Production of human immunodeficiency virus (HIV)-like particles from cells infected with recombinant vaccinia viruses carrying the gag gene of HIV

Anti-monkeypox virus-neutralizing activities of human immunoglobulin manufactured between 1999 and 2021 and derived from donors in the United States and Japan

BACKGROUND AND OBJECTIVES

In May 2022, the United Kingdom reported the first case of chained transmission of the monkeypox (mpox) virus without any known epidemiological links to west or central Africa. The monthly number of mpox patients currently has passed a peak and is declining globally, and infected patients include both non-vaccinated and vaccinated individuals. Herein, the virus-neutralizing (VN) activity against vaccinia viruses, which are considered to cross-react with the mpox virus, in the intravenous immunoglobulin (IVIG) lots derived from donors, including vaccinated Japanese populations, was evaluated to clarify the status of the Japanese blood donor population.

MATERIALS AND METHODS

VN titres against vaccinia and human mpox viruses in IVIG lots derived from donors in Japan and the United States manufactured between 1999 and 2021 and 1995 and 2001, respectively, were evaluated by neutralization testing.

RESULTS

VN titres of IVIG derived from donors in Japan and the United States against vaccinia and mpox viruses showed a slowly decreasing trend between 1999 and 2021.

CONCLUSION

VN titres are expected to decrease in the future since the percentage of vaccinated donors in the donor population seems to have decreased. Therefore, continuous monitoring of VN titres is required.

Analyzing trapped protein complexes by Virotrap and SFINX

The analysis of protein interaction networks is one of the key challenges in the study of biology. It connects genotypes to phenotypes, and disruption of such networks is associated with many pathologies. Virtually all the approaches to the study of protein complexes require cell lysis, a dramatic step that obliterates cellular integrity and profoundly affects protein interactions. This protocol starts with Virotrap, a novel approach that avoids the need for cell homogenization by fusing the protein of interest to the HIV-1 Gag protein, trapping protein complexes in virus-like particles. By using the straightforward filtering index (SFINX), which is a powerful and intuitive online tool (http://sfinx.ugent.be) that enables contaminant removal from candidate lists resulting from mass-spectrometry-based analysis, we provide a complete workflow for researchers interested in mammalian protein complexes. Given direct access to mass spectrometers, researchers can process up to 24 samples in 7 d.

Viruses and Virus-Like Particles in Biotechnology: Fundamentals and Applications

Although viruses are simple biological systems, they are capable of evolving highly efficient techniques for infecting cells, expressing their genomes, and generating new copies of themselves. It is possible to genetically manipulate most of the different classes of known viruses in order to produce recombinant viruses that express foreign proteins. Recombinant viruses have been used in gene therapy to deliver selected genes into higher organisms, in vaccinology and immunotherapy, and as important research tools to study the structure and function of these proteins.

Virus-like particles (VLPs) are multiprotein structures that mimic the organization and conformation of authentic native viruses but lack the viral genome. They have been applied not only as prophylactic and therapeutic vaccines but also as vehicles in drug and gene delivery and, more recently, as tools in nanobiotechnology.

In this chapter, basic and advanced features of viruses and VLPs are presented and their major applications are discussed. The different production platforms based on animal cell technology are explained, and their main challenges and future perspectives are explored. The implications of large-scale production of viruses and VLPs are discussed in the context of process control, monitoring, and optimization. The main upstream and downstream technical challenges are identified and discussed accordingly.

The β-Lactamase Assay: Harnessing a FRET Biosensor to Analyse Viral Fusion Mechanisms

The β-lactamase (BlaM) assay was first revealed in 1998 and was demonstrated to be a robust Förster resonance energy transfer (FRET)-based reporter system that was compatible with a range of commonly-used cell lines. Today, the BlaM assay is available commercially as a kit and can be utilised readily and inexpensively for an array of experimental procedures that require a fluorescence-based readout. One frequent application of the BlaM assay is the measurement of viral fusion—the moment at which the genetic material harboured within virus particles is released into the cytosol following successful entry. The flexibility of the system permits evaluation of not only total fusion levels, but also the kinetics of fusion. However, significant variation exists in the scientific literature regarding the methodology by which the assay is applied to viral fusion analysis, making comparison between results difficult. In this review we draw attention to the disparity of these methodologies and examine the advantages and disadvantages of each approach. Successful strategies shown to render viruses compatible with BlaM-based analyses are also discussed.

Trapping mammalian protein complexes in viral particles

Cell lysis is an inevitable step in classical mass spectrometry–based strategies to analyse protein complexes. Complementary lysis conditions, in situ cross-linking strategies and proximal labelling techniques are currently used to reduce lysis effects on the protein complex. We have developed Virotrap, a viral particle sorting approach that obviates the need for cell homogenization and preserves the protein complexes during purification. By fusing a bait protein to the HIV-1 GAG protein, we show that interaction partners become trapped within virus-like particles (VLPs) that bud from mammalian cells. Using an efficient VLP enrichment protocol, Virotrap allows the detection of known binary interactions and MS-based identification of novel protein partners as well. In addition, we show the identification of stimulus-dependent interactions and demonstrate trapping of protein partners for small molecules. Virotrap constitutes an elegant complementary approach to the arsenal of methods to study protein complexes.

A large portion of the proteome carries out its cellular function as part of macromolecular complexes. Here the authors describe Virotrap, a novel lysis-free approach for the isolation and identification of biologically relevant protein-protein and small molecule-protein interactions.

Mutation of the TYTLE motif in the cytoplasmic tail of the sendai virus fusion protein deeply affects viral assembly and particle production

Enveloped viruses contain glycoproteins protruding from the viral membrane. These proteins play a crucial role in the extra-cellular steps of the virus life cycle, namely attachment to and entry into cells. Their role during the intracellular late phase of virus multiplication has been less appreciated, overlooked by the documented central organizer role of the matrix M protein. Sendai virus, a member of the Paramyxoviridae family, expresses two trans-membrane proteins on its surface, HN and F. In previous work, we have shown that suppression of F in the context of an infection, results in about 70% reduction of virus particle production, a reduction similar to that observed upon suppression of the matrix M protein. Moreover, a TYTLE motif present in F cytoplasmic tail has been proposed essential for virus particle production. In the present work, using original alternate conditional siRNA suppression systems, we generated a double F gene recombinant Sendai virus expressing wt-F and a nonviable mutated TYTLE/5A F protein (F_5A). Suppression of the wild type F gene expression in cells infected with this virus allowed the analysis of F_5A properties in the context of the infection. Coupling confocal imaging analysis to biochemical characterization, we found that F_5A i) was not expressed at the cell surface but restricted to the endoplasmic reticulum, ii) was still capable of interaction with M and iii) had profound effect on M and HN cellular distribution. On the basis of these data, we propose a model for SeV particle formation based on an M/F complex that would serve as nucleation site for virus particle assembly at the cell surface.

