Myeloperoxidase exacerbates secondary injury by generating highly reactive oxygen species and mediating neutrophil recruitment in experimental spinal cord injury

STUDY DESIGN. An animal study using myeloperoxidase-knockout (MPO-KO) mice to examine the in vivo role of myeloperoxidase (MPO) in spinal cord injury (SCI). OBJECTIVE. To clarify the influence of MPO on inflammatory cell infiltration, tissue damage, and functional recovery after SCI. SUMMARY OF BACKGROUND DATA. MPO is considered to be important in spreading tissue damage after SCI because it generates strong neurotoxic oxidant hypochlorous acid (HOCl). However, the direct involvement of MPO in the pathophysiology of SCI remains to be elucidated. METHODS. To compare the inflammatory reaction, tissue damage, and neurological recovery after SCI, a moderate contusion injury was created at the ninth thoracic level in MPO-KO mice and wild-type mice. A HOCl-specific probe solution was injected into the lesion epicenter to assess the spatiotemporal production of MPO-derived HOCl. Inflammatory reactions were quantified by flow cytometry and quantitative real-time polymerase chain reaction, and tissue damage was evaluated by an immunohistochemical analysis. The motor function recovery was assessed by the open-field locomotor score. RESULTS. Prominent production of HOCl was observed during the hyperacute phase of SCI at the lesion site in the wild-type mice; however, little expression was observed in the MPO-KO mice. In this phase, the number of infiltrated neutrophils was significantly reduced in the MPO-KO mice compared with the wild-type mice. In addition, significant differences were observed in the expression levels of proinflammatory cytokines and apoptosis-related genes between 2 groups. In the histological sections, fewer terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling-positive apoptotic cells and more spared myelin were observed at the lesion site in MPO-KO mice. Consistent with these results, better functional recovery was observed in the MPO-KO mice than in the wild-type mice after SCI. CONCLUSION. These results clearly indicated that MPO exacerbated secondary injury and impaired the functional recovery not only by generating strong oxidant HOCl, but also by enhancing neutrophil infiltration after SCI.