Selection Of Thymocytes Research Articles

63P Study of single nucleotide polymorphism of thymocyte selection associated high mobility group box 3 and fibroblast growth factor receptor 2 genes in breast cancer patients

63P Study of single nucleotide polymorphism of thymocyte selection associated high mobility group box 3 and fibroblast growth factor receptor 2 genes in breast cancer patients

PD-1/PD-L1 signaling regulates the thymic niche size of agonistly selected T cells

Abstract Effective peripheral T cell responses rely upon a diverse T cell repertoire, generated during thymic selection. Signaling through the TCR establishes thresholds for thymocyte selection in order to create this repertoire. Because co-stimulation modifies TCR signaling, it may contribute to the regulation of T cell development by determining signal strength thresholds that direct thymocytes into specific lineages. Programmed death-1 (PD-1), a member of the B7/CD28 family of costimulatory/inhibitory receptors, has been implicated in the development of thymocytes because it is expressed in CD4− CD8− double-negative thymocytes during TCRb rearrangement. Further, the ligands for PD-1, PD-L1 and PD-L2, are expressed on thymic epithelial cells. PD-L2 is restricted to the medulla whereas PD-L1 is expressed throughout the thymus. Here, we show that PD-1 modulates agonist selection, a process in which some high-affinity thymocytes will mature into unconventional T cell lineages (e.g. regulatory T cells, NKT cells). Transgenic mice lacking either PD-L1 or PD-1 have increased numbers of agonistly selected T cells compared to wildtype animals. Interestingly, loss of PD-L1 increases the selection of conventional and agonistly selected T cells whereas loss of PD-1 limits the phenotype to agonist lineages. These data demonstrate that PD-1, PD-L1, and possibly PD-L2 interactions, potentially within the thymic microenvironment in which they are expressed, play a role in determining T cell fate and shaping the final T cell repertoire.

Human T cell glycosylation and implications on immune therapy for cancer

ABSTRACT Glycosylation is an important post-translational modification, giving rise to a diverse and abundant repertoire of glycans on the cell surface, collectively known as the glycome. When focusing on immunity, glycans are indispensable in virtually all signaling and cell-cell interactions. More specifically, glycans have been shown to regulate key pathophysiological steps within T cell biology such as T cell development, thymocyte selection, T cell activity and signaling as well as T cell differentiation and proliferation. They are of major importance in determining the interaction of human T cells with tumor cells. In this review, we will describe the role of glycosylation of human T cells in more depth, elaborate on the importance of glycosylation in the interaction of human T cells with tumor cells and discuss the potential of cancer immunotherapies that are based on manipulating the glycome functions at the tumor immune interface. 1,2

Testing the Efficiency and Kinetics of Negative Selection Using Thymic Slices.

Central tolerance is an efficient barrier to autoimmunity and negative selection of self-reactive thymocytes is one of its major manifestations. Because of its importance, negative selection has been studied extensively through numerous in vitro and in vivo approaches that have tremendously increased our understanding of the process. Recently, in situ experimental systems using thymus slices have been developed that combine some of the advantages of in vitro assays such as ease of manipulation and high throughput with the existence of three dimensional mature thymus microenvironment. These approaches offer unprecedented opportunity to study negative selection. Here, we describe how thymic slices can be used to measure the kinetics and magnitude of negative selection. Taking the OT-1/Ova system as an example, we provide detailed guidance on cutting thymic slices, labeling and overlaying thymocytes on them and reading out the extent of negative selection by flow cytometry. The system can easily be adapted to evaluate the effects of various mutations or treatments on negative selection or to study the behavior of different cells in the thymus through time-lapse imaging.

Using Chromatin-Nuclear Receptor Interactions to Quantitate Endocrine, Paracrine, and Autocrine Signaling.

