Intelligent granules: Are eosinophil crystalloid granules inimitable?

Upper panel, The 4 patterns of eosinophil granule mediator deposition after activation: classical exocytosis involving membrane fusion of individual granules with the plasma membrane, leading to mediator secretion; compound exocytosis, whereby granules fuse together before exteriorization of the fused mediators from a single fusion site; piecemeal degranulation involving rapidly mobilizable small secretory vesicles shuttling granule contents to and from the plasma membrane; and cytolysis, in which the cell loses its membrane integrity, releasing intact membrane-bound granules to the extracellular space. Lower panel, Magnified view of an extracellular intact eosinophil granule expressing biologically active receptors for CCL11 (eotaxin and CCR3) and IFN-γ (IFN-γRα). Stimulation with relevant ligands induces the release of stored granule cationic proteins (eosinophil cationic protein [ECP] and eosinophil peroxidase [EPO]), as well as cytokines (IL-4 and IL-6).⁷ A recent study showed that the cysteinyl leukotrienes LTC₄, LTD₄, and LTE₄ stimulated cell-free eosinophil granules to secrete ECP and EPO, but not cytokines, through CysLT1R, CysLT2R, and P2Y12R.⁸