Differential engagement of modules 1 and 4 of vascular cell adhesion molecule-1 (CD106) by integrins α4β1 (CD49d/29) and αMβ2 (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 alpha 4 beta 1 (CD49d/29), whereas adhesion to a construct containing modules 4-7,4-7VCAM-1, is mediated by both alpha 4 beta 1 and alpha M beta 2 (CD11b/18). Inhibitors of phosphoinositide 3-kinase, which block adhesion of eosinophils mediated by alpha M beta 2, 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 alpha M beta 2 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-alpha M beta 2. Adhesion of unstimulated eosinophils to 7d-VCAM-1 under flow conditions was inhibited by anti-alpha 4 or anti-alpha M. IL-5 treatment decreased eosinophil adhesion to 7d-VCAM-1 under flow, and anti-alpha M had the paradoxical effect of increasing adhesion. These results demonstrate that alpha M beta 2 modulates alpha 4 beta 1-mediated eosinophil adhesion to VCAM-1 under both static and flow conditions.