Allelic exchange inFrancisella tularensisusing PCR products

Structural and functional analysis of the Francisella lysine decarboxylase as a key actor in oxidative stress resistance

Francisella tularensis is one of the most virulent pathogenic bacteria causing the acute human respiratory disease tularemia. While the mechanisms underlying F. tularensis pathogenesis are largely unknown, previous studies have shown that a F. novicida transposon mutant with insertions in a gene coding for a putative lysine decarboxylase was attenuated in mouse spleen, suggesting a possible role of its protein product as a virulence factor. Therefore, we set out to structurally and functionally characterize the F. novicida lysine decarboxylase, which we termed LdcF. Here, we investigate the genetic environment of ldcF as well as its evolutionary relationships with other basic AAT-fold amino acid decarboxylase superfamily members, known as key actors in bacterial adaptative stress response and polyamine biosynthesis. We determine the crystal structure of LdcF and compare it with the most thoroughly studied lysine decarboxylase, E. coli LdcI. We analyze the influence of ldcF deletion on bacterial growth under different stress conditions in dedicated growth media, as well as in infected macrophages, and demonstrate its involvement in oxidative stress resistance. Finally, our mass spectrometry-based quantitative proteomic analysis enables identification of 80 proteins with expression levels significantly affected by ldcF deletion, including several DNA repair proteins potentially involved in the diminished capacity of the F. novicida mutant to deal with oxidative stress. Taken together, we uncover an important role of LdcF in F. novicida survival in host cells through participation in oxidative stress response, thereby singling out this previously uncharacterized protein as a potential drug target.

Francisella noatunensis ssp. noatunensis iglC deletion mutant protects adult zebrafish challenged with acute mortality dose of wild-type strain

The intracellular fish pathogen Francisella noatunensis remains an unsolved problem for aquaculture worldwide and an efficient vaccine is needed. In Francisella sp., IglC is an important virulence factor necessary for intracellular growth and escape from phagolysosomes. Deletion of the intracellular growth locus C (iglC) in Francisella sp. causes attenuation, but vaccine potential has only been attributed to ΔiglC from Francisella noatunensis ssp. orientalis, a warm-water fish pathogen. A ΔiglC mutant was constructed in the cold-water fish pathogen F. noatunensis ssp. noatunensis (Fnn), which causes francisellosis in Atlantic cod; the mutant was assessed in primary head kidney leucocytes from Atlantic cod. Fluorescence microscopy revealed reduced growth, while qPCR revealed an initial increase followed by a reduction in mutant genomes. Mutant-infected cod leucocytes presented higher interleukin 1 beta (il1β) and interleukin 8 (il8) transcription than wild-type (WT)-infected cells. Two doses of mutant and WT were tested in an adult zebrafish model whereupon 3 × 109 CFU caused acute disease and 3 × 107 CFU caused low mortality regardless of strain. However, splenomegaly developed only in the WT-infected zebrafish. Immunization with 7 × 106 CFU of Fnn ΔiglC protected zebrafish against challenge with a lethal dose of Fnn WT, and bacterial load was minimized within 28 d. Immunized fish had lower interleukin 6 (il6) and il8 transcription in kidney and prolonged interferon-gamma (ifng) transcription in spleens after challenge compared with non-immunized fish. Our data suggest an immunogenic potential of Fnn ΔiglC and indicate important cytokines associated with francisellosis pathogenesis and protection.

Francisella tularensis IglG Belongs to a Novel Family of PAAR-Like T6SS Proteins and Harbors a Unique N-terminal Extension Required for Virulence

The virulence of Francisella tularensis, the etiological agent of tularemia, relies on an atypical type VI secretion system (T6SS) encoded by a genomic island termed the Francisella Pathogenicity Island (FPI). While the importance of the FPI in F. tularensis virulence is clearly established, the precise role of most of the FPI-encoded proteins remains to be deciphered. In this study, using highly virulent F. tularensis strains and the closely related species F. novicida, IglG was characterized as a protein featuring a unique α-helical N-terminal extension and a domain of unknown function (DUF4280), present in more than 250 bacterial species. Three dimensional modeling of IglG and of the DUF4280 consensus protein sequence indicates that these proteins adopt a PAAR-like fold, suggesting they could cap the T6SS in a similar way as the recently described PAAR proteins. The newly identified PAAR-like motif is characterized by four conserved cysteine residues, also present in IglG, which may bind a metal atom. We demonstrate that IglG binds metal ions and that each individual cysteine is required for T6SS-dependent secretion of IglG and of the Hcp homologue, IglC and for the F. novicida intracellular life cycle. In contrast, the Francisella-specific N-terminal α-helical extension is not required for IglG secretion, but is critical for F. novicida virulence and for the interaction of IglG with another FPI-encoded protein, IglF. Altogether, our data suggest that IglG is a PAAR-like protein acting as a bi-modal protein that may connect the tip of the Francisella T6SS with a putative T6SS effector, IglF.

Francisella tularensis is a highly pathogenic bacterium causing tularemia. Its ability to cause disease is linked to its ability to replicate in the macrophage cytosol. The intracellular life cycle of Francisella is controlled by a type VI secretion system (T6SS), which is thought to inject effectors into the host cell to allow bacterial escape into the host cytosol. The molecular mechanisms behind this process are still largely unclear. In this work, we identify IglG as a protein with two important domains, one conserved in proteins from more than 250 bacterial species (DUF4280, renamed here as PAAR-like domain) and one specific for the Francisella genus. Using protein sequence analysis and three-dimensional structure predictions, comparative modeling and biochemistry approaches, our data demonstrate that IglG is a metal-binding protein that based on its PAAR-like domain might cap the VgrG spike of the T6SS and act as a membrane-puncturing protein. Furthermore, we identified that the Francisella-specific domain is directly involved in forming a protein complex with another virulence protein, IglF. This work, in addition to enhancing the molecular understanding of the Francisella T6SS, defines the features of the conserved DUF4280, a novel PAAR-like domain involved in type VI secretion (T6S) of many bacterial species.

Importance of host cell arginine uptake in Francisella phagosomal escape and ribosomal protein amounts

Upon entry into mammalian host cells, the pathogenic bacterium Francisella must import host cell arginine to multiply actively in the host cytoplasm. We identified and functionally characterized an arginine transporter (hereafter designated ArgP) whose inactivation considerably delayed bacterial phagosomal escape and intracellular multiplication. Intramacrophagic growth of the ΔargP mutant was fully restored upon supplementation of the growth medium with excess arginine, in both F. tularensis subsp. novicida and F. tularensis subsp. holarctica LVS, demonstrating the importance of arginine acquisition in these two subspecies. High-resolution mass spectrometry revealed that arginine limitation reduced the amount of most of the ribosomal proteins in the ΔargP mutant. In response to stresses such as nutritional limitation, repression of ribosomal protein synthesis has been observed in all kingdoms of life. Arginine availability may thus contribute to the sensing of the intracellular stage of the pathogen and to trigger phagosomal egress. All MS data have been deposited in the ProteomeXchange database with identifier PXD001584 (http://proteomecentral.proteomexchange.org/dataset/PXD001584).

Enhancement of vaccine efficacy by expression of a TLR5 ligand in the defined live attenuated Francisella tularensis subsp. novicida strain U112ΔiglB::fljB

Oral vaccination with the defined live attenuated Francisella novicida vaccine strain U112ΔiglB has been demonstrated to induce protective immunity against pulmonary challenge with the highly human virulent Francisella tularensis strain SCHU S4. However, this vaccination regimen requires a booster dose in mice and Exhibits 50% protective efficacy in the Fischer 344 rat model. To enhance the efficacy of this vaccine strain, we engineered U112ΔiglB to express the Salmonella typhimurium FljB flagellin D1 domain, a TLR5 agonist. The U112ΔiglB::fljB strain was highly attenuated for intracellular macrophage replication, and although the FljB protein was expressed within the cytosol, it exhibited TLR5 activation in a TLR5-expressing HEK cell line. Additionally, infection of splenocytes and lymphocytes with U112ΔiglB::fljB induced significantly greater TNF-α production than infection with U112ΔiglB. Oral vaccination with U112ΔiglB::fljB also induced significantly greater protection than U112ΔiglB against pulmonary SCHU S4 challenge in rats. The enhanced protection was accompanied by higher IgG2a production and serum-mediated reduction of Francisella infectivity. Thus, the U112ΔiglB::fljB strain may serve as a potential vaccine candidate against pneumonic tularemia.

