Central tolerance: learning self-control in the thymus

The T-cell antigen receptor: a logical response to an unknown ligand

The immune system can be roughly divided into innate and adaptive compartments. The adaptive compartment includes the B and T lymphocytes, whose antigen receptors are generated by recombination of gene segments. The consequence is that the creation of self-reactive lymphocytes is unavoidable. For the host to remain viable, the immune system has evolved a strategy for removing autoimmune lymphocytes during development. This review discusses how T lymphocytes are generated, how they recognize antigens, and how their antigen receptor directs the removal of self-reactive T cells.

Importance of the CD3γ Ectodomain Terminal β-Strand and Membrane Proximal Stalk in Thymic Development and Receptor Assembly

CD3epsilongamma and CD3epsilondelta are noncovalent heterodimers; each consists of Ig-like extracellular domains associated side-to-side via paired terminal beta-strands that are linked to individual subunit membrane proximal stalk segments. CD3epsilon, CD3gamma, and CD3delta stalks contain the RxCxxCxE motif. To investigate the functional importance of a CD3 stalk and terminal beta-strand, we created a CD3gamma double mutant CD3gamma(C82S/C85S) and a CD3gamma beta-strand triple mutant CD3gamma(Q76S/Y78A/Y79A) for use in retroviral transduction of lymphoid progenitors for comparison with CD3gammawt. Although both mutant CD3gamma molecules reduced association with CD3epsilon in CD3epsilongamma heterodimers, CD3gamma(Q76S/Y78A/Y79A) abrogated surface TCR expression whereas CD3gamma(C82S/C85S) did not. Furthermore, CD3gamma(C82S/C85S) rescued thymic development in CD3gamma(-/-) fetal thymic organ culture. However, the numbers of double-positive and single-positive thymocytes after CD3gamma(C82S/C85S) transduction were significantly reduced despite surface pre-TCR and TCR expression comparable to that of CD3gamma(-/-) thymocytes transduced in fetal thymic organ culture with a retrovirus harboring CD3gammawt cDNA. Furthermore, double-negative thymocyte development was perturbed with attenuated double-negative 3/double-negative 4 maturation and altered surface-expressed CD3epsilongamma, as evidenced by the loss of reactivity with CD3gamma N terminus-specific antisera. Single histidine substitution of either CD3gamma stalk cysteine failed to restore CD3epsilongamma association and conformation in transient COS-7 cell transfection studies. Thus, CD3gamma(C82) and CD3gamma(C85) residues likely are either reduced or form a tight intrachain disulfide loop rather than contribute to a metal coordination site in conjunction with CD3epsilon(C80) and CD3epsilon(C83). The implications of these results for CD3epsilongamma and TCR structure and signaling function are discussed.

CD4 regulatory T cells in human cancer pathogenesis

Over the past decade, there has been an accelerated understanding of immune regulatory mechanisms. Peripheral immune regulation is linked to a collection of specialized regulatory cells of the CD4(+) T cell lineage (i.e., CD4(+) Tregs). This collection consists of Tregs that are either thymically derived (i.e., natural) or peripherally induced. Tregs are important for controlling potentially autoreactive immune effectors and immunity to foreign organisms and molecules. Their importance in maintaining immune homeostasis and the overall health of an organism is clear. However, Tregs may also be involved in the pathogenesis of malignancies as now compelling evidence shows that tumors induce or recruit CD4(+) Tregs to block immune priming and antitumor effectors. Efforts are underway to develop approaches that specifically inhibit the function of tumor-associated Tregs which could lead to an increased capability of the body's immune system to respond to tumors but without off-target immune-related pathologies (i.e., autoimmune disease). In this review, the biology of human CD4(+) Tregs is discussed along with their involvement in malignancies and emerging strategies to block their function.

The unconventional role of LT alpha beta in T cell differentiation

Lymphotoxin (LT)alphabeta, a member of the tumor necrosis factor cytokine superfamily, and its receptor, the LTbeta receptor (LTbetaR), have a well defined role in secondary lymphoid organogenesis but an unexpected function in T cell differentiation. Although earlier studies indicated that conventional T cell subsets were normal in mice deficient in the LTbetaR pathway, accumulating evidence indicates that the LTalphabeta-LTbetaR pathway has a pivotal role in the ontogeny of unconventional T cells, including gammadelta T cells and invariant natural killer T cells. The LTbetaR pathway seems to operate at distinct levels during thymic development. Double positive thymocytes regulate the differentiation of early thymocyte progenitors and gammadelta T cells through a mechanism dependent on LTbetaR. In addition, LTbetaR signaling in thymic stroma was proposed to affect central tolerance to peripherally restricted antigens. These findings highlight the complex cellular crosstalk between lymphoid and stromal compartments during thymic differentiation.

