Allergen-induced airway hyperresponsiveness mediated by cyclooxygenase inhibition is not dependent on 5-lipoxygenase or IL-5, but is IL-13 dependent

Cyclooxygenase (COX) inhibition during allergic sensitization and allergen airway challenge results in augmented allergic inflammation. We hypothesized that this increase in allergic inflammation was dependent on increased generation of leukotrienes that results from COX inhibition, as leukotrienes are important proinflammatory mediators of allergic disease. To test this hypothesis, we allergically sensitized and challenged mice deficient in 5-lipoxygenase (5-LO). We found that 5-LO knockout mice that were treated with a COX inhibitor during allergic sensitization and challenge had significantly increased airway hyperresponsiveness (AHR) (p < 0.01) and airway eosinophilia (p < 0.01) compared with 5-LO knockout mice that were treated with vehicle. The proinflammatory cytokines have also been hypothesized to be critical regulators of airway inflammation and AHR. We found that the increase in airway eosinophilia seen with COX inhibition is dependent on IL-5, whereas the increase in AHR is not dependent on this cytokine. In contrast, the COX inhibition-mediated increase in AHR is dependent on IL-13, but airway eosinophilia is not. These results elucidate the pathways by which COX inhibition exerts a critical effect of the pulmonary allergen-induced inflammatory response and confirm that COX products are important regulators of allergic inflammation.     

Bruton's tyrosine kinase is required for TLR2 and TLR4-induced TNF, but not IL-6, production

Bruton's tyrosine kinase (Btk), the gene mutated in the human immunodeficiency X-linked agammaglobulinemia, is activated by LPS and is required for LPS-induced TNF production. In this study, we have investigated the role of Btk both in signaling via another TLR (TLR2) and in the production of other proinflammatory cytokines such as IL-1beta, IL-6, and IL-8. Our data show that in X-linked agammaglobulinemia PBMCs, stimulation with TLR4 (LPS) or TLR2 (N-palmitoyl-S-[2, 3-bis(palmitoyloxy)-(2R)-propyl]-(R)-cysteine) ligands produces significantly less TNF and IL-1beta than in normal controls. In contrast, a lack of Btk has no impact on the production of IL-6, IL-8, or the anti-inflammatory cytokine, IL-10. Our previous data suggested that Btk lies within a p38-dependent pathway that stabilizes TNF mRNA. Accordingly, TaqMan quantitative PCR analysis of actinomycin D time courses presented in this work shows that overexpression of Btk is able to stabilize TNF, but not IL-6 mRNA. Furthermore, using the p38 inhibitor SB203580, we show that the TLR4-induced production of TNF, but not IL-6, requires the activity of p38 MAPK. These data provide evidence for a common requirement for Btk in TLR2- and TLR4-mediated induction of two important proinflammatory cytokines, TNF and IL-1beta, and reveal important differences in the TLR-mediated signals required for the production of IL-6, IL-8, and IL-10.     

Smad3 is essential for TGF-beta 1 to suppress IL-2 production and TCR-induced proliferation, but not IL-2-induced proliferation

Transforming growth factor-beta1 is essential to maintain T cell homeostasis, as illustrated by multiorgan inflammation in mice deficient in TGF-beta1 signaling. Despite the physiological importance, the mechanisms that TGF-beta1 uses to regulate T cell expansion remain poorly understood. TGF-beta1 signals through transmembrane receptor serine/threonine kinases to activate multiple intracellular effector molecules, including the cytosolic signaling transducers of the Smad protein family. We used Smad3(-/-) mice to investigate a role for Smad3 in IL-2 production and proliferation in T cells. Targeted disruption of Smad3 abrogated TGF-beta1-mediated inhibition of anti-CD3 plus anti-CD28-induced steady state IL-2 mRNA and IL-2 protein production. CFSE labeling demonstrated that TGF-beta1 inhibited entry of wild-type anti-CD3 plus anti-CD28-stimulated cells into cycle cell, and this inhibition was greatly attenuated in Smad3(-/-) T cells. In contrast, disruption of Smad3 did not affect TGF-beta1-mediated inhibition of IL-2-induced proliferation. These results demonstrate that TGF-beta1 signals through Smad3-dependent and -independent pathways to inhibit T cell proliferation. The inability of TGF-beta1 to inhibit TCR-induced proliferation of Smad3(-/-) T cells suggests that IL-2 is not the primary stimulus driving expansion of anti-CD3 plus anti-CD28-stimulated T cells. Thus, we establish that TGF-beta1 signals through multiple pathways to suppress T cell proliferation.     

