The Francisella tularensis pathogenicity island encodes a secretion system that is required for phagosome escape and virulence

Abstract

SummaryFrancisella tularensis causes the human disease tularemia. F. tularensis is able to survive and replicate within macrophages, a trait that has been correlated with its high virulence, but it is unclear the exact mechanism(s) this organism uses to escape killing within this hostile environment. F. tularensis virulence is dependent upon the Francisella pathogenicity island (FPI), a cluster of genes that we show here shares homology with type VI secretion gene clusters in Vibrio cholerae and Pseudomonas aeruginosa. We demonstrate that two FPI proteins, VgrG and IglI, are secreted into the cytosol of infected macrophages. VgrG and IglI are required for F. tularensis phagosomal escape, intramacrophage growth, inflammasome activation and virulence in mice. Interestingly, VgrG secretion does not require the other FPI genes. However, VgrG and other FPI genes, including PdpB (an IcmF homologue), are required for the secretion of IglI into the macrophage cytosol, suggesting that VgrG and other FPI factors are components of a secretion system. This is the first report of F. tularensis FPI virulence proteins required for intramacrophage growth that are translocated into the macrophage.

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

“…Interestingly, FPI-independent secretion of VgrG within macrophages was also detected. In contrast, while the secretion of Hcp (hemolysin-coregulated protein) proteins is common to many bacterial pathogens (18,54,58,66,72,78), secretion of Francisella Hcp/PdpE could not be detected (4).…”

“…While the exact function(s) of this operon remains unknown, iglA and iglB have homologs in many bacterial species that encode type VI secretion systems (T6SSs) (5,24), and their importance for substrate secretion has been experimentally demonstrated in some cases (6,25,61,78). Bioinformatic analyses have identified additional FPI genes with limited homology to conserved T6SS components, including icmF, vgrG, clpV, dotU, and hcp (4,24). Recently, Francisella tularensis subsp.…”

“…novicida was shown to encode a functional T6SS that promoted VgrG (valine-glycine repeat protein G)-and IcmF-dependent translocation of IglI, a protein with no known homologs, into the macrophage cytosol during infection (4). This putative effector protein was shown to be required for phagosome escape, cytosolic replication, induction of inflammasome-dependent interleukin-1␤ (IL-1␤) release, and virulence (4). Interestingly, FPI-independent secretion of VgrG within macrophages was also detected.…”

Subversion of Host Recognition and Defense Systems by Francisella spp

“…Briefly, the FPI proteins IglA and IglB share homology with proteins encoded in T6SS clusters in multiple bacterial species, are required for phagosomal escape, and may form the putative outer tube of the T6SS "needle" (64,65). IglC has been proposed to form the inner tube (64), while IglI and VgrG are secreted and therefore may interact with host proteins during infection (21). In addition, numerous proteins that are not encoded in the FPI have also been implicated in phagosome escape and are reviewed elsewhere (11,49).…”

Degeneration of a CRISPR/Cas system and its regulatory target during the evolution of a pathogen

“…[16][17][18] The FPI encodes a Type VI secretion system that is absolutely essential for intracellular replication and virulence of Francisella spp in mammals. 19 The FPI is flanked by transposable elements that likely facilitated its duplication by non-reciprocal recombination. 17 Duplication of the region may result in an increased gene dosage and/or altered pattern of expression, enhancing the virulence of highly pathogenic strains.…”