Rules of Engagement Enabling Leukocyte Rolling and Adhesion: An In Silico Model Study
Rolling, activation, and adhesion on endothelial surfaces are necessary steps for the proper recruitment of leukocytes from circulating blood to sites of inflammation. Once at the target site, leukocytes help destroy pathogens and decompose damaged tissue. Inflammatory mechanisms, when uncontrolled, are also associated with diseases such as asthma, rheumatoid arthritis, and atherosclerosis. Improved therapeutics can be developed with a better understanding of the molecular-level events that mediate this process.
This dissertation reports the development of a synthetic, in silico model for use as an experimental system for testing the plausibility of mechanistic hypotheses of how molecular components may interact to cause leukocyte behaviors during rolling, activation, and adhesion to endothelial surfaces. Object-oriented software components were designed, instantiated, verified, plugged together, and then operated in ways that can map concretely to mechanisms and processes believed responsible for leukocyte rolling, activation, and adhesion. The result is an in silico analogue of the wet-lab experimental systems to study leukocyte rolling and adhesion; the experimentally measured phenotypic attributes of the analogue can be compared and contrasted to those of leukocytes from the referent systems.
We first demonstrate that the in silico devices can generate behaviors similar to those observed in vitro at the cell-level and population-level, by comparing simulation results with data from three different in vitro experimental conditions. The validated model was then used to test the hypothesized mechanism of LFA-1 integrin clustering in adhesion. The in silico device showed that clustering was not necessary to achieve adhesion as long as densities of LFA-1 and its ligand ICAM-1 were above a critical level. Importantly, at low densities LFA-1 clustering enabled improved efficiency: adhesion exhibited measurable, cell level positive cooperativity.
Our results provide convincing evidence for the feasibility of using synthetic in silico systems to improve our understanding of the molecular- and cellular-level events mediating leukocyte rolling, activation, and adhesion during inflammation. This work represents early, but important advances for future in silico research into plausible causal links between molecular-level events and the variety of systems-level behaviors that distinguish normal leukocyte adhesion from disease-associated adhesion.