Positive selection of thymocytes bearing alpha beta T cell receptors

The thymus road to a T cell: migration, selection, and atrophy

The thymus plays a pivotal role in generating a highly-diverse repertoire of T lymphocytes while preventing autoimmunity. Thymus seeding progenitors (TSPs) are a heterogeneous group of multipotent progenitors that migrate to the thymus via CCR7 and CCR9 receptors. While NOTCH guides thymus progenitors toward T cell fate, the absence or disruption of NOTCH signaling renders the thymus microenvironment permissive to other cell fates. Following T cell commitment, developing T cells undergo multiple selection checkpoints by engaging with the extracellular matrix, and interacting with thymic epithelial cells (TECs) and other immune subsets across the different compartments of the thymus. The different selection checkpoints assess the T cell receptor (TCR) performance, with failure resulting in either repurposing (agonist selection), or cell death. Additionally, environmental cues such as inflammation and endocrine signaling induce acute thymus atrophy, contributing to the demise of most developing T cells during thymic selection. We discuss the occurrence of acute thymus atrophy in response to systemic inflammation. The thymus demonstrates high plasticity, shaping inflammation by abrogating T cell development and undergoing profound structural changes, and facilitating regeneration and restoration of T cell development once inflammation is resolved. Despite the challenges, thymic selection ensures a highly diverse T cell repertoire capable of discerning between self and non-self antigens, ultimately egressing to secondary lymphoid organs where they complete their maturation and exert their functions.

Kras-Deficient T Cells Attenuate Graft-versus-Host Disease but Retain Graft-versus-Leukemia Activity

Acute graft-versus-host disease (aGVHD) is one major serious complication that is induced by alloreactive donor T cells recognizing host Ags and limits the success of allogeneic hematopoietic stem cell transplantation. In the current studies, we identified a critical role of Kras in regulating alloreactive T cell function during aGVHD. Kras deletion in donor T cells dramatically reduced aGVHD mortality and severity in an MHC-mismatched allogeneic hematopoietic stem cell transplantation mouse model but largely maintained the antitumor capacity. Kras-deficient CD4 and CD8 T cells exhibited impaired TCR-induced activation of the ERK pathway. Kras deficiency altered TCR-induced gene expression profiles, including the reduced expression of various inflammatory cytokines and chemokines. Moreover, Kras deficiency inhibited IL-6-mediated Th17 cell differentiation and impaired IL-6-induced ERK activation and gene expression in CD4 T cells. These findings support Kras as a novel and effective therapeutic target for aGVHD.

Plasma membrane sphingomyelin modulates thymocyte development by inhibiting TCR-induced apoptosis

Sphingomyelin (SM) in combination with cholesterol forms specialized membrane lipid microdomains in which specific receptors and signaling molecules are localized or recruited to mediate intracellular signaling. SM-microdomain levels in mouse thymus were low in the early CD4+CD8+ double-positive (DP) stage prior to thymic selection and increased >10-fold during late selection. T-cell receptor (TCR) signal strength is a key factor determining whether DP thymocytes undergo positive or negative selection. We examined the role of SM-microdomains in thymocyte development and related TCR signaling, using SM synthase 1 (SMS1)-deficient (SMS1-/-) mice which display low SM expression in all thymocyte populations. SMS1 deficiency caused reduced cell numbers after late DP stages in TCR transgenic models. TCR-dependent apoptosis induced by anti-CD3 treatment was enhanced in SMS1-/- DP thymocytes both in vivo and in vitro. SMS1-/- DP thymocytes, relative to controls, showed increased phosphorylation of TCR-proximal kinase ZAP-70 and increased expression of Bim and Nur77 proteins involved in negative selection following TCR stimulation. Addition of SM to cultured normal DP thymocytes led to greatly increased surface expression of SM-microdomains, with associated reduction of TCR signaling and TCR-induced apoptosis. Our findings indicate that SM-microdomains are increased in late DP stages, function as negative regulators of TCR signaling and modulate the efficiency of TCR-proximal signaling to promote thymic selection events leading to subsequent developmental stages.

Thymus-specific serine protease, a protease that shapes the CD4 T cell repertoire

The lifespan of T cells is determined by continuous interactions of their T cell receptors (TCR) with self-peptide-MHC (self-pMHC) complexes presented by different subsets of antigen-presenting cells (APC). In the thymus, developing thymocytes are positively selected through recognition of self-pMHC presented by cortical thymic epithelial cells (cTEC). They are subsequently negatively selected by medullary thymic epithelial cells (mTEC) or thymic dendritic cells (DC) presenting self-pMHC complexes. In the periphery, the homeostasis of mature T cells is likewise controlled by the interaction of their TCR with self-pMHC complexes presented by lymph node stromal cells while they may be tolerized by DC presenting tissue-derived self-antigens. To perform these tasks, the different subsets of APC are equipped with distinct combination of antigen processing enzymes and consequently present specific repertoire of self-peptides. Here, we discuss one such antigen processing enzyme, the thymus-specific serine protease (TSSP), which is predominantly expressed by thymic stromal cells. In thymic DC and TEC, TSSP edits the repertoire of peptide presented by class II molecules and thus shapes the CD4 T cell repertoire.

High-Throughput Analysis of the T Cell Receptor Beta Chain Repertoire in PBMCs from Chronic Hepatitis B Patients with HBeAg Seroconversion

T lymphocytes are the most important immune cells that affect both the development and treatment of hepatitis B. We used high-throughput sequencing to determine the diversity in the V and J regions of the TCRβ chain in 4 chronic hepatitis B patients before and after HBeAg seroconversion. Here, we demonstrate that the 4 patients expressed Vβ12-4 at the highest frequencies of 10.6%, 9.2%, 17.5%, and 7.5%, and Vβ28 was the second most common, with frequencies of 7.8%, 6.7%, 5.3%, and 10.9%, respectively. No significant changes were observed following seroconversion. With regard to the Jβ gene, Jβ2-1 was the most commonly expressed in the 4 patients at frequencies of 5.8%, 6.5%, 11.3%, and 7.3%, respectively. Analysis of the V-J region genes revealed several differences, including significant increases in the expression levels of V7-2-01-J2-1, V12-4-J1-1, and V28-1-J1-5 and a decrease in that of V19-01-J2-3. These results illustrate the presence of biased TCRVβ and Jβ gene expression in the chronic hepatitis B patients. TRBVβ12-4, Vβ28, Jβ2-1, V7-2-01-J2-1, V12-4-J1-1, and V28-1-J1-5 may be associated with the development and treatment of CHB.

