HUMAN EOSINOPHILS EXPRESS A RECEPTOR FOR SECRETORY COMPONENT - ROLE IN SECRETORY IGA-DEPENDENT ACTIVATION

The existence of a functional receptor for secretory component (SC) on the eosinophil membrane might explain the preferential degranulation induced by secretory IgA (sIgA) when compared to serum IgA. Indeed, fl ow cytometry analysis revealed that purified human SC could bind to a subpopulation (4-59%) of blood eosinophils purified from 19 patients w ith eosinophilia. Binding of radiolabeled human SC could be competitiv ely inhibited using unlabeled SC or secretory IgA but not with serum I gA or IgG. Immunoprecipitation and immunosorbent chromatography using human SC revealed the presence of a major component at 15 kDa in eosin ophil extracts as well as in culture supernatants but not in neutrophi ls. The 15-kDa protein eluted from the human SC immunosorbent was able to bind to SC or to sIgA but not to serum IgA. Eosinophils preincubat ed with human SC or sIgA released eosinophil cationic protein (ECP) an d eosinophil peroxidase (EPO) after addition of anti-SC or anti-IgA mo noclonal antibody as respective cross-linking reagents. These results indicated that binding of free or complexed SC to human eosinophils co uld induce eosinophil degranulation. Furthermore, the dose-dependent i nhibition by SC of mediator release induced by sIgA but not by serum I gA, suggested that the receptor for SC could be involved in the prefer ential degranulation mediated by sIgA. These results indicate a novel pathway of eosinophil activation and its potential involvement in muco sal immunity, particularly in inflammatory diseases associated with in filtration of eosinophils and the enhanced production of sIgA.