The role of CD28 and CTLA4 in the function and homeostasis of CD4 ⁺CD25⁺ regulatory T cells

CD4⁺CD25⁺ T cells regulate a variety of autoimmune and alloimmune responses including the development of autoimmune diabetes in non-obese diabetic (NOD) mice. We have examined the role of CD28/CTLA4/B7 interactions in the expansion and survival of CD4⁺CD25⁺ regulatory T cells (T_reg) in this setting. CD28/ B7 interactions are essential in the development of T_reg in the thymus and for their survival in the periphery. The CD28-mediated homeostasis of these cells is independent of IL2, OX40, CD40L, and survival factor Bcl-X_L. In addition, analysis of T_reg from CTLA4-deficient mice suggests that CTLA4 expression is not required for their development or function. However, non-activating anti-CTLA4 antibodies blocked the suppressor activity of regulatory cells in vitro. Thus, clinical application of co-stimulatory blockade using agents such as CTLA4Ig in the treatment of autoimmune disease may result in complicated outcomes.