Francisella tularensis LVS initially activates but subsequently down-regulates intracellular signaling and cytokine secretion in mouse monocytic and human peripheral blood mononuclear cells

Francisella tularensis disrupts TLR2-MYD88-p38 signaling early during infection to delay apoptosis of macrophages and promote virulence in the host

Francisella tularensis is a zoonotic pathogen and the causative agent of tularemia. F. tularensis replicates to high levels within the cytosol of macrophages and other host cells while subverting the host response to infection. Critical to the success of F. tularensis is its ability to delay macrophage apoptosis to maintain its intracellular replicative niche. However, the host-signaling pathway(s) modulated by F. tularensis to delay apoptosis are poorly characterized. The outer membrane channel protein TolC is required for F. tularensis virulence and its ability to suppress apoptosis and cytokine expression during infection of macrophages. We took advantage of the F. tularensis ∆tolC mutant phenotype to identify host pathways that are important for activating macrophage apoptosis and that are disrupted by the bacteria. Comparison of macrophages infected with wild-type or ∆tolC F. tularensis revealed that the bacteria interfere with TLR2-MYD88-p38 signaling at early times post infection to delay apoptosis, dampen innate host responses, and preserve the intracellular replicative niche. Experiments using the mouse pneumonic tularemia model confirmed the in vivo relevance of these findings, revealing contributions of TLR2 and MYD88 signaling to the protective host response to F. tularensis, which is modulated by the bacteria to promote virulence. IMPORTANCE Francisella tularensis is a Gram-negative intracellular bacterial pathogen and the causative agent of the zoonotic disease tularemia. F. tularensis, like other intracellular pathogens, modulates host-programmed cell death pathways to ensure its replication and survival. We previously identified the outer membrane channel protein TolC as required for the ability of F. tularensis to delay host cell death. However, the mechanism by which F. tularensis delays cell death pathways during intracellular replication is unclear despite being critical to pathogenesis. In the present study, we address this gap in knowledge by taking advantage of ∆tolC mutants of F. tularensis to uncover signaling pathways governing host apoptotic responses to F. tularensis and which are modulated by the bacteria during infection to promote virulence. These findings reveal mechanisms by which intracellular pathogens subvert host responses and enhance our understanding of the pathogenesis of tularemia.

Francisella tularensis Glyceraldehyde-3-Phosphate Dehydrogenase Is Relocalized during Intracellular Infection and Reveals Effect on Cytokine Gene Expression and Signaling

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is known for its multifunctionality in several pathogenic bacteria. Our previously reported data suggest that the GAPDH homologue of Francisella tularensis, GapA, might also be involved in other processes beyond metabolism. In the present study, we explored GapA's potential implication in pathogenic processes at the host cell level. Using immunoelectron microscopy, we demonstrated the localization of this bacterial protein inside infected macrophages and its peripheral distribution in bacterial cells increasing with infection time. A quantitative proteomic approach based on stable isotope labeling of amino acids in cell culture (SILAC) combined with pull-down assay enabled the identification of several of GapA's potential interacting partners within the host cell proteome. Two of these partners were further confirmed by alternative methods. We also investigated the impact of gapA deletion on the transcription of selected cytokine genes and the activation of the main signaling pathways. Our results show that ∆gapA-induced transcription of genes encoding several cytokines whose expressions were not affected in cells infected with a fully virulent wild-type strain. That might be caused, at least in part, by the detected differences in ERK/MAPK signaling activation. The experimental observations together demonstrate that the F. tularensis GAPDH homologue is directly implicated in multiple host cellular processes and, thereby, that it participates in several molecular mechanisms of pathogenesis.

Macrophages Demonstrate Guanylate-Binding Protein-Dependent and Bacterial Strain-Dependent Responses to Francisella tularensis

Francisella tularensis is a facultative intracellular bacterium and the etiological agent of tularemia, a zoonotic disease. Infection of monocytic cells by F. tularensis can be controlled after activation with IFN-γ; however, the molecular mechanisms whereby the control is executed are incompletely understood. Recently, a key role has been attributed to the Guanylate-binding proteins (GBPs), interferon-inducible proteins involved in the cell-specific immunity against various intracellular pathogens. Here, we assessed the responses of bone marrow-derived murine macrophages (BMDM) and GBP-deficient BMDM to F. tularensis strains of variable virulence; the highly virulent SCHU S4 strain, the human live vaccine strain (LVS), or the widely used surrogate for F. tularensis, the low virulent F. novicida. Each of the strains multiplied rapidly in BMDM, but after addition of IFN-γ, significant GBP-dependent control of infection was observed for the LVS and F. novicida strains, whereas there was no control of the SCHU S4 infection. However, no differences in GBP transcription or translation were observed in the infected cell cultures. During co-infection with F. novicida and SCHU S4, significant control of both strains was observed. Patterns of 18 cytokines were very distinct between infected cell cultures and high levels were observed for almost all cytokines in F. novicida-infected cultures and very low levels in SCHU S4-infected cultures, whereas levels in co-infected cultures for a majority of cytokines showed intermediate levels, or levels similar to those of F. novicida-infected cultures. We conclude that the control of BMDM infection with F. tularensis LVS or F. novicida is GBP-dependent, whereas SCHU S4 was only controlled during co-infection. Since expression of GBP was similar regardless of infecting agent, the findings imply that SCHU S4 has an inherent ability to evade the GBP-dependent anti-bacterial mechanisms.

Guanylate-Binding Proteins Are Critical for Effective Control of Francisella tularensis Strains in a Mouse Co-Culture System of Adaptive Immunity

Francisella tularensis is a Select Agent that causes the severe disease tularemia in humans and many animal species. The bacterium demonstrates rapid intracellular replication, however, macrophages can control its replication if primed and activation with IFN-γ is known to be essential, although alone not sufficient, to mediate such control. To further investigate the mechanisms that control intracellular F. tularensis replication, an in vitro co-culture system was utilized containing splenocytes obtained from naïve or immunized C57BL/6 mice as effectors and infected bone marrow-derived wild-type or chromosome-3-deficient guanylate-binding protein (GBP)-deficient macrophages. Cells were infected either with the F. tularensis live vaccine strain (LVS), the highly virulent SCHU S4 strain, or the surrogate for F. tularensis, F. novicida. Regardless of strain, significant control of the bacterial replication was observed in co-cultures with wild-type macrophages and immune splenocytes, but not in cultures with immune splenocytes and GBP^chr3-deficient macrophages. Supernatants demonstrated very distinct, infectious agent-dependent patterns of 23 cytokines, whereas the cytokine patterns were only marginally affected by the presence or absence of GBPs. Levels of a majority of cytokines were inversely correlated to the degree of control of the SCHU S4 and LVS infections, but this was not the case for the F. novicida infection. Collectively, the co-culture assay based on immune mouse-derived splenocytes identified a dominant role of GBPs for the control of intracellular replication of various F. tularensis strains, regardless of their virulence, whereas the cytokine patterns markedly were dependent on the infectious agents, but less so on GBPs.

Outbreak of Tularemia in a Group of Hunters in Germany in 2018-Kinetics of Antibody and Cytokine Responses

In November 2018, an outbreak of tularemia occurred among hare hunters in Bavaria, Germany. At least one infected hare was confirmed as the source of infection. A number of hunting dogs showed elevated antibody titers to Francisella tularensis, but the absence of titer increases in subsequent samples did not point to acute infections in dogs. Altogether, 12 persons associated with this hare hunt could be diagnosed with acute tularemia by detection of specific antibodies. In nine patients, the antibody and cytokine responses could be monitored over time. Eight out of these nine patients had developed detectable antibodies three weeks after exposure; in one individual the antibody response was delayed. All patients showed an increase in various cytokines and chemokines with a peak for most mediators in the first week after exposure. Cytokine levels showed individual variations, with high and low responders. The kinetics of seroconversion has implications on serological diagnoses of tularemia.

