Process optimization of constitutive human granulocyte-macrophage colony-stimulating factor (hGM-CSF) expression in Pichia pastoris fed-batch culture

Human granulocyte-macrophage colony-stimulating factor (hGM-CSF) is a therapeutically important cytokine that is poorly expressed because of its toxic effects on the host cells. Extracellular expression of hGM-CSF was obtained by cloning its gene in Pichia pastoris under the constitutive glyceraldehyde-3-phosphate dehydrogenase (GAP) promoter with an N-terminal alpha peptide sequence for its extracellular production. The clones obtained were screened for a hyper producer following which media and cultivation conditions were optimized in shake flasks. Batch and fed-batch studies were performed in a bioreactor where different feed compositions were fed exponentially to obtain high biomass concentrations. Feeding of complex media allowed us to maintain a high specific growth rate of 0.2 h(-1) for the longest time period, and a final biomass of 98 g DCW/l was obtained in 34 h. Product formation was found to be growth associated, and the product yield with respect to biomass (Y (P/X)) was similar to 2.5 mg/g DCW. The above fed-batch strategy allowed us to obtain fairly pure glycosylated hGM-CSF at a final product concentration of 250 mg/l in the culture supernatant with a high volumetric productivity of 7.35 mg l(-1) h(-1).