CD4+ and CD8+ T-cell-dependent and -independent host defense mechanisms can operate to control and resolve primary and secondary Francisella tularensis LVS infection in mice

Analyses of human immune responses to Francisella tularensis identify correlates of protection

Francisella tularensis is the etiological agent of the potentially severe infection tularemia. An existing F: tularensis vaccine, the live vaccine strain (LVS), has been used to protect at-risk personnel, but it is not licensed in any country and it has limited efficacy. Therefore, there is a need of a new, efficacious vaccine. The aim of the study was to perform a detailed analysis of the characteristics of the human immune response to F. tularensis, since this will generate crucial knowledge required to develop new vaccine candidates. Nine individuals were administered the LVS vaccine and peripheral blood mononuclear cells (PBMC) were collected before and at four time points up to one year after vaccination. The properties of the PBMC were characterized by flow cytometry analysis of surface markers and intracellular cytokine staining. In addition, the cytokine content of supernatants from F. tularensis-infected PBMC cultures was determined and the protective properties of the supernatants investigated by adding them to cultures with infected monocyte-derived macrophages (MDM). Unlike before vaccination, PBMC collected at all four time points after vaccination demonstrated F. tularensis-specific cell proliferation, cytokine secretion and cytokine-expressing memory cells. A majority of 17 cytokines were secreted at higher levels by PBMC collected at all time points after vaccination than before vaccination. A discriminative analysis based on IFN-γ and IL-13 secretion correctly classified samples obtained before and after vaccination. Increased expression of IFN-γ, IL-2, and MIP-1β were observed at all time points after vaccination vs. before vaccination and the most significant changes occurred among the CD4 transient memory, CD8 effector memory, and CD8 transient memory T-cell populations. Growth restriction of the highly virulent F. tularensis strain SCHU S4 in MDM was conferred by supernatants and protection correlated to levels of IFN-γ, IL-2, TNF, and IL-17. The findings demonstrate that F. tularensis vaccination induces long-term T-cell reactivity, including TEM and TTM cell populations. Individual cytokine levels correlated with the degree of protection conferred by the supernatants. Identification of such memory T cells and effector mechanisms provide an improved understanding of the protective mechanisms against F. tularensis. mechanisms against F. tularensis.

Novel Transcriptional and Translational Biomarkers of Tularemia Vaccine Efficacy in a Mouse Inhalation Model: Proof of Concept

Francisella tularensis subspecies tularensis (Ftt) is extremely virulent for humans when inhaled as a small particle aerosol (<5 µm). Inhalation of ≥20 viable bacteria is sufficient to initiate infection with a mortality rate ≥30%. Consequently, in the past, Ftt became a primary candidate for biological weapons development. To counter this threat, the USA developed a live vaccine strain (LVS), that showed efficacy in humans against inhalation of virulent Ftt. However, the breakthrough dose was fairly low, and protection waned with time. These weaknesses triggered extensive research for better vaccine candidates. Previously, we showed that deleting the clpB gene from virulent Ftt strain, SCHU S4, resulted in a mutant that was significantly less virulent than LVS for mice, yet better protected them from aerosol challenge with wild-type SCHU S4. To date, comprehensive searches for correlates of protection for SCHU S4 ΔclpB among molecules that are critical signatures of cell-mediated immunity, have yielded little reward. In this study we used transcriptomics analysis to expand the potential range of molecular correlates of protection induced by vaccination with SCHU S4 ΔclpB beyond the usual candidates. The results provide proof-of-concept that unusual host responses to vaccination can potentially serve as novel efficacy biomarkers for new tularemia vaccines.

Differential Immune Response Following Intranasal and Intradermal Infection with Francisella tularensis: Implications for Vaccine Development

Francisella tularensis (Ft) is a Gram-negative, facultative intracellular coccobacillus that is the etiological agent of tularemia. Interestingly, the disease tularemia has variable clinical presentations that are dependent upon the route of infection with Ft. Two of the most likely routes of Ft infection include intranasal and intradermal, which result in pneumonic and ulceroglandular tularemia, respectively. While there are several differences between these two forms of tularemia, the most notable disparity is between mortality rates: the mortality rate following pneumonic tularemia is over ten times that of the ulceroglandular disease. Understanding the differences between intradermal and intranasal Ft infections is important not only for clinical diagnoses and treatment but also for the development of a safe and effective vaccine. However, the immune correlates of protection against Ft, especially within the context of infection by disparate routes, are not yet fully understood. Recent advances in different animal models have revealed new insights in the complex interplay of innate and adaptive immune responses, indicating dissimilar patterns in both responses following infection with Ft via different routes. Further investigation of these differences will be crucial to predicting disease outcomes and inducing protective immunity via vaccination or natural infection.

