Toll-like receptor 2 is required for control of pulmonary infection with Francisella tularensis

Pathogenicity and virulence of Francisella tularensis

Tularaemia is a zoonotic disease caused by the Gram-negative bacterium, Francisella tularensis. Depending on its entry route into the organism, F. tularensis causes different diseases, ranging from life-threatening pneumonia to less severe ulceroglandular tularaemia. Various strains with different geographical distributions exhibit different levels of virulence. F. tularensis is an intracellular bacterium that replicates primarily in the cytosol of the phagocytes. The main virulence attribute of F. tularensis is the type 6 secretion system (T6SS) and its effectors that promote escape from the phagosome. In addition, F. tularensis has evolved a peculiar envelope that allows it to escape detection by the immune system. In this review, we cover tularaemia, different Francisella strains, and their pathogenicity. We particularly emphasize the intracellular life cycle, associated virulence factors, and metabolic adaptations. Finally, we present how F. tularensis largely escapes immune detection to be one of the most infectious and lethal bacterial pathogens.

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.

ThioredoxinA1 Controls the Oxidative Stress Response of Francisella tularensis Live Vaccine Strain (LVS)

Francisella tularensis is an intracellular, Gram-negative bacterium known for causing a disease known as tularemia in the Northern Hemisphere. F. tularensis is classified as a category A select agent by the CDC based on its possible use as a bioterror agent. F. tularensis overcomes oxidative stress encountered during its growth in the environment or host macrophages by encoding antioxidant enzymes such as superoxide dismutases, catalase, and alkylhydroperoxy reductase. These antioxidant enzymes are regulated by the oxidative stress response regulator, OxyR. In addition to these antioxidant enzymes, F. tularensis also encodes two thioredoxins, TrxA1 (FTL_0611) and TrxA2 (FTL_1224); however, their role in the oxidative stress response of F. tularensis is not known. This study investigated the role of thioredoxins of F. tularensis in the oxidative stress response and intracellular survival. Our results demonstrate that TrxA1 but not TrxA2 plays a major role in the oxidative stress response of F. tularensis. Most importantly, this study elucidates a novel mechanism through which the TrxA1 of F. tularensis controls the oxidative stress response by regulating the expression of the master regulator, oxyR. Further, TrxA1 is required for the intramacrophage survival and growth of Francisella. Overall, this study describes a novel role of thioredoxin, TrxA1, in regulating the oxidative stress response of F. tularensis. IMPORTANCE The role of thioredoxins in the oxidative stress response of F. tularensis is not known. This study demonstrates that of the two thioredoxins, TrxA1 is vital to counter the oxidative stress in F. tularensis live vaccine strain (LVS). Furthermore, this study shows differences in the well-studied thioredoxins of Escherichia coli. First, the expression of TrxA1 of F. tularensis is independent of the oxidative stress response regulator, OxyR. Second and most importantly, TrxA1 regulates the expression of oxyR and, therefore, the OxyR-dependent oxidative stress response of F. tularensis. Overall, this study reports a novel regulatory role of TrxA1 of F. tularensis in the oxidative stress response.

Critical role of IL-25-ILC2-IL-5 axis in the production of anti-Francisella LPS IgM by B1 B cells

B1 cells, a subset of B lymphocytes whose developmental origin, phenotype, and function differ from that of conventional B2 cells, are the main source of “natural” IgM but can also respond to infection by rapidly producing pathogen-specific IgM directed against T-independent antigens. Francisella tularensis (Ft) is a Gram-negative bacterium that causes tularemia. Infection with Ft Live Vaccine Strain activates B1 cells for production of IgM directed against the bacterial LPS in a process incompletely understood. Here we show that immunization with purified Ft LPS elicits production of LPS-specific IgM and IgG₃ by B1 cells independently of TLR2 or MyD88. Immunization, but not infection, generated peritoneum-resident memory B1 cells that differentiated into LPS-specific antibody secreting cells (ASC) upon secondary challenge. IL-5 was rapidly induced by immunization with Ft LPS and was required for production of LPS-specific IgM. Antibody-mediated depletion of ILC2 indicated that these cells were the source of IL-5 and were required for IgM production. IL-25, an alarmin that strongly activates ILC2, was rapidly secreted in response to immunization or infection and its administration to mice significantly increased IgM production and B1 cell differentiation to ASC. Conversely, mice lacking IL-17RB, the IL-25 receptor, showed impaired IL-5 induction, IgM production, and B1 ASC differentiation in response to immunization. Administration of IL-5 to Il17rb^-/- mice rescued these B1 cells-mediated responses. Il17rb^-/- mice were more susceptible to infection with Ft LVS and failed to develop immunity upon secondary challenge suggesting that LPS-specific IgM is one of the protective adaptive immune mechanisms against tularemia. Our results indicated that immunization with Ft LPS triggers production of IL-25 that, through stimulation of IL-5 release by ILC2, promotes B1 cells activation and differentiation into IgM secreting cells. By revealing the existence of an IL-25-ILC2-IL-5 axis our results suggest novel strategies to improve vaccination against T-independent bacterial antigens.

B1 cells are a subset of B lymphocytes that participate in the immune response to infection by producing antibodies of the IgM class. Here we investigate the mechanisms that control B1 cells activation and production of IgM directed against the lipopolysaccharide (LPS) of Francisella tularensis, a Gram-negative bacterium that causes tularemia. Using a mouse model of tularemia, our results revealed that Francisella LPS elicits production of the cytokine IL-25 that in turn activates blood cells called Innate Lymphoid Cells 2 (ILC2). Once activated, ILC2 produce the cytokine IL-5 that is required for activation of B1 cells and production of IgM. Mice unresponsive to IL-25 are more susceptible to F. tularensis infection. By revealing the existence of an IL-25-ILC2-IL-5 axis our results suggest novel strategies to improve vaccination against bacteria.

Deficiency in CCR2 increases susceptibility of mice to infection with an intracellular pathogen, Francisella tularensis LVS, but does not impair development of protective immunity

CCR2 is the major chemokine receptor that regulates appropriate trafficking of inflammatory monocytes, but the role of this chemokine receptor and its ligands during primary and secondary infection with intracellular infections remains incompletely understood. Here we used murine infection with the Live Vaccine Strain (LVS) of Francisella tularensis to evaluate the role of CCR2 during primary and secondary parenteral responses to this prototype intracellular bacterium. We find that mice deficient in CCR2 are highly compromised in their ability to survive intradermal infection with LVS, indicating the importance of this receptor during primary parenteral responses. Interestingly, this defect could not be readily attributed to the activities of the known murine CCR2 ligands MCP-1/CCL2, MCP-3/CCL7, or MCP-5/CCL12. Nonetheless, CCR2 knockout mice vaccinated by infection with low doses of LVS generated optimal T cell responses that controlled the intramacrophage replication of Francisella, and LVS-immune CCR2 knockout mice survived maximal lethal Francisella challenge. Thus, fully protective adaptive immune memory responses to this intracellular bacterium can be readily generated in the absence of CCR2.

