T-cell receptor ligation by peptide/MHC induces activation of a caspase in immature thymocytes: the molecular basis of negative selection

Targeting regulated cell death (RCD) in hematological malignancies: Recent advances and therapeutic potential

Regulated cell death (RCD) is a form of cell death that can be regulated by numerous biomacromolecules. Accumulating evidence suggests that dysregulated expression and altered localization of related proteins in RCD promote the development of cancer. Targeting subroutines of RCD with pharmacological small-molecule compounds is becoming a promising therapeutic avenue for anti-tumor treatment, especially in hematological malignancies. Herein, we summarize the aberrant mechanisms of apoptosis, necroptosis, pyroptosis, PANoptosis, and ferroptosis in hematological malignancies. In particular, we focus on the relationship between cell death and tumorigenesis, anti-tumor immunotherapy, and drug resistance in hematological malignancies. Furthermore, we discuss the emerging therapeutic strategies targeting different RCD subroutines. This review aims to summarize the significance and potential mechanisms of RCD in hematological malignancies, along with the development and utilization of pertinent therapeutic strategies.

T Cells and Regulated Cell Death: Kill or Be Killed

Cell death plays two major complementary roles in T cell biology: mediating the removal of cells that are targeted by T cells and the removal of T cells themselves. T cells serve as major actors in the adaptive immune response and function by selectively killing cells which are infected or dysfunctional. This feature is highly involved during homeostatic maintenance, and is relied upon and modulated in the context of cancer immunotherapy. The vital recognition and elimination of both autoreactive T cells and cells which are unable to recognize threats is a highly selective and regulated process. Moreover, detection of potential threats will result in the activation and expansion of T cells, which on resolution of the immune response will need to be eliminated. The culling of these T cells can be executed via a multitude of cell death pathways which are used in context-specific manners. Failure of these processes may result in an accumulation of misdirected or dysfunctional T cells, leading to complications such as autoimmunity or cancer. This review will focus on the role of cell death regulation in the maintenance of T cell homeostasis, as well as T cell-mediated elimination of infected or dysfunctional cells, and will summarize and discuss the current knowledge of the cellular mechanisms which are implicated in these processes.

Dying to protect: cell death and the control of T-cell homeostasis

T cells play a critical role in immune responses as they specifically recognize peptide/MHC complexes with their T-cell receptors and initiate adaptive immune responses. While T cells are critical for performing appropriate effector functions and maintaining immune memory, they also can cause autoimmunity or neoplasia if misdirected or dysregulated. Thus, T cells must be tightly regulated from their development onward. Maintenance of appropriate T-cell homeostasis is essential to promote protective immunity and limit autoimmunity and neoplasia. This review will focus on the role of cell death in maintenance of T-cell homeostasis and outline novel therapeutic strategies tailored to manipulate cell death to limit T-cell survival (eg, autoimmunity and transplantation) or enhance T-cell survival (eg, vaccination and immune deficiency).

αβ TCR-mediated recognition: relevance to tumor-antigen discovery and cancer immunotherapy

αβ T lymphocytes sense perturbations in host cellular body components induced by infectious pathogens, oncogenic transformation, or chemical or physical damage. Millions to billions of these lymphocytes are generated through T-lineage development in the thymus, each endowed with a clonally restricted surface T-cell receptor (TCR). An individual TCR has the capacity to recognize a distinct "foreign" peptide among the myriad of antigens that the mammalian host must be capable of detecting. TCRs explicitly distinguish foreign from self-peptides bound to major histocompatibility complex (MHC) molecules. This is a daunting challenge, given that the MHC-linked peptidome consists of thousands of distinct peptides with a relevant nonself target antigen often embedded at low number, among orders of magnitude higher frequency self-peptides. In this Masters of Immunology article, I review how TCR structure and attendant mechanobiology involving nonlinear responses affect sensitivity as well as specificity to meet this requirement. Assessment of human tumor-cell display using state-of-the-art mass spectrometry physical detection methods that quantify epitope copy number can help to provide information about requisite T-cell functional avidity affording protection and/or therapeutic immunity. Future rational CD8 cytotoxic T-cell-based vaccines may follow, targeting virally induced cancers, other nonviral immunogenic tumors, and potentially even nonimmunogenic tumors whose peptide display can be purposely altered by MHC-binding drugs to stimulate immune attack.

Antithymocyte globulin facilitates alloreactive T-cell apoptosis by means of caspase-3: potential implications for monitoring rejection-free outcomes

BACKGROUND

Alloreactive T-cell apoptosis may explain reduced immunosuppression requirements with proapoptotic immunosuppression and among rejection-free recipients. This possibility remains unproven.

METHODS

Apoptotic (caspase-3+, cathepsin B+) and inflammatory (CD154+) T-cell subsets were evaluated before and after adding rabbit antithymocyte globulin (rATG) to mixed lymphocyte co-cultures between human leukocyte antigen-mismatched peripheral blood lymphocytes from healthy adults. In random samples from children with liver (LTx-20) and intestine (ITx-13) transplantation, apoptotic T cells were evaluated for association with rejection-free outcomes using the caspase-3 substrate, phiphilux.

RESULTS

In mixed lymphocyte co-cultures between normal human peripheral blood lymphocytes, (1) frequencies of memory (M) and naive (N) Th and Tc, which expressed activated caspase-3, were enhanced most by the combination of allostimulation and rATG, than either stimulus alone. These findings were confirmed with antibody to activated caspase-3, phiphilux, and terminal deoxynucleotide transferase-mediated dUTP nick-end labeling (TUNEL) assay; (2) frequencies of Th subsets, which expressed activated cathepsin B, were similarly increased with combined stimulation. Tc seemed resistant to cathepsin B activation; (3) with increasing rATG concentrations, proportionately more allospecific CD154+T-cytotoxic memory cells (TcM) survived than TcM, resulting in relative enrichment of allospecific CD154+TcM. In random blood samples, phiphilux+T-cell subset frequencies were higher among 14 rejection-free LTx and ITx recipients and demonstrated a greater increase with ex vivo rATG pretreatment than 19 rejectors. In logistic regression analysis, phiphilux+TcM associated best with rejection-free outcomes with a sensitivity of 57% and a specificity of 89%.