The Wiskott-Aldrich syndrome protein is required for the function of CD4(+)CD25(+)Foxp3(+) regulatory T cells

The Wiskott-Aldrich syndrome, a primary human immunodeficiency, results from defective expression of the hematopoietic-specific cytoskeletal regulator Wiskott-Aldrich syndrome protein (WASP). Because CD4⁺CD25⁺Foxp3⁺ naturally occurring regulatory T (nTreg) cells control autoimmunity, we asked whether colitis in WASP knockout (WKO) mice is associated with aberrant development/function of nTreg cells. We show that WKO mice have decreased numbers of CD4⁺CD25⁺Foxp3⁺ nTreg cells in both the thymus and peripheral lymphoid organs. Moreover, we demonstrate that WKO nTreg cells are markedly defective in both their ability to ameliorate the colitis induced by the transfer of CD45RB^hi T cells and in functional suppression assays in vitro. Compared with wild-type (WT) nTreg cells, WKO nTreg cells show significantly impaired homing to both mucosal (mesenteric) and peripheral sites upon adoptive transfer into WT recipient mice. Suppression defects may be independent of antigen receptor–mediated actin rearrangement because both WT and WKO nTreg cells remodeled their actin cytoskeleton inefficiently upon T cell receptor stimulation. Preincubation of WKO nTreg cells with exogenous interleukin (IL)-2, combined with antigen receptor–mediated activation, substantially rescues the suppression defects. WKO nTreg cells are also defective in the secretion of the immunomodulatory cytokine IL-10. Overall, our data reveal a critical role for WASP in nTreg cell function and implicate nTreg cell dysfunction in the autoimmunity associated with WASP deficiency.

Target-organ specificity of autoimmunity is modified by thymic stroma and bone marrow-derived cells

Physical contact between thymocytes and the thymic stroma is essential for the establishment of self-tolerance, and Aire in thymic epithelial cells plays an important role in this action. As expected, the autoimmune phenotypes of Aire-deficient mice are thymic stroma-dependent. Interestingly, the spectrum of the organs involved differs depending on the genetic background of non-autoimmune-prone mouse strains. Furthermore, deficiency of Aire in an autoimmune-prone strain of NOD also modifies target-cell specificity in the pancreas. In order to clarify the factors that regulate target-organ specificity in Aire-dependent autoimmunity, I have generated both thymic and bone-marrow chimeras, making it possible to evaluate the contribution of thymic stroma and bone-marrow-derived cells to this pathogenic process. The findings suggested that the genetic background of bone-marrow-derived cells contributes to the strain-dependent target-organ specificity of non-autoimmune-prone strains. Furthermore, in a study using NOD mice with a fixed genetic background, thymic stromal cells but not bone-marrow-derived cells were found to be relevant to the Aire-dependent alteration of target-cell specificity in the pancreas. These results clearly underscore the significance of immunological and/or genetic complexity that underlies Aire-deficiency monogenic disease together with critical dialogue between thymic stroma and bone-marrow-derived cells in the organized thymic microenvironment.

The effect of immunosuppressive drug rapamycin on regulatory CD4+CD25+Foxp3+T cells in mice

CD4(+)CD25(+)Regulatory T (Treg) cells are crucial for negatively regulating immune responses. Rapamycin (rapa) is an immunosuppressive agent which is widely used for preventing acute graft rejection in patients and has been used to induce operational tolerance in mouse models. The aim of the present study was to determine the effect of rapa on CD4(+)CD25(+)Foxp3(+)Treg cells in a mouse model. After C57BL/6 mice were intraperitoneally given 1.5 mg/kg/day of rapa for 14 days, the percentages, cell numbers, phenotype and function of CD4(+)CD25(+)Treg cells were determined by flow cytometry as well as the in vitro and in vivo functional assays. The cell numbers of CD4(+) and CD4(+)CD25(+)Treg cell subsets were markedly decreased in rapa-treated mice as reported. However, rapa significantly enhanced the ratios of CD4(+)CD25(+)Treg cells or CD4(+)CD25(+)Foxp3(+)Treg cells to CD4(+)T cells in spleens and thymi of mice (P<0.01) respectively. Furthermore, splenic CD4(+)CD25(+)Treg cells in rapa-treated mice showed immunosuppressive ability on the immune response of T effector cells to alloantigens or mitogen as efficiently as the control CD4(+)CD25(+)Treg cells in vitro and in vivo. Thus, rapa could significantly enhance the percentages of CD4(+)CD25(+)Foxp3(+)Treg cells in the thymus and the periphery while keeping these cells functional, indicating that CD4(+)CD25(+)Treg cells are more resistant to rapa than other CD4(+)T cells. The different effects of rapa on CD4(+)CD25(+)Treg and T effector cells make rapa to be a favorable choice for inducing immune tolerance to self-, allo-, or xeno-antigens.

More promiscuity resulting in more tolerance

Tolerance to self is engendered by multiple mechanisms. Lymph node stromal cells are now found to contribute to self-tolerance by their endogenous expression of peripheral tissue antigens.

