Effector T cell responses can be modulated by competing positive or negative signals transduced by natural killer (NK) cell receptors. This raises the possibility that dominant T cell stimulation might promote autoimmune reactions. In rheumatoid arthritis (RA), the severity of autoimmune and inflammatory joint disease correlates with large numbers of CD4+CD28- T cells, which are scarce in healthy individuals. For poorly defined reasons, these T cells are autoreactive, implying that they may contribute to disease manifestations. CD4+CD28- T cells in peripheral blood and synovial tissue of RA patients were found to express NKG2D, a costimulatory receptor that is absent on normal CD4 T cells. NKG2D was induced by tumor necrosis factor alpha and IL-15, which are abundant in inflamed synovia and RA patient sera. RA synoviocytes aberrantly expressed the stress-inducible MIC ligands of NKG2D, which stimulated autologous CD4+CD28- T cell cytokine and proliferative responses. Peripheral blood serum samples of RA patients contained substantial amounts of synoviocyte-derived soluble MICA, which failed to induce down-modulation of NKG2D because of the opposing activity of tumor necrosis factor alpha and IL-15. These results suggest that a profound dysregulation of NKG2D and its MIC ligands may cause autoreactive T cell stimulation, thus promoting the self-perpetuating pathology in RA and possibly other autoimmune diseases.