Ligation of the β₂ integrin triggers activation and degranulation of human eosinophils

Evidence suggests that cellular adhesion is critical for eosinophil effector functions. Here, we tested the hypothesis that an adhesion molecule, specifically β₂ integrin, participates in intracellular signaling events of eosinophils. Eosinophils stimulated with interleukin (IL)-5 and adherent to protein-coated tissue culture plates via β₂ integrin (CD18) showed tyrosine phosphorylation of a number of proteins. Among these proteins, tyrosine phosphorylation of the 105 kD and 115 kD proteins and the product of the c-cbl protooncogene, Cbl, was specifically inhibited using soluble anti-CD 18 monoclonal antibody (mAb) to block eosinophil cell adhesion. Furthermore, phosphoinositide turnover of IL-5-stimulated adherent eosinophils was also inhibited by anti-CD18 mAb, suggesting that cellular adhesion plays important roles in eosinophil signal transduction. α_Mβ₂ (Mac-1, CD11b/18) was one of the β₂ integrins involved in eosinophil adhesion to protein-coated plates. We found that direct ligation of eosinophil α_Mβ₂ with anti-CD11b mAb coupled to polystyrene microbeads induced tyrosine phosphorylation of a 115 kD protein and Cb1. Furthermore, antiCD11b mAb microbeads induced increases in both phosphoinositide hydrolysis and the eosinophil degranulation response. Control antibodies, such as mouse myeloma IgG1 and anti-HLA class I antigen mAb, did not induce these cellular responses. These results suggest that engagement of β₂ integrin either by cell adhesion or by anti-CD11b mAb triggers activation of an intracellular signaling cascade, including protein tyrosine phosphorylation and phosphoinositide turnover, and subsequent cellular degranulation in human eosinophils. Tyrosine phosphorylation of a 115 kD protein and Cbl may play important roles in adhesion-dependent cellular functions of eosinophils.