A novel transcription factor, T-bet, directs Th1 lineage commitment

Peroxisome proliferator-activated receptor alpha negatively regulates T-bet transcription through suppression of p38 mitogen-activated protein kinase activation

Expression of the nuclear hormone receptor peroxisome proliferator-activated receptor alpha (PPARalpha) in resting lymphocytes was recently established, although the physiologic role(s) played by this nuclear hormone receptor in these cell types remains unresolved. In this study, we used CD4(+) T cells isolated from PPARalpha(-/-) and wild-type mice, as well as cell lines that constitutively express PPARalpha, in experiments designed to evaluate the role of this hormone receptor in the regulation of T cell function. We report that activated CD4(+) T cells lacking PPARalpha produce increased levels of IFN-gamma, but significantly lower levels of IL-2 when compared with activated wild-type CD4(+) T cells. Furthermore, we demonstrate that PPARalpha regulates the expression of these cytokines by CD4(+) T cells in part, through its ability to negatively regulate the transcription of T-bet. The induction of T-bet expression in CD4(+) T cells was determined to be positively influenced by p38 mitogen-activated protein (MAP) kinase activation, and the presence of unliganded PPARalpha effectively suppressed the phosphorylation of p38 MAP kinase. The activation of PPARalpha with highly specific ligands relaxed its capacity to suppress p38 MAP kinase phosphorylation and promoted T-bet expression. These results demonstrate a novel DNA-binding independent and agonist-controlled regulatory influence by the nuclear hormone receptor PPARalpha.

T-bet is required for optimal production of IFN-gamma and antigen-specific T cell activation by dendritic cells

IFN-gamma is well known as the signature cytokine of CD4+ T helper 1, CD8+, and natural killer cells, but recent studies demonstrate that antigen-presenting cells, in particular dendritic cells (DCs), are another potent source for this proinflammatory cytokine. T-bet, a transcription factor that controls IFN-gamma expression in CD4+ T cells, was reported recently to be expressed in human monocytes and myeloid DCs. In this study we investigate the role of T-bet in this important cell type. The development, differentiation, and activation of bone marrow and splenic DCs were unimpaired in mice lacking T-bet. However, T-bet was essential for the optimal production of IFN-gamma by both CD8alpha+ and CD8alpha- DCs. T-bet-deficient DCs were significantly impaired in their capacity to secrete IFN-gamma after both stimulation with IL-12 alone or in combination with IL-18. Further, T-bet-/- DCs were impaired in their ability to activate the T helper 1 program of adoptively transferred antigen-specific T cells in vivo. The rapid up-regulation of T-bet by IFN-gamma in DCs coupled with a function for DC-derived IFN-gamma in T cell activation may constitute a positive feedback loop to maximize type 1 immunity.

Transforming growth factor beta blocks Tec kinase phosphorylation, Ca2+ influx, and NFATc translocation causing inhibition of T cell differentiation

Transforming growth factor (TGF)-beta inhibits T cell proliferation and differentiation. TGF-beta has been shown to inhibit the expression of transcription factors such as GATA-3 and T-bet that play important roles in T cell differentiation. Here we show that TGF-beta inhibits T cell differentiation at a more proximal step. An early event during T cell activation is increased intracellular calcium levels. Calcium influx in activated T cells and the subsequent activation of transcription factors such as NFATc, events essential for T cell differentiation, are modulated by the Tec kinases that are downstream of the T cell receptor and CD28. We show that in stimulated CD4+ T cells, TGF-beta inhibits phosphorylation and activation of the Tec kinase Itk, increase in intracellular Ca2+ levels, NFATc translocation, and activation of the mitogen-activated protein kinase ERK that together regulate T cell differentiation. Our studies suggest that by inhibiting Itk, and consequently Ca2+ influx, TGF-beta limits T cell differentiation along both the Th1 and Th2 lineages.

Analysis of the mobility of signaling molecules in lymphocytes using fluorescence photobleaching techniques

The earliest biochemical events at the plasma membrane that lead to gene activation appear to depend not only on the local concentration of signaling molecules, but also on the mobility of these molecules at the site of signaling. To elucidate the process of signal transduction after receptor engagement in the immune system, it is important to analyze the mobility of signaling molecules in living lymphocytes. Current knowledge of the changes in intracellular localization and dynamic movements of signaling molecules during lymphocyte activation is limited. Here, we describe a method for known as fluorescence recovery after photobleaching, used to measure the diffusion mobility of a signaling molecule in a T cell line after T cell receptor stimulation. This method is a useful tool in studies of spatiotemporal regulation in immunoreceptor signaling.

What it takes to become an effector T cell: the process, the cells involved, and the mechanisms

When activated, CD4(+) T cells differentiate into two major sub-populations differing in their profiles of secreted cytokines. Type One, or TH1, cells secrete IL-2, IFNgamma, and TNFbeta and mediate a cellular immune response. Type Two, or TH2, cells secrete IL-4, IL-5, IL-6, IL-10, and IL-13 and potentiate a humoral response. The nature of any specific immune response depends on the interaction of antigen-presenting cells and T cells. The role of antigen-presenting cells is to respond to the nature of the immune challenge and signal differentiation of CD4(+) T cells. A number of factors are involved in the effector phenotype of T cells-nature and affinity of antigen, co-receptors signals, and cytokine environment. T-cell differentiation is a complex process comprising four defined developmental stages: activation of particular cytokine genes, commitment of the cells, silencing of the opposing cytokine genes, and stabilization of the phenotype. In each of these stages, the cells respond to the products of many signaling cascades from many membrane-bound receptors. The stages in development are mediated by different molecular mechanisms, involving control of gene expression and chromatin remodeling. This review centers on the factors, cellular interactions, and molecular mechanisms involved in the maturation of naïve CD4(+) T lymphocytes into fully effector cells.

