Cellular and peptide requirements for in vitro clonal deletion of immature thymocytes

Functional redundancy between thymic CD8α+ and Sirpα+ conventional dendritic cells in presentation of blood-derived lysozyme by MHC class II proteins

We evaluated the presentation of blood-derived protein Ags by APCs in the thymus. Two conventional dendritic cells (cDCs), the CD8α(+)Sirpα(-)CD11c(hi) (CD8α(+) cDC) and the CD8α(-)Sirpα(+)CD11c(hi) (Sirpα(+) cDC), were previously identified as presenting MHC class II bound peptides from hen egg white lysozyme (HEL) injected intravenously. All thymic APCs acquired the injected HEL, with the plasmacytoid dendritic cell being the best, followed by the Sirpα(+) cDC and the CD8α(+) cDC. Both cDCs induced to similar extent negative selection and regulatory T cells in HEL TCR transgenic mice, indicating a redundant role of the two cDC subsets in the presentation of blood-borne HEL. Immature dendritic cells or plasmacytoid dendritic cells were considerably less efficient. Batf3(-/-) mice, with significantly reduced numbers of CD8α(+) cDCs, were not impaired in HEL presentation by I-A(k) molecules of thymic APCs. Lastly, clodronate liposome treatment of TCR transgenic mice depleted blood APCs including Sirpα(+) cDCs without affecting the number of thymic APCs. In such treated mice, there was no effect on negative selection or regulatory T cells in mice when administering HEL, indicating that the T cell responses were mediated primarily by the cDCs localized in the thymus.

Direct visualization of peptide/MHC complexes at the surface and in the intracellular compartments of cells infected in vivo by Leishmania major

Protozoa and bacteria infect various types of phagocytic cells including macrophages, monocytes, dendritic cells and eosinophils. However, it is not clear which of these cells process and present microbial antigens in vivo and in which cellular compartments parasite peptides are loaded onto Major Histocompatibility Complex molecules. To address these issues, we have infected susceptible BALB/c (H-2d) mice with a recombinant Leishmania major parasite expressing a fluorescent tracer. To directly visualize the antigen presenting cells that present parasite-derived peptides to CD4+ T cells, we have generated a monoclonal antibody that reacts to an antigenic peptide derived from the parasite LACK antigen bound to I-Ad Major Histocompatibility Complex class II molecule. Immunogold electron microscopic analysis of in vivo infected cells showed that intracellular I-Ad/LACK complexes were present in the membrane of amastigote-containing phagosomes in dendritic cells, eosinophils and macrophages/monocytes. In both dendritic cells and macrophages, these complexes were also present in smaller vesicles that did not contain amastigote. The presence of I-Ad/LACK complexes at the surface of dendritic cells, but neither on the plasma membrane of macrophages nor eosinophils was independently confirmed by flow cytometry and by incubating sorted phagocytes with highly sensitive LACK-specific hybridomas. Altogether, our results suggest that peptides derived from Leishmania proteins are loaded onto Major Histocompatibility Complex class II molecules in the phagosomes of infected phagocytes. Although these complexes are transported to the cell surface in dendritic cells, therefore allowing the stimulation of parasite-specific CD4+ T cells, this does not occur in other phagocytic cells. To our knowledge, this is the first study in which Major Histocompatibility Complex class II molecules bound to peptides derived from a parasite protein have been visualized within and at the surface of cells that were infected in vivo.

Xenogeneic beta 2-microglobulin substitution affects functional binding of MHC class I molecules by CD8+ T cells

NK cells and CD8+ T cells bind MHC-I molecules using distinct topological interactions. Specifically, murine NK inhibitory receptors bind MHC-I molecules at both the MHC-I H chain regions and beta2-microglobulin (beta2m) while TCR engages MHC-I molecules at a region defined solely by the class I H chain and bound peptide. As such, alterations in beta2m are not predicted to influence functional recognition of MHC-I by TCR. We have tested this hypothesis by assessing the capability of xenogeneic beta2m to modify the interaction between TCR and MHC-I. Using a human beta2m-transgenic C57BL/6 mouse model, we show that human beta2m supports formation and expression of H-2K(b) and peptide:H-2K(b) complexes at levels nearly equivalent to those in wild-type mice. Despite this finding, the frequencies of CD8+ single-positive thymocytes in the thymus and mature CD8+ T cells in the periphery were significantly reduced and the TCR Vbeta repertoire of peripheral CD8+ T cells was skewed in the human beta2m-transgenic mice. Furthermore, the ability of mouse beta2m-restricted CTL to functionally recognize human beta2m+ target cells was diminished compared with their ability to recognize mouse beta2m+ target cells. Finally, we provide evidence that this effect is achieved through subtle conformational changes occurring in the distal, peptide-binding region of the MHC-I molecule. Our results indicate that alterations in beta2m influence the ability of TCR to engage MHC-I during normal T cell physiology.

Adoptively transferred allergen-specific T cells cause maternal transmission of asthma risk

In addition to genetics and environment, maternal asthma is an identified risk factor for developing the disease during childhood. The mechanisms of this maternal effect remain poorly understood. We tested the role of allergen-specific T cells in the maternal transmission of asthma risk by modifying a model where offspring of asthmatic mothers are more prone to develop asthma after an intentionally suboptimal asthma induction. Normal BALB/c females were injected with allergen-specific T cells from ovalbumin-specific T cell receptor (TCR) transgenic DO11.10 donors before mating. Using the protocol of suboptimal asthma induction, offspring of normal and recipient mothers were tested for their susceptibility to develop asthma. Only pups of recipient mothers showed increased airway responsiveness (Penh), allergic airway inflammation with eosinophilia, and local Th2-skewed cytokine production. Although recipient mothers did not develop asthma, serum levels of interferon-gamma, interleukin (IL)-4, IL-10, and IL-13 were significantly increased during pregnancy. Consistent with this finding, a subset of DO11.10 T cells persisted in the spleen and placenta of expectant recipient mothers. We conclude that allergen-specific T cells are sufficient to orchestrate the maternal transmission of asthma risk. Because overt maternal asthma was not required, our results suggest that similar maternal-fetal interactions may occur in other allergic disorders.

Delineation of Signals Required for Thymocyte Positive Selection

Peptide/MHC complexes capable of inducing positive selection in mouse fetal thymic organ cultures fail to do so in suspension culture. Furthermore, this type of culture does not promote initial stages of differentiation, such as coreceptor down-modulation, unless peptides used for stimulation have (at least) weak agonist activity. We show in this study that signals provided in suspension culture by nonagonist peptide/MHC complexes on the surface of macrophages, even though apparently silent, are sufficient to promote complete phenotypic differentiation when CD4+CD8+ thymocytes are subsequently placed in a proper anatomical setting. Furthermore, the synergistic actions of suboptimal concentrations of phorbol esters and nonagonist peptide/MHC complexes can make the initial stages of positive selection visible, without converting maturation into negative selection. Thus, the correlation between efficiency of positive selection and the degree of coreceptor down-modulation on CD4+CD8+ thymocytes is not linear. Furthermore, these results suggest that the unique role of thymic stromal cells in positive selection is related not to presentation of self-peptide/MHC complexes, but most likely to another ligand.

