Human immunodeficiency virus type 1-specific immunity after genetic immunization is enhanced by modification of Gag and Pol expression

Structural insights into inhibitor mechanisms on immature HIV-1 Gag lattice revealed by high-resolution in situ single-particle cryo-EM

HIV-1 inhibitors, such as Bevirimat (BVM) and Lenacapavir (LEN), block the production and maturation of infectious virions. However, their mechanisms remain unclear due to the absence of high-resolution structures for BVM complexes and LEN's structural data being limited to the mature capsid. Utilizing perforated virus-like particles (VLPs) produced from mammalian cells, we developed an approach to determine in situ cryo-electron microscopy (cryo-EM) structures of HIV-1 with inhibitors. This allowed for the first structural determination of the native immature HIV-1 particle with BVM and LEN bound inside the VLPs at high resolutions. Our findings offer a more accurate model of BVM engaging the Gag lattice and, importantly, demonstrate that LEN not only binds the mature capsid but also targets the immature lattice in a distinct manner. The binding of LEN induces a conformational change in the capsid protein (CA) region and alters the architecture of the Gag lattice, which may affect the maturation process. These insights expand our understanding of the inhibitory mechanisms of BVM and LEN on HIV-1 and provide valuable clues for the design of future inhibitors.

Pseudotyped Viruses

Pseudotyped viruses have been constructed for many viruses. They can mimic the authentic virus and have many advantages compared to authentic viruses. Thus, they have been widely used as a surrogate of authentic virus for viral function analysis, detection of neutralizing antibodies, screening viral entry inhibitors, and others. This chapter reviewed the progress in the field of pseudotyped viruses in general, including the definition and the advantages of pseudotyped viruses, their potential usage, different strategies or vectors used for the construction of pseudotyped viruses, and factors that affect the construction of pseudotyped viruses.

CryoET structures of immature HIV Gag reveal six-helix bundle

Gag is the HIV structural precursor protein which is cleaved by viral protease to produce mature infectious viruses. Gag is a polyprotein composed of MA (matrix), CA (capsid), SP1, NC (nucleocapsid), SP2 and p6 domains. SP1, together with the last eight residues of CA, have been hypothesized to form a six-helix bundle responsible for the higher-order multimerization of Gag necessary for HIV particle assembly. However, the structure of the complete six-helix bundle has been elusive. Here, we determined the structures of both Gag in vitro assemblies and Gag viral-like particles (VLPs) to 4.2 Å and 4.5 Å resolutions using cryo-electron tomography and subtomogram averaging by emClarity. A single amino acid mutation (T8I) in SP1 stabilizes the six-helix bundle, allowing to discern the entire CA-SP1 helix connecting to the NC domain. These structures provide a blueprint for future development of small molecule inhibitors that can lock SP1 in a stable helical conformation, interfere with virus maturation, and thus block HIV-1 infection.

Immunogenicity of HIV-1-Based Virus-Like Particles with Increased Incorporation and Stability of Membrane-Bound Env

An optimal prophylactic vaccine to prevent human immunodeficiency virus (HIV-1) transmission should elicit protective antibody responses against the HIV-1 envelope glycoprotein (Env). Replication-incompetent HIV-1 virus-like particles (VLPs) offer the opportunity to present virion-associated Env with a native-like structure during vaccination that closely resembles that encountered on infectious virus. Here, we optimized the incorporation of Env into previously designed mature-form VLPs (mVLPs) and assessed their immunogenicity in mice. The incorporation of Env into mVLPs was increased by replacing the Env transmembrane and cytoplasmic tail domains with those of influenza haemagglutinin (HA-TMCT). Furthermore, Env was stabilized on the VLP surface by introducing an interchain disulfide and proline substitution (SOSIP) mutations typically employed to stabilize soluble Env trimers. The resulting mVLPs efficiently presented neutralizing antibody epitopes while minimizing exposure of non-neutralizing antibody sites. Vaccination of mice with mVLPs elicited a broader range of Env-specific antibody isotypes than Env presented on immature VLPs or extracellular vesicles. The mVLPs bearing HA-TMCT-modified Env consistently induced anti-Env antibody responses that mediated modest neutralization activity. These mVLPs are potentially useful immunogens for eliciting neutralizing antibody responses that target native Env epitopes on infectious HIV-1 virions.

HIV-1 Envelope Glycoprotein Trafficking through the Endosomal Recycling Compartment Is Required for Particle Incorporation

The human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) encodes specific trafficking signals within its long cytoplasmic tail (CT) that regulate incorporation into HIV-1 particles. Rab11-family interacting protein 1C (FIP1C) and Rab14 are host trafficking factors required for Env particle incorporation, suggesting that Env undergoes sorting from the endosomal recycling compartment (ERC) to the site of particle assembly on the plasma membrane. We disrupted outward sorting from the ERC by expressing a C-terminal fragment of FIP1C (FIP1C_560-649) and examined the consequences on Env trafficking and incorporation into particles. FIP1C_560-649 reduced cell surface levels of Env and prevented its incorporation into HIV-1 particles. Remarkably, Env was trapped in an exaggerated perinuclear ERC in a CT-dependent manner. Mutation of either the Yxxϕ endocytic motif or the YW₇₉₅ motif in the CT prevented Env trapping in the ERC and restored incorporation into particles. In contrast, simian immunodeficiency virus SIVmac239 Env was not retained in the ERC, while substitution of the HIV-1 CT for the SIV CT resulted in SIV Env retention in this compartment. These results provide the first direct evidence that Env traffics through the ERC and support a model whereby HIV-1 Env is specifically targeted to the ERC prior to FIP1C- and CT-dependent outward sorting to the particle assembly site on the plasma membrane.IMPORTANCE The HIV envelope protein is an essential component of the viral particle. While many aspects of envelope protein structure and function have been established, the pathway it follows in the cell prior to reaching the site of particle assembly is not well understood. The envelope protein has a very long cytoplasmic tail that interacts with the host cell trafficking machinery. Here, we utilized a truncated form of the trafficking adaptor FIP1C protein to arrest the intracellular transport of the envelope protein, demonstrating that it becomes trapped inside the cell within the endosomal recycling compartment. Intracellular trapping resulted in a loss of envelope protein on released particles and a corresponding loss of infectivity. Mutations of specific trafficking motifs in the envelope protein tail prevented its trapping in the recycling compartment. These results establish that trafficking to the endosomal recycling compartment is an essential step in HIV envelope protein particle incorporation.

Increasing the CpG dinucleotide abundance in the HIV-1 genomic RNA inhibits viral replication

BACKGROUND

The human immunodeficiency virus type 1 (HIV-1) structural protein Gag is necessary and sufficient to form viral particles. In addition to encoding the amino acid sequence for Gag, the underlying RNA sequence could encode cis-acting elements or nucleotide biases that are necessary for viral replication. Furthermore, RNA sequences that inhibit viral replication could be suppressed in gag. However, the functional relevance of RNA elements and nucleotide biases that promote or repress HIV-1 replication remain poorly understood.

RESULTS

To characterize if the RNA sequence in gag controls HIV-1 replication, the matrix (MA) region was codon modified, allowing the RNA sequence to be altered without affecting the protein sequence. Codon modification of nucleotides (nt) 22-261 or 22-378 in gag inhibited viral replication by decreasing genomic RNA (gRNA) abundance, gRNA stability, Gag expression, virion production and infectivity. Comparing the effect of these point mutations to deletions of the same region revealed that the mutations inhibited infectious virus production while the deletions did not. This demonstrated that codon modification introduced inhibitory sequences. There is a much lower than expected frequency of CpG dinucleotides in HIV-1 and codon modification introduced a substantial increase in CpG abundance. To determine if they are necessary for inhibition of HIV-1 replication, codons introducing CpG dinucleotides were mutated back to the wild type codon, which restored efficient Gag expression and infectious virion production. To determine if they are sufficient to inhibit viral replication, CpG dinucleotides were inserted into gag in the absence of other changes. The increased CpG dinucleotide content decreased HIV-1 infectivity and viral replication.

