Flow cytometric study of cultured mammalian cells

Online flow cytometry for monitoring apoptosis in mammalian cell cultures as an application for process analytical technology

Apoptosis is the main driver of cell death in bioreactor suspension cell cultures during the production of biopharmaceuticals from animal cell lines. It is known that apoptosis also has an effect on the quality and quantity of the expressed recombinant protein. This has raised the importance of studying apoptosis for implementing culture optimization strategies. The work here describes a novel approach to obtain near real time data on proportion of viable, early apoptotic, late apoptotic and necrotic cell populations in a suspension CHO culture using automated sample preparation in conjunction with flow cytometry. The resultant online flow cytometry data can track the progression of apoptotic events in culture, aligning with analogous manual methodologies and giving similar results. The obtained near-real time apoptosis data are a significant improvement in monitoring capabilities and can lead to improved control strategies and research data on complex biological systems in bioreactor cultures in both academic and industrial settings focused on process analytical technology applications.

Automated flow cytometry for monitoring CHO cell cultures

Flow cytometry has been used to accurately monitor cell events that indicate the spatio-temporal state of a bioreactor culture. The introduction of process analytical technology (PAT) has led to process improvements using real-time or semi real-time monitoring systems. Integration of flow cytometry into an automated scheme for improved process monitoring can benefit PAT in bioreactor-based biopharmaceutical productions by establishing optimum process conditions and better quality protocols. Herein, we provide detailed protocols for establishing an automated flow cytometry system that can be used to investigate and monitor cell growth, viability, cell size, and cell cycle data. A method is described for the use of such a system primarily focused on CHO cell culture, although it is foreseen the information gathered from automated flow cytometry can be applied to a variety of cell lines to address both PAT requirements and gain further understanding of complex biological systems.

Nanoparticle uptake measured by flow cytometry

The uptake of nanoparticles by cells is an important factor to assess nanotoxicity. In general, the nanoparticles taken up by the cells have been identified by transmission electron microscope, inductively coupled plasma mass spectrometry, etc.; however, the methods required an immense amount of time and effort. Flow cytometry (FCM) has been used and developed in the fields of biochemistry and clinical hematology, and has advantages to analyze thousands of cells in seconds. We recently clarified that the side-scatter(ed) light of FCM could be used as a guide to measure uptake potential of nanoparticles. Here, we describe the protocol for screening of the uptake potential of nanoparticles using FCM.

Selection methods for high-producing mammalian cell lines

The selection of high-producing mammalian cell lines represents a bottleneck in process development for the production of biopharmaceuticals. Traditional methods are time consuming (development times often exceed six months) and significantly limited by the number of clones that can be feasibly screened. The market for therapeutic proteins is set to double by 2010, so there is an increasing need to develop methods for the selection of mammalian cell lines stably expressing recombinant products at high levels in an efficient, cost-effective and high-throughput manner. Alternatives include higher throughput methods based on flow-cytometric screening and recently developed automated systems for the selection of high-producing cell lines.

Feasibility studies of large scale production of human anti-tetanus toxoid monoclonal antibodies

The feasibility of large scale production of human anti-tetanus toxoid monoclonal antibody for therapeutic use was evaluated using a human heterohybridoma. The effects of duration of subculture, transition from static to agitated culture conditions and the level of serum concentration were studied. The level of antibody secreted by the clone decreased with increasing length of subculture and decreasing serum concentration. The clone exhibited heterogeneity in expression of surface IgG after 2 or 7 weeks of subculture in static culture conditions irrespective of the serum concentration. However, a prolonged duration of subculture (9 weeks) in 3% serum medium had an effect on the expression of surface IgG both in static and agitated culture conditions. With respect to total (surface and intracellular) IgG, two distinct cell populations were observed. On long term subculture (9 weeks) in low serum medium (3% FCS), there was a decrease in the population which was the high synthesizer. In addition, when these cells were cultivated in agitated spinner flasks, a defect in secretion of antibodies was observed. Thus a general fall in the amount of antibody in the supernatant of agitated cultures was due to decrease in antibody synthesis as well as the defect in secretion of antibodies.

