Promoting donor microglial replacement through augmented conditioning or radiation sensitivity

Donor myeloid cells engraft the brain after hematopoietic cell transplantation (HCT), partially replacing native microglia. The conditions which promote or maintain engraftment are not well known. Previously, we demonstrated that mice receiving double HCT using radiation-based conditioning had higher brain engraftment but also mortality of 70%. We now show that mice that undergo double HCT with busulfan preconditioning have nearly 70%-80% brain engraftment and lower mortality of 8%. Using the radiation-sensitive NSG mouse model, we found up to 15% donor myeloid (CX3CR1⁺) brain engraftment after low-dose radiation (450 cGy) vs. 0.1% in C57BL/6 mice treated similarly. NBSGW mice engraft the marrow without the need for conditioning, and we found that in these mice the addition of radiation boosted brain engraftment greater than 10-fold, from 0.3% (no conditioning) to 4.8% after HCT. A further increase in brain engraftment (median 22.5%) was observed when animals were pretreated with PLX3397, a microglia inhibitor, which indicated that myeloid niche clearance promotes brain engraftment. In conclusion, preconditioning and niche clearance play an important role in brain engraftment. New ways of microglia replacement are needed if we are to avoid the use of preconditioning.