Introduction

Dendritic cells

Dendritic cells (DCs) represent a rare population of bone marrow (BM)-derived cells. Their high capacity to present antigens through major histocompatibility complex (MHC) class I molecules to CD8⁺ T cells and MHC class II molecules to CD4⁺ T cells allows them not only to initiate an immune response to exogenous pathogens but also to induce tolerance to self-antigens.¹

DCs are found resident within both primary and secondary lymphoid organs. Tolerance to self-antigens is primarily initiated within the thymus, where T cells develop and differentiate.^{2, 3} Immunity to exogenous pathogens is initiated within the secondary lymphoid tissues, including the spleen and the lymph nodes. The localization of DCs within these organs allows crucial interactions between the antigen-presenting DCs and the T cells to occur. The resulting responses are a combination of direct cell–cell interactions and indirect cell–cytokine interactions between the DCs and the T cells. The nature of these interactions tailors the T-cell response in a way that allows the most appropriate response to be initiated. Furthermore, the heterogeneous nature of the DC population both with respect to surface phenotype and function endows the immune system with a capacity to initiate a wide variety of immune responses.^{4, 5, 6, 7} This system of DC subpopulations has no doubt evolved concurrently with the need to guard us against a diverse array of pathogens, which are also constantly evolving.

T-cell development

T-cell development is a complex process that occurs within the thymus. Hematopoietic precursors seed the thymus and are triggered by a combination of signals provided by the thymic environment to undergo a series of differentiation steps. These include rearrangement and expression of genes encoding the T-cell receptors (TCR), and expressions of both co-receptors CD4⁺ and CD8⁺. TCR gene rearrangement is a random event that allows the generation of a diverse repertoire of TCR specificities capable of recognizing a wide range of antigens and initiating an immune response. Thus, it is essential that the immature thymocytes undergo stringent selection processes to eliminate any self-reactive T cells to prevent autoimmunity. During the progression from an immature CD4⁺8⁺ double-positive thymocyte to a mature CD4⁺ or CD8⁺ T cell, presentation of peptide on MHC class I or MHC class II molecules to the thymocytes results in two developmental fates: (i) positive selection, primarily within the cortex, occurs if the TCR recognizes self-peptide/MHC complexes with a low-intermediate level of avidity, leading to selective survival of these self-MHC-restricted T cells,^{8, 9} and (ii) negative selection, primarily within the medulla, occurs when T cells have a TCR that recognizes self-peptide/MHC complexes with high affinity, leading to the deletion of these T cells by apoptosis, which prevents autoimmune dysfunction.^{10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20} However, some autoreactive thymocytes elude this selection and escape into the periphery. In higher organisms, other non-redundant mechanisms have emerged that counteract this 'leakiness' and control autoreactive T cells. One mechanism of control is through a dedicated lineage of naturally occurring regulatory T cells (T-regs) that act by suppressing the proliferation of naive, self-reactive T cells.^{21, 22, 23, 24} Another mechanism is the deletion or induction of anergy of self-reactive T cells in the periphery, in a process of peripheral tolerance.²⁵

Thus, the thymus is crucial for T-cell education and development including both negative selection and T-reg induction. For central tolerance, a number of different thymic cell types have been implicated in mediating these two processes. In particular, the function of thymic DCs and thymic epithelial cells in both negative selection and T-reg induction has become the focus of recent studies.

In order for tolerance to self-antigens to occur efficiently, it is necessary for all self-antigens, including tissue-specific antigens (TSAs), to be expressed by thymic antigen-presenting cells (APCs). It has been shown clearly that peripheral TSAs can be promiscuously expressed by medullary thymic epithelial cells (mTECs) under the regulation of the transcription factor AIRE. Expression of TSAs within mTECs leads to the deletion of self-reactive thymocytes.^{26, 27, 28, 29, 30} Mutations in the AIRE gene cause a rare disease in humans called autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy, whereas mice lacking AIRE develop organ-specific autoimmunity.^{26, 30} In both cases, autoimmunity is caused by impaired negative selection, which results in the accumulation of undeleted autoreactive T cells. However, although AIRE is important in thymic TSA expression, the expression of some TSAs in a subset of AIRE-deficient mTECs points to other, as-of-yet unidentified factors that may also direct the expression of TSAs.²⁹ Furthermore, mTECs do not express every self-antigen present within the organism. There must be additional mechanisms that broaden the spectrum of self-antigens presented to developing T cells. Indeed, although thymic DCs do not express AIRE,³¹ they have been shown to be important in the negative selection of self-reactive thymocytes in the mouse, and in the induction of T-regs in both the mouse and human systems.

In this commentary, the development and function of DC subsets present in the thymus will be discussed, with particular focus on the effect of migrating DCs on the critical process of T-cell development and differentiation.

Thymic DCs

Thymic DCs can be broadly classified into two subtypes, the plasmacytoid DCs (pDCs), which represent the major type-1 interferon-producing cells,^{32, 33, 34, 35} and the conventional DCs (cDCs), which have a high capacity to process and present antigens through MHC class I and II molecules to T cells.³⁶ The cDCs can be further segregated based on the expression of CD8 and Sirp into two subsets, the CD8^loSirp^+/hi (CD8^loSirp⁺ herein) and CD8^hiSirp^-/lo (CD8^hiSirp^- herein) subsets³⁷ (Figure 1). There are also a number of other known surface molecules that are differentially expressed by the thymic cDC subsets (Table 1).^{38, 39, 40}

Figure 1.