Lipidation of the FPI protein IglE contributes to Francisella tularensis ssp. novicida intramacrophage replication and virulence

Francisella tularensis is a Gram-negative bacterium responsible for the human disease tularemia. The Francisella pathogenicity island (FPI) encodes a secretion system related to type VI secretion systems (T6SS) which allows F. tularensis to escape the phagosome and replicate within the cytosol of infected macrophages and ultimately cause disease. A lipoprotein is typically found encoded within T6SS gene clusters and is believed to anchor portions of the secretion apparatus to the outer membrane. We show that the FPI protein IglE is a lipoprotein that incorporates (3)H-palmitate and localizes to the outer membrane. A C22G IglE mutant failed to be lipidated and failed to localize to the outer membrane, consistent with C22 being the site of lipidation. Francisella tularensis ssp. novicida expressing IglE C22G is defective for replication in macrophages and unable to cause disease in mice. Bacterial two-hybrid analysis demonstrated that IglE interacts with the C-terminal portion of the FPI inner membrane protein PdpB, and PhoA fusion analysis indicated the PdpB C-terminus is located within the periplasm. We predict this interaction facilitates channel formation to allow secretion through this system.

Glutamate utilization couples oxidative stress defense and the tricarboxylic acid cycle in Francisella phagosomal escape

Intracellular bacterial pathogens have developed a variety of strategies to avoid degradation by the host innate immune defense mechanisms triggered upon phagocytocis. Upon infection of mammalian host cells, the intracellular pathogen Francisella replicates exclusively in the cytosolic compartment. Hence, its ability to escape rapidly from the phagosomal compartment is critical for its pathogenicity. Here, we show for the first time that a glutamate transporter of Francisella (here designated GadC) is critical for oxidative stress defense in the phagosome, thus impairing intra-macrophage multiplication and virulence in the mouse model. The gadC mutant failed to efficiently neutralize the production of reactive oxygen species. Remarkably, virulence of the gadC mutant was partially restored in mice defective in NADPH oxidase activity. The data presented highlight links between glutamate uptake, oxidative stress defense, the tricarboxylic acid cycle and phagosomal escape. This is the first report establishing the role of an amino acid transporter in the early stage of the Francisella intracellular lifecycle.

Asparagine assimilation is critical for intracellular replication and dissemination of Francisella

In order to develop a successful infectious cycle, intracellular bacterial pathogens must be able to adapt their metabolism to optimally utilize the nutrients available in the cellular compartments and tissues where they reside. Francisella tularensis, the agent of the zoonotic disease tularaemia, is a highly infectious bacterium for a large number of animal species. This bacterium replicates in its mammalian hosts mainly in the cytosol of infected macrophages. We report here the identification of a novel amino acid transporter of the major facilitator superfamily of secondary transporters that is required for bacterial intracellular multiplication and systemic dissemination. We show that inactivation of this transporter does not affect phagosomal escape but prevents multiplication in the cytosol of all cell types tested. Remarkably, the intracellular growth defect of the mutant was fully and specifically reversed by addition of asparagine or asparagine-containing dipeptides as well as by simultaneous addition of aspartic acid and ammonium. Importantly, bacterial virulence was also restored in vivo, in the mouse model, by asparagine supplementation. This work unravels thus, for the first time, the importance of asparagine for cytosolicmultiplication of Francisella. Amino acid transporters are likely to constitute underappreciated players in bacterial intracellular parasitism.

Kdo hydrolase is required for Francisella tularensis virulence and evasion of TLR2-mediated innate immunity

The highly virulent Francisella tularensis subsp. tularensis has been classified as a category A bioterrorism agent. A live vaccine strain (LVS) has been developed but remains unlicensed in the United States because of an incomplete understanding of its attenuation. Lipopolysaccharide (LPS) modification is a common strategy employed by bacterial pathogens to avoid innate immunity. A novel modification enzyme has recently been identified in F. tularensis and Helicobacter pylori. This enzyme, a two-component Kdo (3-deoxy-d-manno-octulosonic acid) hydrolase, catalyzes the removal of a side chain Kdo sugar from LPS precursors. The biological significance of this modification has not yet been studied. To address the role of the two-component Kdo hydrolase KdhAB in F. tularensis pathogenesis, a ΔkdhAB deletion mutant was constructed from the LVS strain. In intranasal infection of mice, the ΔkdhAB mutant strain had a 50% lethal dose (LD(50)) 2 log(10) units higher than that of the parental LVS strain. The levels of the proinflammatory cytokines tumor necrosis factor alpha (TNF-α) and interleukin-1β (IL-1β) in bronchoalveolar lavage fluid were significantly higher (2-fold) in mice infected with the ΔkdhAB mutant than in mice infected with LVS. In vitro stimulation of bone marrow-derived macrophages with the ΔkdhAB mutant induced higher levels of TNF-α and IL-1β in a TLR2-dependent manner. In addition, TLR2(-/-) mice were more susceptible than wild-type mice to ΔkdhAB bacterial infection. Finally, immunization of mice with ΔkdhAB bacteria elicited a high level of protection against the highly virulent F. tularensis subsp. tularensis strain Schu S4. These findings suggest an important role for the Francisella Kdo hydrolase system in virulence and offer a novel mutant as a candidate vaccine.

IMPORTANCE

The first line of defense against a bacterial pathogen is innate immunity, which slows the progress of infection and allows time for adaptive immunity to develop. Some bacterial pathogens, such as Francisella tularensis, suppress the early innate immune response, killing the host before adaptive immunity can mature. To avoid an innate immune response, F. tularensis enzymatically modifies its lipopolysaccharide (LPS). A novel LPS modification-Kdo (3-deoxy-d-manno-octulosonic acid) saccharide removal--has recently been reported in F. tularensis. We found that the kdhAB mutant was significantly attenuated in mice. Additionally, the mutant strain induced an early innate immune response in mice both in vitro and in vivo. Immunization of mice with this mutant provided protection against the highly virulent F. tularensis strain Schu S4. Thus, our study has identified a novel LPS modification important for microbial virulence. A mutant lacking this modification may be used as a live attenuated vaccine against tularemia.

Cyclic di-GMP stimulates biofilm formation and inhibits virulence of Francisella novicida

Francisella tularensis is a gram-negative bacterium that is highly virulent in humans, causing the disease tularemia. F. novicida is closely related to F. tularensis and exhibits high virulence in mice, but it is avirulent in healthy humans. An F. novicida-specific gene cluster (FTN0451 to FTN0456) encodes two proteins with diguanylate cyclase (DGC) and phosphodiesterase (PDE) domains that modulate the synthesis and degradation of cyclic di-GMP (cdGMP). No DGC- or PDE-encoding protein genes are present in the F. tularensis genome. F. novicida strains lacking either the two DGC/PDE genes (cdgA and cdgB) or the entire gene cluster (strain KKF457) are defective for biofilm formation. In addition, expression of CdgB or a heterologous DGC in strain KKF457 stimulated F. novicida biofilms, even in a strain lacking the biofilm regulator QseB. Genetic evidence suggests that CdgA is predominantly a PDE, while CdgB is predominantly a DGC. The F. novicida qseB strain showed reduced cdgA and cdgB transcript levels, demonstrating an F. novicida biofilm signaling cascade that controls cdGMP levels. Interestingly, KKF457 with elevated cdGMP levels exhibited a decrease in intramacrophage replication and virulence in mice, as well as increased growth yields and biofilm formation in vitro. Microarray analyses revealed that cdGMP stimulated the transcription of a chitinase (ChiB) known to contribute to biofilm formation. Our results indicate that elevated cdGMP in F. novicida stimulates biofilm formation and inhibits virulence. We suggest that differences in human virulence between F. novicida and F. tularensis may be due in part to the absence of cdGMP signaling in F. tularensis.

