Regulated Expression Allows High Level Production and Secretion of HIV-1 gp120 Envelope Glycoprotein in Drosophila Schneider Cells

Drosophila as a Model for Human Viral Neuroinfections

The study of human neurological infection faces many technical and ethical challenges. While not as common as mammalian models, the use of Drosophila (fruit fly) in the investigation of virus–host dynamics is a powerful research tool. In this review, we focus on the benefits and caveats of using Drosophila as a model for neurological infections and neuroimmunity. Through the examination of in vitro, in vivo and transgenic systems, we highlight select examples to illustrate the use of flies for the study of exogenous and endogenous viruses associated with neurological disease. In each case, phenotypes in Drosophila are compared to those in human conditions. In addition, we discuss antiviral drug screening in flies and how investigating virus–host interactions may lead to novel antiviral drug targets. Together, we highlight standardized and reproducible readouts of fly behaviour, motor function and neurodegeneration that permit an accurate assessment of neurological outcomes for the study of viral infection in fly models. Adoption of Drosophila as a valuable model system for neurological infections has and will continue to guide the discovery of many novel virus–host interactions.

HIV-1 Envelope Glycosylation and the Signal Peptide

The RV144 trial represents the only vaccine trial to demonstrate any protective effect against HIV-1 infection. While the reason(s) for this protection are still being evaluated, it serves as justification for widespread efforts aimed at developing new, more effective HIV-1 vaccines. Advances in our knowledge of HIV-1 immunogens and host antibody responses to these immunogens are crucial to informing vaccine design. While the envelope (Env) protein is the only viral protein present on the surface of virions, it exists in a complex trimeric conformation and is decorated with an array of variable N-linked glycans, making it an important but difficult target for vaccine design. Thus far, efforts to elicit a protective humoral immune response using structural mimics of native Env trimers have been unsuccessful. Notably, the aforementioned N-linked glycans serve as a component of many of the epitopes crucial for the induction of potentially protective broadly neutralizing antibodies (bnAbs). Thus, a greater understanding of Env structural determinants, most critically Env glycosylation, will no doubt be of importance in generating effective immunogens. Recent studies have identified the HIV-1 Env signal peptide (SP) as an important contributor to Env glycosylation. Further investigation into the mechanisms by which the SP directs glycosylation will be important, both in the context of understanding HIV-1 biology and in order to inform HIV-1 vaccine design.

A Recombinant Subunit Based Zika Virus Vaccine Is Efficacious in Non-human Primates

Zika Virus (ZIKV), a virus with no severe clinical symptoms or sequelae previously associated with human infection, became a public health threat following an epidemic in French Polynesia 2013-2014 that resulted in neurological complications associated with infection. Although no treatment currently exists, several vaccines using different platforms are in clinical development. These include nucleic acid vaccines based on the prM-E protein from the virus and purified formalin-inactivated ZIKV vaccines (ZPIV) which are in Phase 1/2 clinical trials. Using a recombinant subunit platform consisting of antigens produced in Drosophila melanogaster S2 cells, we have previously shown seroconversion and protection against viremia in an immunocompetent mouse model. Here we demonstrate the efficacy of our recombinant subunits in a non-human primate (NHP) viremia model. High neutralizing antibody titers were seen in all protected macaques and passive transfer demonstrated that plasma from these NHPs was sufficient to protect against viremia in mice subsequently infected with ZIKV. Taken together our data demonstrate the immunogenicity and protective efficacy of the recombinant subunit vaccine candidate in NHPs as well as highlight the importance of neutralizing antibodies in protection against ZIKV infection and their potential implication as a correlate of protection.

Structure-Function Dissection of Pseudorabies Virus Glycoprotein B Fusion Loops

Conserved across the family Herpesviridae, glycoprotein B (gB) is responsible for driving fusion of the viral envelope with the host cell membrane for entry upon receptor binding and activation by the viral gH/gL complex. Although crystal structures of the gB ectodomains of several herpesviruses have been reported, the membrane fusion mechanism has remained elusive. Here, we report the X-ray structure of the pseudorabies virus (PrV) gB ectodomain, revealing a typical class III postfusion trimer that binds membranes via its fusion loops (FLs) in a cholesterol-dependent manner. Mutagenesis of FL residues allowed us to dissect those interacting with distinct subregions of the lipid bilayer and their roles in membrane interactions. We tested 15 gB variants for the ability to bind to liposomes and further investigated a subset of them in functional assays. We found that PrV gB FL residues Trp187, Tyr192, Phe275, and Tyr276, which were essential for liposome binding and for fusion in cellular and viral contexts, form a continuous hydrophobic patch at the gB trimer surface. Together with results reported for other alphaherpesvirus gBs, our data suggest a model in which Phe275 from the tip of FL2 protrudes deeper into the hydrocarbon core of the lipid bilayer, while the side chains of Trp187, Tyr192, and Tyr276 form a rim that inserts into the more superficial interfacial region of the membrane to catalyze the fusion process. Comparative analysis with gBs from beta- and gamma-herpesviruses suggests that this membrane interaction model is valid for gBs from all herpesviruses.IMPORTANCE Herpesviruses are common human and animal pathogens that infect cells by entering via fusion of viral and cellular membranes. Central to the membrane fusion event is glycoprotein B (gB), which is the most conserved envelope protein across the herpesvirus family. Like other viral fusion proteins, gB anchors itself in the target membrane via two polypeptide segments called fusion loops (FLs). The molecular details of how gB FLs insert into the lipid bilayer have not been described. Here, we provide structural and functional data regarding key FL residues of gB from pseudorabies virus, a porcine herpesvirus of veterinary concern, which allows us to propose, for the first time, a molecular model to understand how the initial interactions by gBs from all herpesviruses with target membranes are established.

An Overview and History of Glyco-Engineering in Insect Expression Systems

Insect systems, including the baculovirus-insect cell and Drosophila S2 cell systems are widely used as recombinant protein production platforms. Historically, however, no insect-based system has been able to produce glycoproteins with human-type glycans, which often influence the clinical efficacy of therapeutic glycoproteins and the overall structures and functions of other recombinant glycoprotein products. In addition, some insect cell systems produce N-glycans with immunogenic epitopes. Over the past 20 years, these problems have been addressed by efforts to glyco-engineer insect-based expression systems. These efforts have focused on introducing the capacity to produce complex-type, terminally sialylated N-glycans and eliminating the capacity to produce immunogenic N-glycans. Various glyco-engineering approaches have included genetically engineering insect cells, baculoviral vectors, and/or insects with heterologous genes encoding the enzymes required to produce various glycosyltransferases, sugars, nucleotide sugars, and nucleotide sugar transporters, as well as an enzyme that can deplete GDP-fucose. In this chapter, we present an overview and history of glyco-engineering in insect expression systems as a prelude to subsequent chapters, which will highlight various methods used for this purpose.

HIV-1 Env gp120 structural determinants for peptide triazole dual receptor site antagonism

Despite advances in HIV therapy, viral resistance and side-effects with current drug regimens require targeting new components of the virus. Dual antagonist peptide triazoles (PT) are a novel class of HIV-1 inhibitors that specifically target the gp120 component of the viral spike and inhibit its interaction with both of its cell surface protein ligands, namely the initial receptor CD4 and the co-receptor (CCR5/CXCR4), thus preventing viral entry. Following an initial survey of 19 gp120 alanine mutants by ELISA, we screened 11 mutants for their importance in binding to, and inhibition by the PT KR21 using surface plasmon resonance. Key mutants were purified and tested for their effects on the peptide's affinity and its ability to inhibit binding of CD4 and the co-receptor surrogate mAb 17b. Effects of the mutations on KR21 viral neutralization were measured by single-round cell infection assays. Two mutations, D474A and T257A, caused large-scale loss of KR21 binding, as well as losses in both CD4/17b and viral inhibition by KR21. A set of other Ala mutants revealed more moderate losses in direct binding affinity and inhibition sensitivity to KR21. The cluster of sensitive residues defines a PT functional epitope. This site is in a conserved region of gp120 that overlaps the CD4 binding site and is distant from the co-receptor/17b binding site, suggesting an allosteric mode of inhibition for the latter. The arrangement and sequence conservation of the residues in the functional epitope explain the breadth of antiviral activity, and improve the potential for rational inhibitor development.

