Intracellular trafficking of the alpha(IIb)beta(3) receptor antagonist, abciximab, in normal and Glanzmann's disease megakaryocytes

The alpha(IIb)beta(3) platelet receptor antagonist abciximab (c7E3Fab, ReoP ro(R)) has proved to be effective in preventing arterial thrombosis. Howeve r, its binding capacity to the platelet precursors, megakaryocytes (MKs), w hich also express alpha(IIb)beta(3), is not known, The purpose of this stud y was to establish whether abciximab is able to react with alpha(IIb)beta(3 ) located on human MKs, and to follow its subsequent intracellular traffick ing. MKs were grown from CD34(+) progenitors from normal subjects and from a patient with type I Glanzmann's thrombasthenia, and abciximab was added a t day 10 of culture (4 mu g/ml). Cells were fixed at day 12, cryosectioned, and immunolabelled for abciximab, Labelling was prominent on the MK plasma membrane: it also lined the demarcation membration system. Interestingly, alpha-granule membranes were labelled showing that the antibody was interna lized and further stored into MK secretory granules. Abciximab was also str ongly detected on and in newly-formed platelets. Glanzmann's disease MKs (w hich completely lacked alpha(IIb)beta(3)) were consistently negative, confi rming that the antibody; fragment was specifically interacting with alpha(I Ib)beta(3). In conclusion, this study demon; strated that abciximab: (i) bi nds MK plasma membrane and demarcation membranes, (ii) trafficks into alpha -granules, and (iii) is expressed on and in nascent platelets. These findin gs could be taken in account when monitoring anti-alpha(IIb)beta(3) recepto r therapy.