Inhibition of Histone Deacetylase 6 Reveals a Potent Immunosuppressant Effect in Models of Transplantation.

Background: Current transplant immunosuppression regimens have numerous limitations. Recent evidence suggests histone deacetylase inhibitors (HDACis) may represent a class of drug with immunosuppressive properties. This study compares cyclosporin A (CyA) with the pan-HDACi suberoylanilide hydroxamic acid (SAHA) and a novel HDAC6-specific inhibitor (KA1010) in models of alloreactivity.

Osteoprotegerin-Mediated Homeostasis of Rank+ Thymic Epithelial Cells Does Not Limit Foxp3+ Regulatory T Cell Development.

In the thymus, medullary thymic epithelial cells (mTEC) regulate T cell tolerance via negative selection and Foxp3(+) regulatory T cell (Treg) development, and alterations in the mTEC compartment can lead to tolerance breakdown and autoimmunity. Both the receptor activator for NF-κB (RANK)/RANK ligand (RANKL)/osteoprotegerin (OPG) axis and expression of the transcriptional regulator Aire are involved in the regulation of thymus medullary microenvironments. However, their impact on the mechanisms controlling mTEC homeostasis is poorly understood, as are the processes that enable the thymus medulla to support the balanced production of mTEC-dependent Foxp3(+) Treg.

Differential requirement for CCR4 and CCR7 during the development of innate and adaptive αβT cells in the adult thymus.

αβT cell development depends upon serial migration of thymocyte precursors through cortical and medullary microenvironments, enabling specialized stromal cells to provide important signals at specific stages of their development. Although conventional αβT cells are subject to clonal deletion in the medulla, entry into the thymus medulla also fosters αβT cell differentiation. For example, during postnatal periods, the medulla is involved in the intrathymic generation of multiple αβT cell lineages, notably the induction of Foxp3(+) regulatory T cell development and the completion of invariant NKT cell development.

An essential role for medullary thymic epithelial cells during the intrathymic development of invariant NKT cells.

In the thymus, interactions with both cortical and medullary microenvironments regulate the development of self-tolerant conventional CD4(+) and CD8(+) αβT cells expressing a wide range of αβTCR specificities. Additionally, the cortex is also required for the development of invariant NKT (iNKT) cells, a specialized subset of T cells that expresses a restricted αβTCR repertoire and is linked to the regulation of innate and adaptive immune responses. Although the role of the cortex in this process is to enable recognition of CD1d molecules expressed by CD4(+)CD8(+) thymocyte precursors, the requirements for additional thymus microenvironments during iNKT cell development are unknown.

Differential requirement for CCR4 in the maintenance but not establishment of the invariant Vγ5(+) dendritic epidermal T-cell pool.

Thymocytes expressing the invariant Vγ5 γδT-cell receptor represent progenitors of dendritic epidermal T-cells (DETC) that play an important immune surveillance role in the skin. In contrast to the bulk of αβT-cell development, Vγ5(+) DETC progenitor development occurs exclusively in fetal thymus. Whilst αβT-cell development is known to require chemokine receptor mediated migration through distinct thymus regions, culminating in medullary entry and thymic egress, the importance and control of intrathymic migration for DETC progenitors is unclear.

Mechanisms of thymus medulla development and function.

The development of CD4(+) helper and CD8(+) cytotoxic T-cells expressing the αβ form of the T-cell receptor (αβTCR) takes place in the thymus, a primary lymphoid organ containing distinct cortical and medullary microenvironments. While the cortex represents a site of early T-cell precursor development, and the positive selection of CD4(+)8(+) thymocytes, the thymic medulla plays a key role in tolerance induction, ensuring that thymic emigrants are purged of autoreactive αβTCR specificities. In recent years, advances have been made in understanding the development and function of thymic medullary epithelial cells, most notably the subset defined by expression of the Autoimmune Regulator (Aire) gene.

The thymic medulla is required for Foxp3+ regulatory but not conventional CD4+ thymocyte development.

