CD4/CD8-lineage differentiation in the thymus: from nuclear effectors to membrane signals

Type B thymomas in patients with myasthenia gravis display a distinctive pattern of αβ TCR and IL-7 receptor α expression on CD4+CD8+ thymocytes

Thymoma is closely associated with myasthenia gravis (MG). However, due to the heterogeneity of thymoma and the intricate pathogenesis of MG, it remains unclear why some patients with thymoma develop MG and others do not. In this study, we conducted a comparative phenotype analysis of thymocytes in type B thymomas in patients with MG (MG (+) thymomas) and without MG (MG (-) thymomas) via fluorescence-activated cell sorting (FACS). Our results show that the developmental stages defined by the expression of CD3, CD4, and CD8 were largely maintained in both MG (+) and MG (-) thymomas, with CD4+CD8+ cells constituting the majority of thymocytes in type B thymoma, and no significant difference between this cell population was observed in MG (+) and MG (-) thymomas.We discovered that CD4+CD8+ thymocytes in MG (+) thymomas expressed low levels of αβ TCR and high levels of IL-7 receptor α (IL-7Rα), whereas in MG (-) thymomas, CD4+CD8+ thymocytes exhibited the opposite pattern of αβ TCR and IL-7Rα expression. These results suggest that the positive and negative selection processes of CD4+CD8+ thymocytes might differ between MG (+) thymomas and MG (-) thymomas. The expression of the Helios transcription factor is induced during negative selection and marks a group of T cells that have undergone negative selection and are likely to be deleted due to strong TCR binding with self-peptides/MHC ligands. We observed that the percentage of Helios-positive CD4SP T cells was greater in MG (-) than in MG (+) thymomas. Thus, the differentially regulated selection process of CD4+CD8+ thymocytes, which involves TCR and IL-7/IL-7Rα signaling, is associated with the presence of MG in type B thymomas.

DNA Methylation in Regulatory T Cell Differentiation and Function: Challenges and Opportunities

As a bona fide epigenetic marker, DNA methylation has been linked to the differentiation and function of regulatory T (Treg) cells, a subset of CD4 T cells that play an essential role in maintaining immune homeostasis and suppressing autoimmunity and antitumor immune response. DNA methylation undergoes dynamic regulation involving maintenance of preexisting patterns, passive and active demethylation, and de novo methylation. Scattered evidence suggests that these processes control different stages of Treg cell lifespan ranging from lineage induction to cell fate maintenance, suppression of effector T cells and innate immune cells, and transdifferentiation. Despite significant progress, it remains to be fully explored how differential DNA methylation regulates Treg cell fate and immunological function. Here, we review recent progress and discuss the questions and challenges for further understanding the immunological roles and mechanisms of dynamic DNA methylation in controlling Treg cell differentiation and function. We also explore the opportunities that these processes offer to manipulate Treg cell suppressive function for therapeutic purposes by targeting DNA methylation.

Negative Regulation of Zap70 by Lck Forms the Mechanistic Basis of Differential Expression in CD4 and CD8 T Cells

Lck and Zap70, two non-receptor tyrosine kinases, play a crucial role in the regulation of membrane proximal TCR signaling critical for thymic selection, CD4/CD8 lineage choice and mature T cell function. Signal initiation upon TCR/CD3 and peptide/MHC interaction induces Lck-mediated phosphorylation of CD3 ITAMs. This is necessary for Zap70 recruitment and its phosphorylation by Lck leading to full Zap70 activation. In its native state Zap70 maintains a closed conformation creating an auto-inhibitory loop, which is relieved by Lck-mediated phosphorylation of Y315/Y319. Zap70 is differentially expressed in thymic subsets and mature T cells with CD8 T cells expressing the highest amount compared to CD4 T cells. However, the mechanistic basis of differential Zap70 expression in thymic subsets and mature T cells is not well understood. Here, we show that Zap70 is degraded relatively faster in DP and mature CD4 T cells compared to CD8 T cells, and inversely correlated with relative level of activated Zap70. Importantly, we found that Zap70 expression is negatively regulated by Lck activity: augmented Lck activity resulting in severe diminution in total Zap70. Moreover, Lck-mediated phosphorylation of Y315/Y319 was essential for Zap70 degradation. Together, these data shed light on the underlying mechanism of Lck-mediated differential modulation of Zap70 expression in thymic subsets and mature T cells.

3D Genome Organization as an Epigenetic Determinant of Transcription Regulation in T Cells

In the heart of innate and adaptive immunity lies the proper spatiotemporal development of several immune cell lineages. Multiple studies have highlighted the necessity of epigenetic and transcriptional regulation in cell lineage specification. This mode of regulation is mediated by transcription factors and chromatin remodelers, controlling developmentally essential gene sets. The core of transcription and epigenetic regulation is formulated by different epigenetic modifications determining gene expression. Apart from “classic” epigenetic modifications, 3D chromatin architecture is also purported to exert fundamental roles in gene regulation. Chromatin conformation both facilitates cell-specific factor binding at specified regions and is in turn modified as such, acting synergistically. The interplay between global and tissue-specific protein factors dictates the epigenetic landscape of T and innate lymphoid cell (ILC) lineages. The expression of global genome organizers such as CTCF, YY1, and the cohesin complexes, closely cooperate with tissue-specific factors to exert cell type-specific gene regulation. Special AT-rich binding protein 1 (SATB1) is an important tissue-specific genome organizer and regulator controlling both long- and short-range chromatin interactions. Recent indications point to SATB1’s cooperation with the aforementioned factors, linking global to tissue-specific gene regulation. Changes in 3D genome organization are of vital importance for proper cell development and function, while disruption of this mechanism can lead to severe immuno-developmental defects. Newly emerging data have inextricably linked chromatin architecture deregulation to tissue-specific pathophysiological phenotypes. The combination of these findings may shed light on the mechanisms behind pathological conditions.

