DEVELOPMENT OF A HUMAN INTERLEUKIN-6 RECEPTOR ANTAGONIST

Neutralizing monoclonal antibodies specific for human interleukin-6 (I L-6) bind two distinct sites on the IL-6 protein (sites I and II). The ir interference with IL-6 receptor binding suggested that site I is a receptor-binding site of IL-6, whereas site II is important for signal transduction. Mutagenesis of site II could therefore result in the is olation of IL-6 receptor antagonists. To test this hypothesis, a panel of IL-6 mutant proteins was constructed that did not bind to a site I I-specific monoclonal antibody. One such site II mutant protein (with double substitution of Gln-160 with Glu and Thr-163 with Pro) was foun d to be an antagonist of human IL-6. It was inactive on human CESS cel ls, weakly active on human HepG2 cells, but active on mouse B9 cells. It could specifically antagonize the activity of wild-type IL-6 on CES S and HepG2 cells. The binding affinity of this variant for the 80-kDa IL-6 receptor was similar to that of wild-type IL-6. High affinity bi nding to CESS cells, however, was abolished, suggesting that the mutan t protein is inactive because the complex of the 80-kDa IL-6 receptor and the mutant protein cannot associate with the signal transducer gp1 30. The human IL-6 antagonist protein may be potentially useful as a t herapeutic agent.