Mixed phenotype of blasts in acute megakaryocytic leukaemia and transient abnormal myelopoiesis in Down's syndrome

Blasts from eight cases with acute megakaryoblastic leukaemia (AMKL) and seven with transient abnormal myelopoiesis in Down's syndrome (TAM) were investigated to clarify their phenotypic characteristics. CD41 and CD7 were the most frequently expressed in both disorders. CD41 was positive in six TAM and five AMKL cases, and CD7 was positive in five TAM and five AMKL cases, respectively. CD33 was detected in four TAM and five AMKL cases. Other myeloid‐lineage associated antigens such as CD13 and CD11b could not be found in TAM but were expressed in five AMKL cases. Interestingly, CD56, a neural adhesion molecule, was expressed in three of four TAM and one of five AMKL cases. Cytoplasmic CD3 antigen was also noted in three of five examined cases. A short‐term culture study was conducted on blasts from two TAM cases and five AMKL cases. In two cases in which CD41 was not expressed before culture, the expression of CD41 was enhanced after culture with or without 12–0‐tetradecanoyl‐phorbol‐13‐acetate (TPA). The expression of CD7 remarkably was depressed, while that of CD13 was enhanced after culture with TPA. These findings suggest that blasts of TAM and AMKL originate from very immature cells and represent a mixed phenotype. In the present study, distinction of pheno‐typical differences between blast in TAM and AMKL was not possible.