Internalization of an anti-glycoprotein IIb/IIIa antibody by HEL cells.

Platelets are known to internalize monoclonal antibodies directed against the glycoprotein (GP) IIb/IIIa complex. We investigated whether an antibody directed against this complex (NNKY 2-11) was transported from the surface membrane to the intracellular pool in HEL cells. Flow cytometry showed that the percent binding of NNKY 2-11 to the surface membrane 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 not 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, immunoelectron microscopy demonstrated that GP IIb/IIIa complex antibodies were incorporated into the intracellular pool of HEL cells and were associated with alpha granules. These findings indicated that an anti-GP IIb/IIIa antibody could be internalized by megakaryocytes, as has been previously shown with platelets, suggesting that megakaryocyte GP IIb/IIIa may act as a carrier for various adhesion proteins.