Discovery of a role for Hsp82 in Histoplasma virulence through a quantitative screen for macrophage lethality

The application of forward genetics can reveal new factors required for the virulence of intracellular pathogens. To facilitate such virulence screens, we developed macrophage cell lines with which the number of intact host cells following infection with intracellular pathogens can be rapidly and easily ascertained through the expression of a constitutive lacZ transgene. Using known virulence mutants of Francisella novicida and Histoplasma capsulatum, we confirmed the applicability of these host cells for the quantitative assessment of bacterial and fungal virulence, respectively. To identify new genes required for Histoplasma virulence, we employed these transgenic macrophage cells to screen a collection of individual transfer DNA (T-DNA) insertion mutants. Among the mutants showing decreased virulence in macrophages, we identified an insertion in the locus encoding the Histoplasma Hsp82 homolog. The lesion caused by the T-DNA insertion localizes to the promoter region, resulting in significantly decreased HSP82 expression. Reduced HSP82 expression markedly attenuates the virulence of Histoplasma yeast in vivo. While the HSP82 hypomorph grows normally in vitro at 37°C and under acid and salinity stresses, its ability to recover from high-temperature stress is impaired. These results provide genetic proof of the role of stress chaperones in the virulence of a thermally dimorphic fungal pathogen.

Triggering Ras signalling by intracellular Francisella tularensis through recruitment of PKCα and βI to the SOS2/GrB2 complex is essential for bacterial proliferation in the cytosol

Intracellular proliferation of Francisella tularensis is essential for manifestation of the fatal disease tularaemia, and is classified as a category A bioterrorism agent. The F. tularensis-containing phagosome (FCP) matures into a late endosome-like phagosome with limited fusion to lysosomes, followed by rapid bacterial escape into the cytosol. The Francisella pathogenicity island (FPI) encodes a type VI-like secretion system, and the FPI-encoded IglC is essential for evasion of lysosomal fusion and phagosomal escape. Many host signalling events are likely to be modulated by F. tularensis to render the cell permissive for intracellular proliferation but they are not fully understood. Here we show that within 15 min of infection, intracellular F. tularensis ssp. novicida triggers IglC-dependent temporal activation of Ras, but attached extracellular bacteria fail to trigger Ras activation, which has never been shown for other intracellular pathogens. Intracellular F. tularensis ssp. novicida triggers activation of Ras through recruitment of PKCα and PKCβI to the SOS2/GrB2 complex. Silencing of SOS2, GrB2 and PKCα and PKCβI by RNAi has no effect on evasion of lysosomal fusion and bacterial escape into the cytosol but renders the cytosol non-permissive for replication of F. tularensis ssp. novicida. Since Ras activation promotes cell survival, we show that silencing of SOS2, GrB2 and PKCα and βI is associated with rapid early activation of caspase-3 within 8 h post infection. However, silencing of SOS2, GrB2 and PKCα and βI does not affect phosphorylation of Akt or Erk, indicating that activation of the PI3K/Akt and the Erk signalling cascade are independent of the F. tularensis-triggered Ras activation. We conclude that intracellular F. tularensis ssp. novicida triggers temporal and early activation of Ras through the SOS2/GrB2/PKCα/PKCβI quaternary complex. Temporal and rapid trigger of Ras signalling by intracellular F. tularensis is essential for intracellular bacterial proliferation within the cytosol, and this is associated with downregulation of early caspase-3 activation.

Intra-Vacuolar Proliferation of F. Novicida within H. Vermiformis

Francisella tularensis is a gram negative facultative intracellular bacterium that causes the zoonotic disease tularemia. Free-living amebae, such as Acanthamoeba and Hartmannella, are environmental hosts of several intracellular pathogens. Epidemiology of F. tularensis in various parts of the world is associated with water-borne transmission, which includes mosquitoes and amebae as the potential host reservoirs of the bacteria in water resources. In vitro studies showed intracellular replication of F. tularensis within A. castellanii cells. Whether ameba is a biological reservoir for Francisella in the environment is not known. We used Hartmannella vermiformis as an amebal model system to study the intracellular life of F. novicida. For the first time we show that F. novicida survives and replicates within H. vermiformis. The iglC mutant strain of F. novicida is defective for survival and replication not only within A. castellanii but also in H. vermiformis cells. In contrast to mammalian cells, where bacteria replicate in the cytosol, F. novicida resides and replicates within membrane-bound vacuoles within the trophozoites of H. vermiformis. In contrast to the transient residence of F. novicida within acidic vacuoles prior to escaping to the cytosol of mammalian cells, F. novicida does not reside transiently or permanently in an acidic compartment within H. vermiformis when examined 30 min after initiation of the infection. We conclude that F. tularensis does not replicate within acidified vacuoles and does not escape into the cytosol of H. vermiformis. The Francisella pathogenicity island locus iglC is essential for intra-vacuolar proliferation of F. novicida within H. vermiformis. Our data show a distinct intracellular lifestyle for F. novicida within H. vermiformis compared to mammalian cells.

Tryptophan prototrophy contributes to Francisella tularensis evasion of gamma interferon-mediated host defense

Francisella tularensis is able to survive and replicate within host macrophages, a trait that is associated with the high virulence of this bacterium. The trpAB genes encode the enzymes required for the final two steps in tryptophan biosynthesis, with TrpB being responsible for the conversion of indole to tryptophan. Consistent with this function, an F. tularensis subsp. novicida trpB mutant is unable to grow in defined medium in the absence of tryptophan. The trpB mutant is also attenuated for virulence in a mouse pulmonary model of tularemia. However, the trpB mutant remains virulent in gamma interferon receptor-deficient (IFN-γR(-/-)) mice, demonstrating that IFN-γ-mediated signaling contributes to clearance of the trpB mutant. IFN-γ limits intracellular survival of the trpB mutant within bone marrow-derived macrophages from wild-type but not IFN-γR(-/-) mice. An F. tularensis subsp. tularensis trpB mutant is also attenuated for virulence in mice and survival within IFN-γ-treated macrophages, indicating that tryptophan prototrophy is also important in a human-virulent F. tularensis subspecies. These results demonstrate that trpB contributes to F. tularensis virulence by enabling intracellular growth under IFN-γ-mediated tryptophan limitation.

Genetic manipulation of francisella tularensis

Francisella tularensis is a facultative intracellular pathogen that causes the disease tularemia. F. tularensis subsp. tularensis causes the most severe disease in humans and has been classified as a Category A select agent and potential bioweapon. There is currently no vaccine approved for human use, making genetic manipulation of this organism critical to unraveling the genetic basis of pathogenesis and developing countermeasures against tularemia. The development of genetic techniques applicable to F. tularensis have lagged behind those routinely used for other bacteria, primarily due to lack of research and the restricted nature of the biocontainment required for studying this pathogen. However, in recent years, genetic techniques, such as transposon mutagenesis and targeted gene disruption, have been developed, that have had a dramatic impact on our understanding of the genetic basis of F. tularensis virulence. In this review, we describe some of the methods developed for genetic manipulation of F. tularensis.

