Leukemia cell lines: in vitro models for the study of acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) serves as a paradigm in clinical and biological leukemia research. Firstly, APL represents a model for the new therapeutic approach of differentiation therapy, taking advantage of the ability of APL cells to respond to retinoic acid treatment by terminal differentiation. Secondly, the 15;17 chromosomal translocation specific for APL leads at the molecular genetic level to a chimeric gene fusing the PML and RAR alpha genes and appears to be an instrumental, if not actually the causative event, in the neoplastic process. These unique characteristics of an otherwise rather rare disease have recently attracted intense research interest. As in other types of leukemia where continuous cell lines are powerful research tools, studies using APL-derived cell lines have contributed a large body of relevant data in efforts to unravel the pathobiology and leukemogenesis of APL. Three cell lines have been reported to be derived from patients with APL: HL-60, NB-4 and PL-21. Both HL-60 and PL-21 lack t(15;17) while NB-4 carries this cytogenetic hallmark pathognomonic for APL. Morphological and immunophenotypical examinations of the cell lines do not permit a clear assignment to any stage of myelomonocytic differentiation. Some additional data, such as expression of myeloperoxidase, monocyte-specific esterase and annexin VIII, together with the cytogenetic and molecular biological information, suggest that NB-4 is the only genuine promyelocytic leukemia cell line, whereas HL-60 may represent a discrete stage of differentiation between the late myeloblasts and the promyelocyte; PL-21 has distinct features associated with monocytic cells. These cell lines provide unique in vitro model systems for studying the cellular and molecular events involved in the proliferation and differentiation of normal and leukemic myelomonocytic cells.