Requirement for a complex array of costimulators in the negative selection of autoreactive thymocytes in vivo

Immunotherapy to CD5, a T-cell antigen having roles from development to peripheral function: Future prospective and challenges

CD5 is a pan T-cell marker expressed by all T-cells and a subset of B-cells, i.e., B1a cells. The significance of CD5 is evident from its functions, starting from T-cell development, antigen priming, activation, and effector response to the maintenance of tolerance. Varying CD5 expression and signaling in response to TCR-pMHC complex avidity is associated with thymic selection, competency, and effector response. Altered CD5 expression is associated with immunological and diseased conditions such as CD5-/low infiltrating T-cells in solid tumors, CD5hi T-cells in anergy conditions, CD5-/low phenotype of leukemic T-cells, high CD5 expression by regulatory T-cells, CD5lowphenotype of autoreactive T-cells, etc. A low CD5 expression triggers activation-induced cell death upon antigenic stimulation. There are three forms of CD5: membrane CD5 (mCD5), intracellular CD5 (cCD5) and soluble CD5 (sCD5). mCD5 and cCD5 are generated from conventional and non-conventional mRNA variants, i.e., E1A and E1B, respectively. E1B variant encoding cCD5 is derived from a human endogenous retrovirus segment inserted 8.2 kb upstream to conventional E1A exon. Various conditions, such as leukemia, exposure to hydrocarbon, hypoxia, etc., can trigger E1B transcription and, thus, cCD5 expression. Blocking mCD5 with mAb can restore immune response, effectively targeting cancer. Understanding cCD5, linked to leukemogenesis, can offer new avenues of immunotherapy.

Cellular and molecular signaling towards T cell immunological self-tolerance

The binding of a cognate antigen to T cell receptor (TCR) complex triggers a series of intracellular events controlling T cell activation, proliferation, and differentiation. Upon TCR engagement, different negative regulatory feedback mechanisms are rapidly activated to counterbalance T cell activation, thus preventing excessive signal propagation and promoting the induction of immunological self-tolerance. Both positive and negative regulatory processes are tightly controlled to ensure the effective elimination of foreign antigens while limiting surrounding tissue damage and autoimmunity. In this context, signals deriving from co-stimulatory molecules (i.e., CD80, CD86), co-inhibitory receptors (PD-1, CTLA-4), the tyrosine phosphatase CD45 and cytokines such as IL-2 synergize with TCR-derived signals to guide T cell fate and differentiation. The balance of these mechanisms is also crucial for the generation of CD4+ Foxp3+ regulatory T cells, a cellular subset involved in the control of immunological self-tolerance. This review provides an overview of the most relevant pathways induced by TCR activation combined with those derived from co-stimulatory and co-inhibitory molecules implicated in the cell-intrinsic modulation of T cell activation. In addition to the latter, we dissected mechanisms responsible for T cell-mediated suppression of immune cell activation through regulatory T cell generation, homeostasis, and effector functions. We also discuss how imbalanced signaling derived from TCR and accessory molecules can contribute to autoimmune disease pathogenesis.

The ICOS-ICOSL pathway tunes thymic selection

Negative selection of developing T cells plays a significant role in T cell tolerance to self-antigen. This process relies on thymic antigen presenting cells which express both self-antigens as well as co-signaling molecules. Inducible T cell costimulator (ICOS) belongs to the CD28 family of co-signaling molecules and binds to ICOS ligand (ICOSL). The ICOS signaling pathway plays important roles in shaping the immune response to infections, but its role in central tolerance is less well understood. Here we show that ICOSL is expressed by subsets of thymic dendritic cells and medullary thymic epithelial cells as well as thymic B cells. ICOS expression is upregulated as T cells mature in the thymus and correlates with T cell receptor signal strength during thymic selection. We also provide evidence of a role for ICOS signaling in mediating negative selection. Our findings suggest that ICOS may fine-tune T cell receptor signals during thymic selection contributing to the generation of a tolerant T cell population.

