Glutamine Synthetase Transfected Cells May Avoid Selection by Releasing Glutamine

Abstract

Glutamine synthetase (GS) is a popular marker for selection of plasmid expression in mammalian cell lines. Positive selection is maintained simply by omitting glutamine (gln) from culture medium. We have studied the stability of GS transfectants which produce humanised monoclonal antibodies. Spent culture medium of GS transfectants contained up to 200μM gln which we conclude is released from the cells. Using a new assay to detect intracellular Ig, cells were detected with low levels of Ig after a short time of culture in medium with as little as 60μM added gln and also in hollow fibre cultures fed with gln free medium. These variants comprised up to 50% of the cell population and secreted very little antibody. It may be hard to maintain selection pressure in practical large scale cultures particularly in hollow fibre systems where, with a high cell density, the local gln concentrations may be substantial.