Macrophages and neutrophils in tissue homeostasis and recovery from ischemic injury

Abstract: Neutrophils and macrophages have functions beyond protection against pathogens. The overall aim of the work presented in this thesis was to identify novel tasks for these innate immune cells in maintaining homeostasis.  In the studies presented here, we explored macrophage roles in tissue recovery from ischemic injury and post-natal tissue development, and the origin and recruitment mechanisms of pro-angiogenic neutrophils (PANs) to the site of ischemic injury.In Study I, it was shown that perivascular macrophages at sites of ischemic injury adopt characteristics of mural cell identity.  Combining genetic heritable fate mapping of macrophages with single-cell RNA-sequencing, we were able to demonstrate that macrophages downregulated the expression of myeloid markers, and upregulated those of mural cells. Depletion of macrophages during tissue healing demonstrated that macrophages also adopt important mural cell functions that are crucial for blood vessel maturation after ischemic injury. In Study II, it was shown that perivascular macrophages form cuff-like structures around vessels in the ischemic muscle following ischemia. Using genetically modified mouse models, we showed that these macrophages regulate bloodflow in an inducible NO Synthase (iNOS)-dependent manner, and this could be therapeutically targeted to improve tissue healing through local delivery of plasmid-encoded C-X-C Motif Chemokine Ligand 12 (CXCL12). In Study III, it was shown that recruitment of PANs to the site of ischemic injury is dependent on CD49d signalling, and the spleen contains a peripheral reservoir of PANs which is crucial for achieving adequate accumulation of PANs at the site of ischemic injury. We also showed that the release of splenic PANs in response to ischemic injury relies on sympathetic signalling and downregulation of CXCL12α in the splenic red pulp.In Study IV, it was shown that pancreatic tissue-resident macrophages are important for post-natal islet development, as depletion of macrophages using clodronate liposomes led to impaired glucose tolerance in adult mice. Further, neonatal infection with S. aureus led to reduced number of pancreatic macrophages and interfered with normal post-natal β cell development, leading to impaired glucose tolerance.  In summary, the work presented here expands our understanding on the various roles of macrophages during ischemic injury and tissue development, and significantly advances our understanding of the origins and recruitment mechanisms of pro-angiogenic neutrophils during ischemic injury.