Activation kinetics and off-target effects of thymus-initiated cre transgenes

Generation and characterization of a tamoxifen-inducible lineage tracing tool Cd2-P2A-CreERT2 knock-in mice

Introduction

The new targeted gene editing technologies, such as the CRISPR/Cas system, enable researchers to insert or delete genes at targeted loci efficiently. The Cre-loxp recombination system is widely used to activate or inactivate genes with high spatial and temporal specificity.

Methods

Using the CRISPR/Cas9 system, we inserted the CreERT2 transgene expression cassette into the Cd2 gene locus to generate conditional Cre-driver line Cd2-CreERT2 knock-in mice, which drove the expression of CreERT2 by the endogenous Cd2 promoter. By mating the Cd2-CreERT2 strain with a Rosa26-LSL-tdTomato reporter mouse strain which contains a tdTomato expression fragment blocked with a loxP-flanked STOP cassette (LSL) driven by a CAG promoter, a Cd2-CreERT2;Rosa26-LSL-tdTomato reporter strain was obtained to evaluate the expression pattern of CD2 in different cell types.

Results

After treatment with tamoxifen, the Cd2-CreERT2 knock-in mice were induced to perform efficient recombination at the loxP site following CreERT2 activation and cause the expression of tdTomato fluorescence. The tdTomato and CD2 were expressed in the T cells of peripheral blood, spleen and mesenteric lymph nodes, whereas detected in a low proportion in the B cells. While about 20% of cells labeled with tamoxifen-induced tdTomato were CD2+ monocytes in peripheral blood, 10% of dendritic cells were tdTomato+/CD2+ cells. Tamoxifen-independent expression of tdTomato occurred in approximately 3% of CD2+ macrophages, but in negligible (~0.5%) in CD2+ granulocytes.

Discussion

This work supplied a new transgenic mouse as a valuable tool for lineage tracing in CD2-expressing cells, for conditional mutant studies of immune modulatory effects in a time-dependent manner, and analysis of the potential therapeutic effect of CD2-targeting biologics.

GRP75-dependent mitochondria-ER contacts ensure cell survival during early mouse thymocyte development

Mitochondria and endoplasmic reticulum contacts (MERCs) control multiple cellular processes, including cell survival and differentiation. Based on the observations that MERCs were specifically enriched in the CD4-CD8- double-negative (DN) stage, we studied their role in early mouse thymocyte development. We found that T cell-specific knockout of Hspa9, which encodes GRP75, a protein that mediates MERC formation by assembling the IP3R-GRP75-VDAC complex, impaired DN3 thymocyte viability and resulted in thymocyte developmental arrest at the DN3-DN4 transition. Mechanistically, GRP75 deficiency induced mitochondrial stress, releasing mitochondrial DNA (mtDNA) into the cytosol and triggering the type I interferon (IFN-I) response. The IFN-I pathway contributed to both the impairment of cell survival and DN3-DN4 transition blockage, while increased lipid peroxidation (LPO) played a major role downstream of IFN-I. Thus, our study identifies the essential role of GRP75-dependent MERCs in early thymocyte development and the governing facts of cell survival and differentiation in the DN stage.

Transcriptional rewiring in CD8+ T cells: implications for CAR-T cell therapy against solid tumours

T cells engineered to express chimeric-antigen receptors (CAR-T cells) can effectively control relapsed and refractory haematological malignancies in the clinic. However, the successes of CAR-T cell therapy have not been recapitulated in solid tumours due to a range of barriers such as immunosuppression, poor infiltration, and tumour heterogeneity. Numerous strategies are being developed to overcome these barriers, which include improving culture conditions and manufacturing protocols, implementing novel CAR designs, and novel approaches to engineering the T cell phenotype. In this review, we describe the various emerging strategies to improve CAR T cell therapy for solid tumours. We specifically focus on new strategies to modulate cell function and fate that have precipitated from the growing knowledge of transcriptional circuits driving T cell differentiation, with the ultimate goal of driving more productive anti-tumour T cell immunity. Evidence shows that enrichment of particular phenotypic subsets of T cells in the initial cell product correlates to improved therapeutic responses and clinical outcomes. Furthermore, T cell exhaustion and poor persistence are major factors limiting therapeutic efficacy. The latest preclinical work shows that targeting specific master regulators and transcription factors can overcome these key barriers, resulting in superior T cell therapeutic products. This can be achieved by targeting key transcriptional circuits promoting memory-like phenotypes or sustaining key effector functions within the hostile tumour microenvironment. Additional discussion points include emerging considerations for the field such as (i) targeting permutations of transcription factors, (ii) transient expression systems, (iii) tissue specificity, and (iv) expanding this strategy beyond CAR-T cell therapy and cancer.

RIPK1 protects naive and regulatory T cells from TNFR1-induced apoptosis

The T cell population size is stringently controlled before, during, and after immune responses, as improper cell death regulation can result in autoimmunity and immunodeficiency. RIPK1 is an important regulator of peripheral T cell survival and homeostasis. However, whether different peripheral T cell subsets show a differential requirement for RIPK1 and which programmed cell death pathway they engage in vivo remains unclear. In this study, we demonstrate that conditional ablation of Ripk1 in conventional T cells (Ripk1ΔCD4) causes peripheral T cell lymphopenia, as witnessed by a profound loss of naive CD4+, naive CD8+, and FoxP3+ regulatory T cells. Interestingly, peripheral naive CD8+ T cells in Ripk1ΔCD4 mice appear to undergo a selective pressure to retain RIPK1 expression following activation. Mixed bone marrow chimeras revealed a competitive survival disadvantage for naive, effector, and memory T cells lacking RIPK1. Additionally, tamoxifen-induced deletion of RIPK1 in CD4-expressing cells in adult life confirmed the importance of RIPK1 in post-thymic survival of CD4+ T cells. Ripk1K45A mice showed no change in peripheral T cell subsets, demonstrating that the T cell lymphopenia was due to the scaffold function of RIPK1 rather than to its kinase activity. Enhanced numbers of Ripk1ΔCD4 naive T cells expressed the proliferation marker Ki-67+ despite the peripheral lymphopenia and single-cell RNA sequencing revealed T cell-specific transcriptomic alterations that were reverted by additional caspase-8 deficiency. Furthermore, Ripk1ΔCD4Casp8 ΔCD4 and Ripk1ΔCD4Tnfr1-/- double-knockout mice rescued the peripheral T cell lymphopenia, revealing that RIPK1-deficient naive CD4+ and CD8+ cells and FoxP3+ regulatory T cells specifically die from TNF- and caspase-8-mediated apoptosis in vivo. Altogether, our findings emphasize the essential role of RIPK1 as a scaffold in maintaining the peripheral T cell compartment and preventing TNFR1-induced apoptosis.

