A wild and an attenuated strain of Francisella tularensis differ in susceptibility to hypochlorous acid: a possible explanation of their different handling by polymorphonuclear leukocytes

Structural and functional analysis of the Francisella lysine decarboxylase as a key actor in oxidative stress resistance

Francisella tularensis is one of the most virulent pathogenic bacteria causing the acute human respiratory disease tularemia. While the mechanisms underlying F. tularensis pathogenesis are largely unknown, previous studies have shown that a F. novicida transposon mutant with insertions in a gene coding for a putative lysine decarboxylase was attenuated in mouse spleen, suggesting a possible role of its protein product as a virulence factor. Therefore, we set out to structurally and functionally characterize the F. novicida lysine decarboxylase, which we termed LdcF. Here, we investigate the genetic environment of ldcF as well as its evolutionary relationships with other basic AAT-fold amino acid decarboxylase superfamily members, known as key actors in bacterial adaptative stress response and polyamine biosynthesis. We determine the crystal structure of LdcF and compare it with the most thoroughly studied lysine decarboxylase, E. coli LdcI. We analyze the influence of ldcF deletion on bacterial growth under different stress conditions in dedicated growth media, as well as in infected macrophages, and demonstrate its involvement in oxidative stress resistance. Finally, our mass spectrometry-based quantitative proteomic analysis enables identification of 80 proteins with expression levels significantly affected by ldcF deletion, including several DNA repair proteins potentially involved in the diminished capacity of the F. novicida mutant to deal with oxidative stress. Taken together, we uncover an important role of LdcF in F. novicida survival in host cells through participation in oxidative stress response, thereby singling out this previously uncharacterized protein as a potential drug target.

Contribution of methionine sulfoxide reductase B (MsrB) to Francisella tularensis infection in mice

The essential mechanisms and virulence factors enabling Francisella species to replicate inside host macrophages are not fully understood. Methionine sulfoxide reductase (Msr) is an antioxidant enzyme that converts oxidized methionine into methionine. Francisella tularensis carries msrA and msrB in different parts of its chromosome. In this study, single and double mutants of msrA and msrB were constructed, and the characteristics of these mutants were investigated. The msrB mutant exhibited decreased in vitro growth, exogenous oxidative stress resistance and intracellular growth in macrophages, whereas the msrA mutant displayed little difference with wild-type strain. The double mutant exhibited the same characteristics as the msrB mutant. The bacterial count of the msrB mutant was significantly lower than that of the wild-type strain in the liver and spleen of mice. The bacterial count of the msrA mutant was lower than that of the wild-type strain in the liver, but not in the spleen, of mice. These results suggest that MsrB has an important role in the intracellular replication of F. tularensis in macrophages and infection in mice.

Multifaceted effects of Francisella tularensis on human neutrophil function and lifespan

Francisella tularensis in an intracellular bacterial pathogen that causes a potentially lethal disease called tularemia. Studies performed nearly 100 years ago revealed that neutrophil accumulation in infected tissues correlates directly with the extent of necrotic damage during F. tularensis infection. However, the dynamics and details of bacteria-neutrophil interactions have only recently been studied in detail. Herein, we review current understanding regarding the mechanisms that recruit neutrophils to F. tularensis-infected lungs, opsonization and phagocytosis, evasion and inhibition of neutrophil defense mechanisms, as well as the ability of F. tularensis to prolong neutrophil lifespan. In addition, we discuss distinctive features of the bacterium, including its ability to act at a distance to alter overall neutrophil responsiveness to exogenous stimuli, and the evidence which suggests that macrophages and neutrophils play distinct roles in tularemia pathogenesis, such that macrophages are major vehicles for intracellular growth and dissemination, whereas neutrophils drive tissue destruction by dysregulation of the inflammatory response.

Chlorine disinfection of Francisella tularensis

AIMS

To determine the range of free available chlorine (FAC) required for disinfection of the live vaccine strain (LVS) and wild-type strains of Francisella tularensis.

METHODS AND RESULTS

Seven strains of planktonic F. tularensis were exposed to 0·5 mg·l(-1) FAC for two pH values, 7 and 8, at 5 and 25°C. LVS was inactivated 2 to 4 times more quickly than any of the wild-type F. tularensis strains at pH 8 and 5°C.

CONCLUSIONS

Free available chlorine residual concentrations routinely maintained in drinking water distribution systems would require up to two hours to reduce all F. tularensis strains by 4 log10. LVS was inactivated most quickly of the tested strains.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work provides contact time (CT) values that are useful for drinking water risk assessment and also suggests that LVS may not be a good surrogate in disinfection studies.

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.

