Heterogeneity of thymic dendritic cells

Positive and negative selection of the T cell repertoire: what thymocytes see (and don't see)

The fate of developing T cells is specified by the interaction of their antigen receptors with self-peptide-MHC complexes that are displayed by thymic antigen-presenting cells (APCs). Various subsets of thymic APCs are strategically positioned in particular thymic microenvironments and they coordinate the selection of a functional and self-tolerant T cell repertoire. In this Review, we discuss the different strategies that these APCs use to sample and process self antigens and to thereby generate partly unique, 'idiosyncratic' peptide-MHC ligandomes. We discuss how the particular composition of the peptide-MHC ligandomes that are presented by specific APC subsets not only shapes the T cell repertoire in the thymus but may also indelibly imprint the behaviour of mature T cells in the periphery.

Dendritic cell subset distributions in the aorta in healthy and atherosclerotic mice

Dendritic cells (DCs) can be sub-divided into various subsets that play specialized roles in priming of adaptive immune responses. Atherosclerosis is regarded as a chronic inflammatory disease of the vessel wall and DCs can be found in non-inflamed and diseased arteries. We here performed a systematic analyses of DCs subsets during atherogenesis. Our data indicate that distinct DC subsets can be localized in the vessel wall. In C57BL/6 and low density lipoprotein receptor-deficient (Ldlr^−/−) mice, CD11c⁺ MHCII⁺ DCs could be discriminated into CD103⁻ CD11b⁺F4/80⁺, CD11b⁺F4/80⁻ and CD11b⁻F4/80⁻ DCs and CD103⁺ CD11b⁻F4/80⁻ DCs. Except for CD103⁻ CD11b⁻ F4/80⁻ DCs, these subsets expanded in high fat diet-fed Ldlr^−/− mice. Signal-regulatory protein (Sirp)-α was detected on aortic macrophages, CD11b⁺ DCs, and partially on CD103⁻ CD11b⁻ F4/80⁻ but not on CD103⁺ DCs. Notably, in FMS-like tyrosine kinase 3-ligand-deficient (Flt3l^−/−) mice, a specific loss of CD103⁺ DCs but also CD103⁻ CD11b⁺ F4/80⁻ DCs was evidenced. Aortic CD103⁺ and CD11b⁺ F4/80⁻ CD103⁻ DCs may thus belong to conventional rather than monocyte-derived DCs, given their dependence on Flt3L-signalling. CD64, postulated to distinguish macrophages from DCs, could not be detected on DC subsets under physiological conditions, but appeared in a fraction of CD103⁻ CD11b⁺ F4/80⁻ and CD11b⁺ F4/80⁺ cells in atherosclerotic Ldlr^−/− mice. The emergence of CD64 expression in atherosclerosis may indicate that CD11b⁺ F4/80⁻ DCs similar to CD11b⁺ F4/80⁺ DCs are at least in part derived from immigrated monocytes during atherosclerotic lesion formation. Our data advance our knowledge about the presence of distinct DC subsets and their accumulation characteristics in atherosclerosis, and may help to assist in future studies aiming at specific DC-based therapeutic strategies for the treatment of chronic vascular inflammation.

Thymus and aging: morphological, radiological, and functional overview

Aging is a continuous process that induces many alterations in the cytoarchitecture of different organs and systems both in humans and animals. Moreover, it is associated with increased susceptibility to infectious, autoimmune, and neoplastic processes. The thymus is a primary lymphoid organ responsible for the production of immunocompetent T cells and, with aging, it atrophies and declines in functions. Universality of thymic involution in all species possessing thymus, including human, indicates it as a long-standing evolutionary event. Although it is accepted that many factors contribute to age-associated thymic involution, little is known about the mechanisms involved in the process. The exact time point of the initiation is not well defined. To address the issue, we report the exact age of thymus throughout the review so that readers can have a nicely pictured synoptic view of the process. Focusing our attention on the different stages of the development of the thymus gland (natal, postnatal, adult, and old), we describe chronologically the morphological changes of the gland. We report that the thymic morphology and cell types are evolutionarily preserved in several vertebrate species. This finding is important in understanding the similar problems caused by senescence and other diseases. Another point that we considered very important is to indicate the assessment of the thymus through radiological images to highlight its variability in shape, size, and anatomical conformation.

Fms-like tyrosine kinase 3 ligand-dependent dendritic cells in autoimmune inflammation

Dendritic cells (DCs) are specialized in capture, processing and presentation of antigens to T cells. Depending on the type of DC and its activation state, the interaction of DCs with naive T cells can lead to different types of immune response, or to T-cell tolerance. The existence of many specialized subtypes of DCs with particular functions has raised the need to distinguish DCs formed in steady-state from those produced during an inflammatory response. In patients with autoimmune disease and in experimental animal models of autoimmunity, DCs show abnormalities in both numbers and activation state, expressing immunogenic levels of co-stimulatory molecules and pro-inflammatory cytokines. Initial in vitro studies of cytokines in DC development revealed distinct and important roles for the receptor tyrosine kinases, granulocyte-macrophage colony-stimulating factor (GM-CSF), macrophage colony-stimulating factor (M-CSF, also called CSF1) and fms-like tyrosine kinase 3 ligand (Flt3L) in the generation of DCs. Flt3L is critical for instructing DC generation throughout different organs and regulates DC development from Flt3(+) lymphoid and myeloid-committed progenitors to DCs in vivo. The aim of this review is to provide an overview of the role of Flt3L-dependent DCs in the immunopathogenesis of autoimmunity and chronic inflammation and its potential as therapeutic targets.

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.