Transcriptomic and Metabolic Responses to a Live-Attenuated Francisella tularensis Vaccine

The immune response to live-attenuated Francisella tularensis vaccine and its host evasion mechanisms are incompletely understood. Using RNA-Seq and LC–MS on samples collected pre-vaccination and at days 1, 2, 7, and 14 post-vaccination, we identified differentially expressed genes in PBMCs, metabolites in serum, enriched pathways, and metabolites that correlated with T cell and B cell responses, or gene expression modules. While an early activation of interferon α/β signaling was observed, several innate immune signaling pathways including TLR, TNF, NF-κB, and NOD-like receptor signaling and key inflammatory cytokines such as Il-1α, Il-1β, and TNF typically activated following infection were suppressed. The NF-κB pathway was the most impacted and the likely route of attack. Plasma cells, immunoglobulin, and B cell signatures were evident by day 7. MHC I antigen presentation was more actively up-regulated first followed by MHC II which coincided with the emergence of humoral immune signatures. Metabolomics analysis showed that glycolysis and TCA cycle-related metabolites were perturbed including a decline in pyruvate. Correlation networks that provide hypotheses on the interplay between changes in innate immune, T cell, and B cell gene expression signatures and metabolites are provided. Results demonstrate the utility of transcriptomics and metabolomics for better understanding molecular mechanisms of vaccine response and potential host–pathogen interactions.

Autotransporter-Mediated Display of Complement Receptor Ligands by Gram-Negative Bacteria Increases Antibody Responses and Limits Disease Severity

The targeting of immunogens/vaccines to specific immune cells is a promising approach for amplifying immune responses in the absence of exogenous adjuvants. However, the targeting approaches reported thus far require novel, labor-intensive reagents for each vaccine and have primarily been shown as proof-of-concept with isolated proteins and/or inactivated bacteria. We have engineered a plasmid-based, complement receptor-targeting platform that is readily applicable to live forms of multiple gram-negative bacteria, including, but not limited to, Escherichia coli, Klebsiella pneumoniae, and Francisella tularensis. Using F. tularensis as a model, we find that targeted bacteria show increased binding and uptake by macrophages, which coincides with increased p38 and p65 phosphorylation. Mice vaccinated with targeted bacteria produce higher titers of specific antibody that recognizes a greater diversity of bacterial antigens. Following challenge with homologous or heterologous isolates, these mice exhibited less weight loss and/or accelerated weight recovery as compared to counterparts vaccinated with non-targeted immunogens. Collectively, these findings provide proof-of-concept for plasmid-based, complement receptor-targeting of live gram-negative bacteria.

A predicted Francisella tularensis DXD-motif glycosyltransferase blocks immune activation

Pathogens enhance their survival during infections by manipulating host defenses. Francisella tularensis evades innate immune responses, which we have found to be dependent on an understudied gene ybeX (FTL_0883/FTT_0615c). To understand the function of YbeX, we sought protein interactors in F. tularensis subsp. holarctica live vaccine strain (LVS). An unstudied Francisella protein co-immunoprecipitated with recombinant YbeX, which is a predicted glycosyltransferase with a DXD-motif. There are up to four genomic copies of this gene with identical sequence in strains of F. tularensis pathogenic to humans, despite ongoing genome decay. Disruption mutations were generated by intron insertion into all three copies of this glycosyltransferase domain containing gene in LVS, gdcA1-3. The resulting strains stimulated more cytokines from macrophages in vitro than wild-type LVS and were attenuated in two in vivo infection models. GdcA was released from LVS during culture and was sufficient to block NF-κB activation when expressed in eukaryotic cells. When co-expressed in zebrafish, GdcA and YbeX were synergistically lethal to embryo development. Glycosyltransferases with DXD-motifs are found in a variety of pathogens including NleB, an Escherichia coli type-III secretion system effector that inhibits NF-κB by antagonizing death receptor signaling. To our knowledge, GdcA is the first DXD-motif glycosyltransferase that inhibits NF-κB in immune cells. Together, these findings suggest DXD-motif glycosyltransferases may be a conserved virulence mechanism used by pathogenic bacteria to remodel host defenses.

Innate Immune Recognition: An Issue More Complex Than Expected

Primary interaction of an intracellular bacterium with its host cell is initiated by activation of multiple signaling pathways in response to bacterium recognition itself or as cellular responses to stress induced by the bacterium. The leading molecules in these processes are cell surface membrane receptors as well as cytosolic pattern recognition receptors recognizing pathogen-associated molecular patterns or damage-associated molecular patterns induced by the invading bacterium. In this review, we demonstrate possible sequences of events leading to recognition of Francisella tularensis, present findings on known mechanisms for manipulating cell responses to protect Francisella from being killed, and discuss newly published data from the perspective of early stages of host-pathogen interaction.

Complement Receptor 3-Mediated Inhibition of Inflammasome Priming by Ras GTPase-Activating Protein During Francisella tularensis Phagocytosis by Human Mononuclear Phagocytes

Francisella tularensis is a remarkably infectious facultative intracellular bacterium of macrophages that causes tularemia. Early evasion of host immune responses contributes to the success of F. tularensis as a pathogen. F. tularensis entry into human monocytes and macrophages is mediated by the major phagocytic receptor, complement receptor 3 (CR3, CD11b/CD18). We recently determined that despite a significant increase in macrophage uptake following C3 opsonization of the virulent Type A F. tularensis spp. tularensis Schu S4, this phagocytic pathway results in limited pro-inflammatory cytokine production. Notably, MAP kinase/ERK activation is suppressed immediately during C3-opsonized Schu S4-CR3 phagocytosis. A mathematical model of CR3-TLR2 crosstalk predicted early involvement of Ras GTPase-activating protein (RasGAP) in immune suppression by CR3. Here, we link CR3-mediated uptake of opsonized Schu S4 by human monocytes and macrophages with inhibition of early signal 1 inflammasome activation, evidenced by limited caspase-1 cleavage and IL-18 release. This inhibition is due to increased RasGAP activity, leading to a reduction in the Ras-ERK signaling cascade upstream of the early inflammasome activation event. Thus, our data uncover a novel signaling pathway mediated by CR3 following engagement of opsonized virulent F. tularensis to limit inflammasome activation in human phagocytic cells, thereby contributing to evasion of the host innate immune system.

Francisella tularensis Confronts the Complement System

Francisella tularensis has developed a number of effective evasion strategies to counteract host immune defenses, not the least of which is its ability to interact with the complement system to its own advantage. Following exposure of the bacterium to fresh human serum, complement is activated and C3b and iC3b can be found covalently attached to the bacterial surface. However, the lipopolysaccharide and capsule of the F. tularensis cell wall prevent complement-mediated lysis and endow the bacterium with serum resistance. Opsonization of F. tularensis with C3 greatly increases its uptake by human neutrophils, dendritic cells and macrophages. Uptake occurs by an unusual looping morphology in human macrophages. Complement receptor 3 is thought to play an important role in opsonophagocytosis by human macrophages, and signaling through this receptor can antagonize Toll-like receptor 2-initiated macrophage activation. Complement C3 also determines the survival of infected human macrophages and perhaps other cell types. C3-opsonization of F. tularensis subsp. tularensis strain SCHU S4 results in greatly increased death of infected human macrophages, which requires more than complement receptor engagement and is independent of the intracellular replication by the pathogen. Given its entry into the cytosol of host cells, F. tularensis has the potential for a number of other complement-mediated interactions. Studies on the uptake C3-opsonized adenovirus have suggested the existence of a C3 sensing system that initiates cellular responses to cytosolic C3b present on invading microbes. Here we propose that C3 peptides enter the cytosol of human macrophages following phagosome escape of F. tularensis and are recognized as intruding molecular patterns that signal host cell death. With the discovery of new roles for intracellular C3, a better understanding of tularemia pathogenesis is likely to emerge.