Multiple mechanisms of NADPH oxidase inhibition by type A and type B Francisella tularensis

Ft is a facultative intracellular pathogen that infects many cell types, including neutrophils. In previous work, we demonstrated that the type B Ft strain LVS disrupts NADPH oxidase activity throughout human neutrophils, but how this is achieved is incompletely defined. Here, we used several type A and type B strains to demonstrate that Ft-mediated NADPH oxidase inhibition is more complex than appreciated previously. We confirm that phagosomes containing Ft opsonized with AS exclude flavocytochrome b(558) and extend previous results to show that soluble phox proteins were also affected, as indicated by diminished phosphorylation of p47(phox) and other PKC substrates. However, a different mechanism accounts for the ability of Ft to inhibit neutrophil activation by formyl peptides, Staphylococcus aureus, OpZ, and phorbol esters. In this case, enzyme targeting and assembly were normal, and impaired superoxide production was characterized by sustained membrane accumulation of dysfunctional NADPH oxidase complexes. A similar post-assembly inhibition mechanism also diminished the ability of anti-Ft IS to confer neutrophil activation and bacterial killing, consistent with the limited role for antibodies in host defense during tularemia. Studies of mutants that we generated in the type A Ft strain Schu S4 demonstrate that the regulatory factor fevR is essential for NADPH oxidase inhibition, whereas iglI and iglJ, candidate secretion system effectors, and the acid phosphatase acpA are not. As Ft uses multiple mechanisms to block neutrophil NADPH oxidase activity, our data strongly suggest that this is a central aspect of virulence.

Initial delay in the immune response to Francisella tularensis is followed by hypercytokinemia characteristic of severe sepsis and correlating with upregulation and release of damage-associated molecular patterns

"Francisella tularensis subsp. novicida" intranasal infection causes a rapid pneumonia in mice with mortality at 4 to 6 days with a low dose of bacteria (10(2) bacteria). The short time to death suggests that there is a failure of the innate immune response. As the neutrophil is often the first cell type to infiltrate sites of infection, we focused on the emigration of neutrophils in this infection, as well as cytokines involved in their recruitment. The results indicated that there was a significant delay in the influx of neutrophils into the bronchoalveolar lavage fluid of F. tularensis subsp. novicida-infected mice. The delay in neutrophil recruitment in F. tularensis subsp. novicida-infected mice correlated with a delay in the upregulation of multiple proinflammatory cytokines and chemokines, as well as a delay in caspase-1 activation. Strikingly, the initial delay in the upregulation of cytokines through 1 day postinfection was followed by profound upregulation of multiple cytokines and chemokines to levels consistent with hypercytokinemia described for severe sepsis. This finding was further supported by a bacteremia and the cellular relocalization and release of high-mobility group box-1 and S100A9, both of which are damage-associated molecular pattern molecules and are known to be mediators of severe sepsis.

To activate or not to activate: distinct strategies used by Helicobacter pylori and Francisella tularensis to modulate the NADPH oxidase and survive in human neutrophils

Neutrophils accumulate rapidly at sites of infection, and the ability of these cells to phagocytose and kill microorganisms is an essential component of the innate immune response. Relatively few microbial pathogens are able to evade neutrophil killing. Herein, we describe the novel strategies used by Helicobacter pylori and Francisella tularensis to disrupt neutrophil function, with a focus on assembly and activation of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase.

Resistance of Francisella tularensis strains against reactive nitrogen and oxygen species with special reference to the role of KatG

Francisella tularensis is a facultative intracellular bacterial pathogen capable of proliferating within host macrophages. The mechanisms that explain the differences in virulence between various strains of the species are not well characterized. In the present study, we show that both attenuated (strain LVS) and virulent (strains FSC200 and SCHU S4) strains of the pathogen replicate at similar rates in resting murine peritoneal exudate cells (PEC). However, when PEC were activated by exposure to gamma interferon (IFN-gamma), they killed LVS more rapidly than virulent strains of the pathogen. Addition of N(G)-monomethyl-l-arginine, an inhibitor of inducible nitric oxide synthase, to IFN-gamma-treated PEC, completely inhibited killing of the virulent strains, whereas it only partially blocked the killing of LVS. Similarly, in a cell-free system, SCHU S4 and FSC200 were more resistant to killing by H(2)O(2) and ONOO(-) than F. tularensis LVS. Catalase encoded by katG is a bacterial factor that can detoxify bactericidal compounds such as H(2)O(2) and ONOO(-). To investigate its contribution to the virulence of F. tularensis, katG deletion-containing mutants of SCHU S4 and LVS were generated. Both mutants demonstrated enhanced susceptibility to H(2)O(2) in vitro but replicated as effectively as the parental strains in unstimulated PEC. In mice, LVS-DeltakatG was significantly attenuated compared to LVS whereas SCHU S4-DeltakatG, despite slower replication, killed mice as quickly as SCHU S4. This implies that clinical strains of the pathogen have katG-independent mechanisms to combat the antimicrobial effects exerted by H(2)O(2) and ONOO(-), the loss of which could have contributed to the attenuation of LVS.

