Expression of recombinant capsid proteins of chitta virus, a genogroup II Norwalk virus, and development of an ELISA to detect the viral antigen

The second open reading frame (ORF2) gene of the Chitta virus (CHV) was cloned to construct a recombinant baculovirus. The CHV ORF2 is predicted to encode a capsid protein of 535 amino acids (aa). CHV showed a high aa identity in the capsid region with genogroup II Norwalk virus (NV) (65-85%), but a low aa identity with genogroup I NV (44-46%). Phylogenetic analysis of the ORF2 gene demonstrated that CHV is genetically closely related to the Hawaii virus included in genogroup II NV. The recombinant capsid protein of CHV (rCHV) self-assembled to form empty virus-like particles (VLPs) when expressed in insect cells with the recombinant baculovirus. An enzyme-linked immunosorbent assay (ELISA) based on antisera to rCHV was developed to detect CHV antigen in stools. The antigen ELISA appeared to be highly specific to both rCHV and CHV-like strains. In addition, combined use of antigen ELISAs using antibodies against two antigenically distinct recombinant VLPs, the recombinant Chiba virus (rCV) and recombinant Seto virus (rSEV), enabled us to determine the genetic as well as antigenic relationship among these three viruses.     

Biochemical characterization of a smaller form of recombinant Norwalk virus capsids assembled in insect cells.

The expression of the single capsid protein of Norwalk virus (NV) in Spodoptera frugiperda (Sf9) insect cells infected with recombinant baculovirus results in the assembly of virus-like particles (VLPs) of two sizes, the predominant 38-nm, or virion-size VLPs, and smaller, 23-nm VLPs. Here we describe the purification and biochemical characterization of the 23-nm VLPs. The 23-nm VLPs were purified to 95% homogeneity from the medium of Sf9 cultures by isopycnic CsCl gradient centrifugation followed by rate-zonal centrifugation in sucrose gradients. The compositions of the purified 23- and 38-nm VLPs were compared by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and protein immunoblots. VLPs of both sizes showed a doublet at 58 kDa, the size of the full-length capsid protein. Upon alkaline treatment, the 23-nm VLPs underwent dissociation into soluble intermediates that were able to reassemble into 23- and 38-nm VLPs upon dialysis, suggesting that the assembly of both types of structures has a common pathway. Antigenic and biochemical properties of the 38- and 23-nm VLPs were examined and found to be conserved. Immunoprecipitation assays using polyclonal and monoclonal antibodies indicated that immunodominant epitopes on the capsid protein as well as conformational epitopes are conserved in the two types of particles. The trypsin cleavage site at residue 227 was protected in the assembled particles of both sizes but exposed after alkaline dissociation. These results, and the conservation of the binding activity of both forms of recombinant NV VLPs to cultured cells (L. J. White, J. M. Ball, M. E. Hardy, T. N. Tanaka, N. Kitamoto, and M. K. Estes, J. Virol. 70:6589-6597, 1996), suggest that the tertiary folding of the capsid protein responsible for these properties is conserved in the two structures. We hypothesize that the 23-nm VLPs are formed when 60 units of the NV capsid protein assembles into a structure with T=1 symmetry.     

The 3' end of Norwalk virus mRNA contains determinants that regulate the expression and stability of the viral capsid protein VP1: a novel function for the VP2 protein

