IntroductionT cells, especially cytotoxic T cells (CTLs), play important roles in antitumor immunity. 1 Adoptive transfer of tumor-specific T cells into patients provides a promising means to treat cancer. 2 However, the traditional approaches for obtaining large numbers of tumorspecific T cells are time consuming, laborious, and sometimes difficult because the average frequency of antigen-specific T cells in the periphery is extremely low. [3][4][5] In addition, isolation and expansion of T cells that retain their antigen specificity and function can also be a challenging task. 2 Genetic modification of primary T cells with tumor-specific immunoreceptors, such as full-length T-cell receptors (flTCRs) or chimeric TCR (chTCR) molecules provides a novel way for redirecting T cells against tumor cells. 6-9 This strategy avoids the limitation of low frequency of antigen-specific T cells, allowing for facilitated expansion of tumor-specific T cells to therapeutic doses. Although some success has been reported using flTCR-or chTCR-transduced T cells, several factors, such as incorrect pairing between endogenous and exogenous TCR molecules, variable signaling capacity, and potential immunogenecity of chTCR molecules, may limit the therapeutic efficiency of these approaches. 2,10,11 Therefore, investigation of novel immunoreceptors besides flTCR or chTCR molecules remains an important goal.Natural killer (NK) cells attack tumor and virally infected cells in the absence of major histocompatibility complex (MHC) restriction, using a combination of signals from activating and inhibitory receptors. 12 One of these activating receptors is NKG2D, which is expressed on all NK cells, NKT cells, ␥␦ T cells, and some CD8 ϩ ␣␤ T cells. 12,13 Ligands for mouse NKG2D are Retinoic acid early inducible protein 1 (Rae-1), Murine UL-16-binding proteinlike transcript (Mult-1), and minor histocompatibility antigen H60 (H60), and ligands for human NKG2D include MHC class I chain-related A (MICA), MICB, and several UL-16-binding proteins (ULBPs). 13,14 It has been found that NKG2D ligands are primarily expressed on tumor cells but not on most normal tissues. 13,14 Thus, the NKG2D receptor-NKG2D ligand system provides a relatively specific system for immune cells to recognize tumor cells. Recent data show that Rae-1-and H60-expressing tumor cells can induce robust antitumor effects in an NK cell-mediated and CD8 ϩ T-cell-mediated fashion. 15 However, NKG2D expression on T cells does not induce direct activation of T cells after NKG2D crosslinking, but NKG2D enhances T-cell signaling initiated by TCR crosslinking. 16 This is probably due to the distinct signaling pathway of NKG2D in different cell types. T cells express the adaptor protein Dap10 but lack expression of Dap12, whereas NK cells express both adaptor proteins. 16,17 Dap12 contains an immunoreceptor tyrosine-based activation motif (ITAM) capable of providing primary signals for cell activation. 18 In contrast, Dap10 only contains a Tyr-X-X-Met (YXXM) motif that transduces costimula...