Effects of elevated pCO₂ and/or osmolality on the growth and recombinant tPA production of CHO cells

Carbon dioxide is a by-product of mammalian cell metabolism that will build up in culture if it is not removed from the medium. Increased carbon dioxide levels are generally not a problem in bench-top bioreactors, but inhibitory levels can easily be reached in large-scale vessels, especially if the aeration gas is enriched in oxygen. Due to the equilibrium attained between dissolved CO₂ and bicarbonate, increased pCO₂ is associated with increased osmolality in bioreactors with pH control. While a few preliminary reports indicate that elevated pCO₂ levels can inhibit cell growth and/or recombinant protein production, no comprehensive analysis of the interrelated effects of elevated pCO₂ and osmolality has been published. We have examined the effects of 140, 195, and 250 mm Hg (187, 260, and 333 mbar, respectively) pCO₂ (with and without osmolality control) on the growth of and recombinant tPA production by MT2-1-8 Chinese hamster ovary (CHO) cells. The effects of elevated osmolality were also investigated at the control pCO₂ of 36 mm Hg. Elevated pCO₂ at 310 mOsm/kg osmolality inhibited cell growth in a dose- dependent fashion, with a maximum decrease of 30% in the specific growth rate (μ) at 250 mm Hg. Osmolality alone had no effect on μ, but the combination of elevated pCO₂ and osmolality increased the maximal reduction in μ to 45%. Elevated pCO₂ at 310 mOsm/kg osmolality decreased the specific tPA production rate (q(tPA)) by up to 40% at 250 mm Hg. Interestingly, while increased osmolality decreased q(tPA) significantly at 140 mm Hg pCO₂, it had no effect or even increased q(tPA) at 195 and 250 mm Hg.