Exploitation of host cell biology and evasion of immunity by francisella tularensis

Rickettsia-host interaction: strategies of intracytosolic host colonization

Bacterial infection is a highly complex biological process involving a dynamic interaction between the invading microorganism and the host. Specifically, intracellular pathogens seize control over the host cellular processes including membrane dynamics, actin cytoskeleton, phosphoinositide metabolism, intracellular trafficking and immune defense mechanisms to promote their host colonization. To accomplish such challenging tasks, virulent bacteria deploy unique species-specific secreted effectors to evade and/or subvert cellular defense surveillance mechanisms to establish a replication niche. However, despite superficially similar infection strategies, diverse Rickettsia species utilize different effector repertoires to promote host colonization. This review will discuss our current understandings on how different Rickettsia species deploy their effector arsenal to manipulate host cellular processes to promote their intracytosolic life within the mammalian host.

The Use of Extracellular Membrane Vesicles for Immunization against Francisellosis in Nile Tilapia (Oreochromis niloticus) and Atlantic Cod (Gadus morhua L.)

Francisellosis in fish is caused by the facultative intracellular Gram-negative bacterial pathogens Francisella noatunensis ssp. noatunensis and Francisella orientalis. The disease is affecting both farmed and wild fish worldwide and no commercial vaccines are currently available. In this study, we tested isolated membrane vesicles (MVs) as possible vaccine candidates based on previous trials in zebrafish (Danio rerio) indicating promising vaccine efficacy. Here, the MV vaccine-candidates were tested in their natural hosts, Atlantic cod (Gadus morhua L.) and Nile tilapia (Oreochromis niloticus). Injection of MVs did not display any toxicity or other negative influence on the fish and gene expression analysis indicated an influence on the host immune response. However, unlike in other tested fish species, a protective immunity following vaccine application and immunization period could not be detected in the Atlantic cod or tilapia. Further in vivo studies are required to achieve a better understanding of the development of immunological memory in different fish species.

Cathepsins in Bacteria-Macrophage Interaction: Defenders or Victims of Circumstance?

Macrophages are the first encounters of invading bacteria and are responsible for engulfing and digesting pathogens through phagocytosis leading to initiation of the innate inflammatory response. Intracellular digestion occurs through a close relationship between phagocytic/endocytic and lysosomal pathways, in which proteolytic enzymes, such as cathepsins, are involved. The presence of cathepsins in the endo-lysosomal compartment permits direct interaction with and killing of bacteria, and may contribute to processing of bacterial antigens for presentation, an event necessary for the induction of antibacterial adaptive immune response. Therefore, it is not surprising that bacteria can control the expression and proteolytic activity of cathepsins, including their inhibitors – cystatins, to favor their own intracellular survival in macrophages. In this review, we summarize recent developments in defining the role of cathepsins in bacteria-macrophage interaction and describe important strategies engaged by bacteria to manipulate cathepsin expression and activity in macrophages. Particularly, we focus on specific bacterial species due to their clinical relevance to humans and animal health, i.e., Mycobacterium, Mycoplasma, Staphylococcus, Streptococcus, Salmonella, Shigella, Francisella, Chlamydia, Listeria, Brucella, Helicobacter, Neisseria, and other genera.

Risk1, a Phosphatidylinositol 3-Kinase Effector, Promotes Rickettsia typhi Intracellular Survival

Rickettsia species are Gram-negative obligate intracellular bacteria that infect a wide range of eukaryotes and vertebrates. In particular, human body louse-borne Rickettsia prowazekii and flea-borne Rickettsia typhi have historically plagued humankind and continue to reemerge globally. The unavailability of vaccines and limited effectiveness of antibiotics late in infection place lethality rates up to 30%, highlighting the need to elucidate the mechanisms of Rickettsia pathogenicity in greater detail. Here, we characterize a new effector, Risk1, as a secreted phosphatidylinositol 3-kinase (PI3K) with unique dual class I and class III activities. Risk1 is required for host colonization, and its vacuolar phosphatidylinositol 3-phosphate generation modulates endosomal trafficking to arrest autophagosomal maturation. Collectively, Risk1 facilitates R. typhi growth by altering phosphoinositide metabolism and subverting intracellular trafficking.

