The lymphoid-restricted T-box transcription factor T-bet is known to have a key role in the generation of T helper 1-cell immune responses. Now, an in depth analysis of T-bet-deficient mice, published in Immunity, indicates that this transcription factor also regulates the late stages of development of natural killer (NK) cells and of NKT cells that express the semi-invariant Vα14 T-cell receptor (TCR) — Vα14i NKT cells.

C57BL/6 mice express the NK-cell receptors NK1.1 and Ly49d. Therefore, to assess the role of T-bet in NK-cell development and function, T-bet-deficient mice were backcrossed onto the C57BL/6 background. Flow cytometric analysis indicated that T-bet-deficient mice had reduced numbers of NK cells in the spleen, liver and peripheral blood, and that the remaining cells had an immature phenotype — as determined by reduced expression of CD43, CD11b and DX5, and increased expression of c-Kit — but expressed a normal repertoire of activating and inhibitory NK-cell receptors. Similarly, using α-galactosyl ceramide-loaded CD1d tetramers to identify Vα14i NKT cells, T-bet-deficient mice were shown to have almost no Vα14i NKT cells in the liver, spleen and thymus. By contrast, Vα14i NKT-cell maturation intermediates in the thymus, previously defined as tetramer-positive cells expressing CD44 but not NK1.1 or Ly49 receptors, were detected in T-bet-deficient mice, indicating that, as for NK cells, Vα14i NKT-cell terminal maturation is blocked in the absence of T-bet.

By using wild-type or T-bet-deficient bone marrow to reconstitute wild-type and T-bet-deficient mice, the authors showed that the defect in NK-cell and Vα14i NKT-cell development was intrinsic to T-bet-deficient bone marrow. Furthermore, when a mixture of wild-type and T-bet-deficient bone marrow was used to reconstitute a T-bet-sufficient host, the NK cells and Vα14i NKT cells were largely derived from the wild-type bone marrow, indicating a stem cell intrinsic role for T-bet in the development of the NK-cell and Vα14i NKT-cell lymphoid compartments. Consistent with a cell intrinsic role in terminal maturation, overexpression of T-bet in T-bet-deficient haematopoietic stem cells increased the number and maturation status of NK cells in the spleen.

To further determine the role of T-bet in NK-cell and Vα14i NKT-cell maturation, the authors analysed the effector functions of T-bet-deficient cells. Interleukin-12 (IL-12) and IL-18 stimulation of NK cells results in a rapid burst of interferon-γ (IFN-γ) secretion that was not impaired in T-bet-deficient NK cells. However, T-bet was required to maintain IFN-γ production during prolonged NK-cell stimulation. By contrast, T-bet-deficient Vα14i NKT cells were unable to produce IFN-γ when stimulated in vivo with α-galactosyl ceramide. The authors suggest that this distinct dependence on T-bet for regulating IFN-γ production could result from redundancy between T-box transcription factors, as NK cells but not Vα14i NKT cells express the T-box-family member eomesodermin.

This study identifies a novel role for the transcription factor T-bet, as a regulator of the terminal differentiation of NK cells and Vα14i NKT cells. However, further studies are required to determine whether the molecular role of this transcription factor is the same in these two cell types.