Activation of human eosinophils through leukocyte immunoglobulin-like receptor 7

A dual activation and inhibition role for the paired immunoglobulin-like receptor B in eosinophils

The accumulation of eosinophils in inflammatory foci is a hallmark characteristic of Th2 inflammation. Nevertheless, the expression of inhibitory receptors such as paired immunoglobulin-like receptor B (PIR-B) and their function regulating eosinophil accumulation have received limited attention. We now report that Pirb was up-regulated in an eosinophil-dependent manner in the lungs of allergen-challenged and interleukin (IL)-13-overexpressing mice. Eosinophils expressed high levels of PIR-B, and Pirb(-/-) mice displayed increased gastrointestinal eosinophils. Consistent with these findings, PIR-B negatively regulated eotaxin-dependent eosinophil chemotaxis in vivo and in vitro. Surprisingly, Pirb(-/-) eosinophils and neutrophils had decreased leukotriene B4 (LTB(4))-dependent chemotactic responses in vitro. Furthermore, eosinophil accumulation was decreased in a chitin-induced model, partially dependent on LTB(4). Mechanistic analysis using a miniphosphoproteomic approach revealed that PIR-B recruits activating kinases after LTB(4) but not eotaxin stimulation. Consequently, eotaxin-activated Pirb(-/-) eosinophils displayed markedly increased extracellular signal-related kinase 1 and 2 (ERK1/2) phosphorylation, whereas LTB(4)-activated eosinophils had reduced ERK1/2 phosphorylation. We provide multiple lines of evidence supporting a model in which PIR-B displays opposing but potent regulatory functions in granulocyte activation. These data change the conventional wisdom that inhibitory receptors are restricted to inhibitory signals; we therefore propose that a single receptor can have dual functionality in distinct cell types after unique cellular signals.

Association of LILRA2 (ILT1, LIR7) splice site polymorphism with systemic lupus erythematosus and microscopic polyangiitis

Leukocyte immunoglobulin-like receptors (LILRs) are inhibitory, stimulatory or soluble receptors encoded within the leukocyte receptor complex. Some LILRs are extensively polymorphic, and exhibit evidence for balancing selection and association with disease susceptibility. LILRA2 (LIR7/ILT1) is an activating receptor highly expressed in inflammatory tissues, and is involved in granulocyte and macrophage activation. In this study, we examined the association of LILRA2 and adjacently located LILRA1 with systemic lupus erythematosus (SLE), rheumatoid arthritis (RA) and microscopic polyangiitis (MPA). Polymorphism screening detected a LILRA2 SNP (rs2241524 G>A) that disrupts splice acceptor site of intron 6. Case-control association studies on 273 Japanese SLE, 296 RA, 50 MPA and 284 healthy individuals revealed increase of genotype A/A in SLE (12.1%, odds ratio (OR) 1.82, 95% confidence interval (CI) 1.02-3.24, P=0.041) and in MPA (16.0%, OR 2.52, 95% CI 1.07-5.96, P=0.049) compared with healthy individuals (7.0%). The risk allele caused an activation of a cryptic splice acceptor site that would lead to a novel LILRA2 isoform lacking three amino acids in the linker region (Delta 419-421). Flow cytometry indicated that this isoform was expressed on the surface of monocytes. These findings suggested that LILRA2 Delta 419-421 isoform encoded by the splice site SNP may play a role in SLE and MPA.

Stability engineering, biophysical, and biological characterization of the myeloid activating receptor immunoglobulin-like transcript 1 (ILT1/LIR-7/LILRA2)

Immunoglobulin-like transcript 1 (ILT1/LIR-7/LILRA2/CD85h) is one of the activating receptors in the ILT family whose members have been reported to regulate a broad range of cells involved in the immune response. Although inhibitory ILT receptors have been extensively studied, however, functions and structures of ILT activating receptors have yet to be elucidated. Obtaining of sufficient amount of recombinant proteins is a requisite for the functional and structural studies of a given protein. As a technical bottleneck of the study, extracellular domains of the ILT1 form aggregation during recombinant production in the past efforts. Here, we report the large-scale stable production of ILT1 D1D2 domains through engineering of site-directed mutagenesis (R142C) that introduces a cysteine at amino acid position 142 to form a disulfide bond with the spare cys132 without topological influences of the native protein based on the known structures of the homologous ILT 2/4/11. The recombinant ILT1 D1D2 domains behave as an equilibrium of both stable dimer and monomer in solution and yield ideal crystals for structural determination. The availability of quantities of soluble ILT1 D1D2 domains provides useful reagent for further studies of its detailed structure and functions.