To establish a habitable intracellular niche, various pathogenic bacteria secrete effectors that target intracellular trafficking and modulate phosphoinositide (PI) metabolism. Murine typhus, caused by the obligate intracellular bacterium Rickettsia typhi, remains a severe disease in humans. However, the mechanisms by which R. typhi effector molecules contribute to internalization by induced phagocytosis and subsequent phagosomal escape into the cytosol to facilitate the intracellular growth of the bacteria remain ill-defined. Here, we characterize a new molecule, Risk1, as a phosphatidylinositol 3-kinase (PI3K) secreted effector and the first bacterial secretory kinase with both class I and III PI3K activities. Inactivation of Risk1 PI3K activities reduced the phosphorylation of phosphatidylinositol 4,5-bisphosphate to phosphatidylinositol 3,4,5-trisphosphate within the host, which consequently diminished host colonization by R. typhi. During infection, Risk1 targets the Rab5-EEA1-phosphatidylinositol 3-phosphate [PI(3)P] signaling axis to promote bacterial phagosomal escape. Subsequently, R. typhi undergoes ubiquitination and induces host autophagy; however, maturation to autolysosomes is subverted to support intracellular growth. Intriguingly, only enzymatically active Risk1 binds the Beclin-1 core complex and contributes to R. typhi-induced autophagosome formation. In sum, our data suggest that Risk1, with dual class I and class III PI3K activities, alters host PI metabolism and consequently subverts intracellular trafficking to facilitate intracellular growth of R. typhi.

CD200R deletion promotes a neutrophil niche for Francisella tularensis and increases infectious burden and mortality

Pulmonary immune control is crucial for protection against pathogens. Here we identify a pathway that promotes host responses during pulmonary bacterial infection; the expression of CD200 receptor (CD200R), which is known to dampen pulmonary immune responses, promotes effective clearance of the lethal intracellular bacterium Francisella tularensis. We show that depletion of CD200R in mice increases in vitro and in vivo infectious burden. In vivo, CD200R deficiency leads to enhanced bacterial burden in neutrophils, suggesting CD200R normally limits the neutrophil niche for infection. Indeed, depletion of this neutrophil niche in CD200R^−/− mice restores F. tularensis infection to levels seen in wild-type mice. Mechanistically, CD200R-deficient neutrophils display significantly reduced reactive oxygen species production (ROS), suggesting that CD200R-mediated ROS production in neutrophils is necessary for limiting F. tularensis colonisation and proliferation. Overall, our data show that CD200R promotes the antimicrobial properties of neutrophils and may represent a novel antibacterial therapeutic target.

The authors show that the CD200 receptor (CD200R) promotes effective clearance of pulmonary Francisella tularensis infection in knock out mice. This result is unexpected as CD200R is known to dampen pulmonary immune responses, and these data suggest that the beneficial effect against F. tularensis is due to depletion of a neutrophil niche for the bacterium.

How Viral and Intracellular Bacterial Pathogens Reprogram the Metabolism of Host Cells to Allow Their Intracellular Replication

Viruses and intracellular bacterial pathogens (IBPs) have in common the need of suitable host cells for efficient replication and proliferation during infection. In human infections, the cell types which both groups of pathogens are using as hosts are indeed quite similar and include phagocytic immune cells, especially monocytes/macrophages (MOs/MPs) and dendritic cells (DCs), as well as nonprofessional phagocytes, like epithelial cells, fibroblasts and endothelial cells. These terminally differentiated cells are normally in a metabolically quiescent state when they are encountered by these pathogens during infection. This metabolic state of the host cells does not meet the extensive need for nutrients required for efficient intracellular replication of viruses and especially IBPs which, in contrast to the viral pathogens, have to perform their own specific intracellular metabolism to survive and efficiently replicate in their host cell niches. For this goal, viruses and IBPs have to reprogram the host cell metabolism in a pathogen-specific manner to increase the supply of nutrients, energy, and metabolites which have to be provided to the pathogen to allow its replication. In viral infections, this appears to be often achieved by the interaction of specific viral factors with central metabolic regulators, including oncogenes and tumor suppressors, or by the introduction of virus-specific oncogenes. Less is so far known on the mechanisms leading to metabolic reprogramming of the host cell by IBPs. However, the still scant data suggest that similar mechanisms may also determine the reprogramming of the host cell metabolism in IBP infections. In this review, we summarize and compare the present knowledge on this important, yet still poorly understood aspect of pathogenesis of human viral and especially IBP infections.

