Recombinant Human CD19 in CHO-K1 Cells: Glycosylation Patterns as a Quality Attribute of High Yield Processes

CD19 is an essential protein in personalized CD19-targeting chimeric antigen receptor (CAR)-T cell-based cancer immunotherapies and CAR-T cell functionality evaluation. However, the recombinant expression of this "difficult to-express" (DTE) protein is challenging, and therefore, commercial access to the protein is limited. We have previously described the successful stable expression of our soluble CD19-AD2 fusion protein of the CD19 extracellular part fused with human serum albumin domain 2 (AD2) in CHO-K1 cells. The function, stability, and secretion rate of DTE proteins can be improved by culture conditions, such as reduced temperature and a shorter residence time. Moreover, glycosylation, as one of the most important post-translational modifications, represents a critical quality attribute potentially affecting CAR-T cell effector function and thus impacting therapy's success. In this study, we increased the production rate of CD19-AD2 by 3.5-fold through applying hypothermic culture conditions. We efficiently improved the purification of our his-tagged CD19-AD2 fusion protein via a Ni-NTA-based affinity column using a stepwise increase in the imidazole concentration. The binding affinity to commercially available anti-CD19 antibodies was evaluated via Bio-Layer Interferometry (BLI). Furthermore, we revealed glycosylation patterns via Electrospray Ionization Mass Spectrometry (ESI-MS), and five highly sialylated and multi-antennary N-glycosylation sites were identified. In summary, we optimized the CD19-AD2 production and purification process and were the first to characterize five highly complex N-glycosylation sites.