Differing roles for B7 and intercellular adhesion molecule-1 in negative selection of thymocytes

The role of Toll-like receptors in neuropsychiatric disorders: Immunopathology, treatment, and management

Neuropsychiatric disorders denote a broad range of illnesses involving neurology and psychiatry. These disorders include depressive disorders, anxiety, schizophrenia, bipolar disorder, attention deficit hyperactivity disorder, autism spectrum disorders, headaches, and epilepsy. In addition to their main neuropathology that lies in the central nervous system (CNS), lately, studies have highlighted the role of immunity and neuroinflammation in neuropsychiatric disorders. Toll-like receptors (TLRs) are innate receptors that act as a bridge between the innate and adaptive immune systems via adaptor proteins (e.g., MYD88) and downstream elements; TLRs are classified into 13 families that are involved in normal function and illnesses of the CNS. TLRs expression affects the course of neuropsychiatric disorders, and is influenced during their pharmacotherapy; For example, the expression of multiple TLRs is normalized during the major depressive disorder pharmacotherapy. Here, the role of TLRs in neuroimmunology, treatment, and management of neuropsychiatric disorders is discussed. We recommend longitudinal studies to comparatively assess the cell-type-specific expression of TLRs during treatment, illness progression, and remission. Also, further research should explore molecular insights into TLRs regulation and related pathways.

The Adaptation Model of Immunity: Signal IV Matters Most in Determining the Functional Outcomes of Immune Responses

Current research in immunology and immunotherapy is fully influenced by the self-nonself model of immunity. This theoretical model suggests that alloreactivity results in graft rejection, whereas tolerance toward self-antigens expressed by malignant cells facilitates cancer development. Similarly, breakage of immunological tolerance toward self-antigens results in autoimmune diseases. Accordingly, immune suppression is recommended for the management of autoimmune diseases, allergy, and organ transplantation, whereas immune inducers are used for the treatment of cancers. Although the danger model, the discontinuity model, and the adaptation model are proposed for a better understanding of the immune system, the self-nonself model continues to dominate the field. Nevertheless, a cure for these human diseases remains elusive. This essay discusses current theoretical models of immunity, as well as their impacts and limitations, and expands on the adaptation model of immunity to galvanize a new direction for the treatment of autoimmune diseases, organ transplantation, and cancer.

The adaptation model of immunity: Is the goal of central tolerance to eliminate defective T cells or self-reactive T cells?

The self-non-self model and the danger model are designed to understand how an immune response is induced. These models are not meant to predict if an immune response may succeed or fail in destroying/controlling its target. However, these immunological models rely on either self-antigens or self-dendritic cells for understanding of central tolerance, which have been discussed by Fuchs and Matzinger in response to Al-Yassin. In an attempt to address some questions that these models are facing when it comes to understanding central tolerance, I propose that the goal of negative selection in the thymus is to eliminate defective T cells but not self-reactive T cells. Therefore, any escape from negative selection could increase lymphopenia because of the depletion of defective naïve T cells outside the thymus, as seen in the elderly.

B7-CD28 co-stimulation modulates central tolerance via thymic clonal deletion and Treg generation through distinct mechanisms

The molecular and cellular mechanisms mediating thymic central tolerance and prevention of autoimmunity are not fully understood. Here we show that B7-CD28 co-stimulation and B7 expression by specific antigen-presenting cell (APC) types are required for clonal deletion and for regulatory T (Treg) cell generation from endogenous tissue-restricted antigen (TRA)-specific thymocytes. While B7-CD28 interaction is required for both clonal deletion and Treg induction, these two processes differ in their CD28 signaling requirements and in their dependence on B7-expressing dendritic cells, B cells, and thymic epithelial cells. Meanwhile, defective thymic clonal deletion due to altered B7-CD28 signaling results in the accumulation of mature, peripheral TRA-specific T cells capable of mediating destructive autoimmunity. Our findings thus reveal a function of B7-CD28 co-stimulation in shaping the T cell repertoire and limiting autoimmunity through both thymic clonal deletion and Treg cell generation.

Measuring Thymic Clonal Deletion at the Population Level

Clonal deletion of T cells specific for self-antigens in the thymus has been widely studied, primarily by approaches that focus on a single receptor (using TCR transgenes) or a single specificity (using peptide-MHC tetramers). However, less is known about clonal deletion at the population level. In this article, we report an assay that measures cleaved caspase 3 to define clonal deletion at the population level. This assay distinguishes clonal deletion from apoptotic events caused by neglect and approximates the anatomic site of deletion using CCR7. This approach showed that 78% of clonal deletion events occur in the cortex in mice. Medullary deletion events were detected at both the semimature and mature stages, although mature events were associated with failed regulatory T cell induction. Using this assay, we showed that bone marrow-derived APC drive approximately half of deletion events at both stages. We also found that both cortical and medullary deletion rely heavily on CD28 costimulation. These findings demonstrate a useful strategy for studying clonal deletion within the polyclonal repertoire.

