A cellular automaton model for microcarrier cultures

Attachment of 3T3 and MDBK Cells onto PHEMA-Based Microbeads and their Biologically Modified Forms

Polyhydroxyethylmethacrylate (PHEMA) microbeads in a size range of 150-250 μm were prepared by suspension polymerization in an aqueous phase containing magnesium oxide. Hydroxyl groups were oxidized with NaIO4 and cell adhesive proteins, namely collagen and fibronectin, were immobilized using glutaraldehyde. A spacer-arm, hexamethylene diamine, was used in some cases. Higher amounts of collagen were immobilized, than in fibronectin. The attachment of two cell lines (i.e., 3T3 and MDBK cell lines) on these microbeads with a wide variety of surface properties was studied in vitro culture media. The attachments of both cells, even onto plain microbeads, were significant. Introducing both fibronectin and collagen onto the microbeads caused significant increases in the cell attachment. More cells attached to the microbeads carrying fibronectin covalently attached onto the microbeads through the spacer-arm molecules. Fibronectin was better than collagen for high attachment values. The mathematical model proposed successfully simulated attachment kinetics.

Cell Growth on Poly(EGDMA/HEMA) Based Microbeads

Poly(EGDMA/HEMA) copolymeric microbeads were prepared by suspension polymerization. A comonomer, i.e., HEMA, was included in the formula in order to provide functional hydroxyl groups on the microbead surfaces. Toluene was used in the polymerization formulations to introduce porosity into the matrix. Hydroxyl groups were first oxidized with NaIO4, and then two biological molecules, namely collagen and fibronectin were immobilized by using glutaraldehyde. A spacer-arm, i.e., hexamethylene diamine, was also used in some cases. More protein molecules were immobilized onto more swellable microbeads using spacer-arm. Higher amounts of collagen were immobilized, more than fibronectin immobilization. Growth of two cell lines, 3T3 and MDBK, on these microbeads with a wide variety of surface properties was studied in vitro culture media. Growths of both cells even onto the plain microbeads were significant. More cell proliferation occurred with the more swellable microbeads. More cells proliferated on the microbeads carrying fibronectin covalently attached onto the microbeads through spacer-arm molecules. Fibronectin was better than collagen for promoting high proliferation. The mathematical model proposed successfully simulated the growth kinetics.

Quantification and regulation of cell migration

Cell migration is essential for many physiological and pathological processes that include embryonic development, the immune response, wound healing, angiogenesis, and cancer metastasis. It is also important for emerging tissue engineering applications such as tissue reconstitution and the colonization of biomedical implants. By summarizing results from recent experimental and theoretical studies, this review outlines the role played by growth factors or substrate-adhesion molecules in modulating cell motility and shows that cell motility can be an important factor in determining the rates of tissue formation. The application of cell motility assays and the use of theoretical models for analyzing cell migration and proliferation are also discussed.

Characterization of aggregation and protein expression of bovine corneal endothelial cells as microcarrier cultures in a rotating-wall vessel

Rotating-wall vessels are beneficial to tissue engineering in that the reconstituted tissue formed in these low-shear bioreactors undergoes extensive three-dimensional growth and differentiation. In the present study, bovine corneal endothelial (BCE) cells were grown in a high-aspect rotating-wall vessel (HARV) attached to collagen-coated Cytodex-3 beads as a representative monolayer culture to investigate factors during HARV cultivation which affect three-dimensional growth and protein expression. A collagen type I substratum in T-flask control cultures increased cell density of BCE cells at confluence by 40% and altered the expression of select proteins (43, 50 and 210 kDa). The low-shear environment in the HARV facilitated cell bridging between microcarrier beads to form aggregates containing upwards of 23 beads each, but it did not promote multilayer growth. A kinetic model of microcarrier aggregation was developed which indicates that the rate of aggregation between a single bead and an aggregate was nearly 10 times faster than between two aggregate and 60 times faster than between two single beads. These differences reflect changes in collision frequency and cell bridge formation. HARV cultivation altered the expression of cellular proteins (43 and 70 kDa) and matrix proteins (50, 73, 89 and 210 kDa) relative to controls perhaps due to hypoxia, fluid flow or distortion of cell shape. In addition to the insight that this work has provided into rotating-wall vessels, it could be useful in modeling aggregation in other cell systems, propagating human corneal endothelial cells for eye surgery and examining the response of endothelial cells to reduced shear.

