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

Abstract

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.

Figure 1.

Scatter plots of relations among eosinophil β₁-integrin activation, assessed by reactivity with activation-sensitive anti-β₁ integrin monoclonal antibody (mAb) N29, level of eosinophil-bound P-selectin, and platelet surface P-selectin expression in Severe Asthma Research Program subjects. (A) Eosinophil N29 reactivity versus eosinophil-bound P-selectin. (B) Eosinophil N29 reactivity versus platelet surface P-selectin. (C) Eosinophil-bound P-selectin versus platelet surface P-selectin. Solid symbols, individual subjects; open symbols, means (see Table 3). Correlation coefficients and P values are described in Table 3 and the text. gMCF = geometric mean channel fluorescence.

Figure 2.

Time courses of eosinophil β₁-integrin activation, assessed by reactivity with activation-sensitive anti-β₁ integrin monoclonal antibody (mAb) N29, levels of eosinophil-bound P-selectin and the platelet marker α_IIb integrin, and P-selectin glycoprotein ligand-1 (PSGL-1) expression after whole-lung inhaled antigen challenge in non–Severe Asthma Research Program subjects with mild allergic asthma. (A) Individual time courses of eosinophil N29 reactivity. (B) Individual time courses of eosinophil-bound P-selectin. (C) Individual time courses of eosinophil-bound α_IIb; (D) Individual time courses of eosinophil PSGL-1. (E–H) Means of values (± SEM) of (E) eosinophil N29 reactivity, (F) eosinophil-bound P-selectin, (G) eosinophil-bound α_IIb, and (H) eosinophil PSGL-1 at indicated times. *P ≤ 0.05, **P ≤ 0.01, ***P ≤ 0.001. gMCF = geometric mean channel fluorescence.

Figure 3.

Model of platelet activation, eosinophil β₁-integrin activation, and airway physiology in asthma of increasing severity. Platelet activation and appearance of platelet factor 4 (PF4) in circulation increase with increasing disease severity. P-selectin appears on the surface of platelets upon activation, activated platelets bind to eosinophils via P-selectin, and P-selectin triggers activation of eosinophil β₁-integrins. Eosinophils with activated β₁ integrins persist in the circulation in stable nonsevere asthma but are lost in more severe asthma or during asthma exacerbation. Loss is likely due to interaction of activated α₄β₁ integrin with endothelial cell vascular cell adhesion molecule-1 followed by extravasation in the asthmatic lung. Increased eosinophil β₁-integrin activation is therefore inversely associated with decreased pulmonary function only in the subpopulation with stable nonsevere asthma and minimal eosinophil clearance.