A Soluble Fragment of the Tumor Antigen BCL2-associated Athanogene 6 (BAG-6) Is Essential and Sufficient for Inhibition of NKp30 Receptor-dependent Cytotoxicity of Natural Killer Cells

Background: Cellular ligands of the activating natural killer (NK) cell receptor NKp30 are poorly characterized. Results: The identified domain of the cellular ligand BCL2-associated athanogene 6 (BAG-6) is essential and sufficient to bind NKp30 and inhibits NK cell function. Conclusion: The BAG-6 domain from amino acid 686 to 936 is an important element of BAG-6-dependent tumor immune escape. Significance: This study gives the first molecular insights into BAG-6-mediated inhibition of NKp30-dependent NK cell cytotoxicity. Immunosurveillance of tumor cells depends on NKp30, a major activating receptor of human natural killer (NK) cells. The human BCL2-associated athanogene 6 (BAG-6, also known as BAT3; 1126 amino acids) is a cellular ligand of NKp30. To date, little is known about the molecular details of this receptor ligand system. Within the current study, we have located the binding site of NKp30 to a sequence stretch of 250 amino acids in the C-terminal region of BAG-6 (BAG-6(686-936)). BAG-6(686-936) forms a noncovalent dimer of 57-59 kDa, which is sufficient for high affinity interaction with NKp30 (K-D < 100 nm). As our most important finding, BAG-6(686-936) inhibits NKp30-dependent signaling, interferon- release, and degranulation of NK cells in the presence of malignantly transformed target cells. Based on these data, we show for the first time that BAG-6(686-936) comprises a subdomain of BAG-6, which is sufficient for receptor docking and inhibition of NKp30-dependent NK cell cytotoxicity as part of a tumor immune escape mechanism. These molecular insights provide an access point to restore tumor immunosurveillance by NK cells and to increase the efficacy of cellular therapies.