Differentiation and specification of resident tissue macrophages [SMART-Seq2]

Tissue resident macrophages are functionally diverse cells that share an embryonic mesodermal origin. However, the mechanism(s) that control their specification remain unclear. We performed transcriptional, molecular and in situ spatio-temporal analyses of macrophage development in mice. We report that Erythro-Myeloid Progenitors generate pre-macrophages (pMacs) that simultaneously colonize the head and caudal embryo from embryonic day (E)9.5 in a chemokine-receptor dependent manner, to further differentiate into tissue F4/80+ macrophages. The core macrophage transcriptional program initiated in pMacs, is rapidly diversified in early macrophages as expression of transcriptional regulators becomes tissue-specific. For example, the preferential expression of the transcriptional regulator Id3 initiated in early fetal liver macrophages appears critical for Kupffer cell differentiation, as inactivation of Id3 causes a selective Kupffer cell deficiency that persists in adults. We propose that colonization of developing tissues by differentiating macrophages is immediately followed by their specification as they establish residence, hereby generating the macrophage diversity observed in post-natal tissues.