Eosinophils in the pathogenesis of allergic airways disease

Eosinophils are traditionally thought to form part of the innate immune response against parasitic helminths acting through the release of cytotoxic granule proteins. However, they are also a central feature in asthma. From their development in the bone marrow to their recruitment to the lung via chemokines and cytokines, they form an important component of the inflammatory milieu observed in the asthmatic lung following allergen challenge. A wealth of studies has been performed in both patients with asthma and in mouse models of allergic pulmonary inflammation to delineate the role of eosinophils in the allergic response. Although the long-standing association between eosinophils and the induction of airway hyper-responsiveness remains controversial, recent studies have shown that eosinophils may also promote airway remodelling. In addition, emerging evidence suggests that the eosinophil may also serve to modulate the immune response. Here we review the highly co-ordinated nature of eosinophil development and trafficking and the evolution of the eosinophil as a multi-factoral leukocyte with diverse functions in asthma.

Inhibitory receptors on eosinophils: a direct hit to a possible Achilles heel?

Since their discovery, much data have been accumulated on eosinophil differentiation, morphology, trafficking, and anatomical location(s) in health and disease. Although "classic" activation pathways (such as cytokines, chemokines, proinflammatory components, and adhesion molecules) regulating eosinophil activation have been widely explored, the presence of other activation molecules that might be disease specific is limited. Furthermore, the expression and function of inhibitory receptors on eosinophils have received scant attention. The need to identify new pathways that regulate eosinophil activation is a crucial goal as it can expand our knowledge on this peculiar cell and provide insights into important queries regarding the physiologic function of eosinophils. Over the past several years, it has become increasingly apparent that eosinophils express several receptors belonging to the immunoglobulin superfamily. In this review, we summarize the current knowledge on the expression and function of new pathways that govern eosinophil activation. In addition, we will propose some hypotheses regarding the ability to use these pathways as a future therapeutic approach. In conclusion, we assume that targeting inhibitory receptors on eosinophils may provide opportunities for immunoregulatory therapy in the near future.

CD48 is an allergen and IL-3-induced activation molecule on eosinophils

Eosinophils are involved in a variety of allergic, parasitic, malignant, and idiopathic disorders by releasing a variety of factors including specific granule proteins, lipid mediators, and proinflammatory and immunoregulatory cytokines and chemokines. In addition, they interact with various cell types in the inflamed tissue. Yet, the mechanism of eosinophil activation is still poorly understood. Recently, we described the expression and function of the CD2-subfamily of receptors and especially 2B4 on human eosinophils. In this study we focus on CD48, the high-affinity ligand of 2B4. CD48 is a GPI-anchored protein involved in cellular activation, costimulation, and adhesion, but has not been studied on eosinophils. We demonstrate that human eosinophils from atopic asthmatics display enhanced levels of CD48 expression and that IL-3 up-regulates CD48 expression. Furthermore, cross-linking CD48 on human eosinophils triggers release of eosinophil granule proteins. Assessment of CD48 expression in a murine model of experimental asthma revealed that CD48 is induced by allergen challenge and partially regulated by IL-3. Additionally, anti-IL-3 reduces CD48 expression and the degree of airway inflammation. Thus, CD48 is an IL-3-induced activating receptor on eosinophils, likely involved in promoting allergic inflammation.

Differential secretion of cytokines

Eosinophil granules store a vast array of cytokines and chemokines, many of which possess opposing activities. Specific stimuli can induce the release of entire granules or selective mediators through a process termed piecemeal degranulation. Until recently, the mechanisms that governed the decision to opt for either of these processes were unknown. Recent research has identified a mechanism through which differential secretion occurs during piecemeal degranulation. Eotaxin stimulation of eosinophils induces the selective mobilization of the granule-stored alpha chain of the interleukin-4 (IL-4) receptor into secretory vesicles. This process selectively recruits IL-4 to these vesicles and facilitates its differential secretion. There is also recent evidence for a mechanism of differential mobilization and membrane fusion of secretory vesicles versus granules. These two compartments possess a different set of SNARE and Rab molecules as vesicle fusion and transport-docking proteins, respectively. This presumably allows differential regulation of the processes of mobilization and plasma membrane fusion. These findings provide a model to explain the mechanism by which eosinophils, and likely many other cell types, differentially secrete cytokines and chemokines.