HIV-1 Coreceptor CXCR4 Antagonists Promote Clonal Expansion of Viral Epitope-Specific CD8+ T Cells During Acute SIV Infection in Rhesus Monkeys In Vivo

BACKGROUND

The underlying molecular mechanisms and the kinetics of T cell receptor (TCR) repertoire selection during administration of CXCR4 or CCR5 inhibitors in infection of AIDS viruses in vivo have remained largely unexplored. Viral epitope-specific CD8(+) T lymphocytes play a dominant role in the control of HIV and simian immunodeficiency virus (SIV). We hypothesized that blockade of CXCR4 or CCR5 might influence the clonal expansion of epitope-specific CD8(+) T cells, contributing to antiviral immune responses in vivo.

METHODS

We measured frequencies of the dominant epitope p11C-specific CD8(+) T cells and analyzed the TCR repertoire of those cells in SIV-infected rhesus monkeys treated by CXCR4 or CCR5 inhibitors and vMIP-II, which binds multiple chemokine receptors.

RESULTS

A significantly increase in the levels of epitope-specific CD8(+) T cells was observed after blockade of CXCR4 or CCR5 compared with untreated control groups. Those CD8(+) T cells exhibited selected usage of TCR Vβ families and complementarity-determining region 3 (CDR3) segments. The clonal expansion of distinct Vβ populations could efficiently inhibit SIV replication in vitro, and CXCR4 inhibitor induced more expansion of epitope-specific CD8(+) T cells than CCR5 antagonist (P < 0.01), whereas vMIP-II treatment showed the most marked augmentation of p11C-specific CD8(+) T cells.

CONCLUSIONS

Antagonists of HIV coreceptors, particularly CXCR4, play an important role in the clonal expansion of SIV epitope-specific CD8(+) T cells in vivo, thus inhibitors of chemokine receptors such as CXCR4 or CCR5 may contribute to the ability of epitope-specific CD8(+) T cells to inhibit SIV or HIV infection.

Profiling the repertoire of T-cell receptor beta-chain variable genes in peripheral blood lymphocytes from subjects who have recovered from acute hepatitis B virus infection

The profile of T-cell receptor beta-chain variable (TRBV) genes usually skews in subjects with virus infection or cancer. The gene melting spectral pattern (GMSP) can be used to determine the profile of the TRBV gene family. To explore the portrait of the TRBV family in peripheral blood lymphocytes from subjects who have recovered from acute hepatitis B virus infection (AHI), peripheral blood mononuclear cells (PBMCs) were separated and further sorted into CD4+ and CD8+ T-cell subsets. The molecular features of the TRBV complementary determining region 3 (CDR3) motifs were determined using GMSP analysis. When aGMSP profile showed a single peak, the monoclonally expanded TRBV gene was cloned and sequenced. Skewed expansions of multiple TRBV genes were observed among the CD4+ and CD8+ T-cell subsets and the PBMCs. The frequency of monoclonally expanded TRBV genes in the CD8+ T-cell subset was significantly higher than that of the CD4+ T-cell subset and the PBMCs. Compared to other members of the TRBV gene family, TRBV11, BV15 and BV20 were predominantly expressed in the repertoire of peripheral blood lymphocytes in recovered AHI subjects. The relatively conserved amino acid motifs of TRBV5.1 and BV20 CDR3 were also detected in the CD4+ and CD8+ T-cell subsets. These results demonstrate the presence of multiple biased TRBV families in recovered AHI subjects. TRBV11, BV15 and BV20, especially from the CD8+ T-cell subset, may be relevant to the pathogenesis of subjects with AHI. The preferentially selected TRBV5.1 and BV20 with the relatively conserved CDR3 motif may be potential targets for personalized treatments of chronic HBV infection.

Molecular characterization of T cell receptor beta variable in the peripheral blood T cell repertoire in subjects with active tuberculosis or latent tuberculosis infection

BACKGROUND

T cells are closely linked to the clinical manifestations of subjects with Mycobacterium tuberculosis (MTB) infection. T cell receptor beta variable (TCRBV) is a signal and indicative molecule on the membrane of T lymphocytes, reflecting the composition and specificity of T cells. The molecular profiles of TCRBV in peripheral blood mononuclear cells (PBMCs) and their subpopulations (CD4+ and CD8+ T cells) from subjects with active tuberculosis (TB) or latent TB infection (LTBI) have not been well described.

METHODS

In 42 subjects with active TB or LTBI, PMBCs and their subsets were separated and sorted. The molecular profiles of the TCRBV complementarity determining region 3 (CDR3) in the three cell populations were investigated using our recently developed gene melting spectral pattern (GMSP) assay. The TCRBV members were then cloned and sequenced when their GMSP image profiles showed a single-peak.

RESULTS

The average number of skewed TCRBV molecules in the CD4+ cell subset was significantly higher than that in PBMCs and CD8+ T cells. TCRBV12, BV13.1, BV13.2, and BV24 were expressed more prevalently than other TCRBV gene families in the three cell populations. In addition, relatively conserved amino acid motifs were identified in TCRBV5.1 and BV20 CDR3 in PBMCs and its subsets. The monoclonal TCRBV14 and BV23 expressed were different between active TB and LTBI subjects.

CONCLUSIONS

These results indicate that the T cell immune response is complex and multi-specific in active TB and LTBI subjects. Analysis of TCRBV expression in CD4+ T cells suggest that it could be useful in assessing the composition and status of circulating T cells. Furthermore, the expression of TCRBV14, BV23 and the sequencing of CDR3 amino acid motifs of TCRBV5.1, BV20 could be used in the differential diagnosis and treatment of subjects with active TB or LTBI.

Molecular control of steady-state dendritic cell maturation and immune homeostasis

Dendritic cells (DCs) are specialized sentinels responsible for coordinating adaptive immunity. This function is dependent upon coupled sensitivity to environmental signs of inflammation and infection to cellular maturation-the programmed alteration of DC phenotype and function to enhance immune cell activation. Although DCs are thus well equipped to respond to pathogens, maturation triggers are not unique to infection. Given that immune cells are exquisitely sensitive to the biological functions of DCs, we now appreciate that multiple layers of suppression are required to restrict the environmental sensitivity, cellular maturation, and even life span of DCs to prevent aberrant immune activation during the steady state. At the same time, steady-state DCs are not quiescent but rather perform key functions that support homeostasis of numerous cell types. Here we review these functions and molecular mechanisms of suppression that control steady-state DC maturation. Corruption of these steady-state operatives has diverse immunological consequences and pinpoints DCs as potent drivers of autoimmune and inflammatory disease.

