Characterisation of the development of cardiac lymphatic vessels and their response to myocardial infarction

Thesis

The lymphatic vasculature is a blind-ended network, present in all mammals, which is responsible for immune surveillance, tissue fluid regulation and absorption of lipid-containing chylomicrons into the gut. During pathological conditions lymphatic vessels expand via the process of lymphangiogenesis in order to aid the clearance of inflammation and interstitial fluid. The systemic lymphatic vasculature begins to develop in the mouse embryo around E9-9.5. This process is initiated at three venous sites within the embryo where lymphatic endothelial cells (LECs) migrate away from veins to form lymphatic structures. Since the 17th century it has been known that the lymphatic vasculature is present in the heart, however, until recently little was known regarding its developmental origins or its function during cardiovascular disease. The aims of this thesis were to characterise the development of cardiac lymphatic vessels and their response to adult heart injury, as a means to identify whether the lymphatics reactivate embryonic programs during pathological insult and to establish a potential basis for future therapeutic targeting towards improved repair.

I have characterised the spatiotemporal development of the murine cardiac lymphatics and have performed lineage tracing to determine their cellular origins. This revealed that cardiac LECs are not solely derived from veins, which was confirmed by analysis of conditional PROX1 knockout mouse hearts. Collectively these data refute the previous dogma that all lymphatic vessels are venous-derived. I have also performed studies to assess the roles of lymphatic vessels following myocardial infarction (MI). This revealed that expression of the pro-lymphangiogenic growth factor VEGF-C is reactivated in the heart post-MI, driving cardiac lymphangiogenesis. Treatment with additional recombinant VEGF-C further enhanced the lymphangiogenic response and resulted in improvements in cardiac function. Furthermore, VEGF-C treatment resulted in reduced numbers of macrophages within the heart by day 7 post-MI. This suggests that enhancing lymphangiogenesis leads to increased clearance of the post-MI inflammatory cells, which may modulate levels of inflammation and fibrosis, resulting in an improved outcome. I have also utilised genetic mouse models (LYVE-1 knock-out and PROX1 heterozygous lines) with defects in their lymphatic vasculature. Following MI in LYVE-1 knockout animals, where transmigration of leucocytes into lymphatic vessels is impaired, neutrophils and macrophages were found to accumulate within the heart at day 7 post-injury. Together these data demonstrate that therapeutic targeting of the cardiac lymphatic vasculature may have a significant impact on the resolution of inflammation and the post-MI healing process.

Authors: Norman, S

• DOI: 10.5287/ora-jnbx0pkyg
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English