Basophils are not the key antigen-presenting cells in allergic patients

Cellular activation pathways and interaction networks in vascularized composite allotransplantation

Vascularized composite allotransplantation (VCA) is an evolving field of reconstructive surgery that has revolutionized the treatment of patients with devastating injuries, including those with limb losses or facial disfigurement. The transplanted units are typically comprised of different tissue types, including skin, mucosa, blood and lymphatic vasculature, muscle, and bone. It is widely accepted that the antigenicity of some VCA components, such as skin, is particularly potent in eliciting a strong recipient rejection response following transplantation. The fine line between tolerance and rejection of the graft is orchestrated by different cell types, including both donor and recipient-derived lymphocytes, macrophages, and other immune and donor-derived tissue cells (e.g., endothelium). Here, we delineate the role of different cell and tissue types during VCA rejection. Rejection of VCA grafts and the necessity of life-long multidrug immunosuppression remains one of the major challenges in this field. This review sheds light on recent developments in decoding the cellular signature of graft rejection in VCA and how these may, ultimately, influence the clinical management of VCA patients by way of novel therapies that target specific cellular processes.

Basophils control T cell priming through soluble mediators rather than antigen presentation

Basophils play an important role in the development of type 2 immunity and have been linked to protective immunity against parasites but also inflammatory responses in allergic diseases. While typically classified as degranulating effector cells, different modes of cellular activation have been identified, which together with the observation that different populations of basophils exist in the context of disease suggest a multifunctional role. In this review we aim to highlight the role of basophils play in antigen presentation of type 2 immunity and focus on the contribution basophils play in the context of antigen presentation and T cell priming. We will discuss evidence suggesting that basophils perform a direct role in antigen presentation and relate it to findings that indicate cellular cooperation with professional antigen-presenting cells, such as dendritic cells. We will also highlight tissue-specific differences in basophil phenotypes that might lead to distinct roles in cellular cooperation and how these distinct interactions might influence immunological and clinical outcomes of disease. This review thus aims to consolidate the seemingly conflicting literature on the involvement of basophils in antigen presentation and tries to find a resolution to the discussion whether basophils influence antigen presentation through direct or indirect mechanisms.

SARS-CoV-2 Induces Cytokine Responses in Human Basophils

Basophils play a key role in the orientation of immune responses. Though the interaction of SARS-CoV-2 with various immune cells has been relatively well studied, the response of basophils to this pandemic virus is not characterized yet. In this study, we report that SARS-CoV-2 induces cytokine responses and in particular IL-13, in both resting and IL-3 primed basophils. The response was prominent under IL-3 primed condition. However, either SARS-CoV-2 or SARS-CoV-2-infected epithelial cells did not alter the expression of surface markers associated with the activation of basophils, such as CD69, CD13 and/or degranulation marker CD107a. We also validate that human basophils are not permissive to SARS-CoV-2 replication. Though increased expression of immune checkpoint molecule PD-L1 has been reported on the basophils from COVID-19 patients, we observed that SARS-CoV-2 does not induce PD-L1 on the basophils. Our data suggest that basophil cytokine responses to SARS-CoV-2 might help in reducing the inflammation and also to promote antibody responses to the virus.

Tracing Human IgE B Cell Antigen Receptor-Bearing Cells With a Monoclonal Anti-Human IgE Antibody That Specifically Recognizes Non-Receptor-Bound IgE

Up to 30% of the population suffers from immunoglobulin E (IgE)-mediated allergies. Despite current stepwise gating approaches, the unambiguous identification of human IgE-producing cells by flow cytometry and immunohistology remains challenging. This is mainly due to the scarcity of these cells and the fact that IgE is not only expressed in a membrane-bound form on the surface of IgE-producing cells in form of the B cell antigen receptor (BCR), but is more frequently found on various cell types bound to the low and high affinity receptors, CD23 and FcϵRI, respectively. Here we sought to develop a sequential gating strategy for unambiguous detection of cells bearing the IgE BCR on their surface. To that aim we first tested the monoclonal anti-IgE antibody omalizumab for its ability to discriminate between IgE BCR and receptor-bound IgE using cells producing IgE or bearing IgE bound to CD23 as well as basophils exhibiting FcϵRI receptor-bound IgE. Using flow cytometry, we demonstrated that omalizumab recognized IgE producing cells with a high sensitivity of up to 1 IgE+ cell in 1000 human peripheral blood mononuclear cells (PBMCs). These results were confirmed by confocal microscopy both in cell suspensions as well as in nasal polyp tissue sections. Finally, we established a consecutive gating strategy allowing the clear identification of class-switched, allergen-specific IgE+ memory B cells and plasmablasts/plasma cells in human PBMCs. Birch pollen specific IgE+ memory B cells represented on average 0.734% of total CD19+ B cells in allergic patients after allergen exposure. Thus, we developed a new protocol for exclusive staining of non-receptor bound allergen-specific IgE+ B cell subsets in human samples.

