Probing the receptor-interaction of glycoprotein hormones with monoclonal antibodies

Having recently analyzed with monoclonal antibodies (MCA) the immunologic surface of human chorionic gonadotropin (hCG) as consisting of 9 distinct epitopes exposed on the molecule in a characteristic topographical manner (Schwarz, S., Berger, P., and Wick, G., Endocrinology 118, 189-197, 1986) we now attempted to confirm this result on a more general basis, e.g. by incorporating MCA that were just recently obtained and previously not included. What we were, however, interested most was the question whether the tentative model of the epitope map of hCG could represent a "target" with which hCG-related hormones such as LH, FSH, and TSH (the family of glycoprotein hormones, GPH) would (partially) match. Indeed, repeating various immunizations with GPH of human as well as animal origin revealed a remarkable reproducibility in terms of several anticipated epitope specificities of MCA. This indicates that MCA can be regarded as reliable probes for mapping epitopes and, as we have presumed, of receptor interaction domains of GPH as well. Extending the originally used 2-site MCA binding exclusion approach by an interspecies crossreactivity (Xr) analysis we now are able to refine our epitope model of hCG such that 2 additional epitopes were found which were not previously resolvable. Most surprisingly, two of 5 epitopes on the alpha subunit were now also detected on various non-human GPH, which is in striking contrast to a seemingly well established dogma. Yet all five alpha epitopes of hCG are present on hLH, hFSH, and hTSH as well and arranged in the same spatial relationship to each other as on hCG. Even the 2 conformational epitopes and their close topographical relationship to the alpha-epitopes appear to be remarkably conserved on all human GPH. Among the beta-epitopes we have found one that is not shared by hLH and that - surprisingly - is not the C-terminal peptide (CTP) by which hLH differs from hCG. On the basis of this refined epitope map a way was paved along which it should be feasible to elucidate the sterical relationship of the epitopes to the receptor interaction domain(s) of hCG. To this end the MCA were tested in principally two ways: first, as to which of the 11 MCA with different epitope specificities would be able (or not) to inhibit by preincubation the binding of radiolabeled hCG (or hLH, respectively) to rat testis LH/hCG receptors? Secondly (and inversely) which of the 11 epitopes of hCG would still be accessible to binding by radiolabeled MCA when the (unlabeled) hormone is bound to the receptor?(ABSTRACT TRUNCATED AT 400 WORDS)