The presence of CD14 overcomes evasion of innate immune responses by virulent Francisella tularensis in human dendritic cells in vitro and pulmonary cells in vivo

Production of IFN-γ by splenic dendritic cells during innate immune responses against Francisella tularensis LVS depends on MyD88, but not TLR2, TLR4, or TLR9

Production of IFN-γ is a key innate immune mechanism that limits replication of intracellular bacteria such as Francisella tularensis (Ft) until adaptive immune responses develop. Previously, we demonstrated that the host cell types responsible for IFN-γ production in response to murine Francisella infection include not only natural killer (NK) and T cells, but also a variety of myeloid cells. However, production of IFN-γ by mouse dendritic cells (DC) is controversial. Here, we directly demonstrated substantial production of IFN-γ by DC, as well as hybrid NK-DC, from LVS-infected wild type C57BL/6 or Rag1 knockout mice. We demonstrated that the numbers of conventional DC producing IFN-γ increased progressively over the course of 8 days of LVS infection. In contrast, the numbers of conventional NK cells producing IFN-γ, which represented about 40% of non-B/T IFN-γ-producing cells, peaked at day 4 after LVS infection and declined thereafter. This pattern was similar to that of hybrid NK-DC. To further confirm IFN-γ production by infected cells, DC and neutrophils were sorted from naïve and LVS-infected mice and analyzed for gene expression. Quantification of LVS by PCR revealed the presence of Ft DNA not only in macrophages, but also in highly purified, IFN-γ producing DC and neutrophils. Finally, production of IFN-γ by infected DC was confirmed by immunohistochemistry and confocal microscopy. Notably, IFN-γ production patterns similar to those in wild type mice were observed in cells derived from LVS-infected TLR2, TLR4, and TLR2xTLR9 knockout (KO) mice, but not from MyD88 KO mice. Taken together, these studies demonstrate the pivotal roles of DC and MyD88 in IFN-γ production and in initiating innate immune responses to this intracellular bacterium.

Temporal Requirement for Pulmonary Resident and Circulating T Cells during Virulent Francisella tularensis Infection

The lung is a complex organ with anatomically distinct pools of T cells that play specific roles in combating infection. Our knowledge regarding the generation and/or maintenance of immunity by parenchymal or circulating T cells has been gathered from either persistent (>60 d) or rapidly cleared (<10 d) infections. However, the roles of these distinct T cell pools in infections that are cleared over the course of several weeks are not understood. Clearance of the highly virulent intracellular bacterium Francisella tularensis subspecies tularensis (Ftt) following pulmonary infection of immune animals is a protracted T cell-dependent process requiring ∼30-40 d and serves as a model for infections that are not acutely controlled. Using this model, we found that intranasal vaccination increased the number of tissue-resident CD4⁺ effector T cells, and subsequent challenge of immune mice with Ftt led to a significant expansion of polyfunctional parenchymal CD4⁺ effector T cells compared with the circulating pool. Despite the dominant in vivo response by parenchymal CD4⁺ T cells after vaccination and challenge, circulating CD4⁺ T cells were superior at controlling intracellular Ftt replication in vitro. Further examination in vivo revealed temporal requirements for resident and circulating T cells during Ftt infection. These requirements were in direct contrast to other pulmonary infections that are cleared rapidly in immune animals. The data in this study provide important insights into the role of specific T cell populations that will be essential for the design of novel effective vaccines against tularemia and potentially other agents of pulmonary infection.

GM-CSF has disparate roles during intranasal and intradermal Francisella tularensis infection

