Basophils: what they 'can do' versus what they 'actually do'

Intravital microscopy visualizes innate immune crosstalk and function in tissue microenvironment

Significant advances have been made in the field of intravital microscopy (IVM) on myeloid cells due to the growing number of validated fluorescent probes and reporter mice. IVM provides a visualization platform to directly observe cell behavior and deepen our understanding of cellular dynamics, heterogeneity, plasticity, and cell-cell communication in native tissue environments. This review outlines the current studies on the dynamic interaction and function of innate immune cells with a focus on those that are studied with IVM and covers the advances in data analysis with emerging artificial intelligence-based algorithms. Finally, the prospects of IVM on innate immune cells are discussed.

Basophils from allergy to cancer

Human basophils, first identified over 140 years ago, account for just 0.5-1% of circulating leukocytes. While this scarcity long hampered basophil studies, innovations during the past 30 years, beginning with their isolation and more recently in the development of mouse models, have markedly advanced our understanding of these cells. Although dissimilarities between human and mouse basophils persist, the overall findings highlight the growing importance of these cells in health and disease. Indeed, studies continue to support basophils as key participants in IgE-mediated reactions, where they infiltrate inflammatory lesions, release pro-inflammatory mediators (histamine, leukotriene C₄: LTC₄) and regulatory cytokines (IL-4, IL-13) central to the pathogenesis of allergic diseases. Studies now report basophils infiltrating various human cancers where they play diverse roles, either promoting or hampering tumorigenesis. Likewise, this activity bears remarkable similarity to the mounting evidence that basophils facilitate wound healing. In fact, both activities appear linked to the capacity of basophils to secrete IL-4/IL-13, with these cytokines polarizing macrophages toward the M2 phenotype. Basophils also secrete several angiogenic factors (vascular endothelial growth factor: VEGF-A, amphiregulin) consistent with these activities. In this review, we feature these newfound properties with the goal of unraveling the increasing importance of basophils in these diverse pathobiological processes.

Understanding the bidirectional interactions between two-dimensional materials, microorganisms, and the immune system

Two-dimensional (2D) materials such as the graphene-based materials, transition metal dichalcogenides, transition metal carbides and nitrides (MXenes), black phosphorus, hexagonal boron nitride, and others have attracted considerable attention due to their unique physicochemical properties. This is true not least in the field of medicine. Understanding the interactions between 2D materials and the immune system is therefore of paramount importance. Furthermore, emerging evidence suggests that 2D materials may interact with microorganisms - pathogens as well as commensal bacteria that dwell in and on our body. We discuss the interplay between 2D materials, the immune system, and the microbial world in order to bring a systems perspective to bear on the biological interactions of 2D materials. The use of 2D materials as vectors for drug delivery and as immune adjuvants in tumor vaccines, and 2D materials to counteract inflammation and promote tissue regeneration, are explored. The bio-corona formation on and biodegradation of 2D materials, and the reciprocal interactions between 2D materials and microorganisms, are also highlighted. Finally, we consider the future challenges pertaining to the biomedical applications of various classes of 2D materials.

Prospective selected biomarkers in COVID-19 diagnosis and treatment

COVID-19 has become a global health concern, due to the high transmissible nature of its causal agent and lack of proper treatment. Early diagnosis and nonspecific medical supports of the patients appeared to be effective strategy so far to combat the pandemic caused by COVID-19 outbreak. Biomarkers can play pivotal roles in timely and proper diagnosis of COVID-19 patients, as well as for distinguishing them from other pulmonary infections. Besides, biomarkers can help in reducing the rate of mortality and evaluating viral pathogenesis with disease prognosis. This article intends to provide a broader overview of the roles and uses of different biomarkers in the early diagnosis of COVID-19, as well as in the classification of COVID-19 patients into multiple risk groups.

The Role of Myeloid Cells in Acute Kidney Injury and Kidney Repair

AKI remains highly prevalent, yet no optimal therapy is available to prevent it or promote recovery after initial insult. Experimental studies have demonstrated that both innate and adaptive immune responses play a central role during AKI. In response to injury, myeloid cells are first recruited and activated on the basis of specific signals from the damaged microenvironment. The subsequent recruitment and activation state of the immune cells depends on the stage of injury and recovery, reflecting a dynamic and diverse spectrum of immunophenotypes. In this review, we highlight our current understanding of the mechanisms by which myeloid cells contribute to injury, repair, and fibrosis after AKI.

Difference in Biomarkers Between COVID-19 Patients and Other Pulmonary Infection Patients

Background

The pandemic due to the novel coronavirus disease 2019 (COVID-19) has resulted in an increasing number of patients need to be tested. We aimed to determine if the use of integrated laboratory data can discriminate COVID-19 patients from other pulmonary infection patients.

Methods

This retrospective cohort study was conducted at Kunming Third People’s Hospital in China from January 20 to February 28, 2020. Medical records and laboratory data were extracted and combined for COVID-19 and other pulmonary infection patients on admission. A partial least square discriminant analysis (PLS-DA) model was constructed and calibrated to discriminate COVID-19 from other pulmonary infection patients.

Results

COVID-19 patients diagnosed and treated in Kunming were balanced in terms of sex and covered all age groups. Most of them were mild cases; only five were severe cases. The first two dimensions of the PLS-DA model could classify COVID-19 and other pulmonary infection patients with an accuracy of 96.6% (95.1% in the cross-validation model). Basophil count, the proportion of basophils, prothrombin time, prothrombin time activity, and international normalized ratio were the five most discriminant biomarkers.

Conclusion

Integration of biomarkers can discriminate COVID-19 patients from other pulmonary infections on admission to hospital and thus may be a supplement to nucleic acid tests.

