Emerging roles of basophils in allergic inflammation

Basophils have long been neglected in immunological studies because they were regarded as only minor relatives of mast cells. However, recent advances in analytical tools for basophils have clarified the non-redundant roles of basophils in allergic inflammation. Basophils play crucial roles in both IgE-dependent and -independent allergic inflammation, through their migration to the site of inflammation and secretion of various mediators, including cytokines, chemokines, and proteases. Basophils are known to produce large amounts of IL-4 in response to various stimuli. Basophil-derived IL-4 has recently been shown to play versatile roles in allergic inflammation by acting on various cell types, including macrophages, innate lymphoid cells, fibroblasts, and endothelial cells. Basophil-derived serine proteases are also crucial for the aggravation of allergic inflammation. Moreover, recent reports suggest the roles of basophils in modulating adaptive immune responses, particularly in the induction of Th2 differentiation and enhancement of humoral memory responses. In this review, we will discuss recent advances in understanding the roles of basophils in allergic inflammation.     

The role of mast cells in allergic inflammation

The histochemical characteristics of human basophils and tissue mast cells were described over a century ago by Paul Ehrlich. When mast cells are activated by an allergen that binds to serum IgE attached to their Fc?RI receptors, they release cytokines, eicosanoids and their secretory granules. Mast cells are now thought to exert critical proinflammatory functions, as well as potential immunoregulatory roles, in various immune disorders through the release of mediators such as histamine, leukotrienes, cytokines chemokines, and neutral proteases (chymase and tryptase). The aim of this review is to describe the role of mast cells in allergic inflammation. Mast cells interact directly with bacteria and appear to play a vital role in host defense against pathogens. Drugs, such as glucocorticoids, cyclosporine and cromolyn have been shown to have inhibitory effects on mast cell degranulation and mediator release. This review shows that mast cells play an active role in such diverse diseases as asthma, rhinitis, middle ear infection, and pulmonary fibrosis. In conclusion, mast cells may not only contribute to the chronic airway inflammatory response, remodeling and symptomatology, but they may also have a central role in the initiation of the allergic immune response, that is providing signals inducing IgE synthesis by B-lymphocytes and inducing Th2 lymphocyte differentiation.     

Leukocyte immunoglobulin-like receptors: novel innate receptors for human basophil activation and inhibition

Basophils, recruited from the blood to tissues, have been implicated by their presence in diverse allergic disorders including bronchial asthma, allergic rhinitis, and cutaneous contact hypersensitivity. We hypothesized that like other leukocytes involved in inflammatory responses, basophils would express members of the leukocyte immunoglobulin-like receptor (LIR) family of immuno-regulatory molecules on their cell surface. We identified LIR7, an activating member coupled to the common Fc receptor gamma chain, and LIR3, an inhibitory member containing cytoplasmic immunoreceptor tyrosine-based inhibitory motifs, on these cells from human peripheral blood. Cross-linking of LIR7 resulted in the concentration-dependent net release of histamine (29.8 +/- 10.8%) and cysteinyl leukotrienes (cysLTs) (31.4 +/- 8.7 ng/10(6) basophils) that were maximal at 30 minutes, and of interleukin-4 (IL-4) (410.2 +/- 61.6 pg/10(6) basophils) that was maximal at 4 hours and comparable with the response initiated by cross-linking of the high-affinity receptor for immunoglobulin E (FcepsilonRI). Coligation of LIR3 to LIR7 or to FcepsilonRI by means of a second monoclonal antibody significantly inhibited net histamine release, cysLT production, and IL-4 generation. That LIR3 is profoundly counter-regulatory for both adaptive and innate receptors suggests a broad role in containment of the inflammatory response.     

Mast cells in allergy and host defense.

