Hans-Reimer Rodewald

Cardiac macrophages (cMΦ) are critical for early postnatal heart regeneration and fibrotic repair in the adult heart, but their origins and cellular dynamics during postnatal development have not been well characterized. Tissue macrophages can be derived from embryonic progenitors or from monocytes during inflammation. We report that within the first weeks after birth, the embryo-derived population of resident CX3CR1(+) cMΦ diversifies into MHCII(+) and MHCII(-) cells. Genetic fate mapping demonstrated that cMΦ derived from CX3CR1(+) embryonic progenitors persisted into adulthood but the initially high contribution to resident cMΦ declined after birth. Consistent with this, the early significant proliferation rate of resident cMΦ decreased with age upon diversification into subpopulations. Bone marrow (BM) reconstitution experiments showed monocyte-dependent quantitative replacement of all cMΦ populations. Furthermore, parabiotic mice and BM chimeras of nonirradiated recipient mice revealed a slow but significant donor contribution to cMΦ. Together, our observations indicate that in the heart, embryo-derived cMΦ show declining self-renewal with age and are progressively substituted by monocyte-derived macrophages, even in the absence of inflammation.

Mast cells originate from hematopoietic stem cells, but the mast cell-committed precursor has not been identified. In the study presented here, a cell population in murine fetal blood that fulfills the criteria of progenitor mastocytes was identified. It is defined by the phenotype Thy-1loc-Kithi, contains cytoplasmic granules, and expresses RNAs encoding mast cell-associated proteases but lacks expression of the high-affinity immunoglobulin E receptor. Thy-1loc-Kithi cells generated functionally competent mast cells at high frequencies in vitro but lacked developmental potential for other hematopoietic lineages. When transferred intraperitoneally, this population reconstituted the peritoneal mast cell compartment of genetically mast cell-deficient W/Wv mice to wild-type levels.