ESCRT-independent budding of HIV-1 gag virus-like particles from Saccharomyces cerevisiae spheroplasts

Heterologous expression of HIV-1 Gag in a variety of host cells results in its packaging into virus-like particles (VLPs) that are subsequently released into the extracellular milieu. This phenomenon represents a useful tool for probing cellular factors required for viral budding and has contributed to the discovery of roles for ubiquitin ligases and the endosomal sorting complexes required for transport (ESCRTs) in viral budding. These factors are highly conserved throughout eukaryotes and have been studied extensively in the yeast Saccharomyces cerevisiae, a model eukaryote previously utilized as a host for the production of VLPs. We used heterologous expression of HIV Gag in yeast spheroplasts to examine the role of ESCRTs and associated factors (Rsp5, a HECT ubiquitin ligase of the Nedd4 family; Bro1, a homolog of Alix; and Vps4, the AAA-ATPase required for ESCRT function in all contexts/organisms investigated) in the generation of VLPs. Our data reveal: 1) characterized Gag-ESCRT interaction motifs (late domains) are not required for VLP budding, 2) loss of function alleles of the essential HECT ubiquitin ligase Rsp5 do not display defects in VLP formation, and 3) ESCRT function is not required for VLP formation from spheroplasts. These results suggest that the egress of HIV Gag from yeast cells is distinct from the most commonly described mode of exit from mammalian cells, instead mimicking ESCRT-independent VLP formation observed in a subset of mammalian cells. As such, budding of Gag from yeast cells appears to represent ESCRT-independent budding relevant to viral replication in at least some situations. Thus the myriad of genetic and biochemical tools available in the yeast system may be of utility in the study of this aspect of viral budding.

Virus-like particles in vaccine development

Virus-like particles (VLPs) are multiprotein structures that mimic the organization and conformation of authentic native viruses but lack the viral genome, potentially yielding safer and cheaper vaccine candidates. A handful of prophylactic VLP-based vaccines is currently commercialized worldwide: GlaxoSmithKline's Engerix (hepatitis B virus) and Cervarix (human papillomavirus), and Merck and Co., Inc.'s Recombivax HB (hepatitis B virus) and Gardasil (human papillomavirus) are some examples. Other VLP-based vaccine candidates are in clinical trials or undergoing preclinical evaluation, such as, influenza virus, parvovirus, Norwalk and various chimeric VLPs. Many others are still restricted to small-scale fundamental research, despite their success in preclinical tests. This article focuses on the essential role of VLP technology in new-generation vaccines against prevalent and emergent diseases. The implications of large-scale VLP production are discussed in the context of process control, monitorization and optimization. The main up- and down-stream technical challenges are identified and discussed accordingly. Successful VLP-based vaccine blockbusters are briefly presented concomitantly with the latest results from clinical trials and the recent developments in chimeric VLP-based technology for either therapeutic or prophylactic vaccination.

Viruses and Virus-Like Particles in Biotechnology

Although viruses are simple biological systems, they are capable of evolving highly efficient techniques for infecting cells, expressing their genomes, and generating new copies of themselves. It is possible to genetically manipulate most of the different classes of known viruses in order to produce recombinant viruses that express foreign proteins. Recombinant viruses have been used in gene therapy to deliver selected genes into higher organisms, in vaccinology and immunotherapy, and as important research tools to study the structure and function of these proteins.

Virus-like particles (VLPs) are multiprotein structures that mimic the organization and conformation of authentic native viruses but lack the viral genome. They have been applied not only as prophylactic and therapeutic vaccines but also as vehicles in drug and gene delivery and, more recently, as tools in nanobiotechnology.

In this article, basic and advanced features of viruses and VLPs are presented and their major applications are discussed. The different production platforms based on animal cell technology are explained, and their main challenges and future perspectives are explored. The implications of large-scale production of viruses and VLPs are discussed in the context of process control, monitorization, and optimization. The main upstream and downstream technical challenges are identified and discussed accordingly.

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.

GP64 of group I nucleopolyhedroviruses cannot readily rescue infectivity of group II f-null nucleopolyhedroviruses

The genus Nucleopolyhedrovirus (NPV) of the family Baculoviridae can be subdivided phylogenetically into two groups. The same division can be made on the basis of their budded virus (BV) envelope fusion protein. Group I NPVs are characterized by the presence of a GP64-like major envelope fusion protein, which is involved in viral attachment and the fusion of virus and cell membrane, and is required for budding of progeny nucleocapsids. Group II NPVs have an envelope fusion protein unrelated to GP64, named F. In contrast to GP64, F proteins are found in all baculoviruses, but they are not functional as envelope fusion proteins in group I NPVs. Autographa californica multiple NPV (AcMNPV) lacking GP64 can be pseudotyped by the F protein of Spodoptera exigua multiple NPV (SeMNPV), suggesting that F proteins are functionally analogous to GP64. GP64 homologues are thought to have been acquired by group I NPVs during evolution, thereby giving these viruses a selective advantage and obviating the need for a functional F protein. To address this supposition experimentally, attempts were made to pseudotype a group II NPV, SeMNPV, with GP64. Transfection of an f-null SeMNPV bacmid into Se301 cells did not result in the production of infectious BVs. This defect was rescued by insertion of SeMNPV f, but not by insertion of AcMNPV gp64. This suggests that the functional analogy between GP64 and F is not readily reciprocal and that F proteins from group II NPVs may provide additional functions in BV formation that are lacking in the GP64 type of fusion protein.

Virus-like particles: passport to immune recognition

Virus-like particles (VLPs) are formed by the self-assembly of envelope and/or capsid proteins from many viruses. In many cases such VLPs have structural characteristics and antigenicity similar to the parental virus, and some have already proven successful as vaccines against the cognate virus infection. The structural components of some VLPs have also proven amenable to the insertion or fusion of foreign antigenic sequences, allowing the production of chimeric VLPs exposing the foreign antigen on their surface. Other VLPs have been used as carriers for foreign antigens, including non-protein antigens, via chemical conjugation. This review outlines some of the advantages, disadvantages, and technical considerations for the use of a wide range of VLP systems in vaccine development.

Mobility of human immunodeficiency virus type 1 Pr55Gag in living cells

Human immunodeficiency virus type 1 (HIV-1) assembly requires the converging of thousands of structural proteins on cellular membranes to form a tightly packed immature virion. The Gag polyprotein contains all of the determinants important for viral assembly and must move around in the cell in order to form particles. This work has focused on Gag mobility in order to provide more insights into the dynamics of particle assembly. Key to these studies was the use of several fluorescently labeled Gag derivatives. We used fluorescence recovery after photobleaching as well as photoactivation to determine Gag mobility. Upon expression, Gag can be localized diffusely in the cytoplasm, associated with the plasma membrane, or in virus-like particles (VLPs). Here we show that Gag VLPs are primarily localized in the plasma membrane and do not colocalize with CD63. We have shown using full-length Gag as well as truncation mutants fused to green fluorescent protein that Gag is highly mobile in live cells when it is not assembled into VLPs. Results also showed that this mobility is highly dependent upon cholesterol. When cholesterol is depleted from cells expressing Gag, mobility is significantly decreased. Once cholesterol was replenished, Gag mobility returned to wild-type levels. Taken together, results from these mobility studies suggest that Gag is highly mobile and that as the assembly process proceeds, mobility decreases. These studies also suggest that Gag assembly must occur in cholesterol-rich domains in the plasma membrane.