Isolation of myeloid dendritic cells and epithelial cells from human thymus

In this protocol we provide a method to isolate dendritic cells (DC) and epithelial cells (TEC) from the human thymus. DC and TEC are the major antigen presenting cell (APC) types found in a normal thymus and it is well established that they play distinct roles during thymic selection. These cells are localized in distinct microenvironments in the thymus and each APC type makes up only a minor population of cells. To further understand the biology of these cell types, characterization of these cell populations is highly desirable but due to their low frequency, isolation of any of these cell types requires an efficient and reproducible procedure. This protocol details a method to obtain cells suitable for characterization of diverse cellular properties. Thymic tissue is mechanically disrupted and after different steps of enzymatic digestion, the resulting cell suspension is enriched using a Percoll density centrifugation step. For isolation of myeloid DC (CD11c(+)), cells from the low-density fraction (LDF) are immunoselected by magnetic cell sorting. Enrichment of TEC populations (mTEC, cTEC) is achieved by depletion of hematopoietic (CD45(hi)) cells from the low-density Percoll cell fraction allowing their subsequent isolation via fluorescence activated cell sorting (FACS) using specific cell markers. The isolated cells can be used for different downstream applications.

The dendritic cell lineage: ontogeny and function of dendritic cells and their subsets in the steady state and the inflamed setting

Dendritic cells (DCs) form a remarkable cellular network that shapes adaptive immune responses according to peripheral cues. After four decades of research, we now know that DCs arise from a hematopoietic lineage distinct from other leukocytes, establishing the DC system as a unique hematopoietic branch. Recent work has also established that tissue DCs consist of developmentally and functionally distinct subsets that differentially regulate T lymphocyte function. This review discusses major advances in our understanding of the regulation of DC lineage commitment, differentiation, diversification, and function in situ.

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.

In vivo fate mapping identifies pre-TCRα expression as an intra- and extrathymic, but not prethymic, marker of T lymphopoiesis

A novel pre-TCRα (pTα) reporter mouse reveals that expression of pTα is confined to the T lineage and does not occur on prethymic progenitors.

Expression of the pre–T cell receptor α (pTα) gene has been exploited in previous studies as a molecular marker to identify tiny cell populations in bone marrow (BM) and blood that were suggested to contain physiologically relevant thymus settling progenitors (TSPs). But to what extent these cells genuinely contribute to thymopoiesis has remained obscure. We have generated a novel pTα^iCre knockin mouse line and performed lineage-tracing experiments to precisely quantitate the contribution of pTα-expressing progenitors to distinct differentiation pathways and to the genealogy of mature hematopoietic cells under physiological in vivo conditions. Using these mice in combination with fluorescent reporter strains, we observe highly consistent labeling patterns that identify pTα expression as a faithful molecular marker of T lineage commitment. Specifically, the fate of pTα-expressing progenitors was found to include all αβ and most γδ T cells but, in contrast to previous assumptions, to exclude B, NK, and thymic dendritic cells. Although we could detect small numbers of T cell progenitors with a history of pTα expression in BM and blood, our data clearly exclude these populations as physiologically important precursors of thymopoiesis and indicate that they instead belong to a pathway of T cell maturation previously defined as extrathymic.

Tolerogenic and activatory plasmacytoid dendritic cells in autoimmunity

Plasmacytoid dendritic cells (pDCs) are a particular subset of DCs that link innate and adaptive immunity. They are responsible for the substantial production of type 1 interferon (IFN-I) in response to viral RNA or DNA through activation of TLR7 and 9. Furthermore, pDCs present antigens (Ag) and induce naïve T cell differentiation. It has been demonstrated that pDCs can induce immunogenic T cell responses through differentiation of cytotoxic CD8⁺ T cells and effector CD4⁺ T cells. Conversely, pDCs exhibit strong tolerogenic functions by inducing CD8⁺ T cell deletion, CD4⁺ T cell anergy, and T_reg differentiation. However, since IFN-I produced by pDCs efficiently activates and recruits conventional DCs, B cells, T cells, and NK cells, pDCs also indirectly affect the nature and the amplitude of adaptive immune responses. As a consequence, the precise role of Ag-presenting functions of pDCs in adaptive immunity has been difficult to dissect in vivo. Additionally, different experimental procedures led to conflicting results regarding the outcome of T cell responses induced by pDCs. During the development of autoimmunity, pDCs have been shown to play both immunogenic and tolerogenic functions depending on disease, disease progression, and the experimental conditions. In this review, we will discuss the relative contribution of innate and adaptive pDC functions in modulating T cell responses, particularly during the development of autoimmunity.

Homeostatic properties and phenotypic maturation of murine CD4+ pre-thymic emigrants in the thymus

After a tightly regulated developmental program in the thymus, “mature” single positive (SP) thymocytes leave the thymus and enter the periphery. These newly arrived recent thymic emigrants (RTEs) are phenotypically and functionally immature, and will complete a dynamic maturation in the peripheral lymphoid organs before being licensed to be resident naïve T cells. To study the early events occurring in the RTE maturation process, we identified the phenotype of CD4⁺ pre-RTEs, a population of CD4⁺ SP thymocytes that have acquired the thymus egress capability. Compared to peripheral naïve T cells, CD4⁺ pre-RTEs displayed superior survival capability in lymphoreplete mice and faster proliferation under lymphopenic condition. The differences in Bcl2/Bim expression and/or heightened IL-7 signaling pathway may account for the pre-RTEs’ better responsiveness to homeostatic signals. Qa2, the expression of which indicates the phenotypic maturation of SPs and RTEs, was found to be upregulated in CD4⁺ pre-RTEs in thymic perivascular space. Migratory dendritic cells that surround this region contribute to Qa2 expression in pre-RTEs. The dendritic cell-driven Qa2 induction of CD4⁺ pre-RTEs is independent of MHC class II and Aire molecules.