Positive and negative selection of T cell repertoires during differentiation in allogeneic bone marrow chimeras

T cells acquire immune functions during expansion and differentiation in the thymus. Mature T cells respond to peptide antigens (Ag) derived from foreign proteins when these peptide Ag are presented on the self major histocompatibility complex (MHC) molecules but not on allo-MHC. This is termed self-MHC restriction. On the other hand, T cells do not induce aggressive responses to self Ag (self-tolerance). Self-MHC restriction and self-tolerance are not genetically determined but acquired a posteriori by positive and negative selection in the thymus in harmony with the functional maturation. Allogeneic bone marrow (BM) chimera systems have been a useful strategy to elucidate mechanisms underlying positive and negative selection. In this communication, the contribution of BM chimera systems to the investigation of the world of T-ology is discussed.

Intrathymic expression of neuromuscular acetylcholine receptors and the immunpathogenesis of myasthenia gravis

The thymus has been considered to play an important role in the pathogenesis of myasthenia gravis (MG), an autoimmune disease characterized by skeletal muscle weakness. However, the pathogenic role of the thymus still remains a mystery. The neuromuscular type of acetylcholine receptor (AChR) was the first self-protein associated with a defined autoimmune disease that was found to be expressed by thymic stromal populations. The studies described herein represent our efforts to determine how this "promiscuous" autoantigen expression may be involved in the immunopathogenesis of MG. We review our work, characterizating the expression of the alpha subunit of AChR (AChRalpha) in the thymus, and advance a new hypothesis that examines the intrathymic expression of this autoantigen in disease pathogenesis.

Characterization of phosphotyrosine binding motifs in the cytoplasmic domain of B and T lymphocyte attenuator required for association with protein tyrosine phosphatases SHP-1 and SHP-2

B and T lymphocytes express receptors providing positive and negative co-stimulatory signals. We recently identified a novel co-stimulatory molecule, B and T lymphocyte attenuator (BTLA), which exerts inhibitory effects on B and T lymphocytes. The cytoplasmic domain of murine and human BTLA share three conserved tyrosine-based signaling motifs, a Grb-2 recognition consensus, and two immunoreceptor tyrosine-based inhibitory motifs (ITIMs). Phosphorylation of the cytoplasmic domain of BTLA induced the association with the protein tyrosine phosphatases SHP-1 and SHP-2. Association of SHP-1 and SHP-2 to other receptors can involve recruitment to either a single receptor ITIM or to two receptor ITIMs. Here, we analyzed the requirements of BTLA interaction with SHP-1 and SHP-2 in a series of murine and human BTLA mutants. For human BTLA, mutations of either Y257 or Y282, but not Y226, abrogated association with both SHP-1 and SHP-2. For murine BTLA, mutation of either Y274 or Y299, but not Y245, also abrogated association with both SHP-1 and SHP-2. These results indicate that for both murine and human BTLA, association with SHP-1 or SHP-2 requires both of conserved ITIM motifs and does not involve the conserved Grb-2 consensus. Thus, similar to the bisphosphoryl tyrosine-based activation motif (BTAM) by which the Grb-2 associated binder (Gab1), PDGF receptor, and PECAM-1 recruit SHP-2, BTLA also relies on dual ITIMs for its association with the phosphatases SHP-1 and SHP-2.

A New Model Linking Intrathymic Acetylcholine Receptor Expression and the Pathogenesis of Myasthenia Gravis

The thymus is thought to play an important role in the pathogenesis of myasthenia gravis (MG), an autoimmune disease characterized by skeletal muscle weakness. However, its role remains a mystery. The studies described represent our efforts to determine how intrathymic expression of the neuromuscular type of acetylcholine receptors (nAChRs) is involved in the immunopathogenesis of MG. We review our work characterizing the expression of the alpha subunit of nAChR (nAChRalpha) in the thymus and advance a new hypothesis that examines the intrathymic expression of this autoantigen in disease pathogenesis.

Peripheral antigen-expressing cells and autoimmunity

There is growing evidence that self-molecules with tissue-restricted expression are also expressed at low levels in the thymus, where such expression may affect the development of self-tolerance. Genetic factors may modulate such expression and, in turn, influence susceptibility to autoimmune responses against specific molecules. Contrasting reports exist regarding the phenotype of the cells that express self-molecules in the thymus. Indeed, both bone marrow derived antigen-presenting cells and thymic epithelial cells were reported to express self-molecules with tissue-restricted expression. Further studies of these cells and the mechanisms by which they exert their putative tolerogenic effects clearly are necessary.

Murine Schnurri-2 is required for positive selection of thymocytes

A key step in T cell development involves the positive selection of cells that recognize antigen presented by self-major histocompatibility complex. Yet, the signals that are activated by T cell receptor engagement and lead to cell survival remain unclear. We show here that mice lacking the transcription factor Schnurri-2 (Shn-2), a large metal-finger protein, had severely defective positive selection of CD4+ and CD8+ cells. Drosophila Shn acts as a cofactor of Smad homolog and is required for Decapentaplegic signaling. Vertebrates have at least three Shn orthologs (Shn-1, Shn-2 and Shn-3), which are thought to act as nuclear targets in the bone morphogenetic protein-transforming growth factor-beta-activin signaling pathways. These data raised the possibility that the Smad-Shn-2 complex is involved in the thymic selection of T cells.

Carcinoembryonic Antigen-Related Cell Adhesion Molecule 1 on Murine Dendritic Cells Is a Potent Regulator of T Cell Stimulation

Dendritic cells (DC) are important APCs that play a key role in the induction of an immune response. The signaling molecules that govern early events in DC activation are not well understood. We therefore investigated whether DC express carcinoembryonic Ag-related cell adhesion molecule 1 (CEACAM1, also known as BGP or CD66a), a well-characterized signal-regulating cell-cell adhesion molecule that is expressed on granulocytes, monocytes, and activated T cells and B cells. We found that murine DC express in vitro as well as in vivo both major isoforms of CEACAM1, CEACAM1-L (having a long cytoplasmic domain with immunoreceptor tyrosine-based inhibitory motifs) and CEACAM1-S (having a short cytoplasmic domain lacking phosphorylatable tyrosine residues). Ligation of surface-expressed CEACAM1 on DC with the specific mAb AgB10 triggered release of the chemokines macrophage inflammatory protein 1alpha, macrophage inflammatory protein 2, and monocyte chemoattractant protein 1 and induced migration of granulocytes, monocytes, T cells, and immature DC. Furthermore, the surface expression of the costimulatory molecules CD40, CD54, CD80, and CD86 was increased, indicating that CEACAM1-induced signaling regulates early maturation and activation of dendritic cells. In addition, signaling via CEACAM1 induced release of the cytokines IL-6, IL-12 p40, and IL-12 p70 and facilitated priming of naive MHC II-restricted CD4(+) T cells with a Th1-like effector phenotype. Hence, our results show that CEACAM1 is a signal-transducing receptor that can regulate early maturation and activation of DC, thereby facilitating priming and polarization of T cell responses.