The Transcription Factor PLZF Is Necessary for the Development and Function of Mouse Basophils

Basophils are innate immune cells associated with type 2 immunity, allergic reactions, and host defense against parasite infections. In this study, we show that the transcription factor PLZF, which is known for its essential role in the function and development of several innate lymphocyte subsets, is also important for the myeloid-derived basophil lineage. PLZF-deficient mice had decreased numbers of basophil progenitors in the bone marrow and mature basophils in multiple peripheral tissues. Functionally, PLZF-deficient basophils were less responsive to IgE activation and produced reduced amounts of IL-4. The altered function of basophils resulted in a blunted Th2 T cell response to a protein allergen. Additionally, PLZF-deficient basophils had reduced expression of the IL-18 receptor, which impacted migration to lungs. PLZF, therefore, is a major player in controlling type 2 immune responses mediated not only by innate lymphocytes but also by myeloid-derived cells.

Basophils from allergic rhinitis patients show allergen-specific upregulation of thymic stromal lymphopoietin receptor

BACKGROUND

An allergic rhinitis (AR) diagnosis is based on typical nasal symptoms and allergen sensitization testing. However, it is sometimes difficult to distinguish AR from non-AR, and it is especially difficult to identify the causal allergen for immunotherapy of patients with AR.

OBJECTIVE

To identify differences among patients with AR, subjects with asymptomatic sensitization (AS), and subjects without sensitization (NS) that could facilitate the diagnosis of AR.

METHODS

We used RNA sequencing to examine differential gene expression in unstimulated and allergen-stimulated peripheral basophils from participants with NS, AS, and AR. We selected genes that were upregulated after allergen stimulation and showed differences in expression in patients with AR compared with subjects with AS and NS. Basophil surface expression of 1 gene product was examined by flow cytometry. The usefulness of gene expression in the diagnosis of AR was examined with receiver operating characteristic curves.

RESULTS

Expression of cytokine receptor-like factor 2 and its product, thymic stromal lymphopoietin receptor (TSLPR), was significantly increased in basophils of patients with AR after allergen stimulation. A significantly larger percentage of TSLPR-positive cells was observed after allergen-specific stimulation of basophils from patients with AR compared with subjects with AS. Basophil TSLPR expression was as good as or better than CD203c expression in discriminating between patients with AR and subjects with AS, as judged by receiver operating characteristic curves.

CONCLUSION

Our data suggest that TSLPR expression on basophils was specifically upregulated by allergen stimulation and might be useful for the identification of the causative allergen in patients with AR.

The Role of Norepinephrine and α-Adrenergic Receptors in Acute Stress-Induced Changes in Granulocytes and Monocytes

OBJECTIVE

Acute stress induces redistribution of circulating leucocytes in humans. Although effects on lymphocytes as adaptive immune cells are well understood, the mechanisms underlying stress effects on granulocytes and monocytes as innate immune blood cells are still elusive. We investigated whether the stress hormone norepinephrine (NE) and α-adrenergic receptors (α-ADRs) may play a mediating role.

METHODS

In a stress study, we cross-sectionally tested 44 healthy men for associations between stress-induced NE increases and simultaneous granulocyte and monocyte cell count increases, as measured immediately before and several times after the Trier Social Stress Test. In a subsequent infusion study, 21 healthy men participated in three different experimental trials with sequential infusions of 1- and 15-minute duration with varying substances (saline as placebo, the nonspecific α-ADR blocker phentolamine [2.5 mg/min], and NE [5 μg/min]): trial 1 = saline+saline, trial 2 = saline+NE, trial 3 = phentolamine+NE. Granulocyte and monocyte cell numbers were assessed before, immediately after, 10 minutes, and 30 minutes after infusion procedures.

RESULTS

In the stress study, higher NE related to higher neutrophil stress changes (β = .31, p = .045, R change = .09), but not epinephrine stress changes. In the infusion study, saline+NE induced significant increases in neutrophil (F(3/60) = 43.50, p < .001, η = .69) and monocyte (F(3/60) = 18.56, p < .001, η = .48) numbers compared with saline+saline. With phentolamine+NE, neutrophil (F(3/60) = 14.41, p < .001, η = .42) and monocyte counts (F(2.23/44.6) = 4.32, p = .016, η = .18) remained increased compared with saline+saline but were lower compared with saline+NE (neutrophils: F(3/60) = 19.55, p < .001, η = .494, monocytes: F(3/60) = 2.54, p = .065, η = .11) indicating partial mediation by α-ADRs. Trials did not differ in eosinophil and basophil count reactivity.

CONCLUSIONS

Our findings suggest that NE-induced immediate increases in neutrophil and monocyte numbers resemble psychosocial stress effects and can be reduced by blockade of α-ADRs.

Gastric lesions in dolphins stranded along the Eastern Adriatic coast

Stranded cetaceans are often found with gastric lesions associated with the presence of parasites; most frequently, nematodes of the genus Anisakis and the heterophyd digenean trematode Pholeter gastrophilus. In this study, we present histopathology mainly (but not exclusively) related to these 2 parasite species. Macroscopically, lesions associated with the presence of Anisakis spp. were characterised by the presence of ulcers within the gastric mucosa, while the digenean P. gastrophilus was found within large submucosal fibrotic nodules in the gastric wall. Anisakis-induced alterations included severe ulcerative gastritis with mixed inflammatory infiltrate often associated with colonies of bacteria, and mild to moderate granulomatous gastritis with eosinophilic infiltrate. P. gastrophilus-associated lesions were characterised by fibrogranulomatous gastritis with mixed inflammatory infiltrate. Additionally, immunohistochemical (IHC) analysis of P. gastrophilus lesions was consistent with the histopathologic findings, revealing inflammation-mediated stimulation. IHC-positive localisation of CD3+, iNOS+ and caspase-3+ cells suggests intensive accumulation of cytotoxic T-cells, proinflammatory cytokines and execution-phase of cell apoptosis at the parasitized area. In contrast, mechanical damage, rather than visible inflammatory response could be observed at the site of attachment of Braunina cordiformis recorded in 4 animals. Lesions not associated with the presence of parasites were mostly characterised by focal loss of superficial epithelial cells and accumulation of brown hemosiderin-like pigment or fibrous gastritis with lymphoplasmacytic infiltrate. In light of these results, we argue that observed 'tolerant' host-parasite interactions that led toward gastric lesions do not represent the cause of death and stranding of cetaceans included in this study.

