Aplastic anemia associated with in vitro inhibition of erythropoiesis by bone marrow-adherent cells

In aplastic anemia progenitor cells have a reduced sensitivity to the effects of growth factors

We have recently shown that in patients with aplastic anemia (AA) recovering following immunosuppressive therapy, the persistent reduction in the bone marrow clonogenic potential is unrelated to suppressive effects of the myeloid stroma and intrinsic to the hematopoietic progenitors. We examined the mechanisms of this defect by determining the response of the aplastic CD34+ clonogenic precursors to proliferative signals induced by hematopoietic growth factors and comparing their results with those of a control population. Light density bone marrow mononuclear cells were lymphocyte and monocyte depleted and enhanced for the CD34+ progenitors by immunomagnetic selection. Selected progenitors were then cultured in the mixed colony assay with incremental concentrations of combinations containing erythropoietin (Epo), granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3) and c-kit ligand. Bone marrow from aplastic patients had significantly fewer light density cells displaying the CD34 antigen (mean 0.65%, SD 0.35 vs. 1.62%, SD 1.4; p=0.002). Dose response studies on aplastic CD34+ cells demonstrated that at low concentrations of Epo, IL-3 and GM-CSF, clonogenic growth was significantly impaired but achieved normal values at concentrations giving plateau growth in control cultures. However, for all colony types, responses to effective concentrations of c-kit ligand corresponded with those of controls. These data suggest abnormalities at the receptor or signal transduction levels.

Cytotoxicity in feline leukemia virus subgroup-C infected fibroblasts is mediated by adherent bone marrow mononuclear cells

The pathogenesis of retrovirus-induced erythroid aplasia in cats is unknown. In studies to define mechanisms of cytotoxicity associated with retroviral infections, bone marrow mononuclear cells (BMMC) from healthy specific pathogen-free cats were co-cultured with uninfected feline embryonic fibroblasts (FEA cells) and FEA cells infected with feline leukemia virus (FeLV) of subgroup A (FEA-A) or subgroup C (FEA-C). Moderate to marked cytotoxicity (CPE) developed in co-cultures of BMMC and FEA-C cells on Days 5 to 7 of incubation but not in co-cultures of BMMC and FEA-A or BMMC and uninfected cells (FEA-CT). Cytotoxicity was associated with adherent cells of light density (1.056) from bone marrow and peripheral blood, which were positive for alpha naphthyl butyrate esterase activity. Stimulation of adherent cells with phorbol ester or addition of recombinant human tumor necrosis factor-alpha (rhTNF-alpha) caused similar CPE in FEA-CT cells. The TNF-alpha concentrations in the culture supernatants of BMMC+FEA-C were higher than those of BMMC+FEA-A or BMMC+FEA-CT, and addition of anti-TNF antibodies to the cultures blocked the CPE. These data support the hypothesis that macrophages exposed to FeLV-C cause CPE in co-cultures of BMMC and FEA cells by a mechanism involving TNF-alpha. It is suggested that TNF-alpha may be involved in the suppression of hematopoiesis in cats which develop FeLV-C induced erythroid aplasia.