Direct microglia replacement reveals pathologic and therapeutic contributions of brain macrophages to a monogenic neurological disease

Krabbe disease, also named Globoid Cell Leukodystrophy (GLD) for its distinct lipid- laden macrophages, is a severe leukodystrophy caused by galactosylceramidase (GALC) mutations. Hematopoietic stem cell transplant (HSCT) ameliorates disease and is associated with central nervous system (CNS) engraftment of GALC+ donor macrophages. Yet, the role of macrophages in GLD pathophysiology and HSCT remains unclear. Using single-cell sequencing we revealed early interferon response signatures which precede progressively severe macrophage dyshomeostasis, and identified a molecular signature of globoid cells, which we validated in human brain specimens. Genetic depletion and direct microglia replacement by CNS monocyte injection rapidly replaced >80% of endogenous microglia with healthy monocytes in the twitcher (GalcW355*) mouse, a faithful model of GLD. Perinatal microglia replacement completely normalized transcriptional signatures, rescued histopathology, and doubled average survival. Overall, we uncovered distinct forms of microglia dysfunction, and evidence that direct, CNS-limited microglia replacement improves a monogenic neurodegenerative disease, a promising therapeutic target. Myeloid cells were isolated from Twitcher and WT mouse brains, FACS sorted for CD45, CD11b positivity, and sequenced using 10x chromium controller system. Samples include WT and twitcher animals transplanted with WT monocytes, and unmanipulated WT and Twi animals.