A therapeutic human anti-idiotypic antibody mimics CD55 in three distinct regions

The human anti-idiotypic antibody 105AD7 was isolated from a colorectal can cer patient receiving the anti-tumor antibody 791T/36 for radioimmuno-scint igraphy of liver metastases. We have mapped the binding site of 791T/36 to the first two small consensus repeat (SCR) domains of the complement regula tory protein (CD55) that is overexpressed by a wide range of solid tumors. Cloning of both antigen and anti-idiotype has identified the molecular basi s of their mimicry. Amino acid homology has been identified between three c omplementarity-determining regions of 105AD7 and three regions of CD55 with in the first two SCR domains. 791T/36 and anti-anti-idiotypic (Ab3) polyclo nal antibodies raised against 105AD7 showed specific binding to these pepti des. The antibodies were also found to bind synergistically to combinations of these peptides, indicating cooperativity between the peptides in stabil izing antibody binding. This also implies that the contact face on both CD5 5 antigen and 105AD7 is generated by the cooperation of several peptides po sitioned on two domains in each protein. Thus a human monoclonal anti-idiot ypic antibody generated by a cancer patient is able to show both amino acid and structural homology with the complement regulatory protein CD55. These findings help identify the mechanism by which a human anti-idiotypic antib ody is able to mimic a tumor-associated antigen and stimulate anti-tumor B and T cell responses.