GSK3-mediated instability of tubulin polymers is responsible for the failure of immature CD4+CD8+ thymocytes to polarize their MTOC in response to TCR stimulation

Although mature T cells divide and differentiate when they receive strong TCR stimulation, most immature CD4+CD8+ thymocytes die. The molecular basis for this marked difference in response is not known. Observations that TCR-stimulated CD4+CD8+ thymocytes fail to polarize their microtubule-organizing center (MTOC), one of the first events that occurs upon antigen activation of mature T cells, suggests that TCR signaling routes in immature and mature T cells diverge early and upstream of MTOC polarization. To better understand the source of the divergence, we examined the molecular basis for the difference in TCR-mediated MTOC polarization. We show that unstable microtubules are a feature of immature murine CD4+CD8+ thymocytes, which also exhibit higher levels of glycogen synthase kinase 3 (GSK3) activity, a known inhibitor of microtubule stability. Importantly, CD4+CD8+ thymocytes gained the ability to polarize their MTOC in response to TCR signals when GSK3 activity was inhibited. GSK3 inhibition also abrogated TCR-mediated apoptosis of immature thymocytes. Together, our results suggest that a developmentally regulated difference in GSK3 activity has a major influence on immature CD4+CD8+ thymocyte versus mature T-cell responses to TCR stimulation.

Intercellular MHC transfer between thymic epithelial and dendritic cells

Thymic dendritic cells (DC) and epithelial cells play a major role in central tolerance but their respective roles are still controversial. Epithelial cells have the unique ability to ectopically express peripheral tissue-restricted antigens conferring self-tolerance to tissues. Paradoxically, while negative selection seems to occur for some of these antigens, epithelial cells, contrary to DC, are poor negative selectors. Using a thymic epithelial cell line, we show the functional intercellular transfer of membrane material, including MHC molecules, occurring between epithelial cells. Using somatic and bone marrow chimeras, we show that this transfer occurs efficiently in vivo between epithelial cells and, in a polarized fashion, from epithelial to DC. This novel mode of transfer of MHC-associated, epithelial cell-derived self-antigens onto DC might participate to the process of negative selection in the thymic medulla.

A specific anti-Aire antibody reveals aire expression is restricted to medullary thymic epithelial cells and not expressed in periphery

Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy is an autoimmune disorder caused by mutations in the autoimmune regulator gene AIRE. We examined the expression of Aire in different organs (thymus, spleen, and lymph nodes) in C57BL/6 mice, using a novel rat mAb, specific for murine Aire. Using flow cytometry, directly fluorochrome-labeled mAb revealed Aire expression in a rare thymic cellular subset that was CD45(-), expressed low levels of Ly51, and was high for MHC-II and EpCam. This subset also expressed a specific pattern of costimulatory molecules, including CD40, CD80, and PD-L1. Immunohistochemical analysis revealed that Aire(+) cells were specifically localized to the thymus or, more precisely, to the cortico-medulla junction and medulla, correlating with the site of negative selection. Although in agreement with previous studies, low levels of Aire mRNA was detected in all dendritic cell subtypes however lacZ staining, immunohistochemistry and flow cytometry failed to detect Aire protein. At a cellular level, Aire was expressed in perinuclear speckles within the nucleus. This report provides the first detailed analysis of Aire protein expression, highlighting the precise location at both the tissue and cellular level.

Foxp3+ regulatory T cells promiscuously accept thymic signals critical for their development

Foxp3(+) regulatory T cells develop in the thymus and are essential for maintaining peripheral tolerance to self tissues. We report the critical requirement for CD154 up-regulation specifically on, and during the thymic development of, Foxp3(+) regulatory T cells for the induction of their clonal expansion within the medulla. In the absence of this signal, there was a severe reduction in their thymic generation and output, leading to decreased peripheral numbers. Importantly, CD40 expression on either thymic dendritic or epithelial cells was sufficient to promote the development of normal numbers of Foxp3(+) regulatory T cells. This work suggests that CD154-transduced signals promote Foxp3(+) regulatory T cell development and highlights the plasticity of the thymic stroma for supporting their generation. Crucially, this study demonstrates that Foxp3(+) regulatory T cells can promiscuously accept a single critical signal necessary for their thymic development from different cellular sources, redefining our understanding of their generation.

