IgE(+), Kit(-), I-A/I-E(-) myeloid cells are the initial source of Il-4 after antigen challenge in a mouse model of allergic pulmonary inflammation

Interleukin-33-activated basophils promote asthma by regulating Th2 cell entry into lung tissue

Asthma is characterized by lung eosinophilia, remodeling, and mucus plugging, controlled by adaptive Th2 effector cells secreting IL-4, IL-5, and IL-13. Inhaled house dust mite (HDM) causes the release of barrier epithelial cytokines that activate various innate immune cells like DCs and basophils that can promote Th2 adaptive immunity directly or indirectly. Here, we show that basophils play a crucial role in the development of type 2 immunity and eosinophilic inflammation, mucus production, and bronchial hyperreactivity in response to HDM inhalation in C57Bl/6 mice. Interestingly, conditional depletion of basophils during sensitization did not reduce Th2 priming or asthma inception, whereas depletion during allergen challenge did. During the challenge of sensitized mice, basophil-intrinsic IL-33/ST2 signaling, and not FcεRI engagement, promoted basophil IL-4 production and subsequent Th2 cell recruitment to the lungs via vascular integrin expression. Basophil-intrinsic loss of the ubiquitin modifying molecule Tnfaip3, involved in dampening IL-33 signaling, enhanced key asthma features. Thus, IL-33-activated basophils are gatekeepers that boost allergic airway inflammation by controlling Th2 tissue entry.

Importance of mast cell histamine secretion in IgG-mediated systemic anaphylaxis

BACKGROUND

IgG can mediate murine and human systemic anaphylaxis (SA). The roles of mast cells (MCs) and histamine in IgG-mediated anaphylaxis are controversial for mice and have not been studied in vivo for humans. We are now investigating these issues.

METHODS

Actively or passively sensitized wild-type and immune-deficient mice were induced to develop anaphylaxis by intravenous antigen challenge. Anaphylaxis was characterized by evaluating hypothermia, hypomobility, histamine, and MC protease responses.

RESULTS

In contrast to our previous results with protein-immunized mice from a conventional colony, IgG-mediated passive SA in our specific pathogen-free colony mice depended considerably on histamine produced by connective tissue MCs (CTMCs) in response to FcγRIII crosslinking. This was found for C57BL/6 and young male and female BALB/c mice, including BALB/c mice newly arrived from 3 vendors. IgG-mediated anaphylaxis was less histamine dependent in old than young mice. Although both mucosal MC (MMC) and CTMC responses were severely depleted in c-kit-deficient mice, MMC responses depended considerably more than CTMC responses on c-kit for maintenance. In immunologically naive mice, FcγRIII crosslinking strongly activated a subset of CTMCs but had little ability to activate MMCs. In vivo LPS + poly I:C treatment decreased histamine dependence of IgG-mediated anaphylaxis, while a strong TH2 immune response increased FcγRIII crosslinking-induced MMC activation. IgG-mediated activation of human MCs in reconstituted immunodeficient mice induced histamine-dependent anaphylaxis.

CONCLUSION

IgG-dependent SA can be mediated largely by histamine released by mouse CTMCs and human MCs; histamine dependence is influenced by mouse age, sex, and immune and infectious history, as well as the anaphylaxis model studied.

Roles of basophils and mast cells infiltrating the lung by multiple antigen challenges in asthmatic responses of mice

BACKGROUND AND PURPOSE

Mast cell hyperplasia has been observed in the lungs of mice with experimental asthma, but few reports have studied basophils. Here, we attempted to discriminate and quantify mast cells and basophils in the lungs in a murine asthma model, determine if both cells were increased by multiple antigen challenges and assess the roles of those cells in asthmatic responses.

EXPERIMENTAL APPROACH

Sensitized Balb/c mice were intratracheally challenged with ovalbumin four times. Mast cells and basophils in enzymatically digested lung tissue were detected by flow cytometry. An anti-FcεRI monoclonal antibody, MAR-1, was i.p. administered during the multiple challenges.

KEY RESULTS

The numbers of both mast cells (IgE(+) C-kit(+) ) and basophils (IgE(+) C-kit(-) CD49b(+) ) increased in the lungs after three challenges. Treatment with MAR-1 completely abolished the increases; however, a late-phase increase in specific airway resistance (sRaw), and airway eosinophilia and neutrophilia were not affected by the treatment, although the early-phase increase in sRaw was suppressed. MAR-1 reduced antigen-induced airway IL-4 production. Basophils infiltrating the lung clearly produced IL-4 after antigen stimulation in vitro; however, histamine and murine mast cell protease 1 were not increased in the serum after the challenge, indicating that mast cell activation was not evoked.

CONCLUSION AND IMPLICATIONS

Both mast cells and basophils infiltrated the lungs by multiple intratracheal antigen challenges in sensitized mice. Neither mast cells nor basophils were involved in late-phase airway obstruction, although early-phase obstruction was mediated by basophils. Targeting basophils in asthma therapy may be useful for an early asthmatic response.