The pioneer transcription factors Foxa1 and Foxa2 regulate alternative RNA splicing during thymocyte positive selection

During positive selection at the transition from CD4+CD8+ double-positive (DP) to single-positive (SP) thymocyte, TCR signalling results in appropriate MHC restriction and signals for survival and progression. We show that the pioneer transcription factors Foxa1 and Foxa2 are required to regulate RNA splicing during positive selection of mouse T cells and that Foxa1 and Foxa2 have overlapping/compensatory roles. Conditional deletion of both Foxa1 and Foxa2 from DP thymocytes reduced positive selection and development of CD4SP, CD8SP and peripheral naïve CD4+ T cells. Foxa1 and Foxa2 regulated the expression of many genes encoding splicing factors and regulators, including Mbnl1, H1f0, Sf3b1, Hnrnpa1, Rnpc3, Prpf4b, Prpf40b and Snrpd3. Within the positively selecting CD69+DP cells, alternative RNA splicing was dysregulated in the double Foxa1/Foxa2 conditional knockout, leading to >850 differentially used exons. Many genes important for this stage of T-cell development (Ikzf1-3, Ptprc, Stat5a, Stat5b, Cd28, Tcf7) and splicing factors (Hnrnpab, Hnrnpa2b1, Hnrnpu, Hnrnpul1, Prpf8) showed multiple differentially used exons. Thus, Foxa1 and Foxa2 are required during positive selection to regulate alternative splicing of genes essential for T-cell development, and, by also regulating splicing of splicing factors, they exert widespread control of alternative splicing.

Three functional mutation sites affect the immune response of pigs through altering the expression pattern and IgV domain of the CD4 protein

BACKGROUND

The CD4 protein is an important surface marker of T lymphocytes, which can mediate the antigen presentation process by interacting with MHC II and TCR molecules in human and mouse.

RESULTS

In this study, two haplotypes (A and B) of the CD4 gene were found within Chinese indigenous and Western commercial pig breeds. These two haplotypes were defined by 22 fully linked SNPs in the CDS region of the CD4 gene. The expression level and localization of the CD4 protein were significantly different between haplotypes A and B. Transcriptome analysis revealed that the immune response-related genes and signaling pathways were down-regulated in genotype AA. Finally, three linked functional SNPs were identified, which affected the expression level and membrane localization of the CD4 protein in pigs. These three SNPs led to the replacements of two amino acids in the IgV1 domain of the CD4 protein, and related to the function of the CD4 protein in the immune response.

CONCLUSION

These three linked SNPs were the key functional mutation sites in the CD4 gene, which played important roles in the immune response, and could be utilized as new molecular markers in breeding for disease resistance in pigs.

Single-Cell Transcriptome Profiling of Immune Cell Repertoire of the Atlantic Cod Which Naturally Lacks the Major Histocompatibility Class II System

The Atlantic cod’s unusual immune system, entirely lacking the Major Histocompatibility class II pathway, has prompted intriguing questions about what mechanisms are used to combat bacterial infections and how immunological memory is generated. By single-cell RNA sequencing we here report an in-depth characterisation of cell types found in immune tissues, the spleen and peripheral blood leukocytes of Atlantic cod. Unbiased transcriptional clustering revealed eleven distinct immune cell signatures. Resolution at the single cell level enabled characterisation of the major cell subsets including the cytotoxic T cells, B cells, erythrocytes, thrombocytes, neutrophils, and macrophages. Additionally, to our knowledge we are the first to uncover cell subsets in Atlantic cod which may represent dendritic cells, natural killer-like cells, and a population of cytotoxic cells expressing GATA-3, a master transcription factor of T helper 2 cells. We further identify putative gene markers for each cluster and describe the relative proportions of each cell type in the spleen and peripheral blood leukocytes. Of the major haematopoietic cell populations, the lymphocytes make up 55 and 68% of the spleen and peripheral blood leukocytes respectively, while the myeloid cells make up 45 and 32%. By single-cell analysis, this study provides the most detailed molecular and cellular characterisation of the immune system of the Atlantic cod so far.

Twist2 promotes CD8+ T-cell differentiation by repressing ThPOK expression

CD4/CD8 T-cell lineage differentiation is a key process in immune system development; however, a defined regulator(s) that converts the signal from T-cell receptor and co-receptor complexes into lineage differentiation remains unclear. Here, we show that Twist2 is a critical factor in CD4/CD8 thymocyte differentiation. Twist2 expression is differentially regulated by T-cell receptor signaling, leading to differentiation into the CD4 or CD8 lineage. Forced Twist2 expression perturbed CD4⁺ thymocyte differentiation while enhancing CD8⁺ thymocyte differentiation. Furthermore, Twist2 expression produced mature CD8⁺ thymocytes in B₂m^-/- mice, while its deficiency significantly impaired CD8⁺ cells in MHC class-II^-/- and TCR transgenic mice, favoring CD8 T-cell differentiation. During CD8 lineage differentiation, Twist2 interacted with Runx3 to bind to the silencer region of the ThPOK locus, thereby blocking ThPOK expression. These findings indicate that Twist2 is a part of the transcription factor network controlling CD8 lineage differentiation.

The natural history of naive T cells from birth to maturity

Generating and maintaining a diverse repertoire of naive T cells is essential for protection against pathogens, and developing a mechanistic and quantitative description of the processes involved lies at the heart of our understanding of vertebrate immunity. Here, we review the biology of naive T cells from birth to maturity and outline how the integration of mathematical models and experiments has helped us to develop a full picture of their life histories.

