High yield refolding and purification process for recombinant human interleukin-6 expressed in Escherichia coli

Recombinant human interleukin-6 (hIL-6), a pleiotropic cytokine containing two intramolecular disulfide bonds, was expressed in Escherichia coli as an insoluble inclusion body, before being refolded and purified in high yield providing sufficient qualities for clinical use. Quantitative reconstitution of the native disulfide bonds of hIL-6 from the fully denatured E. coli extracts could be performed by glutathione-assisted oxidation in a completely denaturating condition (6M guanidinium chloride) at protein concentrations higher than 1 mg/mL, preventing aggregation of reduced hIL-6. Oxidation in 6M guanidinium chloride (GdnHCl) required remarkably low concentrations of glutathione (reduced form, 0.01 mM; oxidized form, 0.002 mM) to be added to the solubilized hIL-6 before the incubation at pH 8.5, and 22 degrees C for 16 h. After completion of refolding by rapid transfer of oxidized hIL-6 into acetate buffer by gel filtration chromatography, residual contaminants including endotoxin and E. coli proteins were efficiently removed by successive steps of chromatography. The amount of dimeric hIL-6s, thought to be purification artifacts, was decreased by optimizing the salt concentrations of the loading materials in the ion-exchange chromatography, and gradually removing organic solvents from the collected fractions of the preparative reverse-phase HPLC. These refolding and purification processes, which give an overall yield as high as 17%, seem to be appropriate for the commercial scale production of hIL-6 for therapeutic use.