Imaging Immune Functions Induced by Focused Ultrasound and Microbubble Treatments to Increase Blood-brain Barrier Permeability

Treatment of neurological diseases is limited by the blood-brain barrier (BBB). The BBB controls the exchange of compounds between the brain parenchyma and systemic circulation. One method to bypass the BBB is to use focused ultrasound (FUS). FUS, used in conjunction with preformed microbubbles (MBs), can transiently enhance BBB permeability by increasing transcellular and paracellular transport across the brain endothelium. While FUS+MB BBB treatments have been shown to be an effective method to deliver agents to the brain, the use FUS+MB BBB alone, without additional therapeutics, have also been observed to result in various downstream effects, including the reduction of β-amyloid (Aβ) plaque load in animal models of Alzheimer’s disease. The characteristics of this induced clearance response have yet to be fully elucidated. In this thesis, the time course of the effects of FUS+MB BBB and induced recruitment of peripheral immune cells are investigated using two-photon fluorescence microscopy. Thermal effects of two-photon fluorescence microscopy were assessed to evaluate whether experimental parameters affected the brain milieu. Brain temperatures were moderately hypothermic following cranial window implantation and increased linearly with laser power during imaging. Temporal characteristics of induced Aβ plaque load clearance were investigated by monitoring the size of Aβ plaques following one FUS+MB BBB treatment in the TgCRND8 mouse model of Alzheimer’s disease. Aβ plaque reduction was observed between 2 to 14 days following treatment, and repeated biweekly treatments significantly reduced plaque load. The efficiency of plaque clearance suggested the involvement of peripheral phagocytic immune cells. Blood flow perturbations were observed immediately before and after the onset of sonication, and motile cells were recruited intravascularly within seconds following increase in BBB permeability. Neutrophils were identified to be one of the participating cell types. FUS+MB treated brain hemispheres harboured significantly more neutrophils compared to the contralateral hemispheres, and to control animals. However, the participation of neutrophils in clearing Aβ plaques may be transient, as neutrophil counts in treated brain tissue were not significantly different from control animals following biweekly FUS+MB treatments. The characteristics of immune functions provided in this thesis further inform understanding of biological responses induced by FUS+MB BBB treatments.Ph.D.2021-05-30 00:00:0