Two developmentally distinct populations of dendritic cells inhabit the adult mouse thymus: demonstration by differential importation of hematogenous precursors under steady state conditions

Evaluating in vivo approaches for studying the roles of thymic DCs in T cell development in mice

T cells express an enormous repertoire of T cell receptors, enabling them to recognize any potential antigen. This large repertoire undergoes stringent selections in the thymus, where receptors that react to self- or non-danger-associated- antigens are purged. We know that thymic tolerance depends on signals and antigens presented by the thymic antigen presenting cells, but we still do not understand precisely how many of these cells actually contribute to tolerance. This is especially true for thymic dendritic cells (DC), which are composed of diverse subpopulations that are derived from different progenitors. Although the importance of thymic DCs has long been known, the functions of specific DC subsets have been difficult to untangle. There remains insufficient systematic characterization of the ontogeny and phenotype of thymic APCs in general. As a result, validated experimental models for studying thymic DCs are limited. Recent technological advancement, such as multi-omics analyses, has enabled new insights into thymic DC biology. These recent findings indicate a need to re-evaluate the current tools used to study the function of these cells within the thymus. This review will discuss how thymic DC subpopulations can be defined, the models that have been used to assess functions in the thymus, and models developed for other settings that can be potentially used for studying thymic DCs.

Oral Fc-Coupled Preproinsulin Achieves Systemic and Thymic Delivery Through the Neonatal Fc Receptor and Partially Delays Autoimmune Diabetes

Tolerogenic vaccinations using beta-cell antigens are attractive for type 1 diabetes prevention, but clinical trials have been disappointing. This is probably due to the late timing of intervention, when multiple auto-antibodies are already present. We therefore devised a strategy to introduce the initiating antigen preproinsulin (PPI) during neonatal life, when autoimmunity is still silent and central tolerance mechanisms, which remain therapeutically unexploited, are more active. This strategy employs an oral administration of PPI-Fc, i.e. PPI fused with an IgG Fc to bind the intestinal neonatal Fc receptor (FcRn) that physiologically delivers maternal antibodies to the offspring during breastfeeding. Neonatal oral PPI-Fc vaccination did not prevent diabetes development in PPI T-cell receptor-transgenic G9C8.NOD mice. However, PPI-Fc was efficiently transferred through the intestinal epithelium in an Fc- and FcRn-dependent manner, was taken up by antigen presenting cells, and reached the spleen and thymus. Although not statistically significant, neonatal oral PPI-Fc vaccination delayed diabetes onset in polyclonal Ins2^-/-.NOD mice that spontaneously develop accelerated diabetes. Thus, this strategy shows promise in terms of systemic and thymic antigen delivery via the intestinal FcRn pathway, but the current PPI-Fc formulation/regimen requires further improvements to achieve diabetes prevention.

The thymus medulla and its control of αβT cell development

αβT cells are an essential component of effective immune responses. The heterogeneity that lies within them includes subsets that express diverse self-MHC-restricted αβT cell receptors, which can be further subdivided into CD4⁺ helper, CD8⁺ cytotoxic, and Foxp3⁺ regulatory T cells. In addition, αβT cells also include invariant natural killer T cells that are very limited in αβT cell receptor repertoire diversity and recognise non-polymorphic CD1d molecules that present lipid antigens. Importantly, all αβT cell sublineages are dependent upon the thymus as a shared site of their development. Ongoing research has examined how the thymus balances the intrathymic production of multiple αβT cell subsets to ensure correct formation and functioning of the peripheral immune system. Experiments in both wild-type and genetically modified mice have been essential in revealing complex cellular and molecular mechanisms that regulate thymus function. In particular, studies have demonstrated the diverse and critical role that the thymus medulla plays in shaping the peripheral T cell pool. In this review, we summarise current knowledge on functional properties of the thymus medulla that enable the thymus to support the production of diverse αβT cell types.

Tumor Arrests DN2 to DN3 Pro T Cell Transition and Promotes Its Conversion to Thymic Dendritic Cells by Reciprocally Regulating Notch1 and Ikaros Signaling

Tumor progression in the host leads to severe impairment of intrathymic T-cell differentiation/maturation, leading to the paralysis of cellular anti-tumor immunity. Such suppression manifests the erosion of CD4⁺CD8⁺ double-positive (DP) immature thymocytes and a gradual increase in CD4^-CD8^- double negative (DN) early T-cell progenitors. The impact of such changes on the T-cell progenitor pool in the context of cancer remains poorly investigated. Here, we show that tumor progression blocks the transition of Lin^-Thy1.2⁺CD25⁺CD44⁺c-Kit^lowDN2b to Lin^-Thy1.2⁺CD25⁺CD44^-c-Kit^-DN3 in T-cell maturation, instead leading to DN2-T-cell differentiation into dendritic cells (DC). We observed that thymic IL-10 expression is upregulated, particularly at cortico-medullary junctions (CMJ), under conditions of progressive disease, resulting in the termination of IL-10R^high DN2-T-cell maturation due to dysregulated expression of Notch1 and its target, CCR7 (thus restricting these cells to the CMJ). Intrathymic differentiation of T-cell precursors in IL-10^-/- mice and in vitro fetal thymic organ cultures revealed that IL-10 promotes the interaction between thymic stromal cells and Notch1^low DN2-T cells, thus facilitating these DN2-T cells to differentiate toward CD45⁺CD11c⁺MHC-II⁺ thymic DCs as a consequence of activating the Ikaros/IRF8 signaling axis. We conclude that a novel function of thymically-expressed IL-10 in the tumor-bearing host diverts T-cell differentiation toward a DC pathway, thus limiting the protective adaptive immune repertoire.

