Expression of a functional human interleukin-18 in yeast

The cDNA sequence for mature human interleukin-18 gene (hIL-18) was cloned and then used to transform Saccharomyces cerevisiae. Two different promoters for heterologous expression of hIL-18 were tested: glyceraldehyde-3-phosphate dehydrogenase (GPD) promoter and a yeast hybrid ADH2-GPD promoter consisting of alcohol dehydrogenase II (ADH2) and GPD promoter. Northern blot analysis revealed that, although variation in the expression level of rhIL-18 existed among transformants, the highest expression was obtained by the GPD promoter. Expressed hIL-18 protein (rhIL-18) was successfully secreted into culture medium due to the presence of the signal peptide of rice amylase 1A. It was possible to produce 13 mg of rhIL-18 protein per liter of culture filtrate without any changes in cell growth. Both cell growth and rhIL-18 production reached the peaks after the 3-day cultivation while the accumulation of transgene transcript peaked at 24 h of cultivation. The secreted rhIL-18 had an estimated molecular mass of 18 kDa. The bioassay observing the induction of interferon-γ from the KG-1 cell line indicated that the secreted recombinant rhIL-18 was bioactive and the specific activity of yeast-derived rhIL-18 was enhanced 15 times relative to that of E. coli-derived rhIL-18.