Hormone-activated nuclear receptors (NRs) control myriad cellular processes. The classical paradigm for hormone delivery is secretion from endocrine organs and blood-borne distribution to responding cells. However, many hormones can also be synthesized in the same tissues in which responding cells are found (paracrine signaling). In both endocrine and paracrine signaling, numerous factors affect hormone availability to target cell NRs, including hormone access to and sequestration by carrier proteins, transport across cell membranes, metabolism, and receptor availability. These factors can differ dramatically during development, between anatomical locations, and across cell types, and may cause highly variable responses to the same hormone signal. This has been difficult to study because current approaches are unable to quantify cell-intrinsic exposure to NR hormone ligands, precluding assessment of cell-specific hormone access and signaling. We have used the ligand-dependent interaction of the endogenous glucocorticoid (GC) receptor with chromatin as a biosensor that quantifies systemic access of GCs to cells within tissues at the single cell level, showing that tissues are buffered against circulating GCs. This approach also showed highly targeted paracrine GC signaling within the thymus, where GCs promote the positive selection of thymocytes with moderate affinity for self-antigens and the development of a safe and effective T-cell repertoire. We believe that this and complementary biosensor approaches will be useful to identify endocrine and paracrine target cells in situ and quantify their exposure to hormones regardless of the mode of delivery.

Reduced positive selection of a human TCR in a swine thymus using a humanized mouse model for xenotolerance induction.

Tolerance-inducing approaches to xenotransplantation would be optimal and may be necessary for long-term survival of transplanted pig organs in human patients. The ideal approach would generate donor-specific unresponsiveness to the pig organ without suppressing the patient's normal immune function. Porcine thymus transplantation has shown efficacy in promoting xenotolerance in humanized mice and large animal models. However, murine studies demonstrate that T cells selected in a swine thymus are positively selected only by swine thymic epithelial cells, and therefore, cells expressing human HLA-restricted TCRs may not be selected efficiently in a transplanted pig thymus. This may lead to suboptimal patient immune function. To assess human thymocyte selection in a pig thymus, we used a TCR transgenic humanized mouse model to study positive selection of cells expressing the MART1 TCR, a well-characterized human HLA-A2-restricted TCR, in a grafted pig thymus. Positive selection of T cells expressing the MART1 TCR was inefficient in both a non-selecting human HLA-A2- or swine thymus compared with an HLA-A2+ thymus. Additionally, CD8 MART1 TCRbright T cells were detected in the spleens of mice transplanted with HLA-A2+ thymi but were significantly reduced in the spleens of mice transplanted with swine or HLA-A2- thymi. [Correction added on October 15, 2019, after first online publication: The missing superscript values +, -, and bright have been included in the Results section.] CONCLUSIONS: Positive selection of cells expressing a human-restricted TCR in a transplanted pig thymus is inefficient, suggesting that modifications to improve positive selection of cells expressing human-restricted TCRs in a pig thymus may be necessary to support development of a protective human T-cell pool in future patients.

Adhesion between medullary thymic epithelial cells and thymocytes is regulated by miR-181b-5p and miR-30b*

In this work, we demonstrate that adhesion between medullary thymic epithelial cells (mTECs) and thymocytes is controlled by miRNAs. Adhesion between mTECs and developing thymocytes is essential for triggering negative selection (NS) of autoreactive thymocytes that occurs in the thymus. Immune recognition is mediated by the MHC / TCR receptor, whereas adhesion molecules hold cell-cell interaction stability. Indeed, these processes must be finely controlled, if it is not, it may lead to aggressive autoimmunity. Conversely, the precise molecular genetic control of mTEC-thymocyte adhesion is largely unclear. Here, we asked whether miRNAs would be controlling this process through the posttranscriptional regulation of mRNAs that encode adhesion molecules. For this, we used small interfering RNA to knockdown (KD) Dicer mRNA in vitro in a murine mTEC line. A functional assay with fresh murine thymocytes co-cultured with mTECs showed that single-positive (SP) CD4 and CD8 thymocyte adhesion was increased after Dicer KD and most adherent subtype was CD8 SP cells. Analysis of broad mTEC transcriptional expression showed that Dicer KD led to the modulation of 114 miRNAs and 422 mRNAs, including those encoding cell adhesion or extracellular matrix proteins, such as Lgals9, Lgals3pb, Tnc and Cd47. Analysis of miRNA-mRNA networks followed by miRNA mimic transfection showed that these mRNAs are under the control of miR-181b-5p and miR-30b*, which may ultimately control mTEC-thymocyte adhesion. The expression of CD80 surface marker in mTECs was increased after Dicer KD following thymocyte adhesion. This indicates the existence of new mechanisms in mTECs that involve the synergistic action of thymocyte adhesion and regulatory miRNAs.

