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

Abstract

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.

The Evolving Roles of Cardiac Macrophages in Homeostasis, Regeneration, and Repair

“…Compared to microglia and spleen monocytes/macrophages, cardiac-resident macrophages have enriched expression of signature genes like the folate receptor 2 (Folr2); the lymphatic vessel endothelial hyaluronan receptor 1 (Lyve1); the scavenging receptor for the hemoglobin–haptoglobin complex (CD163); the insulin-like growth factor 1 (Igf1); the mannose receptor 1 (Mrc1 or CD206); resistin-like alpha (Retnla); and the LPS co-receptor CD14, which resemble an alternative activation phenotype that is associated with anti-inflammatory properties and wound resolution [ 53 ]. However, some genes have unique functions which are important in the context of cardiac homeostasis and development, such as Lyve1, implicated in adipose tissue angiogenesis [ 54 ] and lymphangiogenesis in the heart [ 55 , 56 ]; or Igf1, involved in coronary development and maturation during cardiac development [ 48 ]. Furthermore, the high expression of the scavenging receptors Mertk, CD206, CD163, and CD14 suggest a highly phagocytic capacity, which is fundamental to maintain a clean extracellular environment for proper myocardial performance [ 51 ].…”

“…In addition to coronary development, cardiac-resident macrophages are essential for proper lymphangiogenesis and lymphatic vessel maturation in the heart. Cardiac-resident macrophages derived from yolk-sac precursors colocalize with lymphatic endothelial cells (LEC) [ 55 , 56 ] and function as chaperone cells, guiding the LEC to another nearby LEC in order to form lymphatic junctions, extending the lymphatic network of the heart [ 56 ]. Macrophage depletion during early embryogenesis prevents cardiac-resident macrophage seeding resulting in impaired lymphangiogenesis [ 56 ].…”

“…Cardiac-resident macrophages derived from yolk-sac precursors colocalize with lymphatic endothelial cells (LEC) [ 55 , 56 ] and function as chaperone cells, guiding the LEC to another nearby LEC in order to form lymphatic junctions, extending the lymphatic network of the heart [ 56 ]. Macrophage depletion during early embryogenesis prevents cardiac-resident macrophage seeding resulting in impaired lymphangiogenesis [ 56 ]. Importantly, it is proposed that cardiac-resident macrophages secrete hyaluronan, which promotes bridging among LEC in in vitro assays.…”

“…Importantly, it is proposed that cardiac-resident macrophages secrete hyaluronan, which promotes bridging among LEC in in vitro assays. Inhibition of hyaluronan by hyaluronidase impedes sprouting and bridging between LECs [ 56 ]. Interestingly, only cardiac-resident macrophages expressing Lyve1 contribute to lymphangiogenesis [ 55 ], which are cardiac-resident macrophages developed during primitive hematopoiesis in the yolk-sac [ 10 , 17 , 55 , 56 ].…”

The blood vasculature instructs lymphatics patterning in a SOX7 dependent manner

“…It has been established that blood endothelial cells (BECs) produce lymphangiogenic guidance cues such as chemokine receptor 4/chemokine ligand 12, adrenomedullin and semaphorin3G (Cha et al, 2012; Liu et al, 2016; Uchida et al, 2015). In addition to the blood vasculature, other tissues such as the mesenchyme and immune cells play a critical role in informing lymphatic vessel assembly (Cahill et al, 2021; Gordon et al, 2010; Karkkainen et al, 2004), mostly via the production of vascular endothelial growth factor C (VEGFC) (Rauniyar et al, 2018). During the early steps of lymphangiogenesis, VEGFC from the mesenchyme stimulates the exit of specified lymphatic endothelial cell (LEC) progenitors from the cardinal veins (CVs) and inter-somitic veins, and a subsequent dorso-lateral migration to form the lymphatic plexus (Karkkainen et al, 2004).…”