Involvement of growth-related protein in lipopolysaccharide-induced rabbit arthritis: Cooperation between growth-related protein and IL-8, and interrelated regulation among TNFα, IL-1, IL-1 receptor antagonist, IL-8, and growth-related protein

We investigated the functional role of a CXC chemokine, growth-related protein (GRO), in the recruitment of neutrophils in lipopolysaccharide (LPS)- induced rabbit arthritis. The amounts of GRO in the synovial fluids (SF) reached the first peak (major) at 2 hours and the second peak (minor) at 9 hours after injection of LPS into the knee joints. Administration of anti- GRO mouse monoclonal antibody inhibited 54% of the peak leukocyte accumulation at 9 hours (neutrophils greater than 95%), which was similar to the inhibition by anti-IL-8 IgG (48%). Co-administration of these inhibitors increased the inhibition up to 70% at 9 hours and also inhibited 65% of the initial phase of leukocyte infiltration at 2 hours (neutrophils greater than 99%), which was not affected by a single administration of each inhibitor. The amounts of GRO in SF at 2 hours were not altered by either anti-TNFα mAb or anti-IL-8 IgG, but reduced by rabbit recombinant IL-1 receptor antagonist (rrIL-1Ra) by 39%. The inhibition by rrIL-1Ra was augmented further to 59% with coadministered anti-TNFα mAb. In contrast, the amounts of GRO at 9 hours were reduced by rrIL-1Ra by 67%. There was no additional reduction in the amounts of GRO at 9 hours by either combination of rrIL-1Ra with anti- TNFα mAb or anti-IL-8 IgG. Administration of anti-GRO mAb did not alter TNFα or IL-8 contents in SF at their peak (2 hours), but reduced the amounts of IL-1β at 6 hours and IL-1Ra at 9 hours by 42% and 49%, respectively. These results provide evidence for the following: (a) GRO as well as IL-8 are important mediators involved in the recruitment of neutrophils both in the early and the late phase of LPS-induced arthritis, (b) IL-1 produced in the early phase stimulates GRO production, (c) GRO plays a role in the later induction of IL-1β and IL-1Ra, and (d) induction of GRO is not regulated by IL-8.