Andrea J. White

Rank Signaling Links the Development of Invariant γδ T Cell Progenitors and Aire+ Medullary Epithelium

Summary The thymic medulla provides a specialized microenvironment for the negative selection of T cells, with the presence of autoimmune regulator (Aire)-expressing medullary thymic epithelial cells (mTECs) during the embryonic-neonatal period being both necessary and sufficient to establish long-lasting tolerance. Here we showed that emergence of the first cohorts of Aire+ mTECs at this key developmental stage, prior to αβ T cell repertoire selection, was jointly directed by Rankl+ lymphoid tissue inducer cells and invariant Vγ5+ dendritic epidermal T cell (DETC) progenitors that are the first thymocytes to express the products of gene rearrangement. In turn, generation of Aire+ mTECs then fostered Skint-1-dependent, but Aire-independent, DETC progenitor maturation and the emergence of an invariant DETC repertoire. Hence, our data attributed a functional importance to the temporal development of Vγ5+ γδ T cells during thymus medulla formation for αβ T cell tolerance induction and demonstrated a Rank-mediated reciprocal link between DETC and Aire+ mTEC maturation.

Establishment and functioning of intrathymic microenvironments

Summary:  The thymus supports the production of self‐tolerant T cells from immature precursors. Studying the mechanisms regulating the establishment and maintenance of stromal microenvironments within the thymus therefore is essential to our understanding of T‐cell production and ultimately immune system functioning. Despite our ability to phenotypically define stromal cell compartments of the thymus, the mechanisms regulating their development and the ways by which they influence T‐cell precursors are still unclear. Here, we review recent findings and highlight unresolved issues relating to the development and functioning of thymic stromal cells.

Lymphotoxin Signals from Positively Selected Thymocytes Regulate the Terminal Differentiation of Medullary Thymic Epithelial Cells

The thymic medulla represents a key site for the induction of T cell tolerance. In particular, autoimmune regulator (Aire)-expressing medullary thymic epithelial cells (mTECs) provide a spectrum of tissue-restricted Ags that, through both direct presentation and cross-presentation by dendritic cells, purge the developing T cell repertoire of autoimmune specificities. Despite this role, the mechanisms of Aire+ mTEC development remain unclear, particularly those stages that occur post-Aire expression and represent mTEC terminal differentiation. In this study, in mouse thymus, we analyze late-stage mTEC development in relation to the timing and requirements for Aire and involucrin expression, the latter a marker of terminally differentiated epithelium including Hassall’s corpuscles. We show that Aire expression and terminal differentiation within the mTEC lineage are temporally separable events that are controlled by distinct mechanisms. We find that whereas mature thymocytes are not essential for Aire+ mTEC development, use of an inducible ZAP70 transgenic mouse line—in which positive selection can be temporally controlled—demonstrates that the emergence of involucrin+ mTECs critically depends upon the presence of mature single positive thymocytes. Finally, although initial formation of Aire+ mTECs depends upon RANK signaling, continued mTEC development to the involucrin+ stage maps to activation of the LTα–LTβR axis by mature thymocytes. Collectively, our results reveal further complexity in the mechanisms regulating thymus medulla development and highlight the role of distinct TNFRs in initial and terminal differentiation stages in mTECs.

