Recombinant antibody phage library technology provides multiple advantages, including that

human antibodies can be generated against proteins that are highly conserved between

species. We used this technology to isolate and characterize an anti-EphA2 single-chain

antibody. We show that the antibody binds the antigen with 1:1 stoichiometry and has high

specificity for EphA2. The crystal structure of the complex reveals that the antibody targets the

same receptor surface cavity as the ephrin ligand. Specifically, a lengthy CDR-H3 loop protrudes deep into the ligand-binding cavity, with several hydrophobic residues at its tip forming an anchor-like structure buried within the hydrophobic Eph pocket, in a way similar to the ephrin receptor-binding loop in the Eph/ephrin structures. Consequently, the antibody blocks ephrin binding to EphA2. Furthermore, it induces apoptosis and reduces cell proliferation in lymphoma cells lines. Since Ephs are important mediators of tumorigenesis, such antibodies could have applications both in research and therapy.