Francisella tularensis: taxonomy, genetics, and Immunopathogenesis of a potential agent of biowarfare

Tularemia is a zoonosis of humans caused by infection with the facultative intracellular bacterium Francisella tularensis. Interest in F. tularensis has increased markedly in the past few years because of its potential use as an agent of bioterrorism. Five subspecies of this organism are found in the Northern hemisphere, but only F. tularensis subsp. tularensis and subsp. holarctica cause disease in humans. This review summarizes what is known about the pathogenesis of tularemia with a focus on bacterial surface components such as lipopolysaccharide and capsule as well as information obtained from the F. tularensis subsp. tularensis SCHU S4 genome. In particular, the mechanisms of action of recently identified virulence factors are discussed in the context of bacterial replication in macrophages and manipulation of the host inflammatory response. Throughout this report, shared and unique features of F. tularensis subsp. tularensis, subsp. holarctica, and subsp. novicida are discussed.

Francisella tularensis LVS evades killing by human neutrophils via inhibition of the respiratory burst and phagosome escape

Francisella tularensis is a Gram-negative bacterium and the causative agent of tularemia. Recent data indicate that F. tularensis replicates inside macrophages, but its fate in other cell types, including human neutrophils, is unclear. We now show that F. tularensis live vaccine strain (LVS), opsonized with normal human serum, was rapidly ingested by neutrophils but was not eliminated. Moreover, evasion of intracellular killing can be explained, in part, by disruption of the respiratory burst. As judged by luminol-enhanced chemiluminescence and nitroblue tetrazolium staining, neutrophils infected with live F. tularensis did not generate reactive oxygen species. Confocal microscopy demonstrated that NADPH oxidase assembly was disrupted, and LVS phagosomes did not acquire gp91/p22(phox) or p47/p67(phox). At the same time, F. tularensis also impaired neutrophil activation by heterologous stimuli such as phorbol esters and opsonized zymosan particles. Later in infection, LVS escaped the phagosome, and live organisms persisted in the neutrophil cytosol for at least 12 h. To our knowledge, our data are the first demonstration of a facultative intracellular pathogen, which disrupts the oxidative burst and escapes the phagosome to evade elimination inside neutrophils, and as such, our data define a novel mechanism of virulence.

Basis for the failure of Francisella tularensis lipopolysaccharide to prime human polymorphonuclear leukocytes

Francisella tularensis is the intracellular gram-negative coccobacillus that causes tularemia, and its virulence and infectiousness make it a potential agent of bioterrorism. Previous studies using mononuclear leukocytes have shown that the lipopolysaccharide (LPS) of F. tularensis is neither a typical proinflammatory endotoxin nor an endotoxin antagonist. This inertness suggests that F. tularensis LPS does not bind host LPS-sensing molecules such as LPS-binding protein (LBP). Using priming of the polymorphonuclear leukocyte (PMN) oxidase as a measure of endotoxicity, we found that F. tularensis live vaccine strain LPS did not behave like either a classic endotoxin or an endotoxin antagonist in human PMNs, even when the concentration of LBP was limiting. Furthermore, F. tularensis LPS did not compete with a radiolabeled lipooligosaccharide from Neisseria meningitidis for binding to LBP or to the closely related PMN granule protein, bactericidal/permeability-increasing protein. Our results suggest that the inertness of F. tularensis LPS and the resistance of F. tularensis to oxygen-independent PMN killing may result from the inability of F. tularensis LPS to be recognized by these important LPS-sensing molecules of the innate immune system.

Intracellular survival mechanisms of Francisella tularensis, a stealth pathogen

Research on the highly virulent and contagious, facultative intracellular bacterium Francisella tularensis has come into the limelight recently, but still little is known regarding its virulence mechanisms. This review summarizes recent studies on its intramacrophage survival mechanisms, some of which appear to be novel.

Vaccination strategies for Francisella tularensis

Francisella tularensis is the etiologic agent of tularemia, a severe debilitating disease of humans and animals. The low infectious dose of F. tularensis in humans and the relative ease of culture are probably the properties which originally attracted interest in this bacterium as a bioweapon. Even today, F. tularensis is ranked as one of the pathogens most likely to be used as a biological warfare or bioterrorism agent. A live attenuated vaccine (LVS) has been available for over 50 years, but there are shortcomings associated with its use. This vaccine is not fully licensed and does not offer a high level of protection against respiratory challenge. Nevertheless, this vaccine does demonstrate the feasibility of vaccination against tularemia. Protection against tularemia is likely to be dependent on the induction of cellular and humoral immune responses. These types of responses are induced by the LVS vaccine and could also be induced by a rationally attenuated mutant of F. tularensis. Evoking this range of responses with a sub-unit vaccine may be more difficult to achieve, and will be dependent on the use of suitable vaccine delivery systems.

