Direct Effects of Cyclosporin a on Proliferation of Hematopoietic Stem and Progenitor Cells

Abstract

Cyclosporin A (Cy A) has been reported to both stimulate and inhibit bone marrow colony assays in a dose-dependent manner. The observation that anti-γ-IFN antibodies stimulate hematopoiesis to the same degree as Cy A has led several groups to propose that the stimulatory effects of Cy A are due to inhibition of γ-IFN production by T cells. In this study we observed that cultures of highly enriched hematopoietic stem/progenitor cells (HSPC), devoid of CD3/5/8+ T cells, also exhibit enhanced cloning efficiency when cultured in the presence of Cy A. Normal bone marrow cells or Thy-1.1lowSca-1+Linneg HSPC were incubated in methylcellulose cultures and stimulated with various combinations of steel factor (SF), interleukin (IL)-3, IL-6, granulocyte colony stimulating factor (G-CSF), and erythropoietin (EPO) in the presence of increasing concentrations of Cy A. HSPC cultures with SF, IL-3, and IL-6 stimulation and low Cy A concentrations had from 24% to 78% higher cloning efficiencies than did parallel cultures without Cy A, and did not fall below control levels until the Cy A concentration was increased to more than 1.25 μg/ml. The addition of EPO and G-CSF abrogated the Cy A stimulation observed with SF, IL-3, and IL-6. These results were reflected in whole bone marrow, but with a higher range of variability. Cultures in which FK-506 replaced Cy A showed no consistent stimulation or inhibition of colony formation. These studies show that Cy A can stimulate hematopoietic stem cell growth independent of mediation by T cells. Consequently, these results argue for a direct positive effect of Cy A on the signal transduction pathways in HSPC.