Discovery and Characterization of Two Classes of Selective Inhibitors of the Suppressor of the TCR Signaling Family of Proteins

The suppressor of T-cell receptor signaling (Sts) proteins, Sts-1, has recently emerged as a potential immunostimulatory target for drug development. Genetic inactivation of the Sts proteins dramatically increases host survival of systemic infection and leads to improved pathogen clearance. The protein tyrosine phosphatase (PTP) activity of these proteins arises from a C-terminal 2-histidine phosphatase (HP) domain. To identify new inhibitors of the HP activity of Sts-1, we miniaturized a phosphatase assay to a 1536-well format and conducted a 20 580 compound screen. Among the hits were two classes of structurally related compounds, tetracycline variants and sulfonated azo dyes. These hits had low micromolar to nanomolar IC₅₀ values. Orthogonal screening confirmed the validity of these inhibitors and demonstrated that both act competitively on Sts-1 phosphatase activity. When tested on other PTPs, PTP1B and SHP1, it was found that the tetracycline PTP1B, SHP1, the tetracycline variant (doxycycline), and the sulfonated azo dye (Congo red) are selective inhibitors of Sts-1_HP, with selectivity indices ranging from 19 to as high as 200. The planar polyaromatic moieties present in both classes of compounds suggested a common binding mode. The mutation of either tryptophan 494 or tyrosine 596, located near the active site of the protein, reduced the K_i of the inhibitors from 3- to 18-fold, indicating that these residues may help to promote the binding of substrates with aromatic groups. This work provides new insights into substrate selectivity mechanisms and describes two classes of compounds that can serve as probes of function or as a basis for future drug discovery.

Challenges of Francisella classification exemplified by an atypical clinical isolate

The accumulation of sequenced Francisella strains has made it increasingly apparent that the 16S rRNA gene alone is not enough to stratify the Francisella genus into precise and clinically useful classifications. Continued whole-genome sequencing of isolates will provide a larger base of knowledge for targeted approaches with broad applicability. Additionally, examination of genomic information on a case-by-case basis will help resolve outstanding questions regarding strain stratification. We report the complete genome sequence of a clinical isolate, designated here as F. novicida-like strain TCH2015, acquired from the lymph node of a 6-year-old male. Two features were atypical for F. novicida: exhibition of functional oxidase activity and additional gene content, including proposed virulence determinants. These differences, which could potentially impact virulence and clinical diagnosis, emphasize the need for more comprehensive methods to profile Francisella isolates. This study highlights the value of whole-genome sequencing, which will lead to a more robust database of environmental and clinical genomes and inform strategies to improve detection and classification of Francisella strains.

Deletion of the Major Facilitator Superfamily Transporter fptB Alters Host Cell Interactions and Attenuates Virulence of Type A Francisella tularensis

Francisella tularensis is a Gram-negative, facultative, intracellular coccobacillus that can infect a wide variety of hosts. In humans, F. tularensis causes the zoonosis tularemia following insect bites, ingestion, inhalation, and the handling of infected animals. The fact that a very small inoculum delivered by the aerosol route can cause severe disease, coupled with the possibility of its use as an aerosolized bioweapon, has led to the classification of Francisella tularensis as a category A select agent and has renewed interest in the formulation of a vaccine. To this end, we engineered a type A strain SchuS4 derivative containing a targeted deletion of the major facilitator superfamily (MFS) transporter fptB. Based on the attenuating capacity of this deletion in the F. tularensis LVS background, we hypothesized that the deletion of this transporter would alter the intracellular replication and cytokine induction of the type A strain and attenuate virulence in the stringent C57BL/6J mouse model. Here we demonstrate that the deletion of fptB significantly alters the intracellular life cycle of F. tularensis, attenuating intracellular replication in both cell line-derived and primary macrophages and inducing a novel cytosolic escape delay. Additionally, we observed prominent differences in the in vitro cytokine profiles in human macrophage-like cells. The mutant was highly attenuated in the C57BL/6J mouse model and provided partial protection against virulent type A F. tularensis challenge. These results indicate a fundamental necessity for this nutrient transporter in the timely progression of F. tularensis through its replication cycle and in pathogenesis.

