ONLY A SMALL FRACTION OF CELLS PRODUCE ASSEMBLED CAPSIDS DURING TRANSFECTION-BASED MANUFACTURING OF ADENO-ASSOCIATED VIRUS VECTORS

Plasmid transfection of mammalian cells is the dominant platform used to produce adeno‐associated virus (AAV) vectors for clinical and research applications. Low yields from this platform currently make it difficult to supply these activities with adequate material. In an effort to better understand the current limitations of transfection‐based manufacturing, this study examines what proportion of cells in a model transfection produce appreciable amounts of assembled AAV capsid. Using conformation‐specific antibody staining and flow cytometry, we report the surprising result that despite obtaining high transfection efficiencies and nominal vector yields in our model system, only 5%–10% of cells appear to produce measurable levels of assembled AAV capsids. This finding implies that considerable increases in vector titer could be realized through increasing the proportion of productive cells. Furthermore, we suggest that the flow cytometry assay used here to quantify productive cells may be a useful metric for future optimization of transfection‐based AAV vector manufacturing platforms.

On-line monitoring of respiration in recombinant-baculovirus infected and uninfected insect cell bioreactor cultures

Respiration rates in Spodoptera frugiperda (Sf-9) cell bioreactor cultures were successfully measured on-line using two methods: The O(2) uptake rate (OUR) was determined using gas phase pO(2) values imposed by a dissolved oxygen controller and the CO(2) evolution rate (CER) was measured using an infrared detector. The measurement methods were accurate, reliable, and relatively inexpensive. The CER was routinely determined in bioreactor cultures used for the production of several recombinant proteins. Simple linear relationships between viable cell densities and both OUR and CER in exponentially growing cultures were used to predict viable cell density. Respiration measurements were also used to follow the progress of baculoviral infections in Sf-9 cultures. Infection led to increases in volumetric and per-cell respiration rates. The relationships between respiration and several other culture parameters, including viable cell density, cell protein, cell volume, glucose consumption, lactate production, viral titer, and recombinant beta-galactosidase accumulation, were examined. The extent of the increase in CER following infection and the time postinfection at which maximum CER was attained were negatively correlated with the multiplicity of infection (MOI) at multiplicities below the level required to infect all the cells in a culture. Delays in the respiration peak related to the MOI employed were correlated with delays in the peak in recombinant protein accumulation. DO levels in the range 5-100% did not exert any major effects on viable cell densities, CER, or product titer in cultures infected with a baculovirus expressing recombinant beta-galactosidase.

Increased production of active human beta(2)-adrenergic/G(alphas) fusion receptor in Sf-9 cells using nutrient limiting conditions

Using the baculovirus/insect-cell expression vector system, we succeeded in obtaining a high yield of active human beta(2)-adrenergic receptor/G(alphas) fusion protein. This was achieved following high cell density production under nutrient-limiting conditions using a very low multiplicity of infection (MOI). This approach was found to significantly reduce inactive protein accumulation that occurred when production was done using conventional high MOI procedures. The maximum specific and volumetric yields of active receptor using this strategy increased by factors of two- and sixfold, respectively. Our results suggest that the increase in the ratio of active/total protein produced results from production under nutrient limitation. Since low multiplicity of infection offers many advantages for large-scale applications, we suggest that this simple production method should be considered when optimizing expression of G-protein-coupled receptors and other complex proteins.

On-line monitoring of physiological parameters of insect cell cultures during the growth and infection process