Differentiation of monocytes into multinucleated giant bone-resorbing cells: two-step differentiation induced by nurse-like cells and cytokines

Bone resorption in the joints is the characteristic finding in patients with rheumatoid arthritis (RA). Osteoclast-like cells are present in the synovial tissues and invade the bone of patients with RA. The characteristics of these cells are not completely known. In the work reported here, we generated these cells from peripheral-blood monocytes from healthy individuals. The monocytes were co-cultured with nurse-like cells from synovial tissues of patients with RA (RA-NLCs). Within 5 weeks of culture, the monocytes were activated and differentiated into mononuclear cells positive for CD14 and tartrate-resistant acid phosphatase (TRAP). These mononuclear cells then differentiated into multinucleated giant bone-resorbing cells after stimulation with IL-3, IL-5, IL-7, and/or granulocyte-macrophage-colony-stimulating factor. TRAP-positive cells with similar characteristics were found in synovial fluid from patients with RA. These results indicate that multinucleated giant bone-resorbing cells are generated from monocytes in two steps: first, RA-NLCs induce monocytes to differentiate into TRAP-positive mononuclear cells, which are then induced by cytokines to differentiate into multinucleated giant bone-resorbing cells.

The quantity of TCR signal determines positive selection and lineage commitment of T cells

It is generally accepted that the avidity of TCR for self Ag/MHC determines the fate of immature thymocytes. However, the contribution of the quantity of TCR signal to T cell selection has not been well established, particularly in vivo. To address this issue, we analyzed DO-TCR transgenic CD3zeta-deficient (DO-Tg/zetaKO) mice in which T cells have a reduced TCR on the cell surface. In DO-Tg/zetaKO mice, very few CD4 single positive (SP) thymocytes developed, indicating that the decrease in TCR signaling resulted in a failure of positive selection of DO-Tg thymocytes. Administration of the peptide Ag to DO-Tg/zetaKO mice resulted in the generation of functional CD4 SP mature thymocytes in a dose-dependent manner, and, unexpectedly, DO-Tg CD8 SP cells emerged at lower doses of Ag. TCR signal-dependent, sequential commitment from CD8(+) SP to CD4(+) SP was also shown in a class I-restricted TCR-Tg system. These in vivo analyses demonstrate that the quantity of TCR signal directly determines positive and negative selection, and further suggest that weak signal directs positively selected T cells to CD8 lineage and stronger signal to CD4 lineage.

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.

Premature expression of T cell receptor (TCR)alphabeta suppresses TCRgammadelta gene rearrangement but permits development of gammadelta lineage T cells

The T cell receptor (TCR)gammadelta and the pre-TCR promote survival and maturation of early thymocyte precursors. Whether these receptors also influence gammadelta versus alphabeta lineage determination is less clear. We show here that TCRgammadelta gene rearrangements are suppressed in TCRalphabeta transgenic mice when the TCRalphabeta is expressed early in T cell development. This situation offers the opportunity to examine the outcome of gammadelta versus alphabeta T lineage commitment when only the TCRalphabeta is expressed. We find that precursor thymocytes expressing TCRalphabeta not only mature in the alphabeta pathway as expected, but also as CD4(-)CD8(-) T cells with properties of gammadelta lineage cells. In TCRalphabeta transgenic mice, in which the transgenic receptor is expressed relatively late, TCRgammadelta rearrangements occur normally such that TCRalphabeta(+)CD4(-)CD8(-) cells co-express TCRgammadelta. The results support the notion that TCRalphabeta can substitute for TCRgammadelta to permit a gammadelta lineage choice and maturation in the gammadelta lineage. The findings could fit a model in which lineage commitment is determined before or independent of TCR gene rearrangement. However, these results could be compatible with a model in which distinct signals bias lineage choice and these signaling differences are not absolute or intrinsic to the specific TCR structure.

Self/nonself discrimination among immunoregulatory (CD4) T cells

This review covers work on immunological tolerance from 1962 up to the present, focusing on the Th, CD4+ compartment of the immune system. The principle mechanism of tolerance is identified as deletion, occurring centrally and in the periphery. In the periphery, deletion is the normal response of CD4 T cells to soluble monomeric proteins that occurs when activation (mainly of dendritic cells) is avoided. Thus activation and the signals which induce it are crucial to understanding S/NS discrimination, as has long been known. The thymus is important as the site where new T cells first see self-antigens, and as one largely shielded from activation, although deletion in the thymus and the periphery has the same threshold. The relative contribution of dendritic cells and developing T cells to deletion in the thymus remains unclear. Activation induced cell death, containment, anergy and deviation constitute subsidiary mechanisms, and sequestration/neglect is important in limiting the scope of deletion.

Alteration of protein composition in mouse thymocytes by signals through T-cell receptor

To avoid destructive autoimmunity, T-cell precursors (thymocytes) expressing autoreactive T-cell receptor are deleted in the thymus via an apoptotic process by the signals from the T-cell receptor-CD3 complexes. In order to analyze the apoptotic mechanism, we established a cell-free system using the lysates from mouse thymocytes treated in vivo with anti-CD3 monoclonal antibody (mAb). The soluble cytosolic high molecular mass protein fraction from the anti-CD3-treated thymocytes revealed an activity that directly induces nuclear apoptotic morphological changes and DNA fragmentation. This fragmentation activity was not observed in the fraction from the thymocytes without anti-CD3 treatment. Proteins in both fractions were separated by two-dimensional electrophoresis. The silver-stained gels revealed differences in protein spots. These protein spots were identified by database searching of mass spectrometric (MS) and tandem mass spectrometric (MS/MS) data obtained from in-gel tryptic digests of the spots, using an integrated system of liquid chromatography/electrospray ionization/ion-trap mass spectrometry. In this study, the high mobility group protein HMG2 was identified as one of the cytosolic proteins that is increased by the signals from the T-cell receptor, and heterogeneous nuclear ribonucleoprotein A2/B1 and glyceraldehyde 3-phosphate dehydrogenase were found to be decreased by the signals.

Functional comparison of thymic B cells and dendritic cells in vivo

In this report we present a transgenic mouse model in which we targeted gene expression specifically to B-lymphocytes. Using the human CD19 promoter, we expressed major histocompatibility complex class II I-E molecules specifically on B cells of all tissues, but not on other cell types. If only B cells expressed I-E in a class II-deficient background, positive selection of CD4+ T cells could not be observed. A comparison of the frequencies of I-E reactive Vβ5+ and Vβ11+ T cells shows that I-E expression on thymic B cells is sufficient to negatively select I-E reactive CD4+ T cells partially, but not CD8+ T cells. Thus partial negative but no positive selection events can be induced by B-lymphocytes in vivo.

