Pro-inflammatory Aorta-Associated Macrophages Are Involved in Embryonic Development of Hematopoietic Stem Cells

Abstract

Summary Hematopoietic stem cells (HSCs) are generated from specialized endothelial cells of the embryonic aorta. Inflammatory factors are implicated in regulating mouse HSC development, but which cells in the aorta-gonad-mesonephros (AGM) microenvironment produce these factors is unknown. In the adult, macrophages play both pro- and anti-inflammatory roles. We sought to examine whether macrophages or other hematopoietic cells found in the embryo prior to HSC generation were involved in the AGM HSC-generative microenvironment. CyTOF analysis of CD45 + AGM cells revealed predominance of two hematopoietic cell types, mannose-receptor positive macrophages and mannose-receptor negative myeloid cells. We show here that macrophage appearance in the AGM was dependent on the chemokine receptor Cx3cr1. These macrophages expressed a pro-inflammatory signature, localized to the aorta, and dynamically interacted with nascent and emerging intra-aortic hematopoietic cells (IAHCs). Importantly, upon macrophage depletion, no adult-repopulating HSCs were detected, thus implicating a role for pro-inflammatory AGM-associated macrophages in regulating the development of HSCs.

A role for macrophages in hematopoiesis in the embryonic head

“…HCs in the aorta during the time of EHT are decreased/absent in the absence of Cx3cr1, and AGM HPC numbers and HSC in vivo repopulating activity are decreased in Cx3cr1 null embryos. 16 Similarly, we found that HPC numbers in the HBA of Cx3cr1 null embryos were significantly reduced. This is likely to be a consequence of decreased macrophage emigration to this region and is supported by the fact that macrophage (CD45 1 CD11b 1 F4/80 1 ) percentages were significantly (50%) reduced, and percentages of progenitor cells (cKit 1 CD41 low ) were also significantly reduced in the Cx3cr1 null HBA.…”

“…46 Single-cell RNA sequencing of highly enriched HS/PCs from mouse embryonic and fetal stages reveals some low Csf1r expression by these cells, 47 yet Csf1r cell tracing and other studies suggest that Csfr1 is not expressed by HSCs. 16,41 Thus, it is unclear in the HBA whether cells other than macrophages express Csf1r. Our analysis of sorted MacGreen HBA cells revealed that HPCs were distributed in the high-, intermediate-, and negative-expressing fractions (supplemental Figure 4A), and thus, has implications for the use of the in vivo Csf1r targeted DTA ablation model.…”

“…12,13 A surprisingly large number of CD45 1 F4/80 1 macrophages were found to be scattered around the mouse embryonic day (E)10.5 aorta and adjacent to cells of the IAHC. 8,[14][15][16] Recent time-lapse imaging studies show transient interactions of macrophages with IAHC. 16 The earliest embryonic macrophages in the mouse are formed in the yolk sac blood islands beginning at E7.5.…”

“…8,[14][15][16] Recent time-lapse imaging studies show transient interactions of macrophages with IAHC. 16 The earliest embryonic macrophages in the mouse are formed in the yolk sac blood islands beginning at E7.5. 17 In vivo lineage tracing supports a recruitment of these macrophages to head region, where in the brain they become the tissue resident macrophages, "microglia," that persist through all stages of life.…”

Blood making: learning what to put into the dish

“…Moreover, studies suggested a link between the developing nervous system and enhancement of HSC emergence, as shown by the inductive effect of catecholamines in the mouse 85 and neuronal serotonin in the zebrafish 86 . Interestingly, from the yolk sac-derived wave, macrophages 87,88 and neutrophils 89 contribute with local sterile inflammatory signaling that promotes HSC emergence.…”