The Early Dendritic Cell Signaling Induced by Virulent Francisella tularensis Strain Occurs in Phases and Involves the Activation of Extracellular Signal-Regulated Kinases (ERKs) and p38 In the Later Stage

Dendritic cells (DCs) infected by Francisella tularensis are poorly activated and do not undergo classical maturation process. Although reasons of such unresponsiveness are not fully understood, their impact on the priming of immunity is well appreciated. Previous attempts to explain the behavior of Francisella-infected DCs were hypothesis-driven and focused on events at later stages of infection. Here, we took an alternative unbiased approach by applying methods of global phosphoproteomics to analyze the dynamics of cell signaling in primary DCs during the first hour of infection by Francisella tularensis Presented results show that the early response of DCs to Francisella occurs in phases and that ERK and p38 signaling modules induced at the later stage are differentially regulated by virulent and attenuated ΔdsbA strain. These findings imply that the temporal orchestration of host proinflammatory pathways represents the integral part of Francisella life-cycle inside hijacked DCs.

Increased Resistance to Intradermal Francisella tularensis LVS Infection by Inactivation of the Sts Phosphatases

The Suppressor of TCR signaling proteins (Sts-1 and Sts-2) are two homologous phosphatases that negatively regulate signaling pathways in a number of hematopoietic lineages, including T lymphocytes. Mice lacking Sts expression are characterized by enhanced T cell responses. Additionally, a recent study demonstrated that Sts^-/- mice are profoundly resistant to systemic infection by Candida albicans, with resistance characterized by enhanced survival, more rapid fungal clearance in key peripheral organs, and an altered inflammatory response. To investigate the role of Sts in the primary host response to infection by a bacterial pathogen, we evaluated the response of Sts^-/- mice to infection by a Gram-negative bacterial pathogen. Francisella tularensis is a facultative bacterial pathogen that replicates intracellularly within a variety of cell types and is the causative agent of tularemia. Francisella infections are characterized by a delayed immune response, followed by an intense inflammatory reaction that causes widespread tissue damage and septic shock. Herein, we demonstrate that mice lacking Sts expression are significantly resistant to infection by the live vaccine strain (LVS) of F. tularensis Resistance is characterized by reduced lethality following high-dose intradermal infection, an altered cytokine response in the spleen, and enhanced bacterial clearance in multiple peripheral organs. Sts^-/- bone marrow-derived monocytes and neutrophils, infected with F. tularensis LVS ex vivo, display enhanced restriction of intracellular bacteria. These observations suggest the Sts proteins play an important regulatory role in the host response to bacterial infection, and they underscore a role for Sts in regulating functionally relevant immune response pathways.

F. novicida-Infected A. castellanii Does Not Enhance Bacterial Virulence in Mice

Francisella tularensis is a facultative intracellular bacterium that causes tularemia in humans and animals. Epidemiology of tularemia worldwide is often associated with water-borne transmission, which includes mosquitoes and amoebae as the potential host reservoirs of the bacteria in water environment. In vitro studies showed intracellular replication of F. tularensis within Acanthamoeba castellanii and Hartmanella vermiformis cells. While infection of amoeba by Legionella pneumophila has been shown to enhance infectivity of L. pneumophila the role of F. tularensis-infected protozoa in the pathogenesis of tularemia is not known. We used 6 h coculture of A. castellanii and F. novicida for investigation of the effect of inhaled amoeba on the pathogenesis of tularemia on in vivo model. Balb/c mice were infected intratracheally with F. novicida or with F. novicida-infected A. castellanii. Surprisingly, infection with F. novicida-infected A. castellanii did not lead to bronchopneumonia in Balb/c mice, and Francisella did not disseminate into the liver and spleen. Upon inhalation, F. novicida infects a variety of host cells, though neutrophils are the predominant cells early during infection in the lung infiltrates of pulmonary tularemia. The numbers of neutrophils in the lungs of Balb/c mice were significantly lower in the infection of mice with F. novicida-infected A. castellanii in comparison to group of mice infected only with F. novicida. These results demonstrate that following inoculation of mice with F. novicida-infected A. castellanii, mice did not develop tularemia.