Ontogeny of Stromal Organizer Cells during Lymph Node Development

The development of secondary lymphoid organs, such as lymph nodes (LNs), in the embryo results from the reciprocal action between lymphoid tissue inducer (LTi) cells and stromal cells. However, the initial events inducing LN anlagen formation before the LTi stromal cells cross-talk interactions take place are not fully elucidated. In this study, we show that the inguinal LN anlagen in mouse embryos developed from mesenchymal cells surrounding the lymph sacs, spherical structures of endothelial cells that bud from veins. Using inguinal and mesenteric LNs (mLNs), we provide evidence supporting a two-step maturation model for stromal cells: first, ICAM-1−VCAM-1− mesenchymal precursor cells become ICAM-1intVCAM-1int cells, in a process independent of LTi cells and lymphotoxin β receptor (LTβR) signaling. The second step involves the maturation of ICAM-1intVCAM-1int cells to ICAM-1highVCAM-1high mucosal addressin cell adhesion molecule-1+ organizer cells and depends on both LTi cells and LTβR. Addition of αLTβR agonist to LN organ cultures was sufficient to induce ICAM-1intVCAM-1int cells to mature. In LtβR−/− embryos, both inguinal and mLN stromal cells showed a block at the ICAM-1intVCAM-1int stage, and, contrary to inguinal LNs, mLNs persist longer and contained LTi cells, which correlated with the sustained gene expression of Il-7, Cxcl13, and, to a lesser degree, Ccl21. Taken together, these results highlight the importance of the signals and cellular interactions that induce the maturation of stromal cells and ultimately lead to the formation of lymphoid tissues.

OX40 Ligand and CD30 Ligand Are Expressed on Adult but Not Neonatal CD4+CD3− Inducer Cells: Evidence That IL-7 Signals Regulate CD30 Ligand but Not OX40 Ligand Expression

In this report, we have examined the expression of the T cell survival signals, OX40 ligand (OX40L) and CD30 ligand (CD30L) on CD4+CD3−CD11c−B220−IL-7Rα+ inducer cells from birth to adulthood in mice. We found that adult but not neonatal inducer cells expressed high levels of OX40L and CD30L, whereas their expression of TNF-related activation-induced cytokine (TRANCE) and receptor activator of NF-κB (RANK) was comparable. The failure of neonatal inducer cells to express the ligands that rescue T cells helps to explain why exposure to Ag in neonatal life induces tolerance rather than immunity. The expression of OX40L and CD30L on inducer cells increased gradually in the first few weeks of life achieving essentially normal levels around the time mice were weaned. We found that IL-7 signaling through the common cytokine receptor γ-chain was critical for the optimal expression of both TNF-related activation-induced cytokine and CD30L but not OX40L. Furthermore, glucocorticoids, which potently suppress T effector function, did not influence the expression of OX40L and CD30L in the presence of IL-7.

Neonatal and Adult CD4+CD3− Cells Share Similar Gene Expression Profile, and Neonatal Cells Up-Regulate OX40 Ligand in Response to TL1A (TNFSF15)

We report here the quantitative expression of a set of immunity-related genes, including TNF family members, chemokine receptors, and transcription factors, in a CD4+CD3− accessory cell. By correlating gene expression between cell-sorted populations of defined phenotype, we show that the genetic fingerprint of these CD4+CD3− cells is distinct from dendritic cells, plasmacytoid dendritic cells, T cells, B cells, and NK cells. In contrast, it is highly similar to CD4+CD3− cells isolated from embryonic and neonatal tissues, with the exception that only adult populations express OX40L and CD30L. We have previously reported that IL-7 signals regulate CD30L expression. In the present study, we show that both neonatal and adult CD4+CD3− cells express the TNF family member, death receptor 3 (TNFRSF25), and that addition of TL1A (TNFSF15), the ligand for death receptor 3, up-regulates OX40L on neonatal CD4+CD3− cells. Finally, we demonstrate that this differentiation occurs in vivo: neonatal CD4+CD3− cells up-regulate both CD30L and OX40L after adoptive transfer into an adult recipient.

Sequential phases in the development of Aire-expressing medullary thymic epithelial cells involve distinct cellular input

