Intracellular expression of CD61 precedes surface expression

We report a case of acute myeloid leukemia which we have classified as acut e megakaryoblastic leukemia because of intracytoplasmic expression of CD61, Light microscopy demonstrated agranular blasts which, by cytochemical stai ning, were negative for myeloperoxidase. Using flow cytometry, the blast ce lls were seen to be positive for HLA-DR, CD7, CD13, CD33, and CD34. thus re vealing their myeloid origin. Immunophenotyping on fixed blood smears addit ionally showed positive reaction with the CD61 antibody, demonstrating the megakaryoblastic differentiation of the blasts. After permeabilization of t he cell membrane. the intracytoplasmic CD61 expression was confirmed by flo w cytometry. Cytogenetic analysis disclosed a del(7)(q21-22), Our findings suggest that cytoplasmic expression of CD61 may precede the cell-surface ex pression of this antigen and should therefore be investigated in all cases of acute leukemias with undifferentiated morphology and negative cytochemis try to set apart early FAB-M7 from FAB-M0.