Functional comparison of thymic B cells and dendritic cells in vivo

In this report we present a transgenic mouse model in which we targeted gene expression specifically to B-lymphocytes. Using the human CD19 promoter, we expressed major histocompatibility complex class II I-E molecules specifically on B cells of all tissues, but not on other cell types. If only B cells expressed I-E in a class II-deficient background, positive selection of CD4+ T cells could not be observed. A comparison of the frequencies of I-E reactive Vβ5+ and Vβ11+ T cells shows that I-E expression on thymic B cells is sufficient to negatively select I-E reactive CD4+ T cells partially, but not CD8+ T cells. Thus partial negative but no positive selection events can be induced by B-lymphocytes in vivo.

The tyrosine phosphatase SHP-1 influences thymocyte selection by setting TCR signaling thresholds

Modulation of the strength of signals from the TCR determines the outcome of positive and negative selection in thymocyte development. Previous studies have demonstrated that SHP-1 plays a role in determining signal strength from the TCR. Here, we have taken a genetic approach to test whether SHP-1 plays a role in T cell selection in the thymus. Experiments in which a dominant negative mutant of SHP-1 was expressed in the BYDP hybridoma cell line confirmed that SHP-1 regulated TCR signaling in a cell-autonomous manner and suggested that Lck is one of its targets. To examine the role of SHP-1 in T cell development, we crossed the ovalbumin-specific DO11.10 TCR transgene onto the motheaten background, which lacks SHP-1 expression. Analysis of the progeny of these crosses provided evidence that SHP-1 regulates thymocyte selection: (i) flow cytometric analyses revealed alterations in the percentages of thymocyte subpopulations in the me/me background; (ii) ex vivo deletion experiments demonstrated that me/me:Tg thymocytes undergo negative selection at lower concentrations of OVA peptide compared to +/+:Tg thymocytes; and (iii) ex vivo proliferation analyses indicated that me/me:Tg thymocytes were hyper-sensitive to stimulation by the specific OVA peptide. Our observation that the absence of SHP-1 leads to altered selection of TCR transgenic thymocytes demonstrates that SHP-1 regulates the strength of TCR-mediated signals in vivo and, in turn, helps to set the threshold for thymocyte selection.

Limiting TCR Expression Leads to Quantitative But Not Qualitative Changes in Thymic Selection

Thymic selection is controlled in part by the avidity of the interaction between thymocytes and APCs. In agreement, the selective outcome can be modulated by altering the expression levels of selecting ligands on APCs. Here we test the converse proposition, i.e., whether changing TCR levels on thymocytes can alter the selective outcome. To this end, we have generated mice in which all thymocytes express two transgenic TCRs simultaneously (dual TCR-expressing (DTE) mice), the class I-restricted HY TCR and the class II-restricted AND TCR. Due to mutual dilution, surface expression levels of the two individual transgenic TCRs are diminished in DTE relative to single TCR-expressing mice. We find that thymic selection is highly sensitive to these reductions in TCR surface expression. Positive selection mediated by the AND and HY TCRs is severely impaired or abolished, respectively. Negative selection of the HY TCR in male DTE mice is also partly blocked, leading to the appearance of significant numbers of double positive thymocytes. Also, in the periphery of male, but not female, DTE mice, substantial numbers of single positive CD8bright cells accumulate, which are positively selected in the thymus but by a highly inefficient hemopoietic cell-dependent process. Overall our results favor the interpretation that the outcome of thymic selection is not determined solely by avidity and the resulting signal intensity, but is also constrained by other factors such as the nature of the ligand and/or its presentation by different subsets of APCs.

Inactivation of misselected CD8 T cells by CD8 gene methylation and cell death

Misselected CD8 cells that express T cell receptors (TCRs) that do not recognize class I major histocompatibility complex (MHC) protein can emerge from thymic selection. A postthymic quality control mechanism that purges these cells from the repertoire is defined here. The failure of mature CD8 cells to simultaneously engage their TCR and CD8 coreceptor triggers an activation process that begins with inhibition of CD8 gene expression through remethylation and concludes with up-regulation of surface Fas and Fas ligand and cellular apoptosis. Thus, inhibition of a death signal through continued TCR-CD8 coengagement of MHC molecules is a key checkpoint for the continued survival of correctly selected T cells. Molecular defects that prevent delivery of the death signal to mistakenly selected T cells underlie the expansion of double-negative T cells, which is the cellular signature of a subset of systemic autoimmune diseases.

Cutting Edge: Negative Selection of Immature Thymocytes by a Few Peptide-MHC Complexes: Differential Sensitivity of Immature and Mature T Cells

We quantitated the number of peptide-class II MHC complexes required to affect the deletion or activation of 3A9 TCR transgenic thymocytes. Deletion of immature double positive thymocytes was very sensitive, taking place with approximately three peptide-MHC complexes per APC. However, the activation of mature CD4+ thymocytes required 100-fold more complexes per APC. Therefore, a “biochemical margin of safety” exists at the level of the APC. To be activated, autoreactive T cells in peripheral lymphoid tissues require a relatively high level of peptide-MHC complexes.

The extracellular versus intracellular mechanisms of inhibition of TCR-triggered activation in thymocytes by adenosine under conditions of inhibited adenosine deaminase

The absence or low levels of adenosine deaminase (ADA) in humans result in severe combined immunodeficiency (SCID), which is characterized by hypoplastic thymus, T lymphocyte depletion and autoimmunity. Deficiency of ADA causes increased levels of both intracellular and extracellular adenosine, although only the intracellular lymphotoxicity of accumulated adenosine is considered in the pathogenesis of ADA SCID. It is shown that extracellular but not intracellular adenosine selectively inhibits TCR-triggered up-regulation of activation markers and apoptotic events in thymocytes under conditions of ADA deficiency. The effects of intracellular adenosine are dissociated from effects of extracellular adenosine in experiments using an adenosine transporter blocker. We found that prevention of toxicity of intracellular adenosine led to survival of TCR-cross-linked thymocytes in long-term (4 days) assays, but it was not sufficient for normal T cell differentiation under conditions of inhibited ADA. Surviving TCR-cross-linked thymocytes had a non-activated phenotype due to extracellular adenosine-mediated, TCR-antagonizing signaling. Taken together the data suggest that both intracellular toxicity and signaling by extracellular adenosine may contribute to pathogenesis of ADA SCID. Accordingly, extracellular adenosine may act on thymocytes, which survived intracellular toxicity of adenosine during ADA deficiency by counteracting TCR signaling. This, in turn, could lead to failure of positive and negative selection of thymocytes, and to additional elimination of thymocytes or autoimmunity of surviving T cells.

A T cell receptor-specific blockade of positive selection

To investigate the influence of endogenous peptides on the developmental processes that occur during thymocyte selection, we have used monoclonal antibodies that preferentially recognize the major histocompatibility complex (MHC) molecule I-Ek when it is bound to the moth cytochrome c peptide (88-103). One of these antibodies (G35) specifically blocks the positive selection of transgenic thymocytes expressing a T cell receptor that is reactive to this peptide- MHC complex. Furthermore, G35 does not block the differentiation of transgenic T cells bearing receptors for a different I-Ek-peptide complex. This antibody recognizes a subset of endogenous I-Ek-peptide complexes found on a significant fraction of thymic antigen-presenting cells, including cortical and medullary epithelial cells. The sensitivity of G35 to minor alterations in peptide sequence suggests that the thymic peptide-MHC complexes that mediate the positive selection of a particular class II MHC-restricted thymocyte are structurally related to the complexes that can activate it in the periphery.