CONCLUSIONS

The HIV-1 RNA sequence contains low abundance of CpG dinucleotides. Increasing the abundance of CpG dinucleotides inhibits multiple steps of the viral life cycle, providing a functional explanation for why CpG dinucleotides are suppressed in HIV-1.

Siglec-1 initiates formation of the virus-containing compartment and enhances macrophage-to-T cell transmission of HIV-1

HIV-1 particles assemble and bud from the plasma membrane of infected T lymphocytes. Infected macrophages, in contrast, accumulate particles within an apparent intracellular compartment known as the virus-containing compartment or VCC. Many aspects of the formation and function of the VCC remain unclear. Here we demonstrate that VCC formation does not actually require infection of the macrophage, but can be reproduced through the exogenous addition of non-infectious virus-like particles or infectious virions to macrophage cultures. Particles were captured by Siglec-1, a prominent cell surface lectin that attaches to gangliosides on the lipid envelope of the virus. VCCs formed within infected macrophages were readily targeted by the addition of ganglioside-containing virus-like particles to the extracellular media. Depletion of Siglec-1 from the macrophage or depletion of gangliosides from viral particles prevented particle uptake into the VCC and resulted in substantial reductions of VCC volume. Furthermore, Siglec-1-mediated virion capture and subsequent VCC formation was required for efficient trans-infection of autologous T cells. Our results help to define the nature of this intracellular compartment, arguing that it is a compartment formed by particle uptake from the periphery, and that this compartment can readily transmit virus to target T lymphocytes. Inhibiting or eliminating the VCC may be an important component of strategies to reduce HIV transmission and to eradicate HIV reservoirs.

T lymphocytes and macrophages are the two major cell types involved in HIV replication and transmission events. When a T cell is infected, virus particles assemble and bud from the plasma membrane of the cell. In contrast, infected macrophages develop an intracellular collection of viruses termed the virus-containing compartment or VCC. Many aspects of the formation and function of the VCC remain unclear. Here we show that VCC formation does not actually require infection of the macrophage, but can be reproduced through the addition of virus-like particles or infectious virions to macrophages. HIV-1 particles were captured by the cell surface carbohydrate-binding protein Siglec-1, followed by co-migration of Siglec-1 and captured viral particles to the VCC. Depletion of Siglec-1 from the macrophage prevented VCC formation, and inhibited the ability of infected macrophages to transmit HIV to T cells. Our results help to define the origin of this intracellular compartment, arguing that it is a compartment formed by particle uptake from the periphery. Inhibiting or eliminating the VCC may be an important component of strategies to reduce HIV transmission and to eradicate HIV reservoirs.

Mechanistic Studies and Modeling Reveal the Origin of Differential Inhibition of Gag Polymorphic Viruses by HIV-1 Maturation Inhibitors

HIV-1 maturation inhibitors (MIs) disrupt the final step in the HIV-1 protease-mediated cleavage of the Gag polyprotein between capsid p24 capsid (CA) and spacer peptide 1 (SP1), leading to the production of infectious virus. BMS-955176 is a second generation MI with improved antiviral activity toward polymorphic Gag variants compared to a first generation MI bevirimat (BVM). The underlying mechanistic reasons for the differences in polymorphic coverage were studied using antiviral assays, an LC/MS assay that quantitatively characterizes CA/SP1 cleavage kinetics of virus like particles (VLPs) and a radiolabel binding assay to determine VLP/MI affinities and dissociation kinetics. Antiviral assay data indicates that BVM does not achieve 100% inhibition of certain polymorphs, even at saturating concentrations. This results in the breakthrough of infectious virus (partial antagonism) regardless of BVM concentration. Reduced maximal percent inhibition (MPI) values for BVM correlated with elevated EC50 values, while rates of HIV-1 protease cleavage at CA/SP1 correlated inversely with the ability of BVM to inhibit HIV-1 Gag polymorphic viruses: genotypes with more rapid CA/SP1 cleavage kinetics were less sensitive to BVM. In vitro inhibition of wild type VLP CA/SP1 cleavage by BVM was not maintained at longer cleavage times. BMS-955176 exhibited greatly improved MPI against polymorphic Gag viruses, binds to Gag polymorphs with higher affinity/longer dissociation half-lives and exhibits greater time-independent inhibition of CA/SP1 cleavage compared to BVM. Virological (MPI) and biochemical (CA/SP1 cleavage rates, MI-specific Gag affinities) data were used to create an integrated semi-quantitative model that quantifies CA/SP1 cleavage rates as a function of both MI and Gag polymorph. The model outputs are in accord with in vitro antiviral observations and correlate with observed in vivo MI efficacies. Overall, these findings may be useful to further understand antiviral profiles and clinical responses of MIs at a basic level, potentially facilitating further improvements to MI potency and coverage.

The Host RNAs in Retroviral Particles

As they assemble, retroviruses encapsidate both their genomic RNAs and several types of host RNA. Whereas limited amounts of messenger RNA (mRNA) are detectable within virion populations, the predominant classes of encapsidated host RNAs do not encode proteins, but instead include endogenous retroelements and several classes of non-coding RNA (ncRNA), some of which are packaged in significant molar excess to the viral genome. Surprisingly, although the most abundant host RNAs in retroviruses are also abundant in cells, unusual forms of these RNAs are packaged preferentially, suggesting that these RNAs are recruited early in their biogenesis: before associating with their cognate protein partners, and/or from transient or rare RNA populations. These RNAs' packaging determinants differ from the viral genome's, and several of the abundantly packaged host ncRNAs serve cells as the scaffolds of ribonucleoprotein particles. Because virion assembly is equally efficient whether or not genomic RNA is available, yet RNA appears critical to the structural integrity of retroviral particles, it seems possible that the selectively encapsidated host ncRNAs might play roles in assembly. Indeed, some host ncRNAs appear to act during replication, as some transfer RNA (tRNA) species may contribute to nuclear import of human immunodeficiency virus 1 (HIV-1) reverse transcription complexes, and other tRNA interactions with the viral Gag protein aid correct trafficking to plasma membrane assembly sites. However, despite high conservation of packaging for certain host RNAs, replication roles for most of these selectively encapsidated RNAs-if any-have remained elusive.

Identification and Characterization of BMS-955176, a Second-Generation HIV-1 Maturation Inhibitor with Improved Potency, Antiviral Spectrum, and Gag Polymorphic Coverage

BMS-955176 is a second-generation human immunodeficiency virus type 1 (HIV-1) maturation inhibitor (MI). A first-generation MI, bevirimat, showed clinical efficacy in early-phase studies, but ∼50% of subjects had viruses with reduced susceptibility associated with naturally occurring polymorphisms in Gag near the site of MI action. MI potency was optimized using a panel of engineered reporter viruses containing site-directed polymorphic changes in Gag that reduce susceptibility to bevirimat (including V362I, V370A/M/Δ, and T371A/Δ), leading incrementally to the identification of BMS-955176. BMS-955176 exhibits potent activity (50% effective concentration [EC50], 3.9 ± 3.4 nM [mean ± standard deviation]) toward a library (n = 87) of gag/pr recombinant viruses representing 96.5% of subtype B polymorphic Gag diversity near the CA/SP1 cleavage site. BMS-955176 exhibited a median EC50 of 21 nM toward a library of subtype B clinical isolates assayed in peripheral blood mononuclear cells (PBMCs). Potent activity was maintained against a panel of reverse transcriptase, protease, and integrase inhibitor-resistant viruses, with EC50s similar to those for the wild-type virus. A 5.4-fold reduction in EC50 occurred in the presence of 40% human serum plus 27 mg/ml of human serum albumin (HSA), which corresponded well to an in vitro measurement of 86% human serum binding. Time-of-addition and pseudotype reporter virus studies confirm a mechanism of action for the compound that occurs late in the virus replication cycle. BMS-955176 inhibits HIV-1 protease cleavage at the CA/SP1 junction within Gag in virus-like particles (VLPs) and in HIV-1-infected cells, and it binds reversibly and with high affinity to assembled Gag in purified HIV-1 VLPs. Finally, in vitro combination studies showed no antagonistic interactions with representative antiretrovirals (ARVs) of other mechanistic classes. In conclusion, BMS-955176 is a second-generation MI with potent in vitro anti-HIV-1 activity and a greatly improved preclinical profile compared to that of bevirimat.

