Interleukin-2 suppresses activated macrophage intracellular killing activity by inducing macrophages to secrete TGF-beta.

Phorbol myristate acetate (PMA) treatment of an EL-4 thymoma cell line (EL-4FARRAR) induced secretion of a factor that inhibited intracellular killing of Leishmania major amastigotes by activated macrophages. Analysis of the cytokines produced by EL-4 cells after PMA stimulation identified interleukin-2 (IL-2, 2500 U/ml), IL-4 (1280 U/ml), interferon-gamma (IFN-gamma; 100 U/ml), and granulocyte-macrophage colony-stimulating factor (GM-CSF; 50 U/ml). Neither tumor necrosis factor nor transforming growth factor beta (TGF-beta) was detected. Each of the cytokines present in EL-4 fluids was assessed for capacity to activate macrophages for destruction of parasites or to suppress intracellular killing. IFN-gamma and GM-CSF both activated macrophages to kill Leishmania; IL-2 and IL-4 had no activity for induction of this antimicrobial effector function. IL-2 and IL-4 were tested for their capacity to inhibit lymphokine- or IFN-gamma-induced destruction of L. major by macrophages: IL-4 was ineffective, but IL-2 markedly suppressed the activation of macrophages for intracellular killing. Addition of > or = 10 U/ml of IL-2 at the time of infection, or up to 4 h before, blocked up to 100% of the capacity of activated macrophages to kill intracellular amastigotes. Immunoaffinity treatment of EL-4 fluids with anti-IL-2 antibody resulted in > 80% reduction in suppression of intracellular killing. The suppressive effects of IL-2 were not direct, but mediated by TGF-beta. IL-2 induced resident peritoneal macrophages to secrete > 5000 pg/ml TGF-beta 1, a quantity that is > 500-fold higher than constitutive background levels (20-40 pg/ml) and is sufficient to block intracellular killing activities. This increase in secretion of TGF-beta was not dependent increases in TGF-beta 1 mRNA. Treatment of cultures with EL-4 fluids or recombinant IL-2 in the presence of antibody to TGF-beta 1 blocked the suppressive activity of both. Thus, IL-2 was the major suppressor factor in EL-4 fluids, and it acted indirectly through the induction and autocrine action of TGF-beta.