Newly generated thymocytes are not refractory to deletion when the alpha/beta component of the T cell receptor is engaged by the superantigen staphylococcal enterotoxin B

Superantigen Recognition and Interactions: Functions, Mechanisms and Applications

Superantigens are unconventional antigens which recognise immune receptors outside their usual recognition sites e.g. complementary determining regions (CDRs), to elicit a response within the target cell. T-cell superantigens crosslink T-cell receptors and MHC Class II molecules on antigen-presenting cells, leading to lymphocyte recruitment, induction of cytokine storms and T-cell anergy or apoptosis among many other effects. B-cell superantigens, on the other hand, bind immunoglobulins on B-cells, affecting opsonisation, IgG-mediated phagocytosis, and driving apoptosis. Here, through a review of the structural basis for recognition of immune receptors by superantigens, we show that their binding interfaces share specific physicochemical characteristics when compared with other protein-protein interaction complexes. Given that antibody-binding superantigens have been exploited extensively in industrial antibody purification, these observations could facilitate further protein engineering to optimize the use of superantigens in this and other areas of biotechnology.

Interaction between the mouse homologue of CD99 and its ligand PILR as a mechanism of T cell receptor-independent thymocyte apoptosis

Here, we show that the interaction between two membrane proteins, the mouse homologue of CD99 (designated D4) and its ligand, paired immunoglobulin-like type 2 receptor (PILR), is one of the major mechanisms of thymocyte apoptosis. Using the polymeric fusion protein of PILR and IgG1 (PILR-Ig), we demonstrated that D4 ligation in the absence of T cell receptor (TCR) engagement leads to the induction of apoptosis, mainly at the double-positive stage of thymocytes. This was further confirmed by a blocking study in which blocking the interaction between D4 and PILR by soluble D4 protein led to reduced apoptosis in the fetal thymic organ culture with wild type and TCRalpha(-/-) mice. Furthermore, the dissection of intracellular signaling pathway demonstrated that D4 cross-linking led to caspase activation without any change in mitochondrial membrane potential. Based on these data, we propose a mechanism for thymocyte depletion in which the interaction between D4 and PILR delivers an active signal.

Grb2 functions at the top of the T-cell antigen receptor-induced tyrosine kinase cascade to control thymic selection

Grb2 is an adaptor molecule that mediates Ras-MAPK activation induced by various receptors. Here we show that conditional ablation of Grb2 in thymocytes severely impairs both thymic positive and negative selections. Strikingly, the mutation attenuates T-cell antigen receptor (TCR) proximal signaling, including tyrosine phosphorylation of multiple signaling proteins and Ca(2+) influx. The defective TCR signaling can be attributed to a marked impairment in Lck activation. Ectopic expression of a mutant Grb2 composed of the central SH2 and the C-terminal SH3 domains in Grb2(-/-) thymocytes fully restores thymocyte development. Thus, Grb2 plays a pivotal role in both thymic positive and negative selection. It amplifies TCR signaling at the top end of the tyrosine phosphorylation cascade via a scaffolding function.

BH3-only Bcl-2 family member Bim is required for apoptosis of autoreactive thymocytes

During lymphocyte development, the assembly of genes coding for antigen receptors occurs by the combinatorial linking of gene segments. The stochastic nature of this process gives rise to lymphocytes that can recognize self-antigens, thereby having the potential to induce autoimmune disease. Such autoreactive lymphocytes can be silenced by developmental arrest or unresponsiveness (anergy), or can be deleted from the repertoire by cell death. In the thymus, developing T lymphocytes (thymocytes) bearing a T-cell receptor (TCR)-CD3 complex that engages self-antigens are induced to undergo programmed cell death (apoptosis), but the mechanisms ensuring this 'negative selection' are unclear. We now report that thymocytes lacking the pro-apoptotic Bcl-2 family member Bim (also known as Bcl2l11) are refractory to apoptosis induced by TCR-CD3 stimulation. Moreover, in transgenic mice expressing autoreactive TCRs that provoke widespread deletion, Bim deficiency severely impaired thymocyte killing. TCR ligation upregulated Bim expression and promoted interaction of Bim with Bcl-XL, inhibiting its survival function. These findings identify Bim as an essential initiator of apoptosis in thymocyte-negative selection.

