Primary structure of the variable regions encoding antibody to NG2, a tumour-specific antigen on the rat chondrosarcoma HSN. Correlation of idiotypic specificities with amino acid sequences.

Eight syngeneic rat monoclonal antibodies that recognize structurally overlapping epitopes on the chondroitin proteoglycan NG2, a tumour-specific antigen on the chemically induced rat chondrosarcoma HSN, have been analysed for the sequence of their immunoglobulin heavy (H) and light (L) chain variable (V) regions. This analysis defined five groups of antibodies which are very similar for both the H and L chains and revealed that a wide range of different V regions are capable of binding to the same antigenic determinant. However, three mAbs, 11/160, ALN/12/17 and ALN/9/94, which recognize a sequential epitope, were found to use almost identical heavy (V-D-J) and light (V-J) chains in regions demonstrating an exclusivity in specific protein-protein interaction for this particular epitope. Two other mAbs, ALN/11/53 and AL/3/12, used similar V and J segments but totally different D regions. With the exception of the pair ALN/11/53 and AL/3/12, this grouping of antibodies matches that derived from the idiotypic specificity study we have reported previously. The reactivity pattern of Ab1 11/160, ALN/12/17 and ALN/9/94 with six anti-idiotopic mAbs raised against 11/160 demonstrated that the idiotope recognized by Ab2 HIM/3/41 was defined by a single amino acid, Asn, at position 52 within the CDR2 loop of the VH region; whereas the D region of Ab1 ALN/11/53 was implicated as the structural correlate of idiotypy. The substitution of AsnH52 influenced the Id recognition but Ag binding was not affected suggesting that Ab2 HIM/3/41 did not mimic the NG2 Ag.