Basophil-derived IL-6 regulates TH17 cell differentiation and CD4 T cell immunity

Basophils are rare, circulating granulocytes proposed to be involved in T helper (T_H) type 2 immunity, mainly through secretion of interleukin (IL)-4. In addition to IL-4, basophils produce IL-6 and tumor necrosis factor (TNF)-α in response to immunoglobulin E (IgE) crosslinking. Differentiation of T_H17 cells requires IL-6 and transforming growth factor (TGF)-β, but whether basophils play a significant role in T_H17 induction is unknown. Here we show a role for basophils in T_H17 cell development by using in vitro T cell differentiation and in vivo T_H17-mediated inflammation models. Bone marrow derived-basophils (BMBs) and splenic basophils produce significant amounts of IL-6 as well as IL-4 following stimulation with IgE crosslink or cholera toxin (CT). In addition, through IL-6 secretion, BMBs cooperate with dendritic cells to promote T_H17 cell differentiation. In the T_H17 lung inflammation model, basophils are recruited to the inflamed lungs following CT challenge, and T_H17 responses are significantly reduced in the absence of basophils or IL-6. Furthermore, reconstitution with wild-type, but not IL-6-deficient, basophils restored CT-mediated lung inflammation. Lastly, basophil-deficient mice showed reduced phenotypes of T_H17-dependent experimental autoimmune encephalomyelitis. Therefore, our results indicate that basophils are an important inducer of T_H17 cell differentiation, which is dependent on IL-6 secretion.

Comprehensive population-based genome sequencing provides insight into hematopoietic regulatory mechanisms

Genetic variants affecting hematopoiesis can influence commonly measured blood cell traits. To identify factors that affect hematopoiesis, we performed association studies for blood cell traits in the population-based Estonian Biobank using high-coverage whole-genome sequencing (WGS) in 2,284 samples and SNP genotyping in an additional 14,904 samples. Using up to 7,134 samples with available phenotype data, our analyses identified 17 associations across 14 blood cell traits. Integration of WGS-based fine-mapping and complementary epigenomic datasets provided evidence for causal mechanisms at several loci, including at a previously undiscovered basophil count-associated locus near the master hematopoietic transcription factor CEBPA The fine-mapped variant at this basophil count association near CEBPA overlapped an enhancer active in common myeloid progenitors and influenced its activity. In situ perturbation of this enhancer by CRISPR/Cas9 mutagenesis in hematopoietic stem and progenitor cells demonstrated that it is necessary for and specifically regulates CEBPA expression during basophil differentiation. We additionally identified basophil count-associated variation at another more pleiotropic myeloid enhancer near GATA2, highlighting regulatory mechanisms for ordered expression of master hematopoietic regulators during lineage specification. Our study illustrates how population-based genetic studies can provide key insights into poorly understood cell differentiation processes of considerable physiologic relevance.

Cytokine signature and antibody-mediated response against fresh and attenuated Anisakis simplex (L3) administration into Wistar rats: implication for anti-allergic reaction

The third larval stage (L3) of Anisakis simplex (Anisakidae) is one of the zoonotic parasitic nematodes in the musculature and visceral organs of marine fishes belonging to family Moronidae. The consumption of these high-commercial-value fish is widespread in many countries around the Mediterranean Sea including Egypt. The presence of these larvae in fish muscles poses a potential consumer hazard due to the parasite's ability to cause anisakidosis. Forty-two out of 60 (70%) of the European seabass Dicentrarchus labrax were found to be naturally infected by L3 of A. simplex in the form of encapsulated juveniles in the fish musculature. Morphological examination of recovered parasites by light and scanning electron microscopy showed that, in general, all specimens examined closely resembled A. simplex (L3). To evaluate the allergenicity of this nematode, white blood cell count; levels of T helper 1 (Th1) [interferon (IFN)-γ and tumor necrosis factor (TNF)-α)], Th2 [IL-4, IL-5, and IL-6], and Th17 [IL-17] related cytokines; total IgE and IgG antibodies; and nitric oxide (NO) were measured in the plasma of Wistar rats sensitized by oral inoculation with fresh, frozen, and heat-treated A. simplex L3 or rats intraperitoneally injected with L3 crude extract. Rats sensitized with fresh and frozen L3 larvae produced significantly higher levels of IFN-γ, IL-5, IL-17, and total IgE as compared to control rats. Heat-treated larvae administration resulted in a significant rise of IFN-γ, TNF-α, IL-5, and total IgE in comparison to control rats. Intraperitoneal sensitizations enhanced release of IFN-γ, TNF-α, and total IgE. Oral sensitization led to a significant production of NO. Thereby, frozen or cooked larval L3 cannot inhibit the release of Th-related cytokines and IgE, which might impact on the overall anti-parasitic immunity.

Skin thymic stromal lymphopoietin initiates Th2 responses through an orchestrated immune cascade

Thymic stromal lymphopoietin (TSLP) has emerged as a key initiator in Th2 immune responses, but the TSLP-driven immune cascade leading to Th2 initiation remains to be delineated. Here, by dissecting the cellular network triggered by mouse skin TSLP in vivo, we uncover that TSLP-promoted IL-4 induction in CD4(+) T cells in skin-draining lymph nodes is driven by an orchestrated 'DC-T-Baso-T' cascade, which represents a sequential cooperation of dendritic cells (DCs), CD4(+) T cells and basophils. Moreover, we reveal that TSLP-activated DCs prime naive CD4(+) T cells to produce IL-3 via OX40L signalling and demonstrate that the OX40L-IL-3 axis has a critical role in mediating basophil recruitment, CD4(+) T-cell expansion and Th2 priming. These findings thus add novel insights into the cellular network and signal axis underlying the initiation of Th2 immune responses.