Staining an enriched preparation of thymic dendritic cells (DCs) with CD11c and CD45RA reveals the presence of CD11c⁺CD45RA^- cDCs and CD11c^intCD45RA^hi pDCs. The cDC can be further segregated into two distinct subsets based on the expression of CD8 and Sirp into CD8^loSirp^+/hi and CD8^hiSirp^-/lo populations.

Full figure and legend (55K)

Table 1 - Molecules expressed by thymic cDCs.

Full table

Origin of thymic DCs

It has been thought for some time that the DC subsets in the thymus have different developmental origins. Studies have shown that within the thymus, the CD8^hi cDCs but not the CD8^lo cDCs are primarily generated intrathymically during steady-state development from the earliest intrathymic precursors with potentials to develop into T-cell, B-cell, natural killer cells and DC lineages.⁴¹ An extra thymic origin for the CD8^lo cDCs was hypothesized.

This idea has now been formerly shown in three different types of experiments using parabiosis, adoptive cell transfer and fetal thymic grafting. Parabiosis involves joining the blood circulation of two mice and enables an in-depth analysis of the trafficking of cells from the circulation of one mouse into the thymus of the adjoining mouse. These studies showed that there are indeed two developmentally distinct populations of thymic DCs that have different precursors. The first DC subset comprising 50% of total DCs and arising from intrathymic precursors, and the second arising extra thymically from partially differentiated precursors.⁴² At the time these experiments were performed, it was unclear whether the incoming population of cells correlated with the CD8^loSirp⁺ DC subset, as Sirp was not utilized as a marker to segregate the DCs. Subsequent experiment using Sirp as a DC discriminating marker confirmed that the incoming cells included both the pDC population and the CD8^loCD11b⁺Sirp⁺ cDC subset (I Goldschneider, personal communication).

Further characterization of the incoming DCs has been achieved using adoptive transfer studies. In these experiments, total peripheral blood mononucleocytes from a CD45.1 mouse were injected into a recipient CD45.2 mouse, and the phenotype of the DCs migrating into the thymus was determined 3 days later. On the basis of the expression of CD8, Sirp and CD11b, it was clearly shown that the incoming DCs are CD8^loSirp⁺ (Figure 2).⁴³

Figure 2.

White blood cells (20 10⁶) from CD45.1 mice were transferred intravenously into non-irradiated CD45.2 recipients. The phenotype of donor-derived CD11c⁺ cells in the thymus of recipients was determined 3 days post-transfer by staining enriched thymic DCs for CD45.1, CD11c, Sirp and CD8. Gating on recipient-derived CD45.1^-CD11c⁺ cells revealed the presence of both Sirp^hiCD8^lo and Sirp^loCD8^hi cDCs (upper panel). Donor-derived cells were identified as CD45.1⁺CD11c⁺. The donor-derived cDCs were subdivided into subsets based on the expression of Sirp and CD8 (lower panel). The donor-derived DCs were mainly Sirp^hiCD8^lo cDCs.

Full figure and legend (81K)

The final evidence came from experiments using a thymic lobe transplantation experiment. Thymi from day-1 neonatal C57BL/6 (B6) mice were grafted under the kidney capsule of a ubiquitin-green fluorescent protein (GFP) recipient mouse. In this system, recipient-derived cells migrating into the grafted lobes could be distinguished by their expression of GFP. These experiments clearly showed that the incoming GFP⁺ DCs on day 7 post-transplantation included both CD8^loSirp⁺ cDCs and pDCs (Figure 3).

Figure 3.

C57BL/6 (B6) day-1 thymic lobes were grafted under the kidney capsule of ubiquitin-GFP recipient mice. The phenotype of GFP⁺ cells migrating into the thymic lobes 7 days post-transplantation was assessed using flow cytometry. GFP⁺ cells were gated for (left panel), and then analyzed for their CD11c and CD45RA expressions (middle panel). CD11c⁺CD45RA^- cDCs were further analyzed for the expression of Sirp (right panel).

Full figure and legend (53K)

Taken together, it is now clear that there are two distinct DC lineages in the thymus. The CD8^hiSirp^- cDCs comprising 70% of total thymic cDCs are generated intrathymically from the DC/T oligopotent precursor, whereas the CD8^loSirp⁺ cDCs, which contribute to 30% of the cDC pool, migrate from the periphery to the thymus. The pDC, which appear generally to be circulatory DCs, also migrate into the thymus.

Migration of DCs into the thymus has been shown to be dependent on three mechanisms, including P-selectin for DC rolling, VLA-4–VCAM-1 interactions for firm arrest and chemokine/chemokine receptor interactions for migration into the thymus.⁴⁴ Although these experiments utilized total splenic DCs injected intravenously to discern the molecular mechanism required by DCs for thymic homing, it is likely that similar mechanisms are used by blood-borne DCs in their migration into the thymus.

DCs within mouse blood

Given the finding that there are subsets of DCs that migrate into the thymus, it would follow that the precursors of these DCs or fully formed DCs can be found within the blood. A number of studies have addressed this issue.

