Implications of the expression of myeloid markers on normal and leukemic stem cells.

We have recently described the expression of CD33 and other antigens previously thought to be myeloid specific on both acute myeloid leukemia-initiating cells (AML-IC) and normal hematopoietic stem cells (HSC) that are capable of repopulating immuno-deficient mice. Here, we discuss that the presence of myeloid markers on AML-ICs and HSCs makes identification of an antigen for targeted therapy extremely difficult. Xenotransplantation assays should be used wherever possible to test the suitability of candidate antigens for targeted therapy, including the assessment of the effect on normal cells.