The T cell receptor repertoire of CD4-8+ thymocytes is altered by overexpression of the BCL-2 protooncogene in the thymus

BAC-mediated transgenic expression of fluorescent autophagic protein Beclin 1 reveals a role for Beclin 1 in lymphocyte development

Beclin 1/Atg6 is an essential component of the evolutionary conserved PtdIns(3)-kinase (Vps34) protein complex that regulates macroautophagy (autophagy) in eukaryotic cells and also interacts with antiapoptotic Bcl-2 family members, Bcl-2, and Bcl-x(L). To elucidate the physiological function of Beclin 1, we generated transgenic mice producing a green fluorescent Beclin 1 protein (Beclin 1-GFP) under Beclin 1 endogenous regulation. The beclin 1-GFP transgene is functional because it completely rescues early embryonic lethality in beclin 1-deficient mice. The transgenic mice appear normal, with undetected change in basal autophagy levels in different tissues, despite the additional expression of functional Beclin 1-GFP. Staining of Beclin 1-GFP shows mostly diffuse cytoplasmic distribution in various tissues. Detailed analysis of the transgene expression by flow cytometry reveals a Bcl-2-like biphasic expression pattern in developing T and B cells, as well as differential regulation of expression in mature versus immature thymocytes following in vitro stimulation. Moreover, thymocytes expressing high Beclin 1-GFP levels appear increasingly sensitive to glucocorticoid-induced apoptosis in vitro. Our results, therefore, support a role for Beclin 1 in lymphocyte development involving cross talk between autophagy and apoptosis.

Human innate Mycobacterium tuberculosis-reactive alphabetaTCR+ thymocytes

The control of Mycobacterium tuberculosis (Mtb) infection is heavily dependent on the adaptive Th1 cellular immune response. Paradoxically, optimal priming of the Th1 response requires activation of priming dendritic cells with Th1 cytokine IFN-γ. At present, the innate cellular mechanisms required for the generation of an optimal Th1 T cell response remain poorly characterized. We hypothesized that innate Mtb-reactive T cells provide an early source of IFN-γ to fully activate Mtb-exposed dendritic cells. Here, we report the identification of a novel population of Mtb-reactive CD4⁻ αβTCR⁺ innate thymocytes. These cells are present at high frequencies, respond to Mtb-infected cells by producing IFN-γ directly ex vivo, and display characteristics of effector memory T cells. This novel innate population of Mtb-reactive T cells will drive further investigation into the role of these cells in the containment of Mtb following infectious exposure. Furthermore, this is the first demonstration of a human innate pathogen-specific αβTCR⁺ T cell and is likely to inspire further investigation into innate T cells recognizing other important human pathogens.

Mycobacterium tuberculosis (Mtb) infects about one-third of the world's population. Most people who are exposed remain healthy, but control of this intracellular bacterium requires a robust cellular immune response. Production of the pro-inflammatory cytokine IFN-γ from cells in the adaptive immune response is critically important in the immune control of Mtb. However, this cytokine is also essential in initiating an optimal adaptive immune response. We hypothesized that innate cells could provide an early source of IFN-γ to aid in generation of an optimal adaptive immune response. We looked for IFN-γ producing cells in human neonates that were unlikely to have been previously exposed to either Mtb or other environmental mycobacteria. Here, we report the identification of a novel T cell population from the thymus that produces IFN-γ in response to Mtb-infected cells. Mtb-reactive thymocytes are present at high frequencies, are present in nearly all newborns tested, and display characteristics of T cells normally associated with a memory response. This novel innate population of Mtb-reactive cells will drive further investigation into the role of these cells in the containment of Mtb following infectious exposure and is likely to inspire further investigation into innate T cells recognizing other important human pathogens.

Bcl-2 protein family members: versatile regulators of calcium signaling in cell survival and apoptosis

Bcl-2 family members are important regulators of cell survival and cell death. Researchers have focused mainly on mitochondria, where both proapoptotic and antiapoptotic family members function to regulate the release of cytochrome c and other mediators of apoptosis. However, as reviewed here, Bcl-2 family members also operate on another front, the endoplasmic reticulum (ER), to both positively and negatively regulate the release of Ca2+. There is abundant evidence that Ca2+ signals trigger apoptosis in response to a wide variety of agents and conditions. Conversely, Ca2+ signals can also mediate cell survival. Recent findings indicate that Bcl-2 interacts with inositol 1,4,5-trisphosphate (IP3) receptor Ca2+ channels on the ER, regulating their opening in response to IP3- and thus inhibiting IP3-mediated Ca2+ signals that induce apoptosis while enhancing Ca2+ signals that support cell survival.

Bim and Bcl-2 mutually affect the expression of the other in T cells

The life and death of T cells is controlled to a large extent by the relative amounts of Bcl-2-related proteins they contain. The antiapoptotic protein Bcl-2 and the proapoptotic protein Bim are particularly important in this process with the amount of Bcl-2 per cell dropping by about one-half when T cells prepare to die. In this study we show that Bcl-2 and Bim each control the expression of the other. Absence of Bim leads to a drop in the amount of intracellular Bcl-2 protein, while having no effect on the amounts of mRNA for Bcl-2. Conversely, high amounts of Bcl-2 per cell allow high amounts of Bim, although in this case the effect involves increases in Bim mRNA. These mutual effects occur even if Bcl-2 is induced acutely. Thus these two proteins control the expression of the other, at either the protein or mRNA level.

Impact of cellular lifespan on the T cell receptor repertoire

Pro-survival members of the Bcl-2 family are potent inhibitors of cell death and determine the lifespan of immature thymocytes by counteracting the intrinsically active apoptotic program in these cells. BH3-only proteins are potent antagonists of Bcl-2-like molecules and regulate death and survival of lymphocytes during their development and homeostasis. The intrinsic lifespan of CD4(+)8(+) double-positive thymocytes was reported to actively shape the diversity of the immune repertoire, since mice overexpressing Bcl-x(L) were reported to show a bias towards the usage of distal 3' Jalpha elements 1. To gain support for this concept, we analyzed TCRalpha rearrangements in T lymphocytes that show an extended lifespan due to either loss of the BH3-only protein Bim or overexpression of Bcl-2. A minor but reproducible skewing towards the usage of the more distal 3' Jalpha elements was observed in developing thymocytes and mature T cells from bim(-/-) and vav-bcl-2 transgenic mice, indicating that prolonged survival of double-positive thymocytes does have a significant impact on the selected TCRalpha repertoire. However, the changes that we observed were less pronounced than those found in lck-bcl-x(L) transgenic mice, pointing towards qualitative differences between Bcl-2- and Bcl-x(L)-mediated cell death inhibition during T cell development.

