Regulatory T cell differentiation of thymocytes does not require a dedicated antigen-presenting cell but is under T cell-intrinsic developmental control

Macroautophagy substrates are loaded onto MHC class II of medullary thymic epithelial cells for central tolerance

Macroautophagy serves cellular housekeeping and metabolic functions through delivery of cytoplasmic constituents for lysosomal degradation. In addition, it may mediate the unconventional presentation of intracellular antigens to CD4(+) T cells; however, the physiological relevance of this endogenous MHC class II loading pathway remains poorly defined. Here, we characterize the role of macroautophagy in thymic epithelial cells (TECs) for negative selection. Direct presentation for clonal deletion of MHC class II-restricted thymocytes required macroautophagy for a mitochondrial version of a neo-antigen, but was autophagy-independent for a membrane-bound form. A model antigen specifically expressed in Aire(+) medullary TECs (mTECs) induced efficient deletion via direct presentation when targeted to autophagosomes, whereas interference with autophagosomal routing of this antigen through exchange of a single amino acid or ablation of an essential autophagy gene abolished direct presentation for negative selection. Furthermore, when this autophagy substrate was expressed by mTECs in high amounts, endogenous presentation and indirect presentation by DCs operated in a redundant manner, whereas macroautophagy-dependent endogenous loading was essential for clonal deletion at limiting antigen doses. Our findings suggest that macroautophagy supports central CD4(+) T cell tolerance through facilitating the direct presentation of endogenous self-antigens by mTECs.

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.

Regulatory T Cell-Based Immunotherapy

CD4+CD25+ regulatory T (Treg) cells expressing the forkhead box transcription factor Foxp3 have a vital function in the maintenance of immune homeostasis and the prevention of fatal multi-organ autoimmunity throughout life. In the last decade, Foxp3+ Treg cells have raised the hope for novel cell-based therapies to achieve tolerance in clinical settings of unwanted immune responses such as autoimmunity and graft rejection. Conceptually, the antigen-specific enhancement of Treg cell function is of particular importance because such strategies will minimize the requirements for pharmaceutical immunosuppression, sparing desired protective host immune responses to infectious and malignant insults. This chapter discusses current concepts of Treg cell-based immunotherapy with particular emphasis on antigen-specific Treg cell induction from conventional CD4+ T cells to deal with organ-specific autoimmunity.

A broad range of self-reactivity drives thymic regulatory T cell selection to limit responses to self

The degree of T cell self-reactivity considered dangerous by the immune system, thereby requiring thymic selection processes to prevent autoimmunity, is unknown. Here, we analyzed a panel of T cell receptors (TCRs) with a broad range of reactivity to ovalbumin (OVA(323-339)) in the rat insulin promoter (RIP)-mOVA self-antigen model for their ability to trigger thymic self-tolerance mechanisms. Thymic regulatory T (Treg) cell generation in vivo was directly correlated with in vitro TCR reactivity to OVA-peptide in a broad ~1,000-fold range. Interestingly, higher TCR affinity was associated with a larger Treg cell developmental "niche" size, even though the amount of antigen should remain constant. The TCR-reactivity threshold to elicit thymic negative selection and peripheral T cell responses was ~100-fold higher than that of Treg cell differentiation. Thus, these data suggest that the broad range of self-reactivity that elicits thymic Treg cell generation is tuned to secure peripheral tolerance to self.