Lkb1 orchestrates γδ T-cell metabolic and functional fitness to control IL-17-mediated autoimmune hepatitis

γδ T cells play a crucial role in immune surveillance and serve as a bridge between innate and adaptive immunity. However, the metabolic requirements and regulation of γδ T-cell development and function remain poorly understood. In this study, we investigated the role of liver kinase B1 (Lkb1), a serine/threonine kinase that links cellular metabolism with cell growth and proliferation, in γδ T-cell biology. Our findings demonstrate that Lkb1 is not only involved in regulating γδ T lineage commitment but also plays a critical role in γδ T-cell effector function. Specifically, T-cell-specific deletion of Lkb1 resulted in impaired thymocyte development and distinct alterations in γδ T-cell subsets in both the thymus and peripheral lymphoid tissues. Notably, loss of Lkb1 inhibited the commitment of Vγ1 and Vγ4 γδ T cells, promoted the maturation of IL-17-producing Vγ6 γδ T cells, and led to the occurrence of fatal autoimmune hepatitis (AIH). Notably, clearance of γδ T cells or blockade of IL-17 significantly attenuated AIH. Mechanistically, Lkb1 deficiency disrupted metabolic homeostasis and AMPK activity, accompanied by increased mTORC1 activation, thereby causing overactivation of γδ T cells and enhanced apoptosis. Interestingly, activation of AMPK or suppression of mTORC1 signaling effectively inhibited IL-17 levels and attenuated AIH in Lkb1-deficient mice. Our findings highlight the pivotal role of Lkb1 in maintaining the homeostasis of γδ T cells and preventing IL-17-mediated autoimmune diseases, providing new insights into the metabolic programs governing the subset determination and functional differentiation of thymic γδ T cells.

Downregulation of Mirlet7 miRNA family promotes Tc17 differentiation and emphysema via de-repression of RORγt

Environmental air irritants including nanosized carbon black (nCB) can drive systemic inflammation, promoting chronic obstructive pulmonary disease (COPD) and emphysema development. The let-7 microRNA (Mirlet7 miRNA) family is associated with IL-17-driven T cell inflammation, a canonical signature of lung inflammation. Recent evidence suggests the Mirlet7 family is downregulated in patients with COPD, however, whether this repression conveys a functional consequence on emphysema pathology has not been elucidated. Here, we show that overall expression of the Mirlet7 clusters, Mirlet7b/Mirlet7c2 and Mirlet7a1/Mirlet7f1/Mirlet7d, are reduced in the lungs and T cells of smokers with emphysema as well as in mice with cigarette smoke (CS)- or nCB-elicited emphysema. We demonstrate that loss of the Mirlet7b/Mirlet7c2 cluster in T cells predisposed mice to exaggerated CS- or nCB-elicited emphysema. Furthermore, ablation of the Mirlet7b/Mirlet7c2 cluster enhanced CD8+IL17a+ T cells (Tc17) formation in emphysema development in mice. Additionally, transgenic mice overexpressing Mirlet7g in T cells are resistant to Tc17 and CD4+IL17a+ T cells (Th17) development when exposed to nCB. Mechanistically, our findings reveal the master regulator of Tc17/Th17 differentiation, RAR-related orphan receptor gamma t (RORγt), as a direct target of Mirlet7 in T cells. Overall, our findings shed light on the Mirlet7/RORγt axis with Mirlet7 acting as a molecular brake in the generation of Tc17 cells and suggest a novel therapeutic approach for tempering the augmented IL-17-mediated response in emphysema.

KMT2D regulates activation, localization, and integrin expression by T-cells

Individuals with Kabuki syndrome present with immunodeficiency; however, how pathogenic variants in the gene encoding the histone-modifying enzyme lysine methyltransferase 2D (KMT2D) lead to immune alterations remain poorly understood. Following up on our prior report of KMT2D-altered integrin expression in B-cells, we performed targeted analyses of KMT2D's influence on integrin expression in T-cells throughout development (thymocytes through peripheral T-cells) in murine cells with constitutive- and conditional-targeted Kmt2d deletion. Using high-throughput RNA-sequencing and flow cytometry, we reveal decreased expression (both at the transcriptional and translational levels) of a cluster of leukocyte-specific integrins, which perturb aspects of T-cell activation, maturation, adhesion/localization, and effector function. H3K4me3 ChIP-PCR suggests that these evolutionary similar integrins are under direct control of KMT2D. KMT2D loss also alters multiple downstream programming/signaling pathways, including integrin-based localization, which can influence T-cell populations. We further demonstrated that KMT2D deficiency is associated with the accumulation of murine CD8+ single-positive (SP) thymocytes and shifts in both human and murine peripheral T-cell populations, including the reduction of the CD4+ recent thymic emigrant (RTE) population. Together, these data show that the targeted loss of Kmt2d in the T-cell lineage recapitulates several distinct features of Kabuki syndrome-associated immune deficiency and implicates epigenetic mechanisms in the regulation of integrin signaling.

Downregulation of Let-7 miRNA promotes Tc17 differentiation and emphysema via de-repression of RORγt

Environmental air irritants including nanosized carbon black (nCB) can drive systemic inflammation, promoting chronic obstructive pulmonary disease (COPD) and emphysema development. The let-7 family of miRNAs is associated with IL-17-driven T cell inflammation, a canonical signature of lung inflammation. Recent evidence suggests the let-7 family is downregulated in patients with COPD, however, whether this repression conveys a functional consequence on emphysema pathology has not been elucidated. Here we show that overall expression of the let-7 miRNA clusters, let-7b/let-7c2 and let-7a1/let-7f1/let-7d, are reduced in the lungs and T cells of smokers with emphysema as well as in mice with cigarette smoke (CS)- or nCB-elicited emphysema. We demonstrate that loss of the let-7b/let-7c2-cluster in T cells predisposed mice to exaggerated CS- or nCB-elicited emphysema. Furthermore, ablation of the let-7b/let-7c2-cluster enhanced CD8+IL17a+ T cells (Tc17) formation in emphysema development in mice. Additionally, transgenic mice overexpressing let-7 in T cells are resistant to Tc17 and CD4+IL17a+ T cells (Th17) development when exposed to nCB. Mechanistically, our findings reveal the master regulator of Tc17/Th17 differentiation, RAR-related orphan receptor gamma t (RORγt), as a direct target of let-7 miRNA in T cells. Overall, our findings shed light on the let-7/RORγt axis with let-7 acting as a molecular brake in the generation of Tc17 cells and suggests a novel therapeutic approach for tempering the augmented IL-17-mediated response in emphysema.

Zfp281 and Zfp148 control CD4+ T cell thymic development and TH2 functions

How CD4+ lineage gene expression is initiated in differentiating thymocytes remains poorly understood. Here, we show that the paralog transcription factors Zfp281 and Zfp148 control both this process and cytokine expression by T helper cell type 2 (TH2) effector cells. Genetic, single-cell, and spatial transcriptomic analyses showed that these factors promote the intrathymic CD4+ T cell differentiation of class II major histocompatibility complex (MHC II)-restricted thymocytes, including expression of the CD4+ lineage-committing factor Thpok. In peripheral T cells, Zfp281 and Zfp148 promoted chromatin opening at and expression of TH2 cytokine genes but not of the TH2 lineage-determining transcription factor Gata3. We found that Zfp281 interacts with Gata3 and is recruited to Gata3 genomic binding sites at loci encoding Thpok and TH2 cytokines. Thus, Zfp148 and Zfp281 collaborate with Gata3 to promote CD4+ T cell development and TH2 cell responses.

Genetic Strategies to Study T Cell Development

Genetics approaches have been instrumental to deciphering T cell development in the thymus, including gene disruption by homologous recombination and more recently Crispr-based gene editing and transgenic gene expression, especially of specific T cell antigen receptors (TCR). This brief chapter describes commonly used tools and strategies to modify the genome of thymocytes, including mouse strains with lineage- and stage-specific expression of the Cre recombinase used for conditional allele inactivation or expressing unique antigen receptor specificities.

