Platelet peroxidase-positive blast cells in transient myeloproliferative disorder with Down's syndrome.

Transient myeloproliferative disorder accompanied by Down's syndrome has been characterized as exhibiting self-limiting haematological abnormalities. We studied six patients suffering from this disorder in order to clarify the biological nature of their blast cells. Metaphases of leucocytes stimulated with phytohaemagglutinin (PHA) showed trisomy 21 in all patients except one. The exception was constitutionally trisomy 21 mosaic (46,XY = 89/47,XY,+21 = 11). However, metaphases from the peripheral blood cells (blast cells: 70%) without PHA stimulation showed exclusively trisomy 21. Simultaneous examination for morphology and chromosomal analysis on single colonies revealed that granulocyte-macrophage (GM) colonies and an erythroid colony contained only cells with the trisomy 21 karyotype. The blast cells showed positive reactions for platelet peroxidase (PPO) and with monoclonal antibodies against platelet-megakaryocyte antigen (TP 80 and TP 82). In methylcellulose and liquid culture systems, high plating efficiencies were observed, and mainly mature basophils containing histamine developed in the presence of PHA-stimulated leucocyte conditioned medium (PHA-LCM). In vivo, mature neutrophils, basophils, eosinophils or megakaryocytes coexisted with PPO-positive blast cells in the peripheral blood of some patients with this disorder. These findings suggest that transient myeloproliferative disorder is characterized by a proliferation of PPO-positive blast cells with trisomy 21, although some heterogeneity may be seen.