The interaction of BAT3 and Hsp70 was so far only analyzed with respect to the intracellular regulation of apoptosis, although it is known that Hsp70 is involved in the regulation of immune cells in its extracellular, exosomal form. Extracellular BAT3 was precipitated from tumor-cell-derived supernatant upon a non-lethal heat shock using antibodies either binding BAT3 or Hsp70 suggesting that there is a BAT3/Hsp70 complex. Thus, the association of BAT3 and Hsp70 suggests that both factors are associated with Torin 1 exosomes and may act in combination to activate NK cells. It is known that heat shock does not influence the exosomal secretory rate, but it significantly enhances the Hsp70 content of exosomes (+)-JQ1 Epigenetic Reader Domain inhibitor isolated from cell supernatant. Since the exosomal BAT3-expression is also induced in response to heat shock, both factors might act as intracellular danger sensors in a coordinative manner. We subsequently investigated the biological role of BAT3 surface-positive exosomes for NK cell activation. Exosomes with different BAT3 expressions levels were obtained from wildtype 293T cells, upon BAT3-overexpression or siRNAmediated down regulation. NK cells were stimulated with these exosomes and the supernatant was collected for TNF-a and IFN-c ELISA. Wild-type exosomes clearly stimulated the release of the inflammatory cytokines, and as expected, the cytokine release was even increased in response to BAT3- overexpressing exosomes. Interestingly, NK cells treated with BAT3-depleted exosomes failed to produce TNF-a and IFN-c, whereas a robust release was observed with control exosomes derived from control si-RNA transfected cells. Analogous results were obtained with iDC-derived exosomes. Exosomes were purified from untreated iDCs or from heat shock treated iDCs to increase the expression level of BAT3 in the supernatant. The release of TNF-a and IFN-c was in fact stronger in response to heat shock treated exosomes and observed in both allogenic and autologous settings. Thus, the biological activity of exosomes correlated directly with the BAT3 expression level indicating that BAT3 is crucial for the exosomal-dependent activation of NK cells. The data demonstrate a novel role for exosomes released from accessory cells that initiate the innate function of NK cells and may shape the adaptive immune response in a BAT3-dependent manner. It was shown that BAT3 acts as a dendritic cellassociated ligand providing an explanation how and when BAT3 – a intracellular protein – is secreted and engages the NKp30 receptor on effector cells. This is based on the following findings: iDCs and tumor cells release BAT3 surface-positive exosomes; exosomes release was induced by moderate heat shock and correlated with an increase of BAT3 mRNA level; iDCderived BAT3 positive exosomes induce cytokine secretion and BAT3 is critically involved in NK cell-dependent maturation and killing of iDCs.