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Several bacterial pathogens interact with their host through protein secretion effectuated by a type VI secretion system (T6SS). Francisella tularensis is a highly pathogenic intracellular bacterium that causes the disease tularemia. Proteins encoded by the Francisella pathogenicity island (FPI), which constitute a type VI secretion system, are essential for the virulence of the bacterium and a key mechanism behind this is the escape from the phagosome followed by productive cytosolic replication. It has been shown that T6SS in Francisella is distinct since all putative substrates of F. tularensis T6SS, except for VgrG, are unique to the species. Many of the FPI proteins are secreted into the macrophage cytosol and this is dependent on the functional components of DotU, VgrG, IglC and IglG. In addition, PdpC seems to have a regulatory role for the expression of iglABCD. Since previous results showed peculiar phenotypes of the Δ pdp C and Δ igl G mutants in mouse macrophages, their unique behavior was characterized in human monocyte-derived macrophages (HMDM) in this study. Our results show that both Δ pdpC and Δ iglG mutants of the live vaccine strain (LVS) of F. tularensis did not replicate within HMDMs. The Δ pdpC mutant did not escape from the Francisella containing phagosome (FCP), neither caused cytopathogenicity in primary macrophages and was attenuated in a mouse model. Interestingly, the Δ iglG mutant escaped from the HMDMs FCP and also caused pathological changes in the spleen and liver tissues of intradermally infected C57BL/6 mice. The Δ iglG mutant, with its unique phenotype, is a potential vaccine candidate. [ABSTRACT FROM AUTHOR]