluorescence recovery after photobleaching (FRAP) demonstrated that the nucleolar fraction of Gag was highly mobile, indicating that there was rapid exchange with Gag proteins in the nucleoplasm. RSV Gag is targeted to nucleoli by a nucleolar localization signal (NoLS) in the NC domain, and similarly, the human immunodeficiency virus type 1 (HIV-1) NC protein also contains an NoLS consisting of basic residues. Interestingly, co-expression of HIV-1 NC or Rev with HIV-1 Gag resulted in accumulation of Gag in nucleoli. Moreover, a subpopulation of HIV-1 Gag was detected in the nucleoli of HeLa cells stably expressing the entire HIV-1 genome in a Rev-dependent fashion. These findings suggest that the RSV and HIV-1 Gag proteins undergo nucleolar trafficking in the setting of viral infection.

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.

Role of Nucleocytoplasmic RNA Transport during the Life Cycle of Retroviruses

Retroviruses have evolved mechanisms for transporting their intron-containing RNAs (including genomic and messenger RNAs, which encode virion components) from the nucleus to the cytoplasm of the infected cell. Human retroviruses, such as human immunodeficiency virus (HIV) and human T cell leukemia virus type 1 (HTLV-1), encode the regulatory proteins Rev and Rex, which form a bridge between the viral RNA and the export receptor CRM1. Recent studies show that these transport systems are not only involved in RNA export, but also in the encapsidation of genomic RNA; furthermore, they influence subsequent events in the cytoplasm, including the translation of the cognate mRNA, transport of Gag proteins to the plasma membrane, and the formation of virus particles. Moreover, the mode of interaction between the viral and cellular RNA transport machinery underlies the species-specific propagation of HIV-1 and HTLV-1, forming the basis for constructing animal models of infection. This review article discusses recent progress regarding these issues.

cis-Acting determinants of 7SL RNA packaging by HIV-1

The host noncoding RNA 7SL is highly enriched in the virions of retroviruses. We examined the regions of 7SL that mediate packaging by HIV-1. Both the Alu domain and the S domain were sufficient to mediate specific packaging when expressed separately as truncations of 7SL. However, while the Alu domain competed with endogenous 7SL for packaging in proportion to Gag, the S domain was packaged additively, implying that the Alu and S domains are packaged via separate mechanisms and that the Alu domain is packaged by the same mechanism as endogenous 7SL. Further truncations of the Alu domain or mutation of the Alu domain helix 5c region significantly reduced packaging efficiency, implicating helix 5c as critical for packaging, reinforcing the finding that 7SL packaging is highly selective, and confirming that 7SL is not passively acquired. Surprisingly, when the Alu domain was mutated so that it no longer contained a binding site for the SRP protein heterodimer SRP9/14, it was no longer packaged in a competitive manner but instead was packaged additively with endogenous 7SL. These data support a model in which 7SL RNA is packaged via interactions between Gag and a 7SL RNA structure that exists transiently at a discrete stage of SRP biogenesis. Our data further indicate that a secondary "additive" pathway exists that can result in the packaging of certain 7SL derivatives in molar excess to endogenously packaged 7SL.