Loss of long term protection with the inclusion of HIV pol to a DNA vaccine encoding gag

Traditional vaccine strategies that induce antibody responses have failed to protect against HIV infection in clinical trials, and thus cell-mediated immunity is now an additional criterion. Recent clinical trials that aimed to induce strong T cell responses failed to do so. Therefore, to enhance induction of protective T cell responses, it is crucial that the optimum antigen combination is chosen. Limited research has been performed into the number of antigens selected for an HIV vaccine. This study aimed to compare DNA vaccines encoding either a single HIV antigen or a combination of two antigens, using intradermal vaccination of C57BL/6 mice. Immune assays were performed on splenocytes, and in vivo protection was examined by challenge with a chimeric virus, EcoHIV, able to infect mouse but not human leukocytes, at 10 days (short term) and 60 days (long term) post final vaccination. At 60 days there was significantly lower frequency of induced antigen-specific CD8(+) T cells in the spleens of pCMVgag-pol-vaccinated mice compared with mice which received pCMVgag only. Most importantly, short term viral control of EcoHIV was similar for pCMVgag and pCMVgag-pol-vaccinated mice at day 10, but only the pCMVgag-vaccinated significantly controlled EcoHIV at day 60 compared with pCMV-vaccinated mice, showing that control was reduced with the inclusion of the HIV pol gene.

The role of A-kinase anchoring protein 95-like protein in annealing of tRNALys3 to HIV-1 RNA

Background

RNA helicase A (RHA), a DExH box protein, promotes annealing of tRNA^Lys3, a primer for reverse transcription, to HIV-1 RNA and assembles into virus particles. A-kinase anchoring protein 95-like protein (HAP95) is a binding partner of RHA. The role of HAP95 in the annealing of tRNA^Lys3 was examined in this study.

Results

HAP95 associates with the reverse transcriptase region of Pol protein of HIV-1. Decreasing endogenous HAP95 in HIV-1-producing 293T cells by siRNA reduces the amount of tRNA^Lys3 annealed on viral RNA. This defect was further deteriorated by knockdown of RHA in the same cells, suggesting a cooperative effect between these two proteins. Biochemical assay in vitro using purified GST-tagged HAP95 shows that HAP95 may inhibit the activity of RHA.

Conclusion

The results support a hypothesis that HAP95 may transiently block RHA’s activity to protect the annealed tRNA^Lys3 on viral RNA in the cells from removing by RHA during the packaging of RHA into virus particles, thus facilitating the annealing of tRNA^Lys3 to HIV-1 RNA.

The roles of APOBEC3G complexes in the incorporation of APOBEC3G into HIV-1

Background

The incorporation of human APOBEC3G (hA3G) into HIV is required for exerting its antiviral activity, therefore the mechanism underlying hA3G virion encapsidation has been investigated extensively. hA3G was shown to form low-molecular-mass (LMM) and high-molecular-mass (HMM) complexes. The function of different forms of hA3G in its viral incorporation remains unclear.

Methodology/Principal Findings

In this study, we investigated the subcellular distribution and lipid raft association of hA3G using subcellular fractionation, membrane floatation assay and pulse-chase radiolabeling experiments respectively, and studied the correlation between the ability of hA3G to form the different complex and its viral incorporation. Our work herein provides evidence that the majority of newly-synthesized hA3G interacts with membrane lipid raft domains to form Lipid raft-associated hA3G (RA hA3G), which serve as the precursor of mature HMM hA3G complex, while a minority of newly-synthesized hA3G remains in the cytoplasm as a soluble LMM form. The distribution of hA3G among the soluble LMM form, the RA LMM form and the mature forms of HMM is regulated by a mechanism involving the N-terminal part of the linker region and the C-terminus of hA3G. Mutagenesis studies reveal a direct correlation between the ability of hA3G to form the RA LMM complex and its viral incorporation.

Conclusions/Significance

Together these data suggest that the Lipid raft-associated LMM A3G complex functions as the cellular source of viral hA3G.

Breadth of cellular and humoral immune responses elicited in rhesus monkeys by multi-valent mosaic and consensus immunogens

To create an HIV-1 vaccine that generates sufficient breadth of immune recognition to protect against the genetically diverse forms of the circulating virus, we have been exploring vaccines based on consensus and mosaic protein designs. Increasing the valency of a mosaic immunogen cocktail increases epitope coverage but with diminishing returns, as increasingly rare epitopes are incorporated into the mosaic proteins. In this study we compared the immunogenicity of 2-valent and 3-valent HIV-1 envelope mosaic immunogens in rhesus monkeys. Immunizations with the 3-valent mosaic immunogens resulted in a modest increase in the breadth of vaccine-elicited T lymphocyte responses compared to the 2-valent mosaic immunogens. However, the 3-valent mosaic immunogens elicited significantly higher neutralizing responses to Tier 1 viruses than the 2-valent mosaic immunogens. These findings underscore the potential utility of polyvalent mosaic immunogens for eliciting both cellular and humoral immune responses to HIV-1.

Elicitation of anti-1918 influenza virus immunity early in life prevents morbidity and lower levels of lung infection by 2009 pandemic H1N1 influenza virus in aged mice

The Spanish influenza virus pandemic of 1918 was responsible for 40 million to 50 million deaths and is antigenically similar to the swine lineage 2009 pandemic influenza virus. Emergence of the 2009 pandemic from swine into humans has raised the possibility that low levels of cross-protective immunity to past shared epitopes could confer protection. In this study, influenza viruslike particles (VLPs) were engineered to express the hemagglutinin (HA) and genes from the 1918 influenza virus to evaluate the duration of cross-protection to the H1N1 pandemic strain by vaccinating young mice (8 to 12 weeks) and then allowing the animals to age to 20 months. This immunity was long lasting, with homologous receptor-blocking antibodies detected throughout the lifespan of vaccinated mice. Furthermore, the 1918 VLPs fully protected aged mice from 2009 pandemic H1N1 virus challenge 16 months after vaccination. Histopathological assessment showed that aged vaccinated mice had significant protection from alveolar infection but less protection of the bronchial tissue than adult vaccinated mice. Additionally, passive transfer of immune serum from aged vaccinated mice resulted in protection from death but not morbidity. This is the first report describing the lifelong duration of cross-reactive immune responses elicited by a 1918 VLP vaccine in a murine model. Importantly, these lifelong immune responses did not result in decreased total viral replication but did prevent infection of the lower respiratory tract. These findings show that immunity acquired early in life can restrict the anatomical location of influenza viral replication, rather than preventing infection, in the aged.

