Analysis of intracellular IgG secretion in Chinese hamster ovary cells to improve IgG production

COP II-mediated ER-to-Golgi transport is a bottleneck for IgNAR-Fc production in the Chinese hamster ovary cell expression system

The novel heavy-chain antibody known as immunoglobulin new antigen receptor (IgNAR) is derived from cartilaginous fishes such as sharks. IgNAR, which binds to antigens with the high specificity and affinity of a conventional IgG antibody and exhibits high resistance to denaturation, has potential as a next-generation antibody in biopharmaceutical and biotechnological applications. High-level expression of recombinant IgNAR in animal cells has been challenging. In our previous study, IgNAR was expressed as a fusion protein with a human IgG Fc region (IgNAR-Fc) in Chinese hamster ovary (CHO) cells, but did not meet the production level required for further research and application. In this study, we sought to identify the production bottleneck in CHO cells as a first step toward achieving abundant production of IgNAR. Using an established IgG high-production CHO cell line as a comparator, we found that the amounts of intracellular dimeric IgNAR-Fc produced in CHO cells were similar to those of intracellular dimeric IgG. Furthermore, the majority of intracellular IgNAR-Fc was retained in the endoplasmic reticulum (ER) and strongly colocalized to ERGIC-53, the cargo receptor for coat protein complex II (COP II)-coated vesicles. These findings suggest that COP II-mediated ER-to-Golgi transport may represent a bottleneck for IgNAR-Fc production in the CHO cell expression system.

Enhancing the Antibody Production Efficiency of Chinese Hamster Ovary Cells through Improvement of Disulfide Bond Folding Ability and Apoptosis Resistance

The complex structure of monoclonal antibodies (mAbs) expressed in Chinese hamster ovary (CHO) cells may result in the accumulation of unfolded proteins, triggering endoplasmic reticulum (ER) stress and an unfolded protein response (UPR). If the protein folding ability cannot maintain ER homeostasis, the cell will shut down protein translation and ultimately induce apoptosis. We co-overexpressed HsQSOX1b and survivin proteins in the antibody-producing cell line CHO-PAb to obtain a new cell line, CHO-PAb-QS. Compared with CHO-PAb cells, the survival time of CHO-PAb-QS cells in batch culture was extended by 2 days, and the antibody accumulation and productivity were increased by 52% and 45%, respectively. The proportion of (HC-LC)2 was approximately doubled in the CHO-PAb-QS cells, which adapted to the accelerated disulfide bond folding capacity by upregulating the UPR's strength and increasing the ER content. The results of the apoptosis assays indicated that the CHO-PAb-QS cell line exhibited more excellent resistance to apoptosis induced by ER stress. Finally, CHO-PAb-QS cells exhibited mild oxidative stress but did not significantly alter the redox status. This study demonstrated that strategies based on HsQSOX1b and survivin co-overexpression could facilitate protein disulfide bond folding and anti-apoptosis ability, enhancing antibody production efficiency in CHO cell lines.

Sar1A overexpression in Chinese hamster ovary cells and its effects on antibody productivity and secretion

Chinese hamster ovary (CHO) cells are the most widely used for therapeutic antibody production. In cell line development, engineering secretion processes such as folding-related protein upregulation is an effective way of constructing cell lines with high recombinant protein productivity. However, there have been few studies on the transport of recombinant proteins between the endoplasmic reticulum (ER) and the Golgi apparatus. In this study, Sar1A, a protein involved in COPII vesicle formation, was focused on to improve antibody productivity by enhancing COPII vesicle-mediated antibody transport from the ER to the Golgi apparatus, and to clarify its effect on the secretion process. The constructed Sar1A-overexpressing CHO cell lines were batch-cultured, in which they showed an increased specific antibody production rate. The intracellular antibody accumulation and the specific localization of the intracellular antibodies were investigated by chase assay using a translation inhibitor and observed by immunofluorescence-based imaging analysis. The results showed that Sar1A overexpression reduced intracellular antibody accumulation, especially in the ER. The effects of the engineered antibody transport on the antibody's glycosylation profile and the unfolded protein response (UPR) pathway were analyzed by liquid chromatography-mass spectrometry and UPR-related gene expression evaluation, respectively. Sar1A overexpression lowered glycan galactosylation and induced a stronger UPR at the end of the batch culture. Sar1A overexpression enhanced the antibody productivity of CHO cells by modifying their secretion process. This approach could also contribute to the production of not only monoclonal antibodies but also other therapeutic proteins that require transport by COPII vesicles.

