Novel transplantation modalities for generating transcriptionally dependable new microglia from hematopoietic stem and progenitor cells

Recent pre-clinical and clinical evidences indicate that hematopoietic stem and progenitor cells (HSPCs) and/or their progeny can serve as vehicles for therapeutic molecule delivery across the blood brain barrier by contributing to the turnover of myeloid cell populations in the brain. However, the differentiation and functional characteristics of the cells reconstituted after transplantation are still to be determined, and in particular whether bona fide microglia could be reconstituted by the donor cell progeny post-transplant to be assessed. We here firstly demonstrate that HSPC transplantation can generate transcriptionally-dependable new microglia through a stepwise process reminiscent of physiological post-natal microglia maturation. Hematopoietic cells able to generate new microglia upon transplantation into myeloablated recipients are retained within human and murine long-term hematopoietic stem cells (HSCs). Similar transcriptionally dependable new microglia cells can also be generated by intra-cerebral ventricular delivery of HSPCs. Importantly, this novel route is associated to a clinically relevant faster and more widespread microglia replacement compared to systemic HSPC injection. Overall, this work supports the relevance and feasibility of employing HSPCs for renewing brain myeloid and microglia cells with new populations endowed with the ability to exert therapeutic effects in the central nervous system, and identifies novel modalities, such as transplantation of enriched stem cell fractions and direct brain delivery of HSPCs, for increasing the actual contribution of the transplanted cells to microgliosis and their therapeutic activity.