Phenotypes and clinical significance of circulating CD4(+)CD25(+) regulatory T cells (Tregs) in patients with acute-on-chronic liver failure (ACLF)

BACKGROUND

CD4(+)CD25(+) regulatory T cells (Tregs) play an important role in maintaining immunological tolerance to self and foreign antigens. T cell receptors (TCR) reflect the composition and function of T cells. It is not universally agreed that there is a relationship between CD4(+)CD25(+) Treg frequency and the severity of acute-on-chronic liver failure (ACLF). The repertoire of TCR beta chain variable (TCRBV) regions of peripheral Tregs in ACLF patients is not well understood.

METHODS

Human PBMCs were separated and sorted into CD4(+)CD25(+) Treg subsets using density gradient centrifugation and magnetic activated cell sorting (MACS). The CD4(+)CD25(high) Treg frequency in peripheral blood of ACLF and chronic hepatitis B (CHB) patients was measured by flow cytometry. The molecular profiles of TCRBV CDR3 were determined using gene melting spectral pattern (GMSP) analysis. TCRBV gene families were cloned and sequenced when the GMSP profiles showed a single-peak.

RESULTS

CD4(+)CD25(high) Treg prevalence in peripheral blood of ACLF patients is increased significantly compared to healthy donors (HDs) (P < 0.01) and CHB patients (P < 0.01). The prevalence of CD4(+)CD25(high) Tregs in ACLF or CHB patients is positively correlated with HBV DNA load. The TCRBV11, BV13.1, BV18, BV20 are the most prevalent TCRBV in CD4(+)CD25(+) Tregs in ACLF and CHB patients. In addition, the CDR3 motifs were relatively conserved in these four TCRBV gene families.

CONCLUSIONS

The CD4(+)CD25(high) Tregs prevalence in peripheral blood is indicative of disease severity in ACLF or CHB patients. The relatively conserved TCRBV20 CDR3 motif "TGTGHSPLH" and TCRBV11 CDR3 motif "VYNEQ" may be used in helping diagnosis and treat patients with ACLF.

Phospholipase Cγ2 plays a role in TCR signal transduction and T cell selection

One of the important signaling events following TCR engagement is activation of phospholipase Cγ (PLCγ). PLCγ has two isoforms, PLCγ1 and PLCγ2. It is known that PLCγ1 is important for TCR signaling and TCR-mediated T cell selection and functions, whereas PLCγ2 is critical for BCR signal transduction and BCR-mediated B cell maturation and functions. In this study, we report that PLCγ2 was expressed in primary T cells, and became associated with linker for activated T cells and Src homology 2-domain containing leukocyte protein of 76 kDa and activated upon TCR stimulation. PLCγ1/PLCγ2 double-deficient T cells displayed further block from CD4 and CD8 double-positive to single-positive transition compared with PLCγ1 single-deficient T cells. TCR-mediated proliferation was further impaired in PLCγ1/PLCγ2 double-deficient T cells compared with PLCγ1 single-deficient T cells. TCR-mediated signal transduction, including Ca²⁺ mobilization and Erk activation, was further impaired in PLCγ1/PLCγ2 double-deficient relative to PLCγ1 single-deficient T cells. In addition, in HY TCR transgenic mouse model, thymic positive and negative selections were reduced in PLCγ1 heterozygous- and PLCγ2 homozygous-deficient (PLCγ1⁺/⁻PLCγ2⁻/⁻) relative to wild-type, PLCγ2 single-deficient (PLCγ2⁻/⁻), or PLCγ1 heterozygous-deficient (PLCγ1⁺/⁻) mice. Taken together, these data demonstrate that PLCγ2 participates in TCR signal transduction and plays a role in T cell selection.

Molecular features of the complementarity determining region 3 motif of the T cell population and subsets in the blood of patients with chronic severe hepatitis B

BACKGROUND

T cell receptor (TCR) reflects the status and function of T cells. We previously developed a gene melting spectral pattern (GMSP) assay, which rapidly detects clonal expansion of the T cell receptor β variable gene (TCRBV) in patients with HBV by using quantitative real-time reverse transcription PCR (qRT-PCR) with DNA melting curve analysis. However, the molecular profiles of TCRBV in peripheral blood mononuclear cells (PBMCs) and CD8+, CD8- cell subsets from chronic severe hepatitis B (CSHB) patients have not been well described.

METHODS

Human PBMCs were separated and sorted into CD8+ and CD8- cell subsets using density gradient centrifugation and magnetic activated cell sorting (MACS). The molecular features of the TCRBV CDR3 motif were determined using GMSP analysis; the TCRBV families were cloned and sequenced when the GMSP profile showed a single-peak, indicative of a monoclonal population.

RESULTS

The number of skewed TCRBV in the CD8+ cell subset was significantly higher than that of the CD8- cell subset as assessed by GMSP analysis. The TCRBV11 and BV7 were expressed more frequently than other members of TCRBV family in PBMCs and CD8+, CD8- subsets. Also the relatively conserved amino acid motifs were detected in the TCRBV22, BV18 and BV11 CDR3 in PBMCs among patients with CSHB.

CONCLUSIONS

The molecular features of the TCRBV CDR3 were markedly different among PBMCs and CD8+, CD8- cell subsets derived from CSHB patients. Analysis of the TCRBV expression in the CD8+ subset was more accurate in assessing the status and function of circulating T cells. The expression of TCRBV11, BV7 and the relatively conserved CDR3 amino acid motifs could also help to predict and treat patients with CSHB.

Immune regulation through mitochondrion-dependent dendritic cell death induced by T regulatory cells

Dendritic cells (DCs) harbor an active mitochondrion-dependent cell death pathway regulated by Bcl-2 family members and undergo rapid turnover in vivo. However, the functions for mitochondrion-dependent cell death of DCs in immune regulation remain to be elucidated. In this article, we show that DC-specific knockout of proapoptotic Bcl-2 family members, Bax and Bak, induced spontaneous T cell activation and autoimmunity in mice. In addition to a defect in spontaneous cell death, Bax(-/-)Bak(-/-) DCs were resistant to killing by CD4(+)Foxp3(+) T regulatory cells (Tregs) compared with wild-type DCs. Tregs inhibited the activation of T effector cells by wild-type, but not Bax(-/-)Bak(-/-), DCs. Bax(-/-)Bak(-/-) DCs showed increased propensity for inducing autoantibodies. Moreover, the autoimmune potential of Bax(-/-)Bak(-/-) DCs was resistant to suppression by Tregs. Our data suggested that Bax and Bak mediate intrinsic spontaneous cell death in DCs, as well as regulate DC killing triggered by Tregs. Bax- and Bak-dependent cell death mechanisms help to maintain DC homeostasis and contribute to the regulation of T cell activation and the suppression of autoimmunity.

