Immunological properties of a single-chain fragment of the anti-idiotypic antibody ACA125

Vaccination with anti-idiotypic antibodies has been described as a promisin g concept for treatment of several malignant diseases. The murine monoclona l antiidiotypic antibody ACA125 imitates a specific epitope of the tumor-as sociated antigen CA125 expressed by 80% of ovarian carcinomas. In the first clinical trial it could be shown that mAb ACA125 is able to elicit anti-an tiidiotypic antibodies (Ab3) with anti-CA125 specificity in patients with a dvanced ovarian cancer. In order to improve the capabilities of anti-idioty pe vaccines we generated a genetically engineered single-chain fragment (sc Fv) ACA125 composed of heavy- and light-chain variable regions connected by a flexible linker. The antigenicity of scFv ACA125 was demonstrated by imm unizing rats i.p, with scFv or complete mAb in complete/incomplete Freund's adjuvants (CFA/IFA) or precipitated by aluminium hydroxide. Negative contr ol groups included applications of irrelevant mouse IgG or adjuvants alone. Anti-anti-idiotypic antibodies (Ab3) directed against the mAb ACA125 as we ll as specific anti-CA125 antibodies (Ab1') could be detected in all animal s treated with scFv in CFA/IFA. Nevertheless, antibody titers were lower th an when the complete mAb ACA125 was used. Surprisingly, an increase of spec ificity could not be observed in scFv-immunized animals, which had been exp ected because of the lack of heavy- and light-chain constant regions that c ould raise rather unspecific anti-isotypic and anti-allotypic rat anti(mous e Ig) antibodies (RAMA). In contrast, the RAMA responses detected in these rats were even stronger than those following immunization with complete mAb ACA125. In conclusion, the anti-idiotypic scFv ACA125 alone cannot improve the immunogenic features of the corresponding mAb, but provides a useful t ool for the further development of genetic vaccines.