Eosinophil beta 1 integrin activation state correlates with asthma activity in a blind study of inhaled corticosteroid withdrawal

Quality over quantity; eosinophil activation status will deepen the insight into eosinophilic diseases

Eosinophil associated diseases have gained much attention recently because of the introduction of specific eosinophil targeted therapies. These diseases range from acute parasitic infections to chronic inflammatory diseases such as eosinophilic asthma. In eosinophilic asthma an increased eosinophil cell count in peripheral blood is the gold standard for determination of the pheno-/endotype and severity of disease. Despite a broad consensus there is concern on validity of this simple measurement, because the eosinophil compartment is far from homogenous. Multiple tissues harbour non-activated cells under homeostatic conditions and other tissues, normally devoid of eosinophils, become infested with these cells under inflammatory conditions. It will, therefore, be clear that eosinophils become differentially (pre)-activated at different tissue sites in homeostatic and inflammatory conditions. This complexity should be investigated in detail as it is 1) far from clear what the long-term side effects are that are caused by application of eosinophil targeted therapies in a "one size fits all" concept and 2) real-world data of eosinophil targeted therapies in asthma shows a broad variety in the treatment response. This review will focus on complex mechanisms of eosinophil activation in vivo to create a better view on the dynamics of the eosinophil compartment in health and disease both to prevent collateral damage caused by aberrant activation of eosinophils ánd to improve effectiveness of eosinophil targeted treatments.

Characterization of Siglec-8 Expression on Lavage Cells after Segmental Lung Allergen Challenge

BACKGROUND

Siglec-8 is present at a high level on human blood eosinophils and low level on blood basophils. Engagement of Siglec-8 on blood eosinophils causes its internalization and results in death. Siglec-8 is a potential therapeutic target in eosinophilic asthma.

OBJECTIVES

The aim of this study was to determine Siglec-8 levels on eosinophils and basophils recruited during lung inflammation.

METHOD

We analyzed surface Siglec-8 by flow cytometry on cells obtained by bronchoalveolar lavage (BAL) 48 h after segmental lung allergen challenge of human subjects with mild allergic asthma and used confocal microscopy to compare Siglec-8 distribution on BAL and blood eosinophils.

RESULTS

Like their blood counterparts, BAL eosinophils had high unimodal surface Siglec-8, while BAL basophils had lower but detectable surface Siglec-8. BAL macrophages, monocytes, neutrophils, and plasmacytoid dendritic cells did not express surface Siglec-8. Microscopy of freshly isolated blood eosinophils demonstrated homogeneous Siglec-8 distribution over the cell surface. Upon incubation with IL-5, Siglec-8 on the surface of eosinophils became localized in patches both at the nucleopod tip and at the opposite cell pole. BAL eosinophils also had a patchy Siglec-8 distribution.

CONCLUSIONS

We conclude that 48 h after segmental allergen challenge, overall levels of Siglec-8 expression on airway eosinophils resemble those on blood eosinophils, but with a patchier distribution, a pattern consistent with activation. Thus, therapeutic targeting of Siglec-8 has the potential to impact blood as well as lung eosinophils, which may be associated with an improved outcome in eosinophilic lung diseases.

The current and future role of biomarkers in type 2 cytokine-mediated asthma management

Assessment and management of asthma is complicated by the heterogeneous pathophysiological mechanisms that underlie its clinical presentation, which are not necessarily reflected in standardized management paradigms and which necessitate an individualized approach to treatment. This is particularly important with the emerging availability of a variety of targeted forms of therapy that may only be appropriate for use in particular patient subgroups. The identification of biomarkers can potentially aid diagnosis and inform prognosis, help guide treatment decisions and allow clinicians to predict and monitor response to treatment. Biomarkers for asthma have been identified from a variety of sources, including airway, exhaled breath and blood. Biomarkers from exhaled breath include fractional exhaled nitric oxide, measurement of which can help identify patients most likely to benefit from inhaled corticosteroids and targeted anti-immunoglobulin E therapy. Biomarkers measured in blood are relatively non-invasive and technically more straightforward than those measured from exhaled breath or directly from the airway. The most well established of these are the blood eosinophil count and serum periostin, both of which have demonstrated utility in identifying patients most likely to benefit from targeted anti-interleukin and anti-immunoglobulin E therapies, and in monitoring subsequent treatment response. For example, serum periostin appears to be a biomarker for responsiveness to inhaled corticosteroid therapy and may help identify patients as suitable candidates for anti-IL-13 treatment. The use of biomarkers can therefore potentially help avoid unnecessary morbidity from high-dose corticosteroid therapy and allow the most appropriate and cost-effective use of targeted therapies. Ongoing clinical trials are helping to further elucidate the role of established biomarkers in routine clinical practice, and a range of other circulating novel potential biomarkers are currently being investigated in the research setting.

