Cellular inflammatory response after spinal cord injury in Sprague-Dawley and Lewis rats

The distribution of microglia, macrophages, T-lymphocytes, and astrocytes was characterized throughout a spinal contusion lesion in Sprague-Dawley and Lewis rats by using immunohistochemistry. The morphology, spatial localization, and activation state of these inflammatory cells were described both qualitatively and quantitatively at 12 hours, 3, 7, 14, and 28 days after injury. By use of OX42 and ED1 antibodies, peak microglial activation was observed within the lesion epicenter of both rat strains between three and seven days post-injury preceding the bulk of monocyte influx and macrophage activation (seven days). Rostral and caudal to the injury site, microglial activation plateaued between two and four weeks post-injury in the dorsal and lateral funiculi as indicated by morphological transformation and the de-novo expression of major histocompatibility class II (MHC II) molecules. Similar to the timing of microglial reactions, T-lymphocytes maximally infiltrated the lesion epicenter between three and seven days post-injury. Reactive astrocytes, while present in the acute lesion, were more prominent at later survival times (7-28 days). These cells were interspersed with activated microglia but appeared to surround and enclose tissue sites occupied by reactive microglia and phagocytic macrophages. Thus, trauma-induced central nervous system (CNS) inflammation, regardless of strain, occurs rapidly at the site of injury and involves the activation of resident and recruited immune cells. In regions rostral or caudal to the epicenter, prolonged activation of inflammatory cells occurs preferentially in white matter and primarily consists of activated microglia and astrocytes. Differences were observed in the magnitude and duration of macrophage activation between Sprague-Dawley (SD) and Lewis (LEW) rats throughout the lesion. Increased expression of complement type 3 receptors (OX42) and macrophage-activation antigens (ED1) persisted for longer times in LEW rats while expression of MHC class II molecules was attenuated in LEW compared to SD rats at all times examined. Variations in the onset and duration of T-lymphocyte infiltration also were observed between strains with twice as many T-cells present in the lesion epicenter of Lewis rats by 3 days post-injury. These strain-specific findings potentially represent differences in corticosteroid regulation of immunity and may help predict a range of functional neurologic consequences affected by neuroimmune interactions.