CONCLUSION

Rabbit antithymocyte globulin facilitates apoptosis of alloreactive T cells by means of caspase-3 activation, which may explain its steroid-sparing effect in pediatric liver and intestine recipients. Apoptotic susceptibility of T-cytotoxic memory cells, which resist cathepsin B activation, may distinguish rejection-free and rejection-prone liver recipients.

Fine-tuning T cell receptor signaling to control T cell development

T cell development from immature CD4(+)CD8(+) double-positive (DP) thymocytes to the mature CD4 or CD8 single-positive (SP) stage requires proper T cell receptor (TCR) signaling. The current working model of thymocyte development is that the strength of the TCR-mediated signal - from little-or-none, through intermediate, to strong - received by the immature cells determines whether they will undergo death by neglect, positive selection, or negative selection, respectively. In recent years, several developmentally regulated, stage-specifically expressed proteins and miRNAs have been found that act like fine-tuners for signal transduction and propagation downstream of the TCR. This allows them to govern thymocyte positive selection. Here, we summarize recent findings on these molecules and suggest new concepts of TCR positive-selection signaling.

Role of caspase-8 in thymus function

The thymus is the primary organ responsible for de novo generation of immunocompetent T cells that have a diverse repertoire of antigen recognition. During the developmental process, 98% of thymocytes die by apoptosis. Thus apoptosis is a dominant process in the thymus and occurs through either death by neglect or negative selection or through induction by stress/aging. Caspase activation is an essential part of the general apoptosis mechanism, and data suggest that caspases may have a role in negative selection; however, it seems more probable that caspase-8 activation is involved in death by neglect, particularly in glucocorticoid-induced thymocyte apoptosis. Caspase-8 is active in double-positive (DP) thymocytes in vivo and can be activated in vitro in DP thymocytes by T-cell receptor (TCR) crosslinking to induce apoptosis. Caspase-8 is a proapoptotic member of the caspase family and is considered an initiator caspase, which is activated upon stimulation of a death receptor (e.g., Fas), recruitment of the adaptor molecule FADD, and recruitment and subsequent processing of procaspase-8. The main role of caspase-8 seems to be pro-apoptotic and, in this review, we will discuss about the involvement of caspase-8 in (1) TCR-triggered thymic apoptosis; (2) death receptor-mediated thymic apoptosis; and (3) glucocorticoid-induced thymic apoptosis. Regarding TCR triggering, caspase-8 is active in medullary, semi-mature heat-stable antigen(hi) (HAS(hi) SP) thymocytes as a consequence of strong TCR stimulation. The death receptors Fas, FADD, and FLIP are involved upstream of caspase-8 activation in apoptosis; whereas, Bid and HDAC7 are involved downstream of caspase-8. Finally, caspase-8 is involved in glucocortocoid-induced thymocyte apoptosis through an activation loop with the protein GILZ. GILZ activates caspase-8, promoting GILZ sumoylation and its protection from proteasomal degradation.

Assessment of caspase mediated degradation of linker for activation of T cells (LAT) at a single cell level

Caspase/Granzyme B mediated protein degradation is involved in elimination of activated T cell receptor (TCR) signaling molecules during processes of thymocyte selection and maintenance of peripheral homeostasis of T cells. Key components of TCR signaling cassette including LAT undergo biological inactivation in response to pro-apoptotic or anergy inducing environmental stimuli. Although available Western immunoblotting-based techniques are appropriate for detection of protein degradation in bulk populations of target cells, quantitative assessment of this process at a single cell level requires a different approach. Here we report on a novel, flow cytometry-based method for assessment of LAT integrity. This method exploits a loss of an anti-LAT antibody epitope recognition following proteolytic degradation of C-terminal domain of the LAT. We show that the LAT degradation precedes phosphatidylserine translocation to the outer leaflet of the plasma membrane and thus may constitute an early marker of T cell apoptosis. When used in conjunction with multi-parameter flow cytometry, our method revealed that FoxP3(+)CD4(+)CD8(low) thymocytes i.e. precursors of thymus derived CD4(+) regulatory T cells, in contrast to Foxp3(-)CD4(+)CD8(low) thymocytes are resistant to LAT degradation in response to CD3ε crosslinking. This finding can be used as an additional marker for T regulatory cell lineage.

Impaired B cell development and function in the absence of IkappaBNS

IκBNS has been identified as a member of the IκB family of NF-κB inhibitors, which undergoes induction upon TCR signaling. Mice carrying a targeted gene disruption of IκBNS demonstrate dysregulation of cytokines in T cells, macrophages, and dendritic cells. IκBNS mediates both positive and negative gene regulation, depending on individual cell type and/or cytokine. In this study, we demonstrate an additional role for IκBNS in the B cell lineage. B cells from IκBNS knockout (KO) mice were impaired in proliferative responses to LPS and anti-CD40. IgM and IgG3 Igs were drastically reduced in the serum of IκBNS KO mice, although IκBNS KO B cells exhibited a higher level of surface IgM than that found in wild-type mice. Switching to IgG3 was significantly reduced in IκBNS KO B cells. The in vitro induction of plasma cell development demonstrated that progression to Ab-secreting cells was impaired in IκBNS KO B cells. In agreement with this finding, the number of Ab-secreting cells in the spleens of IκBNS KO mice was reduced and production of Ag-specific Igs was lower in IκBNS KO mice after influenza infection as compared with wild-type mice. Additionally, IκBNS KO mice lacked B1 B cells and exhibited a reduction in marginal zone B cells. Thus, IκBNS significantly impacts the development and functions of B cells and plasma cells.