Regulatory T-cell development and function are impaired in mice lacking membrane expression of full length intercellular adhesion molecule-1

To further investigate the contribution of intercellular adhesion molecule-1 (ICAM-1) to adaptive immune responses, we analysed T-cell development and function in mice lacking full-length ICAM-1 (ICAM-1(tm1Jcgr) ). Compared with wild-type (ICAM-1(WT) ) mice, ICAM-1(tm1Jcgr) mice have impaired thymocyte development. Proportions and numbers of double negative, double positive, mature CD4(+) and CD8(+) thymocytes, as well as of regulatory T (Treg) cells were also significantly decreased. In the periphery, ICAM-1(tm1Jcgr) mice had significantly decreased proportions and numbers of naive and activated/memory CD4(+) and CD8(+) T cells, as well as of Treg cells, in lymph nodes but not in the spleen. In vitro activation of CD4(+) and CD8(+) T cells from ICAM-1(tm1Jcgr) mice with anti-CD3 antibodies and antigen-presenting cells (APCs) resulted in a significantly weaker proliferation, whereas proliferation induced with anti-CD3 and anti-CD28 antibody-coated beads was normal. In vivo immunization of ICAM-1(tm1Jcgr) mice resulted in normal generation of specific effector and memory immune responses that protect against a viral challenge. However, contrary to ICAM-1(WT) mice, immunization-induced specific effectors could not eradicate immunogen-expressing tumours. Treg cells from ICAM-1(tm1Jcgr) mice have abnormal activation and proliferation induced by anti-CD3 antibody and APCs, and have markedly decreased suppressive activity in vitro. In contrast to ICAM-1(WT) mice, they were unable to control experimentally induced colitis in vivo. Hence, our results further highlight the pleiotropic role of ICAM-1 in T-cell-dependent immune responses, with a major role in Treg cell development and suppressive function.

A monoclonal antibody against the extracellular domain of mouse and human epithelial V-like antigen 1 reveals a restricted expression pattern among CD4- CD8- thymocytes

Expression of transcripts for the homotypic adhesion protein epithelial V-like antigen 1 (EVA1), also known as myelin protein zero like-2 (Mpzl2), is known to be present in thymic stromal cells. However, protein expression within different thymic subsets, stromal and/or lymphoid, has not been characterized due a lack of specific reagents. To address this, we generated a hybridoma (G9P3-1) secreting a monoclonal antibody (G9P3-1Mab), reactive against both human and mouse EVA1. The G9P3-1Mab was generated by immunizing Mpzl2-deficient gene-targeted mice with the extracellular domain of EVA1, followed by a conventional hybridoma fusion protocol, illustrating the feasibility of using gene-targeted mice to generate monoclonal antibodies with multiple species cross-reactivity. We confirmed expression of EVA1 on cortical and medullary epithelial cell subsets and revealed a restricted pattern of expression on CD4- CD8- double negative (DN) cell subsets, with the highest level of expression on DN3 (CD44(low)CD25(+)) thymocytes. G9P3-1MAb is a valuable reagent to study thymic T cell development and is likely useful for the analysis of pathological conditions affecting thymopoiesis, such as thymic involution caused by stress or aging.

Signals and pathways controlling regulatory T cells

Induction of specific immune tolerance to grafts remains the sought-after standard following transplantation. Defined by expression of the Foxp3 (forkhead box protein 3) transcription factor, the regulatory T-cell (Treg) lineage has been noted to exert potent immunoregulatory functions that contribute to specific graft tolerance. In this review, we discuss the known signals and pathways which govern Treg development, both in the thymus and in peripheral sites, as well as lineage maintenance and homeostasis. In particular, we highlight the roles of T-cell receptor signaling, CD28 costimulation, and signals through phosphatidyl inositol 3-kinase (PI3K) and related metabolic pathways in multiple aspects of Treg biology.

A role for Ly108 in the induction of promyelocytic zinc finger transcription factor in developing thymocytes

The promyelocytic zinc finger transcription factor (PLZF) is required for the development of activated phenotypes in NKT and other innate T lymphocytes. Although strong TCR stimulation has been implicated in the induction of PLZF, the factors regulating PLZF expression are incompletely understood. We show in this study that costimulation of preselection double-positive thymocytes through the signaling lymphocyte activation molecule family receptor Ly108 markedly enhanced PLZF expression compared with that induced by TCR stimulation alone. Costimulation with Ly108 increased expression of early growth response protein (Egr)-2 and binding of Egr-2 to the promoter of Zbtb16, which encodes PLZF, and resulted in PLZF levels similar to those seen in NKT cells. In contrast, costimulation with anti-CD28 failed to enhance Egr-2 binding and Zbtb16 expression. Moreover, mice lacking Ly108 showed decreased numbers of PLZF-expressing CD4(+) T cells. Together, these results support a potential role for Ly108 in the induction of PLZF.

Dock8 mutations cripple B cell immunological synapses, germinal centers and long-lived antibody production

To identify genes and mechanisms involved in humoral immunity, we did a mouse genetic screen for mutations that do not affect the first wave of antibody to immunization but disrupt response maturation and persistence. The first two mutants identified had loss-of-function mutations in the gene encoding a previously obscure member of a family of Rho-Rac GTP-exchange factors, DOCK8. DOCK8-mutant B cells were unable to form marginal zone B cells or to persist in germinal centers and undergo affinity maturation. Dock8 mutations disrupted accumulation of the integrin ligand ICAM-1 in the B cell immunological synapse but did not alter other aspects of B cell antigen receptor signaling. Humoral immunodeficiency due to Dock8 mutation provides evidence that organization of the immunological synapse is critical for signaling the survival of B cell subsets required for long-lasting immunity.

TSCOT+ thymic epithelial cell-mediated sensitive CD4 tolerance by direct presentation

Although much effort has been directed at dissecting the mechanisms of central tolerance, the role of thymic stromal cells remains elusive. In order to further characterize this event, we developed a mouse model restricting LacZ to thymic stromal cotransporter (TSCOT)-expressing thymic stromal cells (TDLacZ). The thymus of this mouse contains approximately 4,300 TSCOT+ cells, each expressing several thousand molecules of the LacZ antigen. TSCOT+ cells express the cortical marker CDR1, CD40, CD80, CD54, and major histocompatibility complex class II (MHCII). When examining endogenous responses directed against LacZ, we observed significant tolerance. This was evidenced in a diverse T cell repertoire as measured by both a CD4 T cell proliferation assay and an antigen-specific antibody isotype analysis. This tolerance process was at least partially independent of Autoimmune Regulatory Element gene expression. When TDLacZ mice were crossed to a novel CD4 T cell receptor (TCR) transgenic reactive against LacZ (BgII), there was a complete deletion of double-positive thymocytes. Fetal thymic reaggregate culture of CD45- and UEA-depleted thymic stromal cells from TDLacZ and sorted TCR-bearing thymocytes excluded the possibility of cross presentation by thymic dendritic cells and medullary epithelial cells for the deletion. Overall, these results demonstrate that the introduction of a neoantigen into TSCOT-expressing cells can efficiently establish complete tolerance and suggest a possible application for the deletion of antigen-specific T cells by antigen introduction into TSCOT+ cells.