Emerging role of human basophil biology in health and disease

Basophils have emerged in recent years as a small but potent subpopulation of leukocytes capable of bridging innate and adaptive immunity. They can be activated through IgE-dependent and IgE-independent mechanisms to release preformed mediators and to produce Th2 cytokines. In addition to their role in protective immunity to helminths, basophils are major participants in allergic reactions as diverse as anaphylaxis and immediate hypersensitivity reactions, late-phase hypersensitivity reactions, and delayed hypersensitivity reactions. Additionally, basophils have been implicated in the pathophysiology of autoimmune diseases such as lupus nephritis and rheumatoid arthritis, and the modulation of immune responses to bacterial infections, as well as being a feature of myelogenous leukemias. Distinct signals for activation, degranulation, transendothelial migration, and immune regulation are being defined, and demonstrate the important role of basophils in promoting a Th2 microenvironment. These mechanistic insights are driving innovative approaches for diagnostic testing and therapeutic targeting of basophils.

IL-4 Derived from Non-T Cells Induces Basophil- and IL-3-independent Th2 Immune Responses

How Th2 immunity develops in vivo remains obscure. Basophils have been considered key innate cells producing IL-4, a cytokine essential for Th2 immunity. Increasing evidence suggests that basophils are dispensable for the initiation of Th2 immunity. In this study, we revisited the role of basophils in Th2 immune responses induced by various types of adjuvants. Mice deficient in IL-3 or IL-3 receptor, in which basophil lymph node recruitment is completely abolished, fully developed wild type level Th2 CD4 T cell responses in response to parasite antigen or papain immunization. Similar finding was also observed in mice where basophils are inducibly ablated. Interestingly, IL-4-derived from non-T cells appeared to be critical for the generation of IL-4-producing CD4 T cells. Other Th2 promoting factors including IL-25 and thymic stromal lymphopoietin (TSLP) were dispensable. Therefore, our results suggest that IL-3- and basophil-independent in vivo Th2 immunity develops with the help of non-T cell-derived IL-4, offering an additional mechanism by which Th2 type immune responses arise in vivo.

GATA-1 regulates the generation and function of basophils

Developmental processes of hematopoietic cells are orchestrated by transcriptional networks. GATA-1, the founding member of the GATA family of transcription factors, has been demonstrated to play crucial roles in the differentiation of erythroid cells, magakaryocytes, eosinophils, and mast cells. However, the role of GATA-1 in basophils remains elusive. Here we show that basophils abundantly express Gata1 mRNAs, and that siRNA-mediated knockdown of Gata1 resulted in impaired production of IL-4 by basophils in response to the stimulation with IgE plus antigens. ΔdblGATA mice that carry the mutated Gata1 promoter and are widely used for functional analysis of eosinophils owing to their selective loss of eosinophils showed a decreased number of basophils with reduced expression of Gata1 mRNAs. The number of basophil progenitors in bone marrow was reduced in these mice, and the generation of basophils from their bone marrow cells in culture with IL-3 or thymic stromal lymphopoietin was impaired. ΔdblGATA basophils responded poorly ex vivo to stimulation with IgE plus antigens compared with wild-type basophils as assessed by degranulation and production of IL-4 and IL-6. Moreover, ΔdblGATA mice showed impaired responses in basophil-mediated protective immunity against intestinal helminth infection. Thus, ΔdblGATA mice showed numerical and functional aberrancy in basophils in addition to the known deficiency of eosinophils. Our findings demonstrate that GATA-1 plays a key role in the generation and function of basophils and underscore the need for careful distinction of the cell lineage responsible for each phenotype observed in ΔdblGATA mice.

Impact of allergic rhinitis and specific subcutaneous immunotherapy on peripheral blood basophils of patients sensitized to Dermatophagoides pteronyssinus

Background

Basophils are important effectors cells in allergic rhinitis (AR) since they are involved in immunoglobulin (Ig) E – mediated inflammation and in the release of pro-inflammatory mediators. Specific subcutaneous immunotherapy (SCIT) provides clear immunologic modulation in some immune cells, however its systemic effects on basophils are not well known.

Methods

Peripheral blood (PB) samples from 43 patients with allergic rhinitis mono-sensitized to Dermatophagoides pteronyssinus (Dpt) [33 of them under SCIT with allergoid Dpt extract, in maintenance dose (SCIT), with evaluation just before SCIT injection (SCIT-T0) and 4 hours later (SCIT-T4) and the other 10 Dpt allergic patients never having, in the past, undergone specific immunotherapy treatment (NSIT)], and 15 healthy age- and gender-matched controls (HG), were analyzed. For each sample, the total (t-IgE) and specific IgE (s-IgE) was performed, as well as, the relative frequency and absolute number of PB basophils and receptor-bound IgE and IgG expression were evaluated by flow cytometry and the Histamine N-methyltransferase (HNMT) and tryptase α/β1 (TPSAB1) gene expression was assessed by real-time PCR.

Results

Higher levels of receptor-bound IgE were observed in SCIT patients, which are correlated with the levels of serum t-IgE and s-IgE, whereas no significant differences were observed for receptor-bound IgG. Regarding HNMT mRNA expression, significantly lower expression levels were detected in AR patients compared to HG, independently of type of therapy. Moreover a negative correlation was found between HNMT gene expression and time under SCIT. Conversely, tryptase gene expression was significantly up-regulated in NSIT when compared to HG; however in SCIT patients, tryptase gene expression was significantly decreased than in NSIT patients. No differences were found for any parameter between SCIT-T0 and SCIT-T4 with exception of a transient increased expression of tryptase in SCIT-T4.

Conclusion

PB basophils from patients with AR show altered functional features, which seems to be influenced by SCIT, suggesting that these cells could be useful to clarify the SCIT triggered mechanisms at a systemic level.

The history of mast cell and basophil research - some lessons learnt from the last century

This year (2013) marks the 50th anniversary of death of Otto Carl Willy Prausnitz (1876-1963) and Heinz Küstner (1897-1963). The two physicians, when working at the Hygiene Institute at the University of Breslau, Germany (Prausnitz was the Head of the Institute), described in 1921 what is still called today the Prausnitz-Küstner or PK reaction showing that allergy could be transferred from the allergic person by transferring serum to a healthy person. Their discovery ended the belief that an anaphylactic/allergic reaction was caused by poisons, but to the contrary showed that the presence of the hypersensitivity factor could be transferred to other people. We know now that this factor is immunoglobulin E (IgE), sensitizing mast cells and basophils to respond to an allergic stimulus. We take this occasion to retrace some of the important discoveries and lessons learnt from the last century relating to the function of these two cell types as effectors of the IgE system and the mediators they produce.