Nitroproteins from a human pituitary adenoma tissue discovered with a nitrotyrosine affinity column and tandem mass spectrometry

The aim of this study was to characterize endogenous nitroproteins, and those proteins that interact with nitroproteins, in a human pituitary nonfunctional adenoma so as to clarify the role of protein nitration in adenomas. A nitrotyrosine affinity column (NTAC) was used to preferentially enrich and isolate endogenous nitroproteins and nitroprotein-protein complexes from a tissue homogenate that was prepared from a human pituitary nonfunctional pituitary adenoma. The preferentially enriched endogenous nitroproteins and nitroprotein-protein complexes were subjected to trypsin digestion, desalination, and tandem mass spectrometry analysis. Nine nitroproteins (Rho-GTPase-activing protein 5, leukocyte immunoglobulin-like receptor subfamily A member 4 precursor, zinc finger protein 432, cAMP-dependent protein kinase type I-beta regulatory subunit, sphingosine-1-phosphate lyase 1, centaurin beta 1, proteasome subunit alpha type 2, interleukin 1 family member 6, and rhophilin 2) and three proteins (interleukin 1 receptor-associated kinase-like 2, glutamate receptor-interacting protein 2, and ubiquitin) that interacted with nitroproteins were discovered. The nitration site of each nitroprotein was located onto the functional domain where nitration occurred, and each nitroprotein was related to a corresponding functional system. Those data indicate that protein nitration might be an important molecular event in the formation of a human pituitary nonfunctional adenoma.

A Common Site of the Fc Receptor γ Subunit Interacts with the Unrelated Immunoreceptors FcαRI and FcϵRI

The transmembrane (TM) region of the Fc receptor-gamma (FcRgamma) chain is responsible for the association of this ubiquitous signal transduction subunit with many immunoreceptor ligand binding chains, making FcRgamma key to a number of leukocyte activities in immunity and disease. Some receptors contain a TM arginine residue that interacts with Asp-11 of the FcRgamma subunit, but otherwise the molecular basis for the FcRgamma subunit interactions is largely unknown. This study reports residues in the TM region of the FcRgamma subunit are important for association with the high affinity IgE receptor FcepsilonRI and a leukocyte receptor cluster member, the IgA receptor FcalphaRI. FcRgamma residue Leu-21 was essential for surface expression of FcepsilonRIalpha/gamma2 and Tyr-8, Leu-14, and Phe-15 contributed to expression. Likewise, detergent-stable FcRgamma association with FcalphaRI was also dependent on Leu-14 and Leu-21 and in addition required residues Tyr-17, Tyr-25, and Cys-26. Modeling the TM regions of the FcRgamma dimer indicated these residues interacting with both FcalphaRI and FcepsilonRI are near the interface between the two FcRgamma TM helices. Furthermore, the FcRgamma residues interacting with FcalphaRI form a leucine zipper-like interface with mutagenesis confirming a complementary interface comprising FcalphaRI residues Leu-217, Leu-220, and Leu-224. The dependence of these two nonhomologous receptor interactions on FcRgamma Leu-14 and Leu-21 suggests that all the associated Fc receptors and the activating leukocyte receptor cluster members interact with this one site. Taken together these data provide a molecular basis for understanding how disparate receptor families assemble with the FcRgamma subunit.

Plasmacytoid dendritic cell-specific receptor ILT7-Fc epsilonRI gamma inhibits Toll-like receptor-induced interferon production

Immunoglobulin-like transcripts are a family of inhibitory and stimulatory cell surface immune receptors. Transcripts for one member of this family, ILT7, are selectively expressed in human plasmacytoid dendritic cells (pDCs). We demonstrate here that ILT7 protein associates with the signal adapter protein Fc epsilonRI gamma to form a receptor complex. Using an anti-ILT7 monoclonal antibody, we show that ILT7 is expressed specifically on human pDCs, but not on myeloid dendritic cells or other peripheral blood leukocytes. Cross-linking of ILT7 resulted in phosphorylation of Src family kinases and Syk kinase and induced a calcium influx in freshly isolated pDCs, which was blocked by Src family and Syk kinases inhibitors, thus indicating the activation of an immunoreceptor-based tyrosine activation motif-mediated signaling pathway. ILT7 cross-linking on CpG or influenza virus-stimulated primary pDCs inhibited the transcription and secretion of type I interferon and other cytokines. Therefore, the ILT7-Fc epsilonRI gamma receptor complex negatively regulates the innate immune functions of human pDCs.