Eosinophil Activation Status in Separate Compartments and Association with Asthma

Asthma is frequently characterized by eosinophil-rich airway inflammation. Airway eosinophilia is associated with asthma exacerbations and likely plays a part in airway remodeling. Eosinophil recruitment from the bloodstream depends on circulating eosinophils becoming activated, which leads to eosinophil arrest on activated endothelium, extravasation, and continued movement through the bronchial tissue by interaction with the extracellular matrix (ECM). Circulating eosinophils can exist at different activation levels, which include non-activated or pre-activated (sensitized or “primed”). Further, the bloodstream may lack pre-activated cells, due to such eosinophils having arrested on endothelium or extravasated into tissue. Increased expression, and in some instances, decreased expression of cell-surface proteins, including CD44, CD45, CD45R0, CD48, CD137, neuropeptide S receptor, cytokine receptors, Fc receptors, and integrins (receptors mediating cell adhesion and migration by interacting with ligands on other cells or in the ECM), and activated states of integrins or Fc receptors on blood eosinophils have been reported to correlate with aspects of asthma. A subset of these proteins has been reported to respond to intervention, e.g., with anti-interleukin (IL)-5. How these surface proteins and the activation state of the eosinophil respond to other interventions, e.g., with anti-IL-4 receptor alpha or anti-IL-13, is unknown. Eosinophil surface proteins suggested to be biomarkers of activation, particularly integrins, and reports on correlations between eosinophil activation and aspects of asthma are described in this review. Intermediate activation of beta1 and beta2 integrins on circulating eosinophils correlates with decreased pulmonary function, airway inflammation, or airway lumen eosinophils in non-severe asthma. The correlation does not appear in severe asthma, likely due to a higher degree of extravasation of pre-activated eosinophils in more severe disease. Bronchoalveolar lavage (BAL) eosinophils have highly activated integrins and other changes in surface proteins compared to blood eosinophils. The activation state of eosinophils in lung tissue, although likely very important in asthma, is largely unknown. However, some recent articles, mainly on mice but partly on human cells, indicate that tissue eosinophils may have a surface phenotype(s) different from that of sputum or BAL eosinophils.

Activation states of blood eosinophils in asthma

Asthma is characterized by airway inflammation rich in eosinophils. Airway eosinophilia is associated with exacerbations and has been suggested to play a role in airway remodelling. Recruitment of eosinophils from the circulation requires that blood eosinophils become activated, leading to their arrest on the endothelium and extravasation. Circulating eosinophils can be envisioned as potentially being in different activation states, including non-activated, pre-activated or 'primed', or fully activated. In addition, the circulation can potentially be deficient of pre-activated or activated eosinophils, because such cells have marginated on activated endothelium or extravasated into the tissue. A number of eosinophil surface proteins, including CD69, L-selectin, intercellular adhesion molecule-1 (ICAM-1, CD54), CD44, P-selectin glycoprotein ligand-1 (PSGL-1, CD162), cytokine receptors, Fc receptors, integrins including αM integrin (CD11b), and activated conformations of Fc receptors and integrins, have been proposed to report cell activation. Variation in eosinophil activation states may be associated with asthma activity. Eosinophil surface proteins proposed to be activation markers, with a particular focus on integrins, and evidence for associations between activation states of blood eosinophils and features of asthma are reviewed here. Partial activation of β1 and β2 integrins on blood eosinophils, reported by monoclonal antibodies (mAbs) N29 and KIM-127, is associated with impaired pulmonary function and airway eosinophilia, respectively, in non-severe asthma. The association with lung function does not occur in severe asthma, presumably due to greater eosinophil extravasation, specifically of activated or pre-activated cells, in severe disease.