Ikaros limits basophil development by suppressing C/EBP-α expression

The Ikaros gene (Ikzf1) encodes a family of zinc-finger transcription factors implicated in hematopoietic cell differentiation. Here we show that Ikaros suppresses the development of basophils, which are proinflammatory cells of the myeloid lineage. In the absence of extrinsic basophil-inducing signals, Ikaros(-/-) (Ik(-/-)) mice exhibit increases in basophil numbers in blood and bone marrow and in their direct precursors in bone marrow and the spleen, as well as decreased numbers of intestinal mast cells. In vitro culture of Ik(-/-) bone marrow under mast cell differentiation conditions also results in predominance of basophils. Basophil expansion is associated with an increase in basophil progenitors, increased expression of Cebpa and decreased expression of mast cell-specifying genes Hes1 and microphthalmia-associated transcription factor (Mitf). Ikaros directly associates with regulatory sites within Cebpa and Hes1 and regulates the acquisition of permissive H3K4 tri-methylation marks at the Cebpa locus and reduces H3K4 tri-methylation at the Hes1 promoter. Ikaros blockade in cultured cells or transfer of Ik(-/-) bone marrow into irradiated Ik(+/+) recipients also results in increased basophils confirming a cell-intrinsic effect of Ikaros on basophil development. We conclude that Ikaros is a suppressor of basophil differentiation under steady-state conditions and that it acts by regulating permissive chromatin modifications of Cebpa.

Selective ablation of mast cells or basophils reduces peanut-induced anaphylaxis in mice

BACKGROUND

Studies with c-kit mutant mast cell (MC)-deficient mice and antibody-mediated depletion of basophils suggest that both MCs and basophils can contribute to peanut-induced anaphylaxis (PIA). However, interpretation of data obtained by using such approaches is complicated because c-kit mutant mice have several phenotypic abnormalities in addition to MC deficiency and because basophil-depleting antibodies can also react with MCs.

OBJECTIVE

We analyzed (1) the changes in the features of PIA in mice after the selective and inducible ablation of MCs or basophils and (2) the possible importance of effector cells other than MCs and basophils in the PIA response.

METHODS

Wild-type and various mutant mice were orally sensitized with peanut extract and cholera toxin weekly for 4 weeks and challenged intraperitoneally with peanut extract 2 weeks later.

RESULTS

Peanut-challenged, MC-deficient Kit(W-sh/W-sh) mice had reduced immediate hypothermia, as well as a late-phase decrease in body temperature that was abrogated by antibody-mediated depletion of neutrophils. Diphtheria toxin-mediated selective depletion of MCs or basophils in Mcpt5-Cre;iDTR and Mcpt8(DTR) mice, respectively, and treatment of wild-type mice with the basophil-depleting antibody Ba103 significantly reduced peanut-induced hypothermia. Non-c-kit mutant MC- and basophil-deficient Cpa3-Cre;Mcl-1(fl/fl) mice had reduced but still significant responses to peanut.

CONCLUSION

Inducible and selective ablation of MCs or basophils in non-c-kit mutant mice can significantly reduce PIA, but partial responses to peanut can still be observed in the virtual absence of both cell types. The neutrophilia in Kit(W-sh/W-sh) mice might influence the responses of these mice in this PIA model.

The role of mast cells in neuroinflammation

Mast cells (MCs) are densely granulated perivascular resident cells of hematopoietic origin and well known for their pathogenetic role in allergic and anaphylactic reactions. In addition, they are also involved in processes of innate and adaptive immunity. MCs can be activated in response to a wide range of stimuli, resulting in the release of not only pro-inflammatory, but also anti-inflammatory mediators. The patterns of secreted mediators depend upon the given stimuli and microenvironmental conditions, accordingly MCs have the ability to promote or attenuate inflammatory processes. Their presence in the central nervous system (CNS) has been recognized for more than a century. Since then a participation of MCs in various pathological processes in the CNS has been well documented. They can aggravate CNS damage in models of brain ischemia and hemorrhage, namely through increased blood-brain barrier damage, brain edema and hemorrhage formation and promotion of inflammatory responses to such events. In contrast, recent evidence suggests that MCs may have a protective role following traumatic brain injury by degrading pro-inflammatory cytokines via specific proteases. In neuroinflammatory diseases such as multiple sclerosis, the role of MCs seems to be ambiguous. MCs have been shown to be damaging, neuroprotective, or even dispensable, depending on the experimental protocols used. The role of MCs in the formation and progression of CNS tumors such as gliomas is complex and both positive and negative relationships between MC activity and tumor progression have been reported. In summary, MCs and their secreted mediators modulate inflammatory processes in multiple CNS pathologies and can thereby either contribute to neurological damage or confer neuroprotection. This review intends to give a concise overview of the regulatory roles of MCs in brain disease.

Antibodies and IL-3 support helminth-induced basophil expansion

Basophils are powerful mediators of Th2 immunity and are present in increased numbers during allergic inflammation and helminth infection. Despite their ability to potentiate Th2 immunity the mechanisms regulating basophil development remain largely unknown. We have found a unique role for isotype-switched antibodies in promoting helminth-induced basophil production following infection of mice with Heligmosomoides polygyrus bakeri or Nippostrongylus brasiliensis. H. polygyrus bakeri-induced basophil expansion was found to occur within the bone marrow, and to a lesser extent the spleen, and was IL-3 dependent. IL-3 was largely produced by CD4(+)CD49b(+)NK1.1(-) effector T cells at these sites, and required the IL-4Rα chain. However, antibody-deficient mice exhibited defective basophil mobilization despite intact T-cell IL-3 production, and supplementation of mice with immune serum could promote basophilia independently of required IL-4Rα signaling. Helminth-induced eosinophilia was not affected by the deficiency in isotype-switched antibodies, suggesting a direct effect on basophils rather than through priming of Th2 responses. Although normal type 2 immunity occurred in the basopenic mice following primary infection with H. polygyrus bakeri, parasite rejection following challenge infection was impaired. These data reveal a role for isotype-switched antibodies in promoting basophil expansion and effector function following helminth infection.