Development of a rapid and convenient method for the quantification of HIV-1 budding

In cells, the expression of Gag protein, one of the major structural proteins of retroviruses, is sufficient for budding virus-like particles (VLPs) from the cell surface. We have previously reported that spheroplasts of Saccharomyces cerevisiae expressing HIV-1 Gag proteins from an episomal plasmid constitutively released a large amount of VLPs into culture media; however, commercially available ELISA kits which detect mature capsid of HIV-1 could not detect uncleaved 55-kDa Gag proteins released from budding yeast. We therefore developed a method to quantitate VLP levels released from budding yeast by using fusion protein from HIV-1 Gag and Firefly Luciferase. This system is useful for screening cellular factor(s) involved in retrovirus budding from S. cerevisiae.

Identification and characterization of HIV-1 epitopes presented by HLA-A*2603: comparison between HIV-1 epitopes presented by A*2601 and A*2603

Human leukocyte antigen (HLA)-A*26 is one of the alleles associated with a slow progression to AIDS. Identification and characterization of HIV-1-specific epitopes presented by this allele are necessary for studies on the immunopathogenesis of AIDS and vaccine development in Asia, where three HLA-A*26 subtypes are frequently found. In the present study, we sought to identify HLA-A*2603-restricted HIV-1 epitopes by using reverse immunogenetics and to compare them with HLA-A*2601-restricted ones recently identified. We found that 31 of 110 HIV-1 peptides bound to HLA-A*2603 and that only two peptides (Gag169-177 and Env63-72) induced specific CD8+T cells by stimulating peripheral blood mononuclear leukocytes from HIV-1-infected individuals carrying HLA-A*2603. The specific cytotoxic T lymphocyte clones killed HIV-1 recombinant vaccinia-infected cells, indicating that these two peptides were naturally occurring peptides presented by HLA-A*2603. Gag169-177-specific CD8+T cells were frequently detected in both HLA-A*2601+ and -A*2603+ individuals with chronic HIV-1 infection, whereas Env63-72-specific ones were frequently detected only in the HLA-A*2603+ individuals. Gag169-177 peptide bound equally to both HLA-A*26 antigens, whereas Env63-72 peptide bound to A*2603 much more strongly than to A*2601. These findings suggest that the relative affinity of these peptides for the HLA-A*26 subtypes determines whether these peptides are recognized as epitopes in HIV-1-infected individuals carrying these alleles.

Molecular and biological characterization of simian-human immunodeficiency virus-like particles produced by recombinant fowlpox viruses

Virus-like particles (VLPs) mimicking the simian-human immunodeficiency virus SHIV89.6P (VLPSHIV) were produced by co-infection of Vero cells with fowlpox SIVgag/pol (FPgag/polSIV) and fowlpox HIV-1env89.6P (FPenv89.6P) recombinant viruses. As a necessary prerequisite for a more efficient vaccine approach, ultrastructural, functional and molecular characterizations of VLP(SHIV) were performed in the SHIV-macaque model to verify the similarity of these particles to SHIV89.6P. Here we show that VLPSHIV can infect T cells by fusion without replication, as demonstrated by the absence of new viral progeny in VLPSHIV-infected C8166 cells. Biochemical characterization showed identical protein profiles of VLPSHIV and SHIV89.6P, and ultrastructural analysis of Vero cells releasing VLPSHIV also confirmed the morphological similarity of these pseudovirions to SHIV89.6P particles. Viral mRNAs were also found packaged inside the core of VLPSHIV by RT-PCR and reverse transcriptase assays.

Involvement of a small GTP binding protein in HIV-1 release

Background

There is evidence suggesting that actin binding to HIV-1 encoded proteins, or even actin dynamics themselves, might play a key role in virus budding and/or release from the infected cell. A crucial step in the reorganisation of the actin cytoskeleton is the engagement of various different GTP binding proteins. We have thus studied the involvement of GTP-binding proteins in the final steps of the HIV-1 viral replication cycle.

Results

Our results demonstrate that virus production is abolished when cellular GTP binding proteins involved in actin polymerisation are inhibited with specific toxins.

Conclusion

We propose a new HIV budding working model whereby Gag interactions with pre-existing endosomal cellular tracks as well as with a yet non identified element of the actin polymerisation pathway are required in order to allow HIV-1 to be released from the infected cell.

Recombinant HIV-1 Pr55gag virus-like particles: potent stimulators of innate and acquired immune responses

Several previous reports have clearly demonstrated the strong effectiveness of human immunodeficiency virus (HIV) Gag polyprotein-based virus-like particles (VLP) to stimulate humoral and cellular immune responses in complete absence of additional adjuvants. Yet, the mechanisms underlying the strong immunogenicity of these particulate antigens are still not very clear. However, current reports strongly indicate that these VLP act as "danger signals" to trigger the innate immune system and possess potent adjuvant activity to enhance the immunogenicity of per se only weakly immunogenic peptides and proteins. Here, we review the current understanding of how various particle-associated substances and other impurities may contribute to the observed immune-activating properties of these complex immunogens.

Virus-like particles as HIV-1 vaccines

Traditional successful antiviral vaccines have relied mostly on live-attenuated viruses. Live-attenuated HIV vaccine candidates are not ideal as they pose risks of reversion, recombination or mutations. Other current HIV vaccine candidates have difficulties generating broadly effective neutralising antibodies and cytotoxic T cell immune responses to primary HIV isolates. Virus-like-particles (VLPs) have been demonstrated to be safe to administer to animals and human patients as well as being potent and efficient stimulators of cellular and humoral immune responses. Therefore, VLPs are being considered as possible HIV vaccines. Chimeric HIV-1 VLPs constructed with either HIV or SIV capsid protein plus HIV immune epitopes and immuno-stimulatory molecules have further improved on early VLP designs, leading to enhanced immune stimulation. The administration of VLP vaccines via mucosal surfaces has also emerged as a promising strategy with which to elicit mucosal and systemic humoral and cellular immune responses. Additionally, new information on antigen processing and the presentation of particulate antigens by dendritic cells (DCs) has created new strategies for improved VLP vaccine candidates. This paper reviews the field of HIV-1 VLP vaccine development, focusing on recent studies that will likely uncover promising prospects for new HIV vaccines.