Thymus-specific serine protease contributes to the diversification of the functional endogenous CD4 T cell receptor repertoire

Thymus-specific serine protease expression in stromal as well as hematopoietic cells in the thymus is needed for diversification of the endogenous repertoire of TCRs specific for a particular protein antigen.

Thymus-specific serine protease (TSSP) is a novel protease that may contribute to the generation of the peptide repertoire presented by MHC class II molecules in the thymus. Although TSSP deficiency has no quantitative impact on the development of CD4 T cells expressing a polyclonal T cell receptor (TCR) repertoire, the development of CD4 T cells expressing the OTII and Marilyn transgenic TCRs is impaired in TSSP-deficient mice. In this study, we assess the role of TSSP in shaping the functional endogenous polyclonal CD4 T cell repertoire by analyzing the response of TSSP-deficient mice to several protein antigens (Ags). Although TSSP-deficient mice responded normally to most of the Ags tested, they responded poorly to hen egg lysozyme (HEL). The impaired CD4 T cell response of TSSP-deficient mice to HEL correlated with significant alteration of the dominant TCR-β chain repertoire expressed by HEL-specific CD4 T cells, suggesting that TSSP is necessary for the intrathymic development of cells expressing these TCRs. Thus, TSSP contributes to the diversification of the functional endogenous CD4 T cell TCR repertoire in the thymus.

Programmed cell death of dendritic cells in immune regulation

Programmed cell death is essential for the maintenance of lymphocyte homeostasis and immune tolerance. Dendritic cells (DCs), the most efficient antigen-presenting cells, represent a small cell population in the immune system. However, DCs play major roles in the regulation of both innate and adaptive immune responses. Programmed cell death in DCs is essential for regulating DC homeostasis and consequently, the scope of immune responses. Interestingly, different DC subsets show varied turnover rates in vivo. The conventional DCs are relatively short-lived in most lymphoid organs, while plasmacytoid DCs are long-lived cells. Mitochondrion-dependent programmed cell death plays an important role in regulating spontaneous DC turnover. Antigen-specific T cells are also capable of killing DCs, thereby providing a mechanism for negative feedback regulation of immune responses. It has been shown that a surplus of DCs due to defects in programmed cell death leads to overactivation of lymphocytes and the onset of autoimmunity. Studying programmed cell death in DCs will shed light on the roles for DC turnover in the regulation of the duration and magnitude of immune responses in vivo and in the maintenance of immune tolerance.

A quantitative real time PCR method to analyze T cell receptor Vbeta subgroup expansion by staphylococcal superantigens

Background

Staphylococcal enterotoxins (SEs), SE-like (SEl) toxins, and toxic shock syndrome toxin-1 (TSST-1), produced by Staphylococcus aureus, belong to the subgroup of microbial superantigens (SAgs). SAgs induce clonal proliferation of T cells bearing specific variable regions of the T cell receptor β chain (Vβ). Quantitative real time PCR (qRT-PCR) has become widely accepted for rapid and reproducible mRNA quantification. Although the quantification of Vβ subgroups using qRT-PCR has been reported, qRT-PCR using both primers annealing to selected Vβ nucleotide sequences and SYBR Green I reporter has not been applied to assess Vβ-dependent expansion of T cells by SAgs.

Methods

Human peripheral blood mononuclear cells were stimulated with various SAgs or a monoclonal antibody specific to human CD3. Highly specific expansion of Vβ subgroups was assessed by qRT-PCR using SYBR Green I reporter and primers corresponding to selected Vβ nucleotide sequences.

Results

qRT-PCR specificities were confirmed by sequencing amplified PCR products and melting curve analysis. To assess qRT-PCR efficiencies, standard curves were generated for each primer set. The average slope and R² of standard curves were -3.3764 ± 0.0245 and 0.99856 ± 0.000478, respectively, demonstrating that the qRT-PCR established in this study is highly efficient. With some exceptions, SAg Vβ specificities observed in this study were similar to those reported in previous studies.

Conclusions

The qRT-PCR method established in this study produced an accurate and reproducible assessment of Vβ-dependent expansion of human T cells by staphylococcal SAgs. This method could be a useful tool in the characterization T cell proliferation by newly discovered SAg and in the investigation of biological effects of SAgs linked to pathogenesis.

Thymoproteasome shapes immunocompetent repertoire of CD8+ T cells

How self-peptides displayed in the thymus contribute to the development of immunocompetent and self-protective T cells is largely unknown. In contrast, the role of thymic self-peptides in eliminating self-reactive T cells and thereby preventing autoimmunity is well established. A type of proteasome, termed thymoproteasome, is specifically expressed by thymic cortical epithelial cells (cTECs) and is required for the generation of optimal cellularity of CD8+ T cells. Here, we show that cTECs displayed thymoproteasome-specific peptide-MHC class I complexes essential for the positive selection of major and diverse repertoire of MHC class I-restricted T cells. CD8+ T cells generated in the absence of thymoproteasomes displayed a markedly altered T cell receptor repertoire that was defective in both allogeneic and antiviral responses. These results demonstrate that thymoproteasome-dependent self-peptide production is required for the development of an immunocompetent repertoire of CD8+ T cells.

Essential role of Notch signaling in effector memory CD8+ T cell-mediated airway hyperresponsiveness and inflammation

Adoptive transfer of in vivo-primed CD8(+) T cells or in vitro-generated effector memory CD8(+) T (T(EFF)) cells restores airway hyperresponsiveness (AHR) and airway inflammation in CD8-deficient (CD8(-/-)) mice. Examining transcription levels, there was a strong induction of Notch1 in T(EFF) cells compared with central memory CD8(+) T cells. Treatment of T(EFF) cells with a gamma-secretase inhibitor (GSI) strongly inhibited Notch signaling in these cells, and after adoptive transfer, GSI-treated T(EFF) cells failed to restore AHR and airway inflammation in sensitized and challenged recipient CD8(-/-) mice, or to enhance these responses in recipient wild-type (WT) mice. These effects of GSI were also associated with increased expression of the Notch ligand Delta1 in T(EFF) cells. Treatment of sensitized and challenged WT mice with Delta1-Fc resulted in decreased AHR and airway inflammation accompanied by higher levels of interferon gamma in bronchoalveolar lavage fluid. These results demonstrate a role for Notch in skewing the T cell response from a T helper (Th)2 to a Th1 phenotype as a consequence of the inhibition of Notch receptor activation and the up-regulation of the Notch ligand Delta1. These data are the first to show a functional role for Notch in the challenge phase of CD8(+) T cell-mediated development of AHR and airway inflammation, and identify Delta1 as an important regulator of allergic airway inflammation.

