Acute promyelocytic leukemia: four distinct patterns by flow cytometry immunophenotyping

A total of 97 acute promyelocytic leukemia (APL) patients with adequate flow cytometry (FC) data, bone marrow aspirates and presence of t(15;17)/PML-RARA by cytogenetics and/or FISH studies were analyzed for immunophenotypic pattern. Leukemic cells had the following phenotype: CD11b^–, CD11c^–, CD13⁺, CD33⁺, CD45⁺, CD64^+/–, CD117⁺, and HLA-DR^–. A subset of cases showed also an expression of CD2, CD4, CD34, and CD56. Based on the immunophenotype and side scatter properties (SSC), four FC patterns were recognized. The majority of cases represented classical (hypergranular) APL and were characterized by high SSC, positive CD117, lack of CD34, heterogeneous CD13, and bright CD33 (pattern 1). Second most common type, corresponding to the hypogranular (microgranular) variant of APL differed from classical APL by low SSC and frequent co-expression of CD2 and CD34 (pattern 2). Rare cases of APL (pattern 3) showed a mixture of neoplastic cells (low SSC/CD2⁺/CD13⁺/CD33⁺/CD34⁺/CD117⁺) and prominent population of benign granulocytes/maturing myeloid precursors (high SSC/CD10^+/–/CD16^+/–/ CD117^–). One case showed two APL populations, one with hypogranular and one with hypergranular characteristics (pattern 4). Apart from a well-known FC pattern of hypergranular APL, we presented less common immunophenotypic variants of APL, which helps to identify an additional group of patients who would benefit from fast confirmatory FISH and/or PCR testing for t(15;17)/PML-RARA.