“
“The role of NK cells in the control of endogenously arising tumors is still unclear. We monitored activation and effector functions

of NK cells in a c-myc-transgenic mouse model of spontaneously arising lymphoma. At early stages, tumors demonstrated reduced MHC class I expression and increased expression of natural killer group 2D ligands (NKG2D-L). NK cells in these tumors showed an activated phenotype that correlated with the loss of tumor MHC class I. With increasing tumor load however, NK-cell effector functions became progressively paralyzed or exhausted. In later stages of disease, tumors re-expressed MHC class I and lost NKG2D-L, suggesting a role of these two signals for NK cell-mediated tumor control. Testing a panel of lymphoma cell lines expressing various MHC class I and NKG2D-L levels suggested that NK cell-dependent tumor control required a priming and a 17-AAG triggering signal that were provided by MHC class I down-regulation and by NKG2D-L, respectively. Deleting either of the “two signals” resulted in tumor escape. At early disease stages, immune stimulation through TLR-ligands in vivo efficiently delayed lymphoma growth in a strictly NK cell-dependent manner. Thus,

NK-receptor coengagement is crucial for NK-cell functions in vivo and especially for NK cell-mediated tumor surveillance. NK cells are effector lymphocytes of the innate immune system, which are capable of recognizing and Flucloronide eliminating virus-infected or malignant cells without prior sensitization. The cytotoxic potential of NK cells depends on direct lytic activity Pifithrin�� and on cytokine expression 1 and is tightly regulated by the balance of positive and negative signals delivered by NK-cell surface receptors 2. Inhibitory receptors interacting with MHC self-molecules interfere with positive signaling, thus

protecting normal tissue from NK-cell attack. As predicted by the “missing self hypothesis”, interaction of NK cells with target cells expressing reduced levels of self MHC, such as virus-infected or tumor cells, ignites the lytic machinery 3–6. Inhibitory receptors of mouse NK cells comprise several Ly49 receptors, CD94/NKG2A 7 or CD48 8. Activating receptors such as Ly49D 9, Ly49H 10 or NKp46 11 recognize nonself molecules that are expressed upon infection. Another type of an activating surface molecule is natural killer group 2D (NKG2D). This receptor recognizes self-molecules when these are overexpressed due to infection or malignant transformation 12. In the mouse, H60, RAE1 and MULT1 were identified as NKG2D ligands (NKG2D-L) 13–15. In summary, the outcome of an NK-cell response is determined by integration of various types of signals arising from sensing distinct self -and nonself-ligands. It is not clear whether single receptors are necessary or sufficient for activating NK cells.