Intracellular trafficking of the α[sub IIb]β[sub 3] receptor antagonist, abciximab, in normal and Glanzmann's disease megakaryocytes.

The αIIbβ3 platelet receptor antagonist abciximab (c7E3Fab, ReoPro®) has proved to be effective in preventing arterial thrombosis. However, its binding capacity to the platelet precursors, megakaryocytes (MKs), which also express αIIbβ3, is not known. The purpose of this study was to establish whether abciximab is able to react with αIIbβ3 located on human MKs, and to follow its subsequent intracellular trafficking. MKs were grown from CD34+ progenitors from normal subjects and from a patient with type I Glanzmann's thrombasthenia, and abciximab was added at day 10 of culture (4 μg/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, α-granule membranes were labelled showing that the antibody was internalized and further stored into MK secretory granules. Abciximab was also strongly detected on and in newly-formed platelets. Glanzmann's disease MKs (which completely lacked αIIbβ3) were consistently negative, confirming that the antibody fragment was specifically interacting with αIIbβ3. In conclusion, this study demonstrated that abciximab: (i) binds MK plasma membrane and demarcation membranes, (ii) trafficks into α-granules, and (iii) is expressed on and in nascent platelets. These findings could be taken in account when monitoring anti-αIIbβ3 receptor therapy. [ABSTRACT FROM AUTHOR]