Interleukin-33 and alveolar macrophages contribute to the mechanisms underlying the exacerbation of IgE-mediated airway inflammation and remodelling in mice

Allergen-specific IgE has long been regarded as a major molecular component of allergic asthma. Additionally, there is increasing evidence of the important roles of interleukin-33 (IL-33) in the disease. Here, we show that IL-33 and alveolar macrophages play essential roles in the exacerbation of IgE-mediated airway inflammation and remodelling. BALB/c mice passively sensitized with ovalbumin (OVA)-specific IgE monoclonal antibody (mAb) were challenged with OVA seven times intratracheally. The seventh challenge exacerbated airway inflammation and remodelling compared with the fourth challenge; furthermore, markedly increased expression of IL-33 in the lungs was observed at the fourth and seventh challenges. When anti-IL-33 or anti-ST2 antibody was administered during the fourth to seventh challenge, airway inflammation and remodelling were significantly inhibited at the seventh challenge. Because increases of IL-33(+) and ST2(+) alveolar macrophages and ST2(+)  CD4(+) T cells in the lungs were observed at the fourth challenge, the roles of macrophages and CD4(+) cells were investigated. Depletion of macrophages by 2-chloroadenosine during the fourth to seventh challenge suppressed airway inflammation and remodelling, and IL-33 production in the lung at the seventh challenge; additionally, anti-CD4 mAb inhibited airway inflammation, but not airway remodelling and IL-33 production. Meanwhile, treatment with 2-chloroadenosine or anti-CD4 mAb decreased IL-33-induced airway inflammation in normal mice; airway remodelling was repressed only by 2-chloroadenosine. These results illustrate that macrophage-derived IL-33 contributes to the exacerbation of IgE-mediated airway inflammation by mechanisms associated with macrophages and CD4(+) cells, and airway remodelling through the activation of macrophages.

Basophils as Key Regulators of Allergic Inflammation and Th2-type Immunity

Basophils have long been suspected as playing more than a bystander role in initiating and maintaining allergic disorders, despite their relatively low numbers compared with other effector cells, such as mast cells and eosinophils. In vitro studies clearly demonstrated their propensity to generate proallergic cytokines, such as interleukin 4 and interleukin 13, as well as histamine and leukotrienes after simulation with allergens and innate IgE-dependent triggers. However, only very recently have mouse basophils been identified as key regulators of allergy in vivo, including orchestrating Th2 immunity to protease allergens in the induction phase. This review highlights these exciting advances that go far in unraveling our understanding of basophil function in the orchestration of allergic inflammation.

Complete dependence on CD4+ cells in late asthmatic response, but limited contribution of the cells to airway remodeling in sensitized mice

It is known that the late asthmatic response (LAR), a characteristic feature of asthma, is closely associated with CD4+ Th2 cell-mediated allergic inflammation. Airway remodeling is also a pathogenesis of asthma, but literature reporting roles of CD4+ cells in the remodeling is controversial. There has been no study that simultaneously assessed the roles of CD4+ cells in both LAR and airway remodeling. Sensitized mice were intratracheally challenged with ovalbumin 4 times. Treatment with an anti-CD4 monoclonal antibody (mAb) before the 1st challenge almost completely abolished increase in CD4+ cells in the tissues after the 4th challenge. The late phase increase in airway resistance after the 4th challenge was also completely inhibited by anti-CD4 mAb. Parameters of airway remodeling, subepithelial fibrosis and epithelial thickening were attenuated by treatment, whereas the inhibition was only 30% - 40%. Bronchial smooth muscle thickening was not affected. Because interleukin (IL)-5 production as well as eosinophilia was effectively suppressed by anti-CD4 mAb, the effect of anti-IL-5 mAb was also examined, resulting in no inhibition of airway remodeling. Collectively, although the LAR was completely dependent on CD4+ cell activation, airway remodeling was only partially dependent on the cell.

Important role of neutrophils in the late asthmatic response in mice

AIMS

Neutrophils have been found increasingly in the lungs of patients with severe asthma; however, it is unclear whether the neutrophils contribute to the induction of the airway obstruction. We determined using a murine model whether neutrophils are involved in the late asthmatic response (LAR), and analyzed mechanisms underlying the antigen-induced airway neutrophilia.

MAIN METHODS

BALB/c mice sensitized by ovalbumin (OVA)+Al(OH)(3) were challenged 4 times by intratracheal administration of OVA. Airway mechanics were measured as specific airway resistance.

KEY FINDINGS

Induction of the LAR after the 4th challenge coincided with airway neutrophilia. In contrast, eosinophil infiltration was established prior to the 4th challenge. A treatment with an anti-Gr-1 monoclonal antibody (mAb) before the 4th challenge selectively suppressed increases in the neutrophil number and myeloperoxidase (MPO) level in bronchoalveolar lavage fluid (BALF), and attenuated the magnitude of LAR by 60-70%. Selective suppression of eosinophilia by anti-IL-5 mAb had little effect on the LAR. The increases in neutrophil number and MPO level were partially inhibited by an anti-CD4 mAb treatment. The CD4(+) cell depletion also significantly inhibited increases in neutrophil chemoattractants, IL-17A, keratinocyte-derived chemokine (KC) and macrophage inflammatory protein (MIP)-2 in BALF. However, blockade of FcγRII/III failed to suppress the neutrophilia.

