Exploration of BAC versus plasmid expression vectors in recombinant CHO cells

Vector engineering approaches are commonly used to increase recombinant protein production in mammalian cells, and among various concepts, bacterial artificial chromosomes (BAC) have been proposed to serve as open chromatin regions to omit chromosome positional effects. For proof of concept, we developed stable recombinant Chinese hamster ovary (CHO) cell lines using different expression vector systems: the plasmid vectors contained the identical expression cassette as the BAC constructs. Two anti-HIV1 antibody derivates served as model proteins (3D6scFc and 2F5scFc) for generation of four stable recombinant CHO cell lines. The BAC-derived clones showed three to four times higher specific productivity, and therefore, gene copy numbers and transcript level were quantified. The active chromatin region provided with the BAC environment significantly improved transcription evidenced with both model proteins. Specific transcription was approximately six times higher from BAC-based vectors compared to the corresponding plasmid vectors for both single-chain fragment crystallizable (scFc) proteins. Our accurate investigations elucidated also differences between translational activities related to the protein of choice. 3D6scFc expressed specifically three to four times more product than 2F5scFc indicating that the product by itself also contributes to enhanced productivity. This study indicated comparable increase of transcription level for both scFc proteins when using the BAC system, but translation, maturation, and secretion of individual proteins seem to be protein specific.