ANALYSIS OF IDIOTOPE STRUCTURE OF OVARIAN-CANCER ANTIBODIES - RECOGNITION OF THE SAME EPITOPE BY 2 MONOCLONAL-ANTIBODIES DIFFERING MAINLY IN THEIR HEAVY-CHAIN VARIABLE SEQUENCES

Two MoAbs, independently raised against ovarian carcinoma cells and re ferred to as OV-TL3 and OV-TL16, display an identical reaction pattern with a membrane-associated protein in both normal and malignant ovari an cells. Also, a similar binding affinity constant and a similar numb er of binding sites per cell indicate that both MoAbs bind to the same antigen. Competition assays reveal that OV-TL16 is able to compete wi th OV-TL3 for binding to OVCAR-3 cells. Epitope mapping using a filame ntous phage hexapeptide epitope library showed that both MoAbs are abl e to select identical phages, suggesting that their epitopes are ident ical or at least overlapping. However, purified polyclonal and monoclo nal anti-idiotypic antibodies directed against OV-TL3 failed to recogn ize the OV-TL16 idiotype, indicating that the structure of the antigen -binding regions of both antibodies is distinct. This was corroborated by molecular cloning and sequencing of the variable heavy (V-H) and l ight (V-L) chain immunoglobulin regions of both MoAbs. The V-H regions of both antibodies were found to be distinct, whereas the V-L regions are almost identical. Computer modelling of the idiotypes suggests th at the complementarity determining regions (CDR), with the exception o f V(H)CDR3, have (almost) identical spatial configurations. Our data i ndicate that, although structurally different in their V-H regions, OV -TL3 and OV-TL16 are able to bind to identical epitopic regions on the antigen, because differences in primary structure do not exclude the formation of sufficient and similar spatial structures for the interac tion with an epitope.