Endotoxin-induced interferon synthesis in macrophage cultures

Pure cultures of murine bone marrow-derived macrophages produce interferon (IFN) after exposure to endotoxin. The levels of endotoxin-induced IFN were enhanced 5- to 20-fold by pretreating (priming) macrophages with murine interferons produced by either NDV-induced L cells, which consisted of a mixture of IFN alpha and IFN beta, or IFN gamma produced by spleen cells stimulated with phytohemagglutinin. Studies conducted on the kinetics of IFN release from endotoxin-induced macrophages demonstrated that peak synthesis occurred within 2-4 hr and was completed 6 hr after the start of treatment. The addition of actinomycin D to macrophages, up to 1 hr after exposure to endotoxin, completely inhibited release of interferon, thus indicating that gene transcription was required for interferon synthesis. The inclusion of cycloheximide in the medium of endotoxin or Poly(I) X Poly(C)-induced macrophages, although inhibiting 90% of protein synthesis, resulted in a superinducing effect, in that induced macrophages treated with cycloheximide produced higher levels of IFN than macrophages not treated with the inhibitor of protein synthesis. Antigenic characterization of macrophage IFNs revealed that endotoxin-induced IFN was neutralized to a higher degree than virus-induced IFNs derived from either macrophages or L cells.