CD95 ligand mediates T-cell receptor-induced apoptosis of a CD4+ CD8+ double positive thymic lymphoma

Tumors in the thymus can be of different cellular origin. Among the most common tumors are thymoma and lymphoma, which are derived from transformed thymic epithelial cells and transformed lymphocytes, respectively. Thymic lymphoma and their response to apoptotic stimuli are poorly characterized. Here, we analyse apoptosis events in the thymic lymphoma cell line Thy278, which expresses cell surface antigens characteristic of immature double positive thymocytes. Upon T-cell receptor (TCR)/CD3 stimulation, Thy278 cells die by apoptosis, similar as primary thymocytes during negative selection. Caspases are crucial for deletion of both Thy278 cells and normal thymocytes. Moreover, we show that deletion of primary thymocytes and Thy278 cells upon CD3 stimulation is considerably impaired by neutralizing CD95L antibody. Thus, our results not only demonstrate that TCR-induced apoptosis is still functional in transformed thymocytes, but also suggest that Thy278 cells are a helpful model for the molecular analysis of negative selection.

Expression of stromelysin-3 (matrix metalloproteinase-11) in macrophages of murine thymus following thymocyte apoptosis

High expression of stromelysin-3 (ST-3), also known as matrix metalloproteinase-11, has been implicated in tumor progression and intense tissue remodeling. Nonetheless, details of the cell type(s) expressing ST-3 are less well defined. Here, we report that ST-3 expression was elevated in mouse thymus following thymocyte apoptosis after administration of anti-CD3 Ab. TUNEL analysis revealed that many thymocytes in the cortical region were induced to apoptotic cell death 14 h after the injection. After an additional 2-6 h, ST-3 expression in the thymus was more apparent. Co-staining analysis by anti-ST-3 and F4/80 Abs showed that most F4/80-positive macrophages were also positive for ST-3. Murine peritoneal macrophages were found to constitutively express ST-3, and exposure to apoptotic cells hardly affected ST-3 expression in the macrophages. Taken together, our results indicate that ST-3 is not involved in the execution process of thymocyte apoptosis, and the increased levels of ST-3 in the thymus may be due to the presence of macrophages responsible for clearance of apoptotic cells.

Efficacy of clonal deletion vs. anergy of self-reactive CD4 T-cells for the prevention and reversal of autoimmune diabetes

The self-reactive CD4 T-cells play an essential role in triggering and sustaining organ-specific autoimmune diseases. Silencing or elimination of these cells can prevent and reverse an autoimmune process. We have previously showed that a single dose-administration of a soluble dimeric MHC II-peptide chimera (DEF) in double-transgenic mice delayed the onset autoimmune diabetes, and restored the euglycemia in already diabetic mice for a period of 1 week. DEF dimer protection relied on induction of anergy of diabetogenic CD4 T-cells in spleen, and stimulation of IL-10-secreting T regulatory type 1 cells in pancreas. Herein, we show that an octameric form of DEF has doubled the period of protection and reversal of disease by clonal deletion of diabetogenic CD4 T-cells in both the thymic and peripheral compartments. Deletion occurred by activation-induced cell death subsequent to repartitioning and signaling of FAS-FADD apoptotic module in the plasma membrane lipid rafts. Our previous and present data indicated first, that DEF valence translates into various effects on the antigen-specific CD4 T-cells, i.e., Th2 immune deviation, anergy, and apoptosis. Second, the present findings argue for a better efficacy of clonal deletion than anergy of diabetogenic CD4 T-cells for the protection and reversal of autoimmune diabetes.

The "fuzzy logic" of the death-inducing signaling complex in lymphocytes

Receptors belonging to the tumor necrosis factor receptor family have long been thought to play an important role in the regulation of immunity. Although this family is composed of a large number of surface receptors that potentiate myriad functions in vivo, a subset is known to directly convey apoptotic signals. One such molecule belonging to this subset is CD95. Ligation of CD95 instigates the formation of a complex known as the "death-inducing signaling complex" or DISC, which is composed of molecules including FADD (Fas associated with death domain) and RIP (receptor-interacting kinase), as well as procaspases-8 and -10, and a caspase-8-like molecule that lacks proteolytic activity called c-FLIP. Although the DISC was initially thought to serve an exclusively proapoptotic role, humans and mice with defects in various components of this complex demonstrate a variety of developmental and hematopoietic defects that are not apparently due to aberrant apoptosis. These findings paint a far more complex picture of the numerous components of the DISC, and provide evidence that these complexes serve nonapoptotic functions. Herein, we summarize the experimental evidence challenging the notion that the DISC imparts an exclusively apoptotic function and provide hypotheses to account for these alternative roles. Rather than operating as a binary system, we propose that the DISCs formed around various DRs transduce signals leading to a variety of cellular fates.

Role of the cytoskeleton during leukocyte responses

The cytoskeleton is a cellular network of structural, adaptor and signalling molecules that regulates most cellular functions that are related to the immune response, including migration, extravasation, antien recognition, activation and phagocytosis by different subsets of leukocytes. Recently, a large number of regulatory elements and structural constituents of the leukocyte cytoskeleton have been identified. In this review, we discuss the composition and regulation of the different cytoskeletal elements and their role in immune responses.

Transcriptional activation of known and novel apoptotic pathways by Nur77 orphan steroid receptor

Nur77 is a nuclear orphan steroid receptor that has been implicated in negative selection. Expression of Nur77 in thymocytes and cell lines leads to apoptosis through a mechanism that remains unclear. In some cell lines, Nur77 was reported to act through a transcription-independent mechanism involving translocation to mitochondria, leading to cytochrome c release. However, we show here that Nur77-mediated apoptosis in thymocytes does not involve cytoplasmic cytochrome c release and cannot be rescued by Bcl-2. Microarray analysis shows that Nur77 induces many genes, including two novel genes (NDG1, NDG2) and known apoptotic genes FasL and TRAIL. Characterization of NDG1 and NDG2 indicates that NDG1 initiates a novel apoptotic pathway in a Bcl-2-independent manner. Thus Nur77-mediated apoptosis in T cells involves Bcl-2 independent transcriptional activation of several known and novel apoptotic pathways.