Norwalk virus (NV) is the prototype strain of a group of noncultivable human caliciviruses responsible for epidemic outbreaks of acute gastroenteritis. The capsid protein VP1 is synthesized from a subgenomic RNA that contains two open reading frames (ORFs), ORF2 and ORF3, and the 3' untranslated region (UTR). ORF2 and ORF3 code for the capsid protein (VP1) and a small structural basic protein (VP2), respectively. We discovered that the yields of virus-like particles (VLPs) composed of VP1 are significantly reduced when this protein is expressed from ORF2 alone. To determine how the 3' terminus of the NV subgenomic RNA regulates VP1 expression, we compared VP1 expression levels by using recombinant baculovirus constructs containing different 3' elements. High VP1 levels were detected by using a recombinant baculovirus that contained ORF2, ORF3, and the 3'UTR (ORF2+3+3'UTR). In contrast, expression of VP1 from constructs that lacked the 3'UTR (ORF2+3), ORF3 (ORF2+3'UTR), or both (ORF2 alone) was highly reduced. Elimination of VP2 synthesis from the subgenomic RNA by mutation resulted in VP1 levels similar to those obtained with the ORF2 construct alone, suggesting a cis role for VP2 in upregulation of VP1 expression levels. Comparisons of the kinetics of RNA and capsid protein expression levels by using constructs with or without ORF3 or the 3'UTR revealed that the 3'UTR increased the levels of VP1 RNA, whereas the presence of VP2 resulted in increased levels of VP1. Furthermore, VP2 increased VP1 stability and protected VP1 from disassembly and protease degradation. The increase in VP1 expression levels caused by the presence of VP2 in cis was also observed in mammalian cells.     

Suitability and perspectives on using recombinant insect cells for the production of virus-like particles

Virus-like particles (VLPs) can be produced in recombinant protein production systems by expressing viral surface proteins that spontaneously assemble into particulate structures similar to authentic viral or subviral particles. VLPs serve as excellent platforms for the development of safe and effective vaccines and diagnostic antigens. Among various recombinant protein production systems, the baculovirus–insect cell system has been used extensively for the production of a wide variety of VLPs. This system is already employed for the manufacture of a licensed human papillomavirus-like particle vaccine. However, the baculovirus–insect cell system has several inherent limitations including contamination of VLPs with progeny baculovirus particles. Stably transformed insect cells have emerged as attractive alternatives to the baculovirus–insect cell system. Different types of VLPs, with or without an envelope and composed of either single or multiple structural proteins, have been produced in stably transformed insect cells. VLPs produced by stably transformed insect cells have successfully elicited immune responses in vivo. In some cases, the yield of VLPs attained with recombinant insect cells was comparable to, or higher than, that obtained by baculovirus-infected insect cells. Recombinant insect cells offer a promising approach to the development and production of VLPs.     

Essential elements of the capsid protein for self-assembly into empty virus-like particles of hepatitis E virus

Hepatitis E virus (HEV) is a noncultivable virus that causes acute liver failure in humans. The virus's major capsid protein is encoded by an open reading frame 2 (ORF2) gene. When the recombinant protein consisting of amino acid (aa) residues 112 to 660 of ORF2 is expressed with a recombinant baculovirus, the protein self-assembles into virus-like particles (VLPs) (T.-C. Li, Y. Yamakawa, K. Suzuki, M. Tatsumi, M. A. Razak, T. Uchida, N. Takeda, and T. Miyamura, J. Virol. 71:7207-7213, 1997). VLPs can be found in the culture medium of infected Tn5 cells but not in that of Sf9 cells, and the major VLPs have lost the C-terminal 52 aa. To investigate the protein requirement for HEV VLP formation, we prepared 14 baculovirus recombinants to express the capsid proteins truncated at the N terminus, the C terminus, or both. The capsid protein consisting of aa residues 112 to 608 formed VLPs in Sf9 cells, suggesting that particle formation is dependent on the modification process of the ORF2 protein. In the present study, electron cryomicroscopy and image processing of VLPs produced in Sf9 and Tn5 cells indicated that they possess the same configurations and structures. Empty VLPs were found in both Tn5 and Sf9 cells infected with the recombinant containing an N-terminal truncation up to aa residue 125 and C-terminal to aa residue 601, demonstrating that the aa residues 126 to 601 are the essential elements required for the initiation of VLP assembly. The recombinant HEV VLPs are potential mucosal vaccine carrier vehicles for the presentation of foreign antigenic epitopes and may also serve as vectors for the delivery of genes to mucosal tissue for DNA vaccination and gene therapy. The results of the present study provide useful information for constructing recombinant HEV VLPs having novel functions.     

