Chemotaxis and activation of human peripheral blood eosinophils induced by pollen-associated lipid mediators

Background: Eosinophil accumulation at sites of allergic inflammation is largely regulated by chemokines and lipid mediators released by a variety of cells of the local microenvironment. Recent studies have shown that pollen grains, apart from their function as allergen carriers, are a rich exogenous source of eicosanoid-like lipid mediators that are rapidly released on contact with the aqueous phase and thus may contribute to the generation of local inflammatory responses. Objective: Here we analyze the biological activity of pollen-associated lipid mediators (PALMs) on peripheral human blood eosinophils. Methods: Human eosinophils were coincubated with pollen grains and analyzed by electron microscopy. The lipid mediator composition of aqueous pollen extracts (APEs) was analyzed by HPLC. Human eosinophils were exposed to APEs or lipid fractions from pollen. Effects on eosinophils were tested by transwell migration and surface expression of CD11b. Results: In vitro experiments showed adhesion of eosinophils to Phleum pratense pollen. In chemotaxis assays eosinophils displayed significant directed migration to APEs. HPLC analysis of APEs from Phleum pratense and Betula alba pollen demonstrated the occurrence of linoleic and alpha-linolenic acid as well as their monohydroxylated derivatives. Moreover, total lipid extracts from pollen and RP-HPLC fractions containing monohydroxylated derivatives of linoleic and alpha-linolenic acid induced similar migratory responses, although to a lesser degree than APEs. In addition, APEs and lipid extracts induced up-regulation of CD11b surface expression and secretion of eosinophil cationic protein. APE-induced chemotaxis was blocked by the leukotriene B-4 receptor antagonist LY293111, suggesting that PALMs may serve as ligands for LTB4 receptors. Conclusion: Pollen grains release lipid mediators that recruit and activate eosinophils in vitro. Similar mechanisms may be effective under natural exposure conditions, in which PALMs may play a role in the recruitment of eosinophils to the site of allergic inflammation.