Regulatory T cells in transplantation tolerance

The identification and characterization of regulatory T (T,,g) cells that can control immune responsiveness to alloantigens have opened up exciting opportunities for new therapies in transplantation. After exposure to alloantigens in vivo, alloantigen-specific immunoregulatory activity is enriched in a population of CD4(+) T cells that express high levels of CD25. In vivo, common mechanisms seem to underpin the activity of CD4(+)CD25(+) T-Reg cells in both naive and manipulated hosts. However, the origin, allorecognition properties and molecular basis for the suppressive activity of CD4(+)CD25(+) T-Reg cells, as well as their relationship to other populations of regulatory cells that exist after transplantation, remain a matter of debate.