Monoclonal antibodies to human growth hormone modulate its biological properties

Previous results indicated that monoclonal antibodies (mAbs) termed mAb AE5, mAb AC8 and mAb F11, recognizing the human growth hormone (hGH) region left exposed after binding to lactogenic, somatogenic and hGH-specific receptors, produce allosteric changes in the hormone which modify its binding properties. To study whether these mAbs could also influence hGH biological activity, experiments were carried out with Nb2 cells, a rat lymphoma cell line which proliferates in the presence of lactogenic hormones. Experiments involving previous binding of the hormone to receptors before adding 125I-mAbs indicated that the hGH domain defined by overlapped epitopes AE5, AC8 and F11 is uncovered in hGH when it is bound to the cell membranes. To reveal any alteration in the hGH molecule induced by the mAbs, preformed 125I-mAb:hGH complexes were added to the cell membranes. Data showed that 125I-mAb AE5:hGH complexes bound better to the receptors than free hormone. On the contrary, hGH previously bound to 125I-mAb AC8 or 125I-mAb F11 was poorly recognized by Nb2 receptors. Furthermore, both mAbs AC8 and F11 strongly inhibited 125I-hGH binding to Nb2 cell membranes and hGH-induced Nb2 cell proliferation whereas mAb AE5 enhanced both hormone binding and hGH mitogenic effect. Additionally, since mAb AC8 is directed towards an epitope shared by hGH and human placental lactogen (hPL), it was also shown that this mAb could impair hPL biological activity even though it recognizes the hPL region left exposed in hPL:Nb2 cell receptor complexes. Data presented in this work suggest that mAbs directed to the hGH or hPL regions unmasked after binding to Nb2 cell receptors produce allosteric alterations in the binding properties of these hormones leading to either enhancement or decrease of their biological activities.