The α-Gal Syndrome and Potential Mechanisms

The α-Gal syndrome is a complex allergic disease characterized by the development of specific IgE antibodies against the carbohydrate galactose-α-1,3-galactose (α-Gal), an oligosaccharide present in cells and tissues of non-primate mammals. Individuals with IgE antibodies to α-Gal suffer from a delayed form of anaphylaxis following red meat consumption. There are several features that make the α-Gal syndrome such a unique allergic disease and distinguish it from other food allergies: (1) symptoms causing IgE antibodies are directed against a carbohydrate moiety, (2) the unusual delay between the consumption of the food and the onset of the symptoms, and (3) the fact that primary sensitization to α-Gal occurs via tick bites. This review takes a closer look at the immune response against α-Gal, in healthy and in α-Gal allergic individuals. Furthermore, the similarities and differences between immune response against α-Gal and against the other important glycan moieties associated with allergies, namely cross-reactive carbohydrate determinants (CCDs), are discussed. Then different mechanisms are discussed that could contribute to the delayed onset of symptoms after consumption of mammalian meat. Moreover, our current knowledge on the role of tick bites in the sensitization process is summarized. The tick saliva has been shown to contain proteins carrying α-Gal, but also bioactive molecules, such as prostaglandin E2, which is capable of stimulating an increased expression of anti-inflammatory cytokines while promoting a decrease in the production of proinflammatory mediators. Together these components might promote Th2-related immunity and trigger a class switch to IgE antibodies directed against the oligosaccharide α-Gal. The review also points to open research questions that remain to be answered and proposes future research directions, which will help to get a better understanding and lead to a better management of the disease.

Is there a Role of Intravenous Immunoglobulin in Immunologic Recurrent Pregnancy Loss?

Recurrent pregnancy loss (RPL) commonly refers to three or more miscarriages that occur before 20 weeks of pregnancy. The immunological cause of RPL could be either an auto- or alloimmune-related event or both. Because of the discovery of immunological abnormalities in RPL patients in clinical practice, several immunomodulatory therapies were introduced to maintain the immune balance at the maternal-fetal interface. Intravenous immunoglobulin (IVIg) is one of the immunomodulators. In recent years, several studies have analyzed the therapeutic effect of IVIg on RPL patients with antiphospholipid syndrome (APS) or unexplained RPL. However, their results are controversial. IVIg can be used in RPL patients with APS who have previously failed in other treatments. It is recommended that IVIg infusion could be considered used before conception in RPL patients who have cellular immune abnormalities such as increased natural killer (NK) cell counts, NK cell cytotoxicity, or increased T helper (Th)1/Th2 ratio, depending on the cut-off values of each hospital. The aim of this review was to summarize the mechanisms, efficacy, pharmacokinetics, and side effects associated with passive immunization using IVIg in immunologic RPL, according to the literature published in recent years. We hope that more obstetricians will be able to understand the timing and indication of IVIg properly in immunologic RPL patients and effectively enhance pregnancy outcomes for mothers and neonates.

Der p 1-based immunotoxin as potential tool for the treatment of dust mite respiratory allergy

Immunotoxins appear as promising therapeutic molecules, alternative to allergen-specific-immunotherapy. In this work, we achieved the development of a protein chimera able to promote specific cell death on effector cells involved in the allergic reaction. Der p 1 allergen was chosen as cell-targeting domain and the powerful ribotoxin α-sarcin as the toxic moiety. The resultant construction, named proDerp1αS, was produced and purified from the yeast Pichia pastoris. Der p 1-protease activity and α-sarcin ribonucleolytic action were effectively conserved in proDerp1αS. Immunotoxin impact was assayed by using effector cells sensitized with house dust mite-allergic sera. Cell degranulation and death, triggered by proDerp1αS, was exclusively observed on Der p 1 sera sensitized-humRBL-2H3 cells, but not when treated with non-allergic sera. Most notably, equivalent IgE-binding and degranulation were observed with both proDerp1αS construct and native Der p 1 when using purified basophils from sensitized patients. However, proDerp1αS did not cause any cytotoxic effect on these cells, apparently due to its lack of internalization after their surface IgE-binding, showing the complex in vivo panorama governing allergic reactions. In conclusion, herein we present proDerp1αS as a proof of concept for a potential and alternative new designs of therapeutic tools for allergies. Development of new, and more specific, second-generation of immunotoxins following proDerp1αS, is further discussed.

Mechanisms of Particles in Sensitization, Effector Function and Therapy of Allergic Disease

Humans have always been in contact with natural airborne particles from many sources including biologic particulate matter (PM) which can exhibit allergenic properties. With industrialization, anthropogenic and combustion-derived particles have become a major fraction. Currently, an ever-growing number of diverse and innovative materials containing engineered nanoparticles (NPs) are being developed with great expectations in technology and medicine. Nanomaterials have entered everyday products including cosmetics, textiles, electronics, sports equipment, as well as food, and food packaging. As part of natural evolution humans have adapted to the exposure to particulate matter, aiming to protect the individual's integrity and health. At the respiratory barrier, complications can arise, when allergic sensitization and pulmonary diseases occur in response to particle exposure. Particulate matter in the form of plant pollen, dust mites feces, animal dander, but also aerosols arising from industrial processes in occupational settings including diverse mixtures thereof can exert such effects. This review article gives an overview of the allergic immune response and addresses specifically the mechanisms of particulates in the context of allergic sensitization, effector function and therapy. In regard of the first theme (i), an overview on exposure to particulates and the functionalities of the relevant immune cells involved in allergic sensitization as well as their interactions in innate and adaptive responses are described. As relevant for human disease, we aim to outline (ii) the potential effector mechanisms that lead to the aggravation of an ongoing immune deviation (such as asthma, chronic obstructive pulmonary disease, etc.) by inhaled particulates, including NPs. Even though adverse effects can be exerted by (nano)particles, leading to allergic sensitization, and the exacerbation of allergic symptoms, promising potential has been shown for their use in (iii) therapeutic approaches of allergic disease, for example as adjuvants. Hence, allergen-specific immunotherapy (AIT) is introduced and the role of adjuvants such as alum as well as the current understanding of their mechanisms of action is reviewed. Finally, future prospects of nanomedicines in allergy treatment are described, which involve modern platform technologies combining immunomodulatory effects at several (immuno-)functional levels.

