Stimulation of human endothelium with IL-3 induces selective basophil accumulation in vitro

The decrease in peripheral blood basophils in a mouse model of IgE-induced inflammation involves their migration to lymph nodes

BACKGROUND

During the active phase of urticaria, a decrease in peripheral blood basophils, known as basopenia, is observed. We previously reported that basopenia occurs as a result of basophils migrating to the skin in a contact dermatitis model where a Th2 response is induced with oxazolone.

OBJECTIVE

Although there is currently no established model for urticaria, given that urticaria is an IgE-mediated immediate-type allergic reaction, we aimed to determine whether an IgE-mediated model could reproduce the decrease in basophils in peripheral blood observed during the active phase of urticaria.

METHODS

Mice were pretreated with 2,4,6-trinitrophenylhaptene (TNP)-specific IgE and basophil dynamics were examined following stimulation with TNP-ovalbumin. Mast cell-deficient WBB6F1-KitW/KitW-v mice were used to investigate the role of mast cells in this IgE-mediated model.

RESULTS

Following stimulation, we observed immediate ear swelling and basopenia after 0.5 hours. However, the number of basophils observed in the skin lesions was low, while a higher number of basophils were observed in the antigen-draining lymph nodes (LN). In mast cell-deficient mice, no increase in basophils in the LN was observed, reflecting reduced antigen influx into the LN, but basophils remained in the skin.

CONCLUSIONS

In the IgE-mediated mouse model, basopenia was observed, which coincided with the induction of inflammation in the skin. The migration of basophils to the LN in this model suggests that the systemic immune system, including the LN, should be considered when exploring the pathogenesis of urticaria in humans.

Granulocyte development, tissue recruitment, and function during allergic inflammation

Granulocytes provide a fast innate response to pathogens and allergens. In allergy and anti-helminth immunity, epithelial cells of damaged barriers release alarmins like IL-25, IL-33, and thymic stromal lymphopoietin (TSLP) but also chemokines like CXCL1 or CCL11 to promote cell recruitment and inflammation. In addition, mast cells positioned at barrier tissue sites also quickly release mediators upon specifically sensing antigens through IgE bound to FcεR1 on their surface. Released mediators induce the recruitment of different granulocytes in a timely ordered manner. First, neutrophils extravasate from the blood vasculature to the side of alarmin release and promote a potent inflammatory response. Alarmins and activated mast cells further promote activation of ILC2s and recruitment of basophils and eosinophils, which inhibit neutrophil recruitment and enhance tissue type 2 immunity. In addition to their potent pro-inflammatory effector functions, granulocytes can also contribute to termination and resolution of inflammation. Here, we summarize the development and tissue recruitment of granulocyte subsets, and describe general effector functions and aspects of their increasingly appreciated role in limiting tissue damage. We further discuss targeting approaches for therapeutic interventions in allergic disorders.

IL-3 produced by T cells is crucial for basophil extravasation in hapten-induced allergic contact dermatitis

Basophils have been recognized as a characterized cellular player for Th2 immune responses implicated in allergic diseases, but the mechanisms responsible for basophil recruitment to allergic skin remain not well understood. Using a hapten fluorescein isothiocyanate (FITC)-induced allergic contact dermatitis (ACD) mouse model, we show that basophils in FITC-treated IL-3-knockout mice are defective in crossing the vascular endothelium to enter the inflamed skin. By generating mice in which IL-3 is selectively ablated in T cells, we further demonstrate that IL-3 produced by T cells mediates basophil extravasation. Moreover, basophils sorted from FITC-treated IL-3-knockout mice exhibit a decreased expression of integrins Itgam, Itgb2, Itga2b and Itgb7, which are potentially implicated in extravasation process. Interestingly, we observed that these basophils had a reduced expression of retinaldehyde dehydrogenase 1 family member A2 (Aldh1a2), an enzyme responsible for the production of retinoic acid (RA), and administration of all-trans RA restored partially the extravasation of basophils in IL-3-knockout mice. Finally, we validate that IL-3 induces the expression of ALDH1A2 in primary human basophils, and provide further evidence that IL-3 stimulation induces the expression of integrins particularly ITGB7 in an RA-dependent manner. Together, our data propose a model that IL-3 produced by T cells activates ALDH1A2 expression by basophils, leading to the production of RA, which subsequently induces the expression of integrins crucially implicated in basophil extravasation to inflamed ACD skin.

