Engagement of alpha4beta7 integrins by monoclonal antibodies or ligands enhances survival of human eosinophils in vitro

Asthma is characterized by an airway inflammatory infiltrate that is rich in eosinophilic leukocytes. Cellular fibronectin and VCAM-1, ligands for alpha4 integrins, are enriched in the fluid of airways of allergic patients subjected to Ag challenge. We therefore hypothesized that ligands of alpha4 integrins can promote eosinophil survival independent of cell adhesion. Cellular fibronectin and VCAM-1 increased viability of human peripheral blood eosinophil in a dose- and time-dependant manner whether the ligand was coated on the culture well or added to the medium at the beginning of the assay. Eosinophils cultured with cellular fibronectin were not adherent to the bottom of culture wells after 3 days. Treatment with mAb Fib 30 to beta7, but not mAb P4C10 or TS2/16 to beta1, increased eosinophil survival. The increased survival of eosinophils incubated with Fib 30 was blocked by Fab fragments of another anti-beta7 mAb, Fib 504. Eosinophils incubated with soluble cellular fibronectin or mAb Fib 30 for 6 h demonstrated a higher level of GM-CSF mRNA than eosinophils incubated with medium alone. Addition of neutralizing mAb to GM-CSF during incubation, but not mAbs to IL-3 or IL-5, reduced the enhancement of eosinophil survival by soluble cellular fibronectin or mAb Fib 30 to control levels. Thus, viability of eosinophils incubated with cellular fibronectin or VCAM-1 is due to engagement, probably followed by cross-linking, of alpha4beta7 by soluble ligand (or mAb) that stimulates autocrine production of GM-CSF and promotes eosinophil survival.