Clinical utility of asthma biomarkers: from bench to bedside

Asthma is a chronic disease characterized by airway inflammation, bronchial hyperresponsiveness, and recurrent episodes of reversible airway obstruction. The disease is very heterogeneous in onset, course, and response to treatment, and seems to encompass a broad collection of heterogeneous disease subtypes with different underlying pathophysiological mechanisms. There is a strong need for easily interpreted clinical biomarkers to assess the nature and severity of the disease. Currently available biomarkers for clinical practice - for example markers in bronchial lavage, bronchial biopsies, sputum, or fraction of exhaled nitric oxide (FeNO) - are limited due to invasiveness or lack of specificity. The assessment of markers in peripheral blood might be a good alternative to study airway inflammation more specifically, compared to FeNO, and in a less invasive manner, compared to bronchoalveolar lavage, biopsies, or sputum induction. In addition, promising novel biomarkers are discovered in the field of breath metabolomics (eg, volatile organic compounds) and (pharmaco)genomics. Biomarker research in asthma is increasingly shifting from the assessment of the value of single biomarkers to multidimensional approaches in which the clinical value of a combination of various markers is studied. This could eventually lead to the development of a clinically applicable algorithm composed of various markers and clinical features to phenotype asthma and improve diagnosis and asthma management.

Anti-IL-5 attenuates activation and surface density of β(2) -integrins on circulating eosinophils after segmental antigen challenge

BACKGROUND

IL-5 activates α(M) β(2) integrin on blood eosinophils in vitro. Eosinophils in bronchoalveolar lavage (BAL) following segmental antigen challenge have activated β(2) -integrins.

OBJECTIVE

To identify roles for IL-5 in regulating human eosinophil integrins in vivo.

METHODS

Blood and BAL eosinophils were analysed by flow cytometry in ten subjects with allergic asthma who underwent a segmental antigen challenge protocol before and after anti-IL-5 administration.

RESULTS

Blood eosinophil reactivity with monoclonal antibody (mAb) KIM-127, which recognizes partially activated β(2) -integrins, was decreased after anti-IL-5. Before anti-IL-5, surface densities of blood eosinophil β(2) , α(M) and α(L) integrin subunits increased modestly post challenge. After anti-IL-5, such increases did not occur. Before or after anti-IL-5, surface densities of β(2) , α(M) , α(L) and α(D) and reactivity with KIM-127 and mAb CBRM1/5, which recognizes high-activity α(M) β(2) , were similarly high on BAL eosinophils 48 h post-challenge. Density and activation state of β(1) -integrins on blood and BAL eosinophils were not impacted by anti-IL-5, even though anti-IL-5 ablated a modest post-challenge increase on blood or BAL eosinophils of P-selectin glycoprotein ligand-1 (PSGL-1), a receptor for P-selectin that causes activation of β(1) -integrins. Forward scatter of blood eosinophils post-challenge was less heterogeneous and on the average decreased after anti-IL-5; however, anti-IL-5 had no effect on the decreased forward scatter of eosinophils in post-challenge BAL compared with eosinophils in blood. Blood eosinophil KIM-127 reactivity at the time of challenge correlated with the percentage of eosinophils in BAL post-challenge.

CONCLUSION AND CLINICAL RELEVANCE

IL-5 supports a heterogeneous population of circulating eosinophils with partially activated β(2) -integrins and is responsible for up-regulation of β(2) -integrins and PSGL-1 on circulating eosinophils following segmental antigen challenge but has minimal effects on properties of eosinophils in BAL. Dampening of β(2) -integrin function of eosinophils in transit to inflamed airway may contribute to the decrease in lung inflammation caused by anti-IL-5.

