Highly active CAR T cells that bind to a juxtamembrane region of mesothelin and are not blocked by shed mesothelin

Mesothelin (MSLN) is a cell-surface protein that is highly expressed by many cancers. Despite many clinical trials, there is not a Food and Drug Administration (FDA)-approved antibody-based therapy targeting MSLN. Shed MSLN is present in high concentrations in tumors and in body fluids and constitutes a major barrier to antibody-based therapies. MSLN is shed from cells by the action of proteases that cut very close to the membrane. We have identified the major protease sites in MSLN and prepared a monoclonal antibody (mAb) 15B6, that binds next to the cell membrane at the protease-sensitive region and inhibits MSLN shedding. We determined the structure of a Fab-MSLN peptide complex and found the antibody binds to residues Y-V-DLSMQEL at the C terminus of MSLN. mAb 15B6 makes very active chimeric antigen receptor (CAR) T cells whose activity is not blocked by shed MSLN. The 15B6 CAR T cells have greatly superior antitumor activity in mice than CAR T cells targeting an epitope in shed MSLN.