The contribution of reactive nitrogen and oxygen species to the killing of Francisella tularensis LVS by murine macrophages

Intracellular killing of Francisella tularensis by macrophages depends on interferon-gamma (IFN-gamma)-induced activation of the cells. The importance of inducible nitric oxide synthase (iNOS) or NADPH phagocyte oxidase (phox) for the cidal activity was studied. Murine IFN-gamma-activated peritoneal exudate cells (PEC) produced nitric oxide (NO), measured as nitrite plus nitrate, and superoxide. When PEC were infected with the live vaccine strain, LVS, of F. tularensis, the number of viable bacteria was at least 1000-fold lower in the presence than in the absence of IFN-gamma after 48 h of incubation. PEC from iNOS-gene-deficient (iNOS-/-) mice killed F. tularensis LVS less effectively than did PEC from wild-type mice. PEC from phox gene-deficient (p47phox-/-) mice were capable of killing the bacteria, but killing was less efficient, although still significant, in the presence of NG-monomethyl-L-arginine (NMMLA), an inhibitor of iNOS. A decomposition catalyst of ONOO-, FeTPPS, completely reversed the IFN-gamma-induced killing of F. tularensis LVS. Under host cell-free conditions, F. tularensis LVS was exposed to S-nitroso-acetyl-penicillamine (SNAP), which generates NO, or 3-morpholinosydnonimine hydrochloride (SIN-1), which generates NO and superoxide, leading to formation of ONOO-. During 6 h of incubation, SNAP caused no killing of F. tularensis LVS, whereas effective killing occurred in the presence of equimolar concentrations of SIN-1. The results suggest that mechanisms dependent on iNOS and to a minor degree, phox, contribute to the IFN-gamma-induced macrophage killing of F. tularensis LVS. ONOO- is likely to be a major mediator of the killing.

Distinct roles of reactive nitrogen and oxygen species to control infection with the facultative intracellular bacterium Francisella tularensis

Reactive nitrogen species (RNS) and reactive oxygen species (ROS) are important mediators of the bactericidal host response. We investigated the contribution of these two mediators to the control of infection with the facultative intracellular bacterium Francisella tularensis. When intradermally infected with the live vaccine strain F. tularensis LVS, mice deficient in production of RNS (iNOS(-/-) mice) or in production of ROS by the phagocyte oxidase (p47(phox-/-) mice) showed compromised resistance to infection. The 50% lethal dose (LD(50)) for iNOS(-/-) mice was <20 CFU, and the LD(50) for p47(phox-/-) mice was 4,400 CFU, compared to an LD(50) of >500,000 CFU for wild-type mice. The iNOS(-/-) mice survived for 26.4 +/- 1.8 days, and the p47(phox-/-) mice survived for 10.1 +/- 1.3 days. During the course of infection, the serum levels of gamma interferon (IFN-gamma) and interleukin-6 were higher in iNOS(-/-) and p47(phox-/-) mice than in wild-type mice. Histological examination of livers of iNOS(-/-) mice revealed severe liver pathology. Splenocytes obtained 5 weeks after primary infection from antibiotic-treated iNOS(-/-) mice showed an in vitro recall response that was similar in magnitude and greater secretion of IFN-gamma compared to cells obtained from wild-type mice. In summary, mice lacking expression of RNS or ROS showed extreme susceptibility to infection with F. tularensis LVS. The roles of RNS and ROS seemed to be distinct since mice deficient in production of ROS showed dissemination of infection and died during the early phase of infection, whereas RNS deficiency led to severe liver pathology and a contracted course of infection.

Tularaemia: bioterrorism defence renews interest in Francisella tularensis

Francisella tularensis is a highly infectious aerosolizable intracellular pathogen that is capable of causing a debilitating or fatal disease with doses as low as 25 colony-forming units. There is no licensed vaccine available. Since the 1950s there has been concern that F. tularensis could be used as a biological threat agent, and it has received renewed attention recently owing to concerns about bioterrorism. The International Conference on Tularaemia in 2003 attracted more than 200 delegates, twice the number of participants as previous meetings. This is a reflection of the increased funding of research on this pathogen, particularly in the United States.

An attenuated strain of the facultative intracellular bacterium Francisella tularensis can escape the phagosome of monocytic cells

The facultative intracellular bacterium Francisella tularensis is a highly virulent and contagious organism, and little is known about its intracellular survival mechanisms. We studied the intracellular localization of the attenuated human vaccine strain, F. tularensis LVS, in adherent mouse peritoneal cells, in mouse macrophage-like cell line J774A.1, and in human macrophage cell line THP-1. Confocal microscopy of infected J774A.1 cells indicated that during the first hour of infection the bacteria colocalized with the late endosomal-lysosomal glycoprotein LAMP-1, but within 3 h this colocalization decreased significantly from approximately 60% to 30%. Transmission electron microscopy revealed that >90% of bacteria were not enclosed by a phagosomal membrane after 2 h of infection, and some bacteria were in vacuoles that were only partially surrounded by a limiting membrane. Similar findings were obtained with all three host cell types. Immunoelectron microscopy performed with an F. tularensis LVS-specific polyclonal rabbit antiserum showed that the antiserum stained a thick, evenly distributed capsule-like material in bacteria grown in broth. In contrast, intracellular F. tularensis LVS cells were only marginally stained with this antiserum. Instead, most of the immunoreactive material was diffusely localized in the phagosomes or was associated with the phagosomal membrane. Our findings indicate that F. tularensis LVS is able to escape from the phagosomes of macrophages via a mechanism that may involve degradation of the phagosomal membrane.