The immunogenic peptide for Th1 development

Th1 cells play a critical role in the induction of cell-mediated immune responses and eradication of intracellular pathogen. The dose and route of immunization of antigen are also determining factors. It remains unclear what types of immunogenic peptide can induce the Th1 development and how it acts to regulate the immune system. Ag85B (also known as alpha antigen or MPT59) has been shown to be the most potent antigen species yet purified in humans and in mice. Strong Th1 responses have been elicited in vitro from PPD(+) asymptomatic individuals and Ag85B-primed cells of C57BL/6 (I-A(b)) mice. Peptide-25 (aa240-254) of Ag85B is a major Th1 cell epitope in I-A(b) mice. Active immunization of C57BL/6 mice with Peptide-25 can induce the development of CDT4(+) TCRVbeta11(+) and CDT4(+) TCRVbeta11(-)Th1 cells that produce IFN-gamma- and TNF-alpha, and protects against subsequent infection with live Mycobacterium tuberculosis H37Rv IFN-gamma. Peptide-25 has a potent adjuvant activity in both humoral and cell-mediated immune responses that is mediated by Th1 cells. We would propose to designate Peptide-25 as "Th1-inducing peptide".

Cutting edge: role of IL-27/WSX-1 signaling for induction of T-bet through activation of STAT1 during initial Th1 commitment

WSX-1 is a member of the class I cytokine receptor family with homology to IL-12R beta 2 and is essential for the initial mounting of Th1 responses. STAT1 interacts with tyrosine-phosphorylated WSX-1, and the conserved tyrosine residue of the cytoplasmic domain of WSX-1 is essential for transcriptional activation of STAT1. IL-27 stimulation induced STAT1 phosphorylation in wild-type but not in WSX-1-deficient naive CD4(+) T cells. Although IL-27 did not directly induce IFN-gamma production by wild-type CD4(+) T cells, IL-12-dependent IFN-gamma production was augmented by IL-27 stimulation in wild-type naive CD4(+) T cells but was impaired in WSX-1-deficient naive CD4(+) T cells. Additionally, IL-27 stimulation induced T-bet and IL-12R beta 2 expression in wild-type, but not in WSX-1-deficient, CD4(+) T cells. Thus, during the initiation of Th1 differentiation, the IL-27/WSX-1 signaling system plays a pivotal role by STAT1-mediated T-bet induction before the IL-12R system.

Integrating systemic information at the molecular level: cross-talk between steroid receptors and cytokine signaling on different target cells

An essential event in immune activation is the increase of cytokines in both plasma and immune tissues. Steroid hormones influence several adaptive responses in both health and disease. Cytokines and steroids have an intimate cross-communication in many systems, making possible a satisfactory adaptive response to environmental changes. The ultimate level of integration of the cytokine-steroids cross-talk is the molecular level. We have demonstrated this in four types of cross-talk mechanisms on different cells in which steroids have major roles: (1) The tumor necrosis factor (TNF)-glucocorticoid receptor (GR) transcriptional interaction in cellular targets of TNF-induced cytotoxicity. TNF potentiates the transactivation activity of GR and the priming with TNF increases the protective action of GR on TNF-induced cytotoxicity. (2) The GR-T cell receptor (TCR) antagonism in GR-TCR-induced T cell apoptosis and its modulation by cAMP. cAMP inhibits the TCR-induced apoptosis through a PKA-CREB-dependent mechanism and potentiates glucocorticoid-induced apoptosis by means of a CREB-independent mechanism. (3) The GR influence on Th1-Th2 cytokine expression and differentiation. Glucocorticoids inhibit the induction of GATA-3 and T-bet transcription factors. (4) The influence of ER/Smad-4 signaling cross-communication on prolactinoma pathogenesis. Physical and functional interactions between Smad-4 and estrogen receptors take place in prolactinoma cells, providing a molecular explanation to link the tumorigenic action of these two important players of prolactinoma pathogenesis. The molecular cross-talk between steroids and transcription factors is the mechanism that provides the basis for the outcome of adaptive responses integrating the systemic information provided by hormones and cytokines.

Cytokines and transcription factors that regulate T helper cell differentiation: new players and new insights

The differentiation of naive CD4+ T cells into subsets of T helper cells is a pivotal process with major implications for host defense and the pathogenesis of immune-mediated diseases. Though the basic paradigm was discovered more than 15 years ago, new discoveries continue to be made that offer fresh insights into the regulation of this process. T helper (TH)1 cells produce interferon (IFN)-gamma, promoting cell-mediated immunity and control of intracellular pathogens. We now know that TH1 differentiation is regulated by transcription factors such as T-bet, Stat1, and Stat4, as well as cytokines such as IL-12, IL-23, IL-27, type I IFNs, and IFN-gamma. In contrast, TH2 cells produce IL-4, which promotes allergic responses and is important in host defense against helminths. The transcription factors Stat6, GATA-3, c-Maf, NFATs, and the cytokine IL-4 promote TH2 differentiation. These key regulators of TH differentiation are the subject of this review.

Interleukin 12-augmented T cell proliferation of peripheral blood mononuclear cells from HIV-seropositive individuals is associated with interleukin 12 receptor beta 2 upregulation

Interleukin 12 (IL-12) production is believed to be impaired in individuals with HIV infection and this impairment manifests early in disease, when the CD4(+) cell counts are within normal values. The reduced antigen-specific and mitogen-stimulated T cell-proliferative responses that occur in HIV infection can be corrected by the addition of recombinant human interleukin 12 (rhIL-12). As the IL-12 receptor (IL-12R) is central to the IL-12 signaling pathway, we examined whether the augmentation of antigen-specific proliferation of HIV(+) peripheral blood mononuclear cells (PBMCs) related to altered IL-12R expression. rhIL-12 augmented antigen-specific proliferation of HIV(+) PBMCs but not of HIV(-) PBMCs. Examination of resting PBMCs from HIV(+) and HIV(-) donors showed that neither of these populations expressed IL-12R beta 1 or IL-12R beta 2 chains on their cell surface as detected by flow cytometry. However, examination of mRNA showed that both IL-12R beta 1 and IL-12R beta 2 mRNAs were markedly reduced in HIV(+) PBMCs when compared with HIV(-) PBMCs. After mitogen activation there was an increase in IL-12R beta 1 expression on the cell surface of HIV(+) and HIV(-) PBMCs and this level was not altered by coculture with rhIL-12 or interferon gamma (IFN-gamma). However, coculture of phytohemagglutinin (PHA)-activated HIV(+) or HIV(-) PBMCs with rhIL-12 (but not IFN-gamma) increased IL-12R beta 2 expression on the cell surface of both populations. Examination at the message level showed a correction of IL-12R beta 1 to normal levels with activation that was further enhanced by rhIL-12 coculture for both the HIV(+) and HIV(-) PBMCs. However, although the level of IL-12R beta 2 for the HIV(+) PBMCs was normalized by PHA, rhIL-12 caused a further augmentation. This information provides a strong link between IL-12R upregulation, and the significant improvement in antigen-specific HIV-proliferative responses seen with the addition of rhIL-12. It also reveals that the dysfunction in IL-12R expression seen in cells from HIV(+) patients occurs at the transcriptional level. In addition, we provide further evidence that IL-12R beta 1 and IL-12R beta 2 regulation in human PBMCs is independent of IFN-gamma.

