Microglial Contribution to Secondary Injury Evaluated in a Large Animal Model of Human Spinal Cord Trauma

Abstract

Spinal cord injury (SCI) in dogs is a well recognized animal model to study pathogenesis and treatment modalities of the debilitating human disease. To define the contributing role of microglial cell activation to the secondary wave following SCI, microglia from 15 dogs with SCI confirmed by imaging, gross, and histopathological examination were isolated and characterized in terms of morphology, immunophenotype, and function ex vivo by flow cytometry, allowing single cell analysis. The results were compared to region-specific findings obtained from healthy control dogs. Light microscopy revealed a significant enhancement of myelinophagia within the traumatized spinal cord of dogs who had had SCI for ≥5 days. Immunophenotypical characterization revealed increased expression of B7-1, B7-2, MHC II, CD1c, ICAM 1, CD14, CD44, and CD45 emphasizing the enhanced function of microglia as co-stimulators of T cells, in leukocyte adhesion and aggregation, and for lipid or glycolipid presentation. In addition, phagocytosis and reactive oxygen species (ROS) generation were significantly increased in dogs with spinal cord trauma. Regional differences within the spinal cord were observed by demonstrating disparities in microglial immunophenotypes in the traumatized cervical compared to the thoracolumbar spinal cord. In contrast to histopathology, microglia activation analyzed on a single cell basis did not depend upon the time span following SCI.