T-helper cell tolerance to ubiquitous nuclear antigens

Systemic autoimmune diseases are characterized by the development of antinuclear autoantibodies. In order to understand the immunologic events leading to the development of such antibodies, knowledge of mechanisms of immune tolerance to nuclear antigens is required. By utilizing adoptive T-cell transfer strategies with transgenic mouse models expressing nuclear neo-self antigens, T-cell tolerance to the lupus-related nuclear antigens human La and nRNP A has been demonstrated. These findings also indicate the existence in normal animals of autoreactive B cells continuously presenting nuclear antigen, suggesting that nuclear antigens are not sequestered from the immune system. Investigations of CD4+ T-cell tolerance to non-nuclear antigens have revealed a number of mechanisms that protect the host from autoreactivity, including autoreactive T-cell deletion, regulatory T-cell development and anergy induction. Recent studies using T-cell receptor and neo-self nuclear antigen transgenic mice are revealing the importance of such mechanisms in maintaining tolerance to nuclear antigens. Mechanisms of tolerogenic antigen presentation, identification of tolerogenic antigen source(s) and the pathways leading to loss of tolerance to nuclear antigens in systemic autoimmune disease states are currently being sought.

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.

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.

Apoptosis during lymphoid development

Recent investigations have provided important insights into how signaling through the antigen receptors determines whether a cell survives or dies. In T cells, Grb2 and MAP kinases play essential roles in differentiating between apoptotic and survival signals. The PTEN phosphatase and Bim, a pro-apoptotic Bcl-2 family member, regulate apoptosis in both T and B cells. In B cells, antigen receptor-mediated death can be rescued by co-stimulation, in which the roles of protein kinase C and BAFF, a TNF family member, have been recently elucidated. In a recently identified mechanism of regulating inflammation, receptors such as c-mer and glycoproteins such as MFG-E8 were found to participate in the clearance of apoptotic cells.

Effects of oestrogen and exercise on caspase-3 activity in primary and secondary lymphoid compartments in ovariectomized mice

This study investigated the effect of oestrogen exposure and exercise on caspase-3 activity, a measure of apoptosis, in lymphocytes from the thymus, spleen, and lymph nodes in ovariectomized mice. Fifty-nine female B6D2F1 mice were randomized to hormone and exercise conditions. Hormone treatment consisted of implantation with oestradiol pellets (0.72 mg oestradiol) or placebo pellets (0 mg) for 21 days following bilateral ovariectomy (OVX). Exercise consisted of a single treadmill exercise bout (26 m min(-1), 6 degrees slope, 90-min) or sedentary condition. Mice were killed and the thymus, spleen and lymph nodes were removed for the determination of caspase-3 expression by enzyme-linked immunosorbent assay (ELISA), serum oestrogen levels by RIA, and tissue weights. Body weights were monitored throughout the study. In the thymus, oestrogen exposure, exercise and both treatments together were associated with higher caspase-3 activity (P < 0.05) and lower thymus weights (P < 0.05). In contrast, oestrogen exposure and exercise treatment were not associated with greater caspase-3 activity or change in tissue weight in secondary lymphoid tissues (spleen, lymph nodes). Oestrogen-replaced OVX mice had a higher concentration of plasma oestradiol than placebo OVX mice (P < 0.05).

CONCLUSION

The results suggest that oestrogen and treadmill exercise are associated with greater apoptosis, as measured by caspase-3 activity, in the thymus but not in the spleen or lymph nodes. Clinical studies will be necessary to determine if women who take oestrogen have higher rates of apoptosis in primary lymphoid tissues and the significance of thymocyte apoptosis for maintenance of cellular immune function during the post-menopausal years.

The ubiquitin-proteasome pathway in thymocyte apoptosis: caspase-dependent processing of the deubiquitinating enzyme USP7 (HAUSP)

Programmed cell death (apoptosis) is crucial for thymocyte development. We analyzed the role of the ubiquitin (Ub)-proteasome pathway in dexamethasone-triggered and TCR-mediated apoptosis in fetal thymic organ culture (FTOC). Proteasome activity was increased in apoptotic thymocytes, as visualized by active-site labeling of proteasomal beta subunits. The activity of deubiquitinating enzymes in murine apoptotic thymocytes was likewise examined by active-site labeling. We show that the deubiquitinating enzyme USP7 (HAUSP) is proteolytically processed upon dexamethasone-, gamma-irradiation-, and antigen-induced cell death. Such processing of HAUSP does not occur in caspase 3-/- thymocytes, or upon pretreatment of wild type thymocytes with the general caspase inhibitor ZVAD-fmk. Thus, our results suggest that thymocyte apoptosis leads to modification of deubiquitinating enzymes by caspase activity and may provide an additional link between the ubiquitin-proteasome pathway and the caspase cascade during programmed cell death.

Involvement of caspace-3 in the cleavage of terminal transferase

To investigate the in vivo role of caspase-3 in Terminal Transferase metabolism DMSO-treated RPMI-8402, a human pre-T cell line was used. In DMSO treated samples (3)H-dGTP incorporation and TdT phosphorylation occurs after 4 hours of treatment. After 8 hours cells undergo TdT proteolysis in addition to its inactivation. The cleavage of TdT into 32- and 58-KDa proteolytic fragments occurred simultaneously with the activation of Caspase-3, but preceded changes associated with the apoptotic process described after 48 hours of treatment. The Caspase-3 peptide inhibitor V, used as a specific inhibitor, prevented TdT proteolysis prolonging its activity and rescued cells from apoptosis. Our experiments suggest that TdT is a nuclear substrate for Caspase-3, the main apoptotic effector protease in many cell types, and that the cleavage of TdT represents a primary step in a signal cascade leading to pre-T cell apoptosis.

