A novel human fibronectin cryptic sequence unmasked by the insertion of the angiogenesis-associated extra type III domain B.

The angiogenesis-associated extra-domain B (EDB) of fibronectin (FN) is a complete type III repeat of 91 amino acids. Its expression is modulated by the alternative splicing pattern of the FN pre-mRNA. FN containing the EDB (B-FN) is undetectable in tissues of healthy adults, with rare exceptions such as the female reproductive system where tissue remodeling and angiogenesis are recurrent physiological processes. On the contrary, B-FN is expressed at high levels in neoplastic tissues and during angiogenesis; consequently, it is considered an excellent marker of angiogenesis. Here, we report on a novel FN cryptic sequence, localized on the FN type III repeat 8 (immediately downstream of the EDB) that is unmasked by the insertion of the EDB. This sequence is specifically recognized by the high-affinity monoclonal antibody, C6, that selectively recognizes B-FN by means of ELISA, immunohistochemical and Western blot assays. The variable regions of C6 were cloned and a divalent covalently linked mini-antibody was generated. Biodistribution studies using the radioiodinated C6 mini-antibody on tumor-bearing mice demonstrated an efficient tumor targeting. This antibody represents a new tool for the study of the potential biological functions of hindered sequences that the inclusion of the EDB renders accessible, and likewise makes its epitope an additional angiogenesis target.