The Role of the Francisella Tularensis Pathogenicity Island in Type VI Secretion, Intracellular Survival, and Modulation of Host Cell Signaling

Francisella tularensis is a highly virulent gram-negative intracellular bacterium that causes the zoonotic disease tularemia. Essential for its virulence is the ability to multiply within host cells, in particular monocytic cells. The bacterium has developed intricate means to subvert host immune mechanisms and thereby facilitate its intracellular survival by preventing phagolysosomal fusion followed by escape into the cytosol, where it multiplies. Moreover, it targets and manipulates numerous host cell signaling pathways, thereby ameliorating the otherwise bactericidal capacity. Many of the underlying molecular mechanisms still remain unknown but key elements, directly or indirectly responsible for many of the aforementioned mechanisms, rely on the expression of proteins encoded by the Francisella pathogenicity island (FPI), suggested to constitute a type VI secretion system. We here describe the current knowledge regarding the components of the FPI and the roles that have been ascribed to them.

3-Deoxy-D-manno-octulosonic acid (Kdo) hydrolase identified in Francisella tularensis, Helicobacter pylori, and Legionella pneumophila

3-Deoxy-D-manno-octulosonic acid (Kdo) is an eight-carbon sugar ubiquitous in Gram-negative bacterial lipopolysaccharides (LPS). Although its biosynthesis is well described, no protein has yet been identified as a Kdo hydrolase. However, Kdo hydrolase enzymatic activity has been detected in membranes of Helicobacter pylori and Francisella tularensis and may be responsible for the removal of side-chain Kdo from the LPS core saccharides. We now report the identification of genes encoding a Kdo hydrolase in F. tularensis Schu S4 and live vaccine strain strains, in H. pylori 26695 strain and in Legionella pneumophila Philadelphia 1 strain. We have renamed the genes kdhA for keto-deoxyoctulosonate hydrolase A. Deletion of kdhA abolished Kdo hydrolase activity in membranes of F. tularensis live vaccine strain. The F. tularensis kdhA mutant synthesized a core oligosaccharide containing a Kdo disaccharide with one of the Kdo residues being a terminal side chain. This side-chain Kdo monosaccharide was absent in the wild-type core oligosaccharide. Expression in Escherichia coli of recombinant KdhA from F. tularensis, H. pylori, and L. pneumophila resulted in a reduction of membrane-associated side-chain Kdo. The identification of this previously faceless enzyme will accelerate study of the biosynthetic basis and biologic impact for postbiosynthetic LPS structural modification.

Characterization of the O-antigen polymerase (Wzy) of Francisella tularensis

The O-antigen polymerase of gram-negative bacteria has been difficult to characterize. Herein we report the biochemical and functional characterization of the protein product (Wzy) of the gene annotated as the putative O-antigen polymerase, which is located in the O-antigen biosynthetic locus of Francisella tularensis. In silico analysis (homology searching, hydropathy plotting, and codon usage assessment) strongly suggested that Wzy is an O-antigen polymerase whose function is to catalyze the addition of newly synthesized O-antigen repeating units to a glycolipid consisting of lipid A, inner core polysaccharide, and one repeating unit of the O-polysaccharide (O-PS). To characterize the function of the Wzy protein, a non-polar deletion mutant of wzy was generated by allelic replacement, and the banding pattern of O-PS was observed by immunoblot analysis of whole-cell lysates obtained by SDS-PAGE and stained with an O-PS-specific monoclonal antibody. These immunoblot analyses showed that O-PS of the wzy mutant expresses only one repeating unit of O-antigen. Further biochemical characterization of the subcellular fractions of the wzy mutant demonstrated that (as is characteristic of O-antigen polymerase mutants) the low molecular weight O-antigen accumulates in the periplasm of the mutant. Site-directed mutagenesis based on protein homology and topology, which was carried out to locate a catalytic residue of the protein, showed that modification of specific residues (Gly(176), Asp(177), Gly(323), and Tyr(324)) leads to a loss of O-PS polymerization. Topology models indicate that these amino acids most likely lie in close proximity on the bacterial surface.

Attenuated Francisella asiatica iglC mutant induces protective immunity to francisellosis in tilapia

Francisella asiatica is a Gram-negative, facultative intracellular bacteria that causes fish francisellosis. Fish francisellosis is a severe sub-acute to chronic granulomatous disease with high mortalities and high infectivity rates in cultured and wild fish. To date, there is no approved vaccine for this widespread emergent disease. The goal of this study was to characterize the efficacy of a defined F. asiatica mutant (ΔiglC) as a live attenuated vaccine against subsequent immersion challenge with the wild-type (WT) organism. In previous work, the ΔiglC was found to be attenuated upon intraperitoneal injection and immersion challenges. In vitro, the ΔiglC exhibited reduced growth in tilapia head-kidney derived macrophages, and was significantly attenuated (p<0.001) as demonstrated by cytopathogenic and apoptosis assays. In this study, the ΔiglC was tested to determine its ability to protect tilapia against challenge with high doses (lethal dose 80) of WT bacteria. Naïve tilapia vaccinated by immersion with a suspension of the ΔiglC and subsequently challenged with WT F. asiatica were protected (90% mean percent survival) from the lethal challenges. F. asiatica-specific antibodies produced in response to immunization with the ΔiglC were subsequently found to protect naïve tilapia against high-dose F. asiatica challenge in passive immunization experiments. Significant protection (p<0.001) was obtained when fish were passively immunized and challenged with 10(4) and 10(5)CFU/fish of WT F. asiatica; but not when challenged with 10(6)CFU/fish. This is the first report of a defined live attenuated strain providing protection against F. asiatica in fish.

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

Francisella 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.

Interaction of Francisella asiatica with tilapia (Oreochromis niloticus) innate immunity

Members of the genus Francisella are facultative intracellular bacteria that cause important diseases in a wide variety of animals worldwide, including humans and fish. Several genes that are important for intramacrophage survival have been identified, including the iglC gene, which is found in the iglABCD operon in the Francisella sp. pathogenicity island (FPI). In the present study, we examined the interaction of wild-type Francisella asiatica and a Delta iglC mutant strain with fish serum and head kidney-derived macrophages (HKDM). Both the wild-type and the mutant strains were resistant to killing by normal and heat-inactivated sera. The wild-type F. asiatica is able to invade tilapia head kidney-derived macrophages and replicate vigorously within them, causing apoptosis and cytotoxicity in the macrophages at 24 and 36 h postinfection. The Delta iglC mutant, however, is defective for survival, replication, and the ability to cause cytotoxicity in HKDM, but the ability is restored when the mutant is complemented with the iglC gene. Uptake by the HKDM was mediated partially by complement and partially by macrophage mannose receptors, as demonstrated by in vitro assays. Light and electron microscopy analysis of the infected macrophages revealed intracellular bacteria present in a tight vacuole at 2 h postinoculation and the presence of numerous bacteria in spacious vacuoles at 12 h postinfection, with some bacteria free in the cytoplasm.

Francisella tularensis T-cell antigen identification using humanized HLA-DR4 transgenic mice

There is no licensed vaccine against the intracellular pathogen Francisella tularensis. The use of conventional mouse strains to screen protective vaccine antigens may be problematic, given the differences in the major histocompatibility complex (MHC) binding properties between murine and human antigen-presenting cells. We used engineered humanized mice that lack endogenous MHC class II alleles but that express a human HLA allele (HLA-DR4 transgenic [tg] mice) to identify potential subunit vaccine candidates. Specifically, we applied a biochemical and immunological screening approach with bioinformatics to select putative F. tularensis subsp. novicida T-cell-reactive antigens using humanized HLA-DR4 tg mice. Cell wall- and membrane-associated proteins were extracted with Triton X-114 detergent and were separated by fractionation with a Rotofor apparatus and whole-gel elution. A series of proteins were identified from fractions that stimulated antigen-specific gamma interferon (IFN-gamma) production, and these were further downselected by the use of bioinformatics and HLA-DR4 binding algorithms. We further examined the validity of this combinatorial approach with one of the identified proteins, a 19-kDa Francisella tularensis outer membrane protein (designated Francisella outer membrane protein B [FopB]; FTN_0119). FopB was shown to be a T-cell antigen by a specific IFN-gamma recall assay with purified CD4(+) T cells from F. tularensis subsp. novicida DeltaiglC-primed HLA-DR4 tg mice and cells of a human B-cell line expressing HLA-DR4 (DRB1*0401) functioning as antigen-presenting cells. Intranasal immunization of HLA-DR4 tg mice with the single antigen FopB conferred significant protection against lethal pulmonary challenge with an F. tularensis subsp. holarctica live vaccine strain. These results demonstrate the value of combining functional biochemical and immunological screening with humanized HLA-DR4 tg mice to map HLA-DR4-restricted Francisella CD4(+) T-cell epitopes.

