Studies on the interleukin-6-type cytokine signal transducer gp130 reveal a novel mechanism of receptor activation by monoclonal antibodies

The transmembrane glycoprotein gp130 belongs to the family of hematopoietic cytokine receptors. It represents the common signal transducing receptor c omponent of the so called interleukin-6-type cytokines. For several cytokin e receptors including gp130 it has been shown that receptor activation cann ot only be achieved by the natural ligand but also by single monoclonal ant ibodies raised against the receptor ectodomain. These findings have been in terpreted in a way that dimerization of cytokine receptors is sufficient fo r receptor activation, Here we show that the recently described gp130-activ ating antibody B-S12 actually consists of two different monoclonal antibodi es. By subcloning of B-S12 the monoclonal antibodies B-S12-A5 and B-S12-G7 were obtained. The individual antibodies are biologically inactive, in comb ination they exert B-Sla-like activity on hepatoma cells. On Ba/F3 cells st ably transfected with gp130 a combination of B-S12-G7 with another monoclon al gp130 antibody, B-P8, is required to stimulate proliferation. Using gp13 0 deletion mutants we show that all three antibodies map to domains 2 and 3 of gp130 which constitute the cytokine binding module. The individual anti bodies inhibit activation of the signal transducer by interleukin-6 and int erfere with binding of interleukin-6 to gp130, Interestingly, the combinati on of B-S12-G7 and a Fab fragment of B-PS retains biological activity. We c onclude from our data that (i) the monoclonal antibodies activate gp130 by mimicking the natural ligand and (ii) enforcement of gp130 dimerization is not sufficient for receptor activation but additional conformational requir ements have to be fulfilled.