The Role of the Cardiac Lymphatics in Heart Repair and Regeneration

Heart attack or myocardial infarction (MI) triggers an immune response, whereby phagocytic cells remove dead tissue and assist with the subsequent remodelling and repair of the infarcted heart. High load and persistence of immune cells, however, contributes to further fibrosis and pathological remodelling and ultimately progression to heart failure. We have shown that in adult mice, MI activates the cardiac lymphatics, which undergo sprouting angiogenesis (lymphangiogenesis) and function to drain the build-up of interstitial fluid (oedema) and traffic immune cells, including macrophages, to mediastinal lymph nodes (MLNs), reducing inflammatory/fibrotic immune cell content and improving cardiac output. Which functional subsets of immune cells/macrophages that are trafficked, versus retained in the heart to elicit improved outcome remains unknown and is the subject of ongoing studies via unbiased single cell transcriptional profiling.

Given the important role of the adult cardiac lymphatics in trafficking macrophages post-MI, we further investigated their role across the so-called regenerative window in neonatal mice (post-natal days 1-7; P1-P7). Mice at P1 fully regenerate their heart following MI in a pro-regenerative macrophage-dependent manner, whereas equivalent injury at P7 leads to scarring driven by pro-fibrotic macrophages. We hypothesised that lymphatics respond and function differently following MI during this regenerative window, to clear macrophage specific subtypes depending upon their requirement for regeneration (P1) or fibrotic repair (P7). Normal lymphatic growth and sprouting is evident in intact neonatal hearts until P16, with strain-dependent developmental differences. Importantly, the maturation status of lymphatic endothelial cell junctions, is altered across the neonatal period via transition from “zipper” (impermeable) to “button”-type (permeable) junctions. Moreover, the response to injury is significantly altered, with limited lymphangiogenesis and decreased clearance of macrophages in P1 compared to P7 mice 7-days post-MI, as determined by adoptive transfer experiments. To gain molecular insight into the mechanisms underpinning lymphatic endothelium-macrophage interactions in P1 versus P7, we have generated unbiased single cell RNA sequencing datasets from samples collected at different time-points after MI. Finally, in mice lacking lymphatic endothelial receptor-1 (Lyve1) that exhibit impaired transmigration of interstitial macrophages to lymphatic vessels, magnetic resonance imaging (MRI) revealed a surprising impaired functional outcome in P1 mice 28 days post-MI. Given our observations that pro-regenerative macrophages at P1 are not trafficked, this suggests a distinct role for Lyve-1 in tissue resident macrophages, consistent with its expression pattern in developing and post-natal hearts: investigation of macrophages-specific deletion of Lyve-1 during neonatal heart regeneration is ongoing.

Collectively, we have revealed that the cardiac lymphatics are essential in clearing immune cells to orchestrate optimal cardiac repair in injured adult mice, whereas they are developmentally compromised for clearance in early neonates which enables retention of pro-regenerative tissue resident macrophages. Further molecular studies to uncover functional subsets of macrophages that are required to be cleared versus retained following adult heart injury and to uncover the molecular mechanisms that lead to the differential response across the neonatal regenerative window may provide therapeutic insights into lymphatic-based immunomodulation of the infarcted heart.

Professor Paul Riley, Institute of Developmental & Regenerative Medicine (IDRM), University of Oxford

Paul Riley is a British Heart Foundation Professor of Regenerative Medicine based at the University of Oxford. He is also the inaugural Director of the Institute of Developmental and Regenerative Medicine which came online in May 2022. He was formerly Professor of Molecular Cardiology at the UCL-Institute of Child Health, London, where he was a principal investigator within the Molecular Medicine Unit for 12 years. Prior to this, he obtained his PhD at UCL and completed post-doctoral fellowships in Toronto and Oxford. In 2008, Professor Riley was awarded an Outstanding Achievement Award by the European Society of Cardiology, in recognition of his team’s discovery that activated epicardial cells can regenerate the adult mammalian heart, and in 2014 he was elected a fellow of the Academy of Medical Sciences. His research interests span across multiple aspects of cardiovascular development and how to restore embryonic potential in the injured/diseased adult heart to facilitate optimal repair and regeneration.