Chronic myeloid leukemia (CML) is a myeloproliferative disorder that results from the clonal expansion of a pluripotent stem cell, characterized by the Philadelphia chromosome (Ph).1The clinical course of CML is generally biphasic, representing an initial chronic phase (CP) and a subsequent blast crisis (BC), which is an inevitably terminal event.1 Although almost all the hematopoietic lineages may be involved in this event, two main forms are recognized as lymphoid and myeloid crisis, and herein the latter includes typical myeloid, erythroid, as well as megakaryocytic phenotype. CML is hypothesized to evolve in a multistep fashion from the preleukemialike CP into the most malignant BC. From this viewpoint,a certain fraction of Ph-positive stem/progenitor cells in CP might acquire additional genetic abnormalities and represent maturation arrest at a given stage of differentiation program. Then, the immunophenotype of blast cells in BC might reflect the antigenic profile of such a fraction of stem/progenitor cells in CP. In this sense, some notice should be given to the 2 recent papers in which it was described that the population of CD34+CD7+cells are higher in CML-CP than in normal donors.2 3
We retrospectively examined the immunophenotype of the blast population in 52 consecutive, well-characterized cases of CML-BC diagnosed in our institute, using multicolor flow cytometric analysis. According to the morphological, conventional cytochemical reactions and the lineage-specific immunological markers such as CD3, CD10, CD13, CD19,CD33, and CD41a, these cases were classified as follows: lymphoid(n = 10; 19.2%), myeloid (n = 39; 75.0%), and undifferentiated(n = 3; 5.7%). Myeloid crisis contains myelomonocytic (n = 1) and megakaryocytic (n = 6) crisis as defined by the expression of CD41a. Flow cytometric analysis was performed in a significant part of our cases according to the method as previously described.4 In brief, diluted bone marrow preparations were directly labeled by a mixture of anti-CD34-phycoerythrin (PE) (clone 8G12; Becton Dickinson, San Jose, CA) and anti-CD7-fluorescein isothiocyanate (FITC) (clone 3A1E-12H7; Coulter Inc., Hialeah, FL), or other pairs of antibodies, along with anti-CD45-peridin chlorophyll αprotein (PerCP) (Becton Dickinson). After labeling and lysing,nucleated cells were analyzed on FACS Calibur (Becton Dickinson) with an argon-ion laser tuned at 488 nm. On a CD45 versus sideward light scatter (SSC) dot plot, blast cells occupy a unique blast region characterized by intermediate CD45 intensity and low SSC. Cells labeled with mouse IgG2b-FITC and IgG1-PE were used as control to define the nonlabeled population. Cells were defined to be positive when 10% or more blasts were labeled with the specific monoclonal antibodies. We found that CD7 expression was detected in 80.6% of myeloid BC and 67.7% of undifferentiated BC cases but in neither cases of lymphoid BC, all of which unequivocally revealed B-lineage or mixed-lineage phenotypes (Table). On the other hand, CD34 was expressed in 75% of lymphoid and 96.4% of myeloid BC (Table). Analysis of the relationship between CD7 and CD34 expression on blasts of myeloid crisis showed their dual expression in 24 of 27 cases analyzed.
Similar to our results, Urbano-Ispizua et al observed CD7 expression in 7 of 26 cases (27%) of myeloid BC,5 and Martin-Henao et al reported that CD7 was expressed in 4 of 5 cases.3 On the contrary, Nair et al did not point out its expression in myeloid BC.6 There are some variations in the level of CD7 expression among the reports. These variations would reflect the differences in detection methods. Both Urbano-Ispizua et al and Nair et al used indirect immunohistochemical methods, while Martin-Henao et al performed flowcytometry using a conventional gating strategy. In this respect, the method we applied makes it possible to accurately estimate the immunophenotype of the blasts even when they represent the minor population. Our results suggest that myeloid BC coexpressing CD7 and CD34 may represent the maturation arrest of immature myeloid progenitor cells when CD7 is transiently expressed.