Analysis of APC types involved in CD4 tolerance and regulatory T cell generation using reaggregated thymic organ cultures

Tolerance to self-Ags is generated in the thymus. Both epithelial and hematopoietic thymic stromal cells play an active and essential role in this process. However, the role of each of the various stromal cell types remains unresolved. To our knowledge, we describe the first comparative analysis of several types of thymic hematopoietic stromal cells (THSCs) for their ability to induce CD4 tolerance to self, in parallel with the thymic epithelium. The THSCs--two types of conventional dendritic cells (cDCs), plasmacytoid dendritic cells, macrophages (MΦs), B lymphocytes, and eosinophils--were first characterized and quantified in adult mouse thymus. They were then examined in reaggregated thymic organ cultures containing mixtures of monoclonal and polyclonal thymocytes. This thymocyte mixture allows for the analysis of Ag-specific events while avoiding the extreme skewing frequently seen in purely monoclonal systems. Our data indicate that thymic epithelium alone is capable of promoting self-tolerance by eliminating autoreactive CD4 single-positive thymocytes and by supporting regulatory T cell (Treg) development. We also show that both non-Treg CD4 single-positive thymocytes and Tregs are efficiently deleted by the two populations of cDCs present in the thymus, as well as to a lesser extent by MΦs. Plasmacytoid dendritic cells, B lymphocytes, and eosinophils were not able to do so. Finally, cDCs were also the most efficient THSCs at supporting Treg development in the thymus, suggesting that although they may share some characteristics required for negative selection with MΦs, they do not share those required for the support of Treg development, making cDCs a unique cell subset in the thymus.

Metallophilic macrophages of the rodent thymus

For a very long time, we studied the metallophilic macrophages of the rodent thymus and in this review our results on morphological, histochemical, enzymehistochemical, immunohistochemical, ultrastructural and functional features of these cells, as well as the molecular regulation of their development, will be presented. Furthermore, the differences between species will also be discussed and the comparisons with similar/related cell types (metallophilic macrophages in the marginal sinus of the spleen, subcapsular sinus of the lymph nodes and germinal centers of secondary lymphoid follicles) will be made. Metallophilic macrophages are strategically positioned in the thymic cortico-medullary zone and are very likely to be involved in: (i) the metabolism, synthesis and production of bioactive lipids, most likely arachidonic acid metabolites, based on their histochemical and enzymehistochemical features, and (ii) the process of negative selection that occurs in the thymus, based on their ultrastructural features and their reactivity after the application of toxic or immunosuppressive/immunomodulatory agents. Taken together, their phenotypic and functional features strongly suggest that metallophilic macrophages play a significant role in the thymic physiology.

Rapid dendritic cell activation and resistance to allotolerance induction in anti-CD154-treated mice receiving CD47-deficient donor-specific transfusion

CD47-SIRPα signaling plays an important role in regulating macrophage and dendritic cell (DC) activation. Here we investigated the role of CD47 expression on donor cells in tolerance induction by combined treatment with donor-specific transfusion (DST) plus anti-CD154 mAb in a mouse model of fully MHC-mismatched heart allotransplantation. The majority of BALB/c recipient mice that received anti-CD154 and CD47(+/+) B6 splenocytes (DST) showed indefinite donor heart survival (median survival time, MST > 150 days). Donor heart survival was improved in anti-CD154-treated BALB/c mice that received CD47(+/-) (MST = 90 days) or CD47(-/-) B6 DST (MST = 42 days) when compared to the nontreated (MST = 7 days) and anti-CD154 alone-treated (MST = 15 days) controls, but significantly reduced when compared to mice receiving anti-CD154 plus CD47(+/+) B6 DST. Recipient mice treated with anti-CD154 plus CD47(-/-) or CD47(+/-) DST also showed significantly increased antidonor, but not anti-third-party, MLR responses compared to those receiving anti-CD154 and CD47(+/+) DST. Furthermore, CD47(-/-) DST induced rapid activation of CD11c(hi)SIRPα(hi)CD8α(-) DCs via a mechanism independent of donor alloantigens. These results demonstrated that CD47 expression on donor cells is essential to the success of tolerance induction by combined therapy with DST and CD40/CD154 blockade.

Novel process of intrathymic tumor-immune tolerance through CCR2-mediated recruitment of Sirpα+ dendritic cells: a murine model

Immune surveillance system can detect more efficiently secretory tumor-specific antigens, which are superior as a target for cancer immunotherapy. On the contrary, immune tolerance can be induced in the thymus when a tumor antigen is massively secreted into circulation. Thus, the secretion of tumor-specific antigen may have contradictory roles in tumor immunity in a context-dependent manner. However, it remains elusive on the precise cellular mechanism of intrathymic immune tolerance against tumor antigens. We previously demonstrated that a minor thymic conventional dendritic cell (cDC) subset, CD8α⁻Sirpα⁺ cDCs, but not the major subset, CD8α⁺Sirpα⁻ cDCs can selectively capture blood-borne antigens and crucially contribute to the self-tolerance. In the present study, we further demonstrated that Sirpα⁺ cDCs can capture a blood-borne antigen leaking inside the interlobular vascular-rich regions (IVRs). Blood-borne antigen selectively captured by Sirpα⁺ cDCs can induce antigen-specific Treg generation or negative selection, depending on the immunogenicity of the presented antigen. Furthermore, CCR2 expression by thymic Sirpα⁺ cDCs and abundant expression of its ligands, particularly, CCL2 by tumor-bearing mice prompted us to examine the function of thymic Sirpα⁺ cDCs in tumor-bearing mice. Interestingly, tumor-bearing mice deposited CCL2 inside IVRs in the thymus. Moreover, tumor formation induced the accumulation of Sirpα⁺ cDCs in IVRs under the control of CCR2-CCL2 axis and enhanced their capacity to take up antigens, resulting in the shift from Treg differentiation to negative selection. Finally, intrathymic negative selection similarly ensued in CCR2-competent mice once the tumor-specific antigen was secreted into bloodstream. Thus, we demonstrated that thymic Sirpα⁺ cDCs crucially contribute to this novel process of intrathymic tumor immune tolerance.