Dendritic cell development: A personal historical perspective

Dendritic cells(DCs) were once considered as a single cell type closely related developmentally to macrophages. Now we recognise several subtypes of DCs and have outlined several different pathways that potentially lead to their development. This article outlines some of the research findings that led to these changes in perspective, from the point of view of one of the participants.

Widespread Cell-Specific Prolactin Receptor Expression in Multiple Murine Organs

The prolactin receptor (Prlr) mediates not only the multiple effects of prolactin, but also those of the placental lactogens and, in humans, some actions of growth hormone. Although Prlr expression has been reported to be widespread in the body, specific cellular expression patterns within tissues are undefined for many organs. One persisting problem in investigating Prlr function is that the protein is difficult to detect using conventional methods. To allow investigation of Prlr expression with a single cell resolution, we have recently developed a knock-in mouse strain in which Cre recombinase is expressed together with the long isoform of the Prlr using an internal ribosome entry site. When crossed to a Cre-dependent reporter mouse strain, Cre-mediated recombination will genetically label cells that acutely express the Prlr as well as cells that have transiently expressed the Prlr during development. We report here the anatomical distribution of cells which express the fluorescent reporter τ green fluorescent protein in a total of 38 organs prepared from young adult male and female Prlr reporter mice. Our results establish a resource for dissecting the functional role of Prlr in multiple murine tissues.

The Lymphatic Fluid

This review will highlight our current understanding of the formation, circulation, and immunological role of lymphatic fluid. The formation of the extracellular fluid depends on the net balance between the hydrostatic and osmotic pressure gradients effective in the capillary beds. Lymph originates from the extracellular fluid and its composition combines the ultrafiltrated plasma proteins with the proteome generated by the metabolic activities of each parenchymal tissue. Several analyses have indicated how the lymph composition reflects the organs' physiological and pathological states. The collected lymphatic fluid moves from the capillaries into progressively larger collectors toward the draining lymph node aided by the lymphangion contractility and unidirectional valves, which prevent backflow. The proteomic composition of the lymphatic fluid is reflected in the MHC II peptidome presented by nodal antigen-presenting cells. Taken together, the past few years have generated new interest in the formation, transport, and immunological role of the lymphatic fluid.

The Importance of Dendritic Cells in Maintaining Immune Tolerance

Immune tolerance is necessary to prevent the immune system from reacting against self, and thus to avoid the development of autoimmune diseases. In this review, we discuss key findings that position dendritic cells (DCs) as critical modulators of both thymic and peripheral immune tolerance. Although DCs are important for inducing both immunity and tolerance, increased autoimmunity associated with decreased DCs suggests their nonredundant role in tolerance induction. DC-mediated T cell immune tolerance is an active process that is influenced by genetic variants, environmental signals, as well as the nature of the specific DC subset presenting Ag to T cells. Answering the many open questions with regard to the role of DCs in immune tolerance could lead to the development of novel therapies for the prevention of autoimmune diseases.

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.

The development and function of dendritic cell populations and their regulation by miRNAs

Dendritic cells (DCs) are important immune cells linking innate and adaptive immune responses. DCs encounter various self and non-self antigens present in the environment and induce different types of antigen specific adaptive immune responses. DCs can be classified into lymphoid tissue-resident DCs, migratory DCs, non-lymphoid resident DCs, and monocyte derived DCs (moDCs). Recent work has also established that DCs consist of developmentally and functionally distinct subsets that differentially regulate T lymphocyte function. The development of different DC subsets has been found to be regulated by a network of different cytokines and transcriptional factors. Moreover, the response of DC is tightly regulated to maintain the homeostasis of immune system. MicroRNAs (miRNAs) are an important class of cellular regulators that modulate gene expression and thereby influence cell fate and function. In the immune system, miRNAs act at checkpoints during hematopoietic development and cell subset differentiation, they modulate effector cell function, and are implicated in the maintenance of homeostasis. DCs are also regulated by miRNAs. In the past decade, much progress has been made to understand the role of miRNAs in regulating the development and function of DCs. In this review, we summarize the origin and distribution of different mouse DC subsets in both lymphoid and non-lymphoid tissues. The DC subsets identified in human are also described. Recent progress on the function of miRNAs in the development and activation of DCs and their functional relevance to autoimmune diseases are discussed.

Dendritic Cells in the Immune System—History, Lineages, Tissues, Tolerance, and Immunity

The aim of this review is to provide a coherent framework for understanding dendritic cells (DCs). It has seven sections. The introduction provides an overview of the immune system and essential concepts, particularly for the nonspecialist reader. Next, the “History” section outlines the early evolution of ideas about DCs and highlights some sources of confusion that still exist today. The “Lineages” section then focuses on five different populations of DCs: two subsets of “classical” DCs, plasmacytoid DCs, monocyte-derived DCs, and Langerhans cells. It highlights some cellular and molecular specializations of each, and also notes other DC subsets that have been proposed. The following “Tissues” section discusses the distribution and behavior of different DC subsets within nonlymphoid and secondary lymphoid tissues that are connected by DC migration pathways between them. In the “Tolerance” section, the role of DCs in central and peripheral tolerance is considered, including their ability to drive the differentiation of different populations of regulatory T cells. In contrast, the “Immunity” section considers the roles of DCs in sensing of infection and tissue damage, the initiation of primary responses, the T-cell effector phase, and the induction of immunological memory. The concluding section provides some speculative ideas about the evolution of DCs. It also revisits earlier concepts of generation of diversity and clonal selection in terms of DCs driving the evolution of T-cell responses. Throughout, this review highlights certain areas of uncertainty and suggests some avenues for future investigation.

