Differential dependence of eosinophil chemotactic responses on phosphoinositide 3-kinase (PI3K)

Background: Control of eosinophil migration to sites of inflammatory responses is a potentially therapeutic intervention in diseases such as bronchial asthma. Chemoattractants, their receptors and the associated signalling pathways may, therefore, be important targets for novel therapeutics. While several potentially important chemoattractants have been identified, the signalling pathways mediating their actions are incompletely understood. Aims of the study: The role of phosphoinositide 3-kinase (PI3K) in responses of human eosinophils to two important eosinophil chemoattractants - platelet-activating factor (PAF) and eotaxin (CCL11) - was studied to determine whether this enzyme activity might be crucial for eosinophil migration. Methods: Eosinophils were isolated from atopic donor blood by immunomagnetic selection. Chemotaxis was assayed in a 96-well blind-chamber cell fluorescence assay. Respiratory burst and leukotriene C-4 secretion were also assayed. Results: Two PI3K inhibitors, wortmannin and LY294002, caused concentration-dependent inhibition of PAF-induced eosinophil chemotaxis (IC50 = 0.54 nM and 0.15 mu M, respectively) but exhibited at least 100-fold lower potency against eotaxin-induced responses (IC50 = 48 nM and > 100 mu M, respectively), indicating that these responses were not dependent upon PI3K. Wortmannin and LY294002 also inhibited PAF induced respiratory burst but not PAF-induced LTC4 secretion. Conclusions: We conclude that PI3K-dependence varies with stimulus and response, and that eotaxin-induced eosinophil migration is not controlled by PI3K. This may indicate a limit to the potential of PI3K inhibitors to suppress tissue eosinophilia in diseases such as asthma.