An immunofluorescent assay for acute promyelocytic leukemia cells

Sequential treatment with all-trans retinoic acid followed by chemotherapy significantly improves the long-term survival of patients who have acute promyelocytic leukemia (APL). Consequently, a simple and accurate test is needed to establish the diagnosis of APL and to identify those patients having a relapse of the disease. We describe an accurate, 2-hour indirect immunofluorescent assay for identifying APL cells in bone marrow specimens. The assay uses the PML (PG-M3) murine monoclonal antibody that is directed against the amino-terminal portion of the PML gene product. We observed a distinctive, finely speckled pattern of fluorescence in the NB4 cell line (a positive control), as well as in 15 clinical specimens that were confirmed to have APL by cytogenetic, cytochemical, and immunophenotypic studies, including four cases of microgranular variant of APL. By contrast, a coarse globular pattern of fluorescence was observed in 53 other clinical specimens that did not contain APL. When we performed dilution studies using artificial mixtures of APL cells with normal bone marrow cells, we detected as few as 5% APL cells in the mixture. Finally, there was complete concordance between the immunofluorescent assay and a polymerase chain reaction-based assay for the PML-retinoic acid receptor alpha chimeric gene in 12 other clinical specimens. We conclude that the immunofluorescent assay for PML protein is a rapid, sensitive, and accurate method for determining the presence of APL cells in clinical specimens. This assay therefore should be considered as a cost-effective alternative to other diagnostic tests, such as karyotyping or polymerase chain reaction, for the diagnostic evaluation of APL.