A vaccine for tularaemia

Francisella tularensis is an intracellular pathogen with a very low infectious dose for humans. Several forms of tularaemia occur, which range from a severely debilitating to a fatal disease. Diagnosis is difficult due to the generalised, nonspecific nature of symptoms and the difficulty in culturing the slow-growing and nutritionally fastidious pathogen. A live attenuated vaccine strain (LVS) has been used in humans as an investigational new drug and does appear to induce a protective response. However, the licensing of this vaccine has not yet been possible. For this reason, modern molecular biology approaches are being used in an attempt to devise replacement vaccines which may be more easily licensed. The approaches which are currently being considered include the production of subunit vaccines and the development of defined isogenic attenuated mutant strains of F. tularensis.

Tularemia

Francisella tularensis is the etiological agent of tularemia, a serious and occasionally fatal disease of humans and animals. In humans, ulceroglandular tularemia is the most common form of the disease and is usually a consequence of a bite from an arthropod vector which has previously fed on an infected animal. The pneumonic form of the disease occurs rarely but is the likely form of the disease should this bacterium be used as a bioterrorism agent. The diagnosis of disease is not straightforward. F. tularensis is difficult to culture, and the handling of this bacterium poses a significant risk of infection to laboratory personnel. Enzyme-linked immunosorbent assay- and PCR-based methods have been used to detect bacteria in clinical samples, but these methods have not been adequately evaluated for the diagnosis of pneumonic tularemia. Little is known about the virulence mechanisms of F. tularensis, though there is a large body of evidence indicating that it is an intracellular pathogen, surviving mainly in macrophages. An unlicensed live attenuated vaccine is available, which does appear to offer protection against ulceroglandular and pneumonic tularemia. Although an improved vaccine against tularemia is highly desirable, attempts to devise such a vaccine have been limited by the inability to construct defined allelic replacement mutants and by the lack of information on the mechanisms of virulence of F. tularensis. In the absence of a licensed vaccine, aminoglycoside antibiotics play a key role in the prevention and treatment of tularemia.

Cytokine expression in the liver of mice infected with a highly virulent strain of Francisella tularensis

Cytokine mRNA expression was determined in the liver of mice subcutaneously inoculated with a lethal dose of the highly virulent strain FSC 041 of Francisella tularensis subvar, tularensis or a sublethal dose of the live vaccine strain of F. tularensis subvar. palaearctica. Expression of mRNA for TNF-alpha, IL-12, IFN-gamma, and IL-10 was demonstrated within 48 h of inoculation, the kinetics being similar irrespective of bacterial strain used. Thus, the expression of a cytokine response believed to be important in the early host defence against live vaccine strain seemed insufficient to prevent the lethality of a more virulent strain.

Modification of neutrophil oxidant production with diphenyleneiodonium and its effect on bacterial killing

Diphenyleneiodonium (DPI), an inhibitor of the NADPH oxidase, has been used to distinguish between oxidative and nonoxidative killing of Staphylococcus aureus and Escherichia coli by neutrophils. The rate of killing of S. aureus was inhibited by 77% in the presence of 10 microM DPI, compared to 81% measured under anaerobic conditions. DPI represents a convenient and accessible alternative to an anaerobic environment or using neutrophils from patients with chronic granulomatous disease, for eliminating oxidative killing. The killing of E. coli was also inhibited by DPI. The effect was more apparent at 30 min than at 10 min, suggesting that E. coli can be killed rapidly by nonoxidative mechanisms that become less efficient at later times. DPI was used at concentrations less than 10 microM to determine how this affected production of the three major neutrophil oxidants, superoxide, hydrogen peroxide, and hypochlorous acid, and to determine the effect of partial inhibition of oxidant production on the killing of S. aureus. Unexpectedly, lower concentrations of DPI (0.1-2 microM) inhibited hydrogen peroxide and hypochlorous acid production 10-30% more than they inhibited superoxide production. Correlation of hydrogen peroxide or hypochlorous acid production with the killing of S. aureus showed that up to 30% inhibition had no effect on the rate of killing, implying that agents that impair neutrophil oxidant production less than this will not compromise bacterial killing. Higher inhibition of oxidant production led to a linear decline in the rate of killing.

Growth of Francisella tularensis LVS in macrophages: the acidic intracellular compartment provides essential iron required for growth

Murine macrophages supported exponential intracellular growth of Francisella tularensis LVS in vitro with a doubling time of 4 to 6 h. LVS was internalized and remained in a vacuolar compartment throughout its growth cycle. The importance of endosome acidification to intracellular growth of this bacterium was assessed by treatment of LVS-infected macrophages with several different lysosomotropic agents (chloroquine, NH4Cl, and ouabain). Regardless of the agent used or its mechanism of action, macrophages treated with agents that blocked endosome acidification no longer supported replication of LVS. Over several experiments for each lysosomotropic agent, the number of CFU of LVS recovered from treated macrophage cultures was equivalent to the input inoculum (approximately 10(4) CFU) at 72 h. In contrast, over 10(8) CFU was consistently recovered from untreated cultures. Pretreatment of macrophages with these endosome acidification inhibitors did not alter their ingestion of bacteria. Further, the effects of the inhibitors were completely reversible: inhibitor-pretreated LVS-infected macrophages washed free of the agent and cultured in medium fully supported LVS growth over 72 h. Endosome acidification is an important cellular event essential for release of iron from transferrin. The growth-inhibitory effects of both chloroquine and NH4Cl were completely reversed by addition of ferric PPi, a transferrin-independent iron source, at a neutral pH but not by addition of excess holotransferrin. Thus, intracellular localization in an acidic vesicle which facilitates the availability of iron essential for Francisella growth is a survival tactic of this bacterium, and iron depletion is one mechanism that macrophages use to inhibit its growth.

