Sara A. Miller

T-bet–dependent S1P5 expression in NK cells promotes egress from lymph nodes and bone marrow

During a screen for ethylnitrosourea-induced mutations in mice affecting blood natural killer (NK) cells, we identified a strain, designated Duane, in which NK cells were reduced in blood and spleen but increased in lymph nodes (LNs) and bone marrow (BM). The accumulation of NK cells in LNs reflected a decreased ability to exit into lymph. This strain carries a point mutation within Tbx21 (T-bet), which generates a defective protein. Duane NK cells have a 30-fold deficiency in sphingosine-1-phosphate receptor 5 (S1P5) transcript levels, and S1P5-deficient mice exhibit an egress defect similar to Duane. Chromatin immunoprecipitation confirms binding of T-bet to the S1pr5 locus. S1P-deficient mice exhibit a more severe NK cell egress block, and the FTY720-sensitive S1P1 also plays a role in NK cell egress from LNs. S1P5 is not inhibited by CD69, a property that may facilitate trafficking of activated NK cells to effector sites. Finally, the accumulation of NK cells within BM of S1P-deficient mice was associated with reduced numbers in BM sinusoids, suggesting a role for S1P in BM egress. In summary, these findings identify S1P5 as a T-bet–induced gene that is required for NK cell egress from LNs and BM.

Coordinated but physically separable interaction with H3K27-demethylase and H3K4-methyltransferase activities are required for T-box protein-mediated activation of developmental gene expression

During cellular differentiation, both permissive and repressive epigenetic modifications must be negotiated to create cell-type-specific gene expression patterns. The T-box transcription factor family is important in numerous developmental systems ranging from embryogenesis to the differentiation of adult tissues. By analyzing point mutations in conserved sequences in the T-box DNA-binding domain, we found that two overlapping, but physically separable regions are required for the physical and functional interaction with H3K27-demethylase and H3K4-methyltransferase activities. Importantly, the ability to associate with these histone-modifying complexes is a conserved function for the T-box family. These novel mechanisms for T-box-mediated epigenetic regulation are essential, because point mutations that disrupt these interactions are found in a diverse array of human developmental genetic diseases.

CCCTC-Binding Factor and the Transcription Factor T-bet Orchestrate T Helper 1 Cell-Specific Structure and Function at the Interferon-γ Locus

How cell type-specific differences in chromatin conformation are achieved and their contribution to gene expression are incompletely understood. Here we identify a cryptic upstream orchestrator of interferon-gamma (IFNG) transcription, which is embedded within the human IL26 gene, compromised of a single CCCTC-binding factor (CTCF) binding site and retained in all mammals, even surviving near-complete evolutionary deletion of the equivalent gene encoding IL-26 in rodents. CTCF and cohesins occupy this element in vivo in a cell type-nonspecific manner. This element is juxtaposed to two other sites located within the first intron and downstream of Ifng, where CTCF, cohesins, and the transcription factor T-bet bind in a T helper 1 (Th1) cell-specific manner. These interactions, close proximity of other elements within the locus to each other and to the gene encoding interferon-gamma, and robust murine Ifng expression are dependent on CTCF and T-bet. The results demonstrate that cooperation between architectural (CTCF) and transcriptional enhancing (T-bet) factors and the elements to which they bind is required for proper Th1 cell-specific expression of Ifng.

T-bet's Ability To Regulate Individual Target Genes Requires the Conserved T-Box Domain To Recruit Histone Methyltransferase Activity and a Separate Family Member-Specific Transactivation Domain

ABSTRACT Appropriate cellular differentiation and specification rely upon the ability of key developmental transcription factors to precisely establish gene expression patterns. These transcription factors often regulate epigenetic events. However, it has been unclear whether this is the only role that they play in functionally regulating developmental gene expression pathways or whether they also participate in downstream transactivation events at the same promoter. The T-box transcription factor family is important in cellular specification events in many developmental systems, and determining the molecular mechanisms by which this family regulates gene expression networks warrants attention. Here, we examine the mechanism by which T-bet, a critical T-box protein in the immune system, influences transcription. T-bet is both necessary and sufficient to induce permissive histone H3-K4 dimethyl modifications at the CXCR3 and IFN-γ promoters. A T-bet structure-function analysis revealed that the conserved T-box domain, with a small C-terminal portion, is required for recruiting histone methyltransferase activity to promoters. Interestingly, this function is conserved in the T-box family and is necessary, but not sufficient, to induce transcription, with an independent transactivation activity also required. The requirement for two separable functional activities may ultimately contribute to the stringent role for T-box proteins in establishing specific developmental gene expression pathways.