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.

Toll-like receptor 2 has a prominent but nonessential role in innate immunity to Staphylococcus aureus pneumonia

Staphylococcus aureus is an important cause of acute bacterial pneumonia. Toll‐like receptor 2 (TLR2) recognizes multiple components of the bacterial cell wall and activates innate immune responses to gram‐positive bacteria. We hypothesized that TLR2 would have an important role in pulmonary host defense against S. aureus. TLR null (TLR2^−/−) mice and wild type (WT) C57BL/6 controls were challenged with aerosolized S. aureus at a range of inocula for kinetic studies of cytokine and antimicrobial peptide expression, lung inflammation, bacterial killing by alveolar macrophages, and bacterial clearance. Survival was measured after intranasal infection. Pulmonary induction of most pro‐inflammatory cytokines was significantly blunted in TLR2^−/− mice 4 and 24 h after infection in comparison with WT controls. Bronchoalveolar concentrations of cathelicidin‐related antimicrobial peptide also were reduced in TLR2^−/− mice. Lung inflammation, measured by enumeration of bronchoalveolar neutrophils and scoring of histological sections, was significantly blunted in TLR2^−/− mice. Phagocytosis of S. aureus by alveolar macrophages in vivo after low‐dose infection was unimpaired, but viability of ingested bacteria was significantly greater in TLR2^−/− mice. Bacterial clearance from the lungs was slightly impaired in TLR2^−/− mice after low‐dose infection only; bacterial elimination from the lungs was slightly accelerated in the TLR2^−/− mice after high‐dose infection. Survival after high‐dose intranasal challenge was 50–60% in both groups. TLR2 has a significant role in early innate immune responses to S. aureus in the lungs but is not required for bacterial clearance and survival from S. aureus pneumonia.

Interleukin 1α (IL-1α) Promotes Pathogenic Immature Myeloid Cells and IL-1β Favors Protective Mature Myeloid Cells During Acute Lung Infection

Bacterial pneumonia is a common risk factor for acute lung injury and sepsis-mediated death, but the mechanisms underlying the overt inflammation and accompanying pathology are unclear. Infiltration of immature myeloid cells and necrotizing inflammation mediate severe pathology and death during pulmonary infection with Francisella tularensis. However, eliciting mature myeloid cells provides protection. Yet, the host factors responsible for this pathologic immature myeloid cell response are unknown. Here, we report that while the influx of both mature and immature myeloid cells is strictly MyD88 dependent, the interleukin 1 (IL-1) receptor mediates an important dual function via its ligands IL-1α and IL-1β. Although IL-1β favors the appearance of bacteria-clearing mature myeloid cells, IL-1α contributes to lung infiltration by ineffective and pathologic immature myeloid cells. Finally, IL-1α and IL-1β are not the sole factors involved, but myeloid cell responses during acute pneumonia were largely unaffected by lung levels of interleukin 10, interleukin 17, CXCL1, granulocyte colony-stimulating factor, and granulocyte-macrophage colony-stimulating factor.

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.

Bacterial lipoproteins and other factors released by Francisella tularensis modulate human neutrophil lifespan: Effects of a TLR1 SNP on apoptosis inhibition

Francisella tularensis infects several cell types including neutrophils, and aberrant neutrophil accumulation contributes to tissue destruction during tularaemia. We demonstrated previously that F. tularensis strains Schu S4 and live vaccine strain markedly delay human neutrophil apoptosis and thereby prolong cell lifespan, but the bacterial factors that mediate this aspect of virulence are undefined. Herein, we demonstrate that bacterial conditioned medium (CM) can delay apoptosis in the absence of direct infection. Biochemical analyses show that CM contained F. tularensis surface factors as well as outer membrane components. Our previous studies excluded roles for lipopolysaccharide and capsule in apoptosis inhibition, and current studies of [¹⁴C] acetate‐labelled bacteria argue against a role for other bacterial lipids in this process. At the same time, studies of isogenic mutants indicate that TolC and virulence factors whose expression requires FevR or MglA were also dispensable, demonstrating that apoptosis inhibition does not require Type I or Type VI secretion. Instead, we identified bacterial lipoproteins (BLPs) as active factors in CM. Additional studies of isolated BLPs demonstrated dose‐dependent neutrophil apoptosis inhibition via a TLR2‐dependent mechanism that is significantly influenced by a common polymorphism, rs5743618, in human TLR1. These data provide fundamental new insight into pathogen manipulation of neutrophil lifespan and BLP function.

Innate Immune Recognition: Implications for the Interaction of Francisella tularensis with the Host Immune System

The intracellular bacterial pathogen Francisella tularensis causes serious infectious disease in humans and animals. Moreover, F. tularensis, a highly infectious pathogen, poses a major concern for the public as a bacterium classified under Category A of bioterrorism agents. Unfortunately, research has so far failed to develop effective vaccines, due in part to the fact that the pathogenesis of intracellular bacteria is not fully understood and in part to gaps in our understanding of innate immune recognition processes leading to the induction of adaptive immune response. Recent evidence supports the concept that immune response to external stimuli in the form of bacteria is guided by the primary interaction of the bacterium with the host cell. Based on data from different Francisella models, we present here the basic paradigms of the emerging innate immune recognition concept. According to this concept, the type of cell and its receptor(s) that initially interact with the target constitute the first signaling window; the signals produced in the course of primary interaction of the target with a reacting cell act in a paracrine manner; and the innate immune recognition process as a whole consists in a series of signaling windows modulating adaptive immune response. Finally, the host, in the strict sense, is the interacting cell.

Necroptotic debris including damaged mitochondria elicits sepsis-like syndrome during late-phase tularemia

Infection with Francisella tularensis ssp. tularensis (Ft) strain SchuS4 causes an often lethal disease known as tularemia in rodents, non-human primates, and humans. Ft subverts host cell death programs to facilitate their exponential replication within macrophages and other cell types during early respiratory infection (⩽72 h). The mechanism(s) by which cell death is triggered remains incompletely defined, as does the impact of Ft on mitochondria, the host cell’s organellar ‘canary in a coal mine’. Herein, we reveal that Ft infection of host cells, particularly macrophages and polymorphonuclear leukocytes, drives necroptosis via a receptor-interacting protein kinase 1/3-mediated mechanism. During necroptosis mitochondria and other organelles become damaged. Ft-induced mitochondrial damage is characterized by: (i) a decrease in membrane potential and consequent mitochondrial oncosis or swelling, (ii) increased generation of superoxide radicals, and (iii) release of intact or damaged mitochondria into the lung parenchyma. Host cell recognition of and response to released mitochondria and other damage-associated molecular patterns engenders a sepsis-like syndrome typified by production of TNF, IL-1β, IL-6, IL-12p70, and IFN-γ during late-phase tularemia (⩾72 h), but are absent early during infection.