Guideline for in vivo assessment of adherent and rolling leukocytes in human skin microvasculature via reflectance confocal videomicroscopy

OBJECTIVE

To develop a guideline that reliably identifies cutaneous adherent and rolling leukocytes from mimicking scenarios via in vivo reflectance confocal videomicroscopy.

METHODS

We used a clinical reflectance confocal microscope, the VivaScope 1500, to acquire 1522 videos of the upper dermal microcirculation from 12 healthy subjects and 60 patients after allogeneic hematopoietic cell transplantation. Blinded to clinical information, two trained raters independently counted the number of adherent and rolling leukocytes in 88 videos. Based on discrepancies in the initial assessments, we developed a guideline to identify both types of leukocyte-endothelial interactions via a modified Delphi method (without anonymity). To test the guideline's ability to improve the inter-rater reliability, the two raters assessed the remaining 1434 videos by using the guideline.

RESULTS

We demonstrate a guideline that consists of definitions, a step-by-step flowchart, and corresponding visuals of adherent and rolling leukocytes and mimicking scenarios. The guideline improved the inter-rater reliability of the manual assessment of both interactions. The intraclass correlation coefficient (ICC) of adherent leukocyte counts increased from 0.056 (95% confidence interval: 0-0.236, n = 88 videos, N = 10 subjects) to 0.791 (0.770-0.809, n = 1434, N = 67). The ICC of rolling leukocyte counts increased from 0.385 (0.191-0.550, n = 88, N = 10) to 0.626 (0.593-0.657, n = 1434, N = 67). Intra-rater ICC post-guideline was 0.953 (0.886-0.981, n = 20, N = 12) and 0.956 (0.894-0.983, n = 20, N = 12) for adherent and rolling, respectively.

CONCLUSION

The guideline aids in the manual identification of adherent and rolling leukocytes via in vivo reflectance confocal videomicroscopy.

The role of basophils in acquired protective immunity to tick infestation

Ticks are blood-feeding ectoparasites that transmit a variety of pathogens to host animals and humans, causing severe infectious diseases such as Lyme disease. In a certain combination of animal and tick species, tick infestation elicits acquired immunity against ticks in the host, which can reduce the ability of ticks to feed on blood and to transmit pathogens in the following tick infestations. Therefore, our understanding of the cellular and molecular mechanisms of acquired tick resistance (ATR) can advance the development of anti-tick vaccines to prevent tick infestation and tick-borne diseases. Basophils are a minor population of white blood cells circulating in the bloodstream and are rarely observed in peripheral tissues under steady-state conditions. Basophils have been reported to accumulate at tick-feeding sites during re-infestation in cattle, rabbits, guinea pigs and mice. Selective ablation of basophils resulted in a loss of ATR in guinea pigs and mice, illuminating the essential role of basophils in the manifestation of ATR. In this review, we discuss the recent advance in the elucidation of the cellular and molecular mechanisms underlying basophil recruitment to the tick-feeding site and basophil-mediated ATR.

Immunobiology of Acquired Resistance to Ticks

Ticks are blood-sucking arthropods of great importance in the medical and veterinary fields worldwide. They are considered second only to mosquitos as vectors of pathogenic microorganisms that can cause serious infectious disorders, such as Lyme borreliosis and tick-borne encephalitis. Hard (Ixodid) ticks feed on host animals for several days and inject saliva together with pathogens to hosts during blood feeding. Some animal species can acquire resistance to blood-feeding by ticks after a single or repeated tick infestation, resulting in decreased weights and numbers of engorged ticks or the death of ticks in subsequent infestations. Importantly, this acquired tick resistance (ATR) can reduce the risk of pathogen transmission from pathogen-infected ticks to hosts. This is the basis for the development of tick antigen-targeted vaccines to forestall tick infestation and tick-borne diseases. Accumulation of basophils is detected in the tick re-infested skin lesion of animals showing ATR, and the ablation of basophils abolishes ATR in mice and guinea pigs, illustrating the critical role for basophils in the expression of ATR. In this review article, we provide a comprehensive overview of recent advances in our understanding of the cellular and molecular mechanisms responsible for the development and manifestation of ATR, with a particular focus on the role of basophils.

