The finite life span of most mature blood cells requires their continuous replacement, a process termed hematopoiesis. New cell production must respond to basal needs and states of increased demand. Red blood cell production can increase >20-fold in response to anemia or hypoxemia, white blood cell production increases dramatically in response to a systemic infection, and platelet production can increase 10- to 20-fold when platelet consumption results in thrombocytopenia.
The regulation of blood cell production is complex. Hematopoietic stem cells are rare bone marrow cells that manifest self-renewal and lineage commitment, resulting in cells destined to differentiate into the nine distinct blood-cell lineages. For the most part, this process occurs in the marrow cavities of the skull, vertebral bodies, pelvis, and proximal long bones; it involves interactions among hematopoietic stem and progenitor cells and the cells and complex macromolecules of the marrow stroma, and is influenced by a number of soluble and membrane-bound hematopoietic growth factors. A number of these hormones and cytokines have been identified and cloned, permitting their production in quantities sufficient for therapeutic use. Clinical applications range from the treatment of primary hematologic diseases to use as adjuncts in the treatment of severe infections and in the management of patients who are undergoing cancer chemotherapy or marrow transplantation.
Hematopoiesis also requires an adequate supply of minerals (e.g., iron, cobalt, and copper) and vitamins (e.g., folic acid, vitamin B12, pyridoxine, ascorbic acid, and riboflavin); deficiencies generally result in characteristic anemias, or, less frequently, a general failure of hematopoiesis (Hoffbrand and Herbert, 1999; Wrighting and Andrews, 2008). Therapeutic correction of a specific deficiency state depends on the accurate diagnosis of the anemic state, knowledge about the correct dose, the use of these agents in various combinations, and the expected response. This chapter deals with the growth factors, vitamins, minerals, and drugs that affect the blood and blood-forming organs.
Modern concepts of hematopoietic cell growth and differentiation arose in the 1950s when cells from the spleen and marrow were shown to play an important role in the restoration of hematopoietic tissue in irradiated animals. In 1961, Till and McCulloch demonstrated that individual hematopoietic cells could form macroscopic hematopoietic colonies in the spleens of irradiated mice. Their work established the concept of discrete hematopoietic stem cells, which can be experimentally identified, albeit in retrospect (i.e., the presence of a multilineage clonal splenic colony appearing 11 days after transplantation implied that a single cell lodged and expanded into several cell lineages). This concept now has been expanded to include normal human marrow cells. Moreover, such cells now can be prospectively identified.
The basis for identifying soluble growth factors was provided by Sachs and independently by Metcalf, who developed clonal, in vitro assays for hematopoietic progenitor cells. Initially, such hematopoietic colonies developed only in the presence of conditioned culture medium from leukocytes or tumor cell lines. Individual growth factors then were isolated based on their activities in clonal in vitro assays. Many of these same assays were instrumental in purifying a hierarchy of progenitor cells committed to individual and combinations of mature blood cells (Akashi, 2000; Nakorn, 2003).
The existence of a circulating growth factor that controls red blood cell development was first suggested by the experiments of Paul Carnot in 1906. He observed an increase in the red-cell count in rabbits injected with serum obtained from anemic animals and postulated the existence of a factor that he called hemopoietin. However, it was not until the 1950s that Reissmann, Erslev, and Jacobsen and coworkers defined the origin and actions of the hormone, now called erythropoietin. Subsequently, extensive studies of erythropoietin were carried out in patients with anemia and polycythemia, leading to the purification of erythropoietin from urine and the subsequent cloning of the erythropoietin gene. The high-level expression of erythropoietin in cell lines has allowed for its purification and use in humans with anemia.
Similarly, the existence of specific leukocyte growth factors was suggested by the capacity of different conditioned culture media to induce the in vitro growth of colonies containing different combinations of granulocytes and monocytes. An activity that stimulated the production of both granulocytes and monocytes was purified from murine lung-conditioned medium, leading to cloning of granulocyte/macrophage colony-stimulating factor (G-CSF), first from mice (Gough et al., 1984) and subsequently from humans (Wong, 1985). Finding an activity that stimulated the exclusive production of neutrophils permitted the cloning of granulocyte colony-stimulating factor (G-CSF) (Welte et al., 1985). Subsequently, a megakaryocyte colony-stimulating factor termed thrombopoietin was purified and cloned (Kaushansky, 1998).
The growth factors that support lymphocyte growth were not identified using in vitro colony-forming assays but rather using assays that measured the capacity of the cytokine to promote lymphocyte proliferation in vitro. This permitted the identification of the growth-promoting properties of interleukin (Il)-7, Il-4, or Il-15 for all lymphocytes, B cells, or NK cells, respectively (Goodwin et al., 1989; Grabstein et al., 1994). Again, recombinant expression of these cDNAs permitted production of sufficient quantities of biologically active growth factors for clinical investigations, allowing for the demonstration of the potential clinical utility of such factors.
Steady-state hematopoiesis encompasses the production of >400 billion blood cells each day. This production is tightly regulated and can be increased several fold with increased demand. The hematopoietic organ also is unique in adult physiology in that several mature cell types are derived from a much smaller number of multipotent progenitors, which develop from a more limited number of pluripotent hematopoietic stem cells. Such cells are capable of maintaining their own number and differentiating under the influence of cellular and humoral factors to produce the large and diverse number ...