PATHOGENESIS OF EXPERIMENTAL RESPIRATORY TULAREMIA IN MONKEYS

Ex vivo infection model for Francisella using human lung tissue

Introduction

Tularemia is mainly caused by Francisella tularensis (Ft) subsp. tularensis (Ftt) and Ft subsp. holarctica (Ftt) in humans and in more than 200 animal species including rabbits and hares. Human clinical manifestations depend on the route of infection and range from flu-like symptoms to severe pneumonia with a mortality rate up to 60% without treatment. So far, only 2D cell culture and animal models are used to study Francisella virulence, but the gained results are transferable to human infections only to a certain extent.

Method

In this study, we firstly established an ex vivo human lung tissue infection model using different Francisella strains: Ftt Life Vaccine Strain (LVS), Ftt LVS ΔiglC, Ftt human clinical isolate A-660 and a German environmental Francisella species strain W12-1067 (F-W12). Human lung tissue was used to determine the colony forming units and to detect infected cell types by using spectral immunofluorescence and electron microscopy. Chemokine and cytokine levels were measured in culture supernatants.

Results

Only LVS and A-660 were able to grow within the human lung explants, whereas LVS ΔiglC and F-W12 did not replicate. Using human lung tissue, we observed a greater increase of bacterial load per explant for patient isolate A-660 compared to LVS, whereas a similar replication of both strains was observed in cell culture models with human macrophages. Alveolar macrophages were mainly infected in human lung tissue, but Ftt was also sporadically detected within white blood cells. Although Ftt replicated within lung tissue, an overall low induction of pro-inflammatory cytokines and chemokines was observed. A-660-infected lung explants secreted slightly less of IL-1β, MCP-1, IP-10 and IL-6 compared to Ftt LVS-infected explants, suggesting a more repressed immune response for patient isolate A-660. When LVS and A-660 were used for simultaneous co-infections, only the ex vivo model reflected the less virulent phenotype of LVS, as it was outcompeted by A-660.

Conclusion

We successfully implemented an ex vivo infection model using human lung tissue for Francisella. The model delivers considerable advantages and is able to discriminate virulent Francisella from less- or non-virulent strains and can be used to investigate the role of specific virulence factors.

Identification of an Attenuated Substrain of Francisella tularensis SCHU S4 by Phenotypic and Genotypic Analyses

Pneumonic tularemia is a highly debilitating and potentially fatal disease caused by inhalation of Francisella tularensis. Most of our current understanding of its pathogenesis is based on the highly virulent F. tularensis subsp. tularensis strain SCHU S4. However, multiple sources of SCHU S4 have been maintained and propagated independently over the years, potentially generating genetic variants with altered virulence. In this study, the virulence of four SCHU S4 stocks (NR-10492, NR-28534, NR-643 from BEI Resources and FTS-635 from Battelle Memorial Institute) along with another virulent subsp. tularensis strain, MA00-2987, were assessed in parallel. In the Fischer 344 rat model of pneumonic tularemia, NR-643 and FTS-635 were found to be highly attenuated compared to NR-10492, NR-28534, and MA00-2987. In the NZW rabbit model of pneumonic tularemia, NR-643 caused morbidity but not mortality even at a dose equivalent to 500x the LD₅₀ for NR-10492. Genetic analyses revealed that NR-10492 and NR-28534 were identical to each other, and nearly identical to the reference SCHU S4 sequence. NR-643 and FTS-635 were identical to each other but were found to have nine regions of difference in the genomic sequence when compared to the published reference SCHU S4 sequence. Given the genetic differences and decreased virulence, NR-643/FTS-635 should be clearly designated as a separate SCHU S4 substrain and no longer utilized in efficacy studies to evaluate potential vaccines and therapeutics against tularemia.

The D-alanyl-d-alanine carboxypeptidase enzyme is essential for virulence in the Schu S4 strain of Francisella tularensis and a dacD mutant is able to provide protection against a pneumonic challenge

Low molecular mass penicillin binding proteins (LMM PBP) are bacterial enzymes involved in the final steps of peptidoglycan biosynthesis. In Escherichia coli, most LMM PBP exhibit dd-carboxypeptidase activity, are not essential for growth in routine laboratory media, and contributions to virulent phenotypes remain largely unknown. The Francisella tularensis Schu S4 genome harbors the dacD gene (FTT_1029), which encodes a LMM PBP with homology to PBP6b of E. coli. Disruption of this locus in the fully virulent Schu S4 strain resulted in a mutant that could not grow in Chamberlain's Defined Medium and exhibited severe morphological defects. Further characterization studies demonstrated that the growth defects of the dacD mutant were pH-dependent, and could be partially restored by growth at neutral pH or fully restored by genetic complementation. Infection of murine macrophage-like cells showed that the Schu S4 dacD mutant is capable of intracellular replication. However, this mutant was attenuated in BALB/c mice following intranasal challenge (LD₅₀ = 603 CFU) as compared to mice challenged with the parent (LD₅₀ = 1 CFU) or complemented strain (LD₅₀ = 1 CFU). Additionally, mice that survived infection with the dacD mutant showed significant protection against subsequent challenge with the parent strain. Collectively, these results indicate that the DacD protein of F. tularensis is essential for growth in low pH environments and virulence in vivo. These results also suggest that a PBP mutant could serve as the basis of a novel, live attenuated vaccine strain.