Activation of naturally occurring lung CD4(+)CD25(+) regulatory T cells requires CD8 and MHC I interaction

Naturally occurring Foxp3(+)CD4(+)CD25(+) T cells (nTregs) isolated from lungs of naive mice regulate allergic airway hyperresponsiveness (AHR) and inflammation. Here, we demonstrate the critical requirement for engagement of MHC class I on CD4(+)CD25(+) T cells by CD8 for the functional activation of these nTregs. Suppression of allergen-induced AHR and inflammation by nTregs was abolished in mice treated with anti-CD8. Correspondingly, decreased levels of IL-10 and TGF-beta and increased levels of Th2 cytokines in bronchoalveolar lavage were detected in these treated mice. Similarly, nTregs isolated from beta2m(-/-) mice or from mice treated with anti-MHC I antibody in vitro before intratracheal transfer failed to modulate AHR or inflammation. Coculture of nTregs with CD8(+) T cells increased IL-10 and TGF-beta. Addition of anti-MHC I or anti-CD8 reduced IL-10 and TGF-beta. These results demonstrate that functional activation of nTregs requires the interaction between MHC I on CD4(+)CD25(+) T cells and CD8.

Homeostasis of memory T cells

The pool of memory T cells is regulated by homeostatic mechanisms to persist for prolonged periods at a relatively steady overall size. Recent work has shown that two members of the common gamma chain (gammac) family of cytokines, interleukin-7 (IL-7) and IL-15, govern homeostasis of memory T cells. These two cytokines work in conjunction to support memory T-cell survival and intermittent background proliferation. Normal animals contain significant numbers of spontaneously arising memory-phenotype (MP) cells, though whether these cells are representative of true antigen-specific memory T cells is unclear. Nevertheless, it appears that the two types of memory cells do not display identical homeostatic requirements. For antigen-specific memory CD8+ T cells, IL-7 is primarily important for survival while IL-15 is crucial for their background proliferation. For memory CD4+ T cells, IL-7 has an important role, whereas the influence of IL-15 is still unclear.

Endogenous TLR ligands and autoimmunity

Based on an evolutionary conserved repertoire Toll-like-receptors (TLRs) donate specificity to innate immune cells. Therefore, TLRs are considered as paradigmatic for "self" versus "non-self" discrimination. This view, however, needs to be modified since TLR's also appear to recognise "endogeneous", that is host-derived ligands, examples being host-derived DNA and -RNA. Here I discuss physiological and pathophysiological consequences of endogeneous ligand-recognition by TLRs. I conclude that endogeneous ligand recognition by TLRs drives sterile inflammation sustained by innate immune cells in certain autoimmune disorders.

Human dendritic cell subsets for vaccination

Protective immunity results from the interplay of antigen (Ag)-nonspecific innate immunity and Ag-specific adaptive immunity. The cells and molecules of the innate system employ non-clonal recognition pathways such as lectins and TLRs. B and T lymphocytes of the adaptive immune system employ clonal receptors recognizing Ag or peptides in a highly specific manner. An essential link between innate and adaptive immunity is provided by dendritic cells (DCs). As a component of the innate immune system, DC organize and transfer information from the outside world to the cells of the adaptive immune system. DC can induce such contrasting states as active immune responsiveness or immunological tolerance. Recent years have brought a wealth of information regarding DC biology and pathophysiology that shows the complexity of this cell system. Thus, presentation of antigen by immature (non-activated) DCs leads to tolerance, whereas mature, antigen-loaded DCs are geared towards the launching of antigen-specific immunity. Furthermore, DCs are composed of multiple subsets with distinct functions at the interface of the innate and adaptive immunity. Our increased understanding of DC pathophysiology will permit their rational manipulation for therapy such as vaccination to improve immunity.

Regulation of mature T cell homeostasis

The overall size and the composition of the mature T cell pool are regulated by homeostatic mechanisms. Recent work has revealed that homeostatic signals are received from contact with two members of the common gamma chain family of cytokines, IL-7 and IL-15, and from self-MHC/peptide ligands. In essence, homeostasis of naïve T cells is regulated by IL-7 and self-MHC/peptide ligands and homeostasis of memory CD8 cells is controlled by IL-7 and IL-15. All of these signals also appear to be important to a varying degree for homeostasis of memory CD4 cells, but the details are less well understood than for other cell type.

Evaluation of the potential immunotoxicity of 3-monochloro-1,2-propanediol in Balb/c mice

3-Monochloro-1,2-propanediol (MCPD) is a well-known by-product of acid-hydrolyzed soy sauce during its manufacturing process. To evaluate the immunotoxicity of MCPD, we investigated its effect on the thymic subset, delayed-type hypersensitivity, mixed-lymphocyte reaction and peritoneal macrophage activity. MCPD was administered by gavage for 14 days at 0, 25, 50, and 100 mg/kg/day to female Balb/c mice. The thymic subsets and annexin-V positive cells in thymic cells were quantified by flow cytometry. Mixed-lymphocyte reaction, delayed-type hypersensitivity and peritoneal macrophage activity were assessed. The mixed-lymphocyte reaction and delayed-type hypersensitivity were not significantly changed. However, there were significant increases in the apoptosis of mice treated with high dose of MCPD compared to the vehicle control. A significant decrease in the CD4+CD8+ thymic subset of mice treated with high dose of MCPD was observed. The activity of peritoneal macrophage was significantly reduced in high dose group. These results indicate that MCPD could modulate the immune function in Balb/c mice.

Directed migration of positively selected thymocytes visualized in real time

Development of many vertebrate tissues involves long-range cell migrations. In most cases, these migrations have been inferred from analysis of single time points and the migration process has not been directly observed and quantitated in real time. In the mammalian adult thymus, immature CD4⁺CD8⁺ double-positive (DP) thymocytes are found in the outer cortex, whereas after T cell antigen receptor (TCR) repertoire selection, CD4⁺CD8^– and CD4^–CD8⁺ single-positive (SP) thymocytes are found in the central medulla. Here we have used two-photon laser-scanning microscopy and quantitative analysis of four-dimensional cell migration data to investigate the movement of thymocytes through the cortex in real time within intact thymic lobes. We show that prior to positive selection, cortical thymocytes exhibit random walk migration. In contrast, positive selection is correlated with the appearance of a thymocyte population displaying rapid, directed migration toward the medulla. These studies provide our first glimpse into the dynamics of developmentally programmed, long-range cell migration in the mammalian thymus.

Two-photon laser-scanning microscopy reveals the change from random motion to directed migration that occurs when thymocytes undergo positive selection.

