Whether hematopoietic stem cells (HSCs) change with aging has been controversial. Previously, we showed that the HSC compartment in young mice consists of distinct subsets, each with predetermined self-renewal and differentiation behavior. Three classes of HSCs can be distinguished based on their differentiation programs: lymphoid biased, balanced, and myeloid biased. We now show that aging causes a marked shift in the representation of these HSC subsets. A clonal analysis of repopulating HSCs demonstrates that lymphoid-biased HSCs are lost and long-lived myeloid-biased HSCs accumulate in the aged. Myeloid-biased HSCs from young and aged sources behave similarly in all aspects tested. This indicates that aging does not change individual HSCs. Rather, aging changes the clonal composition of the HSC compartment. We show further that genetic factors contribute to the age-related changes of the HSC subsets. In comparison with B6 mice, aged D2 mice show a more pronounced shift toward myeloidbiased HSCs with a corresponding reduction in the number of both T-and B-cell precursors. This suggests that low levels of lymphocytes in the blood can be a marker for HSC aging. The loss of lymphoid-biased HSCs may contribute to the impaired immune response to infectious diseases and cancers in the aged. IntroductionTraditionally, tissue stem cells were thought to be immortal and exempt from aging. Like all stem cells, hematopoietic stem cells (HSCs) self-renew, and serial transplantation studies show that populations of HSCs can live much longer than the donor from which they were originally isolated. 1 HSCs express telomerase, necessary for chromosome stability during cell division. 2 Together, these observations supported the idea of an immortal HSC. However, there is increasing evidence that HSCs isolated from aged donors are different from HSCs that are obtained from young donors. 3,4 For example, aged HSCs may have a reduced ability to home to the bone marrow (BM), 5,6 have an altered cell surface phenotype, 5,7 may cycle more rapidly, 5 produce more myeloid cells, [5][6][7][8] and have a different gene expression program than their young counterparts. 9,10 HSCs from aged environments may have slightly less self-renewal capacity than their younger counterparts. 11 An accumulation of DNA damage in HSCs has been invoked to explain an age-related decline in HSCs. 12,13 It should be emphasized that most of these age-related changes are controversial, and contradictory data have been published for almost every aspect of the behavior of aged HSCs. One of the reasons for these inconsistencies might be that most of the information about aged HSCs comes from comparisons of populations of HSCs from aged and young sources. However, it is clear by now that the HSC compartment consists of distinct subsets of HSCs. [14][15][16] We showed that HSCs in these subsets are very different from each other, each possessing distinct selfrenewal capacities, differentiation abilities, life span, and repopulation kinetics. [14][15][16] Our work was...

IntroductionHematopoietic stem cells (HSCs) are capable of generating mature cells of all the hematopoietic lineages. Self-renewal capacity, the ability to proliferate without differentiation, is a similarly important hallmark of HSCs. The undifferentiated status of HSCs is characterized by promiscuous expression of genes, many of which were thought to be lineage specific. [1][2][3] Commitment to differentiation increasingly limits the developmental potential of the original HSCs by regulating the level of expression of lineage-specific genes. Lineage-specific genes then maintain the differentiated state and allow the committed progeny to react to environmental stimuli. Such extrinsic signals promote the survival and expansion of differentiated progeny. The commitment process is accompanied by loss of the ability to self-renew. 4,5 The mechanisms that control self-renewal and commitment are incompletely understood.It has been assumed that all HSCs have similar developmental potential. For example, the usual diagrams of HSC differentiation cascade that can be found in any textbook imply that the commitment process generates a myeloid and a lymphoid progenitor from each HSC. This is thought to assure that these lineages can be produced in equal measure. However, there is evidence that the HSC compartment contains distinct and separable subsets of HSCs that can differ in phenotype and primitiveness. "Primativeness" describes the proliferative capacity of an HSC, where primitive HSCs show delayed onsets of repopulation and then sustain production of mature progeny for a long time. Less primitive HSCs contribute rapidly, albeit transiently, to peripheral hematopoiesis. These types of HSCs can be separated to some extent. For example, HSCs that lack c-kit are more primitive, 6 whereas HSCs that express CD4 7 may be more mature than other types of HSCs. However, differences in primitiveness could be a reflection of the proliferative history of the HSC. 8 According to the generation-age hypothesis, 8 all HSCs may be born with the same level of primitiveness, but each proliferation results in loss of primitiveness. Thus, the subsets defined so far may reflect HSCs with different histories rather than cells with distinct functions. This raises the question of whether there is true heterogeneity in the HSC compartment.To define the composition of the HSC compartment, we developed an efficient approach to isolate clonally derived HSCs. Repopulating HSCs were isolated after limiting-dilution cultures on the stromal cell line S17. 9,10 The simplicity of the approach allowed the analysis of a large number of individual HSC clones. Among the HSC clones, we identified lineage-biased (Lin-bi) HSCs as a distinct class of HSCs. 10 Lin-bi HSCs give rise to cells of all hematopoietic lineages, but they generate noticeably skewed ratios of myeloid and lymphoid cells in the periphery. HSCs with similar properties had been identified previously, though they had not been characterized in detail. [10][11][12][13] The clonal analysis indic...

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