Induction of thymocyte positive selection does not convey immediate resistance to negative selection

The acquisition of functional competence represents a critical phase during intrathymic development of T cells. Thymocytes reaching this stage represent cells which have been positively selected on the basis of major histocompatibility complex reactivity, but which have also been purged of potentially autoreactive T-cell receptor specificities by negative selection. While the developmental window in which thymocytes are subjected to positive selection is now well defined, the precise developmental timing of negative selection, in relation to positive selection events, is less clear. Moreover, the underlying mechanism allowing single-positive thymocytes to respond to T-cell receptor ligation by activation rather than death, remains controversial. Here we have analysed the developmental timing of negative selection in relation to positive selection, using measurement of thymocyte susceptibility to dendritic cell presentation of the superantigen staphylococcal enterotoxin B (SEB). We show that thymocytes which have received initial positive selection signals, namely CD4+ CD8+ CD69+ thymocytes, like their CD4+ CD8+ CD69minus sign precursors, are susceptible to negative selection, indicating that induction of positive selection does not convey immediate resistance to negative selection. In contrast, newly generated CD4+ CD8minus sign CD69+ cells are not only resistant to deletion by SEB, but respond to SEB-mediated T-cell receptor-ligation by activation, indicating that the acquisition of functional competence occurs at the newly generated CD4+ CD8minus sign CD69+ stage. Finally, by using direct retroviral infection of primary CD4+ CD8+ thymocytes, we also show that Notch-1 activation in CD4+ CD8+ thymocytes does not correlate with, nor convey resistance to superantigen-mediated negative selection. Thus, our data suggest that although Notch-1 has been implicated in resistance to thymocyte apoptosis, the acquisition of resistance to negative selection occurs independently of Notch-1 signalling.

Stress-free T-cell development: glucocorticoids are not obligatory

A role for glucocorticoids in thymopoiesis has been suggested by studies using glucocorticoid receptor (GR) anti-sense transgenic mice, glucocorticoid synthesis inhibitors and GR antagonists. Unfortunately, no consensus has been reached on exactly how glucocorticoids influence T-cell development. The most recent approach, using GR knockout (GR(-/-)) mice, indicates that GR signaling is, in fact, dispensable in this entire process.

Intrathymic T cell development and selection proceeds normally in the absence of glucocorticoid receptor signaling

Glucocorticoids are believed to play a role in T cell development and selection, although their precise function is controversial. Glucocorticoid receptor (GR)-deficient mice were used to directly investigate this problem. GR-deficient thymocytes were resistant to dexamethasone-mediated apoptosis, confirming the absence of glucocorticoid responsiveness. An absence of GR signaling had no impact on thymocyte development either in vivo or in vitro. T cell differentiation, including positive selection, was normal as assessed by normal development of CD4+CD8+, alphabetaTCR+CD4+, and alphabetaTCR+CD8+ thymocytes. Negative selection, mediated by the superantigen staphylococcal enterotoxin B (SEB), or anti-CD3/CD28, was also normal in the absence of GR signaling. In contrast to earlier reports, these data demonstrate that GR signaling is not essential for intrathymic T cell development or selection.

Programmed differentiation of murine thymocytes during fetal thymus organ culture

Fetal thymus organ culture (FTOC) has become widely used to investigate the impact of immunomodulators on T cell development. However, these studies have given variable results among different laboratories. In this study, we have found that fetal tissue age and mouse strain differences can affect the development of T cell phenotypes in this system. T cell development in FTOC occurred in two 'waves', defined as peaks of cell recovery. The first wave consisted initially of CD4-CD8- double negative (DN) cells and CD4-CD8+ single positive (SP) T cells expressing gamma delta T cell receptor (TCR). CD4+CD8+ double positive (DP) cells expressing low levels of alpha beta TCR were produced soon thereafter; and these cells dominated the cultures for the balance of the first wave. Prolonged FTOC resulted in the production of another wave of T cells which were relatively enriched for CD4 or CD8 SP cells expressing high levels of alpha beta TCR, as well as DN cells and CD4-CD8+ SP T cells expressing high levels of gamma delta TCR. As defined by cell number and differentiation of alpha beta TCR SP cells, development was delayed in FTOC using fetal thymus tissue from younger fetuses relative to that observed when older fetal thymus tissue was used. The degree of development of T cells in FTOC was also strain dependent. Organ cultures derived from 14 gestation days (gd) C.B-17 scid/scid fetal thymus did not generate TCR-bearing mature SP cells, but they did produce TCR-negative CD4 and CD8 SP cells likely to be precursors of DP thymocytes. Such cultures made from 18 gd tissue did not produce SP cells. Negative selection in FTOC was also evaluated. Mtv-specific V beta 3 cells were deleted in FTOC of C3H/HeN tissue. Deletion occurred only in late FTOC, suggesting a late encounter between the Mtv deleting elements and susceptible T cells during ontogeny. These results show that while FTOC recapitulates normal thymic development by a variety of criteria, results can be influenced by the length of culture, as well as by the age and strain of fetal thymus tissue utilized.