猪圆环病毒2型ORF2基因在昆虫细胞中的表达及其特性

利用Bac-to-Bac杆状病毒表达系统将圆环病毒2型的ORF2全基因克隆到杆状病毒转移载体pFastBac^TM1中,获得重组转移载体pFast-OFR2,再将其转化进含穿梭载体Bacmid的感受态细胞DH10Bac中,发生转座作用,经蓝白菌落筛选得到含ORF2基因的重组穿梭载体Bac-ORF2,以脂质体介导的方法将重组穿梭载体转染sf9细胞,获得重组病毒,命名为Ac-ORF2。间接免疫荧光分析表明,PCV2阳性血清能使Ac-ORF2感染的sf9昆虫细胞呈强的荧光着色; SDS-PAGE与Western-blotting分析可见大小约为28kD的特异性带,表明Ac.ORF2在sf9细胞中成功表达了PCV2-ORF2蛋白。将该表达蛋白纯化并经磷钨酸负染后,通过电镜观察可见形态与PCV2病毒粒子相似的病毒样颗粒(VLPs),其中某些颗粒由于中心浓染而似空衣壳,其直径也为17nm左右。     

密码子优化型HPV16L1基因在两种昆虫细胞中的表达

目的:提高16型人乳头瘤病毒（HPV16）L1基因在杆状病毒昆虫细胞中的表达水平,为研制预防性HPV疫苗奠定基础。方法:根据昆虫细胞密码子偏性对野生型HPV16L1基因进行改造,利用Bac-to-Bac表达系统获得重组杆状病毒,感染昆虫细胞Sf9和High Five。Western blot鉴定表达产物;电镜下观察病毒样颗粒形成。利用ELISA法评价HPV16L1基因的优化效果,探讨L1蛋白表达的最佳条件。结果:在相对分子质量56kDa处出现HPV16L1的特异性条带;电镜下可见病毒样颗粒在昆虫细胞的核内形成;优化型HPV16L1基因的表达水平显著高于野生型。High Five细胞表达的最佳条件为MOI=10,表达时相72h,其L1蛋白表达量至少比Sf9细胞高3倍。结论:密码子优化技术确实能够促进HPV16L1蛋白的高效表达,而High Five细胞表现出的显著优势尤其值得关注。     

HIV—1核蛋白p24在昆虫细胞中的表达

将完整的ＨＩＶ－１ ｐ２４基因克隆到杆状病毒转移质粒中,使用重组转移质粒与野生型杆状病毒ＤＮＡ共转染Ｓｆ９昆虫细胞,经筛选获得带有编码ｐ２４基因的重组杆状病毒。重组杆状病毒感染Ｓｆ９细胞后在细胞中表达了ＨＩＶ核蛋白ｐ２４。其重组蛋白的分子量为２４ｋＤ。此重组糖蛋白在免疫荧光,免疫印染和酶联免疫实验中都能被人ＨＩＶ－１阳性血清和单克隆抗体所识别。     

Expression, processing, and assembly of foot-and-mouth disease virus capsid structures in heterologous systems: induction of a neutralizing antibody response in guinea pigs.

Plasmids containing the foot-and-mouth disease virus structural protein precursor (P1) and 3C protease genes or the P1 gene alone were expressed in Escherichia coli. A recombinant baculovirus containing the P1 gene was also generated and expressed in Spodoptera frugiperda cells. Expression of the P1 and 3C genes in E. coli resulted in efficient synthesis and processing of the structural protein precursor and assembly into 70S empty capsids. This material reacted with neutralizing monoclonal antibodies which recognize only conformational epitopes and elicited a significant neutralizing antibody response in vaccinated guinea pigs. Expression of the P1 gene in E. coli resulted in synthesis of an insoluble product, whereas in insect cells infected with the recombinant baculovirus a soluble product was synthesized. Both soluble and insoluble P1 reacted with a 12S-specific monoclonal antibody, but only soluble P1 elicited a neutralizing antibody response in guinea pigs.     

