Ultrastructural and immunohistochemical studies on myocardial biopsies from a patient with eosinophilic endomyocarditis.

Right ventricular endomyocardial biopsy specimens from a 13-year-old boy with hypereosinophilia were studied by light and electron microscopy using the EG2 monoclonal antibody, which recognizes a common epitope of eosinophil cationic protein and eosinophil protein-X. Although the endocardial layer was of normal thickness, many eosinophils, mononuclear cells, and free eosinophil granules were observed in the endocardium and in the vicinity of degenerated myocardial cells. Under electron microscopy, many of the specific granules in and out of eosinophils had lost their crystalloid internae and displayed reversed density, and there were many degranulated eosinophils with reduced number of granules. Immunohistochemically, large amounts of eosinophil cationic protein and protein-X were observed within cardiocytes when many of them were degenerated. Deposits of the proteins were also found in some small vessels. On electron microscopy, accumulations of gold particles, which bind to eosinophil cationic protein and protein-X, were seen in association with specific granules and on the myofilaments in both degenerated and normal-appearing cardiocytes. The presence of eosinophil cationic proteins within cardiocytes may play an important role in the pathogenesis of eosinophilic endomyocardial disease.