α(M)β(2) integrin-mediated adhesion and motility of IL-5-stimulated eosinophils on periostin

Periostin is an extracellular matrix protein that is up-regulated by T helper cell type 2 cytokines in the asthmatic airway and implicated in mouse studies as promoting eosinophil recruitment. We asked whether periostin modulates eosinophil adhesion and motility in vitro. Periostin adsorbed to polystyrene supported adhesion of purified human blood eosinophils stimulated by IL-5, IL-3, or granulocyte/macrophage colony-stimulating factor, but did not support adhesion of eosinophils treated with IL-4 or IL-13. The degree of adhesion depended on the concentrations of periostin during coating and activating cytokine during the adhesion assay. Both full-length periostin and alternatively spliced periostin, lacking C-terminal exons 17, 18, 19, and 21, supported adhesion. Adhesion was inhibited by monoclonal antibody to α(M) or β(2) integrin subunits, but not by antibodies to other eosinophil integrin subunits. Adsorbed periostin also supported α(M)β(2)-dependent random motility of IL-5-stimulated eosinophils with optimal movement at an intermediate coating concentration. In the presence of IL-5, eosinophils adherent on periostin formed punctate structures positive for filamentous actin, gelsolin, and phosphotyrosine. These structures fit the criteria for podosomes, highly dynamic adhesive contacts that are distinct from classical focal adhesions. The results establish α(M)β(2) (CD11b/CD18, Mac-1) as an adhesive and promigratory periostin receptor on cytokine-stimulated eosinophils, and suggest that periostin may function as a haptotactic stimulus able to guide eosinophils to areas of high periostin density in the asthmatic airway.

Integrin activation States and eosinophil recruitment in asthma

Eosinophil arrest and recruitment to the airway in asthma are mediated, at least in part, by integrins. Eosinophils express α4β1, α6β1, αLβ2, αMβ2, αXβ2, αDβ2, and α4β7 integrins, which interact with counter-receptors on other cells or ligands in the extracellular matrix. Whether a given integrin-ligand pair mediates cell adhesion and migration depends on the activation state of the integrin. Integrins exist in an inactive bent, an intermediate-activity extended closed, and a high-activity extended open conformation. Integrin activation states can be monitored by conformation-specific monoclonal antibodies (mAbs). Studies in mice indicate that both β1 and β2 integrins mediate eosinophil recruitment to the lung. In vitro studies indicate that α4β1 and αMβ2 are the principal integrins mediating eosinophil adhesion, including to vascular cell adhesion molecule-1 and the novel αMβ2 ligand periostin. In vivo, blood eosinophils have intermediate-activity β1 integrins, as judged by mAb N29, apparently resulting from eosinophil binding of P-selectin on the surface of activated platelets, and have a proportion of their β2 integrins in the intermediate conformation, as judged by mAb KIM-127, apparently due to exposure to low concentrations of interleukin-5 (IL-5). Airway eosinophils recovered by bronchoalveolar lavage (BAL) after segmental antigen challenge have high-activity β1 integrins and high-activity αMβ2 that does not require IL-5. Here we review information on how the activation states of eosinophil β1 and β2 integrins correlate with measurements of eosinophil recruitment and pulmonary function in asthma. Blood eosinophil N29 reactivity is associated with decreased lung function under various circumstances in non-severe asthma and KIM-127 with BAL eosinophil numbers, indicating that intermediate-activity α4β1 and αMβ2 of blood eosinophils are important for eosinophil arrest and consequently for recruitment and aspects of asthma.

Biomarkers of therapy responsiveness in asthma: pitfalls and promises

Asthma is one of the most common chronic diseases worldwide. There is a large inter-individual variability in response to asthma treatment. Most patients respond well to standard therapy; however, a small proportion of the patients remain symptomatic despite treatment with high dosages of corticosteroids. Uncontrolled asthma leads to a decreased quality of life. Therefore, it is important to identify individuals who will respond poorly to standard asthma medication, especially to standard maintenance therapy with inhaled corticosteroids, at an early stage. Response to anti-inflammatory therapy is generally monitored by the assessment of clinical symptoms, which only partially correlates with underlying airway inflammation. The identification of specific inflammatory biomarkers might help to guide treatment or predict a corticosteroid response more accurately. Some inflammatory biomarkers are already finding their way into clinical practice (e.g. fraction of nitric oxide in exhaled breath), whereas others are predominantly used as a research tool (e.g. profiles of volatile organic compounds). Currently, there is no inflammatory biomarker used in routine clinical practice to predict a corticosteroid response. More knowledge on the underlying biological mechanism(s) of heterogeneous therapeutic responses could help to identify novel biomarkers. This review will focus on inflammatory patterns and genetic variations that may underlie differences in treatment response in patients with asthma, and will provide an overview of inflammatory biomarkers that could potentially serve as response predictors.

