CXCR2 mediated trafficking of neutrophils and neutrophil extracellular traps are required for myelin clearance after a peripheral nerve injury

Neutrophils are a vital part of the innate immune system. Many of their functions eliminate bacteria & viruses, like neutrophil extracellular traps (NETs), which trap bacteria, enhancing macrophage phagocytosis. It was surprising when it was demonstrated that neutrophils are a part of Wallerian degeneration, a process that is essential for nerve regeneration after a nerve injury. It is not known what signals attract neutrophils into the nerve and how they aid Wallerian degeneration. Neutrophils accumulate in the distal nerve within one day after an injury and are found in the nerve from one to three days. We demonstrate that CXCR2 mediates the trafficking of neutrophils into the distal nerve, and without CXCR2 Wallerian degeneration, as indicated by luxol fast blue staining, was reduced seven days after a sciatic nerve crush or transection injury. NETs were detected in the distal nerve after a sciatic nerve transection. NET formation has been shown to require protein arginine deiminase 4 (PAD4), which citrullinates histone 3. Inhibiting PAD4 reduced NET formation significantly in the distal nerve at two days and myelin clearance at seven days indicating that NETs aid myelin clearance. These results demonstrate another function for NETs other than clearing pathogens. Neutrophils have been detected after injuries to the central nervous system and diseases in humans and animal models. Our results demonstrate neutrophils aid myelin clearance, suggesting a role for their presence in central nervous system injuries and diseases.