Thymus Medulla Formation and Central Tolerance Are Restored in IKKα−/− Mice That Express an IKKα Transgene in Keratin 5+ Thymic Epithelial Cells

Medullary thymic epithelial cells (mTECs) play an essential role in establishing central tolerance due to their unique capacity to present a diverse array of tissue restricted Ags that induce clonal deletion of self-reactive thymocytes. One mTEC subset expresses keratin 5 (K5) and K14, but fails to bind Ulex europaeus agglutinin-1 (UEA-1) lectin. A distinct mTEC subset binds UEA-1 and expresses K8, but not K5 or K14. Development of both mTEC subsets requires activation of the noncanonical NF-kappaB pathway. In this study, we show that mTEC development is severely impaired and autoimmune manifestations occur in mice that are deficient in IkappaB kinase (IKK)alpha, a required intermediate in the noncanonical NF-kappaB signaling pathway. Introduction of an IKKalpha transgene driven by a K5 promoter restores the K5(+)K14(+) mTEC subset in IKKalpha(-/-) mice. Unexpectedly, the K5-IKKalpha transgene also rescues the UEA-1 binding mTEC subset even though K5 expression is not detectable in these cells. In addition, expression of the K5-IKKalpha transgene ameliorates autoimmune symptoms in IKKalpha(-/-) mice. These data suggest that 1) medulla formation and central tolerance depend on activating the alternative NF-kappaB signaling pathway selectively in K5-expressing mTECs and 2) the K5-expressing subset either contains immediate precursors of UEA-1 binding cells or indirectly induces their development.

The role of CD4+ T cell help in cancer immunity and the formulation of novel cancer vaccines

Recent years have seen the unprecedented surge of interest in the role of CD4+ T cells and the role they play in the development of the immune response. In this symposium review, we examine the evidence for this and discuss their functions, particularly in respect to the cancer immunology, including CD4+CD25+ cells (Treg).

Role of B Cells in Pathogenesis of Multiple Sclerosis

Despite the current limited understanding of the etiology of multiple sclerosis (MS), genetic susceptibility and environmental influences are known driving factors. MS is considered a T-cell-mediated disease given the prevalence of T cells in plaques. Plaque formation is characteristic of this disease attributable to immune mechanisms, triggered by an autoimmune attack aimed at antigens in the myelin sheath or oligodendrocyte proteins. The attack consists of the following: The role of the B cells is twofold: first, as autoreactive B cells they produce autoantibodies, secrete cytokines, clonally replicate memory B cells, and long-living plasma cells which serve to advance the diseased state by their constant production of autoantibodies. Second, as antigen-presenting cells they activate the autoreactive T cells. For this reason, the stipulation that T cell is the cornerstone of MS must be reevaluated. Various studies on pathogenesis of MS have indicated that B cells, as the humoral component of the adaptive immune system, are active participants in pathogenesis and lesion maintenance throughout the disease process. The active role of B cells and autoantibodies makes them an encouraging therapeutic target. Advances in the understanding of B-cell development and activity would allow for an enhanced strategy in the design of autoimmune treatment. For this reason, further investigation is necessary to determine whether depletion of B cells or antibodies may restore immune function.

Generation of a tissue-engineered thymic organoid

The thymic microenvironment provides essential support for the generation of a functional and diverse population of human T cells. In particular, the three-dimensional (3D) thymic architecture contributes to critical cell-cell interactions. We report that thymic stroma, arrayed on a synthetic 3D matrix, supports the development of functional human T cells from hematopoietic precursor cells. Newly generated T cells contain T-cell receptor excision circles and are both fully mature and functional. The coculture of T-cell progenitors with thymic stroma can thus be used to generate de novo functional and diverse T-cell populations. This novel tissue engineered thymic system has biological applications for the study of T-lymphopoiesis and self-tolerance as well as potential therapeutic applications including the immune reconstitution of immunocompromised patients and the induction of tolerance in individuals receiving tissue or organ transplants.

Investigating central tolerance with reaggregate thymus organ cultures

In the thymus, immature CD4+8+ thymocytes expressing randomly rearranged T-cell receptor alpha- and beta-chain genes undergo positive and negative selection events based on their ability to recognize self-peptide/major histocompatibility complex (MHC) molecules expressed by thymic stromal cells. In vivo analysis of the role of thymic stromal cells during intrathymic selection is made difficult by the cellular complexity of the thymic microenvironment in the steady-state adult thymus, and by the lack of appropriate targeting strategies to manipulate gene expression in particular thymic stromal compartments. We have shown that the thymic microenvironment can be readily manipulated in vitro through the use of reaggregate thymus organ cultures, which allow the preparation of three-dimensional thymus lobes from defined stromal and lymphoid cells. Although other in vitro systems support some aspects of T-cell development, reaggregate thymus organ culture remains the only in vitro system able to support efficient MHC class I and II-mediated thymocyte selection events, and so can be used as an effective tool to study the cellular and molecular regulation of positive and negative selection in the thymus.

