Ultrastructural localization of the Charcot-Leyden crystal protein (lysophospholipase) to a distinct crystalloid-free granule population in mature human eosinophils

The Charcot-Leyden crystal (CLC) protein is a unique constituent of eosinophils and basophils. This protein forms the hexagonal bipyramidal crystals observed in tissues at sites of eosinophil accumulations, possesses lysophospholipase activity (lysolecithin acylhydrolase E.C.3.1.1.5), and comprises an estimated 7% to 10% of total eosinophil protein. The ultrastructural localization of CLC protein was studied in mature peripheral blood eosinophils from normal donors and from patients with the idiopathic hypereosinophilic syndrome. Subcellular localization was evaluated by immunoelectron microscopy using eosinophils, both from buffy coat and purified cell suspensions, that were fixed by a variety of methods. Immunochemical detection of CLC protein employed rabbit antiserum to eosinophil CLC protein, affinity chromatography-purified monospecific IgG antibodies, and postembedding immunogold techniques. Controls for specificity included (1) omission of the primary antibody to CLC protein and (2) substitution of primary antibody with a nonimmune preimmunization serum, a protein A-purified nonimmune IgG, or a protein A-purified nonreactive IgG prepared from solid-phase CLC protein-Sepharose-absorbed anti-CLC antiserum. CLC protein was localized to a minor (approximately 5%) subpopulation of eosinophil granules. These membrane-bound cytoplasmic granules were large (greater than 0.5 mu), were devoid of crystalloid inclusions, and were morphologically compatible with persisting eosinophil primary granules. The crystalloid-containing, large, specific granules did not stain for CLC protein. Insufficient numbers of small dense granules, lipid bodies, and vesiculotubular structures were present to adequately evaluate their potential as additional sites for the subcellular localization of CLC protein. The cellular specificity of the immunogold localization of CLC protein in the eosinophil was affirmed by the absence of staining in neutrophils and lymphocytes present in the same sections. The ultrastructural immunogold localization of CLC protein (lysophospholipase) to a large, crystalloid-free granule in mature circulating eosinophils supports the persistence of a distinct “primary” granule population that serves as a major intracytoplasmic repository for this enzyme.