Glycosylation in Cell Culture

Glycoengineering of Mammalian Expression Systems on a Cellular Level

Mammalian expression systems such as Chinese hamster ovary (CHO), mouse myeloma (NS0), and human embryonic kidney (HEK) cells serve a critical role in the biotechnology industry as the production host of choice for recombinant protein therapeutics. Most of the recombinant biologics are glycoproteins that contain complex oligosaccharide or glycan attachments representing a principal component of product quality. Both N-glycans and O-glycans are present in these mammalian cells, but the engineering of N-linked glycosylation is of critical interest in industry and many efforts have been directed to improve this pathway. This is because altering the N-glycan composition can change the product quality of recombinant biotherapeutics in mammalian hosts. In addition, sialylation and fucosylation represent components of the glycosylation pathway that affect circulatory half-life and antibody-dependent cellular cytotoxicity, respectively. In this chapter, we first offer an overview of the glycosylation, sialylation, and fucosylation networks in mammalian cells, specifically CHO cells, which are extensively used in antibody production. Next, genetic engineering technologies used in CHO cells to modulate glycosylation pathways are described. We provide examples of their use in CHO cell engineering approaches to highlight these technologies further. Specifically, we describe efforts to overexpress glycosyltransferases and sialyltransfereases, and efforts to decrease sialidase cleavage and fucosylation. Finally, this chapter covers new strategies and future directions of CHO cell glycoengineering, such as the application of glycoproteomics, glycomics, and the integration of 'omics' approaches to identify, quantify, and characterize the glycosylated proteins in CHO cells. Graphical Abstract.

Screening and selection strategy for the establishment of biosimilar to trastuzumab-expressing CHO-K1 cell lines

The high prices of biopharmaceuticals or biologics used in the treatment of many diseases limit the access of patients to these novel therapies. One example is the monoclonal antibody trastuzumab, successfully used for breast cancer treatment. An economic alternative is the generation of biosimilars to these expensive biopharmaceuticals. Since antibody therapies may require large doses over a long period of time, robust platforms and strategies for cell line development are essential for the generation of recombinant cell lines with higher levels of expression. Here, we obtained trastuzumab-expressing CHO-K1 cells through a screening and selection strategy that combined the use of host cells pre-adapted to protein-free media and suspension culture and lentiviral vectors. The results demonstrated that the early screening strategy obtained recombinant CHO-K1 cell populations with higher enrichment of IgG-expressing cells. Moreover, the measurement of intracellular heavy chain polypeptide by flow cytometry was a useful metric to characterize the homogeneity of cell population, and our results suggest this could be used to predict the expression levels of monoclonal antibodies in early stages of cell line development. Additionally, we propose an approach using 25 cm2 T-flasks in suspension and shaking culture conditions as a screening tool to identify high producing cell lines. Finally, trastuzumab-expressing CHO-K1 clones were generated and characterized by batch culture, and preliminary results related to HER2-recognition capacity were successful. Further optimization of elements such as gene optimization, vector selection, type of amplification/selection system, cell culture media composition, in combination with this strategy will allow obtaining high producing clones.

Solid-Phase Enzymatic Remodeling Produces High Yields of Single Glycoform Antibodies

Antibodies are synthesized in mammalian cell culture as heterogeneous mixtures of glycoforms. Production of single glycoforms remains a challenge despite their value as therapeutics. The authors report a method of sequential enzymatic-based changes to antibodies while immobilized on an affinity column. Various antibodies (monoclonal and polyclonal) are isolated on Protein A or G columns and their glycans modified by sequential addition of enzymes for a desired transformation. Galactosylated antibodies (>90% yield) are produced by a one stage reaction process with sialidase to remove any sialic acid residues and addition of galactose with galactosyltransferase and UDP-Gal. Sialylated antibodies (>90%) are produced by a 2 stage conversion involving α(2,3) sialidase and galactosyltransferase followed by treatment with α(2,6) sialyltransferase in the presence of CMP-NANA. By this method, >90% of a disialylated human-llama antibody (EG2-hFc) and equimolar quantities of monosialylated and disialylated forms of human antibodies (αIL8-hFc and human polyclonal) are produced. Such high levels of sialylation are very difficult to obtain by typical cell culture methods. This method of transformation while the antibody is held on a solid-phase column is superior to previous methods because it allows a series of enzymatic steps without the need for intermediate purification. This is an efficient and rapid method to generate therapeutic antibodies with predefined glycosylation profiles. This should also assist in investigating the structure-function relationship of antibody glycans to find the desired glycosylation profile for high functional activity. With further optimization the method can be used to modify antibodies in large-scale manufacturing.

Glycoengineering of CHO Cells to Improve Product Quality

Chinese hamster ovary (CHO) cells represent the predominant platform in biopharmaceutical industry for the production of recombinant biotherapeutic proteins, especially glycoproteins. These glycoproteins include oligosaccharide or glycan attachments that represent one of the principal components dictating product quality. Especially important are the N-glycan attachments present on many recombinant glycoproteins of commercial interest. Furthermore, altering the glycan composition can be used to modulate the production quality of a recombinant biotherapeutic from CHO and other mammalian hosts. This review first describes the glycosylation network in mammalian cells and compares the glycosylation patterns between CHO and human cells. Next genetic strategies used in CHO cells to modulate the sialylation patterns through overexpression of sialyltransfereases and other glycosyltransferases are summarized. In addition, other approaches to alter sialylation including manipulation of sialic acid biosynthetic pathways and inhibition of sialidases are described. Finally, this review also covers other strategies such as the glycosylation site insertion and manipulation of glycan heterogeneity to produce desired glycoforms for diverse biotechnology applications.