Impaired interleukin 4 signaling in T helper type 1 cells

Potential Natural Biomolecules Targeting JAK/STAT/SOCS Signaling in the Management of Atopic Dermatitis

Atopic dermatitis (AD) is a chronic inflammatory skin disease caused by the dysregulation of cytokines and other immune mediators. JAK/STAT is a classical signal transduction pathway involved in various biological processes, and its dysregulation contributes to the key aspects of AD pathogenesis. Suppressor of cytokine signaling (SOCS) proteins negatively regulate the immune-related inflammatory responses mediated by the JAK/STAT pathway. JAK/STAT-mediated production of cytokines including IL-4, IL-13, IL-31, and TSLP inhibits the expression of important skin barrier proteins and triggers pruritus in AD. The expression of SOCS proteins regulates the JAK-mediated cytokines and facilitates maintaining the skin barrier disruptions seen in AD. STATs are crucial in dendritic-cell-activated Th2 cell differentiation in the skin, releasing inflammatory cytokines, indicating that AD is a Th2-mediated skin disorder. SOCS proteins aid in balancing Th1/Th2 cells and, moreover, regulate the onset and maintenance of Th2-mediated allergic responses by reducing the Th2 cell activation and differentiation. SOCS proteins play a pivotal role in inflammatory cytokine-signaling events that act via the JAK/STAT pathway. Therapies relying on natural products and derived biomolecules have proven beneficial in AD when compared with the synthetic regimen. In this review, we focused on the available literature on the potential natural-product-derived biomolecules targeting JAK/STAT/SOCS signaling, mainly emphasizing the SOCS family of proteins (SOCS1, SOCS3, and SOCS5) acting as negative regulators in modulating JAK/STAT-mediated responses in AD pathogenesis and other inflammatory disorders.

A New IRF-1-Driven Apoptotic Pathway Triggered by IL-4/IL-13 Kills Neonatal Th1 Cells and Weakens Protection against Viral Infection

Early life immune responses are deficient in Th1 lymphocytes that compromise neonatal vaccination. We found that IL-4 and IL-13 engage a developmentally expressed IL-4Rα/IL-13Rα1 heteroreceptor to endow IFN regulatory factor 1 (IRF-1) with apoptotic functions, which redirect murine neonatal Th1 reactivation to cell death. IL-4/IL-13-induced STAT6 phosphorylation serves to enhance IRF-1 transcription and promotes its egress from the nucleus. In the cytoplasm, IRF-1 can no longer serve as an anti-viral transcription factor but, instead, colocalizes with Bim and instigates the mitochondrial, or intrinsic, death pathway. The new pivotal function of IRF-1 in the death of neonatal Th1 cells stems from the ability of its gene to bind STAT6 for enhanced transcription and the proficiency of its protein to precipitate Bim-driven apoptosis. This cytokine-induced, IRF-1-mediated developmental death network weakens neonatal Th1 responses during early life vaccination and increases susceptibility to viral infection.

Tregitope-linked Refined Allergen Vaccines for Immunotherapy in Cockroach Allergy

Allergen-specific immunotherapy (AIT) facilitates long-term resolution of allergic morbidity resulting in reduced drug use and increased refractoriness to new sensitization. AIT effectiveness has been demonstrated in seasonal and perennial allergies, and insect stings. However, data and studies in AIT relative to cockroach (CR) allergy are relatively scarce. In this study, mice allergic to American CR (Periplaneta americana) were treated with a liposome (L)-entrapped vaccine made of mouse Tregitope289-Per a 9 of the CR, Tregitope167-Per a 9, or Per a 9 alone - or placebo. Allergic mice that received an individual vaccine intranasally had reduced Th2 response, reduced lung inflammation, and reduced respiratory tissue remodeling. However, only L-Tregitope289-Per a 9 and L-Tregitope167-Per a 9 induced expression of immunosuppressive cytokine genes (IL-10, TGF-β, and IL-35 for L-Tregitope289-Per a 9, and IL-10 and TGF-β for L-Tregitope167-Per a 9) and increment of idoleamine-2,3-dioxygenase 1 (IDO1), indicating that these vaccines caused allergic disease suppression and reversal of respiratory tissue remodeling via generation of regulatory lymphocytes. Liposome entrapped-recombinant Per a 9 (L-Per a 9) did not cause upregulation of immunosuppressive cytokine genes and IDO1 increment; rather, L-Per a 9 induced high expression of IFN-γ in lungs of treated mice, which resulted in mitigation of allergic manifestations. This study provides compelling evidence that both liposome-entrapped vaccines made of single refined major allergen alone and single refined major allergen linked with Tregitopes are effective for reducing allergen-mediated respiratory tissue inflammation and remodeling, but through different mechanisms.

Lymphocyte surface markers and cytokines are suitable for detection and potency assessment of skin-sensitizing chemicals in an in vitro model of allergic contact dermatitis: the LCSA-ly

Allergic contact dermatitis is a widespread health disorder and occupational skin disease. Hence, screening for contact-sensitizing chemicals is highly relevant to toxicology, dermatology, and occupational medicine. The use of animal tests for this purpose is constrained by ethical considerations, need for high-throughput screening, and legislation (e.g., for cosmetics in the European Union). T cell activation is the final and most specific key event of the "adverse outcome pathway" for skin sensitization and therefore a promising target for the development of in vitro sensitization assays. We present a novel in vitro sensitization assay with a lymphocyte endpoint as an add-on to the loose-fit coculture-based sensitization assay (LCSA): the LCSA-ly. While the LCSA measures dendritic cell activation, the LCSA-ly offers the option for an additional lymphocyte endpoint which can be measured concurrently. We incorporated lymphocytes in our previously established coculture of primary human keratinocytes and monocyte-derived dendritic cells and tested nine substances: five sensitizers [2,4-dinitrochlorobenzene (DNCB) 1.25-15 µmol/l, p-phenylenediamine (PPD) 15.6-125 µmol/l, 2-mercaptobenzothiazole (MBT) 50-1000 µmol/l, coumarin, and resorcinol (both: 250-1500 µmol/l)] and four non-sensitizers (monochlorobenzene, caprylic acid, glycerol, and salicylic acid (all: 125-1000 µmol/l)]. DNCB and MBT increased a subset of IL-23 receptor+/IFN-γ receptor 1 (CD119)+ lymphocytes. DNCB, PPD, and MBT enhanced a subunit of the IL-4 receptor (CD124) and a memory marker (CD44) on lymphocytes. Remarkably, DNCB, PPD, and MBT raised IL-4 concentrations in coculture supernatants while IFN-γ levels decreased, which might point to Th2 activation in vitro. Coumarin, resorcinol, and non-sensitizers did not alter any of the tested surface markers or cytokines. IL-17 was not affected by any of the substances. Relative strength of sensitizers according to lymphocyte markers was DNCB > PPD > MBT, which corresponds to earlier results from the LCSA without lymphocyte endpoint, the murine local lymph node assay, and human data. This study is the first to prove the suitability of lymphocyte surface markers for sensitization testing and potency assessment.

