Studies on processing, particle formation, and immunogenicity of the HIV-1 gag gene product: a possible component of a HIV vaccine

Evaluation of Virus-Free Manufacture of Recombinant Proteins Using CRISPR-Mediated Gene Disruption in Baculovirus-Infected Insect Cells

The manufacture and downstream processing of virus-like particles (VLPs) using the baculovirus expression vector system (BEVS) is complicated by the presence of large concentrations of baculovirus particles, which are similar in size and density to VLPs, and consequently are difficult to separate. To reduce the burden of downstream processing, CRISPR-Cas9 technology was used to introduce insertion-deletion (indel) mutations within the Autographa californica multiple nucleopolyhedrovirus (AcMNPV) gp64 open reading frame, which encodes the major envelope protein of AcMNPV. After comfirming the site-specific targeting of gp64 leading to reduced budded virus (BV) release, the gag gene of human immunodeficiency virus type 1 was expressed to produce Gag VLPs. This approach was effective for producing VLPs using the BEVS whilst simultaneously obstructing BV release.

Coupling Microscopy and Flow Cytometry for a Comprehensive Characterization of Nanoparticle Production in Insect Cells

Advancements in the field of characterization techniques have broadened the opportunities to deepen into nanoparticle production bioprocesses. Gag-based virus-like particles (VLPs) have shown their potential as candidates for recombinant vaccine development. However, comprehensive characterization of the production process is still a requirement to meet the desired critical quality attributes. In this work, the production process of Gag VLPs by baculovirus (BV) infection in the reference High Five and Sf9 insect cell lines is characterized in detail. To this end, the Gag polyprotein was fused in frame to the enhanced green fluorescent protein (eGFP) to favor process evaluation with multiple analytical tools. Tracking of the infection process using confocal microscopy and flow cytometry revealed a pronounced increase in the complexity of High Five over Sf9 cells. Cryogenic transmission electron microscopy (cryo-TEM) characterization determined that changes in cell complexity could be attributed to the presence of occlusion-derived BV in High Five cells, whereas Sf9 cells evidenced a larger proportion of the budded virus phenotype (23-fold). Initial evaluation of the VLP production process using spectrofluorometry showed that higher levels of the Gag-eGFP polyprotein were obtained in High Five cells (3.6-fold). However, comparative analysis based on nanoparticle quantification by flow virometry and nanoparticle tracking analysis (NTA) proved that Sf9 cells were 1.7- and 1.5-fold more productive in terms of assembled VLPs, respectively. Finally, analytical ultracentrifugation coupled to flow virometry evidenced a larger sedimentation coefficient of High Five-derived VLPs, indicating a possible interaction with other cellular compounds. Taken together, these results highlight the combined use of microscopy and flow cytometry techniques to improve vaccine development processes using the insect cell/BV expression vector system. © 2020 International Society for Advancement of Cytometry.

Toll-like receptor 3 adjuvant in combination with virus-like particles elicit a humoral response against HIV

Human Immunodeficiency Virus (HIV) Virus-Like Particles (VLPs) composed of HIV_IIIB Gag and HIV_BaL gp120/gp41 envelope are a pseudovirion vaccine capable of presenting antigens in their native conformations. To enhance the immunogenicity of the HIV Env antigen, VLPs were coupled to VesiVax Conjugatable Adjuvant Lipid Vesicles (CALV) containing one of four toll-like-receptor (TLR) ligands, each activating a receptor with distinct cellular localization and downstream pathways. C57BL/6 mice were vaccinated by intranasal prime followed by two sub-cheek boosts and their sera immunoglobulin and neutralizing potency were measured over a duration of 3months after vaccination. PBS control, VLPs alone, CALV+VLPs, and VLPs complexed with CALV and ligands for TLR2 (PAM3CAG), TLR3 (dsRNA), TLR4 (MPLA), or TLR7/8 (resiquimod) were evaluated based on antibody titer, IgG1 and IgG2c class switching, germinal center formation, T follicular cells and potency of neutralizing antibodies. Consistently, the TLR3 ligand dsRNA complexed to CALV and in combination with VLPs (CALV(dsRNA)+VLPs) induced the strongest response. CALV(dsRNA)+VLPs induced the highest titers against the recombinant vaccine antigens clade B Bal gp120 and pr55 Gag. Additionally, CALV(dsRNA)+VLPs induced cross-clade antibodies, represented by high titers of antibody to clade c 96ZM651 gp120. CALV(dsRNA)+VLPs induced predominantly IgG2c over IgG1, a response associated with T helper type 1 (Th1)-like cytokines. In turn, CALV(dsRNA)+VLP immunized mice generated the most potent neutralizing antibodies against HIV strain MN.3. Finally, at time of sacrifice, a significant increase in germinal center B cells and T follicular cells was detected in mice which received CALV(dsRNA)+VLPs compared to PBS. Our results indicate that CALV(dsRNA) is a superior adjuvant for HIV VLPs in generating a Th1-like immunoglobulin profile, while prolonging lymph node germinal centers, T follicular cells and generating neutralizing antibodies to a highly sensitive tier 1A variant of HIV.

