Microglia replacement halts the progression of microgliopathy in mice and humans.

Colony-stimulating factor 1 receptor (CSF1R) is primarily expressed in microglia. Its monoallelic mutation causes CSF1R-associated microgliopathy (CAMP), a major form of adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP) and a fatal neurological disease without clinical cure. We developed mouse models harboring human hotspot mutations of CAMP and replaced CSF1R-deficient microglia with CSF1R-normal cells through microglia replacement by bone marrow transplantation (Mr BMT), which attenuated pathology in mice. We further demonstrated that, in the context of CSF1R deficiency, traditional bone marrow transplantation (tBMT) in ALSP functions similarly to Mr BMT, efficiently replacing microglia and reducing disease progression. We then replaced CSF1R-deficient microglia in eight patients by tBMT. The disease progression was halted during the 24-month follow-up. Together, microglia replacement corrects pathogenic mutations and halts disease progression in mice and humans. Editor's summary: Mutations in the gene encoding colony stimulating factor 1 receptor (CSF1R) lead to a severe neurological disease called adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP), for which there are no treatments. In the brain, CSFR1 is mainly expressed by microglia. Wu et al. developed and used mouse models of ALSP to show that replacing CSF1R-deficient microglia with CSF1R-normal microglia using bone marrow transplantation (BMT) strongly attenuated brain pathology (see the Perspective by Du and Kipnis). Of major clinical relevance, BMT halted disease progression in four individuals, suggesting that this might be an effective therapeutic strategy for treating CSF1R-associated disorders. —Mattia Maroso INTRODUCTION: Microglia are important immune cells in the central nervous system (CNS). The dysfunction of microglia contributes to various CNS disorders. CSF1R is primarily expressed in microglia and is essential for their survival and function. Biallelic CSF1R mutations cause the congenital absence of microglia and are perinatally lethal in both humans and mice, whereas monoallelic mutations cause CSF1R-associated microgliopathy (CAMP), a major form of adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP). ALSP is a fatal disease with no current curative treatment. RATIONALE: Because pathogenic mutations in microglia-specific CSF1R are the cause of ALSP, we reasoned that replacing CSF1R-deficient microglia with wild-type (WT) microglia would halt disease progression. We previously developed efficient strategies for microglia replacement, currently collectively termed microglia intervention strategy for therapy and enhancement by replacement (MISTER), which have since informed and inspired subsequent studies in the field. We thus hypothesized its therapeutic potential for ALSP. RESULTS: Previously used mouse models (e.g., CSF1RWT/KO and CSF1RΔFIRE/ΔFIRE; KO, knockout; FIRE, fms-intronic regulatory element) fail to adequately recapitulate key ALSP phenotypes, highlighting the need for better models and therapeutic approaches. To this end, we generated genuine mouse models for ALSP (CSF1RWT/I792T and CSF1RWT/E631K; I, Ile; T, Thr; E, Glu; K, Lys) based on human hotspot mutations. The newly generated mouse models faithfully recapitulated key features of human ALSP, including microglial number reduction, brain calcification, myelin pathology, axonal swelling, axonal spheroids, motor impairment, and cognitive decline. Next, we replaced mutant microglia of ALSP mice with WT cells using microglia replacement by bone marrow transplantation (Mr BMT), one of the MISTER approaches. Microglia replacement corrected pathogenic Csf1r mutation, substantially halted the brain pathology, rescued neural signal transduction, and improved motor and cognitive function in ALSP mice. Single-cell RNA sequencing showed that Mr BMT cells reshaped abnormal CSF1R signaling pathways, restoring oligodendrocytes to a more normal-like phenotype. Notably, we demonstrated that the therapeutic benefits of Mr BMT stem specifically from the correction of the pathogenic Csf1r mutation in microglia rather than from peripheral macrophage replacement or microglia repopulation alone. In addition, the chemokine receptor CCR2 was identified as essential for successful microglia replacement. Although traditional bone marrow transplantation (tBMT) alone typically does not achieve efficient microglia replacement in healthy brains, the inherent CSF1R deficiency creates a competitive disadvantage for the recipient's resident microglia, enabling tBMT to achieve effective replacement in our ALSP mouse model and confer therapeutic benefits comparable to those of Mr BMT. To validate clinical relevance, we conducted tBMT-based treatment in eight individuals with ALSP. By using noninvasive PET imaging with 18F-FDG, we demonstrated increased glucose metabolism in the brain. Notably, MRI and clinical evaluations indicated halted disease progression, preserved motor function, and stabilized cognitive abilities during a 24-month follow-up. This study also provides a mechanistic explanation for a prior clinical case in which an individual with ALSP, initially misdiagnosed with adult-onset metachromatic leukodystrophy, exhibited long-term stabilization after tBMT. CONCLUSION: Our findings demonstrate that microglia replacement effectively corrects the pathogenic CSF1R mutations in both ASLP mice and humans, halting disease progression and improving neurological function. These results provide compelling preclinical and clinical evidence supporting microglia replacement (MISTER) as a newly developed therapeutic strategy not only for ALSP but potentially for other CNS diseases with microglial dysfunction. Microglia replacement effectively halts the progression of ALSP in mice and human individuals.: (Left) Correction of pathogenic Csf1r mutation by Mr BMT effectively rescues pathological changes in the brain, motor impairment, and cognitive decline in ALSP mice. (Right) tBMT-mediated microglia replacement effectively halts disease progression in individuals with ALSP, with preservation of brain integrity, motor performance, and cognitive function. [ABSTRACT FROM AUTHOR]