NKT Cells in Mice Originate from Cytoplasmic CD3-Positive, CD4-CD8- Double-Negative Thymocytes that Express CD44 and IL-7Rα

Although natural killer T cells (NKT cells) are thought to be generated from CD4⁺CD8⁺ (DP) thymocytes, the developmental origin of CD4⁻CD8⁻ (DN) NKT cells has remained unclear. In this study, we found the level of NK1.1 expression was highest in DN cells, followed by CD4 and CD8 (SP) and DP cells. The level of NK1.1 expression was highest in CD44⁺CD25⁻ (DN₁) cells, after that CD44⁺CD25⁺ (DN₂), finally, CD44⁻CD25⁻ (DN₃) and CD44⁻ CD25⁺ (DN₄) cells. Unexpectedly, cytoplasmic CD3 was not only expressed in SP and DP thymocytes but also in most DN thymocytes at various stages. The mean fluorescence of cytoplasmic and surface CD3 in DN cells was significantly lower than in mature (SP) T and NKT cells in the thymus and spleen. Interestingly, there were more NKT cells in DN-cytoplasmic CD3 expression cells was higher than in DN-surface CD3 expression cells. There were more CD3-NKT cells in DN₁ thymocytes than in TCR-β-NKT cells. NKT cells expressed higher levels of IL-7Rα which was correlated with CD44 expression in the thymus. Our data suggest that T cells and NKT cells follow similar patterns of expression with respect to cytoplasmic and surface CD3. Cytoplasmic CD3 could be used as a marker for early stage T cells. Both cytoplasmic CD3 and surface CD3 were expressed in mature T cells and immature T cells, including the immature cytoplasmic CD3⁺ surface CD3⁻ and surface CD3⁺TCR-β⁻ cells in DN₁-NKT thymocytes. CD44 could be used as an additional marker of NKT cells which may originate from cytoplasmic CD3-positive DN thymocytes that express CD44 and IL-7Rα in mice.

Unraveling the Mechanobiology of the Immune System

Cells respond and actively adapt to environmental cues in the form of mechanical stimuli. This extends to immune cells and their critical role in the maintenance of tissue homeostasis. Multiple recent studies have begun illuminating underlying mechanisms of mechanosensation in modulating immune cell phenotypes. Since the extracellular microenvironment is critical to modify cellular physiology that ultimately determines the functionality of the cell, understanding the interactions between immune cells and their microenvironment is necessary. This review focuses on mechanoregulation of immune responses mediated by macrophages, dendritic cells, and T cells, in the context of modern mechanobiology.

GATA3 as a master regulator and therapeutic target in ovarian high-grade serous carcinoma stem cells

Ovarian high-grade serous carcinoma (HGSC) is the most lethal gynecological malignancy. Prevailing evidences suggest that drug resistance and recurrence of ovarian HGSC are caused by the presence of cancer stem cells. Therefore, targeting cancer stems is appealing, however, all attempts to date, have failed. To circumvent this limit, we analyzed differential transcriptomes at early differentiation of ovarian HGSC stem cells and identified the developmental transcription factor GATA3 as highly expressed in stem, compared to progenitor cells. GATA3 expression associates with poor prognosis of ovarian HGSC patients, and was found to recruit the histone H3, lysine 27 (H3K27) demethylase, UTX, activate stemness markers, and promote stem-like phenotypes in ovarian HGSC cell lines. Targeting UTX by its inhibitor, GSKJ4, impeded GATA3-driven stemness phenotypes, and enhanced apoptosis of GATA3-expressing cancer cells. Combinations of gemcitabine or paclitaxel with GSKJ4, resulted in a synergistic cytotoxic effect. Our findings provide evidence for a new role for GATA3 in ovarian HGSC stemness, and demonstrate that GATA3 may serve as a biomarker for precision epigenetic therapy in the future.

GPR54 deficiency reduces the Treg population and aggravates experimental autoimmune encephalomyelitis in mice

GPR54 is highly expressed in the central nervous system and plays a crucial role in pubertal development. However, GRP54 is also expressed in the immune system, implying possible immunoregulatory functions. Here we investigated the role of GPR54 in T cell and immune tolerance. GPR54 deficiency led to an enlarged thymus, an increased number of thymocytes, and altered thymic micro-architecture starting around puberty, indicating GPR54 function in T-cell development through its regulatory effect on the gonadal system. However, flow cytometry revealed a significant reduction in the peripheral regulatory T cell population and a moderate decrease in CD4 single-positive thymocytes in prepubertal Gpr54^-/- mice. These phenotypes were confirmed in chimeric mice with GPR54 deficient bone marrow-derived cells. In addition, we found elevated T cell activation in peripheral and thymic T cells in Gpr54^-/- mice. When intact mice were immunized with myelin oligodendrocyte glycoprotein, a more severe experimental autoimmune encephalomyelitis (EAE) developed in the Gpr54^-/- mice. Interestingly, aggravated EAE disease was also manifested in castrated and bone marrow chimeric Gpr54^-/- mice compared to the respective wild-type control, suggesting a defect in self-tolerance resulting from GPR54 deletion through a mechanism that bypassed sex hormones. These findings demonstrate a novel role for GPR54 in regulating self-tolerant immunity in a sex hormone independent manner.

Cutting Edge: The Histone Methyltransferase G9a Is Required for Silencing of Helper T Lineage-Associated Genes in Proliferating CD8 T Cells

Helper versus cytotoxic T lineage decision in the thymus has been studied as a model for silencing of alternative lineage genes. Although the transcription factor RUNX3 is required for the initiation of Cd4 silencing in developing CD8 T cells, it is unknown how silencing of Cd4 and other helper T lineage genes is maintained. We show that the histone methyltransferase G9a is necessary for silencing helper T lineage genes in proliferating mouse CD8 T cells. Despite normal initial Cd4 downregulation, G9a-deficient CD8 T cells derepress Cd4 and other helper lineage genes during repeated division in lymphopenia or in response to tumor Ag. However, G9a was dispensable for continued silencing of those genes in CD8 T cells that respond to infection by Listeria monocytogenes These results demonstrate that G9a facilitates maintenance of cellular identity of CD8 T cells during cell division, which is further reinforced by inflammatory signals.

