Role of glycoconjugates and their receptors in human hematopoiesis

Cell-surface receptors and ligands, involved in adhesive interactions between human hematopoietic stem/progenitor cells and components of their microenvironment are important regulators of hematopoiesis. This dissertation investigates the role of glycosylated cell surface ligands (glycoconjugates) and their receptors in the physiology of human hematopoiesis. The experiments show that human bone marrow CD34+ cells attach to immobilized, galactose-containing glycoconjugates, via a specific cell surface receptor - lectin. Isolation of CD34+, Lectin+ cells segregates 100% of clonogenic myeloid cells (including up to 75% of the primitive long term culture-initiating cells). The functional significance of this galactose-mediated cytoadhesion is shown by its obligate requirement for the growth of the newly identified progenitor cell (Burst-Forming Unit with multi- lineage potential; BFU-Mix). Further studies demonstrate that bone marrow CD34+ cells and stromal cells express complementary pairs of selectins and their ligands on their cell-surfaces. Among them, L-selectin and E-selectin-ligand are present on CD34+ cells, and isolation of L-selectin and E-selectin-ligand expressing CD34+ cells segregates 90% of all progenitor cells. A significant enrichment of myeloid progenitor cells is shown in CD34+, L-selectin+, E-selectin-ligand + cells, whereas erythoid progenitors are enriched in CD34 +, Lselecinint, E-selectin-- populations. Another series of experiments show that CD34+ hematopoietic cells both adhere to and are inhibited by "artificial" stromal cells, i.e., CHO cells stably expressing alpha(1--3) fucosyltransferase genes (FucT-III, -IV and VII). These data demonstrate the physiological significance of intercellular interactions involving fucosylated glycoconjugates (on stromal cells) and their receptors (on CD34+ cells). The inhibition manifests itself in decreased proliferation of CD34+ cells and is preferentially targeted at hematopoietic progenitor cells, resulting in an 84% inhibition in progenitor cell proliferation. These data also show that the Lewis X, sialyl Lewis X and VIM-2 (alpha1--3 fucosylated saccharides) act as negative homeostatic regulators. The experiments detailed in this dissertation identify alpha(1--3)-fucosylated glycoconjugates as a part of previously unrecognized inhibitory mechanism directed at maintaining human hematopoietic stem/progenitor cell numbers. These glycoconjugates are thus directly responsible for stromal cell mediated homeostatic control of human myeloid-cell hematopoiesis.