CD9: Differential expression of normal bone marrow cellular components and leukemic myeloid blasts.

Objective: Research on CD9 expression has been extensive in B lymphoblastic leukemia, with fewer studies focusing on acute myeloid leukemia (AML). We investigated the usefulness of CD9 in differentiating normal from abnormal myeloid progenitors, as well as expression in normal cell types and in AML.

Methods: Flow cytometry was used to assess the level of CD9 expression on normal and leukemic myeloid blasts and other normal bone marrow populations. Geometric mean fluorescence intensity levels and expression patterns were compared among cell types and AML subtypes.

Results: In normal subsets (n = 69), the level of CD9 expression was lowest in mature B cells, myeloid blasts, promyelocytes, and neutrophils, with intermediate expression in monocytes and highest in hematogones (stages 1 and 2). Committed myeloid progenitors (CMPs) had lower expression than hematopoietic stem cells (HSCs). CD9 typically has higher expression in AML (n = 58) compared to normal myeloid blasts and promyelocytes, and it is differentially expressed in AML, with the highest expression in PML::RARA AML.

Conclusions: Aberrant CD9 expression can be useful differentiating normal from abnormal myeloid progenitors, with the highest level of expression in AML with PML::RARA in our cohort. There was differential expression between HSCs and CMPs in the small numbers studied. Normal mature B cells can be used as an internal negative control in most cases.