Pyrin-only protein 2 limits inflammation but improves protection against bacteria

Pyrin domain-only proteins (POPs) are recently evolved, primate-specific proteins demonstrated in vitro as negative regulators of inflammatory responses. However, their in vivo function is not understood. Of the four known POPs, only POP2 is reported to regulate NF-κB-dependent transcription and multiple inflammasomes. Here we use a transgenic mouse-expressing POP2 controlled by its endogenous human promotor to study the immunological functions of POP2. Despite having significantly reduced inflammatory cytokine responses to LPS and bacterial infection, POP2 transgenic mice are more resistant to bacterial infection than wild-type mice. In a pulmonary tularaemia model, POP2 enhances IFN-γ production, modulates neutrophil numbers, improves macrophage functions, increases bacterial control and diminishes lung pathology. Thus, unlike other POPs thought to diminish innate protection, POP2 reduces detrimental inflammation while preserving and enhancing protective immunity. Our findings suggest that POP2 acts as a high-order regulator balancing cellular function and inflammation with broad implications for inflammation-associated diseases and therapeutic intervention.

Pyrin-only proteins (POPs) are primate-specific negative regulators of inflammasome activation. Here the authors generate transgenic mice expressing POP2 under the control of the human promoter, and show that POP2 is important for balancing antibacterial inflammatory responses in vivo.

Efficacy of Resistance to Francisella Imparted by ITY/NRAMP/SLC11A1 Depends on Route of Infection

Natural resistance-associated macrophage protein (NRAMP) encoded by the Slc11a1 gene is a membrane-associated transporter of divalent metal ions. Murine Slc11a1 has two known alleles, a functional Slc11a1^Gly169, which is found in DBA2/J, NOD/LtJ, and 129p3/J and related mouse strains, and a non-functional Slc11a1^Asp169, that is found in C56Bl/6J (B6) and BALB/cJ mice. B6 mice congenic for Slc11a1^Gly169 (B6-Slc11a1^G169) are markedly resistant to the intracellular pathogens Salmonella, Leishmania, and Mycobacterium tuberculosis. We examined the host cell response and replication of Francisella in B6-Slc11a1^G169 mice. Bone marrow-derived macrophages from either B6-Slc11a1^G169 or B6 mice were both effectively invaded by Francisella live vaccine strain (LVS). However, at 16 hours post-infection (hpi), the number of LVS bacteria recovered from B6 macrophages had increased roughly 100-fold, while in B6-Slc11a1^G169 mice the number decreased 10-fold. When the mice were challenged intranasally (i.n.) B6 mice lost significant amounts (~15%) of weight, where as B6-Slc11a1^G169 mice lost no weight. Three days after infection in B6-Slc11a1^G169 mice, we failed to recover viable Francisella from the lungs, livers, or spleens. By contrast, B6 mice had bacterial burdens approaching 1 × 10⁶ CFU/organ in all three organs. To further examine the degree of resistance imparted by Slc11a1^Gly169 expression, we challenged mice deficient in TLR2, TLR4, and TLR9, but expressing the functional Slc11a1 (B6-Slc11a1^G169Tlr2/4/9^−/−). Surprisingly, B6-Slc11a1^G169Tlr2/4/9^−/− mice had no notable weight loss. Eighty percent of B6-Slc11a1^G169Tlr2/4/9^−/− mice yielded no detectable Francisella in any organ tested. Additionally, Slc11a1^G169 produced little detectable cytokine either in the lung or serum compared to B6 mice. Mice expressing Slc11a1^Gly169 survived even high doses (~80 LD₅₀) of LVS inoculation. These data taken together serve to highlight that functional Slc11a1^Gly169 can compensate the lack of TLR2/4/9. Thus Slc11a1 is a critical player in murine resistance to pulmonary Francisella infection, but not footpad infection.

FcγR mediates TLR2- and Syk-dependent NLRP3 inflammasome activation by inactivated Francisella tularensis LVS immune complexes

IgG (mAb)-opsonized, inactivated Francisella tularensis LVS (iFt-mAb) enhances TLR2-dependent IL-6 production by macrophages via Fcγ receptors (FcγR). In mice, vaccination with iFt-mAb provides IgA-dependent protection against lethal challenge with Ft LVS. Because inflammasome maturation of IL-1β is thought important for antibody-mediated immunity, we considered the possibility that iFt-mAb elicits an FcγR-dependent myeloid cell inflammasome response. Herein, we find that iFt-mAb enhances macrophage and dendritic cell IL-1β responses in a TLR2- and FcγR-dependent fashion. Although iFt-mAb complexes bind FcγR and are internalized, sensing of cytosolic DNA by absent in melanoma 2 (AIM2) is not required for the IL-1β response. In contrast, ASC, caspase-1, and NLR family pyrin domain-containing 3 (NLRP3) are indispensable. Further, FcγR-mediated spleen tyrosine kinase (Syk) signaling is required for this NLRP3-dependent IL-1β response, but the alternative IL-1β convertase caspase-8 is insufficient. Finally, iFt-mAb-vaccinated wild-type mice exhibit a significant delay in time to death, but IL-1R1- or Nlrp3-deficient mice vaccinated in this way are not protected and lack appreciable Francisella-specific antibodies. This study demonstrates that FcγR-mediated Syk activation leads to NLRP3 inflammasome-dependent IL-1β production in macrophages and suggests that an Nlrp3- and IL-1R-dependent process contributes to the IgA response important for protection against Ft LVS. These findings extend our understanding of cellular responses to inactivated pathogen-opsonized vaccine, establish FcγR-elicited Syk kinase-mediated NLRP3 inflammasome activation, and provide additional insight toward understanding crosstalk between TLR and FcγR signals.

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.

Keyhole limpet hemocyanin induces innate immunity via Syk and Erk phosphorylation

Hemocyanin is an extracellular respiratory protein containing copper in hemolymph of invertebrates, such as Mollusk and Arthropod. Keyhole limpet hemocyanin (KLH) is one of hemocyanins and has many years of experience for vaccine developments and immunological studies in mammals including human. However, the association between KLH and the immune systems, especially the innate immune systems, remains poorly understood. The aim of this study is to clarify the direct effects of KLH on the innate immune systems. KLH activated an inflammation-related transcription factor NF-κB as much as lipopolysaccharide (LPS) in a human monocytic leukemia THP-1 reporter cell line. We have found that the KLH-induced NF-κB activation is partially involved in a spleen tyrosine kinase (Syk) pathway. We have also successfully revealed that an extracellular signal-regulated kinase (Erk), a member of mitogen-activated protein kinases, is located in an upstream of NF-κB activation induced by KLH. Furthermore, a Syk phosphorylation inhibitor partially suppressed the Erk activation in KLH-stimulated THP-1. These results suggest that both Syk and Erk associate with the KLH-induced NF-κB activation in the human monocyte.