Immune responses in mice vaccinated with virus-like particles composed of the GP5 and M proteins of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) induces reproductive failure in sows and respiratory problems in pigs of all ages. Live attenuated and inactivated vaccines are used on swine farms to control PRRSV. However, their protective efficacy against field strains of PRRSV remains questionable. New vaccines have been developed to improve the efficacy of these traditional vaccines. In this study, virus-like particles (VLPs) composed of the GP5 and M proteins of PRRSV were developed, and the capacity of the VLPs to elicit antigen-specific immunity was evaluated. Serum antibody titers and production of cytokines were measured in BALB/C mice immunized intramuscularly three times with different doses (0.5, 1.0, 2.0, and 4.0 μg) of the VLP vaccine. A commercial vaccine consisting of inactivated PRRSV and phosphate-buffered saline (PBS) were used as positive and negative controls, respectively. IgG titers to GP5 were significantly higher in all groups of mice vaccinated with the VLPs than in control mice. Neutralizing antibodies were only detected in mice vaccinated with 2.0 and 4.0 μg of the VLPs. Cytokine levels were determined in cell culture supernatants after in vitro stimulation of splenocytes with the VLPs for 3 days. Mice immunized with 4.0 μg of the VLPs produced a significantly higher amount of interferon-gamma (IFN-γ) than mice immunized with the commercial inactivated PRRSV vaccine and PBS. In contrast, immunization with the commercial vaccine induced higher production of IL-4 and IL-10 in mice than mice vaccinated with VLPs. These data together demonstrate the capacity of VLPs to induce both neutralizing antibodies and IFN-γ in immunized mice. The VLP vaccine developed in this study could serve as a platform for the generation of improved VLP vaccines to control PRRSV.

Generation and characterization of a defective HIV-1 Virus as an immunogen for a therapeutic vaccine

BACKGROUND

The generation of new immunogens able to elicit strong specific immune responses remains a major challenge in the attempts to obtain a prophylactic or therapeutic vaccine against HIV/AIDS. We designed and constructed a defective recombinant virus based on the HIV-1 genome generating infective but non-replicative virions able to elicit broad and strong cellular immune responses in HIV-1 seropositive individuals.

RESULTS

Viral particles were generated through transient transfection in producer cells (293-T) of a full length HIV-1 DNA carrying a deletion of 892 base pairs (bp) in the pol gene encompassing the sequence that codes for the reverse transcriptase (NL4-3/ΔRT clone). The viral particles generated were able to enter target cells, but due to the absence of reverse transcriptase no replication was detected. The immunogenic capacity of these particles was assessed by ELISPOT to determine γ-interferon production in a cohort of 69 chronic asymptomatic HIV-1 seropositive individuals. Surprisingly, defective particles produced from NL4-3/ΔRT triggered stronger cellular responses than wild-type HIV-1 viruses inactivated with Aldrithiol-2 (AT-2) and in a larger proportion of individuals (55% versus 23% seropositive individuals tested). Electron microscopy showed that NL4-3/ΔRT virions display immature morphology. Interestingly, wild-type viruses treated with Amprenavir (APV) to induce defective core maturation also induced stronger responses than the same viral particles generated in the absence of protease inhibitors.

CONCLUSIONS

We propose that immature HIV-1 virions generated from NL4-3/ΔRT viral clones may represent new prototypes of immunogens with a safer profile and stronger capacity to induce cellular immune responses than wild-type inactivated viral particles.

Stability studies of HIV-1 Pr55gag virus-like particles made in insect cells after storage in various formulation media

Background

HIV-1 Pr55^gag virus-like particles (VLPs) expressed by baculovirus in insect cells are considered to be a very promising HIV-1 vaccine candidate, as they have been shown to elicit broad cellular immune responses when tested in animals, particularly when used as a boost to DNA or BCG vaccines. However, it is important for the VLPs to retain their structure for them to be fully functional and effective. The medium in which the VLPs are formulated and the temperature at which they are stored are two important factors affecting their stability.

Findings

We describe the screening of 3 different readily available formulation media (sorbitol, sucrose and trehalose) for their ability to stabilise HIV-1 Pr55^gag VLPs during prolonged storage. Transmission electron microscopy (TEM) was done on VLPs stored at two different concentrations of the media at three different temperatures (4°C, –20°C and −70°C) over different time periods, and the appearance of the VLPs was compared. VLPs stored in 15% trehalose at −70°C retained their original appearance the most effectively over a period of 12 months. VLPs stored in 5% trehalose, sorbitol or sucrose were not all intact even after 1 month storage at the temperatures tested. In addition, we showed that VLPs stored under these conditions were able to be frozen and re-thawed twice before showing changes in their appearance.

Conclusions

Although the inclusion of other analytical tools are essential to validate these preliminary findings, storage in 15% trehalose at −70°C for 12 months is most effective in retaining VLP stability.

Altering an artificial Gagpolnef polyprotein and mode of ENV co-administration affects the immunogenicity of a clade C HIV DNA vaccine

HIV-1 candidate vaccines expressing an artificial polyprotein comprising Gag, Pol and Nef (GPN) and a secreted envelope protein (Env) were shown in recent Phase I/II clinical trials to induce high levels of polyfunctional T cell responses; however, Env-specific responses clearly exceeded those against Gag. Here, we assess the impact of the GPN immunogen design and variations in the formulation and vaccination regimen of a combined GPN/Env DNA vaccine on the T cell responses against the various HIV proteins. Subtle modifications were introduced into the GPN gene to increase Gag expression, modify the expression ratio of Gag to PolNef and support budding of virus-like particles. I.m. administration of the various DNA constructs into BALB/c mice resulted in an up to 10-fold increase in Gag- and Pol-specific IFNγ(+) CD8(+) T cells compared to GPN. Co-administering Env with Gag or GPN derivatives largely abrogated Gag-specific responses. Alterations in the molar ratio of the DNA vaccines and spatially or temporally separated administration induced more balanced T cell responses. Whereas forced co-expression of Gag and Env from one plasmid induced predominantly Env-specific T cells responses, deletion of the only H-2(d) T cell epitope in Env allowed increased levels of Gag-specific T cells, suggesting competition at an epitope level. Our data demonstrate that the biochemical properties of an artificial polyprotein clearly influence the levels of antigen-specific T cells, and variations in formulation and schedule can overcome competition for the induction of these responses. These results are guiding the design of ongoing pre-clinical and clinical trials.