Gli3 in fetal thymic epithelial cells promotes thymocyte positive selection and differentiation by repression of Shh

Gli3 is a Hedgehog (Hh)-responsive transcription factor that can function as a transcriptional repressor or activator. We show that Gli3 activity in mouse thymic epithelial cells (TECs) promotes positive selection and differentiation from CD4⁺ CD8⁺ to CD4⁺ CD8^- single-positive (SP4) cells in the fetal thymus and that Gli3 represses Shh Constitutive deletion of Gli3, and conditional deletion of Gli3 from TECs, reduced differentiation to SP4, whereas conditional deletion of Gli3 from thymocytes did not. Conditional deletion of Shh from TECs increased differentiation to SP4, and expression of Shh was upregulated in the Gli3-deficient thymus. Use of a transgenic Hh reporter showed that the Hh pathway was active in thymocytes, and increased in the Gli3-deficient fetal thymus. Neutralisation of endogenous Hh proteins in the Gli3^-/- thymus restored SP4 differentiation, indicating that Gli3 in TECs promotes SP4 differentiation by repression of Shh Transcriptome analysis showed that Hh-mediated transcription was increased whereas TCR-mediated transcription was decreased in Gli3^-/- thymocytes compared with wild type.

Osteoimmunology in Bone Fracture Healing

PURPOSE OF REVIEW

In the process of bone fracture healing, inflammation is thought to be an essential process that precedes bone formation and remodeling. We review recent studies on bone fracture healing from an osteoimmunological point of view.

RECENT FINDINGS

Based on previous observations that many types of immune cells infiltrate into the bone injury site and release a variety of molecules, recent studies have addressed the roles of specific immune cell subsets. Macrophages and interleukin (IL)-17-producing γδ T cells enhance bone healing, whereas CD8⁺ T cells impair bone repair. Additionally, IL-10-producing B cells may contribute to bone healing by suppressing excessive and/or prolonged inflammation. Although the involvement of other cells and molecules has been suggested, the precise underlying mechanisms remain elusive. Accumulating evidence has begun to reveal the deeper picture of bone fracture healing. Further studies are required for the development of novel therapeutic strategies for bone fracture.

What Happens in the Thymus Does Not Stay in the Thymus: How T Cells Recycle the CD4+-CD8+ Lineage Commitment Transcriptional Circuitry To Control Their Function

MHC-restricted CD4(+) and CD8(+) T cells are at the core of most adaptive immune responses. Although these cells carry distinct functions, they arise from a common precursor during thymic differentiation, in a developmental sequence that matches CD4 and CD8 expression and functional potential with MHC restriction. Although the transcriptional control of CD4(+)-CD8(+) lineage choice in the thymus is now better understood, less was known about what maintains the CD4(+) and CD8(+) lineage integrity of mature T cells. In this review, we discuss the mechanisms that establish in the thymus, and maintain in postthymic cells, the separation of these lineages. We focus on recent studies that address the mechanisms of epigenetic control of Cd4 expression and emphasize how maintaining a transcriptional circuitry nucleated around Thpok and Runx proteins, the key architects of CD4(+)-CD8(+) lineage commitment in the thymus, is critical for CD4(+) T cell helper functions.

Tespa1 regulates T cell receptor-induced calcium signals by recruiting inositol 1,4,5-trisphosphate receptors

Thymocyte-expressed, positive selection-associated 1 (Tespa1) is important in T cell receptor (TCR)-driven thymocyte development. Downstream of the TCR, Tespa1 is a crucial component of the linker for activation of T cells (LAT) signalosome, facilitating calcium signalling and subsequent MAPK activation. However, it is unknown how Tespa1 elicits calcium signalling. Here, we show that inositol 1,4,5-trisphosphate receptor type 1 (IP3R1) is crucial for Tespa1-optimized, TCR-induced Ca²⁺ flux and thymocyte development. Upon TCR stimulation, Tespa1 directly interacts with IP3R1 and recruits it to the TCR complex, where IP3R1 is phosphorylated at Y353 by Fyn. This Tespa1-IP3R1 interaction is mediated by the F187 and F188 residues of Tespa1 and the amino-terminus of IP3R1. Tespa1-F187A/F188A mutant mice phenocopy Tespa1-deficient mice with impaired late thymocyte development due to reduced IP3R1 translocation to the TCR-proximal region. Our work elucidates the function of Tespa1 in T cell development and the regulation of TCR-induced Ca²⁺ signalling through IP3R1.

The thymocyte development protein Tespa1 is known to translate T cell receptor signals by affecting the calcium signalling cascade, but it is not clear how. Here the authors show that Tespa1 recruits IP3R1 to the TCR signalling complex.

Fighting Viral Infections and Virus-Driven Tumors with Cytotoxic CD4+ T Cells

CD4+ T cells have been and are still largely regarded as the orchestrators of immune responses, being able to differentiate into distinct T helper cell populations based on differentiation signals, transcription factor expression, cytokine secretion, and specific functions. Nonetheless, a growing body of evidence indicates that CD4+ T cells can also exert a direct effector activity, which depends on intrinsic cytotoxic properties acquired and carried out along with the evolution of several pathogenic infections. The relevant role of CD4+ T cell lytic features in the control of such infectious conditions also leads to their exploitation as a new immunotherapeutic approach. This review aims at summarizing currently available data about functional and therapeutic relevance of cytotoxic CD4+ T cells in the context of viral infections and virus-driven tumors.

Expression of the SNARE protein SNAP-23 is essential for cell survival

Members of the SNARE-family of proteins are known to be key regulators of the membrane-membrane fusion events required for intracellular membrane traffic. The ubiquitously expressed SNARE protein SNAP-23 regulates a wide variety of exocytosis events and is essential for mouse development. Germline deletion of SNAP-23 results in early embryonic lethality in mice, and for this reason we now describe mice and cell lines in which SNAP-23 can be conditionally-deleted using Cre-lox technology. Deletion of SNAP-23 in CD19-Cre expressing mice prevents B lymphocyte development and deletion of SNAP-23 using a variety of T lymphocyte-specific Cre mice prevents T lymphocyte development. Acute depletion of SNAP-23 in mouse fibroblasts leads to rapid apoptotic cell death. These data highlight the importance of SNAP-23 for cell survival and describe a mouse in which specific cell types can be eliminated by expression of tissue-specific Cre-recombinase.