Involvement of the TCR Cbeta FG loop in thymic selection and T cell function

The asymmetric disposition of T cell receptor (TCR) Cbeta and Calpha ectodomains creates a cavity with a side-wall formed by the rigid Cbeta FG loop. To investigate the significance of this conserved structure, we generated loop deletion (betaDeltaFG) and betawt transgenic (tg) mice using the TCR beta subunit of the N15 CTL. N15betawt and N15betaDeltaFG H-2(b) animals have comparable numbers of thymocytes in S phase and manifest developmental progression through the CD4(-)CD8(-) double-negative (DN) compartment. N15betaDeltaFG facilitates transition from DN to CD4(+)8(+) double-positive (DP) thymocytes in recombinase activating gene (RAG)-2(-/-) mice, showing that pre-TCR function remains. N15betaDeltaFG animals possess approximately twofold more CD8(+) single-positive (SP) thymocytes and lymph node T cells, consistent with enhanced positive selection. As an altered Valpha repertoire observed in N15betaDeltaFG mice may confound the deletion's effect, we crossed N15alphabeta TCR tg RAG-2(-/-) with N15betaDeltaFG tg RAG-2(-/-) H-2(b) mice to generate N15alphabeta RAG-2(-/-) and N15alphabeta.betaDeltaFG RAG-2(-/-) littermates. N15alphabeta.betaDeltaFG RAG-2(-/-) mice show an 8-10-fold increase in DP thymocytes due to reduced negative selection, as evidenced by diminished constitutive and cognate peptide-induced apoptosis. Compared with N15alphabeta, N15alphabeta.betaDeltaFG T cells respond poorly to cognate antigens and weak agonists. Thus, the Cbeta FG loop facilitates negative selection of thymocytes and activation of T cells.

Induction of apoptosis in a human erythroleukemic cell line K562 by tylophora alkaloids involves release of cytochrome c and activation of caspase 3

Tylophora alkaloids are plant products known for their antiasthamatic and antiproliferative activities. The underlying cellular changes resulting from inhibition of proliferation were investigated. Tylophora alkaloids induced apoptosis in K562 cells with characteristic apoptotic features like nuclear condensation, apoptotic body formation, flipping of membrane phosphatidylserine, activation of caspase 3 and release of mitochondrial cytochrome c. These studies suggest that the Tylophora alkaloids, in addition to their antiproliferative effects also induce apoptosis in erythroleukemic cells. These observations imply that Tylophora alkaloids could be useful molecules for their antiproliferative activity and for induction of apoptosis in tumor cells.

The apoptotic protease-activating factor 1-mediated pathway of apoptosis is dispensable for negative selection of thymocytes

Negative selection is a process to delete potentially autoreactive clones in developing thymocytes. Programmed cell death or apoptosis is thought to play an important role in this selection process. In this study, we investigated the role of apoptotic protease-activating factor 1 (Apaf1), a mammalian homologue of CED-4, in programmed cell death during the negative selection in thymus. There was no developmental abnormality in thymocytes from newborn Apaf1(-/-) mice in terms of CD4 and CD8 expression pattern and thymocyte number. Clonal deletion by endogenous male H-Y Ag of Apaf1-deficient thymocytes with transgenic expression of H-Y Ag-specific TCRs (H-Y Tg/Apaf1(-/-) thymocytes) was normally observed in lethally irradiated wild-type mice reconstituted with fetal liver-derived hemopoietic stem cells. Clonal deletion induced in vitro by a bacterial superantigen was also normal in fetal thymic organ culture. Thus, Apaf1-mediated pathway of apoptosis is dispensable for the negative selection of thymocytes. However, H-Y Tg/Apaf1(-/-) thymocytes showed partial resistance to H-Y peptide-induced deletion in vitro as compared with H-Y Tg/Apaf1(+/-) thymocytes, implicating the Apaf1-mediated apoptotic pathway in the negative selection in a certain situation. In addition, the peptide-induced deletion was still observed in H-Y Tg/Apaf1(-/-) thymocytes in the presence of a broad spectrum caspase inhibitor, z-VAD-fmk, suggesting the presence of caspase-independent cell death pathway playing roles during the negative selection. We assume that mechanisms for the negative selection are composed of several cell death pathways to avoid failure of elimination of autoreactive clones.

Peptide-induced negative selection of thymocytes activates transcription of an NF-kappa B inhibitor

Negative selection eliminates thymocytes bearing autoreactive T cell receptors (TCR) via an apoptotic mechanism. We have cloned an inhibitor of NF-kappa B, I kappa BNS, which is rapidly expressed upon TCR-triggered but not dexamethasone- or gamma irradiation-stimulated thymocyte death. The predicted protein contains seven ankyrin repeats and is homologous to I kappa B family members. In class I and class II MHC-restricted TCR transgenic mice, transcription of I kappa BNS is stimulated by peptides that trigger negative selection but not by those inducing positive selection (i.e., survival) or nonselecting peptides. I kappa BNS blocks transcription from NF-kappa B reporters, alters NF-kappa B electrophoretic mobility shifts, and interacts with NF-kappa B proteins in thymic nuclear lysates following TCR stimulation. Retroviral transduction of I kappa BNS in fetal thymic organ culture enhances TCR-triggered cell death consistent with its function in selection.

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.

Inhibitor of apoptosis protein from Orgyia pseudotsugata nuclear polyhedrosis virus provides a costimulatory signal required for optimal proliferation of developing thymocytes

The inhibitors of apoptosis proteins (IAPs) constitute a family of endogenous inhibitors that control apoptosis in the cell by inhibiting caspase processing and activity. IAPs are also implicated in cell division, cell cycle regulation, and cancer. To address the role of IAPs in thymus development and homeostasis, we generated transgenic mice expressing IAP generated from the baculovirus Orgyia pseudotsugata nuclear polyhedrosis virus (OpIAP). Developing thymocytes expressing OpIAP show increased nuclear levels of NF-kappaB and reduced cytoplasmic levels of its inhibitor, IkappaBalpha. In mature thymocytes, OpIAP induces optimal activation and proliferation after TCR triggering in the absence of a costimulatory signal. OpIAP expression in immature thymocytes blocks TCR-induced apoptosis. Taken together, our data illustrate the pleiotropism of OpIAP in vivo.

