Expression, crystallization and X-ray diffraction analysis of a complex between B7-H6, a tumor cell ligand for the natural cytotoxicity receptor NKp30, and an inhibitory antibody.

Natural killer (NK) cells are essential components of the innate immune response to tumors and viral infections. In humans, the activating natural cytotoxicity receptor NKp30 plays a major role in NK cell-mediated tumor cell lysis. NKp30 recognizes the cell-surface protein B7-H6, which is expressed on tumor, but not healthy, cells. A mouse monoclonal antibody (17B1.3) against human B7-H6 has been developed (Kd = 0.2 µM) to investigate NKp30-mediated NK cell activation and to target tumors expressing B7-H6. Surprisingly, 17B1.3 blocks NK cell activation without interfering with the binding of B7-H6 to NKp30. Understanding the inhibitory mechanism of this antibody will require knowing the structure of 17B1.3 bound to B7-H6. The antigen-binding fragment (Fab) of 17B1.3 was expressed by in vitro folding from bacterial inclusion bodies. The extracellular domain of B7-H6 was produced by secretion from baculovirus-infected insect cells. Crystals of the Fab 17B1.3-B7-H6 complex grown by macro-seeding diffracted to 2.5 Å resolution and belonged to space group P2(1)2(1)2(1), with unit-cell parameters a = 89.6, b = 138.0, c = 171.4 Å, α = ß = γ = 90°. Comparison of the Fab 17B1.3-B7-H6 structure with the known NKp30-B7-H6 structure will elucidate the inhibitory mechanism of 17B1.3.