Generating tissue-resident macrophages from pluripotent stem cells: Lessons learned from microglia

Over the past decades, the importance of the immune system in a broad scope of pathologies, has drawn attention towards tissue-resident macrophages, such as microglia in the brain. To enable the study of for instance microglia, it is crucial to recreate in vitro (and in vivo) assays. However, very fast loss of tissue-specific features of primary tissue resident macrophages, including microglia, upon in vitro culture has complicated such studies. Moreover, limited knowledge of macrophage developmental pathways and the role of local 'niche factors', has hampered the generation of tissue-resident macrophages from pluripotent stem cells (PSC). Recent data on the ontogeny of tissue-resident macrophages, combined with bulk and single cell RNAseq studies have identified the distinct origins and gene profile of microglia compared to other myeloid cells. As a result, over the past years, protocols have been published to create hPSC-derived microglia-'like' cells, as these cells are considered potential new therapeutic targets for therapies to treat neurodegenerative diseases. In this review we will provide an overview of different approaches taken to generate human microglia in vitro, taking into account their origin, and resemblance to their in vivo counterpart. Finally, we will discuss cell-extrinsic (culture conditions) and intrinsic factors (transcriptional machinery and epigenetics) that we believe can improve future differentiation protocols of tissue-resident macrophages from stem cells.