Layers of dendritic cell-mediated T cell tolerance, their regulation and the prevention of autoimmunity

The last decades of Nobel prize-honored research have unequivocally proven a key role of dendritic cells (DCs) at controlling both T cell immunity and tolerance. A tight balance between these opposing DC functions ensures immune homeostasis and host integrity. Its perturbation could explain pathological conditions such as the attack of self tissues, chronic infections, and tumor immune evasion. While recent insights into the complex DC network help to understand the contribution of individual DC subsets to immunity, the tolerogenic functions of DCs only begin to emerge. As these consist of many different layers, the definition of a “tolerogenic DC” is subjected to variation. Moreover, the implication of DCs and DC subsets in the suppression of autoimmunity are incompletely resolved. In this review, we point out conceptual controversies and dissect the various layers of DC-mediated T cell tolerance. These layers include central tolerance, Foxp3⁺ regulatory T cells (Tregs), anergy/deletion and negative feedback regulation. The mode and kinetics of antigen presentation is highlighted as an additional factor shaping tolerance. Special emphasis is given to the interaction between layers of tolerance as well as their differential regulation during inflammation. Furthermore, potential technical caveats of DC depletion models are considered. Finally, we summarize our current understanding of DC-mediated tolerance and its role for the suppression of autoimmunity. Understanding the mechanisms of DC-mediated tolerance and their complex interplay is fundamental for the development of selective therapeutic strategies, e.g., for the modulation of autoimmune responses or for the immunotherapy of cancer.

Functional crosstalk between dendritic cells and Foxp3(+) regulatory T cells in the maintenance of immune tolerance

Peripheral immune tolerance requires a controlled balance between the maintenance of self-tolerance and the capacity to engage protective immune responses against pathogens. Dendritic cells (DCs) serve as sentinels of the immune system by sensing environmental and inflammatory signals, and play an essential role in the maintenance of immune tolerance. To achieve this, DC play a key role in dictating the outcome of immune responses by influencing the balance between inflammatory or Foxp3(+) regulatory T (T(reg)) cell responses. At the heart of this immunological balance is a finely regulated DC and T(reg) cell crosstalk whereby T(reg) cells modulate DC phenotype and function, and DC drive the differentiation of Foxp3(+) T(reg) cells in order to control immune responses. This review will focus on recent advances, which highlight the importance of this bidirectional DC and T(reg) cell crosstalk during the induction of tolerance and organ-specific autoimmunity. More specifically, we will discuss how T(reg) cells modulate DC function for the suppression of inflammatory responses and how DC subsets employ diverse mechanisms to drive differentiation of T(reg) cells. Finally, we will discuss the therapeutic potential of tolerogenic DCs for the induction of tolerance in autoimmune diseases.

T-cell tolerance: central and peripheral

Somatic recombination of TCR genes in immature thymocytes results in some cells with useful TCR specificities, but also many with useless or potentially self-reactive specificities. Thus thymic selection mechanisms operate to shape the T-cell repertoire. Thymocytes that have a TCR with low affinity for self-peptide-MHC complexes are positively selected to further differentiate and function in adaptive immunity, whereas useless ones die by neglect. Clonal deletion and clonal diversion (Treg differentiation) are the major processes in the thymus that eliminate or control self-reactive T cells. Although these processes are thought to be efficient, they fail to control self-reactivity in all circumstances. Thus, peripheral tolerance processes exist wherein self-reactive T cells become functionally unresponsive (anergy) or are deleted after encountering self-antigens outside of the thymus. Recent advances in mechanistic studies of central and peripheral T-cell tolerance are promoting the development of therapeutic strategies to treat autoimmune disease and cancer and improve transplantation outcome.

Dll4-Notch signaling in Flt3-independent dendritic cell development and autoimmunity in mice

Delta-like ligand 4 (Dll4)-Notch signaling is essential for T cell development and alternative thymic lineage decisions. How Dll4-Notch signaling affects pro-T cell fate and thymic dendritic cell (tDC) development is unknown. We found that Dll4 pharmacological blockade induces accumulation of tDCs and CD4(+)CD25(+)FoxP3(+) regulatory T cells (T(reg) cells) in the thymic cortex. Both genetic inactivation models and anti-Dll4 antibody (Ab) treatment promote de novo natural T(reg) cell expansion by a DC-dependent mechanism that requires major histocompatibility complex II expression on DCs. Anti-Dll4 treatment converts CD4(-)CD8(-)c-kit(+)CD44(+)CD25(-) (DN1) T cell progenitors to immature DCs that induce ex vivo differentiation of naive CD4(+) T cells into T(reg) cells. Induction of these tolerogenic DN1-derived tDCs and the ensuing expansion of T(reg) cells are Fms-like tyrosine kinase 3 (Flt3) independent, occur in the context of transcriptional up-regulation of PU.1, Irf-4, Irf-8, and CSF-1, genes critical for DC differentiation, and are abrogated in thymectomized mice. Anti-Dll4 treatment fully prevents type 1 diabetes (T1D) via a T(reg) cell-mediated mechanism and inhibits CD8(+) T cell pancreatic islet infiltration. Furthermore, a single injection of anti-Dll4 Ab reverses established T1D. Disease remission and recurrence are correlated with increased T(reg) cell numbers in the pancreas-draining lymph nodes. These results identify Dll4-Notch as a novel Flt3-alternative pathway important for regulating tDC-mediated T(reg) cell homeostasis and autoimmunity.

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.

