Exploring and Modulating Platelet Adhesion and Platelet-Cell Interactions in Health and Disease

Platelets play a major role in vivo in preventing bleeding, hemostasis, and contributing to uncontrolled clotting, thrombosis. Platelets are impacted in health and disease by the other cells they encounter in blood flow, including red blood cells (RBCs), white blood cells (WBCs), and the endothelial cell monolayer lining blood vessel walls. Despite this importance, the effect other cell types have on platelets in disease has yet to be fully explored, in part due to a lack of proper models to study human platelet behavior. In this thesis, we develop new, tunable in vitro methods to study platelet behavior that can be altered depending on disease conditions. We examined platelet adhesion to a confluent endothelial cell monolayer cultured on crosslinked extracellular matrix proteins, which were exposed via manual damage. We attached the damaged endothelial cells to a parallel plate flow chamber and monitored human platelet behavior and adhesion over a wide variety of blood flow conditions. This model was validated using a known anti-platelet compound and we identified two additional novel compounds that significantly reduced platelet adhesion when administered either to blood or to the endothelial cells themselves. Many diseases, including acute lung injury, venous and arterial thrombosis, and sepsis, fall under the umbrella of ‘thromboinflammation,’ when blood clots occur in combination with an overzealous immune cell response. A hallmark of these diseases is the presence of leukocyte-platelet aggregates at the site of inflammation. To capture this phenomenon, we included inflammation of endothelial cells in our flow model to facilitate the adhesion of leukocyte-platelet aggregates. We then utilized model polymeric drug carriers to reduce platelet adhesion to the inflamed endothelium by interfering with platelet-bound leukocytes. Specifically, platelet adhesion in this model decreased by approximately two-fold after 2 µm polystyrene particles were introduced into the system. We verified that this impact of particles is translatable in vivo using a mouse model of systemic inflammation; 2 µm particles significantly reduced platelet adhesion to the mouse mesentery by up to 62% in comparison to non-particle controls. These findings represent a potential new particle-based therapeutic to reduce platelet accumulation in thromboinflammatory diseases by diverting platelet-leukocyte aggregates away from areas of inflammation. Patients with sickle cell disease (SCD) have RBCs that are more stiff than non-SCD controls; we explored the impact that these stiff RBCs have on platelet behavior in blood flow in vitro using SCD patient whole blood samples and a model system where healthy RBCs are artificially stiffened to mimic SCD. The magnitude of SCD platelet adhesion varied greatly depending on patient treatment regimen, though untreated patients had the highest platelet adhesion in comparison to non-SCD controls. Our artificial system allowed us to examine the impact of stiff RBCs on platelet adhesion in a controlled environment, providing knowledge that can help inform how to provide chronic transfusions of RBCs to SCD patients to best reduce excessive platelet adhesion and clotting. We also determined that carbon monoxide releasing molecules (CORMs) reduce excessive platelet adhesion for a subset of patients, a promising new potential therapeutic for SCD. Overall, this work represents new methods to study platelet behavior under different disease conditions as well as novel potential therapeutics to modulate platelet adhesion in thromboinflammation and SCD.PhDChemical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/172653/1/albanka_1.pd