Working toward the future: insights into Francisella tularensis pathogenesis and vaccine development

Francisella tularensis is a facultative intracellular gram-negative pathogen and the etiological agent of the zoonotic disease tularemia. Recent advances in the field of Francisella genetics have led to a rapid increase in both the generation and subsequent characterization of mutant strains exhibiting altered growth and/or virulence characteristics within various model systems of infection. In this review, we summarize the major properties of several Francisella species, including F. tularensis and F. novicida, and provide an up-to-date synopsis of the genes necessary for pathogenesis by these organisms and the determinants that are currently being targeted for vaccine development.

Regulation of apoptosis and anti-apoptosis signalling by Francisella tularensis

Francisella tularensis induces apoptosis within macrophages but the temporal and spatial modulation through activation of caspase-1, caspase-3, and the anti-apoptosis nuclear transcription factor B (NF-kappaB) is not known. Whether escape of the bacteria into the cytosol is sufficient and/or essential for activation of NF-kappaB is not known. Our results show that F. tularensis subsp. novicida induces sustained nuclear translocation of NF-kappaB at early time points after infection of human monocytes derived macrophages (hMDMs). The sustained nuclear translocation of NF-kappaB is defective in the iglC mutant that fails to escape into the cytosol of macrophages. Nuclear translocation of NF-kappaB by the wild type strain is abolished upon treatment with the NF-kappaB inhibitor caffein acid phenyl ester. While the wild type strain triggers caspase-3 and caspase-1 activation by 6 h post-infection the iglC mutant is defective in triggering both caspases. In hMDMs treated with the apoptosis-inducing agent, staurosporin, there is an induction of cell death in the iglC mutant-infected macrophages despite reduced frequency of caspase-1 and caspase-3 activity. The wt-infected macrophages are resistant to cell death-induced agent. We conclude that although caspase-1 and capsase-3 are triggered within F. tularensis-infected hMDMs during early stages of infection, cell death is delayed, which is correlated with simultaneous activation of NF-kappaB.

Utilization of an unstable plasmid and the I-SceI endonuclease to generate routine markerless deletion mutants in Francisella tularensis

We engineered an efficient system to make Francisella tularensis deletion mutations using an unstable, poorly maintained plasmid to enhance the likelihood of homologous recombination. For counterselection, we adapted a strategy using I-SceI, which causes a double-stranded break in the integrated suicide vector, forcing a second recombination to mediate allelic replacement.

Attenuation of the fish pathogen Francisella sp. by mutation of the iglC* gene

Fish francisellosis is an emergent disease caused by gram-negative facultative intracellular bacteria of the genus Francisella. Different strains of the bacterium have caused high mortalities in warmwater and coldwater fish species. Francisella sp. isolates from fish have been found to share more than 97% identity to the human pathogen Francisella tularensis upon 16S ribosomal RNA sequence comparison. Homologue genes of the F. tularensis intracellular growth locus (iglA*, iglB*, iglC*, and iglD*) were identified from LADL 07-285A, a clinical isolate obtained from diseased Nile tilapia Oreochromis niloticus. The iglABCD operon DNA sequence comparison revealed that Francisella LADL 07-285A had 94% identity with F. philomiragia subsp. philomiragia and 83% identity with F. tularensis subsp. novicida U112. The functions of the conserved proteins corresponding to the genes are elusive but appear to be essential for the ability of Francisella sp. to survive within macrophages and cause disease. An insertion mutation was made in the iglC* gene of LADL 07-285A by allelic exchange, and the iglC* mutant was found to be attenuated after intraperitoneal and immersion challenges in Nile tilapia. Laboratory challenge methods for inducing francisellosis in Nile tilapia were evaluated by intraperitoneal injection and immersion with serial dilutions of Francisella LADL 07-285A. The dose lethal to 50% of test fish at 40 d postchallenge was 10(-5.3) (about 1.2 X 10(3) colony-forming units/fish) by intraperitoneal injection and was 10(-1) (2.3 X 10(7) colony-forming units/mL of tank water) by immersion.

Description of two new plasmids isolated from Francisella philomiragia strains and construction of shuttle vectors for the study of Francisella tularensis

Francisella tularensis is the causative agent of tularemia, a zoonotic disease often transmitted to humans by infected animals. The lack of useful specific genetic tools has long hampered the study of F. tularensis subspecies. We identified and characterized two new plasmids, pF242 and pF243, isolated from Francisella philomiragia strains ATCC 25016 and ATCC 25017, respectively. Sequence analysis revealed that pF242 and pF243 are closely related to pC194 and pFNL10 plasmids, respectively. Two generations of pF242- and pF243-based shuttle vectors, harboring several antibiotic resistance markers, were developed. We used the first generation to compare transformation efficiencies in two virulent F. tularensis subspecies. We found that electroporation was more efficient than cryotransformation: almost all vectors tested were successfully introduced by electroporation into Francisella strains with a high level of efficiency. The second generation of shuttle vectors, containing a multiple cloning site and/or gfp gene downstream of Francisella groES promotor, was used for GFP production in F. tularensis. The development of new shuttle vectors offers new perspectives in the genetic manipulation of F. tularensis, helping to elucidate the mechanisms underlying its virulence.

Contribution of citrulline ureidase to Francisella tularensis strain Schu S4 pathogenesis

The citrulline ureidase (CTU) activity has been shown to be associated with highly virulent Francisella tularensis strains, including Schu S4, while it is absent in avirulent or less virulent strains. A definitive role of the ctu gene in virulence and pathogenesis of F. tularensis Schu S4 has not been assessed; thus, an understanding of the significance of this phenotype is long overdue. CTU is a carbon-nitrogen hydrolase encoded by the citrulline ureidase (ctu) gene (FTT0435) on the F. tularensis Schu S4 genome. In the present study, we evaluated the contribution of the ctu gene in the virulence of category A agent F. tularensis Schu S4 by generating a nonpolar deletion mutant, the Deltactu mutant. The deletion of the ctu gene resulted in loss of CTU activity, which was restored by transcomplementing the ctu gene. The Deltactu mutant did not exhibit any growth defect under acellular growth conditions; however, it was impaired for intramacrophage growth in resting as well as gamma interferon-stimulated macrophages. The Deltactu mutant was further tested for its virulence attributes in a mouse model of respiratory tularemia. Mice infected intranasally with the Deltactu mutant showed significantly reduced bacterial burden in the lungs, liver, and spleen compared to wild-type (WT) Schu S4-infected mice. The reduced bacterial burden in mice infected with the Deltactu mutant was also associated with significantly lower histopathological scores in the lungs. Mice infected with the Deltactu mutant succumbed to infection, but they survived longer and showed significantly extended median time to death compared to that shown by WT Schu S4-infected mice. To conclude, this study demonstrates that ctu contributes to intracellular survival, in vivo growth, and pathogenesis. However, ctu is not an absolute requirement for the virulence of F. tularensis Schu S4 in mice.

