Premature Expression of Foxp3 in Double-Negative Thymocytes

Peripheral immune regulation depends on the generation of thymic-derived regulatory T (tT(reg)) cells to maintain self-tolerance and to counterbalance overshooting immune responses. The expression of the T-reg lineage defining transcription factor Foxp3 in developing tT(reg) 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 (beta or gamma delta 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 suggest that Foxp3 expression in these DN cells occurred independently of TCR signaling. Interestingly, these Foxp3+ DN cells were located in a transition state between DN1 and DN2 (CD4(-)CD8(-)CD3(-)TCR(-)CD44(high)CD25(low)). Our results indicate that Tcf7 is involved in preventing the premature expression of Foxp3 in DN thymocytes.