The canonical Wnt signaling pathway plays an important role in lymphopoiesis and hematopoiesis

The evolutionarily conserved canonical Wnt-β-catenin-T cell factor (TCF)/lymphocyte enhancer binding factor (LEF) signaling pathway regulates key checkpoints in the development of various tissues. Therefore, it is not surprising that a large body of gain-of-function and loss-of-function studies implicate Wnt-β-catenin signaling in lymphopoiesis and hematopoiesis. In contrast, recent papers have reported that Mx-Cre-mediated conditional deletion of β-catenin and/ or its homolog γ-catenin (plakoglobin) did not impair hematopoiesis or lymphopoiesis. However, these studies also report that TCF reporter activity remains active in β-catenin- and γ-catenin-deficient hematopoietic stem cells and all cells derived from these precursors, indicating that the canonical Wnt signaling pathway was not abrogated. Therefore, these studies in fact show that the canonical Wnt signaling pathway is important in hematopoiesis and lymphopoiesis, even though the molecular basis for the induction of the reporter activity is currently unknown. In this perspective, we provide a broad background to the field with a discussion of the available data and create a framework within which the available and future studies may be evaluated

T cell factor-1 controls the lifetime of CD4+ CD8+ thymocytes in vivo and distal T cell receptor α-chain rearrangement required for NKT cell development.

Natural killer T (NKT) cells are a component of innate and adaptive immune systems implicated in immune, autoimmune responses and in the control of obesity and cancer. NKT cells develop from common CD4+ CD8+ double positive (DP) thymocyte precursors after the rearrangement and expression of T cell receptor (TCR) Vα14-Jα18 gene. Temporal regulation and late appearance of Vα14-Jα18 rearrangement in immature DP thymocytes has been demonstrated. However, the precise control of lifetime of DP thymocytes in vivo that enables distal rearrangements remains incompletely defined. Here we demonstrate that T cell factor (TCF)-1, encoded by the Tcf7 gene, is critical for the extended lifetime of DP thymocytes. TCF-1-deficient DP thymocytes fail to undergo TCR Vα14-Jα18 rearrangement and produce significantly fewer NKT cells. Ectopic expression of Bcl-xL permits Vα14-Jα18 rearrangement and rescues NKT cell development. We report that TCF-1 regulates expression of RORγt, which regulates DP thymocyte survival by controlling expression of Bcl-xL. We posit that TCF-1 along with its cofactors controls the lifetime of DP thymocytes in vivo

Ectopic expression of Bcl-x_L in developing TCF-1-deficient thymocytes rescues Vα14-Jα18 rearrangements and NKT cells.

<p>(<b>A</b>) Flow cytometry of thymocytes from WT, TCF-1-KO, Bcl-x_L-Tg and TCF-1-KO Bcl-x_L-Tg mice showing CD4⁻CD8⁻ (double-negative, DN), CD4⁺CD8⁺ (double positive, DP), CD4 single-positive (SP) and CD8SP thymocytes. <b>Top</b>, dot plots are representative of at least 4 different experiments. <b>Bottom</b>, graphs with DP cell percentages and cell numbers from 4 mice per group are shown (mean and s.e.m.). Numbers over dot plots refer to total thymocyte cell numbers. (<b>B</b>) Flow cytometry of thymocytes showing percent of gated CD1d-tetramer+ TCRβ+ NKT cells from WT, Bcl-x_L-Tg and TCF-1-KO Bcl-x_L-Tg mice. <b>Top</b>, dot plots are representative of at least 4 different experiments. <b>Bottom</b>, graphs with NKT cell percentages and cell numbers from 4 mice per group are shown (mean and s.e.m.). (<b>C</b>) Relative expression of Vα14-Jα18 rearrangements from WT and TCF-1-KO DP cells (n = 3). (<b>D</b>) Semiquantitative PCR of cDNA (1∶1, 1∶2, 1∶4 dilutions) from DP cells of WT, TCF-1-KO, Bcl-x_L-Tg and TCF-1-KO Bcl-x_L-Tg mice showing Vα14-Jα18 and control Vα14-Cα rearrangements (n = 3). *<i>P</i><.05; ***, <i>P</i><.001.</p

TCF-1-deficient mice have reduced proportion and number of NKT cells.

<p>Flow cytometry of thymocytes showing percent of gated CD1d-tetramer+ TCRβ+ NKT cells from wild-type (WT) and TCF-1 knock-out (KO) mice (<b>A</b>) and from WT, Vα14-Tg and TCF-1-KO Vα14-Tg mice (<b>B</b>). Representative dot plots and graphs with cell percentages and cell numbers from 4–6 mice per group are shown (mean and s.e.m.). Numbers over dot plots refer to total thymocyte cell numbers; *<i>P</i><.05; **, <i>P</i><.01; ***, <i>P</i><.001.</p

TCF-1 is expressed in NKT cells in a developmentally relevant manner.

<p>(<b>A</b>) Flow cytometry of thymocytes (<b>top</b>) and hepatic lymphocytes (<b>bottom</b>) from wild-type (WT) C57BL/6 mice showing TCF-1 intracellular expression versus the control antibody (<b>right</b>) in NKT cells (gated in <b>left</b> as CD1d-tetramer+ TCRβ+ cells, numbers indicate percentage of NKT cells). (<b>B</b>) Flow cytometry of CD1d-tetramer positive thymocytes (<b>top</b>) and hepatic lymphocytes (<b>bottom</b>) from WT mice stained with anti-CD44 and anti-NK1.1 to assess NKT developmental stages 1–3, as depicted in <b>left</b>. <b>Right</b>, TCF-1 intracellular expression among stages. Data are from a representative experiment out of four WT mice analyzed. (<b>C</b>) Flow cytometry of CD1d-tetramer positive thymocytes (<b>top</b>) and hepatic lymphocytes (<b>bottom</b>) from WT mice showing TCF1 intracellular expression versus the control antibody (<b>right</b>) in NKT1 and NKT2 NKT cells (gated in <b>left</b>, numbers indicate percentage of NKT cells). Data are from a representative experiment out of four WT mice analyzed.</p