Requirements for selection of conventional and innate T lymphocyte lineages

Mice deficient in the Tec kinase Itk develop a large population of CD8(+) T cells with properties, including expression of memory markers, rapid production of cytokines, and dependence on Interleukin-15, resembling NKT and other innate T cell lineages. Like NKT cells, these CD8(+) T cells can be selected on hematopoietic cells. We demonstrate that these CD8(+) T cell phenotypes resulted from selection on hematopoietic cells-forcing selection on the thymic stroma reduced the number and innate phenotypes of mature Itk-deficient CD8(+) T cells. We further show that, similar to NKT cells, selection of innate-type CD8(+) T cells in Itk(-/-) mice required the adaptor SAP. Acquisition of their innate characteristics, however, required CD28. Our results suggest that SAP and Itk reciprocally regulate selection of innate and conventional CD8(+) T cells on hematopoietic cells and thymic epithelium, respectively, whereas CD28 regulates development of innate phenotypes resulting from selection on hematopoietic cells.

Vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1) provide co-stimulation in positive selection along with survival of selected thymocytes

T-cell differentiation in the thymus depends on positive selection of CD4+CD8+ double positive (DP) thymocytes by thymic major histocompatibility complex (MHC) molecules. Positive selection allows maturation of only those thymocytes that are capable of self-peptide-MHC recognition. Thymocytes that fail to bind self-peptide-MHC die by apoptosis. An important question in thymocyte differentiation is whether co-stimulation is required for positive selection and on which cells co-stimulatory molecules may be expressed in the thymus. The vascular cell adhesion molecule (VCAM-1) and the intercellular cell adhesion molecule (ICAM-1) are known to be potent co-stimulatory molecules in activation of peripheral T-cells by interacting with the integrins VLA-4 and LFA-1, respectively. We were prompted to investigate whether VCAM-1 and ICAM-1 may also act as co-stimulators during selection of thymocytes. By using recombinant proteins of murine VCAM-1 and ICAM-1 fused to the Fc region of human IgG1 (rVCAM-1, rICAM-1) we examined the capacity of VCAM-1 and ICAM-1 to act as co-stimulatory molecules in positive selection in vitro. Triggering the CD3/TCR complex together with co-stimulation applied by rVCAM-1 or rICAM-1 induced the generation of CD4+ single positive (SP) thymocytes from CD4+CD8+ DP thymocytes whereas either signal alone did not result in generation of CD4+ SP thymocytes. VCAM-1 and ICAM-1 act therefore as co-stimulatory molecules in thymocyte positive selection in vitro. The generation of CD4+ SP cells is accompanied by cell survival both when it was co-stimulated with rVCAM-1 and with rICAM-1. Importantly we show here that VCAM-1 expression in the murine thymus is restricted to cortical F4/80 positive hematopoietic antigen presenting cells (hAPC) present exclusively in the cortex whereas expression of ICAM-1 has been reported on the epithelium both in cortex and medulla. This suggests that not only the cortical epithelium may use the co-stimulatory molecule ICAM-1 to mediate positive selection, but also cortical hAPCs may contribute to positive selection of thymocytes by using the co-stimulator VCAM-1.

Zoned out: functional mapping of stromal signaling microenvironments in the thymus

All hematopoietic cells, including T lymphocytes, originate from stem cells that reside in the bone marrow. Most hematopoietic lineages also mature in the bone marrow, but in this respect, T lymphocytes differ. Under normal circumstances, most T lymphocytes are produced in the thymus from marrow-derived progenitors that circulate in the blood. Cells that home to the thymus from the marrow possess the potential to generate multiple T and non-T lineages. However, there is little evidence to suggest that, once inside the thymus, they give rise to anything other than T cells. Thus, signals unique to the thymic microenvironment compel multipotent progenitors to commit to the T lineage, at the expense of other potential lineages. Summarizing what is known about the signals the thymus delivers to uncommitted progenitors, or to immature T-committed progenitors, to produce functional T cells is the focus of this review.

Toll-like receptors: linking innate and adaptive immunity

Work in recent years has shown an essential role for Toll-like receptors (TLRs) in the activation of innate and adaptive immunity in vertebrate animals. These germ-line encoded receptors, expressed on a diverse variety of cells and tissues, recognize conserved molecular products derived from various classes of pathogens, including Gram-positive and -negative bacteria, DNA and RNA viruses, fungi and protozoa. Ligand recognition induces a conserved host defense program, which includes production of inflammatory cytokines, upregulation of costimulatory molecules, and induction of antimicrobial defenses. Importantly, activation of dendritic cells by TLR ligands is necessary for their maturation and consequent ability to initiate adaptive immune responses. How responses are tailored by individual TLRs to contain specific classes of pathogens is not yet clear.