Basophils in Tumor Microenvironment and Surroundings

Basophils represent approximately 1% of human peripheral blood leukocytes. Their effector functions were initially appreciated in the 1970s when basophils were shown to express the high-affinity receptor (FcεRI) for IgE and to release proinflammatory mediators (histamine and cysteinyl leukotriene C₄) and immunoregulatory cytokines (i.e., IL-4 and IL-13). Basophils in the mouse were subsequently identified and immunologically characterized. There are many similarities but also several differences between human and mouse basophils. Basophil-deficient mice have enabled to examine the in vivo roles of basophils in several immune disorders and, more recently, in tumor immunity. Activated human basophils release several proangiogenic molecules such as vascular endothelial growth factor-A (VEGF-A), vascular endothelial growth factor-B (VEGF-B), CXCL8, angiopoietin 1 (ANGPT1), and hepatocyte growth factor (HGF). On the other side, basophils can exert anti-tumorigenic effects by releasing granzyme B, TNF-α, and histamine. Circulating basophils have been associated with certain human hematologic (i.e., chronic myeloid leukemia) and solid tumors. Basophils have been found in tumor microenvironment (TME) of human lung adenocarcinoma and pancreatic cancer. Basophils played a role in melanoma rejection in basophil-deficient mouse model. By contrast, basophils appear to play a pro-tumorigenic role in experimental and human pancreatic cancer. In conclusion, the roles of basophils in experimental and human cancers have been little investigated and remain largely unknown. The elucidation of the roles of basophils in tumor immunity will demand studies on increasing complexity beyond those assessing basophil density and their microlocalization in TME. There are several fundamental questions to be addressed in experimental models and clinical studies before we understand whether basophils are an ally, adversary, or even innocent bystanders in cancers.

Regulatory T cells induce activation rather than suppression of human basophils

Basophils are a rare granulocyte population that has been associated with allergic and inflammatory responses. It is essential to understand the regulatory mechanisms by which basophils are kept in check, considering the impact of dysregulated basophil function on immune responses under different pathological conditions. Among immunoregulatory cells, CD4⁺CD25⁺FoxP3⁺ regulatory T cells (T_regs) are the key players that maintain immune tolerance. The mechanisms by which T_regs regulate and suppress diverse immune cell subsets have been studied extensively, but the impact of T_regs on basophil functions is not well understood. We report that human basophils are refractory to T_reg-mediated suppression and found that T_regs stimulate resting basophils to induce the expression of activation markers including CD69, CD203c, and CD13 and the release of basophil cytokines including IL-13, IL-8, and IL-4. Mechanistically, T_regs could induce human basophil activation via IL-3 and STAT5 activation, whereas cellular contact was dispensable. Inhibition of either IL-3-IL-3 receptor interactions or STAT5 phosphorylation abrogated T_reg-mediated activation of basophils. These results provide evidence of direct positive effects that human T_regs have on basophil activation and reveal a previously unrecognized feature of this cell subset well known for immunosuppressive functions.

Human mast cells and basophils-How are they similar how are they different?

Mast cells and basophils are key contributors to allergies and other inflammatory diseases since they are the most prominent source of histamine as well as numerous additional inflammatory mediators which drive inflammatory responses. However, a closer understanding of their precise roles in allergies and other pathological conditions has been marred by the considerable heterogeneity that these cells display, not only between mast cells and basophils themselves but also across different tissue locations and species. While both cell types share the ability to rapidly degranulate and release histamine following high-affinity IgE receptor cross-linking, they differ markedly in their ability to either react to other stimuli, generate inflammatory eicosanoids or release immunomodulating cytokines and chemokines. Furthermore, these cells display considerable pharmacological heterogeneity which has stifled attempts to develop more effective anti-allergic therapies. Mast cell- and basophil-specific transcriptional profiling, at rest and after activation by innate and adaptive stimuli, may help to unravel the degree to which these cells differ and facilitate a clearer understanding of their biological functions and how these could be targeted by new therapies.

Molecular Aspects of Allergens and Allergy

Immunoglobulin E (IgE)-associated allergy is the most common immune disorder. More than 30% of the population suffer from symptoms of allergy which are often severe, disabling, and life threatening such as asthma and anaphylaxis. Population-based birth cohort studies show that up to 60% of the world population exhibit IgE sensitization to allergens, of which most are protein antigens. Thirty years ago the first allergen-encoding cDNAs have been isolated. In the meantime, the structures of most of the allergens relevant for disease in humans have been solved. Here we provide an update regarding what has been learned through the use of defined allergen molecules (i.e., molecular allergology) and about mechanisms of allergic disease in humans. We focus on new insights gained regarding the process of sensitization to allergens, allergen-specific secondary immune responses, and mechanisms underlying allergic inflammation and discuss open questions. We then show how molecular forms of diagnosis and specific immunotherapy are currently revolutionizing diagnosis and treatment of allergic patients and how allergen-specific approaches may be used for the preventive eradication of allergy.

