We note, however, that expression LY2606368 clinical trial of RORγ and Runx1, two factors that are essential for NKT cell differentiation 43, was normal in Bcl11bdp−/− mice, indicating that Bcl11b controls NKT cell development independently of these factors. Our expression profiling analyses suggest that Bcl11b is required to prevent premature and inappropriate expression of many genes specifically expressed in mature CD4+ and/or CD8+ T cells. We speculate that Bcl11b may serve as a timing

factor that holds cells in the immature, DP state until a constellation of factors is in place to support SP differentiation. It is likely that the premature SP gene expression program that is induced in the Bcl11b-deficient DP cells reflects both the direct loss of Bcl11b-dependent repression, and the precocious activity of SP-specific transcription factors (such as Klf2, Zbtb7b, Runx3, and Id2). Therefore, our data suggest that correct regulation of SP cell differentiation

involves mechanisms not only to induce cell-specific gene expression programs, but also to prevent these programs from being inappropriately expressed in immature cells. Mechanisms that prevent early expression of differentiation-associated genes have also been described in other systems. For instance, Polycomb-dependent repression has recently been shown to prevent the premature expression of structural genes in differentiating keratinocytes 44. It is of particular interest that that loss of Bcl11b in DP cells expressing low levels of CD3 results in the induction of genes encoding Zbtb7b and VX-765 Runx3, which are required for, and strongly upregulated during, CD4 and CD8 SP differentiation programs, respectively 45, 46. We found that Bcl11b bound to sequences in the regulatory regions of these genes, suggesting that Bcl11b directly represses

expression of Zbtb7b and Runx3 in immature T cells. The regulation of Zbtb7b has been intensively investigated in recent Urease years. Induction of Zbtb7b expression occurs downstream of TCR signaling and requires activation of GATA3 expression 47, whereas Runx3 contributes to Zbtb7b repression in CD8-committed cells 19. The mechanisms that render Zbtb7b silent prior to TCR signaling are less well understood but may in part involve repression by Runx complexes 19. Our present data suggest an essential role for Bcl11b in this early silencing, and thus identify another key player in the regulatory network controlling the dynamic regulation of Zbtb7b during T-cell differentiation. However, our results also raise several questions about how Bcl11b participates in Zbtb7b regulation. It will be important to identify activators responsible for Zbtb7b expression in Bcl11b-deficient DP cells, and determine how Bcl11b antagonizes these activators at the transcriptional level in WT cells.