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.

Functional and Molecular Comparison of Anergic and Regulatory T Lymphocytes

Tolerance in vivo is maintained by multiple mechanisms that function to prevent autoimmunity. An encounter of CD4+ T cells with a circulating self-Ag leads to partial thymic deletion, the development of CD25+ regulatory T cells (Tregs), and functional anergy in the surviving CD25- population. We have compared anergic and regulatory T cells of the same Ag specificity generated in vivo by the systemic self-Ag. Anergic cells are unresponsive to the self-Ag that induces tolerance, but upon transfer into a new host and immunization, anergic cells can induce a pathologic autoimmune reaction against tissue expressing the same Ag. Tregs, in contrast, are incapable of mediating harmful reactions. To define the basis of this functional difference, we have compared gene expression profiles of anergic and regulatory T cells. These analyses show that Tregs express a distinct molecular signature, but anergic cells largely lack such a profile. Anergic cells express transcripts that are associated with effector differentiation, e.g., the effector cytokines IL-4 and IFN-gamma. Anergic cells do not produce these cytokines in response to self-Ag, because the cells exhibit a proximal signaling block in response to TCR engagement. Thus, anergy reflects an aborted activation pathway that can readily be reversed, resulting in pathologic effector cell responses, whereas Treg development follows a distinct developmental pathway that extinguishes effector functions.

EphA Receptors Inhibit Anti-CD3-Induced Apoptosis in Thymocytes

The EphA receptor tyrosine kinases interact with membrane-bound ligands of the ephrin-A subfamily. Interaction induces EphA receptor oligomerization, tyrosine phosphorylation, and, as a result, EphA receptor signaling. EphA receptors have been shown to regulate cell survival, migration, and cell-cell and cell-matrix interactions. However, their functions in lymphoid cells are only beginning to be described. We show in this study that functional EphA receptors are expressed by murine thymocytes, including CD4(+)CD8(+), CD4(+)CD8(-), and CD4(-)CD8(+) subpopulations. We demonstrate that activation of EphA receptors by the ephrin-A1 ligand inhibits the anti-CD3-induced apoptosis of CD4(+)CD8(+) double-positive thymocytes. Furthermore, ephrin-A1 costimulation suppresses up-regulation of both the IL-2R alpha-chain (CD25) and early activation Ag CD69 and can block IL-2 production by CD4(+) single-positive cells. In agreement, EphA receptor activation in thymocytes also inhibits TCR-induced activation of the Ras-MAPK pathway. Our findings suggest that EphA receptor activation is antithetical to TCR signaling in thymocytes, and that the level of engagement by ephrin-A proteins on thymic APCs regulates thymocyte selection.

The timing of TCR alpha expression critically influences T cell development and selection

Sequential rearrangement of the T cell receptor for antigen (TCR) beta and alpha chains is a hallmark of thymocyte development. This temporal control is lost in TCR transgenics because the alpha chain is expressed prematurely at the CD4- CD8- double negative (DN) stage. To test the importance of this, we expressed the HY alpha chain at the physiological CD4+ CD8+ double positive (DP) stage. The reduced DP and increased DN cellularity typically seen in TCR transgenics was not observed when the alpha chain was expressed at the appropriate stage. Surprisingly, antigen-driven selection events were also altered. In male mice, thymocyte deletion now occurred at the single positive or medullary stage. In addition, no expansion of CD8 alpha alpha intestinal intraepithelial lymphocytes (IELs) was observed, despite the fact that HY transgenics have been used to model IEL development. Collectively, these data establish the importance of proper timing of TCR expression in thymic development and selection and emphasize the need to use models that most accurately reflect the physiologic process.

