P5384Postnatal mouse aorta contains yolk sac-derived haemangioblasts with myeloid and endothelial plasticity and vasculogenic capacity
Authors: Williamson A , Toledo D F , Schwarz N , Fernando S , Dimasi C , Liyanage S , Skoumbourdis T , Chandrakanthan V , Di Bartolo B , Tan J T M , Bonder C S , Nicholls S J , Bursill C A , Psaltis P J 
Published: October 2019

Abstract

Background
Macrophages and endothelial cells share an intimate relationship during neovessel formation in different pathophysiological conditions. Recent studies have determined that in some tissues, both cell types are derived embryonically from yolk sac (YS) progenitor cells and are maintained postnatally without contribution from circulating sources. The mechanism by which this local “self-maintenance” occurs is unknown.

Purpose
We previously identified that mouse arteries contain macrophage and endothelial progenitor cells in their adventitial Sca-1+CD45+ compartment. Here we investigated at a clonal level for the existence of postnatal adventitial haemangioblasts and studied their developmental origins.

Methods and results
Single cell digests were prepared from murine aortas to perform colony-forming unit (CFU) assays in methylcellulose. Aortic cells from C57BL/6J mice selectively generated macrophage colonies (CFU-M) which contained progenitor cells that displayed >95% positive for expression of CD45, Sca-1, c-Kit, CX3CR1 and CSF1R, but negative for Lineage markers, as well as mature monocyte/macrophage (CD11b, F4/80) and endothelial (CD144) markers. Secondary replating of CFU-M progenitors from adult aortas revealed their self-renewal capacity, with 1 in 10 cells forming new CFU-M. Lineage mapping using Flt3CrexRosamT/mG mice demonstrated that aortic CFU-M progenitors were FLT3-ve, indicating that they were not derived from definitive bone marrow haematopoiesis. CFU-M prevalence in C57BL/6J aortas was highest in neonatal mice and diminished progressively with increasing age (∼100 per 105 cells at P1, ∼15 at 12w, ∼5 at 52w, P<0.01, n>4/gp), consistent with prenatal seeding. Embryonic profiling determined that CFU-M progenitors first appeared in extra-embryonic yolk sac around E9.5 and in aorta-gonad-mesonephros at E10.5, before the emergence of definitive haematopoietic stem cells. Inducible fate-mapping then confirmed that aortic CFU-M progenitors originated from CX3CR1+ and CSF1R+ cells in E9.5 yolk sac. Both yolk sac and postnatal aortic CFU-M progenitors generated vascular-like networks when cultured in Matrigel in vitro, containing M2-like macrophages (CD11b+F4/80+CD206+) and endothelial cells (CD31+CD144+). They produced similar progeny and rescued adventitial vascular sprouting when seeded around aortic rings whose adventitia had been stripped. Finally, adoptive transfer of CFU-M progenitors into a mouse model of hindlimb ischaemia resulted in 80% augmentation in hindlimb perfusion compared to cell-free control, with de novo transformation of donor cells into macrophages, endothelial cells and perfused neovessels (n=6).

Conclusion
To the best of our knowledge, this is the first ever definitive proof at a clonal level for the existence of haemangioblasts in postnatal tissue. Adventitial haemangioblasts originate from extra-embryonic YS and are a source of vasculogenesis in the arterial wall, relevant to vasa vasorum formation.

Acknowledgement/Funding
NHMRC of Australia (GNT1086796, CDF1161506), NHFA (FLF100412, FLF102056) Royal Australasian College of Physicians