Mast cells have been implicated in the pathogenesis of allergic diseases and in inflammatory responses associated with pathological immune and disease-related processes including fibrosis, autoimmune pathology, and neoplasia. Recent findings in animal models of bacterial infection also suggest that mast cells may have a protective role in host defense against pathogens in innate immunity along with the probable role of mast cells in acquired immunity against parasitic infections. Mast cells are strategically located at the host-environment interface and may provide an early defense against an invading pathogen. Mast cells express an array of adhesion and immune receptors that may assist in the recognition of invading pathogens. When activated, these cells then synthesize and release key immunoregulatory cytokines, one consequence of which is to mobilize a rapid and vigorous inflammatory response. However, although it has been demonstrated that mast cells may have a role in innate immunity in defined in vitro and animal models, it remains to be determined whether mast cells are protective in innate immune responses in humans.     

Recent progress in allergen immunotherapy

The efficacy of allergen immunotherapy for the treatment of allergic rhinoconjunctivitis with or without seasonal bronchial asthma and anaphylaxis caused by the sting of the hymenoptera class of insects has been clearly demonstrated in numerous well-designed, placebo-controlled trials. Immunotherapy whether by subcutaneous injection of allergen extract or by oral/sublingual routes modifies peripheral and mucosal TH2 responses in favour of TH1 responses and augments IL-10 synthesis by TRegs both locally and by peripheral T cells. Recent researches into the cellular and molecular basis of allergic reactions have advanced our understanding of the mechanisms involved in allergic diseases. They have also helped the development of innovative approaches that are likely to further improve the control of allergic responses in the future. Novel approaches to immunotherapy that are currently being explored include the use of peptide-based allergen preparations, which do not bind IgE and therefore do not activate mast cells, but reduce both Th1 and Th2-cytokine synthesis, while increasing levels of IL-10. Alternative strategies include the use of adjuvants, such as nucleotide immunostimulatory sequences derived from bacteria CpG or monophosphoryl lipid A that potentiate Th1 responses. Blocking the effects of IgE using anti-IgE such as omalizumab, a recombinant humanized monoclonal antibody that selectively binds to IgE, has been shown to be a useful strategy in the treatment of allergic asthma and rhinitis. The combination of anti-IgE-monoclonal antibody omalizumab with allergen immunotherapy has proved beneficial for the treatment of allergic diseases, offering improved efficacy, limited adverse effects, and potential immune-modifying effects. This combination may also accelerate the rapidity by which immunotherapy induces TReg cells. If allergic diseases are due to a lack of allergen-specific TReg cells, then effective therapies should target the induction and the development of TReg cells producing cytokines such as IL-10.     

Le basophile humain et la réponse immunitaire

Allergic inflammation is the result of complex immune mechanisms that depends on cytokines produced by Th2 lymphocytes, which gives these cells a pivotal role in the allergic reaction. When activated by allergen, basophils produce IL-4 and IL-13, cytokines key in the allergic inflammatory response. Antigen activation induces expression of these two cytokines by basophils of only about 5–20% of allergic patients; basophils represent 80% of the IL-4 producing cells. Ionomycine can induce the production of these cytokines by basophils from most individuals, regardless of their clinical status. IL-4 production by basophils peaks at 2 h, whereas IL-13 expression is delayed, suggesting that basophils may be involved both in initiation and amplification of the allergic response. Furthermore, the rapid increase in CD40L expression on peripheral blood basophils suggests that they may be involved in IgE production. The CC chemokines increase the production of IL-4 by basophils, and stimulation with a 40-fold lower concentration of antigen then results in an equivalent level of IL-4 production. Diesel exhaust particles induce early IL-4 production by the majority of basophils, regardless of the individual’s allergic status; this effect is dependent on the generation of reactive oxygen species. Taken as a whole, these data add weight to the conclusion that basophils are involved in the pathophysiology of allergic diseases. In addition to their critical role in allergic inflammation, basophils may also be involved in anti-infectious immunity through the nonspecific mechanisms involved in innate immunity     

IL-18, although antiallergic when administered with IL-12, stimulates IL-4 and histamine release by basophils