Noncanonical mode of ERK action controls alternative αβ and γδ T cell lineage fates

Gradations in extracellular regulated kinase (ERK) signaling have been implicated in essentially every developmental checkpoint or differentiation process encountered by lymphocytes. Yet, despite intensive effort, the molecular basis by which differences in ERK activation specify alternative cell fates remains poorly understood. We report here that differential ERK signaling controls lymphoid-fate specification through an alternative mode of action. While ERK phosphorylates most substrates, such as RSK, by targeting them through its D-domain, this well-studied mode of ERK action was dispensable for development of γδ T cells. Instead, development of γδ T cells was dependent upon an alternative mode of action mediated by the DEF-binding pocket (DBP) of ERK. This domain enabled ERK to bind a distinct and select set of proteins required for specification of the γδ fate. These data provide the first in vivo demonstration for the role of DBP-mediated interactions in orchestrating alternate ERK-dependent developmental outcomes.

Transcription analysis of two Eomesodermin genes in lymphocyte subsets of two teleost species

Eomesodermin (Eomes), a T-box transcription factor, is a key molecule associated with function and differentiation of CD8(+) T cells and NK cells. Previously, two teleost Eomes genes (Eomes-a and -b), which are located on different chromosomes, were identified and shown to be expressed in zebrafish lymphocytes. For the present study, we identified these genes in rainbow trout and ginbuna crucian carp. Deduced Eomes-a and -b amino acid sequences in both fish species contain a highly conserved T-box DNA binding domain. In RT-PCR, both Eomes transcripts were readily detectable in a variety of tissues in rainbow trout and ginbuna. The high expression of Eomes-a and -b in brain and ovary suggests involvement in neurogenesis and oogenesis, respectively, while their expression in lymphoid tissues presumably is associated with immune functions. Investigation of separated lymphocyte populations from pronephros indicated that both Eomes-a and -b transcripts were few or absent in IgM(+) lymphocytes, while relatively abundant in IgM(-)/CD8α(+) and IgM(-)/CD8α(-) populations. Moreover, we sorted trout CD8α(+) lymphocytes from mucosal and non-mucosal lymphoid tissues and compared the expression profiles of Eomes-a and -b with those of other T cell-related transcription factor genes (GATA-3, T-bet and Runx3), a Th1 cytokine gene (IFN-γ) and a Th2 cytokine gene (IL-4/13A). Interestingly, the tissue distribution of Eomes-a/b, T-bet, and Runx3 versus IFN-γ transcripts did not reveal simple correlations, suggesting tissue-specific properties of CD8α(+) lymphocytes and/or multiple modes that drive IFN-γ expressions.

Cellular and molecular requirements for the selection of in vitro-generated CD8 T cells reveal a role for Notch

Differentiation of CD8 single-positive (SP) T cells is predicated by the ability of lymphocyte progenitors to integrate multiple signaling cues provided by the thymic microenvironment. In the thymus and the OP9-DL1 system for T cell development, Notch signals are required for progenitors to commit to the T cell lineage and necessary for their progression to the CD4(+)CD8(+) double-positive (DP) stage of T cell development. However, it remains unclear whether Notch is a prerequisite for the differentiation of DP cells to the CD8 SP stage of development. In this study, we demonstrate that Notch receptor-ligand interactions allow for efficient differentiation and selection of conventional CD8 T cells from bone marrow-derived hematopoietic stem cells. However, bone marrow-derived hematopoietic stem cells isolated from Itk(-/-)Rlk(-/-) mice gave rise to T cells with decreased IFN-γ production, but gained the ability to produce IL-17. We further reveal that positive and negative selection in vitro are constrained by peptide-MHC class I expressed on OP9 cells. Finally, using an MHC class I-restricted TCR-transgenic model, we show that the commitment of DP precursors to the CD8 T cell lineage is dependent on Notch signaling. Our findings further establish the requirement for Notch receptor-ligand interactions throughout T cell differentiation, including the final step of CD8 SP selection.

TAF7: traffic controller in transcription initiation

TAF7, a component of the TFIID complex, controls the first steps of transcription. It interacts with and regulates the enzymatic activities of transcription factors that regulate RNA polymerase II progression. Its diverse functions in transcription initiation are consistent with its essential role in cell proliferation.

Thpok-independent repression of Runx3 by Gata3 during CD4+ T-cell differentiation in the thymus

CD4(+) helper T cells are essential for immune responses and differentiate in the thymus from CD4(+) CD8(+) "double-positive" (DP) thymocytes. The transcription factor Runx3 inhibits CD4(+) T-cell differentiation by repressing Cd4 gene expression; accordingly, Runx3 is not expressed in DP thymocytes or developing CD4(+) T cells. The transcription factor Thpok is upregulated in CD4-differentiating thymocytes and required to repress Runx3. However, how Runx3 is controlled at early stages of CD4(+) T-cell differentiation, before the onset of Thpok expression, remains unknown. Here we show that Gata3, a transcription factor preferentially and transiently upregulated by CD4(+) T-cell precursors, represses Runx3 and binds the Runx3 locus in vivo. Accordingly, we show that high-level Gata3 expression and expression of Runx3 are mutually exclusive. Furthermore, whereas Runx3 represses Cd4, we show that Gata3 promotes Cd4 expression in Thpok-deficient thymocytes. Thus, in addition to its previously documented role in promoting CD4-lineage gene-expression, Gata3 represses CD8-lineage gene expression. These findings identify Gata3 as a critical pivot of CD4-CD8 lineage differentiation.

c-Rel: shaping CD4 regulatory T cell development in unexpected ways

In the thymus, sequential antigen and cytokine receptor signals direct the stepwise differentiation of multi-potential CD4+CD8+ thymic precursors into Foxp3+ CD4 regulatory T cells (Tregs). In this Point of View article we discuss our recent findings about how the c-Rel transcription factor orchestrates this developmental process.

