Hox genes were first recognized for their role in embryonic development and may also play important lineage-specific functions in a variety of somatic tissues including the hematopoietic system. We have recently shown that certain members of the Hox A and B clusters, such as HOXB3 and HOXB4, are preferentially expressed in subpopulations of human bone marrow that are highly enriched for the most primitive hematopoietic cell types. To assess the role these genes may play in regulating the proliferation and/or differentiation of such cells, we engineered the overexpression of HOXB4 in murine bone marrow cells by retroviral gene transfer and analyzed subsequent effects on the behavior of various hematopoietic stem and progenitor cell populations both in vitro and in vivo. Serial transplantation studies revealed a greatly enhanced ability of HOXB4.transduced bone marrow cells to regenerate the most primitive hematopoietic stem cell compartment resulting in 50-fold higher numbers of transplantable totipotent hematopoietic stem cells in primary and secondary recipients, compared with serially passaged neo-infected control cells. This heightened expansion in vivo of HOXB4.transduced hematopoietic stem cells was not accompanied by identifiable anomalies in the peripheral blood of these mice. Enhanced proliferation in vitro of day-12 CFU-S and clonogenic progenitors was also documented. These results indicate HOXB4 to be an important regulator of very early but not late hematopoietic cell proliferation and suggest a new approach to the controlled amplification of genetically modified hematopoietic stem cell populations.[Key Words: Homeo box genes~ bone marrow transplantation~ retroviral gene transfer]

Class I homeobox (Hox) genes encode a major group of transcription factors controlling embryonic development and have been implicated in the continuing process of hematopoietic cell differentiation. They are clustered on four chromosomes and, in early development, exhibit spatially restricted expression with respect to their 3'-> 5' chromosomal position. By using an improved PCR-based method for amplifying total cDNA derived from limited cell numbers, we now describe the expression of class I Hox genes in highly purified CD34+ cell subpopulations isolated from normal human bone marrow that represent functionally distinct stem and progenitor cell compartments. Our data indicate that at least 16 different Hox genes, mainly from the A and the B clisters, are expressed in one or more of these subpopulations of human hematopoietic cells. Moreover, markedly elevated expression of some of the Hox genes found at the 3' end of the A and B clusters (e.g., HoxB3) was a unique feature of the subpopulations that contained the most primitive functionally defined cells, whereas genes located in the S' region of each cluster (e.g., HoxA10) were found to be expressed at nearly equal levels in the CD34+ subpopulations analyzed. In contrast to the findings for CD34+ cells, expression of two selected Hox genes, HoxB3 and HoxA10, was virtually extinguished in the CD34-fraction of bone marrow cells. These results demonstrate the expression of a broad range of Hox genes in primitive hematopoietic cells and point to the existence of a regulated program of Hox gene expression during their normal development.

Multipotential hematopoietic progenitor cell lines have been established from nonadherent cell populations removed from continuous mouse bone marrow cultures. Clonal sublines of lines B6SUtA or B6JUt derived from single cells formed mixed colonies containing erythroid cells, neutrophil-granulocytes, and basophil/mast cells in semisolid medium containing erythropoietin and conditioned medium from pokeweed mitogen-stimulated spleen cells. Each of several subclones of cell line Ro cl formed colonies containing eosinophils, neutrophil-granulocytes, and basophil/mast cells in semisolid medium. Multipotentiality was maintained in vitro for over 2 1/2 years. In contrast, cell line 32D formed basophil/mast cell colonies with no detectable differentiation to other pathways. Multipotential cell lines did not produce detectable spleen colonies (CFUs) in vivo, nor did intravenous inoculation of up to 5 X 10(7) cells protect lethally irradiated mice from bone marrow failure. Newborn and adult mice inoculated with 5 X 10(7) cells showed no detectable leukemia or solid tumors after one year. Both multipotential and committed basophil/mast cell lines demonstrated absolute dependence upon a source of a growth factor(s) found in medium conditioned by WEHI-3 cells. These cell lines should be of value in studies of the regulation of hematopoietic stem cell differentiation in vitro.

Chronic myeloid leukemia (CML) has long served as a paradigm for generating new insights into the cellular origin, pathogenesis and improved approaches to treating many types of human cancer. Early studies of the cellular phenotypes and genotypes represented in leukemic populations obtained from CML patients established the concept of an evolving clonal disorder originating in and initially sustained by a rare, multipotent, self-maintaining hematopoietic stem cell (HSC). More recent investigations continue to support this model, while also revealing new insights into the cellular and molecular mechanisms that explain how knowledge of CML stem cells and their early differentiating progeny can predict the differing and variable features of chronic phase and blast crisis. In particular, these emphasize the need for new agents that effectively and specifically target CML stem cells to produce non-toxic, but curative therapies that do not require lifelong treatments.

To develop a purification strategy for isolating the most primitive hematopoietic stem cells present in normal human marrow we have combined cell separation techniques with an assay for cells that initiate sustained hematopoiesis in vitro in the presence of irradiated human marrow adherent cells. These “feeders” were established by subculturing 2- to 6-week-old primary long-term marrow culture adherent layers at a density of 3 x 10(4) irradiated cells per square centimeter. Test “long-term culture (LTC)-initiating cells” were plated on top of the feeders and the cocultures then maintained as standard long-term marrow cultures with half-media changes and removal of half of the nonadherent cells each week. The total number of myeloid, erythroid, and multilineage clonogenic progenitors present after 5 weeks was used to provide a quantitative assessment of the number of LTC-initiating cells originally added. Using this assay, the density, light scatter, and two cell surface antigen properties of LTC- initiating cells have been defined and compared with cells capable of directly forming colonies in methylcellulose. While the majority of the clonogenic cells were found in the high forward light scatter (FLS) “blast” window, LTC-initiating cells had significantly lower FLS properties and in this respect were more similar to lymphocytes. LTC- initiating cells also expressed less HLA-DR antigen than clonogenic cells. The majority of LTC-initiating cells were found in the top 2% of the CD34 (My10) fluorescence profile, whereas clonogenic cells were found throughout the top 5% of the CD34 fluorescence profile. By combining low FLS, low orthogonal light scatter (OLS), low HLA-DR expression, and high CD34 expression, a population could be obtained that was enriched for LTC-initiating cells approximately 800-fold over unseparated marrow. This population contains only 0.06% of the marrow cells and 2% of the total clonogenic cells, but retains 50% to 60% of the LTC-initiating cells present in the original marrow. The ability to purify these two populations independently shows that the LTC and clonogenic assays identify distinct, although not necessarily nonoverlapping cell types in human marrow. Since clonogenic cells are derived from LTC-initiating cells, the LTC assay clearly detects a more primitive population. The availability of a simple approach that allows the purification of such cells by three orders of magnitude in high yield should be useful for the investigation of early events in hematopoiesis as well as for the definitive isolation of human hematopoietic stem cells with long-term in vivo repopulating potential.