Characterization of human immunodeficiency virus type 1 Pr160 gag-pol mutants with truncations downstream of the protease domain

We have constructed a series of HIV-1 Gag-pol mutants by progressive deletion of the pol sequence downstream of the viral protease (PR) domain. Effects of the truncation mutations on virus particle production and Gag particle processing were analyzed. Analysis indicated that removal of the integrase (IN) domain had no major effect on the efficiency of particle processing, but resulted in a marked reduction in virus particle budding. Deletion of both the IN and RNase H domains, however, restored the production of virus particles to wild-type level. The proteolytic processing of virus particle was significantly impaired when the p51RT domain was truncated. All of the truncated Gag-pol proteins could be incorporated into virus particles and demonstrated an immunofluorescence staining pattern similar to that of the wild type (wt). Our data are consistent with the proposal that signals for directing the Gag-pol transport and particle incorporation are determined by its N-terminal Gag domain. Truncated Gag-pol retaining an intact p51RT was able to complement a PR-defective mutant to produce infectious pseudotyped virions, with a virus titer 20-70% of that of wt. Pseudotyped virions produced by the Gag-pol lacking an intact p51RT were noninfectious or poorly infectious. This suggests that an intact p51RT domain is required for the Gag-pol to mediate production of mature infectious virus particles in trans.

The conserved carboxy terminus of the capsid domain of human immunodeficiency virus type 1 gag protein is important for virion assembly and release

The retroviral Gag precursor plays an important role in the assembly of virion particles. The capsid (CA) protein of the Gag molecule makes a major contribution to this process. In the crystal structure of the free CA protein of the human immunodeficiency virus type 1 (HIV-1), 11 residues of the C terminus were found to be unstructured, and to date no information exists on the structure of these residues in the context of the Gag precursor molecule. We performed phylogenetic analysis and demonstrated a high degree of conservation of these 11 amino acids. Deletion of this cluster or introduction of various point mutations into these residues resulted in significant impairment of particle infectivity. In this cluster, two putative structural regions were identified, residues that form a hinge region (353-VGGP-356) and those that contribute to an alpha-helix (357-GHKARVL-363). Overall, mutations in these regions resulted in inhibition of virion production, but mutations in the hinge region demonstrated the most significant reduction. Although all the Gag mutants appeared to have normal Gag-Gag and Gag-RNA interactions, the hinge mutants were characterized by abnormal formation of cytoplasmic Gag complexes. Gag proteins with mutations in the hinge region demonstrated normal membrane association but aberrant rod-like membrane structures. More detailed analysis of these structures in one of the mutants demonstrated abnormal trapped Gag assemblies. These data suggest that the conserved CA C terminus is important for HIV-1 virion assembly and release and define a putative target for drug design geared to inhibit the HIV-1 assembly process.

Human immunodeficiency virus type 1 subtype C Gag virus-like particle boost substantially improves the immune response to a subtype C gag DNA vaccine in mice

Human immunodeficiency virus type 1 (HIV-1) subtype C is the predominant HIV in southern Africa, and is the target of a number of recent vaccine candidates. It has been proposed that a heterologous prime/boost vaccination strategy may result in stronger, broader and more prolonged immune responses. Since HIV-1 Gag Pr55 polyprotein can assemble into virus-like particles (VLPs) which have been shown to induce a strong cellular immune response in animals, we showed that a typical southern African subtype C Pr55 protein expressed in insect cells via recombinant baculovirus could form VLPs. We then used the baculovirus-produced VLPs as a boost to a subtype C HIV-1 gag DNA prime vaccination in mice. This study shows that a low dose of HIV-1 subtype C Gag VLPs can significantly boost the immune response to a single subtype C gag DNA inoculation in mice. These results suggest a possible vaccination regimen for humans.

The central domain of the matrix protein of HIV-1: influence on protein structure and virus infectivity

The central region of the matrix protein p17 of HIV-1 is known to be essential during virus assembly. We substituted alanines for amino acid triplets in this region of p17 (amino acid residues 47 to 55: NPG LLE TSE). Introduction of the respective mutations into the gag-coding sequence of HI-proviruses and subsequent transfection into Cos-7 cells led to particle production and release. Exchange of LLE resulted in the production of non-infectious particles. These residues may be important for correct folding and assembly of the processed matrix protein and the production of infectious HIV. In vitro studies of wild-type and mutated matrix proteins using spectroscopic methods (NMR, fluorescence, CD) yielded detailed data about structure and stability. Two-dimensional NMR spectroscopy showed that wild-type and mutant proteins (p17-NPG and p17-TSE) are well folded. Besides structural changes at the mutated site, chemical shift changes indicate small but significant long range structural rearrangements. The stability against chemically and thermally induced unfolding of the mutants p17-NPG and p17-TSE was slightly decreased, while that of p17-LLE was drastically diminished. The alterations have only a local effect on protein folding for the mutants p17-NPG and p17-TSE, and the globular tertiary structure remains nearly unchanged. For p17-LLE, however, the substitutions seem to trigger significant changes in structural elements.

A block in virus-like particle maturation following assembly of murine leukaemia virus in insect cells

Expression of the murine leukaemia virus (MLV) major Gag antigen p65(Gag) using the baculovirus expression system leads to efficient assembly and release of virus-like particles (VLP) representative of immature MLV. Expression of p180(Gag-Pol), facilitated normally in mammalian cells by readthrough of the p65(Gag) termination codon, also occurs efficiently in insect cells to provide a source of the MLV protease and a pattern of p65(Gag) processing similar to that observed in mammalian cells. VLP release from p180(Gag-Pol)-expressing cells however remains essentially immature with disproportionate levels of the uncleaved p65(Gag) precursor when compared to the intracellular Gag profile. Changing the p65(Gag) termination codon altered the level of p65(Gag) and p180(Gag-Pol) within expressing cells but did not alter the pattern of released VLP, which remained immature. Coexpression of p65(Gag) with a fixed readthrough p180(Gag-Pol) also led to only immature VLP release despite high intracellular protease levels. Our data suggest a mechanism that preferentially selects uncleaved p65(Gag) for the assembly of MLV in this heterologous expression system and implies that, in addition to their relative levels, active sorting of the correct p65(Gag) and p180(Gag-Pol) ratios may occur in producer cells.

Identification and characterization of HLA-A*3303-restricted, HIV type 1 Pol- and Gag-derived cytotoxic T cell epitopes

HLA-A*3303 is one of the common HLA alleles in East and Southeast Asia. Identification of HLA-A*3303-restricted HIV-1 cytotoxic T lymphocyte (CTL) epitopes is therefore required to investigate the immunopathogenesis of AIDS and vaccine development in these areas, where AIDS is rapidly expanding. We attempted to identify HLA-A*3303-restricted CTL epitopes derived from relatively conserved proteins Pol, Gag, and Nef of HIV-1 clade B, using reverse immunogenetics. Ninety-nine 8-mer to 11-mer peptides corresponding to the HLA-A*3303-binding peptide motif were selected from the HIV-1 SF2 sequence. Fifty-two of these 99 peptides bound to HLA-A*3303. Six of these binding peptides induced peptide-specific CTLs in PBMCs from at least one of two HIV-1-seropositive individuals. CTL clones specific for three Pol peptides and one Gag peptide killed HLA-A*3303-restricted target cells infected with HIV-1 recombinant vaccinia, indicating that these peptides were naturally processed HLA-A*3303-restricted CTL epitopes. SF2-Pol 594-602 (FYVDGAANR) and SF2-Gag 144-152 (MVHQAISPR) induced specific CTLs in 5 and 4 of 10 chronically HIV-1-infected individuals, respectively, whereas SF2-Pol 60-70 (TLWQRPLVTIR) and SF2-Pol 934-943 (KIQNFRVYYR) induced specific CTLs in 2 and 1 of 10 chronically HIV-1-infected individuals, respectively. Thus, the former are immunodominant epitopes whereas the latter are not. These epitopes are useful for studies of AIDS immunopathogenesis and vaccine development.

