Title: Myeloid Heterogeneity in the Hippocampus
Description: Historically, the role of immune cells in the nervous system was predominantly examined throughthe lens of disease. In recent years, studies have shown that the complex, orchestrated events of immune activity throughout embryonic and postnatal critical periods are crucial for proper nervous system development. While previous studies have suggested limited immune heterogeneity in the adult brain, the diverse roles of the hippocampus in cognition and pathological development would suggest variation of immune cells in this region. Specifically, the hippocampus is known to be a site of adult neurogenesis. However, fundamental traits of immune cells in this region have not been well characterized. In chapter one, I present a summary of literature that discusses what was previously known of immune regulation of adult neurogenesis during health and disease. In chapter two, I compare different reporter lines and marker genes to evaluate responses in various cell types in the neurogenic niche and in other regions of the brain in the context of injury and pharmacological modulation.

I discuss preliminary evidence suggesting microglial depletion may result in phenotypic changes in astrocytes throughout the hippocampus. In chapter three, I provide evidence of heterogeneity in myeloid-lineage cells in the hippocampus. I leveraged the highthroughput nature of cell suspension based single cell RNA-sequencing to collect transcriptomes of over 20,000 myeloid lineage cells from murine hippocampi. Using a series of bioinformatic techniques, I was able to computationally dissect different populations within this system and found spatial mapping of one distinct subset specifically localized to the neurogenic niche of the hippocampus. The transcriptomic signature of these cells alongside immunoreactivity to candidate genes, and morphological properties of this population resemble those of reactive microglia associated with the restriction of neurodegenerative diseases. In chapter four, I discuss how the immune landscape of the hippocampus responds to perturbation using a model of Focused Ultrasound mediated Blood-Brain Barrier opening.

Subtypes of myeloid lineage cells change in composition and in transcriptomic response. We find distinct, temporally defined transcriptional responses in microglial and macrophage populations, indicating discrete roles for microglia and macrophages in immune activity during the transition from acute to chronic response. Together, these findings point towards diverse properties of microglia in the adult hippocampus.