Protective effects of the Francisella tularensis ΔpdpC mutant against its virulent parental strain SCHU P9 in Cynomolgus macaques

Tularemia is a severe infectious zoonotic disease caused by Francisella tularensis. Although F. tularensis is considered to be a potential biological weapon due to its high infectivity and mortality rate, no vaccine has been currently licensed. Recently, we reported that F. tularensis SCHU P9 derived ΔpdpC strain lacking the pathogenicity determinant protein C gene conferred stable and good protection in a mouse lethal model. In this study, the protective effect of ΔpdpC was evaluated using a monkey lethal model. Two cynomolgus macaques (Macaca fascicularis) intratracheally challenged with the virulent strain SCHU P9 were euthanized on 7 and 11 days post-challenge after the development of severe clinical signs. The bacterial replication in alveolar macrophages and type II epithelial cells in the lungs would cause severe pneumonia accompanied by necrosis. Conversely, two animals subcutaneously immunized with ΔpdpC survived 3 weeks after SCHU P9 challenge. Though one of the two animals developed mild symptoms of tularemia, bacterial replication was limited in the respiratory organs, which may be due to a high level of humoral and cellular immune responses against F. tularensis. These results suggest that the ΔpdpC mutant would be a safe and promising candidate as a live attenuated tularemia vaccine.

A Novel Stochastic Multi-Scale Model of Francisella tularensis Infection to Predict Risk of Infection in a Laboratory

We present a multi-scale model of the within-phagocyte, within-host and population-level infection dynamics of Francisella tularensis, which extends the mechanistic one proposed by Wood et al. (2014). Our multi-scale model incorporates key aspects of the interaction between host phagocytes and extracellular bacteria, accounts for inter-phagocyte variability in the number of bacteria released upon phagocyte rupture, and allows one to compute the probability of response, and mean time until response, of an infected individual as a function of the initial infection dose. A Bayesian approach is applied to parameterize both the within-phagocyte and within-host models using infection data. Finally, we show how dose response probabilities at the individual level can be used to estimate the airborne propagation of Francisella tularensis in indoor settings (such as a microbiology laboratory) at the population level, by means of a deterministic zonal ventilation model.

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

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

A human macrophage-hepatocyte co-culture model for comparative studies of infection and replication of Francisella tularensis LVS strain and subspecies holarctica and mediasiatica

BACKGROUND

Francisella tularensis, a gram-negative bacterium replicates intracellularly within macrophages and efficiently evades the innate immune response. It is able to infect and replicate within Kupffer cells, specialized tissue macrophages of the liver, and to modulate the immune response upon infection to its own advantage. Studies on Francisella tularensis liver infection were mostly performed in animal models and difficult to extrapolate to the human situation, since human infections and clinical observations are rare.

RESULTS

Using a human co-culture model of macrophages and hepatocytes we investigated the course of infection of three Francisella tularensis strains (subspecies holarctica--wildtype and live vaccine strain, and mediasiatica--wildtype) and analyzed the immune response triggered upon infection. We observed that hepatocytes support the intracellular replication of Franciscella species in macrophages accompanied by a specific immune response inducing TNFα, IL-1β, IL-6 and fractalkine (CX3CL1) secretion and the induction of apoptosis.

CONCLUSIONS

We could demonstrate that this human macrophage/hepatocyte co-culture model reflects strain-specific virulence of Francisella tularensis. We developed a suitable tool for more detailed in vitro studies on the immune response upon liver cell infection by F. tularensis.

A dose and time response Markov model for the in-host dynamics of infection with intracellular bacteria following inhalation: with application to Francisella tularensis

In a novel approach, the standard birth–death process is extended to incorporate a fundamental mechanism undergone by intracellular bacteria, phagocytosis. The model accounts for stochastic interaction between bacteria and cells of the immune system and heterogeneity in susceptibility to infection of individual hosts within a population. Model output is the dose–response relation and the dose-dependent distribution of time until response, where response is the onset of symptoms. The model is thereafter parametrized with respect to the highly virulent Schu S4 strain of Francisella tularensis, in the first such study to consider a biologically plausible mathematical model for early human infection with this bacterium. Results indicate a median infectious dose of about 23 organisms, which is higher than previously thought, and an average incubation period of between 3 and 7 days depending on dose. The distribution of incubation periods is right-skewed up to about 100 organisms and symmetric for larger doses. Moreover, there are some interesting parallels to the hypotheses of some of the classical dose–response models, such as independent action (single-hit model) and individual effective dose (probit model). The findings of this study support experimental evidence and postulations from other investigations that response is, in fact, influenced by both in-host and between-host variability.

The in vivo extracellular life of facultative intracellular bacterial parasites: role in pathogenesis

Classically labeled facultative intracellular pathogens are characterized by the ability to have an intracellular phase in the host, which is required for pathogenicity, while capable of extracellular growth in vitro. The ability of these bacteria to replicate in cell-free conditions is usually assessed by culture in artificial bacteriological media. However, the extracellular growth ability of these pathogens may also be expressed by a phase of extracellular infection in the natural setting of the host with pathologic consequences, an ability that adds to the pathogenic potential of the infectious agent. This infective capability to grow in the extracellular sites of the host represents an additional virulence attribute of those pathogens which may lead to severe outcomes. Here we discuss examples of infectious diseases where the in vivo infective extracellular life is well documented, including infections by Francisella tularensis, Yersinia pestis, Burkholderia pseudomallei, Burkholderia cenocepacia, Salmonella enterica serovar Typhimurium and Edwardsiella tarda. The occurrence of a phase of systemic dissemination with extracellular multiplication during progressive infections by facultative intracellular bacterial pathogens has been underappreciated, with most studies exclusively centered on the intracellular phase of the infections. The investigation of the occurrence of a dual lifestyle in the host among bacterial pathogens in general should be extended and likely will reveal more cases of infectious diseases with a dual infective intracellular/extracellular pattern.