Mutation of the dominant endocytosis motif in human immunodeficiency virus type 1 gp41 can complement matrix mutations without increasing Env incorporation

The human immunodeficiency virus type 1 transmembrane glycoprotein (TM) is efficiently endocytosed in a clathrin-dependent manner. Internalization is mediated by a tyrosine-containing motif within the cytoplasmic domain, and replacement of the cytoplasmic tyrosine by cysteine or phenylalanine increased expression of mutant glycoprotein on the surface of transfected cells by as much as 2.5-fold. Because interactions between the cytoplasmic domain of Env and the matrix protein (MA) have been suggested to mediate incorporation of Env in virus particles, we examined whether perturbation of endocytosis would alter incorporation. Proviruses were constructed to contain the wild-type or mutant Env in conjunction with point mutations in MA that had previously been shown to block Env incorporation. These constructs were used to evaluate the effect of glycoprotein endocytosis on incorporation into virus particles and to test the necessity for a specific interaction between Env and MA to mediate incorporation. Viruses produced from transfected 293T cells were used to infect various cell lines, including MAGI, H9, and CEMx174. Viruses encoding both a disrupted endocytosis motif signal and mutations within MA were significantly more infectious in MAGI cells than their counterparts encoding a mutant MA and wild-type Env. This complementation of infectivity for the MA incorporation mutant viruses was not due to increased glycoprotein incorporation into particles but instead reflected an enhanced fusogenicity of the mutated Env proteins. Our findings further support the concept that a specific interaction between the long cytoplasmic domain of TM and MA is required for efficient incorporation of Env into assembling virions. Alteration of the endocytosis signal of Env, and the resulting increase in cell surface glycoprotein, has no effect on incorporation despite demonstrable effects on fusion, virus entry, and infectivity.

Helper plasmids for production of HIV-1-derived vectors

Vectors derived from human immunodeficiency virus type 1 (HIV-1) appear an attractive option for many gene therapy applications. This is due to their ability to transduce noncycling cell populations and to integrate their genome into the host cell chromosome, resulting in the stable genetic modification of the transduced cell. These properties have permitted the direct in vivo transduction of several tissues, including the central nervous system, retina, and liver. However, the pathogenic nature of HIV-1 has raised considerable concerns about the safety of such vector systems. To help address these concerns, we have expressed each of the primary transcriptional units encoding trans functions relevant for vector production in individual plasmid constructs. The gag-pol gene sequence was codon-optimized for expression in mammalian cells resulting in high level Rev/Rev-response element (RRE)-independent expression. Codon optimization of gag-pol also reduces sequence homology with vectors containing gag gene sequences, which results in reduced transfer of biologically active gag-pol sequences to transduced cells. Furthermore, the vif reading frame overlapping the 3' end of the pol coding sequence is destroyed by codon optimization. We have also shown that the Gag and Gag-Pol polyproteins can be efficiently expressed from separate transcriptional units. This has enabled the removal of a cis-acting viral element, the gag-pol translational frameshift sequence, from the vector/packaging system and prevents detectable transfer of biologically active sequences equivalent to the gag-pol gene to transduced cells.

Mutations within the putative membrane-spanning domain of the simian immunodeficiency virus transmembrane glycoprotein define the minimal requirements for fusion, incorporation, and infectivity

The membrane-spanning domain (MSD) of a number of retroviral transmembrane (TM) glycoproteins, including those from the human and simian immunodeficiency viruses (HIV and SIV), have been predicted to contain a charged arginine residue. The wild-type SIV TM glycoprotein is 354 amino acids long. The entire putative cytoplasmic domain of SIV (amino acids 193 to 354) is dispensable for virus replication in vitro, and such truncation-containing viruses are capable of reaching wild-type titers after a short delay. We show here that further truncation of eight additional amino acids to TM185 results in a protein that lacks fusogenicity but is, nevertheless, efficiently incorporated into budding virions. By analyzing a series of nonsense mutations between amino acids 193 and 185 in Env expression vectors and in the SIVmac239 proviral clone, a region of the SIV TM that contains the minimum requirement for glycoprotein-mediated cell-to-cell fusion and that for virus replication was identified. Virus entry and infectivity were evident in truncations to a minimum of 189 amino acids, whereas cell-cell fusion was observed for a protein of only 187 amino acids. Glycoprotein was efficiently incorporated into budding virions in truncations up to 185 amino acids, indicating that such proteins are membrane anchored and are transported to the cell surface. However, truncation of the TM to 180 amino acids resulted in a protein that displays a transport defect and may be retained in the endoplasmic reticulum. Based on our analyses of these mutants, an alternative model for the MSD of SIV is proposed. Our model suggests that membrane-imbedded charged residues can be neutralized by side-chain interactions with lipid polar head groups. As a consequence, the membrane-spanning region can be reduced by more than a helical turn. This new model accounts for the ability of truncations within the predicted MSD to remain membrane anchored and maintain biological activity.

Activation of HIV-1-specific immune responses to an HIV-1 vaccine constructed from a replication-defective adenovirus vector using various combinations of immunization protocols

We constructed a recombinant replication defective adenovirus vector containing the env gene (Ad-Bal) derived from macrophage-trophic HIV-1 (HIV-1 Bal). We then immunized mice with this vector using several administration routes and protocols, and examined the immune response. When the Ad-Bal viral vector (over 1 x 10(7) pfu) was injected subcutaneously, both humoral and cell-mediated immunities were induced. However, immune response induced by the Ad-Bal vector alone was weaker than that induced by the recombinant vaccinia viral vector. We then employed the following three immunization protocols: (l) DNA vaccination followed by immunization with the Ad-Bal; (2) vaccination using the Ad-Bal vector followed by DNA vaccination; and (3) DNA vaccination followed by Ad-Bal infection and passive transfer of dendritic cells (DCs) infected with the Ad-Bal. Among the three protocols, the last gave the strongest humoral and cell-mediated immunity. These results suggest that the combination of DNA vaccination, Ad-Bal vector infection and passive transfer of Ad-Bal-infected DCs can induce strong immunity against HIV-1 Bal.

