Acute model of severe thrombocytopenia. (A) The anti–human GP Ibα mouse monoclonal antibody, LJ-P3, injected into the tail veins of mice expressing human GP Ibα, produced severe thrombocytopenia. When injected into mice expressing the wild-type human GP Ibα subunit (hTg^WT), the platelet count started to return to a normal level after day 7. In mice expressing a truncated form of GP Ibα (hTg^Y605X), the platelet count began to recover by day 4, and by day 7 the platelet counts were near normal levels. A control LJ-P3 injection into nontransgenic mice (nTg^Control) is shown for comparison. (B) The ploidy profile of megakaryocytes is shown before injection of the anti–human monoclonal antibody, LJ-P3 (day 0) and 4 days after injection (day 4). Results were compiled from individual mice, and the mean and SEM are shown (n = 6). *P = .01; **P = .05.