T cell-intrinsic protein kinase D3 is dispensable for the cells' activation

Protein kinases D (PKDs) are implicated in T cell receptor (TCR) signaling. Of the two T cell-expressed isoforms PKD2 and PKD3, however, only the former one is rather well understood in this immune cell type. Recently, we have observed a putative hyper-phenotype of T cells from conventional PKD3-knockout mice, which we explained as a secondary effect due to a skewed T cell compartment from naïve towards effector/memory T cells already under steady state conditions. Nonetheless, to this end it is not clear whether these aberrations are mediated by a T cell-intrinsic or -extrinsic function of PKD3. To address this question, we have investigated mice lacking PKD3 specifically in the T cell compartment. We could show that T cells from CD4-Cre-driven conditional knockout mice did not phenocopy the ones from conventional PKD3-knockout mice. In brief, no skewing in the T cell compartment of peripheral lymphoid organs, no hyper-activation upon stimulation in vitro or in vivo as well as no aberrations in follicular helper T cells (T_FH) upon immunization were observed. Hence, although PKD3 is strongly regulated upon TCR stimulation, in T cells this kinase seems to be dispensable for their activation. The described skewing in the T cell compartment of conventional PKD3-deficient mice seems to be mediated by T cell-extrinsic mechanisms, thus once more emphasizing the importance of cell type-specific mouse models.

ETS2 repressor factor (ERF) is involved in T lymphocyte maturation acting as regulator of thymocyte lineage commitment

Thymocyte differentiation and lineage commitment is regulated by an extensive network of transcription factors and signaling molecules among which Erk plays a central role. However, Erk effectors as well as the molecular mechanisms underlying this network are not well understood. Erf is a ubiquitously expressed transcriptional repressor regulated by Erk-dependent phosphorylation. Here, we investigated the role of Erf in T cell maturation and lineage commitment, using a double-fluorescent Erf-floxed mouse to produce thymus-specific Erf knockouts. We observed significant accumulation of thymocytes in the CD4/CD8 DP stage, followed by a significant reduction in CD4SP cells, a trend for lower CD8SP cell frequency, and an elevated percentage of γδ expressing thymocytes in Erf-deficient mice. Also, an elevated number of CD69⁺ TCRβ⁺ cells indicates that thymocytes undergoing positive selection accumulate at this stage. The expression of transcription factors Gata3, ThPOK, and Socs1 that promote CD4⁺ cell commitment was significantly decreased in Erf-deficient mice. These findings suggest that Erf is involved in T cell maturation, acting as a positive regulator during CD4 and eventually CD8 lineage commitment, while negatively regulates the production of γδ T cells. In addition, Erf-deficient mice displayed decreased percentages of CD4⁺ and CD8⁺ splenocytes and elevated levels of IL-4 indicating that Erf may have an additional role in the homeostasis, differentiation, and immunologic response of helper and cytotoxic T cells in the periphery. Overall, our results show, for the first time, Erf's involvement in T cell biology suggesting that Erf acts as a potential regulator during thymocyte maturation and thymocyte lineage commitment, in γδ T cell generation, as well as in Th cell differentiation.

Arid1a promotes thymocyte development through β-selection-dependent and β-selection-independent mechanisms

Early T-cell development from CD4^-  CD8^- double-negative (DN) stage to CD4⁺  CD8⁺ double-positive (DP) stage in the thymus is regulated through multiple steps involving a batch of sequentially expressed factors. Our preliminary data and a recent report showed that AT-rich interaction domain 1A (Arid1a) is required for the transition from DN to DP stages, but the mechanism is not fully understood. In this study, we consolidated that conditional deletion of Arid1a in T-cell lineage intrinsically caused developmental blocks from DN3 to DN4 stages, as well as from DN4 to DP stages using both in vivo adoptive T-cell transfer model and in vitro culture system. The expression of intracellular TCRβ is significantly decreased in Arid1a-deficient DN4 cells compared with WT cells. OT1 transgenic TCR can rescue the defect in the transition from DN3 to DN4 stages, but not from DN to DP stages. Furthermore, we observed a comparable or stronger proliferation capacity accompanied by a significant increase in cell death in Arid1a^-/- DP cells compared with that in WT controls. RNA-Seq analysis shows a significant enrichment of apoptotic pathway within differentially expressed genes between Arid1a^-/- and WT DP cells, including the upregulation of Bim, Casp3 and Trp53 and the downregulation of Rorc, Bcl-XL and Mcl1. Therefore, our study reveals a novel mechanism that Arid1a controls early T-cell development by maintaining intracellular TCRβ expression-mediated β-selection and activating parallel cell survival pathways.

Glycogen synthase kinase 3 drives thymocyte egress by suppressing β-catenin activation of Akt

Molecular pathways controlling emigration of mature thymocytes from thymus to the periphery remain incompletely understood. Here, we show that T cell–specific ablation of glycogen synthase kinase 3 (GSK3) led to severely impaired thymic egress. In the absence of GSK3, β-catenin accumulated in the cytoplasm, where it associated with and activated Akt, leading to phosphorylation and degradation of Foxo1 and downregulation of Klf2 and S1P₁ expression, thereby preventing emigration of thymocytes. A cytoplasmic membrane-localized β-catenin excluded from the nucleus promoted Akt activation, suggesting a new function of β-catenin independent of its role as a transcriptional activator. Furthermore, genetic ablation of β-catenin, retroviral expression of a dominant negative Akt mutant, and transgenic expression of a constitutively active Foxo1 restored emigration of GSK3-deficient thymocytes. Our findings establish an essential role for GSK3 in thymocyte egress and reveal a previously unidentified signaling function of β-catenin in the cytoplasm.

Nuclear receptor LXRβ controls fitness and functionality of activated T cells

T cells increase cholesterol biosynthesis upon activation to generate substrates for cellular growth and proliferation. The ubiquitously expressed liver X receptor β (LXRβ) encoded by the Nr1h2 gene is a critical regulator of cholesterol homeostasis in mammalian cells; however, its cell-intrinsic role in T cell biology remains poorly understood. We report that ablation of LXRβ in T cells leads to spontaneous T cell activation and T lymphocytopenia. Unexpectedly, analysis of mixed bone marrow chimeric mice revealed a cell-autonomous survival defect that reduced the fitness of LXRβ-deficient effector T cells, suggesting that the heightened immune activation in mice harboring LXRβ-deficient T cells was due to impaired regulatory T (T reg) cell functionality. Indeed, we found that single-copy deletion of Nr1h2 in T reg cells disrupted activated T reg cell metabolism and fitness and resulted in early-onset fatal autoimmune disease. Our study demonstrated an indispensable requirement for T reg cell–intrinsic LXRβ function in immune homeostasis and provides a basis for immunological therapies through targeting of this receptor.

Distal Lck Promoter-Driven Cre Shows Cell Type-Specific Function in Innate-like T Cells

Innate-like T cells, including invariant NKT cells, mucosal-associated invariant T (MAIT) cells, and γ δ T (γδT) cells, are groups of unconventional T lymphocytes. They play important roles in the immune system. Because of the lack of Cre recombinase lines that are specific for innate-like T cells, pan-T cell Cre lines are often used to study innate-like T cells. In this study, we found that distal Lck promoter-driven Cre (dLckCre) in which the distal Lck gene promoter drives Cre expression in the late stage of thymocyte development has limited function in the innate-like T cells using ROSA26floxed-Stop-tdTomato reporter. Innate-like T cells differentiate into mature functional subsets comparable to the CD4+ Th subsets under homeostatic conditions. We further showed that dLckCre-expressing γδT cells are strongly biased toward γδT1 phenotype. Interestingly, the γδT cells residing in the epidermis and comprising the vast majority of dendritic epidermal T cells nearly all express dLckCre, indicating dLckCre is a useful tool for studying dendritic epidermal T cells. Taken together, these data suggest that Lck distal promoter has different activity in the conventional and unconventional T cells. The use of dLCKcre transgenic mice in the innate-like T cells needs to be guided by a reporter for the dLckCre function.