SIGNIFICANCE

These data suggest that neutrophils are key inducers of the LAR, and that the antigen-induced neutrophilia is partially dependent on activated CD4(+) cells that are involved in the production of IL-17A, KC and MIP-2.

CCAAT/enhancer-binding protein alpha (C/EBPalpha) is critical for interleukin-4 expression in response to FcepsilonRI receptor cross-linking

Basophils mediate many of their biological functions by producing IL-4. However, it is unknown how the Il4 gene is regulated in basophils. Here, we report that CCAAT/enhancer-binding protein α (C/EBPα), a major myeloid transcription factor, was highly expressed in basophils. We show that C/EBPα selectively activated Il4 promoter-luciferase reporter gene transcription in response to IgE cross-linking, but C/EBPα did not activate other known Th2 or mast cell enhancers. We found that the PI3K pathway and calcineurin were essential in C/EBPα-driven Il4 promoter-luciferase gene transcription. Our mutation analyses revealed that C/EBPα drove Il4 promoter-luciferase activity depending on its DNA binding domain. Mutation of the C/EBPα-binding site in the Il4 promoter region abolished C/EBPα-driven Il4 promoter-luciferase activity. Our results further showed that a mutation in nuclear factor of activated T cells (NFAT)-binding sites in the Il4 promoter also negated C/EBPα-driven Il4 promoter-luciferase activity. Our study demonstrates that C/EBPα, in cooperation with NFAT, directly regulates Il4 gene transcription.

Basophils are the major producers of IL-4 during primary helminth infection

IL-4 production by leukocytes is a key regulatory event that occurs early in the type 2 immune response, which induces allergic reactions and mediates expulsion of parasites. CD4(+) T cells and basophils are thought to be the key cell types that produce IL-4 during a type 2 response. In this study, we assessed the relative contribution of both CD4(+) T cell- and basophil-IL-4 production during primary and secondary responses to Nippostrongylus brasiliensis using a murine IL-4-enhanced GFP reporter system. During infection, IL-4-producing basophils were detected systemically, and tissue recruitment occurred independent of IL-4/STAT6 signaling. We observed that basophil recruitment to a tissue environment was required for their full activation. Basophil induction in response to secondary infection exhibited accelerated kinetics in comparison with primary infection. However, total basophil numbers were not enhanced, as predicted by previous models of protective immunity. Overall, the induction and migration of IL-4-producing basophils into peripheral tissues was found to be a prominent characteristic of the primary but not memory responses to N. brasiliensis infection, in which CD4(+) T cells were identified as the major source of IL-4. Whereas basophils were the major initial producers of IL-4, we determined that normal Th2 differentiation occurs independently of basophils, and depletion of basophils led to an enhancement of inflammatory cell recruitment to the site of infection.

Polycyclic aromatic hydrocarbons from diesel emissions exert proallergic effects in birch pollen allergic individuals through enhanced mediator release from basophils

BACKGROUND

Diesel exhaust particles (DEPs) act as adjuvants in the immune system and contribute to the increased prevalence and morbidity of asthma and allergic rhinitis. Polycyclic aromatic hydrocarbons (PAHs) are major components of DEPs, which may be involved in the induction and enhancement of proallergic processes. In this study we explored adjuvant effects of DEP-PAHs on activation parameters of human basophils, fostering allergic inflammation through the release of preformed or granule-derived mediators.

METHODS

Heparinized blood samples from birch pollen allergic and control donors were stimulated with Bet v 1, the major allergen of birch pollen grains, alone or together with a mixture of 16 environmental prominent PAHs (EPA-PAH standard). Flow cytometric analysis was performed for quantitative determination of PAH-enhanced basophil activation. To assess direct PAH effects on basophils, enriched cultures from both donor groups were exposed to benzo[a]pyrene (B[a]P) or phenanthrene (Phe), two major DEP-PAHs, with and without allergen. Supernatants were assayed for IL-4 and IL-8 secretion and histamine release by means of ELISA.

RESULTS

At environmental relevant exposure levels EPA-PAH standard synergized with antigen and significantly enhanced basophil activation of all birch pollen allergic individuals up to 95%. Single PAHs significantly drove IL-8 secretion from sensitized basophils of all patients tested, and there was no further enhancement by addition of rBet v 1. B[a]P and Phe also significantly induced IL-4 secretion, a key factor for Th2 development, from purified sensitized basophils in the absence of antigen suggesting an adjuvant role of DEP-PAHs in allergic sensitization. None of the basophil samples from healthy controls showed any PAH effect on mediator release.

CONCLUSION

DEP-PAHs exert proallergic effects on sensitized basophils in an allergen independent fashion, suggesting a potential role of these pollutants for the allergic breakthrough in atopic individuals, who have not developed an allergic disease yet.

