Association between leukemic erythroid progenitors and bone marrow macrophages.

Previous ultrastructural investigations have shown that the erythroblastic island is composed of erythroblasts at different stages of maturation which are intimately associated with a central macrophage. However, it is still unclear at which stage of erythroid differentiation this interaction occurs, mainly because of the lack of purified populations of normal erythroid progenitors [erythroid colony-forming units (CFU-E) and erythroid burst-forming units (BFU-E)] and early precursor cells (proerythroblasts) and because of our limited knowledge of their ultrastructural characteristics. In the present work we analyzed the ultrastructure of CFU-E enriched from normal human bone marrow by avidin-biotin immune rosetting and leukemic blasts of erythroid origin from two patients. Normal and leukemic CFU-Es were defined as glycophorin A (GPA)-negative blasts, devoid of rhopheocytosis, containing some ferritin molecules, either free in the cytoplasm or associated with theta-granules (theta-Gr) in the Golgi zone. Peroxidase activity was detected in the endoplasmic reticulum of these blasts. A preproerythroblast stage was identified, which corresponded to an intermediate phenotype with few GPA sites and rhopheocytosis. In contrast to hemoglobin synthesis, which was absolutely dependent on the presence of erythropoietin (Epo) during culture for 24 hours, ferritin molecules accumulated in the absence of Epo. Interestingly, leukemic CFU-E-like blasts were always in contact with bone marrow macrophages and adhesion between these cell types resisted mechanical dissociation. This result suggests that erythroid progenitors may be part of the erythroblastic island. The mechanisms involved in erythroblast-macrophage binding are still unknown, but the expression by macrophages and erythroid progenitors of receptors for fibronectin and thrombospondin (TSP), as well as their respective ligands in the case of macrophages, suggests that these molecules could be involved in the formation of the erythroblastic island.