Notch-induced endoplasmic reticulum-associated degradation governs mouse thymocyte β-selection

Signals from the pre-T cell receptor and Notch coordinately instruct β-selection of CD4-CD8-double negative (DN) thymocytes to generate αβ T cells in the thymus. However, how these signals ensure a high-fidelity proteome and safeguard the clonal diversification of the pre-selection TCR repertoire given the considerable translational activity imposed by β-selection is largely unknown. Here, we identify the endoplasmic reticulum (ER)-associated degradation (ERAD) machinery as a critical proteostasis checkpoint during β-selection. Expression of the SEL1L-HRD1 complex, the most conserved branch of ERAD, is directly regulated by the transcriptional activity of the Notch intracellular domain. Deletion of Sel1l impaired DN3 to DN4 thymocyte transition and severely impaired mouse αβ T cell development. Mechanistically, Sel1l deficiency induced unresolved ER stress that triggered thymocyte apoptosis through the PERK pathway. Accordingly, genetically inactivating PERK rescued T cell development from Sel1l-deficient thymocytes. In contrast, IRE1α/XBP1 pathway was induced as a compensatory adaptation to alleviate Sel1l-deficiency-induced ER stress. Dual loss of Sel1l and Xbp1 markedly exacerbated the thymic defect. Our study reveals a critical developmental signal controlled proteostasis mechanism that enforces T cell development to ensure a healthy adaptive immunity.

BHLHE40 Regulates IL-10 and IFN-γ Production in T Cells but Does Not Interfere With Human Type 1 Regulatory T Cell Differentiation

Type 1 regulatory T (Tr1) cells are subset of peripherally induced antigen-specific regulatory T cells. IL-10 signaling has been shown to be indispensable for polarization and function of Tr1 cells. However, the transcriptional machinery underlying human Tr1 cell differentiation and function is not yet elucidated. To this end, we performed RNA sequencing on ex vivo human CD49b⁺LAG3⁺ Tr1 cells. We identified the transcription factor, BHLHE40, to be highly expressed in Tr1 cells. Even though Tr1 cells characteristically produce high levels of IL-10, we found that BHLHE40 represses IL-10 and increases IFN-γ secretion in naïve CD4⁺ T cells. Through CRISPR/Cas9-mediated knockout, we determined that IL10 significantly increased in the sgBHLHE40-edited cells and BHLHE40 is dispensable for naïve CD4⁺ T cells to differentiate into Tr1 cells in vitro. Interestingly, BHLHE40 overexpression induces the surface expression of CD49b and LAG3, co-expressed surface molecules attributed to Tr1 cells, but promotes IFN-γ production. Our findings uncover a novel mechanism whereby BHLHE40 acts as a regulator of IL-10 and IFN-γ in human CD4⁺ T cells.

Piezo1 channels restrain regulatory T cells but are dispensable for effector CD4+ T cell responses

T lymphocytes encounter complex mechanical cues during an immune response. The mechanosensitive ion channel, Piezo1, drives inflammatory responses to bacterial infections, wound healing, and cancer; however, its role in helper T cell function remains unclear. In an animal model for multiple sclerosis, experimental autoimmune encephalomyelitis (EAE), we found that mice with genetic deletion of Piezo1 in T cells showed diminished disease severity. Unexpectedly, Piezo1 was not essential for lymph node homing, interstitial motility, Ca²⁺ signaling, T cell proliferation, or differentiation into proinflammatory T helper 1 (T_H1) and T_H17 subsets. However, Piezo1 deletion in T cells resulted in enhanced transforming growth factor-β (TGFβ) signaling and an expanded pool of regulatory T (T_reg) cells. Moreover, mice with deletion of Piezo1 specifically in T_reg cells showed significant attenuation of EAE. Our results indicate that Piezo1 selectively restrains T_reg cells, without influencing activation events or effector T cell functions.

Invalidation of the Transcriptional Modulator of Lipid Metabolism PPARβ/δ in T Cells Prevents Age-Related Alteration of Body Composition and Loss of Endurance Capacity

Anti-inflammatory regulatory T cells (Tregs) are the most metabolically flexible CD4⁺ T cells by using both glycolysis and fatty acid oxidation (FAO) which allow them to migrate in tissues. With aging, Tregs accumulate in secondary lymphoid organs and are involved in impairment of skeletal muscle (SKM) regeneration and mass maintenance. In this study, we showed that a deletion of a FAO modulator, peroxisome proliferator-activated receptor beta/delta (PPARβ/δ), specifically in T cells (KO-T PPARβ/δ), increased the number of CD4⁺ T cells at day 2 following a cardiotoxin-induced SKM regeneration. Older KO-T PPARβ/δ mice maintained a Tregs prevalence in lymph nodes similar to young mice. Surprisingly, KO-T PPARβ/δ mice were protected from the effects of age on lean and fat mass and endurance capacity. Our results lead us to propose an original potential role of T cell metabolism in the effects of aging on the maintenance of body composition and endurance capacity.

From thymus to periphery: Molecular basis of effector γδ-T cell differentiation

The contributions of γδ T cells to immune (patho)physiology in many pre-clinical mouse models have been associated with their rapid and abundant provision of two critical cytokines, interferon-γ (IFN-γ) and interleukin-17A (IL-17). These are typically produced by distinct effector γδ T cell subsets that can be segregated on the basis of surface expression levels of receptors such as CD27, CD44 or CD45RB, among others. Unlike conventional T cells that egress the thymus as naïve lymphocytes awaiting further differentiation upon activation, a large fraction of murine γδ T cells commits to either IFN-γ or IL-17 expression during thymic development. However, extrathymic signals can both regulate pre-programmed γδ T cells; and induce peripheral differentiation of naïve γδ T cells into effectors. Here we review the key cellular events of "developmental pre-programming" in the mouse thymus; and the molecular basis for effector function maintenance vs plasticity in the periphery. We highlight some of our contributions towards elucidating the role of T cell receptor, co-receptors (like CD27 and CD28) and cytokine signals (such as IL-1β and IL-23) in these processes, and the various levels of gene regulation involved, from the chromatin landscape to microRNA-based post-transcriptional control of γδ T cell functional plasticity.

Differential expression of tissue-restricted antigens among mTEC is associated with distinct autoreactive T cell fates

Medullary thymic epithelial cells (mTEC) contribute to the development of T cell tolerance by expressing and presenting tissue-restricted antigens (TRA), so that developing T cells can assess the self-reactivity of their antigen receptors prior to leaving the thymus. mTEC are a heterogeneous population of cells that differentially express TRA. Whether mTEC subsets induce distinct autoreactive T cell fates remains unclear. Here, we establish bacterial artificial chromosome (BAC)-transgenic mouse lines with biased mTEClo or mTEChi expression of model antigens. The transgenic lines support negative selection of antigen-specific thymocytes depending on antigen dose. However, model antigen expression predominantly by mTEClo supports TCRαβ+ CD8αα intraepithelial lymphocyte development; meanwhile, mTEChi-restricted expression preferentially induces Treg differentiation of antigen-specific cells in these models to impact control of infectious agents and tumor growth. In summary, our data suggest that mTEC subsets may have a function in directing distinct mechanisms of T cell tolerance.

