Chimeric organotypic brain slice cultures support the maturation and functional analysis of human iPSC-derived microglia

Microglia, the primary myeloid cells of the brain, are crucial for maintaining brain homeostasis and are implicated in various neuropathological processes. Microglial identity and function are determined by their ontogeny and the brain microenvironment. Therefore, model systems that faithfully reproduce mature human microglial molecular and functional phenotypes are crucial for understanding their role in brain health and disease. Here, we demonstrate that microglial precursors derived from induced pluripotent stem cells (iPSCs) can integrate and mature in murine organotypic brain slices. Importantly, we achieve a near-complete replacement of murine with human microglia and demonstrate that their maturation is driven solely by the tissue environment through previously underappreciated cross-species human colony stimulating factor 1 receptor (hCSF1R) binding of murine interleukin 34 (IL34). The iPSC-derived microglia in the chimeric brain slice cultures (cBSCs) exhibit functional responses and develop transcriptional states reminiscent of adult human microglia while responding to developing -synuclein proteopathy with heightened inflammatory state. Thus, our findings establish cBSCs as a tool for examining human microglia in a physiologically relevant environment, while maintaining the experimental flexibility of an in vitro system for mechanistically investigating neuropathological microglial responses.