Anti-idiotypic antibody as an oestrogen mimetic: removal of Fcfragment converts agonist to antagonist

Previous studies indicated that the anti-idiotypic antibody (clone 1D5) caused an increase in uterine creatine kinase (CK) activity when administered in vivoto immature female rats, indicating that the antibody has oestrogenic-like activity. It was, therefore, of interest to investigate the structural requirements of clone 1D5to act as an oestrogen mimetic in an in vitromodel system. In the present study, the effect of clone 1D5and its proteolytic fragments, F(ab′)2, Fab′ and Fcon CK activity was examined in cultured skeletal cells having functional oestrogen receptor (ER). Incubation of female-derived calvaria cells or epiphyseal cartilage cells with clone 1D5(8·33 nm) or oestradiol (E2) (30 nm) for 24 h caused a significant increase in CK activity, indicating that clone 1D5acted as an agonist. On the other hand, incubation of male-derived calvaria cells devoid of a functional ER with clone 1D5or E2did not have any effect on CK activity. Incubation of female-derived calvaria cells with clone 1D5and E2did not result in any further increase in CK activity, whereas dihydrotestosterone (DHT) did not alter the response to clone 1D5. The CK response to clone 1D5, in female-derived calvaria cells was time- and dose-dependent and could be inhibited in a dose-dependent manner by the oestrogen antagonist tamoxifen. In contrast, the proteolytic fragments of clone 1D5, the F(ab′)2dimer (12 nm) and the Fab′ monomer (24 nm), and the Fcfragment (28 nm) did not have E2-like activity in these cells. However, while the Fab′ monomer or the Fcfragment, as well as clone 1D5, did not affect the response of the female-derived calvaria cells to E2, the F(ab′)2dimer acted like an antagonist and completely inhibited the stimulatory effect of E2or 1D5, but was unable to block the stimulatory effect of DHT on CK in male-derived calvaria cells. Collectively, these results imply that a bivalent antibody is necessary for the observed physiological responses, and that the anti-idiotypic antibody can be converted from an agonist to an antagonist by removal of the Fcportion of the antibody molecule. Furthermore, the anti-idiotypic antibody has an oestrogenic-like effect inhibited by tamoxifen only in skeletal cells capable of responding to E2.Journal of Endocrinology(1995) 145,409–416