Macrophages protect against sensory axon degeneration in diabetic neuropathy

Diabetic peripheral neuropathy (DPN) is a common complication of diabetes, causing sensory loss and debilitating neuropathic pain1,2. Although the onset and progression of DPN have been linked with dyslipidemia and hyperglycemia3, the contribution of inflammation in the pathogenesis of DPN has not been investigated. Here, we use a High Fat High Fructose Diet (HFHFD) to model DPN and its metabolic syndrome in mice. Diabetic mice develop persistent heat hypoalgesia after three months, but a reduction in epidermal skin innervation only manifests at 6 months. Using single-cell sequencing, we find that CCR2+ macrophages infiltrate the sciatic nerves of diabetic mice well before axonal degeneration is detectable. We show that these infiltrating macrophages share gene expression similarities with nerve crush-induced macrophages4 and express neurodegeneration-associated microglia marker genes5 although there is no axon loss or demyelination yet. Inhibiting this macrophage recruitment in diabetic mice by genetically or pharmacologically blocking CCR2 signaling results in a more severe heat hypoalgesia and accelerated skin denervation. These findings reveal a novel neuroprotective recruitment of macrophages into peripheral nerves of diabetic mice that delays the onset of terminal axonal degeneration, thereby reducing sensory loss. Potentiating and sustaining this early neuroprotective immune response in patients represents, therefore, a potential means to reduce or prevent DPN. We fed male C57Bl6 mice on a high fat high fructose diet (HFHFD) or control diet (CD) for 12 weeks. Mice fed HFHFD become diabetic. We FACS sorted Live Cd45+ immune cells from hindpaw skin and sciatic nerves and performed single cell RNA sequencing. We identified infiltration of CCR2+ macrophages in the nerves but not the skin of diabetic mice.