Targeted gene disruption in Francisella tularensis by group II introns

Francisella tularensis is a highly infectious Gram-negative bacterium that is the causative agent of tularemia. Very little is known about the molecular mechanisms responsible for F. tularensis virulence, in part due to the paucity of genetic tools available for the study of F. tularensis. We have developed a gene knockout system for F. tularensis that utilizes retargeted mobile group II introns, or "targetrons". These targetrons disrupt both single and duplicated target genes at high efficiency in three different F. tularensis subspecies. Here we describe in detail the targetron-based method for insertional mutagenesis of F. tularensis genes, which should facilitate a better understanding of F. tularensis pathogenesis. Group II introns can be adapted to inactivate genes in bacteria for which few genetic tools exist, thus providing a powerful tool to study the genetic basis of bacterial pathogenesis.

Characterization of the pathogenicity island protein PdpA and its role in the virulence of Francisella novicida

Francisella tularensis is a highly virulent, intracellular pathogen that causes the disease tularaemia. A research surrogate for F. tularensis is Francisella novicida, which causes a tularaemia-like disease in mice, grows similarly in macrophages, and yet is unable to cause disease in humans. Both Francisella species contain a cluster of genes referred to as the Francisella pathogenicity island (FPI). Pathogenicity determinant protein A (PdpA), encoded by the pdpA gene, is located within the FPI and has been associated with the virulence of Francisella species. In this work we examined the properties of PdpA protein expression and localization as well as the phenotype of a F. novicida pdpA deletion mutant. Monoclonal antibody detection of PdpA showed that it is a soluble protein that is upregulated in iron-limiting conditions and undetectable in an mglA or mglB mutant background. Deletion of pdpA resulted in a strain that was highly attenuated for virulence in chicken embryos and mice.

Intracellular biology and virulence determinants of Francisella tularensis revealed by transcriptional profiling inside macrophages

Summary The highly infectious bacterium Francisella tularensis is a facultative intracellular pathogen, whose virulence requires proliferation inside host cells, including macrophages. Here we have performed a global transcriptional profiling of the highly virulent F. tularensis ssp. tularensis Schu S4 strain during its intracellular cycle within primary murine macrophages, to characterize its intracellular biology and identify pathogenic determinants based on their intracellular expression profiles. Phagocytosed bacteria rapidly responded to their intracellular environment and subsequently altered their transcriptional profile. Differential gene expression profiles were revealed that correlated with specific intracellular locale of the bacteria. Upregulation of general and oxidative stress response genes was a hallmark of the early phagosomal and late endosomal stages, while induction of transport and metabolic genes characterized the cytosolic replication stage. Expression of the Francisella Pathogenicity Island (FPI) genes, which are required for intracellular proliferation, increased during the intracellular cycle. Similarly, 27 chromosomal loci encoding putative hypothetical, secreted, outer membrane proteins or transcriptional regulators were identified as upregulated. Among these, deletion of FTT0383, FTT0369c or FTT1676 abolished the ability of Schu S4 to survive or proliferate intracellularly and cause lethality in mice, therefore identifying novel determinants of Francisella virulence from their intracellular expression profile.

Characterization of rationally attenuated Francisella tularensis vaccine strains that harbor deletions in the guaA and guaB genes

Francisella tularensis, the etiologic agent of tularemia, can cause severe and fatal infection after inhalation of as few as 10 -- 100CFU. F. tularensis is a potential bioterrorism agent and, therefore, a priority for countermeasure development. Vaccination with the live vaccine strain (LVS), developed from a Type B strain, confers partial protection against aerosal exposure to the more virulent Type A strains and provides proof of principle that a live attenuated vaccine strain may be efficacious. However LVS suffers from several notable drawbacks that have prevented its licensure and widespread use. To address the specific deficiencies that render LVS a sub-optimal tularemia vaccine, we engineered F. tularensis LVS strains with targeted deletions in the guaA or guaB genes that encode critical enzymes in the guanine nucleotide biosynthetic pathway. F. tularensis LVSDeltaguaA and LVSDeltaguaB mutants were guanine auxotrophs and were highly attenuated in a mouse model of infection. While the mutants failed to replicate in macrophages, a robust proinflammatory cytokine response, equivalent to that of the parental LVS, was elicited. Mice vaccinated with a single dose of the F. tularensis LVSDeltaguaA or LVSDeltaguaB mutant were fully protected against subsequent lethal challenge with the LVS parental strain. These findings suggest the specific deletion of these target genes could generate a safe and efficacious live attenuated vaccine.

Cellular and humoral immunity are synergistic in protection against types A and B Francisella tularensis

Herein we report studies with a novel combination vaccine that, when administered to mice, conferred protection against highly virulent strains of Francisella tularensis by stimulating both arms of the immune system. Our earlier studies with Ft.LVS::wbtA, an O-polysaccharide (OPS)-negative mutant derived from the available live vaccine strain of F. tularensis (Ft.LVS), elucidated the role of antibodies to the OPS - a key virulence determinant - in protection against virulent type A organisms. However, when expressed on the organism, the OPS enhances virulence. In contrast, in purified form, the OPS is completely benign. We hypothesized that a novel combination vaccine containing both a component that induces humoral immunity and a component that induces cellular immunity to this intracellular microbe would have an enhanced protective capacity over either component alone and would be much safer than the LVS vaccine. Thus we developed a combination vaccine containing both OPS (supplied in an OPS-tetanus toxoid glycoconjugate) to induce a humoral antibody response and strain Ft.LVS::wbtA (which is markedly attenuated by its lack of OPS) to induce a cell-mediated protective response. This vaccine protected mice against otherwise-lethal intranasal and intradermal challenge with wild-type F. tularensis strains Schu S4 (type A) and FSC 108 (type B). These results represent a significant advance in our understanding of immunity to F. tularensis and provide important insight into the development of a safer vaccine effective against infections caused by clinical type A and B strains of F. tularensis.

The early phagosomal stage of Francisella tularensis determines optimal phagosomal escape and Francisella pathogenicity island protein expression

Francisella tularensis is an intracellular pathogen that can survive and replicate within macrophages. Following phagocytosis and transient interactions with the endocytic pathway, F. tularensis rapidly escapes from its original phagosome into the macrophage cytoplasm, where it eventually replicates. To examine the importance of the nascent phagosome for the Francisella intracellular cycle, we have characterized early trafficking events of the F. tularensis subsp. tularensis strain Schu S4 in a murine bone marrow-derived macrophage model. Here we show that early phagosomes containing Schu S4 transiently interact with early and late endosomes and become acidified before the onset of phagosomal disruption. Inhibition of endosomal acidification with the vacuolar ATPase inhibitor bafilomycin A1 or concanamycin A prior to infection significantly delayed but did not block phagosomal escape and cytosolic replication, indicating that maturation of the early Francisella-containing phagosome (FCP) is important for optimal phagosomal escape and subsequent intracellular growth. Further, Francisella pathogenicity island (FPI) protein expression was induced during early intracellular trafficking events. Although inhibition of endosomal acidification mimicked the early phagosomal escape defects caused by mutation of the FPI-encoded IglCD proteins, it did not inhibit the intracellular induction of FPI proteins, demonstrating that this response is independent of phagosomal pH. Altogether, these results demonstrate that early phagosomal maturation is required for optimal phagosomal escape and that the early FCP provides cues other than intravacuolar pH that determine intracellular induction of FPI proteins.

Francisella tularensis: unravelling the secrets of an intracellular pathogen

Francisella tularensis has been recognized as the causative agent of tularaemia for almost a century. Since its discovery in 1911, it has been shown to infect a wide range of hosts, including humans. As early as the 1920s it was suggested to be an intracellular pathogen, but it has proven to be an enigmatic organism, whose interaction with the host has been difficult to elucidate, and we still have a very limited understanding of the molecular mechanisms of virulence. However, the recent availability of genome sequence data and molecular tools has allowed us to start to understand the molecular basis of F. tularensis pathogenicity, and will facilitate the development of a vaccine to protect against infection.

