Studies on the pathogenesis of aplastic anemia.

Three assays were used to study myelopoiesis in 14 patients with aplastic anemia: (1) the soft agar colony assay for granulocyte-monocyte progenitors (CFU-c); (2) coculture of marrow from patients with normal marrows in the CFU-c assay; and (3) culture of marrow pretreated with antithymocyte globulin (ATG) in the CFU-c assay. Marrow from five patients gave low colony counts when cultured alone and suppressed colony formation by normal marrow cells in coculture. Suppressor cells may have caused the aplasia in these patients. Eight patients had low colony formation and no suppression in coculture. These patients may have absent or defective stem cells. Marrow from one patient produced normal colony formation, did not contain suppressor cells and may have a defective hematopoietic environment. Aplastic anemia thus may result from at least three different defects involving (1) the stem cells, (2) the hematopoietic environment or (3) suppressor cells.