STAT3 is indispensable to IL-27-mediated cell proliferation but not to IL-27-induced Th1 differentiation and suppression of proinflammatory cytokine production

IL-27, a member of the IL-6/IL-12 family, activates both STAT1 and STAT3 through its receptor, which consists of WSX-1 and gp130 subunits, resulting in augmentation of Th1 differentiation and suppression of proinflammatory cytokine production. In the present study, we investigated the role of STAT3 in the IL-27-mediated immune functions. IL-27 induced phosphorylation of STAT1, -2, -3 and -5 in wild-type naive CD4+ T cells, but failed to induce that of STAT3 and STAT5 in STAT3-deficient cohorts. IL-27 induced not only proinflammatory responses including up-regulation of ICAM-1, T-box expressed in T cells, and IL-12Rbeta2 and Th1 differentiation, but also anti-inflammatory responses including suppression of proinflammatory cytokine production such as IL-2, IL-4, and IL-13 even in STAT3-deficient naive CD4+ T cells. In contrast, IL-27 augmented c-Myc and Pim-1 expression and induced cell proliferation in wild-type naive CD4+ T cells but not in STAT3-deficient cohorts. Moreover, IL-27 failed to activate STAT3, augment c-Myc and Pim-1 expression, and induce cell proliferation in pro-B BaF/3 transfectants expressing mutant gp130, in which the putative STAT3-binding four Tyr residues in the YXXQ motif of the cytoplasmic region was replaced by Phe. These results suggest that STAT3 is activated through gp130 by IL-27 and is indispensable to IL-27-mediated cell proliferation but not to IL-27-induced Th1 differentiation and suppression of proinflammatory cytokine production. Thus, IL-27 may be a cytokine, which activates both STAT1 and STAT3 through distinct receptor subunits, WSX-1 and gp130, respectively, to mediate its individual immune functions.     

Comparative roles of IL-4, IL-13, and IL-4Ralpha in dendritic cell maturation and CD4+ Th2 cell function

IL-4 and IL-13 play key roles in Th2 immunity and asthma pathogenesis. Although the function of these cytokines is partially linked through their shared use of IL-4Ralpha for signaling, the interplay between these cytokines in the development of memory Th2 responses is not well delineated. In this investigation, we show that both IL-4 and IL-13 influence the maturation of dendritic cells (DC) in the lung and their ability to regulate secretion of IFN-gamma and Th2 cytokines by memory CD4(+) T cells. Cocultures of wild-type T cells with pulmonary DC from allergic, cytokine-deficient mice demonstrated that IL-4 enhanced the capacity of DC to stimulate T cell secretion of Th2 cytokines, whereas IL-13 enhanced the capacity of DC to suppress T cell secretion of IFN-gamma. Because IL-4Ralpha is critical for IL-4 and IL-13 signaling, we also determined how variants of IL-4Ralpha influenced immune cell function. T cells derived from allergic mice expressing a high-affinity IL-4Ralpha variant produced higher levels of IL-5 and IL-13 compared with T cells derived from allergic mice expressing a low-affinity IL-4Ralpha variant. Although DC expressing different IL-4Ralpha variants did not differ in their capacity to influence Th2 cytokine production, they varied in their capacity to inhibit IFN-gamma production by T cells. Thus, IL-4 and IL-13 differentially regulate DC function and the way these cells regulate T cells. The affinity of IL-4Ralpha also appears to be a determinant in the balance between Th2 and IFN-gamma responses and thus the severity of allergic disease.     