Crucial Role for Basophils in Acquired Protective Immunity to Tick Infestation

Ticks are blood-sucking arthropods that can transmit various pathogenic organisms to host animals and humans, causing serious infectious diseases including Lyme disease. Tick feeding induces innate and acquired immune responses in host animals, depending on the combination of different species of animals and ticks. Acquired tick resistance (ATR) can diminish the chance of pathogen transmission from infected ticks to the host. Hence, the elucidation of cellular and molecular mechanism underlying ATR is important for the development of efficient anti-tick vaccines. In this review article, we briefly overview the history of studies on ATR and summarize recent findings, particularly focusing on the role for basophils in the manifestation of ATR. In several animal species, including cattle, guinea pigs, rabbits and mice, basophil accumulation is observed at the tick re-infestation site, even though the frequency of basophils among cellular infiltrates varies in different animal species, ranging from approximately 3% in mice to 70% in guinea pigs. Skin-resident, memory CD4⁺ T cells contribute to the recruitment of basophils to the tick re-infestation site through production of IL-3 in mice. Depletion of basophils before the tick re-infestation abolishes ATR in guinea pigs infested with Amblyomma americanum and mice infested with Haemaphysalis longicornis, demonstrating the crucial role of basophils in the manifestation of ATR. The activation of basophils via IgE and its receptor FcεRI is essential for ATR in mice. Histamine released from activated basophils functions as an important effector molecule in murine ATR, probably through promotion of epidermal hyperplasia which interferes with tick attachment or blood feeding in the skin. Accumulating evidence suggests the following scenario. The 1^st tick infestation triggers the production of IgE against tick saliva antigens in the host, and blood-circulating basophils bind such IgE on the cell surface via FcεRI. In the 2^nd infestation, IgE-armed basophils are recruited to tick-feeding sites and stimulated by tick saliva antigens to release histamine that promotes epidermal hyperplasia, contributing to ATR. Further studies are needed to clarify whether this scenario in mice can be applied to ATR in other animal species and humans.

Skin CD4+ Memory T Cells Play an Essential Role in Acquired Anti-Tick Immunity through Interleukin-3-Mediated Basophil Recruitment to Tick-Feeding Sites

Ticks, blood-sucking arthropods, serve as vectors for transmission of infectious diseases including Lyme borreliosis. After tick infestation, several animal species can develop resistance to subsequent infestations, reducing the risk of transmission. In a mouse model, basophils reportedly infiltrate tick-feeding sites during the second but not first infestation and play a crucial role in the expression of acquired tick resistance. However, the mechanism underlying basophil recruitment to the second tick-feeding site remains ill-defined. Here, we investigated cells and their products responsible for the basophil recruitment. Little or no basophil infiltration was detected in T-cell-deficient mice, and adoptive transfer of CD4⁺ but not CD8⁺ T cells reconstituted it. Il3 gene expression was highly upregulated at the second tick-feeding site, and adoptive transfer of interleukin-3 (IL-3)-sufficient but not IL-3-deficient CD4⁺ T cells conferred the basophil infiltration on T-cell-deficient mice, indicating that the CD4⁺ T-cell-derived IL-3 is essential for the basophil recruitment. Notably, IL-3⁺ resident CD4⁺ memory T cells were detected even before the second infestation in previously uninfested skin distant from the first tick-feeding site. Taken together, IL-3 produced locally by skin CD4⁺ memory T cells appears to play a crucial role in basophil recruitment to the second tick-feeding site.

Basophil Activation-Dependent Autoantibody and Interleukin-17 Production Exacerbate Systemic Lupus Erythematosus

Objective

Autoantibody and inflammatory cytokines play crucial roles in the development of systemic lupus erythematosus (SLE); however, the regulation of their production warrants further investigation. This study aimed to investigate the role of basophil activation in the development of SLE based on studies in patients with SLE and spontaneous lupus-prone MRL-lpr/lpr mice.

Methods

The phenotypes of peripheral basophils and the production of autoantibody and interleukin (IL)-17 in patients with SLE were determined by flow cytometry and enzyme-linked immunosorbent assay, and also their correlations were investigated by statistical analysis. Thereafter, the effect of basophils on autoantibody production by B cells and Th17 differentiation in SLE were evaluated in vitro. Finally, the effect of basophil depletion on the development of autoimmune disorders in spontaneous lupus-prone MRL-lpr/lpr mice was examined.