Expression of recombinant Atlantic salmon serum C-type lectin in Drosophila melanogaster Schneider 2 cells

The Atlantic salmon (Salmo salar) serum lectin (SSL) is a soluble C-type lectin that binds bacteria, including salmon pathogens. This lectin is a cysteine-rich oligomeric protein. Consequently, a Drosophila melanogaster expression system was evaluated for use in expressing SSL. A cDNA encoding SSL was cloned into a vector designed to express it as a fusion protein with a hexahistidine tag, under the control of the Drosophila methallothionein promoter. The resulting construct was stably transfected into Drosophila S2 cells. After CdCl2 induction, transfected S2 cells secreted recombinant SSL into the cell culture medium. A cell line derived from stably transformed polyclonal cell populations expressing SSL was used for large-scale expression of SSL. Recombinant SSL was purified from the culture medium using a two-step purification scheme involving affinity binding to yeast cells and metal-affinity chromatography. Although yields of SSL were very low, correct folding and functionality of the recombinant SSL purified in this manner was demonstrated by its ability to bind to Aeromonas salmonicida. Therefore, Drosophila S2 cells may be an ideal system for the production of SSL if yields can be increased.

HIV-1 virus-like particles produced by stably transfected Drosophila S2 cells: a desirable vaccine component

The development of a successful vaccine against human immunodeficiency virus type 1 (HIV-1) likely requires immunogens that elicit both broadly neutralizing antibodies against envelope spikes and T cell responses that recognize multiple viral proteins. HIV-1 virus-like particles (VLP), because they display authentic envelope spikes on the particle surface, may be developed into such immunogens. However, in one way or the other current systems for HIV-1 VLP production have many limitations. To overcome these, in the present study we developed a novel strategy to produce HIV-1 VLP using stably transfected Drosophila S2 cells. We cotransfected S2 cells with plasmids encoding HIV-1 envelope, Gag, and Rev proteins and a selection marker. After stably transfected S2 clones were established, HIV-1 VLP and their immunogenicity in mice were carefully evaluated. Here, we report that HIV-1 envelope proteins are properly cleaved, glycosylated, and incorporated into VLP with Gag. The amount of VLP released into culture supernatants is comparable to those produced by insect cells infected with recombinant baculoviruses. Moreover, cryo-electron microscopy tomography revealed average 17 spikes per purified VLP, and antigenic epitopes on the spikes were recognized by the broadly neutralizing antibodies 2G12, b12, VRC01, and 4E10 but not by PG16. Finally, mice primed with DNA and boosted with VLP in the presence of CpG exhibited anti-envelope antibody responses, including ELISA-binding, neutralizing, antibody-dependent cell-mediated cytotoxicity and antibody-dependent cell-mediated viral inhibition, as well as envelope and Gag-specific CD8 T cell responses. Thus, we conclude that HIV-1 VLP produced by the S2 expression system has many desirable features to be developed into a vaccine component against HIV-1.

Binding of HIV-1 gp120 glycoprotein to silica nanoparticles modified with CD4 glycoprotein and CD4 peptide fragments

An important step in human immunodeficiency virus infection involves the interaction between the viral envelope glycoprotein gp120 and the human host cell surface receptor CD4. Herein, we describe a CD4-functionalized mesoporous silica-based system to selectively capture HIV-gp120 with high binding efficiency. Using a protection-deprotection strategy developed recently by our group, the external surface of the mesoporous particles was selectively functionalized with soluble CD4 ("sCD4") or an 18-peptide fragment mimicking the gp120 binding region. Confocal microscopy confirmed the CD4 locations and showed that the internal pores can be made accessible after external modification in a controlled manner. An evaluation of the ability of an 18-peptide CD4 fragment versus amide-immobilized sCD4 and sCD4 immobilized through its glycosidic group indicated that while all peptides were selective, the latter method was clearly best, with nearly complete removal of whole gp120 from solution. This study shows, for the first time, that sCD4 bound to mesoporous silica particles actively recognizes and retains high binding affinity for HIV-gp120. It is anticipated that, by proper modification of the accessible internal pores, our methodology can be adopted to develop porous platforms for HIV diagnosis, imaging, drug delivery, and vaccine development.

Drosophila melanogaster S2 cells for expression of heterologous genes: From gene cloning to bioprocess development

In the present review we discuss strategies that have been used for heterologous gene expression in Drosophila melanogaster Schneider 2 (S2) cells using plasmid vectors. Since the growth of S2 cells is not dependent on anchorage to solid substrates, these cells can be easily cultured in suspension in large volumes. The factors that most affect the growth and gene expression of S2 cells, namely cell line, cell passage, inoculum concentration, culture medium, temperature, dissolved oxygen concentration, pH, hydrodynamic forces and toxic metabolites, are discussed by comparison with other insect and mammalian cells. Gene expression, cell metabolism, culture medium formulation and parameters involved in cellular respiration are particularly emphasized. The experience of the authors with the successful expression of a biologically functional protein, the rabies virus glycoprotein (RVGP), by recombinant S2 cells is presented in the topics covered.

The development of recombinant subunit envelope-based vaccines to protect against dengue virus induced disease

Challenges associated with the interference observed between the dengue virus components within early tetravalent live-attenuated vaccines led many groups to explore the development of recombinant subunit based vaccines. Initial efforts in the field were hampered by low yields and/or improper folding, but the use of the Drosophila S2 cell expression system provided a mechanism to overcome these limitations. The truncated dengue envelope proteins (DEN-80E) for all four dengue virus types are expressed in the S2 system at high levels and have been shown to maintain native-like conformation. The DEN-80E proteins are potent immunogens when formulated with a variety of adjuvants, inducing high titer virus neutralizing antibody responses and demonstrating protection in both mouse and non-human primate models. Tetravalent vaccine formulations have shown no evidence of immune interference between the four DEN-80E antigens in preclinical models. Based on the promising preclinical data, the recombinant DEN-80E proteins have now advanced into clinical studies. An overview of the relevant preclinical data for these recombinant proteins is presented in this review.

T cell epitope regions of the P. falciparum MSP1-33 critically influence immune responses and in vitro efficacy of MSP1-42 vaccines

The C-terminal 42 kDa fragments of the P. falciparum Merozoite Surface Protein 1, MSP1-42 is a leading malaria vaccine candidate. MSP1-33, the N-terminal processed fragment of MSP1-42, is rich in T cell epitopes and it is hypothesized that they enhance antibody response toward MSP1-19. Here, we gave in vivo evidence that T cell epitope regions of MSP1-33 provide functional help in inducing anti-MSP1-19 antibodies. Eleven truncated MSP1-33 segments were expressed in tandem with MSP1-19, and immunogenicity was evaluated in Swiss Webster mice and New Zealand White rabbits. Analyses of anti-MSP1-19 antibody responses revealed striking differences in these segments' helper function despite that they all possess T cell epitopes. Only a few fragments induced a generalized response (100%) in outbred mice. These were comparable to or surpassed the responses observed with the full length MSP1-42. In rabbits, only a subset of truncated antigens induced potent parasite growth inhibitory antibodies. Notably, two constructs were more efficacious than MSP1-42, with one containing only conserved T cell epitopes. Moreover, another T cell epitope region induced high titers of non-inhibitory antibodies and they interfered with the inhibitory activities of anti-MSP1-42 antibodies. In mice, this region also induced a skewed TH2 cellular response. This is the first demonstration that T cell epitope regions of MSP1-33 positively or negatively influenced antibody responses. Differential recognition of these regions by humans may play critical roles in vaccine induced and/or natural immunity to MSP1-42. This study provides the rational basis to re-engineer more efficacious MSP1-42 vaccines by selective inclusion and exclusion of MSP1-33 specific T cell epitopes.