RasGRP1 Transmits Prodifferentiation TCR Signaling That Is Crucial for CD4 T Cell Development

TCR signaling plays a governing role in both the survival and differentiation of bipotent double-positive thymocytes into the CD4(+) and CD8(+) single-positive T cell lineages. A central mediator of this developmental program is the small GTPase Ras, emitting cytoplasmic signals through downstream MAPK pathways and eventually affecting gene expression. TCR signal transduction orchestrates the activation of Ras by integrating at least two Ras-guanyl nucleotide exchange factors, RasGRP1 and Sos. In this study, we have characterized the relationship between RasGRP1 function and its potential roles in promoting ERK activity, cell survival, maturation, and lineage commitment. Investigations on RasGRP1(-/-) mice expressing a transgenic (Tg) MHC class II-restricted TCR revealed that the development of CD4 T cells expressing this Tg TCR is completely dependent on RasGRP1. Unexpectedly, a small number of functional CD8 single-positive thymocytes expressing the Tg MHC class II-restricted TCR exists in mutant mice. In addition, RasGRP1(-/-) double-positive thymocytes exhibit marked deficits in TCR-stimulated up-regulation of the positive selection marker CD69 and the antiapoptotic protein Bcl-2, whereas CD5 induction is unaffected. To evaluate the role of RasGRP1 in providing cellular survival signaling, we enforced Bcl-2 expression in RasGRP1(-/-) thymocytes. These studies demonstrate that RasGRP1 function cannot be fully complemented by Tg Bcl-2 expression. Therefore, we propose that RasGRP1 transmits differentiation signaling critically required for CD4 T cell development.

Overexpression of the antiapoptotic protein A1 promotes the survival of double positive thymocytes awaiting positive selection

As it has been shown for Mcl-1, Bcl-xl and Bcl-2, proteins of the Bcl-2 family play a crucial role during T-cell development in the thymus. We here show that the expression of the antiapoptotic gene A1 is specifically enhanced at the DN3/DN4 transition and in DP thymocytes that have been positively selected suggesting that A1 expression might be considered as a transcriptional signature of thymocytes that have received pre-TCR or TCR survival signal. Furthermore, we observed that A1-a overexpression in recombination activation gene 1-deficient mice transgenic for the major histocompatibillity complex class I-restricted F5 TCR enhances cell survival of DP thymocytes and permits accumulation of DP cells awaiting positive selection. However, A1-a overexpression has no effect on negative selection. Therefore, our results suggest that A1 plays a specialized role in allowing survival of DP thymocytes and that its role can be distinguished from that of Mcl-1, Bcl-xl and Bcl-2.

Human high mobility group box transcription factor 1 affects thymocyte development and transgene variegation

It has been shown previously that a human CD2 (hCD2) disabled locus control region (LCR) transgene is unable to establish an open chromatin configuration in all the T cells, and this leads to position effect variegation of the transgene. In this study we show that thymus-specific overexpression of human high mobility group box transcription factor 1 (HBP1), a transcription factor that binds a specific sequence within the hCD2 LCR, affects thymus cellularity as well as the number of CD8(+) thymocytes in two independent transgenic mouse lines and increases the proportion of T cells that fully activate the transgenic locus in hCD2 variegating mice in a sequence-specific dependent manner. This finding suggests that overexpression of HBP1 can affect lineage commitment and can relieve the suppressive influence of heterochromatin, allowing thymocytes to express the variegating target locus more efficiently. These effects could be the result of direct HBP1 action on LCR activity. Alternatively, the extra HBP1 molecules may sequester repressive elements away from the LCR, thus allowing transcription permissive states to form on the transgene locus.

The control of apoptosis in lymphocyte selection

The stochastic nature of rearrangement and diversification of the gene segments encoding immunoglobulins (Igs) and T cell receptors (TCRs) inevitably gives rise to immature B and T lymphocytes that lack antigen receptors or express useless or dangerous (self-antigen-specific) ones. Signaling through antigen receptors promotes survival, proliferative expansion and further differentiation of useful cells and deletion of the useless and dangerous ones. During immune responses, pathogen-specific B and T lymphocytes, as well as cells of the innate immune system, undergo extensive proliferation and develop effector functions, such as antibody secretion, cytotoxicity or cytokine production. To prevent tissue damage by these effector molecules, activated lymphocytes are removed when an infection has been overcome. Together with other mechanisms, including developmental arrest and induction of unresponsiveness (anergy), programmed cell death (apoptosis) of autoreactive lymphocytes safeguards immunological tolerance to self and assists in the development of an effective immune system. We have been investigating the molecular mechanisms that control programmed cell death. This review describes some of our experiments using transgenic and knockout mice, which overexpress or lack apoptosis regulators, that led to discoveries on how life and death decisions are made during development and functioning of the immune system.

Molecular mechanisms of pre-T cell receptor-induced survival

En route to maturing as T cell receptor (TCR) alphabeta-expressing cells, the development of thymocytes is contingent on expression of a pre-TCR complex comprising a TCRbeta chain paired with a surrogate TCRalpha chain, pre-Talpha (pTalpha). The pre-TCR has been proposed to promote cell survival, proliferation, differentiation, and lineage commitment. However, the precise molecular mechanisms governing this variety of effects remain elusive. Here, we present a cellular system designed to biochemically dissect signals elicited upon pre-TCR expression. Using the T cell line 4G4 stably transfected with one of the two known pTalpha isoforms or selective pTalpha deletion mutants and TCRbeta, we were able to observe that expression of a functional pre-TCR complex is sufficient to control the levels of surface Fas protein, the stimulation of mitogen-activated and stress-regulated kinases, and the activation status of the p53 antioncogene. We demonstrate that this regulation has a major impact on the expression of important regulators of apoptosis, such as Bcl-2 family members, and the cell cycle, such as p21(WAF). Furthermore, we show here that cells expressing a functional pre-TCR are more resistant to different types of DNA damage-induced apoptosis and that these effects are contingent on an intact cytoplasmic tail of pTalpha. We finally propose that the presence of a functional pre-TCR complex triggers many intracellular pathways capable of driving and ensuring thymocyte survival in the presence of DNA damage.

Thymocyte development in early growth response gene 1-deficient mice

Early growth response gene 1 (Egr1) codes for a transcriptional regulator that contains a zinc-finger DNA binding domain. Egr1 expression is induced by a variety of extracellular stimuli including TCR-ligand interactions. Its pattern of expression in the thymus and dependence on ERK activation have led to speculation that it has a role in T cell development, but the exact nature of this role has been undefined. To more clearly define the role of Egr1 in thymocyte development, we have analyzed thymocytes from Egr1-deficient mice. We find that thymuses from Egr1-deficient mice contain twice as many cells as age-matched controls, and the increase in thymocyte number is apparent at the early CD4/CD8 double negative stage of development. Subsequent maturation to the CD4/CD8 double positive stage and survival of the double positive cells both appear normal in Egr1-deficient animals. We also find that Egr1 promotes positive selection of both CD4 and CD8 single positive cells without playing a major role in negative selection. Egr1 influences positive selection by enhancing expression of the helix-loop-helix inhibitor Id3 and the anti-apoptosis molecule bcl-2. Thus, Egr1 translates developmental signals into appropriate changes in gene expression at multiple stages of thymocyte development.