Ovariectomy Activates Chronic Low-Grade Inflammation Mediated by Memory T Cells, Which Promotes Osteoporosis in Mice

The loss of estrogen (E₂ ) initiates a rapid phase of bone loss leading to osteoporosis in one-half of postmenopausal women, but the mechanism is not fully understood. Here, we show for the first time how loss of E₂ activates low-grade inflammation to promote the acute phase of bone catabolic activity in ovariectomized (OVX) mice. E₂ regulates the abundance of dendritic cells (DCs) that express IL-7 and IL-15 by inducing the Fas ligand (FasL) and apoptosis of the DC. In the absence of E₂ , DCs become long-lived, leading to increased IL-7 and IL-15. We find that IL-7 and IL-15 together, but not alone, induced antigen-independent production of IL-17A and TNFα in a subset of memory T cells (T_MEM ). OVX of mice with T-cell-specific ablation of IL15RA showed no IL-17A and TNFα expression, and no increase in bone resorption or bone loss, confirming the role of IL-15 in activating the T_MEM and the need for inflammation. Our results provide a new mechanism by which E₂ regulates the immune system, and how menopause leads to osteoporosis. The low-grade inflammation is likely to cause or contribute to other comorbidities observed postmenopause. © 2020 American Society for Bone and Mineral Research.

Transcriptional modulation of the T helper 17/interleukin 17 axis ameliorates renal ischemia-reperfusion injury

BACKGROUND

Signal transducer and activator of transcription 3 (STAT3) is a latent transcription factor critical for T-cell function. Although inhibition of the Janus kinase 2 (JAK2)/STAT3 pathway has been reported to be protective against ischemia-reperfusion injury (IRI), the role of T cell-associated STAT3 in the pathogenesis of renal IRI has not been specifically defined.

METHODS

We induced renal IRI in both mice with T cell-specific STAT3 knockout (Lck-Cre;STAT3flox/flox) and wild-type controls (C57BL/6) and assessed renal damage and inflammation at 48 h after IRI. Human proximal tubular epithelial cells grown under hypoxia were treated with a JAK2 inhibitor, caffeic acid 3,4-dihydroxy-phenylethyl ester, to determine the effect of JAK2/STAT3 inhibition on renal epithelia. Independently, we disrupted Cln 3-requiring 9 (Ctr9) to inhibit T helper 17 (Th17) activation via RNA interference and determined if Ctr9 inhibition aggravates renal injury through upregulated Th17 activation.

RESULTS

The Lck-Cre;STAT3flox/flox mice exhibited significantly reduced kidney damage compared with controls. This protective effect was associated with reduced intrarenal Th17 infiltration and proinflammatory cytokines. Human proximal tubular epithelial cells under hypoxia exhibited significant upregulation of interleukin 17 receptors, and pharmacologic inhibition of JAK2 significantly ameliorated this change. RNA interference with Ctr9 in splenocytes enhanced differentiation into Th17 cells. In vivo knockdown of Ctr9 in mice with renal IRI further aggravated Th17-associated inflammation and kidney injury.

CONCLUSIONS

STAT3 in T cells contributes to renal IRI through Th17 activation. Inhibition of Ctr9 further enhances Th17 activation and aggravates kidney injury, further supporting the role of Th17 cells in renal IRI.

Transforming Growth Factor-beta signaling in αβ thymocytes promotes negative selection

In the thymus, the T lymphocyte repertoire is purged of a substantial portion of highly self-reactive cells. This negative selection process relies on the strength of TCR-signaling in response to self-peptide-MHC complexes, both in the cortex and medulla regions. However, whether cytokine-signaling contributes to negative selection remains unclear. Here, we report that, in the absence of Transforming Growth Factor beta (TGF-β) signaling in thymocytes, negative selection is significantly impaired. Highly autoreactive thymocytes first escape cortical negative selection and acquire a Th1-like-phenotype. They express high levels of CXCR3, aberrantly accumulate at the cortico-medullary junction and subsequently fail to sustain AIRE expression in the medulla, escaping medullary negative selection. Highly autoreactive thymocytes undergo an atypical maturation program, substantially accumulate in the periphery and induce multiple organ-autoimmune-lesions. Thus, these findings reveal TGF-β in thymocytes as crucial for negative selection with implications for understanding T cell self-tolerance mechanisms.

Defective induction of the proteasome associated with T-cell receptor signaling underlies T-cell senescence

The proteasome degradation machinery is essential for a variety of cellular processes including senescence and T-cell immunity. Decreased proteasome activity is associated with the aging process; however, the regulation of the proteasome in CD4⁺ T cells in relation to aging is unclear. Here, we show that defects in the induction of the proteasome in CD4⁺ T cells upon T-cell receptor (TCR) stimulation underlie T-cell senescence. Proteasome dysfunction promotes senescence-associated phenotypes, including defective proliferation, cytokine production and increased levels of PD-1⁺ CD44^High CD4⁺ T cells. Proteasome induction by TCR signaling via MEK-, IKK- and calcineurin-dependent pathways is attenuated with age and decreased in PD-1⁺ CD44^High CD4⁺ T cells, the proportion of which increases with age. Our results indicate that defective induction of the proteasome is a hallmark of CD4⁺ T-cell senescence.

Cosmc is required for T cell persistence in the periphery

T lymphocytes, a key arm of adaptive immunity, are known to dynamically regulate O-glycosylation during T cell maturation and when responding to stimuli; however, the direct role of O-glycans in T cell maturation remains largely unknown. Using a conditional knockout of the gene (C1GalT1C1 or Cosmc) encoding the specific chaperone Cosmc, we generated mice whose T cells lack extended O-glycans (T cell conditional Cosmc knock out or TCKO mice) and homogeneously express the truncated Tn antigen. Loss of Cosmc is highly deleterious to T cell persistence, with near-complete elimination of Cosmc-null T cells from spleen and lymph nodes. Total T cell counts are 20% of wild type (WT), among which only 5% express the truncated glycans, with the remaining 95% consisting of escapers from Cre-mediated recombination. TCKO thymocytes were able to complete thymic maturation but failed to populate the secondary lymphoid organs both natively and upon adoptive transfer to WT recipients. Our results demonstrate that extended O-glycosylation is required for the establishment and maintenance of the peripheral T cell population.

Conditional Deletion of the V-ATPase a2-Subunit Disrupts Intrathymic T Cell Development

Proper orchestration of T lymphocyte development is critical, as T cells underlie nearly all responses of the adaptive immune system. Developing thymocytes differentiate in response to environmental cues carried from cell surface receptors to the nucleus, shaping a distinct transcriptional program that defines their developmental outcome. Our recent work has identified a previously undescribed role for the vacuolar ATPase (V-ATPase) in facilitating the development of murine thymocytes progressing toward the CD4⁺ and CD8⁺ αβ T cell lineages. Vav1^Cre recombinase-mediated deletion of the a2 isoform of the V-ATPase (a2V) in mouse hematopoietic cells leads to a specific and profound loss of peripheral CD4⁺ and CD8⁺ αβ T cells. Utilizing T cell-restricted Lck^Cre and CD4^Cre strains, we further traced this deficiency to the thymus and found that a2V plays a cell-intrinsic role throughout intrathymic development. Loss of a2V manifests as a partial obstruction in the double negative stage of T cell development, and later, a near complete failure of positive selection. These data deepen our understanding of the biological mechanisms that orchestrate T cell development and lend credence to the recent focus on V-ATPase as a potential chemotherapeutic target to combat proliferative potential in T cell lymphoblastic leukemias and autoimmune disease.

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 Ppp2ca ^flox/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.