Cathepsin S dominates autoantigen processing in human thymic dendritic cells

The interaction of developing thymocytes with peptide-MHC complexes on thymic antigen presenting cells (APC) is crucial for T cell development, both for positive selection of "useful" thymocytes as well as negative selection of autoreactive thymocytes to prevent autoimmunity. The peptides presented on MHC II molecules are generated by lysosomal proteases such as the cathepsins. At the same time, lysosomal proteases will also destroy other potential T cell epitopes from self-antigens. This will lead to a lack of presentation on negatively selecting thymic antigen presenting cells and consequently, escape of autoreactive T cells recognizing these epitopes. In order to understand the processes that govern generation or destruction of self-epitopes in thymic APC, we studied the antigen processing machinery and epitope processing in the human thymus. We find that each type of thymic APC expresses a different signature of lysosomal proteases, providing indirect evidence that positive and negative selection of CD4(+) T cells might occur on different sets of peptides, in analogy to what has been proposed for CD8(+) T cells. We also find that myeloid dendritic cells (DC) are more efficient in processing autoantigen than plasmacytoid DC. In addition, we observed that cathepsin S plays a central role in processing of the autoantigens myelin basic protein and proinsulin in thymic dendritic cells. Cathepsin S destroyed a number of known T cell epitopes, which would be expected to result in lack of presentation and consequently, escape of autoreactive T cells. Cathepsin S therefore appears to be an important factor that influences selection of autoreactive T cells.

Dendritic cells and regulation of graft-versus-host disease and graft-versus-leukemia activity

Hematopoietic stem cell transplantation is the only curative treatment for many malignant hematologic diseases, with an often critical graft-versus-leukemia effect. Despite peritransplant prophylaxis, GVHD remains a significant cause of posthematopoietic stem cell transplantation morbidity and mortality. Traditional therapies have targeted T cells, yet immunostimulatory dendritic cells (DCs) are critical in the pathogenesis of GVHD. Furthermore, DCs also have tolerogenic properties. Monitoring of DC characteristics may be predictive of outcome, and therapies that target DCs are innovative and promising. DCs may be targeted in vivo or tolerogenic (tol) DCs may be generated in vitro and given in the peritransplant period. Other cellular therapies, notably regulatory T cells (T(reg)) and mesenchymal stem cells, mediate important effects through DCs and show promise for the prevention and treatment of GVHD in early human studies. Therapies are likely to be more effective if they have synergistic effects or target both DCs and T cells in vivo, such as tolDCs or T(reg). Given the effectiveness of tolDCs in experimental models of GVHD and their safety in early human studies for type 1 diabetes, it is crucial that tolDCs be investigated in the prevention and treatment of human GVHD while ensuring conservation of graft-versus-leukemia effects.

TSLP and immune homeostasis

In an immune system, dendritic cells (DCs) are professional antigen-presenting cells (APCs) as well as powerful sensors of danger signals. When DCs receive signals from infection and tissue stress, they immediately activate and instruct the initiation of appropriate immune responses to T cells. However, it has remained unclear how the tissue microenvironment in a steady state shapes the function of DCs. Recent many works on thymic stromal lymphopoietin (TSLP), an epithelial cell-derived cytokine that has the strong ability to activate DCs, provide evidence that TSLP mediates crosstalk between epithelial cells and DCs, involving in DC-mediated immune homeostasis. Here, we review recent progress made on how TSLP expressed within the thymus and peripheral lymphoid and non-lymphoid tissues regulates DC-mediated T-cell development in the thymus and T-cell homeostasis in the periphery.

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.

Differential processing of self-antigens by subsets of thymic stromal cells

The stromal network of the thymus provides a unique environment that supports the development of mature CD4(+) and CD8(+) T cells expressing a very diverse repertoire of T cell receptors (TCR) with limited reactivity to self-antigens. Thymic cortical epithelial cells (cTECs) are specialized antigen-presenting cells (APCs) that promote the positive selection of developing thymocytes while medullary thymic epithelial cells (mTECs) and thymic dendritic cells (tDCs) induce central tolerance to self-antigens. Recent studies showed that cTECs express a unique set of proteases involved in the generation of self-peptides presented by major-histocompatibility encoded molecules (pMHC) and consequently may express a unique set of pMHC complexes. Conversely, the stromal cells of the medulla developed several mechanisms to mirror as closely as possible the constellation of self-peptides derived from peripheral tissues. Here, we discuss how these different features allow for the development of a highly diverse but poorly self-reactive repertoire of functional T cells.

Hematopoietic chimerism and transplantation tolerance: a role for regulatory T cells

The immunosuppressive regimens currently used in transplantation to prevent allograft destruction by the host's immune system have deleterious side effects and fail to control chronic rejection processes. Induction of donor-specific non-responsiveness (i.e., immunological tolerance) to transplants would solve these problems and would substantially ameliorate patients' quality of life. It has been proposed that bone marrow or hematopoietic stem-cell transplantation, and resulting (mixed) hematopoietic chimerism, lead to immunological tolerance to organs of the same donor. However, a careful analysis of the literature, performed here, clearly establishes that whereas hematopoietic chimerism substantially prolongs allograft survival, it does not systematically prevent chronic rejection. Moreover, the cytotoxic conditioning regimens used to achieve long-term persistence of chimerism are associated with severe side effects that appear incompatible with a routine use in the clinic. Several laboratories recently embarked on different studies to develop alternative strategies to overcome these issues. We discuss here recent advances obtained by combining regulatory T cell infusion with bone-marrow transplantation. In experimental settings, this attractive approach allows development of genuine immunological tolerance to donor tissues using clinically relevant conditioning regimens.

