Treatment of neurologic disorders by brain-wide microglia replacement using a non-conditioning strategy

Genetic evidence indicates that microglial dysfunctions contribute to the development and progression of various neurologic diseases, emphasizing microglia replacement as a promising therapeutic strategy. However, traditional bone marrow transplantation (BMT) aimed at replenishing brain microglia faces challenges due to low efficiency and potential brain injury from preconditioning with irradiation or chemotherapy. Moreover, the BM-derived cells that migrate to the brain fail to replicate the characteristics of resident microglia. Here, we present a simple yet highly effective microglia transplantation strategy devoid of any conditioning, termed "Tri-Cyclic Microglial Depletion for Transplantation" (TCMDT). This approach leverages three cycles of microglial depletion using the CSF1R inhibitor PLX3397, creating a critical window for the efficient engraftment of exogenous microglia. Notably, transplanting primary cultured microglia via the TCMDT strategy achieves their full restoration to the identity and functions of native microglia. To evaluate the therapeutic potential, we applied our strategy to a Sandhoff disease mouse model, a form of neurodegenerative lysosomal storage disorder (LSD) caused by Hexb deficiency. The results revealed that our strategy facilitated the efficient replacement of deficient microglia, leading to a notable decrease in neurodegeneration and an enhancement in motor performance. Similarly, in an Alzheimer's disease (AD) mouse model carrying the Trem2 R47H mutation, our transplantation strategy corrected microglial dysfunction and alleviated AD-related pathology. Overall, our study presents a practical approach for microglia replacement that is simple, efficient, and safe, offering significant therapeutic potential for treating microglia-associated disorders.