DNA vaccination with an insulin construct and a chimeric protein binding to both CTLA4 and CD40 ameliorates type 1 diabetes in NOD mice

Type 1 diabetes (T1D), a T-cell-mediated autoimmune disease, could be attributed to many defects in nonobese diabetic (NOD) mice, including deficient expressions of costimulatory molecules that impair antigen presentation. Thus, this deficient antigen presentation may result in a reduced ability to induce a tolerogenic response through negative selection/regulation of autoreactive T cells. Improperly activated T cells seem to be able to induce autoimmune responses causing diabetes. To re-establish tolerance to autoantigens by modulating costimulation, we constructed and tested a new type of DNA vaccine encoding a membrane-bound preproinsulin (mbPPI) and a chimeric gene vector encoding mutant B7.1/CD40L (mB7.1/CD40L) fusion protein. This mutant B7.1 binds CTLA4 but not CD28. We report that young NOD mice immunized with mbPPI along with mB7.1/CD40L DNA vectors significantly reduced diabetes incidence while treatment with CTLA4/IgG1 exacerbated diabetes. In conclusion, the combination of mbPPI and mB7.1/CD40L was able to protect against autoimmunity and diabetes in NOD mice possibly by promoting a more efficient presentation of autoantigen PPI and inducing specific tolerance to PPI by negatively regulating autoreactive T cells.

The B7/CD28 costimulatory family in autoimmunity

Host defense is dependent on the appropriate induction of immune responses. A central concept in immunology is the ability of the immune system to differentiate foreign from self-antigens. The failure of the immune response to recognize foreign pathogens can result in infection and disease in the host. The inappropriate response of the immune system to self-antigens is equally problematic, leading to autoimmune disease. Central and peripheral tolerance mechanisms control self-reactive T-cell responses and protect peripheral tissues from autoimmune attack. This review examines the roles of B7/CD28 family members, which can augment or antagonize T-cell receptor signaling, in the regulation of central and peripheral T-cell tolerance. We also discuss how B7/CD28 pathways influence both T-cell-intrinsic and -extrinsic mechanisms of regulation.

Ikaros null mice display defects in T cell selection and CD4 versus CD8 lineage decisions

Previous evidence suggested that the hemopoietic-specific nuclear factor Ikaros regulates TCR signaling thresholds in mature T cells. In this study, we test the hypothesis that Ikaros also sets TCR signaling thresholds to regulate selection events and CD4 vs CD8 lineage determination in developing thymocytes. Ikaros null mice were crossed to three lines of TCR-transgenic mice, and positive selection, negative selection, and CD4 vs CD8 lineage decisions were analyzed. Mice expressing a polyclonal repertoire or a MHC class II-restricted TCR transgene exhibited enhanced positive selection toward the CD4 lineage. Moreover, in the absence of Ikaros, CD4 development can occur with decreased thresholds of TCR signaling. In addition, CD4 single-positive thymocytes were detected in MHC class I-restricted TCR-transgenic Ikaros null mice. To assess the role of Ikaros in negative selection, we analyzed deletion of T cells induced by conventional Ag or by endogenous superantigen. Surprisingly, negative selection was impaired in Ikaros null thymocytes despite evidence of high levels of TCR signal and no intrinsic defect in apoptosis ex vivo. To our knowledge, these data identify Ikaros as the first nuclear factor that plays a critical role in regulating negative selection as well as CD4 vs CD8 lineage decisions during positive selection.