Design and immunogenicity assessment of HIV-1 virus-like particles as a candidate vaccine

The rapid growth of the global HIV/AIDS epidemic makes it a high priority to develop an effective vaccine. Since a live attenuated or inactivated HIV vaccine is not likely to be approved for clinical application due to safety concerns, HIV virus like particles (VLPs) offer an attractive alternative because they are considered safer since they lack viral genome. We got a stable eukaryotic cell line by G418 resistance selection, engineered to express the HIV-1 structure protein Gag and Env efficiently and stably. We confirmed the presence of Gag and Env proteins in the cell culture supernatant and that they could self-assemble into VLPs. These VLPs were found to be able to elicit specific humoral and cellular immune response after immunization without any adjuvant.

CD40L-containing virus-like particle as a candidate HIV-1 vaccine targeting dendritic cells

The central role of dendritic cell (DC) in mounting an immune response to a novel antigen is now well established. We sought to demonstrate the use of a particular vaccine strategy based on directing HIV-1 Gag proteins to DCs in conjunction with an activation signal. CD40L was expressed on the surface of virus-like particles (VLPs) to target HIV-1 Gag antigens to the CD40 receptor on DCs, whereas CD40L-CD40 interaction would also result in cellular activation. Multiple CD40L VLP constructs were made and evaluated in vitro and in vivo. Indeed, one VLP that expressed CD40L to the highest level showed greatest capacity to activate DCs in vitro. Correspondingly, this CD40L-VLP also proved to be most immunogenic in mice in raising both humoral and cellular responses to HIV-1 Gag. Confirmatory studies were performed to demonstrate the increased immunogenicity of CD40L-VLP is no longer observed when tested in CD40-/- mice. Our findings lend support to the belief that vaccine strategies that both target and activate DCs could yield a superior immune response.

Live attenuated measles vaccine expressing HIV-1 Gag virus like particles covered with gp160DeltaV1V2 is strongly immunogenic

Although a live attenuated HIV vaccine is not currently considered for safety reasons, a strategy inducing both T cells and neutralizing antibodies to native assembled HIV-1 particles expressed by a replicating virus might mimic the advantageous characteristics of live attenuated vaccine. To this aim, we generated a live attenuated recombinant measles vaccine expressing HIV-1 Gag virus-like particles (VLPs) covered with gp160DeltaV1V2 Env protein. The measles-HIV virus replicated efficiently in cell culture and induced the intense budding of HIV particles covered with Env. In mice sensitive to MV infection, this recombinant vaccine stimulated high levels of cellular and humoral immunity to both MV and HIV with neutralizing activity. The measles-HIV virus infected human professional antigen-presenting cells, such as dendritic cells and B cells, and induced efficient presentation of HIV-1 epitopes and subsequent activation of human HIV-1 Gag-specific T cell clones. This candidate vaccine will be next tested in non-human primates. As a pediatric vaccine, it might protect children and adolescents simultaneously from measles and HIV.

Human immunodeficiency virus type-1 (HIV-1) Pr55gag virus-like particles are potent activators of human monocytes

Human immunodeficiency virus type-1 (HIV-1) Pr55(Gag) virus-like particles (VLP) represent an interesting HIV vaccine component since they stimulate strong humoral and cellular immune responses. We demonstrated that VLP expressed by recombinant baculoviruses activate human PBMC to release pro-inflammatory (lL-6, TNF-alpha), anti-inflammatory (IL-10) and Th1-polarizing (IFN-gamma) cytokines as well as GM-CSF and MIP-1alpha in a dose-and time-dependent manner. Herein, residual baculoviruses within the VLP preparations showed no or minor effects. Monocytes could be identified as a main target for VLP to induce cytokine production. Furthermore, VLP-induced monocyte activation was shown by upregulation of molecules involved in antigen presentation (MHC II, CD80, CD86) and cell adhesion (CD54). Exposure of VLP to serum inactivates its capacity to stimulate cytokine production. In summary, these investigations establish VLP as strong activators of PBMC and monocytes therein, potently enhancing their functionality and potency to promote an efficient immune response. This capacity makes VLP an interesting component of combination vaccines.

Virus-like particles-universal molecular toolboxes

Virus-like particles (VLPs) are highly organised spheres that self-assemble from virus-derived structural antigens. These stable and versatile subviral particles possess excellent adjuvant properties capable of inducing innate and cognate immune responses. Commercialised VLP-based vaccines have been successful in protecting humans from hepatitis B virus (HBV) and human papillomavirus (HPV) infection and are currently explored for their potential to combat other infectious diseases and cancer. Much insight into VLP-mediated immune stimulation and optimised VLP design has been gained from human immunodeficiency virus (HIV)-derived VLPs presenting promising components of current AIDS vaccine approaches. Owing to their unique features, VLPs and virosomes, the in vitro-reconstituted VLP counterparts, have recently gained ground in the field of nanobiotechnology as organic templates for the development of new biomaterials.