Use of intracellular pH and annexin-V flow cytometric assays to monitor apoptosis and its suppression by bcl-2 over-expression in hybridoma cell culture

Accurate identification and quantitation of apoptosis is essential for developing efficient strategies for optimisation of culture viability and productivity in cell lines of industrial significance. We have examined the possibility of using carboxy-seminaphthorhodafluor-1-acetoxymethylester (carboxy SNARF-1-AM), a pH sensitive fluoroprobe and FITC-labelled annexin V (AV), a probe specific to phosphatidylserine exposed on the surface of apoptotic cells, to monitor apoptosis and to determine the relationship between intracellular pH (pHi), apoptosis and cell cycle in hybridoma cells. Temporal changes in the distribution of proliferative capacity (S phase), metabolic activity (pHi), and cell death population dynamics were effectively and reliably determined using flow cytometry. Intracellular acidification was shown to precede the occurrence of apoptosis during batch culture and after treatment with campothecin, staurosporine and under adverse bioreactor conditions such as glutamine deprivation and oxygen deficiency. These results showed that the decrease in pHi can be used as an indicator of cellular deterioration and cell death. AV in combination with propidium iodide permitted the identification of viable, transient apoptotic and necrotic cells in heterogeneous cultures of control (PEF) cells. Hybridoma cells over-expressing bcl-2 were protected from intracellular acidification and phosphatidylserine exposure, which was associated with the suppression of apoptosis in these cells. A decrease in pHi was apparent even before the accumulation of the normally acidic G1 phase and the development of a sub-G1 region, characteristic of apoptotic cell behaviour. The pHi assay can therefore be used as a tool to predict future cell culture performance. reserved.

Decreased mitochondrial function in quiescent cells isolated from multicellular tumor spheroids

Cells in the inner region of multicellular spheroids markedly reduce their oxygen consumption rate, presumably in response to their stressful microenvironment. To determine the mechanism behind this metabolic adaptation, we have investigated relative mitochondrial mass and mitochondrial function in cells isolated from different regions of tumor spheroids by using a combination of mitochondrial-specific fluorescent stains and flow cytometric analysis. Uptake of rhodamine 123 (R123) is driven by the mitochondrial membrane potential and thus reflects mitochondrial activity. Uptake of 10-nonyl-acridine orange (NAO) reflects total mitochondrial mass independently of activity because this compound binds to cardiolipin in the inner mitochondrial membrane. NAO fluorescence per unit cell volume only decreased 10-20% for cells from the inner spheroid region compared with those near the surface. There was greater than a twofold reduction in R123 fluorescence in the inner region cells, however. Thus, tumor cells in spheroids alter their rate of respiration predominately by downregulating mitochondrial function as opposed to degradation of mitochondria. There was a correlation between R123 staining per unit cell volume and the growth fraction of the cells from spheroids, but not for monolayer cultures. We also show a linear correlation between R123 staining and the rate of oxygen consumption for both monolayer- and spheroid-derived cells. After separating the inner region cells from the spheroid and replating them in monolayer culture, the R123 uptake recovered to normal levels prior to entry of the cells into S-phase. This reduction in mitochondrial function in quiescent cells from spheroids can explain the long period required for these cells to re-enter the cell cycle and may have important implications for the regulation of tumor cell oxygenation in vivo.

Correlation between secreted and membrane-bound IgG in mouse myeloma cells transfected with chimeric immunoglobulin heavy and light chain genes

Mouse myeloma cells were transfected with pSV2-gpt and pSV2-neo based immunoglobulin expression vectors. Double transfectants were selected using the xanthine-guanine phosphoribosyl transferase (gpt) and the neomycin (neo) selection marker genes. A broad distribution in the level of mouse-human chimeric IgG expression was observed with series of independently isolated transfectoma clones. The relative amounts of secreted to membrane-bound antibodies correlated closely, which suggested, that fluorescence-activated cell sorting could be a valuable tool for the selection of high-yielding production cell lines. However, a single cycle of cell sorting did not steer the cloning process significantly toward cells that produce enhanced amounts of recombinant IgG. Only in cases in which the polyclonal transfectoma population contained a large percentage of nonproducing cells, these were successfully separated from the IgG-producing cell population. (c) 1996 John Wiley & Sons, Inc.

Optimizing antibody production in batch hybridoma cell culture

Optimizing productivity by hybridoma cells relies partly on developing suitable methods for screening and selection of high producing cultures and on understanding regulation of antibody production. In this study, the behavior of hybridoma cells in batch culture was investigated using flow cytometry, and a simple model for antibody production was used to explain production data obtained from these cultures. Surface antibody fluorescence values were found to closely follow the decreasing trend of specific antibody secretion rate over the course of several batch cultures. Therefore, for the hybridoma cell lines studied here (ATCC HB124 and TIB138), surface immunofluorescence levels can be used to select high producing cells as well as to monitor culture productivity. Surface and intracellular antibody fluorescence values were also found to be correlated for cells exhibiting a bimodal distribution with respect to intracellular antibody content. The population of cells containing a bimodal distribution with respect to intracellular antibody content. The population of cells containing lower levels of intracellular antibody was determined to secrete significantly less antibody than the population possessing high intracellular antibody concentrations. Factors which influence antibody production rates and possible strategies for optimizing monoclonal antibody yield are discussed.