Nurse-like cells from bone marrow and synovium of patients with rheumatoid arthritis promote survival and enhance function of human B cells

Thymic nurse cells are known to interact with T cells and play a role in their functional maturation. However, the role of nurse cells in B cell maturation and differentiation is less well established, especially at extralymphoid sites. To address this issue, nurse-like cell clones from bone marrow and synovial tissue of patients with RA (RA-NLC) were established and characterized. RA-NLC constitutively expressed CD29, CD49c, CD54 (ICAM-1), CD106 (VCAM-1), CD157 (BST-1), and class I MHC molecules, and secreted IL-6, IL-7, IL-8, granulocyte-macrophage colony-stimulating factor (GM-CSF) and granulocyte colony-stimulating factor (G-CSF). Bone marrow-derived and synovial RA-NLC differed in that the former secreted IL-7 and expressed a greater density of CD157 constitutively and after stimulation with IFNgamma, whereas the latter secreted G-CSF and more IL-6. Stimulation of both bone marrow and synovial RA-NLC induced expression of CD40 and class II MHC, but not CD154 (CD40L) or CD35. RA-NLC rescued peripheral B cells from spontaneous apoptosis and promoted survival of B cells for > 4 wk. B cell survival was blocked by antibodies to CD106 or CD157. RA-NLC also increased Ig production from B cells. After long-term culture (4-6 wk) with RA-NLC, but not alone or with fibroblasts, outgrowth of B cells was observed. All B cell lines derived from these cultures had been transformed by EBV, although the RA-NLC themselves were not infected with EBV. Precursor frequency analysis indicated that approximately 1 in 12,500 peripheral B cells could give rise to these EBV-transformed B cell lines upon coculture with RA-NLC. These results indicate that RA-NLC from bone marrow and synovium have the capacity to rescue B cells from spontaneous apoptosis, facilitate Ig production, and promote the outgrowth of EBV-transformed B lymphoblastoid cells. These findings suggest that RA-NLC may play a role in the local and systemic hyperreactivity of B cells characteristic of rheumatoid arthritis.

A novel immunosuppressant, FTY720, increases the efficiency of a superantigen-induced peripheral T-cell deletion whilst inhibiting negative selection in the thymus

A novel immunosuppressant, FTY720, was generated by chemical modification of ISP-I, an immunosuppressive compound purified from culture filtrates of Isaria sinclairii. FTY720 directly induces apoptotic cell death in lymphocytes, which is believed to be the mechanism by which this drug exerts its immunosuppressive effect. We examined the effect of FTY720 treatment on antigen-induced apoptotic cell death in peripheral T cells and thymocytes. A superantigen, staphylococcus enterotoxin B (SEB), induces T-cell antigen receptor (TCR) Vbeta-specific apoptotic cell death in mature T cells in vivo. In this well-documented experimental system, FTY720 administration significantly enhanced the efficiency of superantigen-induced T-cell deletion. We also determined that apoptotic cell death with DNA fragmentation induced in T-hybridoma cells after stimulation in vitro with anti-TCR antibodies was enhanced in the presence of non-cytolytic doses of FTY720. In sharp contrast, negative selection of T cells in the thymus, another example of antigen-induced apoptosis, was found to be inhibited by FTY720 treatment. A rescue effect was observed on clonal deletion in the H-Y-specific TCRalpha beta transgenic male thymus. In a chicken egg albumin (OVA)-specific TCRalphabeta transgenic system, OVA-induced apoptotic cell death of CD4+CD8+ thymocytes was also inhibited by FTY720 injection. Thus, FTY720 increased the susceptibility of mature T cells to TCR-mediated apoptosis but decreased that of immature thymocytes. The results in this report suggest that the potent immunosuppressive effect of FTY720 is, in part, a result of the augmentation of effects on antigen-induced apoptosis in mature T cells, and that two distinct apoptotic cell death pathways are operating in mature and immature T cells.

An acetylcholine receptor alpha subunit promoter confers intrathymic expression in transgenic mice. Implications for tolerance of a transgenic self-antigen and for autoreactivity in myasthenia gravis

Myasthenia gravis (MG) is an autoimmune disease targeting the skeletal muscle acetylcholine receptor (AChR). Although the autoantigen is present in the thymus, it is not tolerated in MG patients. In addition, the nature of the cell bearing the autoantigen is controversial. To approach these questions, we used two lineages of transgenic mice in which the beta-galactosidase (beta-gal) gene is under the control of a 842-bp (Tg1) or a 3300-bp promoter fragment (Tg2) of the chick muscle alpha subunit AChR gene. In addition to expression in muscle cells, thymic expression was observed in both mouse lines (mainly in myoid cells in Tg1 and myoid cells and epithelial cells in Tg2). After challenge with beta-gal, Tg1 mice produced Th2-dependent anti-beta-gal antibodies, while Tg2 mice were almost unresponsive. By contrast, in a proliferation assay both Tg lines were unresponsive to beta-gal. Cells from Tg1 mice produce Th2-dependent cytokine whereas cells from Tg2 mice were nonproducing in response to beta-gal. These data indicate that the level of expression in Tg1 mice could be sufficient to induce tolerance of Th1 cells but not of Th2 cells, while both populations are tolerated in Tg2 mice. These findings are compatible with the hypothesis that AChR expression is not sufficiently abundant in MG thymus to induce a full tolerance.

Positive versus negative signaling by lymphocyte antigen receptors

Antigen receptors on lymphocytes play a central role in immune regulation by transmitting signals that positively or negatively regulate lymphocyte survival, migration, growth, and differentiation. This review focuses on how opposing positive or negative cellular responses are brought about by antigen receptor signaling. Four types of extracellular inputs shape the response to antigen: (a) the concentration of antigen; (b) the avidity with which antigen is bound; (c) the timing and duration of antigen encounter; and (d) the association of antigen with costimuli from pathogens, the innate immune system, or other lymphocytes. Intracellular signaling by antigen receptors is not an all-or-none event, and these external variables alter both the quantity and quality of signaling. Recent findings in B lymphocytes have clearly illustrated that these external inputs affect the magnitude and duration of the intracellular calcium response, which in turn contributes to differential triggering of the transcriptional regulators NF kappa B, JNK, NFAT, and ERK. The regulation of calcium responses involves a network of tyrosine kinases (e.g. lyn, syk), tyrosine or lipid phosphatases (CD45, SHP-1, SHIP), and accessory molecules (CD21/CD19, CD22, FcR gamma 2b). Understanding the biochemistry and logic behind these integrative processes will allow development of more selective and efficient pharmaceuticals that suppress, modify, or augment immune responses in autoimmunity, transplantation, allergy, vaccines, and cancer.

