Regulation of the Signal-Dependent E Protein HEBAlt Through a YYY Motif Is Required for Progression Through T Cell Development

The E protein transcription factors E2A and HEB are critical for many developmental processes, including T cell development. We have shown that the Tcf12 locus gives rise to two distinct HEB proteins, with alternative (HEBAlt) and canonical (HEBCan) N-terminal domains, which are co-expressed during early T cell development. While the functional domains of HEBCan have been well studied, the nature of the HEBAlt-specific (Alt) domain has been obscure. Here we provide compelling evidence that the Alt domain provides a site for the molecular integration of cytokine signaling and E protein activity. Our results indicate that phosphorylation of a unique YYY motif in the Alt domain increases HEBAlt activity by 10-fold, and that this increase is dependent on Janus kinase activity. To enable in vivo studies of HEBAlt in the T cell context, we generated ALT-Tg mice, which can be induced to express a HA-tagged HEBAlt coding cassette in the presence of Cre recombinases. Analysis of ALT-Tg mice on the Vav-iCre background revealed a minor change in the ratio of ISP cells to CD8+ SP cells, and a mild shift in the ratio of T cells to B cells in the spleen, but otherwise the thymus, spleen, and bone marrow lymphocyte subsets were comparable at steady state. However, kinetic analysis of T cell development in OP9-DL4 co-cultures revealed a delay in early T cell development and a partial block at the DN to DP transition when HEBAlt levels or activity were increased. We also observed that HEBCan and HEBAlt displayed significant differences in protein stability that were resolved in the thymocyte context. Finally, a proteomic screen identified STAT1 and Xpo1 as potential members of HEBAlt-containing complexes in thymocytes, consistent with JAK-induced activation of HEBAlt accompanied by translocation to the nucleus. Thus, our results show that the Alt domain confers access to multiple layers of post-translational control to HEBAlt that are not available to HEBCan, and thus may serve as a rheostat to tune E protein activity levels as cells move through different thymic signaling environments during T cell development.

Close Quarters Can Be a Good Fit for Stem Cells to Become T Cells

Human pluripotent stem cells (PSCs) could potentially provide a potentially unlimited supply of T cells. In this issue of Cell Stem Cell, Montel-Hagen et al. (2019) take advantage of a three-dimensional mouse stromal cell-based system that combines mesoderm specification with organoid-induced hematopoietic differentiation to enable commitment, selection, and maturation of conventional human T cells.

A key role for IL-7R in the generation of microenvironments required for thymic dendritic cells

Interleukin-7 receptor (IL-7R) signaling is critical for multiple stages of T-cell development, but a role in the establishment of the mature thymic architecture needed for T-cell development and thymocyte selection has not been established. Crosstalk signals between developing thymocytes and thymic epithelial cell (TEC) precursors are critical for their differentiation into cortical TECs (cTECs) and medullary TECs (mTECs). In addition, mTEC-derived factors have been implicated in the recruitment of thymic dendritic cells (DCs) and intrathymic DC development. We therefore examined corticomedullary structure and DC populations in the thymus of Il7r^−/− mice. Analysis of TEC phenotype and spatial organization revealed a striking shift in the mTEC to cTEC ratio, accompanied by disorganized corticomedullary structure. Several of the thymic subsets known to have DC potential were nearly absent, accompanied by reductions in DC cell numbers. We also examined chemokine expression in the Il7r^−/− thymus, and found a significant decrease in mTEC-derived CCR7 ligand expression, and high levels of cTEC-derived chemokines, including CCL25 and CXCL12. Although splenic DCs were similarly affected, bone marrow (BM) precursors capable of giving rise to DCs were unperturbed. Finally, BM chimeras showed that there was no intrinsic need for IL-7R signaling in the development or recruitment of thymic DCs, but that the provision of wild-type progenitors enhanced reconstitution of thymic DCs from Il7r^−/− progenitors. Our results are therefore supportive of a model in which Il7r-dependent cells are required to set up the microenvironments that allow accumulation of thymic DCs.

HEB plays a critical role in the generation of IL-17 producing Vγ2+ γδ T cells

IL-17 producing Vγ2+ γδ T cells (Vγ2 T cells) form an integral part of the immune system of various lymphoid and mucosal tissues in mice. The γδ T cells acquire the ability to produce IL-17 during their thymic development but the molecular basis of this programming is not well understood. Here, we aim to characterized the role of HEB in the development of IL-17 producing Vγ2 T cells by utilizing HEB conditional knockout mice on a vav-Cre background (HEB conKO). First, we observed that there was a block in the development of Vγ2 T cells in the fetal thymic organ culture (FTOC) from HEB conKO embryos at E14. However, in contrast to the near absence of fetal thymic Vγ2 T cells in FTOC, neonatal thymus of HEB conKO mice supported the development of Vγ2 T cells, albeit at a significantly lower frequency than the wildtype thymus. Further, the frequency of Vγ2 T cells in the thymus of adult mice was the equivalent to that in the wildtype thymus, showing a gradual increase in the development of Vγ2 T cells in the HEB conKO thymus with age. As IL-17 producing γδ T cells have been shown to be generated preferentially during fetal development, we characterized the functional subsets of Vγ2 T cells found in adult HEB conKO mice. In fact, there was a significant reduction in the frequency of RORγt+ Vγ2+ T cells in the thymus of adult HEB conKO mice, as well as in the lungs, spleen and lymph nodes. Furthermore, the HEB-deficient Vγ2 T cells in the lungs, spleen and lymph nodes exhibited a profound deficiency in their ability to produce IL-17 in response to stimulation with IL-1β, IL-23 and IL-21 or PMA/Ionomycin. Collectively, our work shows for the first time that HEB is required for the generation of IL-17 producing Vγ2 T cells for various lymphoid and mucosal tissues.