Plasma cell differentiation and the unfolded protein response intersect at the transcription factor XBP-1

The transcription factor X-box binding protein 1 (XBP-1) is essential for the differentiation of plasma cells and the unfolded protein response (UPR). Here we show that UPR-induced splicing of XBP-1 by the transmembrane endonuclease IRE1 is required to restore production of immunoglobulin in XBP-1-/- mouse B cells, providing an integral link between XBP-1, the UPR and plasma cell differentiation. Signals involved in plasma cell differentiation, specifically interleukin-4, control the transcription of XBP-1, whereas its post-transcriptional processing is dependent on synthesis of immunoglobulins during B cell differentiation. We also show that XBP-1 is involved in controlling the production of interleukin-6, a cytokine that is essential for plasma cell survival. Thus, signals upstream and downstream of XBP-1 integrate plasma cell differentiation with the UPR.

Epigenetic control during lymphoid development and immune responses: aberrant regulation, viruses, and cancer

Methylation of cytosines controls a number of biologic processes such as imprinting and X chromosomal inactivation. DNA hypermethylation is closely associated with transcriptional silencing, while DNA hypomethylation is associated with transcriptional activation. Hypoacetylation of histones leads to compact chromatin with reduced accessibility to the transcriptional machinery. Methyl-CpG binding proteins can recruit corepressors and histone deacetylases; thus, the interplay between these epigenetic mechanisms regulates gene activation. Methylation has been implicated as an important mechanism during immune development, controlling VDJ recombination, lineage-specific expression of cell surface antigens, and transcriptional regulation of cytokine genes during immune responses. Aberrations in epigenetic machinery, either by genetic mutations or by somatic changes such as viral infections, are associated with early alterations in chronic diseases such as immunodeficiency and cancer.

The role of T lymphocytes in the pathogenesis of asthma

There is considerable evidence to support a role for T cells in asthma, particularly the involvement of T(H)2 cells both in atopic allergic asthma and in nonatopic and occupational asthma. There might also be a minor contribution from T(C)2 CD8+ T cells. Several T(H)2 cytokines have the potential to modulate airway inflammation, particularly IL-13, which induces airway hyperresponsiveness independently of IgE and eosinophilia in animal models. The identification of transcription factors controlling T(H)1 and T(H)2 development further support the T(H)2 hypothesis because GATA3 is overexpressed and T-bet is underexpressed in the asthmatic airway. Specific T cell directed immunotherapy might allow induction, modulation, or both of T-cell responses, and elucidation of the mechanisms of regulatory T cells might allow further optimization of immunotherapy. Recent advances in our understanding of dendritic cell function in directing T-cell responses might uncover further therapeutic targets. The efficacy of cyclosporin A and anti-CD4 treatment in patients with chronic severe asthma argues for continued T-cell involvement, but whether remodeling contributes to pathology inaccessible to anti-inflammatory treatment or T-cell immunotherapy will be an important future question.

GATA-3 suppresses Th1 development by downregulation of Stat4 and not through effects on IL-12Rbeta2 chain or T-bet

To further understand the interaction among GATA-3, Stat4, and T-bet in helper T cell development, we first showed that retroviral expression of GATA-3 in developing Th1 cells suppresses Th1 development through downregulation of Stat4 rather through downregulation of the IL-12Rbeta2 chain. Correspondingly, Stat4 levels are greatly suppressed during physiological Th2 development. Then, using cells doubly infected with GFP- and YFP-expressing retroviruses, we showed that retroviral GATA-3 expression in developing Th1 cells does not block Th1 development in cells coexpressing Stat4 but does so in cells coexpressing T-bet. Finally, we showed that retroviral Stat4 expression could facilitate Th2-->Th1 conversion in cells bearing an IL-12Rbeta2 transgene, even in cells lacking T-bet. These findings reassert that Stat4 signaling is a central element of Th1/Th2 development.

Regulation of the generation and maintenance of T-cell memory: a direct, default pathway from effectors to memory cells

Memory T cells are derived directly from effector cells without need for additional antigen, TcR triggering or induced cytokines. A large fraction of effectors can become memory cells without division, supporting a default pathway with little further differentiation. This suggests that the same signals during infection/vaccination determine the extent and nature of both effector and memory cell development.

SWAP-70-like adapter of T cells, an adapter protein that regulates early TCR-initiated signaling in Th2 lineage cells

We describe the isolation of a protein, SWAP-70-like adapter of T cells (SLAT), which is expressed at high levels in thymocytes and differentiated Th2 cells. SLAT expression was upregulated in differentiating Th2 cells and downregulated in Th1 cells. Ectopic SLAT expression exerted positive or negative effects on IL-4 versus IFNgamma induction, respectively. TCR signaling induced translocation of SLAT to the immunological synapse and its association with ZAP-70 kinase. SLAT reduced the association of ZAP-70 with TCR-zeta and interfered with ZAP-70 but not Lck signaling. Consistent with these results, pharmacological inhibition of ZAP-70 also induced Th2 skewing. Thus, SLAT is a protein which plays a role in Th2 development and/or activation, perhaps by interfering with ZAP-70 signaling.

