Microgliosis is a prominent pathological feature in many neurological diseases including multiple sclerosis (MS), a progressive auto‐immune demyelinating disorder. The precise role of microglia, parenchymal central nervous system (CNS) macrophages, during demyelination, and the relative contributions of peripheral macrophages are incompletely understood. Classical markers used to identify microglia do not reliably discriminate between microglia and peripheral macrophages, confounding analyses. Here, we use a genetic fate mapping strategy to identify microglia as predominant responders and key effectors of demyelination in the cuprizone (CUP) model. Colony‐stimulating factor 1 (CSF1), also known as macrophage colony‐stimulating factor (M‐CSF) ‐ a secreted cytokine that regulates microglia development and survival—is upregulated in demyelinated white matter lesions. Depletion of microglia with the CSF1R inhibitor PLX3397 greatly abrogates the demyelination, loss of oligodendrocytes, and reactive astrocytosis that results from CUP treatment. Electron microscopy (EM) and serial block face imaging show myelin sheaths remain intact in CUP treated mice depleted of microglia. However, these CUP‐damaged myelin sheaths are lost and robustly phagocytosed upon‐repopulation of microglia. Direct injection of CSF1 into CNS white matter induces focal microgliosis and demyelination indicating active CSF1 signaling can promote demyelination. Finally, mice defective in adopting a toxic astrocyte phenotype that is driven by microglia nevertheless demyelinate normally upon CUP treatment implicating microglia rather than astrocytes as the primary drivers of CUP‐mediated demyelination. Together, these studies indicate activated microglia are required for and can drive demyelination directly and implicate CSF1 signaling in these events.

Hematopoietic stem cell transplantation (HSCT) can replace endogenous microglia with circulation-derived macrophages but has high mortality. To mitigate the risks of HSCT and expand the potential for microglia replacement, we engineered an inhibitor-resistant CSF1R that enables robust microglia replacement. A glycine to alanine substitution at position 795 of human CSF1R (G795A) confers resistance to multiple CSF1R inhibitors, including PLX3397 and PLX5622. Biochemical and cell-based assays show no discernable gain or loss of function. G795A- but not wildtype-CSF1R expressing macrophages efficiently engraft the brain of PLX3397-treated mice and persist after cessation of inhibitor treatment. To gauge translational potential, we CRISPR engineered human-induced pluripotent stem cell–derived microglia (iMG) to express G795A. Xenotransplantation studies demonstrate that G795A-iMG exhibit nearly identical gene expression to wildtype iMG, respond to inflammatory stimuli, and progressively expand in the presence of PLX3397, replacing endogenous microglia to fully occupy the brain. In sum, we engineered a human CSF1R variant that enables nontoxic, cell type, and tissue-specific replacement of microglia.

Microgliosis is a prominent pathological feature in many neurological diseases including multiple sclerosis (MS). The precise role of microglia during demyelination, and the relative contributions of microglia vs. peripheral macrophages, are incompletely understood. Here, using a genetic fate mapping strategy, we identify microglia as predominant responders and key effectors of demyelination in the cuprizone (CUP) model. Pharmacological depletion of microglia demonstrates these cells are necessary for the demyelination, loss of oligodendrocytes, and reactive astrocytosis normally evident in this model. Electron microscopy (EM) and serial block face imaging show myelin sheaths remain intact in CUP treated mice depleted of microglia. However, these damaged myelin sheaths are lost upon-repopulation of microglia.Injection of colony-stimulating factor-1 to drive focal microgliosis in white matter is sufficient to induce focal demyelination in vivo. These studies indicate activated microglia are required for demyelination that results from primary myelin pathology and are sufficient to induce demyelination directly.Stem Cell and the NIH. DM was a recipient of an NRSA fellowship from NINDS.