Results

The decreased numbers and an increased activation of peripheral basophils were found to be correlated with increased autoantibody production and disease activity in patients with SLE. Correspondingly, in vitro coculture studies showed that basophils obtained from patients with SLE promoted autoantibody production by SLE B cells and promoted Th17 differentiation from SLE naïve CD4⁺ T cells. The decrease of peripheral basophils in patients with SLE might be due to their migration to lymph nodes post their activation mediated by (autoreactive) IgE as supported by their increased CD62L and CCR7 expressions and accumulation in the lymph nodes of MRL-lpr/lpr mice. Furthermore, an increased activation of peripheral basophils was identified in MRL-lpr/lpr mice. Importantly, basophil-depleted MRL-lpr/lpr mice exhibited an extended life span, improved renal function, and lower serum levels of autoantibodies and IL-17, while basophil-adoptive-transferred mice exhibited the opposite results.

Conclusion

These finding suggest that basophil activation-dependent autoantibody and IL-17 production may constitute a critical pathogenic mechanism in SLE.

Antileukemia Efficacy and Mechanisms of Action of SL-101, a Novel Anti-CD123 Antibody Conjugate, in Acute Myeloid Leukemia

Purpose: The persistence of leukemia stem cells (LSC)-containing cells after induction therapy may contribute to minimal residual disease (MRD) and relapse in acute myeloid leukemia (AML). We investigated the clinical relevance of CD34⁺CD123⁺ LSC-containing cells and antileukemia potency of a novel antibody conjugate SL-101 in targeting CD123⁺ LSCs.Experimental Methods and Results: In a retrospective study on 86 newly diagnosed AML patients, we demonstrated that a higher proportion of CD34⁺CD123⁺ LSC-containing cells in remission was associated with persistent MRD and predicted shorter relapse-free survival in patients with poor-risk cytogenetics. Using flow cytometry, we explored the potential benefit of therapeutic targeting of CD34⁺CD38^-CD123⁺ cells by SL-101, a novel antibody conjugate comprising an anti-CD123 single-chain Fv fused to Pseudomonas exotoxin A The antileukemia potency of SL-101 was determined by the expression levels of CD123 antigen in a panel of AML cell lines. Colony-forming assay established that SL-101 strongly and selectively suppressed the function of leukemic progenitors while sparing normal counterparts. The internalization, protein synthesis inhibition, and flow cytometry assays revealed the mechanisms underlying the cytotoxic activities of SL-101 involved rapid and efficient internalization of antibody, sustained inhibition of protein synthesis, induction of apoptosis, and blockade of IL3-induced p-STAT5 and p-AKT signaling pathways. In a patient-derived xenograft model using NSG mice, the repopulating capacity of LSCs pretreated with SL-101 in vitro was significantly impaired.Conclusions: Our data define the mechanisms by which SL-101 targets AML and warrant further investigation of the clinical application of SL-101 and other CD123-targeting strategies in AML. Clin Cancer Res; 23(13); 3385-95. ©2017 AACR.

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.

The deleterious role of basophils in systemic lupus erythematosus

Systemic lupus erythematosus is a complex autoimmune disease of multifactorial origins. All compartments of the immune system appear to be affected, at least in some way, and to contribute to disease pathogenesis. Because of an escape from negative selection autoreactive T and B cells accumulate in SLE patients leading to the production of autoantibodies mainly raised against nuclear components and their subsequent deposition into target organs. We recently showed that basophils, in an IgE and IL-4 dependent manner, contribute to SLE pathogenesis by amplifying autoantibody production. Here, we summarize what we have learned about the deleterious role of basophils in lupus both in a mouse model and in SLE patients. We discuss which possible pathways could be involved in basophil activation and recruitment to secondary lymphoid organs during SLE, and how basophils may amplify autoantibody production.

Enhanced basophil reactivities during severe malaria and their relationship with the Plasmodium falciparum histamine-releasing factor translationally controlled tumor protein

