Role of T-bet in commitment of TH1 cells before IL-12-dependent selection

Sustained NFAT signaling promotes a Th1-like pattern of gene expression in primary murine CD4+ T cells

T cell activation is known to be critically regulated by the extent and duration of TCR-induced signaling pathways. The NFAT family of transcription factors is believed to play an important role in coupling these quantitative differences in TCR-induced signaling events into changes in gene expression. In this study we have specifically investigated the effects of sustained NFAT signaling on T cell activation by introducing a constitutively active mutant version of NFATc1 (caNFATc1) into primary murine CD4(+) T cells and examining its effects on gene expression. We now report that ectopic expression of caNFATc1 partially mimics TCR signaling, resulting in enhanced expression of CD25 and CD40 ligand and down-regulation of CD62L. More importantly, we find that expression of caNFATc1 in T cells maintained under either nonpolarizing or Th1-skewing conditions leads to a marked selective increase in the number of cells expressing the prototypical Th1 cytokine, IFN-gamma. Furthermore, when expressed in Th2-skewed cells, caNFATc1 appears to attenuate Th2 differentiation by decreasing production of IL-4 and promoting the expression of IFN-gamma. Finally, we find that caNFATc1 enhances expression of functional P-selectin glycoprotein ligand-1, up-regulates Fas ligand expression, and increases susceptibility to activation-induced cell death, cellular traits that are preferentially associated with Th1 effector cells. Taken together, these results suggest that sustained NFAT signaling, mediated by ectopic expression of caNFATc1, acts to promote a Th1-like pattern of gene expression and thereby serves to highlight the important relationship between the degree of NFAT signaling and the qualitative pattern of gene expression induced during T cell activation.

Hematopoietic cytokines, transcription factors and lineage commitment

The past two decades have witnessed significant advances in our understanding of the cellular physiology and molecular regulation of hematopoiesis. At the heart of stem cell self-renewal and lineage commitment decisions lies the relative expression levels of lineage-specific transcription factors. The expression of these transcription factors in early stem cells may be promiscuous and fluctuate, but ultimately comes under the influence of extracellular regulatory signals in the form of hematopoietic cytokines. In this review, we first summarize our current understanding of the phenotypic characterization of hematopoietic stem cells. Next, we describe key known transcription factors which govern stem cell self-renewal and lineage commitment decisions. Finally, we review data concerning the role of specific cytokines in influencing these decisions. From this review, a picture emerges in which stem cell fate decisions are governed by the integrated effects of intrinsic transcription factors and external signaling pathways initiated by regulatory cytokines.

The transcription factor T-bet regulates mucosal T cell activation in experimental colitis and Crohn's disease

The balance between pro and antiinflammatory cytokines secreted by T cells regulates both the initiation and perpetuation of inflammatory bowel diseases (IBD). In particular, the balance between interferon (IFN)-gamma/interleukin (IL)-4 and transforming growth factor (TGF)-beta activity controls chronic intestinal inflammation. However, the molecular pathways that evoke these responses are not well understood. Here, we describe a critical role for the transcription factor T-bet in controlling the mucosal cytokine balance and clinical disease. We studied the expression and function of T-bet in patients with IBD and in mucosal T cells in various T helper (Th)1- and Th2-mediated animal models of chronic intestinal inflammation by taking advantage of mice that lack T-bet and retroviral transduction techniques, respectively. Whereas retroviral transduction of T-bet in CD62L(+) CD4(+) T cells exacerbated colitis in reconstituted SCID mice, T-bet-deficient T cells failed to induce colitis in adoptive transfer experiments suggesting that overexpression of T-bet is essential and sufficient to promote Th1-mediated colitis in vivo. Furthermore, T-bet-deficient CD62L(-) CD4(+) T cells showed enhanced protective functions in Th1-mediated colitis and exhibited increased TGF-beta signaling suggesting that a T-bet driven pathway of T cell activation controls the intestinal balance between IFN-gamma/IL-4 and TGF-beta responses and the development of chronic intestinal inflammation in T cell-mediated colitis. Furthermore, TGF-beta was found to suppress T-bet expression suggesting a reciprocal relationship between TGF-beta and T-bet in mucosal T cells. In summary, our data suggest a key regulatory role of T-bet in the pathogenesis of T cell-mediated colitis. Specific targeting of this pathway may be a promising novel approach for the treatment of patients with Crohn's disease and other autoimmune diseases mediated by Th1 T lymphocytes.