Role of B7 in T cell tolerance

The induction of effective immune responses requires costimulation by B7 molecules, and Ag recognition without B7 is thought to result in no response or tolerance. We compared T cell responses in vivo to the same Ag presented either by mature dendritic cells (DCs) or as self, in the presence or absence of B7. We show that Ag presentation by mature B7-1/2-deficient DCs fails to elicit an effector T cell response but does not induce tolerance. In contrast, using a newly developed adoptive transfer system, we show that naive OVA-specific DO11 CD4+ T cells become anergic upon encounter with a soluble form of OVA, in the presence or absence of B7. However, tolerance in DO11 cells transferred into soluble OVA transgenic recipients can be broken by immunization with Ag-pulsed DCs only in B7-deficient mice and not in wild-type mice, suggesting a role of B7 in maintaining tolerance in the presence of strong immunogenic signals. Comparing two double-transgenic models--expressing either a soluble or a tissue Ag--we further show that B7 is not only essential for the active induction of regulatory T cells in the thymus, but also for their maintenance in the periphery. Thus, the obligatory role of B7 molecules paradoxically is to promote effective T cell priming and contain effector responses when self-Ags are presented as foreign.

B7-CD28 Interaction Promotes Proliferation and Survival but Suppresses Differentiation of CD4−CD8− T Cells in the Thymus

Costimulatory molecules play critical roles in the induction and effector function of T cells. More recent studies reveal that costimulatory molecules enhance clonal deletion of autoreactive T cells as well as generation and homeostasis of the CD25(+)CD4(+) regulatory T cells. However, it is unclear whether the costimulatory molecules play any role in the proliferation and differentiation of T cells before they acquire MHC-restricted TCR. In this study, we report that targeted mutations of B7-1 and B7-2 substantially reduce the proliferation and survival of CD4(-)CD8(-) (double-negative (DN)) T cells in the thymus. Perhaps as a result of reduced proliferation, the accumulation of RAG-2 protein in the DN thymocytes is increased in B7-deficient mice, which may explain the increased expression of TCR gene and accelerated transition of CD25(+)CD44(-) (DN3) to CD25(-)CD44(-) (DN4) stage. Qualitatively similar, but quantitatively less striking effects were observed in mice with a targeted mutation of CD28, but not CTLA4. Taken together, our results demonstrate that the development of DN in the thymus is subject to modulation by the B7-CD28 costimulatory pathway.

Linker for Activation of B Cells: A Functional Equivalent of a Mutant Linker for Activation of T Cells Deficient in Phospholipase C-γ1 Binding

Adaptor proteins have important functions in coupling stimulation through immunoreceptors with downstream events. The adaptor linker for activation of B cells (LAB)/non-T cell activation linker (NTAL) is expressed in various immune cell types and has a similar domain structure as linker for activation of T cells (LAT). In this study we generated a LAB transgenic mouse to compare the functional differences between LAB and LAT. A LAB transgene expressed in LAT-deficient T cells was able to restore T cell development. However, these mice developed severe organomegaly with disorganized lymphoid tissues. Lymphocytes from these transgenic mice were hyperactivated, and T cells produced large amounts of type II cytokines. In addition, these activities appeared to be uncoupled from the TCR. An examination of the signaling capabilities of these T cells revealed that LAB resembled a LAT molecule unable to bind phospholipase C-gamma1.

Central tolerance to self-antigen expressed by cortical epithelial cells

The exposure of developing thymocytes to high-affinity self-Ag results in T cell tolerance. A predominant mechanism for this is clonal deletion; though receptor editing, anergy induction, and positive selection of regulatory T cells have also been described. It is unclear what signals are involved in determining different tolerance mechanisms. In particular, OT-I mice displayed receptor editing when the high-affinity self-Ag was expressed in cortical epithelial cells (cEC) using the human keratin 14 promoter. To test the hypothesis that receptor editing is a consequence of a unique instruction given by cEC presenting self-Ag, we created mice expressing the 2C and HY ligands under control of the keratin 14 promoter. Alternatively, we studied the fate of developing T cells in OT-I mice where Ag was presented by all thymic APC. Surprisingly, we found that the tolerance mechanism was not influenced by the APC subset involved in presentation. Clonal deletion was observed in 2C and HY models even when Ag was presented only by cEC; and receptor editing was observed in OT-I mice even when Ag was presented by all thymic APC. These results suggest that different TCRs show intrinsic differences in thymic tolerance mechanism.