Type I interferon licenses enhanced innate recognition and transcriptional responses to Franciscella tularensis live vaccine strain

The innate inflammatory response to Francisella tularensis (Ft) in macrophages is significantly governed by the expression of type I interferon (IFN). Previously, the proteolytic processing and maturation of pro-IL-1β protein was shown to depend upon type I IFN expression. We show in this report that paracrine type I IFN can profoundly enhance innate recognition and TLR-dependent transcriptional responses to Ft infection upstream of its role in inflammasome regulation in both primary human monocyte-derived macrophages and primary murine peritoneal macrophages but not murine bone marrow-derived macrophages. This type I IFN-enhanced response is synergistic with TLR2 transcriptional responses, partially TLR2-independent, but strictly MyD88-dependent.

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.

Entry of Francisella tularensis into Murine B Cells: The Role of B Cell Receptors and Complement Receptors

Francisella tularensis, the etiological agent of tularemia, is an intracellular pathogen that dominantly infects and proliferates inside phagocytic cells but can be seen also in non-phagocytic cells, including B cells. Although protective immunity is known to be almost exclusively associated with the type 1 pathway of cellular immunity, a significant role of B cells in immune responses already has been demonstrated. Whether their role is associated with antibody-dependent or antibody-independent B cell functions is not yet fully understood. The character of early events during B cell–pathogen interaction may determine the type of B cell response regulating the induction of adaptive immunity. We used fluorescence microscopy and flow cytometry to identify the basic requirements for the entry of F. tularensis into B cells within in vivo and in vitro infection models. Here, we present data showing that Francisella tularensis subsp. holarctica strain LVS significantly infects individual subsets of murine peritoneal B cells early after infection. Depending on a given B cell subset, uptake of Francisella into B cells is mediated by B cell receptors (BCRs) with or without complement receptor CR1/2. However, F. tularensis strain FSC200 ΔiglC and ΔftdsbA deletion mutants are defective in the ability to enter B cells. Once internalized into B cells, F. tularensis LVS intracellular trafficking occurs along the endosomal pathway, albeit without significant multiplication. The results strongly suggest that BCRs alone within the B-1a subset can ensure the internalization process while the BCRs on B-1b and B-2 cells need co-signaling from the co receptor containing CR1/2 to initiate F. tularensis engulfment. In this case, fluidity of the surface cell membrane is a prerequisite for the bacteria’s internalization. The results substantially underline the functional heterogeneity of B cell subsets in relation to F. tularensis.

Production of anti-LPS IgM by B1a B cells depends on IL-1β and is protective against lung infection with Francisella tularensis LVS

The role of IL-1β and IL-18 during lung infection with the gram-negative bacterium Francisella tularensis LVS has not been characterized in detail. Here, using a mouse model of pneumonic tularemia, we show that both cytokines are protective, but through different mechanisms. Il-18-/- mice quickly succumb to the infection and showed higher bacterial burden in organs and lower level of IFNγ in BALF and serum compared to wild type C57BL/6J mice. Administration of IFNγ rescued the survival of Il-18-/- mice, suggesting that their decreased resistance to tularemia is due to inability to produce IFNγ. In contrast, mice lacking IL-1 receptor or IL-1β, but not IL-1α, appeared to control the infection in its early stages, but eventually succumbed. IFNγ administration had no effect on Il-1r1-/- mice survival. Rather, Il-1r1-/- mice were found to have significantly reduced titer of Ft LPS-specific IgM. The anti-Ft LPS IgM was generated in a IL-1β-, TLR2-, and ASC-dependent fashion, promoted bacteria agglutination and phagocytosis, and was protective in passive immunization experiments. B1a B cells produced the anti-Ft LPS IgM and these cells were significantly decreased in the spleen and peritoneal cavity of infected Il-1b-/- mice, compared to C57BL/6J mice. Collectively, our results show that IL-1β and IL-18 activate non-redundant protective responses against tularemia and identify an essential role for IL-1β in the rapid generation of pathogen-specific IgM by B1a B cells.

Revisiting the Gram-negative lipoprotein paradigm

The processing of lipoproteins (Lpps) in Gram-negative bacteria is generally considered an essential pathway. Mature lipoproteins in these bacteria are triacylated, with the final fatty acid addition performed by Lnt, an apolipoprotein N-acyltransferase. The mature lipoproteins are then sorted by the Lol system, with most Lpps inserted into the outer membrane (OM). We demonstrate here that the lnt gene is not essential to the Gram-negative pathogen Francisella tularensis subsp. tularensis strain Schu or to the live vaccine strain LVS. An LVS Δlnt mutant has a small-colony phenotype on sucrose medium and increased susceptibility to globomycin and rifampin. We provide data indicating that the OM lipoprotein Tul4A (LpnA) is diacylated but that it, and its paralog Tul4B (LpnB), still sort to the OM in the Δlnt mutant. We present a model in which the Lol sorting pathway of Francisella has a modified ABC transporter system that is capable of recognizing and sorting both triacylated and diacylated lipoproteins, and we show that this modified system is present in many other Gram-negative bacteria. We examined this model using Neisseria gonorrhoeae, which has the same Lol architecture as that of Francisella, and found that the lnt gene is not essential in this organism. This work suggests that Gram-negative bacteria fall into two groups, one in which full lipoprotein processing is essential and one in which the final acylation step is not essential, potentially due to the ability of the Lol sorting pathway in these bacteria to sort immature apolipoproteins to the OM.

IMPORTANCE

This paper describes the novel finding that the final stage in lipoprotein processing (normally considered an essential process) is not required by Francisella tularensis or Neisseria gonorrhoeae. The paper provides a potential reason for this and shows that it may be widespread in other Gram-negative bacteria.

A more flexible lipoprotein sorting pathway

Lipoprotein biogenesis in Gram-negative bacteria occurs by a conserved pathway, each step of which is considered essential. In contrast to this model, LoVullo and colleagues demonstrate that the N-acyl transferase Lnt is not required in Francisella tularensis or Neisseria gonorrhoeae. This suggests the existence of a more flexible lipoprotein pathway, likely due to a modified Lol transporter complex, and raises the possibility that pathogens may regulate lipoprotein processing to modulate interactions with the host.

Divergent functions of Toll-like receptors during bacterial lung infections

Lower respiratory tract infections caused by bacteria are a major cause of death in humans irrespective of sex, race, or geography. Indeed, accumulated data indicate greater mortality and morbidity due to these infections than cancer, malaria, or HIV infection. Successful recognition of, followed by an appropriate response to, bacterial pathogens in the lungs is crucial for effective pulmonary host defense. Although the early recruitment and activation of neutrophils in the lungs is key in the response against invading microbial pathogens, other sentinels, such as alveolar macrophages, epithelial cells, dendritic cells, and CD4(+) T cells, also contribute to the elimination of the bacterial burden. Pattern recognition receptors, such as Toll-like receptors (TLRs) and nucleotide-binding oligomerization domain-like receptors, are important for recognizing and responding to microbes during pulmonary infections. However, bacterial pathogens have acquired crafty evasive strategies to circumvent the pattern recognition receptor response and thus establish infection. Increased understanding of the function of TLRs and evasive mechanisms used by pathogens during pulmonary infection will deepen our knowledge of immunopathogenesis and is crucial for developing effective therapeutic and/or prophylactic measures. This review summarizes current knowledge of the multiple roles of TLRs in bacterial lung infections and highlights the mechanisms used by pathogens to modulate or interfere with TLR signaling in the lungs.

