ANTIIDIOTYPIC ANTIBODY AS AN ESTROGEN MIMETIC - REMOVAL OF F-C FRAGMENT CONVERTS AGONIST TO ANTAGONIST

Previous studies indicated that the anti-idiotypic antibody (clone 1D(5)) caused an increase in uterine creatine kinase (CK) activity when administered in vivo to immature female rats, indicating that the antibody has oestrogenic-like activity. It was, therefore, of interest to investigate the structural requirements of clone 1D(5) to act as an oestrogen mimetic in an in vitro model system. In the present study, the effect of clone 1D(5) and its proteolytic fragments, F(ab')(2), Fab' and F-c on 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 1D(5) (8.33 nM) or oestradiol (E(2)) (30 nM) for 24 h caused a significant increase in CK activity, indicating that clone 1D(5) acted as an agonist. On the other hand, incubation of male-derived calvaria cells devoid of a functional ER with clone 1D(5) or E(2) did not have any effect on CK activity. Incubation of female-derived calvaria cells with clone 1D(5) and E(2) did not result in any further increase in CK activity, whereas dihydrotestosterone (DHT) did not alter the response to clone 1D(5). The CK response to clone 1D(5), 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 1D(5), the F(ab')(2) dimer (12 nM) and the Fab' monomer (24 nM), and the F-c fragment (28 nM) did not have E(2)-like activity in these cells. However, while the Fab' monomer or the F-c fragment, as well as clone 1D(5), did not affect the response of the female-derived calvaria cells to E(2), the F(ab')(2) dimer acted like an antagonist and completely inhibited the stimulatory effect of E(2) or 1D(5), 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 antiidiotypic antibody can be converted from an agonist to an antagonist by removal of the F-c portion 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 E(2).