O-antigen-deficient Francisella tularensis Live Vaccine Strain mutants are ingested via an aberrant form of looping phagocytosis and show altered kinetics of intracellular trafficking in human macrophages

We examined the uptake and intracellular trafficking of F. tularensis Live Vaccine Strain (LVS) and LVS with disruptions of wbtDEF and wbtI genes essential for synthesis of the O antigen of lipopolysaccharide. Unlike parental bacteria, O-antigen-deficient LVS is efficiently killed by serum with intact complement but not by serum lacking terminal complement components. Opsonization of O-antigen-deficient LVS in serum lacking terminal complement components allows efficient uptake of these live bacteria by macrophages. In the presence of complement, whereas parental F. tularensis LVS is internalized within spacious pseudopod loops, mutant LVS is internalized within tightly juxtaposed multiple onion-like layers of pseudopodia. Without complement, both parental and mutant LVSs are internalized within spacious pseudopod loops. Thus, molecules other than O antigen are important in triggering dramatic pseudopod extensions and uptake by spacious pseudopod loops. Following uptake, both parental and mutant LVSs enter compartments that show limited staining for the lysosomal membrane glycoprotein CD63 and little fusion with secondary lysosomes. Subsequently, both parental and mutant LVSs lose their CD63 staining. Whereas the majority of parental LVS escapes into the cytosol by 6 h after uptake, mutant LVS shows a marked lag but does escape by 1 day after uptake. Despite the altered kinetics of phagosome escape, both mutant and parental strains grow to high levels within human macrophages. Thus, the O antigen plays a role in the morphology of uptake in the presence of complement and the kinetics of intracellular growth but is not essential for escape, survival, altered membrane trafficking, or intramacrophage growth.

Innate immune recognition of francisella tularensis: activation of type-I interferons and the inflammasome

Francisella tularensis is an intracellular pathogen that can cause severe disease in a wide range of mammalian hosts. Primarily residing in host macrophages, F. tularensis escapes phagosomal degradation, and replicates in the macrophage cytosol. The macrophage uses a series of pattern recognition receptors to detect conserved microbial molecules from invading pathogens, and initiates an appropriate host response. In the cytosol, F. tularensis is recognized by the inflammasome, a multiprotein complex responsible for the activation of the cysteine protease caspase-1. Caspase-1 activation leads to processing and release of proinflammatory cytokines and host cell death. Here we review recent work on the molecular mechanisms of inflammasome activation by F. tularensis, and its consequences both in vitro and in vivo. Finally, we discuss the coordination between the inflammasome and other cytosolic host responses, and the evidence for F. tularensis virulence factors that suppress inflammasome activation.

Quantification of the relationship between bacterial kinetics and host response for monkeys exposed to aerosolized Francisella tularensis

Francisella tularensis can be disseminated via aerosols, and once inhaled, only a few microorganisms may result in tularemia pneumonia. Effective responses to this threat depend on a thorough understanding of the disease development and pathogenesis. In this study, a class of time-dose-response models was expanded to describe quantitatively the relationship between the temporal probability distribution of the host response and the in vivo bacterial kinetics. An extensive literature search was conducted to locate both the dose-dependent survival data and the in vivo bacterial count data of monkeys exposed to aerosolized F. tularensis. One study reporting responses of monkeys to four different sizes of aerosol particles (2.1, 7.5, 12.5, and 24.0 μm) of the SCHU S4 strain and three studies involving five in vivo growth curves of various strains (SCHU S4, 425, and live vaccine strains) initially delivered to hosts in aerosol form (1 to 5 μm) were found. The candidate models exhibited statistically acceptable fits to the time- and dose-dependent host response and provided estimates for the bacterial growth distribution. The variation pattern of such estimates with aerosol size was found to be consistent with the reported pathophysiological and clinical observations. The predicted growth curve for 2.1-μm aerosolized bacteria was highly consistent with the available bacterial count data. This is the first instance in which the relationship between the in vivo growth of F. tularensis and the host response can be quantified by mechanistic mathematical models.

Environmental and intracellular regulation of Francisella tularensis ripA

Background

Francisella tularensis is a highly virulent, facultative intracellular pathogen and the etiologic agent of the zoonotic disease Tularemia. RipA is a cytoplasmic membrane protein that is conserved among Francisella species and is required for intracellular growth. F. tularensis ripA deletion mutants escape the phagosome of infected cells, but unlike wild type organisms fail to replicate in the host cell cytoplasm.

Results

Further analysis of ripA with respect to environmental effects on the growth of mutant strains and expression levels revealed that RipA is required for optimal growth at pH 7.5 but not pH 6.5. Using a combination of RT-PCR, ripA-lacZ transcriptional and translational fusions, and a RipA-tetracysteine tag fusion protein we found that both ripA transcription and RipA protein levels were elevated in organisms grown at pH 7.5 as compared to organisms grown at pH 5.5. A number of genes, including iglA, that are required for intracellular growth are regulated by the transcriptional regulators MglA and SspA, and are induced upon infection of host cells. We quantified ripA and iglA expression at different stages of intracellular growth and found that the expression of each increased between 1 and 6 hours post infection. Given the similar intracellular expression patterns of ripA and iglA and that MglA and SspA are positive regulators of iglA we tested the impact of mglA and sspA deletions on ripA and iglA expression. In the deletion mutant strains iglA expression was reduced dramatically as expected, however ripA expression was increased over 2-fold.