Immature CD4+CD8+ thymocytes and mature T cells regulate Nur77 distinctly in response to TCR stimulation

The orphan steroid receptor, Nur77, is thought to be a central participant in events leading to TCR-mediated clonal deletion of immature thymocytes. Interestingly, although both immature and mature murine T cell populations rapidly up-regulate Nur77 after TCR stimulation, immature CD4+CD8+ thymocytes respond by undergoing apoptosis, whereas their mature descendants respond by dividing. To understand these developmental differences in susceptibility to the proapoptotic potential of Nur77, we compared its regulation and compartmentalization and show that mature, but not immature, T cells hyperphosphorylate Nur77 in response to TCR signals. Nur77 resides in the nucleus of immature CD4+CD8+ thymocytes throughout the course of its expression and is not found in either the organellar or cytoplasmic fractions. However, hyperphosphorylation of Nur77 in mature T cells, which is mediated by both the MAPK and PI3K/Akt pathways, shifts its localization from the nucleus to the cytoplasm. The failure of immature CD4+CD8+ thymocytes to hyperphosphorylate Nur77 in response to TCR stimulation may be due in part to decreased Akt activity at this developmental stage.

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.

Toll-like receptors: linking innate and adaptive immunity

Detection of and response to microbial infections by the immune system depends largely on a family of pattern-recognition receptors called Toll-like receptors (TLRs). These receptors recognize conserved molecular products derived from various classes of pathogens, including Gram-positive and -negative bacteria, DNA and RNA viruses, fungi and protozoa. Recognition of ligands by TLRs leads to a series of signaling events resulting in induction of acute responses necessary to kill the pathogen. TLRs are also responsible for the induction of dendritic cell maturation, which is responsible and necessary for initiation of adaptive immune responses. Although TLRs control induction of adaptive immunity, it is not clear at this point how responses are appropriately tailored by individual TLRs to the advantage of the host.

The impact of thymic antigen diversity on the size of the selected T cell repertoire

The TCR repertoire of a normal animal is shaped in the thymus by ligand-specific positive- and negative-selection events. These processes are believed to be determined at the single-cell level primarily by the affinity of the TCR-ligand interactions. The relationships among all the variables involved are still unknown due to the complexity of the interactions and the lack of quantitative analysis of those parameters. In this study, we developed a quantitative model of thymic selection that provides estimates of the fractions of positively and negatively selected thymocytes in the cortex and in the medulla, as well as upper-bound ranges for the number of selecting ligands required for the generation of a normal diverse TCR repertoire. Fitting the model to current estimates of positive- and negative-selected thymocytes leads to specific predictions. The results indicate the following: 1) the bulk of thymocyte death takes place in the cortex, and it is due to neglect; 2) the probability of a thymocyte to be negatively selected in the cortex is at least 10-fold lower than in the medulla; 3) <60 ligands are involved in cortical positive selection; and 4) negative selection in the medulla is constrained by a large diversity of selecting ligands on medullary APCs.

Back to central tolerance

The establishment and maintenance of immunological tolerance entails both central and peripheral mechanisms. The latter have been highlighted in the past several years, mostly because of great interest in the activities of regulatory T cells. However, an important role for central tolerance mechanisms has been reemphasized by recent results on human autoimmune diseases, including APECED and type 1 diabetes.

Ligand-dependent regulation of T cell development and activation

Mature CD4+ and CD8 + T lymphocytes develop in the thymus from precursors with diverse clonally distributed receptors, possessing binding sites with negligible, intermediate, or high affinity for selfpeptide major histocompatibility complex (MHC) ligands. Positive- and negative-selection processes acting on this precursor pool yield a peripheral T cell population comprised of cells with receptors (TCR) capable of self-peptide MHC ligand recognition, but largely depleted of those able to mediate overt self-responsiveness. The Lymphocyte Biology Section of the Laboratory of Immunology studies how self-ligand recognition guides T cell development in the thymus and influences the functionality of naive and activated T cells in the periphery. It also seeks to define the molecular basis for the discrimination between self-ligands and foreign antigens that controls T cell activation to effector function. Finally, it uses a combination of conventional cellular immunological methods, biochemical and biophysical studies, and advanced imaging techniques to visualize, quantitate, and model the various steps in the development of primary and memory T cell immune responses.

Thymus cell-cell interactions

The recent advances in molecular biology and genetics, as well as the progress of in vitro techniques, have provided a more coherent image of the thymic function on the molecular level. But they have shifted the attention away from studies on the cellular level, which are necessary to clarify the biological roles of different cell types of the thymic microenvironment. The structure and function of the normal thymus depend on mutual interactions between thymocytes and nonlymphocyte cells. In this review a detailed description of morphological and phenotypic features of both maturing thymocytes and nonlymphocyte cells is given. The recent genetic and biochemical data are presented in conjunction with cytological results to enlighten the thymus cell-cell interactions during thymopoiesis and organization of thymic microstructure. Special emphasis is put on the experimental approaches, which may be used to study the interactions between thymocytes and nonlymphocyte cells in vivo.

Thymic generation and regeneration

The thymus is a complex epithelial organ in which thymocyte development is dependent upon the sequential contribution of morphologically and phenotypically distinct stromal cell compartments. It is these microenvironments that provide the unique combination of cellular interactions, cytokines, and chemokines to induce thymocyte precursors to undergo a differentiation program that leads to the generation of functional T cells. Despite the indispensable role of thymic epithelium in the generation of T cells, the mediators of this process and the differentiation pathway undertaken by the primordial thymic epithelial cells are not well defined. There is a lack of lineage-specific cell-surface-associated markers, which are needed to characterize putative thymic epithelial stem cell populations. This review explores the role of thymic stromal cells in T-cell development and thymic organogenesis, as well as the molecular signals that contribute to the growth and expansion of primordial thymic epithelial cells. It highlights recent advances in these areas, which have allowed for a lineage relationship amongst thymic epithelial cell subsets to be proposed. While many fundamental questions remain to be addressed, collectively these works have broadened our understanding of how the thymic epithelium becomes specialized in the ability to support thymocyte differentiation. They should also facilitate the development of novel, rationally based therapeutic strategies for the regeneration and manipulation of thymic function in the treatment of many clinical conditions in which defective T cells have an important etiological role.