Growth factor independent-1 induced by IL-4 regulates Th2 cell proliferation

IL-4 is important in Th2 differentiation and in cell expansion. Stat6 is necessary and sufficient for both functions. Although Gata3 is critical for Th2 polarization, it is not sufficient to mediate IL-4-driven cell expansion. We report that growth factor independent-1 (Gfi-1), a Stat6-dependent transcriptional repressor, strikingly increases Th2 cell expansion by promoting proliferation and preventing apoptosis. Cells infected with a Gfi-1 retrovirus show striking enhancement of IL-2-induced Stat5 phosphorylation and repression of p27(Kip-1) expression, suggesting a potential mechanism for the Gfi-1 growth effect. The synergy of Gfi-1 and Gata3 provides a mechanism through which IL-4 could selectively promote Th2 cell expansion.

Asthma: T-bet--a master controller?

The transcription factors T-bet and GATA3 are important reciprocal determinants of Th1 and Th2 T helper cell differentiation. Recent evidence suggests that these factors may affect airway immunopathology in asthma.

T-bet regulates IgG class switching and pathogenic autoantibody production

A molecular understanding of the regulation of IgG class switching to IL-4-independent isotypes, particularly to IgG2a, remains largely unknown. The T-box transcription factor T-bet directly regulates Th1 lineage commitment by CD4 T cells, but its role in B lymphocytes has been largely unexplored. We show here a role for T-bet in the regulation of IgG class switching, especially to IgG2a. T-bet-deficient B lymphocytes demonstrate impaired production of IgG2a, IgG2b, and IgG3 and, most strikingly, are unable to generate germ-line or postswitch IgG2a transcripts in response to IFN-gamma. Conversely, enforced expression of T-bet initiates IgG2a switching in cell lines and primary cells. This function contributes critically to the pathogenesis of murine lupus, where the absence of T-bet strikingly reduces B cell-dependent manifestations, including autoantibody production, hypergammaglobulinemia, and immune-complex renal disease and, in particular, abrogates IFN-gamma-mediated IgG2a production. Classical T cell manifestations persisted, including lymphadenopathy and cellular infiltrates of skin and liver. These results identify T-bet as a selective transducer of IFN-gamma-mediated IgG2a class switching in B cells and emphasize the importance of this regulation in the pathogenesis of humorally mediated autoimmunity.

Cutting edge: polarized Th cell response induction by transferred antigen-pulsed dendritic cells is dependent on IL-4 or IL-12 production by recipient cells

To assess the influence of dendritic cell (DC) production of polarizing cytokines on Th2 and Th1 development we transferred Ag-pulsed DC generated from wild-type, IL-4(-/-), or IL-12(-/-) mice into wild-type, IL-4(-/-), or IL-12(-/-) recipients. We found that DC IL-4 was not necessary for Th2 induction and that, surprisingly, DC IL-12 was not an absolute requirement for Th1 development. However, DC IL-12 production facilitated optimal Th1 response development. Critically, recipient ability to produce IL-4 or IL-12 was essential for either Th2 or Th1 development. These data help delineate the source and importance of IL-4 and IL-12 in the process of induction of polarized T cell responses by DC.

Transcription: tantalizing times for T cells

The T helper lymphocyte is responsible for orchestrating an appropriate immune response to pathogens. To do so, it has evolved into two specialized subsets that direct type 1 and type 2 immunity. Here, we discuss the genetic programs that control lineage commitment of progenitor T helper cells along each of these pathways.

