Identification and Hematopoietic Potential of CD45 − Clonal Cells with Very Immature Phenotype (CD45 − CD34 − CD38 − Lin − ) in Patients with Myelodysplastic Syndromes

CD45 is a hematopoietic lineage‐restricted antigen that is expressed on all hematopoietic cells except for some mature cell types. Cells expressing CD45 and CD34 but lacking CD38 and lineage antigens (CD45 + CD34 + CD38 − Lin − cells) are well‐documented hematopoietic stem cells (HSCs), and CD45 + CD34 − CD38 − Lin − cells are probably less mature HSCs. In myelodysplastic syndromes (MDS), the malignant transformation site is a matter of debate, and CD45 + CD34 + CD38 − Lin − HSCs were recently reported to be clonal. In the study reported here, we detected CD45 − CD34 − CD38 − Lin − cells in the peripheral blood and bone marrow of patients with MDS and isolated them by successive application of density centrifugation, magnetic cell sorting, and fluorescence‐activated cell sorting. Fluorescence in situ hybridization showed that CD45 − CD34 − CD38 − Lin − cells had the same chromosomal aberration as the myeloblasts. In addition to CD45 − and CD34 − , they lacked CD117 and CD133 expression. Generally, MDS cells have extremely reduced hematopoietic potential compared with normal hematopoietic cells, but we documented the following in some patients. Freshly isolated CD45 − CD34 − CD38 − Lin − cells did not form any hematopoietic colonies but had long‐term culture‐initiating cell activity. When cocultured with stroma cells, CD45 − CD34 − CD38 − Lin − cells showed only weak potential for proliferation and differentiation, yet they differentiated into CD34 + cells and then mature myeloid cells. This newly identified cell population represents the most immature immunophenotype so far identified in the hematopoietic lineage and is involved in the malignant clone in MDS.