A role for the B7-1/B7-2:CD28/CTLA-4 pathway during negative selection

Although costimulation plays an important role in activating naive T cells, its role in negative selection is controversial. By following thymocyte deletion induced by endogenous superantigens in mice lacking B7-1 and/or B7-2, we have identified a role for both B7-1 and B7-2 in negative selection. Studies using CD28-deficient and CD28/CTLA-4-double-deficient mice have revealed that either CD28 or another as yet undefined coreceptor can mediate these B7-dependent signals that promote negative selection. Finally, CTLA-4 delivers signals that inhibit selection, suggesting that CTLA-4 and CD28 have opposing functions in thymic development. Combined, the data demonstrate that B7-1/B7-2-dependent signals help shape the T cell repertoire.

Con A activates an Akt/PKB dependent survival mechanism to modulate TCR induced cell death in double positive thymocytes

While low avidity ligation of the T cell receptor (TCR) leads to positive selection and further maturation of developing thymocytes providing the immune system with mature CD4(+) and CD8(+) (single positive) T cells, high avidity ligation triggers negative selection by apoptotic cell death and therefore the TCR repertoire is purged of autoreactive T cells. On peripheral T cells, however, high avidity ligation of the TCR triggers activation and survival not death. In the present study we used concanavalin A (Con A) and alpha-CD3 epsilon antibody to investigate a possible survival mechanism in connection with TCR ligation. Con A and alpha-CD3 epsilon were used in the study for the following reasons: (1) they both mimic the effects of high avidity TCR ligation by activating peripheral T cells, and (2) they trigger distinctively different physiological changes in developing thymocytes. While Con A supports events associated with cellular survival, alpha-CD3 epsilon induces apoptotic cell death. In our experimental system the TCR was cross-linked by Con A and alpha-CD3 epsilon in thymocytes of major histocompatibility complex (MHC) deficient thymus organ cultures, where signals from the TCR can be triggered on zero background signal level. We have found that TCR cross-linking by Con A and not by alpha-CD3 epsilon decreases the gene and protein expression of the pro-apoptotic molecule, Bad; and that Con A is capable of the activation of the survival signalling pathway including protein kinase B (Akt/PKB) independently of phosphatidyl inositol kinase (PI3K).

CD28 signal enhances apoptosis of CD8 T cells after strong TCR ligation

High avidity ligation of the TCR induces negative selection in the thymus and can also induce apoptosis of peripheral T cells. Costimulation through CD28 enhances T cell activation and facilitates negative selection in the thymus, but the role of CD28 in peripheral T cell deletional tolerance has not been investigated. We used 2C CD28 wild-type and 2C CD28-deficient strains to assess the effects of CD28 and TCR avidity on peripheral T cell expansion and apoptosis. We compared the activation, division, expansion, and apoptosis of CD28(+/+) and CD28(-/-) 2C cells in response to self-Ag (K(b)), alloantigens with intermediate (K(bm3)), high (L(d)), or very high (L(d) + QL9 peptide) avidity. With intermediate avidity alloantigen, the CD28 signal enhanced T cell activation and expansion. However, when T cells encountered high avidity alloantigen, the CD28 signal reduced T cell expansion and increased apoptosis. These results indicate that the CD28 signal can down-regulate peripheral T cell responses by increasing apoptosis when TCR ligation exceeds a critical threshold.

Cutting edge: identification of the targets of clonal deletion in an unmanipulated thymus

Autoreactive thymocytes can be eliminated by clonal deletion during their development in the thymus. The precise developmental stage(s) at which clonal deletion occurs in a normal thymus has been difficult to assess, in large part because of the absence of a specific marker for TCR-mediated apoptosis. In this report, we reveal that Nur77 expression can be used as a specific marker of clonal deletion in an unmanipulated thymus and directly identify TCRintCD4+CD8+ and semimature CD4+CD8- thymocytes as the principal targets of deletion. These data indicate that clonal deletion normally occurs at a relatively late stage of development, as cells mature from CD4+CD8+ thymocytes to single-positive T cells.

The role of fibroblasts in thymocyte-positive selection

Mice with fibroblast-specific expression of TAP-1 were generated by expressing the TAP-1 transgene under the control of the fibroblast-specific protein (FSP) 1 promoter/enhancer on TAP-1-deficient background. MHC class I expression in primary fibroblast cultures isolated from the resulting strain mimicked that of wild-type counterparts. MHC class I was detected in both types of fibroblasts following treatment with IFN-alphabeta. Positive selection of CD4(-)CD8(+) thymocytes was observed in neither adult nor fetal/neonatal thymus of transgenic mice. IFN-alphabeta-induced expression of MHC class I rescued positive selection of CD4(-)CD8(+) T cells in fetal thymic organ cultures, but not in adult mice. Contrary to previous suggestions, our results indicate a limited role of fibroblasts in promoting positive selection. In addition, the results suggest that positive selection may occur by a different mechanism in fetal vs adult thymus.

The thymus and negative selection

Maintenance of tolerance to self antigens is presumed to reflect a combination of central and peripheral tolerance. For T cells, central tolerance occurs during early T cell development in the thymus and causes cells with strong reactivity to self antigens to be destroyed in situ (negative selection). Here, we summarize evidence that negative selection can occur in the thymic medulla and affects a population of semimature HSA+ T cells. The influence of costimulatory molecules, Fas and cytokines on negative selection is discussed.