Vagaries of fluorochrome reporter gene expression in Foxp3+ regulatory T cells

CD4⁺CD25⁺ regulatory T (Treg) cell lineage commitment and expression of the transcription factor Foxp3 can be induced at the CD4⁺CD8⁺ double-positive (DP) and CD4⁺CD8^? single-positive stages of thymic development, as well as in postthymic CD4⁺ T cells in peripheral lymphoid tissues. The availability of transgenic mice with Foxp3-dependent fluorochrome reporter gene expression has greatly facilitated studies on the intra- and extrathymic generation of murine Foxp3⁺ Treg cells. Here, we performed a comparative analysis of thymic Treg cell development and peripheral compartments of mature Treg cells in various transgenic strains with gene targeted and bacterial artificial chromosome (BAC)-driven Foxp3-fluorochrome expression. These studies revealed a relative deficiency of Foxp3⁺ DP thymocytes selectively in mice with targeted insertion of the fluorochrome reporter gene coding sequence into the endogenous Foxp3 gene. While Foxp3 BAC-driven fluorochrome expression in ex vivo CD4⁺ T cells was found to faithfully reflect Foxp3 protein expression, we provide evidence that Foxp3 BAC transgenesis can result in sizable populations of Foxp3⁺ Treg cells that lack fluorochrome reporter expression. This could be attributed to both timely delayed up-regulation of BAC expression in developing Treg cells and the accumulation of peripheral Foxp3⁺ Treg cells with continuous transcriptional inactivity of the Foxp3 BAC transgene.

Selection of regulatory T cells in the thymus

The generation of regulatory T (T(Reg)) cells in the thymus is crucial for immune homeostasis and self-tolerance. Recent discoveries have revealed the cellular and molecular mechanisms that govern the differentiation of a subset of developing thymocytes into natural T(Reg) cells. Several models, centred on the self-reactivity of the T cell receptor (TCR), have been proposed to explain the generation of a T(Reg) cell population that is cognizant of self. Several molecular pathways link TCR and cytokine signalling with the expression of the T(Reg) cell-associated transcription factor forkhead box P3 (FOXP3). Moreover, interplay between thymocytes and thymic antigen-presenting cells is also involved in T(Reg) cell generation.

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.

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.

B7/CD28 in central tolerance: costimulation promotes maturation of regulatory T cell precursors and prevents their clonal deletion

According to the “two-step model,” the intrathymic generation of CD4⁺ regulatory T (T_reg) cells segregates into a first, T cell receptor (TCR)-driven phase and a second, cytokine-dependent phase. The initial TCR stimulus gives rise to a CD25⁺Foxp3⁻ developmental intermediate. These precursors subsequently require cytokine signaling to establish the mature CD25⁺Foxp3⁺ T_reg cell phenotype. In addition, costimulation via CD28/B7 (CD80/86) axis is important for the generation of a T_reg cell repertoire of normal size. Recent data suggest that CD28 or B7 deficient mice lack CD25⁺Foxp3⁻ T_reg cell progenitors. However, these data leave open whether costimulation is also required at subsequent stages of T_reg differentiation. Also, the fate of “presumptive” T_reg cells carrying a permissive TCR specificity in the absence of costimulation remains to be established. Here, we have used a previously described TCR transgenic model of agonist-driven T_reg differentiation in order to address these issues. Intrathymic adoptive transfer of T_reg precursors indicated that costimulation is dispensable once the intermediate CD25⁺Foxp3⁻ stage has been reached. Furthermore, lack of costimulation led to the physical loss of presumptive T_reg cells rather than their escape from central tolerance and differentiation into the conventional CD4⁺ T cell lineage. Our findings suggest that CD28 signaling does not primarily operate through enhancing the TCR signal strength in order to pass the threshold intensity required to initiate T_reg cell specification. Instead, costimulation seems to deliver unique and qualitatively distinct signals that coordinately foster the developmental progression of T_reg precursors and prevent their negative selection.

Self-antigen presentation by mouse B cells results in regulatory T-cell induction rather than anergy or clonal deletion