IL-18 is a proinflammatory cytokine that plays an important role in natural killer cell activation and T helper 1 (Th1) cell responses. Mast cells and basophils are major inducers and effectors of allergic inflammation. Here we show that basophils and mast cells derived by culture of bone marrow cells with IL-3 for 10 days express IL-18Ralpha chain and that basophils produce large amounts of IL-4 and IL-13 in response to stimulation with IL-3 and IL-18. Injection of IL-12 and IL-18 inhibits IgE production in helminth-infected wild-type mice and abolishes the capacity of their basophils to produce IL-4 and IL-13 in response to stimulation either with IL-3 and IL-18 or with FcepsilonR cross-linkage. By contrast, this combination of cytokines actually increases IgE levels in helminth-infected IFN-gamma(-/-) mice and enhances IL-4 and IL-13 production by their basophils. Furthermore, injection of IL-18 alone enhances basophil production of IL-4 and histamine both in wild-type and IFN-gamma(-/-) mice. Thus, IL-18 has the potential to stimulate basophils but, when given with IL-12, exhibits an antiallergic action in vivo.     

Allergens in the pathogenesis of asthma: potential role of anti-immunoglobulin E therapy

Evidence suggests that allergy is a significant triggering factor in asthma in children and adults alike. In immunoglobulin (Ig) E-mediated allergic reactions, sensitization occurs when allergen-specific B cells are stimulated and switched to IgE antibody production by interleukin (IL)-4 and IL-13 provided by helper T cells type 2 (Th2). The IgE antibodies act by arming cells bearing either the high-affinity (FcepsilonRI) or low-affinity (FcepsilonRII or CD23) receptor. The subsequent interaction of allergen with IgE-FcepsilonRI complexes on mast cells and basophils causes cross-linking of receptors that triggers the release of a variety of inflammatory mediators, cytokines and chemokines. Therefore, the ability to lower circulating free IgE levels is desirable because most individuals are exposed to multiple allergens to which they are sensitive at any given time. Omalizumab (formerly known as rhuMAb-E25) is a recently developed humanized monoclonal anti-IgE antibody directed at the FcepsilonRI binding domain of human IgE. It inhibits binding of IgE to mast cells without provoking mast cell activation. Preliminary clinical data from randomized controlled trials have shown that the addition of omalizumab to standard asthma therapy reduces asthma exacerbations and decreases inhaled corticosteroid and rescue medication use. The compound is also well tolerated. Omalizumab represents a novel therapeutic approach in the management of asthma.     

2型固有淋巴细胞在过敏性疾病发生中的作用

过敏性疾病尤其是支气管哮喘(简称哮喘)一直被认为是由Th2细胞介导的炎症反应.近期研究发现,2型固有淋巴细胞(type-2 innate lymphoid cell,ILC2)作为一种先天性免疫细胞,同样参与哮喘发生的始动环节.该细胞可产生Th2型细胞因子IL-13、IL-5从而应答受损上皮组织释放的IL-33及IL-25,不仅仅作用于固有免疫的初级阶段,还介导获得性免疫相关功能,这使得先天性及获得性两种免疫系统之间存在了某种特殊的联系,也给研究过敏性哮喘发生机制带来了新的思路.     

IgE-independent interleukin-4 expression and induction of a late phase of leukotriene C4 formation in human blood basophils