Different role of Apaf-1 in positive selection, negative selection and death by neglect in foetal thymic organ culture

Apoptotic protease-activating factor 1 (Apaf-1) is a component of the apoptosome which is required for the activation of procaspase-9. As Apaf-1 knockout (KO) (Apaf-1-/-) mice die before birth, the role of Apaf-1 during thymic selection was investigated using 5 day foetal thymic organ culture (FTOC) of thymi obtained at gestational day 15. There was a lower ratio of CD4 single-positive (SP) to CD8 SP cells and decreased apoptosis of CD4+CD8+ (DP) thymocytes from Apaf-1-/- mice compared with wild-type. To determine if these defects resulted in increased production of neglected thymocytes, the Apaf-1-/- mice were crossed with the T-cell receptor (TCR)-alpha-chain KO mice. There was no difference in thymocyte development in the thymi of TCR-alpha-/-Apaf-1-/- and TCR-alpha-/-Apaf-1+/+ mice 5 days after FTOC. To determine if Apaf-1 is involved in apoptosis during death by negative or positive selection, FTOC of the thymus of Apaf-1-/- Db/HY TCR-alphabeta transgenic (Tg) mice was carried out. There was decreased apoptosis of the HY clonal-specific M33+ thymocytes and an increased percentage of the autoreactive CD8+M33+ thymocytes in male, but not female Apaf-1-/- Db/HY TCR Tg mice. Our data suggest that Apaf-1 is not involved in positive selection or death by neglect, but may have a partial role in negative selection during early thymic T-cell development.

A study on CD45 isoform expression during T-cell development and selection events in the human thymus

CD45 molecules are known to appear as various isoforms generated by alternative splicing of variable exons 4, 5, and 6, but the detailed profile of CD45 isoform expression during thymocyte development has not been revealed. We examined the CD45 isoforms expressed in the various human thymocytes' subsets defined by CD3, CD4, and CD8 expressions using RT-PCR and 4-color flow cytometry. RT-PCR study revealed that RABC, RAB, RBC, RB, and R0 isoforms were expressed in thymocytes while any of RAC, RA, or RC isoforms were not detected. RABC, RAB and RBC isoforms were expressed at CD3(-)CD4(-)CD8(-) and CD3(+)CD4(+)CD8(-) stages, but were barely detectable at CD3(-)CD4(+)CD8(+) stage. RB isoform was consistently expressed at a relatively high level at all stages. R0 isoform was expressed at a low level at CD3(-)CD4(-)CD8(-) and CD3(-)CD4(+)CD8(-) stages but upregulated at CD3(+)CD4(+)CD8(+) and CD3(+)CD4(+)CD8(-) stages. In combination with the results obtained by 4-color flow cytometric study, CD45 isoform expression on human thymocytes were determined to be RABC(+)RAB(+/-)RBC(+)RB(+)R0(+/-) at CD3(-)CD4(-)CD8(-) stage, RABC(-)RAB(-)RBC(-)RB(+)R0(+) at CD3(-)CD4(+)CD8(-) and CD3(-)CD4(+)CD8(+) stages, RABC(+/-)RAB(+)RBC(+)RB(++)R0(++) at CD3(+)CD4(+)CD8(+) stage, and RABC(+)RAB(+)RBC(+)RB(++)R0(+) at CD3(+)CD4(+)CD8(-) stage. Bcl-2 expression was upregulated between CD3(-)CD4(+)CD8(+)CD45R0(+) and CD3(+)CD4(+)CD8(+)CD45R0(+) stages. Expression of CD45R0 epitope was upregulated between CD3(-)CD4(+)CD8(+)CD69(-) and CD3(+)CD4(+)CD8(+)CD69(+) stages while CD45RA epitope expression was unchanged. Thus, when thymocytes are positively selected, CD45R0 isoform expression seems to be upregulated while CD45RABC isoform expression stays at a very low level. In summary, various isoforms of CD45 were shown to be tightly regulated during thymocyte development and through the selection process.

Perturbation of B-cell development in mice overexpressing the Bcl-2 homolog A1

Decisions about cell survival or death are central components of adaptive immunity and occur at several levels in immune system development and function. The Bcl-2 family of homologous proteins plays an important role in these decisions in lymphoid cells. Bcl-2, Bcl-xL, and A1 are differentially expressed during B- and T-cell development, and they have shared and distinct roles in regulating cell death. We sought to gain insight into the role of A1 in immune system development and function. A murine A1-a transgene was expressed under the control of the Emu enhancer, and mice with A1 overexpression in B- and T-cell lineages were derived. Thymocytes and early B cells in Emu-A1 mice showed extended survival. B-lineage development was altered, with expansion of the pro-B cell subset at the expense of pre-B cells, suggesting an impairment of the pro- to pre-B-cell transition. This early B-cell phenotype resembled Emu-Bcl-xL mice but did not preferentially rescue cells with completed V(D)J rearrangements of the immunoglobulin heavy chain. In contrast to Emu-Bcl-2 transgenes, A1 expression in pro-B cells did not rescue pre-B-cell development in SCID mice. These studies indicate that A1 protects lymphocytes from apoptosis in vitro but that it has lineage- and stage-specific effects on lymphoid development. Comparison with the effects of Bcl-2 and Bcl-xL expressed under similar control elements supports the model that antiapoptotic Bcl-2 homologs interact differentially with intracellular pathways affecting development and apoptosis in lymphoid 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.

TOX: an HMG box protein implicated in the regulation of thymocyte selection

In the thymus, pre-T cell receptor (pre-TCR)--mediated signaling and then TCR-mediated signaling initiate changes in gene expression that result in the maturation of CD4 and CD8 lineage T cells from common precursors. Using gene chip technology, we isolated a murine gene, designated Tox, that encodes a member of the HMG (high-mobility group) box family of DNA-binding proteins. TOX expression is up-regulated by both pre-TCR and TCR activation of immature thymocytes but not by TCR activation of mature naïve T cells. Transgenic mice that express TOX show expanded CD8+ and reduced CD4+ single positive thymocyte subpopulations. We present evidence here that this phenotype results from a perturbation in lineage commitment due to reduced sensitivity to TCR-mediated signaling. This molecular marker of thymic selection events may therefore play a role in establishing the activation threshold of developing T cells and patterning changes in gene expression.