Recent studies suggest shared pathogenic pathways during malaria and allergy. Indeed, IgE, histamine, and the parasite-derived Plasmodium falciparum histamine-releasing factor translationally controlled tumor protein (PfTCTP) can be found at high levels in serum from patients experiencing malaria, but their relationship with basophil activation remains unknown. We recruited P. falciparum-infected patients in Senegal with mild malaria (MM; n = 19) or severe malaria (SM; n = 9) symptoms and healthy controls (HC; n = 38). Levels of serum IgE, PfTCTP, and IgG antibodies against PfTCTP were determined by enzyme-linked immunosorbent assays (ELISA). Basophil reactivities to IgE-dependent and -independent stimulations were measured ex vivo using fresh blood by looking at the expression level of the basophil activation marker CD203c with flow cytometry. Unstimulated basophils from MM had significantly lower levels of CD203c expression compared to those from HC and SM. After normalization on this baseline level, basophils from SM showed an enhanced reactivity to calcimycin (A23187) and hemozoin. Although SM reached higher median levels of activation after anti-IgE stimulation, great interindividual differences did not allow the results to reach statistical significance. When primed with recombinant TCTP before anti-IgE, qualitative differences in terms of a better ability to control excessive activation could be described for SM. IgE levels were very high in malaria patients, but concentrations in MM and SM were similar and were not associated with basophil responses, which demonstrates that the presence of IgE alone cannot explain the various basophil reactivities. Indeed, PfTCTP could be detected in 32% of patients, with higher concentrations for SM. These PfTCTP-positive patients displayed significantly higher basophil reactivities to any stimulus. Moreover, the absence of anti-PfTCTP IgG was associated with higher responses in SM but not MM. Our results show an association between basophil reactivity and malaria severity and suggest a pathogenic role for plasmodial PfTCTP in the induction of this allergy-like mechanism.

Generation, selection and preclinical characterization of an Fc-optimized FLT3 antibody for the treatment of myeloid leukemia

The therapeutic efficacy of humanized or chimeric second-generation antitumor antibodies is clearly established, but often limited. In recent years, defined modifications of the glycosylation pattern or the amino-acid sequence of the human immunoglobulin G1 Fc part have resulted in the development of third-generation antibodies with improved capability to recruit Fc receptor-bearing effector cells. The first antibodies of this kind, currently evaluated in early clinical trials, are directed against lymphoma-associated antigens. Fc-engineered antibodies targeting myeloid leukemia are not yet available. We here report on the generation and preclinical characterization of an Fc-optimized antibody directed to the FMS-related tyrosine kinase 3 (FLT3), an antigen expressed on the leukemic blasts of all investigated patients with acute myeloid leukemia (AML). This antibody, termed 4G8SDIEM, mediated markedly enhanced cellular cytotoxicity against FLT3-expressing cell lines as well as blasts of AML patients. FLT3 expression levels on AML cells varied between 300 and 4600 molecules/cell and, in most cases, were substantially higher than those detected on normal hematopoietic precursor cells and dendritic cells (approximately 300 molecules/cell). Antibody-mediated cytotoxicity against these normal cells was not detectable. 4G8SDIEM has been produced in pharmaceutical quality in a university-owned production unit and is currently used for the treatment of leukemia patients.

Basophil recruitment and activation in inflammatory skin diseases

BACKGROUND

Basophils are blood leukocytes constituting less than 1% of leukocytes. They share morphological and functional similarities with mast cells, but recent studies indicate that basophils play non-redundant roles via the release of several cytokines and lipid mediators, as well as functioning as antigen presenting cells. However, basophil infiltration into the tissues in human skin diseases remains to be addressed.

METHODS

The infiltration of basophils in 24 skin diseases (136 samples) was immunohistochemically analyzed using basophil-specific BB1 antibody. In addition, activation of blood basophils was examined by assessing CD203c expression with flow cytometry.

RESULTS

Basophils were detected in skin lesions of atopic dermatitis, prurigo, urticaria, bullous pemphigoid, drug eruptions, eosinophilic pustular folliculitis, insect bites, scabies, Henoch-Schönlein purpura and dermatomyositis. While cell densities in urticaria, bullous pemphigoid and eosinophilic pustular folliculitis were prominent, much lower numbers of basophils were seen in lesional skin of atopic dermatitis. Basophils were entirely absent in psoriasis vulgaris, mastocytosis, tumoral lesions, systemic sclerosis, and systemic lupus erythematosus. Levels of CD203c expression on blood basophils from prurigo and urticaria patients were higher than those from healthy donors.

CONCLUSIONS

Basophils infiltrate into skin lesions more commonly than previously thought, and thus they may play important roles in a variety of inflammatory skin diseases.

Understanding eosinophilic esophagitis: the cellular and molecular mechanisms of an emerging disease

Eosinophilic esophagitis (EoE) has been increasingly recognized as a unique clinicopathological entity over the past two decades. In this short time, the mechanisms of a complex disease have begun to emerge. Patient studies suggest that EoE is an immunologic disease related to atopy. At the cellular level, eosinophils, mast cells, and B and T lymphocytes are increased in the esophageal mucosa in a patchy distribution throughout the length of the esophagus. Laboratory investigations have implicated aeroallergens, food allergens, and a unique T helper type 2 cytokine profile. EoE appears to be an antigen-driven hypersensitivity reaction characterized by a mixed IgE-dependent/delayed-type reaction and a distinct cascade of cytokines and growth factors. The causative events that lead to EoE in humans remain unknown.