IL-4/IL-13 Heteroreceptor Influences Th17 Cell Conversion and Sensitivity to Regulatory T Cell Suppression To Restrain Experimental Allergic Encephalomyelitis

IL-4 and IL-13 have been defined as anti-inflammatory cytokines that can counter myelin-reactive T cells and modulate experimental allergic encephalomyelitis. However, it is not known whether endogenous IL-4 and IL-13 contribute to the maintenance of peripheral tolerance and whether their function is coordinated with T regulatory cells (Tregs). In this study, we used mice in which the common cytokine receptor for IL-4 and IL-13, namely the IL-4Rα/IL-13Rα1 (13R) heteroreceptor (HR), is compromised and determined whether the lack of signaling by endogenous IL-4 and IL-13 through the HR influences the function of effector Th1 and Th17 cells in a Treg-dependent fashion. The findings indicate that mice-deficient for the HR (13R^-/-) are more susceptible to experimental allergic encephalomyelitis than mice sufficient for the HR (13R^+/+) and develop early onset and more severe disease. Moreover, Th17 cells from 13R^-/- mice had reduced ability to convert to Th1 cells and displayed reduced sensitivity to suppression by Tregs relative to Th17 effectors from 13R^+/+ mice. These observations suggest that IL-4 and IL-13 likely operate through the HR and influence Th17 cells to convert to Th1 cells and to acquire increased sensitivity to suppression, leading to control of immune-mediated CNS inflammation. These previously unrecognized findings shed light on the intricacies underlying the contribution of cytokines to peripheral tolerance and control of autoimmunity.

On the Role IL-4/IL-13 Heteroreceptor Plays in Regulation of Type 1 Diabetes

Type 1 diabetes (T1D) manifests when the insulin-producing pancreatic β cells are destroyed as a consequence of an inflammatory process initiated by lymphocytes of the immune system. The NOD mouse develops T1D spontaneously and serves as an animal model for human T1D. The IL-4Rα/IL-13Rα1 heteroreceptor (HR) serves both IL-4 and IL-13 cytokines, which are believed to function as anti-inflammatory cytokines in T1D. However, whether the HR provides a responsive element to environmental (i.e., physiologic) IL-4/IL-13 in the regulation of peripheral tolerance and the development of T1D has yet to be defined. In this study, NOD mice deficient for the HR have been generated by means of IL-13Rα1 gene disruption and used to determine whether such deficiency affects the development of T1D. Surprisingly, the findings indicate that NOD mice lacking the HR (13R^-/-) display resistance to T1D as the rise in blood glucose level and islet inflammation were significantly delayed in these HR-deficient relative to HR-sufficient (13R^+/+) mice. In fact, the frequency and spleen-to-pancreas dynamics of both Th1 and Th17 cells were affected in 13R^-/- mice. This is likely due to an increase in the frequency of mTGFβ⁺Foxp3^int regulatory T cells and the persistence of CD206⁺ macrophages in the pancreas as both types of cells confer resistance to T1D upon transfer to 13R^+/+ mice. These findings reveal new insights as to the role environmental IL-4/IL-13 and the HR play in peripheral tolerance and the development of T1D.

Protective role of Th17 cells in pulmonary infection

Th17 cells are characterized as preferential producer of interleukins including IL-17A, IL-17F, IL-21 and IL-22. Corresponding receptors of these cytokines are expressed on number of cell types found in the mucosa, including epithelial cells and fibroblasts which constitute the prime targets of the Th17-associated cytokines. Binding of IL-17 family members to their corresponding receptors lead to modulation of antimicrobial functions of target cells including alveolar epithelial cells. Stimulated alveolar epithelial cells produce antimicrobial peptides and are involved in granulepoesis, neutrophil recruitment and tissue repair. Mucosal immunity mediated by Th17 cells is protective against numerous pulmonary pathogens including extracellular bacterial and fungal pathogens. This review focuses on the protective role of Th17 cells during pulmonary infection, highlighting subset differentiation, effector cytokines production, followed by study of the binding of these cytokines to their corresponding receptors, the subsequent signaling pathway they engender and their effector role in host defense.

Neonatal Basophils Stifle the Function of Early-Life Dendritic Cells To Curtail Th1 Immunity in Newborn Mice

Neonatal immunity exhibits weak Th1 but excessive Th2 responses, and the underlying mechanisms remain elusive. In this article, we show that neonatal basophils readily produce IL-4, a cytokine that proved to be pivotal in shaping the programs of both lymphocyte subsets. Besides promoting Th2 programs, IL-4 is captured by the IL-4 heteroreceptor (IL-4Rα/IL-13Rα1) expressed on dendritic cells and instigates IL-12 downregulation. Under these circumstances, differentiating Th1 cells upregulate IL-13Rα1, leading to an unusual expression of the heteroreceptor, which will serve as a death marker for these Th1 cells during rechallenge with Ag. The resulting Th1/Th2 imbalance impacts childhood immunity culminating in sensitivity to allergic reactions, susceptibility to microbial infection and perhaps poor efficacy of pediatric vaccines.

IFN-γ suppresses permissive chromatin remodeling in the regulatory region of the Il4 gene

In order to develop the most effective T helper type-1 (Th1) immunity, naïve CD4(+) T cells must acquire the capacity to express IFN-γ while silencing T helper type-2 (Th2) cytokine-producing potential. An Il4 gene silencer has been described. However, it is not completely understood how the silencer works. In this study, we examine whether IFN-γ can suppress permissive chromatin remodeling of regulatory region of the Il4 gene. We demonstrate that IFN-γ suppresses H3K4 dimethylation at the intronic enhancer region of the Il4 gene. The IFN-γ-mediated suppression of permissive chromatin remodeling is IFN-γ receptor-, STAT1-, and T-bet-dependent. Our study reveals a novel mechanism of how Th1 cells silence the Il4 gene.

Obesity is associated with activated and insulin resistant immune cells

BACKGROUND

Obesity and type 2 diabetes mellitus are characterized by insulin resistance and 'low-grade inflammation'; however, the pathophysiological link is poorly understood. To determine the relative contribution of obesity and insulin resistance to systemic 'inflammation', this study comprehensively characterized circulating immune cells in different grades of obesity.

