Cyclosporin A Inhibits IgE Receptor Mediated Exocytosis from Rat Basophilic Leukemia Cells Without Inhibiting Phosphatidylinositol Hydrolysis, Ca2+ Uptake, the Rise in Cytosolic Ca2+ Actin Polymerization or Arachidonic Acid Release

Abstract

Cyclosporin A (CSA) is produced by the fungus Tolypocladium inflatum and used clinically as an immunosuppressive drug. Although the mechanism of immunosuppression is unclear, CSA is generally thought to inhibit the immune response by preventing both transcription of T cell derived cytokines and cytotoxic activity in certain subpopulations of T lymphocytes. Here we report the effects of CSA in IgE receptor mediated exocytosis from rat basophilic leukemia (RBL) cells. CSA inhibited IgE receptor mediated and calcium ionophore (A23187) stimulated exocytosis from RBL cells in a dose dependent manner. Half-maximal inhibition was observed at 0.3 μM CSA (≈ 200ng/ml). This value falls within the range of CSA concentrations in the serum of patients undergoing immunosuppressive therapy. CSA also inhibited receptor mediated degranulation of human basophils at similar doses. Inhibition of exocytosis occurred after 2–5 min preincubation with CSA and was not reversed by washing the cells free of CSA prior to triggering. At concentrations that resulted in maximum inhibition of exocytosis, CSA did not inhibit IgE receptor mediated phosphatidylinositol (PI) hydrolysis, release of arachidonic acid metabolites, influx of Ca2+, the rise in cytosolic free Ca2+, or actin polymerization. These data demonstrate that, in addition to T lymphocytes, CSA suppresses the function of mast cells and basophils. Furthermore, these results suggest that early events in signal transduction are not affected by CSA and that its target may be an as yet unidentified late stage in exocytosis.