Robert Dürr

Distributed modeling of human influenza a virus-host cell interactions during vaccine production

This contribution is concerned with population balance modeling of virus–host cell interactions during vaccine production. Replication of human influenza A virus in cultures of adherent Madin–Darby canine kidney (MDCK) cells is considered as a model system. The progress of infection can be characterized by the intracellular amount of viral nucleoprotein (NP) which is measured via flow cytometry. This allows the differentiation of the host cell population and gives rise to a distributed modeling approach. For this purpose a degree of fluorescence is introduced as an internal coordinate which is linearly linked to the intracellular amount of NP. Experimental results for different human influenza A subtypes reveal characteristic dynamic phenomena of the cell distribution like transient multimodality and reversal of propagation direction. The presented population balance model provides a reasonable explanation for these dynamic phenomena by the explicit consideration of different states of infection of individual cells. Kinetic parameters are determined from experimental data. To translate the emerging infinite dimensional parameter estimation problem to a finite dimension the parameters are assumed to depend linearly on the internal coordinate. As a result, the model is able to reproduce all characteristic dynamic phenomena of the considered process for the two examined virus strains and allows deeper insight into the underlying kinetic processes. Thus, the model is an important contribution to the understanding of the intracellular virus replication and virus spreading in cell cultures and can serve as a stepping stone for optimization in vaccine production. Biotechnol. Bioeng. 2013; 110: 2252–2266.

Population balance modeling of biopolymer production in cellular systems

Abstract In this contribution we present a population balance modeling approach for the production of the biopolymer poly(β-hydroxybutyrate) in Ralstonia eutropha. The population balance model is based on a dynamic single cell model, which accounts for cell internal regulation by means of the cybernetic modeling approach. The change of internal coordinates is controlled by cybernetic control variables. Depending on available substrates and internal composition the population balance model is therefore able to switch between growth, synthesis of biopolymer and metabolization of biopolymer. The latter one was neglected in an earlier contribution, but is crucial for overall dynamic behavior. In a first step we present a extended two-dimensional population balance model which includes metabolization of biopolymer and considers the cell internal biopolymer and residual biomass as internal coordinates. The two-dimensional population balance model includes cell internal regulation by means of cybernetic control variables. Since concentration of internal biopolymer and amount of residual biomass are costly to determine, we discuss in a second step a reduction of the two-dimensional to a one-dimensional population model by means of correlating cell size with biopolymer concentration.

Distributed Modeling and Parameter Estimation of Influenza Virus Replication During Vaccine Production

Abstract This contribution is concerned with population balance modeling of influenza virus replication in mammalian cell cultures. The cells are heterogeneous with respect to intracellular compounds like viral proteins. The amount of viral NP protein can be measured directly by means of flow cytometry. The corresponding degree of fluorescence is introduced as internal coordinate for a distributed deterministic modeling approach. The resulting model includes kinetic processes like infection, virus replication and release, apoptosis and cell death. It consists of three partial differential equations describing the distribution dynamics which are coupled to two differential equations that characterize the concentration of active and inactive virions in the medium. Kinetic parameters are determined from experimental data. The parameters are assumed to depend on the internal coordinate. The emerging infinite dimensional parameter estimation problem is translated to a finite dimension using a hermite spline representation of the distributed parameters. Hence the resulting inverse problem can be solved in a weighted nonlinear least squares framework. Spline approaches of different complexity are discussed and the estimation results are compared.

Population balance modelling of influenza virus replication during vaccine prodcution - Influence of apoptosis

Abstract This contribution is concerned with population balance modelling of virus infection in cell cultures during vaccine production. As a model system infection of human influenza A virus in microcarrier cell cultures of Madin-Darby canine kidney (MDCK) cells is considered. Differentiation on the population level is described by a degree of fluorescence, which is proportional to the amount of intracellular viral proteins and can be measured directly using flow cytometry. Additional measurement information allows distinguishing between nonapoptotic and apoptotic cells. Apoptosis or programmed cell death is a natural process during cell development and can be activated by a large variety of external and internal stimuli particularly by viral infection. It invariably leads to cell lysis and has major influence on the process productivity.