Naive CD4+ T cells exhibit distinct expression patterns of cytokines and cell surface molecules on their primary responses to varying doses of antigen

The amount of an Ag used for stimulation affects the type and magnitude of T cell responses. In this study we have investigated the primary response of naive CD4(+) T cells derived from OVA-specific TCR-transgenic mice (OVA23-3) upon stimulation with varying doses of the antigenic peptide, OVA(323-339). IL-4 expression was maximal with 50 nM Ag and decreased significantly with increasing doses. In contrast, IFN-gamma expression, which was also detected at 50 nM Ag, increased with increasing doses. The expression patterns of mRNA for the Th2-specific transcription factors GATA-3 and c-Maf were parallel to that of IL-4. These expression profiles were not altered by the addition of anti-IL-4 plus anti-IL-12 mAbs, suggesting that cytokine receptor signaling is not essential. Naive CD4(+) T cells stimulated with 5 nM Ag elicited IgM secretion from cocultured B cells, whereas those stimulated with 50 nM Ag or more elicited apoptosis of B cells. This may be because at lower doses of Ag (5 nM), naive CD4(+) T cells express CD40 ligand and OX40, whereas at higher doses (50 nM), they express Fas ligand. Clearly, the expression of each type of molecule depends on the Ag dose, and different molecules had different expression patterns. Thus, in the primary response, naive CD4(+) T cells can exhibit different functions depending on the dose of Ag.

The role of Sp1 and NF-kappa B in regulating CD40 gene expression

CD40 is a member of the tumor necrosis factor receptor superfamily and is a key signaling molecule expressed by antigen-presenting cells of the immune system. In a previous paper, we demonstrated that the expression of CD40 is regulated by both post-transcriptional and post-translational processes. In this paper, we show that basal (constitutive) CD40 gene expression is regulated by a TATA-less promoter, with Sp1 as a key transcription factor. Two Sp1 binding regions were identified in the mouse CD40 promoter at positions -59 to -50 and -74 to -66. Surprisingly, Sp1-mediated CD40 transcription was reduced following lipopolysaccharide stimulation and was associated with a time-dependent reduction in Sp1 DNA binding activity. This reduction seemed to be mediated by phosphorylation of the Sp1 molecule. We also show here that CD40 expression in lipopolysaccharide-stimulated cells is up-regulated by NF-kappaB through two distinct sites. One of these sites (-128 to -119) was shown to bind p50 and p65 members of the NF-kappaB family, while the other site (-562 to -553) bound only p65. Transfectants of p65 were generated using RAW 264 cells, and it was shown that the up-regulation of CD40 mRNA expression was dependent on the presence of the p65 molecule.

In the absence of IL-12, CD4(+) T cell responses to intracellular pathogens fail to default to a Th2 pattern and are host protective in an IL-10(-/-) setting

IL-12-deficient mice exposed to nonlethal infections with intracellular pathogens or repeatedly immunized with a pathogen extract developed lowered but nevertheless substantial numbers of IFN-gamma(+) CD4(+) T cells compared to those observed in wild-type animals. Moreover, the CD4(+) responses in these knockout animals failed to default to a Th2 pattern. The protective efficacy of the Th1 cells developing in an IL-12-deficient setting was found to be limited by IL-10 since mice doubly deficient in IL-10 and IL-12 survived, while animals deficient in IL-12 alone succumbed to pathogen challenge. In contrast to IL-12 knockout mice, MyD88-deficient animals exposed to a Th1 microbial stimulus developed a pure Th2 response, arguing that this signaling element plays a more critical function than IL-12 in determining pathogen-induced CD4 polarization.

The role of IL-12 in the induction of intravenous tolerance in experimental autoimmune encephalomyelitis

