Eosinophil crystalloid granules: structure, function, and beyond

Abstract

Eosinophils are granulocytes associated with host defense against parasitic helminths with allergic conditions and more recently, with immunoregulatory responses. Eosinophils are distinguished from leukocytes by their dominant population of cytoplasmic crystalloid (also termed secretory, specific, or secondary) granules 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 function of intracellular receptors and their potential implications in cell signaling. Special focus is provided on how eosinophils might use these intracellular receptors as mechanisms to secrete, selectively and rapidly, cytokines or chemokines and enable cell-free extracellular eosinophil granules to function as independent secretory structures. Potential roles of cell-free eosinophil granules as immune players in the absence of intact eosinophils will also be discussed.

Figure 1.. Eosinophils and their unique crystalloid granules.

Eosinophil crystalloid granules express receptors on their membranes topologically oriented with ligand-binding domains displayed on the outer granule membrane and may be sites of RNA and possibly DNA localization. Eosinophil crystalloid granules contain intragranular membranes that might be involved in granule-stored protein sorting and selective secretion.