PROJECT SUMMARY/ABSTRACT Traumatic spinal cord injury (SCI) is a sudden catastrophic neurologic event resulting in incomplete or complete loss of motor, sensory, and sympathetic function below the level of the injury. Consequently, due to the significant physical, social, and vocational impact of this disease, it is not surprising that it results in a notable decline in patient quality of life and increased socioeconomic burden. SCI results when a mechanical force or forces are imparted on the spinal cord causing compression or transection of the cord. The delivery of this mechanical force leads to the primary injury phase characterized by neuronal and glial necrosis, axonal disruption, loss of myelin, disruption of the blood-spinal cord barrier, hemorrhage, edema, and ischemia. These pathologic processes then trigger a self-perpetuating secondary injury cascade, where among other features, a robust inflammatory response including the release of cytokines and chemokines and infiltration by inflammatory cells such as monocytes, macrophages, and neutrophils is a hallmark. Historically it was believed that there was little interaction between the CNS and the peripheral immune system. However, with the discovery of the glymphatic system, meningeal lymphatic vessels, and that the CNS-adjacent bone marrow supplies the CNS borders with a robust and diverse pool of myeloid cells, this concept of CNS immune privilege has undergone significant revision. We now know that following SCI, the CSF is able to communicate with the CNS-adjacent bone marrow compartment through osseous bone marrow-meninge connections in the skull and vertebrae. This signaling stimulates hematopoiesis and egress of myeloid cells from the bone marrow back to CNS borders including meninges and perivascular spaces, as well as to the parenchyma. Currently, the exact mechanism of CSF signaling to the CNS-adjacent bone marrow remains unknown. It is possible that this signaling is simply through cytokines and chemokines carried by the CSF. As there must be bidirectional movement within the osseous marrow-meninge connections, with CSF moving in one direction and myeloid cells the other, an alternative explanation is that CSF can signal to the bone marrow through flow rate. In Aim 1, we hypothesize that the rate of flow of CSF to the CNS-adjacent bone marrow is capable of regulating myeloid egress to the CNS borders and parenchyma. Further, in many CNS diseases it has been demonstrated that impaired glymphatic CSF flow, waste solute accumulation, and neuroinflammation are key pathologic features that tend to feed forward on one another. Consequently, in Aim 2, we hypothesize that following SCI there will not only be disrupted spinal glymphatic CSF flow, but also CSF flow to the CNS- adjacent bone marrow, and that this will promote increased myeloid cell migration into the CNS borders and parenchyma. It is likely this myeloid infiltration assists with the clearance of accumulated waste solute ...