Dysregulation of apoptosis and a novel mechanism of defective apoptotic signal transduction in human B-cell neoplasms

Apoptotic cell death is essential for normal B-cell development and for shaping the B-cell repertoire. Dysregulation of the Bcl-2 related proteins and alterations of the p53/p14ARF pathway are implicated in the pathogenesis and treatment resistance in human B-cell malignancies. We found a novel mechanism of dysregulated apoptosis in human B lymphoma Raji cells that differs from that of altered Bcl-2 and p53 functions. This cell line was resistant to nuclear apoptosis induced by various stimuli, and neither mitochondrial activation nor activation of caspase-3 led to DNA fragmentation. DNA in purified Raji nuclei was degraded in the presence of lysates from the apoptosis-sensitive cell line HL-60, whereas Raji cell lysates did not induce DNA fragmentation in HL-60 nuclei. Cleavage of ICAD/DFF-45 was normal. These results indicate that the apoptosis signal transduction pathway is defective downstream of caspase-3 in Raji cell cytoplasm. Therefore, exploring the molecular mechanism in this system should provide insight into apoptosis resistance in human B-cell malignancies.

A naturally processed mitochondrial self-peptide in complex with thymic MHC molecules functions as a selecting ligand for a viral-specific T cell receptor

Peptide fragments of self-proteins bound to major histocompatibility complex molecules within the thymus are important for positively selecting T cell receptor (TCR)-bearing CD4(+)CD8(+) double positive (DP) thymocytes for further maturation. The relationship between naturally processed thymic self-peptides and TCR-specific cognate peptides is unknown. Here we employ HPLC purification of peptides released from H-2K(b) molecules of the C57BL/6 thymus in conjunction with mass spectrometry (MS) and functional profiling to identify a naturally processed K(b)-bound peptide positively selecting the N15 TCR specific for the vesicular stomatitis virus octapeptide (VSV8) bound to K(b). The selecting peptide was identified in 1 of 80 HPLC fractions and shown by tandem MS (MS/MS) sequencing to correspond to residues 68-75 of the MLRQ subunit of the widely expressed mitochondrial NADH ubiquinone oxidoreductase (NUbO(68-75)). Of note, the peptide differs at six of its eight residues from the cognate peptide VSV8 and functions as a weak agonist for mature CD8 single positive (SP) N15 T cells, with activity 10,000-fold less than VSV8. In N15 transgenic (tg) recombinase activating gene 2(-/)- transporter associated with antigen processing 1(-/)- fetal thymic organ culture, NUbO(68-75) induces phenotypic and functional differentiation of N15 TCR bearing CD8 SP thymocytes. Failure of NUbO(68-75) to support differentiation of a second K(b)-restricted TCR indicates that its inductive effects are not general.

The history of Z-VAD-FMK, a tool for understanding the significance of caspase inhibition

Dr. Robert Smith is one of the pioneers in histochemistry. One of his most important achievements is the recognition of proteolysis as a major physiological and pathophysiological process. As a consequence, he developed selective fluorogenic and chromogenic substrates and specific inhibitors of proteases that allow the (histochemical) analysis of protease activity. One of the latest successes is the design of Z-VAD-fluoromethylketone (FMK), the specific caspase inhibitor, that is a key compound for studies on apoptosis. Its development was originally meant for therapeutic use but unforeseen cytotoxicity of a metabolic derivative of the FMK compound disabled its potential as a drug. However, as a tool for fundamental research it is a great success. The history of Z-VAD-FMK is an example of the creative brain and the tireless perseverance of Robert Smith for which histochemistry and cytochemistry owes him so much. This history of Z-VAD-FMK is a well-deserved tribute at the occasion of his 70th birthday.

Tumor necrosis factor-related apoptosis-inducing ligand in T cell development: sensitivity of human thymocytes

TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) is a recently identified member of the tumor necrosis factor cytokine superfamily. TRAIL has been shown to induce apoptosis in various tumor cell lines, whereas most primary cells seem to be resistant. These observations have raised considerable interest in the use of TRAIL in tumor therapy. Yet little is known about the physiological function of TRAIL. This is particularly the case in the immune system, where TRAIL has been suggested by some to be involved in target cell killing and lymphocyte death. We have developed a panel of mAbs and soluble proteins to address the role of TRAIL in lymphocyte development. These studies demonstrate activation-induced sensitization of thymocytes to TRAIL-mediated apoptosis and expression of the apoptosis-inducing TRAIL receptors. However, with the use of several model systems, our subsequent experiments rule out the possibility that TRAIL plays a major role in antigen-induced deletion of thymocytes. In contrast to thymocytes, there is no up-regulation of TRAIL receptors in peripheral T cells on activation, which remain resistant to TRAIL. Thus, susceptibility to TRAIL-induced apoptosis is controlled differently by central and peripheral T cells.

A reliable and safe T cell repertoire based on low-affinity T cell receptors

Antigens are presented to T cells as short peptides bound to MHC molecules on the surface of body cells. The binding between MHC/peptides and T cell receptors (TCRs) has a low affinity and is highly degenerate. Nevertheless, TCR-MHC/peptide recognition results in T cell activation of high specificity. Moreover, the immune system is able to mount a cellular response when only a small fraction of the MHC molecules on an antigen-presenting cell is occupied by foreign peptides, while autoimmunity remains relatively rare. We consider how to reconcile these seemingly contradictory facts using a quantitative model of TCR signalling and T cell activation. Taking into account the statistics of TCR recognition and antigen presentation, we show that thymic selection can produce a working T cell repertoire which will produce safe and effective responses, that is, recognizes foreign antigen presented at physiological levels while tolerating self. We introduce "activation curves" as a useful tool to study the repertoire's statistical activation properties.