Intravenous administration of autoantigen is an effective method to induce Ag-specific tolerance against experimental autoimmune encephalomyelitis (EAE). IL-12 is a potent Th1 stimulator and an essential cytokine in the induction of EAE. The role of IL-12 in the induction of i.v. tolerance is not clear. In this study, we induced tolerance by i.v. administering myelin basic protein (MBP) peptide Ac1-11 (MBP1-11) in EAE. We observed significant suppression of IL-12 production by the lymph node cells of MBP1-11-injected mice. To see whether the low level of IL-12 is the cause or effect of tolerance, we administered IL-12 to the EAE mice at the time of i.v. MBP1-11 injection. Exogenous IL-12 abrogated the suppression of clinical and pathological EAE by i.v. tolerance. IL-12 blocked the suppressive effect of i.v. tolerance on the proliferative response to MBP1-11 and MBP1-11-induced production of IL-12 and IFN-gamma. Furthermore, IL-12 completely blocked the i.v. tolerance-induced type 1 T regulatory cell response. These data suggest that i.v. administration of autoantigen results in the suppression of endogenous IL-12 and the consequent switching of the immune response from an immunogenic to a tolerogenic form.

Dysregulation of T lymphocyte function in itchy mice: a role for Itch in TH2 differentiation

Itch is an E3 ubiquitin ligase that is disrupted in nonagouti-lethal or itchy mice. Itch deficiency leads to severe immune and inflammatory disorders and constant itching of the skin. Here we show that Itchminus sign/minus sign T cells show an activated phenotype and enhanced proliferation. Production of the type 2 T helper (TH2) cell cytokines interleukin 4 (IL-4) and IL-5 by Itchminus sign/minus sign T cells was augmented upon stimulation, and the TH2-dependent serum concentrations of immunoglobulin G1 (IgG1) and IgE in itchy mice were also increased. Molecularly, Itch associated with and induced ubiquitination of JunB, a transcription factor that is involved in TH2 differentiation. These results provide a molecular link between Itch deficiency and the aberrant activation of immune responses in itchy mice.

Repeated antigen exposure is necessary for the differentiation, but not the initial proliferation, of naive CD4(+) T cells

The mechanisms that regulate CD4(+) T cells responses in vivo are still poorly understood. We show here that initial Ag stimulation induces in CD4(+) T cells a program of proliferation that can develop, for at least seven cycles of division, in the absence of subsequent Ag or cytokine requirement. Thereafter, proliferation stops but can be reinitiated by novel Ag stimulation. This initial Ag stimulation does not however suffice to induce the differentiation of naive CD4(+) T cells into effector Th1 cells which requires multiple contacts with Ag-loaded APC. Thus, recurrent exposure to both Ag and polarizing cytokines appears to be essential for the differentiation of IFN-gamma-producing cells. Ag and cytokine availability therefore greatly limits the differentiation, but not the initial proliferation, of CD4(+) T cells into IFN-gamma-producing cells.

T-Bet expression and failure of GATA-3 cross-regulation lead to default production of IFN-gamma by gammadelta T cells

gammadelta T cells predominantly produce IFN-gamma upon activation. To determine the basis for default production of IFN-gamma by gammadelta T cells, we analyzed the transcription factors T-box expressed in T cells (T-bet) and GATA-3. T-bet, absent in naive cells, was induced upon TCR signaling, with IFN-gamma production. T-bet also regulated IL-4 synthesis, as gammadelta cells isolated from T-bet-deficient mice displayed enhanced IL-4 levels with reduced IFN-gamma production. Notably, T-bet expression after TCR signaling in gammadelta cells was not down-regulated by IL-4, in conjunction with a higher ratio of T-bet:GATA-3 expression than that found in CD4(+) T cells. Indeed, overexpression of GATA-3 failed to inhibit IFN-gamma secretion in gammadelta cells to the degree seen in CD4(+) T cells. These results indicate that T-bet enhances IFN-gamma secretion and suppresses IL-4 secretion in gammadelta cells, and that GATA-3 fails to counterbalance T-bet-mediated IFN-gamma production, accounting for the default synthesis of IFN-gamma by these T lymphocytes.

Epigenetic regulation of T cell fate and function

During their development, T lymphocytes make sequential cell fate choices: T rather than B lymphocytes, then TCRalphabeta or TCRgammadelta, CD4 or CD8, and Th1 or Th2 lineage. These fate choices require the initiation of new programs of gene expression, and once initiated, these programs must be faithfully propagated in a heritable manner from parental cells to their progeny. With the exception of the T cell receptor, these changes in gene expression occur without a change in information encoded directly in the DNA sequence. Rather, these heritable programs of gene expression are imposed, at least in part, epigenetically through changes in chromatin structure and DNA methylation, allowing T cells to tune the threshold for expression of specific genes.

