Can xenotransplantation of human microglial progenitors model brain-derived microglia and their responses to injury or pathology?
Jonathan Hasselmann1, Morgan Coburn1, Dario Figueroa1, Hayk Davtyan1, Sunil Gandhi1, Mathew Blurton-Jones1 1University of California, Irvine
Much of our current understanding of human microglia biology is confounded by the transcriptomic changes that occur when microglia are studied in-vitro and the need to study model organisms or disease-affected/post-mortem, human samples. Given these challenges, there is a pressing need to develop approaches that allow for direct observation of human microglial function within the CNS. We therefore hypothesized that transplantation of human, stem-cell derived, hematopoietic progenitor cells (HPCs) into the early postnatal mouse brain would result in differentiation of microglia that accurately recapitulate in-vivo human microglial function and gene expression. This was examined by transplanting iPSC-derived HPCs into early postnatal, immunodeficient MITRG mice, expressing humanized versions of key cytokines including CSF1, which were subsequently aged for two to four months. Transplanted mice were then exposed to peripheral lipopolysaccharide injections, laser cell ablation, or transgene-mediated beta-amyloid accumulation. Brains were collected for analysis by immunohistochemistry and/or recovery of human microglia by FACS-sorting followed by RNA isolation for transcriptomic analysis. Robust engraftment of human cells that resemble microglia both morphologically and via marker expression was observed throughout the brain. Furthermore, exposure to insults resulted in microglial activation, proliferation and/or migration to injury sites and pathology. Finally, RNA has been isolated from transplanted, human microglia and is currently undergoing sequencing in parallel with human, ex-vivo samples. These data offer preliminary validation of a first of its kind model of human microglia xenotransplantation. 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 insult and injury. Upcoming transcriptomic analyses will allow for direct comparisons to human microglia and give further insight into the potential of this model to mimic human in-vivo microglial gene expression in the context of normal brain development, aging, and disease.