Chimeric gag-V3 virus-like particles of human immunodeficiency virus induce virus-neutralizing antibodies

Developments in virus-like particle-based vaccines for HIV

Virus-like particles (VLPs) hold great promise for the development of effective and affordable vaccines. VLPs, indeed, are suitable for presentation and efficient delivery to antigen-presenting cells of linear as well as conformational antigens. This will ultimately result in a crosspresentation with both MHC class I and II molecules to prime CD4(+) T-helper and CD8(+) cytotoxic T cells. This review describes an update on the development and use of VLPs as vaccine approaches for HIV.

Baculovirus expression and antigenic characterization of classical swine fever virus E2 proteins

Genes encoding a major structural glycoprotein, E2, of classical swine fever viruses (CSFV) Brescia (subgroup 1.2), Paderborn (subgroup 2.1) and Kanagawa (subgroup 3.4) were constructed by removing the transmembrane domain and adding a C-terminal 6 histidine (His) tag. All the E2 constructs were efficiently expressed in a baculovirus system as 53-kDa glycosylated proteins that were identified in Western blots by their reaction with anti-His and CSFV-specific antibodies. These proteins were used as ELISA antigens to confirm the existence of an antigenic relationship between the viruses using group-specific polyclonal antisera. Antigenic differences were identified by Western blot and ELISA reactivity of the E2 proteins with a panel of monoclonal antibodies. Specifically, one monoclonal antibody (WH303) reacted with all three proteins, two monoclonal antibodies (M1660 and M1665) reacted with only the Brescia E2 protein, and three monoclonal antibodies (M1654, M1664 and M1669) reacted equally well with only Brescia and Kanagawa E2 proteins. Therefore, antibody reactivity profiles, established using recombinant E2 proteins, could be used to quickly identify novel CSFV strains as illustrated in this report with only a limited number of monoclonal antibodies. These proteins could also have added utility in the production of monoclonal antibodies and as critical reagents in diagnostic assays.

Developments in virus-like particle-based vaccines for infectious diseases and cancer

Virus-like particles hold great promise for the development of effective and affordable vaccines. Indeed, virus-like particles are suitable for presentation and efficient delivery of linear as well as conformational antigens to antigen-presenting cells. This will ultimately result in optimal B-cell activation and cross-presentation with both MHC class I and II molecules to prime CD4(+) T-helper as well as CD8(+) cytotoxic T cells. This article provides an update on the development and use of virus-like particles as vaccine approaches for infectious diseases and cancer.

Enterovirus 71 vaccine: close but still far

Background

Enterovirus 71 (EV71), a member of the Enterovirus genus of the Picornaviridae family, is one of the causative pathogens of hand-foot-and-mouth disease (HFMD) and the most common etiological agent isolated from HFMD patients complicated with neurological disorders. EV71 has become an increasingly important neurotropic enterovirus in the post-poliomyelitis eradication era. Effective antiviral agents and vaccines against this virus are currently still under development. We reviewed publications on the development of EV71 vaccines in order to provide an overview of the field.

Methods

Fifty-five articles on EV71 vaccine development, published from 1974 to 2009, were collected from Sun Yat-sen University library and reviewed.

Results

Various types of vaccine have been developed for EV71. In results published to date, all vaccines for EV71 under development appear to elicit an immune response in rodents or in monkeys. According to the established regulatory standards, it may be relatively easy to acquire a license to use the inactivated virus in order to meet the immediate demands for EV71 control . With regard to the attenuated vaccine, it is critical to increase the genetic stability before clinical use, due to the risk of virulent revertants. The virus-like particle (VLP) vaccine, not only conserving the conformational epitopes, but also having no risk of virulent revertants, is another promising vaccine candidate for EV71, but needs further development. The VP1 capsid protein is the backbone antigen protein for developing subunit vaccine and epitope vaccine; these remain viable potential vaccine strategies worthy of further study and development.

Conclusions

The conservation of the three-dimensional structure is important for the EV71 inactivated vaccine and VLP vaccine to induce a strong immune response. To develop EV71 vaccines with a high protection efficacy, strategies such as the use of adjuvant, strong promoters, tissue-specific promoters, and addition of mucosal immune adjuvant should be considered.

Optimization of chimeric HIV-1 virus-like particle production in a baculovirus-insect cell expression system

A baculovirus-insect cell expression system potentially provides the means to produce prophylactic HIV-1 virus-like particle (VLP) vaccines inexpensively and in large quantities. However, the system must be optimized to maximize yields and increase process efficiency. In this study, we optimized the production of two novel, chimeric HIV-1 VLP vaccine candidates (GagRT and GagTN) in insect cells. This was done by monitoring the effects of four specific factors on VLP expression: these were insect cell line, cell density, multiplicity of infection (MOI), and infection time. The use of western blots, Gag p24 ELISA, and four-factorial ANOVA allowed the determination of the most favorable conditions for chimeric VLP production, as well as which factors affected VLP expression most significantly. Both VLP vaccine candidates favored similar optimal conditions, demonstrating higher yields of VLPs when produced in the Trichoplusia ni Pro insect cell line, at a cell density of 1 x 10(6) cells/mL, and an infection time of 96 h post infection. It was found that cell density and infection time were major influencing factors, but that MOI did not affect VLP expression significantly. This work provides a potentially valuable guideline for HIV-1 protein vaccine optimization, as well as for general optimization of a baculovirus-based expression system to produce complex recombinant proteins.

