Tissue-resident macrophages regulate lymphatic vessel growth and patterning in the developing heart.

Macrophages are components of the innate immune system with key roles in tissue inflammation and repair. It is now evident that macrophages also support organogenesis, but few studies have characterized their identity, ontogeny and function during heart development. Here, we show that the distribution and prevalence of resident macrophages in the subepicardial compartment of the developing heart coincides with the emergence of new lymphatics, and that macrophages interact closely with the nascent lymphatic capillaries. Consequently, global macrophage deficiency led to extensive vessel disruption, with mutant hearts exhibiting shortened and mis-patterned lymphatics. The origin of cardiac macrophages was linked to the yolk sac and foetal liver. Moreover, the Cx3cr1 + myeloid lineage was found to play essential functions in the remodelling of the lymphatic endothelium. Mechanistically, macrophage hyaluronan was required for lymphatic sprouting by mediating direct macrophage-lymphatic endothelial cell interactions. Together, these findings reveal insight into the role of macrophages as indispensable mediators of lymphatic growth during the development of the mammalian cardiac vasculature.This work was funded by the British Heart Foundation (chair award CH/11/1/28798 and programme grant RG/08/003/25264 to PRR) and supported by the BHF Oxbridge Centre of Regenerative Medicine (RM/13/3/30159); a Wellcome Trust Doctoral Training Fellowship 106334/Z/14/Z to TJC; a Wellcome Trust Four year PhD Studentship 215103/Z/18/Z to KK; a BHF Intermediate Basic Science Research Fellowship FS/19/31/34158 to JMV; a British Israel Research and Academic Exchange Partnership (BIRAX) Grant 13BX14PRET; a Leducq Foundation Transatlantic Network of Excellence Program 14CVD04 and MRC Unit funding to DGJ.S