ADMINISTRATION OF NEUTRALIZING ANTIBODY AGAINST RABBIT IL-1 RECEPTOR ANTAGONIST EXACERBATES LIPOPOLYSACCHARIDE-INDUCED ARTHRITIS IN RABBITS

Endogenous IL-1 receptor antagonist (IL-1ra) may down-regulate part of IL-1 actions. We examined the participation of endogenous IL-1ra in t he production of IL-1 beta in vitro. Macrophages cultured on adherent IgG produced a 100-fold molar excess of IL-1ra, compared with IL-1 bet a. In the presence of a neutralizing monoclonal antibody (mAb) against rabbit IL-1ra, the production of antigenic IL-1 beta increased by 20- 60%. Since the molar ratio of IL-1ra over IL-1 was 160- to 400-fold in synovial fluid (SF) of lipopolysaccharide (LPS)-induced arthritis, we examined the functional role of endogenous IL-1ra in the regulation o f inflammatory responses. When measured in the presence of anti-IL-1ra mAb, masked IL-1 activity in SF became evident, with a 3- to 4-fold i ncrement. The administration of anti-IL-1ra mAb with LPS into rabbit k nee joints increased the IL-I activity 4-fold and the production of an tigenic IL-1 beta by 30-50%. The treatment also enhanced by 20-40% LPS -induced leukocyte infiltration and protein leakage. Therefore, endoge nous IL-1ra apparently acts as a down-regulating factor for limiting d eleterious effects of IL-1 by masking the biological activity and by i nhibiting the production.