Checks and Balances between Autophagy and Inflammasomes during Infection

Autophagy and inflammasome complex assembly are physiological processes that control homeostasis, inflammation, and immunity. Autophagy is a ubiquitous pathway that degrades cytosolic macromolecules or organelles, as well as intracellular pathogens. Inflammasomes are multi-protein complexes that assemble in the cytosol of cells upon detection of pathogen- or danger-associated molecular patterns. A critical outcome of inflammasome assembly is the activation of the cysteine protease caspase-1, which activates the pro-inflammatory cytokine precursors pro-IL-1β and pro-IL-18. Studies on chronic inflammatory diseases, heart diseases, Alzheimer's disease, and multiple sclerosis revealed that autophagy and inflammasomes intersect and regulate each other. In the context of infectious diseases, however, less is known about the interplay between autophagy and inflammasome assembly, although it is becoming evident that pathogens have evolved multiple strategies to inhibit and/or subvert these pathways and to take advantage of their intricate crosstalk. An improved appreciation of these pathways and their subversion by diverse pathogens is expected to help in the design of anti-infective therapeutic interventions.

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.

Subversion of innate immune responses by Francisella involves the disruption of TRAF3 and TRAF6 signalling complexes

The success of pathogens depends on their ability to circumvent immune defences. Francisella tularensis is one of the most infectious bacteria known. The remarkable virulence of Francisella is believed to be due to its capacity to evade or subvert the immune system, but how remains obscure. Here, we show that Francisella triggers but concomitantly inhibits the Toll-like receptor, RIG-I-like receptor, and cytoplasmic DNA pathways. Francisella subverts these pathways at least in part by inhibiting K63-linked polyubiquitination and assembly of TRAF6 and TRAF3 complexes that control the transcriptional responses of pattern recognition receptors. We show that this mode of inhibition requires a functional type VI secretion system and/or the presence of live bacteria in the cytoplasm. The ability of Francisella to enter the cytosol while simultaneously inhibiting multiple pattern recognition receptor pathways may account for the notable capacity of this bacterium to invade and proliferate in the host without evoking a self-limiting innate immune response.

Expanding Francisella models: Pairing up the soil amoeba Dictyostelium with aquatic Francisella

The bacterial genus Francisella comprises highly pathogenic species that infect mammals, arthropods, fish and protists. Understanding virulence and host defense mechanisms of Francisella infection relies on multiple animal and cellular model systems. In this review, we want to summarize the most commonly used Francisella host model platforms and highlight novel, alternative model systems using aquatic Francisella species. Established mouse and macrophage models contributed extensively to our understanding of Francisella infection. However, murine and human cells display significant differences in their response to Francisella infection. The zebrafish and the amoeba Dictyostelium are well-established model systems for host-pathogen interactions and open up opportunities to investigate bacterial virulence and host defense. Comparisons between model systems using human and fish pathogenic Francisella species revealed shared virulence strategies and pathology between them. Hence, zebrafish and Dictyostelium might complement current model systems to find new vaccine candidates and contribute to our understanding of Francisella infection.

Isolation of F. novicida-Containing Phagosome from Infected Human Monocyte Derived Macrophages

Francisella is a gram-negative bacterial pathogen, which causes tularemia in humans and animals. A crucial step of Francisella infection is its invasion of macrophage cells. Biogenesis of the Francisella-containing phagosome (FCP) is arrested for ~15 min at the endosomal stage, followed by gradual bacterial escape into the cytosol, where the microbe proliferates. The crucial step in pathogenesis of tularemia is short and transient presence of the bacterium within phagosome. Isolation of FCPs for further studies has been challenging due to the short period of time of bacterial residence in it and the characteristics of the FCP. Here, we will for the first time present the method for isolation of the FCPs from infected human monocytes-derived macrophages (hMDMs). For elimination of lysosomal compartment these organelles were pre-loaded with dextran coated colloidal iron particles prior infection and eliminated by magnetic separation of the post-nuclear supernatant (PNS). We encountered the challenge that mitochondria has similar density to the FCP. To separate the FCP in the PNS from mitochondria, we utilized iodophenylnitrophenyltetrazolium, which is converted by the mitochondrial succinate dehydrogenase into formazan, leading to increased density of the mitochondria and allowing separation by the discontinuous sucrose density gradient ultracentrifugation. The purity of the FCP preparation and its acquisition of early endosomal markers was confirmed by Western blots, confocal and transmission electron microscopy. Our strategy to isolate highly pure FCPs from macrophages should facilitate studies on the FCP and its biogenesis.

A mutagenesis-based approach identifies amino acids in the N-terminal part of Francisella tularensis IglE that critically control Type VI system-mediated secretion

The Gram-negative bacterium Francisella tularensis is the etiological agent of the zoonotic disease tularemia. Its life cycle is characterized by an ability to survive within phagocytic cells through phagosomal escape and replication in the cytosol, ultimately causing inflammasome activation and host cell death. Required for these processes is the Francisella Pathogenicity Island (FPI), which encodes a Type VI secretion system (T6SS) that is active during intracellular infection. In this study, we analyzed the role of the FPI-component IglE, a lipoprotein which we previously have shown to be secreted in a T6SS-dependent manner. We demonstrate that in F. tularensis LVS, IglE is an outer membrane protein. Upon infection of J774 cells, an ΔiglE mutant failed to escape from phagosomes, and subsequently, to multiply and cause cytopathogenicity. Moreover, ΔiglE was unable to activate the inflammasome, to inhibit LPS-stimulated secretion of TNF-α, and showed marked attenuation in the mouse model. In F. novicida, IglE was required for in vitro secretion of IglC and VgrG. A mutagenesis-based approach involving frameshift mutations and alanine substitution mutations within the first ∼ 38 residues of IglE revealed that drastic changes in the sequence of the extreme N-terminus (residues 2-6) were well tolerated and, intriguingly, caused hyper-secretion of IglE during intracellular infection, while even subtle mutations further downstream lead to impaired protein function. Taken together, this study highlights the importance of IglE in F. tularensis pathogenicity, and the contribution of the N-terminus for all of the above mentioned processes.