Biological gene delivery vehicles: beyond viral vectors

Gene therapy covers a broad spectrum of applications, from gene replacement and knockdown for genetic or acquired diseases such as cancer, to vaccination, each with different requirements for gene delivery. Viral vectors and synthetic liposomes have emerged as the vehicles of choice for many applications today, but both have limitations and risks, including complexity of production, limited packaging capacity, and unfavorable immunological features, which restrict gene therapy applications and hold back the potential for preventive gene therapy. While continuing to improve these vectors, it is important to investigate other options, particularly nonviral biological agents which include bacteria, bacteriophage, virus-like particles (VLPs), erythrocyte ghosts, and exosomes. Exploiting the natural properties of these biological entities for specific gene delivery applications will expand the repertoire of gene therapy vectors available for clinical use. Here, we review the prospects for nonviral biological delivery vehicles as gene therapy agents with focus on their unique evolved biological properties and respective limitations and potential applications. The potential of these nonviral biological entities to act as clinical gene therapy delivery vehicles has already been shown in clinical trials using bacteria-mediated gene transfer and with sufficient development, these entities will complement the established delivery techniques for gene therapy applications.

Production, characterization and assay application of a purified, baculovirus-expressed, serogroup specific bluetongue virus antigen

The predominant serodiagnostic assay used in many countries to detect bluetongue virus (BTV) infections is a competitive enzyme-linked immunosorbent assay (c-ELISA) which employs two critical reagents: a cell culture-derived BTV antigen and group-specific monoclonal antibody (Mab). Ongoing difficulties have been reported by laboratories in the production and quality control of the native antigen reagent which relies on the presence of adequate molar quantities and appropriate presentation of the major BTV core protein VP7. To address this important issue, a recombinant baculovirus was constructed containing a cDNA copy of genome segment 7 of BTV serotype 11 and used to infect insect cells which, in turn, expressed high levels of theVP7 protein with an estimated molecular mass of 39 kDa. In its purified form, this recombinant protein could be detected by group-specific Mabs designated 3.17.A3 and 8A3B.6 produced against BTV serotypes 1 and 17, respectively, as well as by polyclonal bovine antibodies raised against North American and South African BTV serotypes. No reactivity was observed by Western blot analysis with these two Mabs suggesting that the common antigenic determinants, on the BTV VP7 protein, were mainly conformational. It was interesting to note that the purified recombinant VP7 protein demonstrated a greater degree of reactivity with Mab 8A3B.6 compared to that exhibited with Mab 3.17.A3 when evaluated in an ELISA. Due to its antigenic similarity to the native antigen, the recombinant protein was found to be a suitable replacement for use in a c-ELISA to detect BTV-specific antibodies with the added advantage that it could be consistently produced and was, therefore, amenable to quality control testing for purity, stability and other standards.

Chimaeric HIV-1 subtype C Gag molecules with large in-frame C-terminal polypeptide fusions form virus-like particles

HIV-1 Pr55 Gag virus-like particles (VLPs) are strong immunogens with potential as candidate HIV vaccines. VLP immunogenicity can be broadened by making chimaeric Gag molecules: however, VLPs incorporating polypeptides longer than 200 aa fused in frame with Gag have not yet been reported. We constructed a range of gag-derived genes encoding in-frame C-terminal fusions of myristoylation-competent native Pr55Gag and p6-truncated Gag (Pr50Gag) to test the effects of polypeptide length and sequence on VLP formation and morphology, in an insect cell expression system. Fused sequences included a modified reverse transcriptase-Tat-Nef fusion polypeptide (RTTN, 778 aa), and truncated versions of RTTN ranging from 113 aa to 450 aa. Baculovirus-expressed chimaeric proteins were examined by western blot and electron microscopy. All chimaeras formed VLPs which could be purified by sucrose gradient centrifugation. VLP diameter increased with protein MW, from approximately 100 nm for Pr55Gag to approximately 250 nm for GagRTTN. The presence or absence of the Gag p6 region did not obviously affect VLP formation or appearance. GagRT chimaeric particles were successfully used in mice to boost T-cell responses to Gag and RT that were elicited by a DNA vaccine encoding a GagRTTN polypeptide, indicating the potential of such chimaeras to be used as candidate HIV vaccines.

Hydrophilicity dependent budding and secretion of chimeric HIV Gag-V3 virus-like particles

Virus-like particles (VLPs) of numerous viruses have been considered as possible candidates for vaccine development. We have constructed HIV chimeric genes by coupling the gag gene of HIV-2 with the V3 domain of the gp120 gene of either HIV-1 or HIV-2 and expressed the chimeric genes in SF21 cells using the recombinant baculovirus expression system. Although the level of expression of the chimeric HIV-2 gag gene with the V3 domain of either HIV-1 gp120 (gagC-1V3) or HIV-2 gp120 (gagC-2V3) was high, the VLP assembly and extracellular release of GagC-1V3 was very poor. In contrast, GagC-2V3 chimeric proteins formed VLPs and released efficiently. We have constructed substitution mutants to investigate the effects of the hydrophobic region of the V3 domain of HIV-1 Gp120 (1V3) in VLP assembly and release. The substitution mutant analyses revealed that in replacing the hydrophobic region of the 1V3 in GagC-1V3 with the hydrophilic sequence of the V3 domain of HIV-2 Gp120 (2V3) enhanced the extracellular VLP. We demonstrate here that disruption of the hydrophobic character of the C-terminus of the chimeric protein improves assembly and release of the VLPs. Our results suggest that the poor GagC-1V3 VLP release was attributed to the hydrophobic region in the V3 sequence of the chimeric protein, and that not only the N-terminal myristylation and positively charged domain of the Gag protein functioned as a targeting signal to direct membrane binding, but also that the C-terminal hydrophobic region affected release of chimeric VLPs.

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.

Expression, purification and characterization of enterovirus-71 virus-like particles

AIM: Enterovirus 71 (EV71) has been implicated as the etiological agent responsible for the recent outbreaks of hand, foot and mouth disease associated with severe neurological diseases in the Asia-Pacific region.

