Hematopoietic stem cell transplantation chemotherapy causes microglia senescence and peripheral macrophage engraftment in the brain

Hematopoietic stem cell transplantation (HSCT) is a therapy used for multiple malignant and nonmalignant diseases, with chemotherapy used for pretransplantation myeloablation. The post-HSCT brain contains peripheral engrafted parenchymal macrophages, despite their absence in the normal brain, with the engraftment mechanism still undefined. Here we show that HSCT chemotherapy broadly disrupts mouse brain regenerative populations, including a permanent loss of adult neurogenesis. Microglial density was halved, causing microglial process expansion, coinciding with indicators of broad senescence. Although microglia expressed cell proliferation markers, they underwent cell cycle arrest in S phase with a majority expressing the senescence and antiapoptotic marker p21. In vivo single-cell tracking of microglia after recovery from chemical depletion showed loss of their regenerative capacity, subsequently replaced with donor macrophages. We propose that HSCT chemotherapy causes microglial senescence with a gradual decrease to a critical microglial density, providing a permissive niche for peripheral macrophage engraftment of the brain.