Growth behavior of number distributed adherent MDCK cells for optimization in microcarrier cultures

An assay for measuring the number of adherent cells on microcarriers that is independent from dilution errors in sample preparation was used to investigate attachment dynamics and cell growth. It could be shown that the recovery of seeded cells is a function of the specific rates of cell attachment and cell death, and finally a function of the initial cell-to-bead ratio. An unstructured, segregated population balance model was developed that considers individual classes of microcarriers covered by 1-220 cells/bead. The model describes the distribution of initially attached cells and their growth in a microcarrier system. The model distinguishes between subpopulations of dividing and nondividing cells and describes in a detailed way cell attachment, cell growth, density-dependent growth inhibition, and basic metabolism of Madin-Darby canine kidney cells used in influenza vaccine manufacturing. To obtain a model approach that is suitable for process control applications, a reduced growth model without cell subpopulations, but with a formulation of the specific cell growth rate as a function of the initial cell distribution on microcarriers after seeding was developed. With both model approaches, the fraction of growth-inhibited cells could be predicted. Simulation results of two cultivations with a different number of initially seeded cells showed that the growth kinetics of adherent cells at the given cultivation conditions is mainly determined by the range of disparity in the initial distribution of cells on microcarriers after attachment.

Hybrid cellular automaton modeling of nutrient modulated cell growth in tissue engineering constructs

Mathematic models help interpret experimental results and accelerate tissue engineering developments. We develop in this paper a hybrid cellular automata model that combines the differential nutrient transport equation to investigate the nutrient limited cell construct development for cartilage tissue engineering. Individual cell behaviors of migration, contact inhibition and cell collision, coupled with the cell proliferation regulated by oxygen concentration were carefully studied. Simplified two-dimensional simulations were performed. Using this model, we investigated the influence of cell migration speed on the overall cell growth within in vitro cell scaffolds. It was found that intense cell motility can enhance initial cell growth rates. However, since cell growth is also significantly modulated by the nutrient contents, intense cell motility with conventional uniform cell seeding method may lead to declined cell growth in the final time because concentrated cell population has been growing around the scaffold periphery to block the nutrient transport from outside culture media. Therefore, homogeneous cell seeding may not be a good way of gaining large and uniform cell densities for the final results. We then compared cell growth in scaffolds with various seeding modes, and proposed a seeding mode with cells initially residing in the middle area of the scaffold that may efficiently reduce the nutrient blockage and result in a better cell amount and uniform cell distribution for tissue engineering construct developments.

Optimal design of metabolic flux analysis experiments for anchorage-dependent mammalian cells using a cellular automaton model

Metabolic flux analysis (MFA) is widely used to quantify metabolic pathway activity. Typical applications involve isotopically labeled substrates, which require both metabolic and isotopic steady states for simplified data analysis. For bacterial systems, these steady states are readily achieved in chemostat cultures. However, mammalian cells are often anchorage dependent and experiments are typically conducted in batch or fed-batch systems, such as tissue culture dishes or microcarrier-containing bioreactors. Surface adherence may cause deviations from exponential growth, resulting in metabolically heterogeneous populations and a varying number of cellular "nearest neighbors" that may affect the observed metabolism. Here, we discuss different growth models suitable for deconvoluting these effects and their application to the design and optimization of MFA experiments employing surface-adherent mammalian cells. We describe a stochastic two-dimensional (2D) cellular automaton model, with empirical descriptions of cell number and non-growing cell fraction, suitable for easy application to most anchorage-dependent mammalian cell cultures. Model utility was verified by studying the impact of contact inhibition on the growth rate, specific extracellular flux rates, and isotopic labeling in lactate for MCF7 cells, a commonly studied breast cancer cell line. The model successfully defined the time over which exponential growth and a metabolically homogeneous growing cell population could be assumed. The cellular automaton model developed is shown to be a useful tool in designing optimal MFA experiments.