Cellular and molecular aspects of thymic T-cell education in neuroendocrine self principles. Implications for autoimmunity

Thymic epithelial and nurse cells from different species express a repertoire of neuroendocrine polypeptide precursors. This repertoire exerts a dual role in T-lymphocyte selection according to their status either as cryptocrine signals or as neuroendocrine self-antigens of the peptide sequences that are processed from those precursors then presented to pre-T cells. Thymic neuroendocrine self-antigens correspond to peptide sequences highly conserved throughout evolution of their family. Though thymic MHC class I molecules are involved in the processing of thymic neuroendocrine self-antigens, preliminary data show that their presentation to pre-T cells is not allelically restricted. Thymic T-cell education in neuroendocrine families also implies that the structure of a given family may be presented to pre-T cells. Our studies have evidenced the homology between thymic neuroendocrine-related self-antigens and dominant T-cell epitopes of peripheral neuroendocrine signals (neuroendocrine autoantigens). The biochemical difference between neuroendocrine autoantigens and homologous thymic self-antigens might explain the opposite immune responses evoked by those two types of antigens (activation and memory induction vs. tolerogenic effect). Altogether, these studies support the therapeutic use of thymic neuroendocrine self-antigens in reprogramming the immunological self-tolerance that is broken in autoimmune endocrine diseases like insulin-dependent diabetes type I. As recently stated by P. M. Allen in an important review, the fate of developing T lymphocytes in the thymus is influenced by the numerous types of peptidic interactions within the thymic cellular environment. To define the precise nature of thymic cells and naturally occurring biochemical peptide signals involved in positive and negative selection of immature T cells has become a prominent objective for the future research efforts in thymic physiology. This paper will try to show how thymic neuroendocrine-related peptides synthesized and processed within the thymic microenvironment indeed can play a role both in the development of the peripheral T-cell repertoire and in the death of randomly rearranged, self-reactive T cells.

Aberrant expression of tissue-specific proteins in the thymus: a hypothesis for the development of central tolerance

Herein we present the case for the existence of a thymic cortical epithelial cell that possesses an unusual gene transcription. It produces tissue-specific proteins that have their usual physiological functions outside the thymus and presents them, as well as household proteins, to the differentiating thymocytes. We suggest that this specialized cell enforces tolerance to most self-proteins by causing release of a signal for programmed cell death to thymocytes that express receptors for these self-antigens.

Double-positive T cell receptor(high) thymocytes are resistant to peptide/major histocompatibility complex ligand-induced negative selection

To investigate negative selection events during intrathymic ontogeny, we established T cell receptor (TCR)-transgenic mice [N15tg/RAG-2-/- (H-2b)] expressing a single TCR specific for vesicular stomatitis virus nuclear octapeptide N52-59 (VSV8) in the context of the major histocompatibility complex (MHC) class I molecule, K(b). Administration of VSV8 in vivo induced apoptosis in less than 4 h, deleting the majority of immature double-positive (DP) thymocytes by 24 h. In contrast, DP TCRhigh as well as single-positive (SP) thymocytes were refractory to this death process. Moreover, DP TCRhigh cells differentiated into SP thymocytes in vitro and in vivo, maturing into functional cytotoxic T lymphocytes upon intrathymic transfer to beta RAG 2-/- recipients. Hence, negative selection processes involving MHC-bound peptide ligands are operative only prior to the late DP thymocyte stage in this MHC class I-restricted TCR transgene system.

Thymic nurse cells: their functional ultrastructure

Thymic nurse cells are defined in vitro as multicellular complexes of epithelial cells and thymocytes. Although these structures have been implicated in the intrathymic differentiation of thymocytes, little is known about the biology of this cell complex and about the occurrence of the cells in the thymus in situ. Therefore, to clarify the matter, in this review we have presented characteristics of epithelial cells capable of forming complexes with thymocytes, in light of the literature data and the experience of the authors. The structure of cells within the complexes allowed us to distinguish three types of thymic nurse cells. Three-dimensional reconstruction of the thymus and observations employing TEM and SEM demonstrated the presence of distinct types of complexes in various topographic regions of the thymus. Where possible, the functional relevance of the morphological data was analyzed.

The GTPase Rho has a critical regulatory role in thymus development

The present study employs a genetic approach to explore the role of Rho GTPases in murine thymic development. Inactivation of Rho function in the thymus was achieved by thymic targeting of a transgene encoding C3 transferase from Clostridium botulinum which selectively ADP-ribosylates Rho within its effector domain and thereby abolishes its biological function. Thymi lacking functional Rho isolated from C3 transgenic mice were strikingly smaller and showed a marked (90%) decrease in cellularity compared with their normal litter mates. We also observed a similar decrease in levels of peripheral T cells in C3 transgenic mice. Analysis of the maturation status of thymocytes indicated that differentiation of progenitor cells to mature T cells can occur in the absence of Rho function, and both positive and negative selection of T cells appear to be intact. However, transgenic mice that lack Rho function in the thymus show maturational, proliferative and cell survival defects during T-cell development that severely impair the generation of normal numbers of thymocytes and mature peripheral T cells. The present study thus identifies a role for Rho-dependent signalling pathways in thymocyte development. The data show that the function of Rho GTPases is critical for the proliferative expansion of thymocytes. This defines a selective role for the GTPase Rho in early thymic development as a critical integrator of proliferation and cell survival signals.

Interferons alpha/beta inhibit IL-7-induced proliferation of CD4- CD8- CD3- CD44+ CD25+ thymocytes, but do not inhibit that of CD4- CD8- CD3- CD44- CD25- thymocytes

Type 1 interferons (IFN-alpha/beta) have recently been shown to inhibit interleukin-7 (IL-7)-induced growth and survival of early B-lineage cells. The CD3- CD4- CD8- (triple negative; TN) thymocytes from normal mice strongly proliferated upon stimulation with IL-7 in suspension, culture. Such an IL-7-induced proliferation was suppressed by the addition of IFN-alpha/beta, but a fraction of the TN thymocytes still showed proliferation. The IL-7-induced growth of TN thymocytes from acid mice, which lack the CD44- CD25- subpopulation, was completely inhibited by the addition of IFN-alpha/beta. The IL-7 induced proliferation of CD4- CD8- thymocytes from T-cell receptor (TCR) transgenic mice, the majority of which are CD3+ CD44- CD25-, was resistant to IFN-alpha/beta-mediated suppression. In fetal thymus organ cultures (FTOC), the addition of IL-7 greatly increased the population of CD4- CD8- CD44+ CD25+ thymocytes and IFN-alpha/beta inhibited this IL-7-driven expansion. In contrast, the addition of IL-7 markedly decreased the percentages of CD4- CD8- CD3- CD44- CD25- cells, and IFN-alpha/beta reversed the effect and increased the subpopulations of CD44- CD25+ and CD44- CD25-. Finally, IFN-beta mRNA was found to be expressed in the thymus. The data suggest that type I interferons inhibit IL-7-driven proliferation of TN thymocytes, but do not block the normal differentiation process.