Effect of co-overexpression of the cargo receptor ERGIC-53/MCFD2 on antibody production and intracellular IgG secretion in recombinant Chinese hamster ovary cells

Therapeutic antibodies are attractive biopharmaceuticals because of their high therapeutic effects, fewer side effects, and prolonged half-life in the blood. Chinese hamster ovary (CHO) cells are the most widely used host cell lines to produce therapeutic antibodies in industries. High-producing recombinant CHO cells can be established via overexpression of endogenous proteins. In this study, we focused on the intracellular traffic of an antibody-producing CHO cell line, CHO-HcD6. Assembled antibodies were accumulated in the endoplasmic reticulum (ER) in the cell. We hypothesized that the accumulation was due to the insufficient number of cargo receptors in the cell and focused on a cargo receptor, the ERGIC-53-MCFD2 complex, which transports expressed proteins from the ER to the Golgi apparatus. Overexpression of the cargo receptor transport was expected to improve antibody production. Exogenous ERGIC-53 and MCFD2 were transfected into CHO-HcD6 cells, and overexpressing CHO-HcD6 cells were constructed. As a result of overexpression, antibody productivity increased in batch cultivation. However, the chase assay results and immunofluorescence microscopic observations revealed intracellular IgG accumulation in the overexpressing cells. These results suggest that overexpression of cargo receptors not only promoted extracellular secretion but also enhanced the retention of intracellular antibodies.

Development of a novel capillary electrophoresis method for quantitative measurements of intracellular recombinant protein titer

The measurement of volumetric titer is an integral step in the assessment and selection of a production cell line and cell culture process. The production of monoclonal antibodies (mAbs), a major class of therapeutic proteins, in Chinese Hamster Ovary (CHO) cell lines is challenging due to the clone-to-clone variations in the intrinsic capability to secrete a biologically complex protein. The measurement of intracellular mAb concentration could be a valuable tool to determine the ratio of intracellular to secreted product and be part of the evaluation of potential mAb productive cell lines. High throughput automation is a valuable tool that is used in bioprocess development to reduce work intensive steps. When coupled with the Simple Western (Wes) platform, automated capillary electrophoresis is an efficient method to measure recombinant protein concentration. In this study, we demonstrate the utility of using the automated Wes to rapidly measure intracellular titer and then compare the intracellular titer, volumetric titer and specific productivity between high and low production CHO clones expressing a model human IgG1 mAb.

The secretory pathway - the key for unlocking the potential of Chinese hamster ovary cell factories for manufacturing therapeutic proteins

Mammalian cell factories (in particular the CHO cell system) have been crucial in the rise of biopharmaceuticals. Mammalian cells have compartmentalized organelles where intricate networks of proteins manufacture highly sophisticated biopharmaceuticals in a specialized production pipeline - the secretory pathway. In the bioproduction context, the secretory pathway functioning is key for the effectiveness of cell factories to manufacture these life-changing medicines. This review describes the molecular components and events involved in the secretory pathway, and provides a comprehensive summary of the intracellular steps limiting the production of therapeutic proteins as well as the achievements in engineering CHO cell secretory machinery. We also consider antibody-producing plasma cells (so called "professional" secretory cells) to explore the mechanisms underpinning their unique secretory function/features. Such understandings offer the potential to further enhancement of the current CHO cell production platforms for manufacturing next generation of biopharmaceuticals.