Suppression of ILC2 differentiation from committed T cell precursors by E protein transcription factors

Qian et al. shows that ILC2s can be generated from not only thymic multipotent progenitors but also committed T cell precursors. These processes are greatly suppressed by E protein transcription factors. Thymic ILC2s show functional differences from those made elsewhere.

Current models propose that group 2 innate lymphoid cells (ILC2s) are generated in the bone marrow. Here, we demonstrate that subsets of these cells can differentiate from multipotent progenitors and committed T cell precursors in the thymus, both in vivo and in vitro. These thymic ILC2s exit the thymus, circulate in the blood, and home to peripheral tissues. Ablation of E protein transcription factors greatly promotes the ILC fate while impairing B and T cell development. Consistently, a transcriptional network centered on the ZBTB16 transcription factor and IL-4 signaling pathway is highly up-regulated due to E protein deficiency. Our results show that ILC2 can still arise from what are normally considered to be committed T cell precursors, and that this alternative cell fate is restrained by high levels of E protein activity in these cells. Thymus-derived lung ILC2s of E protein–deficient mice show different transcriptomes, proliferative properties, and cytokine responses from wild-type counterparts, suggesting potentially distinct functions.

FBXO7 sensitivity of phenotypic traits elucidated by a hypomorphic allele

FBXO7 encodes an F box containing protein that interacts with multiple partners to facilitate numerous cellular processes and has a canonical role as part of an SCF E3 ubiquitin ligase complex. Mutation of FBXO7 is responsible for an early onset Parkinsonian pyramidal syndrome and genome-wide association studies have linked variants in FBXO7 to erythroid traits. A putative orthologue in Drosophila, nutcracker, has been shown to regulate the proteasome, and deficiency of nutcracker results in male infertility. Therefore, we reasoned that modulating Fbxo7 levels in a murine model could provide insights into the role of this protein in mammals. We used a targeted gene trap model which retained 4-16% residual gene expression and assessed the sensitivity of phenotypic traits to gene dosage. Fbxo7 hypomorphs showed regenerative anaemia associated with a shorter erythrocyte half-life, and male mice were infertile. Alterations to T cell phenotypes were also observed, which intriguingly were both T cell intrinsic and extrinsic. Hypomorphic mice were also sensitive to infection with Salmonella, succumbing to a normally sublethal challenge. Despite these phenotypes, Fbxo7 hypomorphs were produced at a normal Mendelian ratio with a normal lifespan and no evidence of neurological symptoms. These data suggest that erythrocyte survival, T cell development and spermatogenesis are particularly sensitive to Fbxo7 gene dosage.

VDAC2 enables BAX to mediate apoptosis and limit tumor development

Intrinsic apoptosis is critical to prevent tumor formation and is engaged by many anti-cancer agents to eliminate tumor cells. BAX and BAK, the two essential mediators of apoptosis, are thought to be regulated through similar mechanisms and act redundantly to drive apoptotic cell death. From an unbiased genome-wide CRISPR/Cas9 screen, we identified VDAC2 (voltage-dependent anion channel 2) as important for BAX, but not BAK, to function. Genetic deletion of VDAC2 abrogated the association of BAX and BAK with mitochondrial complexes containing VDAC1, VDAC2, and VDAC3, but only inhibited BAX apoptotic function. Deleting VDAC2 phenocopied the loss of BAX in impairing both the killing of tumor cells by anti-cancer agents and the ability to suppress tumor formation. Together, our studies show that efficient BAX-mediated apoptosis depends on VDAC2, and reveal a striking difference in how BAX and BAK are functionally impacted by their interactions with VDAC2.

Peroxisome Proliferator-Activated Receptor-δ Supports the Metabolic Requirements of Cell Growth in TCRβ-Selected Thymocytes and Peripheral CD4+ T Cells

During T cell development, progenitor thymocytes undergo a large proliferative burst immediately following successful TCRβ rearrangement, and defects in genes that regulate this proliferation have a profound effect on thymus cellularity and output. Although the signaling pathways that initiate cell cycling and nutrient uptake after TCRβ selection are understood, less is known about the transcriptional programs that regulate the metabolic machinery to promote biomass accumulation during this process. In this article, we report that mice with whole body deficiency in the nuclear receptor peroxisome proliferator-activated receptor-δ (PPARδ^mut) exhibit a reduction in spleen and thymus cellularity, with a decrease in thymocyte cell number starting at the double-negative 4 stage of thymocyte development. Although in vivo DNA synthesis was normal in PPARδ^mut thymocytes, studies in the OP9-delta-like 4 in vitro system of differentiation revealed that PPARδ^mut double-negative 3 cells underwent fewer cell divisions. Naive CD4⁺ T cells from PPARδ^mut mice also exhibited reduced proliferation upon TCR and CD28 stimulation in vitro. Growth defects in PPAR-δ-deficient thymocytes and peripheral CD4⁺ T cells correlated with decreases in extracellular acidification rate, mitochondrial reserve, and expression of a host of genes involved in glycolysis, oxidative phosphorylation, and lipogenesis. By contrast, mice with T cell-restricted deficiency of Ppard starting at the double-positive stage of thymocyte development, although exhibiting defective CD4⁺ T cell growth, possessed a normal T cell compartment, pointing to developmental defects as a cause of peripheral T cell lymphopenia in PPARδ^mut mice. These findings implicate PPAR-δ as a regulator of the metabolic program during thymocyte and T cell growth.

Regulation of Hematopoietic Cell Development and Function Through Phosphoinositides

One of the most paramount receptor-induced signal transduction mechanisms in hematopoietic cells is production of the lipid second messenger phosphatidylinositol(3,4,5)trisphosphate (PIP₃) by class I phosphoinositide 3 kinases (PI3K). Defective PIP₃ signaling impairs almost every aspect of hematopoiesis, including T cell development and function. Limiting PIP₃ signaling is particularly important, because excessive PIP₃ function in lymphocytes can transform them and cause blood cancers. Here, we review the key functions of PIP₃ and related phosphoinositides in hematopoietic cells, with a special focus on those mechanisms dampening PIP₃ production, turnover, or function. Recent studies have shown that beyond “canonical” turnover by the PIP₃ phosphatases and tumor suppressors phosphatase and tensin homolog (PTEN) and SH2 domain-containing inositol-5-phosphatase-1 (SHIP-1/2), PIP₃ function in hematopoietic cells can also be dampened through antagonism with the soluble PIP₃ analogs inositol(1,3,4,5)tetrakisphosphate (IP₄) and inositol-heptakisphosphate (IP₇). Other evidence suggests that IP₄ can promote PIP₃ function in thymocytes. Moreover, IP₄ or the kinases producing it limit store-operated Ca²⁺ entry through Orai channels in B cells, T cells, and neutrophils to control cell survival and function. We discuss current models for how soluble inositol phosphates can have such diverse functions and can govern as distinct processes as hematopoietic stem cell homeostasis, neutrophil macrophage and NK cell function, and development and function of B cells and T cells. Finally, we will review the pathological consequences of dysregulated IP₄ activity in immune cells and highlight contributions of impaired inositol phosphate functions in disorders such as Kawasaki disease, common variable immunodeficiency, or blood cancer.

HDAC4 preserves skeletal muscle structure following long-term denervation by mediating distinct cellular responses

BACKGROUND

Denervation triggers numerous molecular responses in skeletal muscle, including the activation of catabolic pathways and oxidative stress, leading to progressive muscle atrophy. Histone deacetylase 4 (HDAC4) mediates skeletal muscle response to denervation, suggesting the use of HDAC inhibitors as a therapeutic approach to neurogenic muscle atrophy. However, the effects of HDAC4 inhibition in skeletal muscle in response to long-term denervation have not been described yet.