Inhibition of inflammatory responses by leukocyte Ig-like receptors

The immune system must effectively regulate the balance between beneficial and detrimental inflammation. This process is achieved in part through cell surface receptors that rapidly integrate activating and inhibitory signals. The inhibitory members of the leukocyte Ig-like receptor (LILR) family, termed LILRBs, are broadly distributed among cell populations in the immune system and potently counterregulate cell activation induced by stimuli of innate and adaptive immune responses. Studies in mice and humans indicate that LILRBs appreciably downregulate harmful inflammatory responses induced by microbial, allergic, and cytotoxic mechanisms. Hence, the LILRBs likely play significant roles in regulating the incidence and severity of many inflammatory diseases, making them potential targets for therapeutic interventions.

Mechanisms of eosinophil cytokine release

Human eosinophils have been demonstrated to contain a multitude of cytokines and chemokines that exist pre-formed within these cells. This content of pre-formed cytokines, with diverse potential biologic activities, provides eosinophils with capabilities distinct from most other leukocytes. The localization of pre-formed cytokines within eosinophils is both within specific granules and associated with substantial numbers of morphologically distinct cytoplasmic vesicles. Stimulation for release of specific cytokines, such as IL-4, leads to a regulated signal transduction cascade, which is dependent on the formation of leukotriene C4 within eosinophils where it acts as an intracrine mediator. IL-4 release occurs selectively and is by means of vesicular transport. The capabilities of eosinophils not only to rapidly release pre-formed cytokines but also to differentially regulate which cytokines are released endow eosinophils with distinct abilities in innate and acquired immunity.

Allergic challenge-elicited lipid bodies compartmentalize in vivo leukotriene C4 synthesis within eosinophils

Eosinophils are an important source of leukotriene (LT)C(4), which can be synthesized within lipid bodies-cytoplasmic organelles where eicosanoid formation may take place. Allergy-driven lipid body formation and function have never been investigated. Here, we studied the in vivo induction and role of lipid bodies within eosinophils recruited to sites of allergic inflammation. Using two murine models of allergic inflammation (asthma and pleurisy), we verified that parallel to the eosinophil influx, allergic challenge also induced lipid body formation within recruited eosinophils. Neutralizing antibodies to eotaxin/CCL11, RANTES/CCL5, or CCR3 partially inhibited lipid body formation within recruited eosinophils in the allergic pleurisy model. Likewise, intrapleural administration of RANTES or eotaxin also induced significant influx of eosinophils loaded with lipid bodies. By immunolabeling, we detected the presence of a key enzyme involved in the leukotriene metabolism-5-lipoxygenase-within eosinophil lipid bodies formed in vivo after allergen challenge. Furthermore, specific immunolocalization of newly formed LTC(4) demonstrated that lipid bodies were the sites of formation of this eicosanoid within infiltrating eosinophils. Therefore, allergic inflammation triggers in vivo formation of new lipid bodies within infiltrating eosinophils, a phenomenon largely mediated by eotaxin/RANTES acting via CCR3 receptors. Such in vivo allergen-driven lipid bodies function as intracellular compartments of LTC(4) synthesis.

The LILR family: modulators of innate and adaptive immune pathways in health and disease

Leukocyte immunoglobulin (Ig)-like receptors [LILRs, also known as Ig-like transcripts (ILTs)] are a family of inhibitory and stimulatory receptors encoded within the leukocyte receptor complex and are expressed by immune cell types of both myeloid and lymphoid lineage. Several members of the LILR family recognize major histocompatibility complex class I. The immunomodulatory role of LILR receptors indicates that they may exert an influence on signaling pathways of both innate and adaptive immune systems. LILR activity can also influence the antigen-presenting properties of macrophages and dendritic cells and may thus play a role in T-cell tolerance. The wide-ranging effects of LILR signaling on immune cell activity imply that these receptors are likely to play an important role in a range of clinical situations including pregnancy, transplantation, and arthritis (including the human leukocyte antigen B27-associated spondyloarthropathies). In this review, we summarize current knowledge on the nature and function of LILRs, focusing on their regulation of immune cell activity and their potential role in disease.