Caspase-1-independent interleukin-1β is required for clearance of Bordetella pertussis infections and whole-cell vaccine-mediated immunity

Whooping cough remains a significant disease worldwide and its re-emergence in highly vaccinated populations has been attributed to a combination of imperfect vaccines and evolution of the pathogen. The focus of this study was to examine the role of IL-1α/β and the inflammasome in generation of the interleukin-1 (IL-1) response, which is required for the clearance of Bordetella pertussis. We show that IL-1β but not IL-1α is required for mediating the clearance of B. pertussis from the lungs of mice. We further found that IL-1β and IL-1R deficient mice, compared to wild-type, have similar but more persistent levels of inflammation, characterized by immune cell infiltration, with significantly increased IFNγ and a normal IL-17A response during B. pertussis infection. Contrary to expectations, the cleavage of precursor IL-1β to its mature form did not require caspase-1 during primary infections within the lung despite being required by bone marrow-derived macrophages exposed to live bacteria. We also found that the caspase-1 inflammasome was not required for protective immunity against a B. pertussis challenge following vaccination with heat-killed whole cell B. pertussis, despite IL-1R signaling being required. These findings demonstrate that caspase-1-independent host factors are involved in the processing of protective IL-1β responses that are critical for bacterial clearance and vaccine-mediated immunity.

TLR2 Signaling is Required for the Innate, but Not Adaptive Response to LVS clpB

Toll-like receptor 2 (TLR2) is the best-characterized pattern-recognition receptor for the highly pathogenic intracellular bacterium, Francisella tularensis. We previously identified a mutant in the live vaccine strain (LVS) of Francisella, LVS clpB, which is attenuated, but induces a protective immune response. We sought to determine whether TLR2 signaling was required during the immune response to LVS clpB. TLR2 knock-out (TLR2 KO) mice previously infected with LVS clpB are completely protected during a lethal challenge with LVS. Furthermore, the kinetics and magnitude of the primary T-cell response in B6 and TLR2 KO mice are similar indicating that TLR2 signaling is dispensable for the adaptive immune response to LVS clpB. TLR2 signaling was important, however, for the innate immune response to LVS clpB. We identified three classes of cytokines/chemokines that differ in their dependence on TLR2 signaling for production on day 3 post-inoculation in the bronchoalveolar lavage fluid. IL-1α, IL-1β, IL-2, IL-17, MIP-1α, and TNF-α production depended on TLR2 signaling, while GM-CSF, IFN-γ, and VEGF production were completely independent of TLR2 signaling. IL-6, IL-12, IP-10, KC, and MIG production were partially dependent on TLR2 signaling. Together our data indicate that the innate immune response to LVS clpB requires TLR2 signaling for the maximal innate response, whereas TLR2 is not required for the adaptive immune response.

FTT0831c/FTL_0325 contributes to Francisella tularensis cell division, maintenance of cell shape, and structural integrity

The Francisella FTT0831c/FTL_0325 gene encodes amino acid motifs to suggest it is a lipoprotein and that it may interact with the bacterial cell wall as a member of the OmpA-like protein family. Previous studies have suggested that FTT0831c is surface exposed and required for virulence of Francisella tularensis by subverting the host innate immune response (M. Mahawar et al., J. Biol. Chem. 287:25216-25229, 2012). We also found that FTT0831c is required for murine pathogenesis and intramacrophage growth of Schu S4, but we propose a different model to account for the proinflammatory nature of the resultant mutants. First, inactivation of FTL_0325 from live vaccine strain (LVS) or FTT0831c from Schu S4 resulted in temperature-dependent defects in cell viability and morphology. Loss of FTT0831c was also associated with an unusual defect in lipopolysaccharide O-antigen synthesis, but loss of FTL_0325 was not. Full restoration of these properties was observed in complemented strains expressing FTT0831c in trans, but not in strains lacking the OmpA motif, suggesting that cell wall contact is required. Finally, growth of the LVS FTL_0325 mutant in Mueller-Hinton broth at 37°C resulted in the appearance of membrane blebs at the poles and midpoint, prior to the formation of enlarged round cells that showed evidence of compromised cellular membranes. Taken together, these data are more consistent with the known structural role of OmpA-like proteins in linking the OM to the cell wall and, as such, maintenance of structural integrity preventing altered surface exposure or release of Toll-like receptor 2 agonists during rapid growth of Francisella in vitro and in vivo.

Comparative review of Francisella tularensis and Francisella novicida

Francisella tularensis is the causative agent of the acute disease tularemia. Due to its extreme infectivity and ability to cause disease upon inhalation, F. tularensis has been classified as a biothreat agent. Two subspecies of F. tularensis, tularensis and holarctica, are responsible for tularemia in humans. In comparison, the closely related species F. novicida very rarely causes human illness and cases that do occur are associated with patients who are immune compromised or have other underlying health problems. Virulence between F. tularensis and F. novicida also differs in laboratory animals. Despite this varying capacity to cause disease, the two species share ~97% nucleotide identity, with F. novicida commonly used as a laboratory surrogate for F. tularensis. As the F. novicida U112 strain is exempt from U.S. select agent regulations, research studies can be carried out in non-registered laboratories lacking specialized containment facilities required for work with virulent F. tularensis strains. This review is designed to highlight phenotypic (clinical, ecological, virulence, and pathogenic) and genomic differences between F. tularensis and F. novicida that warrant maintaining F. novicida and F. tularensis as separate species. Standardized nomenclature for F. novicida is critical for accurate interpretation of experimental results, limiting clinical confusion between F. novicida and F. tularensis and ensuring treatment efficacy studies utilize virulent F. tularensis strains.

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.

