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With the dramatic increase of interest aimed at manipulating and transplanting normal human stem cells for clinical applications and for purging or eradicating leukemic stem cells, there is great need for in vivo assays for normal and leukemic human stem cells. The recent development of methods to transplant human hematopoietic cells into immune-deficient mice provides an important approach to characterize stem cells and to develop animal models for hematopoietic diseases including leukemia

The hematopoietic system is organized as a hierarchy where the majority of the cells are mature, and therefore, need to be continuously replenished from a small pool of immature progenitors. Ultimately, the entire hematopoietic system is derived from stem cells that have extensive proliferative and differentiation capacity and give rise to all myeloid and lymphoid lineages. Insight into the biology of these stem cells is, therefore, fundamental to our understanding of the regulation of normal and abnormal hematopoiesis.

Over the last 40 years the hematopoietic system has provided many of the important paradigms that guide our understanding of stem cell function. Much of our knowledge of the regulation of the hematopoietic system is derived from experiments in the mouse; these studies have involved identification of various classes of progenitor cells, growth factors that stimulate growth and differentiation, and molecular events that underlie the abnormalities that occur in diseases such as leukemia. This information has derived largely from the development of in vivo transplantation assays for normal stem cells and the ability to establish and grow leukemic cells in vitro and in vivo [1]. In contrast, our understanding of the biology of the human hematopoietic system has suffered relative to that in the mouse because of the lack of similar assays for normal stem cells and leukemic cells. Normal and leukemic human cells often appear to have complex growth factor requirements that are not easy to provide in short- or long-term cultures. Furthermore, the difficulties in growing primary human leukemic cells in culture suggest that there are selective processes that may result in alterations of the properties of such cells over time and the resultant cell Unes do not accurately reflect the original disease. In an attempt to develop in vivo animal models for human leukemic cells, a large body of literature has accumulated over the past 20 years on the growth of human tumor xenografts in immune-deficient nude mice [2]. However, the growth of human leukemic cells as an ascites or solid subcutaneous tumor in nude mice does not reflect the normal course of the disease in humans. In addition to leukemic cells, normal human hematopoietic cells have also been introduced into nude mice directly or in diffusion chambers. The transplantation of human bone marrow directly into mice generally yielded inconclusive results [3], while the implantation of diffusion chambers demonstrated the development of human progenitors for as long as 28 days in vivo although it was not possible to distinguish between persistence of progenitors and engraftment of stem cells [4].

Understanding the process of differentiation and development remains as a major challenge in biology. The hematopoietic system offers unique advantages for such studies. The blood-forming system of mice and humans consists of a heterogeneous array of cells, ranging from large numbers of differentiated cells with defined function to rare pluripotent stem cells with considerable developmental and proliferative potential (Till and McCulloch 1980). The reason this complex developmental system has proved to be so malleable for experimentation rests in the availability of quantitative colony assays for committed and multilineage progenitor cell types. Although no colony assays exist for the earliest cells in the murine stem cell hierarchy, they can nevertheless be indirectly assayed by reconstitution of lethally irradiated or genetically deficient mice. A large body of knowledge has accumulated over the past 25 years about the various classes of progenitor and stem cells, as well as the factors and cell-cell interactions that regulate their developmental program. Although in vitro colony assays exist for committed and multilineage human progenitor cells, our understanding of the biology of the human hematopoietic system has suffered relative to that in the mouse because of the lack of an in vivo assay system for human pluripotent stem cells. Several groups have recently reported the engraftment of immunedeficient mice with human lymphoid (Mosier et al 1988; McCune et al 1988) and myeloid hematopoietic cells (Kamel-Reid and Dick 1988). These animals offer an enormous opportunity to examine the human hematopoietic system and may lay the foundation for a human stem cell assay. This paper will briefly review our efforts to establish normal human hematopoietic cells in immunedeficient animals and examine the potential application of this approach to study human leukemic transformation.

Knowledge of the organization and regulation of the human stem cell hierarchy has lagged behind similar understanding in the mouse largely because of the lack of a suitable assay for human stem cells. Several new assays, based on transplantation of human hematopoietic cells into immune-deficient mice, have recently been developed which may detect human stem cells. Combining these assays with the high efficiency gene transfer technologies that have been developed using retrovirus vectors should prove to be a powerful approach to identifying the genetic events that govern the developmental program of human hematopoietic stem cells.