Requirement for sustained MAPK signaling in both CD4 and CD8 lineage commitment: a threshold model

Although there is general agreement that the RAS/MAPK signaling pathway is required for positive selection of CD4 T cells in the thymus, the role of this pathway in CD8 lineage commitment remains controversial. We show here that the differentiation of isolated cultured thymocytes to the CD8 as well as CD4 T cell lineage is sensitive to MEK inhibition and that both CD4 and CD8 thymocyte differentiation requires sustained MEK signaling. However, CD4 lineage commitment is promoted by a stronger stimulus for longer duration than required for CD8 lineage commitment. Interestingly, CD4 lineage commitment is not irreversibly set even after 10 h of signaling, well past early changes in gene expression. These findings are presented in the context of a model of lineage commitment in which a default pathway of CD8 lineage commitment is altered to CD4 commitment if the thymocyte achieves a threshold level of active MAPK within a certain time frame.

Elevated Bcl-2 is not a causal event in the positive selection of T cells

T cell development is characterized by the induction of apoptosis in most immature thymocytes and the rescue from apoptosis of a small proportion of cells by the process of positive selection.Up-regulation of the anti-apoptotic molecule Bcl-2 is associated with thymocytes undergoing positive selection and a bcl-2 transgene promotes the generation of mature T cells. In contrast,mice transgenic for the pro-apoptotic molecule Bax show impaired T cell maturation. We have used fetal thymic organ culture to determine the action of Bcl-2 and Bax on positive selection of thymocytes. Our data show that Bcl-2 and Bax do not alter the number of thymocytes positively selected by a defined peptide ligand. This implies that Bcl-2 and Bax alter the production of mature T cells in vivo by influencing thymocyte viability rather than by direct action on positive selection. It also presents a solution to the 'chicken-and-egg' scenario relating to Bcl-2 up-regulation and positive selection. The data suggest that the up-regulation of Bcl-2 associated with T cell maturation is a consequence of positive selection rather than a cause of it.

Overexpression of Bcl-2 Differentially Restores Development of Thymus-Derived CD4−8+ T Cells and Intestinal Intraepithelial T Cells in IFN-Regulatory Factor-1-Deficient Mice

Mice lacking IFN-regulatory factor (IRF)-1 have reduced numbers of mature CD8+ T cells within the thymus and peripheral lymphoid organs, suggesting a critical role of IRF-1 in CD8(+) T cell differentiation. Here we show that endogenous Bcl-2 expression is substantially reduced in IRF-1(-/-)CD8+ thymocytes and that introduction of a human Bcl-2 transgene driven by Emu or lck promoter in IRF-1(-/-) mice restores the CD8(+) T cell development. Restored CD8+ T cells are functionally mature in terms of allogeneic MLR and cytokine production. In contrast to thymus-derived CD8+ T cells, other lymphocyte subsets including NK, NK T, and TCR-gammadelta(+) intestinal intraepithelial lymphocytes, which are also impaired in IRF-1(-/-) mice, are not rescued by expressing human Bcl-2. Our results indicate that IRF-1 differentially regulates the development of these lymphocyte subsets and that survival signals involving Bcl-2 are critical for the development of thymus-dependent CD8+ T cells.

Thymocyte-targeted overexpression of xiap transgene disrupts T lymphoid apoptosis and maturation

The X-linked inhibitor of apoptosis (XIAP) and other members of the inhibitor of apoptosis (IAP) family can suppress apoptosis induced by a diverse variety of triggers. Functional studies done to date have focused on tissue culture models and adenovirus overexpression of XIAP and other IAP proteins. Here we report the phenotype of an engineered transgenic mouse overexpressing a human IAP, as well as assessing the long-term consequence of IAP overexpression. We document the relative protein expression levels of the endogenous mouse homologue to XIAP, mouse inhibitor of apoptosis (MIAP 3), within thymocyte and T cell subpopulations. The consequence of lymphoid-targeted overexpression of XIAP in transgenic mice suggests a physiological role for the endogenous protein, MIAP3. Xiap-transgenic mice accumulated thymocytes and/or T cells in primary and secondary lymphoid tissue, T cell maturation was perturbed, and transgenic thymocytes resisted a variety of apoptotic triggers both in vitro and in vivo. These observations imply a possible key function for the intrinsic cellular inhibitor XIAP in maintaining the homeostasis of the immune system.

The role of molecules that mediate apoptosis in T-cell selection

The T-cell repertoire is generated as a result of selection processes that happen in the thymus. The molecular basis of these processes is beginning to be elucidated and many of the molecules identified have a role in regulating cell death. These molecules range from orphan nuclear receptors and transcription factors to cyclin-dependent kinase 2 and Bcl2 family members.

BCL-6 protein is expressed in precursor T-cell lymphoblastic lymphoma and in prenatal and postnatal thymus

The organization and expression of the BCL-6 gene in normal and neoplastic thymic T cells has not been fully determined. We examined 8 precursor T-cell lymphoblastic lymphomas (T-LBLs) and found significant BCL-6 expression in 4 cases. Three of the BCL-6(+) cases expressed a common thymocyte phenotype (CD4(+), CD8(+)), and one expressed a precursor thymocyte phenotype (CD4(-), CD8(-)). In 6 cases evaluated, including those expressing BCL-6, molecular analyses demonstrated a germline configuration of the BCL-6 gene and a wild-type BCL-6 gene first exon-intron boundary region. We also evaluated 12 normal prenatal and postnatal thymuses for BCL-6 protein. BCL-6 was expressed by most cortical thymocytes and by scattered medullary thymocytes. BCL-6(+) cortical and medullary thymocytes also expressed CD2, CD3, CD4, CD5, CD7, or CD8. We further analyzed the pattern of BCL-2 and BCL-X(L) expression and their coexpression with BCL-6 in normal thymus and T-LBL and compared it to that of follicle centers of reactive lymph nodes and follicular lymphoma. BCL-6(+) cortical thymocytes coexpressed BCL-X(L) but not BCL-2. All 4 BCL-6(+) T-LBLs and 4 BCL-6(-) T-LBLs coexpressed BCL-2 and BCL-X(L). Conceivably, T-LBLs may arise through clonal expansion of cortical thymocytes normally expressing the BCL-6 protein. The pattern of BCL-6, BCL-2, and BCL-X(L) expression in cortical thymocytes is highly reminiscent of germinal centers, and the abnormal coexpression of BCL-2, BCL-X(L), and BCL-6 in T-LBL is analogous to coexpression in follicle center cell lymphomas, suggesting that coexpression of these anti-apoptotic genes may contribute to the pathogenesis of T-LBL.