T-helper cells can differentiate into at least two subtypes secreting distinct profiles of cytokines, Th1 and Th2, regulating immunoprotection and different immunopathologies. Interleukin-4 (IL-4) is both the product and the inducer of Th2 cells, raising the question whether IL-4 can be produced in response to antigen-independent stimuli. Here we show that human basophils produce IL-4 on stimulation with IL-3 and C5a or C5adesarg in similar amounts as induced by IgE- receptor-cross-linking. C5a-induced IL-4 production requires the presence of IL-3, with little effect of the sequence of stimuli addition. No "Th1-cytokines" (interferon-gamma and IL-2) and even no "Th2-cytokines" (IL-3, IL-5, IL-10, and granulocyte-macrophage colony- stimulating factor) are produced by basophils in response to either IgE- dependent or IgE-independent activation. The generation of leukotriene C4 (LTC4) is regulated in a similar manner. However, C5a induces a rapid, transient burst of leukotriene formation only if added after IL- 3. Interestingly, upon prolonged culture, a late phase of continuous LTC4 production is observed, which also requires two signals (IL-3 and C5a), but rather depends on their continuous presence than on their sequence of action. These data describe an antigen-independent pathway of very restricted IL-4 expression. Thus, basophils must be considered as central immunoregulatory cells of the innate immune system. Furthermore, the results show that LTC4 can also be generated more continuously for many hours, a phenomenon that may be of particular importance in chornic allergic inflammation, such as asthma.     

Mast cell function modulating IgE-mediated allergy

Allergic diseases, such as atopic rhinitis, bronchial asthma and urticaria, are prevalent and increasing in frequency. Mast cells are known to play a central role in the immediate phase reaction of allergic diseases through the IgE-mediated release of a variety of chemical mediators, such as histamine, leukotrienes and prostaglandins. In contrast, T lymphocytes, basophils and eosinophils are thought to be responsible for inducing the late phase response. However, whether the mast cell can be simplistically assigned a role in the immediate phase allergic response and whether mast cells are necessary for the ongoing allergic response, including the development of hyperresponsiveness, remains to be completely studied. In the present article, the author will discuss the integrated roles of mast cells in IgE-mediated allergic inflammation, with specific emphasis on the roles of mast cell-derived cytokines in the late phase allergic response and chronic allergic inflammation.     

Ⅱ型固有淋巴细胞与支气管哮喘的最新研究进展

Ⅱ型固有淋巴细胞(group 2 innate lymphoid cells,ILC2s)是最近才被发现的一类固有淋巴细胞群,并被认为是Th2型细胞因子的主要来源.目前,ILC2s在哮喘中的作用已引起广泛关注.当致敏原刺激气道上皮细胞后,上皮细胞来源的细胞因子IL-33、IL-25、TSLP激活ILC2s,诱导ILC2s产生大量的Th2型细胞因子以及某些生长因子如双调蛋白,这与哮喘的发生以及上皮细胞的修复密切相关.ILC2s不仅能与其他固有免疫细胞相互作用,还可以调控适应性免疫应答,在过敏性炎症反应中起着连接固有免疫与适应性免疫的桥梁作用.本文主要综述ILC2s的调控与功能以及ILC2s在哮喘发病中的最新研究进展.     

Mast cells and basophils

Mast cells and basophils are effector cells in IgE-associated immune responses, such as those that contribute to asthma and other allergic diseases and to host resistance to parasites. Recent work shows that mast cells can also participate in innate immunity to bacterial infection and that the expression of such mast cell-dependent natural immunity can be significantly enhanced by long-term treatment of mice with the kit ligand, stem cell factor. However, mast cells may also influence many other biologic responses, including tissue remodeling and angiogenesis. This review discusses certain recent findings about the differentiation, phenotype, and function of basophils and mast cells, as well as briefly considering evolving concepts about the roles of these cells in health and disease.     

Invariant natural killer T (iNKT) cells in asthma: a novel insight into the pathogenesis of asthma and the therapeutic implication of glycolipid ligands for allergic diseases.

