Studies on the interactions between microglial maturation and perinatal challenges in neuroinflammatory responses

Microglia are crucial for brain development, homeostasis and injury responses. Gene expression studies in the mouse describe microglia as having specific stages of maturation, thus providing them with different functional abilities as their role transitions from brain-building and refinement to maintaining homeostasis. However, when an injury or inflammatory challenge occurs, glia transition to an immune-reactive profile to repair the injury or destroy the pathogen, but how this immune response interacts with stages of microglial development is poorly understood. Perinatal brain injury (PBI) and abnormal brain maturation occur in babies born with intrauterine growth restriction (IUGR). IUGR occurs when there is an inadequate supply of oxygen and nutrients to the fetus due to poor placental function. This can lead to acute or chronic neurodevelopmental deficits, including poor cognitive outcomes and cerebral palsy. Microglial development and immune activation have been characterised in isolation with few studies exploring the dynamic relationship between the two factors. This thesis explored how a compromised fetal environment (growth restriction and immune activation) may interrupt the normal sequence of microglial development in sheep and mouse models. Furthermore, an investigation was undertaken to assess the relationship between disruptions to microglial maturation and the microglial response to injury. Chapters 1 and 2 are the literature review and methods, respectively. In Chapter 3, the outcomes of several studies focused on microglial development and response in several animal models of PBI were compiled and compared. Chapter 3 highlights how microglial development varies across species. In particular, the developmental timeline for microglial maturation was much faster in the mouse compared to the sheep and pig, even when adjusting for gestational length. Furthermore, development-dependent microglial functions were altered following PBI, indicating that depending on the timing of injury, it may result in short or long-term interruptions to overall microglial phenotype and function. Chapter 4 characterised the spatial-temporal distribution of microglia within the developing sheep brain. This included exploring region-dependent changes in microglial proliferation, ramification, and maturation in a species often used as a model of human brain development or PBI. Stages of glial development and morphology in the developing sheep brain were similar, but not identical, to those previously observed in the rodent. Shifts in markers of microglial development and increases in arborisation functional transitions to assist brain development. In Chapters 5 and 6, SUAL was used to model IUGR in the early-mid gestational sheep. SUAL induced-IUGR induced a pro-inflammatory state in microglia residing in specific regions of the grey and white matter following SUAL. SUAL also increased markers of microglial cytotoxicity, especially in the developing grey matter, which was linked to an acceleration in microglial maturation. Building on the sheep studies described in the previous chapters, Chapter 7 characterised interactions between mouse microglial development, sex and immune-reactivity following pro- or anti-inflammatory stimulation.  Specifically, this study investigated the response of mouse microglia at the pre-adult microglia stage – the stage of microglial development expected to be observed in the final trimester of human development and adult microglia. The findings indicated that male pre-adult microglia are more likely to alter their activation phenotype whereas females will change their developmental phenotype in response to inflammatory cytokines. The data also showed that IL-4 was an affective driver against pro-inflammatory stimulation in adult microglia and female pre-adult microglia. This thesis collectively highlights temporal roles of glia, particularly microglia, in the maturing, healthy and injured neonatal brain. Furthermore, this thesis shows that microglia are capable of functionally adapting to a pro- or anti-inflammatory environment. This thesis has contributed to demonstrating how vital microglia are in contributing to and protecting the developing brain, making them ideal candidates for targeted treatment.