Coreceptor reversal in the thymus: signaled CD4+8+ thymocytes initially terminate CD8 transcription even when differentiating into CD8+ T cells

A central paradigm of T cell development is that CD4+8+ (DP) thymocytes differentiate into CD4+ or CD8+ T cells in response to intrathymic signals that extinguish transcription of the inappropriate coreceptor molecule. Contrary to this prevailing paradigm, we now demonstrate that signaled DP thymocytes initially terminate CD8 transcription even when differentiating into CD8+ T cells. Remarkably, thymocytes that have selectively terminated CD8 transcription can be signaled by IL-7 to differentiate into CD8+ T cells by silencing CD4 transcription and reinitiating CD8 transcription, events we refer to as "coreceptor reversal." These observations significantly alter our understanding of CD8+ T cell differentiation and lead to a new perspective ("kinetic signaling") on CD4/CD8 lineage determination in the thymus. These observations also suggest a novel mechanism by which bipotential cells throughout development can determine their appropriate cell fate.

Generation of alphabeta T-cell receptor+ CD4- CD8+ cells in major histocompatibility complex class I-deficient mice upon activation of the aryl hydrocarbon receptor by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Gene-targeted mice lacking the beta2 microglobulin gene (beta2m-/- mice), and hence functional major histocompatibility complex (MHC) class I molecules, do not develop CD4- CD8+ cells. We show here that both in vitro and in vivo treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a trans-activating ligand of the endogenous aryl hydrocarbon receptor (Ah-R), bypasses the need for MHC class I molecules for selection into the CD4- CD8+ cell pool. When beta2m-/- dams were given a single dose of 50 microg of TCDD, approximately 13% of CD4- CD8+ thymocytes could be detected in their newborn pups. In TCDD-exposed fetal thymus organ cultures of beta2m-/- mice, approximately 35% CD4- CD8+ thymocytes were detectable. About 16% of these CD4- CD8+ cells bore the alpha beta T-cell receptor (TCR) and approximately 33% bore CD3. Only a minority of the CD8+ cells were heat-shock antigen positive. The cells possessed killing activity as shown using the 51Cr-release assay comprising gamma delta TCR- CD4- CD8+ thymocytes from 3 to 4-day-old b2m-/- mice. Thus, TCDD leads to a significant increase of mature CD4- CD8+ thymocytes in relative and absolute numbers. High numbers of CD4- CD8+ thymocytes developed also in organ cultures from thymi, lacking both MHC class I and class II molecules, exposed to TCDD. A 10-fold transient increase of Notch1 mRNA in thymocytes from fetal thymus organ culture, exposed for 4 days to TCDD, was detected in CD4+ CD8+ cells compared with controls. We suggest that TCDD affects thymic selection and directs the lineage commitment of CD4+ CD8+ thymocytes towards CD4- CD8+ cells, possibly via up-regulation of the Notch1 gene.

Protein kinase B regulates T lymphocyte survival, nuclear factor kappaB activation, and Bcl-X(L) levels in vivo

The serine/threonine kinase protein kinase B (PKB)/Akt mediates cell survival in a variety of systems. We have generated transgenic mice expressing a constitutively active form of PKB (gag-PKB) to examine the effects of PKB activity on T lymphocyte survival. Thymocytes and mature T cells overexpressing gag-PKB displayed increased active PKB, enhanced viability in culture, and resistance to a variety of apoptotic stimuli. PKB activity prolonged the survival of CD4(+)CD8(+) double positive (DP) thymocytes in fetal thymic organ culture, but was unable to prevent antigen-induced clonal deletion of thymocytes expressing the major histocompatibility complex class I-restricted P14 T cell receptor (TCR). In mature T lymphocytes, PKB can be activated in response to TCR stimulation, and peptide-antigen-specific proliferation is enhanced in T cells expressing the gag-PKB transgene. Both thymocytes and T cells overexpressing gag-PKB displayed elevated levels of the antiapoptotic molecule Bcl-X(L). In addition, the activation of peripheral T cells led to enhanced nuclear factor (NF)-kappaB activation via accelerated degradation of the NF-kappaB inhibitory protein IkappaBalpha. Our data highlight a physiological role for PKB in promoting survival of DP thymocytes and mature T cells, and provide evidence for the direct association of three major survival molecules (PKB, Bcl-X(L), and NF-kappaB) in vivo in T lymphocytes.

TCR v(beta) repertoire restriction and lack of CDR3 conservation implicate TCR-superantigen interactions in promoting the clonal evolution of murine thymic lymphomas

Thymic lymphoma development is a multistage process in which genetic and epigenetic events cooperate in the emergence of a malignant clone. The notion that signaling via TCR-ligand interactions plays a role in promoting the expansion of developing neoplastic clones is a matter of debate. To investigate this issue, we determined the TCR V(beta) repertoire of thymic lymphomas induced in AKR/J mice by either endogenous retroviruses or the carcinogen, N-methyl-N-nitrosourea (MNU). Both spontaneous and MNU-induced lymphomas displayed restricted V(beta) repertoires. However, whereas V(beta)6, V(beta)8 and V(beta)9 were expressed by a greater than expected frequency of MNU-induced lymphomas, V(beta)8, V(beta)7, V(beta)13 and V(beta)14 were over-represented on spontaneous lymphomas. The dissimilar TCR V(beta) profiles indicate that different endogenous ligands promote neoplastic clonal expansion in untreated and MNU-treated mice. Although the nature of these ligands is not clear, the lack of conservation in TCR beta chain CDR3 regions among lymphomas that express the same V(beta) segment suggests that endogenous superantigens (SAG), as opposed to conventional peptide ligands, are likely to be involved in the selection process. The biased representation of lymphomas expressing V(beta)6-, V(beta)7- and V(beta)9-containing TCRs that recognize endogenous SAG is consistent with this hypothesis. The finding that Bcl-2 is expressed at high levels in spontaneous and MNU-induced lymphomas suggests that preneoplastic thymocytes may be resistant to SAG-induced clonal deletion. A working model is presented in which preneoplastic clones expressing TCRs that recognize endogenous SAG are selectively expanded as a consequence of sustained TCR-mediated signaling.

A role for CaMKII in T cell memory

In order to study the role of calcium/calmodulin kinase II (CaMKII) in T cells, we generated transgenic mice expressing CaMKIIgammaB* (T287D), a partially calcium-independent mutant of CaMKIIgammaB. In these mice, the size of the thymus was increased 1.5- to 2-fold, at least in part due to an increase in the lifespan of double-positive (DP) thymocytes. More importantly, there was an increase in the number of T cells in the secondary lymphoid organs that had acquired an antigen-dependent memory phenotype. These T cells were bonafide memory cells as assessed by a variety of criteria. In addition, T cells from wild-type mice acquired calcium-independent CaMKII activity after several rounds of antigen-stimulated division. We propose that CaMKII controls a distinct process of activation-induced cellular differentiation.