人视黄醇结合蛋白4在杆状病毒系统中的表达及其多克隆抗体制备

旨在利用杆状病毒系统表达、制备人视黄醇结合蛋白(RBP4)并检测其免疫原性。将人RBP4基因片段及信号肽SS64片段亚克隆到杆状病毒转移载体pFastBac-dual(pFBd)中,获得相应的重组转移质粒;转化大肠杆菌菌株DH10bac,转座后经筛选获得重组穿梭质粒rbacmid,将重组穿梭质粒转染孔板培养的Sf9细胞,获得含人RBP4表达框的重组杆状病毒,经过扩增获得毒种。毒种感染对数生长期的Sf9细胞并表达人RBP4蛋白(I-RBP4),通过SDS-PAGE和Western blotting对表达蛋白进行检测和鉴定。用毒种感染悬浮培养的Sf9细胞制备一批RBP4蛋白,完成SDS、Western blotting的检测及少量的多抗制备。纯化重组蛋白并与E.coli重组人RBP4(E-RBP4)分别免疫家兔。实验结果,酶切鉴定及测序证实重组转移质粒构建正确;成功构建重组RBP4-bacmid;人RBP4蛋白在昆虫细胞获得高效表达。表达的RBP4蛋白可以分泌到培养基中,分子量约为23 kDa,经过计算表达量为100 mg/L;纯化蛋白免疫兔子制备了多抗血清,血清滴度为1∶100 000,高于原核表达的抗体滴度(1∶10 000),与人体提纯蛋白制备的抗体滴度相近。杆状病毒系统高效表达了人的RBP4蛋白,具有较好的抗原性,并获得高亲和力的抗血清,为下一步的人血RBP4检测试剂盒的制备打下了坚实的基础。     

人乳头瘤病毒16型E6基因的T—A克隆及在真核细胞中的表达

由于ＨＰＶ１６Ｅ６蛋白能诱导机体保护性免疫反应,可作为基因治疗的靶抗原。用杆状病毒昆虫细胞表达系统制备了ＨＰＶ１６Ｅ６基因工程蛋白,拟用于宫颈癌细胞系小鼠模型抗癌的免疫治疗。用ＰＣＲ技术从ＨＰＶ１６基因组中扩增获得转化基因Ｅ６的完整ＯＲＦ,按ＴＡ策略将其克隆到自行制备的杆状病毒转移载体ｐＶＬ１３９３Ｔ尾载体中,置于杆状病毒ＡｃＭＮＰＶＰｏｌｈ晚期启动子控制之下,用此重组转移质粒ｐＶＬ１３９３Ｅ６与杆状病毒ＤＮＡ共转染昆虫细胞Ｓｆ９,经噬斑筛选获得带有编码Ｅ６蛋白基因的重组杆状病毒株,并在昆虫细胞Ｓｆ９中表达为非融合性Ｅ６蛋白。ＳＤＳＰＡＧＥ电泳分析其分子量约为１８ｋＤ,免疫印迹实验表明,此重组蛋白能被兔抗ＨＰＶ１６Ｅ６抗体所识别。     

Rotavirus SA11 genome segment 11 protein is a nonstructural phosphoprotein.

We investigated properties of the rotavirus genome segment 11 protein. A rotavirus SA11 genome segment 11 cDNA which contains the entire coding region was sequenced and inserted into the baculovirus transfer vector pVL941. Recombinants containing gene 11 cDNA were selected, and the gene 11 product expressed in Spodoptera frugiperda cells infected with these recombinants was inoculated into guinea pigs to produce hyperimmune antiserum. Characterization of the antiserum showed that it recognized a primary translation product with a molecular weight of 26,000 (26K protein) in recombinant-infected insect cells, in SA11-infected monkey kidney cells, and in cell-free translation reactions programmed with SA11 mRNA. A modified 28K product was also detected but only in SA11-infected monkey kidney cells. The 26K 28K proteins were shown to be phosphorylated in infected monkey kidney cells, and the 26K protein was phosphorylated in insect cells. We were unable to identify what type of modification caused the molecular weight shift to 28,000 in infected monkey kidney cells. Large amounts of the gene 11 product were detected by immunofluorescence in discrete foci in the cytoplasm of infected monkey kidney cells. Viruses of all known serotypes were also detected by immunofluorescence by using hyperimmune antiserum to the SA11 gene 11 product. The antiserum reacted with particle-depleted cytosol fractions but did not react with purified virus particles by immunoprecipitation or immunoblotting; it also did not neutralize virus infectivity in plaque reduction neutralization assays. Therefore, we conclude that the primary gene 11 product is a nonstructural phosphoprotein which we designated NS26.     