Conclusion

Expression of ripA is required for growth at neutral pH, is pH sensitive, and is responsive to the intracellular environment. The intracellular expression pattern of ripA coincided with iglA, which is positively regulated by MglA and SspA. However, in contrast to their positive impact on iglA expression, MglA and SspA negatively impacted ripA expression in vitro.

Programmed cell death and the pathogenesis of tissue injury induced by type A Francisella tularensis

Francisella tularensis is a highly virulent bacterial species that causes various forms of tularemia in humans. The urgency in understanding the pathogenesis of these diseases has stimulated unprecedented interest in this bacterial species over the past few years. Recent findings underscore a number of important distinctions between the Francisella ssp. and emphasize the importance of using type A F. tularensis strains when characterizing pathophysiological responses that are relevant to the lethal forms of human disease. This review focuses on the mediators of cell death induction in infected tissues and the implications of these processes on the pathophysiological changes observed in various host species.

Identification of migR, a regulatory element of the Francisella tularensis live vaccine strain iglABCD virulence operon required for normal replication and trafficking in macrophages

Francisella tularensis, the etiological agent of tularemia, is capable of infecting a wide range of animals and causes a severe, lethal disease in humans. The pathogen evades killing by cells of the innate immune system utilizing genes encoding a pathogenicity island, including iglABCD, and instead utilizes these cells as a niche for replication and dissemination to other organs within the host. Regulators of the igl genes (e.g., MglA, SspA, FevR and PmrA) have been identified, but environmental stimuli and mechanisms of regulation are as yet unknown and are likely to involve additional gene products. In this work, we more closely examine the roles that environmental iron and the ferric uptake repressor protein (Fur) play in the regulation of the iglABCD operon. We also used a genetic approach to identify and characterize a new regulator of the igl operon, designated migR (macrophage intracellular growth regulator; FTL_1542). Quantitative real-time reverse transcription-PCR in a site-directed migR mutant confirmed the reduction in the number of iglC transcripts in this strain and also demonstrated reduced expression of fevR. Comparison of the migR and fevR mutants in monocyte-derived macrophages (MDMs) and epithelial cell lines revealed a reduced ability for each mutant to grow in MDMs, yet only the fevR mutant exhibited impaired replication in epithelial cell lines. Confocal analysis of infected MDMs revealed that although neither mutant reached the MDM cytosol, the fevR mutant was trapped in lamp-1-positive phagosomes, whereas the migR mutant resided in mature phagolysosomes enriched with both lamp-1 and cathepsin D. Disruption of migR and fevR also impaired the ability of F. tularensis to prevent neutrophil oxidant production. Thus, we have identified migR, a gene that regulates expression of the iglABCD operon and is essential for bacterial growth in MDMs and also contributes to the blockade of neutrophil NADPH oxidase activity.

Pathology of inhalational Francisella tularensis spp. tularensis SCHU S4 infection in African green monkeys (Chlorocebus aethiops)

Tularemia, caused by Francisella tularensis, is a sporadic zoonotic disease with the potential to be an agent of biowarfare or bioterrorism. We describe here the gross, histologic, immunohistochemical, and ultrastructural findings in a group of 5 African green monkeys (AGMs) that received an average inhaled dose of 729 colony-forming units of F. tularensis and died or were euthanatized between days 7 and 11 post infection. Clinical changes were evident by 48 hours post infection, and key physiologic abnormalities included increases in body temperature, heart rate, peak cardiac pressure, and mean blood pressure. Prominent gross changes in all cases included numerous pinpoint to 1-cm, well-demarcated, necrotic foci present consistently in the lungs, mediastinal lymph nodes, and spleen but also seen in the heart, mediastinum, diaphragm, liver, urinary bladder, urethra, and mesentery. The lungs, mediastinal lymph nodes, and spleen were most severely affected, with as much as 50% of the tissue replaced by necrotic foci. Histologic changes in all tissues consisted of well-delineated foci of necrosis and neutrophilic and histiocytic inflammation, with varying amounts of hemorrhage, edema, fibrin, and vasculitis. Some lesions were immature pyogranulomas. Strong immunoreactivity was identified primarily within macrophages. Ultrastructurally, bacteria were present within cytoplasmic vacuoles of alveolar macrophages, many of which were degenerate. In summary, AGMs infected with F. tularensis by aerosol develop lethal multisystemic disease that particularly targets the lungs and lymphoid tissues. Thus, AGMs should serve as a suitable and reliable animal model for further studies of tularemia.