Myeloid dendritic cells exhibit defects in activation and function in patients with multiple sclerosis

BACKGROUND

Regulatory T cells (Tregs) are functionally defective in patients with multiple sclerosis (MS) and this dysfunction is related to an imbalanced composition of naïve and memory Treg subtypes. Several lines of evidence indicate that these abnormalities might result from a premature decline in thymic-dependent Treg neogenesis. Myeloid dendritic cells (mDCs) critically determine Treg differentiation in the thymus, and thymic stromal lymphopoietin receptor (TSLPR) expressed on mDCs is a key component of the signaling pathways involved in this process. TSLPR-expression on mDCs was previously shown to be decreased in MS. We hypothesized that functional alterations in mDCs contribute to aberrant Treg neogenesis and, in turn, to altered Treg homeostasis and function in MS.

METHODS

We recruited blood samples from 20 MS patients and 20 healthy controls to assess TSLPR expression on mDCs ex vivo by flow cytometry and by activating mDCs induced by recombinant TSLP (rhTSLP) in vitro. As previous studies documented normalization of both function and homeostasis of Tregs under immunomodulatory (IM) therapy with interferon-beta (IFN-beta) and glatiramer acetate (GA), we also tested phenotypes and function of mDCs obtained from IM-treated patients (IFN-beta: n=20, GA: n=20).

RESULTS

We found that TSLP-induced mDC activation and effector function in vitro was reduced in MS and correlated with TSLPR-expression levels on mDCs. IM treatment prompted upregulation of TSLPR on mDCs and an increase in TSLP-induced activation of mDCs together with a normalization of Treg homeostasis.

CONCLUSION

The decreased TSLP-induced activation of MS-derived mDCs in vitro, together with the reduced density of TSLPR on the cell surface of mDCs corroborates the hypothesis of mDCs being critically involved in impairing Treg development in MS.

The melting pot of the MHC II peptidome

Recent advances in mass spectrometry technology have facilitated detailed examination of MHC-II immunopeptidomes, for example the repertoires of peptides bound to MHC-II molecules expressed in antigen presenting cells. These studies have deepened our view of MHC-II presentation. Other studies have broadened our view of pathways leading up to peptide loading. Here we review these recent studies in the context of earlier work on conventional and non-conventional MHC-II processing. The message that emerges is that sources of antigen beyond conventional endosomal processing of endocytosed proteins are important for generation of cellular immune responses to pathogens and maintenance of central and peripheral tolerance. The multiplicity of pathways results in a broad MHC II immunopeptidome that conveys the sampled environment to patrolling T cells.

The Dendritic Cell Major Histocompatibility Complex II (MHC II) Peptidome Derives from a Variety of Processing Pathways and Includes Peptides with a Broad Spectrum of HLA-DM Sensitivity

The repertoire of peptides displayed in vivo by MHC II molecules derives from a wide spectrum of proteins produced by different cell types. Although intracellular endosomal processing in dendritic cells and B cells has been characterized for a few antigens, the overall range of processing pathways responsible for generating the MHC II peptidome are currently unclear. To determine the contribution of non-endosomal processing pathways, we eluted and sequenced over 3000 HLA-DR1-bound peptides presented in vivo by dendritic cells. The processing enzymes were identified by reference to a database of experimentally determined cleavage sites and experimentally validated for four epitopes derived from complement 3, collagen II, thymosin β4, and gelsolin. We determined that self-antigens processed by tissue-specific proteases, including complement, matrix metalloproteases, caspases, and granzymes, and carried by lymph, contribute significantly to the MHC II self-peptidome presented by conventional dendritic cells in vivo. Additionally, the presented peptides exhibited a wide spectrum of binding affinity and HLA-DM susceptibility. The results indicate that the HLA-DR1-restricted self-peptidome presented under physiological conditions derives from a variety of processing pathways. Non-endosomal processing enzymes add to the number of epitopes cleaved by cathepsins, altogether generating a wider peptide repertoire. Taken together with HLA-DM-dependent and-independent loading pathways, this ensures that a broad self-peptidome is presented by dendritic cells. This work brings attention to the role of "self-recognition" as a dynamic interaction between dendritic cells and the metabolic/catabolic activities ongoing in every parenchymal organ as part of tissue growth, remodeling, and physiological apoptosis.

The Role of Dendritic Cells in Central Tolerance

Dendritic cells (DCs) play a significant role in establishing self-tolerance through their ability to present self-antigens to developing T cells in the thymus. DCs are predominantly localized in the medullary region of thymus and present a broad range of self-antigens, which include tissue-restricted antigens expressed and transferred from medullary thymic epithelial cells, circulating antigens directly captured by thymic DCs through coticomedullary junction blood vessels, and peripheral tissue antigens captured and transported by peripheral tissue DCs homing to the thymus. When antigen-presenting DCs make a high affinity interaction with antigen-specific thymocytes, this interaction drives the interacting thymocytes to death, a process often referred to as negative selection, which fundamentally blocks the self-reactive thymocytes from differentiating into mature T cells. Alternatively, the interacting thymocytes differentiate into the regulatory T (Treg) cells, a distinct T cell subset with potent immune suppressive activities. The specific mechanisms by which thymic DCs differentiate Treg cells have been proposed by several laboratories. Here, we review the literatures that elucidate the contribution of thymic DCs to negative selection and Treg cell differentiation, and discusses its potential mechanisms and future directions.

