Characterization of the eosinophil granule proteins recognized by the activation-specific antibody EG2

Monoclonal antibodies EG1 and EG2 have the unique ability to distinguish the storage from the secreted forms of the eosinophil cationic protein (ECP). EG2 has been used extensively as a marker for activated, secreting eosinophils, despite the fact that no biochemical differences between the storage and secreted forms of ECP have been identified. We have determined that the activation-specific EG2 detects only one of three distinct glycosylated forms of ECP (18 kDa); in contrast, both EG1 and polyclonal anti-ECP antiserum can detect three glycosylated forms of this protein (18, 20, and 22 kDa). We have also determined that EG2 detects fully deglycosylated ECP as well as fully deglycosylated eosinophil-derived neurotoxin. Our results indicate that activation-specific EG2 recognizes a polypeptide epitope that is masked in the higher-molecular-weight, more heavily glycosylated forms of ECP. These findings suggest that deglycosylation may occur in conjunction with activation and secretion; alternatively, the 18-kDa form of ECP may be present in the storage granule of resting eosinophils but may remain undetected in an inaccessible location or conformation.