Aberrant expression of CD19 as a marker of monocytic lineage in acute myelogenous leukemia.

Immunophenotypic analysis plays a critical role in the diagnosis and classification of acute leukemia. Certain characteristic immunophenotypic patterns have emerged that aid in the classification of acute myelogenous leukemia (AML). We describe a unique pattern of expression of CD19, a B cell-associated cell surface antigen, in cases of AML. We reviewed 59 cases of de novo AML to determine the pattern of CD19 expression in cases of AML using three different CD19 monoclonal antibodies, including B4 (Lytic), B4 89B (Coulter, Miami, Fla) and SJ25-C1 (GenTrak, Plymouth Meeting, Pa). We confirmed the known relationship between CD19 expression and t(8;21)-positive AML M2; in these cases, CD19 was detected with all three antibodies. We also found a unique pattern of CD19 expression in cases of AML with a substantial monocytic-monoblastic component. In 6 of 12 cases of AML M4 or M5, CD19 expression was evident only with the B4 (Lytic) antibody; CD19 expression was not observed using B4 89B or SJ25-C1. We did not observe any recurring chromosomal abnormalities in these cases of CD19-positive AML M4/M5; furthermore, none of these cases demonstrated a t(8;21). Using CD11b, CD14, and other myeloid markers, we found that AML M4 and AML M5 were characterized by dual populations of blasts. With the exception of a case of AML M4 eo, cases of AML M4 were associated with one population of blasts lacking both CD11b and CD14 and a second population with one or both of these antigens. Cases of AML M5 also had dual populations of blasts, but in contrast with AML M4, each population expressed CD11b, CD14, or both. Our findings suggest that specific immunophenotypic patterns, including the unique pattern of CD19 expression with the B4 (Lytic) monoclonal antibody, may prove useful in classifying cases of AML M4 and M5.