Naturally occurring CD4+CD25+ T cells have been shown to suppress immune responses both in vivo and in vitro. Finding a way to harness their regulatory abilities is of particular value in both transplantation and autoimmunity, where unwanted immune reactions need to be eliminated. Tantalizing evidence for their future potential is demonstrated by their ability to cure T cell mediated colitis induced in SCID mice . However, many autoimmune diseases display a strong humoral component, such as rheumatoid arthritis. It is unknown whether CD4+CD25+ T cells are effective in these B cell-mediated autoimmune diseases.