Mitogen-activated protein kinases (MAPKs) are modulated during Francisella tularensis infection, but inhibition of extracellular-signal-regulated kinases (ERKs) is of limited therapeutic benefit

Francisella tularensis is a Gram-negative intracellular bacterium that causes the disease tularemia. The disease can be fatal if left untreated and there is currently no licenced vaccine available; the identification of new therapeutic targets is therefore required. Toll-like receptors represent an interesting target for therapeutic modulation due to their essential role in generating immune responses. In this study, we analysed the in vitro expression of the key mitogen-activated protein kinases (MAPKs) p38, JNK and ERK in murine alveolar macrophages during infection with F. tularensis. The phosphorylation profile of ERK highlighted its potential as a target for therapeutic modulation and subsequently the effect of ERK manipulation was measured in a lethal intranasal F. tularensis in vivo model of infection. The selective ERK1/2 inhibitor PD0325901 was administered orally to mice either pre- or post-challenge with F. tularensis strain LVS. Both treatment regimens selectively reduced ERK expression, but only the pre-exposure treatment produced decreased bacterial burden in the spleen and liver, which correlated with a significant reduction in the pro-inflammatory cytokines IFN-γ, MCP-1, IL-6, and TNF-α. However, no overall improvements in survival were observed for treated animals in this study. ERK may represent a useful therapeutic target where selective dampening of the immune response (to control the damaging pathology seen during infection) is combined with antibiotic treatment required to eradicate bacterial infection. This combination treatment strategy has been shown to be effective in other models of tularemia.

Francisella tularensis - Immune Cell Activator, Suppressor, or Stealthy Evader: The Evolving View from the Petri Dish

One of the hallmarks of pulmonary tularemia, which results from inhalation of Francisella tularensis - a significant bioterrorism concern, is the lack of an acute T_H1-biased inflammatory response in the early phase of disease (days 1–3) despite significant bacterial loads. In an effort to understand this apparent hypo-responsiveness, many laboratories have utilized in vitro cell-based models as tools to probe the nature and consequences of host cell interactions with F. tularensis. The first uses of this model suggested that mammalian host cells recognize this bacterium principally through TLR2 to evoke a robust, classical T_H1-biased cytokine response including TNF, IL-6, IL-1β, and IFN-γ. Others used this model in concert with a variety of non-genetic perturbations of the bacterial-host cell interaction and suggested that F. tularensis actively-suppressed the cellular response. Consistent with this notion, others engaged this model to assess isogenic mutants and, in many cases, found the mutant bacteria to be more pro-inflammatory than their WT counter-parts. Frequently, these observations were interpreted as evidence for the immunosuppressive function of the gene of interest. However, recently appreciated roles of the health of the bacterium and the impact of host factors have refined this model to suggest a “stealthy” mode of bacterial-host cell interaction (rather than one involving active suppression) consistent with the observations during early phase disease.

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.

Identifying Francisella tularensis genes required for growth in host cells

Francisella tularensis is a highly virulent Gram-negative intracellular pathogen capable of infecting a vast diversity of hosts, ranging from amoebae to humans. A hallmark of F. tularensis virulence is its ability to quickly grow to high densities within a diverse set of host cells, including, but not limited to, macrophages and epithelial cells. We developed a luminescence reporter system to facilitate a large-scale transposon mutagenesis screen to identify genes required for growth in macrophage and epithelial cell lines. We screened 7,454 individual mutants, 269 of which exhibited reduced intracellular growth. Transposon insertions in the 269 growth-defective strains mapped to 68 different genes. FTT_0924, a gene of unknown function but highly conserved among Francisella species, was identified in this screen to be defective for intracellular growth within both macrophage and epithelial cell lines. FTT_0924 was required for full Schu S4 virulence in a murine pulmonary infection model. The ΔFTT_0924 mutant bacterial membrane is permeable when replicating in hypotonic solution and within macrophages, resulting in strongly reduced viability. The permeability and reduced viability were rescued when the mutant was grown in a hypertonic solution, indicating that FTT_0924 is required for resisting osmotic stress. The ΔFTT_0924 mutant was also significantly more sensitive to β-lactam antibiotics than Schu S4. Taken together, the data strongly suggest that FTT_0924 is required for maintaining peptidoglycan integrity and virulence.

Inflammasome priming is similar for francisella species that differentially induce inflammasome activation

Inflammasome activation is a two-step process where step one, priming, prepares the inflammasome for its subsequent activation, by step two. Classically step one can be induced by LPS priming followed by step two, high dose ATP. Furthermore, when IL-18 processing is used as the inflammasome readout, priming occurs before new protein synthesis. In this context, how intracellular pathogens such as Francisella activate the inflammasome is incompletely understood, particularly regarding the relative importance of priming versus activation steps. To better understand these events we compared Francisella strains that differ in virulence and ability to induce inflammasome activation for their relative effects on step one vs. step two. When using the rapid priming model, i.e., 30 min priming by live or heat killed Francisella strains (step 1), followed by ATP (step 2), we found no difference in IL-18 release, p20 caspase-1 release and ASC oligomerization between Francisella strains (F. novicida, F. holarctica -LVS and F. tularensis Schu S4). This priming is fast, independent of bacteria viability, internalization and phagosome escape, but requires TLR2-mediated ERK phosphorylation. In contrast to their efficient priming capacity, Francisella strains LVS and Schu S4 were impaired in inflammasome triggering compared to F. novicida. Thus, observed differences in inflammasome activation by F. novicida, LVS and Schu S4 depend not on differences in priming but rather on their propensity to trigger the primed inflammasome.

ModuleBlast: identifying activated sub-networks within and across species

Identifying conserved and divergent response patterns in gene networks is becoming increasingly important. A common approach is integrating expression information with gene association networks in order to find groups of connected genes that are activated or repressed. In many cases, researchers are also interested in comparisons across species (or conditions). Finding an active sub-network is a hard problem and applying it across species requires further considerations (e.g. orthology information, expression data and networks from different sources). To address these challenges we devised ModuleBlast, which uses both expression and network topology to search for highly relevant sub-networks. We have applied ModuleBlast to expression and interaction data from mouse, macaque and human to study immune response and aging. The immune response analysis identified several relevant modules, consistent with recent findings on apoptosis and NFκB activation following infection. Temporal analysis of these data revealed cascades of modules that are dynamically activated within and across species. We have experimentally validated some of the novel hypotheses resulting from the analysis of the ModuleBlast results leading to new insights into the mechanisms used by a key mammalian aging protein.

Live attenuated Francisella novicida vaccine protects against Francisella tularensis pulmonary challenge in rats and non-human primates

Francisella tularensis causes the disease tularemia. Human pulmonary exposure to the most virulent form, F. tularensis subsp. tularensis (Ftt), leads to high morbidity and mortality, resulting in this bacterium being classified as a potential biothreat agent. However, a closely-related species, F. novicida, is avirulent in healthy humans. No tularemia vaccine is currently approved for human use. We demonstrate that a single dose vaccine of a live attenuated F. novicida strain (Fn iglD) protects against subsequent pulmonary challenge with Ftt using two different animal models, Fischer 344 rats and cynomolgus macaques (NHP). The Fn iglD vaccine showed protective efficacy in rats, as did a Ftt iglD vaccine, suggesting no disadvantage to utilizing the low human virulent Francisella species to induce protective immunity. Comparison of specific antibody profiles in vaccinated rat and NHP sera by proteome array identified a core set of immunodominant antigens in vaccinated animals. This is the first report of a defined live attenuated vaccine that demonstrates efficacy against pulmonary tularemia in a NHP, and indicates that the low human virulence F. novicida functions as an effective tularemia vaccine platform.

Francisella tularensis is a bacterium that causes the infectious disease tularemia. F. tularensis has been developed as a biothreat agent, because it causes high morbidity and mortality when spread by aerosol. There is currently no approved vaccine for human use, making mankind vulnerable to the illicit use of this organism. F. tularensis contains a cluster of genes in the Francisella Pathogenicity Island (FPI) that are required for replication inside host macrophages and virulence. In the current study we created a live vaccine strain by inactivating an FPI gene, iglD, in a closely-related species that does not cause disease in humans, F. novicida (Fn iglD). We demonstrate that vaccination with Fn iglD protects against exposure to airborne F. tularensis. Fn iglD vaccination induces antibody and cellular immune responses and protects two different animals, rats and non-human primates, against lethal pulmonary tularemia challenges. These two animal models reflect human sensitivity to F. tularensis. Our results suggest that a vaccine made from the low virulence F. novicida will protect humans against aerosol exposure to this dangerous pathogen.