METHODS: The assembly process was hypothesized to occur via an orchestrated proteolytic processing of the P1 precursor by the viral protease 3CD. To test this hypothesis, we constructed 3 recombinant baculoviruses: Bac-P1 expressing P1; Bac-3CD expressing 3CD; and Bac-P1-3CD co-expressing P1 and 3CD.

RESULTS: Both single infection by Bac-P1-3CD and co-infection by Bac-P1 and Bac-3CD resulted in correct cleavage of P1 to yield individual proteins VP0, VP1 and VP3, while the former approach yielded higher VLP production. In the cells, the structural proteins self-assembled into clusters of virus-like particles (VLP) resembling the authentic EV71 particle aggregates. After ultracentrifugation purification, the dispersed VLPs were indistinguishable from the authentic virus in size, appearance, composition and surface epitopes, as determined by SDS-PAGE, Western blot, transmission electron microscopy and immunogold labeling.

CONCLUSION: Our data, for the first time, suggest that in insect cells EV71 structural proteins adopt a processing and assembly pathway similar to poliovirus assembly. The preservation of particle morphology and composition suggest that the VLP may be a valuable vaccine candidate to prevent EV71 epidemics.

Vaccines for viral and parasitic diseases produced with baculovirus vectors

The baculovirus-insect cell expression system is an approved system for the production of viral antigens with vaccine potential for humans and animals and has been used for production of subunit vaccines against parasitic diseases as well. Many candidate subunit vaccines have been expressed in this system and immunization commonly led to protective immunity against pathogen challenge. The first vaccines produced in insect cells for animal use are now on the market. This chapter deals with the tailoring of the baculovirus-insect cell expression system for vaccine production in terms of expression levels, integrity and immunogenicity of recombinant proteins, and baculovirus genome stability. Various expression strategies are discussed including chimeric, virus-like particles, baculovirus display of foreign antigens on budded virions or in occlusion bodies, and specialized baculovirus vectors with mammalian promoters that express the antigen in the immunized individual. A historical overview shows the wide variety of viral (glyco)proteins that have successfully been expressed in this system for vaccine purposes. The potential of this expression system for antiparasite vaccines is illustrated. The combination of subunit vaccines and marker tests, both based on antigens expressed in insect cells, provides a powerful tool to combat disease and to monitor infectious agents.

Production of a recombinant major inner capsid protein for serological detection of epizootic hemorrhagic disease virus

Constructs of the major core protein, designated VP7, from epizootic hemorrhagic disease virus (EHDV) type 1 were made by amino- or carboxyl-terminal fusion of a six-histidine residue tag to the VP7-1 gene. The resulting fusion proteins were produced in a baculovirus expression system and purified by a rapid, one-step procedure using nickel-nitrilotriacetic acid technology. A high level of VP7-1 protein expression was detected with the N-terminal six-histidine tag fusion construct and was comparable to the level of expression observed with an untagged VP7-1 Bam construct. In contrast, the inclusion of a six-histidine tag at the C terminus adversely affected protein expression. The antigenicity of the N-terminal six-histidine tag EHDV VP7-1 product was identical to that observed with the native virus antigen and untagged EHDV VP7-1 recombinant protein, as determined by reactivity with EHDV specific antibodies in an enzyme-linked immunosorbent assay (ELISA) and Western blot. The high production and purity levels that can be attained for the N-terminal six-histidine tag VP7-1 protein and its reactivity with EHDV-specific sera in a competitive ELISA make it a suitable assay reagent.

Baculovirus as a highly efficient expression vector in insect and mammalian cells

Baculovirus has been widely used for the production of recombinant proteins in insect cells. Since the finding that baculovirus can efficiently transduce mammalian cells, the applications of baculovirus have been greatly expanded. The prospects and drawbacks of baculovirus-mediated gene expression, either in insect or in mammalian cells, are reviewed. Recent progresses in expanding the applications to studies of gene regulation, viral vector preparation, in vivo and ex vivo gene therapy studies, generation of vaccine vectors, etc are discussed and the efforts directed towards overcoming the existing bottlenecks are particularly emphasized.

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.

Budding and secretion of HIV Gag-Env virus-like particles from recombinant human adenovirus infected cells

We have characterized the assembly, budding and extra-cellular release of human immunodeficiency virus (HIV) Gag-Env virus-like particles (VLPs) from human embryonic kidney cells (293 cells expressing the E1a protein of adenovirus) infected with recombinant replication-defective human adenovirus type 5. Recombinant human adenovirus vectors expressing the chimeric Gag-Env protein were constructed by inserting the gag-env fusion gene into the E1a region of the human adenovirus type 5. Biochemical and immunological analyses of VLPs recovered from the culture supernatant revealed that these particles contain the HIV-2 Gag protein and segments of Env protein from the HIV-1 gp120. This chimeric Gag-Env protein interacted with HIV-1 positive patient sera and with HIV-2 Gag monoclonal antibody. Immunoelectron microscopy of the 293 cells infected with the recombinant adenoviruses showed that the HIV Gag-Env antigen is present in the VLPs. Thin-section electron microscopy (EM) revealed that the Gag-Env VLPs bud through the cytoplasmic membrane, as well as through membranes of intracellular vacuoles. The thin-section EM showed that the Gag-Env VLPs have a spherical morphology with an electron-dense ring. The size of VLPs range from 110 to 140 nm in diameter, which is slightly larger than that of the Gag particles without Env protein fusion. Mice immunized with recombinant adenoviruses generated antibodies that specifically reacted with Gag-Env chimeric proteins. Our results support the idea that the replication-defective adenovirus could be used to induce immune responses that might be useful in a vaccine against HIV/AIDS.

