Species-crossreactive scFv against the tumor stroma marker "Fibroblast activation protein" selected by phage display from an immunized FAP(-/-) knock-out mouse

Background: Fibroblast activation protein (FAP) is a type II membrane prote in expressed on tumor stroma fibroblasts in more than 90% of all carcinomas . FAP serves as a diagnostic marker and is potential therapeutic target for treatment of a wide variety of FAP(+) carcinomas. Murine tumor stroma mode ls and FAP-specific antibodies are required to investigate the functional r ole of FAP in tumor biology and its usefulness for drug targeting. We here describe the development of antibodies with crossreactivity for human (hFAP ) and murine FAP (mFAP), which share 89% amino acid identity. Material and Methods: An FAP(-/-) mouse was sequentially immunized with rec ombinant murine and human FAP-CD8 fusion proteins. Immunoglobulin cDNA deri ved from hyperimmune spleen cells was used for the construction of a combin atorial single chain Fv (scFv) library. Phage display selection of FAP-spec ific scFv was performed on immobilized hFAP followed by selection on cells expressing murine FAR Results: High-affinity, species-crossreactive, FAP-specific scFv were isola ted upon sequential phage display selection. A bivalent derivative (minibod y MO36) constructed thereof was applied for immunohistochemical analyses an d allowed detection of FAP expression on stroma cells of different human ca rcinomas as well as on murine host stroma in a tumor xenograft model. Conclusions: MB MO36, derived from phage display selected species crossreac tive scFv, is suitable for tumor stroma targeting and will be a valuable to ol in the analyses of the functional role of FAP in tumor biology as well a s in the evaluation of the suitability of FAP for drug targeting.