Limited niche availability suppresses murine intrathymic dendritic-cell development from noncommitted progenitors

The origins of dendritic cells (DCs) and other myeloid cells in the thymus have remained controversial. In this study, we assessed developmental relationships between thymic dendritic cells and thymocytes, employing retrovirus-based cellular barcoding and reporter mice, as well as intrathymic transfers coupled with DC depletion. We demonstrated that a subset of early T-lineage progenitors expressed CX3CR1, a bona fide marker for DC progenitors. However, intrathymic transfers into nonmanipulated mice, as well as retroviral barcoding, indicated that thymic dendritic cells and thymocytes were largely of distinct developmental origin. In contrast, intrathymic transfers after in vivo depletion of DCs resulted in intrathymic development of non-T-lineage cells. In conclusion, our data support a model in which the adoption of T-lineage fate by noncommitted progenitors at steady state is enforced by signals from the thymic microenvironment unless niches promoting alternative lineage fates become available.

Linkage between dendritic and T cell commitments in human circulating hematopoietic progenitors

The relationships between commitments of dendritic cells (DCs) and T cells in human hematopoietic stem cells are not well understood. In this study, we enumerate and characterize conventional DC and plasmacytoid DC precursors in association with T cell and thymus-derived types of NK cell precursors among CD34(+) hematopoietic progenitor cells (HPCs) circulating in human peripheral blood. By limiting-dilution analyses using coculture with stroma cells expressing Notch1 ligand, the precursor frequencies (PFs) of DCs in HPCs were found to significantly correlate with T cell PFs, but not with NK cell PFs, among healthy donors. Clonal analyses showed that the majority of T/NK dual- and T single-lineage precursors-but only a minority of NK single-lineage precursors-were associated with the generation of DC progenies. All clones producing both DC and T cell progenies were found with monocyte and/or granulocyte progenies, suggesting DC differentiation via myeloid DC pathways. Analyses of peripheral blood HPC subpopulations revealed that the lineage split between DC and T/NK cell progenitor occurs at the stage prior to bifurcation into T and NK cell lineages. The findings suggest a strong linkage between DC and T cell commitments, which may be imprinted in circulating lymphoid-primed multipotent progenitors or in more upstream HPCs.

Maturation and migration of murine CD4 single positive thymocytes and thymic emigrants

T lymphopoiesis in the thymus was thought to be completed once they reach the single positive (SP) stage, when they are “fully mature” and wait to be exported at random or follow a “first in-first out” manner. Recently, accumulating evidence has revealed that newly generated SP thymocytes undergo further maturation in the thymic medulla before they follow a tightly regulated emigrating process to become recent thymic emigrants (RTEs). RTEs in the periphery then experience a post-thymic maturation and peripheral tolerance and eventually become licensed as mature naïve T cells. This review summarizes the recent progress in the late stage T cell development in and outside of the thymus. The regulation of this developmental process is also discussed.

Self-antigen presentation by dendritic cells in autoimmunity

The operation of both central and peripheral tolerance ensures the prevention of autoimmune diseases. The maintenance of peripheral tolerance requires self-antigen presentation by professional antigen presenting cells (APCs). Dendritic cells (DCs) are considered as major APCs involved in this process. The current review discusses the role of DCs in autoimmune diseases, the various factors involved in the induction and maintenance of tolerogenic DC phenotype, and pinpoints their therapeutic capacity as well as potential novel targets for future clinical studies.

Induction of tolerance through mixed chimerism

"Mixed chimerism" refers to a state in which the lymphohematopoietic system of the recipient of allogeneic hematopoietic stem cells comprises a mixture of host and donor cells. This state is usually attained through either bone marrow or mobilized peripheral blood stem cell transplantation. Although numerous treatment regimens have led to transplantation tolerance in mice, the induction of mixed chimerism is currently the only treatment modality that has been successfully extended to large animals and to the clinic. Here we describe and compare the use of mixed chimerism to establish transplantation tolerance in mice, pigs, monkeys, and in the clinic. We also attempt to correlate the mechanisms involved in achieving tolerance with the nature of the tolerance that has resulted in each case.

The lymph self-antigen repertoire

The lymphatic fluid originates from the interstitial fluid which bathes every parenchymal organ and reflects the “omic” composition of the tissue from which it originates in its physiological or pathological signature. Several recent proteomic analyses have mapped the proteome-degradome and peptidome of this immunologically relevant fluid pointing to the lymph as an important source of tissue-derived self-antigens. A vast array of lymph-circulating peptides have been mapped deriving from a variety of processing pathways including caspases, cathepsins, MMPs, ADAMs, kallikreins, calpains, and granzymes, among others. These self peptides can be directly loaded on circulatory dendritic cells and expand the self-antigenic repertoire available for central and peripheral tolerance.

Maturation and emigration of single-positive thymocytes

T lymphopoiesis in the thymus was thought to be completed once it reaches the single positive (SP) stage, a stage when T cells are “fully mature” and waiting to be exported at random or follow a “first-in-first-out” manner. Recent evidence, however, has revealed that the newly generated SP thymocytes undergo a multistage maturation program in the thymic medulla. Such maturation is followed by a tightly regulated emigration process and a further postthymic maturation of recent thymic emigrants (RTEs). This review summarizes recent progress in the late stage T cell development. The regulation of this developmental process is discussed.

Thymus-homing dendritic cells in central tolerance

Central tolerance is critical in establishing a peripheral T-cell repertoire purged of functional autoreactive T cells. One of the major requirements for effective central tolerance is the presentation of self and other innocuous antigens (Ags), including food, gut flora, or airway allergens, to developing T cells in the thymus. This seemingly challenging task can be mediated in some cases by ectopic expression of tissue-specific Ags by thymic epithelial cells or by entry of systemic blood-borne Ags into the thymus. More recently, thymic homing peripheral dendritic cells (DCs) have been proposed as cellular transporters of peripheral tissue-specific Ags or foreign innocuous Ags. The aim of this viewpoint is to discuss the three principal thymic DC populations and their trafficking properties in the context of central tolerance. We will first discuss the importance of peripheral DC trafficking to the thymus and then compare and contrast the three DC subsets. We will describe how they were characterized, describe their trafficking to and their microenvironmental positioning in the thymus, and discuss the functional consequence of thymic trafficking and localization on thymic selection events.

