Direct microglia replacement reveals the impact of ontogeny on murine microglial responses to pathologic amyloid aggregation

Microglia are central to the progression of many neurodegenerative diseases, especially Alzheimer’s Disease (AD). Adoptive transfer of engineered cells has demonstrated clinical benefit in oncology, autoimmune disease, and inherited blood disorders. Replacement of disease-affected microglia thus holds high therapeutic potential in AD, but how microglia replacement, including the source and ontogeny of donor microglia surrogates, fundamentally impacts AD neuropathology, is unresolved. We recently developed a microglia replacement approach based on engineered, inhibitor resistant CSF1R variants, that we here deployed in the 5xFAD model to better understand how microglia replacement impacts neuropathology and associated cell states spurred by aggressive amyloid aggregation. We achieved near-complete, direct microglia replacement prior to plaque accumulation, using ex vivo expanded hematopoietic stem cells (HSCs), Hoxb8 conditionally immortalized macrophages, and primary CD11b+ cells from early postnatal brain as donor sources. Although two months after transplant, CD11b+ brain cells slightly increased plaque burden, and conditionally immortalized macrophages slightly decreased astrogliosis, microglia replacement did not otherwise impact plaque burden, astrogliosis, nor the transcriptional state of astrocytes and oligodendrocytes as measured by single-cell RNA sequencing in 5 month old mice. In stark contrast, we found that donor cell ontogeny highly impacted both homeostatic and amyloid reactive macrophage state. In particular, and unlike transplanted microglia, HSC and Hoxb8 donor populations adopted a disease associated microglia (DAM)-like baseline state, but upon amyloid challenge, failed to effectively induce the complete DAM signature. Overall, these findings reveal large differences in the response of fetal origin compared with adult bone marrow-derived microglia to neuropathology, but that these large differences do not have a commensurate impact on the brain environment, important considerations for the development of microglia replacement therapies and the study of brain myeloid cell infiltration.