Abstract 81: Hemogenic Endocardium Contributes to Cardiac Tissue Macrophages

Transient phase of hematopoiesis in mammalian embryo occur in multiple anatomical sites including yolk-sac, placenta and the aorta-gonad-mesonephros region. A recent report demonstrates that endocardium also contribute to definitive hematopoiesis during embryogenesis. CD41+ hemogenic endocardial cells are enriched in the endocardial cushion in the outflow tract and atrial-ventricular canal at around embryonic day 9.5-11.5. Here we show the hemogenic endocardium as a novel source of cardiac tissue macrophages (cTMs). The fate of endocardial cells were traced using nuclear factor of activated T-cells 1 (Nfatc1)-cre mouse line, a Cre driver specific to the endocardium. Flow-cytometry study using NFATc1 cre/+ ; R26YFP reporter/+ with CD45, CD11b and F4/80 demonstrated that endocardially-derived cardiac tissue macrophages (EcTMs) were found in the heart from embryonic day 10.5 to adult stages. The number of EcTMs gradually expanded from embryonic day 15.5 to postnatal 8 and were maintained until adult stage. Immunofluorescent staining with CD68, F4/80 and CD206 revealed that EcTMs were identified in the cardiac cushion at embryonic day 13.5 and 15.5, and persisted in the valve mesenchyme and atria after birth. To assess their functional potential, we characterized their M1/M2 polarity. Surface marker analyses indicate that EcTMs in embryo were classified exclusively as M2 type whereas non-endocardially-derived cTMs (non-EcTMs) contributed to both M1 and M2 type. EcTMs in postnatal hearts were also predominantly M2 type. Genome-wide transcriptome analysis showed that genes related to antigen presenting and phagocytosis were significantly more upregulated in EcTMs compared to non-EcTMs. These findings indicate that the hemogenic endocardium contributes to M2-type cTMs in endocardial cushion and valve mesenchyme throughout embryogenesis and postnatal heart. Cardiac tissue macrophages migrate to the heart in multiple waves including postnatal bone marrow, fetal liver during embryonic stages, and possibly yolk sac prior to the liver colonization. Our results suggest that the 4 th population of macrophages originate directly from the local hemogenic endocardium.