The new frontier in CHO cell line development: From random to targeted transgene integration technologies

Cell line development represents a crucial step in the development process of a therapeutic glycoprotein. Chinese hamster ovary (CHO) cells are the most frequently employed mammalian host cell system for the industrial manufacturing of biologics. The predominant application of CHO cells for heterologous recombinant protein expression lies in the relative simplicity of stably introducing ectopic DNA into the CHO host cell genome. Since CHO cells were first used as expression host for the industrial production of biologics in the late 1980s, stable genomic transgene integration has been achieved almost exclusively by random integration. Since then, random transgene integration had become the gold standard for generating stable CHO production cell lines due to a lack of viable alternatives. However, it was eventually demonstrated that this approach poses significant challenges on the cell line development process such as an increased risk of inducing cell line instability. In recent years, significant discoveries of new and highly potent (semi)-targeted transgene integration systems have paved the way for a technological revolution in the cell line development sector. These advanced methodologies comprise the application of transposase-, recombinase- or Cas9 nuclease-mediated site-specific genomic integration techniques, which enable a scarless transfer of the transgene expression cassette into transcriptionally active loci within the host cell genome. This review summarizes recent advancements in the field of transgene integration technologies for CHO cell line development and compare them to the established random integration approach. Moreover, advantages and limitations of (semi)-targeted integration techniques are discussed, and benefits and opportunities for the biopharmaceutical industry are outlined.

High-throughput reformatting of phage-displayed antibody fragments to IgGs by one-step emulsion PCR

Single-chain variable fragment (scFv) is the most common format for phage display antibody library. The isolated scFvs need to be reformatted to full-length IgGs for further characterization. High throughput reformatting of scFv to IgG without disrupting VH-VL pairing is of great demanding for exhaustive screening of all antibodies in IgG format. Herein, we developed a strategy based on the overlap extension PCR in emulsion to reformat scFv to IgG while maintain the accuracy and complexity of variable region pairing. Using CD40 as an example target, we reformatted phage display derived CD40 binding scFv library to IgG mammalian display library and isolated high affinity CD40 binding IgGs. This robust and reliable antibody reformatting approach could be integrated into any phage display based antibody drug discovery.

Application of Imaging Flow Cytometry for the Characterization of Intracellular Attributes in Chinese Hamster Ovary Cell Lines at the Single-Cell Level

Biopharmaceutical manufacturing using Chinese hamster ovary (CHO) cells requires the generation of high-producing clonal cell lines. During cell line development, cell cloning using fluorescence-activated cell sorting (FACS) has the potential to combine isolation of single cells with sorting based on specific cellular attributes that correlate with productivity and/or growth, identifying cell lines with desirable phenotypes for manufacturing. This study describes the application of imaging flow cytometry (IFC) to characterize recombinant cell lines at the single-cell level to identify cell attributes predictive of productivity. IFC assays are developed to quantify the organelle content and recombinant heavy-chain (HC) and light-chain (LC) polypeptide as well as messenger RNA (mRNA) amounts in single cells. The assays are then validated against orthogonal standard flow cytometry, western blot, and quantitative reverse transcription polymerase chain reaction (qRT-PCR) methods. The authors describe how these IFC assays may be used in cell line development and show how cellular properties can be correlated with productivity at the single-cell level, allowing the isolation of such cells during the cloning process. From the analysis, HC polypeptide and mRNA are found to be predictive of productivity early in the culture; however, specific organelle content did not show any correlation with productivity.