The Key Events Dose-Response Framework: a foundation for examining variability in elicitation thresholds for food allergens

Food allergies are caused by immunological reactions in individuals sensitized to normal protein components of foods. For any given sensitized individual, the severity of a reaction is generally assumed to be proportional to the dose of allergenic protein. There is substantial clinical evidence that "threshold" doses exist for the elicitation of an allergic reaction; however, the threshold (i.e., lowest dose that elicits a reaction) varies substantially across the sensitized population. Current approaches to protecting sensitized individuals from exposure to food allergens are highly qualitative (i.e., they rely on food avoidance). The Key Events Dose-Response Framework is an analytical approach for refining understanding of the biological basis of the dose-response. Application of this approach to food allergy provides a foundation for a more rigorous quantitative understanding of variability in allergic response. This study reviews the allergic disease process and the current approaches to identifying thresholds for food allergens. The pathway of key biological events occurring between food intake and allergic response is considered, along with factors that may determine the nature and severity of response to food allergens. Data needs, as well as implications for identifying thresholds, and for characterizing variability in thresholds, are also discussed.

IL-3 induces basophil expansion in vivo by directing granulocyte-monocyte progenitors to differentiate into basophil lineage-restricted progenitors in the bone marrow and by increasing the number of basophil/mast cell progenitors in the spleen

Recent work has established important roles for basophils in regulating immune responses. To exert their biological functions, basophils need to be expanded to critical numbers. However, the mechanisms underlying basophil expansion remain unclear. In this study, we established that IL-3 played an important role in the rapid and specific expansion of basophils. We found that the IL-3 complex (IL-3 plus anti-IL-3 Ab) greatly facilitated the differentiation of GMPs into basophil lineage-restricted progenitors (BaPs) but not into eosinophil lineage-restricted progenitors or mast cells in the bone marrow. We also found that the IL-3 complex treatment resulted in approximately 4-fold increase in the number of basophil/mast cell progenitors (BMCPs) in the spleen. IL-3-driven basophil expansion depended on STAT5 signaling. We showed that GMPs but not common myeloid progenitors expressed low levels of IL-3 receptor. IL-3 receptor expression was dramatically up-regulated in BaPs but not eosinophil lineage-restricted progenitors. Approximately 38% of BMCPs expressed the IL-3R alpha-chain. The up-regulated IL-3 receptor expression was not affected by IL-3 or STAT5. Our findings demonstrate that IL-3 induced specific expansion of basophils by directing GMPs to differentiate into BaPs in the bone marrow and by increasing the number of BMCPs in the spleen.

IL-3 is required for increases in blood basophils in nematode infection in mice and can enhance IgE-dependent IL-4 production by basophils in vitro

Basophils represent potential effector and immunoregulatory cells, as well as a potential source of IL-4, during the immune response elicited by infection with the nematode Nippostrongylus brasiliensis (N.b.), and in other settings. However, the factors which regulate the numbers of blood basophils in mice, or the ability of these cells to produce IL-4, are not fully understood. We found that infection of mice with the nematodes N.b. or Strongyloides venezuelensis (S.v.) induced substantial increases in the numbers of blood basophils (to as high as 18% of circulating blood leukocytes). Experiments in IL-3-/- vs IL-3+/+ mice, and in IL-3-treated IL-3-/- mice, showed that essentially all of the increases in blood or bone marrow basophils during N.b. or S.v. infection were IL-3 dependent. Many of the blood, bone marrow or liver-derived basophils from IL-3-/- or IL-3+/+ mice expressed intracellular IL-4 upon stimulation with anti-IgE in vitro. However, after incubation of the cells with exogenous IgE in vitro, blood- or liver-derived basophils from IL-3+/+ mice exhibited higher levels of intracellular IL-4 after stimulation with anti-IgE than did basophils derived from IL-3-/- mice. Thus, IL-3 is a major regulator of the marked increases in blood basophil levels observed during infection of mice with N.b. or S.v. and also can enhance levels of intracellular IL-4 upon activation of basophils with anti-IgE in vitro.

T(H)2 adjuvants: implications for food allergy

A persistent question for immunologists studying allergic disease has been to define the characteristics of a molecule that make it allergenic. There has been substantial progress elucidating mechanisms of innate priming of T(H)2 immunity in the past several years. These accumulating data demonstrate that T(H)2 immunity is actively induced by an array of molecules, many of which were first discovered in the context of antihelminthic immune responses. Similar intrinsic or associated activities are now known to account for the T(H)2 immunogenicity of some allergens, and may prove to play a role for many more. In this review, we discuss what has been discovered regarding molecules that induce innate immune activation and the pathways that promote T(H)2-polarized immune responses generally, and specifically what role these mechanisms may play in food allergy from models of food allergy and the study of T(H)2 gastrointestinal adjuvants.

A mechanism for the initiation of allergen-induced T helper type 2 responses

Both metazoan parasites and simple protein allergens induce T helper type 2 (TH2) immune responses, but the mechanisms by which the innate immune system senses these stimuli are unknown. In addition, the cellular source of cytokines that control TH2 differentiation in vivo has not been defined. Here we showed that basophils were activated and recruited to the draining lymph nodes specifically in response to TH2-inducing allergen challenge. Furthermore, we demonstrate that the basophil was the accessory cell type required for TH2 induction in response to protease allergens. Finally, we show that basophils were directly activated by protease allergens and produced TH2-inducing cytokines, including interleukin 4 and thymic stromal lymphopoietin, which are involved in TH2 differentiation in vivo.