Development of a recombinant tetravalent dengue virus vaccine: immunogenicity and efficacy studies in mice and monkeys

Truncated recombinant dengue virus envelope protein subunits (80E) are efficiently expressed using the Drosophila Schneider-2 (S2) cell expression system. Binding of conformationally sensitive antibodies as well as X-ray crystal structural studies indicate that the recombinant 80E subunits are properly folded native-like proteins. Combining the 80E subunits from each of the four dengue serotypes with ISCOMATRIX adjuvant, an adjuvant selected from a set of adjuvants tested for maximal and long lasting immune responses, results in high titer virus neutralizing antibody responses. Immunization of mice with a mixture of all four 80E subunits and ISCOMATRIX adjuvant resulted in potent virus neutralizing antibody responses to each of the four serotypes. The responses to the components of the tetravalent mixture were equivalent to the responses to each of the subunits administered individually. In an effort to evaluate the potential protective efficacy of the Drosophila expressed 80E, the dengue serotype 2 (DEN2-80E) subunit was tested in both the mouse and monkey challenge models. In both models protection against viral challenge was achieved with low doses of antigen in the vaccine formulation. In non-human primates, low doses of the tetravalent formulation induced good virus neutralizing antibody titers to all four serotypes and protection against challenge with the two dengue virus serotypes tested. In contrast to previous reports, where subunit vaccine candidates have generally failed to induce potent, protective responses, native-like soluble 80E proteins expressed in the Drosophila S2 cells and administered with appropriate adjuvants are highly immunogenic and capable of eliciting protective responses in both mice and monkeys. These results support the development of a dengue virus tetravalent vaccine based on the four 80E subunits produced in the Drosophila S2 cell expression system.

Efficient method for production of high yields of Fab fragments in Drosophila S2 cells

Fab molecules are used as therapeutic agents, and are invaluable tools in structural biology. We report here a method for production of recombinant Fab in Drosophila S2 cells for use in structural biology. Stably transfected S2 cell lines expressing the Fab were created within weeks. The recombinant Fab was secreted, and after affinity and size exclusion chromatography, 16 mg of pure protein were obtained from a liter of cell culture. The Fab was functional and formed a complex with its cognate antigen as demonstrated by co-precipitation and size exclusion chromatography. Biochemical characterization indicated that the Fab from S2 cells is less extensively glycosylated than the Fab obtained by digestion of antibody produced in hybridoma cells, a feature that may be advantageous for the purposes of crystallogenesis. Taken together, obtaining recombinant Fab from the S2 cells has been a faster and considerably more cost-effective method compared with the enzymatic digestion of the monoclonal antibody.

Expression of the hepatitis B virus surface antigen in Drosophila S2 cells

Drosophila melanogaster S2 cells were transfected with a plasmid vector (pAcHBsAgHy) containing the S gene, coding for the hepatitis B virus surface antigen (HBsAg), under control of the constitutive drosophila actin promoter (pAc), and the hygromycin B (Hy) selection gene. The vector was introduced into Schneider 2 (S2) Drosophila cells by DNA transfection and a cell population (S2AcHBsAgHy) was selected by its resistance to hygromycin B. The pAcHBsAgHy vector integrated in transfected S2 cell genome and approximately 1,000 copies per cell were found in a higher HBsAg producer cell subpopulation. The HBsAg production varied in different subpopulations, but did not when a given subpopulation was cultivated in different culture flasks. Higher HBsAg expression was found in S2AcHBsAgHy cells cultivated in Insect Xpress medium (13.5 mug/1E7 cells) and SFX medium (7 mug/1E7 cells) in comparison to SF900II medium (0.6 mug/1E7 cells). An increase of HBsAg was observed in culture maintained under hygromycin selection pressure. Data presented in the paper show that S2AcHBsAgHy cells produce efficiently the HBsAg which is mainly found in the cell supernatant, suggesting that HBsAg is secreted from the cells. The data also show that our approach using the Drosophila expression system is suitable for the preparation of other viral protein preparation.

Structural determinants for affinity enhancement of a dual antagonist peptide entry inhibitor of human immunodeficiency virus type-1

Structure-activity correlations were investigated for substituted peptide conjugates that function as dual receptor site antagonists of HIV-1 gp120. A series of peptide conjugates were constructed via click reaction of both aryl and alkyl acetylenes with an internally incorporated azidoproline 6 derived from the parent peptide 1 (12p1, RINNIPWSEAMM). Compared to 1, many of these conjugates were found to exhibit several orders of magnitude increase in both affinity for HIV-1 gp120 and inhibition potencies at both the CD4 and coreceptor binding sites of gp120. We sought to determine structural factors in the added triazole grouping responsible for the increased binding affinity and antiviral activity of the dual inhibitor conjugates. We measured peptide conjugate potencies in both kinetic and cell infection assays. High affinity was sterically specific, being exhibited by the cis- but not the trans-triazole. The results demonstrate that aromatic, hydrophobic, and steric features in the residue 6 side-chain are important for increased affinity and inhibition. Optimizing these features provides a basis for developing gp120 dual inhibitors into peptidomimetic and increasingly smaller molecular weight entry antagonist leads.

Structure-activity relationships in the binding of chemically derivatized CD4 to gp120 from human immunodeficiency virus

The first step in HIV infection is the binding of the envelope glycoprotein gp120 to the host cell receptor CD4. An interfacial "Phe43 cavity" in gp120, adjacent to residue Phe43 of gp120-bound CD4, has been suggested as a potential target for therapeutic intervention. We designed a CD4 mutant (D1D2F43C) for site-specific coupling of compounds for screening against the cavity. Altogether, 81 cysteine-reactive compounds were designed, synthesized, and tested. Eight derivatives exceeded the affinity of native D1D2 for gp120. Structure-activity relationships (SAR) for derivatized CD4 binding to gp120 revealed significant plasticity of the Phe43 cavity and a narrow entrance. The primary contacts for compound recognition inside the cavity were found to be van der Waals interactions, whereas hydrophilic interactions were detected in the entrance. This first SAR on ligand binding to an interior cavity of gp120 may provide a starting point for structure-based assembly of small molecules targeting gp120-CD4 interaction.

Improving heterologous protein expression in transfected Drosophila S2 cells as assessed by EGFP expression

Drosophila melanogaster S2 cells were co-transfected with plasmid vectors containing the enhanced green fluorescent protein gene (EGFP), under the control of metallothionein promoter (pMt), and the hygromycin selection gene, in view of establishing parameters for optimized gene expression. A protocol of transfection was worked out, leading after hygromycin selection, to approximately 90% of S2MtEGFP fluorescent cells at day 5 after copper sulfate (CuSO(4)) induction. As analyzed by confocal microscopy, S2MtEGFP cell cultures were shown to be quite heterogeneous regarding the intensity and cell localization of fluorescence among the EGFP expressing cells. Spectrofluorimetry kinetic studies of CuSO(4) induced S2MtEGFP cells showed the EGFP expression at 510 nm as soon as 5 h after induction, the fluorescence increasing progressively from this time to attain values of 4.6 x 10(5) counts/s after 72 h of induction. Induction with 700 muM of CuSO(4) performed at the exponential phase of the S2MtEGFP culture (10(6) cells/mL) led to a better performance in terms of cell growth, percent of fluorescent cells and culture intensity of fluorescence. Sodium butyrate (NaBu) treatment of CuSO(4) induced S2MtEGFP cell cultures, although leading to a loss of cell culture viability, increased the percent of EGFP expressing cells and sharply enhanced the cell culture fluorescence intensity. The present study established parameters for improving heterologous protein expression in stably transfected Drosophila S2 cells, as assessed by the EGFP expression.

Transformation of Drosophila cell lines: an alternative approach to exogenous protein expression

Techniques and experimental applications are described for exogenous protein expression in Drosophila cell lines. Ways in which the Drosophila cell lines and the baculovirus expression vector system differ in their applications are emphasized.