Innate immune cells in liver inflammation

Innate immune system is the first line of defence against invading pathogens that is critical for the overall survival of the host. Human liver is characterised by a dual blood supply, with 80% of blood entering through the portal vein carrying nutrients and bacterial endotoxin from the gastrointestinal tract. The liver is thus constantly exposed to antigenic loads. Therefore, pathogenic microorganism must be efficiently eliminated whilst harmless antigens derived from the gastrointestinal tract need to be tolerized in the liver. In order to achieve this, the liver innate immune system is equipped with multiple cellular components; monocytes, macrophages, granulocytes, natural killer cells, and dendritic cells which coordinate to exert tolerogenic environment at the same time detect, respond, and eliminate invading pathogens, infected or transformed self to mount immunity. This paper will discuss the innate immune cells that take part in human liver inflammation, and their roles in both resolution of inflammation and tissue repair.

Activation of B cells by non-canonical helper signals

Cognate interaction between T and B lymphocytes of the adaptive immune system is essential for the production of high-affinity antibodies against microbes, and for the establishment of long-term immunological memory. Growing evidence shows that--in addition to presenting antigens to T and B cells--macrophages, dendritic cells and other cells of the innate immune system provide activating signals to B cells, as well as survival signals to antibody-secreting plasma cells. Here, we discuss how these innate immune cells contribute to the induction of highly diversified and temporally sustained antibody responses, both systemically and at mucosal sites of antigen entry.

Generation and characterization of mouse basophils from bone marrow and purification of basophils from spleen

Basophils are rare circulating granulocytes that originate from progenitor cells in the bone marrow and have been considered important effector cells in IgE-mediated allergic inflammation. Basophils constitute <1% of blood leukocytes and are usually absent or present only in small numbers in tissues. They may, however, be recruited to inflammatory sites when an antigen is present and contribute immediately to hypersensitivity reactions. Basophils can therefore serve as primary effector cells in allergic disorders. Despite a large pool of experimental evidence that has led to the discovery of these functional attributes of basophils, many questions regarding their contribution to these immune responses remain unanswered. This is due, in part, to the lack of methods for generation and purification of basophils and the lack of animal models appropriate for their functional analysis. Recent studies, however, have revealed a role for basophils as antigen-presenting cells that preferentially induce Th2 cells in response to complexes of antigen plus antigen-specific IgE, to protease allergens, or to helminth parasites in vitro and in vivo through the production of "early IL-4" and the presentation to CD4(+) T cells of complexes of peptide plus MHC class II molecules. These findings have uncovered previously unknown functional characteristics of basophils. Knowledge of these and other functional properties of basophils may translate into the design of novel therapeutic strategies for Th2-IgE-mediated diseases, such as bronchial asthma. In this unit, protocols that will enable the study of mouse basophils are described.

Lipoteichoic acid of Staphylococcus aureus enhances IL-6 expression in activated human basophils

At allergic inflammation, cross-linking of FcɛRI with multivalent antigen-bound IgE triggers the signaling pathways via activation of protein kinase C (PKC) and mobilization of intracellular Ca(2+) leading to the production of various mediators such as interleukin-6 (IL-6). Accumulating reports demonstrated that interaction of Toll-like receptor 2 (TLR2) expressed on basophils with a TLR2 ligand, lipoteichoic acid (LTA) of Staphylococcus aureus, exacerbated allergic inflammation. Here, we showed that staphylococcal LTA (Sa.LTA) substantially enhanced IL-6 expression at both the protein and the mRNA levels in the human basophil line, KU812, in the presence of a PKC activator (phorbol 12-myristrate 13-acetate; PMA), and a calcium ionophore (A23187), whereas Sa.LTA alone could not induce IL-6 expression. PMA/A23187 augmented the expression of CD14 and TLR2 on the surface of KU812 cells and concomitantly increased the binding of fluorochrome-labeled Sa.LTA to the cells. Sa.LTA enhanced the phosphorylation of mitogen-activated protein (MAP) kinases in PMA/A23187-stimulated KU812 cells. Notably, Sa.LTA could not enhance PMA/A23187-induced IL-6 expression in the presence of inhibitors of MAP kinases, reactive oxygen species, or protein kinase C. Furthermore, Sa.LTA enhanced the PMA/A23187-increased DNA-binding activities of the transcription factors NF-κB and AP-1. Experiments using human peripheral blood mononuclear cells demonstrated that not only PMA/A23187 but also Sa.LTA increased the intracellular IL-6 expression in the basophil population and Sa.LTA plus PMA/A23187 further enhanced the IL-6 expression. Collectively, these results suggest that Sa.LTA exacerbates allergic inflammation by potentiating IL-6 production from activated basophils.

Understanding the roles of basophils: breaking dawn

Early studies that used parasite-infected interleukin-4 (IL-4) reporter animals led us to identify basophils as the primary source of IL-4 and hence propose the hypothesis that basophils trigger the development of antigen-specific T helper type 2 (Th2) immune responses in vivo. These findings appeared to resolve a long-standing puzzle underlying Th2 immunity, that is, 'what is the source of the initial IL-4 necessary for CD4 T-cell differentiation into Th2 effector cells?'. However, results from extensive investigations of the contribution of basophils to Th2 immunity unveiled some controversial data that cast doubt on the initial hypothesis. In this review, the consensus and the controversy regarding the roles of basophils in infection and immunity, as well as outstanding questions for the future, are discussed.

