Natural killer group 2 member D (NKG2D) expressed in natural killer (NK) cells and T cells is a receptor activated by NKG2D ligands such as MICA and MICB. In patients with gastric cancer, expression of NKG2D in tumor-infiltrating immune cells correlates with good prognosis. Because these tumor-infiltrating cells were mainly CD8⁺ T cells, we hypothesized that NKG2D⁺ T cells had proficient anti-tumor response. In the study, we investigated whether MICA or MICB activated T cells via NKG2D in vitro by using coculture system. Panc1, a human pancreatic cancer cell line, expressed MICA and MICB among eight NKG2D ligands. We sorted cells with low or high NKG2D expression from activated T cells and cocultured each of them with Panc1 cells that had been treated with siRNA of MICA and/or MICB.  After 3 days of coculture, we isolated T cells from coculture and measured interferon gamma (INFg) mRNA as an activation marker. INFg expression of NKG2D^low T cells were augmented by downregulation of MICB, suggesting that MICB was a negative regulator.  In contrast, NKG2D^high T cells remained expressing INFg regardless of downregulation of MICA and/or MICB. These results suggest that NKG2D^high T cells were an elite population that could be activated independently of NKG2D ligands expression in cancer cells.