Allergic bronchial asthma is a complex inflammatory diseases originated from dysregulated immune responses in the respiratory mucosa. The inflammatory state in asthmatic lung is characterized by massive infiltration with eosinophils, lymphocytes, and mast cells in the airway mucosa leading to airway hyperseisitivity, goblet cell hyperplasia and mucus overproduction. The inflammatory process is thought to be the result of intensive T helper (Th) 2-biased immune response. Over the past several years, there has been enormous progress in understanding the mechanisms for development of Th2-biased responses after inhaled exposure to allergens and the characteristics of CD4+ T cells prominently involved in this process. Recently, a new population of T cells, invariant natural killer T (iNKT) cells has been shown to play an important role in the pathogenesis of mouse model of allergic airway inflammation. iNKT cells are one of the most potent immune modulators through a massive production of a various cytokines including IL-4 and IFN-gamma upon activation, and are involved in a variety of immunoregulations including infection, autoimmunity, and tumor surveillance. The potent pathogenic role of iNKT cells in the development of bronchial asthma is due to their ability to produce predominant Th2 cytokines in a given condition. The involvement of iNKT cells in the pathogenesis of asthma might have been underestimated in the past studies demonstrating the involvement of CD4+ T cells in asthma because of the difficulty in the detection of iNKT cells. Meanwhile, growing evidences have demonstrated that iNKT cells could be a promising target for immune-based therapies for autoimmune diseases, tumor, and infection due to the invariance of their TCR usage, the restriction to the evolutionally-conserved non-polymorphic antigen-presenting molecule CD1d, and their outstanding ability to produce both Th1- and Th2-cytokines. In this review, we will overview current understanding of the pathophysiological roles of iNKT cells in asthma. We would also discuss on possible therapeutic approaches to bronchial asthma employing glycolipid ligands for iNKT cells.     

Regulation of Th1 and Th2 immune responses by IL-18

IL-18, which requires cleavage with caspase-1 to become active, was originally discovered as a factor that enhances IFN-gamma production from Th1 cells in the presence of anti-CD3 or anti-TcR Ab. However, it was later shown that IL-12 and IL-18 without TcR engagement can induce IFN-gamma in Th1 cells and nonpolarized T cells. Additional TcR engagement has no effect on this IFN-gamma response. Furthermore, a combination of IL-12 and IL-18 acts on B cells, NK cells, macrophages and dendritic cells to produce IFN-gamma. In contrast, IL-18 without help from IL-12 induces Th2 cytokines in T cells and NK cells. Moreover, IL-18 directly stimulates basophils and mast cells to produce Th2 cytokines and histamine independently of IgE. Most surprisingly, IL-18 causes high-level IgE production when administered to normal mice by causing CD4+ T cells to produce IL-4 and to express CD 40 ligand. We established skin-specific caspase-1 transgenic mice with elevated levels of IL-18 in their sera. We found high serum level of IgE, which is entirely dependent on stat 6 in these transgenic mice. These results indicate that caspase-1/IL-18 may be critically involved in regulation of IgE production in vivo, providing a potential therapeutic target for allergic disorders.     

Toll-like receptors in the respiratory system: Their roles in inflammation

Allergic airway inflammation develops in the context of innate immune cells that express Toll-like receptors (TLRs). TLRs recognize microbial components and evoke diverse responses in immune and other respiratory cells through distinct signaling pathways. Bacterial and viral infection in the airway modulates the extent of allergic inflammation. TLR stimulation controls T helper (Th) 1, Th2, and Th17 cell differentiation, cytokine production in mast cells, and activation of eosinophils via direct and indirect pathways. TLR signals in dendritic cells increase expression of major histocompatibility complex proteins and T-cell coreceptors, resulting in greater T-cell activation with Th1 bias. TLR signals in mast cells increase their release of IL-5, and TLR signals in airway epithelial cells enhance airway generation of proallergic cytokines. Although these responses play an important protective role in infection, they may exacerbate allergic inflammation. Under some conditions, TLR stimulation, especially via TLR9, reduces Th2-dependent allergic inflammation through induction of Th1 responses. Therefore, understanding the regulatory role of TLRs in the pathogenesis of allergic airway inflammation may shed light on improving inflammation control in asthmatic patients.     