Importance of protease cleavage sites within and flanking human immunodeficiency virus type 1 transframe protein p6* for spatiotemporal regulation of protease activation

The human immunodeficiency virus type 1 (HIV-1) protease (PR) has recently been shown to be inhibited by its propeptide p6* in vitro. As p6* itself is a PR substrate, the primary goal of this study was to determine the importance of p6* cleavage for HIV-1 maturation and infectivity. For that purpose, short peptide variants mimicking proposed cleavage sites within and flanking p6* were designed and analyzed for qualitative and quantitative hydrolysis in vitro. Proviral clones comprising the selected cleavage site mutations were established and analyzed for Gag and Pol processing, virus maturation, and infectivity in cultured cells. Amino-terminal cleavage site mutation caused aberrant processing of nucleocapsid proteins and delayed replication kinetics. Blocking the internal cleavage site resulted in the utilization of a flanking site at a significantly decreased hydrolysis rate in vitro, which however did not affect Gag-Pol processing and viral replication. Although mutations blocking cleavage at the p6* carboxyl terminus yielded noninfectious virions exhibiting severe Gag processing defects, mutations retarding hydrolysis of this cleavage site neither seemed to impact viral infectivity and propagation in cultured cells nor seemed to interfere with overall maturation of released viruses. Interestingly, these mutants were shown to be clearly disadvantaged when challenged with wild-type virus in a dual competition assay. In sum, we conclude that p6* cleavage is absolutely essential to allow complete activation of the PR and subsequent processing of the viral precursors.

Reconstruction and function of ancestral center-of-tree human immunodeficiency virus type 1 proteins

The extensive diversity of human immunodeficiency virus type 1 (HIV-1) and its capacity to mutate and escape host immune responses are major challenges for AIDS vaccine development. Ancestral sequences, which minimize the genetic distance to circulating strains, provide an opportunity to design immunogens with the potential to elicit broad recognition of HIV epitopes. We developed a phylogenetics-informed algorithm to reconstruct ancestral HIV sequences, called Center of Tree (COT). COT sequences have potentially significant benefits over isolate-based strategies, as they minimize the evolutionary distances to circulating strains. COT sequences are designed to surmount the potential pitfalls stemming from sampling bias with the consensus method and outlier bias with the most-recent-common-ancestor approach. We computationally derived COT sequences from circulating HIV-1 subtype B sequences for the genes encoding the major viral structural protein (Gag) and two regulatory proteins, Tat and Nef. COT genes were synthesized de novo and expressed in mammalian cells, and the proteins were characterized. COT Gag was shown to generate virus-like particles, while COT Tat transactivated gene expression from the HIV-1 long terminal repeat and COT Nef mediated downregulation of cell surface major histocompatibility complex class I. Thus, retrodicted ancestral COT proteins can retain the biological functions of extant HIV-1 proteins. Additionally, COT proteins were immunogenic, as they elicited antigen-specific cytotoxic T-lymphocyte responses in mice. These data support the utility of the COT approach to create novel and biologically active ancestral proteins as a starting point for studies of the structure, function, and biological fitness of highly variable genes, as well as for the rational design of globally relevant vaccine candidates.

Capsid stability and replication of human immunodeficiency virus type 1 are influenced critically by charge and size of Gag residue 183

Structural data support a model where - following proteolytic cleavage--the amino-terminal domain of human immunodeficiency virus type 1 (HIV-1) capsid protein refolds into a beta-hairpin/helix tertiary structure that is stabilized by a buried salt bridge forming between the positively charged primary imino group of a proline residue and the negatively charged carboxyl group of a conserved aspartate. In order to evaluate the contribution of either side-chain length or charge to the formation of infectious virus capsids, aspartate 183 was substituted for glutamate or asparagine in the viral context. It was found that both modifications abolished infectivity of the corresponding viruses in permissive T lymphocytes, although none of particle assembly and release, RNA encapsidation, incorporation of Env glycoproteins and packaging of cyclophilin A were impaired. However, whereas biophysical analyses of mutant virions yielded wild-type-like particle sizes and densities, electron microscopy revealed aberrant core morphologies that could be attributed to either increased (D183N) or reduced (D183E) capsid stability. Although the two amino acid substitutions had opposing effects upon core stability, both mutants were shown to exhibit a severe block in early reverse transcription, underscoring the importance of correct salt-bridge formation for early steps of virus replication.

Structure and immunogenicity of alternative forms of the simian immunodeficiency virus gag protein expressed using Venezuelan equine encephalitis virus replicon particles

Venezuelan equine encephalitis virus replicon particles (VRP) were engineered to express different forms of SIV Gag to compare expression in vitro, formation of intra- and extracellular structures and induction of humoral and cellular immunity in mice. The three forms examined were full-length myristylated SIV Gag (Gagmyr+), full-length Gag lacking the myristylation signal (Gagmyr-) or a truncated form of Gagmyr- comprising only the matrix and capsid domains (MA/CA). Comparison of VRP-infected primary mouse embryo fibroblasts, mouse L929 cells and primate Vero cells showed comparable expression levels for each protein, as well as extracellular virus-like particles (VRP-Gagmyr+) and distinctive cytoplasmic aggregates (VRP-Gagmyr-) with each cell type. VRP were used to immunize BALB/c mice, and immune responses were compared using an interferon (IFN)-gamma ELISPOT assay and a serum antibody ELISA. Although all three VRP generated similar levels of IFN-gamma-producing cells at 1 week post-boost, at 10 weeks post-boost the MA/CA-VRP-induced response was maintained at a significantly higher level relative to that induced by Gagmyr+-VRP. Antibody responses to MA/CA-VRP and Gagmyr+-VRP were not significantly different.