Multiple mechanisms operate to ensure T-cell tolerance toward self-antigens. Three main processes have been described: clonal deletion, anergy, and deviation to CD4(+) regulatory T cells (Tregs) that suppress autoreactive T cells that have escaped the first 2 mechanisms. Although it is accepted that dendritic cells (DCs) and B cells contribute in maintaining T-cell tolerance to self-antigens, their relative contribution and the processes involved under physiologic conditions remain only partially characterized. In this study, we used different transgenic mouse models to obtain chimeras where a neo self-antigen is expressed by thymic epithelium and/or by DCs or B cells. We found that expression of cognate ligand in the thymus enhances antigen-specific FoxP3(+) cells independently of whether the self-antigen is expressed on thymic epithelium or only on DCs, but not on B cells. On the contrary, self-antigen expression by B cells was very efficient in inducing FoxP3(+) cells in the periphery, whereas self-antigen expression by DC led mainly to deletion and anergy of antigen-specific FoxP3(-) cells. The results presented in this study underline the role of B cells in Treg induction and may have important implications in clinical protocols aimed at the peripheral expansion of Tregs in patients.

Self-tolerance checkpoints in CD4 T cells specific for a peptide derived from the B cell antigen receptor

Linked recognition of Ag by B and T lymphocytes is ensured in part by a state of tolerance acquired by CD4 T cells to germline-encoded sequences within the B cell Ag receptor (BCR). We sought to determine how such tolerance is attained when a peptide from the BCR variable (V) region is expressed by small numbers of B cells as it is in the physiological state. Mixed bone marrow (BM) chimeras were generated using donor BM from mice with B cells that expressed a transgene (Tg)-encoded κ L chain and BM from TCR Tg mice in which the CD4 T cells (CA30) were specific for a Vκ peptide encoded by the κTg. In chimeras where few B cells express the κTg, many CA30 cells were deleted in the thymus. However, a substantial fraction survived to the CD4 single-positive stage. Among single-positive CA30 thymocytes, few reached maturity and migrated to the periphery. Maturation was strongly associated with, and likely promoted by, expression of an endogenous TCR α-chain. CD4(+) CA30 cells that reached peripheral lymphoid tissues were Ag-experienced and anergic, and some developed into regulatory cells. These findings reveal several checkpoints and mechanisms that enforce a state of self-tolerance in developing T cells specific for BCR V region sequences, thus ensuring that T cell help to B cells occurs through linked recognition of foreign Ag.

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.

ATP inhibits the generation and function of regulatory T cells through the activation of purinergic P2X receptors

Extracellular nucleotides are pleiotropic regulators of mammalian cell function. Adenosine triphosphate (ATP) released from CD4(+) helper T cells upon stimulation of the T cell receptor (TCR) contributes in an autocrine manner to the activation of mitogen-activated protein kinase (MAPK) signaling through purinergic P2X receptors. Increased expression of p2rx7, which encodes the purinergic receptor P2X7, is part of the transcriptional signature of immunosuppressive CD4(+)CD25(+) regulatory T cells (T(regs)). Here, we show that the activation of P2X7 by ATP inhibits the suppressive potential and stability of T(regs). The inflammatory cytokine interleukin-6 (IL-6) increased ATP synthesis and P2X7-mediated signaling in T(regs), which induced their conversion to IL-17-secreting T helper 17 (T(H)17) effector cells in vivo. Moreover, pharmacological antagonism of P2X receptors promoted the cell-autonomous conversion of naïve CD4(+) T cells into T(regs) after TCR stimulation. Thus, ATP acts as an autocrine factor that integrates stimuli from the microenvironment and cellular energetics to tune the developmental and immunosuppressive program of the T cell in adaptive immune responses.

Extrathymic generation of regulatory T cells--chances and challenges for prevention of autoimmune disease

Fopx3(+) expressing regulatory T cells (Tregs) function as an indispensable cellular constituent of the immune system by establishing and maintaining immunological self-tolerance. T cell receptor (TCR) ligands of high agonist activity, when applied in vivo under subimmunogenic conditions, convert naive but not activated T cells into stable Tregs expressing Foxp3. Tolerogenic vaccination with strong-agonist mimetopes of self-antigens may function as a safe and highly specific instrument in the prevention of autoimmune disease by promoting self-antigen-specific tolerance. In this review, we address the requirements for generation of dominant tolerance exerted by Foxp3(+) Tregs in autoimmune disease with special focus on type 1 diabetes (T1D). Further understanding of differentiation of T cells into Tregs at the cellular and molecular level will facilitate development of additional tolerogenic vaccination strategies that can be used in prevention as well as therapeutically to combat unwanted immunity.