Differential expression of microRNAs in Francisella tularensis-infected human macrophages: miR-155-dependent downregulation of MyD88 inhibits the inflammatory response

Francisella tularensis is a Gram-negative, facultative intracellular pathogen that replicates in the cytosol of macrophages and is the causative agent of the potentially fatal disease tularemia. A characteristic feature of F. tularensis is its limited proinflammatory capacity, but the mechanisms that underlie the diminished host response to this organism are only partially defined. Recently, microRNAs have emerged as important regulators of immunity and inflammation. In the present study we investigated the microRNA response of primary human monocyte-derived macrophages (MDMs) to F. tularensis and identified 10 microRNAs that were significantly differentially expressed after infection with the live vaccine strain (LVS), as judged by Taqman Low Density Array profiling. Among the microRNAs identified, miR-155 is of particular interest as its established direct targets include components of the Toll-like receptor (TLR) pathway, which is essential for innate defense and proinflammatory cytokine production. Additional studies demonstrated that miR-155 acted by translational repression to downregulate the TLR adapter protein MyD88 and the inositol 5′-phosphatase SHIP-1 in MDMs infected with F. tularensis LVS or the fully virulent strain Schu S4. Kinetic analyses indicated that miR-155 increased progressively 3-18 hours after infection with LVS or Schu S4, and target proteins disappeared after 12–18 hours. Dynamic modulation of MyD88 and SHIP-1 was confirmed using specific pre-miRs and anti-miRs to increase and decrease miR-155 levels, respectively. Of note, miR-155 did not contribute to the attenuated cytokine response triggered by F. tularensis phagocytosis. Instead, this microRNA was required for the ability of LVS-infected cells to inhibit endotoxin-stimulated TNFα secretion 18–24 hours after infection. Thus, our data are consistent with the ability of miR-155 to act as a global negative regulator of the inflammatory response in F. tularensis-infected human macrophages.

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.

Virulent Type A Francisella tularensis actively suppresses cytokine responses in human monocytes

BACKGROUND

Human monocyte inflammatory responses differ between virulent and attenuated Francisella infection.

RESULTS

A mixed infection model showed that the virulent F. tularensis Schu S4 can attenuate inflammatory cytokine responses to the less virulent F. novicida in human monocytes.

CONCLUSION

F. tularensis dampens inflammatory response by an active process.

SIGNIFICANCE

This suppression may contribute to enhanced pathogenicity of F. tularensis. Francisella tularensis is a Gram-negative facultative bacterium that can cause the disease tularemia, even upon exposure to low numbers of bacteria. One critical characteristic of Francisella is its ability to dampen or subvert the host immune response. Previous work has shown that monocytes infected with highly virulent F. tularensis subsp. tularensis strain Schu S4 responded with a general pattern of quantitatively reduced pro-inflammatory signaling pathway genes and cytokine production in comparison to those infected with the less virulent related F. novicida. However, it has been unclear whether the virulent Schu S4 was merely evading or actively suppressing monocyte responses. By using mixed infection assays with F. tularensis and F. novicida, we show that F. tularensis actively suppresses monocyte pro-inflammatory responses. Additional experiments show that this suppression occurs in a dose-dependent manner and is dependent upon the viability of F. tularensis. Importantly, F. tularensis was able to suppress pro-inflammatory responses to earlier infections with F. novicida. These results lend support that F. tularensis actively dampens human monocyte responses and this likely contributes to its enhanced pathogenicity.

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.

Janus kinase 3 activity is necessary for phosphorylation of cytosolic phospholipase A2 and prostaglandin E2 synthesis by macrophages infected with Francisella tularensis live vaccine strain

Francisella tularensis, the causative agent of tularemia, modulates the host immune response to gain a survival advantage within the host. One mechanism of immune evasion is the ability of F. tularensis to induce the synthesis of the small lipid mediator prostaglandin E2 (PGE2), which alters the host T cell response making the host more susceptible to Francisella growth. PGE2 is synthesized by a tightly regulated biosynthetic pathway following stimulation. The synthesis of PGE2 begins with the liberation of arachidonic acid (AA) from membrane phospholipids by cytosolic phospholipase A2 (cPLA2). AA is subsequently converted to the unstable intermediate PGH2 by cyclooxygenase-2 (COX-2), and PGH2 undergoes an isomerization reaction to generate PGE2. Our objective was to identify F. tularensis-activated host signaling pathways that regulate the activity of the enzymes in the PGE2-biosynthetic pathway. In this study, we show that cPLA2, p38 mitogen-activated protein kinase (MAPK), and Janus kinase 3 (JAK3) signaling are necessary for F. tularensis-induced PGE2 production. Inhibition of JAK3 activity reduced the phosphorylation of cPLA2 and COX-2 protein levels. In addition, JAK3 regulates cPLA2 phosphorylation independent of transcription. Moreover, p38 MAPK activity is required for F. tularensis-induced COX-2 protein synthesis, but not for the phosphorylation of cPLA2. This research highlights a unique signaling axis in which JAK3 and p38 MAPK regulate the activity of multiple enzymes of the PGE2-biosynthetic pathway in macrophages infected with F. tularensis.

Adherence and uptake of Francisella into host cells

Francisella tularensis is a highly virulent bacterial pathogen that is easily aerosolized and has a low infectious dose. As an intracellular pathogen, entry of Francisella into host cells is critical for its survival and virulence. However, the initial steps of attachment and internalization of Francisella into host cells are not well characterized, and little is known about bacterial factors that promote these processes. This review highlights our current understanding of Francisella attachment and internalization into host cells. In particular, we emphasize the host cell types Francisella has been shown to interact with, as well as specific receptors and signaling processes involved in the internalization process. This review will shed light on gaps in our current understanding and future areas of investigation.

Assessment of antimicrobial peptide LL-37 as a post-exposure therapy to protect against respiratory tularemia in mice

Early activation of the innate immune response is important for protection against infection with Francisella tularensis live vaccine strain (LVS) in mice. The human cathelicidin antimicrobial peptide LL-37 is known to have immunomodulatory properties, and therefore exogenously administered LL-37 may be suitable as an early post-exposure therapy to protect against LVS infection. LL-37 has been evaluated for immunostimulatory activity in uninfected mice and for activity against LVS in macrophage assays and protective efficacy when administered post-challenge in a mouse model of respiratory tularemia. Increased levels of pro-inflammatory cytokine IL-6, chemokines monocyte chemoattractant protein 1 (MCP-1) and CXCL1 with increased neutrophil influx into the lungs were observed in uninfected mice after intranasal administration of LL-37. Following LVS challenge, LL-37 administration resulted in increased IL-6, IL-12 p70, IFNγ and MCP-1 production, a slowing of LVS growth in the lung, and a significant extension of mean time to death compared to control mice. However, protection was transient, with the LL-37 treated mice eventually succumbing to infection. As this short course of nasally delivered LL-37 was moderately effective at overcoming the immunosuppressive effects of LVS infection this suggests that a more sustained treatment regimen may be an effective therapy against this pathogen.