Identification of molecular determinants from Moloney leukemia virus 10 homolog (MOV10) protein for virion packaging and anti-HIV-1 activity

Discovery of novel antiretroviral mechanism is essential for the design of innovative antiretroviral therapy. Recently, we and others reported that ectopic expression of Moloney leukemia virus 10 (MOV10) protein strongly inhibits retrovirus replication. MOV10, a putative RNA helicase, can be packaged into HIV-1 virions by binding to the nucleocapsid (NC) region of Gag and inhibit viral replication at a postentry step. Here, we report critical determinants for MOV10 virion packaging and antiviral activity. MOV10 has 1,003 amino acids and seven helicase motifs. We found that MOV10 packaging requires the NC basic linker, and Gag binds to the N-terminal amino acids 261-305 region of MOV10. Our predicted MOV10 three-dimensional structure model indicates that the Gag binding region is located in a structurally exposed domain, which spans amino acids 93-305 and is Cys-His-rich. Simultaneous mutation of residues Cys-188, Cys-195, His-199, His-201, and His-202 in this domain significantly compromised MOV10 anti-HIV-1 activity. Notably, although MOV10-Gag interaction is required, it is not sufficient for MOV10 packaging, which also requires its C-terminal all but one of seven helicase motifs. Moreover, we have mapped the minimal MOV10 antiviral region to amino acids 99-949, indicating that nearly all MOV10 residues are required for its antiviral activity. Mutations of residues Cys-947, Pro-948, and Phe-949 at the C terminus of this region completely disrupted MOV10 anti-HIV-1 activity. Taken together, we have identified two critical MOV10 packaging determinants and eight other critical residues for anti-HIV-1 activity. These results provide a molecular basis for further understanding the MOV10 antiretroviral mechanism.

Filamin A protein interacts with human immunodeficiency virus type 1 Gag protein and contributes to productive particle assembly

HIV-1 Gag precursor directs virus particle assembly and release. In a search for Gag-interacting proteins that are involved in late stages of the HIV-1 replication cycle, we performed yeast two-hybrid screening against a human cDNA library and identified the non-muscle actin filament cross-linking protein filamin A as a novel Gag binding partner. The 280-kDa filamin A regulates cortical actin network dynamics and participates in the anchoring of membrane proteins to the actin cytoskeleton. Recent studies have shown that filamin A facilitates HIV-1 cell-to-cell transmission by binding to HIV receptors and coreceptors and regulating their clustering on the target cell surface. Here we report a novel role for filamin A in HIV-1 Gag intracellular trafficking. We demonstrate that filamin A interacts with the capsid domain of HIV-1 Gag and that this interaction is involved in particle release in a productive manner. Disruption of this interaction eliminated Gag localization at the plasma membrane and induced Gag accumulation within internal compartments. Moreover, blocking clathrin-dependent endocytic pathways did not relieve the restriction to particle release induced by filamin A depletion. These results suggest that filamin A is involved in the distinct step of the Gag trafficking pathway. The discovery of the Gag-filamin A interaction may provide a new therapeutic target for the treatment of HIV infection.

Different patterns of expansion, contraction and memory differentiation of HIV-1 Gag-specific CD8 T cells elicited by adenovirus type 5 and modified vaccinia Ankara vaccines

The magnitude and functional quality of antiviral CD8 T cell responses are critical for the efficacy of T cell based vaccines. Here, we investigate the influence of two popular viral vectors, adenovirus type 5 (Ad5) and modified vaccinia Ankara (MVA), on expansion, contraction and memory differentiation of HIV-1 Gag insert-specific CD8 T cell responses following immunization and show different patterns for the two recombinant viral vectors. The Ad5 vector primed 6-fold higher levels of insert-specific CD8 effector T cells than the MVA vector. The Ad5-primed effector cells also underwent less contraction (<2-fold) than the MVA-primed cells (>5-fold). The Ad5-primed memory cells were predominantly CD62L negative (effector memory) whereas the MVA-primed memory cells were predominantly CD62L positive (central memory). Consistent with their memory phenotype, MVA-primed CD8 T cells underwent higher fold expansion than Ad5-primed CD8 T cells following a homologous or heterologous boost. Impressively, the Ad5 boost changed the quality of MVA-primed memory response such that they undergo less contraction with effector memory phenotype. However, the MVA boost did not influence the contraction and memory phenotype of Ad5-primed response. In conclusion, our results demonstrate that vaccine vector strongly influences the expansion, contraction and the functional quality of insert-specific CD8 T cell responses and have implications for vaccine development against infectious diseases.

The cellular source for APOBEC3G's incorporation into HIV-1

Background

Human APOBEC3G (hA3G) has been identified as a cellular inhibitor of HIV-1 infectivity. Viral incorporation of hA3G is an essential step for its antiviral activity. Although the mechanism underlying hA3G virion encapsidation has been investigated extensively, the cellular source of viral hA3G remains unclear.

Results

Previous studies have shown that hA3G forms low-molecular-mass (LMM) and high-molecular-mass (HMM) complexes. Our work herein provides evidence that the majority of newly-synthesized hA3G interacts with membrane lipid raft domains to form Lipid raft-associated hA3G (RA hA3G), which serve as the precursor of the mature HMM hA3G complex, while a minority of newly-synthesized hA3G remains in the cytoplasm as a soluble LMM form. The distribution of hA3G among the soluble LMM form, the RA LMM form and the mature forms of HMM is regulated by a mechanism involving the N-terminal part of the linker region and the C-terminus of hA3G. Mutagenesis studies reveal a direct correlation between the ability of hA3G to form the RA LMM complex and its viral incorporation.

Conclusions

Together these data suggest that the Lipid raft-associated LMM A3G complex functions as the cellular source of viral hA3G.

Coordinate roles of Gag and RNA helicase A in promoting the annealing of formula to HIV-1 RNA

RNA helicase A (RHA) has been shown to promote HIV-1 replication at both the translation and reverse transcription stages. A prerequisite step for reverse transcription involves the annealing of tRNA(3)(Lys), the primer for reverse transcription, to HIV-1 RNA. tRNA(3)(Lys) annealing is a multistep process that is initially facilitated by Gag prior to viral protein processing. Herein, we report that RHA promotes this annealing through increasing both the quantity of tRNA(3)(Lys) annealed by Gag and the ability of tRNA(3)(Lys) to prime the initiation of reverse transcription. This improved annealing is the result of an altered viral RNA conformation produced by the coordinate action of Gag and RHA. Since RHA has been reported to promote the translation of unspliced viral RNA to Gag protein, our observations suggest that the conformational change in viral RNA induced by RHA and newly produced Gag may help facilitate the switch in viral RNA from a translational mode to one facilitating tRNA(3)(Lys) annealing.

Robust vaccine-elicited cellular immune responses in breast milk following systemic simian immunodeficiency virus DNA prime and live virus vector boost vaccination of lactating rhesus monkeys

Breast milk transmission of HIV remains an important mode of infant HIV acquisition. Enhancement of mucosal HIV-specific immune responses in milk of HIV-infected mothers through vaccination may reduce milk virus load or protect against virus transmission in the infant gastrointestinal tract. However, the ability of HIV/SIV strategies to induce virus-specific immune responses in milk has not been studied. In this study, five uninfected, hormone-induced lactating, Mamu A*01(+) female rhesus monkey were systemically primed and boosted with rDNA and the attenuated poxvirus vector, NYVAC, containing the SIVmac239 gag-pol and envelope genes. The monkeys were boosted a second time with a recombinant Adenovirus serotype 5 vector containing matching immunogens. The vaccine-elicited immunodominant epitope-specific CD8(+) T lymphocyte response in milk was of similar or greater magnitude than that in blood and the vaginal tract but higher than that in the colon. Furthermore, the vaccine-elicited SIV Gag-specific CD4(+) and CD8(+) T lymphocyte polyfunctional cytokine responses were more robust in milk than in blood after each virus vector boost. Finally, SIV envelope-specific IgG responses were detected in milk of all monkeys after vaccination, whereas an SIV envelope-specific IgA response was only detected in one vaccinated monkey. Importantly, only limited and transient increases in the proportion of activated or CCR5-expressing CD4(+) T lymphocytes in milk occurred after vaccination. Therefore, systemic DNA prime and virus vector boost of lactating rhesus monkeys elicits potent virus-specific cellular and humoral immune responses in milk and may warrant further investigation as a strategy to impede breast milk transmission of HIV.

