NKG2D is an activating receptor expressed on CD8(+)alphabeta(+) T cells, gammadelta(+) T cells, natural killer (NK) cells, and some CD4(+) T cells. For a long time, the interaction of NKG2D with its ligands (NKG2DLs) MICA, MICB, and ULBP1-3 has been considered a mechanism for recognition and elimination of tumor, infected, or otherwise "stressed" cells. However, a new role for NKG2D as an immunoregulatory receptor is emerging. Here, we show that NKG2D is strongly down-modulated on antigen-activated CD8(+) T cells but only if CD4(+) T cells are present. Down-modulation was caused by soluble factors produced by CD4(+) T cells, and in particular soluble NKG2DLs were found in the supernatants of antigen-activated T-cell cultures. MICB was the ligand released at higher levels when CD4(+) T cells were present in the cell cultures, suggesting that it could be the major player of NKG2D down-modulation. CD8(+) T cells expressing low levels of NKG2D had impaired effector functions, as evaluated by proliferation, cytokine production, and cytotoxicity assays after combined triggering of NKG2D and TCR-CD3 complex. These findings show that activated CD4(+) T cells expressing NKG2DLs can efficiently prevent NKG2D-mediated CD8(+) T-cell functions, and suggest that the NKG2D/NKG2DL interaction can regulate immune responses.