RESUMEN
Culturing CHO (Chinese-hamster ovary) cells at low temperature leads to growth arrest in the G0/G1 phase of the cell cycle and, in many cases, causes an increase in the specific productivity of recombinant protein. Controlled proliferation is often used to increase CHO specific productivity, and thus there is speculation that the enhanced productivity at low temperature is due to G0/G1-phase growth arrest. However, we show that the positive effect of low temperature on recombinant protein production is due to elevated mRNA levels and not due to growth arrest and that a cell line can still exhibit growth-associated productivity at low temperatures. Using a CHO cell producing recombinant human IFN-gamma (interferon-gamma), we show that productivity increases as the percentage of cells in the S phase of the cell cycle increases, at both 32 and 37 degrees C. The increased productivity is due to higher recombinant IFN-gamma mRNA levels. We also show that, for a given cell-cycle distribution, specific productivity increases as the temperature is lowered from 37 to 32 degrees C. Thus specific productivity is maximized when cells are actively growing (high percentage of S-phase cells) and also exposed to low temperature. These findings have important implications for cell-culture optimization and cell-line engineering, providing evidence that a CHO cell line capable of actively growing at low temperature would provide improved total protein production relative to the current growth strategies, namely 37 degrees C active growth or low-temperature growth arrest.