Immature CD4(+)CD8(+) thymocytes form a multifocal immunological synapse with sustained tyrosine phosphorylation

The immunological synapse formed during mature T cell activation consists of a central cluster of TCR and MHC molecules surrounded by a ring of LFA-1 and ICAM-1. We examined synapse formation in thymocytes undergoing activation in a lipid bilayer system by following the movement of fluorescent MHC and ICAM-1 molecules. Immature CD4(+)CD8(+) thymocytes formed a decentralized synapse with multiple foci of MHC accumulation corresponding to areas of exclusion of ICAM-1. The MHC clusters and ICAM-1 holes were mobile and transient and correlated with active and sustained signaling, as shown by staining with antibodies against phosphotyrosine and activated Lck. Our findings show that signaling in immature thymocytes can result from a novel, multifocal pattern of receptor accumulation.

B7-CTLA4 interaction promotes cognate destruction of tumor cells by cytotoxic T lymphocytes in vivo

Costimulatory molecules B7-1 and B7-2 (hereby collectively called B7) interact with CD28 and CTLA4 on T cells and promote antitumor immunity. The function of B7-CTLA4 interaction in antitumor CTL response remains controversial. Here we used CD28(-/-) and CD28(+/-) or CD28(+/+) transgenic mice that express the T-cell receptor specific for an unmutated tumor antigen, P1A, and for tumor cells expressing a CTLA4-specific B7 mutant to evaluate the function of CD28-B7 and CTLA4-B7 interactions in induction and effector phases of antitumor immunity. We report that B7-CD28 and B7-CTLA4 interactions promote tumor rejection. However, this is achieved by distinct mechanisms. B7-CD28 interaction enhances T-cell clonal expansion, though a role for this interaction in the effector phase cannot be ruled out. In contrast, B7-CTLA4 interaction enhances the CTL-mediated destruction of tumors, but not T-cell clonal expansion.

Perinatal blockade of b7-1 and b7-2 inhibits clonal deletion of highly pathogenic autoreactive T cells

A number of in vitro studies have suggested that costimulatory molecules B7-1 and B7-2 and their receptor CD28 can promote clonal deletion, and limited in vivo studies have indicated that CD28 is involved in the clonal deletion of some T cells. However, the significance of B7-mediated clonal deletion in preventing autoimmune diseases has not been studied systematically. Here we report that the perinatal blockade of B7-1 and B7-2 substantially inhibits the clonal deletion of T cells in the thymus and leads to an accumulation of T cells capable of inducing fatal multiorgan inflammation. These results reveal a critical role for costimulatory molecules B7-1 and B7-2 in deleting pathogenic autoreactive T cells in the thymus. The critical role of B7-1 and B7-2 in T cell clonal deletion may explain, at least in part, the paradoxical increase of autoimmune disease in mice deficient for this family of costimulatory molecules, such as cytotoxic T lymphocyte associated molecule 4, CD28, and B7-2. The strong pathogenicity of the self-reactive T cells supports a central hypothesis in immunology, which is that clonal deletion plays an important role in preventing autoimmune diseases.

Aging leads to disturbed homeostasis of memory phenotype CD8(+) cells

Examining the rate of in vivo T cell turnover (proliferation) in aged mice revealed a marked reduction in turnover at the level of memory-phenotype CD44(hi) CD8(+) cells relative to young mice. Based on adoptive transfer experiments, the reduced turnover of aged CD44(hi) CD8(+) cells reflected an inhibitory influence of the aged host environment. Aged CD44(hi) CD8(+) cells also showed poor in vivo responses to IL-15 and IL-15-inducing agents, but responded well to IL-15 in vitro. Two mechanisms could account for the reduced turnover of aged CD44(hi) CD8(+) cells in vivo. First, aging was associated with a prominent and selective increase in Bcl-2 expression in CD44(hi) CD8(+) cells. Hence, the reduced turnover of aged CD44(hi) CD8(+) cells may in part reflect the antiproliferative effect of enhanced Bcl-2 expression. Second, the impaired in vivo response of aged CD44(hi) CD8(+) cells to IL-15 correlated with increased serum levels of type I interferons (IFN-I) and was largely reversed by injection of anti-IFN-I antibody. Hence the selective reduction in the turnover of aged CD44(hi) CD8(+) cells in vivo may reflect the combined inhibitory effects of enhanced Bcl-2 expression and high IFN-I levels.

The thymus and central tolerance

T-cell differentiation in the thymus generates a peripheral repertoire of mature T cells that mounts strong responses to foreign antigens but is largely unresponsive to self-antigens. This state of specific immunological tolerance to self-components involves both central and peripheral mechanisms. Here we review the process whereby many T cells with potential reactivity for self-antigens are eliminated in the thymus during early T-cell differentiation. This process of central tolerance (negative selection) reflects apoptosis and is a consequence of immature T cells receiving strong intracellular signalling through T-cell receptor (TCR) recognition of peptides bound to major histocompatibility complex (MHC) molecules. Central tolerance occurs mainly in the medullary region of the thymus and depends upon contact with peptide-MHC complexes expressed on bone-marrow-derived antigen-presenting cells (APCs); whether tolerance also occurs in the cortex is still controversial. Tolerance induction requires a combination of TCR ligation and co-stimulatory signals. Co-stimulation reflects interaction between complementary molecules on T cells and APCs and probably involves multiple molecules acting in consort, which may account for why deletion of individual molecules with known or potential co-stimulatory function has little or no effect on central tolerance. The range of self-antigens that induce central tolerance is considerable and, via low-level expression in the thymus, may also include tissue-specific antigens; central tolerance to these latter antigens, however, is likely to be limited to high-affinity T cells, leaving low-affinity cells to escape. Tolerance to alloantigens and the possibility of using central tolerance to promote acceptance of allografts are discussed.