In Vivo Overexpression of Dad1, the Defender Against Apoptotic Death-1, Enhances T Cell Proliferation But Does Not Protect Against Apoptosis

The Dad1 protein has been shown to play a role in prevention of apoptosis in certain cell types. Dad1 is also a subunit of the oligosaccharyltransferase enzyme complex that initiates N-linked glycosylation. It is encoded by a gene located adjacent to the TCR α and δ genes on mouse chromosome 14. We have investigated the role of Dad1 during T cell development and activation. We observe that endogenous Dad1 levels are modulated during T cell development to reach maximal expression in mature thymocytes. Transgenic mice that overexpress Dad1 in both the thymus and peripheral immune system have been generated. Apoptosis of thymocytes from such mice is largely unaffected, but peripheral T cells display hyperproliferation in response to stimuli. Therefore, the linkage between the TCR and Dad1 genes may have important consequences for T cell function.

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.

Bad can act as a key regulator of T cell apoptosis and T cell development

Bad is a distant relative of Bcl-2 and acts to promote cell death. Here, we show that Bad expression levels are greatly increased in thymocytes during apoptosis. We generated bad transgenic mice to study the action of upregulated Bad expression on T cell apoptosis. The T cells from these mice are highly sensitive to apoptotic stimuli, including anti-CD95. The numbers of T cells are greatly depleted and the processes of T cell development and selection are perturbed. We show that the proapoptotic function of Bad in primary T cells is regulated by Akt kinase and that Bad overexpression enhances both cell cycle progression and interleukin 2 production after T cell activation. These data suggest that Bad can act as a key regulator of T cell apoptosis and that this is a consequence of its upregulation after exposure to death stimuli.

Increased susceptibility of thymocytes to apoptosis in mice lacking AIM, a novel murine macrophage-derived soluble factor belonging to the scavenger receptor cysteine-rich domain superfamily

Apoptosis of cells must be regulated both positively and negatively in response to a variety of stimuli in the body. Various environmental stresses are known to initiate apoptosis via differential signal transduction cascades. However, induction of signals that may inhibit apoptosis is poorly understood, although a number of intracellular molecules that mediate inhibition of apoptosis have been identified. Here we present a novel murine macrophage-specific 54-kD secreted protein which inhibits apoptosis (termed AIM, for apoptosis inhibitor expressed by macrophages). AIM belongs to the macrophage scavenger receptor cysteine-rich domain superfamily (SRCR-SF), members of which share a highly homologous conserved cysteine-rich domain. In AIM-deficient mice, the thymocyte numbers were diminished to half those in wild-type mice, and CD4/CD8 double-positive (DP) thymocytes were strikingly more susceptible to apoptosis induced by both dexamethasone and irradiation in vivo. Recombinant AIM protein significantly inhibited cell death of DP thymocytes in response to a variety of stimuli in vitro. These results indicate that in the thymus, AIM functions in trans to induce resistance to apoptosis within DP cells, and thus supports the viability of DP thymocytes before thymic selection.

Role of Bcl-2 in αβ T Cell Development in Mice Deficient in the Common Cytokine Receptor γ-Chain: The Requirement for Bcl-2 Differs Depending on the TCR/MHC Affinity

Mice lacking the common cytokine receptor γ-chain (γc) exhibit severely compromised T cell development, with diminished Bcl-2 expression in mature (CD4+ or CD8+) thymocytes and peripheral T cells. Enforced expression of Bcl-2 in these mice partially rescued αβ T cell development but not γδ T cell development. Transgenic expression of the OVA-specific DO11.10 (DO10) TCR also could modestly increase thymocyte numbers, and T cells expressing the transgenic TCR (KJ1-26+ T cells) were found in the periphery. Interestingly, the presence of KJ1-26+ T cells was dependent on the MHC background and was seen in the moderate affinity H-2d/d background but not in the higher affinity H-2d/b background in γc-deficient mice. In contrast, KJ1-26+ T cells exist in the periphery in both the H-2d/d and H-2d/b backgrounds in DO10 transgenic γc wild-type mice. These results suggest that the importance of γc-dependent signals for T cell development differs depending on the affinity of TCR for MHC. Moreover, enforced expression of Bcl-2 had a much greater effect on the development of γc-deficient T cells expressing the DO10 TCR in the high affinity H-2d/b background than in the H-2d/d background, suggesting that γc-dependent Bcl-2 expression influences T cell development in a TCR/MHC-dependent manner.

CD4/CD8 lineage commitment: matching fate with competence

The outcome of positive selection of T lymphocytes is that there is a close match between the lineage adopted by a particular cell (CD4+ or CD8+) and the specificity of the T-cell receptor for the class of Major Histocompatibility Complex molecule recognized. How this match is obtained has been a matter of debate. We review the evidence, from recent and older experiments, that indicates that the process follows a selective logic, rather than an instructive one.

Role of interleukin-7 in T-cell development from hematopoietic stem cells

All lymphocytes are derived from hematopoietic stem cells (HSC). The interleukin-7 receptor (IL-7R) transduces non-redundant signals for both T and B-cell development from HSC. The upregulation of the IL-7R occurs at the stage of the clonogenic common lymphoid progenitor, a recently identified population that can give rise to all lymphoid lineages (T, B and natural killer cells) at a single cell level. The IL-7R plays a critical role in the rearrangement of immunoglobulin heavy chain genes required for B-cell development. IL-7R expression is critically regulated in developing thymocytes; thymocytes that fail the positive selection process downregulate the IL-7R, but those undergoing positive selection upregulate or maintain IL-7R expression. Recent data indicate that IL-7 signaling enhances the survival of developing thymocytes and mature T cells, presumably by its upregulating Bcl-2. Detailed analysis of the signaling cascades activated by the IL-7R may help to reveal the differential roles of IL-7 signaling in T and B-cell development.

The action of Bax and bcl-2 on T cell selection

T cell development and selection in the thymus are shaped by the induction of apoptosis. However, a direct role in T cell development and selection for any of the molecules known to regulate apoptosis has remained controversial. We have studied the effect of bax and bcl-2 transgenes in recombination activation gene 1-deficient (RAG-1(-/-)) mice transgenic for the major histocompatibility complex class I-restricted F5 T cell receptor. Overexpression of a bax transgene in the thymus seriously impairs the production of mature T cells, whereas bcl-2 overexpression greatly promotes it. The effect of bax and bcl-2 overexpression on antigen-induced negative selection was studied using fetal thymic organ cultures. This analysis showed that Bcl-2 strongly inhibits negative selection, whereas Bax does not affect it. Our data directly show that Bcl-2 family members have specific roles in T cell selection and also lend support to the hypothesis that Bax and Bcl-2 can antagonize each other's action in a certain apoptosis pathway while in another they can be functionally nonreciprocal.