Early Interactions of Murine Macrophages with Francisella tularensis Map to Mouse Chromosome 19

Differences among individuals in susceptibility to infectious diseases can be modulated by host genetics. Much of the research in this field has aimed to identify loci within the host genome that are associated with these differences. In mice, A/J (AJ) and C57BL/6J (B6) mice show differential susceptibilities to various pathogens, including the intracellular pathogen Francisella tularensis. Because macrophages are the main initial target during F. tularensis infection, we explored early interactions of macrophages from these two mouse strains with F. tularensis as well as the genetic factors underlying these interactions. Our results indicate that bacterial interactions with bone marrow-derived macrophages (BMDMs) during early stages of infection are different in the AJ and B6 strains. During these early stages, bacteria are more numerous in B6 than in AJ macrophages and display differences in trafficking and early transcriptional response within these macrophages. To determine the genetic basis for these differences, we infected BMDMs isolated from recombinant inbred (RI) mice derived from reciprocal crosses between AJ and B6, and we followed early bacterial counts within these macrophages. Quantitative trait locus (QTL) analysis revealed a locus on chromosome 19 that is associated with early differences in bacterial counts in AJ versus B6 macrophages. QTL analysis of published data that measured the differential susceptibilities of the same RI mice to an in vivo challenge with F. tularensis confirmed the F. tularensis susceptibility QTL on chromosome 19. Overall, our results show that early interactions of macrophages with F. tularensis are dependent on the macrophage genetic background.

IMPORTANCE

Francisella tularensis is a highly pathogenic bacterium with a very low infectious dose in humans. Some mechanisms of bacterial virulence have been elucidated, but the host genetic factors that contribute to host resistance or susceptibility are largely unknown. In this work, we have undertaken a genetic approach to assess what these factors are in mice. Analyzing early interactions of macrophages with the bacteria as well as data on overall susceptibility to infection revealed a locus on chromosome 19 that is associated with both phenotypes. In addition, our work revealed differences in the early macrophage response between macrophages with different genetic backgrounds. Overall, this work suggests some intriguing links between in vitro and in vivo infection models and should aid in further elucidating the genetic circuits behind the host response to Francisella tularensis infection.

Francisella tularensis Catalase Restricts Immune Function by Impairing TRPM2 Channel Activity

As an innate defense mechanism, macrophages produce reactive oxygen species that weaken pathogens and serve as secondary messengers involved in immune function. The Gram-negative bacterium Francisella tularensis utilizes its antioxidant armature to limit the host immune response, but the mechanism behind this suppression is not defined. Here we establish that F. tularensis limits Ca(2+) entry in macrophages, thereby limiting actin reorganization and IL-6 production in a redox-dependent fashion. Wild type (live vaccine strain) or catalase-deficient F. tularensis (ΔkatG) show distinct profiles in their H2O2 scavenging rates, 1 and 0.015 pm/s, respectively. Murine alveolar macrophages infected with ΔkatG display abnormally high basal intracellular Ca(2+) concentration that did not increase further in response to H2O2. Additionally, ΔkatG-infected macrophages displayed limited Ca(2+) influx in response to ionomycin, as a result of ionophore H2O2 sensitivity. Exogenously added H2O2 or H2O2 generated by ΔkatG likely oxidizes ionomycin and alters its ability to transport Ca(2+). Basal increases in cytosolic Ca(2+) and insensitivity to H2O2-mediated Ca(2+) entry in ΔkatG-infected cells are reversed by the Ca(2+) channel inhibitors 2-aminoethyl diphenylborinate and SKF-96365. 2-Aminoethyl diphenylborinate but not SKF-96365 abrogated ΔkatG-dependent increases in macrophage actin remodeling and IL-6 secretion, suggesting a role for H2O2-mediated Ca(2+) entry through the transient receptor potential melastatin 2 (TRPM2) channel in macrophages. Indeed, increases in basal Ca(2+), actin polymerization, and IL-6 production are reversed in TRPM2-null macrophages infected with ΔkatG. Together, our findings provide compelling evidence that F. tularensis catalase restricts reactive oxygen species to temper macrophage TRPM2-mediated Ca(2+) signaling and limit host immune function.

Using host-pathogen protein interactions to identify and characterize Francisella tularensis virulence factors

Background

Francisella tularensis is a select bio-threat agent and one of the most virulent intracellular pathogens known, requiring just a few organisms to establish an infection. Although several virulence factors are known, we lack an understanding of virulence factors that act through host-pathogen protein interactions to promote infection. To address these issues in the highly infectious F. tularensis subsp. tularensis Schu S4 strain, we deployed a combined in silico, in vitro, and in vivo analysis to identify virulence factors and their interactions with host proteins to characterize bacterial infection mechanisms.

