2D). In NK cells from mice with large tumor burdens, by contrast, ex vivo stimulation failed to restore cytotoxicity (Fig. 2D). Taken together, in tumor-bearing λ-myc animals, NK cells became activated but their effector functions were uncoupled from activation. This was not seen in normal selleck control mice, where expression of the activation markers CD45R and CD69 closely correlated with NK-effector functions because injection of DC into WT mice or incubation of normal NK cells with IL-15 in vitro resulted in enhanced cytotoxicity against NK-sensitive targets as well as increased expression of CD45R and CD69 (data not shown). The activation-associated status

of anergy in NK cells from tumor mice was reversible at early stages of disease development and became irreversible at later stages. NK cells might have been paralyzed by developing tumors or exhausted as a consequence of prolonged activation. To identify the lymphoma-derived signals determining NK-cell activation, we tested the lymphomas growing in λ-myc mice for expression of MHC class I and NKG2D-L. At early stages of tumor growth, we observed a decreased expression of MHC class I with a maximum reduction to about 5% as compared with B lymphocytes from PLX3397 normal animals. Furthermore, an induction of NKG2D-L with an

up to tenfold higher level than found on normal B cells was detected (examples in Fig. 3A and B). Therefore, the NK-cell activation observed in tumor mice may be due to lack of inhibitory signals and/or presence of positive signals Paclitaxel in vivo mediated by NKG2D engagement. At later stages of disease development, however, tumors with normally high MHC class I expression and only marginal or absent NKG2D-L expression were detected (data not shown). The absence of NKG2D-L in late-stage lymphomas might suggest a timely limited induction of NKG2D-L as a result of tumor-associated genetic alterations 30 and its progressive down-regulation during disease development. To assess the specific contribution

of missing self and NKG2D-L, respectively, to the NK-cell activation process, we asked whether the activation pattern is quantitatively determined by the phenotype of early-stage tumors. It turned out that NK-cell activation, as determined by CD45R expression, closely correlated with the degree of tumor MHC class I down-regulation (Fig. 3C). In contrast, no significant correlation was found between the NK-cell activation marker and tumor NKG2D-L expression (Fig. 3D). To shed light on the mechanistic background of the correlation detected in vivo we did in vitro incubation experiments using WT NK cells and tumor cells with different MHC class I expression levels. Lymphoma cells were isolated ex vivo and incubated with IFN-γ or left untreated. In response to IFN-γ, tumor cells up-regulated MHC class I expression (Fig. 3E) while NKG2D-L expression remained unaltered (data not shown).