Biomarkers of eosinophil involvement in allergic and eosinophilic diseases: review of phenotypic and serum markers including a novel assay to quantify levels of soluble Siglec-8

There remains considerable controversy in the management of eosinophilic disorders, mainly due to a paucity of information regarding the clinical interpretation of total blood eosinophil counts versus surface activation markers versus eosinophil-derived or eosinophil-influencing mediator levels. Regrettably, few tests have been validated that define a unique clinical or prognostic phenotype that is more useful than simply monitoring total blood eosinophil counts. In this manuscript, phenotypic (cell surface) markers, along with serum and tissue-based markers that have been examined in the context of disease activity, are reviewed. We also report the development of a novel assay for detecting soluble Siglec-8 (sSiglec-8), a protein likely derived largely from eosinophils, as a potential serum biomarker. The assay consists of a competitive ELISA using a recombinant Siglec-8-Fc fusion protein. The goal of this preliminary study was to determine if sSiglec-8 is a useful biomarker that differentiates among patients with various eosinophil-associated diseases. In the final analysis, it is fair to say that further research is sorely needed to fully understand and validate the utility of various biomarkers, including sSiglec-8, before their use in clinical practice can be recommended with confidence.

Allosteric modulation of beta1 integrin function induces lung tissue repair

The cellular cytoskeleton, adhesion receptors, extracellular matrix composition, and their spatial distribution are together fundamental in a cell's balanced mechanical sensing of its environment. We show that, in lung injury, extracellular matrix-integrin interactions are altered and this leads to signalling alteration and mechanical missensing. The missensing, secondary to matrix alteration and cell surface receptor alterations, leads to increased cellular stiffness, injury, and death. We have identified a monoclonal antibody against β1 integrin which caused matrix remodelling and enhancement of cell survival. The antibody acts as an allosteric dual agonist/antagonist modulator of β1 integrin. Intriguingly, this antibody reversed both functional and structural tissue injury in an animal model of degenerative disease in lung.

Platelet activation, P-selectin, and eosinophil β1-integrin activation in asthma

RATIONALE

Eosinophil β1-integrin activation correlates inversely with FEV1 and directly with eosinophil-bound P-selectin in subjects with nonsevere allergic asthma.

OBJECTIVES

Determine the relationships between β1-integrin activation and pulmonary function or eosinophil-bound P-selectin in subjects with asthma of varying severity and discern the source of eosinophil-bound P-selectin.

METHODS

Blood was assayed by flow cytometry for P-selectin and activated β1-integrin on eosinophils and platelets. Plasma was analyzed with ELISA for soluble P-selectin, platelet factor 4, and thrombospondin-1.

MEASUREMENTS AND MAIN RESULTS

Activated β1-integrin correlated with eosinophil-bound P-selectin among all subjects with asthma even though activated β1-integrin was higher in subjects with nonsevere asthma than severe asthma. Activated β1-integrin correlated inversely with FEV1 corrected for FVC only in younger subjects with nonsevere asthma. Paradoxically, platelet surface P-selectin, a platelet activation marker, was low in subjects with severe asthma, whereas plasma platelet factor 4, a second platelet activation marker, was high. Correlations indicated that P-selectin-positive platelets complexed to eosinophils are the major source of the eosinophil-bound P-selectin associated with β1-integrin activation. After whole-lung antigen challenge of subjects with nonsevere asthma, a model of asthma exacerbation known to cause platelet activation, circulating eosinophils bearing P-selectin and activated β1-integrin disappeared.

CONCLUSIONS

The relationship between eosinophil β1-integrin activation and pulmonary function was replicated only for younger subjects with nonsevere asthma. However, we infer that platelet activation and binding of activated platelets to eosinophils followed by P-selectin-mediated eosinophil β1-integrin activation occur in both nonsevere and severe asthma with rapid movement of platelet-eosinophil complexes into the lung in more severe disease.

