IFN-γ inhibits the production of latent transforming growth factor-β1 by mouse inflammatory macrophages

Transforming growth factor (TGF)-β is a multifunctional cytokine, which in mammals exists in three isoforms (TGF-β1, 2 and 3). It is synthesized by a variety of cells including macrophages, and exerts potent immunoregulatory effects such as the inhibition of Th1 development and the suppression or reversal of IFN-γ-induced macrophage activation. In this study we analyzed the effect of IFN-γ on the production of TGF-β1 by thioglycolate-elicited mouse peritoneal macrophages under serum-free conditions. Untreated macrophages released TGF-β1 in its latent form, which became detectable in a capture ELISA specific for active TGF-β1 after acid activation of the culture supernatants. Treatment with IFN-γ reduced the amount of latent TGF-β1 in the culture supernatants in a dose-dependent fashion. The effect of IFN-γ was confirmed by a newly developed Western blot system for the detection of mouse TGF-β1 protein. IFN-γ only weakly (16 – 24 %) reduced the levels TGF-β1 mRNA at early and late time points of stimulation, and no evidence was obtained that IFN-γ suppresses the secretion of latent TGF-β1. Thus, inhibition of TGF-β1 production by IFN-γ is most likely due to decreased synthesis and/or stability of the TGF-β1 protein, and might be important for the generation of fully activated macrophages and a Th1 response.