METHODS

To further study HDAC4 functions in response to denervation, we analyzed mutant mice in which HDAC4 is specifically deleted in skeletal muscle.

RESULTS

After an initial phase of resistance to neurogenic muscle atrophy, skeletal muscle with a deletion of HDAC4 lost structural integrity after 4 weeks of denervation. Deletion of HDAC4 impaired the activation of the ubiquitin-proteasome system, delayed the autophagic response, and dampened the OS response in skeletal muscle. Inhibition of the ubiquitin-proteasome system or the autophagic response, if on the one hand, conferred resistance to neurogenic muscle atrophy; on the other hand, induced loss of muscle integrity and inflammation in mice lacking HDAC4 in skeletal muscle. Moreover, treatment with the antioxidant drug Trolox prevented loss of muscle integrity and inflammation in in mice lacking HDAC4 in skeletal muscle, despite the resistance to neurogenic muscle atrophy.

CONCLUSIONS

These results reveal new functions of HDAC4 in mediating skeletal muscle response to denervation and lead us to propose the combined use of HDAC inhibitors and antioxidant drugs to treat neurogenic muscle atrophy.

Defining the molecular basis of oncogenic cooperation between TAL1 expression and Pten deletion in T-ALL using a novel pro-T-cell model system

T-cell acute lymphoblastic leukemia (T-ALL) is caused by the accumulation of multiple mutations combined with the ectopic expression of transcription factors in developing T cells. However, the molecular basis underlying cooperation between transcription factor expression and additional oncogenic mutations in driving T-ALL has been difficult to assess due to limited robust T-cell model systems. Here we utilize a new ex vivo pro-T-cell model to study oncogenic cooperation. Using a systems biological approach we first dissect the pro-T-cell signaling network driven by interleukin-7, stem cell factor and Notch1 and identify key downstream Akt, Stat, E2f and Myc genetic signaling networks. Next, this pro-T-cell system was used to demonstrate that ectopic expression of the TAL1 transcription factor and Pten deletion are bona-fide cooperating events resulting in an increased stem cell signature, upregulation of a specific E2f signaling network and metabolic reprogramming with higher influx of glucose carbons into the tricarboxylic acid cycle. This ex vivo pro-T-cell system thereby provides a powerful new model system to investigate how normal T-cell signaling networks are perturbed and/or hijacked by different oncogenic events found in T-ALL.

DNA-binding of the Tet-transactivator curtails antigen-induced lymphocyte activation in mice

The Tet-On/Off system for conditional transgene expression constitutes state-of-the-art technology to study gene function by facilitating inducible expression in a timed and reversible manner. Several studies documented the suitability and versatility of this system to trace lymphocyte fate and to conditionally express oncogenes or silence tumour suppressor genes in vivo. Here, we show that expression of the tetracycline/doxycycline-controlled Tet-transactivator, while tolerated well during development and in immunologically unchallenged animals, impairs the expansion of antigen-stimulated T and B cells and thereby curtails adaptive immune responses in vivo. Transactivator-mediated cytotoxicity depends on DNA binding, but can be overcome by BCL2 overexpression, suggesting that apoptosis induction upon lymphocyte activation limits cellular and humoral immune responses. Our findings suggest a possible system-intrinsic biological bias of the Tet-On/Off system in vivo that will favour the outgrowth of apoptosis resistant clones, thus possibly confounding data published using such systems.

Tet-transactivators are used for direct regulation of gene expression, RNA interference and for CRISPR/Cas9-based systems. Here the authors show that DNA-bound Tet-transactivators can induce cell death in antigen-activated lymphocytes in vivo, putting into question the use of, and in vivo data generated with, these molecular tools.

Smyd1C Mediates CD8 T Cell Death via Regulation of Bcl2-Mediated Restriction of outer Mitochondrial Membrane Integrity

The SET and Mynd domain 1 (Smyd1) locus encodes three tissue-restricted isoforms. Two previously characterized isoforms, Smyd1A and Smyd1B, are heart and skeletal muscle-restricted histone methyl transferases. Here we report that a third, non-catalytic isoform, Smyd1C, is expressed predominantly in activated CD8 T cells. While Smyd1C- deficient CD8 T cells undergo activation-induced apoptosis, neither of two classical mechanisms activation-induced cell death nor activated cell autonomous death are utilized. Instead, Smyd1C accumulates within both mitochondria and the immunological synapse where it associates with Bcl-2, FK506-Binding Protein 8/38 (FKBP38) and Calcineurin. This complex maintains Bcl-2 phosphorylation, enhanced mitochondrial localization, and restricted apoptosis of activated CD8 T cells. We suggest that CD8 T cell death is governed, in part, by Smyd1C regulation of Bcl2-mediated restriction of outer mitochondrial membrane integrity.

Grb2-Mediated Recruitment of USP9X to LAT Enhances Themis Stability following Thymic Selection

Themis is a new component of the TCR signaling machinery that plays a critical role during T cell development. The positive selection of immature CD4⁺CD8⁺ double-positive thymocytes and their commitment to the CD4⁺CD8^- single-positive stage are impaired in Themis^-/- mice, suggesting that Themis might be important to sustain TCR signals during these key developmental processes. However, the analysis of Themis mRNA levels revealed that Themis gene expression is rapidly extinguished during positive selection. We show in this article that Themis protein expression is increased in double-positive thymocytes undergoing positive selection and is sustained in immature single-positive thymocytes, despite the strong decrease in Themis mRNA levels in these subsets. We found that Themis stability is controlled by the ubiquitin-specific protease USP9X, which removes ubiquitin K48-linked chains on Themis following TCR engagement. Biochemical analyses indicate that USP9X binds directly to the N-terminal CABIT domain of Themis and indirectly to the adaptor protein Grb2, with the latter interaction enabling recruitment of Themis/USP9X complexes to LAT, thereby sustaining Themis expression following positive selection. Together, these data suggest that TCR-mediated signals enhance Themis stability upon T cell development and identify USP9X as a key regulator of Themis protein turnover.

PTPN2 regulates T cell lineage commitment and αβ versus γδ specification

During early thymocyte development, coordinated JAK/STAT5 and SFK/pre-TCR signaling is critical for T cell lineage commitment and αβ versus γδ specification. Wiede et al. show a role for the tyrosine phosphatase PTPN2 in attenuating SRC family kinase LCK and STAT5 signaling to regulate αβ and γδ T cell development.

In the thymus, hematopoietic progenitors commit to the T cell lineage and undergo sequential differentiation to generate diverse T cell subsets, including major histocompatibility complex (MHC)–restricted αβ T cell receptor (TCR) T cells and non–MHC-restricted γδ TCR T cells. The factors controlling precursor commitment and their subsequent maturation and specification into αβ TCR versus γδ TCR T cells remain unclear. Here, we show that the tyrosine phosphatase PTPN2 attenuates STAT5 (signal transducer and activator of transcription 5) signaling to regulate T cell lineage commitment and SRC family kinase LCK and STAT5 signaling to regulate αβ TCR versus γδ TCR T cell development. Our findings identify PTPN2 as an important regulator of critical checkpoints that dictate the commitment of multipotent precursors to the T cell lineage and their subsequent maturation into αβ TCR or γδ TCR T cells.

