iPS-cell-derived microglia promote brain organoid maturation via cholesterol transfer

Microglia are specialized brain-resident macrophages that arise from primitive macrophages colonizing the embryonic brain¹. Microglia contribute to multiple aspects of brain development, but their precise roles in the early human brain remain poorly understood owing to limited access to relevant tissues^2-6. The generation of brain organoids from human induced pluripotent stem cells recapitulates some key features of human embryonic brain development^7-10. However, current approaches do not incorporate microglia or address their role in organoid maturation^11-21. Here we generated microglia-sufficient brain organoids by coculturing brain organoids with primitive-like macrophages generated from the same human induced pluripotent stem cells (iMac)²². In organoid cocultures, iMac differentiated into cells with microglia-like phenotypes and functions (iMicro) and modulated neuronal progenitor cell (NPC) differentiation, limiting NPC proliferation and promoting axonogenesis. Mechanistically, iMicro contained high levels of PLIN2⁺ lipid droplets that exported cholesterol and its esters, which were taken up by NPCs in the organoids. We also detected PLIN2⁺ lipid droplet-loaded microglia in mouse and human embryonic brains. Overall, our approach substantially advances current human brain organoid approaches by incorporating microglial cells, as illustrated by the discovery of a key pathway of lipid-mediated crosstalk between microglia and NPCs that leads to improved neurogenesis.