NF-kappa B1 is required for optimal CD4+ Th1 cell development and resistance to Leishmania major

The NF-kappaB family of transcription factors regulates the expression of a wide range of immune response genes involved in immunity to pathogens. However, the need for individual family members in regulating innate and adaptive immune responses in vivo has yet to be clearly defined. We investigated the role of NF-kappaB1 in the induction of protective IL-12-dependent Th1 cell responses following infection with the intracellular protozoan parasite Leishmania major. Whereas wild-type C57BL/6 mice controlled parasite replication, NF-kappaB1 knockout (KO) mice were susceptible to infection, developing chronic unresolving lesions associated with persistent parasites. There was a profound defect in Ag-specific CD4(+) T cell proliferation and IFN-gamma production in infected KO mice, although innate responses-including IL-12 production and control of intracellular parasite replication by macrophages-were intact. In vitro polyclonal stimulation of purified naive KO T cells revealed an intrinsic defect in CD4(+) T cell proliferation associated with reduced IL-2 receptor expression, but operating independently of APC function and IL-2 production. Critically, the frequency of proliferating KO CD4(+) T cells secreting IFN-gamma matched that of wild-type cells, suggesting that NF-kappaB1 was not required for efficient transcription of the IFN-gamma gene. Taken together, these results identify a novel role for NF-kappaB1 in CD4(+) T cell proliferation and the development of Th1 cell responses required for protective immunity against intracellular pathogens.

Interleukin-12 and the regulation of innate resistance and adaptive immunity

Interleukin-12 (IL-12) is a heterodimeric pro-inflammatory cytokine that induces the production of interferon-gamma (IFN-gamma), favours the differentiation of T helper 1 (T(H)1) cells and forms a link between innate resistance and adaptive immunity. Dendritic cells (DCs) and phagocytes produce IL-12 in response to pathogens during infection. Production of IL-12 is dependent on differential mechanisms of regulation of expression of the genes encoding IL-12, patterns of Toll-like receptor (TLR) expression and cross-regulation between the different DC subsets, involving cytokines such as IL-10 and type I IFN. Recent data, however, argue against an absolute requirement for IL-12 for T(H)1 responses. Our understanding of the relative roles of IL-12 and other factors in T(H)1-type maturation of both CD4+ and CD8+ T cells is discussed here, including the participation in this process of IL-23 and IL-27, two recently discovered members of the new family of heterodimeric cytokines.

Contrasting alloreactive CD4+ and CD8+ T cells: there's more to it than MHC restriction

Surface expression of CD4 or CD8 is commonly used to identify T-cell subsets that recognize antigen presented by class II MHC or class I MHC, respectively. This holds true for T cells that respond to allogeneic MHC molecules that are directly recognized as foreign, as well as peptides from allogeneic MHC molecules that are indirectly presented by self MHC molecules. CD4 or CD8 expression was initially believed to define cytokine secreting helper T cells or cytotoxic cells, respectively. However, this association of phenotype and function is not absolute, in that CD4+ cells may possess lytic activity and CD8+ cells secrete cytokines, notably IFNgamma. Recently, additional fundamental differences in the immunobiology of these T-cell subsets have been identified. These include differences in costimulatory requirements, cytokine responsiveness, cytokine production, cell survival, and the maintenance of memory. This review will survey these differences, emphasizing alloreactive T-cell responses as well as relevant observations that have been made in other systems.

TH2 dominance and defective development of a CD8+ dendritic cell subset in Id2-deficient mice

BACKGROUND

Although the TH1/TH2 balance is important in many clinical situations, the regulatory mechanisms in vivo have not been well elucidated.

OBJECTIVE

We sought to characterize the immunologic status of mice lacking Id2, an inhibitor of basic helix-loop-helix transcription factors.

METHODS

We analyzed serum immunoglobulin levels, gene-expression profiles in the spleen, TH1/TH2 balance, and dendritic cell (DC) populations of Id2-/- mice.

RESULTS

Serum levels of TH2-mediated IgG1 and IgE were increased more than 10-fold in Id2-/- mice without antigenic stimulation. Gene-expression analysis in Id2-/- splenocytes revealed enhanced expression of TH2-related genes, such as IL-4, and reduced expression of TH1-related genes, including IFN-gamma and IL-12. Intracellular cytokine staining also confirmed that Id2-/- splenic CD4+ T cells are substantially skewed to TH2 cells. However, Id2-/- naive CD4+ T cells differentiated into TH1 cells comparably with wild-type T cells under the appropriate culture conditions. Id2-/- mice displayed a selective and remarkable reduction of the CD8+ DC subset, which is known to induce preferential TH1 differentiation.

CONCLUSION

Id2 is an indispensable regulator of the TH1/TH2 balance, possibly through the proper development of CD8alpha+ DCs, and could be a novel target to treat allergic diseases.

Activated STAT4 and a functional role for IL-12 in human Peyer's patches

T cells in the Peyer's patches (PP) of the human ileum are exposed to a myriad of dietary and bacterial Ags from the gut lumen. Recall proliferative responses to common dietary Ags are readily demonstrable by PP T cells from healthy individuals, and the cytokine response is dominated by IFN-gamma. Consistent with Th1 skewing, PP cells spontaneously secrete IL-12p70, and IL-12p40 protein can be visualized underneath the PP dome epithelium. In this study, we have analyzed IL-12 signaling in PP and investigated whether IL-12 plays a functional role. CD3+ T lymphocytes isolated from PP and adjacent ileal mucosa spontaneously secrete IFN-gamma with negligible IL-4 or IL-5. RNA transcripts for IL-12Rbeta2, the signaling component of the IL-12R, are present in purified CD4+ and CD8+ T PP lymphocytes. Active STAT4, a transcription factor essential for IL-12-mediated Th1 differentiation, is readily detectable in biopsies from PP and ileal mucosa and STAT4-DNA binding activity is demonstrable by EMSA. Nuclear proteins from CD3+ T PP lymphocytes contain STAT4 and T-bet, a transcription factor selectively expressed in Th1 cells. Stimulation of freshly isolated PP cells with staphylococcal enterotoxin B dramatically enhanced the production of IFN-gamma, an effect which was largely inhibited by neutralizing anti-IL-12 Ab. These data show that IL-12 in human PP is likely to be responsible for the Th1-dominated cytokine response of the human mucosal immune system.