HIV type 1 Gag virus-like particle budding from spheroplasts of Saccharomyces cerevisiae

Expression of retroviral Gag protein in yeast has previously shown Gag targeting to the plasma membrane but little or no production of Gag virus-like particles (VLPs). Here we show that, after removal of the cell wall, the expression of HIV type 1 Gag protein in Saccharomyces cerevisiae spheroplasts allowed simultaneous budding of VLPs from the plasma membrane. Our data show that (i) the VLPs released from yeast spheroplasts were spherical and had morphological features, such as membrane apposed electron-dense layers, characteristic of the immature form of HIV particles; (ii) the VLPs were completely enclosed in the plasma membrane derived from yeast, which is denser than that of higher eukaryotic cells; (iii) the VLP Gag shells remained intact after treatment of nonionic detergent; and (iv) the VLPs were released soon after removal of the cell wall and accumulated up to 300 microg/liter of culture. Our results also show that VLP production was abolished by amino acid substitution of the Gag N-terminal myristoylglycine and impaired when Gag C-terminal deletions were extended beyond the nucleocapsid domain. These results were consistent with those obtained previously in higher eukaryotic expression systems, suggesting that similar Gag domains were used for VLP assembly. We suggest that the system described here offers significant advantages for studying host factors required for VLP budding. The system also may be available for production of vector virus-free VLPs for practical applications such as vaccine development.

Proteolytic activity in infected and noninfected insect cells: degradation of HIV-1 Pr55gag particles

In this work the proteolytic activity in the supernatant and inside insect cells in culture was evaluated for different multiplicities of infection (MOI) and times of infection (TOI). Several methods to detect proteolytic activity in insect cells were tested and that using fluorescein thiocyanite-casein as a substrate was chosen. It was observed that infection caused not only a reduction in the concentration of proteases by decreasing their synthesis but also an inhibition of the intracellular proteolytic activity by increasing the intracellular ATP level (measured by in vivo nuclear magnetic resonance, NMR). The maximum proteolytic activity in the supernatant was observed at 72 hpi except when the cells were infected in the late exponential growth phase or with very low MOI, yielding a nonsynchronous infection. The proteolytic degradation of Pr55gag particles was studied during culture and after harvest. In this particular case it was concluded that the supernatant should be stored at low temperature or quickly purified, since the degradation after 24 h is only 3% at 4 degrees C while at 27 degrees C this value rises to 23%. There is a complex relationship between MOI, TOI, proteolytic activity, and product titer and quality. Thus, the optimal conditions for each case will be a compromise between the final product titer, the desired product quality, and operational issues like process time and capacity, requiring proper integration between bioreaction and downstream processing.

Chimeric infectious bursal disease virus-like particles expressed in insect cells and purified by immobilized metal affinity chromatography

Chimeric virus-like particles (VLPs) of infectious bursal disease virus (IBDV) were produced by coinfecting Spodoptera frugiperda (Sf-9) insect cells with two recombinant baculoviruses, vIBD-7 and vEDLH-22. vIBD-7 encodes VP2, VP3, and VP4 of the IBDV structural proteins. vEDLH-22 encodes VP2 with five histidine residues at the carboxy-terminus (VP2H). Coinfection produced hybrid VLPs composed of VP2, VP2H, and VP3. The additional histidine residues on VP2H enabled the efficient purification of VLPs based on immobilized metal affinity chromatography (IMAC). These results demonstrated that the VLPs formed are comprised of chimeric subunits with attached affinity ligands, and further, that sufficient His5 ligand was available for binding to the IMAC metal-chelating resin. Additionally, these novel particles were fully characterized for antigenicity by a series of monoclonal antibodies, and appeared identical to the two wild-type IBDV strains contributing subunits to the chimeric VLP. IMAC purification provides a promising low-cost and simple scheme to purify VLPs as vaccines.

Development of HIV/AIDS vaccine using chimeric gag-env virus-like particles

We attempted to develop a candidate HIV/AIDS vaccine, by using unprocessed HIV-2 gag pr45 precursor protein. We found that a 45 kDa unprocessed HIV-2 gag precursor protein (pr45), with a deletion of a portion of the viral protease, assembles as virus-like particles (VLP). We mapped the functional domain of HIV-2 gag VLP formation in order to find the minimum length of gag protein to form VLP. A series of deletion mutants was constructed by sequentially removing the C-terminal region of HIV-2 gag precursor protein and expressed truncated genes in Spodoptera frugiperda (SF) cells by infecting recombinant baculoviruses. We found that deletion of up to 143 amino acids at the C-terminus of HIV-2 gag, leaving 376 amino acids at the N-terminus of the protein, did not affect VLP formation. There is a proline-rich region at the amino acid positions 373 to 377 of HIV-2 gag, and replacement of these proline residues by site-directed mutagenesis completely abolished VLP assembly. Our data demonstrate that the C-terminal p12 region of HIV-2 gag precursor protein, and zinc finger domains, are dispensable for gag VLP assembly, but the presence of at least one of the three prolines at amino acid positions 373, 375 or 377 of HIV-2NIH-Z is required for VLP formation. Animals immunized with these gag particles produced high titer antibodies and Western blot analyses showed that anti-gag pr45 rabbit sera react with p17, p24 and p55 gag proteins of HIV-1. We then constructed chimeric gag genes, which carry the hypervariable V3 region of HIV-1 gp120, because the V3 loop is known to interact with chemokine receptor as a coreceptor, and known to induce the major neutralizing antibodies and stimulate the cytoxic T lymphocyte responses in humans and mice. We expressed chimeric fusion protein of HIV-2 gag with 3 tandem copies of consensus V3 domain that were derived from 245 different isolates of HIV-1. In addition, we also constructed and expressed chimeric fusion protein that contains HIV-2 gag with V3 domains of HIV-1IIIB, HIV-1MN, HIV-1SF2 and HIV-1RF. The chimeric gag-env particles had a spherical morphology, and the size was slightly larger than that of a gag particle. Immunoprecipitation and Western blot analyses show that these chimeric proteins were recognized by HIV-1 positive human sera and antisera raised against V3 peptides, as well as by rabbit anti-gp120 serum. We obtained virus neutralizing antibodies in rabbits by immunizing these gag-env VLPs. In addition, we found that gag-env chimeric VLPs induce a strong CTL activity against V3 peptide-treated target cells. Our results indicate that V3 peptides from all major clades of HIV-1 carried by HIV-2 gag can be used as a potential HIV/AIDS vaccine.