Importance of PdpC, IglC, IglI, and IglG for modulation of a host cell death pathway induced by Francisella tularensis

Modulation of host cell death pathways appears to be a prerequisite for the successful lifestyles of many intracellular pathogens. The facultative intracellular bacterium Francisella tularensis is highly pathogenic, and effective proliferation in the macrophage cytosol leading to host cell death is a requirement for its virulence. To better understand the prerequisites of this cell death, macrophages were infected with the F. tularensis live vaccine strain (LVS), and the effects were compared to those resulting from infections with deletion mutants lacking expression of either of the pdpC, iglC, iglG, or iglI genes, which encode components of the Francisella pathogenicity island (FPI), a type VI secretion system. Within 12 h, a majority of the J774 cells infected with the LVS strain showed production of mitochondrial superoxide and, after 24 h, marked signs of mitochondrial damage, caspase-9 and caspase-3 activation, phosphatidylserine expression, nucleosome formation, and membrane leakage. In contrast, neither of these events occurred after infection with the ΔiglI or ΔiglC mutants, although the former strain replicated. The ΔiglG mutant replicated effectively but induced only marginal cytopathogenic effects after 24 h and intermediate effects after 48 h. In contrast, the ΔpdpC mutant showed no replication but induced marked mitochondrial superoxide production and mitochondrial damage, caspase-3 activation, nucleosome formation, and phosphatidylserine expression, although the effects were delayed compared to those obtained with LVS. The unique phenotypes of the mutants provide insights regarding the roles of individual FPI components for the modulation of the cytopathogenic effects resulting from the F. tularensis infection.

The Francisella tularensis LVS ΔpdpC mutant exhibits a unique phenotype during intracellular infection

Background

A prerequisite for the virulence of the facultative intracellular bacterium Francisella tularensis is effective intramacrophage proliferation, which is preceded by phagosomal escape into the cytosol, and ultimately leads to host cell death. Many components essential for the intracellular life cycle are encoded by a gene cluster, the Francisella pathogenicity island (FPI), constituting a type VI secretion system.

Results

We characterized the FPI mutant ΔpdpC of the live vaccine strain (LVS) of F. tularensis and found that it exhibited lack of intracellular replication, incomplete phagosomal escape, and marked attenuation in the mouse model, however, unlike a phagosomally contained FPI mutant, it triggered secretion of IL-1β, albeit lower than LVS, and markedly induced LDH release.

Conclusions

The phenotype of the ΔpdpC mutant appears to be unique compared to previously described F. tularensis FPI mutants.

Fine tuning inflammation at the front door: macrophage complement receptor 3-mediates phagocytosis and immune suppression for Francisella tularensis

Complement receptor 3 (CR3, CD11b/CD18) is a major macrophage phagocytic receptor. The biochemical pathways through which CR3 regulates immunologic responses have not been fully characterized. Francisella tularensis is a remarkably infectious, facultative intracellular pathogen of macrophages that causes tularemia. Early evasion of the host immune response contributes to the virulence of F. tularensis and CR3 is an important receptor for its phagocytosis. Here we confirm that efficient attachment and uptake of the highly virulent Type A F. tularensis spp. tularensis strain Schu S4 by human monocyte-derived macrophages (hMDMs) requires complement C3 opsonization and CR3. However, despite a>40-fold increase in uptake following C3 opsonization, Schu S4 induces limited pro-inflammatory cytokine production compared with non-opsonized Schu S4 and the low virulent F. novicida. This suggests that engagement of CR3 by opsonized Schu S4 contributes specifically to the immune suppression during and shortly following phagocytosis which we demonstrate by CD11b siRNA knockdown in hMDMs. This immune suppression is concomitant with early inhibition of ERK1/2, p38 MAPK and NF-κB activation. Furthermore, TLR2 siRNA knockdown shows that pro-inflammatory cytokine production and MAPK activation in response to non-opsonized Schu S4 depends on TLR2 signaling providing evidence that CR3-TLR2 crosstalk mediates immune suppression for opsonized Schu S4. Deletion of the CD11b cytoplasmic tail reverses the CR3-mediated decrease in ERK and p38 activation during opsonized Schu-S4 infection. The CR3-mediated signaling pathway involved in this immune suppression includes Lyn kinase and Akt activation, and increased MKP-1, which limits TLR2-mediated pro-inflammatory responses. These data indicate that while the highly virulent F. tularensis uses CR3 for efficient uptake, optimal engagement of this receptor down-regulates TLR2-dependent pro-inflammatory responses by inhibiting MAPK activation through outside-in signaling. CR3-linked immune suppression is an important mechanism involved in the pathogenesis of F. tularensis infection.

IKKβ in myeloid cells controls the host response to lethal and sublethal Francisella tularensis LVS infection

Background

The NF-κB activating kinases, IKKα and IKKβ, are key regulators of inflammation and immunity in response to infection by a variety of pathogens. Both IKKα and IKKβ have been reported to modulate either pro- or anti- inflammatory programs, which may be specific to the infectious organism or the target tissue. Here, we analyzed the requirements for the IKKs in myeloid cells in vivo in response to Francisella tularensis Live Vaccine Strain (Ft. LVS) infection.

Methods and Principal Findings

In contrast to prior reports in which conditional deletion of IKKβ in the myeloid lineage promoted survival and conferred resistance to an in vivo group B streptococcus infection, we show that mice with a comparable conditional deletion (IKKβ cKO) succumb more rapidly to lethal Ft. LVS infection and are unable to control bacterial growth at sublethal doses. Flow cytometry analysis of hepatic non-parenchymal cells from infected mice reveals that IKKβ inhibits M1 classical macrophage activation two days post infection, which has the collateral effect of suppressing IFN-γ⁺ CD8⁺ T cells. Despite this early enhanced inflammation, IKKβ cKO mice are unable to control infection; and this coincides with a shift toward M2a polarized macrophages. In comparison, we find that myeloid IKKα is dispensable for survival and bacterial control. However, both IKKα and IKKβ have effects on hepatic granuloma development. IKKα cKO mice develop fewer, but well-contained granulomas that accumulate excess necrotic cells after 9 days of infection; while IKKβ cKO mice develop numerous micro-granulomas that are less well contained.

Conclusions

Taken together our findings reveal that unlike IKKα, IKKβ has multiple, contrasting roles in this bacterial infection model by acting in an anti-inflammatory capacity at early times towards sublethal Ft. LVS infection; but in spite of this, macrophage IKKβ is also a critical effector for host survival and efficient pathogen clearance.

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.

Identification of a novel Francisella tularensis factor required for intramacrophage survival and subversion of innate immune response

Francisella tularensis, the causative agent of tularemia, is one of the deadliest agents of biological warfare and bioterrorism. Extremely high virulence of this bacterium is associated with its ability to dampen or subvert host innate immune response. The objectives of this study were to identify factors and understand the mechanisms of host innate immune evasion by F. tularensis. We identified and explored the pathogenic role of a mutant interrupted at gene locus FTL_0325, which encodes an OmpA-like protein. Our results establish a pathogenic role of FTL_0325 and its ortholog FTT0831c in the virulent F. tularensis SchuS4 strain in intramacrophage survival and suppression of proinflammatory cytokine responses. This study provides mechanistic evidence that the suppressive effects on innate immune responses are due specifically to these proteins and that FTL_0325 and FTT0831c mediate immune subversion by interfering with NF-κB signaling. Furthermore, FTT0831c inhibits NF-κB activity primarily by preventing the nuclear translocation of p65 subunit. Collectively, this study reports a novel F. tularensis factor that is required for innate immune subversion caused by this deadly bacterium.