MyD88-dependent immunity to a natural model of vaccinia virus infection does not involve Toll-like receptor 2

Although the pattern recognition receptor Toll-like receptor 2 (TLR2) is typically thought to recognize bacterial components, it has been described to alter the induction of both innate and adaptive immunity to a number of viruses, including vaccinia virus (VACV). However, many pathogens that reportedly encode TLR2 agonists may actually be artifactually contaminated during preparation, possibly with cellular debris or merely with molecules that sensitize cells to be activated by authentic TLR2 agonists. In both humans and mice, the most relevant natural route of infection with VACV is through intradermal infection of the skin. Therefore, we examined the requirement for TLR2 and its signaling adaptor MyD88 in protective immunity to VACV after intradermal infection. We find that although TLR2 may recognize virus preparations in vitro and have a minor role in preventing dissemination of VACV following systemic infection with large doses of virus, it is wholly disposable in both control of virus replication and induction of adaptive immunity following intradermal infection. In contrast, MyD88 is required for efficient induction of CD4 T cell and B cell responses and for local control of virus replication following intradermal infection. However, even MyD88 is not required to induce local inflammation, inflammatory cytokine production, or recruitment of cells that restrict virus from spreading systemically after peripheral infection. Thus, an effective antiviral response does require MyD88, but TLR2 is not required for control of a peripheral VACV infection. These findings emphasize the importance of studying relevant routes of infection when examining innate sensing mechanisms.

IMPORTANCE

Vaccinia virus (VACV) provides the backbone for some of the most widely used and successful viral vaccine vectors and is also related to the human pathogens Cantagalo virus and molluscum contagiosum virus that infect the skin of patients. Therefore, it is vital to understand the mechanisms that induce a strong innate immune response to the virus following dermal infection. Here, we compare the ability of the innate sensing molecule Toll-like receptor 2 (TLR2) and the signaling molecule MyD88 to influence the innate and adaptive immune response to VACV following systemic or dermal infection.

Cas9-dependent endogenous gene regulation is required for bacterial virulence

CRISPR (clustered regularly interspaced short palindromic repeats)–Cas (CRISPR-associated) systems are known to mediate bacterial defence against foreign nucleic acids. We recently demonstrated a non-canonical role for a CRISPR–Cas system in controlling endogenous gene expression, which had not previously been appreciated. In the present article, we describe the studies that led to this discovery, beginning with an unbiased genome-wide screen to identify virulence genes in the intracellular pathogen Francisella novicida. A gene annotated as encoding a hypothetical protein, but which we now know encodes the Cas protein Cas9, was identified as one of the most critical to the ability of F. novicida to replicate and survive during murine infection. Subsequent studies revealed a role for this protein in evasion of the host innate immune response. Specifically, Cas9 represses the expression of a BLP (bacterial lipoprotein) that could otherwise be recognized by TLR2 (Toll-like receptor 2), a host protein involved in initiating an antibacterial pro-inflammatory response. By repressing BLP levels, Cas9 mediates evasion of TLR2, promoting bacterial virulence. Finally, we described the molecular mechanism by which Cas9 functions in complex with two small RNAs to target the mRNA encoding the BLP for degradation. This work greatly broadened the paradigm for CRISPR–Cas function, highlighting a role in gene regulation that could be conserved in numerous bacteria, and elucidating its integral contribution to bacterial pathogenesis.

Serving the new masters - dendritic cells as hosts for stealth intracellular bacteria

Dendritic cells (DCs) serve as the primers of adaptive immunity, which is indispensable for the control of the majority of infections. Interestingly, some pathogenic intracellular bacteria can subvert DC function and gain the advantage of an ineffective host immune reaction. This scenario appears to be the case particularly with so-called stealth pathogens, which are the causative agents of several under-diagnosed chronic diseases. However, there is no consensus how less explored stealth bacteria like Coxiella, Brucella and Francisella cross-talk with DCs. Therefore, the aim of this review was to explore the issue and to summarize the current knowledge regarding the interaction of above mentioned pathogens with DCs as crucial hosts from an infection strategy view. Evidence indicates that infected DCs are not sufficiently activated, do not undergo maturation and do not produce expected proinflammatory cytokines. In some cases, the infected DCs even display immunosuppressive behaviour that may be directly linked to the induction of tolerogenicity favouring pathogen survival and persistence.

Repression of inflammasome by Francisella tularensis during early stages of infection

Francisella tularensis is an important human pathogen responsible for causing tularemia. F. tularensis has long been developed as a biological weapon and is now classified as a category A agent by the Centers for Disease Control because of its possible use as a bioterror agent. F. tularensis represses inflammasome; a cytosolic multi-protein complex that activates caspase-1 to produce proinflammatory cytokines IL-1β and IL-18. However, the Francisella factors and the mechanisms through which F. tularensis mediates these suppressive effects remain relatively unknown. Utilizing a mutant of F. tularensis in FTL_0325 gene, this study investigated the mechanisms of inflammasome repression by F. tularensis. We demonstrate that muted IL-1β and IL-18 responses generated in macrophages infected with F. tularensis live vaccine strain (LVS) or the virulent SchuS4 strain are due to a predominant suppressive effect on TLR2-dependent signal 1. Our results also demonstrate that FTL_0325 of F. tularensis impacts proIL-1β expression as early as 2 h post-infection and delays activation of AIM2 and NLRP3-inflammasomes in a TLR2-dependent fashion. An enhanced activation of caspase-1 and IL-1β observed in FTL_0325 mutant-infected macrophages at 24 h post-infection was independent of both AIM2 and NLRP3. Furthermore, F. tularensis LVS delayed pyroptotic cell death of the infected macrophages in an FTL_0325-dependent manner during the early stages of infection. In vivo studies in mice revealed that suppression of IL-1β by FTL_0325 early during infection facilitates the establishment of a fulminate infection by F. tularensis. Collectively, this study provides evidence that F. tularensis LVS represses inflammasome activation and that F. tularensis-encoded FTL_0325 mediates this effect.

Discordant results obtained with Francisella tularensis during in vitro and in vivo immunological studies are attributable to compromised bacterial structural integrity

Francisella tularensis (Ft) is a highly infectious intracellular pathogen and the causative agent of tularemia. Because Ft can be dispersed via small droplet-aerosols and has a very low infectious dose it is characterized as a category A Select Agent of biological warfare. Respiratory infection with the attenuated Live Vaccine Strain (LVS) and the highly virulent SchuS4 strain of Ft engenders intense peribronchiolar and perivascular inflammation, but fails to elicit select pro-inflammatory mediators (e.g., TNF, IL-1β, IL-6, IL-12, and IFN-γ) within the first ∼72 h. This in vivo finding is discordant with the principally T_H1-oriented response to Ft frequently observed in cell-based studies wherein the aforementioned cytokines are produced. An often overlooked confounding factor in the interpretation of experimental results is the influence of environmental cues on the bacterium's capacity to elicit certain host responses. Herein, we reveal that adaptation of Ft to its mammalian host imparts an inability to elicit select pro-inflammatory mediators throughout the course of infection. Furthermore, in vitro findings that non-host adapted Ft elicits such a response from host cells reflect aberrant recognition of the DNA of structurally-compromised bacteria by AIM2-dependent and -independent host cell cytosolic DNA sensors. Growth of Ft in Muller-Hinton Broth or on Muller-Hinton-based chocolate agar plates or genetic mutation of Ft was found to compromise the structural integrity of the bacterium thus rendering it capable of aberrantly eliciting pro-inflammatory mediators (e.g., TNF, IL-1β, IL-6, IL-12, and IFN-γ). Our studies highlight the profound impact of different growth conditions on host cell response to infection and demonstrate that not all in vitro-derived findings may be relevant to tularemia pathogenesis in the mammalian host. Rational development of a vaccine and immunotherapeutics can only proceed from a foundation of knowledge based upon in vitro findings that recapitulate those observed during natural infection.