A Potential Side Effect of Cyclosporin A: Inhibition of CD4+CD25+ Regulatory T Cells in Mice

BACKGROUND

CD4CD25 regulatory T (Treg) cells are essential for the induction and maintenance of immunologic self-tolerance as well as transplant tolerance. The effects of cyclosporin A (CsA), a widely used immunosuppressive agent, on CD4CD25Treg cells in mice were investigated.

METHODS

Balb/c mice were injected with CsA or control solution for one month. The levels, phenotype, and function of CD4CD25Treg cells in these mice were then assayed.

RESULTS

The percentages and total cell numbers of CD4CD25Treg cells in the peripheral blood and spleen were significantly reduced after the treatment with CsA. The total numbers of CD4CD25Treg cells in the thymus of CsA-treated mice were markedly reduced as compared to the control mice. However, the percentage of CD4CD25Treg cells in the thymus of CsA-treated mice was markedly enhanced. More CD4CD25Treg cells expressing high levels of CD44 and CD45RB, and less CD4CD25Treg cells expressing CD62L were observed in CsA-treated mice, compared with the control mice. CD4CD25Treg cells expressed slightly lower levels of Foxp3 in CsA-treated mice. Furthermore, CsA markedly impaired the immunosuppressive function of CD4CD25Treg cells.

CONCLUSIONS

CsA significantly impaired the development and function of CD4CD25Treg cells. The present studies suggest that CsA may block the potential induction of immune tolerance and increase the susceptibility to develop autoimmune diseases while preventing graft rejection.

NF-kappaB2 is required for the control of autoimmunity by regulating the development of medullary thymic epithelial cells

Medullary thymic epithelial cells function as antigen-presenting cells in negative selection of self-reactive T cell clones, a process essential for the establishment of central self-tolerance. These cells mirror peripheral tissues through promiscuous expression of a diverse set of tissue-restricted self-antigens. The genes and signaling pathways that regulate the development of medullary thymic epithelial cells are not fully understood. Here we show that mice deficient in NF-kappaB2, a member of the NF-kappaB family, display a marked reduction in the number of mature medullary thymic epithelial cells that express CD80 and bind the lectin Ulex europaeus agglutinin-1, leading to a significant decrease in the extent of promiscuous gene expression in the thymus of NF-kappaB2(-/-) mice. Moreover, NF-kappaB2(-/-) mice manifest autoimmunity characterized by multiorgan infiltration of activated T cells and high levels of autoantibodies to multiple organs. A subpopulation of the mice also develops immune complex glomerulonephritis. These findings identify a physiological function of NF-kappaB2 in the development of medullary thymic epithelial cells and, thus, the control of self-tolerance induction.

CD28 ligation costimulates cell death but not maturation of double-positive thymocytes due to defective ERK MAPK signaling

The differentiation of double-positive (DP) CD4(+)CD8(+) thymocytes to single-positive CD4(+) or CD8(+) T cells is regulated by signals that are initiated by coengagement of the Ag (TCR) and costimulatory receptors. CD28 costimulatory receptors, which augment differentiation and antiapoptotic responses in mature T lymphocytes, have been reported to stimulate both differentiation and apoptotic responses in TCR-activated DP thymocytes. We have used artificial APCs that express ligands for TCR and CD28 to show that CD28 signals increase expression of CD69, Bim, and cell death in TCR-activated DP thymocytes but do not costimulate DP thymocytes to initiate the differentiation program. The lack of a differentiation response is not due to defects in CD28-initiated TCR proximal signaling events but by a selective defect in the activation of ERK MAPK. To characterize signals needed to initiate the death response, a mutational analysis was performed on the CD28 cytoplasmic domain. Although mutation of all of CD28 cytoplasmic domain signaling motifs blocks cell death, the presence of any single motif is able to signal a death response. Thus, there is functional redundancy in the CD28 cytoplasmic domain signaling motifs that initiate the thymocyte death response. In contrast, immobilized Abs can initiate differentiation responses and cell death in DP thymocytes. However, because Ab-mediated differentiation occurs through CD28 receptors with no cytoplasmic domain, the response may be mediated by increased adhesion to immobilized anti-TCR Abs.

CD8+CD28- regulatory T lymphocytes prevent experimental inflammatory bowel disease in mice

BACKGROUND & AIMS

Immune responses to innocuous intestinal antigens appear tightly controlled by regulatory T lymphocytes. While CD4+ T lymphocytes have recently attracted the most attention, CD8+ regulatory T-cell populations are also believed to play an important role in control of mucosal immunity. However, CD8+ regulatory T-cell function has mainly been studied in vitro and no direct in vivo evidence exists that they can control mucosal immune responses. We investigated the capacity of CD8+CD28- T cells to prevent experimental inflammatory bowel disease (IBD) in mice.

