Identification and Hematopoietic Potential of CD45-Negative 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 was reported to be committed myeloid progenitors and, more recently, the CD45⁺CD34⁺CD38⁻Lin⁻ HSCs. In this study, we detected CD45⁻CD34⁻CD38⁻Lin⁻ cells in the peripheral blood and bone marrow of MDS patients. 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-negativity, they lacked CD117 and CD133 expressions. Generally, MDS cells have extremely reduced hematopoietic potential compared with normal hematopoietic cells, but we documented the following in some cases. Freshly-isolated CD45⁻CD34⁻CD38⁻Lin⁻ cells did not form any hematopoietic colonies but had long-term culture-initiating cell activity. When these cells were co-cultured with stroma cells, CD45⁻CD34⁻CD38⁻Lin⁻ cells showed only weak potential for proliferation/differentiation, yet differentiated to 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.