Dendritic cell functional improvement in a preclinical model of lentiviral-mediated gene therapy for Wiskott-Aldrich syndrome

Wiskott-Aldrich syndrome (WAS) is a rare X-linked primary immunodeficiency caused by the defective expression of the WAS protein (WASP) in hematopoietic cells. It has been shown that dendritic cells (DCs) are functionally impaired in WAS patients and was^−/− mice. We have previously demonstrated the efficacy and safety of a murine model of WAS gene therapy (GT), using stem cells transduced with a lentiviral vector. The aim of this study was to investigate whether GT can correct DC defects in was^−/− mice. As DCs expressing WASP were detected in the secondary lymphoid organs of the treated mice, we tested the in vitro and in vivo function of bone marrow-derived DCs (BMDCs). The BMDCs showed efficient in vitro uptake of latex beads and Salmonella typhimurium. When BMDCs from the treated mice (GT BMDCs) and the was^−/− mice were injected into wild type hosts, we found a higher number of cells that had migrated to the draining lymph nodes compared to mice injected with was^−/− BMDCs. Finally, we found that OVA-pulsed GT BMDCs or vaccination with anti-DEC205 OVA fusion protein can efficiently induce antigen-specific T cell activation in vivo. These findings show that WAS GT significantly improves DC function, thus adding new evidence of the preclinical efficacy of lentiviral vector-mediated WAS GT.

Intranodal interaction with dendritic cells dynamically regulates surface expression of the co-stimulatory receptor CD226 protein on murine T cells

Dendritic cells (DCs) are the most potent antigen-presenting cells of the immune system. Depending on their maturation status, they prime T cells to induce adaptive immunity or tolerance. DCs express CD155, an immunoglobulin-like receptor binding CD226 present on T and natural killer (NK) cells. CD226 represents an important co-stimulator during T cell priming but also serves as an activating receptor on cytotoxic T and NK cells. Here, we report that cells of the T and NK cell lineage of CD155(-/-) mice express markedly elevated protein levels of CD226 compared with wild type (WT). On heterozygous CD155(+/-) T cells, CD226 up-regulation is half-maximal, implying an inverse gene-dosis effect. Moreover, CD226 up-regulation is independent of antigen-driven activation because it occurs already in thymocytes and naïve peripheral T cells. In vivo, neutralizing anti-CD155 antibody elicits up-regulation of CD226 on T cells demonstrating, that the observed modulation can be triggered by interrupting CD155-CD226 contacts. Adoptive transfers of WT or CD155(-/-) T cells into CD155(-/-) or WT recipients, respectively, revealed that CD226 modulation is accomplished in trans. Analysis of bone marrow chimeras showed that regulators in trans are of hematopoietic origin. We demonstrate that DCs are capable of manipulating CD226 levels on T cells in vivo but not in vitro, suggesting that the process of T cells actively scanning antigen-presenting DCs inside secondary lymphoid organs is required for CD226 modulation. Hence, a CD226 level divergent from WT may be exploited as a sensor to detect abnormal DC/T-cell cross-talk as illustrated for T cells in mice lacking CCR7.

NIK signaling in dendritic cells but not in T cells is required for the development of effector T cells and cell-mediated immune responses

The canonical NF-κB pathway is a driving force for virtually all aspects of inflammation. Conversely, the role of the noncanonical NF-κB pathway and its central mediator NF-κB-inducing kinase (NIK) remains poorly defined. NIK has been proposed to be involved in the formation of T(H)17 cells, and its absence in T(H) cells renders them incapable of inducing autoimmune responses, suggesting a T cell-intrinsic role for NIK. Upon systematic analysis of NIK function in cell-mediated immunity, we found that NIK signaling is dispensable within CD4(+) T cells but played a pivotal role in dendritic cells (DCs). We discovered that NIK signaling is required in DCs to deliver co-stimulatory signals to CD4(+) T cells and that DC-restricted expression of NIK is sufficient to restore T(H)1 and T(H)17 responses as well as cell-mediated immunity in NIK(-/-) mice. When CD4(+) T cells developed in the absence of NIK-sufficient DCs, they were rendered anergic. Reintroduction of NIK into DCs allowed developing NIK(-/-) CD4(+) T cells to become functional effector populations and restored the development of autoimmune disease. Therefore, our data suggest that a population of thymic DCs requires NIK to shape the formation of most αβ CD4(+) T effector lineages during early development.

The earliest intrathymic precursors of CD8α(+) thymic dendritic cells correspond to myeloid-type double-negative 1c cells

The dendritic cells (DCs) present in lymphoid and non-lymphoid organs are generated from progenitors with myeloid-restricted potential. However, in the thymus a major subset of DCs expressing CD8α and langerin (CD207) appears to stand apart from all other DCs in that it is thought to derive from progenitors with lymphoid potential. Using mice expressing a fluorescent reporter and a diphtheria toxin receptor under the control of the cd207 gene, we demonstrated that CD207(+) CD8α(+) thymic DCs do not share a common origin with T cells but originate from intrathymic precursors that express markers that are normally present on all (CD11c(+) and MHCII molecules) or on some (CD207, CD135, CD8α, CX3CR1) DC subsets. Those intrathymic myeloid-type precursors correspond to CD44(+) CD25(-) double-negative 1c (DN1c) cells and are continuously renewed from bone marrow-derived canonical DC precursors. In conclusion, our results demonstrate that the earliest intrathymic precursors of CD8α(+) thymic DCs correspond to myeloid-type DN1c cells and support the view that under physiological conditions myeloid-restricted progenitors generate the whole constellation of DCs present in the body including the thymus.

Autonomous versus dendritic cell-dependent contributions of medullary thymic epithelial cells to central tolerance

Promiscuous expression of 'peripheral' tissue-restricted antigens (TRAs) by medullary thymic epithelial cells (mTECs) is essential for central tolerance. Remarkably, the expression of individual TRAs varies among mTECs and is confined to a perplexingly small number of cells. To reconcile this with the ensuing robust state of tolerance, one might envisage that mTECs serve primarily as an antigen reservoir, whereas tolerogenic recognition of TRAs would ultimately require antigen uptake and presentation by dendritic cells (DCs). Here, we survey the evidence for this 'antigen-spreading' scenario and relate it to findings that document autonomous antigen-presentation by mTECs. We suggest that DC-dependent and autonomous tolerogenic functions of mTECs operate in parallel, and the underlying mechanisms remain to be established.