Naturally occurring deletional mutation in the C-terminal cytoplasmic tail of CCR5 affects surface trafficking of CCR5

CCR5 is an essential coreceptor for the cellular entry of R5 strains of human immunodeficiency virus type 1 (HIV-1). CCR5-893(-) is a single-nucleotide deletion mutation which is observed exclusively in Asians (M. A. Ansari-Lari, et al., Nat. Genet. 16:221-222, 1997). This mutant gene produces a CCR5 which lacks the entire C-terminal cytoplasmic tail. To assess the effect of CCR5-893(-) on HIV-1 infection, we generated a recombinant Sendai virus expressing the mutant CCR5 and compared its HIV-1 coreceptor activity with that of wild-type CCR5. Although the mutant CCR5 has intact extracellular domains, its coreceptor activity was much less than that of wild-type CCR5. Flow cytometric analyses and confocal microscopic observation of cells expressing the mutant CCR5 revealed that surface CCR5 levels were greatly reduced in these cells, while cytoplasmic CCR5 levels of the mutant CCR5 were comparable to that of the wild type. Peripheral blood CD4(+) T cells obtained from individuals heterozygous for this allele expressed very low levels of CCR5. These data suggest that the CCR5-893(-) mutation affects intracellular transport of CCR5 and raise the possibility that this mutation also affects HIV-1 transmission and disease progression.

Organization of immature human immunodeficiency virus type 1

Immature retrovirus particles contain radially arranged Gag polyproteins in which the N termini lie at the membrane and the C termini extend toward the particle's center. We related image features to the polyprotein domain structure by combining mutagenesis with cryoelectron microscopy and image analysis. The matrix (MA) domain appears as a thin layer tightly associated with the inner face of the viral membrane, separated from the capsid (CA) layer by a low-density region corresponding to its C terminus. Deletion of the entire p6 domain has no effect on the width or spacing of the density layers, suggesting that p6 is not ordered in immature human immunodeficiency virus type 1 (HIV-1). In vitro assembly of a recombinant Gag polyprotein containing only capsid (CA) and nucleocapsid (NC) domains results in the formation of nonenveloped spherical particles which display two layers with density matching that of the CA-NC portion of immature HIV-1 Gag particles. Authentic, immature HIV-1 displays additional surface features and an increased density between the lipid bilayers which reflect the presence of gp41. The other internal features match those of virus-like particles.

Differential budding efficiencies of human T-cell leukemia virus type I (HTLV-I) Gag and Gag-Pro polyproteins from insect and mammalian cells

In this study, we examined the ability of human T-cell leukemia virus type I (HTLV-I) Gag and Gag-Pro to assemble immature virus-like particles (VLPs) and bud from insect and mammalian cells. Transmission electron microscopy of insect cells infected with a recombinant baculovirus carrying the entire gag gene revealed that Pr53(Gag) is targeted to the plasma membrane, where it extensively accumulates and forms electron-dense evaginations. However, no particles could be detected either inside the cells or in the culture supernatants. With the Gag-Pro-expressing construct, we observed HTLV-I-specific cytoplasmic proteolysis of the Gag precursor, but again no particle released in the culture supernatants. Transmission electron microscopic analysis of insect cells expressing Gag-Pro polyprotein revealed large vacuoles in the cytoplasm and no budding particles at the plasma membrane. In contrast, human immunodeficiency virus type 1 Gag polyprotein expressed in insect cells is able to release VLPs. These data showed that unlike other retroviruses, Pr53(Gag) is unable to be released as immature VLPs from insect cells. To determine whether the block in particle budding and release is due to an intrinsic property of Pr53(Gag) or the absence of essential cellular factors in insect cells, we expressed Gag and Gag-Pro polyproteins in human 293 cells. The results indicate that Pr53(Gag) and p24 capsid are released within particles into the culture supernatants of human 293 cells. We found that the myristylation of the N-terminal glycine residue is essential for Gag release. Altogether, these results strongly suggest that the proper assembly of HTLV-I particles is dependent on mammalian host cell factors.

The carboxy-terminal fragment of nucleolin interacts with the nucleocapsid domain of retroviral gag proteins and inhibits virion assembly

A yeast two-hybrid screen for cellular proteins that interact with the murine leukemia virus (MuLV) Gag protein resulted in the identification of nucleolin, a host protein known to function in ribosome assembly. The interacting fusions contained the carboxy-terminal 212 amino acids of nucleolin [Nuc(212)]. The nucleocapsid (NC) portion of Gag was necessary and sufficient to mediate the binding to Nuc(212). The interaction of Gag with Nuc(212) could be demonstrated in vitro and was manifested in vivo by the NC-dependent incorporation of Nuc(212) inside MuLV virions. Overexpression of Nuc(212), but not full-length nucleolin, potently and specifically blocked MuLV virion assembly and/or release. A mutant of MuLV, selected to specifically disrupt the binding to Nuc(212), was found to be severely defective for virion assembly. This mutant harbors a single point mutation in capsid (CA) adjacent to the CA-NC junction, suggesting a role for this region in Moloney MuLV assembly. These experiments demonstrate that selection for proteins that bind assembly domain(s) can yield potent inhibitors of virion assembly. These experiments also raise the possibility that a nucleolin-Gag interaction may be involved in virion assembly.

Human immunodeficiency virus type 1 envelope-specific cytotoxic T lymphocytes response dynamics after prime-boost vaccine regimens with human immunodeficiency virus type 1 canarypox and pseudovirions

Virus-specific cytotoxic T lymphocytes (CTLs) may represent significant immune mechanisms in the control of human immunodeficiency virus (HIV) infection and, therefore, CTL induction may be a fundamental goal in the development of an efficacious acquired immunodeficiency syndrome (AIDS) vaccine. In the current study, prime-boost protocols were used to investigate the potential of noninfectious human immunodeficiency virus type 1 (HIV-1) pseudovirions (HIV PSV) in enhancing HIV-specific CTL responses in Balb/c mice primed with the recombinant canarypox vector, vCP205, encoding HIV-1 gp120 (MN strain) in addition to Gag/Protease (HIB strain). The prime-boost immunization regimens were administered intramuscularly and involved injections of vCP205 followed by boosts with HIV PSV. Previous vaccination strategies solely involving vCP205 had induced good cellular immune responses in uninfected human volunteers, despite some limitations. The use of genetically engineered HIV PSV was a logical step in the evaluation of whole noninfectious virus or inactivated virus vaccine strategies, particularly as a potential boosting agent for vCP205-primed recipients. Based on this current study, HIV PSV appeared to have the capability to effectively induce and boost cell-mediated HIV-1-specific responses. In order to observe the immune effects of HIV PSV in a prime-boost immunization strategy, both HIV vaccine immunogens required careful titration in vivo. This suggests that careful consideration should be given to the optimization of immunization protocols destined for human use.

