Neutralizing autoantibodies to interleukin-6 (aAb-IL-6) have been reported in healthy individuals, in patients with autoimmune diseases, and in pharmaceutically prepared pooled IgG (IVIg). We investigated the ability of aAb-IL-6 derived from IVIg to interfere with IL-6 binding to the undifferentiated monocytic cell line U-937. High-affinity aAb-IL-6, primarily of the IgG1 subclass, constituted approximately 1:10(6) of the total IgG in IVIg preparations. IL-6 binding to cellular receptors was strongly inhibited by one class of aAb-IL-6. These antibodies recognized epitope(s) on IL-6 essential for the binding of IL-6 to the alpha subunit of the IL-6 receptor (IL-6R). Another class of aAb-IL-6 recognized epitope(s) on IL-6, which is not essential for the binding to IL-6R but nevertheless important for the formation of high-affinity cellular IL-6 binding. These antibodies presumably interfered with the association of IL-6 receptor beta chains (gp130) with IL-6/IL-6R complexes, implicating that small IL-6/aAb-IL-6 immune complexes bound saturably (low affinity/high capacity) to cellular IL-6 receptors. There was no detectable binding of IL-6 through aAb-IL-6 and Fc receptors on U-937, and IVIg had no direct IL-6 receptor antagonizing activity. Dissociation kinetics of IL-6/aAb-IL-6 complexes at 37 degrees C revealed that IL-6 was liberated from 75% of the aAb-IL-6 with a half-time (t/2) approximately 4 h but bound almost irreversibly to the remaining aAb-IL-6 (t/2 > 20 h). Cellular IL-6 uptake and degradation was suppressed by aAb-IL-6. Taken together, the data suggest that loss of immunologic tolerance against IL-6 might be a novel physiological mechanism by which IL-6 activities are effectively attenuated. Finally, binding of IL-6 in complex with IgG1 aAb-IL-6 on cells expressing IL-6 receptors implicates that such cells could be targets of antibody-dependent immunological reactions, including cytotoxic reactions.