Two different DC precursors have been identified in mouse blood, including a CD11c⁺ immature DC precursor capable of giving rise to CD8^- cDCs in culture and a CD11c^loCD11b^- precursor that has the capacity to produce IFN- upon stimulation, most likely the precursor for pDCs.⁴⁵

Fully formed DCs have also been observed within mouse blood. Analysis of blood DCs has identified the presence of a major population (70–80%) of CD11c⁺CD45RA^-CD8^loSirp⁺ cDCs in both immature MHC class II^lo and mature MHC class II^hi states, and a minor population (20–30%) of CD11c⁺CD45RA^-CD8^hiSirp^- cDCs (Figure 4a).⁴³ The heterogeneity of MHC class II expression on the CD8^loSirp⁺ cDCs suggests that the cDCs within the blood are at different stages of maturation, perhaps reaching their full maturity status only once in the thymus. Indeed, we have shown that the CD8^loSirp⁺ cDCs isolated directly ex vivo from the thymus express very high levels of MHC class II, and are more mature than their resident CD8^hiSirp^- cDC counterparts (Figure 4b).

Figure 4.

(a) The phenotype of CD11c⁺ DCs within mouse blood. PBMCs were depleted of T cells, B cells and macrophages and stained for CD11c, CD45RA, Sirp and MHC class II. CD11c⁺CD45RA^-Sirp⁺ cells were gated for and the expression of MHC class II was analyzed. (b) An enriched preparation of thymic DCs was stained for CD11c, CD45RA, CD8 and Sirp. Conventional DCs were gated as CD11c⁺CD45RA^- and segregated into CD8^loSirp⁺ and CD8^hiSirp^- cDCs. Thymic Sirp⁺ (dark line) and Sirp^- (gray line) cDCs were analyzed for the expression of MHC class II.

Full figure and legend (119K)

Function of thymic DCs

Implications for circulating DCs in negative selection

The function of thymic DCs in T cell development and differentiation is of great interest. As mentioned in the introduction, self-reactive thymocytes that may develop during random generation of the TCR are deleted in the thymus by APCs in a process called negative selection.^{10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20} Early studies suggested that BM-derived cells, DCs, cortical thymic epithelial cells (cTECs) and mTECs all contribute to negative selection.^{12, 46, 47, 48, 49, 50, 51, 52} However, more recent work has demonstrated that the major cell types contributing most significantly to negative selection are the DCs and mTECs, given their higher levels of expression of MHC molecules and of co-stimulatory molecules, both of which are required for efficient negative selection.^{53, 54}

Evidence for an important role for DCs in negative selection came from several studies. We and others have used MHC class II^-/- BM chimeras to asses the contribution of DCs to negative selection.^{55, 56, 57} In the absence of antigen presentation by all DCs through MHC class II (MHC class II^-/- BM chimeras), negative selection of CD4⁺ thymocytes is impaired, as demonstrated by an increase in the number of CD4⁺ thymocytes in MHC class II^-/- chimeras as compared to controls and the observation that the CD4⁺ thymocytes in the MHC class II^-/- chimeras are self-reactive.⁴³

It has been shown that DCs migrating into the thymus from the periphery have the capacity to delete self-reactive thymocytes. DCs pulsed with OVA peptide and injected into the blood stream of OTII transgenic mice (OTII tg), migrated to the thymus where they were able to delete OTII (OVA-specific) CD4⁺ T cells.⁴⁴ In a more physiological experimental set-up, parabiotic mice were used where mice expressing OVA only in cardiomyocytes were joined to OTII transgenic (tg) mice. In this setting, negative selection of OTII⁺ CD4⁺ T cells in the thymus was observed, suggesting that DCs migrating from the heart of the conjoined mouse entered the thymus and initiated selection.⁴⁴

Migrating DCs have also been shown to be important in protection against autoimmunity. In a model of experimental autoimmune encephalomyelitis (EAE), it was demonstrated that enriched preparations of antigen-pulsed thymic DCs, and not splenic DCs, when injected i.v into a mouse suffering from EAE, could migrate to the thymus where they could mediate acquired thymic tolerance. The exact mechanism was not determined in this study. Of most interest, was that the thymus was absolutely required for this protection as thymectomy prevented protection against EAE.⁵⁸

A more recent study showed definitively that BM-derived cells are crucial for the deletion of myelin based protein (MBP) specific T cells, the major cause of EAE. Notably, negative selection of MBP-specific T cells within the thymus could be initiated by BM-derived APCs that themselves were not endogenously expressing MBP. It was shown that BM-derived APCs picked up antigen from the periphery, migrated to the thymus and deleted auto-reactive cells.⁵⁹

In most recent experiments, we used thymic lobe transplantation experiments to address the role of migrating DCs in negative selection in an antigen-specific system. We established that the DCs migrating into the donor thymic lobes from the host were primarily CD8^-Sirp⁺ cDCs. We then grafted OTII tg day-1 thymic lobes under the kidney capsule of CD11c-OVA tg mice where OVA is expressed primarily in DCs. In this system, the effect of in-coming CD11c⁺ OVA expressing DCs on the development of OTII tg CD4⁺ T cells could be assessed. We found that OTII⁺ CD4⁺ T cells were efficiently deleted in this system, compared to control mice where DCs were not expressing OVA (Figure 5a).

Figure 5.