Intrathymic deletion of immature thymocytes that express self‐reactive TCR specificities is essential in the generation of self tolerance. Medullary thymic epithelial cells (mTEC) expressing the transcriptional regulator Aire play a key role in this process by regulating expression of tissue‐restricted antigens to ensure tolerance to peripheral tissues. Here, we have analysed the cellular and molecular requirements for the initial appearance of Aire+ mTEC in the embryonic thymus, in addition to their persistence in the adult thymus. Analysis of thymic ontogeny shows that the emergence of embryonic Aire+ mTEC occurs prior to the appearance of mature thymocytes, and depends upon lymphoid tissue inducer cells expressing retinoic acid receptor‐related orphan receptor γ. In the adult thymus, we show that Aire+ mTEC develop in the absence of thymocyte positive and negative selection and CD40 signalling, but are present at reduced frequency. Collectively these data support a model where the initial differentiation of Aire+ mTEC involves receptor activator of NF‐κB (RANK)‐RANKL interactions with lymphoid tissue inducer cells, with subsequent mTEC turnover and/or survival involving CD40‐mediated signalling following interactions with mature CD4+ thymocytes that express CD40L.

Cutting Edge: Lymphoid Tissue Inducer Cells Maintain Memory CD4 T Cells within Secondary Lymphoid Tissue

Phylogeny shows that CD4 T cell memory and lymph nodes coevolved in placental mammals. In ontogeny, retinoic acid orphan receptor (ROR)γ-dependent lymphoid tissue inducer (LTi) cells program the development of mammalian lymph nodes. In this study, we show that although primary CD4 T cell expansion is normal in RORγ-deficient mice, the persistence of memory CD4 T cells is RORγ-dependent. Furthermore, using bone marrow chimeric mice we demonstrate that LTi cells are the key RORγ-expressing cell type sufficient for memory CD4 T cell survival in the absence of persistent Ag. This effect was specific for CD4 T cells, as memory CD8 T cells survived equally well in the presence or absence of LTi cells. These data demonstrate a novel role for LTi cells, archetypal members of the innate lymphoid cell family, in supporting memory CD4 T cell survival in vivo.

Lymphotoxin-β Receptor Signaling through NF-κB2-RelB Pathway Reprograms Adipocyte Precursors as Lymph Node Stromal Cells

Lymph node development during embryogenesis involves lymphotoxin-β receptor engagement and subsequent differentiation of a poorly defined population of mesenchymal cells into lymphoid tissue organizer cells. Here, we showed that embryonic mesenchymal cells with characteristics of adipocyte precursors present in the microenvironment of lymph nodes gave rise to lymph node organizer cells. Signaling through the lymphotoxin-β receptor controlled the fate of adipocyte precursor cells by blocking adipogenesis and instead promoting lymphoid tissue stromal cell differentiation. This effect involved activation of the NF-κB2-RelB signaling pathway and inhibition of the expression of the key adipogenic factors Pparγ and Cebpα. In vivo organogenesis assays show that embryonic and adult adipocyte precursor cells can migrate into newborn lymph nodes and differentiate into a variety of lymph node stromal cells. Thus, we propose that adipose tissues act as a source of lymphoid stroma for lymph nodes and other lymphoid structures associated with fat.

An Epithelial Progenitor Pool Regulates Thymus Growth

Thymic epithelium provides an essential cellular substrate for T cell development and selection. Gradual age-associated thymic atrophy leads to a reduction in functional thymic tissue and a decline in de novo T cell generation. Development of strategies tailored toward regeneration of thymic tissue provides an important possibility to improve immune function in elderly individuals and increase the capacity for immune recovery in patients having undergone bone marrow transfer following immunoablative therapies. In this study we show that restriction of the size of the functional thymic epithelial progenitor pool affects the number of mature thymic epithelial cells. Using an embryo fusion chimera-based approach, we demonstrate a reduction in the total number of both embryonic and adult thymic epithelium, which relates to the initial size of the progenitor cell pool. The inability of thymic epithelial progenitor cells to undergo sufficient compensatory proliferation to rescue the deficit in progenitor numbers suggests that in addition to extrinsic regulation of thymus growth by provision of growth factors, intrinsic factors such as a proliferative restriction of thymic epithelial progenitors and availability of progenitor cell niches may limit thymic epithelial recovery. Collectively, our data demonstrate an important level of regulation of thymic growth and recovery at the thymic epithelial progenitor level, providing an important consideration for developing methods targeted toward inducing thymic regeneration.