Flow cytometric immunophenotypic features of acute myeloid leukemia with mast cell differentiation

Abstract Objectives Acute myeloid leukemia (AML) with mast cell (MC) differentiation was recently described as an aggressive subgroup of AML cases. The objectives of this study were to assess the flow cytometric immunophenotypic features of AML-MC cases. Methods We characterized the immunophenotypic features of 21 AML-MC cases by flow cytometry and compared them to 20 reactive/regenerating bone marrow specimens. Results The number of MCs detected by flow cytometry in AML-MC cases ranged from 0.4% to 21.1%, with a median of 3.5%, significantly higher than that of normal/reactive bone marrow (BM) (median, 0.01%; range, 0.000%-0.396%; P < .0001). Immunophenotypically, MCs in AML-MC cases demonstrated immaturity, differing from MCs in normal/reactive BMs, including dimmer CD45 (100% vs 0%), lower side scatter (100% vs 0%), more frequent CD34 (81% vs 20%), and CD123 (100% vs 10%) positivity, and more frequent uniform/increased CD38 expression (95% vs 20%) (all P ≤ .0001). CD2 (0/5) and CD25 (2/6, 1 uniform and 1 partial) were assessed in a subset of cases. The myeloblasts in AML-MC were typically CD34+CD117+HLA-DR+ with unusually frequent expression of CD56 (57%, all partial) and CD25 (63%, mostly partial), increased CD117 (62%), and decreased CD38 (86%). The MC percentage determined by flow cytometry correlated well with MCs detected by tryptase immunohistochemistry (r = 0.76, P < .001). Conclusions The MCs in AML-MC cases are characterized by dim CD45, low side scatter, CD34 and CD123 positivity, and uniform and increased CD38 expression. Flow cytometry is an excellent tool for identifying AML-MC cases.