Legionella pneumophila suppresses interleukin-12 production by macrophages. Infect Immun 2001;69:1929–33

In vitro infection of macrophages with Legionella pneumophila induced interleukin-1a (IL-1a), IL-10, mono-cyte chemotactic protein 1 (MCP-1), and MCP-3 but not IL-12. The lipopolysaccharide (LPS)-induced pro-duction of IL-12 was down-regulated by infection with virulent L. pneumophila, but other cytokines were not affected. In contrast, avirulent L. pneumophila or UV-killed, virulent L. pneumophila did not induce any suppression of IL-12. The IL-12 suppression occurred at the level of mRNA accumulation for IL-12 genes in response to LPS stimulation, but the infection induced a marked accumulation of mRNA for both MCP-1 and MCP-3, which are known to suppress IL-12 production in LPS-stimulated macrophages. However, pretreat-ment of macrophages with MCP-1 did not suppress LPS-induced IL-12 production at the concentrations induced by L. pneumophila infection. These results suggest that L. pneumophila selectively suppresses IL-12 production induced by LPS from macrophages in vitro by an MCP-independent mechanism. Legionella pneumophila is a gram-negative, facultative intra-cellular pathogen and is the causative agent of Legionnaires’ disease, a severe form of pneumonia. Once L. pneumophil