Hayakawa and Smyth reported a stronger cytotoxicity in CD27− NK cells compared with CD27+ NK cells, which are in part CXCR3+23. However, only purified CD11b+ NK cells were used in the cytotoxicity assays they performed. CD11b has been associated with elevated levels of cytolytic function of mature NK cells. Whereas all CXCR3− NK cells were CD11b+ and highly cytolytic, a fraction of CXCR3+ NK cells lacked CD11b expression. CXCR3+ NK cells displayed lower cytotoxicity, and this could be due to their different developmental stage. Interestingly, the proliferative response of CXCR3+ NK cells to IL-21 was far greater than that of CXCR3− NK cells. Although inhibition of proliferation of
mouse NK cells by IL-21 has been reported, the effect was not analyzed for different NK-cell subsets 45. For human Selleckchem Panobinostat NK cells we showed that CD56bright NK cells exhibited a strong proliferative response towards IL-21 when combined with IL-2, although IL-21R is equally expressed on CD56dim and Kinase Inhibitor Library cell assay CD56bright NK cells 31. These results also correspond
to our murine data. Compared with CXCR3− NK cells, slightly higher percentages of CXCR3+ NK cells displayed IL-21R expression (data not shown). As shown for the human system, a specific role for STAT proteins can be suggested for the induction of proliferation of murine NK cells by IL-21 and IL-2. The two cytokines may induce the formation of particular STAT protein dimers, which could differentially affect the proliferation of CXCR3− and CXCR3+ NK cells. The combination and properties of STAT complexes still have to be determined in detail. In addition, signaling via IL-21R requires receptor heterodimerization with the γ chain (CD132), which is also shared by IL-2R and IL-15R 46, 47. In humans, the high affinity IL-2R, comprising CD25, CD122 and CD132, is only expressed on CD56bright but not CD56dim NK cells. The stronger proliferation of CXCR3+ NK cells could be due to the higher expression of CD122 on CXCR3+ NK cells (data not shown). In addition, CD11b− NK cells are reported to proliferate faster
than CD11b+ cells in vivo30. Since a fraction of CXCR3+ NK cells was negative for CD11b, it is plausible that these cells proliferate more strongly. A major role of NK cells is to kill malignant tumor cells. Accumulation of NK cells in certain 3-oxoacyl-(acyl-carrier-protein) reductase tumor tissue is dependent on CXCR3 expression and the presence of IFN-γ 28. In this context, CXCR3+ NK cells are probably important for immunosurveillance, since these NK cells are also more potent IFN-γ producers than CXCR3− NK cells when stimulated with IL-12 and IL-18. Regarding cytotoxicity, specific lysis of YAC-1 target cells by CXCR3− NK cells was twice as high as by CXCR3+ NK cells. Degranulation corresponded well to this result in several compartments, corroborating the specific role of CXCR3− NK cells in terms of cytolytic ability.