IglG and IglI of the Francisella Pathogenicity Island Are Important Virulence Determinants of Francisella tularensis LVS

Abstract

The Gram-negative bacterium Francisella tularensis is the causative agent of tularemia, a disease intimately associated with the multiplication of the bacterium within host macrophages. This in turn requires the expression of Francisella pathogenicity island (FPI) genes, believed to encode a type VI secretion system. While the exact functions of many of the components have yet to be revealed, some have been found to contribute to the ability of Francisella to cause systemic infection in mice as well as to prevent phagolysosomal fusion and facilitate escape into the host cytosol. Upon reaching this compartment, the bacterium rapidly multiplies, inhibits activation of the inflammasome, and ultimately causes apoptosis of the host cell. In this study, we analyzed the contribution of the FPI-encoded proteins IglG, IglI, and PdpE to the aforementioned processes in F. tularensis LVS. The ⌬pdpE mutant behaved similarly to the parental strain in all investigated assays. In contrast, ⌬iglG and ⌬iglI mutants, although they were efficiently replicating in J774A.1 cells, both exhibited delayed phagosomal escape, conferred a delayed activation of the inflammasome, and exhibited reduced cytopathogenicity as well as marked attenuation in the mouse model. Thus, IglG and IglI play key roles for modulation of the intracellular host response and also for the virulence of F. tularensis.

Importance of PdpC, IglC, IglI, and IglG for Modulation of a Host Cell Death Pathway Induced by Francisella tularensis

“…We followed bacterial numbers of the ⌬pdpC, ⌬iglC, ⌬iglG, and ⌬iglI mutants and the LVS strain in J774 cells after infection. In agreement with previously published data (20,(23)(24)(25)(26)(28)(29)(30), the ⌬iglC and ⌬pdpC mutants showed very marginal net replication, whereas the ⌬iglG and the ⌬iglI mutants replicated as well as LVS (Table 1). In separate experiments, we observed that the complemented strains showed the same degree of replication as did LVS (not shown).…”

“…Examples of such mutants include those of the iglABCD operon and the core components VgrG and DotU (20)(21)(22)(23)(24)(25). In contrast, we have observed that the ⌬iglI and ⌬iglG mutants showed aberrant intracellular replication and very diminished cytopathogenic effects (26). The ⌬pdpC mutant of the LVS and the highly virulent F. tularensis subsp.…”

“…Previously, we and others have shown that the FPI mutants display a spectrum of intracellular phenotypes, since some replicate as well as the wild type whereas others show compromised or no growth (20,(23)(24)(25)(26)(28)(29)(30). We followed bacterial numbers of the ⌬pdpC, ⌬iglC, ⌬iglG, and ⌬iglI mutants and the LVS strain in J774 cells after infection.…”

“…The bestcharacterized example of such mutants is the ⌬iglC mutant. Not all of the investigated mutants so far fit into this uniform pattern, however, since the ⌬iglI and ⌬iglG mutants of LVS show delayed cytopathogenicity and lack of virulence though more or less intact intracellular replication in J774 cells, whereas only the latter mutant replicated in peritoneal exudate cells and bone marrow-derived macrophages (26). In addition, in a recent study, we observed that the ⌬pdpC mutant of LVS showed a unique phenotype, characterized by a lack of intracellular replication, incomplete phagosomal escape, and marked attenuation in the mouse model; however, unlike a phagosomally contained FPI mutant, it triggered secretion of IL-1␤ and an MOI-dependent release of lactate dehydrogenase (LDH), a marker of membrane damage (27).…”

“…The live vaccine strain was originally obtained from the United States Army Medical Research Institute for Infectious Diseases (USAMRIID), Frederick, MD, USA. The ⌬iglC, ⌬pdpC, ⌬iglG, and ⌬iglI mutants and complemented strains have been described earlier (20,26,27). F. tularensis strains were grown on modified gonococci (GC) agar at 37°C, 5% CO 2 .…”