METHODS

CD8+CD28- regulatory T cells were isolated from unmanipulated mice and tested for their capacity to inhibit T-cell activation in allogeneic mixed lymphocyte cultures in vitro and to prevent IBD induced by injection of CD4+CD45RB(high) cells into syngeneic immunodeficient RAG-2 mutant mice.

RESULTS

CD8+CD28- T lymphocytes inhibited proliferation and interferon gamma production by CD4+ responder T cells in vitro. CD8+CD28- regulatory T cells freshly isolated from spleen or gut efficiently prevented IBD induced by transfer of colitogenic T cells into immunodeficient hosts. Regulatory CD8+CD28- T cells incapable of producing interleukin-10 did not prevent colitis. Moreover, IBD induced with colitogenic T cells incapable of responding to transforming growth factor beta could not be prevented with CD8+CD28- regulatory T cells. CD8+CD28+ T cells did not inhibit in vitro or in vivo immune responses.

CONCLUSIONS

Our findings show that naturally occurring CD8+CD28- regulatory T lymphocytes can prevent experimental IBD in mice and suggest that these cells may play an important role in control of mucosal immunity.

Identification of pre- and postselection TCRalphabeta+ intraepithelial lymphocyte precursors in the thymus

The immune system preserves and makes use of autoreactive lymphocytes with specialized functions. Here we showed that one of these populations, CD8alphaalpha(+)TCRalphabeta(+) intestinal intraepithelial lymphocytes (IELs), arose from a unique subset of double-positive thymocytes. This subset of cells was precommitted to preferentially give rise to CD8alphaalpha(+)TCRalphabeta(+) IELs, but they required exposure to self-agonist peptides. The agonist-selected TCRalphabeta(+) thymocytes are CD4 and CD8 double-negative, and their final maturation, including the induction of CD8alphaalpha expression, appeared to occur only after thymus export in the IL-15-rich environment of the gut. These developmental steps, including precommitment of immature thymocytes, TCR-mediated agonist selection, and postthymic differentiation promoted by cytokines, define a unique pathway for the generation of CD8alphaalpha(+)TCRalphabeta(+) IEL.

Homeostasis of memory T cells

The pool of memory T cells is regulated by homeostatic mechanisms to persist for prolonged periods at a relatively steady overall size. Recent work has shown that two members of the common gamma chain (gammac) family of cytokines, interleukin-7 (IL-7) and IL-15, govern homeostasis of memory T cells. These two cytokines work in conjunction to support memory T-cell survival and intermittent background proliferation. Normal animals contain significant numbers of spontaneously arising memory-phenotype (MP) cells, though whether these cells are representative of true antigen-specific memory T cells is unclear. Nevertheless, it appears that the two types of memory cells do not display identical homeostatic requirements. For antigen-specific memory CD8+ T cells, IL-7 is primarily important for survival while IL-15 is crucial for their background proliferation. For memory CD4+ T cells, IL-7 has an important role, whereas the influence of IL-15 is still unclear.

Partial versus Full Allogeneic Hemopoietic Chimerization Is a Preferential Means to Inhibit Type 1 Diabetes as the Latter Induces Generalized Immunosuppression

In both humans and NOD mice, particular combinations of MHC genes provide the primary risk factor for development of the autoreactive T cell responses causing type 1 diabetes (T1D). Conversely, other MHC variants can confer dominant T1D resistance, and previous studies in NOD mice have shown their expression on hemopoietically derived APC is sufficient to induce disease protection. Although allogeneic hemopoietic chimerization can clearly provide a means for blocking T1D development, its clinical use for this purpose has been obviated by a requirement to precondition the host with what would be a lethal irradiation dose if bone marrow engraftment is not successful. There have been reports in which T1D-protective allogeneic hemopoietic chimerization was established in NOD mice that were preconditioned by protocols not including a lethal dose of irradiation. In most of these studies, virtually all the hemopoietic cells in the NOD recipients eventually converted to donor type. We now report that a concern about such full allogeneic chimeras is that they are severely immunocompromised potentially because their T cells are positively selected in the thymus by MHC molecules differing from those expressed by the APC available in the periphery to activate T cell effector functions. However, this undesirable side effect of generalized immunosuppression is obviated by a new protocol that establishes without a lethal preconditioning component, a stable state of mixed allogeneic hemopoietic chimerism sufficient to inhibit T1D development and also induce donor-specific tolerance in NOD recipients.

Thymic selection threshold defined by compartmentalization of Ras/MAPK signalling

A healthy individual can mount an immune response to exogenous pathogens while avoiding an autoimmune attack on normal tissues. The ability to distinguish between self and non-self is called 'immunological tolerance' and, for T lymphocytes, involves the generation of a diverse pool of functional T cells through positive selection and the removal of overtly self-reactive thymocytes by negative selection during T-cell ontogeny. To elucidate how thymocytes arrive at these cell fate decisions, here we have identified ligands that define an extremely narrow gap spanning the threshold that distinguishes positive from negative selection. We show that, at the selection threshold, a small increase in ligand affinity for the T-cell antigen receptor leads to a marked change in the activation and subcellular localization of Ras and mitogen-activated protein kinase (MAPK) signalling intermediates and the induction of negative selection. The ability to compartmentalize signalling molecules differentially in the cell endows the thymocyte with the ability to convert a small change in analogue input (affinity) into a digital output (positive versus negative selection) and provides the basis for establishing central tolerance.

