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Blood, 15 April 2008, Vol. 111, No. 8, pp. 4413-4414.
CORRESPONDENCE CD150– cells are transiently reconstituting multipotent progenitors with little or no stem cell activityTo the editor:Goodell and colleagues recently published a paper in Blood (Weksberg et al1) that confirmed many of our conclusions from prior studies demonstrating the utility of signaling lymphocytic activation molecule (SLAM) family markers to enhance the purification of mouse hematopoietic stem cells (HSCs).2–5 However, they took issue with 2 conclusions. First, they argued that SLAM family members are not sufficient for the identification of HSCs and second that "a substantial fraction" of HSCs are CD150–, in contrast to our published data. However, the data in the Goodell paper do not support these conclusions. Weksberg et al1 argued that in contrast to our published data,2–5 only a minority of cells expressing SLAM family markers (CD150+CD48–CD41–) expressed the HSC markers c-kit and Sca-1. However, Figure 1B in the Goodell paper shows that they used an inappropriately loose gate that included mainly CD150– cells in the CD150+CD48–CD41– population. Because the gate included 1% of cells in the isotype control and 0.4% of cells in the experimental sample, the authors did not attempt to sort the CD150+CD48–CD41– population characterized in prior studies, which represents only 0.007% of cells.3 The gate used by Weksberg et al1 is different from that illustrated in our papers (compare Figure 2A from Kiel et al2 to Figure 1B in Weksberg et al1). Obviously, SLAM markers will render poorer purity than reported in prior studies if gates are set to include mainly CD150– cells. The only experimental support offered by Weksberg et al1 for the existence of CD150– HSCs was an experiment in which 100 CD150– c-Kit+Lineage–Sca-1+ (KLS) side population (SPKLS) cells were injected into irradiated mice.1 They observed significantly less reconstitution than that observed from CD150+ SPKLS cells but nonetheless concluded that CD150– cells represent "a substantial fraction" of HSCs. However, to test this conclusion it would be necessary to perform a more careful clonal analysis. What percentage of CD150– cells actually give long-term multilineage reconstitution: 1%, 10%, 100%? Without such data it is impossible to know what percentage of HSCs are CD150–, whether such cells represent a significant fraction of HSCs, or whether the HSC activity could be explained by rare contamination from CD150+ cells. Weksberg et al did not even disclose what fraction of mice that received transplants of 100 CD150– SPKLS cells showed long-term multilineage reconstitution. If some did not, this would prove that less than 1% of CD150– SPKLS cells are HSCs, disproving the conclusions of this paper. We have performed clonal analyses of the CD150– fraction of c-kit+Sca-1+CD48– (KS48) or c-kit+Thy-1lowLineage–Sca-1+ (KTLS) cells. We found less than 5% of HSCs in the CD150– fraction of whole bone marrow, KS48, or KTLS cells (Table 1). CD150– cells give transient multilineage reconstitution of irradiated mice. These results have been independently replicated by other labs in papers6,7 not cited by Weksberg et al.1 Our conclusion that CD150– KS48 cells are mainly transiently reconstituting multipotent progenitors is consistent with data in the Goodell paper that showed more rapid proliferation and reduced myeloid reconstitution from CD150– SPKLS cells compared with CD150+ SPKLS cells. Goodell's conclusion that CD150– cells represent a substantial population of rapidly dividing HSCs that can give rise to CD150+ HSCs but that have less reconstituting potential than CD150+ HSCs is internally inconsistent, inconsistent with more rigorous data from multiple labs, and inconsistent with decades of work demonstrating that HSCs are slowly dividing.
Authorship Conflict-of-interest disclosure: The authors declare no competing financial interests. Correspondence: Sean J. Morrison, 5435 Life Sciences Institute, 210 Washtenam Avenue, Howard Hughes Medical Institute and Center for Stem Cell Biology, University of Michigan, Ann Arbor, MI 48109-2216; e-mail: seanjm{at}umich.edu.
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