Genetic lines respond uniquely within the chicken thymic transcriptome to acute heat stress and low dose lipopolysaccharide

Exposure to high temperatures is known to impair immune functions and disease resistance of poultry. Characterizing changes in the transcriptome can help identify mechanisms by which immune tissues, such as the thymus, respond to heat stress. In this study, 22-day-old chickens from two genetic lines (a relatively resistant Fayoumi line and a more susceptible broiler line) were exposed to acute heat stress (35 °C) and/or immune simulation with lipopolysaccharide (LPS; 100 µg/kg). Transcriptome responses in the thymus were identified by RNA-sequencing (RNA-seq). Expression of most genes was unaffected by heat and/or LPS in the Fayoumi line, whereas these treatments had more impact in the broiler line. Comparisons between the broiler and Fayoumi transcriptomes identified a large number of significant genes both at homeostasis and in response to treatment. Functional analyses predicted that gene expression changes impact immune responses, apoptosis, cell activation, migration, and adhesion. In broilers, acute heat stress changed thymic expression responses to LPS and could impact thymocyte survival and trafficking, and thereby contribute to the negative effects of high temperatures on immune responses. Identification of these genes and pathways provides a foundation for testing targets to improve disease resistance in heat-stressed chickens.

Genome association of carcass and palatability traits from Bos indicus-Bos taurus crossbred steers within electrical stimulation status and correspondence with steer temperament 1. Carcass

Genome-wide association analyses were conducted using SNP and carcass traits, with and without electrical stimulation separately, of 350 Bos indicus-Bos taurus steers from a designed mapping population consisting of full-sibling families (embryo transfer) and half-sibling families. Four F1 Nellore–Angus sires had progeny in both types of families. Relationships of carcass traits with multiple assessments of temperament (correlation coefficients of unadjusted or residual values), including aggressiveness (willingness to hit an evaluator), nervousness, flightiness (nervousness with running or other movement), gregariousness (willingness to separate from other steers), and overall temperament (all temperament traits assigned as 1 to 9 values; higher numbers indicate worse or less desirable temperament) at 1 year of age and with overall temperament evaluated immediately prior to slaughter were low (0 < |r| < 0.18). Significant SNP associations were detected (False Discovery Rate controlled at 0.05) on BTA 1 with skeletal maturity (n = 1), and BTA 14 (n = 2) with carcass weight. Candidate genes were (skeletal maturity) phospholipase C like 2 (PLCL2) and (carcass weight) neutral sphingomyelinase activation associated factor (NSMAF) and thymocyte selection associated high mobility group box (TOX). Two SNP with association with marbling score were identified on BTA 12—dachshund family transcription factor 1 (DACH1) was a candidate gene. A region from 24.25 to 38.54 Mb on BTA 23 contained 6 SNP with significant associations with lean or overall maturity, which may be the first reported for these traits. Candidate genes on that chromosome included fibrocystin/polyductin (PKHD1), glial cells missing homolog 1 (GCM1), prolactin (PRL), and ring finger protein 144B (RNF144B). All detections of association were in traits measured on the side with no electrical stimulation; it may be that electrical stimulation removes some of the variation permitting detection of association.

Thymic-specific regulation of TCR signaling by Tespa1.

Double-positive (DP) thymocytes undergo positive selection to become mature single-positive CD4+ and CD8+ T cells in response to T cell receptor (TCR) signaling. Unlike mature T cells, DP cells must respond to low-affinity self-peptide-MHC ligands before full upregulation of their surface TCR expression can occur. Thus, DP thymocytes must be more sensitive to ligands than mature T cells. A number of molecules have been found that are able to enhance the strength of the TCR signal to facilitate positive selection. However, almost all of these molecules are also active in mature T cells. Themis (thymocyte expressed molecule involved in selection) and Tespa1 (thymocyte expressed positive selection associated 1) are two recently discovered molecules essential for optimal TCR signaling and thymocyte development. A deficiency in both molecules leads to defects in positive selection. Here, we compared the relative contributions of Themis and Tespa1 to positive selection in thymocytes. We show that Tespa1 deficiency led to more limited and specific gene expression profile changes in cells undergoing positive selection. In mixed bone marrow transfer experiments, Tespa1-/- cells showed more severe defects in thymocyte development than Themis-/- cells. However, Tespa1-/- cells showed a substantial degree of homeostatic expansion and became predominant in the peripheral lymphoid organs, suggesting that Tespa1 is a thymic-specific TCR signaling regulator. This hypothesis is further supported by our observations in Tespa1 conditional knockout mice, as Tespa1 deletion in peripheral T cells did not affect TCR signaling or cell proliferation. The different regulatory effects of Tespa1 and Themis are in accordance with their nonredundant roles in thymocyte selection, during which Tespa1 and Themis double knockouts showed additive defects.

