Erythroid Progenitor Cells: Their Relevance for the Study of Haematological Disease

Structure and regulation of the erythroid system at the level of progenitor cells

Considerable ground has been covered since the first clonal assays for hemopoietic cells were described. The possibility of studying populations of progenitor cells and the regulatory factors that influence them has already thrown considerable light on our understanding of the structure and physiology of the normal erythroid system and its alterations in disease. The relative importance of humoral and short-range factors and of possible cell-to-cell interactions in the regulation of proliferation and differentiation in the erythroid cell lineage is now being studied actively in several laboratories. The possibility of analyzing possible regulatory networks involving such highly reactive cells as lymphocytes and monocytes-macrophages in defined in vitro conditions now exists. As these studies are being extended to the diseased state, concepts related to alterations in regulatory mechanisms in syndromes with abnormal cell proliferation can be tested. Clinical applications in the treatment of patients with hematological disease are being contemplated. The usefulness of Epo for the treatment of the anemia of renal disease has been demonstrated already.

In vitro drug testing using hemopoietic cells: goals and limitations

In vitro drug sensitivity is one of many biologic variables which may predict in vivo drug response. Even if in vitro assays provide relevant data, for some tumors, variable levels of stem-cell origin, differentiation, tumor heterogeneity, or self renewal may be more important than cytotoxicity to proliferating cells. Although ANLL has been used here frequently as a model, it may not be the most appropriate tumor for study. Unlike many cancers, in ANLL, primary drug resistance is unusual, and in relapse, secondary drug resistance is usually incomplete. It has been suggested that in vitro drug sensitivity predicts remissions for patients who do not die of infection or remain aplastic during induction therapy. However, for the majority of patients, this argument acknowledges the overriding importance of biologic variables other than in vitro drug cytotoxicity. For rapidly growing tumors, such as Burkitt's lymphoma, rapid emergence of drug resistance related to disease burden may be the most important response determinant. Perhaps in other tumors, in vitro drug sensitivity will be an independent variable of overriding importance. To determine the role of in vitro drug testing, trials examining in vitro drug sensitivity must meet stringent criteria. The assays should use well-defined and reproducible cultures and drug exposures. The trials must be large enough, contain homogenously treated patients, and use carefully defined response and survival endpoints. Decision rules derived from such trials must be further tested by prospective evaluation. Investigators conducting these trials must be prepared to search for important in vitro results reflecting tumor biology and to analyze in vitro drug sensitivity as only one continuous variable determining in vivo responses. Such trials will be difficult to conduct and expensive. In the final analysis, in vitro assays may find their most important roles as preclinical drug screens and models for in vitro drug resistance. Further insights into molecular genetics of malignant transformation and drug resistance may make such assays obsolete, but for the present, they provide important insights into tumor variability and mechanisms of drug response.

[Humoral factors in the regulation of cell proliferation in haematopoiesis. II. Erythropoiesis, megakaryopoiesis, multipotent stem cell (author's transl)]

Diffusible factors probably play an important role in the control of cell proliferation during haematopoiesis. Several stimulators of erythropoiesis including erythropoietin are to be found in sera. Inhibitors are found in erythrocyte supernatants, in the urin of newborn children, and in plasma of patients with renal insufficiency. Stimulators of megakaryopoiesis have been shown to be present in conditioned media of lymphoid cells and bone marrow cells. Inhibitors have been detected in various sera. Proliferation of multipotent stem cells is probably also regulated by a humoral mechanism. Stimulators and inhibitors are present in bone marrow extracts. Clinical relevance of these factors is discussed.

Isolation of human haematopoietic progenitor cells using monoclonal antibodies

Studies of the differentiation of human haematopoietic cells have been greatly advanced by the use of in vitro cloning techniques to enumerate the myeloid and erythroid progenitor cells which form colonies in semi-solid medium. The granulocytic/monocytic colony-forming cells (CFU-GM) and erythroid burst-forming cells (BFU-E) are thought to be mononuclear cells, similar in size to medium size lymphocytes, but their ontogenetic relationship to other progenitor cells and the putative pluripotential stem cell remains obscure. This is partly because elucidation of parent-progeny relationships requires the isolation of precursor cells before they acquire mature phenotypic characters. Physical methods have failed to separate CFU-GM cleanly from other cells because of their heterogeneity, and immunologically specific markers for human progenitor cells have not been reported. We report here that a fraction containing all the granulocytic/monocytic and erythroid progenitor cells, as well as cells which may be lymphoid progenitors, can be isolated from bone marrow using a combination of monoclonal antibodies.