Spinal cord injury-induced expression of the immune-regulatory chemokine interleukin-16 caused by activated microglia/macrophages and CD8+ cells

Object. Spinal cord injury (SCI) elicits a strong inflammatory response that readily participates in lipid oxygenation, edema formation, apoptotic cell death, and tissue remodeling. Because cytokines determine the postinjury inflammatory milieu, the authors analyzed the expression of the immunomodulatory chemokine interleukin-16 (IL-16) after SCI. Methods. The authors detected a highly significant, persistent, lesional accumulation of parenchymal IL-16(+) microglia/macrophages, which reached a maximal level 3 days postinjury compared with control rats. The majority of cells that demonstrated positive labeling for IL-16 also had positive labeling for ED1 (> 70%) and OX-8/CD8; these cells exhibited the morphological hallmarks of activated microglia/macrophages and pronounced MHC Class II expression. In contrast to IL-16(+)ED1(+) cells, IL-16(+) microglia/macrophages that coexpressed OX-8 were exclusively seen in the pannecrotic lesion core. In addition, clustering of IL-16(+) cells was observed in perivascular Virchow-Robin-like spaces in areas of the primary injury (lesion core) and in immediately adjacent areas of secondary injury. Furthermore, on Day 3 postinjury, IL-16(+) microglia/macrophages were frequently observed in a perineuronal position. Conclusions. The early lesional accumulation of IL-16(+) microglia/macrophages suggests a role for IL-16 in the early postinjury immune response such as recruitment and activation of immune cells, leading to microvessel clustering and secondary damage progression.