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

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

IMPORTANCE

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

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.

A mucosal subunit vaccine protects against lethal respiratory infection with Francisella tularensis LVS

Francisella tularensis (FT) is a highly virulent pathogen for humans and other mammals. Severe morbidity and mortality is associated with respiratory FT infection and there are concerns about intentional dissemination of this organism. Therefore, FT has been designated a category A biothreat agent and there is a growing interest in the development of a protective vaccine. In the present study, we determine the protective potential of a subunit vaccine comprised of the FT heat shock protein DnaK and surface lipoprotein Tul4 against respiratory infection with the live vaccine strain (LVS) of FT in mice. First, we establish an optimal intranasal immunization regimen in C57BL/6 mice using recombinant DnaK or Tul4 together with the adjuvant GPI-0100. The individual immunization regimens induced robust salivary IgA, and vaginal and bronchoalveolar IgA and IgG antigen-specific antibodies. Serum IgG1 and IgG2c antibody responses were also induced, indicative of a mixed type 2 and type 1 response, respectively. Next, we show that immunization with DnaK and Tul4 induces mucosal and systemic antibody responses that are comparable to that seen following immunization with each antigen alone. This immunization regimen also induced IFN-γ, IL-10 and IL-17A production by splenic CD4(+) T cells in an antigen-specific manner. Importantly, over 80% of the mice immunized with DnaK and Tul4, but not with each antigen alone, were protected against a lethal respiratory challenge with FT LVS. Protection correlated with reduced bacterial burden in the lung, liver and spleen of mice. This study demonstrates the potential of DnaK and Tul4 as protective antigens and lends support to the notion of combining distinct, immunodominant antigens into an effective multivalent tularemia vaccine.

Repression of bacterial lipoprotein production by Francisella novicida facilitates evasion of innate immune recognition

Innate recognition systems, including the Toll-like receptors (TLRs), play a critical role in activating host defences and proinflammatory pathways in response to infection. Pathogens have developed strategies to subvert TLRs in order to survive and replicate within the host. The model intracellular pathogen, Francisella novicida, modulates host defences to promote survival and replication in macrophages. TLR2, which recognizes bacterial lipoproteins (BLPs), is critical for activating host defences and proinflammatory cytokine production in response to Francisella infection. Here we show that the F. novicida protein FTN_0757 acts to repress BLP production, dampening TLR2 activation. The ΔFTN_0757 mutant strain induced robust TLR2-dependent cytokine production in macrophages compared with wild-type bacteria, and produced increased amounts of BLPs. The deletion of one BLP (FTN_1103) from ΔFTN_0757 decreased the total BLP concentration to near wild-type level sand correlated with a decrease in the inductionof TLR2 signalling. The overproduction of BLPs also contributed to the in vivo attenuation of the ΔFTN_0757 mutant, which was significantly rescued when FTN_1103 was deleted. Taken together, these data reveal a novel mechanism of immune evasion by the downregulation of BLP expression to subvert TLR2 activation, which is likely used by numerous other intracellular bacterial pathogens.

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.

Mast cell TLR2 signaling is crucial for effective killing of Francisella tularensis

TLR signaling is critical for early host defense against pathogens, but the contributions of mast cell TLR-mediated mechanisms and subsequent effector functions during pulmonary infection are largely unknown. We have previously demonstrated that mast cells, through the production of IL-4, effectively control Francisella tularensis replication. In this study, the highly human virulent strain of F. tularensis SCHU S4 and the live vaccine strain were used to investigate the contribution of mast cell/TLR regulation of Francisella. Mast cells required TLR2 for effective bacterial killing, regulation of the hydrolytic enzyme cathepsin L, and for coordination and trafficking of MHC class II and lysosomal-associated membrane protein 2. Infected TLR2(-/-) mast cells, in contrast to wild-type and TLR4(-/-) cells, lacked detectable IL-4 and displayed increased cell death with a 2-3 log increase of F. tularensis replication, but could be rescued with rIL-4 treatment. Importantly, MHC class II and lysosomal-associated membrane protein 2 localization with labeled F. tularensis in the lungs was greater in wild-type than in TLR2(-/-) mice. These results provide evidence for the important effector contribution of mast cells and TLR2-mediated signaling on early innate processes in the lung following pulmonary F. tularensis infection and provide additional insight into possible mechanisms by which intracellular pathogens modulate respiratory immune defenses.

The emergence of severe pulmonary hemorrhagic leptospirosis: questions to consider

Since the 1980s, the incidence of severe pulmonary hemorrhage caused by Leptospira spp. infection has increased. The mild, non-specific symptoms or the more classical form of severe disease with hepatorenal manifestations, Weil's syndrome, predominate world-wide. However, several regions of the world have seen increases in numbers of patients with pulmonary hemorrhage attributed to leptospirosis. The reasons behind the emergence of this syndrome, which carries a high mortality rate, are not known. Several avenues for future research may shed light on the mechanisms involved in development of pulmonary hemorrhage, and inform targeted therapeutics to improve outcomes. Possibilities to consider include: (1) emergence of new bacterial strains, (2) acquisition of virulence traits by strains in the endemic regions, (3) changes in underlying health of the affected human populations, and (4) increased recognition of the syndrome and better record keeping by the medical and veterinary communities. Determining the causes of emerging clinical manifestations presents challenges and opportunities for potentially life-saving research into the pathogenesis of a number of infectious diseases, including leptospirosis.

Role of Francisella lipid A phosphate modification in virulence and long-term protective immune responses

Lipopolysaccharide (LPS) structural modifications have been shown to specifically affect the pathogenesis of many gram-negative pathogens. In Francisella, modification of the lipid A component of LPS resulted in a molecule with no to low endotoxic activity. The role of the terminal lipid A phosphates in host recognition and pathogenesis was determined using a Francisella novicida mutant that lacked the 4' phosphatase enzyme (LpxF). The lipid A of this strain retained the phosphate moiety at the 4' position and the N-linked fatty acid at the 3' position on the diglucosamine backbone. Studies were undertaken to determine the pathogenesis of this mutant strain via the pulmonary and subcutaneous routes of infection. Mice infected with the lpxF-null F. novicida mutant by either route survived primary infection and subsequently developed protective immunity against a lethal wild-type (WT) F. novicida challenge. To determine the mechanism(s) by which the host controlled primary infection by the lpxF-null mutant, the role of innate immune components, including Toll-like receptor 2 (TLR2), TLR4, caspase-1, MyD88, alpha interferon (IFN-α), and gamma interferon(IFN-γ), was examined using knockout mice. Interestingly, only the IFN-γ knockout mice succumbed to a primary lpxF-null F. novicida mutant infection, highlighting the importance of IFN-γ production. To determine the role of components of the host adaptive immune system that elicit the long-term protective immune response, T- and B-cell deficient RAG1(-/-) mice were examined. All mice survived primary infection; however, RAG1(-/-) mice did not survive WT challenge, highlighting a role for T and B cells in the protective immune response.