METHODS

Immune cell phenotypes and activation status were analysed by flow cytometry cross-sectionally in morbidly obese (n = 16, body mass index (BMI) 42.2 ± 5.4 kg/m2), overweight (n = 13, BMI 27.4 ± 1.6 kg/m2) and normal weight (n = 12, BMI 22.5 ± 1.9 kg/m2) subjects.

RESULTS

Obese, but not overweight subjects, had increased activation marker expression on neutrophils, monocytes, T-lymphocytes and polarization of T helper cells towards a pro-inflammatory type 1-phenotype (Th1). Th1 numbers correlated positively with the degree of insulin resistance (homeostasis model assessment, p < 0.05). Lymphocytes from obese subjects showed reduced insulin-stimulated AKT-phosphorylation in vitro. Supra-physiological insulin concentrations did not affect T-cell differentiation, which under normal circumstances would promote an anti-inflammatory T helper type 2-phenotype.

CONCLUSIONS

These results show that morbid obesity is characterized by circulating immune cells that are activated and insulin resistant, with the T-cell balance polarized towards a pro-inflammatory Th1 phenotype. The loss of insulin-induced suppression of inflammatory phenotypes in circulating immune cells could contribute to the systemic and adipose tissue inflammation found in morbid obesity.

Sensitivity and resistance to regulation by IL-4 during Th17 maturation

Th17 cells are highly pathogenic in a variety of immune-mediated diseases, and a thorough understanding of the mechanisms of cytokine-mediated suppression of Th17 cells has great therapeutic potential. In this article, we characterize the regulation of both in vitro- and in vivo-derived Th17 cells by IL-4. We demonstrate that IL-4 suppresses reactivation of committed Th17 cells, even in the presence of TGF-β, IL-6, and IL-23. Downregulation of IL-17 by IL-4 is dependent on STAT6 and mediated by inhibition of STAT3 binding at the Il17a promoter. Although Th1 cytokines were shown to induce IFN-γ expression by Th17 cells, IL-4 does not induce a Th2 phenotype in Th17 cells. Suppression by IL-4 is stable and long-lived when applied to immature Th17 cells, but cells that have undergone multiple rounds of stimulation, either in vivo during a Th17-mediated inflammatory disease, or in vitro, become resistant to suppression by IL-4 and lose the ability to signal through IL-4R. Thus, although IL-4 is a potent suppressor of the Th17 genetic program at early stages after differentiation, prolonged stimulation renders Th17 cells impervious to regulatory cytokines.

A continuous T-bet expression is required to silence the interleukin-4-producing potential in T helper type 1 cells

To develop into committed T helper type 1 (Th1) cells, naive CD4(+) T cells not only need to acquire the capacity to produce interferon-gamma (IFN-gamma), but they also need to gain the ability to silence their interleukin-4 (IL-4) -producing potential. How Th1 cells silence their Th2 cytokine-producing potential is an important yet unresolved issue in Th1 immunity. We found that a lack of IL-4 stimulation was not sufficient to silence the IL-4-producing potential in activated CD4(+) T cells and that Th1-promoting factor was required. Although it has been shown that T-bet is a crucial factor in suppressing Il4 gene expression, it is unclear whether a continuous presence of T-bet is required to silence the Il4 gene in Th1 cells. To address this problem, we used an inducible form of T-bet - a T-bet-oestrogen receptor fusion molecule (T-bet-ER). We found that the activation of T-bet during primary or secondary culture was sufficient to silence IL-4-producing potential. On the other hand, the inactivation of T-bet after naïve CD4(+) T cells had differentiated into Th1 cells resulted in derepression of Il4 gene transcription. Additionally, we found that T-bet is required to maintain Ifng expression. Our data demonstrate that the continuous expression of T-bet is required for Th1 cells to silence their IL-4-producing potential.

Neonatal immunity: faulty T-helpers and the shortcomings of dendritic cells

Immunity in the newborn is characterized by minimal T helper (Th)1 function but an excess of Th2 activity. Since Th1 lymphocytes are important to counter microbes and Th2 cells favor allergies, the newborn faces susceptibility to microbial infections and allergic reactions. Delayed maturation of certain dendritic cells leads to limited interleukin (IL)-12 production during the neonatal period. The Th2 cytokine locus of neonatal CD4(+) T cells is poised epigenetically for rapid and robust production of IL-4 and IL-13. Together, these circumstances lead to efficient differentiation of Th2 cells and the expression of an IL-4Ralpha/IL-13Ralpha1 heteroreceptor on Th1 cells. Upon re-challenge, Th2 cells rapidly produce IL-4 which utilizes the heteroreceptor to drive apoptosis of Th1 cells, thus yielding the Th2 bias of neonatal immunity.

IL-23 promotes maintenance but not commitment to the Th17 lineage

IL-23 plays a critical role establishing inflammatory immunity and enhancing IL-17 production in vivo. However, an understanding of how it performs those functions has been elusive. In this report, using an IL-17-capture technique, we demonstrate that IL-23 maintains the IL-17-secreting phenotype of purified IL-17(+) cells without affecting cell expansion or survival. IL-23 maintains the Th17 phenotype over multiple rounds of in vitro stimulation most efficiently in conjunction with IL-1beta. However, in contrast to Th1 and Th2 cells, the Th17 phenotype is not stable and when long-term IL-23-stimulated Th17 cultures are exposed to Th1- or Th2-inducing cytokines, the Th17 genetic program is repressed and cells that previously secreted IL-17 assume the cytokine secreting profile of other Th subsets. Thus, while IL-23 can maintain the Th17 phenotype, it does not promote commitment to an IL-17-secreting lineage.

Delayed maturation of an IL-12-producing dendritic cell subset explains the early Th2 bias in neonatal immunity

Primary neonatal T cell responses comprise both T helper (Th) cell subsets, but Th1 cells express high levels of interleukin 13 receptor α1 (IL-13Rα1), which heterodimerizes with IL-4Rα. During secondary antigen challenge, Th2-produced IL-4 triggers the apoptosis of Th1 cells via IL-4Rα/IL-13Rα1, thus explaining the Th2 bias in neonates. We show that neonates acquire the ability to overcome the Th2 bias and generate Th1 responses starting 6 d after birth. This transition was caused by the developmental maturation of CD8α⁺CD4⁻ dendritic cells (DCs), which were minimal in number during the first few days of birth and produced low levels of IL-12. This lack of IL-12 sustained the expression of IL-13Rα1 on Th1 cells. By day 6 after birth, however, a significant number of CD8α⁺CD4⁻ DCs accumulated in the spleen and produced IL-12, which triggered the down-regulation of IL-13Rα1 expression on Th1 cells, thus protecting them against IL-4–driven apoptosis.