Shelf-Life Extension of Fc-Fused Single Chain Fragment Variable Antibodies by Lyophilization

Generation of sequence defined antibodies from universal libraries by phage display has been established over the past three decades as a robust method to cope with the increasing market demand in therapy, diagnostics and research. For applications requiring the bivalent antigen binding and an Fc part for detection, phage display generated single chain Fv (scFv) antibody fragments can rapidly be genetically fused to the Fc moiety of an IgG for the production in eukaryotic cells of antibodies with IgG-like properties. In contrast to conversion of scFv into IgG format, the conversion to scFv-Fc requires only a single cloning step, and provides significantly higher yields in transient cell culture production than IgG. ScFv-Fcs can be effective as neutralizing antibodies in vivo against a panel of pathogens and toxins. However, different scFv fragments are more heterologous in respect of stability than Fab fragments. While some scFv fragments can be made extremely stable, this may change due to few mutations, and is not predictable from the sequence of a newly selected antibody. To mitigate the necessity to assess the stability for every scFv-Fc antibody, we developed a generic lyophilization protocol to improve their shelf life. We compared long-term stability and binding activity of phage display-derived antibodies in the scFv-Fc and IgG format, either stored in liquid or lyophilized state. Conversion of scFv-Fcs into the full IgG format reduced protein degradation and aggregation, but in some cases compromised binding activity. Comparably to IgG conversion, lyophilization of scFv-Fc resulted in the preservation of the antibodies' initial properties after storage, without any drop in affinity for any of the tested antibody clones.

Overexpression of transcription factor BLIMP1/prdm1 leads to growth inhibition and enhanced secretory capacity in Chinese hamster ovary cells

Chinese hamster ovary (CHO) cells present inherent limitations for processing and secretion of large amounts of recombinant proteins, especially for those requiring complex post-translational processing. To tackle these limitations, we engineered CHO host cells (CHOK1 and CHOS) to overexpress the transcription factor BLIMP1/prdm1 (a master regulator of the highly-secreting phenotype of antibody-producing plasma cells), generating novel CHO cell lines (referred to as CHOB). The CHOB cell lines exhibited decreased cell densities, prolonged stationary phase and arrested cell cycle in G1/G0 phase but simultaneously had significantly greater product titre for recombinant IgG1 (> 2-fold increase) coupled with a significantly greater cell-specific productivities (> 3-fold increase). We demonstrated that the improved productive phenotype of CHOB cells resulted from a series of changes to cell physiology and metabolism. CHOB cells showed a significantly greater ER size and increased protein synthesis and secretion capacity compared to control cells. In addition, CHOB cells presented a metabolic profile that favoured energy production to support increased recombinant protein production. This study indicated that a cell engineering approach based on BLIMP1 expression offers great potential for improving the secretory capacity of CHO cell hosts utilised for manufacture of recombinant biopharmaceuticals. Our findings also provides a greater understanding of the relationship between cell growth and productivity, valuable generic information for improving productive phenotypes for CHO cell lines during industrial cell line development.

Temperature Down-Shift Modifies Expression of UPR-/ERAD-Related Genes and Enhances Production of a Chimeric Fusion Protein in CHO Cells

Low culture temperature enhances the cell-specific productivity of Chinese hamster ovary (CHO) cells expressing varied recombinant (r-) proteins, but the mechanisms remain unclear. Regulation of unfolded protein response (UPR) pathway genes, such as transcriptional regulatory factors and endoplasmic reticulum (ER)-resident proteins, appear to be involved in the improvements of r-protein production under low temperature conditions. The transcriptional regulation of UPR-specific targets is studied in response to decreased culture temperature in relation to production of a difficult-to-express protein. A clonally-derived CHO cell line expressing a chimeric fusion protein (human erythropoietin [hEPO] linked to a murine Fc region, hEPO-Fc) is evaluated in terms of growth, metabolism, r-protein production and UPR-/ER associated degradation (ERAD)-specific gene expression at standard (37 °C) and low (32 °C) temperature in batch and fed-batch systems. Low temperature decreased peak cell density, improved viability, generated cell cycle arrest in the G1 phase and enhanced hEPO-Fc expression in both batch and fed-batch cultures. A low culture temperature significantly upregulated genes encoding UPR-specific transcriptional activators (xbp1s, ddit3, and atf5) and ER-resident proteins (grp78, grp94, trib3, and ero1α), that are associated with folding and processing of proteins within the ER. Further, low culture temperature decreased expression of genes involved in ERAD (edem3, sels, herpud1, and syvn1) indicating a decreased potential for protein degradation.

