CHARACTERIZATION OF THE EOSINOPHIL GRANULE PROTEINS RECOGNIZED BY THEACTIVATION-SPECIFIC ANTIBODY EG2

Monoclonal antibodies EG1. and EG2 have the unique ability to distingu ish the storage from the secreted forms of the eosinophil cationic pro tein (ECP). EG2 has been used extensively as a marker for activated, s ecreting eosinophils, despite the fact that no biochemical differences between the storage and secreted forms of ECP have been identified. W e 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 fo rms 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-sp ecific EG2 recognizes a polypeptide epitope that is masked in the high er-molecular-weight, more heavily glycosylated forms of ECP. These fin dings suggest that deglycosylation may occur in conjunction with activ ation and secretion; alternatively, the 18-kDa form of ECP may be pres ent in the storage granule of resting eosinophils but may remain undet ected in an inaccessible location or conformation.