Author ORCID Identifier
Date of Graduation
Doctor of Philosophy (PhD)
Louise McCullough, MD/PhD
Dianna Milewicz, MD/PhD
Pamela Wenzel, PhD
Dat Tran, MD
Dorothy Lewis, PhD
Ischemic stroke is a major cause of disability and mortality worldwide. As most patients cannot receive the currently approved therapies for ischemic stroke, novel treatments are critically needed. Cerebral ischemia causes irreversible tissue damage, referred to as the “tissue core”, which is surrounded by a salvageable penumbral region. Excitotoxicity, oxidative stress and inflammation can further damage this “tissue-at-risk”, resulting in even greater functional loss and poorer injury outcomes.
Aging represents the single strongest risk factor for high mortality and poor outcome after stroke in patients. This phenotype is also seen in animal models, with aged mice experiencing higher mortality and poorer recovery than their young counterparts. Our lab has recently shown that bone marrow transplantation from young mice into aged mice improves functional outcome after ischemic stroke, whereas aged bone marrow transplantation exacerbates secondary brain hemorrhage in young mice. However, the identity of the deleterious factor present in aged bone marrow remains unknown.
Bone marrow is the main site of neutrophil differentiation, maturation and storage. Neutrophils, innate immune cells with a rich arsenal of anti-bacterial functions, traffic to the brain in large numbers following ischemic stroke. Age has previously been reported to impair neutrophil protective functions (phagocytosis, directed chemotaxis) and exacerbate neutrophil-driven inflammation (increased reactive oxygen species generation, impaired neutrophil clearance). However, there are currently no published studies on the effects of age on neutrophil function and neutrophil-associated brain tissue damage after stroke.
Neutrophils are believed to exacerbate brain tissue injury in ischemic stroke via the release of degradative enzymes and reactive species, leading to blood-brain-barrier breakdown, secondary tissue hemorrhage and direct tissue damage. In light of this data, neutrophil-mediated damage has gained significantly interest as a potential therapeutic target in ischemic stroke.
Despite success in animal models, clinical trials of anti-neutrophil therapies for ischemic stroke have been unsuccessful, due in part to the limited specificity of available treatments and the tendency to test therapeutics in young animals only. We believe this lack of specificity and the failure to include aged animals in pre-clinical studies may have obscured the true contribution of neutrophils to stroke pathology. In recent years, administration of the monoclonal antibody anti-Ly6G has been found to specifically and robustly deplete neutrophils in mice. Encouragingly, our early work confirmed that post-stroke anti-Ly6G treatment results in a transient decrease in circulating neutrophils, making it a promising tool for testing the role of neutrophils in acute ischemic stroke pathology.
In this dissertation, I hypothesized that age enhances the pro-inflammatory functions of neutrophils after ischemic stroke, directly contributing to the poorer outcomes seen in aged animals. I tested this hypothesis by (1) examining the effects of age on neutrophil inflammatory phenotype in vivoand ex vivo, and (2) directly assessing whether neutrophil depletion had a differential treatment effect in young vs. aged animals following ischemic stroke.
ischemic stroke, inflammation, neutrophils, reactive oxygen species, neuroinflammation