Summary Microglia play critical roles in brain development, homeostasis, and neurological disorders. Here, we report that human microglial-like cells (iMGL) can be differentiated from iPSCs to study their function in neurological diseases, like Alzheimer’s disease (AD). We find that iMGLs develop in vitro similarly to microglia in vivo and whole transcriptome analysis demonstrates that they are highly similar to cultured adult and fetal human microglia. Functional assessment of iMGLs reveals that they secrete cytokines in response to inflammatory stimuli, migrate and undergo calcium transients, and robustly phagocytose CNS substrates. iMGLs were used to examine the effects of Aβ fibrils and brain-derived tau oligomers on AD-related gene expression and to interrogate mechanisms involved in synaptic pruning. Furthermore, iMGLs transplanted into transgenic mice and human brain organoids resemble microglia in vivo. Together, these findings demonstrate that iMGLs can be used to study microglial function, providing important new insight into human neurological disease.

We report that prospectively isolated, human CNS stem cells grown as neurospheres (hCNS-SCns) survive, migrate, and express differentiation markers for neurons and oligodendrocytes after longterm engraftment in spinal cord-injured NOD-scid mice. hCNS-SCns engraftment was associated with locomotor recovery, an observation that was abolished by selective ablation of engrafted cells by diphtheria toxin. Remyelination by hCNS-SCns was found in both the spinal cord injury NOD-scid model and myelin-deficient shiverer mice. Moreover, electron microscopic evidence consistent with synapse formation between hCNS-SCns and mouse host neurons was observed. Glial fibrillary acidic protein-positive astrocytic differentiation was rare, and hCNS-SCns did not appear to contribute to the scar. These data suggest that hCNS-SCns may possess therapeutic potential for CNS injury and disease.behavioral assessment ͉ differentiation ͉ stem cell transplantation R ecent studies have used a variety of immortalized, engineered, or isolated rodent-derived precursor͞stem cells transplanted into rodent models of spinal cord injury. Many of these studies focused on cell survival and did not address differentiation, functional recovery, or the causal relationship between successful engraftment and observed behavioral improvements. When differentiation was investigated, embryonic and adult neural stem cells were reported to principally assume glial fibrillary acidic protein (GFAP)-positive astrocytic phenotypes after grafting into nonneurogenic regions of uninjured adult CNS (1, 2) or injured spinal cord (3-5). Furthermore, although in vitro predifferentiation paradigms designed to generate neural lineage restricted precursors successfully generated ␤-tubulin III (Tuj-1)-positive neuronal phenotypes either in vitro or after transplantation into uninjured spinal cord, this commitment was overridden by environmental cues in the injured spinal cord (6).Transplants of human brain-derived stem cells or human spinal cord tissue into injured rat spinal cord have been described (7-9). Moreover, several human cell transplantation paradigms recently have been reported to promote locomotor recovery: human umbilical cell infusion in a rat spinal cord injury model, although only within 3 weeks or less postgrafting (10); neurons differentiated in vitro under retinoic acid from human embryonal teratocarcinoma cells and transplanted into a rat spinal cord injury model (11); human ES cells differentiated in vitro to oligoprogenitors and transplanted into a rat spinal cord injury model (12); and human neural stem͞progenitor cells transplanted into a monkey spinal cord injury model (13). In general, these studies lack some or all of the following: definitive identification of transplanted cells, longterm survival and engraftment data, evidence of differentiation, and͞or direct evidence of functional integration of human cells in the injured spinal cord. The current study addresses three previously unexplored issues in stem cell transplantation research for spinal co...

iPSC-derived microglia offer a powerful tool to study microglial homeostasis and diseaseassociated inflammatory responses. Yet, microglia are highly sensitive to their environment, exhibiting transcriptomic deficiencies when kept in isolation from the brain. Furthermore, species-specific genetic variations demonstrate that rodent microglia fail to fully recapitulate the human condition. To address this, we developed an approach to study human microglia within a surrogate brain environment. Transplantation of iPSC-derived hematopoieticprogenitors into the postnatal brain of humanized, immune-deficient mice results in contextdependent differentiation into microglia and other CNS macrophages, acquisition of an ex vivo human microglial gene signature, and responsiveness to both acute and chronic insults. Most notably, transplanted microglia exhibit robust transcriptional responses to A-plaques that only partially overlap with that of murine microglia, revealing new, human-specific Aresponsive genes. We therefore propose that this chimeric model can provide a powerful new system to examine the in vivo function of patient-derived and genetically-modified microglia.