Changes in cell surface antigen expression during hemopoietic differentiation

Human bone marrow cells were separated on a fluorescence activated cell sorter (FACS) according to their binding of a series of monoclonal antibodies; the positive and negative fractions were cloned for erythroid burst and colony-forming units (BFU-E and CFU-E) and myeloid colony-forming units (CFU-GM), and cytocentrifuge slides were prepared for microscopy of maturing precursors. The pattern of antigen expression on hemopoietic progenitor and precursor populations has been established using antibodies defining blood group (A, I/i), HLA- associated (*A, B, C, DR, DC1), lineage specific, and transferrin receptor antigens. Like monomorphic HLA-DR, the antigen defined by monoclonal antibody OKT10 is expressed on the earliest progenitors and lost during differentiation, suggesting a role in interactions regulating the differentiation of these cells. The HLA-linked DC1 determinant, in contrast to HLA-DR, is not expressed at a detectable level on progenitor cells. Although a lineage-specific early antigen has not been identified, the transferrin receptor is expressed on the majority of erythroid progenitors, but only weakly on myeloid progenitors, and may provide an approach to isolating erythroid progenitors. These and earlier studies with monoclonal antibodies against HLA-DR and glycophorin now provide a detailed "map" of antigen expression during hemopoietic differentiation.     

Decreased hematopoietic accessory cell function following bone marrow transplantation

Hematologic engraftment following bone marrow transplantation requires not only pluripotent stem cells but also functioning accessory cells whose trophic factors support the proliferation and differentiation of stem cells and progenitors to mature blood cells. To better understand the regulation of hematopoiesis following transplantation, we studied hemopoietic accessory cell function in bone marrow transplant recipients 3 weeks to 10 months following transplantation. In general, hematopoietic accessory cell function was decreased following bone marrow transplantation. Sequential fractionation of accessory cells demonstrated that adherent cells often produced near-normal functional burst-promoting activity (BPA) and granulocyte-macrophage colony-stimulating activity (GM-CSA), but Fc receptor-positive (Fc+) cells and T cells uniformly produced greatly reduced BPA and GM-CSA, as compared to transplant donor cells. This cellular deficiency was corrected by soluble burst-promoting activity and granulocyte-macrophage colony-stimulating activity and so appeared to be due to the failure of accessory cells to produce trophic hormones. Limiting-dilution analysis (LDA) for proliferating T-cell precursors demonstrated a greatly reduced frequency in phytohemagglutinin-responsive cells, supporting the role of deficient hematopoietic growth factor production by activated T cells in transplant recipients. This hemopoietic accessory cell defect may thus reflect more generalized lymphocyte dysfunction in these patients. Hematopoiesis following bone marrow transplantation appears to rely upon growth factors released by accessory cells in the adherent layer.     

Dependence of highly enriched human bone marrow progenitors on hemopoietic growth factors and their response to recombinant erythropoietin.

Human bone marrow cells were sequentially fractionated by three negative selection steps to remove adherent cells and Fc receptor-bearing cells, followed by immune adsorption (panning) to deplete maturing cells that react with a panel of monoclonal antibodies. This nonadherent Fc receptor and antibody negative fraction could be further enriched by a positive selection "panning" step, using an antibody to HLA-DR antigen; 12-27% of the cells formed erythroid burst-forming unit (BFU-E), erythroid colony-forming unit, granulocyte-monocyte colony-forming unit, and erythroid and granulocyte and/or monocyte colony-forming unit-derived colonies with recovery of 0.5-1% of the cells and 20-100% of the colony-forming cells. Sequential fractionation resulted in increasing dependence of a subset of BFU-E-derived colonies on exogenous burst-promoting activity (BPA) for proliferation in culture, but the most enriched progenitor fraction still contained a proportion of accessory cell or BPA-independent BFU-E that responded to either natural or biosynthetic erythropoietin when added to cultures on day 0 in the absence of BPA. If the addition of erythropoietin was delayed until day 3, the data suggest that this population of BFU-E either died or became unresponsive to erythropoietin. Delayed addition of erythropoietin to cultures of enriched progenitors provided a sensitive BPA assay, since BPA-independent but erythropoietin-responsive BFU-E were eliminated. The surviving BFU-E that were dependent for their proliferation on the presence of both BPA and erythropoietin showed a characteristic dose response to increasing BPA concentrations.     

Stromal cell-associated erythropoiesis

A novel cover slip-transfer culture system was designed to study the functional roles of stromal cells in hemopoiesis, particularly erythropoiesis. Human bone marrow stromal cell colonies were allowed to develop on small glass cover slips in liquid medium. The cover slips, along with the stromal cell colonies and progenitors attached to them were then transferred to a new tissue culture dish and overlaid with methylcellulose culture medium. No exogenous colony-stimulating factors except erythropoietin were supplied. Large erythroid bursts, comprising multiple subcolonies, developed on the stromal cells. In order to determine if stromal fibroblasts together with erythropoietin and serum proteins could support erythroid development, human bone marrow cells depleted of monocytes, macrophages, and T lymphocytes were allowed to adhere to monolayers of a homogeneous fibroblastoid human stromal cell strain ST-1 grown on cover slips. The cover slips were then washed to remove nonadherent cells, transferred to a new culture dish, and overlaid with methylcellulose culture medium containing fetal calf serum and erythropoietin. In this modified system as well, primitive erythroid progenitors migrated extensively on and within the stroma to form huge colonies of hemoglobinized erythroblasts that proceeded to enucleate. Our results indicate that (1) ST-1 cells together with serum proteins and erythropoietin can support the development of large erythroid bursts; (2) erythroid progenitors and precursors adhere to and migrate on and within the extracellular matrix elaborated by ST-1 cells; (3) erythroid progenitors are more adherent to the ST-1 cells or the extracellular matrix than are the more mature cells and possibly the myeloid progenitors.