Specific lysis of YAC-1 targets by freshly isolated CD70-Tg and WT spleen NK cells was comparable at 4 wk of age. However, at 6 and 8 wk of age for spleen and at all analysed time points for liver CD70-Tg NK cells, cytotoxic capacities were significantly increased, with most pronounced differences

evidenced in liver (Fig. 5A). As lysis of YAC-1 targets is highly dependent upon NKG2D receptor presence 33 and the secretion of granzyme B 4, we analysed the expression of both proteins. In accordance to their elevated YAC-1 cytotoxicity, both liver and spleen NK cells of CD70-Tg mice showed higher NKG2D and granzyme B expression (Fig. 5B and C and data not shown). Cytotoxicity was also analysed using cytokine-stimulated NK cells from spleen and liver of 8-wk-old WT and CD70-Tg mice. Again NK cells from CD70-Tg mice displayed significant higher cytotoxicity compared with WT NK cells. find more Differences in cytotoxicity between CD70-Tg versus Staurosporine ic50 WT for cytokine-stimulated liver NK cells were comparable to freshly isolated cells. For cytokine-stimulated spleen cells,

differences were higher compared with freshly isolated cells (Fig. 5D). Finally, spleen and liver NK cells were tested for IFN-γ production. Upon IL-12 and IL-18 stimulation, spleen NK cells from CD70-Tg mice produced significantly less IFN-γ compared with WT NK cells. No differences in cytokine production were found for liver NK cells (Fig. 5E). When NK cells Adenosine triphosphate were stimulated through NK1.1, again CD70-Tg NK cells from spleen produced lower IFN-γ levels compared with WT NK cells, whereas no differences were

observed for liver NK cells (Fig. 5F). To test whether the evidenced effects on the NK cell population in CD70-Tg mice are indeed due to the continuous triggering of CD27 by CD70 phenotypical and functional assays were conducted in CD70-Tg×CD27−/− mice and compared with WT and CD70-Tg mice. The severely reduced NK cell numbers in spleen and liver of CD70-Tg mice were normalized in CD70-Tg×CD27−/− mice (Fig. 6A and E). The spleen and liver expression of CD43 and CD11b, which was significantly down-regulated in NK cells of CD70-Tg mice, was normal in NK cells from CD70-Tg×CD27−/− mice (Fig. 6B and F). Also, spleen and liver NK cells from CD70-Tg×CD27−/− mice showed equal levels of expression of CD69 and Ly49D compared with WT mice (Fig. 6C and D, and G and H). In addition to the NK cell number and phenotype, the functional capacities of NK cells in CD70-Tg mice were directly affected through continuous CD27–CD70 interaction, as IFN-γ production and YAC-1 specific cytotoxicity reached WT levels in NK cells from CD70-Tg×CD27−/− mice (Fig. 6I and J). CD27 is a unique TNFR family member as it is the only receptor of this family that is constitutively expressed on freshly isolated NK cells 31. The present study is the first to investigate the possible effects of continuous in vivo triggering of CD27 on NK cells.