Dendritic cell development: a choose-your-own-adventure story

Dendritic cells (DCs) are essential components of the immune system and contribute to immune responses by activating or tolerizing T cells. DCs comprise a heterogeneous mixture of subsets that are located throughout the body and possess distinct and specialized functions. Although numerous defined precursors from the bone marrow and spleen have been identified, emerging data in the field suggests many alternative routes of DC differentiation from precursors with multilineage potential. Here, we discuss how the combinatorial expression of transcription factors can promote one DC lineage over another as well as the integration of cytokine signaling in this process.

Early T-cell progenitors are the major granulocyte precursors in the adult mouse thymus

The mouse thymus supports T-cell development, but also contains non-T-cell lineages such as dendritic cells, macrophages, and granulocytes that are necessary for T-cell repertoire selection and apoptotic thymocyte clearance. Early thymic progenitors (ETPs) are not committed to the T-cell lineage, as demonstrated by both in vitro and in vivo assays. Whether ETPs realize non-T-cell lineage potentials in vivo is not well understood and indeed is controversial. In the present study, we investigated whether ETPs are the major precursors of any non-T-lineage cells in the thymus. We analyzed the development of these populations under experimental circumstances in which ETPs are nearly absent due to either abrogated thymic settling or inhibition of early thymic development by genetic ablation of IL-7 receptorα or Hes1. Results obtained using multiple in vivo approaches indicate that the majority of thymic granulocytes derive from ETPs. These data indicate that myelolymphoid progenitors settle the thymus and thus clarify the pathways by which stem cells give rise to downstream blood cell lineages.

Development of antigen cross-presentation capacity in dendritic cells

Cross-presentation by dendritic cells (DCs) of exogenous antigens on MHC class I is important for the generation of immune responses to intracellular pathogens, as well as for maintenance of self tolerance. In mice, the CD8(+) DC lineage is specialised for this role. However, DCs of this lineage are not born with cross-presentation capacity. Several studies have demonstrated that it must be induced as a later developmental step by cytokines such as granulocyte macrophage colony-stimulating factor (GM-CSF), or by microbial products such as toll-like receptor (TLR) ligands. Increased cross-presentation capacity is thus induced in peripheral CD8 lineage DCs during inflammation or infection. However, this capacity is already fully developed in steady-state thymic CD8(+) DCs, in accordance with their role in the deletion of self-reactive developing T cells.

Plasmacytoid dendritic cells transport peripheral antigens to the thymus to promote central tolerance

Central tolerance can be mediated by peripheral dendritic cells (DCs) that transport innocuous antigens (Ags) to the thymus for presentation to developing T cells, but the responsible DC subsets remained poorly defined. Immature plasmacytoid DCs (pDCs) express CCR9, a chemokine receptor involved in migration of T cell precursors to the thymus. We show here that CCR9 mediated efficient thymic entry of endogenous or i.v. transfused pDCs. pDCs activated by Toll-like receptor (TLR) ligands downregulated CCR9 and lost their ability to home to the thymus. Moreover, endogenous pDCs took up subcutaneously injected fluorescent Ag and, in the absence of TLR signals, transported Ag to the thymus in a CCR9-dependent fashion. Injected, Ag-loaded pDCs effectively deleted Ag-specific thymocytes, and this thymic clonal deletion required CCR9-mediated homing and was prevented by infectious signals. Thus, peripheral pDCs can contribute to immune tolerance through CCR9-dependent transport of peripheral Ags and subsequent deletion of Ag-reactive thymocytes.

T Cell Adaptive Immunity Proceeds through Environment-Induced Adaptation from the Exposure of Cryptic Genetic Variation

Evolution is often characterized as a process involving incremental genetic changes that are slowly discovered and fixed in a population through genetic drift and selection. However, a growing body of evidence is finding that changes in the environment frequently induce adaptations that are much too rapid to occur by an incremental genetic search process. Rapid evolution is hypothesized to be facilitated by mutations present within the population that are silent or "cryptic" within the first environment but are co-opted or "exapted" to the new environment, providing a selective advantage once revealed. Although cryptic mutations have recently been shown to facilitate evolution in RNA enzymes, their role in the evolution of complex phenotypes has not been proven. In support of this wider role, this paper describes an unambiguous relationship between cryptic genetic variation and complex phenotypic responses within the immune system. By reviewing the biology of the adaptive immune system through the lens of evolution, we show that T cell adaptive immunity constitutes an exemplary model system where cryptic alleles drive rapid adaptation of complex traits. In naive T cells, normally cryptic differences in T cell receptor reveal diversity in activation responses when the cellular population is presented with a novel environment during infection. We summarize how the adaptive immune response presents a well studied and appropriate experimental system that can be used to confirm and expand upon theoretical evolutionary models describing how seemingly small and innocuous mutations can drive rapid cellular evolution.