The love-hate relationship between bacterial polysaccharides and the host immune system

This article explores the fascinating relationship between the mammalian immune system and the bacteria that are present in the mammalian gut. Every human is an ecosystem that hosts 10(13)-10(14) bacteria. We review the evidence that immunomodulatory molecules produced by commensal bacteria in the gut have a beneficial influence on the development of certain immune responses, through eliciting the clonal expansion of CD4(+) T-cell populations. This process seems to contribute to the overall health of the host by offering protection against various diseases and might provide supporting evidence at a molecular level for the 'hygiene hypothesis' of allergic immune disorders.

Strong-arming immune regulation: suppressing regulatory T-cell function to treat cancers

In recent years there has been an accelerated understanding of immune regulatory mechanisms. Much of this immune regulation is linked to a collection of specialized regulatory cells of the T-cell lineage (Tregs). This collection consists of Tregs that are either thymically derived or peripherally induced. Tregs are important for controlling potentially autoreactive immune effectors and immune responses to foreign organisms and molecules. Their importance in maintaining immune homeostasis and the overall health of an organism cannot be overstated. However, there is a dark side, and Tregs may also be involved in the pathogenesis of malignancies. Evidence shows that tumors induce or recruit Tregs to block antitumor effectors. Thus, there are efforts underway to identify approaches that specifically inhibit the function of intratumoral Tregs, which could lead to increased immunity to tumors without off-target immune-related pathologies (i.e., autoimmune disease). In this review, the biology of Tregs is discussed along with their involvement in malignancies and emerging strategies to block their function.

Agonist ligands expressed by thymic epithelium enhance positive selection of regulatory T lymphocytes from precursors with a normally diverse TCR repertoire

CD4+CD25+ regulatory T lymphocytes play a crucial role in inhibition of autoimmune pathology. In accordance with this physiological role, it is now well established that the repertoire of these lymphocytes is strongly enriched in autospecific cells. However, despite extensive investigation, the thymic mechanisms involved in development of regulatory T cells remain incompletely defined. To address the issue of selection of regulatory T cell precursors in mice with a naturally diverse TCR repertoire, we have analyzed development of superantigen-specific regulatory T cells in hemopoietic chimeras in which endogenous super-antigens are exclusively presented by thymic epithelial cells. Our results demonstrate that recognition of agonist ligands expressed by thymic epithelium does not lead to deletion but substantially enhances development of mature regulatory T cells. Interestingly, also development of a small subpopulation of CD25-expressing T cells lacking expression of the transcription factor Foxp3, thought to be autospecific, is enhanced by expression of the agonist ligand on thymic epithelium. Based on quantitative arguments, we propose that commitment to the regulatory T cell lineage is not dictated by the specificity of precursors, but that recognition of the agonist ligand expressed by thymic epithelium substantially enhances their positive selection.

T-cell immunity to the folate receptor alpha is prevalent in women with breast or ovarian cancer

PURPOSE

Studies have demonstrated that the generation of immunity to tumor antigens is associated with improved prognosis for many cancers. A candidate antigen is the folate receptor alpha (FRalpha), which is overexpressed in breast and ovarian cancers. Our goal in this study was to attain a better understanding of the extent of endogenous FRalpha immunity.

METHODS

Using a CD4+ T cell epitope prediction algorithm, we predicted promiscuous epitopes of FRalpha, and tested for immunity in 30 breast (n = 17) or ovarian (n = 13) cancer patients and 18 healthy donors using enzyme-linked immunospot analysis.

RESULTS

Fourteen peptides were predicted, seven each from the carboxy- and amino-terminus halves of the protein. More than 70% of patients demonstrated immunity to at least one FRalpha peptide. Patients responded to an average of 3 +/- 0.5 peptides, whereas healthy donors responded to 1 +/- 0.4 peptides (P = .004). Five peptides were recognized by more than 25% of patients. Responses to three peptides were higher (P < .05) in patients than in healthy donors, suggesting augmented immunity. Compared with healthy individuals, patients developed higher immunity to the amino-terminus half of the receptor (P = .03). There was no difference between each group in the responses to nonspecific (P = .2) and viral stimuli (P = .5). Lastly, patients demonstrated elevated levels of FRalpha antibodies consistent with a coordinated immune response.

CONCLUSION

These findings demonstrate that the FRalpha is a target of the immune system in breast and ovarian cancer patients. Understanding which antigens are targeted by the immune system may be important for prognosis or immune-based therapies.