Expression of Human Recombinant CD23 in Insect Cells

Abstract

Human CD23 (low affinity receptor for IgE) has been expressed in insect cells (Sf9) using the baculovirus expression system and the baculovirus transfer vector pAc373. Insect cells infected with a recombinant baculovirus coding for CD23 synthesized a polypeptide not found in wild-type infected insect cells that had antigenic properties similar to natural CD23 produced in RPMI 8866 cells. Surface expression of recombinant CD23 was demonstrated by its ability to bind IgE. Recombinant CD23 expressed in insect cells had a slightly lower molecular weight (4 3 kDa) than that of natural CD23 (4 5 kDa) from RPMI 8866 cells as detected by SDS-PAGE followed by Western-blotting. Affinity-purified recombinant CD23 from in-fected insect cells showed B-cell growth promoting activity. These observations demonstrate for the first time that biologically active recombinant CD23 can be produced by the baculovirus expression system, thus providing a useful source of recombinant material to elucidate the biological functions of CD23.     

Structural requirements for the assembly of Norwalk virus-like particles

Norwalk virus (NV) is the prototype strain of a group of human caliciviruses responsible for epidemic outbreaks of acute gastroenteritis. While these viruses do not grow in tissue culture cells or animal models, expression of the capsid protein in insect cells results in the self-assembly of recombinant NV virus-like particles (rNV VLPs) that are morphologically and antigenically similar to native NV. The X-ray structure of the rNV VLPs has revealed that the capsid protein folds into two principal domains: a shell (S) domain and a protruding (P) domain (B. V. V. Prasad, M. E. Hardy, T. Dokland, J. Bella, M. G. Rossmann, and M. K. Estes, Science 286:287-290, 1999). To investigate the structural requirements for the assembly of rNV VLPs, we performed mutational analyses of the capsid protein. We examined the ability of 10 deletion mutants of the capsid protein to assemble into VLPs in insect cell cultures. Deletion of the N-terminal 20 residues, suggested by the X-ray structure to be involved in a switching mechanism during assembly, did not affect the ability of the mutant capsid protein to self-assemble into 38-nm VLPs with a T=3 icosahedral symmetry. Further deletions in the N-terminal region affected particle assembly. Deletions in the C-terminal regions of the P domain, involved in the interactions between the P and S domains, did not block the assembly process, but they affected the size and stability of the particles. Mutants carrying three internal deletion mutations in the P domain, involved in maintaining dimeric interactions, produced significantly larger 45-nm particles, albeit in low yields. The complete removal of the protruding domain resulted in the formation of smooth particles with a diameter that is slightly smaller than the 30-nm diameter expected from the rNV structure. These studies indicate that the shell domain of the NV capsid protein contains everything required to initiate the assembly of the capsid, whereas the entire protruding domain contributes to the increased stability of the capsid by adding intermolecular contacts between the dimeric subunits and may control the size of the capsid.     

Increase of host cell longevity by the expression of 30K protein originating from silkworm hemolymph in an insect cell–baculovirus system

A recombinant baculovirus was constructed by the homologous recombination between wild-type AcMNPV DNA and a baculovirus transfer vector containing a gene coding for the 30K protein originating from silkworm hemolymph. The 30K protein was successfully expressed in Sf9 cells infected with the recombinant baculovirus (AcMNPV/30K). To investigate the effect produced by the expression of the 30K protein, host cell viability after infection was compared with that of Sf9 cells infected with AcMNPV/β-gal. The viability of the cells infected with AcMNPV/β-gal began to decrease exponentially 3 days after infection, whereas that of the cells infected with AcMNPV/30K remained at a high level until 5 days after infection. This indicates that the 30K protein increases cell longevity after viral infection. This increased cell longevity is considered to be due to the inhibition of host cell apoptosis induced by a baculovirus, and the extent of apoptosis was measured by the flow cytometric method. The percentage of the sub-G1 fraction, which represents the extent of apoptosis, was decreased by the expression of the 30K protein. This indicates that the expression of the 30K protein in insect cells increases host cell longevity by inhibiting apoptosis.     