Transcriptional priming of intrathymic precursors for dendritic cell development

Specialized dendritic cells (DCs) within the thymus are crucial for the deletion of autoreactive T cells. The question of whether these cells arise from intrathymic precursors with T-cell potential has been hotly debated, and the regulatory pathways and signals that direct their development remain unclear. Here, we compared the gene expression profiles of thymic DC subsets with those of four early thymic precursor subsets: early T-cell precursors (ETPs), double-negative 1c (DN1c), double-negative 1d (DN1d) and double-negative 1e (DN1e) subsets. We found that the DN1d subset expressed Spi-B, HEBCan, Ccr7 and Ccr4, similar to thymic plasmacytoid DCs, whereas the DN1e subset expressed Id2, Ccr7 and Ccr4, similar to thymic conventional DCs. The expression of Ccr7 and Ccr4 in DN1d and DN1e cells suggested that they might be able to migrate towards the medulla (low in Dll proteins) and away from the cortex (high in Dll proteins) where early T-cell development occurs. We therefore assessed the sensitivity of developing DC precursors to Dll-Notch signaling, and found that high levels of Dll1 or Dll4 were inhibitory to DC development, whereas medium levels of Dll4 allowed DC development but not myeloid development. To evaluate directly the lineage potential of the ETP, DN1d and DN1e subsets, we injected them into nonirradiated congenic hosts intrathymically or intravenously, and found that they were all able to form medullary DCs in vivo. Therefore, DN1d and DN1e cells are transcriptionally primed to home to the thymus, migrate into DC-permissive microenvironments and develop into medullary DCs.

A missing link in thymic dendritic cell development

Negative selection in the thymus prevents the generation of self-reactive T cells through the deletion of thymocytes with high affinity for self-antigens. Within the thymus, self-antigens are presented by thymic epithelial cells and DCs. Both cell types can mediate negative selection, although the relative contribution of each cell type remains elusive. Similar to DCs of other lymphoid organs, thymic DCs come in different flavors. Over the past years, various lines of evidence have emerged that either favor a common origin for some thymic DCs and thymocytes or, conversely, indicate the existence of separate intrathymic T lineage and DC precursors. In this issue of the European Journal of Immunology, a study reports the identification of an intrathymic DC precursor that is likely to be unrelated to the earliest physiological T-cell progenitors. Thus this intrathymic DC precursor may constitute a "missing link" between extrathymic DC precursor-types, which are able to generate DCs in secondary lymphoid organs and intrathymic DCs, and supports the notion that intrathymic DCs and thymocytes arise from different precursors.

NFIL3/E4BP4 is a key transcription factor for CD8α⁺ dendritic cell development

Antigen presentation by mature dendritic cells (DCs) is the first step for initiating adaptive immune responses. DCs are composed of heterogeneous functional subsets; however, the molecular mechanisms that regulate differentiation of specific DC subsets are not understood. Here, we report that the basic leucine zipper transcription factor NFIL3/E4BP4 is essential for the development of CD8α(+) conventional DCs (cDCs). Nfil3(-/-) mice specifically lack CD8α(+) cDCs but not CD8α(-) cDCs or plasmacytoid DCs in lymphoid tissues. Flt3 ligand-dependent generation of CD8α(+) cDCs in lymphoid tissues and CD8α(+)-equivalent cDCs from Nfil3(-/-) bone marrow cells was also impaired. NFIL3 regulates CD8α(+) cDC development in part through Batf3 expression. Importantly, Nfil3(-/-) mice exhibited impaired cross-priming of CD8(+) T cells against cell-associated antigen, a process normally performed by CD8α(+) cDCs, and failed to produce IL-12 after TLR3 stimulation. Thus, NFIL3 plays an essential role in the development of CD8α(+) cDCs.

The role of the thymus in tolerance

The thymus serves as the central organ of immunologic self-nonself discrimination. Thymocytes undergo both positive and negative selection, resulting in T cells with a broad range of reactivity to foreign antigens but with a lack of reactivity to self-antigens. The thymus is also the source of a subset of regulatory T cells that inhibit autoreactivity of T-cell clones that may escape negative selection. As a result of these functions, the thymus has been shown to be essential for the induction of tolerance in many rodent and large animal models. Proper donor antigen presentation in the thymus after bone marrow, dendritic cell, or solid organ transplantation has been shown to induce tolerance to allografts. The molecular mechanisms of positive and negative selection and regulatory T-cell development must be understood if a tolerance-inducing therapeutic intervention is to be designed effectively. In this brief and selective review, we present some of the known information on T-cell development and on the role of the thymus in experimental models of transplant tolerance. We also cite some clinical attempts to induce tolerance to allografts using pharmacologic or biologic interventions.

Dysregulation of thymic clonal deletion and the escape of autoreactive T cells

Events ongoing in the thymus are critical for deleting developing thymocytes specific for tissue antigens, and establishing self-tolerance within the T cell compartment. Aberrant thymic negative selection, however, is believed to generate a repertoire with increased self-reactivity, which in turn can contribute to the development of T cell-mediated autoimmunity. In this review, mechanisms that regulate the efficacy of negative selection and influence the deletion of autoreactive thymocytes will be discussed.

The regulatory role of dendritic cells in the induction and maintenance of T-cell tolerance

The induction and maintenance of T-cell tolerance to tissue antigens is essential to prevent autoimmunity. Combinations of central and peripheral mechanisms act in parallel to inactivated, eliminated or control autoreactive T cells. Both centrally and peripherally, a key requirement for self-tolerance is the presentation of self-antigens in a correct context. There is now evidence to suggest that dendritic cells (DCs) play a fundamental role in the development of central and peripheral tolerance. In this review, we summarize recent progress toward the definition of the multiple roles of DCs in these processes. We will also discuss the association between defects in the DC compartment and the development of autoimmune responses, with particular reference to DC deregulation in the context of type I diabetes.