Recombinant attenuated Listeria monocytogenes vaccine expressing Francisella tularensis IglC induces protection in mice against aerosolized Type A F. tularensis

Fransicella tularensis, the causative agent of tularemia, is in the top category (Category A) of potential agents of bioterrorism. To develop a safer vaccine against aerosolized F. tularensis, we have employed an attenuated Listeria monocytogenes, which shares with F. tularensis an intracellular and extraphagosomal lifestyle, as a delivery vehicle for F. tularensis antigens. We constructed recombinant L. monocytogenes (rLm) vaccines stably expressing seven F. tularensis proteins including IglC (rLm/iglC), and tested their immunogenicity and protective efficacy against lethal F. tularensis challenge in mice. Mice immunized intradermally with rLm/iglC developed significant cellular immune responses to F. tularensis IglC as evidenced by lymphocyte proliferation and CD4+ and CD8+ T-cell intracellular expression of interferon gamma. Moreover, mice immunized with rLm/iglC were protected against lethal challenge with F. tularensis LVS administered by the intranasal route, a route chosen to mimic airborne infection, and, most importantly, against aerosol challenge with the highly virulent Type A F. tularensis SchuS4 strain.

Genome-wide screen in Francisella novicida for genes required for pulmonary and systemic infection in mice

Francisella tularensis is a gram-negative, highly infectious, aerosolizable facultative intracellular pathogen that causes the potentially life-threatening disease tularemia. To date there is no approved vaccine available, and little is known about the molecular mechanisms important for infection, survival, and dissemination at different times of infection. We report the first whole-genome screen using an inhalation mouse model to monitor infection in the lung and dissemination to the liver and spleen. We queried a comprehensive library of 2,998 sequence-defined transposon insertion mutants in Francisella novicida strain U112 using a microarray-based negative-selection screen. We were able to track the behavior of 1,029 annotated genes, equivalent to a detection rate of 75% and corresponding to approximately 57% of the entire F. novicida genome. As expected, most transposon mutants retained the ability to colonize, but 125 candidate virulence genes (12%) could not be detected in at least one of the three organs. They fell into a variety of functional categories, with one-third having no annotated function and a statistically significant enrichment of genes involved in transcription. Based on the observation that behavior during complex pool infections correlated with the degree of attenuation during single-strain infection we identified nine genes expected to strongly contribute to infection. These included two genes, those for ATP synthase C (FTN_1645) and thioredoxin (FTN_1415), that when mutated allowed increased host survival and conferred protection in vaccination experiments.

Evasion of complement-mediated lysis and complement C3 deposition are regulated by Francisella tularensis lipopolysaccharide O antigen

The bacterium Francisella tularensis (Ft) is a potential weapon of bioterrorism when aerosolized. Macrophage infection is necessary for disease progression and efficient phagocytosis by human macrophages requires serum opsonization by complement. Microbial complement activation leads to surface deposition of a highly regulated protein complex resulting in opsonization or membrane lysis. The nature of complement component C3 deposition, i.e., C3b (opsonization and lysis) or C3bi (opsonization only) fragment deposition, is central to the outcome of activation. In this study, we examine the mechanisms of Ft resistance to complement-mediated lysis, C3 component deposition on the Ft surface, and complement activation. Upon incubation in fresh nonimmune human serum, Schu S4 (Ft subsp. tularensis), Fn (Ft subsp. novicida), and LVS (Ft subsp. holarctica live vaccine strain) were resistant to complement-mediated lysis, but LVSG and LVSR (LVS strains altered in surface carbohydrate structures) were susceptible. C3 deposition, however, occurred on all strains. Complement-susceptible strains had markedly increased C3 fragment deposition, including the persistent presence of C3b compared with C3bi, which indicates that C3b inactivation results in survival of complement-resistant strains. C1q, an essential component of the classical activation pathway, was necessary for lysis of complement-susceptible strains and optimal C3 deposition on all strains. Finally, use of Francisella LPS mutants confirmed O Ag as a major regulator of complement resistance. These data provide evidence that pathogenic Francisella activate complement, but are resistant to complement-mediated lysis in part due to limited C3 deposition, rapid conversion of surface-bound C3b to C3bi, and the presence of LPS O Ag.

RipA, a cytoplasmic membrane protein conserved among Francisella species, is required for intracellular survival

Francisella tularensis is a highly virulent bacterial pathogen that invades and replicates within numerous host cell types, including macrophages and epithelial cells. In an effort to better understand this process, we screened a transposon insertion library of the F. tularensis live vaccine strain (LVS) for mutant strains that invaded but failed to replicate within alveolar epithelial cell lines. One such strain isolated from this screen contained an insertion in the gene FTL_1914, which is conserved among all sequenced Francisella species yet lacks significant homology to any gene with known function. A deletion strain lacking FTL_1914 was constructed. This strain did not replicate in either epithelial or macrophage-like cells, and intracellular replication was restored by the wild-type allele in trans. Based on the deletion mutant phenotype, FTL_1914 was termed ripA (required for intracellular proliferation, factor A). Following uptake by J774.A1 cells, F. tularensis LVS Delta ripA colocalized with LAMP-1 then escaped the phagosome at the same rate and frequency as wild-type LVS-infected cells. Electron micrographs of the F. tularensis LVS Delta ripA mutant demonstrated the reentry of the mutant bacteria into double membrane vacuoles characteristic of autophagosomes in a process that was not dependent on replication. The F. tularensis LVS Delta ripA mutant was significantly impaired in its ability to persist in the lung and in its capacity to disseminate and colonize the liver and spleen in a mouse model of pulmonary tularemia. The RipA protein was expressed during growth in laboratory media and localized to the cytoplasmic membrane. Thus, RipA is a cytoplasmic membrane protein conserved among Francisella species that is required for intracellular replication within the host cell cytoplasm as well as disease progression, dissemination, and virulence.

