INCREASED CIRCULATING INTERLEUKIN-6 (IL-6) ACTIVITY IN ENDOTOXIN-CHALLENGED MICE PRETREATED WITH ANTI-IL-6 ANTIBODY IS DUE TO IL-6 ACCUMULATED IN ANTIGEN-ANTIBODY COMPLEXES

Mice pretreated with monoclonal anti-interleukin-6 (IL-6) antibody and then challenged with lipopolysaccharide (LPS), paradoxically develop higher levels of circulating biological IL-6 activity, as measured by the hybridoma growth promotion assay, than mice similarly challenged b ut not pretreated with antibody. Here we provide evidence that this in creased biological activity was entirely accounted for by the presence of increased amounts of IL-6 protein, which could be isolated by immu noaffinity chromatography and subsequently visualized after gel electr ophoresis. Chromatography on a protein G matrix and a sandwich ELISA a llowed to demonstrate that all IL-6 present in the serum was in the fo rm of antigen-antibody complexes. Serum samples of antibody-treated an imals which contained the highest biological activity typically contai ned near equimolar concentrations of IL-6 and antibody. In vitro neutr alization tests with pure antibody an d IL-6 demonstrated that, with b oth antibodies tested, more than 1000-fold molar excess of antibody is needed for neutralization in the hybridoma growth assay. It is conclu ded that increased biological activity in serum of the anti-IL-6 antib ody-treated mice is due to sequestration of the endogenous IL-6 in the form of antigen-antibody complexes which, due to the lack of sufficie nt antibody excess, produce nearly full activity in tle hybridoma grow th assay.