TRANSFORMING GROWTH-FACTOR-BETA-1 AND GAMMA-INTERFERON PROVIDE OPPOSING SIGNALS TO LIPOPOLYSACCHARIDE-ACTIVATED MOUSE MACROPHAGES

Bacterial lipopolysaccharides (LPS) are potent inducers of macrophage activation, leading to the production of a number of proinflammatory m ediators. Although several cytokines that prime macrophages for enhanc ed LPS-triggered responses have been identified, far less is known reg arding the role that cytokines play in do,vn-regulating macrophage res ponses to LPS. This study was designed to determine the effects of rec ombinant transforming growth factor beta 1 (rTGF-beta 1) on macrophage activation by LPS. Pretreatment of either mouse peritoneal macrophage s or cells of the RAW 264.7 macrophage-like cell line with rTGF-beta 1 inhibited their ability to produce both tumor necrosis factor alpha ( TNF-alpha) and nitric oxide (NO) in response to LPS. These inhibitory effects were reversed by increasing the concentration of LPS or by pri ming cells with optimal concentrations of recombinant gamma interferon (rIFN-gamma). Pretreatment of cells with rTGF-beta 1 had only a modes t inhibitory effect on the expression of TNF-alpha mRNA. By contrast, the expression of mRNA for the inducible form of nitric oxide synthase (iNOS), which is responsible for NO production in activated; macropha ges, was significantly inhibited by rTGF-beta 1 pretreatment. Thus, rT GF-beta 1-dependent suppression of macrophage TNF-alpha biosynthesis w as manifest at a posttranscriptional level, whereas the inhibition of NO production correlated with a direct effect on iNOS gene expression. Importantly, both of these suppressive effects of rTGF-beta 1 were re versed by exposing the cells to priming concentrations of rIFN-gamma. As with NO production, immunocytochemical analysis of iNOS expression in LPS-stimulated macrophages revealed that rIFN-gamma and rTGF-beta 1 had antagonistic effects, with the former increasing, and the latter reducing, the number of iNOS-expressing cells induced by LPS. These da ta suggest that a balance between the priming effects of IFN-gamma and the inhibitory effects of TGF-beta 1 can determine the overall level of macrophage activation induced by LPS.