Effect of Crohn's Disease on Bone Metabolism In Vitro: A Role for Interleukin-6†

Circulating proinflammatory cytokines may be involved in osteopenia associated with Crohn's disease (CD). Therefore, the effect of interleukin (IL)-6, IL-1β, and tumor necrosis factor (TNF) α contained in Crohn's serum on bone formation was examined in a bone organ culture system. Initially, serum levels of IL-6, IL-1β, and TNF-α were determined by ELISA in newly diagnosed, untreated children with CD and healthy age-matched controls. Serum IL-6 levels were significantly higher in patients with CD than in controls (23.9 ± 2.8 pg/ml vs. 0.7 pg/ml ± 0.2; p < 0.001), whereas IL-1β and TNF-α serum levels were not. In the organ culture studies, 20-day-old fetal rat parietal bones were incubated for 96 h with CD or control serum, serum preincubated with a neutralizing antibody to each cytokine or a nonimmune immunoglobulin control, and with IL-6. Bone formation measured by assaying calcium content and dry weight was significantly decreased in bones exposed to Crohn's serum. Light microscopy of the bones treated with CD serum revealed a discontinuous, uneven mineralized bone matrix and disorganized osteoblasts with altered morphology. Incubation with an antibody that neutralized IL-6 activity prevented the change in osteoblast and bone morphology. TNF-α and IL-1β antibodies had no apparent effects. Collagen synthesis and DNA content were not affected by CD serum. Also, addition of IL-6 to the culture medium decreased mineralization. These results suggest that IL-6 is a mediator of the effects of Crohn's serum on in vitro mineralization and may be a contributing factor to the osteopenia associated with CD.