Clonal Features of Hematopoietic Stem and Progenitor Cells in Aging

It is traditionally thought that Hematopoietic Stem Cells (HSCs) maintain blood homeostasis through long-term self-renewal and multilineage differentiation. This concept, however, is challenged by two recent studies in which the fundamental features of unperturbed hematopoiesis are evaluated by different approaches of lineage tracing. Both the kinetic analysis of HSC output by the Rodewald group and our clonal analysis with transposon barcoding suggest a dominant role of non-transplantable short-term HSCs and progenitors, but not the long-term HSCs, in driving native blood cell production. In addition, our longitudinal analysis of peripheral blood demonstrates extensive clonal succession in granulocyte production. These findings collectively suggest a distinct mechanism of native hematopoiesis that differs significantly from what has been learned in transplantation experiments. At the same time, they bring to light new questions regarding the ultimate fate of the progenitor population and the exact contribution of HSCs under normal physiological conditions. To address these questions, we examined clonal features of HSCs and progenitors in aged mice. Our results show a progressive reduction in clonal complexity and a concurrent increase in clonal stability when blood granulocytes are analyzed up to a hundred ten weeks after transposon barcoding. As time elapses, clonal overlapping between granulocytes and B cells become much more extensive, suggesting an increased tendency toward multilineage differentiation during aging. Analysis of stem and progenitor cells in bone marrow of aged mice reveals prevalent lineage output by multipotent progenitors (MPPs), whereas a lower fraction of HSC clones are found to produce mature progeny. While this overall pattern of differentiation is reminiscent of what has been observed in young and middle-aged animals, a two-fold increase in HSC clonal output was observed in these old mice, indicating their increased contribution to blood cell production. A comparison of clonal compositions in blood and marrow cell populations demonstrates an MPP origin of stable peripheral blood clones, and a smaller fraction of these clones can even be traced back to HSCs. These observations hence suggest extensive self-renewal and asymmetric cell division of these two cell populations in aging. Taken together, our results indicate that the aged hematopoietic system is characterized by reduced clonal complexity, increased clonal persistence, and HSC activation. The higher propensity to self-renewal during aging may also explain the elevated risk of malignant transformation in the elderly population.