Improvement of Intracellular Traffic System by Overexpression of KDEL Receptor 1 in Antibody-Producing CHO Cells

The localization of soluble endoplasmic reticulum (ER) chaperones in the cell organelle is mediated by the C-terminal KDEL (lysine, aspartic acid, glutamic acid and leucine) motif. This motif is recognized by the KDEL receptor, a seven-transmembrane protein that cycles between the ER and cis-Golgi to capture missorted KDEL chaperones from post-ER compartments in a pH-dependent manner. The KDEL receptor's target chaperones have a substantial role in protein folding and assembly. In this study, the gene expression level of KDEL receptor 1 shows a moderate upregulation during either ER stress or growth of Chinese hamster ovary (CHO) cells in batch culture, while the ER chaperones show higher upregulation. This might indicate the possibility of saturation of the ER retention machinery or at least hindered retention during late stage batch culture in recombinant CHO cells. KDELR1 is overexpressed in a monoclonal antibody-producing CHO cell line to improve the intracellular chaperone retention rate in the ER. An increase in the specific productivity of IgG1 by 13.2% during the exponential phase, and 23.8% in the deceleration phase of batch culture is observed. This is the first study to focus on the ER retention system as a cell engineering target for enhancing recombinant protein production.

Analysis of the immunoglobulin G (IgG) secretion efficiency in recombinant Chinese hamster ovary (CHO) cells by using Citrine-fusion IgG

Biopharmaceuticals represented by immunoglobulin G (IgG) are produced by the cultivation of recombinant animal cells, especially Chinese hamster ovary (CHO) cells. It is thought that the intracellular secretion process of IgG is a bottleneck in the production of biopharmaceuticals. Many studies on the regulation of endogenous secretory protein expression levels have shown improved productivity. However, these strategies have not universally improved the productivity of various proteins. A more rational and efficient establishment of high producer cells is required based on an understanding of the secretory processes in IgG producing CHO cells. In this study, a CHO cell line producing humanized IgG1, which was genetically fused with fluorescent proteins, was established to directly analyze intracellular secretion. The relationship between the amount of intracellular and secreted IgG was analyzed at the single cell level by an automated single-cell analysis and isolation system equipped with dual color fluorescent filters. The amounts of intracellular and secreted IgG showed a weak positive correlation. The amount of secreted IgG analyzed by the system showed a weak negative linear correlation with the specific growth of isolated clones. An immunofluorescent microscopy study showed that the established clones could be used to analyze the intracellular secretion bottleneck. This is the first study to report the use of fluorescent protein fusion IgG as a tool to analyze the secretion of recombinant CHO cells.

Secretion analysis of intracellular "difficult-to-express" immunoglobulin G (IgG) in Chinese hamster ovary (CHO) cells

The Chinese hamster ovary (CHO) cell line is the most widely used host cell for therapeutic antibody production. Although its productivity has been improved by various strategies to satisfy the growing global demand, some difficult-to-express (DTE) antibodies remain at low secretion levels. To improve the production of various therapeutic antibodies, it is necessary to determine possible rate-limiting steps in DTE antibody secretion in comparison with other high IgG producers. Here, we analyzed the protein secretion process in CHO cells producing the DTE immunoglobulin G (IgG) infliximab. The results from chase assays using a translation inhibitor revealed that infliximab secretion could be nearly completed within 2 h, at which time the cells still retained about 40% of heavy chains and 65% of light chains. Using fluorescent microscopy, we observed that these IgG chains remained in the endoplasmic reticulum and Golgi apparatus. The cells inefficiently form fully assembled heterodimer IgG by making LC aggregates, which may be the most serious bottleneck in the production of DTE infliximab compared with other IgG high producers. Our study could contribute to establish the common strategy for constructing DTE high-producer cells on the basis of rate-limiting step analysis.