Delta‐like 4‐mediated Notch signaling is required for early T‐cell development in a three‐dimensional thymic structure

Abstract

Delta-like 4 (Dll4)-mediated Notch signaling is critical for specifying T-cell fate, but how Dll4-mediated Notch signaling actually contributes to T-cell development in the thymus remains unclear. To explore this mechanism in the thymic three-dimensional structure, we performed fetal thymus organ culture using Dll4-deficient mice. DN1a/b+DN2mt cells, which had not yet committed to either the αβ T or γδ T/NK cell lineage, did not differentiate into the αβ T-cell lineage in Dll4-deficient thymus despite the lack of cell fate conversion into other lineages. However, DN3 cells efficiently differentiated into a later developmental stage of αβ T cells, the double-positive (DP) stage, although the proliferation was significantly impaired during the differentiation process. These findings suggest that the requirement for Notch signaling differs between the earliest and pre-TCR-bearing precursors and that continued Notch signaling is required for proper differentiation with active proliferation of αβ T lineage cells. Furthermore, we showed that Notch signaling increased the c-Myc expression in DN3 cells in the thymus and that its overexpression rescued the proliferation and differentiation of DN3 cells in the Dll4-null thymus. Therefore, c-Myc plays a central role in the transition from stage DN3 to DP as a downstream target of Notch signaling.Keywords: c-Myc r Dll4 r Notch signal r Thymus r T-cell development Additional supporting information may be found in the online version of this article at the publisher's web-site

Pregnancy‐induced thymic involution is associated with suppression of chemokines essential for T‐lymphoid progenitor homing

“…It is well known that the differentiation of thymic epithelial cells is governed by different ligands of TNFR family members, RANKL, LTβ, and CD40L and, accordingly, the KO models of the respective receptors or signaling pathways show severe defects in TEC development, as well as a failure in central tolerance . Likewise, the signals from thymic stromal cells are required for the thymocyte progenitor seeding, proliferation, migration, and differentiation . Here we showed that pregnancy‐induced thymic involution is accompanied by a profound reduction in all major nonlymphoid cell subsets, which is at least partly a result of decreased cell proliferation.…”

“…In order to define whether pregnancy‐induced involution may result from a decreased production of stromal cell‐derived factors required for the functional thymocyte progenitor niche, we examined the expression levels of IL‐7, Kitl, and Dll4 in different nonlymphoid cell subsets. Clearly, all three mediators showed a substantial decrease at late pregnancy (Fig.…”

“…While the interactions between cortical thymic epithelial cells (cTECs) and thymocytes are responsible for TCR‐mediated positive selection, the medullary thymic epithelial cells (mTECs) and dendritic cells (DCs) express and present self‐antigens and co‐stimulatory molecules to the developing thymocytes and play a central role in negative selection and the induction of central tolerance. In addition, it has been shown that TECs and fibroblasts produce many soluble factors and transmembrane proteins such as IL‐7, Kitl, and Dll4 that are essential for thymocyte proliferation, differentiation, and survival whereas thymocyte‐produced factors such as RANKL, CD40, and LTβ are critical for the proper differentiation of TECs . Thus, it is well established that bidirectional crosstalk is required for the proper function of both lymphoid as well as nonlymphoid thymic cells.…”

EphB receptors, mainly EphB3, contribute to the proper development of cortical thymic epithelial cells

“…Our current results do not demonstrate changes in the IL7 values in mutant TECs, but preliminary studies show significantly reduced IL7 receptor a chain (IL7Ra) transcripts in both EphB2¡/¡ and EphB3¡/¡ FTOCs at E13.5 and E15.5 (data not shown), as previously reported in thymocytes with specific deletion of ephrin-B1 and ephrin-B2 genes. 34 On the other hand, the expression of Dll4, a ligand of Notch1, necessary for the differentiation of DN thymocytes 26 is reduced in EphB2and EphB3-deficient thymuses, significantly at E15.5.…”

“…Finally, we evaluated whether, in turn, mutant cTECs were capable of producing two factors, IL7 and Dll4, necessary for the proper development of both DN and DP thymocytes that occurs in the thymic cortex. 26 IL7 expression was lower, but not significantly, in mutant TECs compared to WT values at both E12.5 and E15.5 (Fig. 8), whereas that of Dll4, a Notch ligand, was significantly lower in both EphB2¡/¡ and EphB3¡/¡ TECs at E15.5 but not at E12.5 (Fig.…”