Overexpression of the anti-apoptotic oncogene, bcl-2, in the thymus does not prevent thymic atrophy induced by estradiol or 2,3,7, 8-tetrachlorodibenzo-p-dioxin

Dexamethasone (Dex), estradiol (E2), and 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) all affect the immune system, causing immunosuppression and thymic atrophy. It is still uncertain how and where these compounds act to induce thymic atrophy. However, it has been suggested that these compounds may have similar actions and targets, i.e., apoptosis of immature thymocytes for Dex and TCDD and preferential targeting of double-positive cells by Dex and E2. The lckpr-bcl-2 transgenic mouse has been shown to be protected against Dex-induced thymic atrophy. We used this murine model to determine if bcl-2 expression would also protect against E2- and TCDD-induced thymic atrophy. Our results indicate that, although the bcl-2 transgenic (TG+) mice were fully protected from atrophy induced by a single dose of Dex, atrophy was still induced in these mice following treatment with E2 or TCDD. Phenotypic analysis of thymocytes from TG- and TG+ mice also showed distinct consequences of atrophy induced by Dex, E2, and TCDD. Finally, since there are alternative pathways for apoptosis that are bcl-2 independent, both TG- and TG+ thymocytes were examined directly for indications of apoptosis using the TUNEL assay. After TCDD and E2 treatment there were no detectable signs of apoptosis in either TG- or TG+ mice even at early time points and at elevated dose levels. These results indicate that there are distinct mechanisms for the actions of Dex, E2, and TCDD in the thymus and that apoptosis is not a key mechanism of E2- and TCDD-induced thymic atrophy.

BCL-2 family: regulators of cell death

An expanding family of BCL-2 related proteins share homology, clustered within four conserved regions, namely BCL-2 homology (BH1-4) domains, which control the ability of these proteins to dimerize and function as regulators of apoptosis. Moreover, BCL-XL, BCL-2, and BAX can form ion-conductive pores in artificial membranes. The BCL-2 family, comprised of both pro-apoptotic and anti-apoptotic members, acts as a checkpoint upstream of CASPASES and mitochondrial dysfunction. BID and BAD possess the minimal death domain BH3, and the phosphorylation of BAD connects proximal survival signals to the BCL-2 family. BCL-2 and BCL-XL display a reciprocal pattern of expression during lymphocyte development. Gain- and loss-of-function models revealed stage-specific roles for BCL-2 and BCL-XL. BCL-2 can rescue maturation at several points of lymphocyte development. The BCL-2 family also reveals evidence for a cell-autonomous coordination between the opposing pathways of proliferation and cell death.

The role of the bcl-2/ced-9 gene family in cancer and general implications of defects in cell death control for tumourigenesis and resistance to chemotherapy

Cell production within an organ is determined by the rate of immigration, proliferation, differentiation, emigration and death of cells. Abnormalities in any one of these processes will disturb normal control of cell production, thereby eliciting hyperplasia can be an early event in neoplasia. Cell death, apoptosis, is a physiological process responsible for removing unwanted cells. It is used in multi-cellular organisms for tissue remodelling during embryogenesis, regulation of cell turnover and as a defence strategy against invading pathogens. In this review article we describe the role of the bcl-2/ced-9 gene family in cancer and discuss the general implications of defects in the apoptosis program for tumourigenesis and resistance of cancer cells to chemotherapy in light of current knowledge of the molecular mechanisms of cell death.

Bcl-2 rescues T lymphopoiesis, but not B or NK cell development, in common gamma chain-deficient mice

The common cytokine receptor gamma chain (gamma(c)) is an indispensable subunit for the formation of lymphoid-related cytokine receptors, including IL-7 and IL-15 receptors, that mediate nonredundant or critical signals for the differentiation of T and B cells and natural killer (NK) cells, respectively. We introduced the bcl-2 transgene driven by E mu or H-2K promoters into gamma(c)-deficient mice that lack all three lymphoid subclasses. The forced expression of Bcl-2 restored all stages of T lymphopoiesis, but not B or NK cell development, indicating that a primary function of gamma(c)-mediated signals in the T lineage might be to maintain cell survival. Therefore, the development of T, B, and NK cells may be influenced by distinct intracytoplasmic signaling cascades that are activated by coupling of gamma(c)-related receptors.

Differential effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin, bis(tri-n-butyltin) oxide and cyclosporine on thymus histophysiology

Recent advances in the histophysiology of the normal thymus have revealed its complex architecture, showing distinct microenvironments at the light and electron microscopic level. The epithelium comprising the major component of the thymic stroma is not only involved in the positive selection of thymocytes, but also in their negative selection. Dendritic cells, however, are more efficient than epithelial cells in mediating negative selection. Thymocytes are dependent on the epithelium for normal development. Conversely, epithelial cells need the presence of thymocytes to maintain their integrity. The thymus rapidly responds to immunotoxic injury. Both the thymocytes and the nonlymphoid compartment of the organ can be targets of exposure. Disturbance of positive and negative thymocyte selection may have a major impact on the immunological function of the thymus. Suppression of peripheral T-cell-dependent immunity as a consequence of thymus toxicity is primarily seen after perinatal exposure when the thymus is most active. Autoimmunity may be another manifestation of chemically mediated thymus toxicity. Although the regenerative capacity of thymus structure is remarkable, it remains to be clarified whether this also applies to thymus function. In-depth mechanistic studies on chemical-induced dysfunction of the thymus have been conducted with the environmental contaminants 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and bis(tri-n-butyltin)oxide (TBTO) as well as the pharmaceutical immunosuppressant cyclosporine (CsA). Each of these compounds exerts a differential effect on the morphology of the thymus, depending on the cellular targets for toxicity. TCDD and TBTO exposure results in cortical lymphodepletion, albeit by different mechanisms. An important feature of TCDD-mediated thymus toxicity is the disruption of epithelial cells in the cortex. TBTO primarily induces cortical thymocyte cell death. In contrast CsA administration results in major alterations in the medulla, the cortex remaining largely intact. Medullary epithelial cells and dendritic cells are particularly sensitive to CsA. The differential effects of these three immunotoxicants suggest unique susceptibilities of the various cell types and regions that make up the thymus.

