Eosinophils are granulocytic leukocytes important in allergy and immune responses to parasites, which are stimulated by such factors as interferon-γ (IFN-γ) and the chemokine CCL11 to secrete effector proteins, including cytokines, cationic proteins, and hydrolytic enzymes, found in intracellular stores known as granules. In addition, cytolysis of eosinophils can result in the release of intact granules, which are deposited in tissues; however, the function of these intact granules is unclear. Neves et al. prepared intact granules from human eosinophils and performed flow cytometric and electron microscopic analyses to demonstrate their lack of contamination by secretory vesicles or plasma membrane. In response to either IFN-γ or CCL11, the granules released eosinophil cationic protein (ECP) and β-hexosaminidase, as well as various cytokines, and flow cytometry studies showed that the IFN-γ receptor and CCR3, a G protein–coupled receptor for CCL11, were present on their outer surface. Inhibitors of tyrosine kinases, protein kinase C (PKC), and p38 mitogen-activated protein kinase (MAPK) inhibited IFN-γ-dependent release of effector proteins from the granules, whereas the p38 MAPK inhibitor and pertussis toxin, an inhibitor of Gi-coupled CCR3, blocked CCL11-dependent granule content release. Western blotting showed the increased phosphorylation, and thus activation, of PKC and p38 MAPK in granules in response to either stimulus compared with that in untreated granules. Finally, brefeldin A, an inhibitor of vesicular secretion, dose-dependently blocked IFN-γ- and CCL11-dependent release of ECP and β-hexosaminidase from granules. Together, these data suggest that functional signaling pathways in extracellular, intact eosinophil granules mediate the secretion of granule proteins in response to IFN-γ and CCL11, which the authors suggest may contribute to eosinophil-dependent immune responses.
J. S. Neves, S. A. C. Perez, L. A. Spencer, R. C. N. Melo, L. Reynolds, I. Ghiran, S. Mahmudi-Azer, S. O. Odemuyiwa, A. M. Dvorak, R. Moqbel, P. F. Weller, Eosinophil granules function extracellularly as receptor-mediated secretory organelles. Proc. Natl. Acad. Sci. U.S.A. 105, 18478–18483 (2008). [Abstract] [Full Text]