The Notch regulator Numb links the Notch and TCR signaling pathways

Both the Notch and TCR signaling pathways play an important role in T cell development, but the links between these signaling pathways are largely unexplored. The adapter protein Numb is a well-characterized inhibitor of Notch and also contains a phosphotyrosine binding domain, suggesting that Numb could provide a link between these pathways. We explored this possibility by investigating the physical interactions among Notch, Numb, and the TCR signaling apparatus and by examining the consequences of a Numb mutation on T cell development. We found that Notch and Numb cocluster with the TCR at the APC contact during Ag-driven T cell-APC interactions in both immature and mature T cells. Furthermore, Numb coimmunoprecipitates with components of the TCR signaling apparatus. Despite this association, T cell development and T cell activation occur normally in the absence of Numb, perhaps due to the expression of the related protein, Numblike. Together our data suggest that Notch and TCR signals may be integrated at the cell membrane, and that Numb may be an important adapter in this process.

Immunotherapy via dendritic cells

The immune system evolved to protect us from microbes. The antigen (Ag)-nonspecific innate immunity and Ag-specific adaptive immunity synergize to eradicate the invading pathogen through cells, such as dendritic cells (DCZ7) and lymphocytes, and through their effector proteins including antimicrobial peptides, complement, and antibodies. Its intrinsic complexity renders the immune system prone to dysfunction including cancer, autoimmunity, chronic inflammation and allergy. DCs are unique in their capacity to induce and regulate immune responses and are therefore attractive candidates for immunotherapy. However, DCs consist of distinct subsets with common as well as unique functions that lead to distinct types of immune responses. Therefore, understanding DC heterogeneity and their role in immunopathology is critical to design better strategies for immunotherapy. Indeed, what we learn from studying autoimmunity will help us induce strong vaccine specific immunity, either protective, as in the case of microbes, or therapeutic, as in the case of tumors.

Mouse lysozyme-M knockout mice reveal how the self-determinant hierarchy shapes the T cell repertoire against this circulating self antigen in wild-type mice

We have studied T cell tolerance to defined determinants within ML-M using wild-type (WT; ML-M(+/+)) and LysMcre (ML-M(-/-)) C3H (H-2(k)) mice to determine the relative contribution of ML-M-derived epitopes vs those from other self Ags in selection of the ML-M-specific T cell repertoire. ML-M was totally nonimmunogenic in WT mice, but was rendered immunogenic in LysMcre mice. Most of the response to ML-M in LysMcre mice was directed to the immunodominant determinant region 105-119. This determinant is spontaneously displayed (without adding exogenous ML-M) by macrophages of WT, but not LysMcre, mice and is stimulatory for peptide 105-119 (p105-119)-primed T cells. Moreover, neonatal tolerization of LysMcre mice with p105-119 or ML-M abrogated the T cell response to subsequent challenge with ML-M or p105-119. Furthermore, p95-109 and p110-125 of ML-M were immunogenic in LysMcre mice, but not in WT mice, thereby representing subdominant, tolerance-inducing epitopes of ML-M. As expected, the T cell repertoire to cryptic ML determinants in WT mice was also intact in LysMcre mice. Furthermore, the pattern of response to the related homologue of ML-M, hen eggwhite lysozyme, was similar in these two groups of mice. Thus, several codominant T cell determinants within ML-M contribute significantly to tolerance induction, and the anti-cryptic T cell repertoire to ML-M was positively selected on non-ML-M self ligands. These results reveal that the induction of self tolerance to a multideterminant protein follows the quantitative hierarchy of self-determinant expression and are of relevance in understanding the pathogenesis of autoimmunity.

Autoimmunity through cytokine-induced dendritic cell activation

We propose a model where autoimmunity can be viewed as a dynamic system driven by opposite vectors IFN-alpha/beta and TNF. These cytokines drive differentiation of distinct types of DCs, TNF-DCs, or IFN-DCs, which present different antigens leading to distinct autoimmune responses. When balanced, both cytokines synergize in protective immunity. When one of the cytokines prevails, autoimmunity occurs, Type I interferons (IFN-alpha/beta) playing a major role in systemic lupus erythematosus (SLE) and TNF playing a major role in rheumatoid arthritis. This model complements the Type 1/Type 2 paradigm. Therefore, immunity can be viewed as a dynamic system driven by two sets of opposite vectors: IFN-alpha/beta/TNF and IFN-gamma/IL-4.

Production of Ribosome Components in Effector CD4+ T Cells Is Accelerated by TCR Stimulation and Coordinated by ERK-MAPK

Effector CD4+ T cells rapidly activate high-level cytokine expression following TCR stimulation. Consistent with accelerated protein production in these cells, global mRNA profiles revealed that, after cytokines, the most impressive cluster of activated genes encode rRNA-maturation factors. Activation of these genes was ERK-MAPK dependent, accompanied by increased rRNA transcription and faster maturation kinetics, and much greater in effector CD4+ T cells than in naive cells. Ribosomal protein subunit (RPS) synthesis was also ERK-MAPK dependent and increased to match rRNA production, but without evident increase in RPS mRNA. Instead, stimulation promoted polysome loading of RPS mRNA via cis-acting, 5'-terminal oligopyrimidines. These results demonstrate how, in response to extracellular signals, effector CD4+ T cells coordinately increase multiple ribosomal components to accommodate burgeoning cytokine production.

Modeling TCR signaling complex formation in positive selection

T cell receptor signaling in the thymus can result in positive selection, and hence progressive maturation to the CD4(+)8(-) or CD4(-)8(+) stage, or induction of apoptosis by negative selection. Although it is poorly understood how TCR ligation at the CD4(+)8(+) stage can lead to such different cell fates, it is thought that the strength of signal may play a role in determining the outcome of TCR signaling. In this study, we have characterized the formation of an active signaling complex in thymocytes undergoing positive selection as a result of interaction with thymic epithelial cells. Although this signaling complex involves redistribution of cell surface and intracellular molecules, reminiscent of that observed in T cell activation, accumulation of GM1-containing lipid rafts was not observed. However, enforced expression of the costimulatory molecule CD80 on thymic epithelium induced GM1 polarization in thymocytes, and was accompanied by reduced positive selection and increased apoptosis. We suggest that the presence or absence of CD80 costimulation influences the outcome of TCR signaling in CD4(+)8(+) thymocytes through differential lipid raft recruitment, thus determining overall signal strength and influencing developmental cell fate.