Interleukin-10-Secreting regulatory T cells in allergy and asthma

Allergic diseases, including asthma, are chronic inflammatory disorders originating from an aberrant immune response to innocuous antigens in our environment (allergens). In susceptible individuals, sensitization to allergen leads to the induction of allergen-specific Thelper type 2 (Th2) responses and immunoglobulin E (IgE) production. Subsequent challenge with allergen results in IgE-mediated mast cell activation and the recruitment and activation of effector cells, leading to clinical symptoms of disease. In this review, we discuss evidence that the anti-inflammatory cytokine interleukin-10 (IL-10) offers therapeutic promise for the control of asthma and allergy. We highlight the potential role of IL-10 secretion by a specialized T-cell subset, T regulatory cells, to prevent allergic inflammation in healthy individuals and to provide long-term relief from disease symptoms in allergic patients.     

Innate-Type and Acquired-Type Allergy Regulated by IL-33

We propose two types of allergic response: IgE-dependent and IgE-independent, and designate these as 'acquired-type allergy' and 'innate-type allergy', respectively. IL-33 stimulates both innate (basophils, mast cells, or group 2 innate lymphoid cells) and acquired (Th2 cells) allergy-related cells to induce and/or augment Th2 cytokine production, which leads to eosinophilic inflammation in vivo. Thus, IL-33 is an essential regulator for both 'innate-type allergy' and 'acquired-type allergy', and might be an attractive therapeutic target for allergic diseases.     

Human basophils and eosinophils are the direct target leukocytes of the novel IL-1 family member IL-33

In mice, interleukin-18 (IL-18) regulates Th1- or Th2-type immune responses depending on the cytokine environment and effector cells involved, and the ST2-ligand, IL-33, primarily promotes an allergic phenotype. Human basophils, major players in allergic inflammation, constitutively express IL-18 receptors, while ST2 surface expression is inducible by IL-3. Unexpectedly, freshly isolated basophils are strongly activated by IL-33, but, in contrast to mouse basophils, do not respond to IL-18. IL-33 promotes IL-4, IL-13 and IL-8 secretion in synergy with IL-3 and/or FcepsilonRI-activation, and enhances FcepsilonRI-induced mediator release. These effects are similar to that of IL-3, but the signaling pathways engaged are distinct because IL-33 strongly activates NF-kappaB and shows a preference for p38 MAP-kinase, while IL-3 acts through Jak/Stat and preferentially activates ERK. Eosinophils are the only other leukocyte-type directly activated by IL-33, as evidenced by screening of p38-activation in peripheral blood cells. Only upon CD3/CD28-ligation, IL-33 weakly enhances Th2 cytokine expression by in vivo polarized Th2 cells. This study on primary human cells demonstrates that basophils and eosinophils are the only direct target leukocytes for IL-33, suggesting that IL-33 promotes allergic inflammation and Th2 polarization mainly by the selective activation of these specialized cells of the innate immune system.     

Novel Biologicals for the Treatment of Allergic Diseases and Asthma

Purpose of Review

The development of biological therapies has rapidly progressed during the last few years, and major advances were reported for the treatment of allergic diseases, such as atopic dermatitis, allergic rhinitis, urticaria, food allergy, and asthma. Here, we review biologicals targeting the type 2 immune response involving Th2 cells, type 2 innate lymphoid cells, natural killer T cells, mast cells, basophils, and epithelial cells, such as IL-4, IL-5, IL-13, IL-31, tumor necrosis factor alpha (TNF-α), and thymic stromal lymphopoietin (TSLP).

Recent Findings

The biologicals that have been currently approved for asthma are omalizumab targeting IgE and reslizumab and mepolizumab targeting interleukin (IL)-5. Many other monoclonal antibodies are currently in various phases of clinical development. The new biological therapies for allergic diseases will eventually be tailored to the endotypes of these diseases and the identification of novel biomarkers.

Summary

Further development of novel biologicals for the treatment of allergic diseases and asthma will be possible upon improved understanding of mechanisms of allergic diseases. Accordingly, further refinement of endotypes of allergen-specific and non-specific type 2 immune response and related inflammatory mediators is needed for optimal treatment of allergic diseases.