Results

We initially used comparative genomics and literature to identify and select a set of 49 putative and known virulence factors for analysis. Each protein was then subjected to proteome-scale yeast two-hybrid (Y2H) screens with human and murine cDNA libraries to identify potential host-pathogen protein-protein interactions. Based on the bacterial protein interaction profile with both hosts, we selected seven novel putative virulence factors for mutant construction and animal validation experiments. We were able to create five transposon insertion mutants and used them in an intranasal BALB/c mouse challenge model to establish 50 % lethal dose estimates. Three of these, ΔFTT0482c, ΔFTT1538c, and ΔFTT1597, showed attenuation in lethality and can thus be considered novel F. tularensis virulence factors. The analysis of the accompanying Y2H data identified intracellular protein trafficking between the early endosome to the late endosome as an important component in virulence attenuation for these virulence factors. Furthermore, we also used the Y2H data to investigate host protein binding of two known virulence factors, showing that direct protein binding was a component in the modulation of the inflammatory response via activation of mitogen-activated protein kinases and in the oxidative stress response.

Conclusions

Direct interactions with specific host proteins and the ability to influence interactions among host proteins are important components for F. tularensis to avoid host-cell defense mechanisms and successfully establish an infection. Although direct host-pathogen protein-protein binding is only one aspect of Francisella virulence, it is a critical component in directly manipulating and interfering with cellular processes in the host cell.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2351-1) contains supplementary material, which is available to authorized users.

Cholera toxin B induced activation of murine macrophages exposed to a fixed bacterial immunogen

OBJECTIVES

Previous studies have demonstrated that intranasal administration of inactivated (fixed) Francisella tularensis (iFt) live vaccine strain (LVS) in conjunction with the mucosal adjuvant, cholera toxin B (CTB), provides full protection against subsequent lethal challenge with Ft LVS and partial protection against the more virulent Ft SchuS4 strain. Understanding the mechanisms of CTB-induced immune stimulation that confer protection against Ft will be valuable to the development of an effective vaccine against this highly virulent fatal pathogen. In this study, an in vitro system was utilized to further elucidate the immunologic adjuvant effect of CTB when administered with the fixed bacterial immunogen iFt.

METHODS

The murine macrophage cell line (RAW264.7) was treated with combinations of iFt and CTB. The treated RAW264.7 cells and their supernatants were collected and assessed for cell surface marker expression and cytokine secretion. In addition, the ability of RAW264.7 cells to present bacterial antigens (iFt or LVS) to an Ft-specific T-cell hybridoma cell line, following exposure to CTB, was analyzed.

RESULTS

We found that RAW264.7 cells responded to treatment with iFt + CTB by an increased secretion of the proinflammatory cytokines interleukin 6 and tumor necrosis factor α and upregulation of the surface expression of toll-like receptor 4 and the costimulatory molecules CD80 and CD86. Furthermore, the experimental vaccine treatment iFt + CTB enhanced the ability of macrophages to present iFt antigens to an FT-specific T-cell hybridoma cell line, although they failed to do so with LVS.

CONCLUSION

The adjuvant CTB administered in conjunction with iFt showed evidence of enhancing an antigen-specific proinflammatory response in vitro. These observations allow us to define, in part, the mechanisms of immune activation conferred by mucosal administration of iFt + CTB against lethal F. tularensis challenge.

The complex amino acid diet of Francisella in infected macrophages

Francisella tularensis, the agent of the zoonotic disease tularemia, is a highly infectious bacterium for a large number of animal species and can be transmitted to humans by various means. The bacterium is able to infect a variety of cell types but replicates in mammalian hosts mainly in the cytosol of infected macrophages. In order to resist the stressful and nutrient-restricted intracellular environments, it encounters during its systemic dissemination, Francisella has developed dedicated stress resistance mechanisms and adapted its metabolic and nutritional needs. Recent data form our laboratory and from several other groups have shown that Francisella simultaneously relies on multiple host amino acid sources during its intracellular life cycle. This review will summarize how intracellular Francisella use different amino acid sources, and their role in phagosomal escape and/or cytosolic multiplication and systemic dissemination. We will first summarize the data that we have obtained on two amino acid transporters involved in Francisella phagosomal escape and cytosolic multiplication i.e., the glutamate transporter GadC and the asparagine transporter AnsP, respectively. The specific contribution of glutamate and asparagine to the physiology of the bacterium will be evoked. Then, we will discuss how Francisella has adapted to obtain and utilize host amino acid resources, and notably the contribution of host transporters and autophagy process in the establishment of a nutrient-replete intracellular niche.