The TCR C beta FG loop regulates alpha beta T cell development

The TCRbeta chain constant domain contains an unusually elongated, solvent-exposed FG loop. This structural element forms one component of an alphabeta TCR cavity against which CD3epsilongamma may abut to facilitate Ag-specific signaling. Consistent with this notion, in the present study we show that N15alphabeta TCR transfectants expressing a FG loop-deleted chain (betaDeltaFG) stimulate less tyrosine protein phosphorylation than those bearing a wild-type beta-chain (betawt) upon TCR cross-linking. Furthermore, coimmunoprecipitation studies suggest a weakened association between the CD3epsilongamma heterodimer and the beta-chain in TCR complexes containing the betaDeltaFG variant. To further investigate the biologic role of the Cbeta FG loop in development, we competitively reconstituted the thymus of Ly5 congenic or RAG-2-/- mice using bone marrow cells from betawt or betaDeltaFG transgenic C57BL/6 (B6) mice. Both betawt and betaDeltaFG precursor cells generate thymocytes representative of all maturational stages. However, betaDeltaFG-expressing thymocytes dominate during subsequent development, resulting in an excess of betaDeltaFG-expressing peripheral T cells with reduced proliferative and cytokine production abilities upon TCR stimulation. Collectively, our results show that the unique Cbeta FG loop appendage primarily controls alphabeta T cell development through selection processes.

Metallophilic macrophages are lacking in the thymus of lymphotoxin-beta receptor-deficient mice

Lymphotoxin-beta receptor (LTbetaR) axis plays a crucial role in development and compartmentalization of peripheral lymphatic organs. But, it is also required for the appropriate function and maintenance of structural integrity of the thymus: in LTbetaR-deficient animals the clonal deletion of autoreactive lymphocytes is impaired and differentiation of thymic medullary epithelial cells is disturbed. In this study, using several markers, we showed that thymic metallophilic macrophages were lacking in LTbetaR-deficient mice. In tumor necrosis factor receptor-I (p55)-deficient mice (which we used as positive control) thymic metallophilic cells were located, similarly as in normal mice, in the thymic cortico-medullary zone at the junction of cortex and medulla. These findings show that LTbetaR is necessary for maintenance of metallophilic macrophages in the thymus and provide further evidence that these cells may represent a factor involved in thymic negative selection.

A central role for central tolerance

Recent elucidation of the role of central tolerance in preventing organ-specific autoimmunity has changed our concepts of self/nonself discrimination. This paradigmatic shift is largely attributable to the discovery of promiscuous expression of tissue-restricted self-antigens (TRAs) by medullary thymic epithelial cells (mTECs). TRA expression in mTECs mirrors virtually all tissues of the body, irrespective of developmental or spatio-temporal expression patterns. This review summarizes current knowledge on the cellular and molecular regulation of TRA expression in mTECs, outlines relevant mechanisms of antigen presentation and modes of tolerance induction, and discusses implications for the pathogenesis of autoimmune diseases and other biological processes such as fertility, pregnancy, puberty, and tumor defense.

Peripheral tolerance via the anterior chamber of the eye: role of B cells in MHC class I and II antigen presentation

Ags introduced into the anterior chamber (AC) of the eye induce a form of peripheral immune tolerance termed AC-associated immune deviation (ACAID). ACAID mitigates ocular autoimmune diseases and promotes corneal allograft survival. Ags injected into the AC are processed by F4/80(+) APCs, which migrate to the thymus and spleen. In the spleen, ocular APCs induce the development of Ag-specific B cells that act as ancillary APCs and are required for ACAID induction. In this study, we show that ocular-like APCs elicit the generation of Ag-specific splenic B cells that induce ACAID. However, direct cell contact between ocular-like APCs and splenic B cells is not necessary for the induction of ACAID B cells. Peripheral tolerance produced by ACAID requires the participation of ACAID B cells, which induce the generation of both CD4(+) regulatory T cells (Tregs) and CD8(+) Tregs. Using in vitro and in vivo models of ACAID, we demonstrate that ACAID B cells must express both MHC class I and II molecules for the generation of Tregs. These results suggest that peripheral tolerance induced through the eye requires Ag-presenting B cells that simultaneously present Ags on both MHC class I and II molecules.

Organ specific autoantigens and the autoreactiveT cell repertoire: the case of myelin oligodendrocyte glycoprotein

There is strong evidence that immunological self tolerance critically relies on the elimination of potentially autoaggressive T lymphocyte clones from the emerging immune repertoire during intrathymic T cell differentiation. These 'forbidden' T cells are deleted as a result of a confrontation with their specific self antigen as presented on medullary stroma cells. But this purging mechanism is remarkably leaky, allowing numerous autoreactive T cells to join the healthy immune repertoire. A paper in this issue of the European Journal of Immunology studies the effect of organ-specific autoantigen expression on the cognate T cell repertoire. Myelin oligodendrocyte glycoprotein (MOG), a putative autoantigen in human multiple sclerosis, is used as a model self antigen. T cell receptor profiles in wild-type mice were compared with those in MOG-knock-out mice. Surprisingly, significant differences were not found suggesting that, in this particular case, autoantigen expression does not affect the autoreactive T cell repertoire.