IL-4 induces differentiation and expansion of Th2 cytokine-producing eosinophils

Innate effector cells that produce Th2-type cytokines are critical in Th2 cell-mediated immune responses. However, it is not known how these cells acquire the ability to produce Th2 cytokines. IL-4 is a potent inducer that directs differentiation of naive CD4(+) T cells into CD4(+) Th2 effector cells. To determine whether IL-4 can induce differentiation and expansion of Th2 cytokine-producing innate cells, we used mice whose il-4 gene was replaced by a knock-in green fluorescence protein (gfp) gene. We found that, directly ex vivo, IL-4 increased the number of GFP(+) cells in the airway and the lung tissue in an Ag-specific manner. The majority of GFP(+) cells were eosinophils, suggesting that IL-4 plays a pivotal role in expanding IL-4-producing eosinophils in vivo. IL-4-producing eosinophils showed some unique features compared with IL-4-producing CD4(+) T cells. They exhibited biallelic expression of the il-4 gene when stimulated and were more dominant IL-4- and IL-5-producing cells. Furthermore, we show that IL-4 drove bone marrow progenitor cells to differentiate into Th2 cytokine-producing eosinophils in vitro. These results strongly suggest IL-4 is a potent factor in directing bone marrow progenitor cells to differentiate into Th2 cytokine-producing eosinophils.     

Cutting edge: differential expression of chemokines in Th1 and Th2 cells is dependent on Stat6 but not Stat4

The in vivo function of Th cell subsets is largely dependent on the ability of differentiated CD4+ T cells to be recruited to specific sites and secrete restricted sets of cytokines. In this paper we demonstrate that Th1 and Th2 cells secrete discrete patterns of chemokines, small m.w. cytokines that function as chemoattractants in inflammatory reactions. Th2 cells secrete macrophage-derived chemokine and T cell activation gene 3, and acquisition of this pattern of expression is dependent on Stat6. In contrast, Th1 cells secrete lymphotactin and RANTES, though unlike IFN-gamma, expression of these chemokines is independent of Stat4. We further show that supernatants from activated Th2 cells preferentially induce the chemotaxis of Th2 over Th1 cells, corresponding with Stat6-dependent expression of CCR4 and CCR8 in Th2 cells. These data provide the basis for restricted and direct T cell-mediated cellular recruitment to sites of inflammation.     

IL-10 reduces Th2 cytokine production and eosinophilia but augments airway reactivity in allergic mice

Mononuclear phagocytes can interact with mesenchymal cells and extracellular matrix components that are crucial for connective tissue rearrangement. We asked whether blood monocytes can alter matrix remodeling mediated by human lung fibroblasts cultured in a three-dimensional collagen gel. Blood monocytes from healthy donors (>95% pure) were cast into type I collagen gels that contained lung fibroblasts. Monocytes in coculture inhibited the fibroblast-mediated gel contractility in a time- and concentration-dependent manner. The concentration of PGE(2), a well-known inhibitor of gel contraction, was higher (P < 0.01) in media from coculture; this media attenuated fibroblast gel contraction, whereas conditioned media from either cell type cultured alone did not. Three-dimensional cultured monocytes responded to conditioned media from cocultures by producing interleukin-1beta and tumor necrosis factor-alpha, whereas fibroblasts increased synthesis of PGE(2). Antibodies to interleukin-1beta and tumor necrosis factor-alpha blocked the monocyte inhibitory effect and reduced the amount of PGE(2) produced. The ability of monocytes to block the fibroblast contraction of matrix may be an important mechanism in regulating tissue remodeling.     

Lipopolysaccharide-induced suppression of airway Th2 responses does not require IL-12 production by dendritic cells

The prevalence of atopic asthma, a Th2-dependent disease, is reaching epidemic proportions partly due to improved hygiene in industrialized countries. There is an inverse correlation between the level of environmental endotoxin exposure and the prevalence of atopic sensitization. As dendritic cells (DC) have been implicated in causing sensitization to inhaled Ag, we studied the effect of endotoxin on Th2 development induced by bone marrow DC in vitro and by intratracheal injection in vivo, with particular emphasis on the role played by the polarizing cytokine IL-12. Bone marrow-derived DC stimulated with Escherichia coli O26:B6 LPS produced IL-12p70 for a limited period of time, after which production became refractory to further stimulation with CD40 ligand, a phenomenon previously called "exhaustion." The level of IL-12 production of DC did not correlate with Th1 development, as exhausted OVA-pulsed DC were still capable of shifting the cytokine pattern of responding OVA-specific Th cells toward Th1 in vitro and in vivo. When mice were first immunized by intratracheal injection of OVA-DC and subsequently challenged with OVA aerosol, prior in vitro stimulation of DC with LPS reduced the development of airway eosinophilia and Th2 cytokine production. Most surprisingly, the capacity of LPS to reduce Th2-dependent eosinophilic airway inflammation was IL-12-independent altogether, as IL-12p40 knockout DC had a similar reduced capacity to prime for Th2 responses. These results suggest that LPS reduces sensitization to inhaled Ag by reducing DC-driven Th2 development, but that IL-12 is not necessary for this effect.     

