Early Age Efferocytosis Directs Macrophage Arachidonic Acid Metabolism for Tissue Regeneration

In response to organ injury in adults, macrophages often promote scarring, yet during early life are required for tissue regeneration. To elucidate mechanisms underlying age-associated regeneration, we compared the macrophage injury response in newborns versus adult hearts. Single cell analysis revealed an accumulation of tissue-resident macrophages in neonates that were selectively polarized for apoptotic cell recognition and uptake (efferocytosis). Ablation of the apoptotic cell recognition receptor Mertk in newborns prevented cardiac regeneration. These findings could be attributed to reprogramming of macrophage gene expression that was required for biosynthesis of the eicosanoid Thromboxane A2, which unexpectedly activated parenchymal cell proliferation. Markers of Thromboxane A2 production were suppressed in adult macrophages after efferocytosis. Moreover, macrophage-neighboring neonatal cardiomyocytes expressed the Thromboxane A2 receptor, whose activation induced a metabolic shift supporting cellular proliferation. Our data reveal a fundamental age-defined macrophage response in which lipid mitogens produced during efferocytosis support receptor-mediated tissue regeneration.