Fate-mapping studies have demonstrated the prenatal origin of macrophages in many tissues and their ability to undergo population expansion and be maintained in situ. In Immunity, Mann and colleagues characterize macrophage populations in the myocardium at steady state and after cardiac stress. Four subsets of cardiac-resident macrophages exist separately from the blood and are renewed through in situ proliferation at steady state. Most adult cardiac macrophages are established embryonically, with a substantial contribution from the yolk sac. In a model of cardiac infarction, macrophage numbers increase through both the in situ proliferation of resident macrophages and the recruitment of blood Ly6Chi monocytes, which differentiate into all four macrophage subsets to different degrees. Transcriptional analysis shows that monocyte-derived macrophages coordinate cardiac inflammation, while both blood-derived macrophages and embryo-derived macrophages have a role in antigen sampling.

Immunity 40, 91–104 (2014)