Influence of alterations in culture condition and changes in perfusion parameters on the retention performance of a 20 mum spinfilter during a perfusion cultivation of a recombinant CHO cell line in pilot scale.

Since 1969 much attention has been devoted to the useof spinfilter systems for retention of mammalian cellsin continuous perfusion cultivations. Previousinvestigations dealt with hydrodynamic conditions,fouling processes and upscaling. But hydrodynamicconditions and fouling processes seem to have asecondary importance in spinfilter performance duringauthentic perfusion cultivations. Obviously,alterations in culture condition are more relevantespecially during long-term processes. Therefore, ourpratical approach focussed on the performance qualityof a commercially available 20 mum spinfilterduring a perfusion cultivation of a recombinant CHOcell line in pilot scale regarding the followingissues: 1) retention of viable cells in thebioreactor; 2) removal of dead cells and cell debrisfrom the bioreactor; 3) alterations in culturecondition; and 4) changes in perfusion mode.Furthermore, we tested the performance of 20 mumspinfilters in 2 and 100 l pilot scale using solidmodel particles instead of cells. Our investigationsshowed that retention of viable cells in pilot scalewas independent of spinfilter rotation velocity andperfusion rate; the retention increased from 75 to 95%corresponding to operation time, enlarging celldiameter and enhanced formation of aggregates in theculture during the perfusion cultivation. By means ofthe Cell Counter and Analyzer System (CASY) anoperation cut off of 13 mum was determined forthis spinfilter. Using solid model particles in 2 lscale, optimal retention was achieved at a tip speedof 0.43 m s(-1) (141 rpm) - furtherenhancement of spinfilter rotation velocity up to0.56 m s(-1) (185 rpm) decreased the retentionrapidly. In pilot scale best retention performance wasobtained with tip speeds of 0.37 m s(-1)(35 rpm) and 1.26 m s(-1) (120 rpm). Hence,significant retention in pilot scale could already beachieved with low agitation. Therefore, the additionof shear force protectives could be avoided so thatthe purification of the target protein from thesupernatant would be facilitated.

Pilot-scale production of murine monoclonal antibodies in agitated, ceramic-matrix or hollow-fiber cell culture systems.

The purpose of this research was to compare three bioreactor systems for the pilot-scale production of monoclonal antibodies (MAbs). We needed to produce gram quantities of murine MAbs against human prostatic acid phosphatase for use in fragmentation, radiolabeling and in vivo radio-imaging studies. The stable hybridoma cell line secreting IgG1 antibodies was chosen for production. Of the available bioreactor systems, we chose to test an agitating 30-liter bioreactor in repeated batch mode, a ceramic-matrix bioreactor in both repeated batch and continuous perfusion modes and a hollow-fiber bioreactor in continuous perfusion mode. The highest cultured MAb yield, 151 +/- 126 mg/day (mean +/- SD, n = 22), was achieved in the 30-liter bioreactor in repeated batch mode with the MAb being harvested in a large volume of medium, giving a reactor productivity of 4.3 +/ 3.4 mg/liter/day (mean +/- SD, n = 22). The most concentrated MAb was harvested from the continuously perfused hollow-fiber bioreactor, which had the highest reactor productivity, 307 +/ 142 mg/liter/day (mean +/- SD, n = 47) and an average rate of MAb production of 55.3 +/- 25.7 mg/day (mean +/- SD, n = 47). Taking the use of serum into consideration, the cost of MAb production was lowest in the continuously fed and harvested ceramic-matrix and hollow-fiber cell culture systems. A compact blood glucose meter proved to be a novel and suitable device for the rapid monitoring of glucose concentrations in hybridoma cultures.

Agitation, aeration and perfusion modules for cell culture bioreactors.