Bcl-2 rescues T lymphopoiesis in interleukin-7 receptor-deficient mice

Mice lacking functional IL-7 or IL-7R alpha genes are severely deficient in developing thymocytes, T cells, and B cells. IL-7 and IL-7 receptor functions are believed to result in lymphoid cell proliferation and cell maturation, implying signal transduction pathways directly involved in mitogenesis and elaboration of developmentally specific new gene programs. Here, we show that enforced expression of the bcl-2 gene in T-lymphoid cells (by crossing in the Emu-bcl-2 transgene) in IL-7R alpha-deficient mice results in a significant restoration of thymic positive selection and T cell numbers and function. We propose cell survival signals to be the principal function of IL-7R engagement in thymic and T cell development.

Bax alpha perturbs T cell development and affects cell cycle entry of T cells

Bax alpha can heterodimerize with Bcl-2 and Bcl-X(L), countering their effects, as well as promoting apoptosis on overexpression. We show that bax alpha transgenic mice have greatly reduced numbers of mature T cells, which results from an impaired positive selection in the thymus. This perturbation in positive selection is accompanied by an increase in the number of cycling thymocytes. Further to this, mature T cells overexpressing Bax alpha have lower levels of p27Kip1 and enter S phase more rapidly in response to interleukin-2 stimulation than do control T cells, while the converse is true of bcl-2 transgenic T cells. These data indicate that apoptotic regulatory proteins can modulate the level of cell cycle-controlling proteins and thereby directly impact on the cell cycle.

Inhibition of Nur77/Nurr1 leads to inefficient clonal deletion of self-reactive T cells

The Nur77/Nurr1 family of DNA binding proteins has been reported to be required for the signal transduction of CD3/T cell receptor (TCR)-mediated apoptosis in T cell hybridomas. To determine the role of this family of DNA-binding proteins in thymic clonal deletion, transgenic (Tg) mice bearing a dominant negative mutation were produced. The transgene consisted of a truncated Nur77 (deltaNur77) gene encoding the DNA-binding domain of Nur77 ligated to a TCR-beta enhancer resulting in early expression in thymocytes. Apoptosis of CD4+CD8+ thymocytes mediated by CD3/TCR signaling was greatly inhibited in the deltaNur77 Tg mice, compared with non-Tg littermates, after treatment with anti-CD3 or anti-TCR antibody in vivo and in vitro. Clonal deletion of self-reactive T cells was investigated in deltaNur77-Db/HY TCR-alpha/beta double Tg mice. There was a five-fold increase in the total number of thymocytes expressing self-reactive Db/HY TCR-alpha/beta in the deltaNur77-TCR-alpha/beta double Tg male mice. Deficient clonal deletion of self-reactive thymocytes was demonstrated by a 10-fold increase in the CD4+CD8+ thymocytes that expressed Tg TCR-alpha/beta. There was an eightfold increase in the CD8+, Db/HY TCR-alpha/beta T cells in the lymph nodes (LN) of delta Nur77-Db/HY TCR-alpha/beta double Tg compared with Db/HY TCR-alpha/beta Tg male mice. In spite of defective clonal deletion, the T cells expressing the Tg TCR were functionally anergic. In vivo analysis revealed increased activation and apoptosis of T cells associated with increased expression of Fas and Fas ligand in LN of deltaNur77-Db/HY TCR-alpha/beta double male mice. These results indicate that inhibition of Nur77/Nurr1 DNA binding in T cells leads to inefficient thymic clonal deletion, but T cell tolerance is maintained by Fas-dependent clonal deletion in LN and spleen.

Evidence for the promotion of positive selection of thymocytes by Ah receptor agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a ligand for the arylhydrocarbon receptor (Ah receptor), abundant in the murine thymus. In the thymus immunocompetent T cells develop. Upon exposure of murine fetal thymi in organ cultures to TCDD the distribution of mature and immature thymocytes is skewed towards apparently mature, prospective cytotoxic cells of the CD4-CD8+T cell receptor+ phenotype. The normally abundant CD4+ CD8+ cells are decreased. Proliferation of the most immature thymocyte subpopulations is inhibited and maturation of thymocytes appears accelerated by TCDD. Eventually the thymocyte number is significantly decreased. Selective treatment of stroma cells showed them to be the primary target cells of TCDD action. Thymus stroma plays a pivotal role in thymocyte maturation and is indispensable for the selection of thymocytes bearing T cell receptors specific for foreign antigen in the context of self. We tested whether the effects of TCDD on thymocyte differentiation and maturation has further consequences for the selection processes by analysing (a) the repertoire of V beta genes used as a measure for negative selection and (b) the expression of CD69 and bcl-2 by thymocytes as a parameter of positive selection. Our data indicate that TCDD does not cause gross disturbance of negative selection but provide evidence for more cells auditioning for positive selection by TCDD exposure.

The Emu-bcl-2 transgene enhances antigen-induced germinal center formation in both BALB/c and SJL mice but causes age-dependent germinal center hyperplasia only in the lymphoma-prone SJL strain

Emu-bcl-2 transgenic and littermate control BALB/c and SJL mice were immunized in the front footpads with trinitrophenylated Brucella abortus and the germinal center (GC) response in draining brachial lymph nodes was studied by staining with peanut agglutinin peroxidase and methyl green. Although the GCs induced were not larger in transgenic than in control young mice, there was a significant increase in the percentage of B cell follicles exhibiting GCs 7 to 8 days after primary and secondary antigen injections in the transgenic mice of both strains. In addition, glucocorticosteroid injected on day 7 after the primary injection caused a marked decrease in GCs in littermate controls but had no effect in the bcl-2 transgenic SJL mice. Antibody production to B. abortus was only slightly higher in transgenic than in control mice, but anti-TNP immunoglobulin M and G titers were significantly enhanced in the transgenic mice. The bcl-2 transgenic SJL mice, older than 6 months, showed the spontaneous appearance of large numbers of peanut agglutinin-binding GCs that greatly varied in size and were located without regard for the normal lymph node structure or follicle localization. This GC hyperplasia was seen in a large percent of the older transgenic SJL mice and never in similarly aged normal SJL or BALB/c mice with and without the bcl-2 transgene. Frank lymphomatous transformation of peanut agglutinin-binding germinal center-like areas was seen in lymph nodes and Peyer's patches of some of the older bcl-2 transgenic SJL mice. These results suggest that the tendency of SJL mice to develop GC-derived lymphomas synergizes with the presence of the bcl-2 transgene to cause the development of GC hyperplasia.

Positive selection of T cells

In the past year, significant technical developments have provided the opportunity to investigate the more mechanistic features of positive selection. Major progress has been made in determining the structure and function of the early pre-T cell receptor, in defining cell types that mediate positive selection, and in analyzing the contribution of MHC and co-receptors to CD4/CD8 lineage commitment. The most revealing studies have been those addressing the role of peptides in thymic selection.