Construction and characterization of recombinant VLPs and Semliki-Forest virus live vectors for comparative evaluation in the SHIV monkey model

For testing of recombinant virus-like particles (VLPs) in the SHIV monkey model, SIVmac239 Pr56gag precursor-based pseudovirions were modified by HIV-1 gp160 derived peptides. First, well-characterized epitopes from the HIV-1 envelope glycoprotein were inserted into the Pr56gag precursor by replacing defined regions that were shown to be dispensable for virus particle formation. Expression of these chimeric proteins in a baculovirus expression system resulted in efficient assembly and release of non-infectious, hybrid VLPs. In a second approach the HIV-1IIIB external glycoprotein gp120 was covalently linked to an Epstein-Barr virus derived transmembrane domain. Coexpression of the hybrid envelope derivative with the Pr56gag precursor yielded recombinant SIV derived Pr56gag particles with the HIV-1 gp120 firmly anchored on the VLP surface. Immunization of rhesus monkeys with either naked VLPs or VLPs adsorbed to alum induced substantial serum antibody titers and promoted both T helper cell and cytotoxic T lymphocyte responses. Furthermore, priming macaques with the corresponding set of recombinant Semliki-Forest viruses tended to enhance the immunological outcome. Challenge of the immunized monkeys with chimeric SHIV resulted in a clearly accelerated reduction of the plasma viremia as compared to control animals.

Tagging the human immunodeficiency virus gag protein with green fluorescent protein. Minimal evidence for colocalisation with actin

The assembly and budding of human immunodeficiency virus type 1, encoded solely in the Gag protein precursor Pr55Gag, occur at the plasma membrane of infected cells. However, little is known about the routing of the Gag molecule from its site of synthesis in the cytoplasm to the site of budding, with past studies suggesting that the cytoskeleton, particularly actin, may be involved in the translocation. We have constructed a T7 promoter-driven gag gene fusion with green fluorescent protein (GFP) that expresses Gag-GFP in both cells and supernatant. The distribution of Gag-GFP was the same as Gag only, suggesting that cellular routing was not affected by fusion to GFP, and using colabelling techniques, Gag-GFP was shown to have no particular colocalisation with actin. After detergent extraction of expressing cells, Gag and Gag-GFP remained cell associated, whereas GFP only was wholly released. These data suggest that Gag may associate with other cytoskeletal components or, perhaps more likely, that a partial assembly to a large-molecular-weight intermediate occurs before localisation at the plasma membrane.

Core particles of hepatitis B virus as carrier for foreign epitopes

To be effective as vaccines, most monomeric proteins and peptides either require chemical coupling to high molecular weight carriers or application together with adjuvants. More recently, recombinant DNA techniques have been used to insert foreign epitopes into proteins with inherent multimerization capacity, such as particle-forming viral capsid or envelope proteins. The core protein of hepatitis B virus (HBcAg), because of its unique structural and immunological properties, has gained widespread interest as a potential antigen carrier. Foreign sequences of up to approximately 40 amino acid residues at the N terminus, 50 or 100 amino acids in the central immunodominant c/e 1 epitope region of HBcAg, and up to 100 or even more residues at the C terminus, did not interfere with particle formation. The humoral immunogenicity of inserted epitopes is determined by the immunogenicity of the peptide itself and its surface exposure, and is influenced by the route of application. The probably flexible and surface-exposed c/e1 region emerged as the most promising insertion site. When applied together with adjuvants approved for human and veterinary use, or even without adjuvants, such chimeric particles induced B and T cell immune responses against the inserted epitopes. In some cases neutralizing antibodies, cytotoxic T cells and protection against challenge with the intact pathogen were demonstrated. Major factors for the potentiated immune response against the foreign epitopes are the multimeric structure of chimeric HBcAg that results in a high epitope density per particle, and the provision of T cell help by the carrier moiety. Beyond its use as subunit vaccine, chimeric HBcAg produced in attenuated Salmonella strains may be applicable as live vaccine.

Induction of V3-specific cytotoxic T lymphocyte responses by HIV gag particles carrying multiple immunodominant V3 epitopes of gp120