The CD8+ dendritic cell subset

Mouse lymphoid tissues contain a subset of dendritic cells (DCs) expressing CD8 alpha together with a pattern of other surface molecules that distinguishes them from other DCs. These molecules include particular Toll-like receptor and C-type lectin pattern recognition receptors. A similar DC subset, although lacking CD8 expression, exists in humans. The mouse CD8(+) DCs are non-migrating resident DCs derived from a precursor, distinct from monocytes, that continuously seeds the lymphoid organs from bone marrow. They differ in several key functions from their CD8(-) DC neighbors. They efficiently cross-present exogenous cell-bound and soluble antigens on major histocompatibility complex class I. On activation, they are major producers of interleukin-12 and stimulate inflammatory responses. In steady state, they have immune regulatory properties and help maintain tolerance to self-tissues. During infection with intracellular pathogens, they become major presenters of pathogen antigens, promoting CD8(+) T-cell responses to the invading pathogens. Targeting vaccine antigens to the CD8(+) DCs has proved an effective way to induce cytotoxic T lymphocytes and antibody responses.

Thymic stromal lymphopoietin-activated plasmacytoid dendritic cells induce the generation of FOXP3+ regulatory T cells in human thymus

Human thymus contains major dendritic cell (DC) subsets, myeloid DCs (mDCs), and plasmacytoid DCs (pDCs). We previously showed that mDCs, educated by thymic stromal lymphopoietin (TSLP) produced by the epithelial cells of the Hassall's corpuscles, induced differentiation of CD4(+)CD25(-) thymocytes into Forkhead Box P3(+) (FOXP3(+)) regulatory T cells (T(R)) within the medulla of human thymus. In this study, we show that pDCs expressed the TSLP receptor and IL-7 receptor alpha complexes upon activation and became responsive to TSLP. TSLP-activated human pDCs secrete macrophage-derived chemokine CCL-22 and thymus- and activation-regulated chemokine CCL-17 but not Th1- or Th2-polarizing cytokines. TSLP-activated pDCs induced the generation of FOXP3(+) T(R) from CD4(+)CD8(-)CD25(-) thymocytes, which could be strongly inhibited by Th1-polarizing cytokine IL-12 or Th2-polarizing cytokine IL-4. Interestingly, the FOXP3(+) T(R) induced by the TSLP-pDCs expressed more IL-10 but less TGF-beta than that induced by the TSLP-mDCs. These data suggest that TSLP expressed by thymic epithelial cells can activate mDCs and pDCs to positively select the FOXP3(+) T(R) with different cytokine production potential in human thymus. The inability of TSLP to induce DC maturation without producing Th1- or Th2-polarizing cytokines may provide a thymic niche for T(R) development.

Review of murine dendritic cells: types, location, and development

Dendritic cells (DCs) are key coordinators of the immune response, governing the choice between tolerance and immunity. DCs are professional antigen-presenting cells capable of presenting antigen on MHC molecules and priming CD4 and CD8 T-cell responses. They form a heterogeneous group of cells based on phenotype, location, and function. In this review, murine DCs will be discussed regarding their function with special emphasis on their tissue distribution. Recent findings on DC homeostasis during cancer progression will be presented. Finally, the developmental pathways leading to DC differentiation from their precursors will be summarized.

The impact of negative selection on thymocyte migration in the medulla

Developing thymocytes are screened for self-reactivity before exiting the thymus, but how thymocytes scan the medulla for self-antigens is unclear. Using two-photon microscopy, we observed that medullary thymocytes migrated rapidly and made frequent, transient contacts with dendritic cells. In the presence of a negative selecting ligand, thymocytes slowed, became confined to areas of approximately 30 microns in diameter, and had increased contact with dendritic cells surrounding confinement zones. One third of polyclonal medullary thymocytes also exhibited confined, slower migration, and may correspond to auto-reactive thymocytes. Our data suggest that many auto-reactive thymocytes do not undergo immediate arrest and death upon encounter with a negative selecting ligand, but rather adopt an altered migration program while remaining within the medullary microenvironment.

Thymus-homing peripheral dendritic cells constitute two of the three major subsets of dendritic cells in the steady-state thymus

Many dendritic cells (DCs) in the normal mouse thymus are generated intrathymically from common T cell/DC progenitors. However, our previous work suggested that at least 50% of thymic DCs originate independently of these progenitors. We now formally demonstrate by parabiotic, adoptive transfer, and developmental studies that two of the three major subsets of thymic DCs originate extrathymically and continually migrate to the thymus, where they occupy a finite number of microenvironmental niches. The thymus-homing DCs consisted of immature plasmacytoid DCs (pDCs) and the signal regulatory protein α–positive (Sirpα⁺) CD11b⁺ CD8α⁻ subset of conventional DCs (cDCs), both of which could take up and transport circulating antigen to the thymus. The cDCs of intrathymic origin were mostly Sirpα⁻ CD11b⁻ CD8α^hi cells. Upon arrival in the thymus, the migrant pDCs enlarged and up-regulated CD11c, major histocompatibility complex II (MHC II), and CD8α, but maintained their plasmacytoid morphology. In contrast, the migrant cDCs proliferated extensively, up-regulated CD11c, MHC II, and CD86, and expressed dendritic processes. The possible functional implications of these findings are discussed.

Dendritic cell homeostasis

Dendritic cells (DCs) are a heterogeneous fraction of rare hematopoietic cells that coevolved with the formation of the adaptive immune system. DCs efficiently process and present antigen, move from sites of antigen uptake to sites of cellular interactions, and are critical in the initiation of immune responses as well as in the maintenance of self-tolerance. DCs are distributed throughout the body and are enriched in lymphoid organs and environmental contact sites. Steady-state DC half-lives account for days to up to a few weeks, and they need to be replaced via proliferating hematopoietic progenitors, monocytes, or tissue resident cells. In this review, we integrate recent knowledge on DC progenitors, cytokines, and transcription factor usage to an emerging concept of in vivo DC homeostasis in steady-state and inflammatory conditions. We furthermore highlight how knowledge of these maintenance mechanisms might impact on understanding of DC malignancies as well as posttransplant immune reactions and their respective therapies.