Protein phosphatase 2A has an essential role in promoting thymocyte survival during selection

The development of thymocytes to mature T cells in the thymus is tightly controlled by cellular selection, in which only a small fraction of thymocytes equipped with proper quality of TCRs progress to maturation. It is pivotal to protect the survival of the few T cells, which pass the selection. However, the signaling events, which safeguard the cell survival in thymus, are not totally understood. In this study, protein Ser/Thr phosphorylation in thymocytes undergoing positive selection is profiled by mass spectrometry. The results revealed large numbers of dephosphorylation changes upon T cell receptor (TCR) activation during positive selection. Subsequent substrate analysis pinpointed protein phosphatase 2A (PP2A) as the enzyme responsible for the dephosphorylation changes in developing thymocytes. PP2A catalytic subunit α (Ppp2ca) deletion in the T cell lineage in Ppp2caflox/flox-Lck-Cre mice (PP2A cKO) displayed dysregulated dephosphorylation of apoptosis-related proteins in double-positive (DP) cells and caused substantially decreased numbers of DP CD4+ CD8+ cells. Increased levels of apoptosis in PP2A cKO DP cells were found to underlie aberrant thymocyte development. Finally, the defective thymocyte development in PP2A cKO mice could be rescued by either Bcl2 transgene expression or by p53 knockout. In summary, our work reveals an essential role of PP2A in promoting thymocyte development through the regulation of cell survival.

Long-term survival and differentiation of human thymocytes in human thymus-grafted immunodeficient mice.

Aim: Thymus transplants have produced encouraging clinical outcomes in achieving thymopoiesis and T-cell development. This study was aimed to investigate whether human thymus contains self-renewing lymphoid progenitors capable of maintaining long-term T-cell development. Materials & methods: Immunodeficient mice were transplanted with human thymic tissue along with autologous GFP-expressing or allogeneic CD34+ cells and followed for human thymopoiesis and T-cell development from the thymic progenitors versus CD34+ cells, which can be distinguished by GFP or HLA expression. Results: In both models, long-term thymopoiesis and T-cell development from the thymic grafts were detected. In these mice, human thymic progenitor-derived T cells including CD45RA+CD31+CD4+ new thymic emigrants were persistently present in the periphery throughout the observation period (32weeks). Conclusion: The results indicate that human thymus contains long-lived lymphoid progenitors that can maintain durable thymopoiesis and T-cell development.

Sexual dimorphism in rat thymic involution: a correlation with thymic oxidative status and inflammation.

The study investigated mechanisms underlying sex differences in thymic involution in Dark Agouti rats. Adverse effects of aging on thymus were more pronounced in males than in females. Thymi from old males exhibited more prominent: (i) fibro-adipose degeneration which correlated with greater intensity of thymic oxidative stress and enhanced thymic TGF-β and IL-6 expression and (ii) decline in thymopoiesis, as suggested by the number of the most mature CD4+CD8-/CD4-CD8+ single positive (SP) TCRαβhigh thymocytes. The greater accumulation of adipose tissue in old male thymus was linked with greater age-related increase in thymic expression of PPARγ and STAT3, a transcription factor regulating the expression of PPARγ downstream genes, in male than in female rats. In aged thymi of both sexes the early CD4-CD8- double negative (DN) stage of thymocyte development was affected, so relative accumulation of the least mature CD45RC+CD2- cells followed by decreased frequency of their DN and CD4+CD8+ double positive (DP) TCRαβ- descendants was observed. Additionally, in old males, because of the increased thymic expression of Nur77, a nuclear receptor involved in negative selection, and decreased CD90 (a negative regulator of thymocyte selection threshold) MFI on DP TCRαβint thymocytes, less efficient positive/more efficient negative selection was found. Moreover, in male rats, thymocyte post-selection differentiation/maturation was skewed towards CD4-CD8+ SP TCRαβhigh cells compared with age-matched females, reflecting, at least partly, greater IL-15 expression in their thymi. The study indicated mechanisms underlying sex-based differences in age-related thymic changes and consequently necessity of sex-specific approaches in designing strategies to rejuvenate thymus.