7SL RNA is retained in HIV-1 minimal virus-like particles as an S-domain fragment

HIV-1 is known to package several small cellular RNAs in addition to its genome. Previous work consistently demonstrated that the host structural RNA 7SL is abundant in HIV-1 virions but has yielded conflicting results regarding whether 7SL is present in minimal, assembly-competent virus-like particles (VLPs). Here, we demonstrate that minimal HIV-1 VLPs retain 7SL RNA primarily as an endoribonucleolytic fragment, referred to as 7SL remnant (7SLrem). Nuclease mapping showed that 7SLrem is a 111-nucleotide internal portion of 7SL, with 5' and 3' ends corresponding to unpaired loops in the 7SL two-dimensional structure. Analysis of VLPs comprised of different subsets of Gag domains revealed that all NC-positive VLPs contained intact 7SL while the presence of 7SLrem correlated with the absence of the NC domain. Because 7SLrem, which maps to the 7SL S domain, was not detectable in infected cells, we propose a model whereby the species recruited to assembling VLPs is intact 7SL RNA, with 7SLrem produced by an endoribonuclease in the absence of NC. Since recruitment of 7SL RNA was a conserved feature of all tested minimal VLPs, our model further suggests that 7SL's recruitment is mediated, either directly or indirectly, through interactions with conserved features of all tested VLPs, such as the C-terminal domain of CA.

Heterologous HIV-based lentiviral/adenoviral vectors immunizations result in enhanced HIV-specific immunity

Viral vectors are considered as one of the major means for the induction of strong immune responses against recombinant antigens by genetic immunization. Among these, lentiviral vectors are particularly attractive vehicles, as they can infect a wide variety of cells and can transduce replicating as well as non-replicating cells. We have engineered VRX1023, an HIV-1-based lentiviral vector (LV) vaccine candidate, to deliver HIV-1 Gag, Pol and Rev antigens under control of the native LTR promoter. While VRX1023 has been shown to elicit strong cell-mediated and humoral immunity as a stand-alone vaccine, we report here its combination in a heterologous prime-boost approach. Its combination with an adenovirus serotype 5 (Ad5)-based vector in the mouse model increased the frequency and polyfunctionality of HIV-specific CD4+ and CD8+ T cells. Homologous prime-boost regimens induced high levels of anti-vector neutralizing antibodies in Ad5-immunized mice, whereas the VSV-G-pseudotyped VRX1023 LV elicited low levels of anti-lentiviral vector neutralization. In addition, the heterologous prime-boost strategy resulted in a 5-fold reduction in Ad5-specific vector neutralization as compared to Ad5 homologous immunization. In conclusion, this study demonstrates that LV and Ad5 vector candidates can be combined in a heterologous immunization regimen, yielding dramatically improved immunogenicity while overcoming anti-vector immunity. These findings may have implications for the development of HIV vaccine regimens in populations with elevated Ad5 seroprevalence or when repeated vector administrations are required.

Human immunodeficiency virus-like particles with consensus envelopes elicited broader cell-mediated peripheral and mucosal immune responses than polyvalent and monovalent Env vaccines

Envelope (Env) sequences from human immunodeficiency virus (HIV) strains can vary by 15-20% within a single clade and as much as 35% between clades. Previous AIDS vaccines based upon a single isolate often could not elicit protective immune responses against heterologous viral challenges. In order to address the vast sequence diversity in Env sequences, consensus sequences were constructed for clade B and clade C envelopes and delivered to the mouse lung mucosa on the surface of virus-like particles (VLP). Consensus sequences decrease the genetic difference between the vaccine strain and any given viral isolate. The elicited immune responses were compared to a mixture of VLPs with Envs from primary viral isolates. This polyvalent vaccine approach contains multiple, diverse Envs to increase the breadth of epitopes recognized by the immune response and thereby increase the potential number of primary isolates recognized. Both consensus and polyvalent clade B Env VLP vaccines elicited cell-mediated immune responses that recognized a broader number of clade B Env peptides than a control monovalent Env VLP vaccine in both the systemic and the mucosal immune compartments. All three clade C Env vaccine strategies elicited similar responses to clade C peptides. However, both the consensus B and C Env VLP vaccines were more effective at eliciting cross-reactive cellular immune responses to epitopes in other clades. This is the first study to directly compare the breadth of cell-mediated immune responses elicited by consensus and polyvalent Env vaccines.

gp340 promotes transcytosis of human immunodeficiency virus type 1 in genital tract-derived cell lines and primary endocervical tissue

The human scavenger receptor gp340 has been identified as a binding protein for the human immunodeficiency virus type 1 (HIV-1) envelope that is expressed on the cell surface of female genital tract epithelial cells. This interaction allows such epithelial cells to efficiently transmit infective virus to susceptible targets and maintain viral infectivity for several days. Within the context of vaginal transmission, HIV must first traverse a normally protective mucosa containing a cell barrier to reach the underlying T cells and dendritic cells, which propagate and spread the infection. The mechanism by which HIV-1 can bypass an otherwise healthy cellular barrier remains an important area of study. Here, we demonstrate that genital tract-derived cell lines and primary human endocervical tissue can support direct transcytosis of cell-free virus from the apical to basolateral surfaces. Further, this transport of virus can be blocked through the addition of antibodies or peptides that directly block the interaction of gp340 with the HIV-1 envelope, if added prior to viral pulsing on the apical side of the cell or tissue barrier. Our data support a role for the previously described heparan sulfate moieties in mediating this transcytosis but add gp340 as an important facilitator of HIV-1 transcytosis across genital tract tissue. This study demonstrates that HIV-1 actively traverses the protective barriers of the human genital tract and presents a second mechanism whereby gp340 can promote heterosexual transmission.

Japanese encephalitis virus-based replicon RNAs/particles as an expression system for HIV-1 Pr55 Gag that is capable of producing virus-like particles

Ectopic expression of the structural protein Pr55(Gag) of HIV-1 has been limited by the presence of inhibitory sequences in the gag coding region that must normally be counteracted by HIV-1 Rev and RRE. Here, we describe a cytoplasmic RNA replicon based on the RNA genome of Japanese encephalitis virus (JEV) that is capable of expressing HIV-1 gag without requiring Rev/RRE. This replicon system was constructed by deleting all three JEV structural protein-coding regions (C, prM, and E) from the 5'-proximal region of the genome and simultaneously inserting an HIV-1 gag expression cassette driven by the internal ribosome entry site of encephalomyocarditis virus into the 3'-proximal noncoding region of the genome. Transfection of this JEV replicon RNA led to expression of Pr55(Gag) in the absence of Rev/RRE in the cytoplasm of hamster BHK-21, human HeLa, and mouse NIH/3T3 cells. Production of the Pr55(Gag) derived from this JEV replicon RNA appeared to be increased by approximately 3-fold when compared to that based on an alphavirus replicon RNA. Biochemical and morphological analyses demonstrated that the Pr55(Gag) proteins were released into the culture medium in the form of virus-like particles. We also observed that the JEV replicon RNAs expressing the Pr55(Gag) could be encapsidated into single-round infectious JEV replicon particles when transfected into a stable packaging cell line that provided the three JEV structural proteins in trans. This ectopic expression of the HIV-1 Pr55(Gag) by JEV-based replicon RNAs/particles in diverse cell types may represent a useful molecular platform for various biological applications in medicine and industry.