Basophil-derived mouse mast cell protease 11 induces microvascular leakage and tissue edema in a mast cell-independent manner

Mouse mast cell protease 11 (mMCP-11) is the most recently identified member of the mouse mast cell tryptase family. This tryptase is preferentially produced by basophils in contrast to other members that are expressed by mast cells but not basophils. Although blood-circulating basophils have long been considered as minor and redundant relatives of tissue-resident mast cells, recent studies illustrated that basophils and mast cells play distinct roles in vivo. To explore the in vivo role of basophil-derived mMCP-11, here we prepared recombinant mMCP-11 and its protease-dead mutant. Subcutaneous injection of the wild-type mMCP-11 but not the mutant induced edematous skin swelling with increased microvascular permeability in a dose-dependent manner. No apparent infiltration of proinflammatory cells including neutrophils and eosinophils was detected in the skin lesions. The cutaneous swelling was abolished by the pretreatment of mice with indomethacin, a cyclooxygenase inhibitor, suggesting the major contribution of prostaglandins to the microvascular leakage. Of note, the cutaneous swelling was elicited even in mast cell-deficient mice, indicating that mast cells are dispensable for the mMCP-11-induced cutaneous swelling. Thus, basophil-derived mMCP-11 can induce microvascular leakage via prostaglandins in a mast cell-independent manner, and may contribute to the development of basophil-mediated inflammatory responses.

Innate immunity in allergic disease

The innate immune system consists of multiple cell types that express germline-encoded pattern recognition receptors that recognize pathogen-associated molecular patterns (PAMPs) or danger-associated molecular patterns (DAMPs). Allergens are frequently found in forms and mixtures that contain PAMPs and DAMPs. The innate immune system is interposed between the external environment and the internal acquired immune system. It is also an integral part of the airways, gut, and skin. These tissues face continuous exposure to allergens, PAMPs, and DAMPs. Interaction of allergens with the innate immune system normally results in immune tolerance but, in the case of allergic disease, this interaction induces recurring and/or chronic inflammation as well as the loss of immunologic tolerance. Upon activation by allergens, the innate immune response commits the acquired immune response to a variety of outcomes mediated by distinct T-cell subsets, such as T-helper 2, regulatory T, or T-helper 17 cells. New studies highlighted in this review underscore the close relationship between allergens, the innate immune system, and the acquired immune system that promotes homeostasis versus allergic disease.

Newly appreciated roles for basophils in allergy and protective immunity

Basophils are evolutionarily conserved in many animal species, in spite of the fact that they account for <1% of peripheral blood leukocyte. This suggests that basophils have an indispensable and nonredundant role in vivo, even though they show some phenotypic similarity with tissue-resident mast cells. However, their functional significance remained uncertain long after Paul Ehrlich discovered them as blood-circulating cells with basophilic granules more than 130 years ago. The study of basophils has been far behind that of mast cells, owing to the rarity of basophils and the paucity of tools for their detection and functional analysis. Recent development of novel analytical tools, including basophil-depleting antibodies and genetically engineered mice deficient only in basophils, has greatly advanced basophil research and illuminated previously unrecognized roles of basophils. We now appreciate that basophils and mast cells play distinct roles in immune responses. Basophils have crucial roles in the development of acute and chronic allergic responses, the protective immunity against ecto- and endoparasites, and the regulation of acquired immunity, including the augmentation of humoral memory responses and the initiation of Th2 responses. Thus, basophils are no longer the neglected minority and are key players in the immune system.

Host basophils are dispensable for induction of donor T helper 2 cell differentiation and severity of experimental graft-versus-host disease

Host hematopoietic-derived antigen-presenting cells are important for induction of graft-versus-host disease (GVHD). The relative importance of various subsets of hematopoietic-derived antigen-presenting cells is not well understood. Recent data suggest that basophils can function as antigen-presenting cells and induce T helper 2 (Th2) lymphocyte responses. We investigated the role of host basophils in the induction of donor T cell responses and GVHD after allogeneic bone marrow transplantation. Elimination of host basophils did not alter the severity of GVHD-induced mortality across multiple clinically relevant models of allogeneic bone marrow transplantation. Furthermore, induction of donor T cell proliferation and Th2 polarization was not altered significantly after depletion of host basophils. Our results demonstrate that, in contrast to their role in inducing Th2 responses in certain contexts, basophils are dispensable for the induction of donor Th2 responses and for the severity of GVHD.

Basophils, IgE, and autoantibody-mediated kidney disease

Basophils are of interest in immunology due to their ability to produce a Th2-signature cytokine, IL-4, following activation. A new understanding of the role of basophils in immunity shows novel functions at a cellular level through which basophils influence adaptive immunity. This review summarizes new advances in basophil biology and discusses new roles for basophils in human disease, especially in the mediation of the pathogenesis of lupus nephritis. Recently, basophils have been shown to contribute to self-reactive Ab production in systemic lupus erythematosus and may enhance pre-existing loss of B cell tolerance, suggesting that basophils, IL-4, and IgE mediate the pathogenesis of lupus nephritis by promoting the Th2 environment and activating autoreactive B cells. In addition to envisaging exciting therapeutic prospects, these novel findings open the way for the study of basophils in other autoimmune and renal diseases.

NK cell-depleting anti-asialo GM1 antibody exhibits a lethal off-target effect on basophils in vivo

NK cells are innate immune lymphocytes and play a key role in both innate and adaptive immunity. Their pivotal functions in vivo have been illustrated in mice by means of their ablation with NK cell-depleting Abs, particularly anti-asialo GM1 (ASGM1). In this study, we show that the whole population of basophils constitutively expresses ASGM1 as well as CD49b (DX5) as does the NK cell population and was ablated in vivo by anti-ASGM1 as efficiently as by a basophil-depleting anti-FcεRIα Ab. Anti-ASGM1-mediated basophil depletion was operative as for NK cell depletion in various mouse strains, irrespective of NK1 allotype and MHC H2 haplotype, including C57BL/6, BALB/c, C3H, and A/J mice. These results identified basophils as a previously unrecognized target of anti-ASGM1-mediated cell depletion and raised concern about possible contribution of basophils, rather than or in addition to NK cells, to some of phenotypes observed in anti-ASGM1-treated mice. Indeed, regardless of the presence or absence of NK cells in mice, anti-ASGM1 treatment abolished the development of IgE-mediated chronic cutaneous allergic inflammation as efficiently as did the treatment with basophil-depleting Ab. Given the fact that basophils have recently been shown to play crucial roles in a variety of immune responses, our finding of the off-target effect on basophils issues a grave warning about the use of anti-ASGM1 and underscores the need for careful interpretation of phenotypes observed in anti-ASGM1-treated mice.