Our laboratory has employed in vitro and in vivo mouse models based on Francisella tularensis Live Vaccine Strain (LVS)-induced protection to elucidate immune correlates for intracellular bacteria. Among the effectors found was GM-CSF, a pleiotropic cytokine that is integral to the development and proliferation of myeloid cells, including alveolar macrophages. GM-CSF has roles in resistance to primary murine infection with several intracellular pathogens, but its role during Francisella infection is unknown. Francisella is an intracellular pathogen that infects lungs after inhalation, primarily invading alveolar macrophages. Here we show that GM-CSF has route-dependent roles during primary infection of mice with LVS. GM-CSF deficient (GM-CSF KO) mice were slightly more susceptible than wild type to intradermal infection, but had increased resistance to intranasal infection. Similarly, these mice had increased resistance to pulmonary infection with virulent F. tularensis (SchuS4). LVS-vaccinated GM-CSF KO mice had normal adaptive immune responses, as measured by T cell activities after LVS intradermal or intranasal vaccination, and survived lethal secondary LVS challenge. GM-CSF KO mice also had robust humoral responses, producing elevated levels of serum antibodies following LVS vaccination compared to wild type mice. Taken together, our data demonstrates that the absence of GM-CSF improves resistance to pulmonary, but not intradermal, infection with Francisella.

Dendritic cell functions: Learning from microbial evasion strategies

Dendritic cells (DCs) are specialized antigen presenting cells (APC) that are fundamental to initiate both immunity and tolerance. DCs play a 'sentinel' role to protect our body from potential pathogens and induce tolerogenic responses toward harmless antigens. The flexibility of DCs or macrophages to adapt to the environment and to respond accordingly can be hijacked by pathogens for their own interest to transform a potentially immunogenic APC into a tolerogenic cell with clear consequences in pathogen clearance. While these immune evasion mechanisms can be detrimental for the host, they can highlight important molecular pathways in DCs necessary for their function. In this review we will mention several mechanisms employed by pathogens to evade DC patrolling function.

Successful protection against tularemia in C57BL/6 mice is correlated with expansion of Francisella tularensis-specific effector T cells

Francisella tularensis is an intracellular, Gram-negative bacterium that causes the fatal disease tularemia. Currently, there are no licensed vaccines for tularemia and the requirements for protection against infection are poorly defined. To identify correlates of vaccine-induced immunity against tularemia, we compared different strains of the live vaccine strain (LVS) for their relative levels of virulence and ability to protect C57BL/6 mice against challenge with virulent F. tularensis strain SchuS4. Successful vaccination, as defined by survival of C57BL/6 mice, was correlated with significantly greater numbers of effector T cells in the spleen and lung. Further, lung cells and splenocytes from fully protected animals were more effective than lung cells and splenocytes from vaccinated but nonimmune animals in limiting intracellular replication of SchuS4 in vitro. Together, our data provide a unique model to compare efficacious vaccines to nonefficacious vaccines, which will enable comprehensive identification of host and bacterial components required for immunization against tularemia.

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.

Alternative activation of macrophages and induction of arginase are not components of pathogenesis mediated by Francisella species

Virulent Francisella tularensis ssp tularensis is an intracellular, Gram negative bacterium that causes acute lethal disease following inhalation of fewer than 15 organisms. Pathogenicity of Francisella infections is tied to its unique ability to evade and suppress inflammatory responses in host cells. It has been proposed that induction of alternative activation of infected macrophages is a mechanism by which attenuated Francisella species modulate host responses. In this report we reveal that neither attenuated F. tularensis Live Vaccine Strain (LVS) nor virulent F. tularensis strain SchuS4 induce alternative activation of macrophages in vitro or in vivo. LVS, but not SchuS4, provoked production of arginase1 independent of alternative activation in vitro and in vivo. However, absence of arginase1 did not significantly impact intracellular replication of LVS or SchuS4. Together our data establish that neither induction of alternative activation nor expression of arginase1 are critical features of disease mediated by attenuated or virulent Francisella species.

Lipids derived from virulent Francisella tularensis broadly inhibit pulmonary inflammation via toll-like receptor 2 and peroxisome proliferator-activated receptor α

Francisella tularensis is a Gram-negative facultative intracellular pathogen that causes an acute lethal respiratory disease in humans. The heightened virulence of the pathogen is linked to its unique ability to inhibit Toll-like receptor (TLR)-mediated inflammatory responses. The bacterial component and mechanism of this inhibition are unknown. Here we show that lipids isolated from virulent but not attenuated strains of F. tularensis are not detected by host cells, inhibit production of proinflammatory cytokines by primary macrophages in response to known TLR ligands, and suppress neutrophil recruitment in vivo. We further show that lipid-mediated inhibition of inflammation is dependent on TLR2, MyD88, and the nuclear hormone and fatty acid receptor peroxisome proliferator-activated receptor α (PPARα). Pathogen lipid-mediated interference with inflammatory responses through the engagement of TLR2 and PPARα represents a novel manipulation of host signaling pathways consistent with the ability of highly virulent F. tularensis to efficiently evade host immune responses.