1,25-dihydroxyvitamin D3 inhibits IFN-gamma and IL-4 levels during in vitro polarization of primary murine CD4+ T cells

Following their activation, naive CD4+ T cells can differentiate into one of two effector cell subsets, Th1 and Th2. These two subsets have different cytokine secretion patterns and thus mediate separate arms of the immune response. It has been established that the fat-soluble vitamin D(3) metabolite 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) and its nuclear receptor, the vitamin D receptor, play an important role in the immune system primarily through the transcriptional inhibition of cytokine genes that either are required for Th1 differentiation or are products of differentiated Th1 cells. Therefore, we wanted to test directly the ability of 1,25(OH)(2)D(3) to alter the Th differentiation process. Our results indicate that 1,25(OH)(2)D(3) inhibits not only the Th1 cytokine IFN-gamma but also the Th2 cytokine IL-4 in naive CD62 ligand+CD4+ T cells during their in vitro polarization. This effect is most dramatic when the ligand is present from the onset of the differentiation process. If the ligand is added after the polarization has ensued, the inhibition is significantly diminished. In activated (CD62 ligand-CD4+) T cells, 1,25(OH)(2)D(3) is still able to inhibit IFN-gamma but has no effect on IL-4 production. Our results also indicate that inhibition of these two cytokines in naive cells by vitamin D receptor and its ligand is neither a result of a cell cycle block nor an inhibition of Th1 or Th2 transcription factor expression but, rather, at least in the case of Th2 differentiation, an attenuation of IL-4 transcription by the receptor.

Development of spontaneous airway changes consistent with human asthma in mice lacking T-bet

Human asthma is associated with airway infiltration by T helper 2 (TH2) lymphocytes. We observed reduced expression of the TH1 transcription factor, T-bet, in T cells from airways of patients with asthma compared with that in T cells from airways of nonasthmatic patients, suggesting that loss of T-bet might be associated with asthma. Mice with a targeted deletion of the T-bet gene and severe combined immunodeficient mice receiving CD4+ cells from T-bet knockout mice spontaneously demonstrated multiple physiological and inflammatory features characteristic of asthma. Thus, T-bet deficiency, in the absence of allergen exposure, induces a murine phenotype reminiscent of both acute and chronic human asthma.

Distinct effects of T-bet in TH1 lineage commitment and IFN-gamma production in CD4 and CD8 T cells

T-bet is a member of the T-box family of transcription factors that appears to regulate lineage commitment in CD4 T helper (TH) lymphocytes in part by activating the hallmark TH1 cytokine, interferon-gamma (IFN-gamma). IFN-gamma is also produced by natural killer (NK) cells and most prominently by CD8 cytotoxic T cells, and is vital for the control of microbial pathogens. Although T-bet is expressed in all these cell types, it is required for control of IFN-gamma production in CD4 and NK cells, but not in CD8 cells. This difference is also apparent in the function of these cell subsets. Thus, the regulation of a single cytokine, IFN-gamma, is controlled by distinct transcriptional mechanisms within the T cell lineage.

IL-23: a cytokine that acts on memory T cells

The newly discovered cytokine interleukin (IL)-23 shares some in vivo functions with IL-12, including the activation of the transcription factor STAT4 (signal tranducer and activator of transcription-4). Indeed, the receptors for each appear to share one subunit, but also have at least one distinct subunit. Frucht discusses the similarities of IL-12 and IL-23 and the effects that distinguish one from the other. In contrast to IL-12, IL-23 appears to participate in the proliferative signal in memory T cells. More functions that distinguish IL-23 from IL-12 are likely to be uncovered as soon as the other component(s) of the IL-23 receptor are molecularly cloned and characterized.