Crossreactive public TCR sequences undergo positive selection in the human thymic repertoire.

We investigated human T-cell repertoire formation using high throughput TCRβ CDR3 sequencing in immunodeficient mice receiving human hematopoietic stem cells (HSCs) and human thymus grafts. Replicate humanized mice generated diverse and highly divergent repertoires. Repertoire narrowing and increased CDR3β sharing was observed during thymocyte selection. While hydrophobicity analysis implicated self-peptides in positive selection of the overall repertoire, positive selection favored shorter shared sequences that had reduced hydrophobicity at positions 6 and 7 of CDR3βs, suggesting weaker interactions with self-peptides than unshared sequences, possibly allowing escape from negative selection. Sharing was similar between autologous and allogeneic thymi and occurred between different cell subsets. Shared sequences were enriched for allo-crossreactive CDR3βs and for Type 1 diabetes-associated autoreactive CDR3βs. Single-cell TCR-sequencing showed increased sharing of CDR3αs compared to CDR3βs between mice. Our data collectively implicate preferential positive selection for shared human CDR3βs that are highly cross-reactive. While previous studies suggested a role for recombination bias in producing "public" sequences in mice, our study is the first to demonstrate a role for thymic selection. Our results implicate positive selection for promiscuous TCRβ sequences that likely evade negative selection, due to their low affinity for self-ligands, in the abundance of "public" human TCRβ sequences.

Proteolytic dynamics of human 20S thymoproteasome

An efficient immunosurveillance of CD8+ T cells in the periphery depends on positive/negative selection of thymocytes and thus on the dynamics of antigen degradation and epitope production by thymoproteasome and immunoproteasome in the thymus. Although studies in mouse systems have shown how thymoproteasome activity differs from that of immunoproteasome and strongly impacts the T cell repertoire, the proteolytic dynamics and the regulation of human thymoproteasome are unknown. By combining biochemical and computational modeling approaches, we show here that human 20S thymoproteasome and immunoproteasome differ not only in the proteolytic activity of the catalytic sites but also in the peptide transport. These differences impinge upon the quantity of peptide products rather than where the substrates are cleaved. The comparison of the two human 20S proteasome isoforms depicts different processing of antigens that are associated to tumors and autoimmune diseases.

CD301b+ SIRPα+ dendritic cells play a non-redundant role in inducing clonal deletion in the thymus medulla

Abstract The thymus contains an assortment of antigen presenting cells (APC) that have distinct locations and antigen processing capabilities. This suggests that they play non-redundant roles in mediating thymocyte selection. Many APC subsets were shown to be capable of driving clonal deletion, however these studies relied primarily on T cell receptor transgenic models. We therefore still do not understand the extent to which distinct APC contribute to clonal deletion in the polyclonal repertoire. Here, we assessed the contribution of different APC subsets to clonal deletion using a cleaved caspase 3-based assay paired with cell type ablation or deficiency. Total deletion frequencies were not altered in the absence of B cells, pDC, or XCR1+ cDC1. In an effort to eliminate SIRPα+ cDC2, we discovered that a substantial proportion of thymic SIRPα+ DC express the lectin CD301b. CD301b+ DCs were localized exclusively within the thymus medulla and depended on IL-4R□. Deficiency of these IL-4/IL-13 signaled DC resulted in a measurable reduction in clonal deletion. These data suggest that CD301b+ SIRPα+ DC represent a distinct population of antigen presenting cells within the thymus and that they are essential for non-redundant deletional tolerance in the polyclonal repertoire.