Th2 cell therapy of established acute graft-versus-host disease requires IL-4 and IL-10 and is abrogated by IL-2 or host-type antigen-presenting cells

Delayed donor Th2 cell infusion permits a graft-versus-tumor (GVT) effect to occur with subsequent amelioration of established graft-versus-host disease (GVHD). Relative to GVHD controls (B6-into-BALB/c model), recipients of delayed Th2 cells (day 14 post-BMT) had increased survival (3/3 experiments [exp]; each exp P < .0001) and reduced GVHD by histology analysis 5 days post-Th2 infusion without increased tumor burden (3 of 3 exp; each exp P < or = .02). Th2 cell-mediated amelioration of GVHD was associated with greatly reduced allospecific IFN-gamma secretion, in vivo augmentation of allospecific IL-4 and IL-10 secretion, and reduction in donor CD8(+) T cell number post-BMT (3 of 3 exp; each comparison, P < or = .003). To better understand the molecular mechanism of this GVHD therapy, Th2 cells were generated from wild-type (WT), IL-4 deficient (KO), or IL-10 KO donors: remarkably, recipients of IL-4 or IL-10 KO Th2 cells had no survival advantage, no improvement in GVHD by histology, no reduction in CD8(+) T cell expansion post-BMT, and no in vivo shift toward type II cytokines. We reasoned that IL-2 and alloantigen availability may be limiting factors for Th2 cell therapy, and as such, evaluated whether coadministration of IL-2 or coinfusion of host-type antigen-presenting cells (APC) might intensify the anti-GVHD effect. However, contrary to these hypotheses, concomitant IL-2 therapy or APC administration fully abrogated the Th2 cell-mediated survival advantage and histology-defined GVHD reduction, reduced Th2 cell expansion in vivo while promoting CD8(+) T cell expansion from cells originating from the initial allograft, and impaired type II polarization in vivo. In conclusion, Th2 cell therapy can rapidly ameliorate severe GVHD via IL-4 and IL-10 mediated mechanisms, and potentially, via IL-2 consumption and APC modulation mechanisms.

Interferon-gamma enhances human eosinophil effector functions induced by granulocyte-macrophage colony-stimulating factor or interleukin-5

T helper (Th) 2-type cytokines play a dominant role in allergic inflammation. Accumulating evidence suggests that Th1-type cytokines antagonize Th2-type cytokine responses; however, recent studies demonstrate that Th1 cytokines might enhance Th2 immune responses. We examined whether interferon (IFN)-gamma, a representative Th1 cytokine, modifies the effector functions of human eosinophils stimulated by granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin (IL)-5. GM-CSF and IL-5 have significant functional homology, and contribute to the regulation of Th2 immunity. After the pretreatment of eosinophils with IFN-gamma, GM-CSF- or IL-5-induced eosinophil functions were examined, including superoxide anion generation, degranulation, adhesion, expression of GM-CSF receptor (R), IL-5R, or CD11b, and phosphorylation of intracellular signaling molecules. Superoxide anion generation was measured using the cytochrome c reduction method. Degranulation and cell adhesion were evaluated based on eosinophil-derived neurotoxin (EDN) contents in supernatants or adherent cells. Phosphorylation of signaling molecules was analyzed using a multiplex beads array system. Preincubation with IFN-gamma resulted in enhanced GM-CSF- or IL-5-induced superoxide anion generation and degranulation of human eosinophils, whereas stimulus-induced eosinophil adhesion was unaffected. In addition, IFN-gamma did not influence the expression of GM-CSFR, IL-5R, and CD11b. Furthermore, IFN-gamma upregulated GM-CSF- or IL-5-induced phosphorylation of extracellular signal-regulated kinase (ERK), p38 mitogen-activated protein kinase (MAPK), c-Jun N-terminal kinase (JNK), and activating transcription factor (ATF)-2. Finally, we confirmed that MAPK inhibitors blocked the enhancement of stimuli-induced superoxide anion generation of IFN-gamma treated eosinophils. In conclusion, IFN-gamma might upregulate ERK, p38, or JNK/ATF-2 phosphorylation induced by GM-CSF or IL-5, leading to enhanced cytokine-induced eosinophil superoxide generation and degranulation.

Emergence of the Th17 pathway and its role in host defense

Recently, a paradigm shift has emerged in T-cell-mediated adaptive immunity. On the heels of the discovery of T cells with immunosuppressive function, so-called regulatory T cells (Tregs), the diversity of effector cells has expanded to include a third helper T cell, termed Th17. The appreciation that Th17 cells are products of a distinct effector pathway depended critically on observations made during investigations of mouse models of autoimmunity, advanced by discovery of the cytokines IL-17 and IL-23. These studies understandably led investigators to highlight the role played by Th17 cells in autoimmunity. Yet while the dysfunctional behavior of this phenotype as a contributor to inflammatory disease remains a central issue, this pathway evolved to meet a need for host protection against potential pathogens. It has become apparent that the Th17 pathway promotes host defense against certain extracellular bacteria and fungi, but more recent studies also implicate a role in protection against some protozoa and viruses. Here we review the experimental history that ultimately uncovered the existence and nature of Th17 cells, and then turn the reader's attention to what is currently known about Th17 cells as a bulwark against pathogens.

Role of suppressor of cytokine signaling in ocular allergy

PURPOSE OF REVIEW

The goal of this article is to evaluate developments in the knowledge of suppressor of cytokine signaling (SOCS) protein ocular allergy and the potential of SOCS proteins as targets for therapeutic strategies.

RECENT FINDINGS

The family of proteins designated SOCS proteins plays an important role in Th2-mediated allergic responses through the control of the balance between Th1 and Th2 cells. SOCS3 and SOCS5 are predominantly expressed in Th2 and Th1 cells, respectively, and they reciprocally inhibit the Th1 and Th2 differentiation processes. SOCS3 is highly expressed at the disease site of allergic conjunctivitis, and T-cell-specific expression of SOCS3 deteriorates clinical and pathological features of allergic conjunctivitis. Reduction of the expression level or inhibition of function clearly reduces the severity of allergic conjunctivitis. On the other hand, constitutive expression of SOCS5, a specific inhibitor of IL-4 signaling, results in reduced eosinophil infiltration. Moreover, negative regulation of the Th2-mediated response by dominant-negative SOCS3 and SOCS5 reduced the incidence of allergic conjunctivitis in a mouse model.

SUMMARY

The present article summarizes recent findings in terms of a role of SOCS protein as a negative regulator in ocular allergy and its clinical application.