Selective effect of mercury on Th2-type cytokine production in humans

Mercury induces autoimmune disease and increases IL-4 production and IgE levels in certain rodent strains. The object of this study was to determine if mercury was capable of inducing Th2 cytokine production in human leucocytes. Human peripheral blood mononuclear cells (PBMC) were incubated with PMA/ionomycin or Con A in the presence or absence of methyl mercury (CH(3)Hg) or mercuric chloride (HgCl(2)). IL-4 and gamma-IFN were measured by ELISA.

RESULTS

IL-4 production significantly increased at low concentrations of CH(3)Hg (0.5 uM, p < 0.01), while gamma-IFN production was suppressed starting at CH(3)Hg 2 uM (p = 0.004). Inorganic mercury (HgCl(2)) increased IL-4 only at concentrations 10-20 times higher than CH(3)Hg. These findings suggest a mechanism by which mercury could trigger or potentiate TH2 cytokine production in humans.

Induction of Th2 type immunity in a mouse system reveals a novel immunoregulatory role of basophils

While production of cytokines such as IL-12 by activated dendritic cells supports development of Th1 type immunity, a source of early IL-4 that is responsible for Th2 immunity is not well understood. We now show that coculture of basophils could promote a robust Th2 differentiation upon stimulation of naive CD4 T cells primarily via IL-4. Th2 promotion by basophils was also observed even when naive CD4 T cells were stimulated in a Th1-promoting condition or when fully differentiated Th1 phenotype effector CD4 T cells were restimulated. IL-4–deficient basophils failed to induce Th2 differentiation but suppressed Th1 differentiation. It was subsequently revealed that the IL-4–deficient basophils must engage cell-to-cell contact to exert the inhibitory effect on Th1 differentiation. Stimulation of naive CD4 T cells within an in vivo environment of increased basophil generation supported development of Th2 type immunity. Taken together, our results suggest that basophils may provide an important link for the development of Th2 immunity.

The 21st century renaissance of the basophil? Current insights into its role in allergic responses and innate immunity

Basophils and mast cells express all the three subchains of the high-affinity immunoglobulin E (IgE) receptor Fc epsilon RI and contain preformed histamine in the cytoplasmic granules. However, it is increasingly clear that these cells play distinct roles in allergic inflammatory disease. Despite their presence throughout much of the animal kingdom, the physiological function of basophils remains obscure. As rodent mast cells are more numerous than basophils, and generate an assortment of inflammatory cytokines, basophils have often been regarded as minor players in allergic inflammation. In humans, however, basophils are the prime early producers of interleukin (IL)-4 and IL-13, T helper (Th)2-type cytokines crucial for initiating and maintaining allergic responses. Basophils also express CD40 ligand which, in combination with IL-4 and IL-13, facilitates IgE class switching in B cells. They are the main cellular source for early IL-4 production, which is vital for the development of Th2 responses. The localization of basophils in various tissues affected by allergic inflammation has now been clearly demonstrated by using specific staining techniques and the new research is shedding light on their selective recruitment to the tissues. Finally, recent studies have shown that basophil activation is not restricted to antigen-specific IgE crosslinking, but can be caused in non-sensitized individuals by a growing list of parasitic antigens, lectins and viral superantigens, binding to non-specific IgE antibodies. This, together with novel IgE-independent routes of activation, imparts important new insights into the potential role of basophils in both adaptive and innate immunity.

Itk and Th2 responses: action but no reaction

The Tec family tyrosine kinase, Itk, was initially characterized as a crucial component of T-cell receptor signaling pathways resulting in phospholipase C-gamma1 activation and actin polymerization. In 1999, a seminal report by Fowell, Locksley and colleagues demonstrated that, in CD4+ T cells, Itk-dependent signals are differentially required for T-helper (Th)2 versus Th1 differentiation and effector function. These findings launched a series of in vitro and in vivo studies addressing the molecular defects of Itk-/- CD4+ T cells, and the impaired immune responses of intact Itk-deficient mice. While demonstrating a bias against Th2 differentiation, overall these experiments have indicated that the most significant failing is an inability of Itk-/- CD4+ T cells to produce Th2 cytokines in a recall response, rather than an absolute defect in Th2 differentiation by T cells lacking Itk. In this review, we discuss the pathways by which Itk might impact the differentiation of Th cells.

Basophils: a potential liaison between innate and adaptive immunity

Activation of innate immunity is closely associated to development of protective adaptive immune response. Significant advances have been made to reveal such links between innate immunity and Th1 type adaptive immune responses. By contrast, the role of innate immunity in the development of Th2 type adaptive immune responses is still not well understood. Production of IL-4, a key cytokine in the induction of Th2 immunity, by innate type cells represents an attractive mechanism for such an innate link to Th2 immunity. We have recently reported that in the course of infection with the intestinal nematode, Nippostrongylus brasiliensis, a robust basophil accumulation in the liver/spleen occurs and that these basophils display enhanced IL-4 production. Thus, the basophils is an attractive candidate to mediate the innate-adaptive link for Th2 responses and understanding the control of the tissue homing patterns and cytokine responses of basophils in the course of infections may shed important light on the in vivo induction of Th2 adaptive immunity.