DotU and VgrG, core components of type VI secretion systems, are essential for Francisella LVS pathogenicity

The Gram-negative bacterium Francisella tularensis causes tularemia, a disease which requires bacterial escape from phagosomes of infected macrophages. Once in the cytosol, the bacterium rapidly multiplies, inhibits activation of the inflammasome and ultimately causes death of the host cell. Of importance for these processes is a 33-kb gene cluster, the Francisella pathogenicity island (FPI), which is believed to encode a type VI secretion system (T6SS). In this study, we analyzed the role of the FPI-encoded proteins VgrG and DotU, which are conserved components of type VI secretion (T6S) clusters. We demonstrate that in F. tularensis LVS, VgrG was shown to form multimers, consistent with its suggested role as a trimeric membrane puncturing device in T6SSs, while the inner membrane protein DotU was shown to stabilize PdpB/IcmF, another T6SS core component. Upon infection of J774 cells, both ΔvgrG and ΔdotU mutants did not escape from phagosomes, and subsequently, did not multiply or cause cytopathogenicity. They also showed impaired activation of the inflammasome and marked attenuation in the mouse model. Moreover, all of the DotU-dependent functions investigated here required the presence of three residues that are essentially conserved among all DotU homologues. Thus, in agreement with a core function in T6S clusters, VgrG and DotU play key roles for modulation of the intracellular host response as well as for the virulence of F. tularensis.

A Francisella tularensis live vaccine strain that improves stimulation of antigen-presenting cells does not enhance vaccine efficacy

Vaccination is a proven strategy to mitigate morbidity and mortality of infectious diseases. The methodology of identifying and testing new vaccine candidates could be improved with rational design and in vitro testing prior to animal experimentation. The tularemia vaccine, Francisella tularensis live vaccine strain (LVS), does not elicit complete protection against lethal challenge with a virulent type A Francisella strain. One factor that may contribute to this poor performance is limited stimulation of antigen-presenting cells. In this study, we examined whether the interaction of genetically modified LVS strains with human antigen-presenting cells correlated with effectiveness as tularemia vaccine candidates. Human dendritic cells infected with wild-type LVS secrete low levels of proinflammatory cytokines, fail to upregulate costimulatory molecules, and activate human T cells poorly in vitro. One LVS mutant, strain 13B47, stimulated higher levels of proinflammatory cytokines from dendritic cells and macrophages and increased costimulatory molecule expression on dendritic cells compared to wild type. Additionally, 13B47-infected dendritic cells activated T cells more efficiently than LVS-infected cells. A deletion allele of the same gene in LVS displayed similar in vitro characteristics, but vaccination with this strain did not improve survival after challenge with a virulent Francisella strain. In vivo, this mutant was attenuated for growth and did not stimulate T cell responses in the lung comparable to wild type. Therefore, stimulation of antigen-presenting cells in vitro was improved by genetic modification of LVS, but did not correlate with efficacy against challenge in vivo within this model system.

Francisella tularensis RipA protein topology and identification of functional domains

Francisella tularensis is a Gram-negative coccobacillus and is the etiological agent of the disease tularemia. Expression of the cytoplasmic membrane protein RipA is required for Francisella replication within macrophages and other cell types; however, the function of this protein remains unknown. RipA is conserved among all sequenced Francisella species, and RipA-like proteins are present in a number of individual strains of a wide variety of species scattered throughout the prokaryotic kingdom. Cross-linking studies revealed that RipA forms homoligomers. Using a panel of RipA-green fluorescent protein and RipA-PhoA fusion constructs, we determined that RipA has a unique topology within the cytoplasmic membrane, with the N and C termini in the cytoplasm and periplasm, respectively. RipA has two significant cytoplasmic domains, one composed roughly of amino acids 1 to 50 and the second flanked by the second and third transmembrane domains and comprising amino acids 104 to 152. RipA functional domains were identified by measuring the effects of deletion mutations, amino acid substitution mutations, and spontaneously arising intragenic suppressor mutations on intracellular replication, induction of interleukin-1β (IL-1β) secretion by infected macrophages, and oligomer formation. Results from these experiments demonstrated that each of the cytoplasmic domains and specific amino acids within these domains are required for RipA function.

The biochemical properties of the Francisella pathogenicity island (FPI)-encoded proteins IglA, IglB, IglC, PdpB and DotU suggest roles in type VI secretion

The Francisella pathogenicity island (FPI) encodes proteins thought to compose a type VI secretion system (T6SS) that is required for the intracellular growth of Francisella novicida. In this work we used deletion mutagenesis and genetic complementation to determine that the intracellular growth of F. novicida was dependent on 14 of the 18 genes in the FPI. The products of the iglABCD operon were localized by the biochemical fractionation of F. novicida, and Francisella tularensis LVS. Sucrose gradient separation of water-insoluble material showed that the FPI-encoded proteins IglA, IglB and IglC were found in multiple fractions, especially in a fraction that did not correspond to a known membrane fraction. We interpreted these data to suggest that IglA, IglB and IglC are part of a macromolecular structure. Analysis of published structural data suggested that IglC is an analogue of Hcp, which is thought to form long nano-tubes. Thus the fractionation properties of IglA, IglB and IglC are consistent with the current model of the T6SS apparatus, which supposes that IglA and IglB homologues form an outer tube structure that surrounds an inner tube composed of Hcp (IglC) subunits. Fractionation of F. novicida expressing FLAG-tagged DotU (IcmH homologue) and PdpB (IcmF homologue) showed that these proteins localize to the inner membrane. Deletion of dotU led to the cleavage of PdpB, suggesting an interaction of these two proteins that is consistent with results obtained with other T6SSs. Our results may provide a mechanistic basis for many of the studies that have examined the virulence properties of Francisella mutants in FPI genes, namely that the observed phenotypes of the mutants are the result of the disruption of the FPI-encoded T6SS structure.

Host-adaptation of Francisella tularensis alters the bacterium's surface-carbohydrates to hinder effectors of innate and adaptive immunity

BACKGROUND

The gram-negative bacterium Francisella tularensis survives in arthropods, fresh water amoeba, and mammals with both intracellular and extracellular phases and could reasonably be expected to express distinct phenotypes in these environments. The presence of a capsule on this bacterium has been controversial with some groups finding such a structure while other groups report that no capsule could be identified. Previously we reported in vitro culture conditions for this bacterium which, in contrast to typical methods, yielded a bacterial phenotype that mimics that of the bacterium's mammalian, extracellular phase.

METHODS/FINDINGS

SDS-PAGE and carbohydrate analysis of differentially-cultivated F. tularensis LVS revealed that bacteria displaying the host-adapted phenotype produce both longer polymers of LPS O-antigen (OAg) and additional HMW carbohydrates/glycoproteins that are reduced/absent in non-host-adapted bacteria. Analysis of wildtype and OAg-mutant bacteria indicated that the induced changes in surface carbohydrates involved both OAg and non-OAg species. To assess the impact of these HMW carbohydrates on the access of outer membrane constituents to antibody we used differentially-cultivated bacteria in vitro to immunoprecipitate antibodies directed against outer membrane moieties. We observed that the surface-carbohydrates induced during host-adaptation shield many outer membrane antigens from binding by antibody. Similar assays with normal mouse serum indicate that the induced HMW carbohydrates also impede complement deposition. Using an in vitro macrophage infection assay, we find that the bacterial HMW carbohydrate impedes TLR2-dependent, pro-inflammatory cytokine production by macrophages. Lastly we show that upon host-adaptation, the human-virulent strain, F. tularensis SchuS4 also induces capsule production with the effect of reducing macrophage-activation and accelerating tularemia pathogenesis in mice.