Potential antiinflammatory role of insulin via the preferential polarization of effector T cells toward a T helper 2 phenotype

Hyperglycemia in critical illness is a common complication and a strong independent risk factor for morbidity and death. Intensive insulin therapy decreases this risk by up to 50%. It is unclear to what extent this benefit is due to reversal of glucotoxicity or to a direct effect of insulin, because antiinflammatory effects of insulin have already been described, but the underlying mechanisms are still poorly understood. The insulin receptor is expressed on resting neutrophils, monocytes, and B cells, but is not detectable on T cells. However, significant up-regulation of insulin receptor expression is observed on activated T cells, which suggests an important role during T cell activation. Exogenous insulin in vitro induced a shift in T cell differentiation toward a T helper type 2 (Th2)-type response, decreasing the T helper type 1 to Th2 ratio by 36%. This result correlated with a corresponding change in cytokine secretion, with the interferon-gamma to IL-4 ratio being decreased by 33%. These changes were associated with increased Th2-promoting ERK phosphorylation in the presence of insulin. Thus, we demonstrate for the first time that insulin treatment influences T cell differentiation promoting a shift toward a Th2-type response. This effect of insulin in changing T cell polarization may contribute to its antiinflammatory role not only in sepsis, but also in chronic inflammation associated with obesity and type 2 diabetes.

Suppressor of cytokine signaling 3 (SOCS3) in Th2 cells evokes Th2 cytokines, IgE, and eosinophilia

Atopic dermatitis, allergic rhinitis, and bronchial asthma are allergic immune disorders characterized by a predominance of T helper 2 (Th2) cells, the resulting elevation of allergen-specific immunoglobulin E (IgE), and mast cell- and eosinophil-associated inflammation. The cytokine environment at the site of the initial antigen stimulation determines the direction of helper T-cell differentiation into Th1 or Th2 cells. Therefore, negative regulators of cytokine signaling, suppressors of cytokine signaling (SOCS) proteins, play an important role in Th2-mediated allergic responses through the control of the balance between Th1 and Th2 cells. SOCS3 and SOCS5 are predominantly expressed in Th2 and Th1 cells, respectively, and they reciprocally inhibit the Th1 and Th2 differentiation processes. In this article, we discuss the role of SOCS3 and SOCS5 proteins in atopic asthma and allergic conjunctivitis and explore the potential of SOCS proteins as targets for therapeutic strategies in allergic disorders.

The key regulators of adult T helper cell responses, STAT6 and T-bet, are established in early life in mice

Murine neonatal immunity is typically Th2 biased. This is characterized by high-level IL-4 production at all phases of the immune response and poor IFN-gamma memory responses. The differential expression of Th1/Th2 cytokines by neonates and adults could arise if the critical regulators of Th differentiation and function, STAT6 and T-bet, operate differently during the neonatal period. To test this idea, the Th cell responses of wild-type, T-bet-deficient, or STAT6-deficient mice were compared in vitro and in vivo. The absence of these factors had similar qualitative effects on the development of effector function in neonates and adults, i.e., if a Th lineage was inhibited or enhanced in adult animals, a similar phenomenon was observed in neonates. However, there was a striking difference observed in the in vivo Th1 memory responses of STAT6-deficient mice initially immunized as neonates. Antigen-specific IFN-gamma production was increased 50-100-fold in STAT6-deficient neonates, achieving levels similar to those of STAT6-deficient adults. These findings demonstrate that STAT6 and T-bet signals are central in shaping Th responses in wild-type neonates, as in adult mice, and that the master regulators of Th cell development and function are already firmly established in early life.

The role of interferon-gamma on immune and allergic responses

Allergic diseases have been closely related to Th2 immune responses, which are characterized by high levels of interleukin (IL) IL-4, IL-5, IL-9 and IL-13. These cytokines orchestrate the recruitment and activation of different effector cells, such as eosinophils and mast cells. These cells along with Th2 cytokines are key players on the development of chronic allergic inflammatory disorders, usually characterized by airway hyperresponsiveness, reversible airway obstruction, and airway inflammation. Accumulating evidences have shown that altering cytokine-producing profile of Th2 cells by inducing Th1 responses may be protective against Th2-related diseases such as asthma and allergy. Interferon-gamma (IFN-gamma), the principal Th1 effector cytokine, has shown to be crucial for the resolution of allergic-related immunopathologies. In fact, reduced production of this cytokine has been correlated with severe asthma. In this review, we will discuss the role of IFN-gamma during the generation of immune responses and its influence on allergic inflammation models, emphasizing its biologic properties during the different aspects of allergic responses.

IFN-gamma suppresses STAT6 phosphorylation by inhibiting its recruitment to the IL-4 receptor

Polarized Th1 cells show a stable phenotype: they become insensitive to IL-4 stimulation and lose the potential to produce IL-4. Previously, we reported that IFN-gamma played a critical role in stabilizing Th1 phenotype. However, the mechanism by which IFN-gamma stabilizes Th1 phenotype is not clear. In this study, we compared STAT6 phosphorylation in wild-type (WT) and IFN-gamma receptor knockout (IFNGR(-/-)) Th1 cells. We found a striking diminution of STAT6 phosphorylation in differentiated WT Th1 cells, but not in differentiated IFNGR(-/-) Th1 cells. The impairment of STAT6 phosphorylation in differentiated WT Th1 cells was not due to a lack of IL-4R expression or phosphorylation. Jak1 and Jak3 expression and phosphorylation were comparable in both cell types. No differential expression of suppressor of cytokine signaling 1 (SOCS1), SOCS3, or SOCS5 was observed in the two cell types. In addition, Src homology 2-containing phosphatase mutation did not affect IL-4-induced STAT6 phosphorylation in differentiated Th1 cells derived from viable motheaten (me(v)/me(v)) mice. These results led us to focus on a novel mechanism. By using a pulldown assay, we observed that STAT6 in WT Th1 cells bound less effectively to the phosphorylated IL-4R/GST fusion protein than that in IFNGR(-/-) Th1 cells. Our results suggest that IFN-gamma may suppress phosphorylation of STAT6 by inhibiting its recruitment to the IL-4R.

IL-4-induced GATA-3 expression is a time-restricted instruction switch for Th2 cell differentiation

An initial activation signal via the TCR in a restricted cytokine environment is critical for the onset of Th cell development. Cytokines regulate the expression of key transcriptional factors, T-bet and GATA-3, which instruct the direction of Th1 and Th2 differentiation, through changes in chromatin conformation. In this study, we investigated the kinetics of IL-4-mediated signaling in a transgenic mouse, expressing human IL-4R on a mouse IL-4alphaR-deficient background. These experiments, allowing induction with human IL-4 at defined times, demonstrated that an IL-4 signal was required at the early stage of TCR-mediated T cell activation for lineage commitment to Th2, along with structural changes in chromatin, which take place in the conserved noncoding sequence-1 and -2 within the IL-4 locus. At later times, however, IL-4 failed to promote efficient Th2 differentiation and decondensation of chromatin, even though GATA-3 was clearly induced in the nuclei by IL-4 stimulation. Moreover, IL-4-mediated Th2 instruction was independent from cell division mediated by initial TCR stimulation. The role of IL-4 signaling may have a time restriction during Th2 differentiation. In late stages of initial T cell activation, the chromatin structure of the IL-4 locus retains condensation state. These results demonstrate that IL-4-induced GATA-3 expression is time-restriction switch for Th2 differentiation.