Transforming growth factor-beta in T-cell biology

Strict control of T-cell homeostasis is required to permit normal immune responses and prevent undesirable self-targeted responses. Transforming growth factor-beta (TGF-beta) has been shown to have an essential role in that regulation. Owing to its broad expression, and inhibitory effects on multiple cell types of the immune system, TGF-beta regulation is complex. Through advances in cell-specific targeting of TGF-beta signalling in vivo, the role of TGF-beta in T-cell regulation has become clearer. Recent in vitro studies provide a better understanding of how TGF-beta regulates T-cell homeostasis, through multiple mechanisms involving numerous cell types.

Th1 and Th2 cytokines regulate proteoglycan-specific autoantibody isotypes and arthritis

BALB/c mice immunized with human cartilage proteoglycan (PG) develop arthritis accompanied by the production of autoantibodies to mouse cartilage PG. To determine whether the autoantibody isotype contributes to the onset and severity of arthritis, PG-specific serum IgG1 (Th2, IL-4-cytokine-supporting) and IgG2a (Th1, IFN-gamma-controlling) concentrations were monitored during immunization with PG in IL-4-deficient and IFN-gamma-deficient mice. Paradoxically, despite elevated IFN-gamma, the PG-specific IgG1 isotype was significantly higher than the PG-specific IgG2a response, and the PG-specific IgG1 isotype was independent of IL-4. In contrast, the serum concentration of PG-specific IgG2a isotype was six times higher in IL-4-deficient mice than in wild-type controls. Moreover, the high concentration of PG-specific IgG2a isotype in IL-4-deficient mice corresponded to an increased severity of arthritis. The concentration of PG-specific IgG2a isotype was lower in IFN-gamma-deficient mice than in wild-type mice, and the incidence and severity of arthritis also were significantly lower. Concentrations of PG-specific IgG2a isotype autoantibody correlated with the onset and severity of arthritis, suggesting a pathological role of this isotype, probably locally in the joint.

T-bet is rapidly induced by interferon-gamma in lymphoid and myeloid cells

Differentiation of naive CD4(+) T cells into IFN-gamma-producing T helper 1 (T(H)1) cells is pivotal for protective immune responses against intracellular pathogens. T-bet, a recently discovered member of the T-box transcription factor family, has been reported to play a critical role in this process, promoting IFN-gamma production. Although terminal T(H)1 differentiation occurs over days, we now show that challenge of mice with a prototypical T(H)1-inducing stimulus, Toxoplasma gondii soluble extract, rapidly induced IFN-gamma and T-bet; T-bet induction was substantially lower in IFN-gamma-deficient mice. Naive T cells expressed little T-bet, but this transcription factor was induced markedly by the combination of IFN-gamma and cognate antigen. Human myeloid antigen-presenting cells showed T-bet induction after IFN-gamma stimulation alone, and this induction was antagonized by IL-4 and granulocyte/macrophage colony-stimulating factor. Although T-bet was induced rapidly and directly by IFN-gamma, it was not induced by IFN-alpha, lipopolysaccharide, or IL-1, indicating that this action of IFN-gamma was specific. Moreover, T-bet induction was dependent on Stat1 but not Stat4. These data argue for a model in which IFN-gamma gene regulation involves an autocrine loop, whereby the cytokine regulates a transcription factor that promotes its own production. These findings substantially alter the current view of T-bet in IFN-gamma regulation and promotion of cell-mediated immune responses.

Cell cycle controlling the silencing and functioning of mammalian activators

Naïve CD4(+) helper T (T(H)) cells respond to stimulation by terminally differentiating into two mature classes, T(H)1 cells, which express interferon gamma (IFN-gamma), and T(H)2 cells, which express interleukin 4 (IL-4). The transcriptional activators T-bet and Gata-3 mediate commitment to the T(H)1 and T(H)2 fates, respectively, including chromatin remodeling of signature genes. The cytokine IL-12 fosters growth of committed T(H)1 cells, while IL-4 fosters growth of committed T(H)2 cells. IL-12 and IL-4 also play critical roles in commitment by promoting transcriptional silencing of Gata-3 and T-bet, respectively. We now show that both T-bet and Gata-3 are induced in a cell cycle-independent manner in bipotent progenitor cells. In contrast, both lineage-restricted gene induction by the activator proteins and heritable silencing of the transcription of each activator, the hallmarks of terminal differentiation, are cell cycle dependent. We found that cells that cannot cycle remain uncommitted and bipotent in response to the most polarizing signals for maturation. These results provide mechanistic insight into a mammalian model of terminal differentiation by illustrating that cell cycle-coupled epigenetic effects, as originally described in yeast, may represent an evolutionarily conserved strategy for organizing signaling and cell fate.