Thymic and peripheral apoptosis of antigen-specific T cells might cooperate in establishing self tolerance

Aside from CD4+CD8+ double-positive (DP) thymocytes, the subpopulations of T lineage cells affected by negative selection are unknown. To address whether this process occurs in more mature cell types, we have compared the responses of purified single-positive (SP) murine thymocytes and peripheral T cells to the superantigen staphylococcal enterotoxin B (SEB) utilizing as antigen-presenting cells (APC) a fibroblast cell line expressing transfected I-Ek class II molecules. Whereas approximately 70% of SEB-reactive SP thymocytes, either CD4+ or CD8+, undergo programmed cell death (apoptosis) and, therefore, negative selection, CD4+ and CD8+ antigen-specific peripheral T cells are predominantly activated and proliferate to APC+SEB. Thus, mature thymocytes and peripheral T cells, with identical patterns and levels of expression of CD4, CD8 and T cell receptor (TCR), are programmed to elicit different responses following TCR stimulation. Unexpectedly, however activation of peripheral T cells was preceded by deletion of a large fraction of V beta 8+ T lymphocytes (SEB specific). This surprising phenomenon was also observed in in vivo studies: in fact, administration of SEB to adult mice resulted in depletion of the majority of antigen-specific T cells from the peripheral lymphoid tissues analyzed (lymph nodes and spleen). This depletion is the consequence of deletion as indicated by program cell death of V beta 8+ T cells and is followed by proliferation of the remaining SEB-reactive T cells. Clonal elimination of peripheral T cells may represent a mechanism by which tolerance to self antigens never expressed in and/or exported to the thymus is achieved.

Apoptosis and T-cell repertoire selection in the thymus

Thymic tolerance depends on induction of apoptosis (programmed cell death) in immature thymocytes by antigen/MHC complexes on dendritic cells (and possibly other bone marrow-derived APCs). Interactions with antigen/MHC complexes on thymic epithelial cells promote maturation of double-positive thymocytes to single-positive cells. However, the nature of the antigen/MHC complexes on thymic epithelial cells is unknown, and if the stromal cell interaction model as just outlined is correct, then presumably these complexes must be different from those presented on dendritic cells, otherwise all cells signaled for positive selection would be subject to negative selection by similar complexes on APCs. The nature of the signals provided by thymic epithelial cells versus dendritic cells is unknown and may provide a key to understanding the processes of positive and negative selection within the thymus. Clearly the intense selection of the T-cell repertoire within the thymus explains the high level of cell death observed within the immature thymocyte compartment. Such intensive selection shapes the T-cell repertoire in a way that provides an explanation for the genetic basis of immune responsiveness and for the susceptibility of certain individuals to autoimmunity.

Why is clonal deletion of neonatal thymocytes defective?

A major mechanism for establishing tolerance to some murine self antigens is clonal deletion of self reactive T cells in the thymus. This mechanism is responsible for the near absence of T cells displaying particular T cell receptor (TcR) V beta in strains of mice that express the major histocompatibility complex class II E molecule and a protein encoded within the 3' open reading frame (ORF) of certain endogenous mammary tumor viruses (Mtv). However, clonal deletion does not operate in these same strains during the first few days after birth. This defect could be explained by a difference in any (or any combination of) the three elements involved: the T cell, the thymic stromal cell(s) or the antigen. We have explored these different possibilities and have come to the conclusion that a lack of antigen is the most likely explanation. Yet, neonatal and adult thymi have quite similar levels of messenger ribonucleic acid corresponding to Mtv 3' ORF.

Studies on T cell maturation on defined thymic stromal cell populations in vitro

We describe an in vitro system in which positive selection of developing T cells takes place on defined stromal cell preparations, which include major histocompatibility complex class II+ epithelial cells but exclude cells of bone marrow origin. In this system, maturation of double-positive T cell receptor negative (TCR-), CD4+8+ thymocytes into single-positive TCR+, CD4+ and CD8+ cells takes place together with the development of functional competence. As in vivo, this maturation is associated with the upregulation of TCR levels as cells progress from double-positive to single-positive status. We also show that class II+ epithelial cells in these cultures are less efficient than dendritic cells in mediating the deletion (negative selection) of V beta 8+ cells by the superantigen staphylococcal enterotoxin B. For the first time, this approach provides a model in which the cellular interactions involved in both positive and negative selection can be studied under controlled in vitro conditions.