(a) Thymic lobes from OTII transgenic (tg) CD45.2⁺ mice crossed with CD45.1⁺ WT mice were grafted under the kidney capsule of CD45.2⁺ CD11cOVA tg or WT recipients. The phenotype of CD45.1⁺ thymocytes from the grafted OTII tg thymic lobes were analyzed 10–12 days post-transplantation. The total number of CD45.1⁺CD4⁺V2⁺V5⁺ cells (OTII) and CD45.1⁺CD4⁺V2⁺V5⁺CD25⁺Foxp3⁺ T-regs was calculated in OTII lobes grafted into WT or CD11c-OVA tg recipients. One experiment representative of three is shown (error bars, s.d.) (n=6–8). ^*P<0.05. (b) The number of CD4⁺CD25⁺Foxp3⁺ T-regs in the individual thymus of the WT and MHC class II^-/- BM chimeric mice (n=20–24). ^**P<0.001. (c) Purified thymic DC subsets (CD45.2) were co-cultured with sorted CD4⁺CD25^- thymocytes (CD45.1) for 5 days in the presence of IL-7. The T-regs generated were analyzed by staining the cells for CD45.1, CD4, CD25 and Foxp3.

Full figure and legend (89K)

Taken together, these results suggest that negative selection of CD4⁺ thymocytes can be achieved by the migrating CD8^loSirp⁺ cDCs.

Implications for circulating DCs in T-reg induction

Self-reactive T cells not deleted in the thymus can be controlled in the periphery by T-regs, which suppress the aberrant proliferation of these 'escapees'. Naturally occurring T-regs develop within the thymus, and can be seen to egress 3 days after birth.⁶⁰ The induction of T-regs in the thymus requires presentation of self-peptide on MHC class II molecules by APCs. In early studies, first with birds⁶¹ and then with mice,^{62, 63} transplantation of the histoincompatible thymic epithelium through skin grafting (before hematopoietic cell colonization) resulted in the development of T cells capable of suppressing the immune response to skin grafts from the same, but not different donors. This suppression was transplantable with the CD4⁺ T cells from these mice, strongly suggesting that epithelial cells alone could instruct the development of a T-reg population. In more recent experiments, peptides have been selectively expressed within the radioresistant compartment of the thymus, and the induction of TCR-transgenic T-reg cells specific for the peptide has been clearly observed.^{22, 64, 65} From these experiments, it was concluded that expression of antigen by epithelial cells was sufficient for the occurrence of T-reg induction.

Given that DCs in the thymus can also present antigen to developing thymocytes, it was important to dissect the contribution of DCs from that of mTECs in the induction of CD4⁺CD25⁺Foxp3⁺ T-regs. As expression of MHC class II on DCs is essential for antigen presentation and for the induction of thymic-derived T-regs,^{22, 64, 66, 67, 68, 69} we constructed MHC class II^-/- BM chimeras that enabled us to discern the contribution of DCs to T-reg induction. A significant reduction in T-reg numbers of 30% in MHC class II^-/- chimeras compared with WT controls was observed, showing an important role for DCs in T-reg induction (Figure 5b). The capacity of thymic DC subsets to induce T-regs were compared using an in vitro assay where DC subsets were placed directly in contact with CD4⁺CD25^- thymocytes and co-cultured for 5 days. The Sirp⁺ thymic cDCs were clearly the most efficient inducers of polyclonal T-regs in this syngeneic co-culture system compared with the Sirp^- cDCs and pDCs (Figure 5c).

We once again used the thymic lobe transplantation system to ask whether migrating DCs also had the capacity to induce T-regs in an antigen-specific manner. In OTII tg lobes that were grafted under the kidney capsule of recipient CD11c-OVA tg mice, a clear induction of OTII⁺CD4⁺Foxp3⁺ T-regs was observed. This was in addition to the negative selection seen (discussed above) (Figure 5a).

Thus, taken together, it is likely that the migratory CD8^loSirp⁺ cDC subset can efficiently induce negative selection and T-reg induction in the thymus. However, this raises the question of the potential consequences of deleting T cells during an infection that would potentially be beneficial and required for the clearance of the pathogen, or in inducing pathogen-specific T-regs that may dampen a necessary immune response. In a recent study, purified splenic DCs when injected intravenously showed a reduced capacity to home to the thymus if they had been pre-activated with lipopolysaccharide from Escherichia coli.⁴⁴ This reduced homing capacity was specific for the thymus, suggesting that the migration properties of DCs do alter after activation. However, the physiological relevance of this is unclear, as in this study, large numbers of splenic DCs were injected, which may not represent the true migrating DCs under normal conditions.

Translation into the human system: human DCs in the blood and thymus

DCs have been identified within the human thymus. Studies have identified three DC subtypes including the pDCs that were CD11c^-CD123⁺, and two 'conventional' DCs, the CD11c⁺HLA-DR⁺CD11b⁺ and CD11c⁺HLA-DR⁺CD11b^- subsets.^{70, 71} One functional distinction between mouse DC subtypes is in their ability to secrete bioactive IL12p70 in response to TLR stimulation. It is the mouse CD8⁺CD11b^- population that is the superior producer of this cytokine. It can be noted that the human CD11b^- thymic DCs also showed this functional capability compared with the CD11b⁺ DCs, thus correlating functionally and phenotypically with mouse lymphoid-resident DC subtypes.

Studies on human thymic DC development also suggest a close correlation with the mouse system. It was shown that the early human thymic T-cell precursors could differentiate into DCs in vitro in the presence of cytokines,⁷² suggesting an intrathymic origin for some human thymic DCs.