Aire-dependent production of XCL1 mediates medullary accumulation of thymic dendritic cells and contributes to regulatory T cell development

Dendritic cells (DCs) in the thymus (tDCs) are predominantly accumulated in the medulla and contribute to the establishment of self-tolerance. However, how the medullary accumulation of tDCs is regulated and involved in self-tolerance is unclear. We show that the chemokine receptor XCR1 is expressed by tDCs, whereas medullary thymic epithelial cells (mTECs) express the ligand XCL1. XCL1-deficient mice are defective in the medullary accumulation of tDCs and the thymic generation of naturally occurring regulatory T cells (nT reg cells). Thymocytes from XCL1-deficient mice elicit dacryoadenitis in nude mice. mTEC expression of XCL1, tDC medullary accumulation, and nT reg cell generation are diminished in Aire-deficient mice. These results indicate that the XCL1-mediated medullary accumulation of tDCs contributes to nT reg cell development and is regulated by Aire.

Cytokine crosstalk for thymic medulla formation

The medullary microenvironment of the thymus plays a crucial role in the establishment of self-tolerance through the deletion of self-reactive thymocytes and the generation of regulatory T cells. Crosstalk or bidirectional signal exchanges between developing thymocytes and medullary thymic epithelial cells (mTECs) contribute to the formation of the thymic medulla. Recent studies have identified the molecules that mediate thymic crosstalk. Tumor necrosis factor superfamily cytokines, including RANKL, CD40L, and lymphotoxin, produced by positively selected thymocytes and lymphoid tissue inducer cells promote the proliferation and differentiation of mTECs. In return, CCR7 ligand chemokines produced by mTECs facilitate the migration of positively selected thymocytes to the medulla. The cytokine crosstalk between developing thymocytes and mTECs nurtures the formation of the thymic medulla and thereby regulates the establishment of self-tolerance.

Functional redundancy between thymic CD8α+ and Sirpα+ conventional dendritic cells in presentation of blood-derived lysozyme by MHC class II proteins

We evaluated the presentation of blood-derived protein Ags by APCs in the thymus. Two conventional dendritic cells (cDCs), the CD8α(+)Sirpα(-)CD11c(hi) (CD8α(+) cDC) and the CD8α(-)Sirpα(+)CD11c(hi) (Sirpα(+) cDC), were previously identified as presenting MHC class II bound peptides from hen egg white lysozyme (HEL) injected intravenously. All thymic APCs acquired the injected HEL, with the plasmacytoid dendritic cell being the best, followed by the Sirpα(+) cDC and the CD8α(+) cDC. Both cDCs induced to similar extent negative selection and regulatory T cells in HEL TCR transgenic mice, indicating a redundant role of the two cDC subsets in the presentation of blood-borne HEL. Immature dendritic cells or plasmacytoid dendritic cells were considerably less efficient. Batf3(-/-) mice, with significantly reduced numbers of CD8α(+) cDCs, were not impaired in HEL presentation by I-A(k) molecules of thymic APCs. Lastly, clodronate liposome treatment of TCR transgenic mice depleted blood APCs including Sirpα(+) cDCs without affecting the number of thymic APCs. In such treated mice, there was no effect on negative selection or regulatory T cells in mice when administering HEL, indicating that the T cell responses were mediated primarily by the cDCs localized in the thymus.

Regulatory T-cell differentiation versus clonal deletion of autoreactive thymocytes

The concept of clonal deletion of immune cells that carry an autoreactive antigen receptor was a central prediction of Burnet's clonal selection theory. A series of classical experiments in the late 1980s revealed that certain immature thymocytes upon encounter of 'self' are indeed removed from the T-cell repertoire before their release into the blood circulation. A second essential cornerstone of immunological tolerance, not anticipated by Burnett, has more recently surfaced through the discovery of Foxp3(+) regulatory T cells (Treg). Intriguingly, it appears that the expression of an autoreactive T-cell receptor is a shared characteristic of T cells that are subject to clonal deletion as well as of those deviated into the Treg lineage. This is all the more striking as Treg differentiation for the most part branches off from mainstream CD4T cell development during thymocyte maturation in the thymus, that is, it may neither temporally nor spatially be separated from clonal deletion. This raises the question of how an apparently identical stimulus, namely the encounter of 'self' during thymocyte development, can elicit fundamentally different outcomes such as apoptotic cell death on the one hand or differentiation into a highly specialized T-cell lineage on the other hand. Here, we will review the T-cell intrinsic and extrinsic factors that have been implicated in intrathymic Treg differentiation and discuss how these parameters may determine whether an autoreactive major histocompatibility complex class II-restricted thymocyte is deviated into the Treg lineage or subject to clonal deletion.

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.

A follicular dendritic cell sarcoma of the mediastinum with immature T cells and association with myasthenia gravis

Follicular dendritic cell (FDC) sarcoma is a very rare neoplasm showing morphologic and phenotypic features of FDCs. It occurs primarily in lymph nodes but also in extranodal sites. So far, there have been no reports on FDC sarcoma associated with myasthenia gravis. In the following we will present a case of an FDC tumor of the mediastinum associated with paraneoplastic myasthenia gravis in a 39-year-old man. The tumor contained a major proportion of immature T cells, which may be connected to this patient's very unusual clinical presentation with autoimmune phenomena. Extranodal FDC sarcomas still seem hardly noticed, and their clinical and pathologic characteristics remain to be better defined.

