Eosinophil crystalloid granules: structure, function, and beyond

Eosinophils are granulocytes associated with host de-fense against parasitic helminths with allergic condi-tions and more recently, with immunoregulatory re-sponses. Eosinophils are distinguished from leukocytes by their dominant population of cytoplasmic crystalloid (also termed secretory, specific, or secondary) gran-ules that contain robust stores of diverse, preformed cationic proteins. Here, we provide an update on our knowledge about the unique and complex structure of human eosinophil crystalloid granules. We discuss their significance as rich sites of a variety of receptors and review our own recent research findings and those of others that highlight discoveries concerning the func-tion of intracellular receptors and their potential impli-cations in cell signaling. Special focus is provided on how eosinophils might use these intracellular receptors as mechanisms to secrete, selectively and rapidly, cy-tokines or chemokines and enable cell-free extracellu-lar eosinophil granules to function as independent se-cretory structures. Potential roles of cell-free eosinophil granules as immune players in the absence of intact eosinophils will also be discussed. J. Leukoc. Biol. 92: 281–288; 2012