Virus-like particles: designing an effective AIDS vaccine

Viruses that infect eukaryotic organisms have the unique characteristic of self-assembling into particles. The mammalian immune system is highly attuned to recognizing and attacking these viral particles following infection. The use of particle-based immunogens, often delivered as live-attenuated viruses, has been an effective vaccination strategy for a variety of viruses. The development of an effective vaccine against the human immunodeficiency virus (HIV) has proven to be a challenge, since HIV infects cells of the immune system causing severe immunodeficiency resulting in the syndrome known as AIDS. In addition, the ability of the virus to adapt to immune pressure and reside in an integrated form in host cells presents hurdles for vaccinologists to overcome. A particle-based vaccine strategy has promise for eliciting high titer, long-lived, immune responses to a diverse number of viral epitopes against different HIV antigens. Live-attenuated viruses are effective at generating both cellular and humoral immune responses. However, while these vaccines stimulate immunity, challenged animals rarely clear the viral infection and the degree of attenuation directly correlates with protection from disease. Further, a live-attenuated vaccine has the potential to revert to a pathogenic form. Alternatively, virus-like particles (VLPs) mimic the viral particle without causing an immunodeficiency disease. VLPs are self-assembling, non-replicating, non-pathogenic particles that are similar in size and conformation to intact virions. A variety of VLPs for lentiviruses are currently in preclinical and clinical trials. This review focuses on our current status of VLP-based AIDS vaccines, regarding issues of purification and immune design for animal and clinical trials.

Elicitation of immunity to HIV type 1 Gag is determined by Gag structure

The gag gene of the human immunodeficiency virus type 1 (HIV-1) encodes for viral proteins that self-assemble into viral particles. The primary Gag gene products (capsid, matrix, and nucleocapsid) elicit humoral and cellular immune responses during natural infection, and these proteins are included in many preclinical and clinical HIV/AIDS vaccines. However, the structure (particulate or soluble) of these proteins may influence the immunity elicited during vaccination. In this study, mice were inoculated with four different HIV-1 Gag vaccines to compare the elicitation of immune responses by the same Gag immunogen presented to the immune system in different forms. The immunity elicited by particles produced in vivo by DNA plasmid (pGag) was compared to these same proteins retained intracellularly (pGag(DMyr)). In addition, the elicitation of anti- Gag immunity by Gag(p55) virus-like particles (VLPs) or soluble, nonparticulate Gag(p55) proteins was compared. Enhanced cellular responses, but almost no anti-Gag antibodies, were elicited with intracellularly retained Gag proteins. In contrast, DNA vaccines expressing VLPs elicited both anti-Gag antibodies and cellular responses. Mice vaccinated with purified Gag(p55) VLPs elicited robust humoral and cellular immune responses, which were significantly higher than the immunity elicited by soluble, nonparticulate Gag(p55) protein. Overall, purified particles of Gag effectively elicited the broadest and highest titers of anti-Gag immunity. The structural form of Gag influences the elicited immune responses and should be considered in the design of HIV/AIDS vaccines.

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.

Expression and characterization of Gag protein of HIV-1CN in Pichia pastoris

To express the core protein of HIV-1 of Chinese prevalent strain (HIV-1(CN)) in Pichia pastoris, the full-length gag gene was inserted into the secretory expression vector pHILS1. Linearized recombinant plasmid pHILGAG by SalI was electrotransformed into the yeast strain GS115, and the yeast transformants were identified by PCR. To induce the interest protein to be expressed, the PCR positive transformants were inoculated in the medium of BMGY and BMMY, mRNA of the strain was detected by RT-PCR, and the expressed protein was analyzed by SDS-PAGE, Western blotting and thin layer scanning. mRNA (1.3kb) was amplified by RT-PCR. SDS-PAGE and Western blotting analysis showed that the molecular mass of the expressed protein was 55kDa, which was similar to the expected value, and the expressed protein could react with McAb to HIV-1 p24. Thin layer scanning analysis demonstrated that the whole amount of the expressed protein was approximately 13% of the soluble protein in the supernatant. The recombinant yeast had good genetic stability. The optimal expression conditions of the engineering yeast were as follows: BMMY medium, 80-90% of dissolved oxygen, 1% methanol, and 3-day-cultivation course. Gag proteins were expressed under the optimal expression condition and purified via gel filtration chromatography. The purity of the interest protein was up to 85%. After the purified proteins were inoculated into BALB/c mice, the anti-HIV-1 antibodies in the immunized mice could be detected by Western blotting.

