Design and development of antibody tools for characterizing the structure and function of cancer-related serine proteases

Proteases play important roles in human biology and disease, and the development of selective tools is essential for studying their function and structure. Members of the same protease family have structurally similar active sites and closely-related substrate specificities. This makes targeting individual proteases particularly challenging. We are interested in using recombinant antibody technology to generate protease-specific tools that are inhibitory, selective, and compatible for translational applications. The focus of our work has been on the design and development of antibodies to study serine proteases involved in cancer pericellular proteolysis. The type II transmembrane serine protease (TTSP) family includes members, matriptase, hepsin, TMPRSS2, TMPRSS4, and DESC1, which are involved in this process. Using a naïve phage display antibody library, we identified a new inhibitory antibody A11 against matriptase. A11 uses a novel reverse binding motif found in a long antibody loop to inhibit matriptase in a mechanism different from another antibody E2 that uses a substrate-like binding motif. We designed synthetic antibody libraries around these two motifs to create biased libraries for identifying antibody-based inhibitors of serine protease targets more efficiently than other general approaches. The libraries were validated against matriptase and tested against a second protease target TMPRSS2, chosen for its increasing importance in aggressive and metastatic prostate cancer. Multiple expression systems were tested to produce reagent quantities of active TMPRSS2 for antibody selection as well as future structural efforts. Using recombinant protein expressed from Pichia pastoris with our libraries, we identified and characterized a number of inhibitory TMPRSS2 antibodies and observed a higher ratio of inhibitors selected from the biased libraries. These antibodies were used in flow cytometry and immunofluorescence microscopy applications to follow TMPRSS2 expression and localization in prostate cancer cells. Since TMPRSS2 is an androgen receptor regulated gene, we explored using our antibodies for monitoring androgen receptor signaling, critical for prostate cancer cell survival and treatment resistance mechanisms, for in vivo molecular imaging. The work presented here advances our knowledge of protease-targeting mechanisms of antibodies, demonstrates the importance of selective tools for studying proteases, and contributes to ongoing work in understanding the role of TTSPs in cancer