Lactobacilli and Streptococci Activate NF-κB and STAT Signaling Pathways in Human Macrophages

Gram-positive bacteria induce the production of several cytokines in human leukocytes. The molecular mechanisms involved in Gram-positive bacteria-induced cytokine production have been poorly characterized. In this work we demonstrate that both nonpathogenic Lactobacillus rhamnosus GG and pathogenic Streptococcus pyogenes (group A streptococci) induce NF-kappa B and STAT DNA-binding activity in human primary macrophages as analyzed by EMSA. NF-kappa B activation was rapid and was not inhibited by a protein synthesis inhibitor cycloheximide, suggesting that these bacteria could directly activate NF-kappa B. STAT1, STAT3, and IFN regulatory factor-1 DNA binding was induced by both bacteria with delayed kinetics compared with NF-kappa B. In addition, streptococci induced the formation of IFN-alpha-specific transcription factor complex and IFN-stimulated gene factor-3 (ISGF3). STAT1 and STAT3 activation and ISGF3 complex formation were inhibited by cycloheximide or by neutralization with IFN-alpha/beta-specific Abs. Streptococci were more potent than lactobacilli in inducing STAT1, ISGF3, and IFN regulatory factor-1 DNA binding. Accordingly, only streptococci induced IFN-alpha production. The activation of the IFN-alpha signaling pathway by streptococci could play a role in the pathogenesis of these bacteria. These results indicate that extracellular Gram-positive bacteria activate transcription factors involved in cytokine signaling by two mechanisms: directly, leading to NF-kappa B activation, and indirectly via cytokines, leading to STAT activation.