IL-13-induced airway hyperreactivity during respiratory syncytial virus infection is STAT6 dependent

Airway damage and hyperreactivity induced during respiratory syncytial virus (RSV) infection can have a prolonged effect in infants and young children. These infections can alter the long-term function of the lung and may lead to severe asthma-like responses. In these studies, the role of IL-13 in inducing and maintaining a prolonged airway hyperreactivity response was examined using a mouse model of primary RSV infection. Using this model, there was evidence of significant airway epithelial cell damage and sloughing, along with mucus production. The airway hyperreactivity response was significantly increased by 8 days postinfection, peaked during days 10-12, and began to resolve by day 14. When the local production of Th1- and Th2-associated cytokines was examined, there was a significant increase, primarily in IL-13, as the viral response progressed. Treatment of RSV-infected mice with anti-IL-13 substantially inhibited airway hyperreactivity. Anti-IL-4 treatment had no effect on the RSV-induced responses. Interestingly, when IL-13 was neutralized, an early increase in IL-12 production was observed within the lungs, as was a significantly lower level of viral Ags, suggesting that IL-13 may be regulating an important antiviral pathway. The examination of RSV-induced airway hyperreactivity in STAT6(-/-) mice demonstrated a significant attenuation of the response, similar to the anti-IL-13 treatment. In addition, STAT6(-/-) mice had a significant alteration of mucus-producing cells in the airway. Altogether, these studies suggest that a primary factor leading to chronic RSV-induced airway dysfunction may be the inappropriate production of IL-13.     

Murine cytomegalovirus infection alters Th1/Th2 cytokine expression, decreases airway eosinophilia, and enhances mucus production in allergic airway disease

Concomitant infection of murine CMV (MCMV), an opportunistic respiratory pathogen, altered Th1/Th2 cytokine expression, decreased bronchoalveolar lavage (BAL) fluid eosinophilia, and increased mucus production in a murine model of OVA-induced allergic airway disease. Although no change in the total number of leukocytes infiltrating the lung was observed between challenged and MCMV/challenged mice, the cellular profile differed dramatically. After 10 days of OVA-aerosol challenge, eosinophils comprised 64% of the total leukocyte population in BAL fluid from challenged mice compared with 11% in MCMV/challenged mice. Lymphocytes increased from 11% in challenged mice to 30% in MCMV/challenged mice, and this increase corresponded with an increase in the ratio of CD8(+) to CD4(+)TCRalphabeta lymphocytes. The decline in BAL fluid eosinophilia was associated with a change in local Th1/Th2 cytokine profiles. Enhanced levels of IL-4, IL-5, IL-10, and IL-13 were detected in lung tissue from challenged mice by RNase protection assays. In contrast, MCMV/challenged mice transiently expressed elevated levels of IFN-gamma and IL-10 mRNAs, as well as decreased levels of IL-4, IL-5, and IL-13 mRNAs. Elevated levels of IFN-gamma and reduced levels of IL-5 were also demonstrated in BAL fluid from MCMV/challenged mice. Histological evaluation of lung sections revealed extensive mucus plugging and epithelial cell hypertrophy/hyperplasia only in MCMV/challenged mice. Interestingly, the development of airway hyperresponsiveness was observed in challenged mice, not MCMV/challenged mice. Thus, MCMV infection can modulate allergic airway inflammation, and these findings suggest that enhanced mucus production may occur independently of BAL fluid eosinophilia.     

Necrosis is associated with IL-6 production but apoptosis is not

Due to loss of cell membrane integrity, necrotic cells passively release several cytosolic factors that can activate antigen presenting cells and other immune cells. In contrast, cells dying by apoptosis do not induce an inflammatory response. Here we show that necrotic cell death induced by several stimuli, such as TNF, anti-Fas or dsRNA, coincides with NF-kappaB-and p38MAPK-mediated upregulation and secretion of the pro-inflammatory cytokine IL-6. This event is greatly reduced or absent in conditions of apoptotic cell death induced by the same stimuli. This demonstrates that besides the capacity of necrotic cells to induce an inflammatory response due to leakage of cellular contents, necrotic dying cells themselves are involved in the expression and secretion of inflammatory cytokines. Moreover, inhibition of NF-kappaB and p38MAPK activation does not affect necrotic cell death in all conditions tested. This suggests that the activation of inflammatory pathways is distinct from the activation of necrotic cell death sensu strictu.     