Activation of beta1 integrins on blood eosinophils by P-selectin

Activation of β(1) integrins of blood eosinophils, assessed by mAb N29, correlates inversely with FEV(1) in two paradigms for studying control of human asthma. We asked whether P-selectin causes eosinophil β(1) integrin activation and results in increased adhesivity. By dual-label flow cytometry, eosinophils with high levels of surface-associated P-selectin had higher reactivity with the activation-sensitive anti-β(1) mAbs N29, 8E3, and 9EG7 than eosinophils with no or with a low-level of surface-associated P-selectin. Among patients with nonsevere asthma, surface P-selectin correlated with N29, 8E3, and 9EG7 signals. By immunofluorescence microscopy, surface-associated P-selectin was present in patches on eosinophils, some of which stained for the platelet marker thrombospondin-1. Activated β(1) and P-selectin partially colocalized on eosinophils. Soluble P-selectin added to whole blood enhanced activation of eosinophil β(1), but not β(2), integrins. In contrast, IL-5 activated eosinophil β(2), but not β(1), integrins. Eosinophils that did not attach to vascular cell adhesion molecule-1 (VCAM-1) in a static adhesion assay had a lower N29 signal than the original population. Soluble P-selectin added to whole blood enhanced eosinophil adhesion to VCAM-1. These findings are compatible with a scenario whereby P-selectin, on eosinophil-associated activated platelets or acquired from plasma or from prior interactions with endothelial cells or platelets, activates eosinophil α(4)β(1) integrin and stimulates eosinophils to adhere to VCAM-1 and move to the airway in asthma.

Up-regulation and activation of eosinophil integrins in blood and airway after segmental lung antigen challenge

We hypothesized that there are clinically relevant differences in eosinophil integrin expression and activation in patients with asthma. To evaluate this, surface densities and activation states of integrins on eosinophils in blood and bronchoalveolar lavage (BAL) of 19 asthmatic subjects were studied before and 48 h after segmental Ag challenge. At 48 h, there was increased expression of alpha(D) and the N29 epitope of activated beta(1) integrins on blood eosinophils and of alpha(M), beta(2), and the mAb24 epitope of activated beta(2) integrins on airway eosinophils. Changes correlated with the late-phase fall in forced expiratory volume in 1 s (FEV(1)) after whole-lung inhalation of the Ag that was subsequently used in segmental challenge and were greater in subjects defined as dual responders. Increased surface densities of alpha(M) and beta(2) and activation of beta(2) on airway eosinophils correlated with the concentration of IL-5 in BAL fluid. Activation of beta(1) and beta(2) on airway eosinophils correlated with eosinophil percentage in BAL. Thus, eosinophils respond to an allergic stimulus by activation of integrins in a sequence that likely promotes eosinophilic inflammation of the airway. Before challenge, beta(1) and beta(2) integrins of circulating eosinophils are in low-activation conformations and alpha(D)beta(2) surface expression is low. After Ag challenge, circulating eosinophils adopt a phenotype with activated beta(1) integrins and up-regulated alpha(D)beta(2), changes that are predicted to facilitate eosinophil arrest on VCAM-1 in bronchial vessels. Finally, eosinophils present in IL-5-rich airway fluid have a hyperadhesive phenotype associated with increased surface expression of alpha(M)beta(2) and activation of beta(2) integrins.

Homeostatic intracellular-free Ca2+ is permissive for Rap1-mediated constitutive activation of alpha4 integrins on eosinophils

Although much progress has been made in understanding the molecular mechanisms underlying agonist-induced "inside-out" activation of integrins, little is known about how basal levels of integrin function are maintained. This is particularly important for nonactivated eosinophils, where intermediate activation of alpha(4)beta(1) integrin supports recruitment to endothelial cells under flow conditions. Depletion of intracellular Ca(2+) and pharmacological inhibition of phospholipase C (but not other intracellular signaling molecules, including PI3K, ERK1/2, p38 MAPK, and tyrosine kinase activity) abrogated basal alpha(4) integrin activity in nonactivated eosinophils. Basal alpha(4) integrin activation was associated with activation of the small GTPase Rap1, a known regulator of agonist-induced integrin function. Basal Rap activation was dependent upon phospholipase C, but not intracellular Ca(2+). However, depletion of intracellular Ca(2+) in CD34(+) hematopoietic progenitor cells abolished RapV12-mediated induction of alpha(4) integrin activity. Thus, residual Rap activity or constitutively active Rap activity in Ca(2+)-depleted cells is not sufficient to induce alpha(4) integrin activation. These data suggest that activation of functional alpha(4) integrin activity in resting eosinophils is mediated by Rap1 provided that the intracellular-free Ca(2+) is at a normal homeostatic concentration.