Four distinct DC subsets have also been identified in human blood. Using CD11c and HLA-DR to identify blood DCs, three clear subsets of 'conventional' CD11c⁺HLA-DR⁺ DCs that are CD16⁺CD33^lo, BDCA1⁺CD33⁺ and BDCA3⁺CD33⁺ have been identified. In addition to these 'cDCs', pDCs that are CD11c^-CD123⁺ have been observed.⁷³ However, given that the markers used to segregate human DCs are distinct from those used for mouse DC subset discrimination, it is not clear which blood DC represents the migrating DCs identified in the mouse.

Finally, the role of human thymic DCs in mediating central tolerance has not been clearly shown. However, recently, the capacity for human thymic DCs in T-reg induction in vitro has been shown. Human thymic DCs when isolated ex vivo had a strong capacity to induce T-regs from a population of CD4⁺ thymocytes.⁶⁹ Of interest, there appeared to be two distinct types of DCs within the human thymus, an immature and an activated DC type. The activated DCs were the ones observed to be co-localized with the T-regs within the medullary regions of the thymus and thus thought to be the DCs involved in this induction process. Furthermore, blockage of the co-stimulatory molecule CD86 in in vitro co-cultures with DCs and CD4⁺ T cells prevented the induction of T-regs. These findings were consistent with those observed in the mouse thymus, where it was the CD8^-Sirp⁺ cDCs that were phenotypically more mature and had the highest capacity to induce T-regs in vitro (Figure 5c).

Overall, there are many similarities between the DCs found within the human thymus with those identified in the mouse. It appears that they may share similarities in their developmental origins, and in their functions with respect to T-cell development and differentiation. Further investigations in this area are required to address these questions. It would be of great interest to ascertain whether DCs migrate into the human thymus, as they do in the mouse system.

Concluding remarks

Overall, it appears that circulating DCs are important in two different arms of immune tolerance induction. First, in initiating central tolerance through negative selection and the second through the induction of T-regs. Although mTECs do express a large number of TSAs, there are other antigens not expressed by this cell type. There is now clear evidence showing that DCs have the capacity to migrate from the periphery to the thymus, bringing with them peripheral self-antigens. The question of whether DC circulation is altered during a pathogen infection remains open and requires further investigation. Finally, pDCs are also circulatory, but their role in the thymus has not been identified. This would be of interest for future studies.