Cell population dynamics modulate the rates of tissue growth processes

The development and testing of a discrete model describing the dynamic process of tissue growth in three-dimensional scaffolds is presented. The model considers populations of cells that execute persistent random walks on the computational grid, collide, and proliferate until they reach confluence. To isolate the effect of population dynamics on tissue growth, the model assumes that nutrient and growth factor concentrations remain constant in space and time. Simulations start either by distributing the seed cells uniformly and randomly throughout the scaffold, or from an initial condition designed to simulate the migration and cell proliferation phase of wound healing. Simulations with uniform seeding show that cell migration enhances tissue growth by counterbalancing the adverse effects of contact inhibition. This beneficial effect, however, diminishes and disappears completely for large migration speeds. By contrast, simulations with the "wound" seeding mode show a continual enhancement of tissue regeneration rates with increasing cell migration speeds. We conclude that cell locomotory parameters and the spatial distribution of seed cells can have profound effects on the dynamics of the process and, consequently, on the pattern and rates of tissue growth. These results can guide the design of experiments for testing the effectiveness of biomimetic modifications for stimulating tissue growth.

Evaluation of production parameters with the vaccinia virus expression system using microcarrier attached HeLa cells

Parameters that affect production of the recombinant reporter protein, EGFP, in the T7 promoter based VOTE vaccinia virus-HeLa cell expression system were examined. Length of infection phase, inducer concentration, and timing of its addition relative to infection were evaluated in 6-well plate monolayer cultures. One hour infection with 1.0 mM IPTG added at the time of infection provided a robust process. For larger scale experiments, anchorage-dependent HeLa cells were grown on 5 g/L Cytodex 3 microcarriers. The change to this dynamic culture environment, with cell-covered microcarriers suspended in culture medium in spinner flasks, suggested a re-examination of the multiplicity of infection (MOI) for this culture type that indicated a need for an increase in the number of virus particles per cell to 5.0, higher than that needed for complete infection in monolayer tissue flask culture. Additionally, dissolved oxygen level and temperature during the protein production phase were evaluated for their effect on EGFP expression in microcarrier spinner flask culture. Both increased dissolved oxygen, based on surface area to volume (SA/V) adjustments, and decreased temperature from 37 to 31 degrees C showed increases in EGFP production over the course of the production phase. The level of production achieved with this system reached approximately 17 microg EGFP/10(6) infected cells.

Attachment of 3T3 and MDBK cells onto poly(EGDMA/HEMA) based microbeads and their biologically modified forms

Poly(EGDMA/HEMA) based microbeads were prepared by suspension polymerization. A comonomer, i.e., 2-hydroxyethylmethacrylate (HEMA) was included in the recipe in order to have functional hydroxyl groups on the microbead surfaces. Toluene was used in the polymerization formulations to introduce porosity into the matrix. Hydroxyl groups were first oxidized with NaIO4, and then two biological molecules, namely collagen and fibronectin were immobilized by using glutaraldehyde. A spacer-arm, i.e., hexamethylene diamine, was also used in some cases. More protein molecules were immobilized onto more swellable microbeads using spacer-arm. Higher amounts of collagen were immobilized, more than fibronectin immobilization. Attachment of two cell lines (i.e., 3T3 and MDBK cell lines) on these microbeads with a wide variety of surface properties was studied in vitro culture media. Attachments of both cells even onto the plain microbeads were significant. More cells did attach to more swellable microbeads. Introducing both fibronectin and collagen onto the microbeads caused significant increase in the cell attachment. More cells attached to the microbeads carrying fibronectin covalently attached onto the microbeads through the spacer-arm molecules. Fibronectine was better than collagen for high attachment values. The mathematical model proposed successfully simulated attachment kinetics.