Host-pathogen interactions and immune evasion strategies in Francisella tularensis pathogenicity

Francisella tularensis is an intracellular Gram-negative bacterium that causes life-threatening tularemia. Although the prevalence of natural infection is low, F. tularensis remains a tier I priority pathogen due to its extreme virulence and ease of aerosol dissemination. F. tularensis can infect a host through multiple routes, including the intradermal and respiratory routes. Respiratory infection can result from a very small inoculum (ten organisms or fewer) and is the most lethal form of infection. Following infection, F. tularensis employs strategies for immune evasion that delay the immune response, permitting systemic distribution and induction of sepsis. In this review we summarize the current knowledge of F. tularensis in an immunological context, with emphasis on the host response and bacterial evasion of that response.

Monocyte/macrophage inflammatory response pathways to combat Francisella infection: possible therapeutic targets?

Francisella tularensis can bypass and suppress host immune responses, even to the point of manipulating immune cell phenotypes and intercellular inflammatory networks. Strengthening these responses such that immune cells more readily identify and destroy the bacteria is likely to become a viable (and perhaps necessary) strategy for combating infections with Francisella, especially given the likelihood of antibiotic resistance in the foreseeable future. Monocytes and macrophages offer a niche wherein Francisella can invade and replicate, resulting in substantially higher bacterial load that can overcome the host. As such, understanding their responses to Francisella may uncover potential avenues of therapy that could promote a lowering of bacterial burden and clearance of infection. These response pathways include Toll-like Receptor 2 (TLR2), the caspase-1 inflammasome, Interferons, NADPH oxidase, Phosphatidylinositide 3-kinase (PI3K), and the Ras pathway. In this review we summarize the literature pertaining to the roles of these pathways during Francisella infection, with an emphasis on monocyte/macrophage responses. The therapeutic targeting of one or more such pathways may ultimately become a valuable tool for the treatment of tularemia, and several possibilities are discussed.

Novel catanionic surfactant vesicle vaccines protect against Francisella tularensis LVS and confer significant partial protection against F. tularensis Schu S4 strain

Francisella tularensis is a Gram-negative immune-evasive coccobacillus that causes tularemia in humans and animals. A safe and efficacious vaccine that is protective against multiple F. tularensis strains has yet to be developed. In this study, we tested a novel vaccine approach using artificial pathogens, synthetic nanoparticles made from catanionic surfactant vesicles that are functionalized by the incorporation of either F. tularensis type B live vaccine strain (F. tularensis LVS [LVS-V]) or F. tularensis type A Schu S4 strain (F. tularensis Schu S4 [Schu S4-V]) components. The immunization of C57BL/6 mice with "bare" vesicles, which did not express F. tularensis components, partially protected against F. tularensis LVS, presumably through activation of the innate immune response, and yet it failed to protect against the F. tularensis Schu S4 strain. In contrast, immunization with LVS-V fully protected mice against intraperitoneal (i.p.) F. tularensis LVS challenge, while immunization of mice with either LVS-V or Schu S4-V partially protected C57BL/6 mice against an intranasal (i.n.) F. tularensis Schu S4 challenge and significantly increased the mean time to death for nonsurvivors, particularly following the i.n. and heterologous (i.e., i.p./i.n.) routes of immunization. LVS-V immunization, but not immunization with empty vesicles, elicited high levels of IgG against nonlipopolysaccharide (non-LPS) epitopes that were increased after F. tularensis LVS challenge and significantly increased early cytokine production. Antisera from LVS-V-immunized mice conferred passive protection against challenge with F. tularensis LVS. Together, these data indicate that functionalized catanionic surfactant vesicles represent an important and novel tool for the development of a safe and effective F. tularensis subunit vaccine and may be applicable for use with other pathogens.