IgE and FcepsilonRI regulation

The high-affinity immunoglobulin (Ig)E receptor, FcepsilonRI, regulates the action of mast cells and basophils and therefore, regulates the expression of atopic disease. There have been several recent observations that demonstrate new behaviors for this receptor. The control of FcepsilonRI expression, control of cell function by FcepsilonRI, and expression of FcepsilonRI on other cell types are important new areas of understanding currently being explored.

Lung inflammatory responses

Inflammation is an important manifestation of respiratory disease in domestic animals. The respiratory system is mucosal in nature and has specific defense mechanisms used to control invasion by microbes and environmental elements. Inflammation can be beneficial or detrimental to the host. This article broadly discusses the primary mediators and mechanisms of inflammation within the respiratory tract of domestic animals. The role of cells, chemokines, cytokines and mediators in both acute and chronic inflammation are addressed. The pathogenesis of the initial insult determines the type of inflammation that will be induced, whether it is acute, chronic or allergic in origin. Maintenance of the microenvironment of cytokines and chemokines is critical for pulmonary homeostasis. Uncontrolled inflammation in the respiratory tract can be life threatening to the animal. The understanding of the mechanisms of inflammation, whether due to microbes or through inappropriate immune activation such as those occurring with allergies, is required to develop successful intervention strategies and control respiratory disease in animals.

Immunostimulatory oligonucleotides block allergic airway inflammation by inhibiting Th2 cell activation and IgE-mediated cytokine induction

A single treatment with a CpG-containing immunostimulatory DNA sequence (ISS) given before allergen challenge can inhibit T helper type 2 cell (Th2)–mediated airway responses in animal models of allergic asthma; however, the mechanism of this inhibition remains largely undefined. Here, we demonstrate that airway delivery of ISS before allergen challenge in Th2-primed mice acts in two distinct ways to prevent the allergic responses to this challenge. The first is to prevent induction of cytokines from allergen-specific Th2 cells, as demonstrated by the nearly complete inhibition of Th2 cytokine production, Th2-dependent functional responses, and gene induction patterns. ISS inhibits the Th2 response by rendering lung antigen-presenting cells (APCs) unable to effectively present antigen to Th2 cells, but not to Th1 cells. This loss of APC function correlates with a reduced expression of costimulatory molecules, including programmed cell death ligand (PD-L)1, PD-L2, CD40, CD80, CD86, and inducible T cell costimulator, and of major histocompatibility complex class II on CD11c⁺APCs from the airways of ISS-treated mice. The second important action of ISS is inhibition of immunoglobulin E–dependent release of Th2 cytokines, especially interleukin 4, from basophils and/or mast cells in the airways of Th2-primed mice. Thus, inhibition by ISS of allergic responses can be explained by two novel mechanisms that culminate in the inhibition of the principal sources of type 2 cytokines in the airways.

IgE and Fc{epsilon}RI regulation

A central feature of allergic reactions is the aggregation of the high-affinity IgE receptor, FcepsilonRI, to initiate a change in the behavior of the cell expressing the receptor. It is now clear that a number of cell types can express this receptor, which broadens the biology that revolves around IgE antibody. It is also quite clear that the presence of monomeric IgE antibody alters the expression of FcepsilonRI. There remains considerable uncertainty about the importance of independent regulation of the FcepsilonRIbeta subunit or its splice variant beta(T), in terms of regulating both expression and function of FcepsilonRI. There is also only primitive understanding of the role of various polymorphisms in the subunit genes on the atopic phenotype. There are, however, many efforts being made to resolve these issues and to discover other factors that regulate expression of this receptor. Of particular interest for understanding the variation in expression in atopy among patients, the role of this receptor on non-mast cell/basophils will be important to elucidate.

Induction of a late asthmatic response associated with airway inflammation in mice

To investigate mechanisms underlying the late asthmatic response, we developed a murine model using repetitive intratracheal antigen challenge. BALB/c mice sensitized by i.p. injection with ovalbumin+alum were challenged with ovalbumin intratracheally 4 times. The 1st challenge induced early airway obstruction peaking at 30 min but without a late response; however, the 4th challenge caused not only early but also late airway obstruction at 2-8 h. Eosinophils, and CD4+ and CD8+ T lymphocytes were increased in the airway before the 4th but not before the 1st-3rd challenges. The numbers of IgE+/CD117+ (mast) cells were also increased in the lung before the 4th challenge. Levels of Th2 cytokines were also increased in the airway. Daily administration of dexamethasone during the challenge period suppressed all these inflammatory events. Thus, this experimental late asthmatic response is associated with Th2 cytokine production from inflammatory cells recruited as a consequence of the 1st-3rd challenges.