CD69 expression induced by thapsigargin, phorbol ester and ouabain on thymocytes is dependent on external Ca2+ entry

In the present work murine thymocytes exposed to Thapsigargin (TG 10, 20 and 50 nM), Phorbol-12,13,20-triacetate (TPA16 nM) and Ouabain (OUA100 nM) exhibited an increased expression of CD69, a molecule related to cellular activation and associated to Ca(++) influx in other systems. The kinetics of CD69 appearance depended on the stimuli and dose used. TG 50 nM induced an increased expression by 6 h whereas with lower doses (10 and 20 nM) an increase was detected at 18 h. TPA maximal increase was evident at 6 h. OUA lead to an observable increase at 18 h. However, in the case of TPA or TG the presence of the stimuli was only necessary for the first 2 h of culture, whereas OUA needed to be present during the whole assay. It was also demonstrated that Ca(++) influx was an essential feature, as EGTA diminished or abolished CD69 increased expression. Nevertheless, EGTA was only capable of this effect when present at the time of the stimuli. No correlation of CD69 expression with thymocyte death was observed. Similarly, the agents under study did not promote the maturation from double-positive into single-positive thymocytes. TPA and Thapsigargin were capable of decreasing the level of CD4 molecules on the cell surface, probably due to the loss of these molecules. OUA, on the other hand, did not modify CD4/CD8 expression on these cells.

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

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

Thymic function and allogeneic T-cell responses in stem-cell transplantation

The thymus is the primary site of T-cell production early in life, and has now been shown to continue to function in both healthy and immunocompromised individuals late into life. Positive and negative selection occurring in the thymus are two of the most important processes that govern the development and specificity of peripheral T cells, including their restriction to self HLA and their ability to respond in an alloreactive manner. In the chimeric state that follows successful allogeneic stem-cell transplants, the specificity of alloreactive cells may be governed by either host- or recipient-derived cellular elements, as well as maturing lymphoid cells, which are, in turn, derived from donor stem cells or host cells surviving transplant conditioning. The ability to measure recent thymic emigrants via the detection of T-cell receptor excision circles has facilitated studies of thymic function in immunodeficient individuals, including HIV-1 infected subjects and recipients of autologous or allogeneic stem-cell transplant (SCT). These studies have now demonstrated that thymic function is likely to play a beneficial role in immune reconstitution in these settings, but have yet to clearly demonstrate what clinical variables are the most important determinants of thymic persistence. It is also not yet clear how much the degree of thymic function following allogeneic SCT influences the alloreactive T-cell repertoire, although studies in animal models and early clinical studies suggest that GvHD results in thymic injury and dysfunction. Future studies will further clarify how thymic function shapes the repertoire of T cells that mediate alloreactivity, as well as protective pathogen-specific immune responses, following SCT. Finally, these studies will also demonstrate whether endogenous mediators of thymic function could be selectively applied to regulate post-SCT thymic function and alloreactivity.

Dynamics of thymocyte-stromal cell interactions visualized by two-photon microscopy

Thymocytes are selected to mature according to their ability to interact with self major histocompatibility complex (MHC)-peptide complexes displayed on the thymic stroma. Using two-photon microscopy, we performed real-time analysis of the cellular contacts made by developing thymocytes undergoing positive selection in a three-dimensional thymic organ culture. A large fraction of thymocytes within these cultures were highly motile. MHC recognition was found to increase the duration of thymocyte-stromal cell interactions and occurred as both long-lived cellular associations displaying stable cell-cell contacts and as shorter, highly dynamic contacts. Our results identify the diversity and dynamics of thymocyte interactions during positive selection.

E protein function in lymphocyte development

Lymphocytes arise from hematopoietic stem cells through the coordinated action of transcription factors. The E proteins (E12, E47, HEB and E2-2) have emerged as key regulators of both B and T lymphocyte differentiation. This review summarizes the current data and examines the various functions of E proteins and their antagonists, Id2 and Id3, throughout lymphoid maturation. Beyond an established role in B and T lineage commitment, E proteins continue to be essential at subsequent stages of development. E protein activity regulates the expression of surrogate and antigen receptor genes, promotes Ig and TCR rearrangements, and coordinates cell survival and proliferation with developmental progression in response to TCR signaling. Finally, this review also discusses the role of E47 as a tumor suppressor.

Positive selection of self-MHC-reactive T cells by individual peptide-MHC class II complexes

If T cells require specific interactions with MHC-bound peptides during positive selection, then the specificities of T cells selected by one peptide should be distinct from those selected by another. We have examined positive selection of CD4 T cells in four strains of mice, each overexpressing a different peptide-1-A(b)(A(b)) complex. We show that a subset of CD4 T cells is selected by the overexpressed peptide and that the specificities of the CD4 T cells, as measured by reactivity to wild-type antigen-presenting cells, vary greatly depending on which peptide is overexpressed. These differences in specificity are mediated through positive selection not negative selection. Each of the four peptide-A(b) complexes appears to adopt a different conformation, and these differences correlate with the differences in reactivity. Our results suggest that individual peptide-MHC complexes positively select different subsets of self-MHC-reactive T cells and that the conformation of the peptide-MHC complex may contribute to this process.

Reversible CD8 expression induced by common cytokine receptor gamma chain-dependent cytokines in a cloned CD4(+) T(h)1 cell line

T cells that are intrathymically lineage committed are believed to maintain their CD4 or CD8 co-receptor expression. Here, we investigated whether intrathymic lineage commitment involves irreversible genetic modification or whether co-receptor expression can be reprogrammed depending on external stimuli. The CD4(+) T(h)1 clone 2D6 established from splenic T cells as an IL-12-dependent line survived in culture with IL-2, IL-7 or IL-15 alone. Surprisingly, CD8 expression occurred in 2D6 cells upon replacement of IL-12 with any one of the three cytokines that stimulate the common cytokine receptor gamma chain, yielding CD4(+)CD8(+) 2D6 cells. CD8 expression declined when IL-2 was replaced with IL-12 and CD8 induction was inhibited when IL-12 was included in IL-2 or IL-7 culture. Our observations show that even a lineage-committed mature T cell can be reprogrammed for co-receptor expression in response to particular external stimuli.

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

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

Phorbol esters and cAMP differentially regulate the expression of CD4 and CD8 in human thymocytes

BACKGROUND

Intrathymic development and selection of the T lymphocyte repertoire is restricted by the interactions of the T cell antigen receptor and CD4 or CD8 co-receptors with self major histocompatibility complex molecules. Positive or negative selection depends on a tight regulatory control of CD4 and CD8 expression. Determining the intracellular signals that differentially regulate the expression of CD4 and CD8 is important to understand the mechanisms that are implicated in selection of single positive CD4+CD8- or CD4-CD8+.

RESULTS

The present study shows that stimulation of human thymocytes by phorbol esters or cAMP result in a differential regulation of CD4 and CD8 expression, both at the mRNA and cell surface glycoprotein level.