Influence of polypeptide size and intracellular sorting on the induction of epitope-specific CTL responses by DNA vaccines in a mouse model

We have analysed the influence of size, intracellular localisation, and sorting of various human immunodeficiency virus type 1 (HIV-1)-derived Gag and Env polypeptides containing well defined H2(d)-restricted cytotoxic T lymphocyte (CTL) epitopes on the induction of a humoral and cellular immune response after DNA vaccination. Thus, expression vectors were generated based on RNA- and codon-optimised genes encoding (i). budding competent full-length Gag, (ii). a myristylation defect mutant GagMyr(-), (iii). the isolated p24 capsid moiety of Gag as well as variants of these proteins, which were C-terminally fused HIV gp120-derived V3 epitope (R10I), respectively. These constructs were compared to different minitopes each encoding one of the H2(d)-restricted Gag epitopes A9I and E10F or the V3 epitope R10I that were directly linked to the C-terminus of an Ad2-E3 protein-derived ER signal peptide. Immunological evaluation of these constructs in BALB/c mice revealed that both, the budding competent as well as the intracellular Gag proteins were-irrespective of their molecular weights-equally efficient in the priming of Gag-specific humoral and cellular immune responses. In addition, the capacity of these constructs to stimulate Gag-specific humoral as well as H2-K(d) and H2-L(d) restricted cellular immune responses was not influenced by C-terminal fusion of the immunodominant H2-D(d) restricted V3 epitope. Chimeric GagV3 polyproteins encoding all three major CTL epitopes within a continuous polyprotein were more efficient to stimulate epitope-specific cellular immune responses than the selected minitopes. In addition, the minitopes failed to induce epitope-specific antibody responses. These results clearly show the advantages of complex polypeptides over minitopes regarding the induction of strong humoral and cellular immune responses.

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.

Control of human immunodeficiency virus type-1 protease activity in insect cells expressing Gag-Pol rescues assembly of immature but not mature virus-like particles

Expression of human immunodeficiency virus type 1 (HIV-1) Gag protein in insect cells using baculovirus vectors leads to the abundant production of virus-like particles (VLPs) that represent the immature form of the virus. When Gag-Pol is included, however, VLP production is abolished, a result attributed to premature protease activation degrading the intracellular pool of Gag precursor before particle assembly can occur. As large-scale synthesis of mature noninfectious VLPs would be useful, we have sought to control HIV protease activity in insect cells to give a balance of Gag and Gag-Pol that is compatible with mature particle formation. We show here that intermediate levels of protease activity in insect cells can be attained through site-directed mutagenesis of the protease and through antiprotease drug treatment. However, despite Gag cleavage patterns that mimicked those seen in mammalian cells, VLP synthesis exhibited an essentially all-or-none response in which VLP synthesis occurred but was immature or failed completely. Our data are consistent with a requirement for specific cellular factors in addition to the correct ratio of Gag and Gag-Pol for assembly of mature retrovirus particles in heterologous cell types.

Proline residues in the HIV-1 NH2-terminal capsid domain: structure determinants for proper core assembly and subsequent steps of early replication

Recent analyses suggest that the p24 capsid (p24(CA)) domain of the HIV-1 group-specific antigen (Gag) may be divided into two structurally and functionally distinct moieties: (i) an amino-terminal portion, previously shown to bind the cellular chaperone cyclophilin A, and (ii) a carboxy-terminal domain, known to contribute to the interaction of the Gag and Gag-Pol precursors during the early assembly process. In order to gain deeper insight into the role of the amino-terminal domain of the p24(CA) protein during viral replication, eight highly conserved proline residues known to promote turns and to terminate alpha-helices within the p24 tertiary structure were replaced by a leucine residue (P-position-L). Following transfection of the proviral constructs in COS7 cells, the majority of the mutants resembled wild-type viruses with respect to the assembly and release of virions. However, although the released particles contained wild-type levels of genomic viral RNA, the mature products of the Gag and Gag-Pol polyproteins as well as the Env glycoproteins-all of them, except mutant P225L-were either noninfectious or severely affected in their replicative capacity. Entry assays monitoring the process of viral DNA synthesis led to the classification of selected provirus mutants into four different phenotypes: (i) mutant P225L was infectious and allowed complete reverse transcription including formation of 2-LTR circles; (ii) mutants P149L, P170L, and P217L failed to form 2-LTR circles; (iii) mutant P222L displayed a severe defect in binding and incorporating cyclophilin A into virions, was delayed with respect to DNA polymerization, and failed to form a 2-LTR replication intermediate; and (iv) mutant P133L was unable even to synthesize a first-strand cDNA product. All replication-defective mutants were characterized by severe alterations in the stability of virion cores, which were in two cases reflected by visible changes in the core morphology. These results suggest that proline residues in the NH(2)-terminal capsid domain represent critical structure determinants for proper formation of functional virion cores and subsequent stages of early replication.

Increased expression and immunogenicity of sequence-modified human immunodeficiency virus type 1 gag gene

A major challenge for the next generation of human immunodeficiency virus (HIV) vaccines is the induction of potent, broad, and durable cellular immune responses. The structural protein Gag is highly conserved among the HIV type 1 (HIV-1) gene products and is believed to be an important target for the host cell-mediated immune control of the virus during natural infection. Expression of Gag proteins for vaccines has been hampered by the fact that its expression is dependent on the HIV Rev protein and the Rev-responsive element, the latter located on the env transcript. Moreover, the HIV genome employs suboptimal codon usage, which further contributes to the low expression efficiency of viral proteins. In order to achieve high-level Rev-independent expression of the Gag protein, the sequences encoding HIV-1(SF2) p55(Gag) were modified extensively. First, the viral codons were changed to conform to the codon usage of highly expressed human genes, and second, the residual inhibitory sequences were removed. The resulting modified gag gene showed increases in p55(Gag) protein expression to levels that ranged from 322- to 966-fold greater than that for the native gene after transient expression of 293 cells. Additional constructs that contained the modified gag in combination with modified protease coding sequences were made, and these showed high-level Rev-independent expression of p55(Gag) and its cleavage products. Density gradient analysis and electron microscopy further demonstrated that the modified gag and gag protease genes efficiently expressed particles with the density and morphology expected for HIV virus-like particles. Mice immunized with DNA plasmids containing the modified gag showed Gag-specific antibody and CD8(+) cytotoxic T-lymphocyte (CTL) responses that were inducible at doses of input DNA 100-fold lower than those associated with plasmids containing the native gag gene. Most importantly, four of four rhesus monkeys that received two or three immunizations with modified gag plasmid DNA demonstrated substantial Gag-specific CTL responses. These results highlight the useful application of modified gag expression cassettes for increasing the potency of DNA and other gene delivery vaccine approaches against HIV.