CONCLUSION

F. tularensis undergoes host-adaptation which includes production of multiple capsular materials. These capsules impede recognition of bacterial outer membrane constituents by antibody, complement, and Toll-Like Receptor 2. These changes in the host-pathogen interface have profound implications for pathogenesis and vaccine development.

IFN-β mediates suppression of IL-12p40 in human dendritic cells following infection with virulent Francisella tularensis

Active suppression of inflammation is a strategy used by many viral and bacterial pathogens, including virulent strains of the bacterium Francisella tularensis, to enable colonization and infection in susceptible hosts. In this study, we demonstrated that virulent F. tularensis strain SchuS4 selectively inhibits production of IL-12p40 in primary human cells via induction of IFN-β. In contrast to the attenuated live vaccine strain, infection of human dendritic cells with virulent SchuS4 failed to induce production of many cytokines associated with inflammation (e.g., TNF-α and IL-12p40). Furthermore, SchuS4 actively suppressed secretion of these cytokines. Assessment of changes in the expression of host genes associated with suppression of inflammatory responses revealed that SchuS4, but not live vaccine strain, induced IFN-β following infection of human dendritic cells. Phagocytosis of SchuS4 and endosomal acidification were required for induction of IFN-β. Further, using a defined mutant of SchuS4, we demonstrated that the presence of bacteria in the cytosol was required, but not sufficient, for induction of IFN-β. Surprisingly, unlike previous reports, induction of IFN-β by F. tularensis was not required for activation of the inflammasome, was not associated with exacerbation of inflammatory responses, and did not control SchuS4 replication when added exogenously. Rather, IFN-β selectively suppressed the ability of SchuS4-infected dendritic cells to produce IL-12p40. Together, these data demonstrated a novel mechanism by which virulent bacteria, in contrast to attenuated strains, modulate human cells to cause disease.

Chlamydia trachomatis induces anti-inflammatory effect in human macrophages by attenuation of immune mediators in Jurkat T-cells

The chronic course of Chlamydia trachomatis infection is subtle with no obvious unusual inflammatory change. The reason for this is not clear. The data reported here explain how macrophage usual inflammatory response switches to anti-inflammatory response during C. trachomatis infection of mixed culture of macrophages and Jurkat T-cells. We assessed the establishment of productive infection in individual or mixed cell culture models, determined the status of C. trachomatis in the cells by monitoring HSP-60:MOMP or the proportions of the estimated IFUs that shed HSP-60 or MOMP. Also, the specific time-course expression of IL-12, IL-10 and IFN-γ or IL-12R, IL-10R, and IFN-γ-R during infection of cell models was assessed. Finally, the early events in cytokine elaboration in circumstances of varying intracellular Ca²⁺ levels were determined. There was evidence of productive infection in all individual and mixed cell culture models. The shedding of HSP-60 was highest in THP-1/Jurkat mixed cell culture model. The proportions of IFU that shed HSP-60 was heightened in infected THP-1/Jurkat mixed culture model, while the proportion of IFU that shed MOMP was higher in infected macrophage/Jurkat mixed culture and infected macrophages only. There was profound early elaboration of IL-10, varying significantly from IL-12 and IFN-γ in all infected individual or mixed cell culture models except in the case of Jurkat; where all cytokine elaboration was downregulated. The receptor to IL-10 was upregulated in infected macrophage/Jurkat cells and THP-1/Jurkat cells compared with other models in which IL-12 and IFN-γ receptors were more expressed. There was no observed significant change in cytokine in any model following the impairment of intracellular Ca²⁺ except in the case of macrophage/Jurkat cell model in which IL-12 and IL-10 were upregulated in 1h or 3 h, respectively. The implication of these findings is that C. trachomatis mediates a switch from inflammatory to anti-inflammatory function in macrophages due to downregulation of the regulatory cytokine, IFN-γ in Jurkat cells, culminating in C. trachomatis chronic course.

Development of tolerogenic dendritic cells and regulatory T cells favors exponential bacterial growth and survival during early respiratory tularemia

Tularemia is a vector-borne zoonosis caused by Ft, a Gram-negative, facultative intracellular bacterium. Ft exists in two clinically relevant forms, the European biovar B (holarctica), which produces acute, although mild, self-limiting infections, and the more virulent United States biovar A (tularensis), which is often associated with pneumonic tularemia and more severe disease. In a mouse model of tularemia, respiratory infection with the virulence-attenuated Type B (LVS) or highly virulent Type A (SchuS4) strain engenders peribronchiolar and perivascular inflammation. Paradoxically, despite an intense neutrophilic infiltrate and high bacterial burden, T(h)1-type proinflammatory cytokines (e.g., TNF, IL-1β, IL-6, and IL-12) are absent within the first ∼72 h of pulmonary infection. It has been suggested that the bacterium has the capacity to actively suppress or block NF-κB signaling, thus causing an initial delay in up-regulation of inflammatory mediators. However, our previously published findings and those presented herein contradict this paradigm and instead, strongly support an alternative hypothesis. Rather than blocking NF-κB, Ft actually triggers TLR2-dependent NF-κB signaling, resulting in the development and activation of tDCs and the release of anti-inflammatory cytokines (e.g., IL-10 and TGF-β). In turn, these cytokines stimulate development and proliferation of T(regs) that may restrain T(h)1-type proinflammatory cytokine release early during tularemic infection. The highly regulated and overall anti-inflammatory milieu established in the lung is permissive for unfettered growth and survival of Ft. The capacity of Ft to evoke such a response represents an important immune-evasive strategy.

Administration of a nitric oxide donor inhibits mglA expression by intracellular Francisella tularensis and counteracts phagosomal escape and subversion of TNF-α secretion

Francisella tularensis is a highly virulent intracellular bacterium capable of rapid multiplication in phagocytic cells. Previous studies have revealed that activation of F. tularensis-infected macrophages leads to control of infection and reactive nitrogen and oxygen species make important contributions to the bacterial killing. We investigated the effects of adding S-nitroso-acetyl-penicillamine (SNAP), which generates nitric oxide, or 3-morpholinosydnonimine hydrochloride, which indirectly leads to formation of peroxynitrite, to J774 murine macrophage-like cell cultures infected with F. tularensis LVS. Addition of SNAP led to significantly increased colocalization between LAMP-1 and bacteria, indicating containment of F. tularensis in the phagosome within 2 h, although no killing occurred within 4 h. A specific inhibitory effect on bacterial transcription was observed since the gene encoding the global regulator MglA was inhibited 50-100-fold. F. tularensis-infected J774 cells were incapable of secreting TNF-α in response to Escherichia coli LPS but addition of SNAP almost completely reversed the suppression. Similarly, infection with an MglA mutant did not inhibit LPS-induced TNF-α secretion of J774 cells. Strong staining of nitrotyrosine was observed in SNAP-treated bacteria, and MS identified nitration of two ribosomal 50S proteins, a CBS domain pair protein and bacterioferritin. The results demonstrated that addition of SNAP initially did not affect the viability of intracellular F. tularensis LVS but led to containment of the bacteria in the phagosome. Moreover, the treatment resulted in modification by nitration of several F. tularensis proteins.

IglG and IglI of the Francisella pathogenicity island are important virulence determinants of Francisella tularensis LVS

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