Molecular mechanisms by which T-bet regulates T-helper cell commitment

Summary:  Current research suggests that a number of newly identified T‐helper cell subsets retain a degree of context‐dependent plasticity in their signature cytokine expression patterns. To understand this process, a major challenge is to determine the molecular mechanisms by which lineage‐defining transcription factors regulate gene expression profiles in T‐helper cells. This mechanistic information will aid in our interpretation of whether a T‐helper cell state that expresses or retains the capacity to re‐express a combination of lineage‐defining transcription factors will have a stable or more flexible gene expression profile. Studies examining the developmental T‐box transcription factor T‐bet demonstrate the powerful information that is gained from combining in vivo analysis with basic biochemical and molecular mechanism approaches. Significantly, T‐bet’s ability to physically recruit epigenetic modifying complexes, in particular a Jmjd3 H3K27‐demethylase and a Set7/9 H3K4‐methyltransferase complex, to its target genes allows T‐bet to effectively reverse and establish new epigenetic states. This observation suggests that until T‐bet is permanently extinguished, T‐helper cells will retain some plasticity toward a T‐helper 1‐like program. Therefore, insight into the complexity of T‐helper cell commitment decisions will be aided by determining the molecular mechanisms for lineage‐defining transcription factors.

Common themes emerge in the transcriptional control of T helper and developmental cell fate decisions regulated by the T-box, GATA and ROR families

Cellular differentiation requires the precise action of lineage‐determining transcription factors. In the immune system, CD4+ T helper cells differentiate into at least three distinct effector lineages, T helper type 1 (Th1), Th2 and Th17, with the fate of the cell at least in part determined by the transcription factors T‐box expressed in T cells (T‐bet), GATA‐3 and retinoid‐related orphan receptor γt (RORγt), respectively. Importantly, these transcription factors are members of larger families that are required for numerous developmental transitions from early embryogenesis into adulthood. Mutations in members of these transcription factor families are associated with a number of human genetic diseases due to a failure in completing lineage‐specification events when the factor is dysregulated. Mechanistically, there are both common and distinct functional activities that are utilized by T‐box, GATA and ROR family members to globally alter the cellular gene expression profiles at specific cell fate decision checkpoints. Therefore, understanding the molecular events that contribute to the ability of T‐bet, GATA‐3 and RORγt to define T helper cell lineages can provide valuable information relevant to the establishment of other developmental systems and, conversely, information from diverse developmental systems may provide unexpected insights into the molecular mechanisms utilized in T helper cell differentiation.

Restricted MHC-peptide repertoire predisposes to autoimmunity.

MHC molecules associated with autoimmunity possess known structural features that limit the repertoire of peptides that they can present. Such limitation gives a selective advantage to TCRs that rely on interaction with the MHC itself, rather than with the peptide residues. At the same time, negative selection is impaired because of the lack of negatively selecting peptide ligands. The combination of these factors may predispose to autoimmunity. We found that mice with an MHC class II–peptide repertoire reduced to a single complex demonstrated various autoimmune reactions. Transgenic mice bearing a TCR (MM14.4) cloned from such a mouse developed severe autoimmune dermatitis. Although MM14.4 originated from a CD4+ T cell, dermatitis was mediated by CD8+ T cells. It was established that MM14.4+ is a highly promiscuous TCR with dual MHC class I/MHC class II restriction. Furthermore, mice with a limited MHC–peptide repertoire selected elevated numbers of TCRs with dual MHC class I/MHC class II restriction, a likely source of autoreactivity. Our findings may help to explain the link between MHC class I responses that are involved in major autoimmune diseases and the well-established genetic linkage of these diseases with MHC class II.

Cell Type-Specific Induction and Inhibition of Apoptosis by Herpes Simplex Virus Type 2 ICP10

ABSTRACT Herpes simplex virus (HSV) inhibits apoptosis induced by external stimuli in epithelial cells. In contrast, apoptosis is the primary outcome in HSV-infected lymphocytes. Here, we show that HSV type 2 (HSV-2) gene expression appears to be necessary for the induction of apoptosis in Jurkat cells, a T-cell leukemia line. HSV-2 ICP10 gene expression is sufficient to induce apoptosis in Jurkat cells, while its expression protects epithelial HEp-2 cells from apoptosis triggered by cycloheximide and tumor necrosis factor alpha. Thus, the effect of HSV-2 gene expression on the cellular apoptotic pathway appears to depend on the specific cell type.

An essential interaction between T-box proteins and histone-modifying enzymes.

Cellular differentiation requires precisely coordinated events to induce developmentally appropriate gene expression profiles. Lineage-defining transcription factors are responsible for establishing cell-type specific gene expression patterns during development. Recently, we reported a novel mechanism by which the T-box transcription factor T-bet interacts with JMJD3, an H3K27-demethylase, and Set7/9, an H3K4-methyltransferase (Genes Dev. 2008. 22: 2980-2993). Importantly, separable contact points in the T-box DNA binding domain mediate these interactions. Due to the highly conserved nature of the contact residues, these represent common interactions for the T-box family. Therefore, studies examining the molecular mechanisms that account for the ability of T-bet to regulate Ifng and Cxcr3, prototypic CD4+ Th1 genes, have provided novel insight into essential regulatory events that occur at diverse developmental transitions. In this article, we discuss the implications for these findings as well as explore the role epigenetic mechanisms may play in the development of human genetic diseases that are caused by T-box mutations, including congenital heart defects, cleft palate, pituitary deficiencies, and Ulnar-mammary syndrome.