How to deal with polarized Th2 cells: exploring the Achilles' heel

The central effector cells in the pathogenesis of atopic allergic diseases are type 2 T helper (Th2) cells, which display an aberrant cytokine profile dominated by type 2 cytokines. Initial reports from mouse studies indicated that established and committed Th2 cells are stable and unsusceptible to modulation. However, there is a growing awareness that in humans, established effector Th2 cells are more flexible and can be reverted to predominant Th1 phenotypes. In fact, the Th1-driving cytokine interleukin (IL)-12 is the crucial factor in this respect. IL-12 is mainly produced by dendritic cells (DC), which can be primed for high or low IL-12 production, depending on inflammatory and/or microbial signals they encounter during their residence in the peripheral tissues. Accordingly, both the regulation of and the priming for IL-12 production in DC form ideal targets for therapeutic intervention. The development of new therapies for atopic allergy now focuses on local IL-12-promoting substances to target both the development of new Th2 cells and the persistent population of established allergen-specific Th2 cells.

Induction of cytotoxic T lymphocyte antigen 4 (CTLA-4) restricts clonal expansion of helper T cells

Cytotoxic T lymphocyte antigen (CTLA)-4 plays an essential role in immunologic homeostasis. How this negative regulator of T cell activation executes its functions has remained controversial. We now provide evidence that CTLA-4 mediates a cell-intrinsic counterbalance to restrict the clonal expansion of proliferating CD4(+) T cells. The regulation of CTLA-4 expression and function ensures that, after approximately 3 cell divisions of expansion, most progeny will succumb to either proliferative arrest or death over the ensuing three cell divisions. The quantitative precision of the counterbalance hinges on the graded, time-independent induction of CTLA-4 expression during the first three cell divisions. In contrast to the limits imposed on unpolarized cells, T helper type 1 (Th1) and Th2 effector progeny may be rescued from proliferative arrest by interleukin (IL)-12 and IL-4 signaling, respectively, allowing appropriately stimulated progeny to proceed to the stage of tissue homing. These results suggest that the cell-autonomous regulation of CTLA-4 induction may be a central checkpoint of clonal expansion of CD4(+) T cells, allowing temporally and spatially restricted growth of progeny to be dictated by the nature of the threat posed to the host.

A call to arms: the cytokine selection service

The process by which naïve T helper (T(H)) cells differentiate into the T(H)1 and T(H)2 subtypes has been well studied. However, there remain some unresolved issues pertaining to the requirements for the initial step of T(H) cell differentiation. Much debate exists about whether the roles of cytokines include the forcing of the initial steps of differentiation on naïve T(H) cells, termed "instruction," or whether cytokines act in a supportive role, termed "selection," whereby newly differentiating T(H) cells are given the proper signals for survival and proliferation. A recent paper by Mullen et al., which helps delineate the role of cytokines in T(H)1 cell development, is addressed by Nelson; it appears that cytokines act in the selection stage of T(H) cell maturation.

The art of the probable: system control in the adaptive immune system

The immune system provides very effective host defense against infectious agents. Although many details are known about the cells and molecules involved, a broader "systems engineering" view of this complex system is just beginning to emerge. Here the argument is put forward that stochastic events, potent amplification mechanisms, feedback controls, and heterogeneity arising from spatially dispersed cell interactions give rise to many of the gross properties of the immune system. A better appreciation of these underlying features will not only add to our basic understanding of how immunity develops or goes awry, but also illuminate new directions for manipulating the system in prophylactic and therapeutic settings.