Recently,major advances have been made in our understanding of a naturally occurring lymphocyte subset called regulatory T (Treg) cells that are used by the immune system to perform antigen- or tissuespecific immunosuppression[1-6].They play indispensable roles in maintaining health since their deficiency in mice or humans causes fatal inflammatory diseases[1-6].They may also be directly involved in the development of autoimmune diseases since their numbers and/or functions are reduced in patients studied by several groups[7-11].More importantly,a number of laboratories including ours have shown that adoptive transfer of antigen-specific Treg cells effectively prevents,and ameliorates ongoing,experimental models of autoimmune diseases such as multiple sclerosis[12-20].