Effort to develop a vaccine to prevent infection of human immunodeficiency virus (HIV) have focused on the induction of neutralizing antibodies. In our previous study, we reported that chimeric gag-env virus-like particles (VLPs) induce neutralizing antibodies which block HIV infection. In addition to the neutralizing antibodies, the cytotoxic T-lymphocyte (CTL) response is considered to be another major immune defense mechanism required for recovery from many different viral infections. In the present study, we have constructed chimeric fusion proteins using HIV-2 gag precursor protein with (1) four neutralizing epitopes from HIV-1 gp160; (2) three tandem copies of consensus V3 domain, which have been derived from 245 different isolates of HIV-1 and carries both the principal neutralizing determinant (PND) and CTL epitopes; and (3) V3 domains from HIV-1IIIB, HIV-1MN, HIV-1RF, and HIV-1SF2. These chimeric fusion proteins were expressed in a large quantity within insect cells, and released as VLPs into the cell culture medium. The purified gag-env VLPs from all three constructs appear to be spherical particles similar to immature HIV but slightly larger than the gag VLPs. Immunoprecipitation analysis showed that the chimeric proteins were recognized not only by HIV-1 positive patient sera, but also by monoclonal and polyclonal antisera raised against V3 peptides of HIV-1IIIB, HIV-1MN, HIV-1RF, and the gp120 antiserum against HIV-1SF2. Balb/C mice immunized with these chimeric VLPs successfully induced CTL activity against V3 peptide-stimulated target cells. In addition, a high degree of cross-reactivity was observed among the four different strains of HIV-1 V3 domain, indicating that the tandem multiple consensus V3 peptide sequence carried by HIV-2 gag can be used as a potential HIV vaccine against various HIVs.

Expression of human immunodeficiency virus genes using baculovirus expression system

The structural protein genes of HIV-1 and HIV-2 have been expressed in Spodoptera frugiperda (SF) cells using baculovirus expression system. The noncoding flanking sequences of HIV structural genes were removed and a putative ribosome binding site was placed in front of the open reading frame of each gene by using crossover linker mutagenesis. The coding sequences of the gag, pol, env, and vif proteins were inserted into Autographa californica nuclear polyhedrosis virus (AcNPV) so that HIV genes were under the control of the AcNPV polyhedrin promoter. All recombinant AcNPV-infected SF cells express high levels of HIV structural proteins. Detailed strategies of recombinant AcNPV construction for high level protein expression are presented.

Chimeric HIV-1 virus-like particles containing gp120 epitopes as a result of a ribosomal frameshift elicit Gag- and SU-specific murine cytotoxic T-lymphocyte activities

Insect cell expression of the HIV-1 Gag precursor protein by recombinant baculoviruses results in the assembly and budding of noninfectious virus-like particles (VLPs). The VLPs resemble immature virus in ultrastructural morphology and can be purified by conventional retroviral techniques. The virus-like appearance of the particles suggested that they could be used to package additional peptides. The retroviral frameshift mechanism was used to translate the pol gene products by expressing additional genetic information as chimeric Gag-Pol fusion proteins. Sequences encoding the carboxyl 65% of the HIV-1 surface glycoprotein (gp120, SU) were inserted into the Gag-Pol reading frame immediately downstream of the Gag stop codon. The assembly and budding of large quantities of Gag and chimeric Gag-SU VLPs were observed by standard transmission electron microscopy. The presence of gp120 epitopes in the Gag-SU VLPs was confirmed by immunoelectron microscopy and Western blot analysis using monoclonal anti-gp120 antibodies. Mice inoculated with the Gag-SU pseudovirions developed cytotoxic lymphocyte responses to both HIV-1 Gag and Env epitopes yet humoral immune responses only to Gag epitopes. The chimeric Gag-SU particles may have applications as vaccines or immunotherapeutic treatments for HIV-1 infection. In addition, the frameshift mechanism can be applied to the packaging of other viral or cellular proteins.

Increased incorporation of chimeric human immunodeficiency virus type 1 gp120 proteins into Pr55gag virus-like particles by an Epstein-Barr virus gp220/350-derived transmembrane domain

Noninfectious Pr55gag virus-like particles containing high quantities of oligomeric human immunodeficiency virus type 1 (HIV-1) envelope (Env) proteins represent potential candidate immunogens for a vaccine against HIV-1 infection. Thus, chimeric env genes were constructed encoding the HIV-1 exterior glycoprotein gp120 which was covalently linked at different C-terminal positions to a transmembrane domain (TM) from the Epstein-Barr virus (EBV) major Env glycoprotein gp220/ 350. All chimeric Env-TM polypeptides as well as the wild-type HIV Env proteins were equally produced and incorporated at the outer surface of insect cells using the baculovirus expression system. In the presence of coexpressed HIV Pr55gag polyproteins significantly decreased amounts of wild-type Env proteins were presented at the cell surface, whereas the membrane incorporation of the Env-TM chimeras was not affected. Biochemical and immunoelectron microscopical analysis of particles that were efficiently released from these cells displayed the incorporation of both wild-type Env and chimeric Env-TM proteins on the surface of VLPs. However, the quantities of particle-associated chimeric Env-TM proteins exceeded those of incorporated wild-type Env proteins by a factor of 5-10. Chemical cross-linking and subsequent polyacrylamide gel electrophoresis of VLP-entrapped Env proteins revealed that the chimeric Env-TM proteins form homodimers and a higher-order oligomer, similar to that observed for wild-type Env proteins. Thus, the results of this study clearly demonstrate that the replacement of the gp41 transmembrane protein of gp160 by a heterologous, EBV gp220/350-derived membrane anchor provides an effective strategy to incorporate high quantities of oligomeric HIV gp120 proteins on the surface of Pr55gag virus-like particles.

