Single-cell mRNA analysis of colon phagocyte heterogeneity identifies two major macrophage developmental pathways

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Abstract Tissue macrophages (MPs) develop distinctive functions due to unique transcriptional programs driven by local environmental ques. In the intestine, it is known that MPs are heterogeneous cells that differentiate from blood monocytes and embryonic precursors, and are essential for homeostasis with the vast microbiome through their ability to kill invading microbes, clear apoptotic cells, and produce regulatory cytokines. However, the full extent of MP heterogeneity, their developmental relationships, and how the intestinal microbiota affects MP generation is not known. Here, we performed single cell RNA sequencing of colonic myeloid cells from specific pathogen free (SPF) and germ free (GF) C57BL/6 mice and found extensive heterogeneity of colon MP, with at least six different identifiable populations, with less heterogeneity of dendritic cells (DCs). Unsupervised modeling of developmental pathways predicted two major trajectories from common CCR2+precursors resulting in CD11c+CD9+MMRintCD121b+and CD11c−CD9intMMRhiCD121b−MP populations. The generation of both mature populations of colon MPs was impaired in germ-free mice, which coincided with the appearance of a dominant unique population of MPs, whereas the generation of DC populations in GF mice were unaffected or increased. Furthermore, the two major MP populations were clearly distinct from other populations regarding their gene expression profile, localization within the lamina propria, and ability to phagocytose macromolecules from the blood. These data uncover the diversity of intestinal myeloid cell populations, highlight the importance of microbiota on the unique developmental as well as anatomical and functional fates of colon MPs.