Bcl11b (also known as Ctip2) is highly and specifically expressed within T cells, and to a lesser extent in NK cells 20, suggesting that Bcl11b could function

as a T-cell-specific regulator. Bcl11b has been shown to bind to GC-rich target sequences, and is involved mostly in gene repression 21–23. It recruits the class III histone deacetylase SIRT1 22 and/or the class I histone deacetylases to promoters 23, 24. Genetic analyses have shown that Bcl11b is crucial at several stages of T-cell development. Germline deletion of Bcl11b results in a complete block of T-cell differentiation at the DN stage, associated with impaired TCRβ rearrangement 25. Bcl11b inactivation at the DP stage strongly blocks the maturation of DP thymocytes into SP cells and impairs positive selection, possibly through defective

TCR signaling 26. Here, we further investigated Bcl11b PS341 function in T cells by generating new T-cell-specific deletions of this gene. We previously generated a germline deletion of exon 4 of the Bcl11b locus, Bcl11bL−/L−27, which is lethal just after birth 27. These mice exhibited a tenfold decrease in thymic cellularity (0.9±0.2×106 RGFP966 cells for Bcl11bL−/L− versus 9.3±2.3×106 cells for Bcl11bL−/+ or Bcl11b+/+ mice). The majority of Bcl11bL−/L− thymocytes were large cells lacking CD4 and CD8 expression, whereas a smaller proportion expressed CD8 (Supporting Information Fig. 1A). Bcl11bL−/L− thymocytes lacked αβTCR but most expressed γδTCR, including those expressing Neratinib mw CD8 ( Supporting Information Fig. 1A, and data not shown). To circumvent the perinatal lethality and to analyze the role of Bcl11b in adult T cells, we combined the floxed Bcl11b alleles (Bcl11bL2/L2) with a transgenic allele expressing Cre recombinase under the transcriptional control of the Lck promoter, which initiates T-cell-specific expression in DN2 and DN3 cells 28. Bcl11bL2/L2Lckcre/+ mice appeared healthy and indistinguishable from littermates and were analyzed at 6 wk of age. The thymuses from these mice were very small and contained low numbers of thymocytes (an average of 3×105 cells; control

littermates had an average of >108 cells). T cells from Lck-Cre-deleted mice exhibited a phenotype reminiscent of that found in null newborn mice: most cells were large DN (48%) or CD8+ (30%) cells, and few DP cells (10%) were detected (Supporting Information Fig. 1B). In addition, as was observed in Bcl11bL−/L− newborns, a large proportion of cells, including most CD8+ cells, expressed γδTCR ( Supporting Information Fig. 1B; 46% of total thymocytes on average). Although these γδTCR+ cells were present in absolute numbers similar to WT, the phenotype of these cells was clearly abnormal, as CD8-expressing TCRγδ+ cells were not detected in control mice (Supporting Information Figs. 1B and 2). These data confirm that Bcl11b acts early in T cells to promote differentiation toward the αβ lineage.