References

1. Heath WR, Belz GT, Behrens GM, Smith CM, Forehan SP, Parish IA et al. Cross-presentation, dendritic cell subsets, and the generation of immunity to cellular antigens. Immunol Rev 2004; 199: 9–26. | Article | PubMed | ISI | ChemPort |
2. Miller JF. Recovery of leukaemogenic agent from nonleukaemic tissues of thymectomized mice. Nature 1960; 187: 703. | Article | PubMed | ISI | ChemPort |
3. Miller JF. Immunological function of the thymus. Lancet 1961; 2: 748–749. | Article | PubMed | ISI | ChemPort |
4. Maldonado-Lopez R, Moser M. Dendritic cell subsets and the regulation of Th1/Th2 responses. Semin Immunol 2001; 13: 275–282. | Article | PubMed | ISI | ChemPort |
5. Moser M, Murphy KM. Dendritic cell regulation of TH1–TH2 development. Nat Immunol 2000; 1: 199–205. | Article | PubMed | ISI | ChemPort |
6. Pulendran B, Smith JL, Caspary G, Brasel K, Pettit D, Maraskovsky E et al. Distinct dendritic cell subsets differentially regulate the class of immune response in vivo. Proc Natl Acad Sci USA 1999; 96: 1036–1041. | Article | PubMed | ChemPort |
7. Soares H, Waechter H, Glaichenhaus N, Mougneau E, Yagita H, Mizenina O et al. A subset of dendritic cells induces CD4+ T cells to produce IFN-gamma by an IL-12-independent but CD70-dependent mechanism in vivo. J Exp Med 2007; 204: 1095–1106. | Article | PubMed | ISI | ChemPort |
8. Janeway Jr CA. Thymic selection: two pathways to life and two to death. Immunity 1994; 1: 3–6. | Article | PubMed | ChemPort |
9. von Boehmer H. Positive selection of lymphocytes. Cell 1994; 76: 219–228. | Article | PubMed | ChemPort |
10. Berg LJ, Fazekas de St Groth B, Pullen AM, Davis MM. Phenotypic differences between alpha beta versus beta T-cell receptor transgenic mice undergoing negative selection. Nature 1989; 340: 559–562. | Article | PubMed | ISI | ChemPort |
11. Bill J, Kanagawa O, Woodland DL, Palmer E. The MHC molecule I-E is necessary but not sufficient for the clonal deletion of V beta 11-bearing T cells. J Exp Med 1989; 169: 1405–1419. | Article | PubMed | ISI | ChemPort |
12. Fowlkes BJ, Schwartz RH, Pardoll DM. Deletion of self-reactive thymocytes occurs at a CD4+8+ precursor stage. Nature 1988; 334: 620–623. | Article | PubMed | ISI | ChemPort |
13. Kappler JW, Roehm N, Marrack P. T cell tolerance by clonal elimination in the thymus. Cell 1987; 49: 273–280. | Article | PubMed | ISI | ChemPort |
14. Kappler JW, Staerz U, White J, Marrack PC. Self-tolerance eliminates T cells specific for Mls-modified products of the major histocompatibility complex. Nature 1988; 332: 35–40. | Article | PubMed | ISI | ChemPort |
15. Kisielow P, Bluthmann H, Staerz UD, Steinmetz M, von Boehmer H. Tolerance in T-cell-receptor transgenic mice involves deletion of nonmature CD4+8+ thymocytes. Nature 1988; 333: 742–746. | Article | PubMed | ISI | ChemPort |
16. MacDonald HR, Schneider R, Lees RK, Howe RC, Acha-Orbea H, Festenstein H et al. T-cell receptor V beta use predicts reactivity and tolerance to Mlsa-encoded antigens. Nature 1988; 332: 40–45. | Article | PubMed | ChemPort |
17. Pircher H, Burki K, Lang R, Hengartner H, Zinkernagel RM. Tolerance induction in double specific T-cell receptor transgenic mice varies with antigen. Nature 1989; 342: 559–561. | Article | PubMed | ISI | ChemPort |
18. Pullen AM, Marrack P, Kappler JW. The T-cell repertoire is heavily influenced by tolerance to polymorphic self-antigens. Nature 1988; 335: 796–801. | Article | PubMed | ISI | ChemPort |
19. Sha WC, Nelson CA, Newberry RD, Kranz DM, Russell JH, Loh DY. Positive and negative selection of an antigen receptor on T cells in transgenic mice. Nature 1988; 336: 73–76. | Article | PubMed | ISI | ChemPort |
20. Vacchio MS, Hodes RJ. Selective decreases in T cell receptor V beta expression. Decreased expression of specific V beta families is associated with expression of multiple MHC and non-MHC gene products. J Exp Med 1989; 170: 1335–1346. | Article | PubMed | ChemPort |
21. Itoh M, Takahashi T, Sakaguchi N, Kuniyasu Y, Shimizu J, Otsuka F et al. Thymus and autoimmunity: production of CD25+CD4+ naturally anergic and suppressive T cells as a key function of the thymus in maintaining immunologic self-tolerance. J Immunol 1999; 162: 5317–5326. | PubMed | ISI | ChemPort |
22. Jordan MS, Boesteanu A, Reed AJ, Petrone AL, Holenbeck AE, Lerman MA et al. Thymic selection of CD4+CD25+ regulatory T cells induced by an agonist self-peptide. Nat Immunol 2001; 2: 301–306. | Article | PubMed | ISI | ChemPort |
23. Sakaguchi S. Naturally arising CD4+ regulatory t cells for immunologic self-tolerance and negative control of immune responses. Annu Rev Immunol 2004; 22: 531–562. | Article | PubMed | ISI | ChemPort |
24. Sakaguchi S, Sakaguchi N, Asano M, Itoh M, Toda M. Immunologic self-tolerance maintained by activated T cells expressing IL-2 receptor alpha-chains (CD25). Breakdown of a single mechanism of self-tolerance causes various autoimmune diseases. J Immunol 1995; 155: 1151–1164. | PubMed | ISI | ChemPort |
25. Steinman RM, Hawiger D, Liu K, Bonifaz L, Bonnyay D, Mahnke K et al. Dendritic cell function in vivo during the steady state: a role in peripheral tolerance. Ann N Y Acad Sci 2003; 987: 15–25. | PubMed | ISI | ChemPort |
26. Pitkanen J, Peterson P. Autoimmune regulator: from loss of function to autoimmunity. Genes Immun 2003; 4: 12–21. | Article | PubMed | ISI | ChemPort |