Gene expression following low dose inhalational Francisella tularensis (SchuS4) exposure in Balb/c mice and the potential role of the epithelium and cell adhesion

Interactions between Francisella tularensis and the host are slowly being elucidated. Microarray technology was used to further characterise the response of Balb/c mice after inhalation of the virulent F. tularensis, SchuS4. The validated array data revealed changes in expression of 476 genes across a 96 h time course following infection (p ≤ 0.05). These data confirm down-regulation of the toll-like receptor pathway (TLR3, 4, 5, 7 and 8), and the induction of IFN-γ inducible genes (T-cell specific GTPase, β2 microglobulin and interleukin 21). The overall response appears to be two staged with an initial up-regulation of genes involved in apoptosis, TNFα production and antigen presentation. This is followed by a large alteration of expression at 96 h as the host succumbs to infection. A key regulatory time-point has been identified at 24 h post challenge, where several transcriptional events may predicate the progression of infection; these include transcriptional regulators of inflammation and proteolytic pathways. Pathway analysis indicates a novel role for cell-cell adhesion and extracellular matrix modulation in infection. Transcripts representing cellular junctions, focal adhesion and adherens junctions changed following infection. Additionally, aspects of extracellular matrix remodelling have been confirmed at the protein level, suggesting an important role of the respiratory epithelium in host response to F. tularensis warranting further study.

Francisella tularensis reveals a disparity between human and mouse NLRP3 inflammasome activation

Pathogen-triggered activation of the inflammasome complex leading to caspase-1 activation and IL-1β production involves similar sensor proteins between mouse and human. However, the specific sensors used may differ between infectious agents and host species. In mice, Francisella infection leads to seemingly exclusive activation of the Aim2 inflammasome with no apparent role for Nlrp3. Here we examine the IL-1β response of human cells to Francisella infection. Francisella strains exhibit differences in IL-1β production by influencing induction of IL-1β and ASC transcripts. Unexpectedly, our results demonstrate that Francisella activates the NLRP3 inflammasome in human cells. Francisella infection of THP-1 cells elicits IL-1β production, which is reduced by siRNA targeting of NLRP3. Moreover, in reconstituted 293T cells, Francisella triggers assembly of the NLRP3 inflammasome complex. In addition, inhibitors of reactive oxygen species, cathepsin B, and K(+) efflux pathways, known to specifically influence NLRP3, substantially but not completely impair the Francisella-elicited IL-1β response, suggesting the involvement of another inflammasome pathway. Finally, shRNA targeting of NLRP3 and AIM2 reveals that both pathways contribute to the inflammasome response. Together these results establish NLRP3 as a cytosolic sensor for Francisella in human cells, a role not observed in mouse.

Uncoupling of Pyrin-only protein 2 (POP2)-mediated dual regulation of NF-κB and the inflammasome

Activation of transcription factor NF-κB and inflammasome-directed caspase-1 cleavage of IL-1β are key processes in the inflammatory response to pathogen or host-derived signals. Pyrin-only proteins (POPs) are restricted to Old World monkeys, apes, and humans and have previously been shown to impair inflammasome assembly and/or NF-κB p65 transcriptional activity in transfected epithelial cells. However, the biological role of POP2 and the molecular basis for its observed functions are not well understood. In this report we demonstrate that POP2 regulates TNFα and IL-1β responses in human monocytic THP-1 cells and in stable transfectants of mouse J774A.1 macrophages. Deletion analysis of POP2 revealed that the first α-helix (residues 1-19) is necessary and sufficient for both inflammasome and NF-κB inhibitory functions. Further, key acidic residues Glu(6), Asp(8), and Glu(16), believed critical for Pyrin/Pyrin domain interaction, are important for inflammasome inhibition. Moreover, these mutations did not reduce the effect of POP2 upon NF-κB, indicating that the inflammasome and NF-κB inhibitory properties of POP2 can be uncoupled mechanistically. Collectively, these data demonstrate that POP2 acts as a regulator of inflammatory signals and exerts its two known functions through distinct modalities employed by its first α-helix.

Host Defense and the Airway Epithelium: Frontline Responses That Protect against Bacterial Invasion and Pneumonia

Airway epithelial cells are the first line of defense against invading microbes, and they protect themselves through the production of carbohydrate and protein matrices concentrated with antimicrobial products. In addition, they act as sentinels, expressing pattern recognition receptors that become activated upon sensing bacterial products and stimulate downstream recruitment and activation of immune cells which clear invading microbes. Bacterial pathogens that successfully colonize the lungs must resist these mechanisms or inhibit their production, penetrate the epithelial barrier, and be prepared to resist a barrage of inflammation. Despite the enormous task at hand, relatively few virulence factors coordinate the battle with the epithelium while simultaneously providing resistance to inflammatory cells and causing injury to the lung. Here we review mechanisms whereby airway epithelial cells recognize pathogens and activate a program of antibacterial pathways to prevent colonization of the lung, along with a few examples of how bacteria disrupt these responses to cause pneumonia.

Elevated AIM2-mediated pyroptosis triggered by hypercytotoxic Francisella mutant strains is attributed to increased intracellular bacteriolysis

Intracellular bacterial pathogens Francisella novicida and the Live Vaccine Strain (LVS) are recognized in the macrophage cytosol by the AIM2 inflammasome, which leads to the activation of caspase-1 and the processing and secretion of active IL-1β, IL-18 and pyroptosis. Previous studies have reported that F. novicida and LVS mutants in specific genes (e.g. FTT0584, mviN and ripA) induce elevated inflammasome activation and hypercytotoxicity in host cells, leading to the proposal that F. novicida and LVS may have proteins that actively modulate inflammasome activation. However, there has been no direct evidence of such inflammasome evasion mechanisms. Here, we demonstrate for the first time that the above mutants, along with a wide range of F. novicida hypercytotoxic mutants that are deficient for membrane-associated proteins (ΔFTT0584, ΔmviN, ΔripA, ΔfopA and ΔFTN1217) or deficient for genes involved in O-antigen or LPS biosynthesis (ΔwbtA and ΔlpxH) lyse more intracellularly, thus activating increased levels of AIM2-dependent pyroptosis and other innate immune signalling pathways. This suggests that an inflammasome-specific evasion mechanism may not be present in F. novicida and LVS. Furthermore, future studies may need to consider increased bacterial lysis as a possible cause of elevated stimulation of multiple innate immune pathways when the protein composition or surface carbohydrates of the bacterial membrane is altered.