Natural-killer (NK)-cells are lymphocytes of the innate immune system, which directly attack tumor and virus-infected cells. They provide a link between innate and adaptive immunity through crosstalk with dendritic cells (DCs) and mediate T-cell activation. Among the activating and inhibitory receptors that regulate NK cell activity, the orphan NKp30 receptor (NCR3, CD337) plays a special role as NKp30 does not only induce target cell lysis but is also crucial for the interaction between NK cells and dendritic cells. However, so far the cellular ligands for NKp30 have remained elusive.

Using a yeast two hybrid approach with the extracellular NKp30 sequence as bait we were able to isolate a putative NKp30 ligand from a tumor cDNA library. Sequence analysis revealed that the cDNA encoded for BAT3 (HLA-B-associated transcript 3) which originally had been cloned from the human major histocompatibility complex by chromosome walking and mapped to chromosome 6p21.3 within the HLA complex. BAT3 was described as a nuclear protein characterized by an N-terminal ubiquitin-like region, a polyproline stretch and a highly conserved BAG-(Bcl-2-associated athanogene) domain. Until now, the function of BAT3 in mammals is not clearly defined. With BAT3-specific antibodies and by means of laser scanner microscopy and Western Blotting we demonstrate that tumor cell lines and immature as well as mature DCs express BAT3. Upon co-cultivation with NK cells or exposure to stress signals such as heat shock, we observed that BAT3 translocates from the nucleus to the cell-surface and the protein is released from tumor cells into the extracellular environment. BAT3 binds directly and specifically to NKp30 on NK cells and triggers NK cell-mediated cytokine release and NKp30-dependent cytotoxicity. The inhibition of endogenous BAT3 using polyclonal antibodies prevents tumor rejection in vivo, demonstrating that BAT3 is necessary for the tumor rejection in a xenograft tumor model.

Thus, BAT3 is the long sought cellular NKp30 ligand and exhibits a novel mechanism by which a nuclear protein activates NK cells via NKp30-engagement after its release to the cell-surface and the cell environment. The isolation of the first cellular NKp30-ligand provides the basis for a better molecular understanding of NK cell-regulation and allows the development of clinical applications targeting NKp30 for the immunotherapy.

Disclosure: No relevant conflicts of interest to declare.

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