The insect metalloproteinase inhibitor gene of the lepidopteran Galleria mellonella encodes two distinct inhibitors

The insect metalloproteinase inhibitor (IMPI) from the greater wax moth, Galleria mellonella, represents the first and to date only specific inhibitor of microbial metalloproteinases reported from animals. Here, we report on the characterization including carbohydrate analysis of two recombinant constructs encoded by impi cDNA either upstream or downstream of the furin cleavage site identified. rIMPI-1, corresponding to native IMPI purified from hemolymph, is encoded by the N-terminal part of the impi sequence, whereas rIMPI-2 is encoded by its C-terminal part. rIMPI-1 is glycosylated at N48 with GlcNAc2Man3, showing fucosylation to different extents. Similarly, rIMPI-2 is glycosylated at N149 with GlcNAc2Man3, but is fully fucosylated. rIMPI-1 represents a promising template for the design of second-generation antibiotics owing to its specific activity against thermolysin-like metalloproteinases produced by human pathogenic bacteria such as Vibrio vulnificus. In contrast, rIMPI-2 does not inhibit bacterial metalloproteinases, but is moderately active against recombinant human matrix metalloproteinases (MMPs). Both microbial metalloproteinases and MMPs induce expression of the impi gene when injected into G. mellonella larvae. These findings provide evidence that the impi gene encodes two distinct inhibitors, one inhibiting microbial metalloproteinases and contributing to innate immunity, the other putatively mediating regulation of endogenous MMPs during metamorphosis.

Preparation and immunogenic properties of a recombinant West Nile subunit vaccine

While several West Nile vaccines are being developed, none are yet available for humans. In this study aimed at developing a vaccine for humans, West Nile virus (WNV) envelope protein (E) and non-structural protein 1 (NS1) were produced in the Drosophila S2 cell expression system. The C-terminal 20% of the E protein, which contains the membrane anchor portion, was deleted, thus allowing for efficient secretion of the truncated protein (80E) into the cell culture medium. The proteins were purified by immunoaffinity chromatography (IAC) using monoclonal antibodies that were flavivirus envelope protein group specific (for the 80E) or flavivirus NS1 group specific (for NS1). The purified proteins were produced in high yield and used in conjunction with adjuvant formulations to vaccinate mice. The mice were tested for both humoral and cellular immune responses by a plaque reduction neutralization test and ELISA, and by lymphocyte proliferation and cytokine production assays, respectively. The results revealed that the 80E and the NS1 proteins induced both high-titered ELISA and neutralizing antibodies in mice. Splenocytes from immunized mice, cultured in vitro with the vaccine antigens as stimulants, showed excellent proliferation and production of cytokines (IFN-gamma, IL-4, IL-5, and IL-10). The level of antigen-stimulated lymphocyte proliferation and cytokine production was comparable to the level obtained from mitogen (phytohemagglutinin or pokeweed) stimulation, indicating a robust cellular response as well. These findings are encouraging and warrant further in vivo studies to determine the protective efficacy of the WNV vaccine candidate.

Effects of signal peptide exchange on HIV-1 glycoprotein expression and viral infectivity in mammalian cells

In certain cell systems, exchange of the human immunodeficiency virus (HIV) Env signal peptide (SP) sequence with that of heterologous SPs has been shown to increase gp120 transport and secretion. Here we demonstrate that exchange of the HIV-Env-SP with those from erythropoietin or tissue plasminogen activator in the proviral context does not increase wild-type membrane-bound Env expression or incorporation into released virions. In fact, virion infectivity was decreased. These infectivity decreases were largely due to effects on Env transport and/or function and only to a minor extent to cis effects as a result of the sequence exchanges themselves. Thus, in fact, it is not advantageous to employ heterologous SPs to achieve high-level expression of functional cell surface membrane- or virion-associated HIV-Env.

Expression and purification of sea raven type II antifreeze protein from Drosophila melanogaster S2 cells

We present a system for the expression and purification of recombinant sea raven type II antifreeze protein, a cysteine-rich, C-type lectin-like globular protein that has proved to be a difficult target for recombinant expression and purification. The cDNAs encoding the pro- and mature forms of the sea raven protein were cloned into a modified pMT Drosophila expression vector. These constructs produced N-terminally His(6)-tagged pro- and mature forms of the type II antifreeze protein under the control of a metallothionein promoter when transfected into Drosophila melanogaster S2 cells. Upon induction of stable cell lines the two proteins were expressed at high levels and secreted into the medium. The proteins were then purified from the cell medium in a simple and rapid protocol using immobilized metal affinity chromatography and specific protease cleavage by tobacco etch virus protease. The proteins demonstrated antifreeze activity indistinguishable from that of wild-type sea raven antifreeze protein purified from serum as illustrated by ice affinity purification, ice crystal morphology, and their ability to inhibit ice crystal growth. This expression and purification system gave yields of 95 mg/L of fully active mature sea raven type II AFP and 9.6 mg/L of the proprotein. This surpasses all previous attempts to express this protein in Escherichia coli, baculovirus-infected fall armyworm cells and Pichia pastoris and will provide sufficient protein for structural analysis.

Characterization of antibody responses to purified HIV-1 gp120 glycoproteins fused with the molecular adjuvant C3d

The HIV-1 exterior envelope glycoprotein gp120 binds receptor (CD4) and co-receptors (CCR5/CXCR4) and is a major target for neutralizing antibodies. The two functionally conserved regions of gp120 involved in receptor binding are conformational in nature. It is likely that the elicitation of neutralizing antibodies to these targets will benefit by presentation of these sites to the humoral immune system under physiologic conditions. Initially, we investigated the ability of the molecular adjuvant C3d to enhance antibody responses to variant gp120 glycoproteins in phosphate-buffered saline (PBS). We utilized a gp120 variant glycoprotein deleted of N- and C-terminal sequences (gp120DeltaC1/C5) originally designed to eliminate immunodominant, non-neutralizing epitopes and characterized this protein when fused to two C3d elements (gp120DeltaC1/C5(C3d)(2)). In PBS, the gp120DeltaC1/C5(C3d)(2) proteins are able to elicit gp120 binding antibodies more efficiently than gp120 lacking C3d moieties. We then asked if we could observe C3d-enhanced immunogenicity of gp120 in the presence of the classical oil-in-water adjuvant, Ribi. In the presence of the Ribi, which contains the TLR-4 agonist monophospholipid A (MPL), antibodies elicited by the gp120DeltaC1/C5(C3d)(2) were of higher titer than those elicited by the identical protein in PBS. To determine if the elicited secondary response was due to a synergy between the C3d repeats and the Ribi, we then inoculated gp120DeltaC1/C5 protein in Ribi and observed that similar titers of anti-gp120 antibodies were elicited in comparison to the gp120DeltaC1/C5(C3d)(2) protein also inoculated in Ribi adjuvant. In Ribi, there was a small but consistent increase in gp120-specific antibody titer of a gp120DeltaC1/C5(C3d)(2) prime followed by two gp120DeltaC1/C5 boosts compared to three inoculations of either the gp120DeltaC1/C5 proteins or the gp120DeltaC1/C5(C3d)(2) proteins alone. We conclude that the molecular adjuvant C3d demonstrates utility in conditions where physiologic presentation of native protein structures is desired, but may have less benefit in the context of a relatively potent protein adjuvant such as Ribi.

An evaluation of dengue type-2 inactivated, recombinant subunit, and live-attenuated vaccine candidates in the rhesus macaque model

The safety, immunogenicity, and protective efficacy of two non-replicating antigen-based vaccines and one live-attenuated virus (LAV) vaccine for dengue type-2 (dengue-2) virus were evaluated in the rhesus macaque model. The non-replicating vaccines consisted of whole, purified inactivated virus (PIV) and a recombinant subunit protein containing the amino-(N)-terminal 80% of envelope protein (r80E), each formulated with one of five different adjuvants. Each formulation was administered to three animals on a 0, 3-month schedule. Following the primary immunizations, 37 of 39 animals demonstrated dengue-2 virus neutralizing antibodies. After the booster immunizations all animals had dengue neutralizing antibodies with peak titers ranging from 1:100 to 1:9700. The highest neutralizing antibody titers were observed in the groups that received r80E antigen formulated with AS04, AS05, or AS08 adjuvant, and PIV formulated with AS05 or AS08 adjuvant. These newer adjuvants are based on alum, fraction QS-21 of saponin, and monophosphoryl lipid A (MPL). Protection was evaluated by dengue-2 virus challenge 2 months after the booster by the measurement of circulating virus (viremia) and post-challenge immune responses. Several groups exhibited nearly complete protection against viremia by bioassay, although there was evidence for challenge virus replication by Taqmantrade mark and immunological assays. None of the vaccines conferred sterile immunity.

