Generation of human high-affinity antibodies specific for the fibroblast activation protein by guided selection

Four completely human antibody derivatives [single-chain-antibody fragments (scFvs)] with specificity for the general tumor stroma marker fibroblast a ctivation protein (FAP) were isolated by guided selection. Highly diverse I gG, IgM and IgD isotypes comprising heavy-chain variable domain libraries w ere generated using cDNAs derived from diverse lymphoid organs of a multitu de of donors. Three of the human scFvs were converted into bivalent minibod ies and expressed in eukaryotic cells for further functional characterizati on. Binding-competition studies and analysis by fluorescence-activated cell sorting showed high-affinity binding (10-20 nm) for two clones and recogni tion of the same epitope as the murine guiding antibody. The minibodies wer e successfully used for immunohistology of a variety of human carcinoma bio psies, revealing specific staining of stromal fibroblasts. Therefore, they should be suitable for in vivo diagnostic and tumor-targeting studies and, because of their completely human origin, be superior to murine or humanize d antibody derivatives.