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.

Francisella tularensis SchuS4 and SchuS4 lipids inhibit IL-12p40 in primary human dendritic cells by inhibition of IRF1 and IRF8

Induction of innate immunity is essential for host survival of infection. Evasion and inhibition of innate immunity constitute a strategy used by pathogens, such as the highly virulent bacterium Francisella tularensis, to ensure their replication and transmission. The mechanism and bacterial components responsible for this suppression of innate immunity by F. tularensis are not defined. In this article, we demonstrate that lipids enriched from virulent F. tularensis strain SchuS4, but not attenuated live vaccine strain, inhibit inflammatory responses in vitro and in vivo. Suppression of inflammatory responses is associated with IκBα-independent inhibition of NF-κBp65 activation and selective inhibition of activation of IFN regulatory factors. Interference with NF-κBp65 and IFN regulatory factors is also observed following infection with viable SchuS4. Together these data provide novel insight into how highly virulent bacteria selectively modulate the host to interfere with innate immune responses required for survival of infection.

Francisella tularensis subsp. tularensis induces a unique pulmonary inflammatory response: role of bacterial gene expression in temporal regulation of host defense responses

Pulmonary exposure to Francisella tularensis is associated with severe lung pathology and a high mortality rate. The lack of induction of classical inflammatory mediators, including IL1-β and TNF-α, during early infection has led to the suggestion that F. tularensis evades detection by host innate immune surveillance and/or actively suppresses inflammation. To gain more insight into the host response to Francisella infection during the acute stage, transcriptomic analysis was performed on lung tissue from mice exposed to virulent (Francisella tularensis ssp tularensis SchuS4). Despite an extensive transcriptional response in the lungs of animals as early as 4 hrs post-exposure, Francisella tularensis was associated with an almost complete lack of induction of immune-related genes during the initial 24 hrs post-exposure. This broad subversion of innate immune responses was particularly evident when compared to the pulmonary inflammatory response induced by other lethal (Yersinia pestis) and non-lethal (Legionella pneumophila, Pseudomonas aeruginosa) pulmonary infections. However, the unique induction of a subset of inflammation-related genes suggests a role for dysregulation of lymphocyte function and anti-inflammatory pathways in the extreme virulence of Francisella. Subsequent activation of a classical inflammatory response 48 hrs post-exposure was associated with altered abundance of Francisella-specific transcripts, including those associated with bacterial surface components. In summary, virulent Francisella induces a unique pulmonary inflammatory response characterized by temporal regulation of innate immune pathways correlating with altered bacterial gene expression patterns. This study represents the first simultaneous measurement of both host and Francisella transcriptome changes that occur during in vivo infection and identifies potential bacterial virulence factors responsible for regulation of host inflammatory pathways.

B1a cells enhance susceptibility to infection with virulent Francisella tularensis via modulation of NK/NKT cell responses

B1a cells are an important source of natural Abs, Abs directed against T-independent Ags, and are a primary source of IL-10. Bruton's tyrosine kinase (btk) is a cytoplasmic kinase that is essential for mediating signals from the BCR and is critical for development of B1a cells. Consequentially, animals lacking btk have few B1a cells, minimal Ab responses, and can preferentially generate Th1-type immune responses following infection. B1a cells have been shown to aid in protection against infection with attenuated Francisella tularensis, but their role in infection mediated by fully virulent F. tularensis is not known. Therefore, we used mice with defective btk (CBA/CaHN-Btk(XID)/J [XID mice]) to determine the contribution of B1a cells in defense against the virulent F. tularensis ssp. tularensis strain SchuS4. Surprisingly, XID mice displayed increased resistance to pulmonary infection with F. tularensis. Specifically, XID mice had enhanced clearance of bacteria from the lung and spleen and significantly greater survival of infection compared with wild-type controls. We revealed that resistance to infection in XID mice was associated with decreased numbers of IL-10-producing B1a cells and concomitant increased numbers of IL-12-producing macrophages and IFN-γ-producing NK/NKT cells. Adoptive transfer of wild-type B1a cells into XID mice reversed the control of bacterial replication. Similarly, depletion of NK/NKT cells also increased bacterial burdens in XID mice. Together, our data suggest B cell-NK/NKT cell cross-talk is a critical pivot controlling survival of infection with virulent F. tularensis.