27. Liston A, Lesage S, Wilson J, Peltonen L, Goodnow CC. Aire regulates negative selection of organ-specific T cells. Nat Immunol 2003; 4: 350–354. | Article | PubMed | ISI | ChemPort |
28. Liston A, Gray DH, Lesage S, Fletcher AL, Wilson J, Webster KE et al. Gene dosage—limiting role of Aire in thymic expression, clonal deletion, and organ-specific autoimmunity. J Exp Med 2004; 200: 1015–1026. | Article | PubMed | ISI | ChemPort |
29. Derbinski J, Gabler J, Brors B, Tierling S, Jonnakuty S, Hergenhahn M et al. Promiscuous gene expression in thymic epithelial cells is regulated at multiple levels. J Exp Med 2005; 202: 33–45. | Article | PubMed | ISI | ChemPort |
30. Anderson MS, Venanzi ES, Klein L, Chen Z, Berzins SP, Turley SJ et al. Projection of an immunological self shadow within the thymus by the aire protein. Science (New York, NY) 2002; 298: 1395–1401. | ChemPort |
31. Hubert FX, Kinkel SA, Webster KE, Cannon P, Crewther PE, Proietto AI et al. A specific anti-Aire antibody reveals aire expression is restricted to medullary thymic epithelial cells and not expressed in periphery. J Immunol 2008; 180: 3824–3832. | PubMed | ChemPort |
32. Diebold SS, Kaisho T, Hemmi H, Akira S, Reis e Sousa C. Innate antiviral responses by means of TLR7-mediated recognition of single-stranded RNA. Science (New York, NY) 2004; 303: 1529–1531. | ChemPort |
33. Jarrossay D, Napolitani G, Colonna M, Sallusto F, Lanzavecchia A. Specialization and complementarity in microbial molecule recognition by human myeloid and plasmacytoid dendritic cells. Eur J Immunol 2001; 31: 3388–3393. | Article | PubMed | ISI | ChemPort |
34. Kadowaki N, Antonenko S, Liu YJ. Distinct CpG DNA and polyinosinic–polycytidylic acid double-stranded RNA, respectively, stimulate CD11c- type 2 dendritic cell precursors and CD11c+ dendritic cells to produce type I IFN. J Immunol 2001; 166: 2291–2295. | PubMed | ISI | ChemPort |
35. Krug A, Rothenfusser S, Hornung V, Jahrsdörfer B, Blackwell S, Ballas ZK et al. Identification of CpG oligonucleotide sequences with high induction of IFN-alpha/beta in plasmacytoid dendritic cells. Eur J Immunol 2001; 31: 2154–2163. | Article | PubMed | ISI | ChemPort |
36. Steinman RM. The dendritic cell system and its role in immunogenicity. Annu Rev Immunol 1991; 9: 271–296. | Article | PubMed | ISI | ChemPort |
37. Lahoud MH, Proietto AI, Gartlan KH, Kitsoulis S, Curtis J, Wettenhall J et al. Signal regulatory protein molecules are differentially expressed by CD8^- dendritic cells. J Immunol 2006; 177: 372–382. | PubMed | ChemPort |
38. Caminschi I, Proietto AI, Ahmet F, Kitsoulis S, Shin Teh J, Lo JC et al. The dendritic cell subtype restricted C-type lectin Clec9A is a target for vaccine enhancement. Blood 2008; 112: 3264–3273. | Article | PubMed | ChemPort |
39. Vremec D, Pooley J, Hochrein H, Wu L, Shortman K. CD4 and CD8 expression by dendritic cell subtypes in mouse thymus and spleen. J Immunol 2000; 164: 2978–2986. | PubMed | ISI | ChemPort |
40. Vremec D, Shortman K. Dendritic cell subtypes in mouse lymphoid organs: cross-correlation of surface markers, changes with incubation, and differences among thymus, spleen, and lymph nodes. J Immunol 1997; 159: 565–573. | PubMed | ISI | ChemPort |
41. Wu L, Vremec D, Ardavin C, Winkel K, Süss G, Georgiou H et al. Mouse thymus dendritic cells: kinetics of development and changes in surface markers during maturation. Eur J Immunol 1995; 25: 418–425. | Article | PubMed | ChemPort |
42. Donskoy E, Goldschneider I. Two developmentally distinct populations of dendritic cells inhabit the adult mouse thymus: demonstration by differential importation of hematogenous precursors under steady state conditions. J Immunol 2003; 170: 3514–3521. | PubMed | ChemPort |
43. Proietto AI, van Dommelen S, Zhou P, Rizzitelli A, D'Amico A, Steptoe RJ et al. Dendritic cells in the thymus contribute to T regulatory cell induction. PNAS 2008; (in press).
44. Bonasio R, Scimone ML, Schaerli P, Grabie N, Lichtman AH, von Andrian UH. Clonal deletion of thymocytes by circulating dendritic cells homing to the thymus. Nat Immunol 2006; 7: 1092–1100. | Article | PubMed | ChemPort |
45. O'Keeffe M, Hochrein H, Vremec D, Scott B, Hertzog P, Tatarczuch L et al. Dendritic cell precursor populations of mouse blood: identification of the murine homologues of human blood plasmacytoid pre-DC2 and CD11c+ DC1 precursors. Blood 2003; 101: 1453–1459. | Article | PubMed | ChemPort |
46. Kisielow P, Teh HS, Bluthmann H, von Boehmer H. Positive selection of antigen-specific T cells in thymus by restricting MHC molecules. Nature 1988; 335: 730–733. | Article | PubMed | ISI | ChemPort |
47. Finkel TH, Cambier JC, Kubo RT, Born WK, Marrack P, Kappler JW. The thymus has two functionally distinct populations of immature alpha beta + T cells: one population is deleted by ligation of alpha beta TCR. Cell 1989; 58: 1047–1054. | Article | PubMed | ChemPort |
48. Baldwin TA, Sandau MM, Jameson SC, Hogquist KA. The timing of TCR alpha expression critically influences T cell development and selection. J Exp Med 2005; 202: 111–121. | Article | PubMed | ISI | ChemPort |
49. Speiser DE, Kolb E, Schneider R, Pircher H, Hengartner H, MacDonald HR et al. Tolerance to Mlsa by clonal deletion of V beta 6+ T cells in bone marrow and thymus chimeras. Thymus 1989; 13: 27–33. | PubMed | ChemPort |