Soluble mimetics of human immunodeficiency virus type 1 viral spikes produced by replacement of the native trimerization domain with a heterologous trimerization motif: characterization and ligand binding analysis

The human immunodeficiency virus type 1 (HIV-1) exterior envelope glycoprotein, gp120, mediates binding to the viral receptors and, along with the transmembrane glycoprotein gp41, is a major target for neutralizing antibodies. We asked whether replacing the gp41 fusion/trimerization domain with a stable trimerization motif might lead to a more stable gp120 trimer that would be amenable to structural and immunologic analysis. To obtain stable gp120 trimers, a heterologous trimerization motif, GCN4, was appended to the C terminus of YU2gp120. Biochemical analysis indicated that the gp120-GCN4 trimers were superior to gp140 molecules in their initial homogeneity, and trilobed structures were observable by electron microscopy. Biophysical analysis of gp120-GCN4 trimers by isothermal titration calorimetry (ITC) and ultracentrifugation analyses indicated that most likely two molecules of soluble CD4 could bind to one gp120-GCN4 trimer. To further examine restricted CD4 stoichiometric binding to the gp120-GCN4 trimers, we generated a low-affinity CD4 binding trimer by introducing a D457V change in the CD4 binding site of each gp120 monomeric subunit. The mutant trimers could definitively bind only one soluble CD4 molecule, as determined by ITC and sedimentation equilibrium centrifugation. These data indicate that there are weak interactions between the gp120 monomeric subunits of the GCN4-stabilized trimers that can be detected by low-affinity ligand sensing. By similar analysis, we also determined that removal of the variable loops V1, V2, and V3 in the context of the gp120-GCN4 proteins allowed the binding of three CD4 molecules per trimer. Interestingly, both the gp120-GCN4 variants displayed a restricted stoichiometry for the CD4-induced antibody 17b of one antibody molecule binding per trimer. This restriction was not evident upon removal of the variable loops V1 and V2 loops, consistent with conformational constraints in the wild-type gp120 trimers and similar to those inherent in the functional Env spike. Thus, the gp120-GCN4 trimers demonstrate several properties that are consistent with some of those anticipated for gp120 in the context of the viral spike.

Factors limiting the immunogenicity of HIV-1 gp120 envelope glycoproteins

Efficient immune responses to HIV-1 gene products are essential elements to the development and design of an effective vaccine. Ideally, both humoral and cellular responses will be optimally elicited. It is therefore important to elucidate any factors that might limit the immunogenicity of HIV-1 proteins that are likely to be included in an effective vaccine. Since the HIV-1 exterior envelope glycoprotein gp120 is a major target for neutralizing antibodies, it is a virtual certainty that this gene product will be a component of any vaccine that seeks to elicit neutralizing antibody responses from the host humoral immune system. We report here the testing of several HIV-1 gp120 variants derived from a primary isolate that appears deficient in eliciting immune responses at both the level of CD4+ help and consequently in the generation of high-affinity IgG antibody responses in small animals. Factors limiting an effective immune response include (a) envelope glycoprotein strain variation decreasing functional T-cell help, (b) alteration of the glycosylation patterns of gp120 by expression in different cell types, and (c) the native structure of gp120 itself, which may limit the elicitation of effective T-cell help during natural infection or during parenteral immunization in adjuvant. Such limiting factors and others should be considered in the design and testing of gp120-based immunogens in small animals and possibly in primates as well.

Enzyme-linked immunosorbent assays using recombinant envelope protein expressed in COS-1 and Drosophila S2 cells for detection of West Nile virus immunoglobulin M in serum or cerebrospinal fluid

Humans infected with West Nile virus (WNV) develop immunoglobulin M (IgM) antibodies soon after infection. The microtiter-based assays for WNV IgM antibody detection used by most state public health and reference laboratories utilize WNV antigen isolated from infected Vero cells or recombinant envelope protein produced in COS-1 cells. Recombinant antigen produced in COS-1 cells was used to develop a WNV IgM capture enzyme immunoassay (EIA). A supplementary EIA using WNV envelope protein expressed in Drosophila melanogaster S2 cells was also developed. Both assays detected WNV IgM in the sera of experimentally infected rhesus monkeys within approximately 10 days postinfection. Human sera previously tested for WNV IgM at a state public health laboratory (SPHL) were evaluated using both EIAs. Among the sera from 20 individuals with laboratory-confirmed WNV infection (i.e., IgM-positive cerebrospinal fluid [CSF]) that were categorized as equivocal for WNV IgM at the SPHL, 19 were IgM positive and one was negative by the new EIAs. Of the 19 IgM-positive patients, 15 were diagnosed with meningitis or encephalitis; the IgM-negative patient was not diagnosed with neurological disease. There was 100% agreement between the EIAs for the detection of WNV IgM. CSF samples from 21 individuals tested equivocal for WNV IgM at the SPHL; all 21 were positive in both bead assays, and 16 of these patients were diagnosed with neurological disease. These findings demonstrate that the new EIAs accurately identify WNV infection in individuals with confirmed WNV encephalitis and that they exhibit enhanced sensitivity over that of the microtiter assay format.

Characterization of the outer domain of the gp120 glycoprotein from human immunodeficiency virus type 1

The core of the gp120 glycoprotein from human immunodeficiency virus type 1 (HIV-1) is comprised of three major structural domains: the outer domain, the inner domain, and the bridging sheet. The outer domain is exposed on the HIV-1 envelope glycoprotein trimer and contains binding surfaces for neutralizing antibodies such as 2G12, immunoglobulin G1b12, and anti-V3 antibodies. We expressed the outer domain of HIV-1(YU2) gp120 as an independent protein, termed OD1. OD1 efficiently bound 2G12 and a large number of anti-V3 antibodies, indicating its structural integrity. Immunochemical studies with OD1 indicated that antibody responses against the outer domain of the HIV-1 gp120 envelope glycoprotein are rare in HIV-1-infected human sera that potently neutralize the virus. Surprisingly, such outer-domain-directed antibody responses are commonly elicited by immunization with recombinant monomeric gp120. Immunization with soluble, stabilized HIV-1 envelope glycoprotein trimers elicited antibody responses that more closely resembled those in the sera of HIV-1-infected individuals. These results underscore the qualitatively different humoral immune responses elicited during natural infection and after gp120 vaccination and help to explain the failure of gp120 as an effective vaccine.

Expression analysis of a modified factor X in stably transformed insect cell lines

A modified Factor X protein was combined with a cellulose-binding domain tag and expressed in insect cell lines. The protein, CBDFX, was expressed and secreted into the medium. Stable, transformed Hi5 and Sf9 insect cell lines were generated and tested for production of secreted CBDFX. The highest Sf9 and Hi5 CBDFX-producing cell lines were scaled up to 2-liter fermentors to evaluate production of this recombinant protein. Secreted protein production levels reached 4 mg/liter for the stable, transformed Hi5 cell line and 18 mg/liter for the stable, transformed Sf9 cell line. The protein was properly processed as determined by amino terminal sequencing and bound well to the cellulose substrate Avicel. In addition the activated recombinant CBDFX(a) was capable of recognizing and efficiently processing a Factor X cleavage site.