Proteomic analysis of bronchoalveolar lavage fluid proteins from mice infected with Francisella tularensis ssp. novicida

Francisella tularensis causes the zoonosis tularemia in humans and is one of the most virulent bacterial pathogens. We utilized a global proteomic approach to characterize protein changes in bronchoalveolar lavage fluid from mice exposed to one of three organisms, F. tularensis ssp. novicida, an avirulent mutant of F. tularensis ssp. novicida (F.t. novicida-ΔmglA), and Pseudomonas aeruginosa. The composition of bronchoalveolar lavage fluid (BALF) proteins was altered following infection, including proteins involved in neutrophil activation, oxidative stress, and inflammatory responses. Components of the innate immune response were induced including the acute phase response and the complement system; however, the timing of their induction varied. F. tularensis ssp. novicida infected mice do not appear to have an effective innate immune response in the first hours of infection; however, within 24 h, they show an upregulation of innate immune response proteins. This delayed response is in contrast to P. aeruginosa infected animals which show an early innate immune response. Likewise, F.t. novicida-ΔmglA infection initiates an early innate immune response; however, this response is diminished by 24 h. Finally, this study identifies several candidate biomarkers, including Chitinase 3-like-1 (CHI3L1 or YKL-40) and peroxiredoxin 1, that are associated with F. tularensis ssp. novicida but not P. aeruginosa infection.

Low dose vaccination with attenuated Francisella tularensis strain SchuS4 mutants protects against tularemia independent of the route of vaccination

Tularemia, caused by the Gram-negative bacterium Francisella tularensis, is a severe, sometimes fatal disease. Interest in tularemia has increased over the last decade due to its history as a biological weapon. In particular, development of novel vaccines directed at protecting against pneumonic tularemia has been an important goal. Previous work has demonstrated that, when delivered at very high inoculums, administration of live, highly attenuated strains of virulent F. tularensis can protect against tularemia. However, lower vaccinating inoculums did not offer similar immunity. One concern of using live vaccines is that the host may develop mild tularemia in response to infection and use of high inoculums may contribute to this issue. Thus, generation of a live vaccine that can efficiently protect against tularemia when delivered in low numbers, e.g. <100 organisms, may address this concern. Herein we describe the ability of three defined, attenuated mutants of F. tularensis SchuS4, deleted for FTT0369c, FTT1676, or FTT0369c and FTT1676, respectively, to engender protective immunity against tularemia when delivered at concentrations of approximately 50 or fewer bacteria. Attenuated strains for use as vaccines were selected by their inability to efficiently replicate in macrophages in vitro and impaired replication and dissemination in vivo. Although all strains were defective for replication in vitro within macrophages, protective efficacy of each attenuated mutant was correlated with their ability to modestly replicate and disseminate in the host. Finally, we demonstrate the parenteral vaccination with these strains offered superior protection against pneumonic tularemia than intranasal vaccination. Together our data provides proof of principle that low dose attenuated vaccines may be a viable goal in development of novel vaccines directed against tularemia.

Cd14 SNPs regulate the innate immune response

CD14 is a monocytic differentiation antigen that regulates innate immune responses to pathogens. Here, we show that murine Cd14 SNPs regulate the length of Cd14 mRNA and CD14 protein translation efficiency, and consequently the basal level of soluble CD14 (sCD14) and type I IFN production by murine macrophages. This has substantial downstream consequences for the innate immune response; the level of expression of at least 40 IFN-responsive murine genes was altered by this mechanism. We also observed that there was substantial variation in the length of human CD14 mRNAs and in their translation efficiency. sCD14 increased cytokine production by human dendritic cells (DCs), and sCD14-primed DCs augmented human CD4T cell proliferation. These findings may provide a mechanism for exploring the complex relationship between CD14 SNPs, serum sCD14 levels, and susceptibility to human infectious and allergic diseases.

