Modeling neutrophil chemotaxis in multiple chemoattractant gradients

Neutrophils constitute the largest class of leukocytes and are the front line of cellular immune defense. They are able to sense and migrate up concentration gradients of chemoattractants in search of primary sites of inflammation in a process termed chemotaxis. Chemoattractants include formylated peptides, complement factors, and various chemokines. Each chemoattractant binds to a specific receptor that activates a number of responses including chemotaxis. While there is much information on the molecular interactions and signaling pathways that respond to these stimuli, how the pathways process multiple signals to effect migration in the appropriate direction is not understood. The primary motivation behind the work presented here is to understand the engineering rules by which neutrophils combine multiple signals to choose a direction to move and efficiently locate a target in a complex environment.; For this purpose, we have developed an assay system and novel image processing techniques, along with a modeling framework in which to compare the results. We used the micropipette assay for generating the gradients and tracked changes in cell densities over time. This represents the first use of the micropipette assay toward studies of neutrophil chemotaxis in multiple chemoattractant gradients. The modeling framework is based on an Ornstein-Uhlenbeck (OU) process which has been successfully used to describe neutrophil migration paths in uniform chemoattractant concentrations. We modified the OU process by including a term to describe chemotactic bias resulting from chemoattractant gradients. We assumed that this bias is the result of a vector sum of multiple gradients as sensed by neutrophils through their receptors. In developing the model we compared experimental results and applied these results toward parameter estimation and model validation.; Using this framework, we were able to relate experimentally observable cell migration paths to the physical principles involved in receptor-ligand binding. Our main results quantify signal processing and prioritization based on binding parameters such as receptor quantities and dissociation constants. Overall, this dissertation represents a tight coupling of experimental and modeling techniques toward understanding how neutrophil chemotaxis has been engineered by evolution.