Characterization of the receptor-ligand pathways important for entry and survival of Francisella tularensis in human macrophages

Inhalational pneumonic tularemia, caused by Francisella tularensis, is lethal in humans. F. tularensis is phagocytosed by macrophages followed by escape from phagosomes into the cytoplasm. Little is known of the phagocytic mechanisms for Francisella, particularly as they relate to the lung and alveolar macrophages. Here we examined receptors on primary human monocytes and macrophages which mediate the phagocytosis and intracellular survival of F. novicida. F. novicida association with monocyte-derived macrophages (MDM) was greater than with monocytes. Bacteria were readily ingested, as shown by electron microscopy. Bacterial association was significantly increased in fresh serum and only partially decreased in heat-inactivated serum. A role for both complement receptor 3 (CR3) and Fcgamma receptors in uptake was supported by studies using a CR3-expressing cell line and by down-modulation of Fcgamma receptors on MDM, respectively. Consistent with Fcgamma receptor involvement, antibody in nonimmune human serum was detected on the surface of Francisella. In the absence of serum opsonins, competitive inhibition of mannose receptor (MR) activity on MDM with mannan decreased the association of F. novicida and opsonization of F. novicida with lung collectin surfactant protein A (SP-A) increased bacterial association and intracellular survival. This study demonstrates that human macrophages phagocytose more Francisella than monocytes with contributions from CR3, Fcgamma receptors, the MR, and SP-A present in lung alveoli.

Mac-1+ cells are the predominant subset in the early hepatic lesions of mice infected with Francisella tularensis

The cell composition of early hepatic lesions of experimental murine tularemia has not been characterized with specific markers. The appearance of multiple granulomatous-necrotic lesions in the liver correlates with a marked increase in the levels of serum alanine transferase and lactate dehydrogenase. Francisella tularensis, detected by specific antibodies, can be first noted by day 1 and becomes associated with the lesions by 5 days postinoculation. These lesions become necrotic, with some evidence of in situ apoptosis. The lesions do not contain B, T, or NK cells. Rather, the lesions are largely composed of two subpopulations of Mac-1(+) cells that are associated with the bacteria. Gr-1(+) Mac-1(+) immature myeloid cells and major histocompatibility complex class II-positive (MHC-II(+)) Mac-1(+) macrophages were the most abundant cell phenotypes found in the granuloma and are likely major contributors in controlling the infection in its early stages. Our findings have shown that there is an early development of hepatic lesions where F. tularensis colocalizes with both Gr-1(+) Mac-1(+) and MHC-II(+) Mac-1(+) cells.

Francisella tularensis enters macrophages via a novel process involving pseudopod loops

Intracellular bacterial pathogens employ a variety of strategies to invade their eukaryotic host cells. From an ultrastructural standpoint, the processes that bacteria employ to invade their host cells include conventional phagocytosis, coiling phagocytosis, and ruffling/triggered macropinocytosis. In this paper, we describe a novel process by which Francisella tularensis, the agent of tularemia, enters host macrophages. F. tularensis is a remarkably infectious facultative intracellular bacterial parasite--as few as 10 bacteria can cause life-threatening disease in humans. However, the ultrastructure of its uptake and the receptor mechanisms that mediate its uptake have not been reported previously. We have used fluorescence microscopy and electron microscopy to examine the adherence and uptake of a virulent recent clinical isolate of F. tularensis, subspecies tularensis, and the live vaccine strain (LVS), subspecies holarctica, by human macrophages. We show here that both strains of F. tularensis enter human macrophages by a novel process of engulfment within asymmetric, spacious pseudopod loops, a process that differs ultrastructurally from all previously described uptake mechanisms. We demonstrate also that adherence and uptake of F. tularensis by macrophages is strongly dependent upon complement receptors and upon serum with intact complement factor C3 and that uptake requires actin microfilaments. These findings have significant implications for understanding the intracellular biology and virulence of this extremely infectious pathogen.

Virulent and avirulent strains of Francisella tularensis prevent acidification and maturation of their phagosomes and escape into the cytoplasm in human macrophages

Francisella tularensis, the agent of tularemia, is an intracellular pathogen, but little is known about the compartment in which it resides in human macrophages. We have examined the interaction of a recent virulent clinical isolate of F. tularensis subsp. tularensis and the live vaccine strain with human macrophages by immunoelectron and confocal immunofluorescence microscopy. We assessed the maturation of the F. tularensis phagosome by examining its acquisition of the lysosome-associated membrane glycoproteins (LAMPs) CD63 and LAMP1 and the acid hydrolase cathepsin D. Two to four hours after infection, vacuoles containing live F. tularensis cells acquired abundant staining for LAMPs but little or no staining for cathepsin D. However, after 4 h, the colocalization of LAMPs with live F. tularensis organisms declined dramatically. In contrast, vacuoles containing formalin-killed bacteria exhibited intense staining for all of these late endosomal/lysosomal markers at all time points examined (1 to 16 h). We examined the pH of the vacuoles 3 to 4 h after infection by quantitative immunogold staining and by fluorescence staining for lysosomotropic agents. Whereas phagosomes containing killed bacteria stained intensely for these agents, indicating a marked acidification of the phagosomes (pH 5.5), phagosomes containing live F. tularensis did not concentrate these markers and thus were not appreciably acidified (pH 6.7). An ultrastructural analysis of the F. tularensis compartment revealed that during the first 4 h after uptake, the majority of F. tularensis bacteria reside within phagosomes with identifiable membranes. The cytoplasmic side of the membranes of approximately 50% of these phagosomes was coated with densely staining fibrils of approximately 30 nm in length. In many cases, these coated phagosomal membranes appeared to bud, vesiculate, and fragment. By 8 h after infection, the majority of live F. tularensis bacteria lacked any ultrastructurally discernible membrane separating them from the host cell cytoplasm. These results indicate that F. tularensis initially enters a nonacidified phagosome with LAMPs but without cathepsin D and that the phagosomal membrane subsequently becomes morphologically disrupted, allowing the bacteria to gain direct access to the macrophagic cytoplasm. The capacity of F. tularensis to alter the maturation of its phagosome and to enter the cytoplasm is likely an important element of its capacity to parasitize macrophages and has major implications for vaccine development.