Generation of a convalescent model of virulent Francisella tularensis infection for assessment of host requirements for survival of tularemia

Francisella tularensis is a facultative intracellular bacterium and the causative agent of tularemia. Development of novel vaccines and therapeutics for tularemia has been hampered by the lack of understanding of which immune components are required to survive infection. Defining these requirements for protection against virulent F. tularensis, such as strain SchuS4, has been difficult since experimentally infected animals typically die within 5 days after exposure to as few as 10 bacteria. Such a short mean time to death typically precludes development, and therefore assessment, of immune responses directed against virulent F. tularensis. To enable identification of the components of the immune system that are required for survival of virulent F. tularensis, we developed a convalescent model of tularemia in C57Bl/6 mice using low dose antibiotic therapy in which the host immune response is ultimately responsible for clearance of the bacterium. Using this model we demonstrate αβTCR⁺ cells, γδTCR⁺ cells, and B cells are necessary to survive primary SchuS4 infection. Analysis of mice deficient in specific soluble mediators shows that IL-12p40 and IL-12p35 are essential for survival of SchuS4 infection. We also show that IFN-γ is required for survival of SchuS4 infection since mice lacking IFN-γR succumb to disease during the course of antibiotic therapy. Finally, we found that both CD4⁺ and CD8⁺ cells are the primary producers of IFN-γand that γδTCR⁺ cells and NK cells make a minimal contribution toward production of this cytokine throughout infection. Together these data provide a novel model that identifies key cells and cytokines required for survival or exacerbation of infection with virulent F. tularensis and provides evidence that this model will be a useful tool for better understanding the dynamics of tularemia infection.

Francisella infection triggers activation of the AIM2 inflammasome in murine dendritic cells

The intracellular bacterium Francisella tularensis is the causative agent of tularemia, a potentially fatal disease. In macrophages, Francisella escapes the initial phagosome and replicates in the cytosol, where it is detected by the cytosolic DNA sensor AIM2 leading to activation of the AIM2 inflammasome. However, during aerosol infection, Francisella is also taken up by dendritic cells. In this study, we show that Francisella novicida escapes into the cytosol of bone marrow-derived dendritic cells (BMDC) where it undergoes rapid replication. We show that F. novicida activates the AIM2 inflammasome in BMDC, causing release of large amounts of IL-1β and rapid host cell death. The Francisella Pathogenicity Island is required for bacterial escape and replication and for inflammasome activation in dendritic cells. In addition, we show that bacterial DNA is bound by AIM2, which leads to inflammasome assembly in infected dendritic cells. IFN-β is upregulated in BMDC following Francisella infection, and the IFN-β signalling pathway is partially required for inflammasome activation in this cell type. Taken together, our results demonstrate that F. novicida induces inflammasome activation in dendritic cells. The resulting inflammatory cell death may be beneficial to remove the bacterial replicative niche and protect the host.

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.

Immunity to Francisella

In recent years, studies on the intracellular pathogen Francisella tularensis have greatly intensified, generating a wealth of new information on the interaction of this organism with the immune system. Here we review the basic elements of the innate and adaptive immune responses that contribute to protective immunity against Francisella species, with special emphasis on new data that has emerged in the last 5 years. Most studies have utilized the mouse model of infection, although there has been an expansion of work on human cells and other new animal models. In mice, basic immune parameters that operate in defense against other intracellular pathogen infections, such as interferon gamma, TNF-α, and reactive nitrogen intermediates, are central for control of Francisella infection. However, new important immune mediators have been revealed, including IL-17A, Toll-like receptor 2, and the inflammasome. Further, a variety of cell types in addition to macrophages are now recognized to support Francisella growth, including epithelial cells and dendritic cells. CD4⁺ and CD8⁺ T cells are clearly important for control of primary infection and vaccine-induced protection, but new T cell subpopulations and the mechanisms employed by T cells are only beginning to be defined. A significant role for B cells and specific antibodies has been established, although their contribution varies greatly between bacterial strains of lower and higher virulence. Overall, recent data profile a pathogen that is adept at subverting host immune responses, but susceptible to many elements of the immune system's antimicrobial arsenal.