T-cell autonomous death induced by regeneration of inert glucocorticoid metabolites

Glucocorticoids (GC) are essential regulators of T-cell development and function. Activation of the immune system increases systemic adrenal-derived GC levels which downregulate immune activity as part of a negative feedback control system. Increasing evidence shows, however, that GC can also be derived from extra-adrenal sources such as the thymus or intestine, thus providing local control of GC-mediated effects. The thymus reportedly produces GC, but whether thymic epithelial cells or thymocytes produce GC acting either in an autocrine or paracrine fashion is not clear. We studied the expression of two main enzymes involved in de novo GC synthesis, CYP11A1 and CYP11B1, as well as the expression and activity of HSD11B1, an enzyme catalyzing interconversion of inert GC metabolites with active GC. While we found no evidence of de novo GC synthesis in both thymocytes and peripheral T cells, abundant regeneration of GC from the inactive metabolite 11-dehydrocorticosterone was detectable. Irrespective of their maturation stage, T cells that produced GC in this manner undergo autonomous cell death as this was blocked when glucocorticoid receptor-deficient T cells were treated with GC metabolites. These results indicate that both immature and mature T cells possess the capacity to undergo apoptosis in response to intrinsically generated GC. Consequently, positive selection of thymocytes, as well as survival of peripheral T cells may depend on TCR-induced escape of otherwise HSD11B1-driven autonomous T-cell death.

Progressive Changes in CXCR4 Expression That Define Thymocyte Positive Selection Are Dispensable For Both Innate and Conventional αβT-cell Development

The ordered migration of immature thymocytes through thymic microenvironments generates both adaptive MHC restricted αβT-cells and innate CD1d-restricted iNKT-cells. While several chemokine receptors and ligands control multiple stages of this process, their involvement during early thymocyte development often precludes direct analysis of potential roles during later developmental stages. For example, because of early lethality of CXCR4-/- mice, and stage-specific requirements for CXCR4 in thymus colonisation and pre-TCR mediated selection, its role in thymic positive selection is unclear. Here we have examined CXCR4-CXCL12 interactions during the maturation of CD4+CD8+ thymocytes, including downstream stages of iNKT and αβT-cell development. We show CXCL12 expression is a common feature of cortical thymic epithelial cells, indicating widespread availability throughout the cortex. Moreover, CXCR4 expression by CD4+CD8+ pre-selection thymocytes is progressively downregulated following both MHC and CD1d-restricted thymic selection events. However, using CD4Cre-mediated deletion to bypass its involvement in CD4-CD8- thymocyte development, we show CXCR4 is dispensable for the maintenance and intrathymic positioning of CD4+CD8+ thymocytes, and their ability to generate mature αβT-cells and CD1d-restricted iNKT-cells. Collectively, our data define dynamic changes in CXCR4 expression as a marker for intrathymic selection events, and show its role in T-cell development is restricted to pre-CD4+CD8+ stages.

A Histone Methyltransferase ESET Is Critical for T Cell Development

ESET/SETDB1, one of the major histone methyltransferases, catalyzes histone 3 lysine 9 (H3K9) trimethylation. ESET is critical for suppressing expression of retroviral elements in embryonic stem cells; however, its role in the immune system is not known. We found that thymocyte-specific deletion of ESET caused impaired T cell development, with CD8 lineage cells being most severely affected. Increased apoptosis of CD8 single-positive cells was observed, and TCR-induced ERK activation was severely inhibited in ESET(-/-) thymocytes. Genome-wide comprehensive analysis of mRNA expression and H3K9 trimethylation revealed that ESET regulates expression of numerous genes in thymocytes. Among them, FcγRIIB, whose signaling can inhibit ERK activation, was strongly and ectopically expressed in ESET(-/-) thymocytes. Indeed, genetic depletion of FcγRIIB in ESET(-/-) thymocytes rescued impaired ERK activation and partially restored defective positive selection in ESET(-/-) mice. Therefore, impaired T cell development in ESET(-/-) mice is partly due to the aberrant expression of FcγRIIB. Collectively, to our knowledge, we identify ESET as the first trimethylated H3K9 histone methyltransferase playing a crucial role in T cell development.

lck-Driven Cre Expression Alters T Cell Development in the Thymus and the Frequencies and Functions of Peripheral T Cell Subsets

Conditional gene targeting using the bacteriophage-derived Cre recombinase is widely applied for functional gene studies in mice. Mice transgenic for Cre under the control of the lck gene promoter are used to study the role of loxP-targeted genes in T cell development and function. In this article, we show a striking 65% reduction in cellularity, preferential development of γδ versus αβ T cells, and increased expression of IL-7R in the thymus of mice expressing Cre under the proximal lck promoter (lck-cre(+) mice). The transition from CD4/CD8 double-negative to double-positive cells was blocked, and lck-cre(+) double-positive cells were more prone to apoptosis and showed higher levels of Cre expression. Importantly, numbers of naive T cells were reduced in spleens and lymph nodes of lck-cre(+) mice. In contrast, frequencies of γδ T cells, CD44(+)CD62L(-) effector T cells, and Foxp3(+) regulatory T cells were elevated, as was the frequency of IFN-γ-secreting CD4(+) and CD8(+) T cells. A literature survey of 332 articles that used lck-cre(+) mice for deletion of floxed genes indicated that results are statistically influenced by the control used (lck-cre(+) or lck-cre(-)), more frequently resembling the lck-cre(+) phenotype described in this article if lck-cre(-) controls were used. Altogether, care should be taken when interpreting published results and to properly control targeted gene deletions using the lck-cre(+) strain.

Genetic Tools to Study T Cell Development

Genetics tools, and especially the ability to enforce, by transgenesis, or disrupt, by homologous recombination, gene expression in a cell-specific manner, have revolutionized the study of immunology and propelled the laboratory mouse as the main model to study immune responses. Perhaps more than any other aspect of immunology, the study of T cell development has benefited from these technologies. This brief chapter summarizes genetic tools specific to T cell development studies, focusing on mouse strains with lineage- and stage-specific expression of the Cre recombinase, or expressing unique antigen receptor specificities. It ends with a broader discussion of strategies to enforce ectopic lineage and stage-specific gene expression.

A monoallelic-to-biallelic T-cell transcriptional switch regulates GATA3 abundance

Ku et al. show that loss of one Gata3 allele leads to diminished expansion of immature T cells as well as aberrant induction of myeloid transcription factor PU.1. Gata3 is monoallelically expressed in hematopoietic stem cells and early T-cell progenitors. Half of the developing cells switch to biallelic Gata3 transcription abruptly at midthymopoiesis.

Protein abundance must be precisely regulated throughout life, and nowhere is the stringency of this requirement more evident than during T-cell development: A twofold increase in the abundance of transcription factor GATA3 results in thymic lymphoma, while reduced GATA3 leads to diminished T-cell production. GATA3 haploinsufficiency also causes human HDR (hypoparathyroidism, deafness, and renal dysplasia) syndrome, often accompanied by immunodeficiency. Here we show that loss of one Gata3 allele leads to diminished expansion (and compromised development) of immature T cells as well as aberrant induction of myeloid transcription factor PU.1. This effect is at least in part mediated transcriptionally: We discovered that Gata3 is monoallelically expressed in a parent of origin-independent manner in hematopoietic stem cells and early T-cell progenitors. Curiously, half of the developing cells switch to biallelic Gata3 transcription abruptly at midthymopoiesis. We show that the monoallelic-to-biallelic transcriptional switch is stably maintained and therefore is not a stochastic phenomenon. This unique mechanism, if adopted by other regulatory genes, may provide new biological insights into the rather prevalent phenomenon of monoallelic expression of autosomal genes as well as into the variably penetrant pathophysiological spectrum of phenotypes observed in many human syndromes that are due to haploinsufficiency of the affected gene.