For an optimized bioreactor design which is adapted to the cultivation of sensitive animal cells different modular bioreactor components for gentle agitation, sufficient aeration and long-term perfusion were developed and investigated with respect to their suitability from laboratory to production scale. Aeration systems have been designed for both shear sensitive cells and cells which tolerate bubbles. The systems are based on either membranes for bubble-free aeration or stainless steel sparger systems. They were characterized by determination of their oxygen transfer capacity and optimized in cultivation processes of different cell lines under process conditions such as batch and perfusion mode. Different impellers for suspension cells and cells grown on carriers were investigated for their suitability to ensure homogeneous gentle mixing. A large pitch blade impeller as well as a novel 3-blade segment impeller are appropriate for homogeneous mixing at low shear rates. Especially with the 3-blade segment impeller fluid mechanical stress can be reduced at a given stirrer speed which is advantageous for the cultivation of cells attached to microcarriers or extremely shear sensitive suspension cells. However, our results indicate that shear sensitivity of animal cells has been generally overestimated. Continuous perfusion of both suspension cell cultures and cells cultivated on microcarriers could be successfully performed over extended periods of time using stainless steel spinfilters with appropriate pore sizes and systems based on microporous hydrophilic membranes. Spinfilters are suitable cell retention systems for technical scale bioreactors allowing continuous perfusion cultures of suspension cells (pore size 10 to 20 microns) as well as anchorage dependent cells grown on microcarriers (pore size 75 microns) over six weeks to 3 months. Applying the developed modules for agitation, aeration and perfusion process adapted bioreactor set-ups can be realized which ensure optimum growth and product formation conditions in order to maximize cell and product yields.

On-line monitoring of monoclonal antibody formation in high density perfusion culture using FIA.

An automated flow injection system for on-line analysis of proteins in real fermentation fluids was developed by combining the principles of stopped-flow, merging zones flow injection analysis (FIA) with antigen-antibody reactions. IgG in the sample reacted with its corresponding antibody (a-IgG) in the reagent solution. Formation of insoluble immunocomplexes resulted in an increase of the turbidity which was determined photometrically. This system was used to monitor monoclonal antibody production in high cell density perfusion culture of hybridoma cells. Perfusion was performed with a newly developed static filtration unit equipped with hydrophilic microporous tubular membranes. Different sampling devices were tested to obtain a cell-free sample stream for on-line product analysis of high molecular weight (e.g., monoclonal antibodies) and low molecular weight (e.g., glucose, lactate) medium components. In fermentation fluids a good correlation (coefficient: 0.996) between the FIA method and an ELISA test was demonstrated. In a high density perfusion cultivation process mAb formation was successfully monitored on-line over a period of 400 h using a reliable sampling system. Glucose and lactate were measured over the same period of time using a commercially available automatic analyser based on immobilized enzyme technology.

Multistage production of Autographa californica nuclear polyhedrosis virus in insect cell cultures.

The aim of our study was to establish an efficient system for the in vitro production of the insect pathogenic Autographa californica nuclear polyhedrosis virus in a Spodoptera frugiperda cell line. We optimized cultivation conditions for cell proliferation as well as for virus replication in a 1.5 litre stirred tank bioreactor. Cell and virus propagation were found to be optimal at a constant oxygen tension of 40%. In order to provide sufficient nutrients during virus synthesis filtration and perfusion devices were connected to the bioreactor. A virus production procedure in a repeated batch mode by using a two stage bioreactor system is described. Stage I was optimized for cell production and stage II for virus production.

Purification of proteins from cell culture supernatants.

Desired proteins excreted by animal cells usually reach rather low concentrations in the culture supernatant and have to be purified from an excess of serum proteins which are added to the animal cell culture to maintain its viability and/or productivity. Defined media offer among others the advantage of an easier purification procedure. In addition lysis of cells may contribute also to the complexity of the protein mixture encountered. If fetal calf serum or new born calf serum is used in cultivation, bovine serum albumin and globulins will be the most abundant protein in the supernatant. The interaction of albumin especially with hydrophobic proteins represents significant problems for the effective purification of minor constituent. Isolation of a desired protein follows the general scheme: concentration: by precipitation, ultrafiltration, batch adsorption or partition in aqueous phase system; enrichment: by chromatography or partition; high resolution purification: by chromatography and/or immuno adsorption; final concentration and finishing: pyrogen removal, sterilization, formulation. Biochemical engineering aspects of proteins purification are discussed using human interferon-beta produced in a recombinant mouse cell line as an example. The process developed encloses extraction of interferon-beta from the production medium in aqueous two-phase systems and subsequent recovery of interferon-beta from the top phase by high pressure liquid chromatography.