Epigenetic adaptation drives monocyte differentiation into microglia-like cells upon engraftment into the retina

The identification of specific markers for microglia has been a long-standing challenge. Recently, markers such as P2ry12, TMEM119, and Fcrls have been proposed as microglia-specific and widely used to explore microglial functions within various central nervous system (CNS) contexts. The specificity of these markers was based on the assumption that circulating monocytes retain their distinct signatures even after infiltrating the CNS. However, recent findings reveal that infiltrating monocytes can adopt microglia-like characteristics while maintaining a pro-inflammatory profile upon permanent engraftment in the CNS.In this study, we utilize bone marrow chimeras, single-cell RNA sequencing, ATAC-seq, flow cytometry, and immunohistochemistry to demonstrate that engrafted monocytes acquire expression of established microglia markers P2ry12, TMEM119, Fcrls and the pan-myeloid marker Iba1, which has been commonly mischaracterized as microglia-specific. These changes are accompanied by alterations in chromatin accessibility and shifts in chromatin binding motifs that are indicative of microglial identity. Moreover, we show that engrafted monocytes dynamically regulate the expression of CX3CR1, CCR2, Ly6C, and transcription factors PU.1, CTCF, RUNX, AP-1, CEBP, and IRF2, all of which are crucial for shaping microglial identity. This study is the first to illustrate that engrafted monocytes in the retina undergo both epigenetic and transcriptional changes, enabling them to express microglia-like signatures. These findings highlight the need for future research to account for these changes when assessing the roles of monocytes and microglia in CNS pathology.