Transfer of immunity against lethal murine Francisella infection by specific antibody depends on host gamma interferon and T cells

Both serum and spleen cells from mice immune to Francisella tularensis transfer protection to naive recipients. Here we characterize the mechanism of protection induced by transfer of immune mouse serum (IMS). IMS obtained 4 weeks after intradermal infection with 10(3) bacteria of the live vaccine strain (LVS) contained high levels of immunoglobulin G2 (IgG2a) and IgM (end point titers, 1:16,600 and 1:7,200, respectively) and little IgG1, IgG2b, or IgG3. LVS-specific antibodies were detected 5 days after intradermal infection, and reached peak levels by 2 weeks postinfection. Only sera obtained 10 days or more after sublethal infection, when IgG titers peaked, transferred protection against a challenge of 100 50% lethal doses (LD50s). Purified high-titer IgG anti-LVS antibody but not IgM anti-LVS antibody was responsible for transfer of protection against an intraperitoneal challenge of up to 3,000 LD50s. IMS had no direct toxic effects on LVS and did not affect uptake or growth of bacteria in association with peritoneal cells. One day after LVS infection, liver, spleen, and lung tissue from mice treated with IMS contained 1 to 2 log units fewer bacteria than did tissue from mice treated with normal mouse serum or phosphate-buffered saline. Between 2 and 4 days after infection, however, bacterial growth rates in tissues were similar in both serum-protected mice and unprotected mice. Bacterial burdens in IMS-treated, LVS-infected mice declined in infected tissues after day 5, whereas control animals died. This lag phase suggested that development of a host response was involved in complete bacterial clearance. In fact, transfer of IMS into normal recipients that were simultaneously treated with anti-gamma interferon and challenged with LVS did not protect mice from death. Further, transfer of IMS into athymic nu/nu mice did not protect against LVS challenge; protection was, however, reconstituted by transfer of normal T cells into nu/nu mice. Thus, "passive" transfer of protection against LVS with specific antibody is not passive but depends on a host T-cell response to promote clearance of systemic infection and protection against lethal disease.

Passive protection of mice against lethal Francisella tularensis (live tularemia vaccine strain) infection by the sera of human recipients of the live tularemia vaccine

The relative role that humoral immunity plays in protection against infection with the intracellular bacterium, Francisella tularensis, remains controversial. Cellular immunity is thought to play the major and perhaps only role. The authors, in this article, investigate the immunologic and protective properties of immune serum collected from human recipients of the live tularemia vaccine (LVS). Sera of recipients of the vaccine demonstrated reactivity with the vaccine strain by enzyme-linked immunosorbent assay and Western blot analysis. This reactivity appeared to be directed primarily against the lipopolysaccharide of LVS and demonstrated complete cross-reactivity with fully virulent F. tularensis (Schu4). Pooled immune sera protected mice fully against a 10,000 LD50 challenge with the LVS strain relative to non-immune sera. The protection was abrogated by dilution or preadsorption with the LVS strain but not by preadsorption with Escherichia coli, which suggests specificity of protection. The authors conclude that antibodies to the LVS strain of F. tularensis are generated by live vaccination in humans and play a significant role in protection of mice against lethal challenge with the same organism. These antibodies crossreact completely with fully virulent F. tularensis, but whether they play a role in protection against fully virulent human tularemia strains requires further experimentation.

Neutrophils are critical for host defense against primary infection with the facultative intracellular bacterium Francisella tularensis in mice and participate in defense against reinfection

It is generally believed that immunity to experimental infection with the facultative intracellular bacterium Francisella tularensis is an example of T-cell-mediated immunity that is expressed by activated macrophages and mediated by Francisella-specific T cells. According to the results presented herein, neutrophils are also essential for defense against primary infection with this organism. It is shown that mice depleted of neutrophils by treatment with the granulocyte-specific monoclonal antibody RB6-8C5 are rendered defenseless against otherwise sublethal doses of F. tularensis LVS inoculated intravenously or intradermally. In neutrophil-depleted mice, the organism grew progressively in the livers, spleens, and lungs to reach lethal numbers, whereas infection was resolved in normal mice. Although neutrophils were found to resistance to reinfection, their participation was less important. The results suggest that neutrophils are needed for defense against primary infection because they serve to restrict the growth of F. tularensis before it reaches numbers capable of overwhelming a developing specific immune response. The exact way that neutrophils achieve this is not clear at this time, although it is probable that they contribute in ways other than by ingesting and killing the bacterium.

