Hypergranular Non-M3 Acute Myelogenous Leukemia: A Potential Pitfall in Flow Cytometric Diagnosis.

Background

Flow cytometry (FC) is an important tool in the diagnosis, subclassification and clinical management of acute leukemias. CD45 vs. orthogonal or side scatter (SSC) gating criteria allows easy identification of non-M3 blasts, which form a coherent population in the low SSC/moderate CD45 blast gate. The vast majority of acute myeloid leukemias (AML) of non-M3 type express pan-myeloid antigens CD13 and CD33, HLA-DR, blastic markers CD34 and CD117. AML M3 (acute promyelocytic leukemia, APL) often lacks CD34 and HLA-DR expression, and has characteristically high SSC/moderate CD45 expression, which overlaps with the granulocyte gate, but is CD117-positive. We report nine cases of non-M3 AML (hypergranular AML) with increased side scatter, moderate CD45 expression, and lack of expression of blastic markers, which mimic APL.

Design

Multiparameter 4-color FC analysis was performed on peripheral blood and/or bone marrow aspirate samples using the following antibody panel: CD2, CD3, CD4, CD5, CD7, CD8, CD10, CD11b, CD11c, CD13, CD14, CD16, CD19, CD20, CD33, CD34, CD38, CD45, CD56, CD64, CD79a, CD117, TdT, and HLA-DR. Fresh bone marrow aspirate and/or peripheral blood films and cytospin preparations from FC samples were stained with Wright Giemsa and reviewed in each case.

Results

Instead of a coherent population of blasts in the low SSC/moderate CD45 blast gate, the blasts in all 9 cases displayed increased granularity, which placed them in the granulocyte gate on FC display. The immunophenotype was as follows: HLA-DR+ in 5/9, CD4+ in 6/9, CD10+ in 1/9, CD11b+ in 4/9, CD11c+ in 8/9, CD13+ in 8/9, CD14+ in 0/9, CD16+ in 0/9, CD19+ in 0/9, CD33+ in 9/9, CD34+ in 0/9, CD38+ in 9/9, CD56+ in 5/9, CD64+ in 9/9 and CD117+ in 0/9 cases. The cytologic preparations revealed type II and type III cytomorphology in all 9 cases; none contained Auer rods.

Conclusion

Hypergranular non-M3 AML is difficult to diagnose by FC by pattern recognition because the blasts overlap with maturing granulocytes by SSC/CD45 gating criteria, and are not located in the blast gate. In this respect, they mimic APL and regenerative marrows, including those following G-CSF therapy. Correlation with cytomorphology and the expression of CD10, CD11b, CD16 and CD56 helps to differentiate between hypergranular AML, benign processes, APL, and MDS. Specifically, benign maturing myeloid cells show graduating positivity for CD10, CD11b and CD16, and are negative for CD56. This is the inverse pattern of the majority of cases of hypergranular AML. These cases differ from APL by expression of HLA-DR (in a majority of cases), lack of CD117, and different cytomorphology. Myelodysplastic syndromes, which may display similar aberrant myeloid expression of CD10, CD11b, CD16 and CD56, are most often low in SSC, and blastic cells will be <20% cytologically on smear review.