Identification of prostate specific membrane antigen (PSMA) as the target of monoclonal antibody 107-1A4 by...

Recently we described the generation of the prostate tissue-specific monoclonal antibody (MAb) 107-1A4, its expression pattern and preliminary targeting of human prostate cancer xenografts. In this report we demonstrate that the target antigen for MAb 107-1A4 is prostate-specific membrane antigen (PSMA) using immunoaffinity absorption followed by SDS-PAGE and mass spectrometric analysis of peptides produced by in-gel tryptic digestion. The identity of the antigen has been confirmed by Western blots using MAbs of known specificity. MAb 107-1A4 is not reactive on Western blots. The conformational epitope for 107-1A4 is on the extracellular domain of PSMA. In competition studies, the binding of MAb 107-1A4 to LNCaP cells is inhibited by itself but not by any other of several other anti-PSMA MAbs, suggesting that the epitope may be unique. These results suggest that 107-1A4 is reactive to a conformational epitope in the external domain of PSMA that is unique among the panel of anti-PSMA MAbs in this study. Furthermore this work demonstrates the ability of mass spectroscopy to elucidate antibody-ligand interaction.