Delayed functional maturation of natural regulatory T cells in the medulla of postnatal thymus: role of TSLP

BACKGROUND

Generation of functional (CD4+)(CD8-)CD25+ regulatory T cells (Treg) in the murine thymus depends on FoxP3. Removal of the thymus from neonatal mice has been shown to result in a multiple organ autoimmune disease phenotype that can be prevented by introducing the FoxP3+ Treg population to the animal. It has therefore, been proposed that functional FoxP3+ Treg cells are not made in the neonatal thymus; however, it remains unclear when and where functional (FoxP3+)(CD4+)(CD8-)CD25+ thymocytes are generated in postnatal thymus.

RESULTS

We report that neither FoxP3 mRNA nor protein is expressed in (CD4+)(CD8-)CD25+, or (CD4+)(CD8-)CD25- thymocytes until 3-4 days post birth, despite the presence of mature (CD4+)(CD8-)CD25+/- thymocytes in the thymus by 1-2 days after birth. (FoxP3-)(CD4+)(CD8-)CD25+ thymocytes from day 2 newborn mice show no Treg activity. Interestingly, we are able to detect low numbers of FoxP3+ thymocytes dispersed throughout the medullary region of the thymus as early as 3-4 days post birth. Expression of FoxP3 is induced in embryonic day 17 fetal thymus organ culture (FTOC) after 4-6 days of in vitro culture. Treatment of FTOCs with thymic stromal derived lymphopoietin (TSLP) enhanced expression of FoxP3, and blocking the TSLP receptor reduces FoxP3 expression in FTOC. Furthermore, TSLP stimulates FoxP3 expression in purified (CD4+)CD8- thymocytes, but not in (CD4+)CD8+, (CD4-)CD8+ and (CD4-)CD8- thymocytes.

CONCLUSION

Expression of FoxP3 or Treg maturation is ontogenically distinct and kinetically delayed from the generation of (CD4+)(CD8-)CD25+ or (CD4+)(CD8-)CD25- thymocytes in the postnatal thymus. TSLP produced from medullary thymic epithelia cells (mTEC) contributes to the expression of FoxP3 and the maturation of natural regulatory T cells. Overall, these results suggest that the development of Treg cells requires paracrine signaling during late stages of thymocyte maturation that is distinct from signaling during positive or negative selection.

Remnant epitopes, autoimmunity and glycosylation

The role of extracellular proteolysis in innate and adaptive immunity and the interplay between cytokines, chemokines and proteinases are gradually becoming recognized as critical factors in autoimmune processes. Many of the involved proteinases, including those of the plasminogen activator and matrix metalloproteinase cascades, and also several cytokines and chemokines, are glycoproteins. The stability, interactions with inhibitors or receptors, and activities of these molecules are fine-controlled by glycosylation. We studied gelatinase B or matrix metalloproteinase-9 (MMP-9) as a glycosylated enzyme involved in autoimmunity. In the joints of rheumatoid arthritis patients, CXC chemokines, such as interleukin-8/CXCL8, recruit and activate neutrophils to secrete prestored neutrophil collagenase/MMP-8 and gelatinase B/MMP-9. Gelatinase B potentiates interleukin-8 at least tenfold and thus enhances neutrophil and lymphocyte influxes to the joints. When cartilage collagen type II is cleaved at a unique site by one of several collagenases (MMP-1, MMP-8 or MMP-13), it becomes a substrate of gelatinase B. Human gelatinase B cleaves the resulting two large collagen fragments into at least 33 peptides of which two have been shown to be immunodominant, i.e., to elicit activation and proliferation of autoimmune T cells. One of these two remnant epitopes contains a glycan which is important for its immunoreactivity. In addition to the role of gelatinase B as a regulator in adaptive immune processes, we have also demonstrated that it destroys interferon-beta, a typical innate immunity effector molecule and therapeutic cytokine in multiple sclerosis. Furthermore, glycosylated interferon-beta, expressed in Chinese hamster ovary cells, was more resistant to this proteolysis than recombinant interferon-beta from bacteria. These data not only prove that glycosylation of proteins is mechanistically important in the pathogenesis of autoimmune diseases, but also show that targeting of glycosylated proteinases or the use of glycosylated cytokines seems also critical for the treatment of autoimmune diseases.

Co-evolution of a primordial peptide-presentation system and cellular immunity

How did early vertebrates survive when their lymphocytes began to use antigen receptors with random specificities, despite their potential for extensive self-reactivity? Here, I propose that the quality-control mechanisms that tame self-reactivity in the adaptive immune system were derived, at least in part, from an ancient mechanism that guided sexual selection on the basis of evaluating genetic relatedness.