MyD88 regulation of Fusarium keratitis is dependent on TLR4 and IL-1R1 but not TLR2

The fungal pathogens Fusarium solani and Fusarium oxysporum cause severe corneal disease in the United States and worldwide and were the causative organisms in a recent outbreak of contact lens-associated keratitis. To characterize innate immunity in Fusarium keratitis, we developed a murine model in which conidia are injected into the corneal stroma. Immunocompetent C57BL/6 mice rapidly developed severe corneal opacification associated with neutrophil infiltration and clearance of Fusarium hyphae. In contrast, neutrophil infiltration was delayed in MyD88-/- mice, resulting in uncontrolled growth of Fusarium hyphae in the corneal stroma and anterior chamber, and eventually resulting in corneal perforation. Corneal opacification scores in TLR2-/-, TLR4-/-, and TLR2/4-/- mice were similar to those of C57BL/6 mice; however, TLR4-/- and TLR2/4-/- mice had impaired antifungal responses. The phenotype of infected IL-1R1-/- mice was similar to that of MyD88-/- mice, with uncontrolled fungal growth resulting in corneal perforation. IL-1R1-/- mice also produced significantly less CXCL1/KC in the corneal stroma compared with C57BL/6 mice consistent with delayed neutrophil recruitment to the corneal stroma. Together, these findings indicate that IL-1R1 and MyD88 regulate CXC chemokine production and neutrophil recruitment to the cornea, and that TLR4 has an important role in controlling growth and replication of these pathogenic fungi.     

Helicobacter pylori-induced mucosal inflammation is Th1 mediated and exacerbated in IL-4, but not IFN-gamma, gene-deficient mice

To elucidate the pathogenesis of Helicobacter pylori-associated gastritis, we studied immune responses of C57BL/6J wild-type (WT), SCID, and gene deficient (IFN-gamma-/- and IL-4-/-) mice following infection with a pathogenic isolate of H. pylori (SPM326). During early infection in WT mice, mononuclear and polymorphonuclear cells accumulated in the gastric lamina propria, and the numbers of cells in the inflamed mucosa expressing IFN-gamma, but not IL-4, mRNA rose significantly (p < 0.005), consistent with a local Th1 response. Splenic T cells from the same infected WT mice produced high levels of IFN-gamma, no detectable IL-4, and low amounts of IL-10 following in vitro H. pylori urease stimulation, reflecting a systemic Th1 response. Infected C57BL/6J SCID mice did not develop gastric inflammation despite colonization by many bacteria. Infected C57BL/10J and BALB/c mice also did not develop gastric inflammation and displayed a mixed Th1/Th2 splenic cytokine profile. These data imply a major role for the Th1 cytokine IFN-gamma in H. pylori-associated gastric inflammation in C57BL/6J mice. Compared with WT animals, infected IL-4-/- animals had more severe gastritis and higher levels of IFN-gamma production by urease-stimulated splenocytes (p < 0.01), whereas IFN-gamma-/- mice exhibited no gastric inflammation and higher levels of IL-4 production by stimulated splenocytes. These findings establish C57BL/6J mice as an important model for H. pylori infection and demonstrate that up-regulated production of IFN-gamma, in the absence of the opposing effects of IL-4 (and possibly IL-10), plays a pivotal role in promoting H. pylori-induced mucosal inflammation.     

IL-4 inhibits IL-2 synthesis and IL-2-induced up-regulation of IL-2R alpha but not IL-2R beta chain in CD4+ human T cells.