The culture of chicken embryo fibroblast cells on microcarriers to produce infectious bursal disease virus

The cultures of chicken embryo fibroblast (CEF) cells in flasks, spinner bottles, and bioreactors were studied. The growth and metabolism characteristics of CEF cells and the feasibility of the CEF cell culture in bioreactor were investigated. The plating process of the CEF cells on GT-2 microcarriers in spinner bottles was studied, and a plating kinetic model was presented. The culture of CEF cells in 1.5 L CelliGen bioreactor to produce infectious bursal disease virus (IBDV) had met success. Whereas the additive microcarriers were fed during the culture, the cell density was increased 10 times as against seed cells adhering to microcarriers and the virus titer was as high as 7.5. All the aforementioned experimental results have laid the foundation for high density culture of CEF cells and process scale-up.

On-line control of an immobilized hybridoma culture with multi-channel flow injection analysis

An immobilized hybridoma cell line was cultivated at controlled glucose and glutamine concentrations. On-line analysis of the substrates was carried out with a multi-channel flow injection analysis system. The analysis system also determined on-line the lactate and ammonium concentration. The substrate concentrations were controlled using an adaptive-control strategy. This strategy consisted of the estimation of the real-time concentrations and volumetric substrate consumption rates by an Extended Kalman Filter, and a minimum variance controller, which used the estimated parameters to set the feed rates of the substrates. The closed-loop control was used to start-up two cultures with either glucose or glutamine as control-substrate for the medium feed rate. The controller kept the concentration of the control-substrate constant by enhancing the medium feed rate simultaneously to the increasing volumetric consumption rate of the substrate. When glutamine was used as control-substrate, the glucose concentration remained relatively constant, whereas the glutamine concentration decreased during the start-up at a constant glucose concentration. This indicates that glutamine is consumed faster than glucose and will be a better control-substrate to avoid limitation during the start-up of a culture with the applied hybridoma cell line. During the colonization of the microcarriers, the yield of ammonium on glutamine decreased from 0.80 to 0.55 (mol mol-1), indicating a change in the glutamine metabolism. The yield of lactate on glucose stayed constant for both experiments. During long-term culture of more than 800 h, the controller kept both the glucose and glutamine concentrations constant at perfusion rates between 0.50 h-1 and 0.15 h-1. The medium, glucose and glutamine feed rate were independently controlled. Both the specific glutamine and glucose consumption rates remained constant for all perfusion rates, which was probably as a result of the constant concentrations. The specific monoclonal antibody production rate decreased with the perfusion rate decreasing from 0.40 h-1 to 0.20 h-1. The immobilized-cell concentration decreased only at the lowest perfusion rate. Both effects could not be explained directly by the increasing ammonium and lactate concentrations nor by the decreasing amino-acid concentrations.

Cell cycle dynamics of microcarrier cultures

In anchorage-dependent cell microcarrier cultures knowledge of the cell's growth kinetics is necessary in order to design and successfully operate bioreactors, particularly on a large scale. However, in addition to growth kinetics, an understanding of the physiological state of the culture is also important. In this paper the cell cycle progression of Vero and MRC-5 microcarrier cultures have been observed utilizing a flow cytometer. Flow cytometry analysis enabled the differentiation of the various phases of the cell cycle as the culture moved from initial inoculation to the stationary, or confluent stage. Not only was the flow cytometer able to distinguish contact inhibited cells from noncontact inhibited cells, but the measured fraction of contact inhibition cells were found to be in agreement with fractions predicted from a previously developed cellular automation model for microcarrier cultures. Further, the data from the stationary phase was used to quantify the death rate in microcarrier cultures.