Crystal structure of neutralizing antibody / IL-17 complex
with H/D-exchange identified IL-17 epitope red highlighted

While an X-ray co-crystal of the antigen:antibody complex remains the gold standard of epitope determination, it is technically challenging, tedious and not always feasible due to the difficulty of obtaining high quality well diffracting crystals. In the absence of a crystal complex, hydrogen/deuterium exchange (e.g. DXMS, H/D-Ex, etc.) is the next best available option. ExSAR employs hydrogen/deuterium exchange mass spectrometry as a tool to identify binding interfaces. Differences in the rate of exchange serve to highlight the location of an epitope. Hydrogen/deuterium exchange (H/D-Ex) was utilized to identify the epitope of a potent neutralizing antibody to IL-17, a homodimeric cytokine involved in a number of pro-inflammatory signaling pathways. Illustrated in red in the above figure are areas where exchange was significantly reduced upon complex formation. Pink highlighted regions indicate areas that were moderately affected upon complex formation. The epitope was subsequently validated by the X-ray crystal structure of the complex.