Absence of CD47 in vivo influences thymic dendritic cell subset proportions but not negative selection of thymocytes

CD47 is a ubiquitously expressed molecule which has been attributed a role in many cellular processes. Its role in preventing cellular phagocytosis has defined CD47 as an obligatory self-molecule providing a 'don't-eat-me-signal'. Additionally, CD47-CD172a interactions are important for cellular trafficking. Yet, the contribution of CD47 to T cell stimulation remains controversial, acting sometimes as a co-stimulator and sometimes as an inhibitor of TCR signalling or peripheral T cell responses. Most of the experiments leading to this controversy have been carried in in vitro systems. Moreover, the role of CD47 on thymocyte differentiation, which precisely relies on TCR signal strength, has not been evaluated. Here, we examine the in vivo role of CD47 in T cell differentiation using CD47-deficient mice. We find that, in the absence of CD47, thymocyte positive and negative selection processes are not altered. Indeed, our data demonstrate that the absence of CD47 does not influence the strength of TCR signalling in thymocytes. Furthermore, in agreement with a role for CD47-CD172a interactions in CD172a(+) dendritic cell migration, we report a reduced proportion of thymic dendritic cells expressing CD172a in CD47-deficient mice. As the total proportion of dendritic cells is maintained, this creates an imbalance in the proportion of CD172a(+) and CD172a(low) dendritic cells in the thymus. Together, these data indicate that the altered proportion of thymic dendritic cell subsets does not have a primordial influence on thymic selection processes.

TSLP in epithelial cell and dendritic cell cross talk

Dendritic cells (DCs) are professional antigen-presenting cells that have the ability to sense infection and tissue stress, sample and present antigen to T lymphocytes, and instruct the initiation of different forms of immunity and tolerance. The functional versatility of DCs depends on their remarkable ability to translate collectively the information from the invading microbes, as well as their resident tissue microenvironments. Recent progress in understanding Toll-like receptor (TLR) biology has illuminated the mechanisms by which DCs link innate and adaptive antimicrobial immune responses. However, how tissue microenvironments shape the function of DCs has remained elusive. Recent studies of TSLP (thymic stromal lymphopoietin), an epithelial cell-derived cytokine that strongly activates DCs, provide strong evidence at a molecular level that epithelial cells/tissue microenvironments directly communicate with DCs, the professional antigen-presenting cells of the immune system. We review recent progress on how TSLP expressed within thymus and peripheral lymphoid and nonlymphoid tissues regulates DC-mediated central tolerance, peripheral T cell homeostasis, and inflammatory Th2 responses.

Thymocyte-dendritic cell interactions near sources of CCR7 ligands in the thymic cortex

Little is known about the dynamics of the interactions between thymocytes and other cell types, as well as the spatiotemporal distribution of thymocytes during positive selection in the microenvironment of the cortex. We used two-photon laser scanning microscopy of the mouse thymus to visualize thymocytes and dendritic cells (DCs) and to characterize their interactions in the cortex. We show that thymocytes make frequent contacts with DCs in the thymic cortex and that these associations increase when thymocytes express T cell receptors that mediate positive selection. We also show that cortical DCs and the chemokine CCL21 expression are closely associated with capillaries throughout the cortex. The overexpression of the chemokine receptor CCR7 in thymocytes results in an increase in DC-thymocyte interactions, while the loss of CCR7 in the background of a positive-selecting TCR reduces the extent of DC-thymocyte interactions. These observations identify a vasculature-associated microenvironment within the thymic cortex that promotes interactions between DCs and thymocytes that are receiving positive selection signals.

Dendritic cells in the thymus contribute to T-regulatory cell induction

Central tolerance is established through negative selection of self-reactive thymocytes and the induction of T-regulatory cells (T(R)s). The role of thymic dendritic cells (TDCs) in these processes has not been clearly determined. In this study, we demonstrate that in vivo, TDCs not only play a role in negative selection but in the induction of T(R)s. TDCs include two conventional dendritic cell (DC) subtypes, CD8(lo)Sirpalpha(hi/+) (CD8(lo)Sirpalpha(+)) and CD8(hi)Sirpalpha(lo/-) (CD8(hi)Sirpalpha(-)) [corrected] which have different origins. We found that the CD8(hi)Sirpalpha(+) DCs represent a conventional DC subset that originates from the blood and migrates into the thymus. Moreover, we show that the CD8(lo)Sirpalpha(+) DCs demonstrate a superior capacity to induce T(R)s in vitro. Finally, using a thymic transplantation system, we demonstrate that the DCs in the periphery can migrate into the thymus, where they efficiently induce T(R) generation and negative selection.

The impact of circulating dendritic cells on the development and differentiation of thymocytes

Central tolerance is established through the negative selection of self-reactive thymocytes and the induction of T-regulatory cells (T-regs). A role for thymic epithelial cells in mediating both negative selection and T-reg induction has been clearly shown. The role of thymic dendritic cells (DCs) in these processes has not been clearly determined but has been the focus of recent studies. Thymic DCs include two conventional DC (cDC) subtypes, CD8(lo)Sirpalpha(hi/+) (CD8(lo)Sirpalpha(+) herein) and CD8(hi)Sirpalpha(lo/-) (CD8(hi)Sirpalpha(-) herein). It has been shown that these DC subsets have distinct developmental origins, the CD8(hi)Sirpalpha(-) cDCs developing intra-thymically and the CD8(lo)Sirpalpha(+) migrating into the thymus from the periphery. Furthermore, an important role for thymic DCs in the induction of T-regs has been shown. In this review, the role of DCs in the development and education of T cells in the thymus will be reviewed, with emphasis on the role of circulatory DCs in mediating these processes.