A key role of the thymic medulla is to negatively select autoreactive CD4(+) and CD8(+) thymocytes, a process important for T cell tolerance induction. However, the involvement of the thymic medulla in other aspects of αβ T cell development, including the generation of Foxp3(+) natural regulatory T cells (nTreg cells) and the continued maturation of positively selected conventional αβ T cells, is unclear. We show that newly generated conventional CD69(+)Qa2(-) CD4 single-positive thymocytes mature to the late CD69(-)Qa2(+) stage in the absence of RelB-dependent medullary thymic epithelial cells (mTECs).

Normal T cell selection occurs in CD205-deficient thymic microenvironments.

The thymus imparts a developmental imprint upon T cells, screening beneficial and self-tolerant T cell receptor (TCR) specificities. Cortical thymic epithelial cells (CTEC) present self-peptide self-MHC complexes to thymocytes, positively selecting those with functional TCRs. Importantly, CTEC generate diverse self-peptides through highly specific peptide processing.

Generation of both cortical and Aire(+) medullary thymic epithelial compartments from CD205(+) progenitors.

In the adult thymus, the development of self-tolerant thymocytes requires interactions with thymic epithelial cells (TECs). Although both cortical and medullary TECs (cTECs/mTECs) are known to arise from common bipotent TEC progenitors, the phenotype of these progenitors and the timing of the emergence of these distinct lineages remain unclear. Here, we have investigated the phenotype and developmental properties of bipotent TEC progenitors during cTEC/mTEC lineage development.

Developmentally regulated availability of RANKL and CD40 ligand reveals distinct mechanisms of fetal and adult cross-talk in the thymus medulla.

T cell tolerance in the thymus is a key step in shaping the developing T cell repertoire. Thymic medullary epithelial cells play multiple roles in this process, including negative selection of autoreactive thymocytes, influencing thymic dendritic cell positioning, and the generation of Foxp3(+) regulatory T cells. Previous studies show that medullary thymic epithelial cell (mTEC) development involves hemopoietic cross-talk, and numerous TNFR superfamily members have been implicated in this process.

Rank signaling links the development of invariant γδ T cell progenitors and Aire(+) medullary epithelium.

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.

A novel method to allow noninvasive, longitudinal imaging of the murine immune system in vivo.

In vivo imaging has revolutionized understanding of the spatiotemporal complexity that subserves the generation of successful effector and regulatory immune responses. Until now, invasive surgery has been required for microscopic access to lymph nodes (LNs), making repeated imaging of the same animal impractical and potentially affecting lymphocyte behavior. To allow longitudinal in vivo imaging, we conceived the novel approach of transplanting LNs into the mouse ear pinna.

Multiple suppression pathways of canonical Wnt signalling control thymic epithelial senescence.

Members of the Wnt family of secreted glyco-lipo-proteins affect intrathymic T-cell development and are abundantly secreted by thymic epithelial cells (TECs) that create the specific microenvironment for thymocytes to develop into mature T-cells. During ageing, Wnt expression declines allowing adipoid involution of the thymic epithelium leading to reduced naïve T-cell output. The protein kinase C (PKC) family of serine-threonine kinases is involved in numerous intracellular biochemical processes, including Wnt signal transduction.

Wnt-4 protects thymic epithelial cells against dexamethasone-induced senescence.

Glucocorticoids are widely used immunosuppressive drugs in treatment of autoimmune diseases and hematological malignancies. Glucocorticoids are particularly effective immune suppressants, because they induce rapid peripheral T cell and thymocyte apoptosis resulting in impaired T cell-dependent immune responses. Although glucocorticoids can induce apoptotic cell death directly in developing thymocytes, how exogenous glucocorticoids affect the thymic epithelial network that provides the microenvironment for T cell development is still largely unknown.

Clonal analysis reveals uniformity in the molecular profile and lineage potential of CCR9(+) and CCR9(-) thymus-settling progenitors.

The entry of T cell progenitors to the thymus marks the beginning of a multistage developmental process that culminates in the generation of self-MHC-restricted CD4(+) and CD8(+) T cells. Although multiple factors including the chemokine receptors CCR7 and CCR9 are now defined as important mediators of progenitor recruitment and colonization in both the fetal and adult thymi, the heterogeneity of thymus-colonizing cells that contribute to development of the T cell pool is complex and poorly understood. In this study, in conjunction with lineage potential assays, we perform phenotypic and genetic analyses on thymus-settling progenitors (TSP) isolated from the embryonic mouse thymus anlagen and surrounding perithymic mesenchyme, including simultaneous gene expression analysis of 14 hemopoietic regulators using single-cell multiplex RT-PCR.