Important and specific role for basophils in acute allergic reactions

IgE‐mediated allergic reactions involve the activation of effector cells, predominantly through the high‐affinity IgE receptor (FcεRI) on mast cells and basophils. Although the mast cell is considered the major effector cell during acute allergic reactions, more recent studies indicate a potentially important and specific role for basophils and their migration which occurs rapidly upon allergen challenge in humans undergoing anaphylaxis. We review the evidence for a role of basophils in contributing to clinical symptoms of anaphylaxis and discuss the possibility that basophil trafficking during anaphylaxis might be a pathogenic (to target organs) or protective (preventing degranulation in circulation) response. Finally, we examine the potential role of basophils in asthma exacerbations. Understanding the factors that regulate basophil trafficking and activation might lead to new diagnostic and therapeutic strategies in anaphylaxis and asthma.

Granulocytes: New Members of the Antigen-Presenting Cell Family

Granulocytes, the most abundant types of leukocytes, are the first line of defense against pathogen invasion. However, the plasticity and diversity of granulocytes have been increasingly revealed, especially with regard to their versatile functions in orchestrating adaptive immune responses. A substantial body of recent evidence demonstrates that granulocytes can acquire the function as antigen-presenting cells under pathological or inflammatory conditions. In addition, they can acquire surface expression of MHC class II and costimulatory molecules as well as T cell stimulatory behavior when cultured with selected cytokines. The classic view of granulocytes as terminally differentiated, short-lived phagocytes is therefore changing to phenotypically and functionally heterogeneous cells that are engaged in cross-talk with other leukocyte populations and provide an additional link between innate and adaptive immunity. In this brief review, we summarize the current knowledge on the antigen-presenting capacity of granulocyte subsets (neutrophils, eosinophils, and basophils). Underlying mechanisms, relevant physiological significance and potential controversies are also discussed.

Transcription factor RBP-J-mediated signalling regulates basophil immunoregulatory function in mouse asthma model

Basophils (BA) play an important role in the promotion of aberrant T helper type 2 (Th2) immune responses in asthma. It is not only the effective cell, but also modulates the initiation of Th2 immune responses. We earlier demonstrated that Notch signalling regulates the biological function of BAin vitro. However, whether this pathway plays the same role in vivo is not clear. The purpose of the present study was to investigate the effect of Notch signalling on BA function in the regulation of allergic airway inflammation in a murine model of asthma. Bone marrow BA were prepared by bone marrow cell culture in the presence of recombinant interleukin-3 (rIL-3; 300 pg/ml) for 7 days, followed by isolation of the CD49b⁺ microbeads. The recombination signal binding protein J (RBP-J^-/- ) BA were co-cultured with T cells, and the supernatant and the T-cell subtypes were examined. The results indicated disruption of the capacity of BA for antigen presentation alongside an up-regulation of the immunoregulatory function. This was possibly due to the low expression of OX40L in the RBP-J^-/- BA. Basophils were adoptively transferred to ovalbumin-sensitized recipient mice, to establish an asthma model. Lung pathology, cytokine profiles of brobchoalveolar fluid, airway hyperactivity and the absolute number of Th1/Th2 cells in lungs were determined. Overall, our results indicate that the RBP-J-mediated Notch signalling is critical for BA-dependent immunoregulation. Deficiency of RBP-J influences the immunoregulatory functions of BA, which include activation of T cells and their differentiation into T helper cell subtypes. The Notch signalling pathway is a potential therapeutic target for BA-based immunotherapy against asthma.

The role of rare innate immune cells in Type 2 immune activation against parasitic helminths

The complexity of helminth macroparasites is reflected in the intricate network of host cell types that participate in the Type 2 immune response needed to battle these organisms. In this context, adaptive T helper 2 cells and the Type 2 cytokines interleukin (IL)-4, IL-5, IL-9 and IL-13 have been the focus of research for years, but recent work has demonstrated that the innate immune system plays an essential role. Some innate immune cells that promote Type 2 immunity are relatively abundant, such as macrophages and eosinophils. However, we now appreciate that more rare cell types including group 2 innate lymphoid cells, basophils, mast cells and dendritic cells make significant contributions to these responses. These cells are found at low frequency but they are specialized to their roles - located at sites such as the skin, lung and gut, where the host combats helminth parasites. These cells respond rapidly and robustly to worm antigens and worm-induced damage to produce essential cytokines, chemokines, eicosanoids and histamine to activate damaged epithelium and to recruit other effectors. Thus, a greater understanding of how these cells operate is essential to understand how the host protects itself during helminth infection.

Basophils, high-affinity IgE receptors, and CCL2 in human anaphylaxis

Background

The role of basophils in anaphylaxis is unclear.

Objective

We sought to investigate whether basophils have an important role in human anaphylaxis.

Methods

In an emergency department study we recruited 31 patients with acute anaphylaxis, predominantly to Hymenoptera venom. We measured expression of basophil activation markers (CD63 and CD203c); the absolute number of circulating basophils; whole-blood FCER1A, carboxypeptidase A3 (CPA3), and L-histidine decarboxylase (HDC) gene expression; and serum markers (CCL2, CCL5, CCL11, IL-3, and thymic stromal lymphopoietin) at 3 time points (ie, during the anaphylactic episode and in convalescent samples 7 and 30 days later). We recruited 134 patients with Hymenoptera allergy and 76 healthy control subjects for comparison. We then investigated whether the changes observed during venom-related anaphylaxis also occur during allergic reactions to food in 22 patients with peanut allergy undergoing double-blind, placebo-controlled food challenge to peanut.

