Thymocyte and thymic epithelial cell (TEC) development are interdependent processes. Although lineage relationships among progressively maturing thymocyte subsets have been characterized, the developmental relationships among TEC subsets are obscure. Because epithelial cells express distinct keratin (K) species as a function of differentiation stage and proliferative status, we used K expression patterns to identify mouse TEC subsets and determine their lineage relationships. CD25ϩ pre-T cells that productively rearrange the TCR␤ locus and express pre-TCR͞CD3 complexes proliferate and differentiate to the CD4 ϩ CD8 ϩ double-positive (DP) stage. The DN to DP transition is accompanied by loss of CD25 expression, prohibition of TCR␤ locus rearrangements, and induction of TCR␣ locus rearrangements. Signaling through the ␣␤TCR͞CD3 complexes on DP thymocytes mediates the positive and negative selection processes that shape the T-cell repertoire. DP thymocytes that are positively selected by self-peptide͞major histocompatibility complex molecules presented on cortical epithelial cells terminate CD8 or CD4 expression and migrate to the thymic medulla.Thymic epithelial cells (TECs) are not only involved in the selection of DP cells, but also promote differentiation of early DN thymocyte precursors. Nude mice that are unable to generate a normal thymic epithelial compartment because of an inactivating whn gene mutation have a primitive thymic anlage devoid of T cells (3). Furthermore, recent studies have shown that the maturation of DN precursors in thymic organ culture requires the presence of major histocompatibility complex class II ϩ TECs (4-6). Although TECs are known to play a critical role in T-cell maturation, the factors that govern TEC development are incompletely understood, particularly in comparison to the well-characterized T-cell developmental process. Nevertheless, it is clear that the establishment of normal thymic architecture and thus, TEC differentiation, depends on thymocyte͞TEC interactions (7,8). This interdependence is apparent in mice that express a human CD3 (hCD3) transgene, which prevents T-cell maturation beyond the primitive CD44 ϩ CD25 Ϫ

Thymic epithelial cells (TECs) in adult mice have been classified into distinct subsets based on keratin expression profiles. To explore the emergence of TEC subsets during ontogeny, we analyzed keratin 8 and keratin 5 expression at several stages of fetal development in normal C57BL/6J mice. In addition, thymic epithelial development and compartmentalization were explored in recombination-activating gene 2/common cytokine receptor γ-chain-deficient and Ikaros-null mice that sustain early and profound blocks in thymocyte differentiation. The results demonstrate that initial patterning of the thymic epithelial compartment as defined by differential keratin expression does not depend on inductive signals from hematopoietic cells. However, thymocyte-derived signals are required during late fetal stages for continued development and maintenance of TEC subsets in the neonate and adult.

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...