Contribution of resident macrophages to the fetal liver hematopoietic stem cell niche

Abstract

Tissue-resident macrophages are a heterogeneous population of phagocytes found in almost all tissues of the body. Recent studies indicate that most resident macrophages originate from yolk sac progenitors and are maintained by local proliferation independent of hematopoietic stem cells (HSC). It is known that HSC can quickly respond to organismal demands. During embryogenesis, the fetal liver supports the HSC with a unique microenvironment called the “HSC niche” that helps their expansion and maintenance. One of the most important aspects of a niche is cell-cell interactions that can happen either directly or through signaling. In the fetal liver, hepatic macrophages are known to participate in the maturation of erythroblasts. However, whether resident macrophages can also contribute to the fetal liver HSC niche remains enigmatic. Furthermore, the macrophages heterogeneity across different organs is essential for tissue-specific niches and cellular interactions during tissue development as well as tissue homeostasis. However, their heterogeneity within one organ, such as the fetal liver, especially during development, has not been addressed in detail. <br /> In this dissertation, using single-cell RNA-sequencing, it was shown that hepatic macrophages are heterogeneous and have different subpopulations that can have different functions. This heterogeneity was further confirmed through multi-color flow-cytometry and unsupervised clustering of cells using computational methods. Investigating the ontogeny of the identified clusters using three fate-mapping models revealed that all macrophage clusters originated from the yolk sac and are not HSC derived. <br /> Further, immunofluorescent-labeled sections from the fetal liver could show that while many of the macrophages serve as a platform for erythroblasts maturation in the fetal liver, yet some of them also interact with HSC. To test the functionality of hepatic macrophages in HSC, conditional mouse models were developed that lead to the depletion of macrophages in the fetal liver. The results indicated that hepatic macrophages are responsible for erythroblast maturation, as enucleation of red blood cells is not efficient when macrophages are lacking. Further, it was shown that macrophages are part of the HSC niche since stem cells have a differentiation bias towards the myeloid linage in knock-out embryos. These results were further confirmed by performing a bulk RNA- sequencing using HSC from embryos of mouse models. <br /> All in all, the findings of this dissertation support the hypothesis that hepatic macrophages play a crucial role in the development and maintenance of fetal liver hematopoiesis and provide evidences for their heterogeneity within the fetal liver.