Comparative and functional evaluation of in vitro generated to ex vivo CD8 T cells

The generation of the cytotoxic CD8 T cell response is dependent on the functional outcomes imposed by the intrathymic constraints of differentiation and self-tolerance. Although thymic function can be partly replicated in vitro using OP9-DL1 cell cultures to yield CD8 αβ TCR-bearing cells from hematopoietic progenitor cells, a comprehensive and functional assessment of entirely in vitro generated CD8 T cells derived from bone marrow hematopoietic stem cells has not been established and remains controversial. In this study, we demonstrate that a phenotypic, molecular, and functional signature of in vitro derived CD8 T cells is akin to that of ex vivo CD8 T cells, although several significant differences were also observed. Transfer of in vitro derived CD8 T cells into syngeneic and immunodeficient host mice showed no graft-versus-host response, whereas a robust homeostatic proliferation was observed, respectively. These findings, along with a diverse and broad TCR repertoire expressed by the in vitro derived CD8 T cells, allowed for the successful generation of Ag-specific T cells to be obtained from an entirely in vitro generated CD8 T cell pool. These findings support the use of Ag-specific in vitro derived effector CD8 T cells for immune reconstitution approaches, which would be amenable to further tailoring for their use against viral infections or malignancies.

NF-κB subunit specificity in hemopoiesis

Although the diverse functions served by the nuclear factor-κB (NF-κB) pathway in virtually all cell types are typically employed to deal with stress responses, NF-κB transcription factors also play key roles in the development of hemopoietic cells. This review focuses on how NF-κB transcription factors control various aspects of thymic T-cell and myeloid cell differentiation that include its roles in hemopoietic precursors, conventional αβ T cells, CD4(+) regulatory T cells, natural killer T cells, γδ T cells, macrophages, and dendritic cells.

Tespa1 is involved in late thymocyte development through the regulation of TCR-mediated signaling

Signaling via the T cell antigen receptor (TCR) during the CD4(+)CD8(+) double-positive developmental stage determines thymocyte selection and lineage commitment. Here we describe a previously uncharacterized T cell-expressed protein, Tespa1, with critical functions during the positive selection of thymocytes. Tespa1(-/-) mice had fewer mature thymic CD4(+) and CD8(+) T cells, which reflected impaired thymocyte development. Tespa1 associated with the TCR signaling components PLC-γ1 and Grb2, and Tespa1 deficiency resulted in attenuated TCR signaling, as reflected by defective activation of the Erk-AP-1 and Ca(2+)-NFAT pathways. Our findings demonstrate that Tespa1 is a component of the TCR signalosome and is essential for T cell selection and maturation through the regulation of TCR signaling during T cell development.

The general transcription factor TAF7 is essential for embryonic development but not essential for the survival or differentiation of mature T cells

TAF7, a component of the TFIID complex that nucleates the assembly of transcription preinitiation complexes, also independently interacts with and regulates the enzymatic activities of other transcription factors, including P-TEFb, TFIIH, and CIITA, ensuring an orderly progression in transcription initiation. Since not all TAFs are required in terminally differentiated cells, we examined the essentiality of TAF7 in cells at different developmental stages in vivo. Germ line disruption of the TAF7 gene is embryonic lethal between 3.5 and 5.5 days postcoitus. Mouse embryonic fibroblasts with TAF7 deleted cease transcription globally and stop proliferating. In contrast, whereas TAF7 is essential for the differentiation and proliferation of immature thymocytes, it is not required for subsequent, proliferation-independent differentiation of lineage committed thymocytes or for their egress into the periphery. TAF7 deletion in peripheral CD4 T cells affects only a small number of transcripts. However, T cells with TAF7 deleted are not able to undergo activation and expansion in response to antigenic stimuli. These findings suggest that TAF7 is essential for proliferation but not for proliferation-independent differentiation.

ATP-dependent chromatin remodeling in T cells

One of the best studied systems for mammalian chromatin remodeling is transcriptional regulation during T cell development. The variety of these studies have led to important findings in T cell gene regulation and cell fate determination. Importantly, these findings have also advanced our knowledge of the function of remodeling enzymes in mammalian gene regulation. First we briefly present biochemical and cell-free analysis of 3 types of ATP dependent remodeling enzymes (SWI/SNF, Mi2, and ISWI) to construct an intellectual framework to understand how these enzymes might be working. Second, we compare and contrast the function of these enzymes during early (thymic) and late (peripheral) T cell development. Finally, we examine some of the gaps in our present understanding.

The NF-κB1 transcription factor prevents the intrathymic development of CD8 T cells with memory properties

The role of specific members of the NF-κB family of transcription factors in CD8 T-cell selection and development is largely unknown. Here, we show that mice lacking NF-κB1 develop a unique population of conventional CD8 single-positive (SP) thymocytes with memory T cell-like properties that populate peripheral immune organs. Development of this memory-like population is not due to PLZF(+) thymocytes and instead coincides with changes in CD8 T-cell selection. These include a reduction in the efficiency of negative selection and a dependence on MHC class Ia or Ib expressed by haematopoietic cells. These findings indicate that NF-κB1 regulates multiple events in the thymus that collectively inhibit the excess development of CD8(+) thymocytes with memory cell characteristics.

NK cell development, homeostasis and function: parallels with CD8⁺ T cells

Natural killer (NK) cells survey host tissues for signs of infection, transformation or stress and, true to their name, kill target cells that have become useless or are detrimental to the host. For decades, NK cells have been classified as a component of the innate immune system. However, accumulating evidence in mice and humans suggests that, like the B and T cells of the adaptive immune system, NK cells are educated during development, possess antigen-specific receptors, undergo clonal expansion during infection and generate long-lived memory cells. In this Review, we highlight the many stages that an NK cell progresses through during its remarkable lifetime, discussing similarities and differences with its close relative, the cytotoxic CD8(+) T cell.

