NKG2D is a key homodimeric activation receptor expressed on the cell surface of almost all NK cells, 
cells, some cytolytic CD8+
ß T cells and NKT cells, and a small subset of CD4+
ß T cells (15). Several ligands that bind to NKG2D are members of the MHC class Ib family (5, 6). In humans, the polymorphic MHC class I chainrelated molecules (MIC) A and MICB can be recognized by NKG2D (3, 7). Although MIC molecules have not been found in mice, the retinoic acid early inducible-1 (Rae-1) gene products UL16-binding protein-like transcript 1 (Mult1) and a distantly related minor histocompatibility Ag, H60, have been reported as NKG2D ligands in mice (5, 6, 8, 9).
The immune system responds to "stressed self," and stress signals include molecules released by dying cells such as putative endogenous ligands for TLRs (10, 11), uric acid (12), and surface molecules, including MHC Class Ib gene products, that are up-regulated by heat shock, retinoic acid, IFN-
, TLR signaling, growth factors, viral infection, DNA damage, and UV irradiation (1317). In particular, unlike conventional MHC class I, the MHC class Ib MIC proteins display up-regulated surface expression on stressed cells and are frequently overexpressed by tumors that are reportedly infiltrated by larger numbers of lymphocytes than are MIC tumors (18, 19). In the mouse, the expression of the MHC Class Ib molecules, Rae-1, and H60 is negligible in normal skin but is strongly induced by skin painting with chemical carcinogens (20). Natural or induced expression of NKG2D ligands markedly enhances the sensitivity of tumor cells to NK cells in vitro (25, 8, 21). Expression of NKG2D ligands by tumor cells also results in immune destruction in vivo, and the ectopic expression of NKG2D ligands, Rae-1, and H60 in several tumor cell lines results in the rejection of the tumor cells expressing normal levels of MHC class I molecules (6, 22). Immune depletion and other experiments showed that rejection was dependent on NK cells and/or CD8+ T cells and perforin (22, 23). Our more recent study has illustrated that some cytokines mediate their antitumor activity, largely via the NKG2DNKG2D ligand pathway (24).
However, despite the assumption that NKG2D-mediated engagement of stress-induced ligands may be a key aspect of tumor immune surveillance (20, 25), no study has ever evaluated the importance of the pathway in de novo tumorigenesis. We show the importance of the NKG2D activation receptor in controlling the natural and activated host response to spontaneous malignancy.
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Results and Discussion
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NKG2D neutralization enhances methylcholanthrene (MCA)-induced sarcoma formation in mice
Neutralizing anti-NKG2D mAbs have the ability to enhance the growth and metastasis of tumors that ectopically or endogenously express NKG2D ligands (22, 23). To evaluate the importance of the NKG2D pathway in de novo tumorigenesis, B6 mice were inoculated with increasing doses of MCA and treated for 8 wk with control or anti-NKG2D mAb (Fig. 1 A). At all MCA doses examined, mice treated with anti-NKG2D mAb had a greater incidence of fibrosarcoma than control Igtreated mice. Similar increased incidence was obtained when treating mice for more extended periods of time with anti-NKG2D mAb (day 0140; unpublished data). Sarcoma formation was also assessed in BALB/c mice inoculated with MCA. In concert with the findings in B6 mice, neutralization of NKG2D increased sarcoma formation in BALB/c mice (Fig. 1 B). Thus, in two different strains of mice, the activation receptor NKG2D controls the initiation of MCA-induced sarcoma.
M.J. Smyth is supported by a National Health and Medical Research Council of Australia (NH&MRC) Principal Research Fellowship. E. Cretney is supported by the Cancer Council Victoria Postdoctoral Cancer Research Fellowship. Y. Hayakawa is supported by a Cancer Research Institute Postdoctoral Fellowship. The project was supported by a Program Grant from the NH&MRC.
The authors have no conflicting financial interests.
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