(233c) Multi-Scale Agent-Based Modeling of Human Endotoxemia

The systemic inflammatory response syndrome (SIRS) often accompanies critical illness but is evoked by many stimuli e.g. infection, trauma, invasive surgery and biological stressors in general [1]. It often progresses to severe cases of sepsis with a substantial morbidity and mortality where bacterial growth is found and organ failure occurs [2, 3]. Despite increasing knowledge about pathophysiological pathways and processes involved in sepsis as well as promising results on animal studies and preclinical trials [4, 5], the molecular mechanisms and physiological significance of the systemic inflammatory response are still not fully understood. In order to learn more about the mechanisms associated with the host inflammatory responses, the human endotoxemia model, an in vivo model of systemic inflammation in which a single intravenous bolus of E.coli endotoxin (LPS) is given to healthy human subjects, has been employed. The model results in many similar physiological host responses that characterize Gram-negative infection [6], providing an invaluable source for the systemic identification of biological features representing the complex dynamics of a host undergoing inflammatory responses [7, 8].

With the assumption that transcriptional signatures can be able to characterize the cellular dynamics in response to the inflammatory agent, high-dimensional transcriptional human endotoxemia data were decomposed into significantly co-expressed clusters [9], three out of which are selected as representatives for common expression profiles of inflammatory biomarkers including pro-inflammatory, anti-inflammatory, and bio-energetic transcriptional patterns. Additionally, the activation for ?translating' from extra-cellular signals to intra-cellular responses is assumed to be governed by signaling cascades activated with transcription factors (e.g. NFkB [10, 11]) that drive the expression of relevant genes. In an attempt towards a better understanding of the molecular mechanisms as well as the complex dynamics of the host inflammatory responses, we previously developed a semi-mechanistic indirect response model using the ordinary differential equation (ODE) system [12]. However, it is fully deterministic with respect to their behavior given a certain set of initial conditions and assumes the homogeneity and perfect mixing within compartments as well as ignores the spatial aspect [13, 14]. Thus, we here explore an alternative approach ? agent-based model (ABM) which can naturally account for more biological phenomena e.g. stochasticity [15, 16], heterogeneity [17, 18].

ABM is an object-oriented, rule-based, discrete, and stochastic modeling method [19, 20]. Interactions between agents are nonlinear, stochastic, spatial, and are described with multiple compartments and asynchronous movements. However, since the model is relied on an ill-defined principle of ?emergence', the ABM is not a ?mathematical model' per se, and thus is not subjected to formal analysis and ?solved' [20]. One of the most important issues is the estimation of model parameters. With the assumption that the default system production of a component type is approximately equal to its degradation in the normal (healthy/steady) state, we present a novel technique based on the pharmacokinetic principles for tuning parameters including the population sizes and the synthesis rates of component types. As such, in a healthy system the total number of individuals of each component type will not change significantly over the time. After all parameters are identified to maintain the system in the healthy state, circadian rhythms are incorporated and then LPS is putatively injected with various doses for qualitatively examining the profiles of representative responses.

In conclusion, the aim of this study is two folds: (i) develop a ?better' in silico agent-based model that can provide more insights into the human endotoxemia, and (ii) utilize the created model to study the individual cellular dynamics and especially ensemble patterns that lead to distinct outcomes under the administration of different LPS doses.

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