Roles of integrin activation in eosinophil function and the eosinophilic inflammation of asthma

Eosinophilic inflammation is a characteristic feature of asthma. Integrins are highly versatile cellular receptors that regulate extravasation of eosinophils from the postcapillary segment of the bronchial circulation to the airway wall and airspace. Such movement into the asthmatic lung is described as a sequential, multistep paradigm, whereby integrins on circulating eosinophils become activated, eosinophils tether in flow and roll on bronchial endothelial cells, integrins on rolling eosinophils become further activated as a result of exposure to cytokines, eosinophils arrest firmly to adhesive ligands on activated endothelium, and eosinophils transmigrate to the airway in response to chemoattractants. Eosinophils express seven integrin heterodimeric adhesion molecules: alpha 4 beta 1 (CD49d/29), alpha 6 beta 1 (CD49f/29), alpha M beta 2 (CD11b/18), alpha L beta 2 (CD11a/18), alpha X beta 2 (CD11c/18), alpha D beta2 (CD11d/18), and alpha 4 beta 7 (CD49d/beta 7). The role of these integrins in eosinophil recruitment has been elucidated by major advances in the understanding of integrin structure, integrin function, and modulators of integrins. Such findings have been facilitated by cellular experiments of eosinophils in vitro, studies of allergic asthma in humans and animal models in vivo, and crystal structures of integrins. Here, we elaborate on how integrins cooperate to mediate eosinophil movement to the asthmatic airway. Antagonists that target integrins represent potentially promising therapies in the treatment of asthma.

Differential engagement of modules 1 and 4 of vascular cell adhesion molecule-1 (CD106) by integrins alpha4beta1 (CD49d/29) and alphaMbeta2 (CD11b/18) of eosinophils

We have studied adhesion of eosinophils to various forms of vascular cell adhesion molecule 1 (VCAM-1, CD106), an integrin counter-receptor implicated in eosinophil recruitment to the airway in asthma. Full-length 7d-VCAM-1, with seven immunoglobulin-like modules, contains integrin-binding sites in modules 1 and 4. The alternatively spliced six-module protein, 6d-VCAM-1, lacks module 4. In static assays, unactivated purified human blood eosinophils adhered similarly to recombinant soluble human 6d-VCAM-1 and 7d-VCAM-1 coated onto polystyrene microtiter wells. Further experiments, however, revealed differences in recognition of modules 1 and 4. Antibody blocking indicated that eosinophil adhesion to 6d-VCAM-1 or a VCAM-1 construct containing only modules 1-3, 1-3VCAM-1, is mediated by alpha4beta1 (CD49d/29), whereas adhesion to a construct containing modules 4-7, 4-7VCAM-1, is mediated by bothalpha4beta1 andalphaMbeta2 (CD11b/18). Inhibitors of phosphoinositide 3-kinase, which block adhesion of eosinophils mediated by alphaMbeta2, blocked adhesion to 4-7VCAM-1 but had no effect on adhesion to 6d-VCAM-1. Consistent with the antibody and pharmacological blocking experiments, eosinophilic leukemic cell lines lacking alphaMbeta2 did not adhere to 4-7VCAM-1 but did adhere to 6d-VCAM-1 or 1-3VCAM-1. Activation of eosinophils by interleukin (IL)-5 enhanced static adhesion to 6d-VCAM-1, 7d-VCAM-1, or 4-7VCAM-1; IL-5-enhanced adhesion to all 3 constructs was blocked by anti-alphaMbeta2. Adhesion of unstimulated eosinophils to 7d-VCAM-1 under flow conditions was inhibited by anti-alpha4 or anti-alphaM. IL-5 treatment decreased eosinophil adhesion to 7d-VCAM-1 under flow, and anti-alphaM had the paradoxical effect of increasing adhesion. These results demonstrate that alphaMbeta2 modulatesalpha4beta1-mediated eosinophil adhesion to VCAM-1 under both static and flow conditions.