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

Abstract

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.

Figures

Figure 1. Integrins of eosinophils

Schematic of the seven heterodimeric integrin adhesion receptors expressed by eosinophils. Functions and ligands assigned to integrins have been deduced in various assays employing eosinophils. Asterisks represent subunits that contain the I (Insert)-domain. Subunits that are underlined contain the I-like domain. ICAM, intercellular adhesion molecule; MAdCAM-1, mucosal addressin cell adhesion molecule-1; FG, fibrinogen; LN, laminin; VCAM, vascular cell adhesion molecule; VN, vitronectin.

Figure 2. Structure and schematic of VCAM-1 splice forms

Crystal structure of the first two N-terminal immunoglobulin (Ig)-like modules of 6d- and 7d-VCAM-1 [75]. The crystal structure is color coded to match the schematic of VCAM-1 modules depicted to the right. In the crystal structure, module 1 contains an IDSPL loop (highlighted in black) that is recognized by α4β1 of eosinophils. A second IDSPL motif recognized by α4β1 of eosinophils is present in module 4 of 7d-VCAM-1, which is also recognized by αMβ2. Module 4 is absent from 6d-VCAM-1, the likely result of exon skipping during mRNA splicing [78]. Two intra-domain disulfide bonds are present in modules 1 and 4 while modules 2, 3, 6, and 7 are predicted to contain only one such disulfide. The disulfide is missing from module 5. Individual modules have been color coded based on amino acid sequence identity: modules 1 and 4 = 73%, modules 2 and 5 = 60%, modules 3 and 6 = 60%. The internal homology and similarity of intron sizes between modules 1–3 and 4–6 suggests that modules 4–6 may have arisen in evolution by gene duplication of ancestral modules 1–3 [163]. The figure was created with RasMol.

Figure 3. Antibody probes of α4β1 conformation and affinity

Schematic of three putative conformations of α4β1 that might be expressed by eosinophils as modeled on the crystal structures of αVβ3 and αIIbβ3 and adapted from Luo BH et al. [164]. The bent, closed form of α4β1 is presumably not recognized by any of the activation-sensitive antibodies (left). Extension of α4β1 reveals the epitope in the β1 PSI domain recognized by N29 - this conformation is presumed to most closely depict the conformation of α4β1 of blood or airway eosinophils (middle). Further activation results in swing-out of the hybrid domain and exposure of the epitope recognized by HUTS-21, along with separation of the integrin legs and exposure of the epitope recognized by 9EG7 (right). The latter form of α4β1 is presumably present on Jurkat, EoL-3, or Mn2+- or PMA-activated AML14.3D10 eosinophilic cells [30].

Figure 4. I-domain of αMβ2

Schematic (left) of the I-domain of αMβ2 color coded to match the crystal structure (right) [86]. α-helices-1, 3, and -7 of the I-domain (shown in green) undergo conformational changes in response to αMβ2 activation [84]. Residues recognized by CBRM1/5 are labeled and highlighted in black. The figure was created with PyMol.