A novel role for macrophages in peripheral nerve regeneration

Peripheral nerves are one of the few adult tissues which can regenerate following injury, and macrophages have many important roles in this multicellular process. Following nerve injury, regrowing axons must traverse tissue termed the ‘nerve bridge’, which forms spontaneously between the nerve stumps (proximal and distal), to reconnect with their original tissue target. Previously, we found that hypoxic macrophages in the bridge induce the formation of a polarised vasculature which dedifferentiated Schwann cells subsequently use as a scaffold to migrate along, taking axons with them into the distal stump. Here, macrophages together with Schwann cells function to clear debris and remodel the environment to facilitate axonal regeneration, demonstrating distinct functions for macrophages within discrete areas of the regenerating nerve. This thesis aims to characterise nerve macrophage populations and to determine whether there is a specific Schwann cell chemoattractant within the bridge. Resident nerve macrophages displayed a distinct gene expression pattern compared to other resident macrophage populations. Moreover, we found two distinct nerve resident populations which can be distinguished by CX3CR1 expression and physiological location. In the injured nerve, the origin and phenotype of macrophage populations is currently unknown. We determined that the majority of macrophages in the bridge and distal stump are monocyte-derived. In further characterisation of bridge macrophages, we have identified an intrinsic ability to differentially sense hypoxia. Here we show data to support a novel role for hypoxic bridge macrophages in promoting Schwann cell migration. Using in vitro chemotaxis assays, we found that hypoxic macrophages are able to induce Schwann cell migration and an unbiased screen identified the chemoattractant factor as CCL3. CCL3 is able to induce Schwann cell migration in chemotactic assays, and knockdown studies showed that CCL3 is the primary chemoattractant secreted by hypoxic macrophages. We also present preliminary in vivo experiments investigating the role of CCL3 following injury, as well as in models of nerve repair. A Schwann cell chemoattractant factor has wide therapeutic implications in peripheral nerve injury, as well as potential uses in the treatment of aberrant nerve growth and tumour spread.