Increased synthesis of DnaK, GroEL, and GroES homologs by Francisella tularensis LVS in response to heat and hydrogen peroxide

The response of the facultative intracellular bacterium Francisella tularensis LVS to stress was assayed by pulse-labeling with [35S]methionine followed by two-dimensional gel electrophoresis and autoradiography. A temperature increase from 37 to 42 degrees C or exposure to 5 mM hydrogen peroxide induced increased syntheses of at least 15 proteins. Among these proteins were a 75-, a 60-, and a 10-kDa protein. By N-terminal sequence analysis, these three proteins were found to be extensively homologous to the highly conserved chaperone proteins DnaK, GroEL, and GroES of Escherichia coli. Antibodies specific to the DnaK homolog of E. coli reacted with the 75-kDa protein, and antibodies to the GroEL homolog of Legionella micdadei reacted with the 60-kDa protein. A readiness to respond to hydrogen peroxide with synthesis of the chaperone components may be fundamental to the intracellular survival of pathogens such as F. tularensis, which are exposed to oxidative stress while invading the host macrophages.

Salmonella enteritidis immune leukocyte-stimulated soluble factors: effects on increased resistance to Salmonella organ invasion in day-old Leghorn chicks

Cytokines, derived from either concanavalin A-stimulated Salmonella enteritidis-immune chicken T lymphocytes [SE-immune Lymphocyte Stimulated Soluble Factor (LSSF)] or lipopolysaccharide-stimulated SE-immune chicken macrophages [SE-immune Macrophage Stimulated Soluble Factor (MSSF)], were evaluated for their ability to increase resistance to SE organ invasion in day-old Leghorn chicks. In Trial 1, day of hatch chicks were injected i.p. with either SE-immune LSSF or SE-nonimmune LSSF (control). In Trial 2, chicks were similarly injected with either SE-immune MSSF, SE-nonimmune MSSF, or SE-immune LSSF (positive control). Thirty minutes postinjection, all chicks were gavaged with an invasive dose of SE. Twenty-four hours later, livers and spleens from all chicks were cultured for SE. In Trial 1, SE-immune LSSF caused a rapid and marked protection (P < .01) against SE infection as determined by the number of chicks that were culture positive regardless of challenge dose. In Trial 2, SE-immune MSSF was not associated with protection against SE organ infection. These experiments demonstrate that SE-immune LSSF, but not MSSF, are able to confer protection against SE organ invasion in day-old Leghorn chicks. Thus, it appears that the stimulated immune T cell, and not the macrophage, is responsible for producing the soluble products that protected the chicks.

Activation of macrophages for destruction of Francisella tularensis: identification of cytokines, effector cells, and effector molecules

Francisella tularensis live vaccine strain (LVS) was grown in culture with nonadherent resident, starch-elicited, or Proteose Peptone-elicited peritoneal cells. Numbers of bacteria increased 4 logs over the input inoculum in 48 to 72 h. Growth rates were faster in inflammatory cells than in resident cells: generation times for the bacterium were 3 h in inflammatory cells and 6 h in resident macrophages. LVS-infected macrophage cultures treated with lymphokines did not support growth of the bacterium, although lymphokines alone had no inhibitory effects on replication of LVS in culture medium devoid of cells. Removal of gamma interferon (IFN-gamma) by immunoaffinity precipitation rendered lymphokines ineffective for induction of macrophage anti-LVS activity, and recombinant IFN-gamma stimulated both resident and inflammatory macrophage populations to inhibit LVS growth in vitro. Inflammatory macrophages were more sensitive to effects of IFN-gamma: half-maximal activity was achieved at 5 U/ml for inflammatory macrophages and 20 U/ml for resident macrophages. IFN-gamma-induced anti-LVS activity correlated with the production of nitrite (NO2-), an oxidative end product of L-arginine-derived nitric oxide (NO). Anti-LVS activity and nitrite production were both completely inhibited by the addition of either the L-arginine analog NG-monomethyl-L-arginine or anti-tumor necrosis factor antibodies to activated macrophage cultures. Thus, macrophages can be activated by IFN-gamma to suppress the growth of F. tularensis by generation of toxic levels of NO, and inflammatory macrophages are substantially more sensitive to activation activities of IFN-gamma for this effector reaction than are more differentiated resident cells.

The requirement for gamma interferon in resistance of mice to experimental tularemia

The role of gamma interferon (IFN-gamma) in the host response to experimental tularemia was evaluated in a murine model. C57BL/6 strain mice were given a series of daily intravenous injections of 10(6) units (U) recombinant murine IFN-gamma prior to infection with Francisella tularensis LVS. Three days later, the number of bacteria in the tissues of IFN-gamma-treated mice was found to be less than that in control mice by a factor of 10-20. The effect of IFN-gamma on anti-tularemic resistance was dependent upon the administered dose, with as little as 10(4) U/mouse/day inducing a significant level of enhanced resistance. IFN-gamma was also effective in enhancing resistance to tularemia in the A/J mouse strain which, in comparison with the C57BL/6 strain, is more susceptible to infection. When C57BL/6 mice were treated with a monoclonal antibody directed against murine IFN-gamma, the number of Francisella recovered from their tissues 6 days following infection was increased by as much as 15 times, in comparison with control mice. The results of these experiments clearly indicate that the resolution of experimental murine tularemia is dependent, at least in part, on the participation of IFN-gamma.