Interaction of recombinant norwalk virus particles with the 105-kilodalton cellular binding protein, a candidate receptor molecule for virus attachment

Norwalk virus (NV), responsible for outbreaks of acute gastroenteritis, comprises the species of the genus Norwalk-like viruses in the family Caliciviridae. Although the study of the molecular biology of NV has been hampered by a lack of culture systems or small experimental animal models, virus-like particles (VLPs) generated with recombinant baculoviruses harboring the capsid protein gene of NV provide a useful tool for investigating NV-cell interactions. In this study, the attachment of the recombinant VLPs derived from the Ueno virus (UEV), a strain belonging to the genogroup II NVs, to mammalian and insect cells was examined. Kinetic analyses of the binding of the recombinant VLPs of the UEV (rUEVs) to Caco-2 cells demonstrated that the binding was specific and occurred in a dose-dependent manner. Approximately 7.5% of the prebound rUEVs were internalized into the Caco-2 cells. Enzymatic and chemical modification of Caco-2 cell surface molecules suggested that the binding was directly mediated by a protein-protein interaction. A virus overlay protein-binding assay (VOPBA) indicated that rUEVs appeared to bind to a 105-kDa molecule, designated as the NV attachment (NORVA) protein. Furthermore, the assay indicated that its native conformational structure was indispensable for the binding activity. In Caco-2 cells, the NORVA protein was detected when VOPBA was carried out with the VLPs from Seto and Funabashi viruses, which are serologically different NVs from UEV, used as probes. The binding of rUEVs to NORVA protein was also observed in six mammalian cell lines other than Caco-2. These data suggest that the attachment of NV to mammalian cells is mediated by NORVA protein, which is ubiquitously expressed in the mammalian cells. The present study is the first report on the role of the cellular molecule in the binding of recombinant VLPs of NV.     

Efficient assembly and release of SARS coronavirus-like particles by a heterologous expression system

Virus-like particles (VLPs) produced by recombinant expression of the major viral structural proteins could be an attractive method for severe acute respiratory syndrome (SARS) control. In this study, using the baculovirus system, we generated recombinant viruses that expressed S, E, M and N structural proteins of SARS-CoV either individually or simultaneously. The expression level, size and authenticity of each recombinant SARS-CoV protein were determined. In addition, immunofluorescence and FACS analysis confirmed the cell surface expression of the S protein. Co-infections of insect cells with two recombinant viruses demonstrated that M and E could assemble readily to form smooth surfaced VLPs. On the other hand, simultaneous high level expression of S, E and M by a single recombinant virus allowed the very efficient assembly and release of VLPs. These data demonstrate that the VLPs are morphological mimics of virion particles. The high level expression of VLPs with correct S protein conformation by a single recombinant baculovirus offers a potential candidate vaccine for SARS.     

Production of recombinant snakehead rhabdovirus: the NV protein is not required for viral replication

Snakehead rhabdovirus (SHRV) affects warm water fish in Southeast Asia and belongs to the genus Novirhabdovirus by virtue of its nonvirion gene (NV). Because SHRV grows best at temperatures between 28 and 31 degrees C, we were able to use the T7 expression system to produce viable recombinant SHRV from a cloned cDNA copy of the viral genome. Expression of a positive-strand RNA copy of the 11, 550-nucleotide SHRV genome along with the viral nucleocapsid (N), phosphoprotein (P), and polymerase (L) proteins resulted in the generation of infectious SHRV in cells preinfected with a vaccinia virus vector for T7 polymerase expression. Recombinant virus production was verified by detection of a unique restriction site engineered into the SHRV genome between the NV and L genes. Since we were now able to begin examining the function of the NV gene, we constructed a recombinant virus containing a nonsense mutation located 22 codons into the coding sequence of the NV protein. The NV knockout virus was produced at a concentration as high as that of wild-type virus in cultured fish cells, and the resulting virions appeared to be identical to the wild-type virions in electron micrographs. These initial studies suggest that NV has no critical function in SHRV replication in cultured fish cells.