A cell-line-specific defect in the intracellular transport and release of assembled retroviral capsids

Retrovirus assembly involves a complex series of events in which a large number of proteins must be targeted to a point on the plasma membrane where immature viruses bud from the cell. Gag polyproteins of most retroviruses assemble an immature capsid on the cytoplasmic side of the plasma membrane during the budding process (C-type assembly), but a few assemble immature capsids deep in the cytoplasm and are then transported to the plasma membrane (B- or D-type assembly), where they are enveloped. With both assembly phenotypes, Gag polyproteins must be transported to the site of viral budding in either a relatively unassembled form (C type) or a completely assembled form (B and D types). The molecular nature of this transport process and the host cell factors that are involved have remained obscure. During the development of a recombinant baculovirus/insect cell system for the expression of both C-type and D-type Gag polyproteins, we discovered an insect cell line (High Five) with two distinct defects that resulted in the reduced release of virus-like particles. The first of these was a pronounced defect in the transport of D-type but not C-type Gag polyproteins to the plasma membrane. High Five cells expressing wild-type Mason-Pfizer monkey virus (M-PMV) Gag precursors accumulate assembled immature capsids in large cytoplasmic aggregates similar to a transport-defective mutant (MA-A18V). In contrast, a larger fraction of the Gag molecules encoded by the M-PMV C-type morphogenesis mutant (MA-R55W) and those of human immunodeficiency virus were transported to the plasma membrane for assembly and budding of virions. When pulse-labeled Gag precursors from High Five cells were fractionated on velocity gradients, they sedimented more rapidly, indicating that they are sequestered in a higher-molecular-mass complex. Compared to Sf9 insect cells, the High Five cells also demonstrate a defect in the release of C-type virus particles. These findings support the hypothesis that host cell factors are important in the process of Gag transport and in the release of enveloped viral particles.

Induction of immune responses to SIV antigens by mucosally administered vaccines

In an attempt to develop an immunization strategy to induce mucosal and circulatory antibodies against SIV antigens, we have investigated the potential of attenuated recombinant vaccinia virus to deliver SIV antigens (gp160 of SIVmac239) to mucosal surfaces of mice. After systemic or mucosal (intragastric, intranasal, or intrarectal) immunization with vaccinia virus-SIV Env recombinants the immune responses against the envelope glycoprotein of SIV, as well as against vaccinia virus antigens, were assessed by ELISA of serum, saliva, and intestinal and vaginal secretions. All immunization routes induced specific antibody titers against gp160 in both serum and external secretions. Recall responses against SIV were found to be acquired after administration of SIVmac239 Env and Gag antigens in a virus-like particle (VLP) form by the same mucosal routes as those used for the priming with recombinant vaccinia virus. The results obtained demonstrate the potential of vaccinia virus recombinants to elicit a primary immune response at mucosal surfaces, which could be enhanced by delivering the same antigen in the form of VLPs.

An endoplasmic reticulum retrieval signal partitions human foamy virus maturation to intracytoplasmic membranes

Among all retroviruses, foamy viruses (FVs) are unique in that they regularly mature at intracytoplasmic membranes. The envelope glycoprotein of FV encodes an endoplasmic reticulum (ER) retrieval signal, the dilysine motif (KKXX), that functions to localize the human FV (HFV) glycoprotein to the ER. This study analyzed the function of the dilysine motif in the context of infectious molecular clones of HFV that encoded mutations in the dilysine motif. Electron microscopy (EM) demonstrated virion budding both intracytoplasmically and at the plasma membrane for the wild-type and mutant viruses. Additionally, mutant viruses retained their infectivity, but viruses lacking the dilysine signal budded at the plasma membrane to a greater extent than did wild-type viruses. Interestingly, this relative increase in budding across the plasma membrane did not increase the overall release of viral particles into cell culture media as measured by protein levels in viral pellets or infectious virus titers. We conclude that the dilysine motif of HFV imposes a partial restriction on the site of viral maturation but is not necessary for viral infectivity.

Accumulation of specific amino acid substitutions in HLA-B35-restricted human immunodeficiency virus type 1 cytotoxic T lymphocyte epitopes

HLA is one of the genetic factors that influence the clinical course of HIV-1 infection, and patients with HLA-B35 are prone to rapid disease progression. Nine viral epitopes that are recognized by cytotoxic T lymphocytes (CTLs) in an HLA-B35-restricted manner were determined. To examine how HIV-1 sequences are selected by CTLs in vivo, we sequenced the nine CTL epitopes of the virus in patient plasma. Here we show that certain amino acid substitutions at three epitopes were observed with significantly higher frequency in HLA-B35-positive patients than in HLA-B35-negative patients. By performing experiments with CTL clones established from the HLA-B35-positive patients, it was determined that one of the three substitutions was probably an escape mutation. However, concerning the other two epitopes, representative CTL clones killed target cells pulsed with mutant peptides as efficiently as those pulsed with wild-type peptides, suggesting that CTLs that can be established in vitro are not functioning properly in vivo. Amino acid sequence drift in all HLA-B35-restricted epitopes was rare during the observation period (1 year). Our results may have relevance in understanding the rapid clinical progression in HLA-B35-positive patients.

Analysis of human lymphotropic T-cell virus type II-like particle production by recombinant baculovirus-infected insect cells

The molecular processes involved in retrovirus assembly and budding formation remain poorly understood. The gag-pro-pol genes of human lymphotropic T-cell virus type II (HTLV-II) are translated into Gag, Gag-Pro, or Gag-Pro-Pol by frameshift events. In the present study, we investigated the roles of the gag, pro, and pol regions of HTLV-II in viral particle formation using recombinant baculoviruses. In this study we could successfully produce mature HTLV-II viral particles containing core structures using a construct expressing the entire gag-pro-pol region. We also investigated the role of the pol region in particle formation. Deletion of the pol region affects viral particle assembly or release very little, indicating that the gag-pro region is sufficient for viral particle formation and maturation. Expression of the Gag proteins alone or Gag proteins with inactivated viral proteases (Pro) resulted in the formation of viral particles; however, these particles did not contain core structures. These results suggest the intracellular expression of Gag with Pro of HTLV-II is essential for the production of mature virus particles, whereas that of Pol is not.

Identification of multiple HIV-1 CTL epitopes presented by HLA-B*5101 molecules

We attempted to identify and characterize HIV-1 CTL epitopes presented by HLA-B51 which is associated with a slow progression to AIDS. HLA-B*5101 stabilization assay showed that 33 out of 172 HIV-1 peptides carrying HLA-B*5101 anchor residues bound to HLA-B*5101. Seven peptides were suggested as HIV-1 CTL epitopes presented by HLA-B*5101 because the specific CTL was induced for these peptides in PBMC from three HIV-1 seropositive individuals carrying HLA-B51 by stimulation with HLA-B*5101 binding peptides. Analysis of these epitopes using the specific CTL clones confirmed that six of seven HIV-1 peptides are epitopes presented by HLA-B*5101. Three epitopes presented by HLA-B*5101 are highly conserved among the clade B strain, suggesting that the specific CTL for these epitopes might play an important role in recognition of HIV-1 infected cells. These epitopes will be useful to analyze CTL responses in HIV-1 infected individuals.