Peptone, a low-cost growth-promoting nutrient for intensive animal cell culture

The effect of addition of peptone to serum-free and serum supplemented media for the growth of hybridoma cells in various systems was studied. Supplementation of defined medium with either proteose peptone or meat peptone resulted in significant increases in cell number and specific monoclonal antibody production in batch culture system. Other peptones were either inactive or less effective. In continuous culture, using medium supplemented with new born calf serum, the addition of peptone resulted in 125% and 150% increases in cell and antibody concentrations respectively. Similar increase in cell number (128%) was also obtained in spin-filter perfusion culture when medium was supplemented with peptone. By comparison, the substitution of a defined 1 x MEM amino acids mixture resulted in only a 50% increase. At higher perfusion rates the cell number maintained in steady state using peptone supplement could be increased to 1.3 x 10(7) cells ml-1 while the serum concentration was reduced from 5% to 1% at a perfusion rate of 2.5 volumes per day.

A flow cytometric study of hydrodynamic damage to mammalian cells

Flow cytometry has been used to study the mechanisms of damage to mammalian cells by hydrodynamic forces. Cell damage resulted from the stresses created both by bubble entrainment and by bubble bursting caused by vortex formation in highly agitated culture. Damage to the antigen molecules on the cell surface together with increasing leakage and passive transport were observed. Pluronic F-68 reduced the efflux of fluorescein out of cells suggesting the possibility of plugging damaged parts of the membrane or change in membrane molecular organisation. Surface-associated immunoglobulin molecules were also protected by Pluronic F-68. The loss of plasma membrane integrity was followed by the destruction of cytoplasmic matrix. The nuclei were last to be destroyed. The fraction of metabolically active cells was drastically decreased by the intense hydrodynamic forces. However the mitochondrial transmembrane potential, which is linked to the ATP requirements of cells was well as the mean cell size of the live cell population, remained unaffected by the stressful hydrodynamic conditions. F-Actin content peaked during the early exponential phase and declined during the mid and late exponential and death phases of batch culture. The increase of actin during exponential phase was faster in stationary culture than in agitated culture. The increase was associated with the proliferative rate. Furthermore, the response to hydrodynamic forces was not related either to cell size difference or to the stage in the cell cycle.

Comparison of a quadroma and its parent hybridomas in fed batch culture

A quadroma (#22 x 63), formed by the fusion of two hybridomas, and its parent hybridomas (#22 and FMC 63) were each grown in fed batch cultures in order to examine the change in antibody productivity over time of the quadroma compared to its parent hybridomas. The growth rate, glucose uptake rate and lactate production rate of the quadroma were found to be intermediate between those of its parent cells of origin. The specific antibody productivity and internal antibody content of the quadroma followed the same decreasing trends over time as those seen in both parent hybridomas. Losses in specific antibody production rate and antibody content, however, occurred at a faster rate for the quadroma than for either of its parent hybridomas. Although the growth of a non-producing subpopulation is presumed to account for the drop in antibody production, there was no direct correlation between the percentage of high antibody containing cells, as determined by flow cytometry, and the specific antibody production rate.

Immuno- and flow cytometric analytical methods for biotechnological research and process monitoring

In this article, the applications of immunoanalysis and flow cytometry for research and process monitoring in biotechnology are discussed. Brief reviews of the two analytical methods are followed by descriptions of actual applications in various areas of biotechnology. In the case of immunoanalysis, emphasis is placed on systems for on-line bioprocess monitoring, and examples are given for a thermostable pullulanase, a mouse IgG, and antithrombin III. Although flow cytometry is not currently an on-line analytical technique, its value as an off-line method is illustrated by examples of the measurement of shear stress effects, lipid content, and sterol content.

Specific monoclonal antibody productivity and the cell cycle-comparisons of batch, continuous and perfusion cultures

A selection of mouse hybridoma cell lines showed a variation of approximately two orders of magnitude in intracellular monoclonal antibody contents. The different levels directly influenced apparent specific monoclonal antibody productivity during the death phase but not during the growth phase of a batch culture. The pattern of changes in specific productivity during culture remained basically similar even though at different levels for all cell lines tested. Arresting the cells in the G1 phase using thymidine increased the specific productivity, cell volume and intracellular antibody content but at the same time led to decreased viability. In continuous culture DNA synthesis decreased with decreasing dilution rate though without an accompanying change in cell cycle and cell size distributions. The data shows both the decrease in viability and intracellular antibody content to be important factors which influence the negative association between specific antibody productivity and growth rate. In high cell density perfusion culture, when the cell cycle was prolonged by slow growth, viability was low and dead, but not lysed, cells were retained in the system, the specific antibody productivity was nearly two fold higher than that obtained in either batch or continuous cultures. The results imply that the prolongation of G1 phase and the increase in death rate of cells storing a large amount of antibody together cause an apparent increase in specific antibody productivity.