Impaired accumulation and function of memory CD4 T cells in human IL-12 receptor beta 1 deficiency

Defects in IL-12 production or IL-12 responsiveness result in a vulnerability to infection with non-viral intracellular organisms, but the immunological mechanisms responsible for this susceptibility remain poorly understood. We present an immunological analysis of a patient with disseminated Salmonella enteritidis and a homozygous splice acceptor mutation in the IL-12Rbeta1-chain gene. This mutation resulted in the absence of IL-12Rbeta1 protein on PBMC and an inability of T cells to specifically bind IL-12 or produce IFN-gamma in response to either IL-12 or IL-23. The accumulation of memory (CD45R0(high)) CD4 T cells that were CCR7(high) (putative central memory cells) was normal or increased for age. Central memory CD4 T cells of the patient and age-matched controls were similar in having a low to undetectable capacity to produce IFN-gamma after polyclonal stimulation. In contrast, the patient had a substantial decrease in the number of CCR7(neg/dull) CD45R0(high) memory CD4 T cells (putative effector memory cells), and these differed from control cells in having a minimal ability to produce IFN-gamma after polyclonal stimulation. Importantly, tetanus toxoid-specific IFN-gamma production by PBMC from the patient was also significantly reduced compared with that in age-matched controls, indicating that signaling via the IL-12Rbeta1-chain is generally necessary for the in vivo accumulation of human memory CD4 T cells with Th1 function. These results are also consistent with a model in which the IL-12Rbeta1 subunit is necessary for the conversion of central memory CD4 T cells into effector memory cells.

Memory and flexibility of cytokine gene expression as separable properties of human T(H)1 and T(H)2 lymphocytes

CD4+ T cell priming under T helper type I (T(H)1) or T(H)2 conditions gives rise to polarized cytokine gene expression. We found that in these conditions human naive T cells acquired stable histone hyperacetylation at either the Ifng or Il4 promoter. Effector memory T cells showed polarized cytokine gene acetylation patterns in vivo, whereas central memory T cells had hypoacetylated cytokine genes but acquired polarized acetylation and expression after appropriate stimulation. However, hypoacetylation of the nonexpressed cytokine gene did not lead to irreversible silencing because most T(H)1 and T(H)2 cells acetylated and expressed the alternative gene when stimulated under opposite T(H) conditions. Such cytokine flexibility was absent in a subset of T(H)2 cells that failed to up-regulate T-bet and to express interferon-gamma when stimulated under T(H)1 conditions. Thus, most human CD4+ T cells retain both memory and flexibility of cytokine gene expression.

A unique role for IL-23 in promoting cellular immunity

Recent discoveries of interleukin (IL)-23, its receptor, and its signal-transduction pathway add to our understanding of cellular immunity. IL-23 is a heterodimer, comprising IL-12 p40 and the recently cloned IL-23-specific p19 subunit. IL-23 uses many of the same signal-transduction components as IL-12, including IL-12Rbeta1, Janus kinase 2, Tyk2, signal transducer and activator of transcription (Stat)1, Stat3, Stat4, and Stat5. This may explain the similar actions of IL-12 and IL-23 in promoting cellular immunity by inducing interferon-gamma production and proliferative responses in target cells. Additionally, both cytokines promote the T helper cell type 1 costimulatory function of antigen-presenting cells. IL-23 does differ from IL-12 in the T cell subsets that it targets. Whereas IL-12 acts on naïve CD4+ T cells, IL-23 preferentially acts on memory CD4+ T cells. This review summarizes recent advances regarding IL-23, providing a functional and mechanistic basis for the unique niche that IL-23 occupies in cellular immunity.

Cutting edge: distal regulatory elements are required to achieve selective expression of IFN-gamma in Th1/Tc1 effector cells

Using a transgenic approach, we analyzed the contribution of introns located within the IFN-gamma gene and distal regulatory regions to IFN-gamma gene expression. Intron 1 and 3 from the IFN-gamma gene displayed strong enhancer activity. This activity appeared to be dependent upon integration into the genome but resulted in a loss of Th1 selectivity. We also found that distal regulatory elements are not required for high level expression of the human IFN-gamma gene, but rather for cell lineage-specific expression. An 8.6-kb human IFN-gamma transgene was sufficient to yield high level expression but a 191-kb IFN-gamma transgene with approximately 90 kb of flanking 5' and 3' sequence was necessary to achieve both high level and Th1 selective expression of human IFN-gamma.

Th2 cell-specific cytokine expression and allergen-induced airway inflammation depend on JunB

Naïve CD4+ T cells differentiate into effector T helper 1 (Th1) or Th2 cells, which are classified by their specific set of cytokines. Here we demonstrate that loss of JunB in in vitro polarized Th2 cells led to a dysregulated expression of the Th2-specific cytokines IL-4 and IL-5. These cells produce IFN-gamma and express T-bet, the key regulator of Th1 cells. In line with the essential role of Th2 cells in the pathogenesis of allergic asthma, mice with JunB-deficient CD4+ T cells exhibited an impaired allergen-induced airway inflammation. This study demonstrates novel functions of JunB in the development of Th2 effector cells, for a normal Th2 cytokine expression pattern and for a complete Th2-dependent immune response in mice.

Cysteinyl leukotriene receptors

Cysteinyl leukotrienes (CysLTs) are important inflammatory mediators in asthma and allergic disorders. Two types of CysLT receptors, CysLT(1) and CysLT(2), which were originally defined pharmacologically based on their sensitivity to CysLT(1) specific antagonists, are responsible for most of the known CysLT biological actions. The regulation of CysLT receptor expression and signaling in disease processes is largely unclear. Recent molecular cloning of both receptor subtypes from several different species will greatly facilitate future research in understanding CysLT signal transduction mechanisms. Expression of the relatively better-studied CysLT(1) is verified in lung tissues and peripheral blood cells. Elucidating how this receptor mediates airway inflammation will deepen our understanding of asthma etiology. On the other hand, detection of CysLT(2) in the heart, brain, and adrenal glands will inject new excitement into the search for novel CysLT functions. This review summarizes receptor cloning, ligand binding, expression, signaling, and functions in an effort to bridge early pharmacological studies to future studies at the molecular level.

