RESEARCH ARTICLE Premature Expression of Foxp3 in Double- Negative Thymocytes

Peripheral immune regulation depends on the generation of thymic-derived regulatory T (tTreg) cells to maintain self-tolerance and to counterbalance overshooting immune re-sponses. The expression of the Treg lineage defining transcription factor Foxp3 in develop-ing tTreg cells depends on TCR signaling during the thymic selection process of these T cells. In this study, we surprisingly identify Foxp3+ immature thymocytes at the double-negative (DN) stage in transcription factor 7 (Tcf7)-deficient mice. These Foxp3+ cells did not express a TCR (β or γδ chains), CD3 or CD5 and therefore these cells were true DN cells. Further investigation of this phenomenon in a transgenic TCR model showed that Foxp3-expressing DN cells could not respond to TCR stimulation in vivo. These data sug-gest that Foxp3 expression in these DN cells occurred independently of TCR signaling. In-terestingly, these Foxp3+ DN cells were located in a transition state between DN1 and DN2 (CD4-CD8-CD3-TCR-CD44highCD25low). Our results indicate that Tcf7 is involved in preventing the premature expression of Foxp3 in DN thymocytes