Tissue LyC6− Macrophages Are Generated in the Absence of Circulating LyC6− Monocytes and Nur77 in a Model of Muscle Regeneration

Abstract

There are several open questions regarding the origin, development, and differentiation of subpopulations of monocytes, macrophages (MFs), and dendritic cells. It is a particularly intriguing question how circulating monocyte subsets develop and contribute to the generation of steady-state and inflammatory tissue MF pools and which transcriptional mechanisms contribute to these processes. In this study, we took advantage of a genetic model in which LyC6− circulating monocyte development is severely diminished due to the lack of the nuclear receptor, NUR77. We show that, in a mouse model of skeletal muscle injury and regeneration, the accumulation of leukocytes and the generation of LyC6+ and LyC6− MF pools are intact in the absence of circulating LyC6− blood monocytes. These data suggest that NUR77, which is required for LyC6− blood monocyte development, is expressed but not critically required for LyC6+ to LyC6− tissue MF specification. Moreover, these observations support a model according to which tissue macrophage subtype specification is distinct from that of circulating monocytes. Lastly, our data show that in the used sterile inflammation model tissue LyC6− MFs are derived from LyC6+ cells.