Anti-Interleukin-6 and Soluble Interleukin-6 Receptor in Systemic Lupus Erythematosus

IL-6, soluble IL-6 receptor and IgG with anti-IL-6 activity were measured in the plasma of 14 lupus patients and 10 normal subjects. The capacity of peripheral blood mononuclear cells to spontaneously produce IL-6 in vitro was also measured. Our results indicate that IL-6 plasma levels in patient plasma as measured by ELISA were not different from normal but that supernatant levels were significantly lower than normal (P < 0.05). In vitro IgG production was comparable for all lupus patients' cells irrespective of the in vitro IL-6 levels. Plasma soluble IL-6 receptor levels directly correlated with IL-6 production capacity of SLE cells and the ratio of soluble receptor to anti-IL-6 directly correlated with IL-6 production in patients but not in normals. Inhibition assays demonstrated competition between anti-IL-6 and soluble receptors for IL-6 and the inhibition by plasma of IL-6 binding to monoclonal anti-IL-6. We believe the interaction of anti-IL-6 and IL-6 receptor with IL-6 may contribute to the homeostasis in IL-6 activity in vivo and skewing of the soluble receptor/anti-IL-6 ration may contribute to the lupus disease process.