Characterization of human immunodeficiency virus type-1 (HIV-1) particles that express protease-reverse transcriptase fusion proteins

We have selectively mutagenized specific residues at the junction between the protease (PR) and reverse transcriptase (RT) genes of human immunodeficiency virus type 1 (HIV-1) to study the effects of PR-RT fusion proteins in the context of a full-length, infectious proviral construct. Mutant viruses derived from COS-7 cells transfected with this construct were analyzed in regard to each of viral replication, maturation, and infectivity. Immunoblot analysis revealed that the mutation prevented cleavage between the PR and RT proteins and that both existed as a PR-RT fusion protein in each of cellular and viral lysates. Interestingly, intracellular PR that existed within the PR-RT fusion protein remained functionally active, whereby HIV-1 precursor proteins were processed efficiently. Furthermore, the RT component of the fusion protein also retained its enzymatic activity as shown in RT assays. Electron microscopy revealed that the mutant viruses containing the PR-RT fusion protein possessed wild-type morphology. These viruses also displayed wild-type sensitivities to inhibitors of each of the HIV-1 PR and RT activities. However, viruses containing the PR-RT fusion protein were 20 times less infectious than wild-type viruses. This defect was further pronounced when mutated Gag-Pol proteins were overexpressed as a consequence of an additional mutation that interfered with frameshifting. Thus, unlike cleavage site mutations at the N terminus of PR, a cleavage site mutation between PR and RT did not affect the enzymatic activities of either PR or RT and viruses containing PR-RT fusion proteins were viable.

The matrix protein of HIV-1 is not sufficient for assembly and release of virus-like particles

The matrix (MA) proteins of human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) are known to be important for the targeting and assembly of lentiviral proteins. The objective of the present study was to determine whether the MA protein of HIV-1 was sufficient for particle assembly and release. Eukaryotic expression of wild-type HIV-1 Gag-Pol, HIV-1 MA alone, or SIV MA alone was analyzed with radio-immunoprecipitation, density centrifugation, and a protease protection assay. Cells that expressed HIV-1 Gag-Pol or SIV MA alone released virus-like particles (VLPs) with sucrose gradient densities of 1.15 or 1.12 g/ml, respectively. The MA and/or capsid proteins in these particles were protected from protease degradation, indicating the presence of a protective outer membrane. Expression of HIV-1 MA protein alone resulted in release of MA which pelleted through a 20% sucrose cushion but failed to enter a 20-60% sucrose gradient and was not protected from protease degradation. The MA protein of SIV was previously reported to be sufficient for production of VLPs (S. A. Gonzalez, H, K, Affrachino, H. R. Gelderblom, and A. Burney. Virology 194, 548-556, 1993; V. Liska, D. Spehner, M. Mehtali, D. Schmitt, A. Kirn, and A. M. Aubertin. J. Gen. Virol. 75, 2955-2962, 1994). Our study confirmed that result, but indicated that the MA protein of HIV-1 was not sufficient to assemble and release VLPs.

N-Terminal extension of human immunodeficiency virus capsid protein converts the in vitro assembly phenotype from tubular to spherical particles

Expression of retroviral Gag polyproteins is sufficient for morphogenesis of virus-like particles with a spherical immature protein shell. Proteolytic cleavage of Gag into the matrix (MA), capsid (CA), nucleocapsid (NC), and p6 domains (in the case of human immunodeficiency virus [HIV]) leads to condensation to the mature cone-shaped core. We have analyzed the formation of spherical or cylindrical particles on in vitro assembly of purified HIV proteins or inside Escherichia coli cells. CA protein alone yielded cylindrical particles, while all N-terminal extensions of CA abolished cylinder formation. Spherical particles with heterogeneous diameters or amorphous protein aggregates were observed instead. Extending CA by 5 amino acids was sufficient to convert the assembly phenotype to spherical particles. Sequences C-terminal of CA were not required for sphere formation. Proteolytic cleavage of N-terminally extended CA proteins prior to in vitro assembly led to the formation of cylindrical particles, while proteolysis of in vitro assembly products caused disruption of spheres but not formation of cylinders. In vitro assembly of CA and extended CA proteins in the presence of cyclophilin A (CypA) at a CA-to-CypA molar ratio of 10:1 yielded significantly longer cylinders and heterogeneous spheres, while higher concentrations of CypA completely disrupted particle formation. We conclude that the spherical shape of immature HIV particles is determined by the presence of an N-terminal extension on the CA domain and that core condensation during virion maturation requires the liberation of the N terminus of CA.

Cytotoxic T cells and neutralizing antibodies induced in rhesus monkeys by virus-like particle HIV vaccines in the absence of protection from SHIV infection

HIV Pr55gag has in the absence of other viral components the capacity to self assemble in budding noninfectious virus-like particles (VLP). The immunological spectrum of the HIV-1IIIB gag-derived VLP was expanded either by stable anchoring of chimeric modified gp 120 on the surface of the VLP (type 1) or by replacing sequences of the Pr55gag precursor by the V3 loop and a linear portion of the CD4 binding domain (type 2). This noninfectious antigen delivery system was evaluated for immunogenicity and efficacy in rhesus macaques without adjuvants. Intramuscular immunization with both types of VLP induced high titers of gag-specific antibodies ranging from 1/8000 to 1/510,000 for type 1 VLP and from 1/4000 to 1/16,000 for type 2 VLP. Only animals immunized with type 1 VLP developed substantial endpoint titers of env-specific antibodies (1/2000-1/32,000) with a neutralizing capacity at serum dilutions of 1/32-1/128. Gag- and env-specific cytotoxic T lymphocyte (CTL) activity was induced by both types of VLP at similar levels. Four weeks after the last immunization animals were challenged intravenously with 20 MID50 of the cell free homologous envelope simian/HIV-1IIIB chimeric challenge stock Despite HIV-1-specific neutralizing and CTL responses, all vaccinated animals became infected.

The molecular basis of HIV capsid assembly

In common with many aspects of the HIV life cycle, the assembly of the virus particle has been the subject of intense investigation over recent years. Study of the subject is facilitated by the fact that only a single gene product, the Pr55 Gag protein, is required for virus assembly. A combination of site directed mutagenesis, biochemical characterisation and structural studies have led to a picture of the overall architecture of the particle, the partial structure of Pr55, and the subdomains involved in oligomerisation. Copyright 1998 John Wiley & Sons, Ltd.

Analysis of the assembly function of the human immunodeficiency virus type 1 gag protein nucleocapsid domain

Previous studies have shown that in addition to its function in specific RNA encapsidation, the human immunodeficiency virus type 1 (HIV-1) nucleocapsid (NC) is required for efficient virus particle assembly. However, the mechanism by which NC facilitates the assembly process is not clearly established. Formally, NC could act by constraining the Pr559gag polyprotein into an assembly-competent conformation or by masking residues which block the assembly process. Alternatively, the capacity of NC to bind RNA or make interprotein contacts might affect particle assembly. To examine its role in the assembly process, we replaced the NC domain in Pr55gag with polypeptide domains of known function, and the chimeric proteins were analyzed for their abilities to direct the release of virus-like particles. Our results indicate that NC does not mask inhibitory domains and does not act passively, by simply providing a stable folded monomeric structure. However, replacement of NC by polypeptides which form interprotein contacts permitted efficient virus particle assembly and release, even when RNA was not detected in the particles. These results suggest that formation of interprotein contacts by NC is essential to the normal HIV-1 assembly process.