2B4 (CD244) is expressed and functional on human eosinophils

Eosinophils are present in parasitic, allergic, various immunological, and malignant disorders as well as in a variety of idiopathic hypereosinophilic syndromes. However, their exact role in some of these conditions remains elusive. They can be activated both in vivo and in vitro by various agonists, such as Igs, lipid mediators, and cytokines. By phenotyping the surface of the eosinophils, it may be possible to better define their function(s) in different pathophysiological settings. In the present work we screened eosinophils with a panel of Abs recognizing CD2 subfamily receptors usually present on a number of hemopoietic cells. We have demonstrated that human peripheral blood eosinophils, but not basophils or neutrophils, express NTB-A. In addition eosinophils express 2B4, CD84, CD58, and CD48, but not signaling lymphocytic activation molecule or CD2, on their surface (FACS). Cross-linking of 2B4 on eosinophils elicited a significant release of eosinophil peroxidase (30 min), IFN-gamma, and IL-4 (18 h). Moreover, activation of eosinophils via 2B4 induced eosinophil-mediated cytotoxicity toward two malignant cell lines, i.e., mouse mastocytoma P815 and EBV-infected 721.221 B cell lines. Cross-linking of 2B4 on the surface of eosinophils or pervenadate treatment elicited ERK and tyrosine phosphorylation, respectively. Furthermore, we showed that eosinophils express slam-associated protein. The demonstration that human eosinophils express a functional 2B4 receptor indicates a broader role for these cells in health and disease.

Heterogeneous expression of apolipoprotein-E by human macrophages

Apolipoprotein-E (apoE) is expressed at high levels by macrophages. In addition to its role in lipid transport, macrophage-derived apoE plays an important role in immunoregulation. Previous studies have identified macrophage subpopulations that differ substantially in their ability to synthesize specific cytokines and enzymes, however, potential heterogeneous macrophage apoE expression has not been studied. Here we examined apoE expression in human THP-1 macrophages and monocyte-derived macrophages (MDM). Using immunocytochemistry and flow cytometry methods we reveal a striking heterogeneity in macrophage apoE expression in both cell types. In phorbol-ester-differentiated THP-1 macrophages, 5% of the cells over-expressed apoE at levels more than 50-fold higher than the rest of the population. ApoE over-expressing THP-1 macrophages contained condensed/fragmented nuclei and increased levels of activated caspase-3 indicating induction of apoptosis. In MDM, 3-5% of the cells also highly over-expressed apoE, up to 50-fold higher than the rest of the population; however, this was not associated with obvious nuclear alterations. The apoE over-expressing MDM were larger, more granular, and more autofluorescent than the majority of cells and they contained numerous vesicle-like structures that appeared to be coated by apoE. Flow cytometry experiments indicated that the apoE over-expressing subpopulation of MDM were positive for CD14, CD11b/Mac-1 and CD68. These observations suggest that specific macrophage subpopulations may be important for apoE-mediated immunoregulation and clearly indicate that subpopulation heterogeneity should be taken into account when investigating macrophage apoE expression.

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

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

Eosinophils: 'new' roles for 'old' cells

Prominent blood and tissue eosinophilia is manifested in a number of inflammatory states, particularly in allergic diseases. Eosinophils are a source of numerous cytokines and growth factors, thus in principle they can display both pro-inflammatory and anti-inflammatory activities as well as immunoregulatory ones. In this review, we will discuss the cross-talk between eosinophils and other cell types that they come in contact with in the inflammatory milieu, such as mast cells, fibroblasts and endothelial cells. 'New' roles for eosinophils in cancer and novel activatory signals will also be described.

5-lipoxygenase and FLAP

The initial steps in the biosynthesis of leukotrienes from arachidonic acid are carried out by the enzyme 5-lipoxygenase (5-LO). In intact cells, the helper protein 5-LO activating protein (FLAP) is necessary for efficient enzyme utilization of endogenous substrate. The last decade has witnessed remarkable progress in our understanding of these two proteins. Here we review the molecular and cellular aspects of the expression, function, and regulation of 5-LO and FLAP.

Eosinophils and cysteinyl leukotrienes

Eosinophils are the main source of the cysteinyl leukotrienes, LTC(4)/D(4)/E(4), which are lipid mediators that play major roles in the pathogenesis of asthma and other forms of allergic inflammation. Here, we review the mechanisms governing eosinophil LTC(4) synthesis, focusing on the distinct intracellular domains that regulate eicosanoid formation and function within eosinophils. Cysteinyl leukotrienes exert their actions by engaging specific receptors. As recently shown, eosinophils express CysLT1 and CysLT2, the only cloned receptors for cysteinyl leukotrienes. Therefore, here we also present some of the new findings regarding the paracrine/autocrine activation of these CysLT receptors on eosinophils, and discuss some data on novel intracrine effects of LTC(4) triggered by a putative third CysLT receptor expressed intracellularly within eosinophils.