Glycoconjugate vaccine using a genetically modified O antigen induces protective antibodies to Francisella tularensis

Francisella tularensis is the causative agent of tularemia, a category A bioterrorism agent. The lipopolysaccharide (LPS) O antigen (OAg) of F. tularensis has been considered for use in a glycoconjugate vaccine, but conjugate vaccines tested so far have failed to confer protection necessary against aerosolized pulmonary bacterial challenge. When F. tularensis OAg was purified under standard conditions, the antigen had a small molecular size [25 kDa, low molecular weight (LMW)]. Using milder extraction conditions, we found the native OAg had a larger molecular size [80 kDa, high molecular weight (HMW)], and in a mouse model of tularemia, a glycoconjugate vaccine made with the HMW polysaccharide coupled to tetanus toxoid (HMW-TT) conferred better protection against intranasal challenge than a conjugate made with the LMW polysaccharide (LMW-TT). To further investigate the role of OAg size in protection, we created an F. tularensis live vaccine strain (LVS) mutant with a significantly increased OAg size [220 kDa, very high molecular weight (VHMW)] by expressing in F. tularensis a heterologous chain-length regulator gene (wzz) from the related species Francisella novicida Immunization with VHMW-TT provided markedly increased protection over that obtained with TT glycoconjugates made using smaller OAgs. We found that protective antibodies recognize a length-dependent epitope better expressed on HMW and VHMW antigens, which bind with higher affinity to the organism.

Vaccine-Mediated Mechanisms Controlling Replication of Francisella tularensis in Human Peripheral Blood Mononuclear Cells Using a Co-culture System

Cell-mediated immunity (CMI) is normally required for efficient protection against intracellular infections, however, identification of correlates is challenging and they are generally lacking. Francisella tularensis is a highly virulent, facultative intracellular bacterium and CMI is critically required for protection against the pathogen, but how this is effectuated in humans is poorly understood. To understand the protective mechanisms, we established an in vitro co-culture assay to identify how control of infection of F. tularensis is accomplished by human cells and hypothesized that the model will mimic in vivo immune mechanisms. Non-adherent peripheral blood mononuclear cells (PBMCs) were expanded with antigen and added to cultures with adherent PBMC infected with the human vaccine strain, LVS, or the highly virulent SCHU S4 strain. Intracellular numbers of F. tularensis was followed for 72 h and secreted and intracellular cytokines were analyzed. Addition of PBMC expanded from naïve individuals, i.e., those with no record of immunization to F. tularensis, generally resulted in little or no control of intracellular bacterial growth, whereas addition of PBMC from a majority of F. tularensis-immune individuals executed static and sometimes cidal effects on intracellular bacteria. Regardless of infecting strain, statistical differences between the two groups were significant, P < 0.05. Secretion of 11 cytokines was analyzed after 72 h of infection and significant differences with regard to secretion of IFN-γ, TNF, and MIP-1β was observed between immune and naïve individuals for LVS-infected cultures. Also, in LVS-infected cultures, CD4 T cells from vaccinees, but not CD8 T cells, showed significantly higher expression of IFN-γ, MIP-1β, TNF, and CD107a than cells from naïve individuals. The co-culture system appears to identify correlates of immunity that are relevant for the understanding of mechanisms of the protective host immunity to F. tularensis.

Inflammasome-Independent NLRP3 Restriction of a Protective Early Neutrophil Response to Pulmonary Tularemia