The subversion of the immune system by francisella tularensis

Francisella tularensis is a highly virulent bacterial pathogen and the causative agent of tularemia. Perhaps the most impressive feature of this bacterium is its ability to cause lethal disease following inoculation of as few as 15 organisms. This remarkable virulence is, in part, attributed to the ability of this microorganism to evade, disrupt, and modulate host immune responses. The objective of this review is to discuss the mechanisms utilized by F. tularensis to evade and inhibit innate and adaptive immune responses. The capability of F. tularensis to interfere with developing immunity in the host was appreciated decades ago. Early studies in humans were the first to demonstrate the ability of F. tularensis to suppress innate immunity. This work noted that humans suffering from tularemia failed to respond to a secondary challenge of endotoxin isolated from unrelated bacteria. Further, anecdotal observations of individuals becoming repeatedly infected with virulent strains of F. tularensis suggests that this bacterium also interferes with the generation of adequate adaptive immunity. Recent advances utilizing the mouse model for in vivo studies and human cells for in vitro work have identified specific bacterial and host compounds that play a role in mediating ubiquitous suppression of the host immune response. Compilation of this work will undoubtedly aid in enhancing our understanding of the myriad of mechanisms utilized by virulent F. tularensis for successful infection, colonization, and pathogenesis in the mammalian host.

Role of neutrophils and NADPH phagocyte oxidase in host defense against respiratory infection with virulent Francisella tularensis in mice

Francisella tularensis subspecies (subsp.) tularensis is a CDC Category A biological warfare agent and inhalation of as few as 15 bacilli can initiate severe disease. Relatively little is known about the cellular and molecular mechanisms of host defense against respiratory infection with subsp. tularensis. In this study, we examined the role of neutrophils and NADPH phagocyte oxidase in host resistance to pulmonary infection in a mouse intranasal infection model. We found that despite neutrophil recruitment to the lungs and increased concentrations of neutrophil-chemotactic chemokines (KC, MIP-2 and RANTES) in the bronchoalveolar lavage fluid following intranasal inoculation of the pathogen, neither depletion of neutrophils nor enhancement of their recruitment into the lungs had any impact on bacterial burdens or survival rate/time. Nevertheless, mice deficient in NADPH phagocyte oxidase (gp91(phox⁻/⁻)) did exhibit higher tissue and blood bacterial burdens and succumbed to infection one day earlier than wild-type C57BL/6 mice. These results imply that although neutrophils are not a major effector cell in defense against subsp. tularensis infection, NADPH phagocyte oxidase does play a marginal role.

Long lived protection against pneumonic tularemia is correlated with cellular immunity in peripheral, not pulmonary, organs

Protection against the intracellular bacterium Francisella tularensis within weeks of vaccination is thought to involve both cellular and humoral immune responses. However, the relative roles for cellular and humoral immunity in long lived protection against virulent F. tularensis are not well established. Here, we dissected the correlates of immunity to pulmonary infection with virulent F. tularensis strain SchuS4 in mice challenged 30 and 90 days after subcutaneous vaccination with LVS. Regardless of the time of challenge, LVS vaccination protected approximately 90% of SchuS4 infected animals. Surprisingly, control of bacterial replication in the lung during the first 7 days of infection was not required for survival of SchuS4 infection in vaccinated mice. Control and survival of virulent F. tularensis strain SchuS4 infection within 30 days of vaccination was associated with high titers of SchuS4 agglutinating antibodies, and IFN-γ production by multiple cell types in both the lung and spleen. In contrast, survival of SchuS4 infection 90 days after vaccination was correlated only with IFN-γ producing splenocytes and activated T cells in the spleen. Together these data demonstrate that functional agglutinating antibodies and strong mucosal immunity are correlated with early control of pulmonary infections with virulent F. tularensis. However, early mucosal immunity may not be required to survive F. tularensis infection. Instead, survival of SchuS4 infection at extended time points after immunization was only associated with production of IFN-γ and activation of T cells in peripheral organs.