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.

Anti-P30-52 monoclonal antibody cross-reacted to Env V3 and inhibited the viral multiplication of HIV-1-infected MT-4 cells

It is well known that the anti-p17 antibody titer decreases with the disease progression among human immunodeficiency virus type 1 (HIV-1) carriers. We previously established several murine anti-p17 monoclonal antibodies (MAbs) to investigate the immunological role of p17, and to further characterize these MAbs, we examined the anti-p17 antibody titer in serum of a patient who was a long-term nonprogressor with hemophilia, and found that the antibody for the p17-derivative peptide from amino acid residues 30 to 52 (P30-52) cross-reacted to the third variable region of the envelope glycoprotein of HIV-1, Env V3. In the present study, we primed mice with P30-52 to establish anti-P30-52 MAbs (P30-52 MAbs), and examined their affinity and whether they suppressed the viral multiplication of HIV-1-infected MT-4 (HTLV-1-transformed CD4+ T-cell line) cells, in a TCID50 assay. At the same time, an anti-Env V3 MAb (Env V3 MAb) was also established and examined as above. The IgM-type P30-52 MAb and Env V3 MAb showed heteroclitic binding, and the IgM-type P30-52 MAb inhibited the viral multiplication. We also found that an increase of fragmented DNA of HIV-1-infected MT-4 cells co-cultured with P30-52 MAbs. Because DNA fragmentation is one of the features of programmed cell death, the viral multiplication may be suppressed by the apoptosis of HIV-1-infected MT-4 cells co-cultured with P30-52 MAbs. Though the relationship between cross-reactivity and the inhibition mechanism of multiplication of HIV-1 is unclear, P30-52 of p17 may well be a useful region of viral proteins for the development of therapeutic and vaccination strategies.

Correlates of protection, antigen delivery and molecular epidemiology: basics for designing an HIV vaccine

Major obstacles in the development of HIV vaccines are the high variability of the virus and its complex interaction with the immune system. Recent studies demonstrated, that CTLs recognizing highly conserved epitopes in the group-specific antigen are capable of controlling HIV-replication in long-term nonprogressors. Necessary consequences for novel vaccine concepts are the presentation of a large repertoire of antigenic sites as well as the stimulation of different effectors of the immune system. Accordingly, different types of recombinant HIV-1 virus-like particles (VLPs) have been constructed stimulating the induction of neutralizing antibodies and HIV-specific CD8-positive CTL responses in preclinical studies. With respect to future vaccine trials, HIV vaccine formulations may need to be tailored to the local strains circulating within a geographical region. The expert group of the joint United Nations Programme on AIDS recently identified Yunnan, a southwestern province of China, as a region, in which the HIV epidemic is starting to gain speed, resembling to the situation in Thailand 10 years ago. A molecular clone of a representative virus strain is now available for the development of innovative antigen delivery systems aiming to be evaluated in future clinical vaccine trials throughout this area.

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.

Recognition of Two Overlapping CTL Epitopes in HIV-1 p17 by CTL from a Long-Term Nonprogressing HIV-1-Infected Individual

HIV-1 infection has been shown to elicit strong CTL responses in some infected persons, but few data are available regarding the relationship between targeted epitopes and in vivo viral quasispecies. In this study, we examined the CTL response in a person infected for 15 yr with a CD4 count persistently >500 cells/μl. The dominant in vivo activated CTL response was directed against two overlapping Gag CTL epitopes in an area of p17 known to be essential for viral replication. The 9-mer SLYNTVATL (amino acids 77–85) was recognized in conjunction with HLA-A2, whereas the overlapping 8-mer TLYCVHQR (amino acids 83–91) was recognized by HLA-A11-restricted CTL. Analysis of in vivo virus sequences both in PBMC and plasma revealed the existence of sequence variation in this region, which did not affect viral replication in vitro, but decreased recognition by the A11-restricted CTL response, with maintenance of the A2-restricted response. These results indicate that an essential region of the p17 protein can be simultaneously targeted by CTL through two different HLA molecules, and that immune escape from CTL recognition can occur without impairing viral replication. In addition, they demonstrate that Ag processing can allow for presentation of overlapping epitopes in the same infected cell, which can be affected quite differently by sequence variation.

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.

Cryo-electron microscopy reveals ordered domains in the immature HIV-1 particle

BACKGROUND

Human immunodeficiency virus type 1 (HIV-1) is the causative agent of AIDS and the subject of intense study. The immature HIV-1 particle is traditionally described as having a well ordered, icosahedral structure made up of uncleaved Gag protein surrounded by a lipid bilayer containing envelope proteins. Expression of the Gag protein in eukaryotic cells leads to the budding of membranous virus-like particles (VLPs).