Transcription initiation platforms and GTF recruitment at tissue-specific enhancers and promoters

Recent work has shown that RNA polymerase (Pol) II can be recruited to and transcribe distal regulatory regions. Here we analyzed transcription initiation and elongation through genome-wide localization of Pol II, general transcription factors (GTFs) and active chromatin in developing T cells. We show that Pol II and GTFs are recruited to known T cell-specific enhancers. We extend this observation to many new putative enhancers, a majority of which can be transcribed with or without polyadenylation. Importantly, we also identify genomic features called transcriptional initiation platforms (TIPs) that are characterized by large areas of Pol II and GTF recruitment at promoters, intergenic and intragenic regions. TIPs show variable widths (0.4-10 kb) and correlate with high CpG content and increased tissue specificity at promoters. Finally, we also report differential recruitment of TFIID and other GTFs at promoters and enhancers. Overall, we propose that TIPs represent important new regulatory hallmarks of the genome.

LKB1 regulates TCR-mediated PLCγ1 activation and thymocyte positive selection

The serine/threonine kinase LKB1 is a tumour suppressor that regulates cell growth, polarity, and proliferation in many different cell types. We previously demonstrated that LKB1 controls thymocyte survival via regulation of AMPK activation. In this study, we show that LKB1 was also involved in thymocyte positive selection through regulation of T cell receptor (TCR) signalling. Both Lck-Cre- and CD4-Cre-mediated deletion of LKB1 impaired the generation of mature CD4 and CD8 single positive (SP) thymocytes that might have resulted from the attenuated tyrosine phosphorylation of phospholipase C-γ 1 (PLCγ1) in the absence of LKB1. We found that LKB1 was directly phosphorylated by Lck at tyrosine residues 36, 261, and 365 and predominately interacted with LAT and PLCγ1 following TCR stimulation. Loss of LKB1 led to impaired recruitment of PLCγ1 to the LAT signalosome. Correlatively, LKB1-deficient thymocytes failed to upregulate lineage-specifying factors, and to differentiate into SP thymocytes even if their impaired survival was rescued. These observations indicated that LKB1 is a critical component involved in TCR signalling, and our studies provide novel insights into the mechanisms of LKB1-mediated thymocyte development.

TCF1 and beta-catenin regulate T cell development and function

T cell factor-1 (TCF1) critically regulates T cell development. However, signals that control TCF1 function in developing and mature T cells remain unknown. TCF1 along with beta-catenin activates gene transcription and in cooperation with Groucho family of proteins mediates gene repression. It has been established that the beta-catenin-dependent gene expression is often downstream of the canonical Wnt signaling pathway. We have genetically manipulated the beta-catenin gene and generated mutant mice that have shown an essential role for beta-catenin and TCF1 during pre-T cell receptor (TCR) and TCR-dependent stages of T cell development. We have also demonstrated a function for TCF1 and beta-catenin downstream of TCR signaling in the differentiation of mature CD4 T cells into T helper lineages.

SAP protein-dependent natural killer T-like cells regulate the development of CD8(+) T cells with innate lymphocyte characteristics

CD8(+) T cells are selected via low-affinity interaction with MHC class I molecules on thymic epithelial cells (TECs). However, compromised T cell receptor signaling was proposed to force CD8(+) T cell selection on hematopoietic cells through a SLAM-associated protein (SAP)-dependent mechanism similar to NKT cells. The outcome is an unconventional CD8(+) T cell with phenotypic and functional characteristics of innate lymphocytes. Here we showed that Id3(-/-) CD8(+) T cells had an innate-like phenotype and required SAP for their development. However, like conventional CD8(+) T cells, Id3(-/-) CD8(+) thymocytes were selected on TECs. The requirement for SAP and the innate-like phenotype was not intrinsic to Id3(-/-) CD8(+) thymocytes. Rather, an expanded population of NKT-like cells induced the innate phenotype on CD8(+) T cells through production of interleukin-4. Our findings reveal that accumulation of NKT-like cells promotes conventional CD8(+) thymocytes to acquire innate lymphocyte characteristics.

T-cell signaling regulated by the Tec family kinase, Itk

The Tec family tyrosine kinases regulate lymphocyte development, activation, and differentiation. In T cells, the predominant Tec kinase is Itk, which functions downstream of the T-cell receptor to regulate phospholipase C-gamma. This review highlights recent advances in our understanding of Itk kinase structure and enzymatic regulation, focusing on Itk protein domain interactions and mechanisms of substrate recognition. We also discuss the role of Itk in the development of conventional versus innate T-cell lineages, including both alphabeta and gammadelta T-cell subsets. Finally, we describe the complex role of Itk signaling in effector T-cell differentiation and the regulation of cytokine gene expression. Together, these data implicate Itk as an important modulator of T-cell signaling and function.

An active CD8alpha/pMHCI interaction is required for CD8 single positive thymocyte differentiation

Recognition of viral antigenic peptides bound to major histocompatibility complex class I molecules (MHCI) by TCR is critical for initiating the responses of CD8(+) T cells that ultimately lead to elimination of virus-infected cells. This antigen recognition is enhanced by the CD8 coreceptor through its interaction with the peptide-MHCI complexes (pMHCI). Mouse CD8alphabeta can form two different complexes with pMHCI via either the CD8alpha- or CD8beta-dominated interaction. To understand the functional significance of these complexes in vivo, we generated Tg mice carrying a variant CD8alphabeta (CD8alpha(m3)beta) capable of forming only the CD8beta-dominated CD8alphabeta/pMHCI complex. These mice show sub-optimal thymic differentiation with reduced populations of CD8(+) single-positive thymocytes. Tg CD8(+) T cells exhibit a compromised developmental capacity when competing with CD8(+) T cells from B6 mice in mixed bone marrow chimera experiments. However, once these CD8(+) T cells have emigrated to the peripheral lymphoid organs, they exhibit normal effector function against viral infection. Our observations indicate that, in addition to the CD8 activity conferred by CD8beta-dominated CD8alphabeta/pMHCI complexes, full thymocyte differentiation requires additional coreceptor activities conferred by CD8alphaalpha and/or CD8alphabeta with CD8alpha-dominated CD8/pMHCI complexes.

