ABSTRACT / PROJECT SUMMARY Rapid-stretch peripheral nerve injuries are among the most devastating neurologic injuries, often with complete loss of functional use of a limb. Nerve regeneration, i.e., the cascade of regenerative changes after injury, commonly fails in rapid-stretch injuries. Instead, a neuroma forms, where abundant scar tissue replaces the normal pathway for nerve regeneration. The majority of our scientific understanding on nerve injury has been developed using techniques that are easy to perform, such as crush and transection, however these injuries demonstrate better nerve regeneration than is often seen clinically. Pathophysiologic understanding of failed regeneration and neuroma formation is lacking, thus treatment options for these severe nerve injuries are limited and have not changed in decades. We have sought to create a biofidelic model for investigating the mechanisms that lead to neuroma formation. We developed a technique for producing rapid-stretch nerve injuries that demonstrates consistent biomechanical injury, specific patterns of acute histology and reproducible functional deficits and pathology. The degree of stretch injury robustly predicts the injury outcome. The injury patterns we have observed closely mimic human nerve injury patterns, including the specific histopathology of the neuroma-in-continuity. Nerve regeneration is initiated with participation of the immune system, particularly through the involvement of macrophages. Abrogating inflammation can compromise regeneration – yet several immunosuppressive agents have been demonstrated to improve nerve regeneration. Our central hypothesis is that nerve injury severity determines the response of the immune system, which then determines the severity of neuroma formation. Aim 1 investigates several hypotheses on the participation of the immune system through immunohistology, flow cytometry and gene transcription data. Aim 2 mechanistically investigates the role of macrophages, neutrophils and T-cells in the formation of neuromas after injury. The goal of this R21 proposal is to develop robust preliminary research for the pursuit of R01 or similar grant for further investigation into the inflammatory pathways associated with neuroma formation. We believe that neuroma formation is analogous to other wound healing environments, particularly with ischemia and inflammation synergistically increasing intraneural scar formation.