Basophils induce Th2 immunity: is this final answer?

Despite that basophils represent less than 0.5% of circulating leukocytes, recent studies have begun to unveil their potent immunoregulatory functions, i.e., induction of Th2 immunity. It is believed that basophils are capable of doing so primarily by secreting key Th2-inducing cytokines, namely IL-4 and Thymic Stromal Lymphopoietin (TSLP), and by functioning as professional antigen presenting cells. However, we have recently demonstrated that Th2 immunity can develop in the absence of basophils or IL-4 during helminth infection. In this review, how basophils may (and may not) contribute to the development of Th2 immunity in vivo is discussed.

Mast cells in allergy and infection: versatile effector and regulatory cells in innate and adaptive immunity

Mast cells are widely distributed in tissues, particularly near surfaces exposed to the environment. Mast cells can be activated to secrete diverse mediators and cytokines by IgE and specific Ag and many other stimuli, including products derived from either pathogens or the host during innate immune responses. Although mast cells are best known for their role in IgE-associated allergic disorders, mast cells can also exacerbate models of autoimmunity, enhance the sensitization and/or effector phases of certain cutaneous contact hypersensitivity responses, and increase inflammation and mortality during some severe bacterial infections. In other settings, however, mast cells can limit inflammation and tissue injury: mast cells promote host resistance in certain models of bacterial or parasite infection, limit pathology during some acquired immune responses to environmental Ag, including examples of severe contact hypersensitivity, and have adjuvant-like properties that can enhance the development of protective immunity against pathogens. These and other findings suggest that mast cells occupy a critical niche at the interface of innate and acquired immunity, where, depending on circumstances that remain to be fully understood, mast cells may function to perturb or help to restore homeostasis (or both), with consequences that can either promote health or contribute to disease.

Mice that "conditionally" lack basophils, AT LAST

Basophils are the least abundant granulocytes found in the circulation. Until recently, their functions were poorly understood. In the past few years, the list of basophil functions in the context of immunity has dramatically increased. Thus, the need for basophil-deficient animal models to confirm these findings is imperative. In this issue of the JCI, Wada and colleagues introduce the first mouse model in which basophils are conditionally ablated in vivo. Using this model, they then uncover a nonredundant role for basophils in acquired immunity against tick infection.

Selective ablation of basophils in mice reveals their nonredundant role in acquired immunity against ticks

Ticks are ectoparasitic arthropods that can transmit a variety of microorganisms to humans and animals during blood feeding, causing serious infectious disorders, including Lyme disease. Acaricides are pharmacologic agents that kill ticks. The emergence of acaricide-resistant ticks calls for alternative control strategies for ticks and tick-borne diseases. Many animals develop resistance to ticks after repeated infestations, but the nature of this acquired anti-tick immunity remains poorly understood. Here we investigated the cellular and molecular mechanisms underlying acquired resistance to Haemaphysalis longicornis ticks in mice and found that antibodies were required, as was IgFc receptor expression on basophils but not on mast cells. The infiltration of basophils at tick-feeding sites occurred during the second, but not the first, tick infestation. To assess the requirement for basophil infiltration to acquired tick resistance, mice expressing the human diphtheria toxin receptor under the control of the mast cell protease 8 (Mcpt8) promoter were generated. Diphtheria toxin administration to these mice selectively ablated basophils. Diphtheria toxin-mediated basophil depletion before the second tick infestation resulted in loss of acquired tick resistance. These data provide the first clear evidence, to our knowledge, that basophils play an essential and nonredundant role in antibody-mediated acquired immunity against ticks, which may suggest new strategies for controlling tick-borne diseases.

Basophils as T(h)2-inducing antigen-presenting cells

Recent studies have revealed a role for basophils as antigen-presenting cells that preferentially induce T(h)2 cells in response to complexes of antigen plus antigen-specific IgE, to protease allergens or to helminthic parasites in vitro and in vivo through the production of 'early IL-4' and the presentation to CD4(+) T cells of complexes of peptide plus MHC class II. These findings have revealed previously unknown functions of basophils and will potentially aid the design of novel therapeutic strategies for T(h)2-IgE-mediated diseases such as bronchial asthma.

IgE, mast cells, basophils, and eosinophils

IgE, mast cells, basophils, and eosinophils are essential components of allergic inflammation. Antigen-specific IgE production, with subsequent fixation of IgE to FcepsilonRI receptors on mast cells and basophils, is central to the initiation and propagation of immediate hypersensitivity reactions. Mast cells, basophils, and eosinophils are central effector cells in allergic inflammation, as well as in innate and adaptive immunity. This review highlights what is known about these components and their roles in disease pathogenesis.

Cutting edge: basophils are transiently recruited into the draining lymph nodes during helminth infection via IL-3, but infection-induced Th2 immunity can develop without basophil lymph node recruitment or IL-3

Basophils are recognized as immune modulators through their ability to produce IL-4, a key cytokine required for Th2 immunity. It has also recently been reported that basophils are transiently recruited into the draining lymph node (LN) after allergen immunization and that the recruited basophils promote the differentiation of naive CD4 T cells into Th2 effector cells. Using IL-3(-/-) and IL-3Rbeta(-/-) mice, we report in this study that the IL-3/IL-3R system is absolutely required to recruit circulating basophils into the draining LN following helminth infection. Unexpectedly, the absence of IL-3 or of basophil LN recruitment played little role in helminth-induced Th2 immune responses. Moreover, basophil depletion in infected mice did not diminish the development of IL-4-producing CD4 T cells. Our results reveal a previously unknown role of IL-3 in recruiting basophils to the LN and demonstrate that basophils are not necessarily associated with the development of Th2 immunity during parasite infection.