Foxp3+ CD25+ regulatory T cells specific for a neo-self-antigen develop at the double-positive thymic stage.

Thymus-derived regulatory T cells (Tregs) expressing CD4, CD25, and the transcription factor Foxp3 play major roles in preventing autoimmunity. The Treg population is enriched in T cells expressing high-avidity self-reactive T cell receptors, and thymic epithelial cells expressing self-antigens (Ag) have been implicated in their induction and/or selection. However, the thymic selection events leading to Treg lineage commitment remain unclear. We followed the thymic development of self-Ag-specific Tregs in double-transgenic mice coexpressing a neo-self-Ag, hemagglutinin (HA) under the control of a neural tissue-specific promoter, and a transgenic class II-restricted T cell antigen receptor specific for HA111-119. Our data show that the promiscuous expression of the HA transgene in thymic epithelial cells is involved in the selective induction and/or expansion of HA-specific Foxp3(+) Treg thymic precursors as early as the double-positive stage.