From receptor recognition mechanisms to bioinspired mimetic antagonists in HIV-1/cell docking

Understanding the ways in which two or more proteins interact may give insight into underlying binding and activation mechanisms in biology, methods for protein separation and structure-based antagonism. This review describes ways in which protein recognition has been explored in our laboratory for the HIV-1/cell entry process. Initial contact between an HIV-1 virion particle and a human cell occurs between gp120 (an HIV-1 envelope protein) and CD4 (a human extracellular signaling protein). This interaction leads to a sequence of events which includes a conformational change in gp120, fusion of the HIV-1 and cellular membranes and eventual infection of the cell. Using an optical biosensor and a reporter antibody, we have been able to measure the conformational change in gp120 that occurs upon CD4 binding. We also have used this biosensor system to characterize CD4 mimetics, obtained by peptide synthesis in miniprotein scaffolds. Phage display techniques have been employed to identify novel miniprotein sequences. The combination of biosensor interaction kinetics analysis and phage display provides a useful approach for understanding the recognition mechanisms involved in the HIV/cell docking process. This approach may also be useful in investigating other protein complexes of importance in health and disease.

Energetics of the HIV gp120-CD4 binding reaction

HIV infection is initiated by the selective interaction between the cellular receptor CD4 and gp120, the external envelope glycoprotein of the virus. We used analytical ultracentrifugation, titration calorimetry, and surface plasmon resonance biosensor analysis to characterize the assembly state, thermodynamics, and kinetics of the CD4-gp120 interaction. The binding thermodynamics were of unexpected magnitude; changes in enthalpy, entropy, and heat capacity greatly exceeded those described for typical protein-protein interactions. These unusual thermodynamic properties were observed with both intact gp120 and a deglycosylated and truncated form of gp120 protein that lacked hypervariable loops V1, V2, and V3 and segments of its N and C termini. Together with previous crystallographic studies, the large changes in heat capacity and entropy reveal that extensive structural rearrangements occur within the core of gp120 upon CD4 binding. CD spectral studies and slow kinetics of binding support this conclusion. These results indicate considerable conformational flexibility within gp120, which may relate to viral mechanisms for triggering infection and disguising conserved receptor-binding sites from the immune system.

HIV-1 gp120 and chemokines activate ion channels in primary macrophages through CCR5 and CXCR4 stimulation

HIV type 1 (HIV-1) uses the chemokine receptors CCR5 and CXCR4 as coreceptors for entry into target cells. Here we show that the HIV-1 envelope gp120 (Env) activates multiple ionic signaling responses in primary human macrophages, which are important targets for HIV-1 in vivo. Env from both CCR5-dependent JRFL (R5) and CXCR4-dependent IIIB (X4) HIV-1 opened calcium-activated potassium (K(Ca)), chloride, and calcium-permeant nonselective cation channels in macrophages. These signals were mediated by CCR5 and CXCR4 because macrophages lacking CCR5 failed to respond to JRFL and an inhibitor of CXCR4 blocked ion current activation by IIIB. MIP-1beta and SDF-1alpha, chemokine ligands for CCR5 and CXCR4, respectively, also activated K(Ca) and Cl(-) currents in macrophages, but nonselective cation channel activation was unique to gp120. Intracellular Ca(2+) levels were also elevated by gp120. The patterns of activation mediated by CCR5 and CXCR4 were qualitatively similar but quantitatively distinct, as R5 Env activated the K(Ca) current more frequently, elicited Cl(-) currents that were approximately 2-fold greater in amplitude, and elevated intracellular Ca(+2) to higher peak and steady-state levels. Env from R5 and X4 primary isolates evoked similar current responses as the corresponding prototype strains. Thus, the interaction of HIV-1 gp120 with CCR5 or CXCR4 evokes complex and distinct signaling responses in primary macrophages, and gp120-evoked signals differ from those activated by the coreceptors' chemokine ligands. Intracellular signaling responses of macrophages to HIV-1 may modulate postentry steps of infection and cell functions apart from infection.

Protein expression in Drosophila Schneider cells

We expressed recombinant secreted, membrane, and cytosolic proteins in stably transfected Drosophila Schneider (SL-3) cells. To allow easy cloning of N- and C-terminal fusion proteins containing epitope- and His-tags for the detection of recombinant proteins and purification by affinity chromatography we constructed new expression vectors. To exemplify the general applicability of protein expression in Schneider cells we characterized the expression system with respect to inducibility, localization of the recombinant proteins, yields of purified proteins, and presence of posttranslational and cotranslational modifications. Secreted proteins became quantitatively N-glycosylated in SL-3 cells and the N-glycan of a Golgi-resident membrane protein was found to be Endo-H-resistant. Myristoylation of AnxXIIIb, a member of the annexin family, could be demonstrated and glycosylphosphatidylinositol-anchored proteins containing their lipid anchor were expressed efficiently in SL-3 cells. Since generation of stable cell lines and mass culture of SL-3 cells is cheap and easy, they provide an attractive eukaryotic expression system.

Effects of soluble CD4 on simian immunodeficiency virus infection of CD4-positive and CD4-negative cells

A soluble form of the CD4 receptor (sCD4) can either enhance or inhibit the infection of cells by simian immunodeficiency virus (SIV) and human immunodeficiency virus. We investigated the basis for these varying effects by studying the entry of three SIV isolates into CD4-positive and CD4-negative cells expressing different chemokine receptors. Infection of CD4-negative cells depended upon the viral envelope glycoproteins and upon the chemokine receptor, with CCR5 and gpr15 being more efficient than STRL33. Likewise, enhancement of infection by sCD4 was observed when CCR5- and gpr15-expressing target cells were used but not when those expressing STRL33 were used. The sCD4-mediated enhancement of virus infection of CD4-negative, CCR5-positive cells was related to the sCD4-induced increase in binding of the viral gp120 envelope glycoprotein to CCR5. Inhibitory effects of sCD4 could largely be explained by competition for virus attachment to cellular CD4 rather than other detrimental effects on virus infectivity (e.g., disruption of the envelope glycoprotein spike). Consistent with this, the sCD4-activated SIV envelope glycoprotein intermediate on the virus was long-lived. Thus, the net effect of sCD4 on SIV infectivity appears to depend upon the degree of enhancement of chemokine receptor binding and upon the efficiency of competition for cellular CD4.

Construction of modular and versatile plasmid vectors for the high-level expression of single or multiple genes in insects and insect cell lines

We have constructed a series of plasmid vectors for the expression of foreign genes in insects or insect cell lines. We incorporated the Drosophila hsp70 and actin 5C promoters, as well as the hr5 enhancer-driven baculovirus ie1 promoter, into plasmids that allow convenient cloning of heterologous genes into multiple cloning sites. We combined these promoters with either a short, double poly-adenylation site derived from the Heliothis virescens p63 chaperonin gene, or with a fusion of the small t intron with the early 3' untranslated region and poly-adenylation sites of SV40. Unique eight base cutter restriction sites flanking the promoters and poly-adenylation sequences make it possible to transfer the entire transcription units into other sequence contexts, for example, into transposable elements or into other plasmids bearing selectable marker genes. It is also convenient to combine two of our transcription units on the same plasmid in order to express multiple genes simultaneously. To test the ability of our vectors to drive expression of reporter genes, luciferase derivatives were made of the expression plasmids and introduced into Aedes albopictus C6/36 cells by electroporation or into Anopheles gambiae embryos by biolistic particle bombardment. All three promoters directed high levels of luciferase expression. However, there were differences in their relative activities in the two experimental systems. In C6/36 cells, the actin 5C and hr5-ie1 promoters were significantly more active than the hsp70 promoter. In Anopheles embryos, hsp70 and actin 5C had maximal activities, while hr5-ie1 was weaker. We also found that the constructs containing the SV40 small t intron and early 3' untranslated region sequences had higher expression levels than their counterparts containing the Heliothis poly-adenylation sequence. Our most active construct combines the actin 5C promoter with the SV40 intron and 3' untranslated region sequences. This vector was also used to drive expression of a visible marker, the enhanced green fluorescent protein gene, resulting in readily visible green fluorescent protein expression in C6/36 cells.

