Expression and characterization of a soluble rubella virus E1 envelope protein

The key role of rubella virus glycoproteins in the formation of immune response, and perspectives on their use in the development of new recombinant vaccines

Rubella is a highly contagious viral disease which is mostly threatens to women of reproductive age. Existent live attenuated vaccines are effective enough, but have some drawbacks and are unusable for a certain group of people, including pregnant women and people with AIDS and other immunodeficiency. Thereby the development of alternative non-replicating, recombinant vaccines undoubtedly is needed. This review discusses the protein E1 and E2 role in formation of immune response and perspectives in development of new generation recombinant vaccines using them.

Preparation of recombinant viral glycoproteins for novel and therapeutic antibody discovery

Neutralizing antibodies are a critical component in the protection or recovery from viral infections. In the absence of available vaccines or antiviral drugs for many important human viral pathogens, the identification and characterization of new human monoclonal antibodies (hmAbs) that are able to neutralize viruses offers the possibility for effective pre- and/or post-exposure therapeutic modalities. Such hmAbs may also help in our understanding of the virus entry process, the mechanisms of virus neutralization, and in the eventual development of specific entry inhibitors, vaccines, and research tools. The majority of the more recently developed antiviral hmAbs have come from the use of antibody phage-display technologies using both naïve and immune libraries. Many of these agents are also enveloped viruses possessing important neutralizing determinants within their membrane-anchored envelope glycoproteins, and the use of recombinant, soluble versions of these viral glycoproteins is often critical in the isolation and development of antiviral hmAbs. This chapter will detail several methods that have been successfully employed to produce, purify, and characterize soluble and secreted versions of several viral envelope glycoproteins which have been successfully used as antigens to capture and isolate human phage-displayed monoclonal antibodies.

Expression and characterization of rubella virus glycoprotein E1 in yeast cells

OBJECTIVES

To express E1 glycoprotein of rubella virus (RuV) strain JR23 in yeast and develop a diagnostic assay using expressed E1 protein as coating antigen in comparison with other diagnostic assays.

METHODS

cDNA of E1 open reading frame of RuV was PCR amplified using plasmid pMD18-T-E1 as template and cloned into plasmid pBluscriptII SK+. After being confirmed by PCR, restriction endonuclease digestion and sequencing, pBluscriptII SK(+)-E1 plasmid DNA was digested by restriction endonuclease EcoR I and Xba I, and a fragment of 1.5 kb was isolated and cloned into a yeast expression pGAPZ(alpha)A, resulting in pGAPZ(alpha)A-E1. After confirmation by sequencing, pGAPZ(alpha)A- E1 was transformed into yeast GS115 cells with LiCl method. E1 protein expression in GS115 was analyzed by SDS-PAGE and Western blot. An indirect ELISA was developed using the recombinant E1 protein as coating antigen for detecting RuV E1 antibodies in 90 serum samples. To compare the specificity, sensitivity and reproducibility of the assay with other methods, the same serum samples were also assayed by RuV culture medium as coating antigen (Jingmei kit and German RECI kit). Statistical analyses were performed to compare the differences among these methods and to determine which coating antigen source, the recombinant E1 protein or RuV-infected culture medium, is more suitable for the assay.

RESULTS

A fragment of 1.5 kb, corresponding to the full open reading frame of E1, was PCR amplified and cloned in yeast expression vector. The clone was confirmed by restriction digestion, PCR and sequencing. E1 as a secretive protein was successfully expressed by GS115. Its molecular weight was about 58 kDa. SDS-PAGE showed that the recombinant protein was expressed efficiently and constantly in Pichia pastoris GS115 cells. The expression level reached a peak 48 h after culturing and stabilized thereafter. E1 protein was detected in both supernatant and cells. Western blot showed that the secretive E1 protein in the supernatants could react with both the anti-RuV-positive serum and a monoclonal antibody against E1. However, E1 protein derived from cells could only react with the anti-RuV-positive serum, polyclonal antibody, but not the monoclonal antibody. Compared with the German RECI kit, the sensitivity, specificity, and accordance rate of the assay using recombinant E1 protein as coating antigen were 67.11, 71.43 and 67.78%, respectively, while those of the assay using RuV-infected culture medium as coating antigen were 50, 78.57 and 54.44%, respectively. Compared with the German RECI kit, the sensitivity, specificity, and accordance rate of the ELISA assay using the Jingmei kit were 84.71, 71.43 and 82.22%, respectively. The data indicated that recombinant E1 protein derived from the yeast expression system can serve as a better source than RuV-infected cell medium as coating antigen for detecting antibodies against RuV in the indirect ELISA assay. Statistical analysis of the data generated from two independent experiments using recombinant E1 protein as coating antigen indicated that the assay was very consistent with no statistically significant difference between the two experiments (p > 0.05). 76 out of 90 serum samples were detected positive using the German RECI kit, while 68, 55 and 41 samples were positive using the Jingmei kit, recombinant E1 and RuV-infected cell medium, respectively. Statistical analyses indicated that the positive rates were significantly different among all four assays (p < 0.05) except for one pair (German RECI kit and the Jingmei kit: p > 0.05). Comparing the positive rates obtained from the assay using recombinant E1 and that using RuV-infected cell medium, it seems that the recombinant E1 protein is a better source than RuV culture medium as coating antigen in the indirect ELISA assay for detection of RuV antibody.

CONCLUSIONS

The recombinant yeast expression vector of RuV E1 glycoprotein was constructed successfully. The E1 protein as a secretive protein was successfully expressed by GS115 and maintained its antigenicity very well. As coating antigen, the recombinant E1 protein served a better source than RuV culture medium in the indirect ELISA method for the detection of RuV antibody.

