Embryonic Immune Cells Remodel the Heart

Intercellular genetic tracing of cardiac endothelium in the developing heart

Cardiac resident macrophages play vital roles in heart development, homeostasis, repair, and regeneration. Recent studies documented the hematopoietic potential of cardiac endothelium that supports the generation of cardiac macrophages and peripheral blood cells in mice. However, the conclusion was not strongly supported by previous genetic tracing studies, given the non-specific nature of conventional Cre-loxP tracing tools. Here, we develop an intercellular genetic labeling system that can permanently trace heart-specific endothelial cells based on cell-cell interaction in mice. Results from cell-cell contact-mediated genetic fate mapping demonstrate that cardiac endothelial cells do not exhibit hemogenic potential and do not contribute to cardiac macrophages or other circulating blood cells. This Matters Arising paper is in response to Shigeta et al. (2019), published in Developmental Cell. See also the response by Liu and Nakano (2023), published in this issue.

Lineage tracing clarifies the cellular origin of tissue-resident macrophages in the developing heart

Tissue-resident macrophages play essential functions in the maintenance of tissue homeostasis and repair. Recently, the endocardium has been reported as a de novo hemogenic site for the contribution of hematopoietic cells, including cardiac macrophages, during embryogenesis. These observations challenge the current consensus that hematopoiesis originates from the hemogenic endothelium within the yolk sac and dorsal aorta. Whether the developing endocardium has such a hemogenic potential requires further investigation. Here, we generated new genetic tools to trace endocardial cells and reassessed their potential contribution to hematopoietic cells in the developing heart. Fate-mapping analyses revealed that the endocardium contributed minimally to cardiac macrophages and circulating blood cells. Instead, cardiac macrophages were mainly derived from the endothelium during primitive/transient definitive (yolk sac) and definitive (dorsal aorta) hematopoiesis. Our findings refute the concept of endocardial hematopoiesis, suggesting that the developing endocardium gives rise minimally to hematopoietic cells, including cardiac macrophages.

IL-18 Promotes Erythrophagocytosis and Erythrocyte Degradation by M1 Macrophages in a Calcific Microenvironment

BACKGROUND

Calcific aortic-valve disease (CAVD) is the most common cause of aortic valve replacement in developed countries. Intraleaflet hemorrhage has been found to be positively correlated with the progression of CAVD. Although most research has focused on erythrocyte degradation products, which promote progression of CAVD, the process of erythrophagocytosis and erythrocyte degradation by macrophages in intraleaflet hemorrhage areas has remained unexplored.

METHODS

The erythrocyte degradation products of aortic valve specimens were detected by Perls' staining and quantified. The gene and protein expression levels of interleukin (IL)-18 in THP-1-polarized macrophages cultured in osteogenic medium were tested. We also quantified the iron and heme degraded by macrophages and analyzed the expression of ferroportin (FPN) and heme oxygenase-1 (HO-1) in the osteogenic medium. Furthermore, we tested the mRNA and protein levels of osteoblast markers in valve interstitial cells after co-culture with M1 macrophages treated with IL-18 and erythrocytes.

RESULTS

Our experiments demonstrated that IL-18 activates HO-1 and FPN to promote erythrophagocytosis and erythrocyte degradation by macrophages in a calcific microenvironment via p38 and Erk1/2. We also found that the calcific microenvironment promotes IL-18 mRNA and protein expression in THP-1-polarized macrophages.

CONCLUSIONS

In conclusion, IL-18 promotes M1 macrophage-mediated erythrophagocytosis and erythrocyte degradation by regulating the activation of HO-1 and FPN via p38 and Erk1/2 in a calcific microenvironment.