There is growing evidence to suggest a regulatory role of IL-4 in the immune system affecting both proliferation and lymphokine production. In the present work we have analyzed the effect of IL-4 on IL-2 and IFN-gamma synthesis by stimulating CD4+ human T cells (+10% accessory cells) with Con A in the presence of several doses (1 to 100 U/ml) of human rIL-4. The results showed an impaired IL-2 and IFN-gamma synthesis in the presence of IL-4. This inhibition was dose dependent and was evident only when IL-4 was added in the first 2 h of culture. Moreover, the external addition of IL-2 did not revert the inhibitory effect of IL-4 on IL-2 and IFN-gamma synthesis induced by Con A. We have also analyzed the effect of IL-4 on the expression of both alpha- and beta-chains of the IL-2R. Although the expression of IL-2R alpha mRNA was not modified after 6 h in culture in the presence of IL-4, a decrease was observed at 24 and 48 h. The addition of rIL-2 showed that the inhibition in IL-2R alpha expression could be explained by an impairment in the up-regulatory signal transmitted through the IL-2R. In addition to this, IL-4 did not modify the IL-2R beta mRNA expression at 6 and 24 h although a decreased expression was observed at 48 h which could be explained by the defective IL-2 production. The differential effect of IL-4 on the up-regulatory effect of IL-2 in the expression of IL-2R alpha and IL-2R beta suggest the existence of different regulatory mechanisms acting on the expression of both chains.     

Complement activation product C5a is a selective suppressor of TLR4-induced, but not TLR3-induced, production of IL-27(p28) from macrophages

There is accumulating evidence that the complement activation product, C5a, can orchestrate cellular immune functions. IL-27(p28/EBI3) is an emerging key player essential for regulating inflammatory responses and T cells. In this article, we report that C5a robustly suppressed IL-27(p28) gene expression and release in peritoneal macrophages. These cells from C57BL/6J mice abundantly produced IL-27(p28) after engagement of either the TLR3 (polyinosinic-polycytidylic acid) or TLR4 (LPS) receptor. Genetic deficiency of either TLR4 or LBP completely incapacitated the ability of macrophages to secrete IL-27(p28) in response to LPS. IL-27(p28)-producing macrophages also expressed the C5aR receptor, thus displaying an IL-27(p28)(+)F4/80(+)C5aR(+) phenotype. C5a suppressed IL-27(p28) in LPS-stimulated macrophages via interactions with the C5aR receptor rather than the C5L2 receptor. After endotoxemia, C5aR(-/-) mice displayed higher plasma levels of IL-27(p28) compared with C57BL/6J mice. C5a did not affect the release of IL-27(p28) or the frequency of IL-27(p28)(+)F4/80(+) macrophages after engagement of TLR3. Mechanistically, LPS activated both the NF-κB and the PI3K/Akt pathways, whereas C5a activated only the PI3K/Akt pathway. Engagement of PI3K/Akt was inhibitory for IL-27(p28) production, because PI3K/Akt pharmacologic blockade resulted in increased amounts of IL-27(p28) and reversed the suppressive effects of C5a. Blockade of PI3K/Akt in endotoxemic C57BL/6J mice resulted in higher generation of IL-27(p28). In contrast, the PI3K/Akt pathway was not involved in TLR3-mediated release of IL-27(p28). These data provide new evidence about how complement activation may selectively interfere with production of T cell regulatory cytokines by APCs in the varying contexts of either bacterial (TLR4 pathway) or viral (TLR3 pathway) infection.     

Inducible costimulator regulates Th2-mediated inflammation, but not Th2 differentiation, in a model of allergic airway disease

A novel costimulatory molecule expressed on activated T cells, inducible costimulator (ICOS), and its ligand, B7-related protein-1 (B7RP-1), were recently identified. ICOS costimulation leads to the induction of Th2 cytokines without augmentation of IL-2 production, suggesting a role for ICOS in Th2 cell differentiation and expansion. In the present study, a soluble form of murine ICOS, ICOS-Ig, was used to block ICOS/B7RP-1 interactions in a Th2 model of allergic airway disease. In this model, mice are sensitized with inactivated Schistosoma mansoni eggs and are subsequently challenged with soluble S. mansoni egg Ag directly in the airways. Treatment of C57BL/6 mice with ICOS-Ig during sensitization and challenge attenuated airway inflammation, as demonstrated by a decrease in cellular infiltration into the lung tissue and airways, as well as by a decrease in local IL-5 production. These inhibitory effects were not due to a lack of T cell priming nor to a defect in Th2 differentiation. In addition, blockade of ICOS/B7RP-1 interactions during ex vivo restimulation of lung Th2 effector cells prevented cytokine production. Thus, blockade of ICOS signaling can significantly reduce airway inflammation without affecting Th2 differentiation in this model of allergic airway disease.