RESULTS

We have used cryo-electron microscopy of VLPs from insect cells and lightly fixed, immature HIV-1 particles from human lymphocytes to determine their organization. Both types of particle were heterogeneous in size, varying in diameter from 1200-2600 A. Larger particles appeared to be broken into semi-spherical sectors, each having a radius of curvature of approximately 750 A. No evidence of icosahedral symmetry was found, but local order was evidenced by small arrays of Gag protein that formed facets within the curved sectors. A consistent 270 A radial density was seen, which included a 70 A wide low density feature corresponding to the carboxy-terminal portion of the membrane attached matrix protein and the amino-terminal portion of the capsid protein.

CONCLUSIONS

Immature HIV-1 particles and VLPs both have a multi-sector structure characterized, not by an icosahedral organization, but by local order in which the structures of the matrix and capsid regions of Gag change upon cleavage. We propose a model in which lateral interactions between Gag protein molecules yields arrays that are organized into sectors for budding by RNA.

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.

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.

Sequence-specific resonance assignments of the 1H-NMR spectra and structural characterization in solution of the HIV-1 transframe protein p6

The frameshift protein p6* encoded directly upstream of the protease in the human immunodeficiency virus type 1 (HIV-1) pol reading frame is thought to be a natural inhibitor of protease activation and to play a role in the polyprotein processing of Gag and Gag-Pol precursors. To allow structural characterization of the p6* transframe protein, the p6* coding region was cloned into the vector pGEX-KG and expressed in Escherichia coli as a fusion protein with glutathione S-transferase (GST) under the control of the tac promoter. Thrombin cleavage of the construct resulted in a 70-amino-acid polypeptide which is extended by two additional residues at the N-terminus compared to the natural p6* sequence. The native purification procedure including an affinity and a size-exclusion chromatography step yielded sufficient amounts of highly pure protein suitable for NMR spectroscopy. Fluorescence, circular dichroism and 1H-NMR spectroscopy were applied to characterize the structure of protein. Two-dimensional NMR spectra provided essentially complete sequence-specific resonance assignments at pH 5.9. Although there is evidence for a helix-forming tendency in the N-terminus of the protein, the experiments indicate that p6* has no overall stable secondary or tertiary structure with the single tryptophan exposed in aqueous solution. However, the results reported herein open the way to characterize further the interaction of p6* with the HIV-1 protease in structural and functional in vitro studies.

Efficient particle formation can occur if the matrix domain of human immunodeficiency virus type 1 Gag is substituted by a myristylation signal

Lentiviruses, such as human immunodeficiency virus type 1 (HIV-1), assemble at and bud through the cytoplasmic membrane. Both the matrix (MA) domain of Gag and its amino-terminal myristylation have been implicated in these processes. We have created HIV-1 proviruses lacking the entire matrix domain of gag which either lack or contain an amino-terminal myristate addition sequence at the beginning of the capsid domain. Myristate- and matrix-deficient [myr(-)MA(-)] viruses produced after transient transfection are still able to assemble into particles, although the majority do not form at the plasma membrane or bud efficiently. Myristylation of the amino terminus of the truncated Gag precursor permits a much more efficient release of the mutant virions. While myr(-)MA(-) particles were inefficient in proteolytic processing of the Gag precursor, myristylation enabled efficient proteolysis of the mutant Gag. All matrix-deficient viruses are noninfectious. Particles produced by matrix-deficient mutants contain low levels of glycoproteins, indicating the importance of matrix in either incorporation or stable retention of Env. Since matrix-deficient viruses contain a normal complement of viral genomic RNA, a role for MA in genomic incorporation can be excluded. Contrary to previous reports, the HIV-1 genome does not require sequences between the 5' splice donor site and the gag start codon for efficient packaging.

Cell line-dependent release of HIV-like gag particles after infection of mammalian cells with recombinant vaccinia viruses

We investigated the production of Gag particles by Vero, CV-1, or 1D cells infected with different vaccinia virus recombinants expressing HIV gag or gag-pol genes. Immunoblots of (centrifuged) culture media from 1D cells infected with vMM5, a vaccinia virus recombinant expressing the HIV-2 gag-pol genes, revealed the presence of abundant particles that contained (mostly processed) Gag antigens. In contrast, Gag particles were found only in low amounts in the culture medium from Vero cells infected with the same HIV gag-pol vaccinia virus recombinant; the Gag precursor remained associated with the infected Vero cells and was efficiently processed. This low excretion of Gag particles after infection of Vero cells with vMM5 was also demonstrated by assays of reverse transcriptase activity in the pellet of centrifuged culture medium. Cell fractionation showed that Gag proteins were predominantly found in the membrane fraction from both 1D and Vero cells. Electron microscopy observations of 1D or of Vero cells infected with vMM5 vaccinia virus recombinant revealed in both cases the presence of particles budding at the plasma membrane. However, the shape of the budding particles was different in the two cell lines, with immature forms present in the membrane from the infected Vero cells. An inefficient excretion of Gag particles was also observed after infection of Vero cells with different vaccinia virus recombinants expressing either an uncleaved HIV-2 Gag protein or the HIV-1 gag-pol genes, as judged both by immunoblot and reverse transcriptase activity assays.(ABSTRACT TRUNCATED AT 250 WORDS)