Francisella tularensis selectively induces proinflammatory changes in endothelial cells

Naturally acquired infections with Francisella tularensis, the bacterial agent of tularemia, occur infrequently in humans. However, the high infectivity and lethality of the organism in humans raise concerns that it might be exploited as a weapon of bioterrorism. Despite this potential for illicit use, the pathogenesis of tularemia is not well understood. To examine how F. tularensis interacts with cells of its mammalian hosts, we tested the ability of a live vaccine strain (LVS) to induce proinflammatory changes in cultured HUVEC. Living F. tularensis LVS induced HUVEC to express the adhesion molecules VCAM-1 and ICAM-1, but not E-selectin, and to secrete the chemokine CXCL8, but not CCL2. Stimulation of HUVEC by the living bacteria was partially suppressed by polymyxin B, an inhibitor of LPS, but did not require serum, suggesting that F. tularensis LVS does not stimulate endothelium through the serum-dependent pathway that is typically used by LPS from enteric bacteria. In contrast to the living organisms, suspensions of killed F. tularensis LVS acquired the ability to increase endothelial expression of both E-selectin and CCL2. Up-regulation of E-selectin and CCL2 by the killed bacteria was not inhibited by polymyxin B. Exposure of HUVEC to either live or killed F. tularensis LVS for 24 h promoted the transendothelial migration of subsequently added neutrophils. These data indicate that multiple components of F. tularensis LVS induce proinflammatory changes in endothelial cells in an atypical manner that may contribute to the exceptional infectivity and virulence of this pathogen.

Experimental tularemia in mice challenged by aerosol or intradermally with virulent strains of Francisella tularensis: bacteriologic and histopathologic studies

BALB/c and C57BL/6 mice were challenged by aerosol or intradermally with low doses ( approximately 10-20 colony forming units) of virulent type A and type B strains of the facultative intracellular pathogen, Francisella tularensis, and the course of infection was monitored. Both mouse strains were equally susceptible to infection, but type A strains reached lethal numbers a few days earlier than type B strains regardless of challenge route. BALB/c mice showed overt signs of infection for several days, whereas C57BL/6 mice remained asymptomatic until a few hours before death. Histological changes were extensive and severe in the liver and spleen, but much more limited in the lungs, even in mice challenged by aerosol. Thus, it appears that regardless of the route of infection, systemic rather than pulmonary infection was the likely cause of death following low dose challenge with virulent F. tularensis.

A case of primary tularemic pneumonia presenting with necrotizing mediastinal and hilar lymph nodes

Tularemia is an unusual disease caused by the gram-negative coccobacillus Francisella tularensis. The clinical features of the disease depend on the route of inoculation. Ulceroglandular and typhoidal forms have been recognized as occurring in tularemia, however primary or secondary pneumonic infections have also been reported. Symptoms, laboratory markers and radiological features are non-specific in tularemic pneumonia. Diagnosis is made on clinical grounds and evidence of elevated agglutinating antibodies to F. tularensis (> 1:128). We report a case of primary tularemic pneumonia presenting with pulmonary infiltrates and necrotizing mediastinal and hilar lymph nodes in an otherwise healthy subject from a non-endemic area. Diagnosis of tularemia was obtained serologically, and antibiotic therapy with doxycycline and streptomycin resolved symptoms and radiological abnormalities. We suggest that tularemia should be considered in the differential diagnosis of pneumonia with mediastinal and/or hilar lymphadenopathy.

Will the enigma of Francisella tularensis virulence soon be solved?

Francisella tularensis is one of the most infectious bacterial pathogens known and is the causative agent of the zoonotic disease tularemia. In spite of the importance of this pathogen little is known about its virulence mechanisms. However, it is clear that the bacterium is an intracellular pathogen, replicating mainly in macrophages, with replication in amoebae also having been reported. The genome sequence of a high virulence strain of F. tularensis is close to completion and when available, will stimulate further research into virulence mechanisms.

Different host defences are required to protect mice from primary systemic vs pulmonary infection with the facultative intracellular bacterial pathogen, Francisella tularensis LVS

Francisella tularensis is a zoonotic, facultative intracellular bacterial pathogen capable of initiating infection, tularemia, via multiple routes including dermal micro-abrasions and inhalation. Mouse models of systemically-initiated infection with F. tularensis LVS have been used extensively to reveal potential host defence mechanisms against the pathogen. Such studies have demonstrated the critical need for neutrophils and interferon-gamma (IFN-gamma) to combat the early stages of primary experimental tularaemia initiated by systemic routes. Surprisingly, however, the present study shows that these defences appear not to combat early pulmonary tularaemia initiated by inhalation of the pathogen into the lower airways. The results imply that the effectiveness of particular anti-bacterial host defences vary with invasion site. Thus, it is impossible to predict effective host defence mechanisms against inhalation-initiated tularaemia from current knowledge of anti- Francisella defences that have been shown to combat systemically-initiated infection.