Molecular and cellular control of T1/T2 immunity at the interface between antimicrobial defense and immune pathology

The immune system evolved to rapidly recognize infectious threats and promptly mobilize cellular effectors to the infection site. Establishment of a robust T1-type immunity is a prerequisite for effective defense against most viruses and intracellular bacteria. However, accumulating evidence shows that T1 and T2 responses during such infections are not mutually exclusive. A possibility may be that the dual T1-T2 nature of antiviral immune responses is merely a byproduct of less than perfect crossregulatory mechanisms. Herein, we discuss molecular and cellular mechanisms of T-cell differentiation along with recent evidence supporting the hypothesis that rather than representing an epiphenomenon, coinduction of virus-specific T2 cells plays a significant homeostatic role. Thus, molecular pathways that regulate IL-4 production during influenza virus infection monitor T1-mediated immune responses in vital organs such as lungs and prevent immune pathology that may otherwise interfere with recovery from disease. Such evidence suggests that coinduction of T2 immunity maintains immune homeostasis during T1-mediated defense reactions. Finally, we outline implications on the earlier concept of T1/T2 dichotomy, supporting a model in which these two subsets, rather than being mutually antagonistic, together facilitate the recovery from infection.

Th1-dominant shift of T cell cytokine production, and subsequent reduction of serum immunoglobulin E response by administration in vivo of plasmid expressing Txk/Rlk, a member of Tec family tyrosine kinases, in a mouse model

BACKGROUND

Th1 and Th2 cells, resulting from antigenic stimulation in the presence of IL-12 and IL-4, respectively, are implicated in the pathology of various diseases including allergic and autoimmune diseases. Txk/Rlk is a member of Tec family tyrosine kinases. We reported that Txk acts as a Th1-specific transcription factor in the T lymphocytes.

OBJECTIVE

In this study we have asked whether administration of txk expression plasmid brings about a Th1/Th2 shift in vivo of the mice, and subsequent reduction of circulating IgE.

METHODS

Mice were administered a txk expression plasmid with hemagglutinating virus of Japan (HVJ) envelope vector. Txk expressions in spleen cells were assessed by immunoblotting and immunocytochemical staining. Cytokine productions by the spleen cells and serum Ig concentrations were studied by ELISA.

RESULTS

Administration of a txk expression plasmid with HVJ vector induced expression of Txk in the spleen cells. The spleen cells showed enhanced Th1-specific cytokine production; spleen cells from the txk administered mice produced more IFN-gamma as compared with those from control plasmid-administered mice in an antigen-specific manner. IL-2 and IL-4 secretions of the spleen cells were comparable between the two mouse groups. Txk administration did not reduce serum IgG concentration. It markedly reduced total IgE level and an IgG1/IgG2a ratio, reflection of Th1/Th2 balance, in sera. Furthermore, txk administration reduced ovalbumin (OVA)-specific IgE levels in sera of the OVA sensitized mice.

CONCLUSION

Thus, Txk enhances IFN-gamma secretion and thus modulates Th1/Th2 cytokine balance, leading to reduction of serum IgE.

IL-4 Utilizes an Alternative Receptor to Drive Apoptosis of Th1 Cells and Skews Neonatal Immunity toward Th2

Primary neonatal Th1 cells develop alongside of Th2 upon priming of the newborn but undergo apoptosis upon recall with antigen. These Th1 cells were isolated, and their death was correlated with elevated IL-13Ralpha1 chain expression. Strikingly, neutralization of Th2s' IL-4 reduced apoptosis, sustained recall responses, and the live Th1 cells displayed a decrease in IL-13Ralpha1 expression. Blockade of IL-13Ralpha1 or IL-4Ralpha also restores recall and secondary Th1 responses. Adult T cells primed within the neonatal environment did not upregulate IL-13Ralpha1 chain or undergo apoptosis and developed recall Th1 responses. These observations indicate that developmental expression of IL-13Ralpha1 along with IL-4Ralpha provides a receptor through which IL-4 induces death of Th1 cells and skews neonatal immunity toward Th2.

Molecular mechanisms regulating Th1 immune responses

The T helper lymphocyte is responsible for orchestrating the appropriate immune response to a wide variety of pathogens. The recognition of the polarized T helper cell subsets Th1 and Th2 has led to an understanding of the role of these cells in coordinating a variety of immune responses, both in responses to pathogens and in autoimmune and allergic disease. Here, we discuss the mechanisms that control lineage commitment to the Th1 phenotype. What has recently emerged is a rich understanding of the cytokines, receptors, signal transduction pathways, and transcription factors involved in Th1 differentiation. Although the picture is still incomplete, the basic pathways leading to Th1 differentiation can now be understood in in vitro and a number of infection and disease models.

Reciprocal expression of TRAIL and CD95L in Th1 and Th2 cells: role of apoptosis in T helper subset differentiation

Upon activation, naïve T helper cells can differentiate into two major distinct subsets, T helper 1 (Th1) and T helper 2 (Th2), as defined by their effector functions and cytokine secretion patterns. Cytokine milieu and costimulatory molecules have been shown to play an essential role in determining T helper differentiation. However, it is still unclear how the effects of signals of costimulatory molecules and cytokines are exerted during T helper differentiation. We show evidence suggesting that while cytokine signals initiate the differentiation program, the selective action of death effectors determines the end point balance of differentiating T helper subsets. We examined the expression of TNF-related apoptosis-inducing ligand (TRAIL) and CD95L in cloned and in vitro differentiated Th1 and Th2 cells. We found that activation-induced expression of TRAIL is exclusively observed in Th2 clones and primary T helper cells differentiated under the Th2 condition, while the expression of CD95L is mainly in Th1 cells. Furthermore, these two subsets exhibit distinct susceptibilities to TRAIL- and CD95L-mediated apoptosis. Th2 cells are more resistant to either TRAIL- or CD95L-induced apoptosis than Th1 cells. More importantly, both Th1 and Th2 cells could induce apoptosis in labeled Th1 but not Th2 cells. Blocking TRAIL and CD95L significantly enhance IFN-gamma production in vitro. Likewise, young MRL/MpJ-lpr/lpr mice also showed more Th1 response to ovalbumin immunization as compared to MRL/MpJ+/+. Therefore, apoptosis mediated by CD95L and TRAIL is critical in determining the fate of differentiating T helper cells.

