Project Summary Chronic neuroinflammation plays a central role in spinal cord injury (SCI) and SCI-induced secondary damage. Although peripheral immune cells such as macrophages (Mφ) and the resident microglia-mediated neuroinflammatory cascade have been implicated in SCI, the mechanisms of peripheral Mφ and residential microglia cross talk and how their interaction controls microglia-mediated neuroinflammation remains largely unknown. This gap in our knowledge is a significant barrier to mitigating inflammation-induced secondary damage in SCI. We have shown that peripheral bone marrow-derived macrophages (BMDMɸ) migrate to the epicenter of the injured core, where they engulf myelin debris to become pro-inflammatory myelin-laden macrophages (Mye-Mϕ), which occupy the entire epicenter of the injured area indefinitely. In contrast, residential microglia are largely excluded from the injury epicenter, but are in close contact with Mye-Mϕ and remain chronically activated, suggesting that: 1) BMDMφ, not microglia, may be the major scavenger cells for myelin debris clearance from the lesion center, and 2) the cause of chronic microglial activation in the injured spinal cord is constantly present. We also demonstrated that myelin debris contains significant quantities of microRNAs (myelin-enriched miRs) and Mye-Mφ secrete exosomes that contain abundant myelin-enriched miRs, which are distinct from naïve-Mφ secreted exosomes. We further showed that these Mye-Mφ-derived exosomes can transfer to microglia, promoting additional inflammatory responses in microglia. Consequently, our central hypothesis is that infiltrated peripheral BMDMφ engulf myelin debris and associated miRs and secrete exosomal myelin-enriched miRs, which are then transferred to adjacent microglia to promote microglia-mediated neuroinflammation in SCI. We will test our hypothesis by completing the following specific aims: 1) Investigate how peripheral BMDMφ regulate microglia-mediated neuroinflammation. 2) Investigate whether targeting exosome-mediated communication between Mye-Mφ and microglia influences microglial activation. This research is innovative because exosomes are a unique way of exchanging integrated signals, and targeting exosomes may represent a therapeutic strategy more advantageous than classical approaches aimed at neutralizing single inflammatory molecules in SCI. This work is significant because our study can not only be applied to SCI but also to other demyelinating diseases that generate myelin debris such as stroke and multiple sclerosis, which account for 80% of the sources of paralysis. Our research will have the positive impact of generating novel therapeutic targets for SCI treatment.