CD1d expression level in tumor cells is an important determinant for anti-tumor immunity by natural killer T cells

Invariant natural killer T (iNKT) cells are thought to regulate anti-tumor immunity. Human iNKT (i.e. Vα24+ NKT) cells have been reported to recognize CD1d on target cells and show cytotoxicity directly on the target cells in vitro. However, the anti-tumor effect of mouse iNKT (i.e. Vα14+ NKT) cells has been repeatedly reported to be dependent on the activity of natural killer (NK) cells via interferon-γ, with no evidence of direct cytotoxicity. In the present study, we report that in vitro cytolysis of EL-4 mouse lymphoblastic lymphoma cells by Vα24+ NKT cells and in vivo eradication of these cells are both dependent on the level of CD1d expression on the tumor cell surface. These observations possibly suggest that direct cytotoxicity of tumor cells by iNKT cells is common to both humans and mice, and that the high expression level of CD1d may be a predictor whether the tumor is a good target of iNKT cells.