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.

Allergen-specific basophil suppression associated with clinical tolerance in patients with milk allergy

BACKGROUND

Children with milk allergy who tolerate heat-denatured milk (HM) have less severe reactions and outgrow the condition earlier than those who react to HM, which might be related to differences in IgE-dependent effector cell function.

OBJECTIVE

We sought to apply a novel assay to test the hypothesis that HM-tolerant children have suppressed IgE-mediated basophil responses.

METHODS

Allergic, HM-tolerant, outgrown, or control subjects were defined based on oral food challenges. Whole blood cells were stimulated in vitro with a range of milk allergen doses in the presence or absence of autologous serum or with dilutions of autologous serum. Activated basophils were identified by means of flow cytometry as CD63(bright)CD123+CD203c+HLA-DR(-)CD41a(-).

RESULTS

HM-tolerant subjects' basophils were significantly less responsive to milk allergen stimulation at all doses than were basophils from HM-reactive (allergic) individuals. In the absence of autologous serum, HM-tolerant subjects' basophils were significantly more reactive at low allergen concentrations. To a lesser extent, autologous serum also inhibited IL-3- and anti-IgE-induced, but not N-formyl-methionyl-leucyl-phenylalanine-induced, responses. The allergen-specific responsiveness of HM-tolerant subjects' basophils increased with dilution of autologous serum with normal pooled serum.

CONCLUSION

Children with milk allergy with a favorable prognosis have evidence of extrinsically suppressed allergen-specific effector cell reactivity.

Human basophils and cytokines/chemokines

Basophils comprise the smallest population in human peripheral blood leukocytes. The role of basophils in the pathogenesis of allergic diseases has long been obscure, although their accumulation and activation in tissues have suggested their potential importance. Recent advances in the field of basophil biology have indicated that cytokines and chemokines are the primary regulators of basophil functions. In addition, various functions of these cells seem differently modulated. The evidence strongly supports the notion that basophils exposed to these substances and allergens will behave as unique effector cells that presumably play proinflammatory roles in type I allergic reactions.

Hematopoietic progenitor cells and interleukin-stimulated endothelium: expansion and differentiation of myeloid precursors

BACKGROUND

Cytokine-stimulated endothelial cells (EC) propagate hematopoietic progenitor cell (HPC) expansion. However, the effects on the functional capacities of cultured progenitors have not been evaluated. HPC were assessed by flow cytometry, colony and cobblestone assays and long-term cultures (LTC) after culturing in the supernatant of EC stimulated by IL-1beta, IL-3 or IL-6.

RESULTS

EC incubation with IL-6 did not improve cell expansion in comparison to non-stimulated EC supernatant, while the HPCs' phenotype and functional capacities were retained. In contrast, IL-1beta and IL-3 stimulation resulted in a 10- and 100-fold increase in cell numbers with more than 90% of these cells being CD33(+). Plating efficiencies and LTC initiating cells were greatest in IL-6 supernatants, whereas the highest numbers of burst-forming units were observed using IL-3. IL-1beta supernatants diminished the number of 5-week cobblestone-areas, whereas the number of 2-week cobblestone areas remained equal to freshly isolated HPC. Fewer 2-week cobblestones and greater amounts of 5-week cobblestones were observed with IL-6 and IL-3. Expanded progenitors from all interleukin conditions were further matured into functional granulocytes.

CONCLUSION

IL-1beta and IL-3 stimulated endothelium induces proliferation and differentiation of myeloid precursors, while IL-6 treatment induced a benefit of HPC survival.

Basophils and type 2 immunity

PURPOSE OF REVIEW

Mechanisms involved in the development of in-vivo type 2 immunity are poorly defined. Basophils are potent IL-4-producing cells and may contribute to the process of polarizing immune responses.

RECENT FINDINGS

Although basophils represent fewer than 0.5% of blood leukocytes, their frequency dramatically increases under certain circumstances, particularly Th2-related responses including parasitic infection and allergic inflammation. Recent studies proposed the hypothesis that basophils could contribute to the development of type 2 immunity by providing initial IL-4 important in T cell polarization and by recruiting other effector cells such as eosinophils or neutrophils. Multiple stimuli of IgE-dependent and IgE-independent pathways that lead to release of cytokines and mediators from activated basophils have been identified. In addition, progenitors that differentiate into mature basophils have recently been identified.