A capsule-deficient mutant of Francisella tularensis LVS exhibits enhanced sensitivity to killing by serum but diminished sensitivity to killing by polymorphonuclear leukocytes

The live vaccine strain (LVS) of Francisella tularensis is killed by human polymorphonuclear leukocytes as a result of strictly oxygen-dependent mechanisms (S. Löfgren, A. Tärnvik, M. Thore, and J. Carlsson, Infect. Immun. 43:730-734, 1984). We now report that a capsule-deficient (Cap-) mutant of LVS survives in the leukocytes. In contrast to the encapsulated parent strain, the Cap- mutant was avirulent in mice and was susceptible to the bactericidal effect of nonimmune human serum. The mutant was killed by serum as a result of activation of the classical pathway of complement by naturally occurring immunoglobulin M. This killing by serum was mitigated by the presence of human polymorphonuclear leukocytes. After opsonization in complement component C5-deficient nonimmune serum, the Cap- mutant was ingested and survived in the leukocytes. Under these conditions, the parent strain was killed. The leukocytes responded to both the parent and the Cap- strain with a very low chemiluminescent response. Only the response to the parent strain was inhibited by superoxide dismutase. When the Cap- mutant was opsonized with immunoglobulin G, it induced a higher and superoxide dismutase-inhibitable chemiluminescent response and was killed by the leukocytes. In conclusion, the capsule of F. tularensis LVS seemed to protect this organism against the bactericidal effect of serum. When deprived of the capsule, the organism failed to induce an antimicrobial response in polymorphonuclear leukocytes and survived in the leukocytes. Survival in phagocytes is a key characteristic of intracellular parasites. The Cap- mutant of F. tularensis may become a useful tool in experiments to explain the differences between pathways of ingestion of intracellular parasites, evidenced by the death or survival of the parasite.

In vitro susceptibility of fungi to killing by neutrophil granulocytes discriminates between primary pathogenicity and opportunism

Pathogenic fungi, according to their propensity to cause infection of apparently normal individuals, can be grouped into either primary pathogens (e.g., Coccidioides, Histoplasma, Paracoccidioides, Blastomyces, and Sporothrix) or opportunists (e.g., Candida, Mucoraceae, Aspergillus spp., Petriellidium, and Trichosporon). There is, however, no unifying concept explaining the difference between the virulence of the two fungal categories. Previously we have speculated that neutrophils are the common denominator of the high natural resistance to opportunistic fungi. Accordingly, we then compared the susceptibility to killing by neutrophil granulocytes of Histoplasma, Blastomyces, Paracoccidioides, and Sporothrix with that of 14 opportunistic fungi. We found the four virulent dimorphic yeasts, in contrast to opportunistic fungi, to be resistant to killing by neutrophils. Virulent dimorphic yeasts were ingested by neutrophils, and triggered a respiratory burst comparably to opportunists but were less susceptible to hydrogen peroxide, suggesting that differences in the susceptibility to microbicidal products of leukocytes may explain the difference in virulence.

Anaerobic phagocytosis, killing, and degradation of Streptococcus pneumoniae by human peripheral blood leukocytes

Encapsulated Streptococcus pneumoniae of serotypes 2, 9N, 14, 21, and 23F and an unencapsulated variant of type 2 pneumococci were efficiently phagocytosed by both aerobically and anaerobically incubated human leukocytes. In the presence of O2, the pneumococci rapidly lost their viability, whereas during anaerobiosis, killing was considerably delayed. Type 14 pneumococci radiolabeled with [14C]choline or [14C]ethanolamine for cell wall teichoic acid, [14C]uracil for nucleic acids, or [14C]arachidonic acid for unsaturated cytoplasmic membrane lipids were used in studies of the fate of bacterial macromolecules after phagocytosis. The degradation of teichoic acid, RNA, and DNA during anaerobiosis approached that recorded in air at 60 min of incubation (45 to 70% and 55 to 75%, respectively). In contrast, the marked loss of [14C]arachidonic acid from pneumococcal membrane lipids observed in aerobic leukocytes did not occur during anaerobic incubation. Hence, lipid peroxidation could be involved in the rapid aerobic leukocyte killing of pneumococci, whereas a different leukocyte function of as yet unknown nature appears to be responsible for the killing seen in anaerobiosis. Autolysis-resistant type 14 pneumococci were obtained by substituting ethanolamine for choline in a defined culture medium. Differences between such bacteria and normal (autolytic) pneumococci in their killing and degradation by leukocytes were not detected in either the presence or the absence of O2. The aerobic and anaerobic handling of phagocytosed pneumococci by human blood leukocytes thus proceeded independently of the bacterial autolytic system.