The envelope glycoprotein ectodomains determine the efficiency of CD4+ T lymphocyte depletion in simian-human immunodeficiency virus-infected macaques

CD4+ T lymphocyte depletion in human immunodeficiency virus type 1 (HIV-1)-infected humans underlies the development of acquired immune deficiency syndrome. Using a model in which rhesus macaques were infected with chimeric simian-human immunodeficiency viruses (SHIVs), we show that both the level of viremia and the structure of the HIV-1 envelope glycoprotein ectodomains individually contributed to the efficiency with which CD4(+) T lymphocytes were depleted. The envelope glycoproteins of recombinant SHIVs that efficiently caused loss of CD4(+) T lymphocytes exhibited increased chemokine receptor binding and membrane-fusing capacity compared with those of less pathogenic viruses. These studies identify the HIV-1 envelope glycoprotein ectodomains as determinants of CD4(+) T lymphocyte loss in vivo and provide a foundation for studying pathogenic mechanisms.

A series of broad host range shuttle vectors for constitutive and inducible expression of heterologous proteins in insect cell lines

A series of shuttle vectors have been constructed that allow expression of heterologous proteins in either dipteran or lepidopteran insect cell lines. Constitutive expression in a broad range of host cells is mediated by the Orgyia pseudotsugata multicapsid nucleopolyhedrosis virus (OpMNPV) immediate-early 2 (ie2) promoter. Alternatively, if inducible expression is required, for example to express cytotoxic proteins, a vector has been constructed that uses the Drosophila metallothionein (Mtn) promoter for metal-inducible protein expression in dipteran cell lines. A chimeric synthetic bacterial-OpMNPV ie promoter-Zeocin resistance gene cassette has been included to facilitate cloning in E. coli as well as the generation of stably transformed insect cell lines. The utility of the system is demonstrated by the constitutive and inducible expression of the highly processed glycosylphosphatidylinositol-anchored glycoprotein, human melanotransferrin, in transformed insect cell lines.

Structural characterization of the N-linked oligosaccharides derived from HIVgp120 expressed in lepidopteran cells

The oligosaccharides of recombinant HIV gp120 expressed in lepidopteran Sf9 cells were analysed after hydrazine release by gel permeation and high pH anion exchange chromatography. N-Linked glycans were exclusively of the oligomannose series and no evidence for charged complex or hybrid type glycans was found. However a glycosylation reaction similar to those found in vertebrates was evident. The major glycoform of gp120, that comprised 30% of all the species analysed, was structurally identified by exoglycosidase digestion and found to be a core fucosylated structure, Manalpha1,6(Manalpha1,3)Manbeta1,4GlcNAc(Fucalpha1+ ++,6)GlcNAc. Further confirmation of the ability of lepidopteran cells to fucosylate N-linked glycans was provided by an in vitro analysis of this reaction using authentic oligosaccharide substrates.

Investigation of human cyclooxygenase-2 glycosylation heterogeneity and protein expression in insect and mammalian cell expression systems

Human cyclooxygenase-2 (hCox-2) is a key enzyme in the biosynthesis of prostaglandins and the target of nonsteroidal anti-inflammatory drugs. Recombinant hCox-2 overexpressed in a vaccinia virus (VV)-COS-7 system comprises two glycoforms. Removal of the N-glycosylation consensus sequence at Asn580 (N580Q and S582A mutants) resulted in the expression of protein comprising a single glycoform, consistent with the partial N-glycosylation at this site in the wild-type (WT) enzyme. The specific cyclooxygenase activities of the purified WT and N580Q mutant were equivalent (40 +/- 3 mumol O2/min/mg) and titrations with diclofenac showed no difference in inhibitor sensitivities of WT and both mutants. Results of the expression of WT and N580Q hCox-2 in a Drosophila S2 cell system were also consistent with the N-glycosylation at this site, but low levels of activity were obtained. High levels of N-glycosylation heterogeneity are observed in hCox-2 expressed using recombinant baculovirus (BV) in Sf9 cells. Expression of a double N-glycosylation site mutant in Sf9 cells, N580Q/N592Q, resulted in a decrease in glycosylation but no clear decrease in heterogeneity, indicating that the high degree of N-glycosylation heterogeneity observed with the BV-Sf9 system is not due to partial glycosylation of both Asn580 and Asn592. N-linked oligosaccharide profiling of purified VV and BV WT and S582A mutant hCox-2 showed the presence of high mannose structures, (Man)n (GlcNAc)2, n = 9, 8, 7, 6. The S582A mutant was the most homogeneous with (Man)9(GlcNAc)2 comprising greater than 50% of oligosaccharides present. Analysis of purified VV WT and S582A mutant hCox-2 by liquid chromatography-electrospray ionization-mass spectrometry showed an envelope of peaks separated by approximately 160 Da, corresponding to differences of a single monosaccharide. The difference between the highest mass peaks of the two envelopes, of approximately 1500 Da, is consistent with the wild-type enzyme containing an additional high mannose oligosaccharide.

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.

Expression of human dopamine beta-hydroxylase in Drosophila Schneider 2 cells

Human dopamine beta-hydroxylase (DBH) has been expressed in transformed Drosophila Schneider 2 (S2) cells with yields of > 16 mg/l. Most of the activity was found in the culture fluid. Similarly, human neuroblastoma cells also secrete native DBH into the medium, but at a much lower level than recombinant Drosophila cells. We have purified native and recombinant human DBH by a modified purification procedure using SP-Sepharose, lentil lectin-Sepharose and gel-filtration chromatography and carried out studies to compare the two enzymes. Two variants of human DBH that differ by a single amino acid (either serine or alanine) at position 304 were expressed in Drosophila cells, purified, and found to have no significant difference in enzyme activity. The molecular mass of human DBH monomer has been determined from SDS/PAGE to be 73 kDa, but the recombinant DBH from Drosophila is smaller at 66 kDa. The difference may be due to glycosylation as deglycosylated enzymes from both sources are identical in size (61 kDa). The Km of tyramine for native and recombinant human enzymes are virtually the same but higher than bovine DBH by about 3-fold. Likewise, the inhibition of native and recombinant human DBH by fusaric acid and SKF102698 is not significantly different but IC50 values are 2-3-fold higher than that for the bovine enzyme. These results strongly support the conclusion that recombinant human DBH from Drosophila S2 cells can be used in place of human neuroblastoma-derived DBH for drug screening, characterization of the enzyme's physicochemical properties, and determination of structure-function relationships. The Drosophila expression system has thus provided a convenient source for large quantities of human DBH enzyme.

Sera from HIV-1 infected individuals in all stages of disease preferentially recognize the V3 loop of the prototypic macrophage-tropic glycoprotein gp120 ADA

The outer membrane glycoprotein gp120 and the transmembrane glycoprotein gp41 are predominant targets of the humoral immune response to infection by human immunodeficiency virus type 1. The third hypervariable region (V3 loop) is the principal neutralizing domain and is the primary target of neutralizing antibodies directed against the envelope proteins of HIV-1. The V3 loop is also the major determinant for HIV-1 cell-specific tropism. To further characterize the humoral immune response directed against the gp120 envelope proteins, we expressed two prototypic gp120 envelope proteins (LAI/HXB2 and ADA) and chimeric gp120 envelope proteins in stable transfected Drosophila melanogaster Schneider 2 cells. Sera from four infected adults over the course of infection [McNearney et al. (1992) Proc. natn. Acad. Sci. U.S.A. 89, p. 10,242] were assayed for reactivity with the respective envelope proteins. Sera obtained at early stages preferentially recognized the gp120 envelope protein ADA, whereas in later stages of infection the sera showed diminished reactivity with both gp120 LAI/HXB2 and gp120 ADA. Chimeric envelope proteins revealed that the humoral response was directed primarily against the V3 loop of gp120 ADA. Furthermore, 22 sera from HIV-1 infected individuals in different stages of the disease were tested. Reactivity of sera with the gp120 envelope protein ADA was seven-fold higher than with the gp120 envelope protein LAI/HXB2. Our results suggest that the humoral immune response is preferentially elicited against the V3 loop of the prototypic macrophage-tropic gp120 envelope protein ADA.

