Neonatal microglia replacement in mice modulates seizure severity in adulthood

Microglia replacement therapy, where endogenous brain macrophages are depleted and replaced by adoptively transferred surrogates, holds promise for treating pediatric neurologic diseases, but little is known about how early-life microglia replacement impacts the brain. We sought to investigate how early postnatal microglia depletion and adoptive macrophage transfer, essential components of microglia replacement, durably impact neural circuits in a mouse model. Using both pharmacologic and genetic models, postnatal microglia depletion worsened adult seizure severity, mortality, and neuropathology in a chemical seizure model. Replacement of endogenous microglia by adoptive transfer of monocytes rescued this effect, while transfer of authentic microglia from a donor mouse did not, and even worsened seizure phenotypes. RNA sequencing of transplanted microglia, monocyte-derived surrogates, and endogenous microglia revealed distinct state changes across groups in response to chemically induced seizures, demonstrating that both ontogeny and adoptive transfer significantly impact resident macrophage responses to the excitotoxic brain environment. In sum, we established models for neonatal microglia depletion and replacement, then applied them to identify durable impacts of depletion and reconstitution on the brain environment. We ultimately identified differential responses of macrophages to excitotoxic challenge based on their ontogeny, underscoring focus areas for ongoing preclinical development of microglia replacement therapies.