Negative control of basophil expansion by IRF-2 critical for the regulation of Th1/Th2 balance

Although basophils are known to produce interleukin 4 (IL-4), the roles of these cells have been documented only in mice infected with parasites or in the effector phase of allergic inflammations. Here we show that naive mice lacking the transcription factor, interferon regulatory factor 2 (IRF-2), exhibited signal transducer and activator of transcription 6 (Stat6)–independent expansion of basophils in the periphery. IRF-2 appeared to act autonomously in the cells to negatively regulate the expansion of, but not cytokine production by, basophils. Spontaneous Th2 polarization of CD4+ T cells was observed in these mice and the genetic reduction of basophil numbers by mutating the Kit gene abolished such a polarization in vivo. We also found that both basophils and IL-4 derived from them were indeed essential for Th2 development under neutral conditions in vitro. Furthermore, neutralization of IL-3 abolished IL-4 production by basophils during Th1/Th2 differentiation cultures and subsequent Th2 development. These results indicated that basophils acted as a cellular converter to turn the neutral IL-3 into the Th2-inducing IL-4 during the initiation of Th1/Th2 differentiation. Thus, the negative regulatory role of IRF-2 on the basophil population size is critically important for preventing excess Th2 polarization and the Th1/Th2 balance in naive animals.

Induction and inhibition of the Th2 phenotype spread: implications for childhood asthma

The interactions between genetic and environmental factors play a major role in the development of childhood asthma. We hypothesized that a pre-existing Th2/asthmatic response can promote Th2 responses to newly encountered Ags (i.e., phenotype spread). To test this hypothesis, we developed a mouse model in which the requirements for the induction and inhibition of phenotype spread to a clinically relevant neo-allergen (i.e., ragweed) were investigated. Our results indicate that 1) phenotype spread to the neo-allergen can be induced only within the first 8 h after a bronchial challenge with the first Ag (OVA); 2) Th2 differentiation of naive CD4(+) T cells occurs in bronchial lymph nodes; 3) trafficking of naive CD4(+) T cells to local lymph nodes and IL-4 produced by OVA-activated Th2 cells play essential roles in the differentiation of naive CD4(+) T cells to Th2 cells; and 4) suppression of the production of chemokines involved in the homing of naive CD4(+) T and Th2 cells to bronchial lymph nodes by a TLR9 agonist inhibited phenotype spread and abrogated the consequent development of experimental asthma. These findings provide a mechanistic insight into Th2 phenotype spread and offer an animal model for testing relevant immunomodulatory interventions.

Pharmacological targeting of anaphylatoxin receptors during the effector phase of allergic asthma suppresses airway hyperresponsiveness and airway inflammation

Airway hyperresponsiveness and airway inflammation are hallmarks of allergic asthma, the etiology of which is crucially linked to the presence of Th2 cytokines. A role for the complement anaphylatoxins C3a and C5a in allergic asthma was suggested, as deficiencies of the C3a receptor (C3aR) and of complement factor C5 modulate airway hyperresponsiveness, airway inflammation, and Th2 cytokine levels. However, such models do not allow differentiation of effects on the sensitization phase and the effector phase of the allergic response, respectively. In this study, we determined the role of the anaphylatoxins on the effector phase of asthma by pharmacological targeting of the anaphylatoxin receptors. C3aR and C5a receptor (C5aR) signaling was blocked using the nonpeptidic C3aR antagonist SB290157 and the neutralizing C5aR mAb 20/70 in a murine model of Aspergillus fumigatus extract induced pulmonary allergy. Airway hyperresponsiveness was substantially improved after C5aR blockade but not after C3aR blockade. Airway inflammation was significantly reduced in mice treated with the C3aR antagonist or the anti-C5aR mAb, as demonstrated by reduced numbers of neutrophils and eosinophils in bronchoalveolar lavage fluid. Of note, C5aR but not C3aR inhibition reduced lymphocyte numbers in bronchoalveolar lavage fluid. Cytokine levels of IL-5 and IL-13 in bronchoalveolar lavage fluid were not altered by C3aR or C5aR blockade. However, blockade of both anaphylatoxin receptors markedly reduced IL-4 levels. These data suggest an important and exclusive role for C5aR signaling on the development of airway hyperresponsiveness during pulmonary allergen challenge, whereas both anaphylatoxins contribute to airway inflammation and IL-4 production.

Basophils produce IL-4 and accumulate in tissues after infection with a Th2-inducing parasite

Using mice in which the eGfp gene replaced the first exon of the Il4 gene (G4 mice), we examined production of interleukin (IL)-4 during infection by the intestinal nematode Nippostrongylus brasiliensis (Nb). Nb infection induced green fluorescent protein (GFP)pos cells that were FcepsilonRIpos, CD49bbright, c-kitneg, and Gr1neg. These cells had lobulated nuclei and granules characteristic of basophils. They were found mainly in the liver and lung, to a lesser degree in the spleen, but not in the lymph nodes. Although some liver basophils from naive mice express GFP, Nb infection enhanced GFP expression and increased the number of tissue basophils. Similar basophil GFP expression was found in infected Stat6-/- mice. Basophils did not increase in number in infected Rag2-/- mice; Rag2-/- mice reconstituted with CD4 T cells allowed significant basophil accumulation, indicating that CD4 T cells can direct both tissue migration of basophils and enhanced IL-4 production. IL-4 production was immunoglobulin independent and only partially dependent on IL-3. Thus, infection with a parasite that induces a "Th2-type response" resulted in accumulation of tissue basophils, and these cells, stimulated by a non-FcR cross-linking mechanism, are a principal source of in vivo IL-4 production.