Comparative phosphoproteomics reveals components of host cell invasion and post-transcriptional regulation during Francisella infection

Francisella tularensis is a facultative intracellular bacterium that causes the deadly disease tularemia. Most evidence suggests that Francisella is not well recognized by the innate immune system that normally leads to cytokine expression and cell death. In previous work, we identified new bacterial factors that were hyper-cytotoxic to macrophages. Four of the identified hyper-cytotoxic strains (lpcC, manB, manC, and kdtA) had an impaired lipopolysaccharide (LPS) synthesis and produced an exposed lipid A lacking the O-antigen. These mutants were not only hyper-cytotoxic but also were phagocytosed at much higher rates compared with the wild type parent strain. To elucidate the cellular signaling underlying this enhanced phagocytosis and cell death, we performed a large-scale comparative phosphoproteomic analysis of cells infected with wild-type and delta-lpcC F. novicida. Our data suggest that not only actin but also intermediate filaments and microtubules are important for F. novicida entry into the host cells. In addition, we observed differential phosphorylation of tristetraprolin, a key component of the mRNA-degrading machinery that controls the expression of a variety of genes including many cytokines. Infection with the delta-lpcC mutant induced the hyper-phosphorylation and inhibition of tristetraprolin, leading to the production of cytokines such as IL-1beta and TNF-alpha that may kill the host cells by triggering apoptosis. Together, our data provide new insights for Francisella invasion and a post-transcriptional mechanism that prevents the expression of host immune response factors that control infection by this pathogen.

Temporal transcriptional response during infection of type II alveolar epithelial cells with Francisella tularensis live vaccine strain (LVS) supports a general host suppression and bacterial uptake by macropinocytosis

Pneumonic tularemia is caused by inhalation of Francisella tularensis, one of the most infectious microbes known. We wanted to study the kinetics of the initial and early interactions between bacterium and host cells in the lung. To do this, we examined the infection of A549 airway epithelial cells with the live vaccine strain (LVS) of F. tularensis. A549 cells were infected and analyzed for global transcriptional response at multiple time points up to 16 h following infection. At 15 min and 2 h, a strong transcriptional response was observed including cytoskeletal rearrangement, intracellular transport, and interferon signaling. However, at later time points (6 and 16 h), very little differential gene expression was observed, indicating a general suppression of the host response consistent with other reported cell lines and murine tissues. Genes for macropinocytosis and actin/cytoskeleton rearrangement were highly up-regulated and common to the 15 min and 2 h time points, suggesting the use of this method for bacterial entry into cells. We demonstrate macropinocytosis through the uptake of FITC-dextran and amiloride inhibition of Francisella LVS uptake. Our results suggest that macropinocytosis is a potential mechanism of intracellular entry by LVS and that the host cell response is suppressed during the first 2-6 h of infection. These results suggest that the attenuated Francisella LVS induces significant host cell signaling at very early time points after the bacteria's interaction with the cell.

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.

Francisella-arthropod vector interaction and its role in patho-adaptation to infect mammals

Francisella tularensis is a Gram-negative, intracellular, zoonotic bacterium, and is the causative agent of tularemia with a broad host range. Arthropods such as ticks, mosquitoes, and flies maintain F. tularensis in nature by transmitting the bacteria among small mammals. While the tick is largely believed to be a biological vector of F. tularensis, transmission by mosquitoes and flies is largely believed to be mechanical on the mouthpart through interrupted feedings. However, the mechanism of infection of the vectors by F. tularensis is not well understood. Since F. tularensis has not been localized in the salivary gland of the primary human biting ticks, it is thought that bacterial transmission by ticks is through mechanical inoculation of tick feces containing F. tularensis into the skin wound. Drosophila melanogaster is an established good arthropod model for arthropod vectors of tularemia, where F. tularensis infects hemocytes, and is found in hemolymph, as seen in ticks. In addition, phagosome biogenesis and robust intracellular proliferation of F. tularensis in arthropod-derived cells are similar to that in mammalian macrophages. Furthermore, bacterial factors required for infectivity of mammals are often required for infectivity of the fly by F. tularensis. Several host factors that contribute to F. tularensis intracellular pathogenesis in D. melanogaster have been identified, and F. tularensis targets some of the evolutionarily conserved eukaryotic processes to enable intracellular survival and proliferation in evolutionarily distant hosts.