Modulation of T cell cytokine production by interferon regulatory factor-4

Production of cytokines is one of the major mechanisms employed by CD4(+) T cells to coordinate immune responses. Although the molecular mechanisms controlling T cell cytokine production have been extensively studied, the factors that endow T cells with their ability to produce unique sets of cytokines have not been fully characterized. Interferon regulatory factor (IRF)-4 is a lymphoid-restricted member of the interferon regulatory factor family of transcriptional regulators, whose deficiency leads to a profound impairment in the ability of mature CD4(+) T cells to produce cytokines. In these studies, we have investigated the mechanisms employed by IRF-4 to control cytokine synthesis. We demonstrate that stable expression of IRF-4 in Jurkat T cells not only leads to a strong enhancement in the synthesis of interleukin (IL)-2, but also enables these cells to start producing considerable amounts of IL-4, IL-10, and IL-13. Transient transfection assays indicate that IRF-4 can transactivate luciferase reporter constructs driven by either the human IL-2 or the human IL-4 promoter. A detailed analysis of the effects of IRF-4 on the IL-4 promoter reveals that IRF-4 binds to a site adjacent to a functionally important NFAT binding element and that IRF-4 cooperates with NFATc1. These studies thus support the notion that IRF-4 represents one of the lymphoid-specific components that control the ability of T lymphocytes to produce a distinctive array of cytokines.

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.

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.

Expression of the suppressor of cytokine signaling-5 (SOCS5) negatively regulates IL-4-dependent STAT6 activation and Th2 differentiation

The development of helper T (Th) cell subsets, which secrete distinct cytokines, plays an important role in determining the type of immune response. The IL-4-mediated Janus kinase-signal transducer and activator of transcription signaling pathway is crucial for mediating Th2 cell development. Notably, this pathway is selectively impaired in Th1 cells, although the molecular basis of this impairment remains unclear. We show here that during Th1 differentiation a reduction in the association of Janus kinase 1 with the IL-4 receptor (IL-4R) correlated with the appearance of the suppressor of cytokine signaling-5 (SOCS5). SOCS5 protein was preferentially expressed in committed Th1 cells and interacted with the cytoplasmic region of the IL-4Ralpha chain irrespective of receptor tyrosine phosphorylation. This unconventional interaction of SOCS5 protein with the IL-4R resulted in the inhibition of IL-4-mediated signal transducer and activator of transcription-6 activation. T cells from transgenic mice constitutively expressing SOCS5 exhibited a significant reduction of IL-4-mediated Th2 development. Therefore, the induced SOCS5 protein in Th1 differentiation environment may play an important role by regulating Th1 and Th2 balance.

Dysregulated T helper cell differentiation in the absence of interferon regulatory factor 4

Certain IFN regulatory factor (IRF) transcription factors indirectly influence T helper (Th) cell differentiation by regulating the production of IL-12. Here, we show that IRF4 directly regulates Th cell differentiation in vitro and in vivo during murine leishmaniasis. In the absence of IRF4, IL-12-induced Th1 cell differentiation was compromised, while IL-4 failed to induce Th2 cell differentiation. Instead, IL-4 tended to induce Th1 cells, defined by production of IFN-gamma and TNF. Although early IL-4 signaling was normal in IRF4(-/-) Th cells, the protein GATA-3, a transcription factor critical for Th2 development, was not up-regulated following IL-4 treatment. Retroviral overexpression of GATA-3 rescued Th2 differentiation. Therefore, IRF4 deficiency manifests itself as severely dysregulated Th cell differentiation.

The biological outcome of CD40 signaling is dependent on the duration of CD40 ligand expression: reciprocal regulation by interleukin (IL)-4 and IL-12

CD40 ligand (CD154) expression on activated T cells can be separated into an early TCR-dependent phase, which occurs between 0 and 24 h after activation, and a later extended phase, which occurs after 24 h and is reciprocally regulated by the cytokines IL-4 and IL-12. IL-4 represses, whereas IL-12 sustains CD154 expression. Consistent with this, Th1, but not Th2, cells express CD154 for extended periods. Differences in the duration of CD154 expression have important biological consequences because sustained, but not transient, expression of CD154 on activated T cells can prevent B cell terminal differentiation. Thus, the differential ability of Th cells to sustain CD154 expression is an important part of their helper function and should influence the activities of other CD40-expressing cell types.

Overexpression of insulin receptor substrate-1, but not insulin receptor substrate-2, protects a T cell hybridoma from activation-induced cell death

The insulin receptor substrate (IRS) family of signaling molecules is expressed in lymphocytes, although their functions in these cells is largely unknown. To investigate the role of IRS in the protection of T cells from activation-induced cell death (AICD), we transfected the T cell hybridoma A1.1, which is IL-4 responsive but lacks expression of IRS family members with cDNA encoding IRS1 or IRS2. Stimulation of these clones with immobilized anti-CD3-induced expression of CD69 to the same level as the parental A1.1 cells. However, the A1.1 IRS1-expressing cells were markedly resistant to AICD, while the A1.1 IRS2-expressing cells were not. Inhibition of phosphatidylinositol 3'-kinase in the A1.1 IRS1-expressing cells did not abrogate their resistance to AICD. Fas mRNA was induced similarly by anti-CD3 in A1.1, A1.1 IRS1-expressing, and A1.1 IRS2-expressing cells. However, induction of Fas ligand (FasL) mRNA and functional FasL protein was delayed and decreased in IRS1-expressing cells, but not in IRS2-expressing cells. The induction of transcription from a 500-bp FasL promoter and a minimal 16-mer early growth response element linked to luciferase was also impaired in the IRS1-expressing cells. These results suggest that overexpression of IRS1, but not IRS2, protects A1.1 cells from AICD by diminishing FasL transcription through a pathway that is independent of the tyrosine phosphorylation of IRS1 and phosphatidylinositol 3'-kinase activity.