Results

The number of circulating basophils was significantly lower during anaphylaxis (median, 3.5 cells/μL) than 7 and 30 days later (17.5 and 24.7 cells/μL, P < .0001) and compared with those in patients with venom allergy and healthy control subjects (21 and 23.4 cells/μL, P < .0001). FCER1A expression during anaphylaxis was also significantly lower than in convalescent samples (P ≤ .002) and control subjects with venom allergy (P < .0001). CCL2 levels (but not those of other serum markers) were significantly higher during anaphylaxis (median, 658 pg/mL) than in convalescent samples (314 and 311 pg/mL at 7 and 30 days, P < .001). Peanut-induced allergic reactions resulted in a significant decrease in circulating basophil counts compared with those in prechallenge samples (P = .016), a decrease in FCER1A expression (P = .007), and an increase in CCL2 levels (P = .003).

Conclusions

Our findings imply an important and specific role for basophils in the pathophysiology of human anaphylaxis.

The Evolution of Human Basophil Biology from Neglect towards Understanding of Their Immune Functions

Being discovered long ago basophils have been neglected for more than a century. During the past decade evidence emerged that basophils share features of innate and adaptive immunity. Nowadays, basophils are best known for their striking effector role in the allergic reaction. They hence have been used for establishing new diagnostic tests and therapeutic approaches and for characterizing natural and recombinant allergens as well as hypoallergens, which display lower or diminished IgE-binding activity. However, it was a long way from discovery in 1879 until identification of their function in hypersensitivity reactions, including adverse drug reactions. Starting with a historical background, this review highlights the modern view on basophil biology.

Roles of basophils and mast cells in cutaneous inflammation

Mast cells and basophils are associated with T helper 2 (Th2) immune responses. Newly developed mast cell-deficient mice have provided evidence that mast cells initiate contact hypersensitivity via activating dendritic cells. Studies using basophil-deficient mice have also revealed that basophils are responsible for cutaneous Th2 skewing to haptens and peptide antigens but not to protein antigens. Recently, several studies reported the existence of innate lymphoid cells (ILCs), which differ from classic T cells in that they lack the T cell receptor. Mast cells and basophils can interact with ILCs and play some roles in the pathogenesis of Th2 responses. Basophil-derived interleukin (IL)-4 enhances the expression of the chemokine CCL11, as well as IL-5, IL-9, and IL-13 in ILC2s, leading to the accumulation of eosinophils in allergic reactions. IL-33-stimulated mast cells can play a regulatory role in the development of ILC2-mediated non-antigen-specific protease-induced acute inflammation. In this review, we discuss the recent advances in our understanding of mast cells and basophils in immunity and inflammation.

The expression of CD123 can decrease with basophil activation: implications for the gating strategy of the basophil activation test

Background

Basophil activation test (BAT) reproduces IgE-mediated allergic reactions in vitro and has been used as a diagnostic test. Different markers can be used to identify basophils in whole blood and have implications for the outcome of the test. We aimed to assess changes in the expression of CD123 and HLA-DR following basophil activation and to select the best gating strategy for BAT using these markers.

Methods

BAT was performed in whole blood from 116 children. Peanut extract, anti-IgE, anti-FcεRI or formyl-methionyl-leucyl-phenylalanin (fMLP) was used for stimulation. Surface expression of CD123, HLA-DR, CD63 and CD203c was evaluated by flow cytometry.

Results

In some cases, gating on CD123+/HLA-DR− led to the loss-to-analysis of basophils in conditions where basophils were activated. Adding CD203c as an identification marker restored the cell number. Basophils remained HLA-DR-negative with activation. CD123 expression decreased following stimulation with fMLP (n = 116, p < 0.001), anti-IgE (n = 104, p < 0.001) and peanut (n = 42, p < 0.001). The decrease in the mean fluorescence intensity of CD123 correlated with the up-regulation of basophil activation markers, CD63 (r_s = −0.31, p < 0.001) and CD203c (r_s = −0.35, p < 0.001). BAT to peanut gating basophils on CD203c+/CD123+/HLA-DR− reduced the false-negatives (1 vs. 5 %) and showed a higher diagnostic accuracy compared to using CD123+/HLA-DR− (97 vs. 91 %). CD203c+ appeared as an alternative gating strategy allowing two-colour BAT.

Conclusions

Basophils of a subset of patients down-regulate CD123 with activation. The use of CD203c before gating on CD123+/HLA-DR− cells or in isolation ensures the identification of the entire basophil population and accurate assessment of basophil activation, with important diagnostic implications.

Contribution of Basophils to Cutaneous Immune Reactions and Th2-Mediated Allergic Responses

Basophils are potent effector cells of innate immunity and also play a role in T helper 2 (Th2)-mediated allergic responses. But, although their in vitro functions are well studied, their in vivo functions remain largely unknown. However, several mouse models of basophil depletion have recently been developed and used to investigate basophil functions. For example, in a croton oil-induced model of irritant contact dermatitis in conditionally basophil-depleted transgenic mice, we found that basophils rapidly infiltrate inflamed skin and subsequently induce infiltration of eosinophils. We also showed that basophils induce Th2 skewing upon epicutaneous sensitization with various haptens and peptide antigens. Intriguingly, basophils also promoted Th2 polarization upon protein antigen exposure in the presence of dendritic cells (DCs). The dermal DC subset associated with Th2 skewing was recently identified as CD301b⁺ DC. Such studies with basophil-deficient mouse models have significantly improved our understanding of the mechanisms involved in human immune-related diseases. In this review, we will focus on the relative contribution of basophils and DCs to Th2-mediated allergic responses.