Type 2 immunity reflects orchestrated recruitment of cells committed to IL-4 production

Using IL-4 reporter mice we identified eosinophils, basophils, and Th2 cells as the three IL-4-producing cell types that appear in the lungs of mice infected with the migrating intestinal helminth, Nippostrongylus brasiliensis. Eosinophils were most prevalent, peaking by approximately 1000-fold on day 9 after infection, with Th2 cells and basophils at 3- and 10-fold lower numbers, respectively. Eosinophil and basophil expansion in blood in response to parasites and their capacity for IL-4 expression required neither Stat6 nor T cells. Th2 induction and expansion in draining lymph nodes was also Stat6 independent. In contrast, eosinophil (and Th2 cell) recruitment to the lung was dependent on Stat6 expression by a bone marrow-derived tissue resident cell, whereas basophil recruitment was Stat6 and IL-4/IL-13 independent but T cell dependent. Primary type 2 immune responses in the lung represent the focal recruitment and activation of discrete cell populations from the blood that have previously committed to express IL-4.

Identification of granulocyte subtype–selective receptors and ion channels by using a high-density oligonucleotide probe array

BACKGROUND

During inflammation, neutrophils, basophils, and eosinophils release cell type-specific mediators and proteases through signaling molecules, such as G protein-coupled receptors and ion channels. As such, ion channels and receptors, including G protein-coupled receptors, are common drug targets.

OBJECTIVE

We sought to identify, for the first time, ion channels and receptors preferentially expressed by each granulocyte subtype.

METHODS

Using GeneChip, we compared approximately 20,000 transcripts present in 7 leukocyte types, platelets, mast cells, and fibroblasts to identify granulocyte subtype-selective transcripts for receptors and ion channels. Granulocyte subtype-selective transcripts were chosen on the basis of several conditions, such as the transcript having a 5-fold or greater expression level compared with the maximum level of other leukocytes.

RESULTS

Fifty-one transcripts were chosen to be preferentially expressed by each granulocyte subtype. Seventeen of the 51 transcripts have not been previously reported as granulocyte subtype selective. Among the 17 receptors and ion channels, 6 were basophil selective, eosinophil selective, or both and were not highly expressed by other organs, indicating that they might be potential targets for antiallergy drugs.

CONCLUSION

Use of this database of potential cell type-selective drug targets should minimize the efforts required for pharmaceutical development.

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.

F(ab)'2-mediated neutralization of C3a and C5a anaphylatoxins: a novel effector function of immunoglobulins

High-dose intravenous immunoglobulin (IVIG) prevents immune damage by scavenging complement fragments C3b and C4b. We tested the hypothesis that exogenous immunoglobulin molecules also bind anaphylatoxins C3a and C5a, thereby neutralizing their pro-inflammatory effects. Single-cell calcium measurements in HMC-1 human mast cells showed that a rise in intracellular calcium caused by C3a and C5a was inhibited in a concentration-dependent manner by IVIG, F(ab)2-IVIG and irrelevant human monoclonal antibody. C3a- and C5a-induced thromboxane (TXB2) generation and histamine release from HMC-1 cells and whole-blood basophils were also suppressed by exogenous immunoglobulins. In a mouse model of asthma, immunoglobulin treatment reduced cellular migration to the lung. Lethal C5a-mediated circulatory collapse in pigs was prevented by pretreatment with F(ab)2-IVIG. Molecular modeling, surface plasmon resonance (SPR) and western blot analyses suggested a physical association between anaphylatoxins and the constant region of F(ab)2. This binding could interfere with the role of C3a and C5a in inflammation.

Advances in mechanisms of allergy

This review summarizes selected "mechanisms of allergy" articles appearing between 2001 and 2002 in the Journal of Allergy and Clinical Immunology (JACI). Papers chosen include those dealing with human eosinophil and basophil biology from life to death, as well as studies with animal models of allergic disease, including knock-out mice, Brown Norway rats, and ovalbumin (OVA)-sensitized and challenged BALB/c mice, that further our understanding of mechanisms of allergic diseases. When appropriate, articles from other journals have been included to supplement the topics being presented.

4. IgE, mast cells, basophils, and eosinophils

IgE, mast cells, basophils, and eosinophils constitute essential elements in allergic inflammation. Allergen-specific IgE, synthesized in response to allergens in the environment and in susceptible individuals, becomes fixed to high-affinity receptors on cellular membranes, especially of mast cells and basophils. If these receptor-bound IgE molecules are aggregated on reexposure to specific allergen, these mast cells and basophils produce mediators that result in the allergic response. Principal among the cells drawn to sites of mediator release is the eosinophil.