Effects of medium quality on the expression of human interleukin-2 at high cell density in fermentor cultures of Escherichia coli K-12.

We examined the ability of transformed Escherichia coli cells in fermentor cultures to accumulate interleukin-2 (IL-2) intracellularly under temperature-regulated control of the phage lambda pL promoter. Induction of expression was undertaken at different culture optical densities, and specific IL-2 accumulation was found to decrease with increasing cell density at induction. Induction at higher culture optical densities was also accompanied by decreased growth during induction and increased acetate accumulation in the culture medium. Experiments were undertaken to study the effect of replacing spent medium by perfusion with fresh medium both before induction and during IL-2 expression at high cell density. Improved IL-2 expression was seen only when perfusion was continued past 1.6 h after the start of induction, and it was accompanied by a significant reduction in acetate buildup. Further improvements were not seen when perfusion was continued beyond hour 3 of induction. Replenishing medium components and decreasing the concentration of diffusible inhibitors before induction did not alleviate acetate buildup, growth limitation, or limitation of IL-2 synthesis. These results suggested that accumulation of diffusible inhibitors such as acetate during induction may be a significant factor limiting IL-2 expression in high-density cultures, but other factors intrinsic to the organism or the protein also played a major role.