Human iPSC-derived microglia assume a primary microglia-like state after transplantation into the neonatal mouse brain

Significance Microglia, the resident immune cells of the brain, are known to be crucial for normal brain function as well as contributing to neurodegenerative disease. However, microglial cells are very sensitive to culture conditions and have been shown to lose their key properties when grown in vitro. In this study, we developed a model in which we transplant immature microglial cells derived from human stem cells into the brains of neonatal mice. By growing microglia in the environment of the mouse brain, the stem cell-derived microglial cells acquire key aspects of microglial morphology and gene expression which cannot be attained in culture, and are therefore a better representation of human microglial cells for the purpose of studying human disease. Microglia are essential for maintenance of normal brain function, with dysregulation contributing to numerous neurological diseases. Protocols have been developed to derive microglia-like cells from human induced pluripotent stem cells (hiPSCs). However, primary microglia display major differences in morphology and gene expression when grown in culture, including down-regulation of signature microglial genes. Thus, in vitro differentiated microglia may not accurately represent resting primary microglia. To address this issue, we transplanted microglial precursors derived in vitro from hiPSCs into neonatal mouse brains and found that the cells acquired characteristic microglial morphology and gene expression signatures that closely resembled primary human microglia. Single-cell RNA-sequencing analysis of transplanted microglia showed similar cellular heterogeneity as primary human cells. Thus, hiPSCs-derived microglia transplanted into the neonatal mouse brain assume a phenotype and gene expression signature resembling that of resting microglia residing in the human brain, making chimeras a superior tool to study microglia in human disease.