Genetic engineering of a modified herpes simplex virus 1 vaccine vector

The herpes simplex virus 1 (HSV-1) d106 mutant virus is a multiple immediate-early gene deletion mutant virus that has been effective as an AIDS vaccine vector in rhesus macaques (Kaur A, Sanford HB, Garry D, Lang S, Klumpp SA, Watanabe D, et al. Ability of herpes simplex virus vectors to boost immune responses to DNA vectors and to protect against challenge by simian immunodeficiency virus. Virology 2007;357:199-214). Further analysis of this vector is needed to advance development into clinical trials. In this study we have defined the precise nature of the multiple IE gene mutations in the d106 viral genome and have used this information to construct a new transfer plasmid for gene transfer into d106. We tested the effect of an additional mutation in the U(L)41 gene on d106 immunogenicity and found that it did not improve the efficacy of the d106 vector, in contrast with results from other studies with U(L)41 gene mutants. The safety profile of d106 was improved by generating a new vector strain, d106S, with increased sensitivity to acyclovir. Finally, we have constructed a d106S recombinant vector that expresses the HIV clade C envelope protein. The d106S HIVenvC recombinant has retained the sensitivity to acyclovir, indicating that this phenotype is a stable property of the d106S vector.

Characterization of a myristoylated, monomeric HIV Gag protein

The process of HIV assembly requires extensive homomultimerization of the Gag polyprotein on cellular membranes to generate the nascent particle bud. Here we generated a full-length, monomeric Gag polyprotein bearing mutations that eliminated multimerization in living cells as indicated by fluorescence resonance energy transfer (FRET). Monomeric Gag resembled non-myristoylated Gag in its weak membrane binding characteristics and lack of association with detergent-resistant membranes (DRMs or lipid rafts). Monomeric Gag failed to assemble virus-like particles, but was inefficiently rescued into particles by wildtype Gag through the influence of the matrix domain. The subcellular distribution of monomeric Gag was remarkably different than either non-myristoylated Gag or wildtype Gag. Monomeric Gag was found on intracellular membranes and at the plasma membrane, where it highlighted plasma membrane extensions and ruffles. This study indicates that monomeric Gag can traffic to assembly sites in the cell, where it interacts weakly with membranes.

The A-rich RNA sequences of HIV-1 pol are important for the synthesis of viral cDNA

The bias of A-rich codons in HIV-1 pol is thought to be a record of hypermutations in viral genomes that lack biological functions. Bioinformatic analysis predicted that A-rich sequences are generally associated with minimal local RNA structures. Using codon modifications to reduce the amount of A-rich sequences within HIV-1 genomes, we have reduced the flexibility of RNA sequences in pol to analyze the functional significance of these A-rich 'structurally poor' RNA elements in HIV-1 pol. Our data showed that codon modification of HIV-1 sequences led to a suppression of virus infectivity by 5-100-fold, and this defect does not correlate with, viral entry, viral protein expression levels, viral protein profiles or virion packaging of genomic RNA. Codon modification of HIV-1 pol correlated with an enhanced dimer stability of the viral RNA genome, which was associated with a reduction of viral cDNA synthesis both during HIV-1 infection and in a cell free reverse transcription assay. Our data provided direct evidence that the HIV-1 A-rich pol sequence is not merely an evolutionary artifact of enzyme-induced hypermutations, and that HIV-1 has adapted to rely on A-rich RNA sequences to support the synthesis of viral cDNA during reverse transcription, highlighting the utility of using 'structurally poor' RNA domains in regulating biological process.

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.

Generation of functionally active HIV-1 specific CD8+ CTL in intestinal mucosa following mucosal, systemic or mixed prime-boost immunization

Gastrointestinal and vaginal mucosa are major sites of entry in natural HIV infection and therefore the preferred sites to elicit high-avidity CD8+ CTL by vaccination. We directly compare systemic and mucosal immunization in mice after DNA priming and boosting with rgp160 env expressed either in MVA or Ad for their ability to induce mucosal as well as systemic HIV-specific CTL. The optimal CTL response in the gut mucosa was observed after priming with the HIV-1 gp160 env DNA vaccine and boosting with rMVA or rAd encoding the same envelope gene all administered intrarectally (IR). Maximum levels of high-avidity CD8+ T cells were seen in intestinal lamina propria following this regimen. When the prime and boost routes were distinct, the delivery site of the boost had a greater impact than the DNA priming. IM DNA prime and IR rMVA boost were more effective than IR DNA prime and IM rMVA boost for eliciting mucosal CD8+ T-cell avidity. A systemic DNA-prime-followed by systemic rMVA boost induced high levels of high-avidity CD8+ T cells systemically, but responses were undetectable in mucosal sites. A single systemic immunization with rMVA was sufficient to induce high-avidity IFN-gamma secreting CD8+ T cells in systemic organs, whereas a single mucosal immunization with rMVA was not sufficient to elicit high-avidity CD8+ T cells in mucosa. Thus, a heterologous mucosal DNA prime-viral vectored boost strategy was needed. The requirement for a heterologous DNA prime-recombinant viral boost strategy for generation of high-avidity CD8+ T cells in mucosal sites in mice may be more stringent than for the induction of high-avidity CD8+ T cells in systemic compartments.

Interactions of reverse transcriptase sequences in Pol with Gag and LysRS in the HIV-1 tRNALys3 packaging/annealing complex

During HIV-1 assembly, tRNA(Lys3), the primer for reverse transcriptase (RT) in HIV-1, is selectively packaged into the virus due to a specific interaction between Gag and lysyl-tRNA synthetase (LysRS). However, while Gag alone will incorporate LysRS, tRNA(Lys3) packaging also requires the presence of RT thumb domain sequences in GagPol. The formation of a tRNA(Lys3) packaging/annealing complex involves an interaction between Gag/GagPol/viral RNA and LysRS/tRNA(Lys), and herein, we have investigated whether the transfer of tRNA(Lys3) from LysRS to RT sequences in Pol by a currently unknown mechanism is facilitated by an interaction between LysRS and Pol. We demonstrate that, in addition to its interaction with Gag, LysRS also interacts with sequences within the connection/RNaseH domains in RT. However, cytoplasmic Gag/Pol interactions, detected by either coimmunoprecipitation or incorporation of Pol into Gag viral-like particles, were found to be insensitive to the overexpression or underexpression of LysRS, indicating that a Gag/LysRS/RT interaction is not essential for Gag/Pol interactions. Based on this and previous work, including the observation that the RT connection domain is not required for tRNA(Lys3) packaging, but is required for tRNA(Lys3) annealing, a model is proposed for a tRNA(Lys3) packaging/annealing complex in which the interaction of Gag with Pol sequences during early viral assembly facilitates the retention in budding viruses of both tRNA(Lys3) and early Pol processing intermediates, with tRNA(Lys3) annealing to viral RNA further facilitated by the LysRS/RT interaction.