165 Contribution of TOX1 and STAT3 pathways to the development of cutaneous T-cell lymphoma (CTCL)

Thymocyte selection associated high mobility group box 1 (Tox1), a transcription factor that is essential to establish the CD4+ lineage, is overexpressed in malignant cells from patients with CTCL. Knockdown of Tox1 leads to decreased malignant cell viability, while treatment with HDAC inhibitors results in normalization of Tox1expression. Another gene which is consistently overexpressed in patient samples is signal transducers and activators of transcription3 (Stat3), a transcription factor essential for the differentiation of Th17 and follicular helper T cells. Treating CTCL cell lines with a STAT3 inhibitor results in decreased cell number and increased apoptosis, highlighting the importance of this pathway for malignant cell survival. Importantly, the Koralov lab has developed a mouse model which constitutively expresses a hyperactive STAT3 allele, STAT3C, that recapitulates several key features of MF. To examine the contribution of TOX1overexpression to CTCL pathogenesis, we have introduced Tox1 cDNA downstream of a floxed stop cassette into the ubiquitously expressed Rosa26 locus of C57Bl/6J embryonic stem (ES) cells. We have screened the ES clones to validate the presence of the correctly targeted allele, and we have treated ES clones with TAT-Cre to delete the floxed stop cassette and drive subsequent expression of Tox1 cDNA. We have now generated R26Tox1stopflmice using tetraploid complementation to generate 100% ES cell derived animals. These animals will be crossed to CD4Cre and CD4Cre STAT3Cstopflstrains, thus enabling us to study the contribution of Tox1 overexpression to T cell lymphomagenesis and allowing us to examine synergy between Tox1 overexpression and hyperactive STAT3 signaling in CTCL pathogenesis. We hope that these mice will pave the way to a better understanding of this enigmatic malignancy and allow us to develop a relevant small animal model of this disease.

MiR-181a/b-1 controls thymic selection of Treg cells and tunes their suppressive capacity.

The interdependence of selective cues during development of regulatory T cells (Treg cells) in the thymus and their suppressive function remains incompletely understood. Here, we analyzed this interdependence by taking advantage of highly dynamic changes in expression of microRNA 181 family members miR-181a-1 and miR-181b-1 (miR-181a/b-1) during late T-cell development with very high levels of expression during thymocyte selection, followed by massive down-regulation in the periphery. Loss of miR-181a/b-1 resulted in inefficient de novo generation of Treg cells in the thymus but simultaneously permitted homeostatic expansion in the periphery in the absence of competition. Modulation of T-cell receptor (TCR) signal strength in vivo indicated that miR-181a/b-1 controlled Treg-cell formation via establishing adequate signaling thresholds. Unexpectedly, miR-181a/b-1–deficient Treg cells displayed elevated suppressive capacity in vivo, in line with elevated levels of cytotoxic T-lymphocyte–associated 4 (CTLA-4) protein, but not mRNA, in thymic and peripheral Treg cells. Therefore, we propose that intrathymic miR-181a/b-1 controls development of Treg cells and imposes a developmental legacy on their peripheral function.

Instructional signals for nascent T cells

Thymic Selection Signals relayed through the T cell receptor (TCR)–CD3 complex are critical to determining whether immature T cells in the thymus undergo positive or negative selection. Positive selection enables T cell survival and subsequent export to the periphery, whereas negative selection leads to T cell death. Neier et al. developed a proteomics-based approach to profile proteins bound to the TCR–CD3 complex and compared differences between signaling associated with thymocyte survival and signaling associated with death. The full range of cellular outcomes after thymic selection could be attributed to quantitative differences in a shared core set of biochemical signals. Thus, differences in the affinity of TCRs for peptide–major histocompatibility complex complexes are transduced into downstream signals that are quantitatively variable, but qualitatively similar. Sci. Immunol. 4 , eaal2201 (2019).

Cutting Edge: HDAC3 Protects Double-Positive Thymocytes from P2X7 Receptor-Induced Cell Death.

Intricate life-versus-death decisions are programmed during T cell development, and the regulatory mechanisms that coordinate their activation and repression are still under investigation. In this study, HDAC3-deficient double-positive (DP) thymocytes exhibit a severe decrease in numbers. The thymic cortex is rich in ATP, which is released by macrophages that clear apoptotic DP thymocytes that fail to undergo positive selection. We demonstrate that HDAC3 is required to repress expression of the purinergic receptor P2X7 to prevent DP cell death. HDAC3-deficient DP thymocytes upregulate the P2X7 receptor, increasing sensitivity to ATP-induced cell death. P2rx7/HDAC3-double knockout mice show a partial rescue in DP cell number. HDAC3 directly binds to the P2rx7 enhancer, which is hyperacetylated in the absence of HDAC3. In addition, RORγt binds to the P2rx7 enhancer and promotes P2X7 receptor expression in the absence of HDAC3. Therefore, HDAC3 is a critical regulator of DP thymocyte survival and is required to suppress P2X7 receptor expression.