Generation and characterization of an attenuated mutant in a response regulator gene of Francisella tularensis live vaccine strain (LVS)

Francisella tularensis is a zoonotic bacterium that must exist in diverse environments ranging from arthropod vectors to mammalian hosts. To better understand how virulence genes are regulated in these different environments, a transcriptional response regulator gene (genome locus FTL0552) was deleted in F. tularensis live vaccine strain (LVS). The FTL0552 deletion mutant exhibited slightly reduced rates of extracellular growth but was unable to replicate or survive in mouse macrophages and was avirulent in the mouse model using either BALB/c or C57BL/6 mice. Mice infected with the FTL0552 mutant produced reduced levels of inflammatory cytokines, exhibited reduced histopathology, and cleared the bacteria quicker than mice infected with LVS. Mice that survived infection with the FTL0552 mutant were afforded partial protection when challenged with a lethal dose of the virulent SchuS4 strain (4 of 10 survivors, day 21 postinfection) when compared to naive mice (0 of 10 survivors by day 7 postinfection). Microarray experiments indicate that 148 genes are regulated by FTL0552. Most of the genes are downregulated, indicating that FTL0552 controls transcription of genes in a positive manner. Genes regulated by FTL0552 include genes located within the Francisella pathogenicity island that are essential for intracellular survival and virulence of F. tularensis. Further, a mutant in FTL0552 or the comparable locus in SchuS4 (FTT1557c) may be an alternative candidate vaccine for tularemia.

Characterization of fig operon mutants of Francisella novicida U112

Francisella species secrete a polycarboxylate siderophore that resembles rhizoferrin to acquire ferric iron. Several of the Francisella siderophore synthesis genes are contained in a Fur-regulated operon (designated fig or fsl) comprised of at least seven ORFs including fur. Reverse transcriptase-PCR showed transcriptional linkage between figD and figE and between figE and figF. Mutations were constructed in four of these ORFs (figB, figC, figD, and figE) in Francisella novicida U112. All four of these new mutants and a F. novicida figA mutant grew at rates comparable to that of wild type under iron-replete conditions but growth of all five mutants was stunted in iron-limiting media. When ferric rhizoferrin was added to the iron-limited media, growth of the figA, figB, figC, and figD mutants was restored to levels similar to those obtained in iron-replete media. However, this exogenously added siderophore could not rescue the figE mutant. When Chrome Azurol S assays were used to measure siderophore production, the figA, figB, and figC mutants were markedly deficient in their ability to synthesize siderophore whereas the figD and figE mutants produced siderophore at levels equivalent to the wild-type parent strain.

CD4+ T cells are required during priming but not the effector phase of antibody-mediated IFN-gamma-dependent protective immunity against pulmonary Francisella novicida infection

We have previously demonstrated the protective efficacy of intranasal vaccination with a defined Francisella tularensis subsp. novicida DeltaiglC mutant (KKF24) against pulmonary F. novicida U112 challenge. In this study, we further characterized the mechanisms of KKF24-induced immunity. Intranasally vaccinated KKF24 C57BL/6 major histocompatibility class (MHC) class II-/- mice produced minimal antigen-specific interferon (IFN)-gamma and serum antibodies and were highly susceptible (0% survival) to F. novicida challenge, compared to MHC class I-/- or wild-type mice (both 100% survival). Protective immunity could be transferred by immune serum into recipient wild type, but not IFN-gamma-/- mice. The protective effect of KKF24 vaccination against the respiratory F. novicida U112 challenge was not abrogated by anti-CD4 neutralizing antibody treatment and was not conferred by adoptive transfer of KKF24-specific CD4+ T cells. The protective effect of antibody was partially dependent upon Fc receptor-mediated clearance. Taken together, our data indicate that CD4+ T cells are required for priming, but not during the effector phase, of anti-KKF24 antibody-mediated IFN-gamma-dependent immunity against pulmonary F. novicida infection.

Acquisition of the vacuolar ATPase proton pump and phagosome acidification are essential for escape of Francisella tularensis into the macrophage cytosol

The Francisella tularensis-containing phagosome (FCP) matures to a late-endosome-like phagosome prior to bacterial escape into the cytosols of macrophages, where bacterial proliferation occurs. Our data show that within the first 15 min after infection of primary human monocyte-derived macrophages (hMDMs), approximately 90% of the FCPs acquire the proton vacuolar ATPase (vATPase) pump and the lysomotropic dye LysoTracker, which concentrates in acidic compartments, similar to phagosomes harboring the Listeria monocytogenes control. The acquired proton vATPase pump and lysomotropic dye are gradually lost by 30 to 60 min postinfection, which coincides with bacterial escape into the cytosols of hMDMs. Colocalization of phagosomes harboring the iglD mutant with the vATPase pump and the LysoTracker dye was also transient, and the loss of colocalization was faster than that observed for the wild-type strain, which is consistent with the faster escape of the iglD mutant into the macrophage cytosol. In contrast, colocalization of both makers with phagosomes harboring the iglC mutant was persistent, which is consistent with fusion to the lysosomes and failure of the iglC mutant to escape into the macrophage cytosol. We have utilized a fluorescence microscopy-based phagosome integrity assay for differential labeling of vacuolar versus cytosolic bacteria, using antibacterial antibodies loaded into the cytosols of live hMDMs. We show that specific inhibition of the proton vATPase pump by bafilomycin A1 (BFA) blocks rapid bacterial escape into the cytosols of hMDMs, but 30% to 50% of the bacteria escape into the cytosol by 6 to 12 h after BFA treatment. The effect of BFA on the blocking of bacterial escape into the cytosol is completely reversible, as the bacteria escape after removal of BFA. We also show that the limited fusion of the FCP to lysosomes is not due to failure to recruit the late-endosomal fusion regulator Rab7. Therefore, within few minutes of its biogenesis, the FCP transiently acquires the proton vATPase pump to acidify the phagosome, and this transient acidification is essential for subsequent bacterial escape into the macrophage cytosol.

Identification of differentially regulated francisella tularensis genes by use of a newly developed Tn5-based transposon delivery system

Francisella tularensis is the etiologic agent of an intracellular systemic infection of the lymphatic system in humans called tularemia. The organism has become the subject of considerable research interest due to its classification as a category A select agent by the CDC. To aid genetic analysis of this pathogen, we have constructed a temperature-sensitive Tn5-based transposon delivery system that is capable of generating chromosomal reporter fusions with lacZ or luxCDABE, enabling us to monitor gene expression. Transposition is catalyzed by the hyperactive Tn5 transposase, whose expression is driven by the Francisella groES promoter. When high-temperature selection (42 degrees C) is applied to a bacterial culture carrying the transposon delivery plasmid, approximately 0.1% of the population is recovered with Tn5 insertions in the chromosome. Nucleotide sequence analysis of a sample of mutants revealed that the insertions occur randomly throughout the chromosome. The kanamycin-selectable marker of the transposon is also flanked by FLP recombination target sequences that allow deletion of the antibiotic resistance gene when desired. This system has been used to generate transposon mutant libraries for the F. tularensis live vaccine strain as well as two different virulent F. tularensis strains. Chromosomal reporters delivered with the transposon were used to identify genes upregulated by growth in Chamberlain's defined medium. Genes in the fsl operon, reported to be involved in iron acquisition, as well as genes in the igl gene cluster were among those identified by the screen. Further experiments implicate the ferric uptake regulator (Fur) protein in the negative regulation of fsl but not igl reporters, which occurs in an iron-dependent manner. Our results indicate that we have created a valuable new transposon that can be used to identify and characterize virulence genes in F. tularensis strains.

