Jonathan Hasselmann, Morgan Coburn, Dario Figueroa, Christina Tu, Hayk Davtyan, Sunil Gandhi, Mathew Blurton-Jones

Department of Neurobiology and Behavior and Sue and Bill Gross Stem Cell Research Center, University of California, Irvine

 

Microglia are strongly implicated in virtually every age-related neurodegenerative disorder including Alzheimer’s Disease (AD).   Yet, studies of human microglia have been hindered by the difficulty in obtaining these cells and the need to examine them either in vitro or within post-mortem specimens. This problem was further highlighted by recent findings that isolated human microglia undergo rapid transcriptional changes with in vitro culture. Thus, there is pressing need to develop new approaches to directly observe and examine human microglial function within the ageing CNS. To address this need, we hypothesized that transplantation of human iPSC-derived hematopoietic progenitor cells (HPCs) into the early postnatal brain of immune-deficient mice would result in differentiation of functional microglia that more accurately recapitulate in vivo human microglia biology. GFP-expressing iPSC-derived HPCs were transplanted into P1 immunodeficient MITRG mice that express humanized versions of key cytokines that facilitate long-term survival and differentiation of human myeloid cells. Transplanted mice were aged for 2-6 months and then exposed to either intrinsic or extrinsic insults including peripheral LPS, laser cell ablation, or transgenic-mediated production of beta-amyloid plaques. Brains were then collected for analysis by immunohistochemistry and recovery of human microglia by FACs sorting for GFP expression. Robust engraftment of human cells that morphologically closely resemble microglia was observed throughout the brain and these cells stained for multiple microglia markers including P2RY12, Iba1, CD11b, Pu.1, and TMEM119. Furthermore, exposure to insults resulted in human microglial activation, proliferation, and/or migration to injury sites and pathology. In addition RNA sequencing of FACS-sorted human microglia recovered from the mouse brain revealed numerous changes in the microglial transcriptome in response to LPS and the adoption of a more ‘in vivo-like’ transcriptional profile following transplantation. Both histological and functional analyses show that transplantation of human HPCs into the early postnatal mouse brain results in robust engraftment of microglia which actively monitor the CNS and rapidly respond to insults or injury.  Taken together these data suggest that this model can be used to advance our understanding of human microglial biology in the context of normal brain development, aging, and disease.