Interleukin-6 production is upregulated by interaction between disc tissue and macrophages

Study Design. Interleukin (IL)-6 production was investigated using a coculture system of disc tissue and macrophages. Objectives. The purpose of this study was to investigate the interaction between intervertebral disc tissue and macrophages in terms of IL-6 production. Summary of Background Data. IL-6 production is observed in human herniated disc specimens, and there is a correlation between IL-6 production and neurologic symptoms. However, the mechanism of IL-6 production in the herniated disc is not clear. Materials and Methods. Coccygeal intervertebral discs and exudated peritoneal macrophages were obtained from male Sprague-Dawley rats. Macrophages and intervertebral disc without endplates were cocultured in a serum-free medium. Fat tissue culture with or without macrophages, intervertebral disc alone, and macrophages alone were used for controls. The supernatant fluid of the culture was utilized for the enzyme-linked immunosorbent assay. The precipitations of macrophages and disc coculture were used for semiquantitative RT-PCR for IL-6. Immunohistochemical staining for IL-6 and the macrophages marker (ED2) were also carried out using disc tissue cultured with macrophages. Results. IL-6 production level was significantly increased in the coculture of intervertebral disc and macrophages (P < 0.01). However, there was no significant production of IL-6 in the control groups. The precipitations from coculture of macrophages and disc expressed IL-6 mRNA in semiquantitative RT-PCR. Immunohistochemical staining revealed most IL-6 producing cells were also positive for ED2, which adheres to or infiltrates the peripheral area of the nucleus pulposus. Conclusions. Our results demonstrated that interaction between disc tissue and macrophage is necessary for upregulation of IL-6 production. Immunohistochemical staining also indicated that infiltrated macrophages played a major role in production of IL-6, suggesting that infiltration of macrophages into herniated disc material may be a trigger for IL-6 production and associated neurologic symptoms.