Progressive differentiation and selection of the fittest in the immune response

T cells are stimulated by stochastic exposure to antigen-presenting cells and cytokines. We review evidence that the level of signal that is accumulated determines progression through hierarchical thresholds for proliferation and differentiation, leading to the generation of various intermediates and effector T cells. These cells are then selected to enter the memory pool according to their fitness--that is, their capacity to access and use survival signals. We suggest that the intermediates that are generated by antigenic stimulation of T and B cells persist as central memory cells, which can mount secondary responses to antigen and maintain appropriate levels of effector cells and antibodies throughout the lifetime of an individual.

T-bet as a possible therapeutic target in autoimmune disease

The prominent role of pro-inflammatory cytokines produced by T helper-1 (T(H1)) cells in regulating autoimmune responses in vitro and in vivo has been demonstrated. Recent observations of T cell polarisation by regulatory transcription factors--especially T-bet (T-box expressed in T cells)--raise the question of their influence in controlling autoimmune diseases. Here, the authors summarise recent observations of the role of T-bet in controlling chronic inflammatory and autoimmune diseases and discuss the implications of these findings for future therapeutic approaches.

IFN-gamma represses IL-4 expression via IRF-1 and IRF-2

Polarization of CD4(+) T helper cells toward either a Th1 or Th2 response can significantly influence host immunity to pathogens. IL-4 and IFN-gamma are the signature cytokines of Th2 and Th1 cells, respectively. IFN-gamma was shown to assist Th1 development by promoting IL-12 and IL-12 receptor expression. So far, direct influence of Th2 cytokine expression by IFN-gamma has not been described. We show here that IFN-gamma directly suppresses IL-4 gene expression. IRF-1 and IRF-2 induced by IFN-gamma bind to three distinct IL-4 promoter sites and function as transcriptional repressors. Our data demonstrate a direct negative feedback of IFN-gamma on expression of the Th2 cytokine gene IL-4 and, thus, provide evidence for another important mechanism by which IFNgamma assists Th1 and attenuates Th2 responses.

A genomic view of helper T cell subsets

Genomic-scale gene expression profiling in combination with the availability of a draft sequence of the human genome is beginning to revolutionize the way immunology is done. The possibility of measuring levels of gene expression for tens of thousands of genes simultaneously and in a quantitative fashion aids in the definition of a comprehensive molecular phenotype of cells and cellular processes of the immune system in health and disease. T helper lymphocytes are an essential element of appropriate immune responses to pathogens. To achieve effective immunity, T helper cells differentiate into at least two specialized subsets that direct type 1 and type 2 immune responses. Here, I discuss recent progress that has been made in our understanding of the genetic program that controls the development and functional properties of helper T cell subsets.

The lineage decisions of helper T cells

After encountering antigen, helper T (T(H)) cells undergo differentiation to effector cells, which can secrete high levels of interferon-gamma, interleukin-4 (IL-4), IL-10 and other immunomodulators. How T(H) cells acquire, and remember, new patterns of gene expression is an area of intensive investigation. The process is remarkably plastic, with cytokines being key regulators. Extrinsic signals seem to be integrated into cell-intrinsic programming, in what is becoming an intriguing story of regulated development. We summarize the latest insights into mechanisms that govern the lineage choices that are made during T(H)-cell responses to foreign pathogens.

Identification of highly expressed genes in peripheral blood T cells from patients with atopic dermatitis

BACKGROUND

Analysis of genes that are differentially expressed in patients with atopic dermatitis (AD) and normal individuals will provide important information on the underlying molecular pathogenetic mechanisms of AD.

METHODS

Transcript of freshly isolated peripheral blood T cells from 59 individuals were analyzed with a fluorescent differential display (FDD) method. Ninety-two differentially expressed genes were identified in this manner. Additionally, real-time quantitative RT-PCR was employed to investigate the expression of the FDD-selected genes and also genes related to T cell function.

RESULTS

A number of genes, including CC chemokine receptor 4, T cell-specific tyrosine kinase (Emt/Itk), integrin beta1, integrin alpha6, IQGAP1 and MAR/SAR DNA-binding protein (SATB1), were shown to be more highly expressed in patients with moderate and/or severe AD than in controls or patients with mild AD. Because the products of these upregulated genes influence chemotaxis, adhesion, migration and Th2 polarization, it is suggested that in more severe AD, circulating T cells may function differently in this regard. Several other genes, the role of which in T cell function is currently unknown, were also found to be differentially expressed in AD. These included the heat shock protein 40 and vasopressin-activated calcium-mobilizing receptor 1.

CONCLUSION

The upregulated genes identified in this work may serve as useful markers for moderate to severe AD as opposed to normal or mild AD and also as markers indicating progression to more severe AD. Further functional characterization will provide a better understanding of the pathophysiology of circulating T cells in AD.

Controlling CD4 gene expression during T cell lineage commitment

T cell lineage commitment as the double-positive (DP) thymocyte matures into the single-positive (SP) T cell requires the irreversible repression or maintenance of CD4 gene expression. Signals transmitted from the T cell antigen receptor (TCR) during thymic selection are believed to be linked to the transcriptional regulation of the CD4 gene; thus, a study of the factors that control CD4 gene expression may lead to further insight into the molecular mechanisms that drive T cell development. This review discusses the work conducted to date to identify and characterize the transcriptional control elements in the CD4 locus and the factors that mediate their function. From these studies, it is clear that the molecular mechanisms controlling CD4 gene expression are very complex and are controlled by many different signals as the thymocyte develops.

Cytokine memory of T helper lymphocytes

Memory is one of the key features of the adaptive immune system. Specific T and B lymphocytes are primed for a particular antigen and upon challenge with it will react faster than naive lymphocytes. They also memorize the expression of key effector molecules, in particular cytokines, which determine the type and scale of an immune reaction. While in primary activations differential expression of cytokine genes is dependent on antigen-receptor signaling and differentiation signals, in later activations the expression is triggered by antigen-receptor signaling and dependent on the cytokine memory. The molecular basis of the cytokine memory implies differential expression of transcription factors and epigenetic modifications of cytokine genes and gene loci. GATA-3 for Th2 and T-bet for Th1 cells expressing interleukin-4 or interferon-gamma, respectively, are prime candidates for key transcription factors of cytokine memory. The essential role of epigenetic modifications is suggested by the requirement of DNA synthesis for the establishment of a cytokine memory in Th lymphocytes. At present the molecular link between transcription factors and epigenetic modifications of cytokine genes in the establishment and maintenance of cytokine memory is not clear. The initial cytokine memory is not stable against adverse differentiation signals, while in repeatedly stimulated lymphocytes it is stabilized by a variety of mechanisms.