Comparison of immunogenicity, protective efficacy of single and cocktail DNA vaccine of Brugia malayi abundant larval transcript (ALT-2) and thioredoxin peroxidase (TPX) in mice

Although DNA vaccines have several advantages over conventional vaccines, antibody production and protection are often not adequate, particularly in single plasmid vaccine formulation. In the present study we evaluated the efficacy of a cocktail vaccine based on plasmids encoding larval (L3) stage-specific Brugia malayi abundant larval transcript (BmALT-2) and antioxidant detoxification enzyme B. malayi thioredoxin peroxidase (BmTPX) to induce antibodies, protective efficacy and cell-mediated immune response in mice. Mice immunized with cocktail DNA vaccines containing the pVAX ALT-2+TPX developed higher titers of anti-BmALT-2+TPX (1:5000) antibodies, compared to the mice immunized with single DNA vaccine of pVAX ALT-2 or pVAX TPX (1:2000). Correlating with this, the mice administered with cocktail vaccine induced up to 78% of cytotoxicity against B. malayi mf. This cytotoxicity was high compared to 34% induced by the pVAX-ALT2 or 37% by pVAX-TPX. Moreover, cocktail vaccination of mice resulted in significantly higher level of cellular proliferative response associated with raised levels of IFN-gamma that skewed towards Th1 type of response compared to vaccination using either of the components. Taken together, these data suggest that the combination of two or more antigens maybe an effective vaccine development strategy to improve protection and immunogenicity against human lymphatic filariasis.

The incorporation of APOBEC3 proteins into murine leukemia viruses

APOBEC3 proteins represent a group of potent intrinsic inhibitors of retroviral replication. Murine APOBEC3 (mA3) is able to inhibit HIV-1, whereas it is inactive against Moloney murine leukemia virus (MLV). In this work, we present evidence showing that mA3, compared to hA3G, is incorporated inefficiently into MLV, while the abilities of mA3 and hA3G to be packaged into HIV-1 are similar. The nucleocapsid (NC) domain of HIV-1 is capable of facilitating the interaction of mA3 with HIV-1 Gag, and thereby the incorporation of mA3 into HIV-1. Swapping studies of the NC domains in HIV-1 and MLV indicate that MLV NC domain is responsible for viral exclusion of mA3, due to its inability to facilitate the mA3/Gag interaction. The interaction between mA3 and HIV-1 Gag is mediated by the linker region between two zinc coordination motifs in mA3, similar to what has been found for the incorporation of hA3G into both HIV-1 and MLV. These results suggest that the interaction between NC domains and the linker regions might represent a common mechanism for viral incorporation of APOBEC3 proteins, and the inefficient incorporation of endogenous mA3 appears to be a strategy by which MLV escapes the inhibitory effect of mA3.

Insulin-like growth factor II mRNA binding protein 1 associates with Gag protein of human immunodeficiency virus type 1, and its overexpression affects virus assembly

The assembly of human immunodeficiency virus type 1 (HIV-1) particles is driven by viral Gag protein. This function of Gag not only benefits from its self-multimerization property but also depends on its interaction with a number of cellular factors such as TSG101 and ALIX/AIP1 that promote virus budding and release from cell surfaces. However, interaction with Gag also allows some cellular factors such as APOBEC3G and Trim5alpha to access viral replication machinery and block viral replication. In this study, we report a new HIV-1 Gag-binding factor named insulin-like growth factor II mRNA binding protein 1 (IMP1). Gag-IMP1 interaction requires the second zinc finger of the nucleocapsid (NC) domain of Gag and the KH3 and KH4 domains of IMP1. A fourfold reduction of HIV-1 infectivity was seen with overexpression of the wild-type IMP1 and its mutant that is able to interact with Gag but not with overexpression of IMP1 mutants exhibiting Gag-binding deficiency. The decreased viral infectivity was further shown as a result of diminished viral RNA packaging, abrogated Gag processing on the cellular membranes, and impeded maturation of virus particles. Together, these results demonstrate that IMP1 interacts with HIV-1 Gag protein and is able to block the formation of infectious HIV-1 particles.

The requirement of the DEAD-box protein DDX24 for the packaging of human immunodeficiency virus type 1 RNA

RNA helicases play important roles in RNA metabolism. Human immunodeficiency virus type 1 (HIV-1) does not carry its own RNA helicase, the virus thus needs to exploit cellular RNA helicases to promote the replication of its RNA at various steps such as transcription, folding and transport. In this study, we report that knockdown of a DEAD-box protein named DDX24 inhibits the packaging of HIV-1 RNA and thus diminishes viral infectivity. The decreased viral RNA packaging as a result of DDX24-knockdown is observed only in the context of the Rev/RRE (Rev response element)-dependent but not the CTE (constitutive transport element)-mediated nuclear export of viral RNA, which is explained by the specific interaction of DDX24 with the Rev protein. We propose that DDX24 acts at the early phase of HIV-1 RNA metabolism prior to nuclear export and the consequence of this action extends to the viral RNA packaging stage during virus assembly.

High-level expression from two independent expression cassettes in replication-incompetent adenovirus type 35 vector

Replication-incompetent adenovirus type 35 (rAd35) represents a potent vaccine carrier that elicits strong, antigen-specific T- and B-cell responses in diverse preclinical models. Moreover, Ad35 is rare in human populations, resulting in the absence of neutralizing antibodies against this carrier, in contrast to the commonly used rAd5. Therefore, rAd35 is being investigated as a vaccine carrier for a number of diseases for which an effective vaccine is needed, including malaria, AIDS and tuberculosis. However, it can be perceived that effective immunization will require insertion of multiple antigens into adenoviral vectors. We therefore wanted to create rAd35 vectors carrying double expression cassettes, to expand within one vector the number of insertion sites for foreign DNA encoding antigenic proteins. We show that it is possible to generate rAd35 vectors carrying two cytomegalovirus promoter-driven expression cassettes, provided that the polyadenylation signals in each expression cassette are not identical. We demonstrate excellent rAd35 vector stability and show that expression of a transgene is not influenced by the presence of a second expression cassette. Moreover, by using two model vaccine antigens, i.e. the human immunodeficiency virus-derived Env-gp120 protein and the Plasmodium falciparum-derived circumsporozoite protein, we demonstrate that potent T- and B-cell responses are induced to both antigens expressed from a single vector. Such rAd35 vectors thus expand the utility of rAd35 vaccine carriers for the development of vaccines against, for example, malaria, AIDS and tuberculosis.

APOBEC3G multimers are recruited to the plasma membrane for packaging into human immunodeficiency virus type 1 virus-like particles in an RNA-dependent process requiring the NC basic linker

APOBEC3G is an endogenous host restriction factor that inhibits human immunodeficiency virus (HIV) replication. The antiviral activity of APOBEC3G is dependent upon its incorporation into the virus particle. The mechanisms governing incorporation of APOBEC3G into virus particles are not completely understood. In particular, some investigators have reported that APOBEC3G interacts directly with the nucleocapsid (NC) subunit of Gag, while others have found that an RNA intermediate is required for Gag-APOBEC3G interactions. In this study, we confirmed the RNA dependence of APOBEC3G packaging and performed detailed mapping of the determinants within NC that are required for virion incorporation. Surprisingly, APOBEC3G packaging did not correlate well with the presence of the N-terminal "I," or interaction, domain within NC. Specifically, Gag constructs containing only the N-terminal region of NC packaged minimal amounts of APOBEC3G, while significant levels of APOBEC3G packaging were achieved with Gag constructs containing the basic linker region of NC. Furthermore, membrane-binding experiments revealed that the basic linker region was essential for the membrane association of APOBEC3G in a Gag-APOBEC3G complex. Fluorescence resonance energy transfer was detected between labeled APOBEC3G in cells and in particles, indicating that APOBEC3G is packaged as a multimer that is bound to packaged RNA. Regions of APOBEC3G-Gag colocalization at the plasma membrane were detected that were distinct from the punctate cytoplasmic bodies where APOBEC3G accumulates within the cell. Together, our results indicate that APOBEC3G multimerizes in an RNA-dependent fashion and that RNA-APOBEC3G multimers are recruited to the plasma membrane and subsequently into virion particles by Gag.

