The orphan nuclear receptor Ear-2 (Nr2f6) is a novel negative regulator of T cell development

We describe a novel role for the orphan nuclear receptor Ear-2 in regulating T cell development. Retrovirus-mediated overexpression of Ear-2 (EAR-2++) in a bone marrow (BM) transplantation assay resulted in limited T cell development and a greater than tenfold decrease in thymus size and cellularity relative to controls. Ear-2-transduced murine BM hematopoietic stem cells (HSCs) in OP9-DL1 cultures showed a proliferation deficit during days 1-5 after induction of differentiation, which corresponded to increased expression of the cell cycle regulators p21 (cdkn1a) and p27 (cdkn1b), as well as increased expression of Hes1, Notch3, Egr1, and Scl (Tal1) and decreased expression of Gli1, Gfi-1, HoxA9, PU.1, Nrarp, and Tcf1. In addition, there was a block in differentiation at the DN4 to double-positive (DP) transition accompanied by an increase in apoptosis, similar to the deficit seen in the RORγt null mouse. Gene expression profiling revealed that, like the RORγt-deficient mouse, EAR-2++ DP cells had decreased expression of BclXL and increased expression of the proapoptosis gene Bad. In addition, EAR-2++ DP cells had decreased expression of Bcl11b, PU.1, and HoxA9, and increased expression of Id2. Based on these findings, we conclude that EAR-2++ cells were able to migrate to, but not fully repopulate, the thymus because of a cell-intrinsic defect in the proliferation of DN1 cells followed by a block in differentiation from the DN4 to DP stage of T cell development. We conclude that Ear-2 is a novel negative regulator of T-cell development and that downregulation of Ear-2 is indispensable for the proliferation of DN1 cells and the survival of DN4-DP cells.