Basophils in inflammation

Basophils are functionally closely related to mast cells. Both cell types express the high-affinity IgE receptor (FcεRI) and rapidly release preformed mediator from intracellular stores upon IgE-mediated activation. However, in contrast to mast cells basophils finish their maturation in the bone marrow and have a lifespan of only 2-3 days. Basophil numbers increase in response to IL-3 or TSLP and migrate into tissues to promote type 2 immune responses. Here we review recent advances regarding the pro- and anti-inflammatory functions of basophils in murine models and human allergic inflammation of the skin, lung and intestine.

Basophils are inept at promoting human Th17 responses

Basophils are the rare granulocytes and play an important role in the polarization of Th2 responses and protection against helminth parasites. In addition, basophils contribute to the pathogenesis of several diseases such as asthma, chronic allergy and lupus. Notably, Th17 cells are also implicated in the pathogenesis of these diseases suggesting that basophils support the activation and expansion of this subset of CD4(+) T cells. Therefore, we explored whether basophils promote the expansion of human Th17 cells. We show that basophils lack the capacity to expand Th17 cells and to induce the secretion of Th17 cytokines either directly or indirectly via antigen presenting cells such as monocytes. As human basophils lack HLA-DR and co-stimulatory molecules, their inability to confer T cell receptor- and co-stimulatory molecule-mediated signals to CD4(+) T cells might explain the lack of Th17 responses when memory CD4(+) T cells were co-cultured with basophils.

Atypical MHC class II-expressing antigen-presenting cells: can anything replace a dendritic cell?

Dendritic cells, macrophages and B cells are regarded as the classical antigen-presenting cells of the immune system. However, in recent years, there has been a rapid increase in the number of cell types that are suggested to present antigens on MHC class II molecules to CD4(+) T cells. In this Review, we describe the key characteristics that define an antigen-presenting cell by examining the functions of dendritic cells. We then examine the functions of the haematopoietic cells and non-haematopoietic cells that can express MHC class II molecules and that have been suggested to represent 'atypical' antigen-presenting cells. We consider whether any of these cell populations can prime naive CD4(+) T cells and, if not, question the effects that they do have on the development of immune responses.

Basophils have emerged as a key player in immunity

Basophils had long been neglected in immunological studies, because of their paucity and phenotypic similarity with tissue-resident mast cells. However, recent development of analytical tools has cast new light on this neglected minority, and revealed previously unappreciated roles of basophils, distinct from those of mast cells, in various immune responses. Primary function of basophils appears to be the protection against infections with parasites, including ticks and helminths. This is why basophils are evolutionally conserved well in many animal species, albeit a small number. Nevertheless, basophils sometimes exert host-deleterious functions in immunological disorders such as allergy. Here we summarize recent advance in our understanding of basophil ontogeny and their in vivo roles under physiological and pathological conditions.

Potential non-T cells source of interleukin-4 in food allergy

BACKGROUND

Recently, a study from the Consortium of Food Allergy Research (CoFAR) showed that allergen-induced IL-4 expression in CD25(+) mononuclear cells was increased in allergic patients. However, they did not find the expected increase in GATA-3 expression, suggesting that allergen-induced IL-4 might not be of T-cell origin. We sought to determine whether other cell types were responsible for the increased IL-4 expression in the CD25(+) cell population.

METHODS

Comparing six allergic patients and six healthy controls, we analyzed the CD25(+) isolated population from PBMC for the presence of potential IL-4-expressing non-T cells. We also compared spontaneous expression levels of surface markers (CD203c, CD63, CD25, and HLA-DR) on basophils from whole blood of 42 peanut-allergic patients and from 12 non-atopic controls. Expression of these markers was also evaluated following basophil activation in eight peanut-allergic patients selected from the previous cohort.

RESULTS

In addition to CD4(+) T cells, a substantial proportion of non-T cells were found in the CD25(+) -isolated cell population: basophils, NK, and NK-T cells with a mean percentage ± s.e.m. of 5.24 ± 0.63%, 6.65 ± 1.01%, and 6.01 ± 1.04%, respectively. The majority of these cells exhibited positive intracytoplasmic staining for IL-4. Expression of CD63 and CD25 was significantly higher in allergic patients compared with controls (p < 0.05). Interestingly, we found a significantly higher proportion of activated basophils expressing HLA-DR, compared with non-activated basophils (p < 0.05).

CONCLUSIONS

Our results support the suggested key role of non-T cells secreting IL-4 in food allergy, particularly basophils, which may also play a central role in antigen presentation.