UEA-I-binding to thymic medullary epithelial cells selectively reduces numbers of cortical TCRalphabeta+ thymocytes in FTOCs

Thymic medullary epithelial cells (TMECs) constitute a major stromal cell type, the function of which is incompletely understood. Some TMECs express L-fucose-glycosylated proteins on their plasma membrane; these have been shown to specifically bind the lectin UEA-I. We exploited this observation to investigate the consequences of in situ blockage of TMECs in FTOCs by UEA-I. In UEA-I-treated FTOCs, we noted a decreased cellularity among TCRalphabeta+ but not TCRgammadelta+ cells. In fact, CD3- and CD3lo cortical cells were markedly depleted, while CD3hi cells were unaffected. Since the affected cell subsets are in a different compartment from that where UEA-I binding occurs, it is likely that the effect is mediated through a soluble factor. Two possible mechanisms are proposed: a reduced activation of either TMECs or of medullary thymocytes which normally bind to them, results in lowered production of soluble factors responsible for cortical thymocyte proliferation. Alternately, the binding of UEA-I to TMECs could activate the latter to produce signals inhibitory to cortical thymocytes.

Immature CD4+CD8+ thymocytes do not polarize lipid rafts in response to TCR-mediated signals

TCR-mediated stimulation induces activation and proliferation of mature T cells. When accompanied by signals through the costimulatory receptor CD28, TCR signals also result in the recruitment of cholesterol- and glycosphingolipid-rich membrane microdomains (lipid rafts), which are known to contain several molecules important for T cell signaling. Interestingly, immature CD4(+)CD8(+) thymocytes respond to TCR/CD28 costimulation not by proliferating, but by dying. In this study, we report that, although CD4(+)CD8(+) thymocytes polarize their actin cytoskeleton, they fail to recruit lipid rafts to the site of TCR/CD28 costimulation. We show that coupling of lipid raft mobilization to cytoskeletal reorganization can be mediated by phosphoinositide 3-kinase, and discuss the relevance of these findings to the interpretation of TCR signals by immature vs mature T cells.

Opening a window on thymic positive selection: developmental changes in the influence of cosignaling by integrins and CD28 on selection events induced by TCR engagement

How TCR and non-TCR signals are integrated by thymocytes to generate a decision to undergo either positive or negative selection remains incompletely understood. Recent evidence suggests that TCR signal transduction changes its quality during thymocyte maturation, but whether the contributions of various cosignaling or costimulatory pathways to thymocyte selection also are modified during development is unclear. Questions also remain about the possible selective roles of specific costimulatory pathways in induction of differentiation vs death among thymocytes at any given stage of maturity. To address these issues, a quantitative in vitro analysis of initiation of CD4+CD8+ thymocyte differentiation as measured by CD69 up-regulation/coreceptor down-modulation was conducted in parallel with an analysis of induction of death. Using transfected cells varying in their surface display of ICAM-1 or B7.1 along with antibody blocking experiments, we demonstrate here that ICAM-1 provides a selective boost to signaling for differentiation without substantially affecting induction of death among CD4+CD8+ cells, a property that is lost as thymocytes mature further. In contrast, B7 engagement enhances both cell activation and death in parallel. Based on these data, we propose that the high level of ICAM-1 on cortical epithelial cells plays a special role in opening a window between TCR signaling for differentiation vs death, permitting efficient initiation of positive selection on epithelial ligands. In contrast, late CD28-dependent cosignaling on hemopoietic cells in the medulla would help enforce negative selection by augmenting the effects of TCR engagement by low levels of high affinity ligands.

Transcriptional control of T cell development

Transcriptional control of T cell development is a complex and rapidly moving area of investigation. Recent advances reveal critical roles for several transcription factors in T cell commitment, differentiation and selection. In particular, new roles for E proteins as well as members of the Notch signaling pathway have been described. Additionally, a unique function of Ikaros in chromatin remodeling reveals a novel mechanism by which transcriptional control may be exerted.

The thymus and central tolerance

T cell tolerance to self-components occurs largely in the thymus during early differentiation and leads to death (negative selection) of T cells with overt autoreactivity. In this article we review the evidence that negative selection in mice occurs mainly in the medulla at the level of a population of semimature T cells. The role of Fas and several costimulatory molecules on negative selection and the inhibitory role of certain cytokines are discussed.

Disruption of positive selection of thymocytes causes autoimmunity

To differentiate into T cells, immature thymocytes must engage, through their antigen-specific T-cell receptor, peptides derived from self proteins presented by cortical epithelial cells in the thymus, a process called positive selection. Despite this requirement for self-recognition during development, mature T cells do not normally show autoreactivity. Mice injected in the thymus with procainamide-hydroxylamine, a metabolite of procainamide, develop autoimmune features resembling drug-induced lupus. Here, we show that when thymocytes undergo positive selection in the presence of procainamide-hydroxylamine, they fail to establish unresponsiveness to low affinity selecting self antigens, resulting in systemic autoimmunity.

Abnormal thymic expression of epithelial cell adhesion molecule (EP-CAM) in New Zealand Black (NZB) mice

New Zealand Black (NZB) mice have been well documented to have a variety of thymic epithelial cell microenvironmental abnormalities, including disruption of corticoepithelial cell networks and medullary cell clusters. These abnormalities of the thymic stromal network are particularly important because similar observations have been noted in other models of murine lupus. Thymic epithelial cells, a key component of the microenvironment, play an important role in selection of the mature T cell receptor repertoire. Recently, a homotypic calcium-independent human and murine epithelial cell adhesion molecule, Ep-CAM, has been described which is located at the thymocyto-cortical cell junction. The function of Ep-CAM is still unclear but its unique location within the thymus suggests that it is critical in the process of providing maturation signals. Consequently, we examined the thymic expression of Ep-CAM in a series of autoimmune prone mice by thymic distribution of Ep-CAM in NZB, NZW, NZB/W, BXSB-Yaa, MRL- lpr/lpr, C3H- gld/gld and the control strains BALB/c, C57BL6, C3H and MRL(+/+), by immunohistology and flow cytometry. Interestingly, NZB mice are similar to control mice from day 4 to 2 weeks of age, having a very low expression of Ep-CAM at the thymocyto-cortical junction. In control strains, there is a marked increased in expression of Ep-CAM beginning at 5 weeks of age. In contrast, NZB mice fail to show significant expression of Ep-CAM even well into adulthood. This abnormality of NZB mice was also noted in NZB/W F1 and BXSB mice, but not MRL- lpr/lpr or C3H- gld/gld mice. Given the potential importance of Ep-CAM in thymic selection, this study provides important evidence that a defective stromal microenvironment is likely to be of etiological significance in the susceptibility of NZB to autoimmune disease.