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.

Wnt4 and LAP2alpha as pacemakers of thymic epithelial senescence.

Age-associated thymic involution has considerable physiological impact by inhibiting de novo T-cell selection. This impaired T-cell production leads to weakened immune responses. Yet the molecular mechanisms of thymic stromal adipose involution are not clear.

Splenic stromal cells mediate IL-7 independent adult lymphoid tissue inducer cell survival.

Lymphoid tissue inducer cells (LTi) play an important role in the development of lymphoid tissue in embryos. Adult CD4(+)CD3(-) LTi-like cells present a similar phenotype and gene expression to their embryonic counterpart and have important roles in CD4(+) T-cell memory and lymphoid tissue recovery following viral infection. However, adult LTi-like cells are heterogeneous populations and the factors that regulate their survival and accumulation within secondary lymphoid organs remain unclear, in particular whether the T-zone stroma is involved.

Absence of thymus crosstalk in the fetus does not preclude hematopoietic induction of a functional thymus in the adult.

Cortical and medullary thymic epithelial cells provide essential signals for a normal programme of T-cell development. Current models of thymus development suggest that thymocyte-derived signals play an important role in establishing thymic microenvironments, a process termed thymus crosstalk. Studies on CD3epsilontg26 mice lacking intrathymic T-cell progenitors provided evidence that normal development of the thymic cortex depends upon thymocyte-derived signals.

A roadmap for thymic epithelial cell development.

Thymic epithelial cells (TEC) are essential components of the thymus that guide and control the development and TCR repertoire selection of T cells. Previously, TEC have been considered as postmitotic cells that, once generated during ontogeny, were maintained in their mature state. Recently, it has become clear that TEC can be generated from common or committed medullary and cortical TEC progenitor cells in ontogeny, that stages of immature and mature TEC are phenotypically separable, and that TEC undergo a rapid turnover in a matter of a few weeks.

Checkpoints in the development of thymic cortical epithelial cells.

In the thymus, interactions between immature thymocytes and thymic epithelial cells (TECs) regulate the development and selection of self-tolerant MHC-restricted T cells. Despite the importance of cortical (cTEC) and medullary (mTEC) thymic epithelial cells in fostering T cell production, events in TEC development are still unclear. Although precursor-product relationships during mTEC development have been reported, and some genetic regulators of mTEC development have been identified, stages in cTEC development occurring downstream of recently identified bipotent cTEC/mTEC progenitors remain poorly defined.

Transplantation of embryonic spleen tissue reveals a role for adult non-lymphoid cells in initiating lymphoid tissue organization.

In this report we describe a transplantation system where embryonic spleens are grafted into adult hosts. This model can be used to analyze the cellular and molecular requirements for the development and organization of splenic microenvironments. Whole embryonic day 15 (ED15) spleens, grafted under the kidney capsule of adult mice, were colonized by host-derived lymphocytes and DC and developed normal splenic architecture.

Reaggregate thymus cultures.

Stromal cells within lymphoid tissues are organized into three-dimensional structures that provide a scaffold that is thought to control the migration and development of haemopoeitic cells. Importantly, the maintenance of this three-dimensional organization appears to be critical for normal stromal cell function, with two-dimensional monolayer cultures often being shown to be capable of supporting only individual fragments of lymphoid tissue function. In the thymus, complex networks of cortical and medullary epithelial cells act as a framework that controls the recruitment, proliferation, differentiation and survival of lymphoid progenitors as they undergo the multi-stage process of intrathymic T-cell development.

Preparation of 2-dGuo-treated thymus organ cultures.

In the thymus, interactions between developing T-cell precursors and stromal cells that include cortical and medullary epithelial cells are known to play a key role in the development of a functionally competent T-cell pool. However, the complexity of T-cell development in the thymus in vivo can limit analysis of individual cellular components and particular stages of development. In vitro culture systems provide a readily accessible means to study multiple complex cellular processes.