Lymphocyte-based model systems for allergy research: a historic overview

During the last decades, a multitude of studies applying distinct in vitro and in vivo model systems have contributed greatly to our better understanding of the initiation and regulation of inflammatory processes leading to allergic diseases. Over the years, it has become evident that among lymphocytes, not only IgE-producing B cells and allergy-orchestrating CD4(+) helper cells but also cytotoxic CD8(+) T cells, γδ-T cells and innate lymphoid cells, as well as regulatory lymphocytes, might critically shape the immune response towards usually innocuous allergens. In this review, we provide a historic overview of pioneering work leading to the establishment of important lymphocyte-based model systems for allergy research. Moreover, we contrast the original findings with our currently more refined knowledge to appreciate the actual validity of the respective models and to reassess the conclusions obtained from them. Conflicting studies and interpretations are identified and discussed. The tables are intended to provide an easy overview of the field not only for scientists newly entering the field but also for the broader readership interested in updating their knowledge. Along those lines, herein we discuss in vitro and in vivo approaches to the investigation of lymphocyte effector cell activation, polarization and regulation, and describe depletion and adoptive transfer models along with gene knockout and transgenic (tg) methodologies. In addition, novel attempts to establish humanized T cell antigen receptor tg mouse models for allergy research are described and discussed.

The role of eosinophils and basophils in allergic diseases considering genetic findings

PURPOSE OF REVIEW

Basophils and eosinophils represent less than 1 and 5% of white blood cells, respectively. Their role in asthma and allergic inflammation remains incompletely defined. The present review addresses recent advances regarding the role of these two cell populations in allergic inflammation and asthma regarding both biological and genetic point of view.

RECENT FINDINGS

Regarding eosinophils, the role of interleukin(IL)-25, IL-33 and thymic stromal lymphoprotein (TSLP) have been evidenced, and activation states of eosinophil β1 and β2 integrins have been found to correlate with the measurement of eosinophil recruitment and pulmonary function in asthma. New insights into the biology of basophils concern their role as regulators of Th2 cell response through IL-4 expression or the differentiation of monocytes to macrophages, and their population heterogeneity in human. The transcription factor PU.1 was reported to be involved in controlling transcription of specific genes both in eosinophils and basophils. Candidate genetic studies on eosinophils have explored genes involved in the intracellular calcium influx and apoptosis. At the genome-wide level, studies identified genetic variants belonging to IL1RL1, TSLP and IL-33, and four loci with pleiotropic effects on eosinophil and basophil counts [GATA2 (3q21), MHC (6p21), HBS1L-MYB (6q23), and ERG (21q22)].

SUMMARY

Recent findings from biological and genetic studies on eosinophils and basophils highlight the role of epithelial cell-derived cytokines such as TSLP and IL-33 in asthma and allergic diseases.

Identification and quantification of basophils in the airways of asthmatics following segmental allergen challenge

During asthma attacks, allergens activate sensitized basophils in the lung, thereby aggravating symptoms. Due to the paucity of basophils in bronchial lavage fluid and the lack of specific basophil detection and quantification methods, basophil-directed research in these samples was hampered in the past. This study aimed to establish and validate a flow cytometry-based basophil detection and quantification method for human basophils from bronchoalveolar lavage (BAL) and blood as a prerequisite for a better understanding of their pathogenic contribution and subtyping of asthma phenotypes. BAL and blood leukocytes from seasonal asthmatics were analyzed by flow cytometry. Chipcytometry, a highly sensitive single-cell analysis method, was used to validate the staining panel for basophils. Cell differentials of May-Grünwald-Giemsa-stained cytospins were used to compare basophil percentages. BAL basophils are identifiable as CD123(+) HLA-DR(-) CD3(-) CD14(-) CD19(-) CD20(-) CD56(-) cells in flow cytometrical analysis. Their identity was validated by Chipcytometry. CD203c was highly expressed by BAL basophils, whereas it was expressed at variable levels on blood basophils. The two quantification methods correlated, although more basophils were detected by flow cytometry. Furthermore, the increase in basophil percentages in the lung correlated with the decrease in the basophil percentages in the blood after allergen challenge. We here validated a reliable basophil quantification method, which is independent of the cell's activation and degranulation state. The results obtained with this method indicate that basophils are directly recruited from the blood circulation to the airway lumen.

Basophils and allergic inflammation

Basophils were discovered by Paul Ehrlich in 1879 and represent the least abundant granulocyte population in mammals. The relative rarity of basophils and their phenotypic similarities with mast cells resulted in this cell lineage being historically overlooked, both clinically and experimentally. However, recent studies in human subjects and murine systems have shown that basophils perform nonredundant effector functions and significantly contribute to the development and progression of TH2 cytokine-mediated inflammation. Although the potential functions of murine and human basophils have provoked some controversy, recent genetic approaches indicate that basophils can migrate into lymphoid tissues and, in some circumstances, cooperate with other immune cells to promote optimal TH2 cytokine responses in vivo. This article provides a brief historical perspective on basophil-related research and discusses recent studies that have identified previously unappreciated molecules and pathways that regulate basophil development, activation, and function in the context of allergic inflammation. Furthermore, we highlight the unique effector functions of basophils and discuss their contributions to the development and pathogenesis of allergic inflammation in human disease. Finally, we discuss the therapeutic potential of targeting basophils in preventing or alleviating the development and progression of allergic inflammation.

MHC class II expression in human basophils: induction and lack of functional significance

The antigen-presenting abilities of basophils and their role in initiating a Th2 phenotype is a topic of current controversy. We aimed to determine whether human basophils can be induced to express MHC Class II and act as antigen presenting cells for T cell stimulation. Isolated human basophils were exposed to a panel of cytokines and TLR-ligands and assessed for MHC Class II expression. MHC Class II was expressed in up to 17% of isolated basophils following incubation with a combination of IL-3, IFN-γ and GM-CSF for 72 hours. Costimulatory molecules (CD80 and CD86) were expressed at very low levels after stimulation. Gene expression analysis of MHC Class II-positive basophils confirmed up-regulation of HLA-DR, HLA-DM, CD74 and Cathepsin S. However, MHC Class II expressing basophils were incapable of inducing antigen-specific T cell activation or proliferation. This is the first report of significant cytokine-induced MHC Class II up-regulation, at both RNA and protein level, in isolated human basophils. By testing stimulation with relevant T cell epitope peptide as well as whole antigen, the failure of MHC Class II expressing basophils to induce T cell response was shown not to be solely due to inefficient antigen uptake and/or processing.