T-cell receptor downregulation by ceramide-induced caspase activation and cleavage of the zeta chain

Regulation of T-cell receptor (TCR) cell surface expression levels is probably an important mechanism by which T-cell responsiveness is controlled. Previously, two distinct pathways for TCR downregulation have been described. One is dependent on protein kinase C (PKC) and the leucine-based receptor-sorting motif (L-based motif) of the CD3 gamma chain but independent of tyrosine kinases, whereas the other is dependent on the tyrosine kinase activation but independent of the PKC and the CD3 gamma L-based motif. In this study, we describe a new pathway for TCR downregulation distinct from both the PKC/CD3 gamma L-based motif-dependent and the tyrosine kinase-dependent pathways. This pathway is dependent on ceramide-induced activation of caspases and correlate with caspase-mediated cleavage of the zeta chain. Thus, a 10--15% downregulation of the TCR was induced following the treatment of the T cells with ceramide for 4 h. A close correlation between TCR downregulation, caspase activation, and cleavage of the zeta chain was found. Furthermore, the caspase inhibitors abolished the cleavage of the zeta chain and TCR downregulation in parallel with the inhibition of the caspase activity.

Possible involvement of cyclophilin B and caspase-activated deoxyribonuclease in the induction of chromosomal DNA degradation in TCR-stimulated thymocytes

TCR engagement of immature CD4(+)CD8(+) thymocytes induces clonal maturation (positive selection) as well as clonal deletion (negative selection) in the thymus. However, the cell death execution events of thymocytes during the negative selection process remain obscure. Using a cell-free system, we identified two different DNase activities in the cytosol of in vivo anti-TCR-stimulated murine thymocytes: one that induced chromosomal DNA fragmentation, which was inhibited by an inhibitor of caspase-activated DNase, and another that induced plasmid DNA degradation, which was not inhibited by an inhibitor of caspase-activated DNase. We purified the protein to homogeneity that induced plasmid DNA degradation from the cytosol of anti-CD3-stimulated thymocytes and found that it is identical with cyclophilin B (Cyp B), which was reported to locate in endoplasmic reticulum. Ab against Cyp B specifically inhibited the DNA degradation activity in the cytosol of anti-CD3-stimulated thymocytes. Furthermore, recombinant Cyp B induced DNA degradation of naked nuclei, but did not induce internucleosomal DNA fragmentation. Finally, we demonstrated that TCR engagement of a murine T cell line (EL4) with anti-CD3/CD28 resulted in the release of Cyp B from the microsome fraction to the cytosol/nuclear fraction. Our data strongly suggest that both active caspase-activated DNase and Cyp B may participate in the induction of chromosomal DNA degradation during cell death execution of TCR-stimulated thymocytes.

Caspase enzyme activity is not essential for apoptosis during thymocyte development

Caspases, a family of cysteine proteases, are critical mediators of apoptosis. To address the importance of caspases in thymocyte development, we have generated transgenic mice that express the baculovirus protein p35, a viral caspase inhibitor, specifically in the thymus. p35 expression inhibited Fas (CD95)-, CD3-, or peptide-induced caspase activity in vitro and conferred resistance to Fas-induced apoptosis. However, p35 did not block specific peptide-induced negative selection in OT1 and HY TCR transgenic mouse models. Even the potent pharmacological caspase inhibitor zVAD-FMK (benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl-ketone) could not prevent peptide-induced deletion of OT1 thymocytes, although it improved basal thymocyte survival in vitro. Moreover, the developmental block observed in rag1-/- thymocytes, which lack pre-TCR signaling, was also not rescued by p35 expression. These results indicate that caspase-independent signal transduction pathways can mediate thymocyte death during normal T cell development.

Glucocorticoid-induced, caspase-dependent organ apoptosis early after burn injury

Immune suppression and increased apoptotic loss of circulating lymphocytes have been reported after burn injury. However, little is known about the underlying mechanisms responsible for the increased apoptosis of lymphoid and parenchymal cells in solid organs and the role played by inflammatory mediators, such as tumor necrosis factor-alpha (TNF-alpha) and Fas ligand (FasL), as well as by glucocorticoids. To evaluate the role of endogenously produced glucocorticoids and FasL, mice subjected to a 20% steam burn were pretreated with a glucocorticoid receptor antagonist (mifepristone) or a neutralizing murine Fas fusion protein. Three and twenty-four hours after burn injury, histological analysis, caspase-3 activity, and in situ terminal deoxynucleotidyl transferase dUTP nick-end labeling staining and phenotyping of lymphocyte populations for apoptosis were evaluated. Burn injury increased the number of apoptotic cells and caspase-3 activity in thymus and spleen, but not in other solid organs. Increased apoptosis was seen in several T and B cell populations from both thymus and spleen. Mifepristone pretreatment significantly reduced the apoptosis and caspase-3 activity after burn injury, whereas blocking FasL activity had only minimal effects. We conclude that corticosteroids, and not FasL, are primarily responsible for the increased caspase-3 activity and apoptosis in thymus and spleen cell populations early after burn injury.

MEK activity regulates negative selection of immature CD4+CD8+ thymocytes

CD4+CD8+ thymocytes are either positively selected and subsequently mature to CD4 single positive (SP) or CD8 SP T cells, or they die by apoptosis due to neglect or negative selection. This clonal selection is essential for establishing a functional self-restricted T cell repertoire. Intracellular signals through the three known mitogen-activated protein (MAP) kinase pathways have been shown to selectively guide positive or negative selection. Whereas the c-Jun N-terminal kinase and p38 MAP kinase regulate negative selection of thymocytes, the extracellular signal-regulated kinase (ERK) pathway is required for positive selection and T cell lineage commitment. In this paper, we show that the MAP/ERK kinase (MEK)-ERK pathway is also involved in negative selection. Thymocytes from newborn TCR transgenic mice were cultured with TCR/CD3epsilon-specific Abs or TCR-specific agonist peptides to induce negative selection. In the presence of the MEK-specific pharmacological inhibitors PD98059 or UO126, cell recovery was enhanced and deletion of DP thymocytes was drastically reduced. Furthermore, development of CD4 SP T cells was blocked, but differentiation of mature CD8 SP T cells proceeded in the presence of agonist peptides when MEK activity was blocked. Thus, our data indicate that the outcome between positively and negatively selecting signals is critically dependent on MEK activity.