Recombinant human immunodeficiency Pr55gag virus-like particles presenting chimeric envelope glycoproteins induce cytotoxic T-cells and neutralizing antibodies

Very recently, we demonstrated that the replacement of the human immunodeficiency virus type-1 (HIV-1) gp41 transmembrane protein by an Epstein-Barr virus gp220/350-derived membrane anchor resulted in the incorporation of chimeric envelope (Env) oligomers into Pr55gag virus-like particles (VLPs), exceeding that of wild-type gp160 by a factor of 10. In this study, we examined the immunostimulatory properties of Pr55gag VLPs to both (i) chimeric HIV-1 gp120 external envelope proteins and (ii) full-length gp160 presented on the outer surface of the particles. Immunization studies carried out with VLPs presenting different derivatives of the chimeric and wild-type Env proteins elicited a consistent anti-Pr55gag as well as anti-Env antibody response in complete absence of additional adjuvants. In both cases, the immune sera exhibited an in vitro neutralizing activity against homologous HIV-1 infection in MT4 cells. Noteworthy, these VLPs were also capable of inducing a strong CD8+ cytotoxic T-cell (CTL) response in immunized BALB/c mice that was directed toward a known CTL epitope in the third variable domain V3 of the gp120 external glycoprotein. However, the induction of V3-loop-specific CTLs critically depended on the amounts of Env proteins that were presented by the Pr55gag VLPs. Moreover, the CD8+ CTL response was not significantly altered by adsorbing the VLPs to alum or by repeated booster immunizations. These results illustrate that Pr55gag VLPs provide a safe and effective means of enhancing neutralizing humoral responses to particle-entrapped gp120 proteins and are also capable of delivering these proteins to the MHC class I antigen processing and presentation pathway. Therefore, antigenically expanded Pr55gag VLPs represent an attractive approach in the design of vaccines for which specific stimulation of neutralizing antibodies and cytotoxic effector functions to complex glycoproteins is desired.

Relationship between antigenicity and immunogenicity of chimeric hepatitis B virus core particles carrying HIV type 1 epitopes

We have developed a comparative study of antigenic and immunogenic properties of selected immunodominant HIV-1 epitopes from p24 and gp120 proteins added to C-terminally truncated hepatitis B virus (HBV) core protein and exposed on the surface of chimeric core particles. Inserted p24 (121-210) and gp120/MN (306-328) epitopes induced the appropriate humoral and cellular immune responses against HIV-1. Superficially exposed region 160-192 of p24 also showed maximal B cell immunogenicity whereas buried region 148-162 induced maximal T cell response. Both recombinant proteins were also able to be recognized in vitro by T lymphocytes of HIV-1 asymptomatic carriers.

The neutralization relationship of HIV type 1, HIV type 2, and SIVcpz is reflected in the genetic diversity that distinguishes them

Neutralizing antibody (NA) patterns in the sera of individuals naturally infected with human immunodeficiency virus (HIV) type 1, HIV-2, and the simian immunodeficiency virus (SIVcpz) to their homologous and heterologous isolates were determined in a peripheral blood mononuclear cell-based neutralization assay. We examined the role of the V3 loop of HIV-1 and SIVcpz in neutralization and the cross-reactivities among them. Cross-neutralization by sera of humans and chimpanzees naturally infected, respectively, with HIV-1 and SIVcpz isolates was more extensive than the infrequent and low-titer cross-neutralizations observed between HIV-1 and HIV-2. Neutralization of 9 of the 16 HIV-1 isolates by 9 of 10 HIV-2 and all 3 SIVcpz antibody-positive sera were weak and sporadic (titer, 1:10-1:160). Twelve of 15 HIV-1 sera neutralized the 2 SIVcpz isolates with titers of 1:10-1:320 but only sporadically neutralized the 6 HIV-2 isolates (titers: 1:10-1:20). The majority of HIV-1 and SIVcpz sera bound to the V3 peptides although their binding capacity did not readily reflect their neutralizing capacity. The HIV-2 sera did not or only weakly bound to the V3 peptides. These results suggest that HIV-1 and SIVcpz share some structural and functional similarities that set them apart from HIV-2.

Effects of inefficient cleavage of the signal sequence of HIV-1 gp 120 on its association with calnexin, folding, and intracellular transport

The HIV-1 envelope glycoprotein gp120 displays inefficient intracellular transport, which is caused by its retention in the endoplasmic reticulum. Coexpression in insect cells (Sf9) of HIV-1 gp120 with calnexin has shown that their interaction was modulated by the signal sequence of HIV-1 gp120. gp120, with its natural signal sequence, showed a prolonged association with calnexin with a t1/2 of greater than 20 min. Replacement of the natural signal sequence with the signal sequence from mellitin led to a decreased time of association of gp120 with calnexin (t1/2 < 10 min). These different times of calnexin association coincided both with the folding of gp120 as measured by the ability of bind CD4 and with endoplasmic reticulum to Golgi transport as analyzed by the acquisition of partial endoglycosidase H resistance. Using a monospecific antibody to the HIV-1 gp120 natural signal peptide, we showed that calnexin associated with N-glycosylated but uncleaved gp120. Only after dissociation from calnexin was gp120 cleaved, but very inefficiently. Only the small proportion of signal-cleaved gp120 molecules acquired transport competence and were secreted. This is the first report demonstrating the effect of the signal sequence on calnexin association.

Location, exposure, and conservation of neutralizing and nonneutralizing epitopes on human immunodeficiency virus type 2 SU glycoprotein

Eleven rat monoclonal antibodies (MAbs) that recognize the SU glycoprotein of human immunodeficiency virus type 2 (HIV-2) ROD were produced and characterized. Binding sites for eight of these MAbs were mapped to epitopes within the Cl, V1/V2, C2, and V3 envelope regions. The three other MAbs defined at least two conformation-dependent, strain-specific epitopes outside Vl/V2, V3, and the CD4-binding site. The MAbs were used to probe the tertiary structure of oligomeric envelope glycoprotein expressed on the surfaces of infected cells. Epitopes at the apices of V2 and V3 were exposed on the native molecule, whereas other epitopes on V1/V2, Cl, and C2 were hidden. The MAbs defined three neutralization targets on exposed domains: two linear epitopes in the V2 and the V3 loops and one conformational epitope outside V1, V2, and V3.

