CD133 Is a Positive Marker Of Human Cord Blood-Derived CD34-Negative Hematopoietic Stem Cells

We have previously identified very primitive human cord blood (CB)-derived CD34-negative (CD34^-) severe combined immunodeficiency (SCID)- repopulating cells (SRCs) using the intra-bone marrow injection (IBMI) method (Blood 2003:101;2924). A series of our studies suggests that the identified CD34^- SRCs are a distinct class of primitive hematopoietic stem cell (HSC) and that they are at the apex of human HSC hierarchy. Recently, we developed a high-resolution purification method for primitive CD34^- SRCs using 18 lineage (Lin)-specific antibodies, which can enrich CD34^- SRC at 1/1,000 level (Exp Hematol 2011: 39:203). In the present study, we tried to identify the positive marker of CD34^- SRCs in order to further purify and characterize the CD34^- SRCs (HSCs).

First, we extensively analyzed candidate positive markers, including known HSC markers and various adhesion molecules by FACS using highly purified CB-derived 18Lin^-CD34^+/- cells. Finally, we identified CD133 as a positive marker of human CB-derived CD34^- SRCs. Then, CB-derived 18Lin^- CD34^+/-CD133^+/- cells were sorted by FACS, and hematopoietic stem/progenitor cell (HSPC) capacities of these four fractions of cells were extensively investigated. HSPC capacities were evaluated using (1) colony-forming cell (CFC) assays, (2) measurement of maintenance/production of CD34⁺ cell capacities in co-cultures with human bone marrow-derived mesenchymal stromal cells (BM-MSCs) (Blood 2010:24:162), (3) SRC activities using NOG mice, (4) limiting dilution analyses (LDA) to determine the SRC frequency in the 18Lin^-CD34^-CD133⁺ fractions, and (5) comparison of gene expression profiles between 18Lin^-CD34^+/-CD133^+/- cells by real-time RT-PCR.

Seventy-five percent of 18Lin^-CD34⁺ and 13.5% of 18Lin^-CD34^- cells highly expressed CD133. In the CFC assays, the plating efficiencies of 18Lin^-CD34⁺CD133⁺, CD34⁺CD133^-, CD34^-CD133⁺ and CD34^-CD133^- cells were 57%, 65%, 39% and 19%, respectively. Interestingly, most of 18Lin^-CD34^-CD133^+/- cells formed erythroid-bursts (71% and 73%) and erythro/megakaryocytes-containing mixed colonies (25% and 27%). On the contrary, they formed few granulocyte/macrophage colonies (4.2% and 0%). Then, we co-cultured these four fractions of cells with human BM-MSCs. One thousand of 18Lin^-CD34^+/-CD133^+/- cells were seeded into each well and cells were co-cultured for 7 days in the presence of SCF+TPO+FL+IL-3+IL-6 +G-CSF. Both the 18Lin^-CD34^-CD133^+/- cells produced CD34⁺ cells. However, the percentage and absolute number of CD34⁺ cells produced from 18Lin^-CD34^-CD133⁺ cells (31.7 % and 3.2 x 10⁴ cells) were greater than those of 18Lin^-CD34^-CD133^- cells (13.2 % and 0.4 x 10⁴ cells). In addition, both the 18Lin^-CD34^- CD133^+/- cells generated higher percentages (13.5 % and 11.5%) of CD41⁺ cells compared to those of the 18Lin^- CD34⁺CD133^+/- (1.8% and 4.2%) cells. Collectively, 18Lin^-CD34^+/-CD133^+/- cells showed different in vitro lineage differentiation potentials. Then, these four fractions of cells were transplanted into NOG mice by IBMI. We performed primary and secondary transplantations for up to 36 weeks. In the results, all of the mice received 18Lin^-CD34⁺CD133⁺ cells (n = 5) or 18Lin^-CD34^-CD133⁺ cells (n = 9) showed primary and secondary human CD45⁺ cell repopulations. However, neither 18Lin^-CD34⁺CD133^- cells nor 18Lin^-CD34^-CD133^- cells showed human cell repopulations (n = 6 in each group). These results clearly demonstrated that the CD133 expression clearly segregated SRC activities in the 18Lin^-CD34^+/- cells. Moreover, LDA demonstrated that the frequency of SRCs in the 18Lin^-CD34^-CD133⁺ fraction was 1/142. Interestingly, HSC self-renewal maintenance genes, such as Notch1, HoxB4, HoxA9, and Bmi-1, were highly expressed in both 18Lin^-CD34^+/-CD133⁺ cells.

These results clearly demonstrated that CD133 is a positive marker of human CB-derived CD34^- SRCs (HSCs). Furthermore, CD133 segregated SRC activities of 18Lin^-CD34^- as well as 18Lin^-CD34⁺ cells in its positive fractions. More importantly, these findings suggest that number of CD133⁺ cells in cord blood units is a more appropriate marker to detect/predict HSC potentials in cord blood stem cell transplantation in comparison to currently used CD34⁺ cell numbers.

No relevant conflicts of interest to declare.