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Hematopoietic stem cells (HSCs) show pronounced heterogeneity in self-renewal and differentiation behavior, which is reflected in their repopulation kinetics. Here, a single-cell-based mathematical model of HSC organization is used to examine the basis of HSC heterogeneity. Our modeling results, which are based on the analysis of limiting dilution competitive repopulation experiments in mice, demonstrate that small quantitative but clonally fixed differences of cellular properties are necessary and sufficient to account for the observed functional heterogeneity. The model predicts, and experimental data validate, that competitive pressures will amplify small clonal differences into large changes in the number of differentiated progeny. We further predict that the repertoire of HSC clones will evolve over time. Last, our results suggest that larger differences in cellular properties have to be assumed to account for genetically determined differences in HSC behavior as observed in different inbred mice strains. The model provides comprehensive systemic and quantitative insights into the clonal heterogeneity among HSCs with potential applications in predicting the behavior of malignant and/or genetically modified cells.

The question whether stem cells age remains an enigma. Traditionally, aging was thought to change the properties of hematopoietic stem cells (HSC). We discuss here a new model of stem cell aging that challenges this view. It is now well-established that the HSC compartment is heterogeneous, consisting of epigenetically fixed subpopulations of HSC that differ in self-renewal and differentiation capacity. New data show that the representation of these HSC subsets changes during aging. HSC that generate lymphocyte-rich progeny are depleted, while myeloid-biased HSC are enriched in the aged HSC compartment. Myeloid-biased HSC, even when isolated from young donors, have most of the characteristics that had been attributed to aged HSC. Thus, the distinct behavior of the HSC isolated from aged hosts is due to the accumulation of myeloid-biased HSC. By extension this means that the properties of individual HSC are not substantially changed during the lifespan of the organism and that aged hosts do not contain many aged HSC. Myeloid-biased HSC give rise to mature cells slowly but contribute for a long time to peripheral hematopoiesis. We propose that such slow, "lazy" HSC are less likely to be transformed and therefore may safely sustain hematopoiesis for a long time.

Hematopoietic stem cells (HSC) show heterogeneous behavior even when isolated as phenotypically homogeneous populations. The cellular and molecular mechanisms that control the generation of diversity (GOD) in the HSC compartment are not well understood, but have been the focus of much debate. There is increasing evidence that the most important HSC functions, self-renewal and differentiation, are epigenetically preprogrammed and therefore predictable. Indeed, recent data show that the adult HSC compartment consists of a limited number of functionally distinct subsets of HSC. This contradicts older models of HSC behavior, which postulated a single type of HSC that can be continuously molded into different subtypes of HSC. We propose a clonal diversity model where the adult HSC compartment consists of a fixed number of different types of HSC, each with epigenetically preprogrammed behavior. Aging or disease may change the overall function of the HSC population. The model predicts that these changes reflect the relative composition of the HSC subsets, rather than changes in individual HSC. This view has implications for using HSC in experimental and clinical settings. Selection for the appropriate subsets of HSC, rather than attempts to force HSC to adjust, should improve their utility in transplantation and gene transfer applications.

The use of bone marrow transplantation (BMT) for the treatment of genetic diseases with neurologic involvement has yielded mixed results. We have employed a mouse model of Lesch-Nyhan disease (LND) to assess the efficacy of BMT in ameliorating the neurologic manifestations of the disease. Adult HPRT-deficient mice exhibit a measurable decrease in striatal dopamine levels and a hypersensitivity to amphetamine. Marrow-ablated adult HPRT-deficient mice were transplanted with marrow from congenic HPRT-expressing mice. BMT altered neither the neurochemical nor the behavioral phenotypes in either HPRT-positive or HPRT-deficient mice. Barring any important species differences, these results suggest that BMT in its present form may not be an effective therapy for Lesch-Nyhan syndrome.

Nearly all hematopoietic cells in mammals derive from precursors that undergo much or all of their development in the bone marrow. In vitro models for many lineages are available and represent modifications of the original bone marrow culture system designed by Dexter and Lajtha (1) and described elsewhere in this book. In this chapter, we describe a second bone marrow culture system, first reported in 1982 (2), that provides an in vitro environment selectively supporting long-term proliferation and differentiation of early B lymphocyte lineage cells. This method can be used to obtain heterogeneous populations of immature precursors of the B cell lineage greatly enriched from other hematopoietic cell types. Clonal populations can also be obtained by extension of this method to limiting dilution culture.

Two novel early B lymphocyte precursor populations have been identified by their capacity to differentiate in Whitlock-Witte bone marrow cultures. Cells expressing neither the B lineage antigen B220 nor Thy-1 contain committed B cell precursors which differentiate in short-term culture into pre-B and B cells. The other population expresses low levels of Thy-1, and lacks B220 as well as the T cell markers L3T4 and Lyt-2. The Thy-1+ cells which initiate long-term B cell cultures contain clonogenic B cell precursors at a frequency of 1 in 11, a 100-fold enrichment over unseparated bone marrow. Thy-1+ cells are also highly enriched for myeloid-erythroid precursors (CFU-S). Thy-1+ cells allow long-term survival of lethally irradiated mice and fully reconstitute the hematopoietic system, including T and B lymphocyte compartments. These results indicate that this population (approximately 0.1% of bone marrow) may contain the pluripotent hematopoietic stem cell.

Bone marrow stromal cell lines have been isolated that directly support B lymphopoiesis in vitro. Single B-lineage precursors proliferate and differentiate on certain of these stromal cell lines to establish long-term B-lineage cultures. These lymphopoietic stromal cells produce novel soluble factors that support proliferation of in vitro established pre-B cell populations. Lymphoid populations established on lymphopoietic stromal cell lines lack surface Ig-bearing cells, but give rise to surface Ig+ cells when transferred to mixed bone marrow feeder layers. Several stromal lines expressed a B-lineage neoplasia marker detected by the monoclonal antibody MAb6C3. Remarkably, only the 6C3Aghi stromal lines supported long-term proliferation of B-lineage cells. We propose that the 6C3 antigen-bearing molecule may play a role in stromal cell-dependent, pre-B cell proliferation, as well as in neoplastic proliferation of pre-B leukemias.