Expanding regulators

Until now, harnessing the potent suppressive activity of regulatory T (T_Reg) cells for the treatment of autoimmune disease and transplant rejection has been limited by the low numbers and diverse antigen specificity of these cells in vivo. In The Journal of Experimental Medicine, two papers describe in vitro methods for clonally expanding antigen-specific CD4⁺CD25⁺ T_Reg cells that effectively suppress disease in a mouse model of autoimmune diabetes.

In the report by Jeffrey Bluestone and colleagues, CD4⁺CD25⁺ T_Reg cells were purified from nonobese diabetic (NOD) mice or islet-antigen-specific TCR-transgenic mice (BDC2.5 mice) and expanded more than 200 fold in the presence of co-immobilized CD3- and CD28-specific antibodies plus a high dose of interleukin-2 (IL-2). The expanded CD4⁺CD25⁺ T_Reg-cell populations maintained antigen specificity and a T_Reg-cell phenotype — expressing high levels of CD25, CD62L, SOCS2, PD1, CTLA4, TRAIL and the T_Reg-cell lineage marker FOXP3. Expanded T_Reg-cell populations suppressed the in vitro proliferation of effector T cells more efficiently than T_Reg cells that were freshly isolated from NOD mice. The induction of suppressor activity was antigen specific and mediated through cell–cell interactions, and did not depend on the immunosuppressive cytokines IL-10 and transforming growth factor-β for effector function in vitro.

In contrast to the effective suppressive activity of both the expanded T_Reg-cell lines in vitro, they had marked differences in effectiveness in vivo. Expanded BDC2.5 T_Reg cells efficiently suppressed diabetes induced in lymphopenic NOD mice by the transfer of diabetogenic polyclonal cells or BDC2.5 effector T cells. By contrast, expanded polyclonal NOD T_Reg cells did not inhibit the transfer of disease, even when high numbers of T_Reg cells were used. Moreover, the BDC2.5 T_Reg cells efficiently blocked the development of spontaneous diabetes in T_Reg-deficient, CD28-deficient NOD mice.

In a similar study by Ralph Steinman and colleagues, CD4⁺CD25⁺ T_Reg-cell populations were expanded in vitro using dendritic cells (DCs) isolated from NOD mice and pulsed with specific autoantigen. Similarly, they found that such antigen-specific T_Reg cells were more suppressive than freshly isolated T_Reg cells in vitro and efficiently prevented diabetes caused by diabetogenic T cells in NOD mice, even when low numbers of T_Reg cells were transferred.

For T_Reg cells to be of therapeutic use in patients with autoimmune diabetes, they must be able to block ongoing disease. To test this, Bluestone and colleagues expanded BDC2.5 T_Reg cells and transferred them into NOD mice with recent disease onset or chronic diabetes, together with a syngeneic islet-cell transplant. BDC2.5 T_Reg cells reversed diabetes in both settings and restored long-term immune homeostasis. Using a different model, Steinman and colleagues also showed that expanded T_Reg cells blocked ongoing disease, as they were effective even when administered 2 weeks after the transfer of diabetogenic T cells.

So, both these studies achieve effective in vitro expansion of T_Reg-cell populations that have enhanced suppressive function in vivo. As such, this work provides hope that the suppressive activity of T_Reg cells can be harnessed to treat autoimmune disease in humans in an antigen-specific manner.