Flow-cytometry and cell sorting: an efficient approach to investigate productivity and cell physiology in mammalian cell factories.

The performance of cell lines used for the production of biotherapeutic proteins typically depends on the number of cells in culture, their specific growth rate, their viability and the cell specific productivity (qP). Therefore both cell line development and process development are trying to (a) improve cell proliferation to reduce lag-phase and achieve high number of cells; (b) delay cell death to prolong the production phase and improve culture longevity; (c) and finally, increase qP. All of these factors, when combined in an optimised process, concur to increase the final titre and yield of the recombinant protein. As cellular performance is at the centre of any improvement, analysis methods that enable the characterisation of individual cells in their entirety can help in identifying cell types and culture conditions that perform exceptionally well. This observation of cells and their complexity is reflected by the term "cytomics" and flow cytometry is one of the methods used for this purpose. With its ability to analyse the distribution of physiological properties within a population and to isolate rare outliers with exceptional properties, flow cytometry ideally complements other methods used for optimisation, including media design and cell engineering. In the present review we describe approaches that could be used, directly or indirectly, to analyse and sort cellular phenotypes characterised by improved growth behaviour, reduced cell death or high qP and outline their potential use for cell line and process optimisation.