Thymocyte apoptosis

Apoptosis is the fate of most thymocytes. Many molecules participate in the decision of whether a thymocyte is to live or to die, including cell surface receptors, such as the T cell receptor for antigen, Notch-1, and costimulatory receptors, ligand-regulated nuclear transcription factors such as the glucocorticoid receptor, signaling, and effector proteases, and direct regulators of the apoptotic machinery such IAPs. In this review we discuss recent data concerning these molecules and pathways and their implication for understanding the mechanisms underlying thymocyte death, survival, and the generation of inmmunocompetent T cells.

Receptor avidity and costimulation specify the intracellular Ca2+ signaling pattern in CD4(+)CD8(+) thymocytes

Thymocyte maturation is governed by antigen-T cell receptor (TCR) affinity and the extent of TCR aggregation. Signals provided by coactivating molecules such as CD4 and CD28 also influence the fate of immature thymocytes. The mechanism by which differences in antigen-TCR avidity encode unique maturational responses of lymphocytes and the influence of coactivating molecules on these signaling processes is not fully understood. To better understand the role of a key second messenger, calcium, in governing thymocyte maturation, we measured the intracellular free calcium concentration ([Ca2+]i) response to changes in TCR avidity and costimulation. We found that TCR stimulation initiates either amplitude- or frequency-encoded [Ca2+]i changes depending on (a) the maturation state of stimulated thymocytes, (b) the avidity of TCR interactions, and (c) the participation of specific coactivating molecules. Calcium signaling within immature but not mature thymocytes could be modulated by the avidity of CD3/CD4 engagement. Low avidity interactions induced biphasic calcium responses, whereas high avidity engagement initiated oscillatory calcium changes. Notably, CD28 participation converted the calcium response to low avidity receptor engagement from a biphasic to oscillatory pattern. These data suggest that calcium plays a central role in encoding the nature of the TCR signal received by thymocytes and, consequently, a role in thymic selection.

Strong TCR Ligation Without Costimulation Causes Rapid Onset of Fas-Dependent Apoptosis of Naive Murine CD4+ T Cells

Activation-induced cell death of T cells typically occurs late in the primary response after a prior proliferative response. Here, we describe a novel form of cell death in which purified naive murine CD4+ cells undergo apoptosis within 18 h in vitro after strong TCR ligation. Such rapid-onset TCR-mediated death of T cells does not involve cell division and is Fas-dependent, inhibited by CD28 (and IL-6) costimulation and enhanced by IL-4 and IL-7; by contrast, spontaneous death of CD4+ cells cultured alone is Fas-independent and inhibited by IL-4 and IL-7. TCR-mediated Fas-dependent death of CD4+ cells is prevented by combined TCR/Fas ligation and by drugs that inhibit calcineurin-dependent signaling and mitogen-activated protein kinase MEK1 activation.

Several different cell surface molecules control negative selection of medullary thymocytes

Repeated attempts to show that costimulation for negative selection is controlled by a single cell surface molecule have been unsuccessful. Thus, negative selection may involve multiple cell surface molecules acting in consort. In support of this idea, we show here that at least three cell surface molecules, namely CD28, CD5, and CD43, contribute to Fas-independent negative selection of the tolerance-susceptible population of heat-stable antigen (HSA)hiCD4+8- cells found in the medulla. The costimulatory function of these three molecules can be blocked by certain cytokines, IL-4 and IL-7, and coinjecting these cytokines with antigen in vivo abolishes negative selection; Fas-dependent negative selection, however, is maintained. The results suggest that efficient negative selection requires the combined functions of at least four cell surface molecules: CD28, CD5, CD43, and Fas.

Selection of the T cell repertoire

Advances in gene technology have allowed the manipulation of molecular interactions that shape the T cell repertoire. Although recognized as fundamental aspects of T lymphocyte development, only recently have the mechanisms governing positive and negative selection been examined at a molecular level. Positive selection refers to the active process of rescuing MHC-restricted thymocytes from programmed cell death. Negative selection refers to the deletion or inactivation of potentially autoreactive thymocytes. This review focuses on interactions during thymocyte maturation that define the T cell repertoire, with an emphasis placed on current literature within this field.

Divergent changes in the sensitivity of maturing T cells to structurally related ligands underlies formation of a useful T cell repertoire

CD4+ CD8+ thymocyte differentiation requires TCR signaling induced by self-peptide/MHC ligands. Nevertheless, the resulting mature T cells are not activated by these self-complexes, whereas foreign ligands can be potent stimuli. Here, we show that the signaling properties of TCR change during thymocyte maturation, differentially affecting responses to related peptide/MHC molecule complexes and contributing to this discrimination. Weak agonists for CD4+ CD8+ thymocytes lose potency during development, accompanied by a change in TCR-associated phosphorylation from an agonist to a partial agonist/antagonist pattern. In contrast, sensitivity to strong agonists is maintained, along with full signaling. This yields a mature T cell pool highly responsive to foreign antigen while possessing a wide margin of safety against activation by self-ligands.