A 46-year-old male with myelofibrosis developed immune thrombocytopenia (ITP) on day 199 after allogeneic bone marrow transplantation from an HLA-matched unrelated donor. The platelet count was stable around 3×104/μl after engraftment, but on day 199 the platelet count suddenly decreased to 0.7×104/μl. His white blood cell count and hemoglobin did not decrease. While his myelofibrosis was persistent, chimerism study in the peripheral blood showed a complete donor pattern, denying the possibility of secondary graft failure. Splenomegaly did not worsen. Because of unresponsiveness to platelet transfusion, we examined antibody against platelet. Although anti-HLA antibody was negative, anti-GP IIb/IIIa antibody was detected and PAIgG also increased. A diagnosis of ITP was made based on these findings. He was treated with prednisolone at a dose of 1 mg/kg and the platelet count gradually increased from 7 days after treatment. Almost all cases of ITP after allogeneic hematopoietic stem cell transplantation result from antecedent GVHD or CMV infection or T cell depletion, and there is often resistance against standard ITP treatments. This case was not accompanied by any other immune response and showed good response to prednisolone. We report this case with several speculations about mechanisms and treatments of ITP after allogeneic transplantation.