The transcription factor GATA3 actively represses RUNX3 protein-regulated production of interferon-gamma

The transcription factor GATA3 is crucial for the differentiation of naive CD4(+) T cells into T helper 2 (Th2) cells. Here, we show that deletion of Gata3 allowed the appearance of interferon-gamma (IFN-gamma)-producing cells in the absence of interleukin-12 (IL-12) and IFN-gamma. Such IFN-gamma production was transcription factor T-bet independent. Another T-box-containing transcription factor Eomes, but not T-bet, was induced both in GATA3-deficient CD4(+) T cells differentiated under Th2 cell conditions and in Th2 cells with enforced Runx3 expression, contributing to IFN-gamma production. GATA3 overexpression blocked Runx3-mediated Eomes induction and IFN-gamma production, and GATA3 protein physically interacted with Runx3 protein. Furthermore, we found that Runx3 directly bound to multiple regulatory elements of the Ifng gene and that blocking Runx3 function in either Th1 or GATA3-deficient "Th2" cells results in diminished IFN-gamma production by these cells. Thus, the Runx3-mediated pathway, actively suppressed by GATA3, induces IFN-gamma production in a STAT4- and T-bet-independent manner.

The zinc-finger protein MAZR is part of the transcription factor network that controls the CD4 versus CD8 lineage fate of double-positive thymocytes

The CD4 versus CD8 lineage specification of thymocytes is linked to coreceptor expression. The transcription factor MAZR has been identified as an important regulator of Cd8 expression. Here we show that variegated CD8 expression by loss of Cd8 enhancers was reverted in MAZR-deficient mice, which confirms that MAZR negatively regulates the Cd8 loci during the transition to the double-positive (DP) stage. Moreover, loss of MAZR led to partial redirection of major histocompatibility complex (MHC) class I-restricted thymocytes into CD4(+) helper-like T cells, which correlated with derepression of Th-POK, a central transcription factor for helper-lineage development. MAZR bound the silencer of the gene encoding Th-POK, which indicated direct regulation of this locus by MAZR. Thus, MAZR is part of the transcription factor network that regulates the CD8 lineage differentiation of DP thymocytes.

Foxp1 is an essential transcriptional regulator for the generation of quiescent naive T cells during thymocyte development

Proper thymocyte development is required to establish T-cell central tolerance and to generate naive T cells, both of which are essential for T-cell homeostasis and a functional immune system. Here we demonstrate that the loss of transcription factor Foxp1 results in the abnormal development of T cells. Instead of generating naive T cells, Foxp1-deficient single-positive thymocytes acquire an activated phenotype prematurely in the thymus and lead to the generation of peripheral CD4(+) T and CD8(+) T cells that exhibit an activated phenotype and increased apoptosis and readily produce cytokines upon T-cell receptor engagement. These results identify Foxp1 as an essential transcriptional regulator for thymocyte development and the generation of quiescent naive T cells.

The transcription factor Ets1 is important for CD4 repression and Runx3 up-regulation during CD8 T cell differentiation in the thymus

The transcription factor Ets1 contributes to the differentiation of CD8 lineage cells in the thymus, but how it does so is not understood. In this study, we demonstrate that Ets1 is required for the proper termination of CD4 expression during the differentiation of major histocompatability class 1 (MHC I)–restricted thymocytes, but not for other events associated with their positive selection, including the initiation of cytotoxic gene expression, corticomedullary migration, or thymus exit. We further show that Ets1 promotes expression of Runx3, a transcription factor important for CD8 T cell differentiation and the cessation of Cd4 gene expression. Enforced Runx3 expression in Ets1-deficient MHC I–restricted thymocytes largely rescued their impaired Cd4 silencing, indicating that Ets1 is not required for Runx3 function. Finally, we document that Ets1 binds at least two evolutionarily conserved regions within the Runx3 gene in vivo, supporting the possibility that Ets1 directly contributes to Runx3 transcription. These findings identify Ets1 as a key player during CD8 lineage differentiation and indicate that it acts, at least in part, by promoting Runx3 expression.

CD44 expression positively correlates with Foxp3 expression and suppressive function of CD4+ Treg cells

Background

CD4⁺CD25⁺ regulatory T (T_reg) cells develop in the thymus and can suppress T cell proliferation, modulated by Foxp3 and cytokines; however, the relevance of CD44 in T_reg cell development is less clear. To address this issue, we analyzed Foxp3 expression in CD44⁺ T_reg cells by using multiple parameters, measured the levels of the immunoregulatory cytokine interleukin (IL)-10 in various thymocyte subsets, and determined the suppressor activity in different splenic T_reg cell populations.

Results

Within mouse thymocytes, we detected T_reg cells with two novel phenotypes, namely the CD4⁺CD8^-CD25⁺CD44⁺ and CD4⁺CD8^-CD25⁺CD44^- staining features. Additional multi-parameter analyses at the single-cell and molecular levels suggested to us that CD44 expression positively correlated with Foxp3 expression in thymocytes, the production of IL-10, and T_reg activity in splenic CD4⁺CD25⁺ T cells. This suppressive effect of T_reg cells on T cell proliferation could be blocked by using anti-IL-10 neutralizing antibodies. In addition, CD4⁺CD25⁺CD44⁺ T_reg cells expressed higher levels of IL-10 and were more potent in suppressing effector T cell proliferation than were CD4⁺CD25⁺CD44^- cells.

Conclusion

This study indicates the presence of two novel phenotypes of T_reg cells in the thymus, the functional relevance of CD44 in defining T_reg cell subsets, and the role of both IL-10 and Foxp3 in modulating the function of T_reg cells.

Reviewers

This article was reviewed by Dr. M. Lenardo, Dr. L. Klein & G. Wirnsberger (nominated by Dr. JC Zungia-Pfluker), and Dr. E.M. Shevach.