Targeted expression of major histocompatibility complex (MHC) class II molecules demonstrates that dendritic cells can induce negative but not positive selection of thymocytes in vivo

It is well established that lymphoid dendritic cells (DC) play an important role in the immune system. Beside their role as potent inducers of primary T cell responses, DC seem to play a crucial part as major histocompatibility complex (MHC) class II+ "interdigitating cells" in the thymus during thymocyte development. Thymic DC have been implicated in tolerance induction and also by some authors in inducing major histocompatibility complex restriction of thymocytes. Most of our knowledge about thymic DC was obtained using highly invasive and manipulatory experimental protocols such as thymus reaggregation cultures, suspension cultures, thymus grafting, and bone marrow reconstitution experiments. The DC used in those studies had to go through extensive isolation procedures or were cultured with recombinant growth factors. Since the functions of DC after these in vitro manipulations have been reported to be not identical to those of DC in vivo, we intended to establish a system that would allow us to investigate DC function avoiding artificial interferences due to handling. Here we present a transgenic mouse model in which we targeted gene expression specifically to DC. Using the CD 11c promoter we expressed MHC class II I-E molecules specifically on DC of all tissues, but not on other cell types. We report that I-E expression on thymic DC is sufficient to negatively select I-E reactive CD4+ T cells, and to a less complete extent, CD8+ T cells. In contrast, it only DC expressed I-E in a class II-deficient background, positive selection of CD4+ T cells could not be observed. Thus negative, but not positive, selection events can be induced by DC in vivo.

Cellular interactions in thymocyte development

Interactions between stromal cells and thymocytes play a crucial role in T cell development. The thymic stroma is complex and consists of epithelial cells derived from the pharyngeal region during development, together with macrophages and dendritic cells of bone marrow origin. In addition, fibroblasts and matrix molecules permeate the whole framework. It is now apparent that these individual stromal components play specialized roles at different stages of T cell differentiation. Thus, at the early CD4-8- stage of development, T cell precursors require fibroblast as well as epithelial cell interactions. Later, at the CD4+8+ stage, as well as providing low avidity TCR/MHC-peptide interactions, thymic epithelial cells have been shown to possess unique properties essential for positive selection. Dendritic cells, on the other hand, are probably efficient mediators of negative selection, but they may not be solely responsible for this activity. Alongside the functional roles of stromal cells, considerable progress is being made in unraveling the nature of the signaling pathways involved in T cell development. Identification of the pre-T cell receptor (pre-TCR) and associated signaling molecules marks an important advance in understanding the mechanisms that control gene rearrangement and allelic exclusion. In addition, a better understanding of the signaling pathways that lead to positive selection on the one hand and negative selection on the other is beginning to emerge. Many issues remain unresolved, and some are discussed in this review. What, for example, is the nature of the chemotactic factor(s) that attract stem cells to the thymus? What is the molecular basis of the essential interactions between early thymocytes and fibroblasts, and early thymocytes and epithelial cells? What is special about cortical epithelial cells in supporting positive selection? These and other issues are ripe for analysis and can now be approached using a combination of modern molecular and cellular techniques.

MHC class II-specific T cells can develop in the CD8 lineage when CD4 is absent

The generation of mature CD4 T cells from CD4+CD8+ precursor thymocytes usually requires corecognition of class II MHC by a TCR and CD4, while the production of mature CD8 T cells requires corecognition of class I MHC by a TCR and CD8. To assess the role of the CD4 coreceptor in development and lineage commitment, we generated CD4-deficient mice expressing a transgenic class II-specific TCR. Surprisingly, in the absence of CD4 a large number of T cells mature, but these cells appear in the CD8 lineage. Thus, when CD4 is present, the majority of immature T cells with this class II-specific TCR choose the CD4 lineage but develop in the CD8 pathway when CD4 is absent. The results indicate that even for TCRs that are not dependent on coreceptor for MHC recognition, the coreceptor can influence the lineage choice. These findings are considered in terms of a quantitative signaling model for CD4/CD8 lineage commitment.

Presentation of a self-peptide for in vivo tolerance induction of CD4+ T cells is governed by a processing factor that maps to the class II region of the major histocompatibility complex locus

Self-proteins are regularly processed for presentation to autoreactive T cells in association with both class I and class II major histocompatibility complex (MHC) molecules. The presentation of self-peptides plays a crucial role in the acquisition of T cell repertoire during thymic selection. We previously reported that the self-MHC class I peptide Ld 61-80 was immunogenic in syngeneic B10.A mice (H-2a). We showed that despite its high affinity for self-MHC class II molecules, Ld 61-80 peptide failed to induce elimination of autoreactive CD4+ T cells, presumably due to incomplete processing and presentation in the B10.A's developing thymus (cryptic-self peptide). In this report, we showed that the cryptic phenotype was not an intrinsic property of the self-peptide Ld 61-80 since it was found to be naturally presented and subsequently tolerogenic in BALB/c mice (H-2d) (dominant self-peptide). In addition, the self-peptide Ld 61-80 was found to be immunogenic in different H-2a mice while it was invariably tolerogenic in H-2d mice regardless of their background genes. We observed that Ld 61-80 bound equally well to H-2d and H-2k MHC class II molecules. Also, no correlation was found between the quantity of self-Ld protein and the tolerogenicity of Ld 61-80. Surprisingly, Ld 61-80 was not naturally presented in (H-2d x H-2a) F1 mice, indicating that the H-2a MHC locus contained a gene that impaired the presentation of the self-peptide. Analyses of T cell responses to the self-peptide in several H-2 recombinant mice revealed that the presentation of Ld 61-80 was controlled by genes that mapped to a 170-kb portion of the MHC class II region. This study shows that (a) endogenously processed self-peptides presented by MHC class II molecules are involved in shaping the CD4+ T cell repertoire in the thymus; (b) The selection of self-peptides for presentation by MHC class II molecules to nascent autoreactive T cells is influenced by nonstructural MHC genes that map to a 170-kb portion of the MHC class II region; and (c) the MHC locus of H-2a mice encodes factors that prevent or abrogate the presentation by MHC class II molecules of the self-peptide Ld 61-80. These findings may have important implications for understanding the molecular mechanisms involved in T cell repertoire acquisition and self-tolerance induction.

T-B cell interaction inhibits spontaneous apoptosis of mature lymphocytes in Bcl-2-deficient mice

Bcl-2 expression is tightly regulated during lymphocyte development. Mature lymphocytes in Bcl-2-deficient mice show accelerated spontaneous apoptosis in vivo and in vitro. Stimulation of Bcl-2-deficient lymphocytes by anti-CD3 antibody inhibited the spontaneous apoptosis not only in T cells but also in B cells. The rescue of B cells was dependent on the presence of T cells, mainly through CD40L and interleukin (IL)-4. Furthermore, we generated Bcl-2-deficient mice transgenic for a T cell receptor or an immunoglobulin, both specific for chicken ovalbumin, to test for antigen-specific T-B cell interaction in the inhibition of the spontaneous apoptosis. The initial T cell activation by antigenic peptides presented by B cells suppressed apoptosis in T cells. Subsequently, T cells expressed CD40L and released ILs, leading to the protection of B cells from spontaneous apoptosis. These results suggest that the antiapoptotic signaling via CD40 or IL-4 may be largely independent of Bcl-2. Engagement of the Ig alone was not sufficient for the inhibition of B cell apoptosis. Thus, the physiological role of Bcl-2 in mature lymphocytes may be to protect cells from spontaneous apoptosis and to extend their lifespans to increase the opportunity for T cells and B cells to interact with each other and specific antigens in secondary lymphoid tissues. Bcl-2, however, appears to be dispensable for survival once mature lymphocytes are activated by antigen-specific T-B cell collaboration.

