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-4(FARRAR)) induced secretion of a factor that inhibited intracell ular killing of Leishmania major amastigotes by activated macrophages. Analysis of the cytokines produced by EL-4 cells after PMA stimulatio n identified interleukin-2 (IL-2, 2500 U/mI), IL-4 (1280 U/ml), interf eron-gamma (IFN-gamma; 100 U/ml), and granulocyte-macrophage colony-st imulating factor (GM-CSF; 50 U/ml). Neither tumor necrosis factor nor transforming growth factor beta (TGF-beta) was detected. Each of the c ytokines 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 Leish mania; IL-2 and lL-4 had no activity for induction of this antimicrobi al effector function. IL-2 and IL-4 were tested for their capacity to inhibit lymphokine- or IFN-gamma-induced destruction of L. major by ma crophages: lL-4 was ineffective, but IL-2 markedly suppressed the acti vation of macrophages for intracellular killing. Addition of greater t han or equal to 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 t o kill intracellular amastigotes. Immunoaffinity treatment of EL-4 flu ids with anti-IL-2 antibody resulted in >80% reduction in suppression of intracelluIar killing. The suppressive effects of IL-2 were not dir ect, but mediated by TGF-beta. IL-2 induced resident peritoneal macrop hages 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 suffic ient to block intracellular killing activities. This increase in secre tion of TGF-beta was not dependent increases in TGF-beta 1 mRNA. Treat ment 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. Th us, IL-2 was the major suppressor factor in EL-4 fluids, and it acted indirectly through the induction and autocrine action of TGF-beta.