50. Speiser DE, Lees RK, Hengartner H, Zinkernagel RM, MacDonald HR. Positive and negative selection of T cell receptor V beta domains controlled by distinct cell populations in the thymus. J Exp Med 1989; 170: 2165–2170. | Article | PubMed | ChemPort |
51. Ramsdell F, Lantz T, Fowlkes BJ. A nondeletional mechanism of thymic self tolerance. Science (New York, NY) 1989; 246: 1038–1041. | ChemPort |
52. Fukushi N, Wang BY, Arase H, Ogasawara K, Good RA, Onoe K. Cell components required for deletion of an autoreactive T cell repertoire. Eur J Immunol 1990; 20: 1153–1160. | Article | PubMed | ChemPort |
53. Hare KJ, Pongracz J, Jenkinson EJ, Anderson G. Modeling TCR signaling complex formation in positive selection. J Immunol 2003; 171: 2825–2831. | PubMed | ChemPort |
54. Jenkinson EJ, Anderson G, Moore NC, Smith CA, Owen JJ. Positive selection by purified MHC class II+ thymic epithelial cells in vitro: costimulatory signals mediated by B7 are not involved. Dev Immunol 1994; 3: 265–271. | Article | PubMed | ISI | ChemPort |
55. Gallegos AM, Bevan MJ. Central tolerance to tissue-specific antigens mediated by direct and indirect antigen presentation. J Exp Med 2004; 200: 1039–1049. | Article | PubMed | ISI | ChemPort |
56. Gao EK, Lo D, Sprent J. Strong T cell tolerance in parent—F1 bone marrow chimeras prepared with supralethal irradiation. Evidence for clonal deletion and anergy. J Exp Med 1990; 171: 1101–1121. | Article | PubMed | ChemPort |
57. van Meerwijk JP, Marguerat S, Lees RK, Germain RN, Fowlkes BJ, MacDonald HR. Quantitative impact of thymic clonal deletion on the T cell repertoire. J Exp Med 1997; 185: 377–383. | Article | PubMed | ChemPort |
58. Khoury SJ, Gallon L, Chen W, Betres K, Russell ME, Hancock WW et al. Mechanisms of acquired thymic tolerance in experimental autoimmune encephalomyelitis: thymic dendritic-enriched cells induce specific peripheral T cell unresponsiveness in vivo. J Exp Med 1995; 182: 357–366. | Article | PubMed | ChemPort |
59. Huseby ES, Sather B, Huseby PG, Goverman J. Age-dependent T cell tolerance and autoimmunity to myelin basic protein. Immunity 2001; 14: 471–481. | Article | PubMed | ISI | ChemPort |
60. Fontenot JD, Dooley JL, Farr AG, Rudensky AY. Developmental regulation of Foxp3 expression during ontogeny. J Exp Med 2005; 202: 901–906. | Article | PubMed | ISI | ChemPort |
61. Ohki H, Martin C, Corbel C, Coltey M, Le Douarin NM. Tolerance induced by thymic epithelial grafts in birds. Science (New York, NY) 1987; 237: 1032–1035. | ChemPort |
62. Modigliani Y, Coutinho A, Pereira P, Le Douarin N, Thomas-Vaslin V, Burlen-Defranoux O et al. Establishment of tissue-specific tolerance is driven by regulatory T cells selected by thymic epithelium. Eur J Immunol 1996; 26: 1807–1815. | Article | PubMed | ISI | ChemPort |
63. Salaun J, Bandeira A, Khazaal I, Calman F, Coltey M, Coutinho A et al. Thymic epithelium tolerizes for histocompatibility antigens. Science (New York, NY) 1990; 247: 1471–1474. | ChemPort |
64. Apostolou I, Sarukhan A, Klein L, von Boehmer H. Origin of regulatory T cells with known specificity for antigen. Nat Immunol 2002; 3: 756–763. | Article | PubMed | ISI | ChemPort |
65. Aschenbrenner K, D'Cruz LM, Vollmann EH, Hinterberger M, Emmerich J, Swee LK et al. Selection of Foxp3+ regulatory T cells specific for self antigen expressed and presented by Aire+ medullary thymic epithelial cells. Nat Immunol 2007; 8: 351–358. | Article | PubMed | ISI | ChemPort |
66. Bensinger SJ, Bandeira A, Jordan MS, Caton AJ, Laufer TM. Major histocompatibility complex class II-positive cortical epithelium mediates the selection of CD4(+)25(+) immunoregulatory T cells. J Exp Med 2001; 194: 427–438. | Article | PubMed | ISI | ChemPort |
67. Fontenot JD, Rasmussen JP, Williams LM, Dooley JL, Farr AG, Rudensky AY. Regulatory T cell lineage specification by the forkhead transcription factor foxp3. Immunity 2005; 22: 329–341. | Article | PubMed | ISI | ChemPort |
68. Bochtler P, Wahl C, Schirmbeck R, Reimann J. Functional adaptive CD4 Foxp3T cells develop in MHC class II-deficient mice. J Immunol 2006; 177: 8307–8314. | PubMed | ChemPort |
69. Watanabe N, Wang YH, Lee HK, Ito T, Cao W, Liu YJ. Hassall's corpuscles instruct dendritic cells to induce CD4+CD25+ regulatory T cells in human thymus. Nature 2005; 436: 1181–1185. | Article | PubMed | ISI | ChemPort |
70. Vandenabeele S, Hochrein H, Mavaddat N, Winkel K, Shortman K. Human thymus contains 2 distinct dendritic cell populations. Blood 2001; 97: 1733–1741. | Article | PubMed | ISI | ChemPort |
71. Bendriss-Vermare N, Barthelemy C, Durand I, Bruand C, Dezutter-Dambuyant C, Moulian N et al. Human thymus contains IFN-alpha-producing CD11c(-), myeloid CD11c(+), and mature interdigitating dendritic cells. J Clin Invest 2001; 107: 835–844. | Article | PubMed | ISI | ChemPort |
72. Kelly KA, Lucas K, Hochrein H, Metcalf D, Wu L, Shortman K. Development of dendritic cells in culture from human and murine thymic precursor cells. Cell Mol Biol (Noisy-le-Grand, France) 2001; 47: 43–54. | ChemPort |
73. MacDonald KP, Munster DJ, Clark GJ, Dzionek A, Schmitz J, Hart DN. Characterization of human blood dendritic cell subsets. Blood 2002; 100: 4512–4520. | Article | PubMed | ISI | ChemPort |