A monoclonal antibody recognizing an epitope shared by receptors for growth hormone, prolactin, interleukin 2 and interleukin 6

Monoclonal antibody (MAb) termed R7B4 was generated throughout the idiotypi c-antiidiotypic network from mice immunized with human and bovine growth ho rmones (GH). The Ab was selected on the basis that it did not recognize hum an GH (hGH) neither insolubilized nor in solution but inhibited I-125-hGH b inding to receptors from rat and rabbit liver and from Nb2-cell membranes. Since it inhibited Nb2-cell mitogenesis stimulated by hGH, prolactins or pl acental lactogens, MAb R7B4 behaved as an antagonist of lactogenic hormones . Furthermore, the Ab impaired proliferative activity of interleukin 2 (IL- 2) on Nb2 cells as well as growth of 7TD1 cells, an interleukin 6 (IL-6) de pendent hybridoma not expressing GH receptors. Biotin-labeled MAb R7B4 specifically bound to rat liver microsomes, and the Ab was able to recognize Nb2 and 7TD1-cell membranes as shown by flow cyto metry experiments. However, MAb binding was not hampered by hGH, indicating that the Ab did not mimic GH binding site to receptors. Immunoblot assays indicated that rat and rabbit liver as well as Nb2-cells membrane antigens recognized by MAb R7B4 were similar to those revealed by a MAb directed to prolactin receptors. In addition, MAb R7B4 was able to detect two bands pro bably corresponding to the somatogenic receptor in rabbit liver microsomes as well as three different proteins in 7TD1-cells showing molecular weights similar to those of the IL-6 receptor complex. Results suggest that MAb R7B4 is directed to an epitope shared by receptors for lactogenic and somatogenic hormones, IL-2 and IL-6. To our knowledge, these data are the first experimental evidence of the existence of structur al similarity between some of the receptors grouped in the cytokine recepto r superfamily.