Altered IL-4 mRNA stability correlates with Th1 and Th2 bias and susceptibility to hypersensitivity pneumonitis in two inbred strains of mice

Previously, we have shown in a model of hypersensitivity pneumonitis that Th1-biased C57BL/6 mice are susceptible and Th2-biased DBA/2 mice are resistant to disease. We also showed that this was explained in part by differential regulation of IL-12 by IL-4. For these reasons, we postulated that C57BL/6 and DBA/2 mice differentially express IL-4. In this study, we show that C57BL/6 immune cells express Th2 but not Th1 cytokines at lower levels than DBA/2 cells. We also found that C57BL/6 splenocytes exhibit decreased mRNA stability of Th2 cytokines, relative to DBA/2 splenocytes. Stability of IL-2 and IFN-gamma were similar in the two strains of mice. Differences in Th2 cytokine mRNA stability between C57BL/6 and DBA/2 cells were not due to sequence polymorphism at specific regions of the IL-4/IL-13 locus. Furthermore, expression of Th1- and Th2-specific transcription factors T-bet and GATA-3, as well as the nuclear factor of activated T cells transcription factor, NFATc, was not significantly different between the two mice. Our data suggest that decreased mRNA stability of Th2 cytokines in C57BL/6 splenocytes may underlie the differential susceptibility to hypersensitivity pneumonitis between C57BL/6 and DBA/2 mice. Moreover, our results indicate that regulation of mRNA stability may serve as an important mechanism underlying Th1/Th2 immune polarization.

Identification of a conserved GATA3 response element upstream proximal from the interleukin-13 gene locus

Differentiation of naive CD4 T cells into type 2 helper (Th2) cells is accompanied by chromatin remodeling of Th2 cytokine gene loci. Hyperacetylation of histone H3 on nucleosomes associated with the interleukin (IL)-4, IL-13 and IL-5 genes was observed in developing Th2 cells but not in Th1 cells. Histone hyperacetylation on IL-5 gene-associated nucleosomes was Th2-specific but occurred with delayed kinetics, and hyperacetylation on RAD50 gene-associated nucleosomes was T cell antigen receptor stimulation-dependent but not Th2-specific. The induction of the Th2-specific histone hyperacetylation was STAT6- and GATA3-dependent, and interestingly, it was accompanied by the expression of intergenic transcripts within the IL-13 and IL-4 gene loci. A conserved GATA3 response element (CGRE) containing four GATA consensus sequences was identified 1.6 kbp upstream from the IL-13 gene, corresponding with the 5'-border of the Th2-specific histone hyperacetylation region. The CGRE was shown to bind to GATA3, histone acetyltransferase complexes including CBP/p300, and RNA polymerase II. Also, the CGRE showed a significant enhancing effect on the Th2 cytokine gene promoters. Thus, the CGRE may play a crucial role for GATA3-mediated targeting and downstream spreading of core histone hyperacetylation within the IL-13 and IL-4 gene loci.

Type 1 and type 2 responses in regulation of Ig isotype expression in cattle

Regulation of humoral immune responses is multifactorial involving appropriate activation, costimulation and the presence of specific soluble factors. Polarized type 1 or type 2 humoral responses in the laboratory mouse have been linked to expression of specific cytokines and thus can be used to provide insight into the type of response generated by infection. For example, IFN-gamma has been linked to IgG2a and IgG3 production, IL-4 to IgG1 and IgE production and TGF-beta to IgA production. Unlike the laboratory mouse, generally housed under defined conditions, highly skewed isotype expression patterns generally occur in cattle in chronic infections. A few examples of polarized responses have been noted in chronic experimental or naturally occurring infections including F. hepatica, M. paratuberculosis, C. parvum and B. abortus. In vitro studies using purified bovine B cells and various forms of costimulation and cytokines have demonstrated that isotype responses can be polarized under certain experimental conditions in vitro. That is, IgG1 expression is positively regulated by IL-4 and IgG2 expression is positively regulated by IFN-gamma. Other as yet unidentified factors may play pivotal roles in regulating humoral immune responses in large ruminant species in vivo. This possibility is best exemplified by recent studies using DNA vaccines in cattle that have been demonstrated in the mouse to be generally polarizing to a type 1 response. Surprisingly, studies in cattle using plasmid DNA as vaccination material show an almost exclusive IgG1 response. Based on a number of studies using T cell clones and various biological assays, it is clear that the classical roles of many cytokines in the laboratory mouse do not extrapolate entirely or at all to cattle. Thus, the design of adjuvants and immune modulators should be based on studies done in cattle or using bovine cells. Based on studies to date, several "holes" in the cytokine repertoire exist and these roles may be assumed by unique factors or activities of other known cytokines.

Murine neonatal CD4+ lymph node cells are highly deficient in the development of antigen-specific Th1 function in adoptive adult hosts

It is well established that murine neonates are biased toward Th2 responses. Th2-dominant responses are observed following immunization with a variety of Ags, using different carrier/adjuvant systems, and are seen in both BALB/c and C57BL/6 mice. Therefore, Th2 skewing appears to be a universal phenomenon unique to the neonatal period. One important question about this phenomenon is whether these responses are due to T cell intrinsic properties or are regulated by the neonatal environment. Here we have addressed this issue by transferring neonatal or adult CD4(+) lymph node cells to adoptive adult recombinase-activating gene 2(-/-) hosts and studied the development of Th responses. Neonatal CD4(+) cells were highly deficient in the development of both primary and secondary Ag-specific Th1 responses. This did not appear to be due to anergy of a developed population, since exogenous IL-2 only marginally increased production of the Th1 cytokine IFN-gamma. This profound Th1 deficiency was observed despite similar proliferation by neonatal and adult cells within the recombinase-activating gene 2(-/-) hosts. Moreover, neonatal CD4(+) cells up-regulated activation markers in a manner similar to adult CD4(+) cells. Therefore, although their proliferation and phenotypic maturation proceeded normally, neonatal CD4(+) cells appeared to be intrinsically deficient in the functional maturation of Th1 lineage cells. These results offer a candidate explanation for the reduced graft-vs-host responses observed following transplantation of cord blood cells or murine neonatal lymphoid cells to allogeneic adult hosts.

