Systemic Lupus Erythematosus (SLE) is an autoimmune disease characterized by the production of autoantibodies particularly to nuclear antigens and by an abnormal production of proinflammatory cytokines. In the present study, we measured the levels of the proinflammatory cytokine IL-18 and its natural inhibitor, the IL-18 binding protein (IL-18BP), in sera of SLE patients at various stages of the disease. This is the first study to present IL-18BP levels in sera of SLE patients as well as the calculated, biologically active, free IL-18 concentrations that are most probably more relevant to the pathology of SLE. Sera from 48 unselective SLE patients (total of 195 samples) were obtained longitudinally with a mean follow-up period of 11.1 +/- 8.9 years and were compared to sera from 100 healthy volunteers. Circulating levels of IL-18, IL-18BP and free IL-18 in the SLE patients were significantly higher than the levels of healthy controls (5 fold, 6 fold and 3 fold for IL-18, IL-18BP and free IL-18, respectively) and correlated with disease activity as scored by SLEDAI-2K. Furthermore, these levels during active disease (SLEDAI-2K > or = 6) were higher compared to the levels measured in the sera of the same patients during remission or during mild disease (SLEDAI-2K 0-5). The high levels of IL-18 and IL-18BP in sera of active SLE patients suggest their possible role in the pathogenesis and course of the disease. However, despite the elevated levels of IL-18BP during active disease, free IL-18 remained more than 2 fold higher than the levels in healthy controls suggesting a potential benefit of administration of exogenous IL-18BP as a novel therapeutic approach for active SLE.
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