Positive and negative selection of T cells

A functional immune system requires the selection of T lymphocytes expressing receptors that are major histocompatibility complex restricted but tolerant to self-antigens. This selection occurs predominantly in the thymus, where lymphocyte precursors first assemble a surface receptor. In this review we summarize the current state of the field regarding the natural ligands and molecular factors required for positive and negative selection and discuss a model for how these disparate outcomes can be signaled via the same receptor. We also discuss emerging data on the selection of regulatory T cells. Such cells require a high-affinity interaction with self-antigens, yet differentiate into regulatory cells instead of being eliminated.

Loss of Ly-6A.2 expression on immature developing T cells in the thymus is necessary for their normal growth and generation of the Vbeta T-cell repertoire

Stage-specific expression of a number of cell-surface and signaling proteins is critical for normal development of T cells in the thymus. Equally important may be the loss of expression/signaling of developmentally regulated proteins for proper transitioning of developing T cells into thymic subsets. Ly-6A.2 exhibits a regulated pattern of expression on T cells maturing in the thymus, and dysregulating its expression results in arrest of developing T cells within the CD3-CD4-CD8- triple negative (TN) stage where the normal expression of Ly-6A.2 is extinguished. To further characterize the mechanisms underlying this block, we examined whether cell signaling and/or cell adhesion properties of the Ly-6A.2 molecule influenced the block in T-cell development. Analysis of bone marrow chimeras generated by injecting CFSE-labeled Ly-6A.2 transgenic bone marrow cells into irradiated syngeneic non-transgenic mice revealed normal trafficking of developing T cells from the cortex into the medulla. Production of LAT but not p56lck was diminished in CD4-CD8- DN cells from Ly-6A.2 dysregulated mice when compared with control littermates. Dysregulated expression of Ly-6A.2 did not suppress endogenous TCR-Vbeta expression. Finally, dysregulated expression of Ly-6A.2 enhanced apoptosis of an immature CD4+CD8+ (DP) subset of developing cells and altered the selected TCR-Vbeta repertoire. Taken together, these observations indicate that the termination of Ly-6A.2 expression and signaling within the CD4-CD8-CD3- subset of developing T cells is an important checkpoint during normal thymic development.

Constitutively active protein kinase B enhances Lck and Erk activities and influences thymocyte selection and activation

Protein kinase B (PKB), a serine threonine kinase is critically involved in cellular proliferation and survival. To characterize its role in T cell development in vivo, we have analyzed transgenic mice that express a membrane-targeted constitutively active version of PKB (myr PKB) in thymocytes and peripheral T cells. We report that myr PKB renders proliferative responses of thymocytes more sensitive to TCR signals by increased and sustained activation of Src kinase Lck and the extracellular signal-regulated kinase/mitogen-activated protein kinase pathway. In addition, the proliferative response of myr PKB T cells is relatively independent of calcium mobilization and calcineurin activity. We also find that myr PKB enhances phosphorylation of glycogen synthase kinase 3, a negative regulator of NFAT and T cell activation, and the recruitment of the adapter protein Cbl-c. Interestingly, we demonstrate that upon TCR/CD3 stimulation of wild-type T cells PKB is translocated into lipid rafts, adding a new role for PKB in TCR-initiated signalosome formation in T cell activation. Localization of transgenic PKB in lipid rafts could contribute to the higher TCR sensitivity of myr PKB thymocytes which is reflected in an increase in positive selection toward the CD4 lineage and variable effects on negative selection depending on the model system analyzed. Thus, our observations clearly indicate a cross-talk between PKB and important signaling molecules downstream of TCR that modulate the thresholds of thymocyte selection and T cell activation.

