Intracellular replication of Edwardsiella tarda in murine macrophage is dependent on the type III secretion system and induces an up-regulation of anti-apoptotic NF-κB target genes protecting the macrophage from staurosporine-induced apoptosis

Edwardsiella tarda is a pathogen with a broad host range that infects both animals and humans. Resistance to phagocytic killing may be involved in the pathogenicity of this bacterium. Here we show that intracellular replication of E. tarda in murine macrophages is dependent on the type III secretion system and induces an anti-apoptotic effect by up-regulating anti-apoptotic NF-κB target genes. The wild-type strain replicates within the phagosomal membrane of macrophages; whereas the type III mutant does not. Microarray analysis shows the mRNA expression level of NF-κB target genes (e.g. pro-inflammatory cytokines and anti-apoptotic genes) in macrophages infected with the wild-type strain were up-regulated compared to macrophages infected with the type III mutant. Up-regulation of Bcl2a1a, Bcl2a1b, cIAP-2, and TRAF1 genes induced expression of anti-apoptotic proteins to protect macrophages from apoptosis induced by staurosporine. Further, this protection was inhibited by adding kamebakaurin, an inhibitor of NF-κB activation and was confirmed using an NF-κB reporter gene assay. Up-regulation of anti-apoptotic NF-κB target genes is responsible for the anti-apoptotic activity of E. tarda and is required for intracellular replication in murine macrophages.