SUMMARY

The current review revisits basophils with the goal of providing insights into understanding unappreciated roles of basophils in vivo.

Honeybee venom secretory phospholipase A2 induces leukotriene production but not histamine release from human basophils

The role of basophils in an anaphylactic response is well recognized but is usually masked by mast cells, which contain similar mediators for the induction of generalized vasodilatation and laryngeal constriction. The rapid onset of systemic anaphylactic symptoms, particularly in insect stings and ingested food, suggest that basophils, a circulating pool of cells containing histamine and other potent mediators such as leukotrienes, may be more involved in systemic anaphylaxis than originally thought. We wished to examine if secretory phospholipase A2, a systemic allergen found in honey bee venom (HBV-sPLA2) may activate basophils directly leading to rapid systemic mediator release. Basophils were isolated from human blood and stimulated with increasing concentrations of HBV-sPLA2. We found that physiological concentrations of HBV-sPLA2 induce rapid leukotriene C4 production from purified human basophils within 5 min, while interleukin (IL)-4 expression and production was induced at later time-points. Histamine release was not induced, signifying that HBV-sPLA2 did not induce generalized degranulation. Surface expression of CD63, CD69 and CD11b were up-regulated following HBV-sPLA2 treatment. Stimulation of basophils with anti-immunoglobulin E (IgE) following treatment with HBV-sPLA2 did not induce more leukotriene release. To investigate the mechanism of leukotriene production, 9-12 octadecadiynioc acid, a cyclooxygenase-1 (COX-1) and 15-lipoxygenase inhibitor, was used and this abrogated leukotriene production. These results indicate that HBV-sPLA2 can directly activate human basophils in vitro to induce leukotriene production.

Formaldehyde exposure induces airway inflammation by increasing eosinophil infiltrations through the regulation of reactive oxygen species production

Formaldehyde (FA) is a well-known cytotoxic irritant to the airways, but the mechanism of airway inflammation due to FA has not been clarified. In the present study, C57BL/6 mice were exposed to two concentrations (5 and 10ppm) of FA for 6h/day, 5days/week, for 2 weeks. The FA-exposed mice had much higher number of CCR3(+) eosinophils than control mice, and showed upregulated gene expression of CC-chemokine receptor-3 (CCR3), eotaxin and intercellular adhesion molecules-1 (ICAM-1) as well as an increased expression of proinflammatory and Th2 cytokines, such as interleukin (IL)-1β, IL-4 and IL-5. In addition, FA exposure revealed a considerable increase in the serum levels of IgG1, IgG3, IgA and IgE compared to controls. Histopathological analysis of the lung tissues demonstrated eosinophils and mononuclear cell infiltration of the alveolar cell walls and alveolar spaces. Gene expression of thioredoxin (TRX), redox-regulating antioxidant proteins, was markedly suppressed in FA-exposed mice, and thereby intracellular ROS levels were increased along with increased FA concentration. These results were consistent with an increase in the number of CCR3-expressing eosinophils, and indicate that FA-induced ROS was generated from eosinophils recruited to the inflammatory sites of the airways.

Heme oxygenase-1 inhibits cytokine production by activated mast cells

Heme oxygenase-1 (HO-1) is thought to contribute to host defense reactions against various stresses. In addition, recent reports have suggested that HO-1 modulates immunocyte activation and functions. HO-1 suppresses mast cell degranulation, but whether HO-1 suppresses cytokine synthesis as well is not yet known. We examined whether rat HO-1 cDNA transfected rat basophilic leukemia (RBL)-2H3 cells have altered cytokine production in response to stimulation with anti-ovalbumin (OA) serum/OA compared to Mock transfected RBL-2H3 cells. HO-1 inhibited anti-OA serum/OA-induced IL-3 and TNF-alpha production. Inhibition of HO-1 activity by Zn (II) protoporphyrin IX, a specific HO-1 inhibitor, prevented the suppression of TNF-alpha production. The cytokine inhibition by HO-1 was associated with selective suppression of the DNA-binding activity of AP-1 transcription factors. The suppression of mast cell cytokine production by HO-1 may be an important aspect of the processes that lead to resolution of allergic inflammation.

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

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