The characterization of host cell protein (HCP) content during the production of therapeutic recombinant proteins is an important aspect in the drug development process. Despite this, key components of the HCP profile and how this changes with processing has not been fully investigated. Here we have investigated the supernatant HCP profile at different times throughout culture of a null and model GS-CHO monoclonal antibody producing mammalian cell line grown in fed-batch mode. Using 2D-PAGE and LC-MS/MS we identify a number of intracellular proteins (e.g., protein disulfide isomerise; elongation factor 2; calreticulin) that show a significant change in abundance relative to the general increase in HCP concentration observed with progression of culture. Those HCPs that showed a significant change in abundance across the culture above the general increase were dependent on the cell line examined. Further, our data suggests that the majority of HCPs in the supernatant of the cell lines investigated here arise through lysis or breakage of cells, associated with loss in viability, and are not present due to the secretion of protein material from within the cell. SELDI-TOF and principal components analysis were also investigated to enable rapid monitoring of changes in the HCP profile. SELDI-TOF analysis showed the same trends in the HCP profile as observed by 2D-PAGE analysis and highlighted biomarkers that could be used for process monitoring. These data further our understanding of the relationship between the HCP profile and cell viability and may ultimately enable a more directed development of purification strategies and the development of cell lines based upon their HCP profile.

Recombinant protein products such as monoclonal antibodies (mAbs) for use in the clinic must be clear of host cell impurities such as host cell protein (HCP), DNA/RNA, and high molecular weight immunogenic aggregates. Despite the need to remove and monitor HCPs, the nature, and fate of these during downstream processing (DSP) remains poorly characterized. We have applied a proteomic approach to investigate the dynamics and fate of HCPs in the supernatant of a mAb producing cell line during early DSP including centrifugation, depth filtration, and protein A capture chromatography. The primary clarification technique selected was shown to influence the HCP profile that entered subsequent downstream steps. MabSelect protein A chromatography removed the majority of contaminating proteins, however using 2D-PAGE we could visualize not only the antibody species in the eluate (heavy and light chain) but also contaminant HCPs. These data showed that the choice of secondary clarification impacts upon the HCP profile post-protein A chromatography as differences arose in both the presence and abundance of specific HCPs when depth filters were compared. A number of intracellularly located HCPs were identified in protein A elution fractions from a Null cell line culture supernatant including the chaperone Bip/GRP78, heat shock proteins, and the enzyme enolase. We demonstrate that the selection of early DSP steps influences the resulting HCP profile and that 2D-PAGE can be used for monitoring and identification of HCPs post-protein A chromatography. This approach could be used to screen cell lines or hosts to select those with reduced HCP profiles, or to identify HCPs that are problematic and difficult to remove so that cell-engineering approaches can be applied to reduced, or eliminate, such HCPs.

Protein A chromatography is a critical and 'gold-standard' step in the purification of monoclonal antibody (mAb) products. Its ability to remove >98% of impurities in a single step alleviates the burden on subsequent process steps and facilitates the implementation of platform processes, with a minimal number of chromatographic steps. Here, we have evaluated four commercially available protein A chromatography matrices in terms of their ability to remove host cell proteins (HCPs), a complex group of process related impurities that must be removed to minimal levels. SELDI-TOF MS was used as a screening tool to generate an impurity profile fingerprint for each resin and indicated a number of residual impurities present following protein A chromatography, agreeing with HCP ELISA. Although many of these were observed for all matrices there was a significantly elevated level of impurity binding associated with the resin based on controlled pore glass under standard conditions. Use of null cell line supernatant with and without spiked purified mAb demonstrated the interaction of HCPs to be not only with the resin back-bone but also with the bound mAb. A null cell line column overload and sample enrichment method before 2D-PAGE was then used to determine individual components associated with resin back-bone adsorption. The methods shown allow for a critical analysis of HCP removal during protein A chromatography. Taken together they provide the necessary process understanding to allow process engineers to identify rational approaches for the removal of prominent HCPs.

We have developed a simple and robust transient expression system utilizing the 25 kDa branched cationic polymer polyethylenimine (PEI) as a vehicle to deliver plasmid DNA into suspension-adapted Chinese hamster ovary cells synchronized in G2/M phase of the cell cycle by anti-mitotic microtubule disrupting agents. The PEI-mediated transfection process was optimized with respect to PEI nitrogen to DNA phosphate molar ratio and the plasmid DNA mass to cell ratio using a reporter construct encoding firefly luciferase. Optimal production of luciferase was observed at a PEI N to DNA P ratio of 10:1 and 5 mug DNA 10(6) cells(-1). To manipulate transgene expression at mitosis, we arrested cells in G2/M phase of the cell cycle using the microtubule depolymerizing agent nocodazole. Using secreted human alkaline phosphatase (SEAP) and enhanced green fluorescent protein (eGFP) as reporters we showed that continued inclusion of nocodazole in cell culture medium significantly increased both transfection efficiency and reporter protein production. In the presence of nocodazole, greater than 90% of cells were eGFP positive 24 h post-transfection and qSEAP was increased almost fivefold, doubling total SEAP production. Under optimal conditions for PEI-mediated transfection, transient production of a recombinant chimeric IgG4 encoded on a single vector was enhanced twofold by nocodazole, a final yield of approximately 5 microg mL(-1) achieved at an initial viable cell density of 1 x 10(6) cells mL(-1). The glycosylation of the recombinant antibody at Asn297 was not significantly affected by nocodazole during transient production by this method.

