Expression, purification and characterization of the recombinant chimeric IgE Fc-fragment opossum–human–opossum (OSO), an active immunotherapeutic vaccine component

The active vaccine component recombinant chimeric IgE Fc-fragment opossum–human–opossum (OSO) has been expressed in CHO-K1 cells. It contains two identical polypeptide chains with 338 amino acid residues in each chain connected by two disulfide bridges. The cell lines were adapted to suspension culture in a serum-free medium. An expression level of 60 mg/L was obtained after 8 days in a shaking flask at a temperature of 31.5 °C. The OSO protein has been purified to homogeneity by a combination of three chromatographic steps. Virus inactivation and reduction by solvent detergent treatment and nano-filtration were included in the process. The residual host cell protein content was less than 50 ng/mg OSO as analyzed by ELISA. Purity was analyzed by SDS–PAGE under reducing and non-reducing conditions and was estimated by densitometry to be above 99.0%. The dimer content was less than 0.1% as estimated by analytical size exclusion chromatography. The molecular mass, as estimated by SDS–PAGE, is 90 kDa. A value of around 74 kDa was calculated from its amino acid composition. This indicates that the protein is heavily glycosylated containing around 18% carbohydrate. Isoelectric focusing in polyacrylamide gel disclosed a ladder type band pattern with pI values in the pH-range 7.0–8.3, indicating a variation in the sialic acid content. The OSO protein is not stable at temperatures above 40 °C and at pH values below 4 indicating that virus inactivation by incubating the protein solution at higher temperature or at lower pH is not possible.