Eosinophil granule proteins: form and function

Abstract

Experimental and clinical data strongly support a role for the eosinophil in the pathogenesis of asthma, allergic and parasitic diseases, and hypereosinophilic syndromes, in addition to more recently identified immunomodulatory roles in shaping innate host defense, adaptive immunity, tissue repair/remodeling, and maintenance of normal tissue homeostasis. A seminal finding was the dependence of allergic airway inflammation on eosinophil-induced recruitment of Th2-polarized effector T-cells to the lung, providing a missing link between these innate immune effectors (eosinophils) and adaptive T-cell responses. Eosinophils come equipped with preformed enzymatic and nonenzymatic cationic proteins, stored in and selectively secreted from their large secondary (specific) granules. These proteins contribute to the functions of the eosinophil in airway inflammation, tissue damage, and remodeling in the asthmatic diathesis. Studies using eosinophil-deficient mouse models, including eosinophil-derived granule protein double knock-out mice (major basic protein-1/eosinophil peroxidase dual gene deletion) show that eosinophils are required for all major hallmarks of asthma pathophysiology: airway epithelial damage and hyperreactivity, and airway remodeling including smooth muscle hyperplasia and subepithelial fibrosis. Here we review key molecular aspects of these eosinophil-derived granule proteins in terms of structure-function relationships to advance understanding of their roles in eosinophil cell biology, molecular biology, and immunobiology in health and disease.

Keywords: Asthma; Crystal Structure; Enzyme Structure; Eosinophil; Eosinophil Granule Proteins; Inflammation.

FIGURE 1.

Structural representations of the human eosinophil granule proteins showing their location and known functions. The cationic granule proteins shown are: MBP-1 (Protein Data Bank (PDB) code 1H8U); EDN (RNase-2) (PDB code 1HI2); ECP (RNase-3) (PDB code 1QMT); and EPX/EPO) (based on molecular modeling using a myeloperoxidase structure, PDB code 1D2V). Also shown is CLC/Gal-10 (PDB code 1LCL), which is mainly cytosolic but also present in a small residual population of primary large core-less granules in the mature eosinophil. CLC/Gal-10 forms the hexagonal bipyramidal crystals (arrow) considered a hallmark of eosinophilic inflammation in tissues and body fluids in eosinophil-associated diseases.

FIGURE 2.

Molecular details of the functional sites identified thus far. A, MBP-1 carbohydrate-binding region with bound heparin disaccharide and sulfate ions (PDB code 2BRS). B, EDN ribonucleolytic active site with bound inhibitor bis(adenosine)-5′-pentaphosphate (PDB code 1HI5). C, ECP ribonucleolytic active site with bound adenosine-2′,5′-diphosphate (PDB code 1H1H). D, CLC/Gal-10 carbohydrate recognition domain with bound mannose (PDB code 1QKQ).

FIGURE 3.

Mechanisms of eosinophil degranulation. Eosinophils release their secondary (specific) granule contents, including the granule cationic proteins and other immunomodulatory mediators, by four different mechanisms. In classical exocytosis, mainly seen at sites of bacterial infection, the contents of single granules are released by fusion of the granule membrane with the plasma membrane lipid bilayer. In compound exocytosis, mainly seen with eosinophil degranulation onto helminth parasites, a number of granules first coalesce and fuse, and the cationic protein contents are then released through a single fusion pore at the plasma membrane. In piecemeal degranulation, mainly seen in eosinophil inflammatory responses in human tissues, secretory vesicles bud from granules, capturing granule matrix and/or core contents, and are targeted to the plasma membrane in a fashion analogous to the release of neurotransmitters from neurons. Differential secretion of the cationic proteins from the matrix (EPX, EDN/RNase-2, ECP/RNase-3) or core (MBP-1, MBP-2) of the granule has been shown to occur, leaving coreless granules with intact matrix or intact cores with no matrix in the cell. In cytolysis, intact whole granules are deposited in tissues and body fluids after disruption of the plasma membrane due to eosinophil necrosis.