CD4 T cells selected by antigen under Th2 polarizing conditions favor an elongated TCR alpha chain complementarity-determining region 3

The affinity of the MHC/peptide/TCR interaction is thought to be one factor determining the differentiation of CD4+ T cells into Th1 or Th2 phenotypes. To study whether CD4+ cells generated under conditions favoring Th1 or Th2 responses select structurally different TCRs, Th1 and Th2 clones and lines were generated from nonobese diabetic and nonobese diabetic H2-E transgenic mice against the peptides proteolipoprotein 56-70, glutamic acid decarboxylase(65) 524-543, and heat shock protein-60 peptides 168-186 and 248-264. Th1/Th2 polarization allowed the generation of clones and lines with fixed peptide specificity and class II restriction but differing in Th1/Th2 phenotype in which the impact on TCR selection and structure could be studied. The Th2 clones tended to use longer TCR complementarity-determining region (CDR)3alpha loops than their Th1 counterparts. This trend was confirmed by analyzing TCRalpha transcripts from Th1 and Th2 polarized, bulk populations. Molecular modeling of Th1- and Th2-derived TCRs demonstrated that Th2 CDR3alpha comprised larger side chain residues than Th1 TCRs. The elongated, bulky Th2 CDR3alpha loops may be accommodated at the expense of less optimal interactions between the MHC class II/peptide and other CDR loops of the TCR. We propose that CD4+ T cells selected from the available repertoire under Th2 polarizing conditions tend to have elongated TCR CDR3alpha loops predicted to alter TCR binding, reducing contact at other interfaces and potentially leading to impeded TCR triggering.

Influence of parasite load on the ability of type 1 T cells to control Leishmania major infection

BALB/c mice infected with Leishmania major developed a type 2 immune response which failed to control parasite replication. We found that scid mice that received splenocytes from BALB/c mice that had been infected for 3 weeks with L. major (a type 2 cell population) and that were subsequently infected with L. major were protected when they were treated with interleukin 12 (IL-12). In contrast, IL-12 was ineffective at protecting BALB/c mice infected for 3 weeks, suggesting that a high parasite load regulates the development of protective immunity. To determine how this regulation operates, we performed a series of adoptive transfers of naïve, type 1 or type 2 splenocytes into scid mice. The recipient scid mice were infected either for 5 weeks prior to cell transfer (and thus had a high parasite load) or at the time of cell transfer. scid mice that were infected for 5 weeks and received a type 1 cell population were able to cure their lesions. However, when 5-week-infected scid mice received both type 1 and 2 cell populations, they were unable to control their infections. In contrast, the same type 1 and 2 cells transferred to naïve scid mice, which were subsequently infected, provided protection. In addition, we found that naïve cells mediated protection in scid mice with established lesions. These results show that high parasite numbers do not block type 1 protective responses or the development of type 1 responses. Instead, the influence of a high parasite load is dependent on the presence of a type 2 cell population.

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.

Helper T cell differentiation, inside and out

The past year has seen remarkable progress in the field of helper T cell differentiation, including discovery of a novel transcription factor as well as a novel cytokine receptor of the Th1 lineage. The year has also brought new perspectives on the genetic and epigenetic control of gene expression. It is likely that mechanisms of immunity will continue to provide insight into the general problem of cellular decision-making.

CD8+ T cells control the TH phenotype of MBP-reactive CD4+ T cells in EAE mice

Trimolecular interactions between the T cell antigen receptor and MHC/peptide complexes, together with costimulatory molecules and cytokines, control the initial activation of naive T cells and determine whether the helper precursor cell differentiates into either T helper (TH)1 or TH2 effector cells. We now present evidence that regulatory CD8(+) T cells provide another level of control of TH phenotype during further evolution of immune responses. These regulatory CD8(+) T cells are induced by antigen-triggered CD4(+) TH1 cells during T cell vaccination and, in vitro, distinguish mature TH1 from TH2 cells in a T cell antigen receptor Vbeta-specific and Qa-1-restricted manner. In vivo, protection from experimental autoimmune encephalomyelitis (EAE) induced by T cell vaccination depends on CD8(+) T cells, and myelin basic protein-reactive TH1 Vbeta8(+) clones, but not TH2 Vbeta8(+) clones, used as vaccine T cells, protect animals from subsequent induction of EAE. Moreover, in vivo depletion of CD8(+) T cells during the first episode of EAE results in skewing of the TH phenotype toward TH1 upon secondary myelin basic protein stimulation. These data provide evidence that CD8(+) T cells control autoimmune responses, in part, by regulating the TH phenotype of self-reactive CD4(+) T cells.

Induction of type 2 activity in adult human CD8(+) T cells by repeated stimulation and IL-4

Repeated administration or chronic presence of antigen during CD4(+) T cell activation and a cytokine milieu enriched in IL-4 favour the generation and maintenance of a T(h)2 population. However, there is little data on how these factors affect adult human CD8(+) T cell functions. We established in vitro conditions to culture purified human CD8(+) T cells, and investigated how repeated stimulation and exogenous IL-4 modulated their functions. Repeated TCR-CD3 stimulation of CD8(+) T cells increased the number of CD25-, CD30- and CD40 ligand-expressing cells, and their capacity to secrete IL-4 and IL-5. In addition, repeatedly stimulated CD8(+) T cells had cytotoxic activity and provided help to resting B cells for IgE synthesis. The presence of exogenous IL-4 during repeated stimulation further increased the number of CD25(+) and CD30(+) CD8(+) T cells, up-regulated the number of IL-5(+) cells, and increased IL-5 levels released. These observations demonstrate that repeated TCR-CD3 stimulation of normal human CD8(+) T cells favoured the growth of cells with a type 2 phenotype and that this was further amplified by the presence of IL-4. These mechanisms may be important in virus-induced lung eosinophilic inflammation in healthy subjects and virus-induced exacerbations of asthma.

NF-kappa B/Rel participation in the lymphokine-dependent proliferation of T lymphoid cells

Proliferative responses of lymphoid cells to IL-2 and IL-4 depend on activation of the cells, but the mechanism(s) by which activation enhances cellular competence to respond to cytokines is not fully understood. The NF-kappaB/Rel family represents one signal transduction pathway induced during such activation. We show in this study that inhibition of NF-kappaB through the expression of an IkappaBalpha (inhibitory protein that dissociates from NF-kappaB) mutant refractory to signal-induced degradation (IkappaBalpha(DeltaN)) interfered with the acquisition of competence to proliferate in response to IL-4 as well as IL-2. Thymocytes and T cells from IkappaBalpha(DeltaN) transgenic mice expressed normal levels of IL-2R subunits. However, transgenic cells exhibited a dramatic defect in Stat5A activation treatment with IL-2, and a similar defect was observed for IL-4-induced Stat5. In contrast, T lymphoid cells with inhibition of NF-kappaB showed normal insulin receptor substrate-2 phosphorylation and only a modest decrease in Stat6 activation and insulin receptor substrate-1 phosphorylation after IL-4 stimulation. These results indicate that the NF-kappaB/Rel/IkappaBalpha system can regulate cytokine receptor capacitation through effects on the induction of downstream signaling by the Stat transcription factor family.