Baculovirus-mediated large-scale expression and purification of a polyhistidine-tagged rubella virus capsid protein

The capsid protein of rubella virus was produced in baculovirus-infected Spodoptera frugiperda insect cells, with a polyhistidine affinity tag at the carboxy terminus. The RV capsid recombinant protein was produced in a 10-liter bioreactor and purified, under nondenaturing conditions, using immobilized metal-ion affinity chromatography. Immunoblot analyses indicated that the purified recombinant protein was intact and migrated with the expected molecular weight. The final yield was 5 mg of purified protein per liter of cell culture. Surface plasmon resonance was used to investigate the antigenic potential of the histidine tagged capsid protein in an antigen-antibody interaction study. A specific interaction between the two proteins was shown. Our results suggest that this strategy should be useful in interaction studies of other virus-specific proteins and antibodies.

Synthesis of soluble rubella virus spike proteins in two lepidopteran insect cell lines: large scale production of the E1 protein

The two envelope glycoproteins of rubella virus (RV), E1 of 58 kDa and E2 of 42-47 kDa, were individually expressed in lepidopteran Spodoptera frugiperda as well as in Trichoplusia ni insect cells using baculovirus vectors. The authentic signal sequences of E1 and E2 were replaced with the honeybee melittin signal sequence, allowing efficient entrance into the secretory pathway of the insect cell. In addition, the hydrophobic transmembrane anchors at the carboxyl termini of E1 and E2 proteins were removed to enable secretion rather than maintenance in the cellular membranes. Synthesis of the recombinant proteins in the absence and presence of tunicamycin revealed that both E1 and E2 were glycosylated with apparent molecular weights of 52 kDa and 37 kDa, respectively. Recombinant E2 appeared to be partially secreted, whereas E1 was essentially found inside the infected insect cell. The E1 protein was produced in large scale using a 10-1 bioreactor and serum-free medium (SFM). Purification of the recombinant protein product was performed from cytoplasmic extracts by ammonium sulphate precipitation followed by Concanavalin A affinity chromatography. This type of purified recombinant viral glycoproteins may be useful not only in diagnostic medicine or for immunization, but should enable studies designed to solve the structure of the virus particle.

Cell surface expression of a functional rubella virus E1 glycoprotein by addition of a GPI anchor

Rubella virus (RV) envelope glycoproteins E1 and E2 are targeted to the Golgi as heterodimers. While E2 contains a transmembrane Golgi retention signal, E1 is arrested in a pre-Golgi compartment in the absence of E2, and appears to require heterodimerization in order to reach the Golgi. Various forms of E1 with deletions in the ectodomain or lacking the cytoplasmic (CT) and transmembrane (TM) domains, as well as the 29 C-terminal amino acid residues of the ectodomain were also retained intracellularly. We therefore investigated the possibility of targetting E1 to the plasma membrane by addition of a glycosylphosphatidylinositol (GPI) anchor. We found that E1GPI was transported to the cell surface where it retained the hemadsorption activity characteristic of the wild-type E1/E2 heterodimer. Furthermore, coexpression of a mammalian GPI-specific phospholipase D (GPI-PLD) resulted in the release of E1GPI and in constitutive expression of a soluble form of E1. This study thus demonstrates that the GPI anchor has a dominant effect over the E1 pre-Golgi retention signal and that E1 is sufficient for hemadsorption.

Detection of rubella virus-specific immunoglobulin M antibodies with a baculovirus-expressed E1 protein

The structural proteins of rubella virus (RV) were expressed in insect cells by using the baculovirus expression vector system. The recombinant E1 envelope glycoprotein was purified by immunoaffinity chromatography and used to detect RV-specific immunoglobulin M antibodies in a time-resolved fluoroimmunoassay. Correlation analysis between the reactivities of antibodies against this recombinant E1 and the reactivities against authentic RV antigen shows that purified E1 can detect RV antibodies of the immunoglobulin M type.

Evaluation of Cobas Core Rubella IgG EIA recomb, a new enzyme immunoassay based on recombinant rubella-like particles

Cobas Core Rubella IgG EIA recomb (Roche), a new, commercially available rubella enzyme immunoassay using recombinant rubella-like particles, was compared with a hemagglutination inhibition assay (Rubenosticon; Organon Teknika) and two whole-virus enzyme immunoassays (IMx [Abbott] and Platelia [Sanofi Diagnostics Pasteur]). Compared with those of these reference assays, the relative sensitivities of Cobas Core Rubella IgG recomb were 100, 94, and 95.9%, with specificities amounting to 80.8, 98, and 98.2%, respectively.

Expression and characterization of secreted forms of rubella virus E2 glycoprotein in insect cells

Two different forms of rubella virus E2 glycoproteins were expressed in insect cells: intact wild-type E2 and a soluble form of E2 (E2 delta Tm) glycoprotein, in which the C-terminal membrane-anchor domain was deleted. E2 delta Tm behaved as a secretory protein and was secreted abundantly (5 mg/liter) from insect cells. In contrast to wild-type E2 (36 kDa), E2 delta Tm was secreted into the media and was detected as two species (33 and 30 kDa). Lectin binding assays in conjunction with glycosidase analyses revealed that both intracellular wild-type E2 and E2 delta Tm contained only N-linked glycans, while the two secreted forms of E2 delta Tm were found to differ in their glycosylation, with the 30-kDa form having only N-linked glycans while the 33-kDa species had both N-linked and O-linked glycans. The secreted E2 delta Tm species were purified by precipitation between 20 and 40% saturation with (NH4)2SO4 and retained full antigenicity. The levels of antibodies elicited in mice immunized with purified E2 delta Tm showed that the immunogenicity of secreted E2 delta Tm compared favorably to that of natural virion E2.