Modified spinning top homogeneous spray apparatus for use in experimental respiratory disease studies

The May spinning top generator was adapted to a modified Henderson tube for producing large aerosol particles (>4 mum) to obtain almost exclusive upper respiratory tract deposition of infectious aerosols in exposed mice. The system was installed in a biological safety cabinet to permit experimentation with pathogens. A novel mechanism utilizing parts from a machinists micrometer and the mechanical stage from a light microscope was developed for the spinning top generator as a means for precisely positioning the liquid feed needle. Aerosol light-scatter properties were continuously analyzed to provide relative measures of particle size distribution and aerosol concentration. When mice were exposed to influenza virus aerosols in which none of the virus was contained in particles with aerodynamic diameters <4 mum, essentially all of the virus was deposited in the upper respiratory tract tissues.

Pathogenesis of tularemia in monkeys aerogenically exposed to Francisella tularensis 425

The pathogenesis of tularemia was studied in groups of rhesus monkeys (Macaca mulatta) that inhaled graded 10-fold doses ranging from 10 through 10(6) organisms of Francisella tularensis 425, a strain highly virulent for the white mouse but of reduced virulence for the domestic rabbit. Mean incubation periods ranged from 3 to 6 days followed by acute illness lasting 5 to 11 days with subsequent recovery of most animals. The higher inhaled doses resulted in shorter incubation periods, longer and more severe acute illnesses, and 18% mortality at the highest dose. Strain 425 multiplied in the lungs, disseminated to the regional lymph nodes, and became systemic. Maximal bacterial populations in tissues were reached by the 7th day after exposure of the animals regardless of the number of organisms inhaled. F. tularensis was no longer recoverable from any of six tissues examined 2 months after exposure. The most significant tissue changes occurred in the lungs; these consisted of foci of liquefaction necrosis, lobular consolidation, and pleural effusion and adhesions. The data indicate that the inhaled dose of strain 425 determined the maximal growth of the organism in the lungs which in turn influenced the severity of the usually self-limiting pneumonia and systemic infection. Although the monkey is less resistant to tularemia than is man, this laboratory animal when infected with F. tularensis 425 provides a useful model for the self-limiting type of human pulmonary tularemia usually observed in Europe and Asia but to a lesser extent in North America.

Experimental tularemia in Macaca mulatta: relationship of aerosol particle size to the infectivity of airborne Pasteurella tularensis

Ninety-six Macaca mulatta were exposed to aerosol particles containing Pasteurella tularensis. Four different aerosols were employed that contained particle size distributions with median diameters of 2.1, 7.5, 12.5, or 24.0 mum. Size distributions were calculated only for those particles observed by phase microscope to contain organisms. Animals exposed to particles whose median diameters were either 2.1 or 7.5 mum were all infected and showed extensive infection of the lower respiratory tract, evidenced by large patches of consolidation with many necrotic foci on the surface. Death occurred in these animals 4 to 8 days after exposure. Monkeys exposed to 12.5- or 24.0-mum median diameter particles presented involvement of the cervical and mandibular lymph nodes, evidenced by swelling and abscess formation. Thirty-eight of the 45 animals in this group were infected. Those animals succumbing to the disease died from 8 to 21 days after exposure. The respiratory LD(50) values increased from 14 to 4,447 cells as the median diameter was raised from 2.1 to 24.0 mum.

In vitro interactions between rabbit alveolar macrophages and Pasteurella tularensis

Nutter, J. E. (Fort Detrick, Frederick, Md.), and Q. N. Myrvik. In vitro interactions between rabbit alveolar macrophages and Pasteurella tularensis. J. Bacteriol. 92:645-651. 1966.-Rabbit alveolar macrophages were successfully employed in a study of host cell-Pasteurella tularensis interactions in vitro. Under cell culture conditions in which inhibitory antibiotics were not employed and small infection ratios were used, the relative in vivo virulence of two strains of P. tularensis was duplicated. As a consequence of intracellular multiplication, normal macrophages were killed in relation to the virulence of the strain employed. Alveolar macrophages were also collected from immune rabbits, and macrophage mortality and bacterial growth were significantly suppressed below levels observed with normal macrophage preparations. The effect of immune serum could only be ascribed a minor role in the observed reactions. A marked intravenous toxicity of P. tularensis for the rabbit was observed with both virulent and attenuated strains. The toxicity was possessed only by viable preparations and could be elicited in animals immune to virulent challenge.

Evaluation of slit sampler in quantitative studies of bacterial aerosols

Quantitative studies were conducted to evaluate the efficiency of the slit sampler in collecting airborne Serratia marcescens and Bacillus subtilis var. niger, and to compare it with the collecting efficiency of the all-glass impinger AGI-30. The slit sampler was approximately 50% less efficient than the AGI-30. This ratio remained the same whether liquid or dry cultures were disseminated when the sample was taken at 2 min of aerosol cloud life. At 30 min of aerosol cloud life, this ratio was approximately 30% for B. subtilis var. niger. S. marcescens recoveries by the slit sampler were, however, only 17% lower than the AGI-30 at 30 min of cloud age, indicating a possible interaction involving the more labile vegetative cells, aerosol age, and method of collection.