Animal Cell Expression Systems

The glycan profile of therapeutic recombinant proteins such as monoclonal antibodies is a critical quality attribute, which affects the efficacy of the final product. The cellular glycosylation process during protein expression is dependent upon a number of factors such as the availability of substrates in the media, the intracellular content of nucleotide sugars, and the enzyme repertoire of the host cells. In order to control the variability of glycosylation it is important to understand the critical process parameters and their acceptable range of values to enable reproducible production of proteins with a predetermined glycan profile providing the desired biological function or therapeutic effect. The depletion of critical nutrients such as glucose or galactose, which may occur toward the end of a culture process, can lead to truncated glycans. Terminal galactosylation and sialyation are particularly variable but may be controlled by the presence of some key media components. Ammonia accumulation, pH, and dissolved oxygen levels are also known to be key bioprocess parameters that affect the glycosylation of recombinant proteins. Specific enzyme inhibitors can be added to the media to drive the formation of selected and predetermined glycan profiles. Various attempts have been made to predict the glycan profiles of cellular expressed proteins and have led to metabolic models based upon knowledge of metabolic flux and the kinetics of individual glycosylation reactions.In contrast to single recombinant proteins, the glycan profiles of viral vaccines are far more complex and difficult to predict. The example of influenza A virus shows that hemagglutinin, the major antigenic determinant, has three to nine N-glycans, which may influence the antigenicity and efficacy of the vaccine. Glycosylation of the influenza A virus has been largely unmonitored in the past as production has been from eggs, where glycan profiles of antigens are difficult if not impossible to control. Over the past decade, however, there have been various commercial influenza vaccines made available from cell technology using animal host cells. Analysis of glycosylation control shows that the type of host cell has the greatest influence on the final analyzed glycan profile. Other factors such as the virus strain, the cultivation system, or various process parameters have been shown to have only a minor effect on the glycosylation pattern. We predict that the analysis of glycan profiles in viral vaccines will become increasingly important in the development and consistent manufacturing of safe and potent vaccines. Graphical Abstract.