Objective. To assess the role of the activating receptor NKG2D in arthritis.Methods. Levels of NKG2D and its ligands were determined by fluorescence-activated cell sorting, realtime polymerase chain reaction, and immunohistochemistry in rheumatoid arthritis (RA) synovial membrane tissue and in paw tissue from arthritic mice. Arthritis was induced in DBA/1 mice by immunization with type II collagen, and mice were treated intraperitoneally with a blocking anti-NKG2D antibody (CX5) on days 1, 5, and 8 after clinical onset and were monitored for 10 days.Results. We demonstrated expression of NKG2D Natural killer (NK) cells are lymphocytes of the innate immune system with both cytotoxic and cytokineproducing effector functions, regulated by a repertoire of cell receptors, including activating receptors NKG2D, CD16 (Fc␥ receptor III), NKp30, NKp44, and NKp46; killer cell immunoglobulin-like receptor; and the inhibitory receptors CD94/NKG2A and LY49 (mouse) (for review, see ref. 1). The role of NK cells in the defense against infections and cancer has been studied extensively (1), but their role in autoimmunity and chronic