Mechanisms and potential therapeutic targets in allergic inflammation: recent insights

Deeper insight into pathogenetic pathways and into the biological effects of immunomodulatory agents will help to optimize or adopt therapeutic strategies for atopic disorders. In this article, we highlight selected findings of potential therapeutic relevance that emerged from recent mechanistic studies with focus on molecular and cellular aspects of allergic inflammation. Furthermore, the often complex mechanisms of action of pleiotropic immunomodulatory agents, such as glucocorticoids, vitamin D, or intravenous immunoglobulin (IVIG), are discussed, as their dissection might reveal targets for novel therapeutics or lead to a more rational use of these compounds. Besides reporting novel evidence, this article points to areas of current debate or uncertainty and aims at stimulating scientific discussion and experimental work.

Activation of human basophils by combined toll-like receptor- and FcεRI-triggering can promote Th2 skewing of naive T helper cells

Basophils are mostly known for their involvement in allergic reactions. Recent studies in mice indicate a role for basophils in the induction of adaptive immunity, especially T helper 2 (Th2) responses. Therefore, it would be highly important to understand how basophils respond to pathogen-associated molecules, such as ligands for toll-like receptors (TLRs), and if the basophils could promote Th2 responses via these stimuli. To this end, the activation of basophils via TLRs in combination with activation via IgE was studied, as well as its effect on T helper cell skewing. Using quantitative PCR, we demonstrated the presence of mRNA for TLRs 1-8 in human basophils. Basophils responded to TLR triggering with differential cytokine production, but not with degranulation. Simultaneous triggering of TLRs and IgE led to synergy in production of IL-4, IL-8, IL-13, and RANTES. Furthermore, the synergistic effects on basophils mediated by IgE and TLR-4 triggering allowed robust Th2 skewing upon activation of naïve human CD4⁺ T cells. Our data show that human basophils respond to TLR ligands in synergy with IgE-mediated activation and that the cytokines produced can promote Th2 differentiation. These results indicate a role for basophils in the regulation of T-cell responses in humans.

Protective and pathological roles of mast cells and basophils

Mast cells and basophils are potent effector cells of the innate immune system, and they have both beneficial and detrimental functions for the host. They are mainly implicated in pro-inflammatory responses to allergens but can also contribute to protection against pathogens. Although both cell types were identified more than 130 years ago by Paul Ehrlich, their in vivo functions remain poorly understood. The precursor cell populations that give rise to mast cells and basophils have recently been characterized and isolated. Furthermore, new genetically modified mouse strains have been developed, which enable more specific targeting of mast cells and basophils. Such advances offer new opportunities to uncover the true in vivo activities of these cells and to revisit their previously proposed effector functions.

Circulating human basophils lack the features of professional antigen presenting cells

Recent reports in mice demonstrate that basophils function as antigen presenting cells (APC). They express MHC class II and co-stimulatory molecules CD80 and CD86, capture and present soluble antigens or IgE-antigen complexes and polarize Th2 responses. Therefore, we explored whether human circulating basophils possess the features of professional APC. We found that unlike dendritic cells (DC) and monocytes, steady-state circulating human basophils did not express HLA-DR and co-stimulatory molecules CD80 and CD86. Basophils remained negative for these molecules following stimulation with soluble Asp f 1, one of the allergens of Aspergillus fumigatus; Bet v 1, the major birch allergen; TLR2-ligand or even upon IgE cross-linking. Unlike DC, Asp f 1-pulsed basophils did not promote Th2 responses as analyzed by the secretion of IL-4 in the basophil-CD4⁺ T cell co-culture. Together, these results demonstrate the inability of circulating human basophils to function as professional APC.

Update in clinical allergy and immunology

In the recent years, a tremendous body of studies has addressed a broad variety of distinct topics in clinical allergy and immunology. In this update, we discuss selected recent data that provide clinically and pathogenetically relevant insights or identify potential novel targets and strategies for therapy. The role of the microbiome in shaping allergic immune responses and molecular, as well as cellular mechanisms of disease, is discussed separately and in the context of atopic dermatitis, as an allergic model disease. Besides summarizing novel evidence, this update highlights current areas of uncertainties and debates that, as we hope, shall stimulate scientific discussions and research activities in the field.

Recent developments in basophil research: do basophils initiate and perpetuate type 2 T-helper cell responses?

Basophils account for only 0.1-1% of all peripheral blood leukocytes. They were considered to be a redundant cell type for a long time. However, several findings show a non-redundant role for basophils in type 2 T-helper cell (Th2) immune responses in helminth infections, allergy and autoimmunity. Both immunoglobulin-E-dependent and -independent pathways have been described to contribute to basophil activation. In addition, several recent studies reported that basophils can function as antigen-presenting cells and are important in the initiation of Th2 immune responses. However, there are also conflicting studies that do not corroborate the importance of basophils in Th2 immune responses. This review discusses the role of basophils in Th2 immune responses in view of these recent findings.