On-line monitoring of insect cell cultures used for the production of recombinant proteins with the baculovirus expression vector system (BEVS) provides valuable tools for the optimization, operation, and control of the production process. The relative permittivity (epsilon') and CO(2) evolution rates (CER) were measured on-line using the biomass monitor and the infrared CO(2) analyzer, respectively. The growth and infection phases of two different cell lines, Spodoptera frugiperda (Sf-9) and Trichoplusia ni(High-5), were monitored using the above measurements. These in turn were correlated to the progress of the culture by using the off-line measurements of protein produced, virus titer, and biovolume, which is the product of viable cell density and mean cell volume. The epsilon', CER, and the biovolume profiles were closely matched during the growth phase of cells when grown in a batch or fed batch culture. The relationship became more complex when the cultures were either in stationary phase or in the postinfection phase. The epsilon' profile was found to be a good indicator of the process of synchronous baculoviral infection, showing a plateau between 18 and 24 h postinfection (hpi), the period during which budded virus is produced, and a peak at approximately 48 hpi correlated to the onset of accelerated cell lysis. The CER profile continues to increase after the growth period with a peak around the 24 hpi period, after which there is a decline in the profile corresponding to release of virus as seen from virus titer determinations. This was examined for Sf-9 cultures under conditions of cell densities from 3 to 50 x 10(6) cells/mL and MOI values ranging from 0.001 to 1000. The profiles were found to be similar also in the case of the High-5 cells. Thus both measurements give reliable information regarding the physiological status of the cells as seen from their correlation to virus and protein production. A further combination of these with the off-line measured parameters such as the biovolume and metabolite concentrations can give a more detailed understanding of the process and help in the better design and automation of these processes.

Enhanced growth of Sf-9 cells to a maximum density of 5.2 x 10(7) cells per mL and production of beta-galactosidase at high cell density by fed batch culture

Significant improvement in cell growth and protein production has been achieved in Sf-9 insect cell cultures using pulse additions of multicomponent nutrient feed concentrates (Bédard et al., 1994; Chan et al., 1998). The present work focuses on investigating an alternative feeding strategy wherein the nutrients are fed in a semi continuous manner. Fed batch culture experiments were carried out to compare the two different feeding strategies, pulse and semi continuous and a process developed to achieve a cell density of 5.2 x 10(7) cells/mL of Sf-9 cells in a 3.5 L bioreactor. Production of recombinant protein beta-galactosidase was carried out by infecting the cells with baculovirus at a MOI of 10 at cell densities of 17 x 10(6)cells/mL. Specific productivity could be maintained at cell densities as high as 14 x 10(6) cells/mL. The results presented indicate that the feeding method can provide significant improvements in the performance with a reduction in the amount of total nutrients added. On-line monitoring of the culture using the capacitance probe showed that the capacitance probe can be used successfully to monitor the biomass and infection process even at higher cell densities.

Understanding factors that limit the productivity of suspension-based perfusion cultures operated at high medium renewal rates

One of the key parameters in perfusion culture is the rate of medium replacement (D). Intensifying D results in enhanced provision of nutrients, which can lead to an increase in the viable cell density (X(v)). The daily MAb production of hybridoma cells can thus be increased proportionally without modifying the bioreactor scale, provided that both viable cell yield per perfusion rate (Y(Xv/D)) and specific MAb productivity (q(MAb)) remain constant at higher D. To identify factors prone to limit productivity in perfusion, a detailed kinetic analysis was carried out on a series of cultures operated within a D range of 0.48/4.34 vvd (volumes of medium/reactor volume/day) in two different suspension-based systems. In the Celligen/vortex-flow filter system, significant reductions in Y(Xv/D) and q(MAb) resulting from the use of gas sparging were observed at D > 1.57 vvd (X(v) > 15 x 10(6) cells/mL). Through glucose supplementation, we have shown that the decrease in Y(Xv/D) encountered in presence of sparging was not resulting from increased cellular destruction or reduced cell growth, but rather from glucose limitation. Thus, increases in hydrodynamic shear stress imparted to the culture via intensification of gas sparging resulted in a gradual increase in specific glucose consumption (q(glc)) and lactate production rates (q(lac)), while no variations were observed in glutamine-consumption rates. As a result, while glutamine was the sole limiting-nutrient under non-sparging conditions, both glutamine and glucose became limiting under sparging conditions. Although a reduction in q(MAb) was observed at high-sparging rates, inhibition of MAb synthesis did not result from direct impact of bubbles, but was rather associated with elevated lactate levels (25-30 mM), resulting from shear stress-induced increases in q(lac), q(glc), and Y(lac/glc). Deleterious effects of sparging on Y(Xv/D) and q(MAb) encountered in the Celligen/vortex-flow filter system were eliminated in the sparging-free low-shear environment of the Chemap-HRI/ultrasonic filter system, allowing for the maintenance of up to 37 x 10(6) viable cells/mL. A strategy aimed at reducing requirements for sparging in large-scale perfusion cultures by way of a reduction in the oxygen demand using cellular engineering is discussed.