Thymic and peripheral differentiation of regulatory T cells

The development of regulatory T (Treg) cells is essential for the maintenance of immune tolerance and homeostasis. Here, we review recent studies that have advanced our understanding of Treg cell differentiation. In the thymus, TCR specificity to self-antigen appears to be a primary determinant for Treg cell lineage commitment, with c-Rel being an important factor that links T cell receptor (TCR) engagement and Foxp3 expression, along with cytokines and costimulatory molecules. It is also clear that postthymic events shape the peripheral Treg cell population. This includes preferential maintenance of Treg cells specific to self-antigens presented in the periphery, as well as the de novo generation of Treg cells from conventional Foxp3(-) T cells. The process of peripheral Treg cell differentiation shares some features with thymic Treg cell development, but there are notable differences. Together, thymic and peripheral Treg cell differentiation appear to generate an "imprint" of both self- and foreign antigens in the peripheral Treg cell population to provide dominant tolerance.

Becoming self-aware: the thymic education of regulatory T cells

The generation of Foxp3(+) regulatory T (Treg) cells in the thymus is essential for immune homeostasis. In the past several years, substantial progress has been made in understanding the mechanisms by which a minor portion of developing thymocytes are selected to become Treg cells. Although previously controversial, recent data support the importance of TCR specificity as a primary determinant for selecting self-reactive thymocytes to become Treg cells in a multi-step process involving cytokines, co-stimulatory molecules, and a variety of antigen-presenting cells. Importantly, the antigenic niche for Treg cell development appears to be typically quite small, implying the recognition of tissue-specific, rather than ubiquitous, self-antigens. Finally, it appears that an NF-κB transcription factor, c-Rel, may be the link between TCR recognition and the induction of Foxp3 expression, which is required for the function and stability of the natural Treg cell population.

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.

Cutting edge: Intrathymic differentiation of adaptive Foxp3+ regulatory T cells upon peripheral proinflammatory immunization

Thymocytes differentiate into CD4(+) Foxp3(+) regulatory T cells (T(R)) upon interaction between their TCR and peptide-MHC II complexes locally expressed in the thymus. Conversion of naive CD4(+) T cells into T(R) can additionally take place in the periphery under noninflammatory conditions of Ag encounter. In this study, making use of TCR transgenic models naturally devoid of Foxp3(+) cells, we report de novo generation of T(R) upon a single footpad injection of Ag mixed with a classic proinflammatory adjuvant. Abrupt T(R) differentiation upon immunization occurred intrathymically and was essential for robust tolerance induction in a mouse model of spontaneous encephalomyelitis. This phenomenon could be attributed to a specific feature of thymocytes, which, in contrast to mature peripheral CD4(+) T cells, were insensitive to the inhibitory effects of IL-6 on the induction of Foxp3 expression. Our findings uncover a pathway for T(R) generation with major implications for immunity and tolerance induction.

PI3 kinase signalling blocks Foxp3 expression by sequestering Foxo factors

Expression of the regulatory T (T reg) cell–associated transcription factor Foxp3 can be induced by signals from the T cell receptor (TCR), interleukin-2 (IL-2), and transforming growth factor (TGF)-β. These signals are integrated by a network involving phosphatidylinositol 3 kinase (PI3K), protein kinase B (PKB; here referred to as Akt), and the mammalian target of rapamycin (mTOR). New studies show that the Foxo proteins Foxo1 and Foxo3a, which are inactivated by Akt, drive Foxp3 expression. These studies therefore explain the negative regulation of Foxp3 by PI3K signaling, and add Foxo proteins to the growing list of nuclear factors capable of modulating Foxp3 expression.