CONCLUSIONS

The differential regulation of CD4 and CD8 gene expression suggests that the selective activation of protein kinase C (PKC) and cAMP-dependent protein kinases (PKA) may be required for the selection of single positive CD4+CD8- and CD4-CD8+ cells during Intrathymic differentiation.

The generation of mature, single-positive thymocytes in vivo is dysregulated by CD69 blockade or overexpression

During development in the thymus, mature CD4+ or CD8+ cells are derived from immature CD4+CD8+ cells through a series of selection events. One of the hallmarks of this maturation process is the expression of CD69, which first appears on thymocytes as they begin positive selection. We have used blockade and overexpression of CD69 to determine the role of CD69 in thymocyte development. Blockade of CD69 led to a reduction in single-positive cells and a concomitant increase in double-positive cells in the thymus. Overexpression of a CD69 transgene in the thymus resulted in a dramatic increase in both CD8SP and CD4SP cells. Coexpression with a TCR transgene demonstrated that both positive and negative selection were enhanced by the increased levels of CD69 on thymocytes. Finally, mice overexpressing CD69 displayed a sharp reduction in the number of T cells in the spleen and lymph node. Taken as a whole, these data suggest the involvement of CD69 in the process of selection and maturation during the trafficking of thymocytes to the medulla.

Deletion of the CD4 silencer element supports a stochastic mechanism of thymocyte lineage commitment

The mechanism of T cell lineage commitment remains controversial; to examine it we deleted the CD4-silencer element in the germ line of a mouse using a combination of gene targeting and Cre/LoxP-mediated recombination. We found that these mice were unable to extinguish CD4 expression either in immature thymocytes or mature CD8+ cytotoxic T cells (CTLs), which resulted in the development of major histocompatibility complex class II-restricted double-positive CTLs in the periphery. This finding strongly supports a stochastic over an instructive mechanism of coreceptor down-regulation.

Epigenetic silencing of CD4 in T cells committed to the cytotoxic lineage

The process of thymocyte development culminates in the maturation of helper (CD4+) and cytotoxic (CD8+) T cells from their common precursors, the CD4+CD8+ double-positive cells. A crucial step during lineage specification is the termination of expression of either the CD4 or the CD8 coreceptor. A silencer element within the first intron of the CD4 gene is sufficient for CD4 transcriptional repression in cells of the cytotoxic lineage, as well as in thymocytes at earlier stages of differentiation. Here we show that the function of the CD4 silencer is required only at distinct stages of development. Its deletion before the initiation of lineage specification resulted in CD4 derepression throughout thymocyte differentiation. By contrast, once cells committed to the cytotoxic CD8+ lineage, the CD4 locus remained silent through subsequent mitoses, even when the silencer element was excised. The epigenetic inheritance of the silenced CD4 locus was not affected by the inhibition of DNA methylation or histone deacetylation, and may thus involve other mechanisms that ensure a stable state of gene expression.

The critical role of LIGHT, a TNF family member, in T cell development

Negative selection refers to the selective deletion of autoreactive thymocytes but its molecular events have not been well defined. In this study, we demonstrate that a cellular ligand for herpes virus entry mediator and lymphotoxin receptor (LIGHT), a newly identified member of the TNF superfamily, may play a critical role in negative selection. Using TCR transgenic mice, we find that the blockade of LIGHT signaling in vitro and in vivo prevents negative selection induced by peptide and intrathymically expressed Ags, resulting in the rescue of thymocytes from apoptosis. Furthermore, the thymi of LIGHT transgenic mice show severe atrophy with remarkably reduced CD4(+)CD8(+) double-positive cells caused by increased apoptosis, suggesting that LIGHT can delete immature T cells in vivo. Taken together, these results demonstrate a critical role of LIGHT in thymic negative selection of the T cell repertoire.

Enforced expression of GATA-3 during T cell development inhibits maturation of CD8 single-positive cells and induces thymic lymphoma in transgenic mice

The zinc finger transcription factor GATA-3 is of critical importance for early T cell development and commitment of Th2 cells. To study the role of GATA-3 in early T cell development, we analyzed and modified GATA-3 expression in vivo. In mice carrying a targeted insertion of a lacZ reporter on one allele, we found that GATA-3 transcription in CD4(+)CD8(+) double-positive thymocytes correlated with the onset of positive selection events, i.e., TCRalphabeta up-regulation and CD69 expression. LacZ expression remained high ( approximately 80% of cells) during maturation of CD4 single-positive (SP) cells in the thymus, but in developing CD8 SP cells the fraction of lacZ-expressing cells decreased to <20%. We modified this pattern by enforced GATA-3 expression driven by the CD2 locus control region, which provides transcription of GATA-3 throughout T cell development. In two independent CD2-GATA3-transgenic lines, approximately 50% of the mice developed thymic lymphoblastoid tumors that were CD4(+)CD8(+/low) and mostly CD3(+). In tumor-free CD2-GATA3-transgenic mice, the total numbers of CD8 SP cells in the thymus were within normal ranges, but their maturation was hampered, as indicated by increased apoptosis of CD8 SP cells and a selective deficiency of mature CD69(low)HSA(low) CD8 SP cells. In the spleen and lymph nodes, the numbers of CD8(+) T cells were significantly reduced. These findings indicate that GATA-3 supports development of the CD4 lineage and inhibits maturation of CD8 SP cells in the thymus.

The TCR repertoire of an immunodominant CD8+ T lymphocyte population

The TCR repertoire of an epitope-specific CD8(+) T cell population remains poorly characterized. To determine the breadth of the TCR repertoire of a CD8(+) T cell population that recognizes a dominant epitope of the AIDS virus, the CD8(+) T cells recognizing the tetrameric Mamu-A*01/p11C(,CM) complex were isolated from simian immunodeficiency virus (SIV)-infected Mamu-A*01(+) rhesus monkeys. This CD8(+) T cell population exhibited selected usage of TCR V beta families and complementarity-determining region 3 (CDR3) segments. Although the epitope-specific CD8(+) T cell response was clearly polyclonal, a dominance of selected V beta(+) cell subpopulations and clones was seen in the TCR repertoire. Interestingly, some of the selected V beta(+) cell subpopulations and clones maintained their dominance in the TCR repertoire over time after infection with SIV of macaques. Other V beta(+) cell subpopulations declined over time in their relative representation and were replaced by newly evolving clones that became dominant. The present study provides molecular evidence indicating that the TCR repertoire shaped by a single viral epitope is dominated at any point in time by selected V beta(+) cell subpopulations and clones and suggests that dominant V beta(+) cell subpopulations and clones can either be stable or evolve during a chronic infection.