INTERNALIZATION OF AN ANTI-GLYCOPROTEIN IIB IIIA ANTIBODY BY HEL CELLS/

Platelets are known to internalize monoclonal antibodies directed agai nst the glycoprotein (GP) IIb/IIIa complex. We investigated whether an antibody directed against this complex (NNKY 2-11) was transported fr om the surface membrane to the intracellular pool in HEL cells. Flow c ytometry showed that the percent binding of NNKY 2-11 to the surface m embrane of HEL cells was decreased after incubation for 24 h compared with 1 h, while the binding of an anti-GPIb antibody (NNKY 5-5) did no t change. It did not seem likely that the GP IIb/IIIa complex antibody was shed from the surface membrane of the HEL cells during incubation , because the medium conditioned by incubation with these cells for 24 h showed almost no binding to washed platelets. In addition, immunoel ectron microscopy demonstrated that GP IIb/IIIa complex antibodies wer e incorporated into the intracellular pool of HEL cells and were assoc iated with alpha granules. These findings indicated that an anti-GP II b/IIIa antibody could be internalized by megakaryocytes, as has been p reviously shown with platelets, suggesting that megakaryocyte GP IIb/I IIa may act as a carrier for various adhesion proteins.