Heat shock induces apoptosis in mouse thymocytes and protects them from glucocorticoid-induced cell death

Thymocyte death is a complex phenomenon under the control of different signals and stimuli. We evaluated the effect of elevated temperature (heat shock, HS) on mouse thymocyte apoptosis. Incubation of thymocytes at 43 degrees C for 20 min induced DNA fragmentation and cell death, but it was also able to decrease the apoptosis induced by dexamethasone (DEX), TPA or Ca2+ ionophore. The anti-apoptotic effect was correlated with induction of heat shock proteins (HSPs) and abolished by protein synthesis inhibition. On the other hand, HS-induced unlike DEX-induced apoptosis was not inhibited by protein synthesis and mRNA transcription inhibitors, the PKC inhibitors H-7 and staurosporine, or interleukin-4 (IL-4), but only by Zn2+. These results suggest that HS interferes in thymocyte death by either inducing or inhibiting thymocyte apoptosis and that the induction process mechanisms are different from those of GCH.

Developmental regulation of thymocyte susceptibility to deletion by "self"-peptide

The specificity of the T cell receptor (TCR) repertoire for foreign peptide bound to self-major histocompatibility complex (MHC) molecules is determined in large part by positive and negative selection processes in the thymus, yet the mechanisms of these selection events remain unknown. Using in vitro organ culture of thymi isolated from mice transgenic for a TCR-alpha/beta specific for cytochrome c peptide bound to I-Ek, we analyzed the developmental timing of negative selection (deletion). On the basis of the experiments described below, we conclude that all CD4+8+ thymocytes, and only CD4+8+ thymocytes, are susceptible to negative selection mediated by the cytochrome c peptide antigen. First, we found that deletion of thymocytes resulting from addition of the cytochrome c peptide to the thymic organ cultures can occur at the earliest stage of TCR, CD4, and CD8 coexpression. Second, we found that CD4+8+ thymocytes isolated from positively selecting or nonselecting MHC haplotypes were equally efficiently deleted in vitro, suggesting that positive selection is not a prerequisite for deletion. Third, we examined the effects of TCR/ligand avidity on the developmental timing of deletion by varying the concentration of cytochrome c peptide added to the organ cultures. We detected deletion only at the CD4+8+ stage: intermediate concentrations of peptide that resulted in partial deletion of CD4+8+ cells did not eliminate the appearance of mature CD4+8- cells. Finally, we found that CD4+8- thymocytes were resistant to deletion as well as activation by peptide antigen added to the intact organ cultures. Nevertheless, the CD4+8- thymocytes isolated from the peptide-treated organ cultures responded vigorously to peptide presented by spleen cells in vitro. Thus, the T cells were tolerant of (but not anergized by) self-antigen encountered in thymic organ culture. Together, these results indicate that thymocytes susceptible to negative selection are not developmentally distinct from those susceptible to positive selection, and further, that the thymic microenvironment plays a role in regulating the outcome of TCR/ligand interactions.

Interleukins modulate glucocorticoid-induced thymocyte apoptosis

Glucocorticoid hormones, calcium ionophores and anti-CD3 monoclonal antibodies induce apoptosis in mouse thymocytes. This type of cell death, which is characterized by an extensive DNA fragmentation into oligonucleosomal subunits, occurs in the intrathymic process of negative selection, and is involved in the deletion of autoreactive T-cells during thymic maturation. A number of cytokines are able to modulate apoptosis, and interleukins, including interleukin-1, interleukin-2, and interleukin-4, play a crucial role in thymic maturation and T-cell development. We tested the effects of several cytokines on the glucocorticoid hormone-induced apoptosis of mouse thymocytes in vitro, and demonstrated that interleukin-1 alpha, interleukin-2, and interleukin-4 inhibit the apoptosis induced by dexamethasone, but that interleukin-3 and interleukin-6 exert no noteworthy effect. Dose-response experiments indicated that interleukin-4 is more potent than interleukin-1 alpha and interleukin-2 in inhibiting dexamethasone-induced apoptosis. Furthermore, interleukin-4 fully inhibited the DNA fragmentation induced by the protein kinase-C activator 12-O-tetradecanoylphorbol-13-acetate, but was ineffective against apoptosis induced by the calcium ionophore A23187. These results suggest that interleukins regulate the thymic selection process by acting as modulators of the negative selection process.

Pidotimod stimulates natural killer cell activity and inhibits thymocyte cell death

Experiments were performed to analyze the possible effect of the immunomodulating agent Pidotimod (3-L-pyroglutamyl-L-thiazolidine-4-carboxylic acid) on mouse Natural Killer (NK) cell activity and glucocorticoid hormone(GCH)-induced thymocyte apoptosis. The results indicate that in vivo treatment with Pidotimod (200 mg/Kg ip for 5 days) causes a significant increase in NK activity and in vitro treatment produces a significant reduction of dexamethasone-induced thymocyte apoptosis. This inhibition appears to be dose-dependent and is also evident against TPA or Ca(++)ionophore-induced apoptosis.