Plasmacytoid dendritic cells resident in human thymus drive natural Treg cell development

The generation of natural regulatory T cells (nTregs) is crucial for the establishment of immunologic self-tolerance and the prevention of autoimmunity. Still, the origin of nTregs and the mechanisms governing their differentiation within the thymus are poorly understood, particularly in humans. It was recently shown that conventional dendritic cells (cDCs) in human thymus were capable of inducing nTreg differentiation. However, the function of plasmacytoid DCs (pDCs), the other major subset of thymic DCs, remains unknown. Here we report that pDCs resident in the human thymus, when activated with CD40 ligand (CD40L) plus interleukin-3, efficiently promoted the generation of CD4+CD25+Foxp3+ nTregs from autologous thymocytes. The progenitors of these nTregs were selectively found within CD4+CD8+ thymocytes that had accomplished positive selection, as judged by their CD69hiTCRhi phenotype. Supporting the involvement of the CD40-CD40L pathway in pDC-induced nTreg generation, we show that positively selected CD4+CD8+ progenitors specifically transcribed CD40L in vivo and up-regulated CD40L expression on T-cell receptor engagement, thereby promoting the activation of pDCs. Finally, evidence is provided that nTregs primed by pDCs displayed reciprocal interleukin-10/transforming growth factor-β cytokine expression profiles compared with nTregs primed by cDCs. This functional diversity further supports a nonredundant tolerogenic role for thymic pDCs in the human thymus.

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.

CD28 facilitates the generation of Foxp3(-) cytokine responsive regulatory T cell precursors

The T cell costimulatory molecule CD28 plays an important role in the thymic generation of Foxp3(+) regulatory T cells (Tregs) essential for the maintenance of self-tolerance. In this study, we show that a cell-intrinsic signal from CD28 is involved in the generation of cytokine-responsive Foxp3(-) precursors using studies of mixed bone marrow chimeras as well as TCR-specific generation of Foxp3(+) cells using intrathymic transfer of TCR-transgenic thymocytes expressing a natural Treg TCR. Contrary to a previous report, the analysis of CD28 mutant knockin mice revealed that this cell-intrinsic signal is only partially dependent on the Lck-binding PYAP motif. Surprisingly, even though the absence of CD28 resulted in a 6-fold decrease in thymic Tregs, the TCR repertoires of CD28-deficient and sufficient cells were largely overlapping. Thus, these data suggest that CD28 does not operate by markedly enlarging the repertoire of TCRs available for Treg development, but rather by improving the efficiency of Treg development of thymocytes expressing natural Treg TCRs.

Regulation in chronic obstructive pulmonary disease: the role of regulatory T-cells and Th17 cells

COPD (chronic obstructive pulmonary disease) is an inflammatory disorder of the airways, which is associated with irreversible airway obstruction. The pathological hallmarks of COPD are destruction of the lung parenchyma (pulmonary emphysema), inflammation of the central airways (chronic bronchitis) and inflammation of the peripheral airways (respiratory bronchiolitis). Tobacco smoking is established as the main aetiological factor for COPD. A maladaptive modulation of inflammatory responses to inhalation of noxious particles and gases is generally accepted as being a key central pathogenic process; however, the precise regulatory mechanisms of the disease are poorly understood. Two cell types are known to be important in immune regulation, namely regulatory T-cells and the newly identified Th17 (T-helper 17) cells. Both types of cells are subsets of CD4 T-lymphocytes and modulate the immune response through secretion of cytokines, for example IL (interleukin)-10 and IL-17 respectively. The present review will begin by describing the current understanding of inflammatory cell involvement in the disease process, and then focus on the possible role of subsets of regulatory and helper T-cells in COPD.

Kickstarting Foxp3 with c-Rel

In this issue of Immunity, reports by Long et al. (2009) and Ruan et al. (2009) suggest that the transcription factor NF-kappaB-c-Rel is an important molecular mechanism by which T cell receptor-specificity for self-antigens instructs the selection of Foxp3(+) regulatory T cells.

