A20 Restrains Thymic Regulatory T Cell Development

Maintaining immune tolerance requires the production of Foxp3-expressing regulatory T (T(reg)) cells in the thymus. Activation of NF-κB transcription factors is critically required for T(reg) cell development, partly via initiating Foxp3 expression. NF-κB activation is controlled by a negative feedback regulation through the ubiquitin editing enzyme A20, which reduces proinflammatory signaling in myeloid cells and B cells. In naive CD4(+) T cells, A20 prevents kinase RIPK3-dependent necroptosis. Using mice deficient for A20 in T lineage cells, we show that thymic and peripheral T(reg) cell compartments are quantitatively enlarged because of a cell-intrinsic developmental advantage of A20-deficient thymic T(reg) differentiation. A20-deficient thymic T(reg) cells exhibit reduced dependence on IL-2 but unchanged rates of proliferation and apoptosis. Activation of the NF-κB transcription factor RelA was enhanced, whereas nuclear translocation of c-Rel was decreased in A20-deficient thymic T(reg) cells. Furthermore, we found that the increase in T(reg) cells in T cell–specific A20-deficient mice was already observed in CD4(+) single-positive CD25(+) GITR(+) Foxp3(−) thymic T(reg) cell progenitors. T(reg) cell precursors expressed high levels of the tumor necrosis factor receptor superfamily molecule GITR, whose stimulation is closely linked to thymic T(reg) cell development. A20-deficient T(reg) cells efficiently suppressed effector T cell–mediated graft-versus-host disease after allogeneic hematopoietic stem cell transplantation, suggesting normal suppressive function. Holding thymic production of natural T(reg) cells in check, A20 thus integrates T(reg) cell activity and increased effector T cell survival into an efficient CD4(+) T cell response.