Pathogenesis of autoimmunity after xenogeneic thymus transplantation

Thymus transplantation is a promising strategy to induce xenotolerance, but may also induce an autoimmune syndrome (AIS). The pathogenesis of this AIS was explored using nude rats as recipients. Thymus grafts consisted of fetal hamster thymic tissue with or without mixing with fetal rat tissue such as thymus, thyroid, salivary gland, and heart. All hamster thymus recipients died of AIS within 2-3 mo. In most recipients of xenothymus mixed with rat tissues such as thymus, thyroid, and salivary gland, but not heart, AIS was prevented, indicating an insufficient presence of rat epithelial cell Ags within the xenothymus. AIS could be transferred to control nude rats by whole splenocytes or by splenocyte subpopulations such as CD3(+), CD3(-), and B lymphocytes, but not by non-T, non-B cells from AIS animals. This transfer could be suppressed by cotransferring either CD4(+) or CD8(+) lymphocytes from euthymic rats, but not by splenocytes from recipients of syngeneic or xenogeneic thymus mixed with rat tissue, indicating a defective generation of regulatory lymphocytes. As for CD4(+) regulatory cells this defect was probably qualitative, because the percentages of CD4(+)CD25(+) or CD4(+)CD45RC(low) populations were normal after xenothymus transplantation. As for the CD8(+) regulatory cells, the defect was quantitative, as CD8(+) cell levels always remained low. The latter was related to the nonvascularized nature of thymus grafts. In conclusion, AIS after xenothymus transplantation in nude rats is due to a combination of insufficient intrathymic presence of host-type epithelial cell Ags and a defective generation of regulatory T lymphocytes.

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.

Negative selection--clearing out the bad apples from the T-cell repertoire

Dead cells are a prominent feature of the thymic landscape as only 5% of developing thymocytes are exported as mature T cells. The remaining thymocytes die by one of two mechanisms; most thymocytes die because they are not positively selected and do not receive a survival signal, whereas a minority of thymocytes undergo T-cell receptor (TCR)-mediated apoptosis, a process known as negative selection. Negative selection is extremely important for establishing a functional immune system, as it provides an efficient mechanism for ridding the T-cell repertoire of self-reactive and potentially autoimmune lymphocytes. This review discusses several cellular and molecular aspects of negative selection.

CD5 inhibits signaling at the immunological synapse without impairing its formation

Physiologically, Ag detection by T cells occurs at the immunological synapse (IS) formed at the interface with an APC. CD5 is considered as an inhibitory molecule for Ag receptor-mediated signals in T cells. However, the influence of CD5 at the IS on synapse formation and functioning has not yet been reported. We demonstrate here that CD5 is recruited and tightly colocalized with CD3 in different human and murine IS. Following transfection in a CD5-negative T cell line of CD5 fused to the green fluorescent protein, we show that CD5 recruitment includes a fast Ag-independent and a slower Ag-dependent component. In video-imaging recordings of doubly transfected cells, the movements of CD3 and CD5 show similar kinetics, and the amount of CD3 recruited to the synapse is unaffected by CD5 expression. Moreover, APC-T cell adhesion is unchanged in CD5-expressing cells. Despite this, the extent of tyrosine phosphorylation at the synapse and the amplitude of calcium responses induced by Ag recognition are both decreased by CD5. These inhibitions increase with CD5 membrane levels. They also requires the pseudo-immunoreceptor tyrosine-based activation motif expressed in the cytoplasmic domain of the molecule. Thus, CD5 is rapidly recruited at the IS and lowers the T cell response elicited by Ag presentation by targeting downstream signaling events without affecting IS formation.

