The gel microdrop secretion assay: Identification of a low productivity subpopulation arising during the production of human antibody in CHO cells

The long-term stability of high-level expression is the most important factor to consider when choosing cell lines for the expression of recombinant proteins. Declining volumetric yields in large-scale fermentation can be caused by changes affecting the cell population as a whole such as loss in viability, depletion of nutrients or accumulation of metabolites affecting cell growth. Alternatively, genetic instability may lead to the outgrowth of a less productive, metabolically favored sub-population. Currently a variety of parameters are measured to monitor the condition of cells in fermenters including glucose uptake, lactate accumulation and oxygen consumption; in addition, periodic viable cell counts allow the determination of the growth rate and viability of the population. All of these methods measure the condition of the cell population as a whole and changes must involve a significantly large proportion of the total culture in order to be detectable. Here we report on a method that allows the evaluation of the productivity of individual cells. Using the gel microdrop secretion assay, we detected the appearance of a sub-population of cells with lower productivity. Subsequent analysis of the culture confirmed the existence of lower productivity cells with a lower vector copy number. Therefore, the single cell secretion assay proved to be a rapid method to detect and isolate a low productivity variant of the producer cell line.