Prothymosin alpha fragmentation in apoptosis

We observed fragmentation of an essential proliferation-related human nuclear protein prothymosin alpha in the course of apoptosis induced by various stimuli. Prothymosin alpha cleavage occurred at the DDVD(99) motif. In vitro, prothymosin alpha could be cleaved at D(99) by caspase-3 and -7. Caspase hydrolysis disrupted the nuclear localization signal of prothymosin alpha and abrogated the ability of the truncated protein to accumulate inside the nucleus. Prothymosin alpha fragmentation may therefore be proposed to disable intranuclear proliferation-related function of prothymosin alpha in two ways: by cleaving off a short peptide containing important determinants, and by preventing active nuclear uptake of the truncated protein.

Early activation of caspases during T lymphocyte stimulation results in selective substrate cleavage in nonapoptotic cells

Apoptosis induced by T cell receptor (TCR) triggering in T lymphocytes involves activation of cysteine proteases of the caspase family through their proteolytic processing. Caspase-3 cleavage was also reported during T cell stimulation in the absence of apoptosis, although the physiological relevance of this response remains unclear. We show here that the caspase inhibitor benzyloxycarbonyl (Cbz)-Val-Ala-Asp(OMe)-fluoromethylketone (zVAD) blocks proliferation, major histocompatibility complex class II expression, and blastic transformation during stimulation of peripheral blood lymphocytes. Moreover, T cell activation triggers the selective processing and activation of downstream caspases (caspase-3, -6, and -7), but not caspase-1, -2, or -4, as demonstrated even in intact cells using a cell-permeable fluorescent substrate. Caspase-3 processing occurs in different T cell subsets (CD4(+), CD8(+), CD45RA(+), and CD45RO(+)), and in activated B lymphocytes. The pathway leading to caspase activation involves death receptors and caspase-8, which is also processed after TCR triggering, but not caspase-9, which remains as a proenzyme. Most importantly, caspase activity results in a selective substrate specificity, since poly(ADP-ribose) polymerase (PARP), lamin B, and Wee1 kinase, but not DNA fragmentation factor (DFF45) or replication factor C (RFC140), are processed. Caspase and substrate processing occur in nonapoptotic lymphocytes. Thus, caspase activation is an early and physiological response in viable, stimulated lymphocytes, and appears to be involved in early steps of lymphocyte activation.

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.

A portrait of the Bcl-2 protein family: life, death, and the whole picture

The Bcl-2 family of proteins are important regulators of cell death. They are comprised of two opposing factions, the proapoptotic versus the antiapoptotic members. Both are required for normal development and cellular homeostasis of the immune system and other tissues. However, in certain circumstances they may participate in the development of disease.

B Cell Apoptosis Triggered by Antigen Receptor Ligation Proceeds Via a Novel Caspase-Dependent Pathway

In contrast to positive signaling leading to proliferation, the mechanisms involved in negative signaling culminating in apoptosis after B cell Ag receptor (BCR) ligation have received little study. We find that apoptosis induced by BCR cross-linking on EBV-negative mature and immature human B cell lines involves the following sequential, required events: a cyclosporin A-inhibitable, likely calcineurin-mediated step; and activation of caspase-2, -3, and -9. Caspase-2 is activated early and plays a major role in the apoptotic pathway, while caspase-9 is activated later in the apoptotic pathway and most likely functions to amplify the apoptotic signal. Caspase-8 and -1, which are activated by ligation of the CD95 and TNF-R1 death receptors, are not involved. Apoptosis induced by BCR ligation thus proceeds via a previously unreported intracellular signaling pathway.

Application of a fluorometric assay to detect caspase activity in thymus tissue undergoing apoptosis in vivo

To date, in vivo apoptosis within the thymus has been assessed using morphological criteria and/or detection of a DNA ladder indicative of oligonucleosomal fragmentation of the DNA. Here, we have used a fluorometric method to investigate activation of the caspase protease family in the thymus following in vivo induction of apoptosis by injection of the synthetic glucocorticoid hydrocortisone. Cleavage of DEVD-MCA by caspase-3 and other group II caspases releases free MCA which can be detected fluorimetrically. We demonstrate a time-dependent increase in DEVD-MCA cleavage activity within this tissue indicating the activation of caspase-3 like enzymes. This activity was inhibited by the specific group II caspase inhibitor DEVD-CHO. The interpretation of increased caspase activity was confirmed by immunoblot analysis to reveal cleavage of the caspase-3 substrate, fodrin. In addition, agarose gel electrophoresis of the DNA yielded a ladder pattern, confirming the occurrence of apoptosis. This study demonstrates that DEVD-MCA cleavage activity may be a useful quantitative method for the analysis of apoptosis in thymus tissue. It is a relatively rapid procedure not requiring thymocyte isolation or gel electrophoresis and detects fairly early biochemical changes occurring during apoptosis. In the present study we have used this method to demonstrate the involvement of caspases in thymocyte apoptotic death induced in vivo by glucocorticoids. Thus, measurement of caspase activity in thymus tissue may have applications for studying the in vivo effects of immunotoxicants.

The central effectors of cell death in the immune system

The immune system relies on cell death to maintain lymphoid homeostasis and avoid disease. Recent evidence has indicated that the caspase family of cysteine proteases is a central effector in apoptotic cell death and is absolutely responsible for many of the morphological features of apoptosis. Cell death, however, can occur through caspase-independent and caspase-dependent pathways. In the case of cells that are irreversibly neglected or damaged, death occurs even in the absence of caspase activity. In contrast, healthy cells require caspase activation to undergo cell death induced by surface receptors. This review summarizes the current understanding of these two pathways of cell death in the immune system.

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.