Autonomous role of medullary thymic epithelial cells in central CD4(+) T cell tolerance

Medullary thymic epithelial cells (mTECs) serve an essential function in central tolerance by expressing peripheral-tissue antigens. These antigens may be transferred to and presented by dendritic cells (DCs). Therefore, it is unclear whether mTECs, in addition to being an antigen reservoir, also serve a mandatory function as antigen-presenting cells. Here we diminished major histocompatibility complex (MHC) class II on mTECs through transgenic expression of a 'designer' microRNA specific for the MHC class II transactivator CIITA (called 'C2TA' here). This resulted in an enlarged polyclonal CD4(+) single-positive compartment and, among thymocytes specific for model antigens expressed in mTECs, enhanced selection of regulatory T cells (T(reg) cells) at the expense of deletion. Our data document an autonomous contribution of mTECs to both dominant and recessive mechanisms of CD4(+) T cell tolerance and support an avidity model of T(reg) cell development versus deletion.

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.

Thymus-blood protein interactions are highly effective in negative selection and regulatory T cell induction

Using hen egg-white lysozyme, the effect of blood proteins on CD4 thymic cells was examined. A small fraction of i.v. injected hen egg-white lysozyme rapidly entered the thymus into the medulla. There it was captured and presented by dendritic cells (DCs) to thymocytes from two TCR transgenic mice, one directed to a dominant peptide and a second to a poorly displayed peptide, both presented by MHC class II molecules I-A(k). Presentation by DC led to negative selection and induction of regulatory T cells, independent of epithelial cells. Presentation took place at very low levels, less than 100 peptide-MHC complexes per DC. Such low levels could induce negative selection, but even lower levels could induce regulatory T cells. The anatomy of the thymus-blood barrier, the highly efficient presentation by DC, together with the high sensitivity of thymic T cells to peptide-MHC complexes, results in blood protein Ags having a profound effect on thymic T cells.

Development of monocytes, macrophages, and dendritic cells

Monocytes and macrophages are critical effectors and regulators of inflammation and the innate immune response, the immediate arm of the immune system. Dendritic cells initiate and regulate the highly pathogen-specific adaptive immune responses and are central to the development of immunologic memory and tolerance. Recent in vivo experimental approaches in the mouse have unveiled new aspects of the developmental and lineage relationships among these cell populations. Despite this, the origin and differentiation cues for many tissue macrophages, monocytes, and dendritic cell subsets in mice, and the corresponding cell populations in humans, remain to be elucidated.

Regeneration of dendritic cells in aged mice

Age-related thymic involution causes a decreased output of thymocytes from the thymus, thereby resulting in impairment of T cell-mediated immunity. While alterations in the T cell and non-haematopoietic stromal compartments have been described, the effects of thymic involution on thymic dendritic cells (DC) are not clearly known. Thymic DC play an essential role in shaping T cell-mediated immune responses by deleting self-reactive thymocytes to establish central tolerance and by inducing regulatory T-cell (Treg) development. It is therefore important to assess the prevalence of and alterations to thymic DC with age, as this may impact on their function. We assessed the numbers and proportions of the three distinct subsets of thymic DC in ageing mice, and showed that these subsets are differentially regulated. This is expected as thymic DC subsets have different origins of development. We further assessed the responses of thymic DC in a regenerative environment, such as that induced by sex-steroid ablation (SSA), and clearly showed that, consistent with global thymus regrowth, all three DC populations increased in numbers and regained their relative proportions to thymocytes after an initial lag period. These findings are important for the clinical translation of thymic regenerative approaches, and indicate that SSA facilitates the maintenance of critical processes such as negative selection and Treg induction through promoting thymic DC regeneration.

Crucial contribution of thymic Sirp alpha+ conventional dendritic cells to central tolerance against blood-borne antigens in a CCR2-dependent manner

Thymic dendritic cells (DCs) as well as thymic epithelial cells are presumed to be major sentinels in central tolerance by inducing the apoptosis of autoreactive T progenitor cells. The thymic DC population is composed of heterogeneous subsets including CD11c(+)B220(+) plasmacytoid DCs, CD11c(+)B220(-)CD8alpha(+) signal regulatory protein alpha (Sirpalpha)(-) and CD11c(+)B220(-)CD8alpha(-)Sirpalpha(+) conventional DCs (cDCs). However, the distinctive role of each DC subset remains undefined. We show herein that Sirpalpha(+) cDCs, a minor subpopulation, was disseminated in the thymic cortical area with some of them uniquely localized inside perivascular regions and nearby small vessels in the thymus. The Sirpalpha(+) but not Sirpalpha(-) cDC subset can selectively capture blood-circulating Ags. Moreover, in CCR2-deficient mice, the thymic Sirpalpha(+) cDC subset, but not other thymic cell components, was moderately decreased especially in the perivascular regions. Concomitantly, these mice exhibited a modest impairment in intrathymic negative selection against blood-borne Ags, with the reduced capacity to uptake blood-borne Ags. Given their intrathymic cortical localization, CD11c(+)B220(-)CD8alpha(-)Sirpalpha(+) cDCs can have a unique role in the development of central tolerance against circulating peripheral Ags, at least partially in a CCR2-dependent manner.

Learning to be tolerant: how T cells keep out of trouble

A pool of immature T cells with a seemingly unrestricted repertoire of antigen specificities is generated life-long in the thymus. Amongst these cells are, however, thymocytes that express a strongly self-reactive antigen receptor and hence hold the potential to trigger autoimmunity. To prevent such an outcome, the thymus employs several independent but functionally related strategies that act in parallel to enforce self-tolerance. The deletion of strongly self-reactive thymocytes and the generation of regulatory T cells constitute the two most efficient mechanisms to induce and maintain immunological tolerance. Thymic epithelial cells of the medulla express for this purpose tissue-restricted self-antigens. This review will focus on the cellular and molecular mechanisms operative in the thymus to shape a repertoire of mature T cells tolerant to self-antigens.

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.