Analysis of the inflammatory cytokine network among TNFα, IL-1β, IL-1 receptor antagonist, and IL-8 in LPS-induced rabbit arthritis

We investigated the cytokine network in rabbit lipopolysaccharide (LPS)- induced arthritis, using inhibitors against homologous TNFα, IL-1β, and IL- 8. Rabbits were intraarticularly injected with LPS (10 ng) and cytokine inhibitors (10μg each), and the concentrations of each cytokine in the synovial fluids were measured. Maximum levels of TNFα and IL-8 were detected at 2 hours after LPS-injection, whereas IL-1β and IL-1 receptor antagonist (IL-1Ra) were detected at 6 and 9 hours, respectively. By immunohistochemistry, synovial lining cells were positive for TNFα and IL- 8, and infiltrating leukocytes were positive for IL-1α and IL-1Ra. The effects of cytokine inhibitors on the release of each cytokine were then investigated. The maximum levels of TNFα and IL-8 were not affected by blocking the activities of other cytokines. In contrast, the peak concentration of IL-1β was reduced by anti-TNFα monoclonal Ab (mAb), IL- 1Ra or anti-IL-8 IgG. Peak concentrations of IL-1Ra were reduced by anti- TNFα mAb or anti-IL-8 IgG. Anti-TNFα mAb, IL-1Ra, and anti-IL-8 IgG reduced the recruitment of leukocytes into the joint cavity, and the effect of anti- IL-8 IgG was less than that of anti-TNFα mAb plus IL-1Ra. The initial phase of the leukocyte influx was not inhibited. These results provide new evidence that IL-8 as well as TNFα are the most proximal cytokines and induce subsequent production of IL-1β and IL-1Ra. The data also raise the possibility that factor(s) other than IL-8 may be involved in the leukocyte influx in LPS-induced arthritis.