Phase 1 safety and immunogenicity evaluation of a multiclade HIV-1 candidate vaccine delivered by a replication-defective recombinant adenovirus vector

BACKGROUND

The development of an effective human immunodeficiency virus (HIV) vaccine is a high global priority. Here, we report the safety, tolerability, and immunogenicity of a replication-defective recombinant adenovirus serotype 5 (rAd5) vector HIV-1 candidate vaccine.

METHODS

The vaccine is a mixture of 4 rAd5 vectors that express HIV-1 subtype B Gag-Pol fusion protein and envelope (Env) from subtypes A, B, and C. Healthy, uninfected adults were randomized to receive 1 intramuscular injection of placebo (n=6) or vaccine at dose levels of 10(9) (n=10), 10(10) (n=10), or 10(11) (n=10) particle units and were followed for 24 weeks to assess immunogenicity and safety.

RESULTS

The vaccine was well tolerated but was associated with more reactogenicity at the highest dose. At week 4, vaccine antigen-specific T cell responses were detected in 28 (93.3%) and 18 (60%) of 30 vaccine recipients for CD4(+) and CD8(+) T cells, respectively, by intracellular cytokine staining assay and in 22 (73%) of 30 vaccine recipients by enzyme-linked immunospot assay. Env-specific antibody responses were detected in 15 (50%) of 30 vaccine recipients by enzyme-linked immunosorbant assay and in 28 (93.3%) of 30 vaccine recipients by immunoprecipitation followed by Western blotting. No neutralizing antibody was detected.

CONCLUSIONS

A single injection induced HIV-1 antigen-specific CD4(+) T cell, CD8(+) T cell, and antibody responses in the majority of vaccine recipients. This multiclade rAd5 HIV-1 vaccine is now being evaluated in combination with a multiclade HIV-1 DNA plasmid vaccine.

Effect of promoter strength on protein expression and immunogenicity of an HSV-1 amplicon vector encoding HIV-1 Gag

Helper-free herpes simplex virus type-1 (HSV-1) amplicon vectors elicit robust immune responses to encoded proteins, including human immunodeficiency virus type-1 (HIV-1) antigens. To improve this vaccine delivery system, seven amplicon vectors were constructed, each encoding HIV-1 Gag under the control of a different promoter. Gag expression levels were analyzed in murine and human cell lines, as well as in biopsied tissue samples from injected mice; these data were then compared with Gag-specific T cell responses in BALB/c mice. The magnitude of the amplicon-induced immune response was found to correlate strongly with the level of Gag production both in vitro and in vivo. Interestingly, the best correlation of the strength of the amplicon-induced immune response was with antigen expression in cultured DC rather than expression at the tissue site of injection or in cultured cell lines. These findings may have implications for the generation of improved HSV-1 amplicon vectors for HIV-1 vaccine delivery.

Phase 1 safety and immunogenicity evaluation of a multiclade HIV-1 DNA candidate vaccine

BACKGROUND

Gene-based vaccine delivery is an important strategy in the development of a preventive vaccine for acquired immunodeficiency syndrome (AIDS). Vaccine Research Center (VRC) 004 is the first phase 1 dose-escalation study of a multiclade HIV-1 DNA vaccine.

METHODS

VRC-HIVDNA009-00-VP is a 4-plasmid mixture encoding subtype B Gag-Pol-Nef fusion protein and modified envelope (Env) constructs from subtypes A, B, and C. Fifty healthy, uninfected adults were randomized to receive either placebo (n=10) or study vaccine at 2 mg (n=5), 4 mg (n=20), or 8 mg (n=15) by needle-free intramuscular injection. Humoral responses (measured by enzyme-linked immunosorbant assay, Western blotting, and neutralization assay) and T cell responses (measured by enzyme-linked immunospot assay and intracellular cytokine staining after stimulation with antigen-specific peptide pools) were measured.

RESULTS

The vaccine was well tolerated and induced cellular and humoral responses. The maximal CD4(+) and CD8(+) T cell responses occurred after 3 injections and were in response to Env peptide pools. The pattern of cytokine expression by vaccine-induced HIV-specific T cells evolved over time, with a diminished frequency of interferon- gamma -producing T cells and an increased frequency of interleukin-2-producing T cells at 1 year.

CONCLUSIONS

DNA vaccination induced antibody to and T cell responses against 3 major HIV-1 subtypes and will be further evaluated as a potential component of a preventive AIDS vaccine regimen.

Membrane embedded HIV-1 envelope on the surface of a virus-like particle elicits broader immune responses than soluble envelopes

Virally regulated HIV-1 particles were expressed from DNA plasmids encoding Gag, protease, reverse transcriptase, Vpu, Tat, Rev, and Env. The sequences for integrase, Vpr, Vif, Nef, and the long terminal repeats (LTRs) were deleted. Mutations were engineered into the VLP genome to produce particles deficient in activities associated with viral reverse transcriptase, RNase H, and RNA packaging. Each plasmid efficiently secreted particles from primate cells in vitro and particles were purified from the supernatants and used as immunogens. Mice (BALB/c) were vaccinated intranasally (day 1 and weeks 3 and 6) with purified VLPs and the elicited immunity was compared to particles without Env (Gag(p55)), to soluble monomeric Env(gp120), or to soluble trimerized Env(gp140). Only mice vaccinated with VLPs had robust anti-Env cellular immunity. In contrast, all mice had high titer anti-Env serum antibody (IgG). However, VLP-vaccinated mice had antisera that detected a broader number of linear Env peptides, had anti-Env mucosal IgA and IgG, as well as higher titers of serum neutralizing antibodies. VLPs elicited high titer antibodies that recognized linear regions in V4-C5 and the ectodomain of gp41, but did not recognize V3. These lentiviral VLPs are effective mucosal immunogens that elicit broader immunity against Env determinants in both the systemic and mucosal immune compartments than soluble forms of Env.

Biodistribution of DNA plasmid vaccines against HIV-1, Ebola, Severe Acute Respiratory Syndrome, or West Nile virus is similar, without integration, despite differing plasmid backbones or gene inserts

The Vaccine Research Center has developed a number of vaccine candidates for different diseases/infectious agents (HIV-1, Severe Acute Respiratory Syndrome virus, West Nile virus, and Ebola virus, plus a plasmid cytokine adjuvant-IL-2/Ig) based on a DNA plasmid vaccine platform. To support the clinical development of each of these vaccine candidates, preclinical studies have been performed in mice or rabbits to determine where in the body these plasmid vaccines would biodistribute and how rapidly they would clear. In the course of these studies, it has been observed that regardless of the gene insert (expressing the vaccine immunogen or cytokine adjuvant) and regardless of the promoter used to drive expression of the gene insert in the plasmid backbone, the plasmid vaccines do not biodistribute widely and remain essentially in the site of injection, in the muscle and overlying subcutis. Even though approximately 10(14) molecules are inoculated in the studies in rabbits, by day 8 or 9 ( approximately 1 week postinoculation), already all but on the order of 10(4)-10(6) molecules per microgram of DNA extracted from tissue have been cleared at the injection site. Over the course of 2 months, the plasmid clears from the site of injection with only a small percentage of animals (generally 10-20%) retaining a small number of copies (generally around 100 copies) in the muscle at the injection site. This pattern of biodistribution (confined to the injection site) and clearance (within 2 months) is consistent regardless of differences in the promoter in the plasmid backbone or differences in the gene insert being expressed by the plasmid vaccine. In addition, integration has not been observed with plasmid vaccine candidates inoculated i.m. by Biojector 2000 or by needle and syringe. These data build on the repeated-dose toxicology studies performed (see companion article, Sheets et al., 2006) to demonstrate the safety and suitability for investigational human use of DNA plasmid vaccine candidates for a variety of infectious disease prevention indications.