Targeted inactivation of francisella tularensis genes by group II introns

Studies of the molecular mechanisms of pathogenesis of Francisella tularensis, the causative agent of tularemia, have been hampered by a lack of genetic techniques for rapid targeted gene disruption in the most virulent subspecies. Here we describe efficient targeted gene disruption in F. tularensis utilizing mobile group II introns (targetrons) specifically optimized for F. tularensis. Utilizing a targetron targeted to blaB, which encodes ampicillin resistance, we showed that the system works at high efficiency in three different subspecies: F. tularensis subsp. tularensis, F. tularensis subsp. holarctica, and "F. tularensis subsp. novicida." A targetron was also utilized to inactivate F. tularensis subsp. holarctica iglC, a gene required for virulence. The iglC gene is located within the Francisella pathogenicity island (FPI), which has been duplicated in the most virulent subspecies. Importantly, the iglC targetron targeted both copies simultaneously, resulting in a strain mutated in both iglC genes in a single step. This system will help illuminate the contributions of specific genes, and especially those within the FPI, to the pathogenesis of this poorly studied organism.

Characterization and application of a glucose-repressible promoter in Francisella tularensis

Francisella tularensis, the causative agent of tularemia, is a category A biodefense agent. The examination of gene function in this organism is limited due to the lack of available controllable promoters. Here, we identify a promoter element of F. tularensis LVS that is repressed by glucose (termed the Francisella glucose-repressible promoter, or FGRp), allowing the management of downstream gene expression. In bacteria cultured in medium lacking glucose, this promoter induced the expression of a red fluorescent protein allele, tdtomato. FGRp activity was used to produce antisense RNA of iglC, an important virulence factor, which severely reduced IglC protein levels. Cultivation in glucose-containing medium restored IglC levels, indicating the usefulness of this promoter for controlling both exogenous and chromosomal gene expression. Moreover, FGRp was shown to be active during the infection of human macrophages by using the fluorescence reporter. In this environment, the FGRp-mediated expression of antisense iglC by F. tularensis LVS resulted in reduced bacterial fitness, demonstrating the applicability of this promoter. An analysis of the genomic sequence indicated that this promoter region controls a gene, FTL_0580, encoding a hypothetical protein. A deletion analysis determined the critical sites essential for FGRp activity to be located within a 44-bp region. This is the first report of a conditional promoter and the use of antisense constructs in F. tularensis, valuable genetic tools for studying gene function both in vitro and in vivo.

A conserved and immunodominant lipoprotein of Francisella tularensis is proinflammatory but not essential for virulence

Francisella tularensis is a highly virulent bacterium that causes tularemia, a disease that is often fatal if untreated. A live vaccine strain (LVS) of this bacterium is attenuated for virulence in humans but produces lethal disease in mice. F. tularensis has been classified as a Category A agent of bioterrorism. Despite this categorization, little is known about the components of the organism that are responsible for causing disease in its hosts. Here, we report the deletion of a well-characterized lipoprotein of F. tularensis, designated LpnA (also known as Tul4), in the LVS. An LpnA deletion mutant was comparable to the wild-type strain in its ability to grow intracellularly and cause lethal disease in mice. Additionally, mice inoculated with a sublethal dose of the mutant strain were afforded the same protection against a subsequent lethal challenge with the LVS as were mice initially administered a sublethal dose of the wild-type bacterium. The LpnA-deficient strain showed an equivalent ability to promote secretion of chemokines by human monocyte-derived macrophages as its wild-type counterpart. However, recombinant LpnA potently stimulated primary cultures of human macrophages in a Toll-like receptor 2-dependent manner. Although human endothelial cells were also activated by recombinant LpnA, their response was relatively modest. LpnA is clearly unnecessary for multiple functions of the LVS, but its inflammatory capacity implicates it and other Francisella lipoproteins as potentially important to the pathogenesis of tularemia.

Construction of targeted insertion mutations in Francisella tularensis subsp. novicida

Francisella tularensis is one of the most deadly bacterial agents, yet most of the genetic determinants of pathogenesis are still unknown. We have developed an efficient targeted mutagenesis strategy in the model organism F. tularensis subsp. novicida by utilizing universal priming of optimized antibiotic resistance cassettes and splicing by overlap extension (SOE). This process enables fast and efficient construction of targeted insertion mutations in F. tularensis subsp. novicida that have characteristics of nonpolar mutations; optimized targeted mutagenesis strategies will promote the study of this mysterious bacterium and facilitate vaccine development against tularemia. Moreover the general strategy of gene disruption by PCR-based antibiotic resistance cassette insertion is broadly applicable to many bacterial species.

Identification of Francisella tularensis Himar1-based transposon mutants defective for replication in macrophages

Francisella tularensis, the etiologic agent of tularemia in humans, is a potential biological threat due to its low infectious dose and multiple routes of entry. F. tularensis replicates within several cell types, eventually causing cell death by inducing apoptosis. In this study, a modified Himar1 transposon (HimarFT) was used to mutagenize F. tularensis LVS. Approximately 7,000 Km(r) clones were screened using J774A.1 macrophages for reduction in cytopathogenicity based on retention of the cell monolayer. A total of 441 candidates with significant host cell retention compared to the parent were identified following screening in a high-throughput format. Retesting at a defined multiplicity of infection followed by in vitro growth analyses resulted in identification of approximately 70 candidates representing 26 unique loci involved in macrophage replication and/or cytotoxicity. Mutants carrying insertions in seven hypothetical genes were screened in a mouse model of infection, and all strains tested appeared to be attenuated, which validated the initial in vitro results obtained with cultured macrophages. Complementation and reverse transcription-PCR experiments suggested that the expression of genes adjacent to the HimarFT insertion may be affected depending on the orientation of the constitutive groEL promoter region used to ensure transcription of the selective marker in the transposon. A hypothetical gene, FTL_0706, postulated to be important for lipopolysaccharide biosynthesis, was confirmed to be a gene involved in O-antigen expression in F. tularensis LVS and Schu S4. These and other studies demonstrate that therapeutic targets, vaccine candidates, or virulence-related genes may be discovered utilizing classical genetic approaches in Francisella.

Identification of an orphan response regulator required for the virulence of Francisella spp. and transcription of pathogenicity island genes

Francisella tularensis is a category A agent of biowarfare/biodefense. Little is known about the regulation of virulence gene expression in Francisella spp. Comparatively few regulatory factors exist in Francisella, including those belonging to two-component systems (TCS). However, orphan members of typical TCS can be identified. To determine if orphan TCS members affect Francisella gene expression, a gene encoding a product with high similarity to the Salmonella PmrA response regulator (FTT1557c/FNU0663.2) was deleted in Francisella novicida (a model organism for F. tularensis). The F. novicida pmrA mutant was defective in survival/growth within human and murine macrophage cell lines and was 100% defective in virulence in mice at a dose of up to 10(8) CFU. In addition, the mutant strain demonstrated increased susceptibility to antimicrobial peptide killing, but no differences were observed between the lipid A of the mutant and the parental strain, as has been observed with pmrA mutants of other microbes. The F. novicida pmrA mutant was 100% protective as a single-dose vaccine when challenge was with 10(6) CFU of F. novicida but did not protect against type A Schu S4 wild-type challenge. DNA microarray analysis identified 65 genes regulated by PmrA. The majority of these genes were located in the region surrounding pmrA or within the Francisella pathogenicity island (FPI). These FPI genes are also regulated by MglA, but MglA does not regulate pmrA, nor does PmrA regulate MglA. Thus, the orphan response regulator PmrA is an important factor in controlling virulence in F. novicida, and a pmrA mutant strain is an effective vaccine against homologous challenge.

An in vitro model system used to study adherence and invasion of Francisella tularensis live vaccine strain in nonphagocytic cells

In observing Francisella tularensis interactions with nonphagocytic cell lines in vitro, we noted significant adherence, invasion, and intracellular growth of the bacteria within these cells. F. tularensis live vaccine strain invasion of nonprofessional phagocytic cells is inhibited by cytochalasin D and nocodazole, suggesting that both the actin and microtubule cytoskeletons are important for invasion.