Resident immune cells in mucosal barriers of vertebrates, such as tissue-resident macrophages (TRMs), are highly heterogeneous populations that play crucial roles in maintenance of tissue homeostasis, which still lack comprehensive understanding. Here I report the identification and characterization of three novel non-hematopoietic derived immune cells in three major mucous barrier tissues-epidermis, gill and intestine of zebrafish. By using IR-LEGO system, I firstly identify two morphological similar mpeg1⁺ populations in these tissues, one is widely distributed conventional TRMs generated from AGM, the other is locally restricted mpeg1⁺ cells with non-hematopoietic origin. RNA-sequencing analysis of these two populations reveals that locally restricted mpeg1⁺ populations in al...[ Read more ]

Resident immune cells in mucosal barriers of vertebrates, such as tissue-resident macrophages (TRMs), are highly heterogeneous populations that play crucial roles in maintenance of tissue homeostasis, which still lack comprehensive understanding. Here I report the identification and characterization of three novel non-hematopoietic derived immune cells in three major mucous barrier tissues-epidermis, gill and intestine of zebrafish. By using IR-LEGO system, I firstly identify two morphological similar mpeg1⁺ populations in these tissues, one is widely distributed conventional TRMs generated from AGM, the other is locally restricted mpeg1⁺ cells with non-hematopoietic origin. RNA-sequencing analysis of these two populations reveals that locally restricted mpeg1⁺ populations in all three tissues are myeloidlike cells (MLCs) which share the highest similarities with TRMs in transcriptome levels. Intriguingly, further fate mapping studies demonstrate that MLCs in epidermis are generated from ectoderm while those in gill and intestine are derived from endoderm. Unlike TRMs, MLCs in three tissues respond neither to tissue injury nor bacterial infection, but rather sample soluble antigens from external environment, which are depend on tight-junction mediated formation of transepithelial protrusions. After soluble antigen uptake, MLCs in epidermis transfer those antigens to TRMs (Langerhans Cells) through apoptosis-phagocytosis axis, for which we designate this MLCs as “metaphocyte”. This phenomenon suggests an immunological role of metaphocytes. In line with this, MLCs in gill also participate in regulating interferon productions upon poly(I:C) stimulation. My studies document the existence of non-hematopoietic derived myeloid-like cells that manifest distinct function from conventional TRMs in mucosal barriers and open a new paradigm for investigation the heterogeneities of resident immune cells of vertabrates.