Thymocyte deletion can bias Treg formation toward low-abundance self-peptide

Autoreactive CD4+ T cells can undergo deletion and/or become CD25+Foxp3+ Treg as they develop intrathymically, but how these alternative developmental fates are specified based on interactions with self-peptide(s) is not understood. We show here that thymocytes expressing an autoreactive TCR can be subjected to varying degrees of deletion that correlate with the amount of self-peptide. Strikingly, among thymocytes that evade deletion, similar proportions acquire Foxp3 expression. These findings provide evidence that Foxp3+ Treg can develop among members of a cohort of autoreactive thymocytes that have evaded deletion by a self-peptide, and that deletion and Treg formation can act together to bias the Treg repertoire toward low-abundance self-peptide(s).

Thymic selection and lineage commitment of CD4(+)Foxp3(+) regulatory T lymphocytes

Regulatory T lymphocytes play a central role in the control of a variety of immune-responses. Their absence in humans and in experimental animal models leads to severe autoimmune and inflammatory disorders. Consistent with their major role in prevention of autoimmune pathology, their repertoire is enriched in autospecific cells. Probably the majority of regulatory T cells develop in the thymus. How T cell-precursors choose between the conventional versus regulatory T cell lineages remains an unanswered question. More is known about selection of regulatory T cell precursors. Positive selection of these cells is favored by high affinity interactions with MHC class II/peptide ligands expressed by thymic epithelial or dendritic cells. They are also known to be relatively resistant to negative selection. These two parameters allow for the generation of the autoreactive regulatory T cell repertoire, and clearly distinguish selection-criteria of conventional versus regulatory T cell-precursors. It will now be important to elucidate the molecular mechanisms involved in the intrathymic choice of the regulatory T cell-lineage.

Regulatory T cells reinforce intestinal homeostasis

Regulatory T cells help maintain intestinal homeostasis by preventing inappropriate innate and adaptive immune responses. CD4(+) T cells that express Foxp3 and Tr1-like cells that produce IL-10 comprise the major regulatory populations in the intestine. CD4(+)Foxp3(+) T cells play an important functional role in promoting tolerance of the flora and dietary proteins. Tr1-like cells can be generated in conditions that also promote effector T cell responses and may serve a similar function. In this review, we discuss the signals specific to the gastrointestinal tract that support both regulatory cell types and their distinct modes of action in the mesenteric lymph nodes and intestinal tissues. Dysregulation of intestinal immune homeostasis occurs in inflammatory bowel disease and can also be observed in graft-versus-host disease, tumor immunotherapy regimens, and acute HIV infection.

The thymic medulla: a unique microenvironment for intercellular self-antigen transfer

Central tolerance is shaped by the array of self-antigens expressed and presented by various types of thymic antigen-presenting cells (APCs). Depending on the overall signal quality and/or quantity delivered in these interactions, self-reactive thymocytes either apoptose or commit to the T regulatory cell lineage. The cellular and molecular complexity underlying these events has only recently been appreciated. We analyzed the ex vivo presentation of ubiquitous or tissue-restricted self-antigens by medullary thymic epithelial cells (mTECs) and thymic dendritic cells (DCs), the two major APC types present in the medulla. We found that the ubiquitously expressed nuclear neo-self-antigen ovalbumin (OVA) was efficiently presented via major histocompatibility complex class II by mTECs and thymic DCs. However, presentation by DCs was highly dependent on antigen expression by TECs, and hemopoietic cells did not substitute for this antigen source. Accordingly, efficient deletion of OVA-specific T cells correlated with OVA expression by TECs. Notably, OVA was only presented by thymic but not peripheral DCs. We further demonstrate that thymic DCs are constitutively provided in situ with cytosolic as well as membrane-bound mTEC-derived proteins. The subset of DCs displaying transferred proteins was enriched in activated DCs, with these cells being most efficient in presenting TEC-derived antigens. These data provide evidence for a unique, constitutive, and unidirectional transfer of self-antigens within the thymic microenvironment, thus broadening the cellular base for tolerance induction toward promiscuously expressed tissue antigens.