Characterization of human immunodeficiency virus type 1 gp120 binding to liposomes containing galactosylceramide

Human immunodeficiency virus type 1 (HIV-1) infects some cell types which lack CD4, demonstrating that one or more alternative viral receptors exist. One such receptor is galactosylceramide (GalCer), a glycosphingolipid distributed widely in the nervous system and in colonic epithelial cells. Using a liposome flotation assay, we found that the HIV-1 surface glycoprotein, gp120, quantitatively bound to liposomes containing GalCer but not to liposomes containing phospholipids and cholesterol alone. Binding was saturable and was inhibited by preincubating liposomes with anti-GalCer antibodies. We observed less efficient binding of gp120 to liposomes containing lactosylceramide, glucosylceramide, and galactosylsulfate, whereas no binding to liposomes containing mixed gangliosides, psychosine, or sphingomyelin was detected. Binding to GalCer was rapid, largely independent of temperature and pH, and stable to conditions which remove most peripheral membrane proteins. By contrast, gp120 bound to lactosylceramide could be removed by 2 M potassium chloride or 3 M potassium thiocyanate, demonstrating a less stable interaction. Removal of N-linked oligosaccharides on gp120 did not affect binding efficiency. However, as previously observed for CD4 binding, heat denaturation of gp120 prevented binding to GalCer. Finally, binding was critically dependent on the concentration of GalCer in the target membrane, suggesting that binding to glycolipid-rich domains occurs and that GalCer conformation may be important for gp120 recognition.

HIV-1 envelope-elicited neutralizing antibody titres correlate with protection and virus load in chimpanzees

In an attempt to compare the protective effect of vaccination with two forms of envelope antigens, and to define immunological correlates of protection against HIV infection, chimpanzees were vaccinated with either recombinant gp160 or gp120. Homologous HIV challenge was performed 3 weeks after the fourth immunization. The animal with the highest level of serum neutralizing antibodies (gp160 immunogen) was protected against HIV infection. All other chimpanzees became infected, but displayed various levels of infected PBMCs. The postchallenge data gave rise to the following conclusions: (1) protection correlated with the level of the serological immune response, but not with the nature of immunogen (gp120 versus gp160); (2) the virus-neutralizing titre at day of challenge correlated with protection from infection; (3) the relative magnitude of the lymphoproliferative T-cell response at day of challenge did not correlate with any protective effect; (4) the peak numbers of virus-infected PBMCs in vaccinated animals were lower than those observed in control animals, and this effect was correlated with the intensity of the antibody response at day of challenge. This raises the possibility that a beneficial effect of HIV vaccination may be achieved in a situation where sterile immunity is not consistently obtained.

A cis-acting repressive sequence that overlaps the Rev-responsive element of human immunodeficiency virus type 1 regulates nuclear retention of env mRNAs independently of known splice signals

The Rev protein of human immunodeficiency virus type 1 (HIV-1) binds to an RNA structure, the Rev-responsive element (RRE), to enhance expression of the viral structural genes by relieving the nuclear sequestration of incompletely spliced viral transcripts. It has been suggested that nuclear retention of these mRNAs, in mammalian cells, is due to the activity of either cis-acting repressive sequence elements or to inefficient splicing signals. Expression of the HIV-1 envelope gene in transfected Drosophila cells is also dependent upon Rev coexpression and, hence, the mechanism of nuclear retention and Rev regulation are highly conserved. Here we use the Drosophila system to identify a major cis-acting repressive sequence element that overlaps the RRE and is responsible for the nuclear entrapment and Rev-dependent expression of HIV-1 env mRNAs. Moreover, the splice signals spanning env are not required for nuclear retention or Rev-dependent trans-activation of env mRNAs. We suggest that the RRE and its associated RNA structure are necessary for both the repressive and known trans-activation effects of Rev regulation.

Multiepitope polypeptide of the HIV-1 envelope induces neutralizing monoclonal antibodies against V3 loop

A gene encoding a multiepitope polypeptide (MEP) has been synthesized. It contains the information for (1) an 11-amino acid (aa) epitope from the C1 region of gp120 of HIV-1 and (2) 3 epitopes of 15 amino acids each, from the central part of the V3 loop of isolates MN, SC, and WMJII. These four segments are linked by the short spacer peptide AGGGA. This gene was cloned in a plasmid vector and expressed in Escherichia coli as a fusion product with a 62-aa fragment of human IL-2. The recombinant protein TAB1 was purified by washed pellet procedures and reversed-phase HPLC. TAB1 was recognized in ELISAs by 25 of 27 sera from seropositive individuals. Mice were immunized and several hybridomas were obtained. Two of them secrete monoclonal antibodies that react with synthetic peptides from isolates MN, WMJI, WMJIII, and SC with an affinity constant in the range of 10(8) M-1. They also recognized peptides from isolates SF2 and WMJII, but at much lower affinity. The results obtained from peptide ELISAs indicate that the putative epitope recognized by these MAbs lies within the sequence IHIGPGRAFYT. Classic neutralization assays demonstrated that MAb 2C4 neutralizes 50% of the MN isolate at 0.6 micrograms/ml but fails to neutralize IIIB and SF2 strains. The presence of antibodies directed against every one of the component peptides in the sera of rabbits immunized with TAB1 was also documented.

Use of the recombinant chimera proteins, LacZ-Env and Gag-Env, for immunological studies on HIV-1 infection

To use Env proteins as antigens for detection of the human immunodeficiency virus type-1 (HIV-1) antibodies, we attempted to overexpress the Env proteins in Escherichia coli. To study the epitopes in the Env proteins recognized by the sera of HIV carriers, various regions of the proviral DNA encoding the Env region were fused to the 3' end of the lacZ gene. The immunoblotting analysis of the LacZ-Env(512-611) and LacZ-Env(721-826) proteins with the 41 positive sera revealed that the former and the latter immunologically reacted with 100 and 78% of the sera, respectively. To avoid rare false-positive reactions due to the LacZ moiety of the fusion protein, we attempted to express the Env(512-611) alone or Gag-Env(512-611) under the control of bacteriophage T7 promoter. Although we could express only a low level of the Env(512-611) peptide in E. coli, we succeeded in producing large amounts of the Gag(121-406)-Env(512-611) and Gag(308-406)-Env(512-611) proteins as chimeric proteins. Both of these chimera proteins strongly reacted with the 41 positive sera. We purified these proteins and analyzed the immunological reactivity by dot blot with the 60 positive sera and the 84 normal sera. As little as 20 ng of the dotted proteins was enough for the reaction with the positive sera, whereas as much as 320 ng of them did not show false-positive reactions with the normal sera. We obtained highly purified Gag-Env proteins with highly specific seroreactivity, which should be useful for diagnosis and prognosis.

A human lymphoid recombinant cell line with functional human immunodeficiency virus type 1 envelope

Our goal has been to develop a safe and effective system that would allow us to explore the functions of the human immunodeficiency virus (HIV) envelope. We have generated a human lymphoid cell line (TF228.1.16) that stably expresses functional HIV envelope proteins on its cell surface, and therefore closely mimics the viral envelope and virus-infected cells. The TF228.1.16 line forms syncytia with human cells of the CD4+ phenotype and provides a facile virus-free cell-based assay for examining the mechanism of syncytia formation and for evaluating novel agents that may disrupt this process. The TF228.1.16 cells also provide an opportunity to present the HIV envelope proteins to the immune system in cellular form. In vitro immunization of human peripheral blood mononuclear cells (PBMC) and in vivo immunization of rhesus monkeys with this reagent results in the production of antibodies with neutralizing (anti-syncytia) activities. When the HIV envelope is expressed against the background of human lymphoid cells, it may exhibit immune protection with unique properties that have not yet been explored. Our results indicate that a virus-free cell system can play an important role in exploring the biology and function of HIV-envelope proteins without the interference of other viral components present in infected cells. This paper discusses these results, and examines the potential use of TF228.1.16 as a vaccine.