On-line monitoring of the progress of infection in Sf-9 insect cell cultures using relative permittivity measurements

The use of on-line relative permittivity (epsilon') measurements for monitoring cultures of Sf-9 cells was evaluated in a batch culture and a batch infected with a baculovirus expressing beta-galactosidase. It was found that viable cell density and volume essentially accounted for all the variation in epsilon' in both non-infected and synchronously infected cultures, indicating that the epsilon' of a cell suspension was sensitive only to changes in the viable cell population. Additionally the parameter provided clearly defined signposts of the progress of the infection.

Identification and crystallization of a protease-resistant core of calnexin that retains biological activity

Calnexin is a molecular chaperone that facilitates folding of glycoproteins in the endoplasmic reticulum (ER). The cloned lumenal domain of canine calnexin, cnxDeltaTMC, retains its biological activity without the transmembrane and cytosolic region. For the purpose of structure determination we generated a crystallizable core by mild proteolysis and identified its termini by N-terminal sequencing and molecular mass determination. A truncated gene was cloned accordingly. Its product, cnxDeltaN25C15, was purified to apparent homogeneity and crystallized. This truncated variant remains biologically active as shown by its binding to monoglucosylated oligosaccharides and functional interaction with ERp57. A heavy atom derivative was identified.

Use of the Centritech Lab centrifuge for perfusion culture of hybridoma cells in protein-free medium

As part of an effort to develop a suspension-culture perfusion-based process with high flow rate without the fouling and antibody retention inherent to filter-based cell-separation devices, we have evaluated and contributed to the development of the Centritech Lab centrifuge for the perfusion culture of hybridoma cells in protein-free medium. Culture start-ups showed that cell growth and monoclonal-antibody (MAb) production rates were similar in both a spinner flask and continuous centrifugation coupled to a bioreactor. The centrifuge efficiently separated viable cells from dead ones. Viable-cell recoveries were never below 98%, whereas dead-cell recoveries were usually around 80%. The cell content of the centrifuge supernatant and concentrate was strongly determined by the total amount of cells, viable and dead, in the culture broth, but an influence of the centrifugation parameters (feed rate, times of separation and discharge, and rotor speed) was observed. This understanding of the separation process inside the centrifuge is important and may apply to other similar devices. Monoclonal antibodies were not retained in the bioreactor during centrifugation perfusion. However, whereas similar growth rates were obtained in perfusion cultures using either continuous centrifugation or filtration, MAb concentrations were 35% lower in the former case. Utilization of the centrifuge in an intermittent fashion decreased the daily cell residence time outside the bioreactor, the daily pelleted-cell residence time in the centrifuge, and the frequency of cell passage to the centrifuge. This led to higher viable-cell numbers in the bioreactor and an accompanying increase in MAb concentrations, 225-250 mg of IgM L-1, equal to the performance of filter-based perfusion systems with the same cell line. It was hypothesized that having cells periodically packed at the bottom of the centrifuge insert (up to 800 x 10(6) cells mL-1) is deleterious to the culture by exposing the pelleted cells to prolonged nutrient limitations.

Improved yields of the extracellular domain of the epidermal growth factor receptor produced using the baculovirus expression system by medium replacement following infection

The extracellular domain of the epidermal growth factor receptor (EGFR) was expressed using the baculovirus expression vector system. The maximum level of the EGFR extracellular domain secreted into the medium in Sf-9 (Spodoptera frugiperda or fall army-worm) cell batch culture was approximately 2.5 micrograms ml-1. In order to increase this yield, a process was developed that included the following sequence of steps: batch growth to maximum cell density, infection of the cells with recombinant virus, and replacement of spent medium. By using this process, the specific yield of recombinant protein, which in batch culture drops when infection is carried out at densities greater than 3 x 10(6) cells ml-1, can be maintained at a maximum in cultures infected at densities of 10(7) cells ml-1 or greater. The process, when applied to 3-1 and 11-1 bioreactor cultures, allowed a maximum volumetric yield of triple the maximum value attainable in batch culture. Spent-medium analysis indicates that medium replacement provides certain nutrients that could otherwise be limiting for recombinant protein production.