Abstract
Macrophages are innate immune cells present in all tissues, in which they participate in immune responses and maintenance of tissue homeostasis. They develop either from embryonic precursors or from circulating monocytes, and their functions are in part dictated by their origin. We previously observed robust monocyte recruitment and contribution to the macrophage pool in brown adipose tissue. In particular, monocytes were predicted to give rise to two phenotypically distinct macrophage subsets identifiable by CD206 or CD226 expression. In the present study, we investigated monocyte differentiation pathways in brown adipose tissue and the function of monocyte-derived macrophages. We found that bone marrow monocytes highly contributed to the CD226 (+) macrophage population while the CD206 (+) population contained mainly yolk sac-derived cells. Fate mapping analysis revealed a low contribution of GMP- and a high contribution of MDP-derived monocytes to the CD226 (+) macrophage subset. Importantly, adoptive transfer experiments demonstrate that MDP- but not GMP-derived monocytes are pre-conditioned to give rise to CD226 (+) macrophages. Using meta-analysis of single cell RNA-sequencing data, we found that MDP-derived CD226 (+) macrophages were present in several tissues including peritoneal cavity, adrenal glands and all adipose depots, with a particular enrichment in beige and brown fat. A similar macrophage subset was identified in humans. Functionally, while depletion of CD206 (+) macrophages using anti-CD115 blocking antibodies led to decreased adipose triglyceride content, genetic depletion of CD226 (+) macrophages caused the opposite phenotype. We thus identify CD226 (+) MDP-derived macrophages as a new myeloid cell type conserved across tissues and tied to lipid metabolism homeostasis.