Effective, broad spectrum control of virulent bacterial infections using cationic DNA liposome complexes combined with bacterial antigens

Protection against virulent pathogens that cause acute, fatal disease is often hampered by development of microbial resistance to traditional chemotherapeutics. Further, most successful pathogens possess an array of immune evasion strategies to avoid detection and elimination by the host. Development of novel, immunomodulatory prophylaxes that target the host immune system, rather than the invading microbe, could serve as effective alternatives to traditional chemotherapies. Here we describe the development and mechanism of a novel pan-anti-bacterial prophylaxis. Using cationic liposome non-coding DNA complexes (CLDC) mixed with crude F. tularensis membrane protein fractions (MPF), we demonstrate control of virulent F. tularensis infection in vitro and in vivo. CLDC+MPF inhibited bacterial replication in primary human and murine macrophages in vitro. Control of infection in macrophages was mediated by both reactive nitrogen species (RNS) and reactive oxygen species (ROS) in mouse cells, and ROS in human cells. Importantly, mice treated with CLDC+MPF 3 days prior to challenge survived lethal intranasal infection with virulent F. tularensis. Similarly to in vitro observations, in vivo protection was dependent on the presence of RNS and ROS. Lastly, CLDC+MPF was also effective at controlling infections with Yersinia pestis, Burkholderia pseudomallei and Brucella abortus. Thus, CLDC+MPF represents a novel prophylaxis to protect against multiple, highly virulent pathogens.

Conventional treatment of bacterial infections typically includes administration of antibiotics. However, many pathogens have developed spontaneous resistance to commonly used antibiotics. Development of new compounds that stimulate the host immune system to directly kill bacteria by mechanisms different from those utilized by antibiotics may serve as effective alternatives to antibiotic therapy. In this report, we describe a novel compound capable of controlling infections mediated by different, unrelated bacteria via the induction of host derived reactive oxygen and reactive nitrogen species. This compound is comprised of cationic liposome DNA complexes (CLDC) and crude membrane preparations (MPF) obtained from attenuated Francisella tularensis Live Vaccine Strain (LVS). Pretreatment of primary mouse or human cells limited replication of virulent F. tularensis, Burkholderia pseudomallei, Yersinia pestis and Brucella abortus in vitro. CLDC+MPF was also effective for controlling lethal pulmonary infections with virulent F. tularensis. Thus, CLDC+MPF represents a novel antimicrobial for treatment of lethal, acute, bacterial infections.

MiR-155 induction by F. novicida but not the virulent F. tularensis results in SHIP down-regulation and enhanced pro-inflammatory cytokine response

The intracellular gram-negative bacterium Francisella tularensis causes the disease tularemia and is known for its ability to subvert host immune responses. Previous work from our laboratory identified the PI3K/Akt pathway and SHIP as critical modulators of host resistance to Francisella. Here, we show that SHIP expression is strongly down-regulated in monocytes and macrophages following infection with F. tularensis novicida (F.n.). To account for this negative regulation we explored the possibility that microRNAs (miRs) that target SHIP may be induced during infection. There is one miR that is predicted to target SHIP, miR-155. We tested for induction and found that F.n. induced miR-155 both in primary monocytes/macrophages and in vivo. Using luciferase reporter assays we confirmed that miR-155 led to down-regulation of SHIP, showing that it specifically targets the SHIP 3'UTR. Further experiments showed that miR-155 and BIC, the gene that encodes miR-155, were induced as early as four hours post-infection in primary human monocytes. This expression was dependent on TLR2/MyD88 and did not require inflammasome activation. Importantly, miR-155 positively regulated pro-inflammatory cytokine release in human monocytes infected with Francisella. In sharp contrast, we found that the highly virulent type A SCHU S4 strain of Francisella tularensis (F.t.) led to a significantly lower miR-155 response than the less virulent F.n. Hence, F.n. induces miR-155 expression and leads to down-regulation of SHIP, resulting in enhanced pro-inflammatory responses. However, impaired miR-155 induction by SCHU S4 may help explain the lack of both SHIP down-regulation and pro-inflammatory response and may account for the virulence of Type A Francisella.