Essential role for ZAP-70 in both positive and negative selection of thymocytes

During thymic development, T cells that can recognize foreign antigen in association with self major histocompatibility complex (MHC) are selected for survival (positive selection) and autoreactive T cells are eliminated (negative selection). Both of these selective events are mediated by interaction between the T-cell receptor (TCR) and the peptide-MHC complex. But the signalling pathways that lead to cell survival or to cell death are still unclear. ZAP-70 is a protein tyrosine kinase (PTK) that is associated with the TCR signalling subunits (CD3 and zeta) and is expressed in T cells and natural killer cells. It has been shown that ZAP-70 plays a crucial role in T-cell activation and development. Here we show that mice lacking ZAP-70 had neither CD4 nor CD8 single-positive T cells, but human ZAP-70 reconstituted both CD4 and CD8 single-positive populations. Moreover, ZAP-70-/- thymocytes were not deleted by peptide antigens. Natural killer cell function was intact in the absence of ZAP-70. These data suggest that ZAP-70 is a central signalling molecule during thymic selection for CD4 and CD8 lineage.

Murine nursing thymic epithelial cell lines capable of inducing thymocyte apoptosis express the self-superantigen Mls-1a

Two cloned thymic epithelial cell (TEC) lines, D2.TEC-A3 and AKR TEC-K1, were established from minor lymphocyte-stimulating (Mls)-1a-positive normal, 4-week-old DBA/2 (H-2d, Mls-1a2a) mice and AKR (H-2k, Mls-1a2b) mice, respectively. Both cell lines were MHC class I and class II (both I-A and I-E) positive without stimulation by interferon-gamma. They were capable of infolding immature thymocytes to form thymic nurse cells (TNC; we call this type of TEC "nursing TEC") and induced apoptosis with DNA fragmentation in immature thymocytes. Using a primary Mls mixed lymphocyte reaction (MLR) we demonstrated that self-superantigen Mls-1a was expressed on these cloned nursing TEC lines. D2.TEC-A3 cells stimulated nylon-wool-purified splenic T cells obtained from H-2d-compatible BALB/c (Mls-1b2a) and B10.D2 (Mls-1b2b) mice with a maximal response at a stimulator:responder ratio of 1:40 after 4 days of the coculture. AKR TEC-K1 cells also stimulated purified T cells from H-2k-compatible C3H/He mice (Mls-1b2a) in a similar manner. The Mls MLR induced by the nursing TEC lines was completely inhibited in the presence of anti-mouse I-A and anti-mouse I-E monoclonal antibodies. These results suggest that nursing TEC/TNC could be involved in negative selection due to apoptosis.

Early molecular events induced by T cell receptor (TCR) signaling in immature CD4+ CD8+ thymocytes: increased synthesis of TCR-alpha protein is an early response to TCR signaling that compensates for TCR-alpha instability, improves TCR assembly, and parallels other indicators of positive selection

Differentiation of immature CD4+ CD8+ thymocytes into mature CD4+ or CD8+ T cells occurs within the thymus and is dependent upon expression of antigen receptor complexes (T cell receptor [TCR]) containing clonotypic alpha/beta proteins. We have recently found that CD4+ CD8+ thymocytes express low levels of surface TCR because of limitations placed on TCR assembly by the instability of nascent TCR-alpha proteins within the endoplasmic reticulum (ER) of immature thymocytes. Because TCR-alpha/beta expression increases during development, a molecular mechanism must exist for increasing the number of assembled TCR complexes present in immature CD4+ CD8+ thymocytes that have been signaled to differentiate into mature T cells, although no such mechanism has yet been described. In the current report we have examined the molecular consequences of intracellular signals generated by engagement of surface TCR complexes on immature CD4+ CD8+ thymocytes. Isolated TCR engagement generated signals that increased TCR-alpha RNA levels and increased synthesis of TCR-alpha proteins, which, in turn, significantly increased assembly of complete TCR-alpha/beta complexes in CD4+ CD8+ thymocytes. Increased TCR-alpha protein levels in TCR-signaled CD4+ CD8+ thymocytes was the result of increased synthesis and not increased stability of TCR-alpha proteins, indicating that TCR engagement compensates for, but does not correct, the inherent instability of TCR-alpha proteins in the ER of immature thymocytes. Consistent with the delivery by TCR engagement of a positive selection signal, TCR engagement also increased CD5 expression, decreased RAG-1 expression, and decreased CD4/CD8 coreceptor expression in immature CD4+ CD8+ thymocytes. These data identify amplified TCR-alpha expression as an initial response of immature CD4+ CD8+ thymocytes to TCR-mediated positive selection signals and provide a molecular basis for increased surface TCR density on developing thymocytes undergoing selection events within the thymus.

Genetic determinants of feline leukemia virus-induced lymphoid tumors: patterns of proviral insertion and gene rearrangement

The genetic basis of feline leukemia virus (FeLV)-induced lymphoma was investigated in a series of 63 lymphoid tumors and tumor cell lines of presumptive T-cell origin. These were examined for virus-induced rearrangements of the c-myc, flvi-2 (bmi-1), fit-1, and pim-1 loci, for T-cell receptor (TCR) gene rearrangements, and for the presence of env recombinant FeLV (FeLV-B). The myc locus was most frequently affected in naturally occurring lymphomas (32%; n = 38) either by transduction (21%) or by proviral insertion (11%). Proviral insertions were also common at flvi-2 (24%). The two other loci were occupied in a smaller number of the naturally occurring tumors (fit-1, 8%; pim-1, 5%). Examination of the entire set of tumors showed that significant numbers were affected at two (19%) or three (5%) of the loci. Occupation of the fit-1 locus was observed most frequently in tumors induced by FeLV-myc strains, while flvi-2 insertions occurred with similar frequency in the presence or absence of obvious c-myc activation. These results suggest a hierarchy of mutational events in the genesis of feline T-cell lymphomas by FeLV and implicate insertion at fit-1 as a late progression step. The strongest links observed were with T-cell development, as monitored by rearrangement status of the TCR beta-chain gene, which was positively associated with activation of myc (P < 0.001), and with proviral insertion at flvi-2 (P = 0.02). This analysis also revealed a genetically distinct subset of thymic lymphomas with unrearranged TCR beta-chain genes in which the known target loci were involved very infrequently. The presence of env recombinant FeLV (FeLV-B) showed a negative correlation with proviral insertion at fit-1, possibly due to the rapid onset of these tumors. These results shed further light on the multistep process of FeLV leukemogenesis and the relationships between lymphoid cell maturation and susceptibility to FeLV transformation.