A minimal IFN-gamma promoter confers Th1 selective expression

Th1 and Th2 cells differentiate from naive precursors to effector cells that produce either IFN-gamma or IL-4, respectively. To identify transcriptional paths leading to activation and silencing of the IFN-gamma gene, we analyzed transgenic mice that express a reporter gene under the control of the 5' IFN-gamma promoter. We found that as the length of the promoter is increased, -110 to -225 to -565 bp, the activity of the promoter undergoes a transition from Th1 nonselective to Th1 selective. This is due, at least in part, to a T box expressed in T cells-responsive unit within the -565 to -410 region of the IFN-gamma promoter. The -225 promoter is silent when compared with the -110 promoter and silencing correlates with Yin Yang 1 binding to the promoter. The p38 mitogen-activated protein kinase signaling pathway, which also regulates IFN-gamma gene transcription, regulates the -70- to -44-bp promoter element. Together, the results demonstrate that a minimal IFN-gamma promoter contains a T box expressed in T cells responsive unit and is sufficient to confer Th1 selective expression upon a reporter.

Cutting edge: the differential involvement of the N-finger of GATA-3 in chromatin remodeling and transactivation during Th2 development

The development of Th subset is accompanied by subset-specific chromatin remodeling of cytokine gene loci. In this study, we show that the C-terminal, but not the N-terminal zinc finger (N-finger) of GATA-3 mediates the association with the IL-4/IL-13 intergenic DNase I hypersensitive site and the induction of an extended DNase I hypersensitivity on the IL-4/IL-13 locus. Consistently, deletion of the transactivation domains or the C-finger, but not the N-finger, abrogated the induction of IL-4 and IL-13 as well as the down-regulation of IFN-gamma. In contrast, the N-finger of GATA-3 was indispensable for the binding to the IL-5 promoter and the induction of IL-5. The selective use of the N-finger may underlie the differential roles of GATA-3 in the induction of IL-4, IL-13, and IL-5.

Activated STAT4 has an essential role in Th1 differentiation and proliferation that is independent of its role in the maintenance of IL-12R beta 2 chain expression and signaling

In this study we demonstrated that CD4(+) T cells from STAT4(-/-) mice exhibit reduced IL-12R expression and poor IL-12R signaling function. This raised the question of whether activated STAT4 participates in Th1 cell development mainly through its effects on IL-12 signaling. In a first approach to this question we determined the capacity of CD4(+) T cells from STAT4(-/-) bearing an IL-12Rbeta2 chain transgene (and thus capable of normal IL-12R expression and signaling) to undergo Th1 differentiation when stimulated by Con A and APCs. We found that such cells were still unable to exhibit IL-12-mediated IFN-gamma production. In a second approach to this question, we created Th2 cell lines (D10 cells) transfected with STAT4-expressing plasmids with various tyrosine-->phenylalanine mutations and CD4(+) T cell lines from IL-12beta2(-/-) mice infected with retroviruses expressing similarly STAT4 mutations that nevertheless express surface IL-12Rbeta2 chains. We then showed that constructs that were unable to support STAT4 tyrosine phosphorylation (in D10 cells) as a result of mutation were also incapable of supporting IL-12-induced IFN-gamma production (in IL-12Rbeta2(-/-) cells). Thus, by two complementary approaches we demonstrated that activated STAT4 has an essential downstream role in Th1 cell differentiation that is independent of its role in the support of IL-12Rbeta2 chain signaling. This implies that STAT4 is an essential element in the early events of Th1 differentiation.

Interleukin 21 is a T helper (Th) cell 2 cytokine that specifically inhibits the differentiation of naive Th cells into interferon gamma-producing Th1 cells

The cytokine potential of developing T helper (Th) cells is directly shaped both positively and negatively by the cytokines expressed by the effector Th cell subsets. Here we find that the recently identified cytokine, interleukin (IL)-21, is preferentially expressed by Th2 cells when compared with Th1 cells generated in vitro and in vivo. Exposure of naive Th precursors to IL-21 inhibits interferon (IFN)-gamma production from developing Th1 cells. The repression of IFN-gamma production is specific in that the expression of other Th1 and Th2 cytokines is unaffected. IL-21 decreases the IL-12 responsiveness of developing Th cells by specifically reducing both signal transducer and activator of transcription 4 protein and mRNA expression. These results suggest that Th2 cell-derived IL-21 regulates the development of IFN-gamma-producing Th1 cells which could serve to amplify a Th2 response.

The role of transforming growth factor beta in atherosclerosis: novel insights and future perspectives

PURPOSE OF REVIEW

Atherosclerosis is a disease of the arterial wall that seems to be tightly modulated by the local inflammatory balance. Transforming growth factors beta 1, 2 and 3 are cytokines/growth factors with broad activities on cells and tissues in the cardiovascular system, and have been suggested to play a role in the pathogenesis of atherosclerosis.

RECENT FINDINGS

In the present review, we discuss recent developments in the role of transforming growth factor beta in the regulation of the immuno-inflammatory balance that modulates atherosclerosis. Such studies strongly suggest that the inhibition of endogenous transforming growth factor beta signalling favours the development of atherosclerotic lesions with an increased inflammatory component (T cells and macrophages) and decreased collagen content, features that are characteristic of unstable atherosclerotic plaques.

SUMMARY

Transforming growth factor beta is identified as a critical modulator of the immuno-inflammatory balance in atherosclerosis, and a crucial plaque-stabilizing factor. Future studies should aim at defining the precise molecular mechanisms responsible for this protective effect, and developing immunomodulatory strategies based on the promotion of transforming growth factor beta activity (T regulatory T helper type 3 cells) to limit disease complications.