Flow cytometric immunofluorescence assay for detection of antibodies to human immunodeficiency virus type 1 using insoluble precursor forms of recombinant polyproteins as carriers and antigens

A new serological assay, the recombinant flow cytometric immunofluorescence assay (r-FIFA), was developed for the early detection of human immunodeficiency virus type 1 (HIV-1) antibodies by using recombinant insoluble forms of HIV-1 Gag-p45, Gag-gp41 chimeric protein, gp160, Po197 polyprotein as antigens and autologous carriers through flow cytometry. These recombinant proteins were expressed in insect cells by a baculovirus expression system. Eight anti-HIV-1 seroconversion panels, a low-titer anti-HIV-1 panel from Boston Biomedica Inc. (BBI), and three HIV-1 seroconversion specimens from the Provincial Health Laboratory of Ontario, Toronto, Ontario, Canada (PHL), were tested and analyzed by r-FIFA. In sensitivity comparisons between r-FIFA and tests licensed by the U.S. Food and Drug Administration, which were used to test all of the HIV-1 panels from BBI, detection of HIV-1 antibody by r-FIFA was on average greater than 20 days earlier than that by enzyme immunoassay. The sensitivity of r-FIFA has permitted the detection of HIV-1-specific immunoglobulin G (IgG), IgM, and IgA antibodies during seroconversion. A kinetic analysis of HIV-1 antibody production of r-FIFA has shown that either IgG or IgM, or both, can be detected, depending on the phase and type of the immune response in the HIV-1-infected individual. Both primary and secondary immune responses were observed during this period. The r-FIFA results suggest that implementation of r-FIFA may significantly reduce the "window" period from the time of infection to the time of seroconversion, with earlier detection of antibodies after initial infection. This would also make it possible for us to understand the immune response and the precise mechanisms of immunopathogenesis in the early period of HIV-1 infection.

Release of virus-like particles from cells infected with poliovirus replicons which express human immunodeficiency virus type 1 Gag

The effectiveness of attenuated poliovirus vaccines when given orally to induce both systemic and mucosal immune responses against poliovirus has resulted in an effort to develop poliovirus-based vectors to express foreign proteins. We have previously described the construction of poliovirus genomes (referred to as replicons) in which the complete human immunodeficiency virus type 1 (HIV-1) gag gene was substituted for the capsid gene (P1) (D.C. Porter, D.C. Ansardi, and C.D. Morrow, J. Virol. 69:1548-1555, 1995). Infection of cells with encapsidated replicons resulted in the expression of a 55-kDa protein. To further characterize the biological features of the HIV-1 Gag proteins expressed in cells infected with encapsidated replicons, we utilized biochemical analysis and electron microscopy. Expression of the 55-kDa protein in cells infected with encapsidated replicons resulted in myristylation of the Pr55gag protein. The Gag precursor protein was released from infected cells; analysis on sucrose density gradients revealed that the precursor sedimented at a density consistent with that of an HIV-1 virus-like particle. Analysis of replicon-infected cells by electron microscopy demonstrated the presence of condensed structures at the plasma membrane and the release of virus-like particles. These studies demonstrate that poliovirus-based vectors can be used to express foreign proteins which require posttranslational modifications, such as myristylation, and assemble into higher-order structures, providing a foundation for the future use of poliovirus replicons as vaccine vectors.

Assembly and immunogenicity of chimeric Gag-Env proteins derived from the human immunodeficiency virus type 1

We evaluated the potential of the precursor Gag protein (Pr55) of the human immunodeficiency virus type 1 (HIV-1) as a carrier for the presentation of envelope epitopes. Recombinant chimeric core-envelope protein-expressing constructs were derived by deletion of regions within the gag gene, especially of regions encoding p24 capsid epitopes. Sequences encoding either the principal neutralization determinant (PND) and/or the CD4-binding domains (CD4BS) were then inserted. Deletion of residues 196-226 within the p24 capsid protein did not prevent self-assembly into virus-like particles (VLPs) whereas deletion of residues 299-328 completely abolished VLP formation. Thus the major homology region (MHR) and proximal sequences are required for capsid assembly. An immunization study in mice showed that assembled chimeric proteins elicited strong anti-Gag, weak anti-envelope, and no neutralizing humoral responses. Nonassembled chimeric proteins were poor immunogens. Mapping of Pr55 antigenic sites using sera from immunized mice and peptides overlapping the entire Gag precursor showed that p24 capsid and p17 matrix epitopes presented to the immune system differed from the mature form (p24 or p17) and the multimeric immature form (Pr55).

Viral liposomes released from insect cells infected with recombinant baculovirus expressing the matrix protein of vesicular stomatitis virus

The matrix (M) protein of vesicular stomatitis virus (VSV) has been found to promote assembly and budding of virions as well as down-regulating of VSV transcription. Large quantities of M protein can be produced in insect cells infected with recombinant baculovirus expressing the VSV M gene under control of the polyhedrin promoter. Analysis by pulse-chase experiments and density gradient centrifugation revealed that the [35S]methionine-labeled M protein synthesized in insect cells is released into the extracellular medium in association with lipid vesicles (liposomes). Electron microscopy and immunogold labeling showed that M protein expressed in insect cells induced the formation on plasma membrane of vesicles containing M protein, which are released from the cell surface in the form of liposomes. The baculovirus vector itself or recombinants expressing VSV glycoprotein (G) or nucleocapsid (N) protein did not produce the formation of vesicles in infected cells. The baculovirus-expressed M protein retains biological activity as demonstrated by its capacity to inhibit transcription when reconstituted with VSV nucleocapsids in vitro. These data suggest that M protein has the capacity to associate with the plasma membrane of infected cells and, in so doing, causes evagination of the membrane to form a vesicle which is released from the cell. This observation leads to the postulate, which requires further proof, that the VSV M protein can induce the formation and budding of liposomes from the cell membrane surface.