Epithelial cells in the thymus produce IL-7, an essential cytokine that promotes the survival, differentiation, and proliferation of thymocytes. We identified IL-7-expressing thymic epithelial cells (TECs) throughout ontogeny and in the adult mouse thymus by in situ hybridization analysis. IL-7 expression is initiated in the thymic fated domain of the early primordium by embryonic day 11.5 and is expressed in a Foxn1-independent pathway. Marked changes occur in the localization and regulation of IL-7-expressing TECs during development. IL-7-expressing TECs are present throughout the early thymic rudiment. In contrast, a major population of IL-7-expressing TECs is localized to the medulla in the adult thymus. Using mouse strains in which thymocyte development is arrested at various stages, we show that fetal and postnatal thymi differ in the frequency and localization of IL-7-expressing TECs. Whereas IL-7 expression is initiated independently of hemopoietic-derived signals during thymic organogenesis, thymocyte-derived signals play an essential role in regulating IL-7 expression in the adult TEC compartment. Moreover, different thymocyte subsets regulate the expression of IL-7 and keratin 5 in adult cortical epithelium, suggesting that despite phenotypic similarities, the cortical TEC compartments of wild-type and RAG-1−/− mice are developmentally and functionally distinct.

The persistence of CD4 expression is a key event distinguishing the differentiation of MHC class II-restricted thymocytes into CD4 T cells from that of MHC class I-restricted thymocytes into CD8 T cells. The zinc finger transcription factor Zbtb7b (or cKrox or Thpok) is normally expressed in MHC class II-restricted thymocytes and promotes CD4 lineage choice. When expressed in MHC class I-restricted cells, Zbtb7b redirects these cells from their normal CD8 fate to CD4 differentiation, implying that it promotes, directly or not, sustained CD4 expression; the present study has investigated the mechanism of this effect. We demonstrate that, although Zbtb7b does not enhance CD4 expression on its own, it antagonizes the CD4 repression mediated by the transcription factor Runx3, which is normally up-regulated during CD8 differentiation and promotes CD4 silencing. Zbtb7b also antagonizes CD4 repression by the related protein Runx1, which is expressed in CD4 lineage cells. This antagonism is observed both in vitro and in vivo, is transcriptional, and requires domains of Zbtb7b that are essential to its ability to promote CD4 differentiation in vivo. Furthermore, Zbtb7b fails to antagonize Runx in cells treated with histone deacetylase inhibitors, suggesting that Zbtb7b acts by reducing the expression of thus far unknown factors that cooperate with Runx molecules to repress CD4. These findings demonstrate that the transcription factor Zbtb7b promotes CD4 expression by antagonizing Runx-mediated CD4 repression.

To study interleukin-7 (IL-7) in early thymocyte development, we generated mice transgenic (Tg) for the IL-7 gene under control of the lck proximal promoter. Founder line TgA, with the lowest level of IL-7 overexpression, showed enhanced ␣␤ T-cell development. In contrast, in the highest overexpressing founder line, TgB, ␣␤ T-cell development was disturbed with a block at the earliest intrathymic precursor stage. This was due to decreased progenitor proliferation as assessed by Ki-67 staining and in vivo bromodeoxyuridine (BrdU) incorporation. Bcl-2 was upregulated in T-cell-committed progenitors in all Tg lines, and accounted for greater numbers of double positive (DP), CD4 single positive (SP), and CD8SP thymocytes in TgA mice where, in contrast to TgB mice, thymocyte progenitor proliferation was normal. Mixed marrow chimeras using TgB ؉ and congenic mice as donors, and experiments using anti-IL-7 monoclonal antibody (MAb) in vivo, confirmed the role of IL-7 protein in the observed TgB phenotype. In conclusion, at low Tg overexpression, IL-7 enhanced ␣␤ T-cell development by increasing thymocyte progenitor survival, while at high overexpression IL-7 reduces their proliferation, inducing a dramatic block in DP production. These results show for the first time in vivo a dose effect of IL-7 on ␣␤ T-cell development and have implications for IL-7 in the clinical setting. IntroductionInterleukin-7 (IL-7) is a nonredundant cytokine in thymic development. It has been implicated in both proliferation and survival of early T cells. [1][2][3] After the transition from the multipotent to the T-cell-committed stage, thymocyte progenitors become dependent on IL-7 for normal cell cycle progression and cell survival through inhibition of apoptosis via up-regulation of the Bcl-2 expression. 1 Consequently, in several mouse models of IL-7 signal disruption such as IL-7 Ϫ/Ϫ , 2 IL-7 receptor ␣ Ϫ/Ϫ (IL-7R␣ Ϫ/Ϫ ), 3,4 ␥c Ϫ/Ϫ , 5-7 Jak3 Ϫ/Ϫ , [8][9][10] and Jak1 Ϫ/Ϫ , 11 progression beyond double negative-2 (DN2) stage is severely diminished. However, despite its role on proliferation, studies evaluating the need for IL-7 during lymphocyte development led to the conclusion that the primary role of this cytokine was rather in maintaining cell survival. 12 Additionally, IL-7 controls T-cell receptor ␥ (TCR␥) rearrangement by regulating locus accessibility, 13 such that ␥␦ T-cell production is abrogated in the absence of IL-7 signaling, 4,5,9,13-15 demonstrating a complete reliance on IL-7 by this lineage.Many in vitro studies have shown an effect of IL-7 on thymocyte progenitors. [16][17][18][19] However, the effect of IL-7 on ␣␤ T-cell development yielded somewhat conflicting results. Varas et al observed, with rat fetal thymic organ culture (FTOC) grown in the presence of 2000 U/mL IL-7, an enhancement of ␣␤ thymocyte maturation. 19 In contrast, Plum et al's study, which used mouse FTOC treated with different doses of human recombinant IL-7 (rIL-7, 100-5000 U/mL), showed significantly lower numbers of ␣␤ T cells with increasing I...