In this study we describe optimization of polyethylenimine (PEI)-mediated transient production of recombinant protein by CHO cells by facile manipulation of a chemically defined culture environment to limit accumulation of nonproductive cell biomass, increase the duration of recombinant protein production from transfected plasmid DNA, and increase cell-specific production. The optimal conditions for transient transfection of suspension-adapted CHO cells using branched, 25 kDa PEI as a gene delivery vehicle were experimentally determined by production of secreted alkaline phosphatase reporter in static cultures and recombinant IgG4 monoclonal antibody (Mab) production in agitated shake flask cultures to be a DNA concentration of 1.25 microg 10(6) cells(-1) mL(-1) at a PEI nitrogen:DNA phosphate ratio of 20:1. These conditions represented the optimal compromise between PEI cytotoxicity and product yield with most efficient recombinant DNA utilization. Separately, both addition of recombinant insulin-like growth factor (LR3-IGF) and a reduction in culture temperature to 32 degrees C were found to increase product titer 2- and 3-fold, respectively. However, mild hypothermia and LR3-IGF acted synergistically to increase product titer 11-fold. Although increased product titer in the presence of LR3-IGF alone was solely a consequence of increased culture duration, a reduction in culture temperature post-transfection increased both the integral of viable cell concentration (IVC) and cell-specific Mab production rate. For cultures maintained at 32 degrees C in the presence of LR3-IGF, IVC and qMab were increased 4- and 2.5-fold, respectively. To further increase product yield from transfected DNA, the duration of transgene expression in cell populations maintained at 32 degrees C in the presence of LR3-IGF was doubled by periodic resuspension of transfected cells in fresh media, leading to a 3-fold increase in accumulated Mab titer from approximately 13 to approximately 39 mg L(-1). Under these conditions, Mab glycosylation at Asn297 remained essentially constant and similar to that of the same Mab produced by stably transfected GS-CHO cells. From these data we suggest that the efficiency of transient production processes (protein output per rDNA input) can be significantly improved using a combination of mild hypothermia and growth factor(s) to yield an extended "activated hypothermic synthesis".

This article describes how a combination of an ultra scale-down (USD) shear device feeding a microwell centrifugation plate may be used to provide a prediction of how mammalian cell broth will clarify at scale. In particular a method is described that is inherently adaptable to a robotic platform and may be used to predict how the flow rate and capacity (equivalent settling area) of a centrifuge and the choice of feed zone configuration may affect the solids carry over in the supernatant. This is an important consideration as the extent of solids carry over will determine the required size and lifetime of a subsequent filtration stage or the passage of fine particulates and colloidal material affecting the performance and lifetime of chromatography stages. The extent of solids removal observed in individual wells of a microwell plate during centrifugation is shown to correlate with the vertical and horizontal location of the well on the plate. Geometric adjustments to the evaluation of the equivalent settling area of individual wells (Sigma(M)) results in an improved prediction of solids removal as a function of centrifuge capacity. The USD centrifuge settling characteristics need to be as for a range of equivalent flow rates as may be experienced at an industrial scale for a machine of different shear characteristics in the entry feed zone. This was shown to be achievable with two microwell-plate based measurements and the use of varying fill volumes in the microwells to allow the rapid study of a fivefold range of equivalent flow rates (i.e., at full scale for a particular industrial centrifuge) and the effect of a range of feed configurations. The microwell based USD method was used to examine the recovery of CHO-S cells, prepared in a 5 L reactor, at different points of growth and for different levels of exposure to shear post reactor. The combination of particle size distribution measurements of the cells before and after shear and the effect of shear on the solids remaining after centrifugation rate provide insight into the state of the cells throughout the fermentation and the ease with which they and accumulated debris may be removed by continuous centrifugation. Hence bioprocess data are more readily available to help better integrate cell culture and cell removal stages and resolve key bioprocess design issues such as choice of time of harvesting and the impact on product yield and contaminant carry over. Operation at microwell scale allows data acquisition and bioprocess understanding over a wide range of operating conditions that might not normally be achieved during bioprocess development.

The manufacture of complex therapeutic proteins using mammalian cells is well established, with several strategies developed to improve productivity. The application of sustained mild hypothermic conditions during culture has been associated with increases in product titer and improved product quality. However, despite associated cell physiological effects, very few studies have investigated the impact on downstream processing (DSP). Characterization of cells grown under mild hypothermic conditions demonstrated that the stationary phase was prolonged by delaying the onset of apoptosis. This enabled cells to maintain viability for extended periods and increase volumetric productivity from 0.74 to 1.02 g L−1. However, host cell proteins, measured by ELISA, increased by ∼50%, attributed to the extended time course and higher peak and harvest cell densities. The individual components making up this impurity, as determined by SELDI-TOF MS and 2D-PAGE, were shown to be largely comparable. Under mild hypothermic conditions, cells were less shear sensitive than those maintained at 37°C, enhancing the preliminary primary recovery step. Adaptive changes in membrane fluidity were further investigated by adopting a pronounced temperature shift immediately prior to primary recovery and the improvement observed suggests that such a strategy may be implementable when shear sensitivity is of concern. Early and late apoptotic cells were particularly susceptible to shear, at either temperature, even under the lowest shear rate investigated. These findings demonstrate the importance of considering the impact of cell culture strategies and cell physiology on DSP, by implementing a range of experimental methods for process characterization. © 2013 American Institute of Chemical Engineers Biotechnol. Prog., 29:688–696, 2013

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