Molecular mechanism for the action of the anti-CD44 monoclonal antibody MEM-85.
The hyaluronate receptor CD44 plays role in cell adhesion and migration and is involved in tumor metastasis. The extracellular domain of CD44 comprises the hyaluronate-binding domain (HABD) and the membrane-proximal stem region; the short intracellular portion interacts with adaptor proteins and triggers signaling pathways. Binding of hyaluronate to CD44 HABD induces an allosteric conformational change, which results in CD44 shedding. A poorly characterized epitope in human CD44 HABD is recognized by the murine monoclonal antibody MEM-85, which cross-blocks hyaluronate binding to CD44 and also induces CD44 shedding. MEM-85 is of therapeutic interest, as it inhibits growth of lung cancer cells in murine models. In this work, we employed a combination of biophysical methods to determine the MEM-85 binding epitope in CD44 HABD and to provide detailed insight into the mechanism of MEM-85 action. In particular, we constructed a single-chain variable fragment (scFv) of MEM-85 as a tool for detailed characterization of the CD44 HABD-antibody complex and identified residues within CD44 HABD involved in the interaction with scFv MEM-85 by NMR spectroscopy and mutational analysis. In addition, we built a rigid body model of the CD44 HABD-scFv MEM-85 complex using a low-resolution structure obtained by small-angle X-ray scattering. The MEM-85 epitope is situated in the C-terminal part of CD44 HABD, rather than the hyaluronate-binding groove, and the binding of MEM-85 induces a structural reorganization similar to that induced by hyaluronate. Therefore, the mechanism of MEM-85 cross-blocking of hyaluronate binding is likely of an allosteric, relay-like nature.