Microglia are the tissue-resident macrophages of the central nervous system. Important in brain health and homeostasis, these immune cells play critical roles in neurological injury and disease, making them attractive targets for cell-based therapies. When depleted, the brain’s microglial niche can be reconstituted by surrogate macrophages - a process termed “microglia replacement”. Endogenous microglia can be replaced by macrophages from different derivations, including cells from the yolk sac, the bone marrow, or generated induced pluripotent stem cell (iPSC) lines. While yolk sac- and bone marrow- derived (BMD) cells are difficult to transduce and finite in number, iPSC-derived cells are expensive to create and difficult to maintain. To expand the microglia replacement tool kit, we here introduce a novel model of microglia replacement by way of estrogen-regulated (ER) homeobox B8 (Hoxb8) conditionally immortalized macrophages - cells that are readily manipulable, economical, and infinite in number. In this thesis, we first deeply characterize ER-Hoxb8 macrophages in vitro and after transplantation into the microglia-deficient brain, finding that they are similar to primary BMD macrophages in transcriptional identity and in engraftment potential. We secondly utilized this model to investigate the role that Adar1 plays in macrophage identity and function, as well as the intrinsic contribution of macrophages to Aicardi-Goutières Syndrome (AGS), a brain-predominant pediatric interferonopathy. While we find that Adar1 is required for macrophage expansion and brain engraftment, we discover that these phenotypes are likely not dependent on interferon production. Furthermore, we determine that microglia are likely a driving cell of AGS pathology. Collectively, these results offer fundamental insights into macrophage function, microglia replacement dynamics, and therapeutic strategies for AGS. In conclusion, we harness the power of ER-Hoxb8 conditionally immortalized macrophages to study microglia replacement and to provide insight into macrophage function and contribution to brain health and disease.