Acquired amegakaryocytic thrombocytopenic purpura: a syndrome of diverse etiologies

The possible pathogenetic mechanisms responsible for the production of acquired amegakaryocytic thrombocytopenic purpura (AATP) were investigated in a group of patients with this disorder. Absence of megakaryocytes and small platelet glycoprotein-bearing mononuclear cells, as determined by immunochemical staining of patient marrows with an antisera to platelet glycoproteins, suggested that the defect in AATP occurs in an early progenitor cell of the megakaryocytic lineage. Using an in vitro clonal assay system for negakaryocytic progenitor cells or megakaryocyte colony-forming units (CFU-M), the proliferative capacity of AATP marrow cells was then assessed. Bone marrow cells from three of four patients formed virtually no megakaryocyte colonies, suggesting that in these individuals the AATP was due to an intrinsic defect in the CFU-M. Bone marrow cells from an additional patient, however, formed 12% of the normal numbers of colonies, providing evidence for at least partial integrity of the CFU-M compartment in this patient. Serum specimens from all six patients were screened for their capacity to alter in vitro megakaryocyte colony formation. Five of six sera enhanced colony formation in a stepwise fashion, demonstrating appropriately elevated levels of megakaryocyte colony-stimulating activity. The serum of the patient with partial integrity of the CFU-M compartment, however, stimulated colony formation only at low concentrations. At higher concentrations, this patient's serum actually inhibited the number of colonies cloned, suggesting the presence of a humoral inhibitor to CFU-M. Serum samples from all patients were further screened for such humoral inhibitors of megakaryocyte colony formation using a cytotoxicity assay. The patient whose serum was inhibitory to CFU-M at high concentrations was indeed found to have a complement-dependent serum IgG inhibitor that was cytotoxic to allogeneic and autologous marrow CFU-M but did not alter erythroid colony formation. These-studies suggest that AATP can be due to at least two mechanisms: either an intrinsic effect at the level of the CFU-M or a circulating cytotoxic autoantibody directed against the CFU-M.

Assay of an activity in the serum of patients with disorders of thrombopoiesis that stimulates formation of megakaryocytic colonies

We have recently described an in vitro clonal assay system for human megakaryocyte-progenitor cells or megakaryocytic colony-forming units (CFU-M). Serum specimens from patients with quantitative platelet disorders were screened for the capacity to alter in vitro megakaryocyte-colony formation. Serum from 11 patients with hypomegakaryocytic thrombocytopenia significantly enhanced the formation of CFU-M-derived colonies (200 to 1840 per cent). Neither serum from eight patients with thrombocytopenia and normal or increased numbers of marrow megakaryocytes nor serum from 11 patients with thrombocytosis altered colony formation. This stimulatory activity has been termed megakaryocytic-colony-stimulating activity (Meg-CSA). The number of megakaryocytic colonies formed was directly proportional to the quantity of stimulatory serum added. Meg-CSA levels appeared to be inversely related to marrow megakaryocyte numbers. The variations in Meg-CSA levels that were detected in different disease states suggest that alterations in the production of this stem-cell regulator have physiologic importance.

Erythropoiesis during an erythroblastic transformation of chronic myelocytic leukemia

The requirement of erythropoiesis for erythropoietin were studied in a patient with Ph chronic myelocytic leukemia who had undergone an erythroblastic transformation. Transfusions resulted in a suppression of erythropoiesis. Plasma clot culture studies indicated that both the CFU-E and BFU-E in the peripheral blood of this patient were dependent upon erythropoietin for their differentiation and proliferation. Neither of these committed erythroid stem cells was cloned in the absence of erythropoietin. These studies suggest that the proliferation and differentiation of erythroid stem cells during the erythroblastic crisis of this disorder remain dependent upon physiologic regulators.