LIGHT (a cellular ligand for herpes virus entry mediator and lymphotoxin receptor)-mediated thymocyte deletion is dependent on the interaction between TCR and MHC/self-peptide

Negative selection serves as a major mechanism to maintain self-tolerance. We previously reported that LIGHT (a cellular ligand for herpes virus entry mediator and lymphotoxin receptor), a TNF family member, plays an important role in thymocyte development via promoting apoptosis of double-positive thymocytes. Here, we demonstrated that LIGHT-mediated deletion of thymocyte requires the strong interaction of TCR with MHC/self-peptide. Transgenic mice overexpressing LIGHT in thymocytes were bred with a transgenic mouse line expressing a TCR recognizing the H-Y male Ag in the context of H-2b class I MHC molecules. In male H-Y/LIGHT double-transgenic mice, more efficient negative selection of H-Y T cells occurred, and total thymocyte number was further reduced compared with H-Y/negative littermates. In contrast, the presence of LIGHT transgene had no evident impact on the thymocyte development of female H-Y/LIGHT double-transgenic mice. Taken together, LIGHT plays a role in negative selection of thymocytes via inducing the apoptosis of thymocytes bearing high affinity TCR during negative selection.

The transcriptional repressor Gfi1 affects development of early, uncommitted c-Kit+ T cell progenitors and CD4/CD8 lineage decision in the thymus

In the thymus, several steps of proliferative expansion and selection coordinate the maturation of precursors into antigen-specific T cells. Here we identify the transcriptional repressor Gfi1 as an important regulator of this maturation process. Mice lacking Gfi1 show reduced thymic cellularity due to an increased cell death rate, lack of proliferation, and a differentiation block in the very early uncommitted CD4-/CD8-/c-Kit+ cytokine-dependent T cell progenitors that have not yet initiated VDJ recombination. In addition, Gfi1-deficient mice show increased major histocompatibility complex class I-restricted positive selection and develop significantly more CD8+ cells suggesting a requirement of Gfi1 for a correct CD4/CD8 lineage decision. Absence of Gfi1 correlates with high level expression of the genes for lung Krüppel-like factor (LKLF), inhibitor of DNA binding (Id)1 and Id2, suggesting the existence of new regulatory pathways in pre-T cell development and thymic selection in which Gfi1 acts upstream of LKLF as well as the E-proteins, which are negatively regulated by Id1 and Id2.

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.

Generation of mice deficient for macrophage galactose- and N-acetylgalactosamine-specific lectin: limited role in lymphoid and erythroid homeostasis and evidence for multiple lectins

Macrophage receptors function in pattern recognition for the induction of innate immunity, in cellular communication to mediate the regulation of adaptive immune responses, and in the clearance of some glycosylated cells or glycoproteins from the circulation. They also function in homeostasis by initiating the engulfment of apoptotic cells. Evidence has suggested that macrophage receptors function to recognize cells that are destined for programmed cell death but not yet overtly apoptotic. We have examined the function of a macrophage receptor specific for unsialylated glycoproteins, known as the mouse macrophage galactose- and N-acetylgalactosamine-specific lectin (mMGL) (Ii et al., J. Biol. Chem. 265:11295-11298, 1990; Sato et al., J. Biochem. [Tokyo] 111:331-336, 1992; Yamamoto et al., Biochemistry 33:8159-8166, 1994). With targeted disruption, we tested whether mMGL is necessary for macrophage function, controlled thymic development, the loss of activated CD8 T cells, and the turnover of red blood cells. Evidence indicates that mMGL may play a nonessential role in several of these macrophage functions. Experiments are presented that indicate the existence of another galactose- and N-acetylgalactosamine-recognizing lectin distinct from mMGL. This may explain the absence of a strong phenotype in mMGL-deficient mice.

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.

The GTPase Rac-1 controls cell fate in the thymus by diverting thymocytes from positive to negative selection

The positive selection of CD4 or CD8 single-positive mature peripheral T lymphocytes and the deletion of self-reactive cells are crucial for central tolerance in the peripheral immune system. Previously, the guanine nucleotide binding protein Rac-1 has been shown to control pre-T cell development. The present report now describes the actions of Rac-1 in thymocyte selection. The study reveals that this molecule has the striking and unique ability to efficiently divert cells from positive selection into a pathway of negative selection and deletion. The ability of Rac-1 to switch thymocytes from a destiny of positive to negative selection identifies this molecule as a critical regulator of the developmental processes in T cells that are essential for immune homeostasis.