Class II–Associated Invariant Chain Peptide Expression Represents a Novel Parameter for Flow Cytometric Detection of Acute Promyelocytic Leukemia

Because of severe bleeding complications, patients with acute promyelocytic leukemia (APL) have to be treated with all- trans retinoic acid immediately following diagnosis. In addition to morphology, flow cytometry contributes to a rapid detection of APL according to phenotypic characteristics of leukemic cells. In some patients, these analyses are inconclusive or even contradictory to diagnosis. Previously, we showed the clinical and functional impact of class II–associated invariant chain peptide (CLIP) in acute myeloid leukemia (AML). This study focuses on the analysis of CLIP expression on leukemic cells to characterize HLA-DR–negative AML, including APL. We demonstrate exclusive and significant CLIP expression in all cases of typical and variant APL, as compared to other HLA-DR–negative non–APL-type AML. CLIP appears to be a highly sensitive and specific flow cytometric marker, resolving discrepant identification of both genetic subgroups. Our findings show the additive value of CLIP analysis for a fast and unequivocal recognition of APL by flow cytometry in conjunction with morphology.