Francisella tularensis (Ft) causes a frequently fatal, acute necrotic pneumonia in humans and animals. Following lethal Ft infection in mice, infiltration of the lungs by predominantly immature myeloid cells and subsequent myeloid cell death drive pathogenesis and host mortality. However, following sub-lethal Ft challenge, more mature myeloid cells are elicited and are protective. In addition, inflammasome-dependent IL-1β and IL-18 are important for protection. As Nlrp3 appears dispensable for resistance to infection with Francisella novicida, we considered its role during infection with the virulent Type A strain SchuS4 and the attenuated Type B live vaccine strain LVS. Here we show that both in vitro macrophage and in vivo IL-1β and IL-18 responses to Ft LVS and SchuS4 involve both the Aim2 and Nlrp3 inflammasomes. However, following lethal infection with Francisella, IL-1r-, Caspase-1/11-, Asc- and Aim2-deficient mice exhibited increased susceptibility as expected, while Nlrp3-deficient mice were more resistant. Despite reduced levels of IL-1β and IL-18, in the absence of Nlrp3, Ft infected mice have dramatically reduced lung pathology, diminished recruitment and death of immature myeloid cells, and reduced bacterial burden in comparison to wildtype and inflammasome-deficient mice. Further, increased numbers of mature neutrophil appear in the lung early during lethal Ft infection in Nlrp3-deficient mice. Finally, Ft infection induces myeloid and lung stromal cell death that in part requires Nlrp3, is necrotic/necroptotic in nature, and drives host mortality. Thus, Nlrp3 mediates an inflammasome-independent process that restricts the appearance of protective mature neutrophils and promotes lethal necrotic lung pathology.

Progress, challenges, and opportunities in Francisella vaccine development

Renewed interest in Francisella tularensis has resulted in substantial new information about its pathogenesis and immunology, along with development of useful animal models. While understanding of protective immunity against Francisella remains incomplete, data in both animals and humans suggest that inducing T cell-mediated immunity is crucial for successful vaccination with current candidates such as the Live Vaccine Strain (LVS), with specific antibodies and immune B cells playing supporting roles. Consistent with this idea, recent results indicate that measurements of T cell functions and relative gene expression by immune T cells predict vaccine-induced protection in animal models. Because field trials of new vaccines will be difficult to design, using such measurements to derive potential correlates of protection may be important to bridge between animal efficacy studies and people.

Gallium Potentiates the Antibacterial Effect of Gentamicin against Francisella tularensis

The reasons why aminoglycosides are bactericidal have not been not fully elucidated, and evidence indicates that the cidal effects are at least partly dependent on iron. We demonstrate that availability of iron markedly affects the susceptibility of the facultative intracellular bacterium Francisella tularensis strain SCHU S4 to the aminoglycoside gentamicin. Specifically, the intracellular depots of iron were inversely correlated to gentamicin susceptibility, whereas the extracellular iron concentrations were directly correlated to the susceptibility. Further proof of the intimate link between iron availability and antibiotic susceptibility were the findings that a ΔfslA mutant, which is defective for siderophore-dependent uptake of ferric iron, showed enhanced gentamicin susceptibility and that a ΔfeoB mutant, which is defective for uptake of ferrous iron, displayed complete growth arrest in the presence of gentamicin. Based on the aforementioned findings, it was hypothesized that gallium could potentiate the effect of gentamicin, since gallium is sequestered by iron uptake systems. The ferrozine assay demonstrated that the presence of gallium inhibited >70% of the iron uptake. Addition of gentamicin and/or gallium to infected bone marrow-derived macrophages showed that both 100 μM gallium and 10 μg/ml of gentamicin inhibited intracellular growth of SCHU S4 and that the combined treatment acted synergistically. Moreover, treatment of F. tularensis-infected mice with gentamicin and gallium showed an additive effect. Collectively, the data demonstrate that SCHU S4 is dependent on iron to minimize the effects of gentamicin and that gallium, by inhibiting the iron uptake, potentiates the bactericidal effect of gentamicin in vitro and in vivo.

In vivo mechanisms involved in enhanced protection utilizing an Fc receptor-targeted mucosal vaccine platform in a bacterial vaccine and challenge model

Targeting antigens (Ag) to Fcγ receptors (FcγR) intranasally (i.n.) enhances immunogenicity and protection against intracellular and extracellular pathogens. Specifically, we have demonstrated that targeting fixed (inactivated) Francisella tularensis (iFT) organisms to FcR in mice i.n., with MAb-iFT immune complexes, enhances F. tularensis-specific immune responses and protection against F. tularensis challenge. Furthermore, traditional adjuvant is not required. In addition, we have demonstrated that the increased immunogenicity following the targeting of iFT to FcR is due, in part, to enhanced dendritic cell (DC) maturation, enhanced internalization, and processing and presentation of iFT by DCs, as well as neonatal FcR (FcRn)-enhanced trafficking of iFT from the nasal passage to the nasal mucosa-associated lymphoid tissue (NALT). Using this immunization and challenge model, we expanded on these studies to identify specific in vivo immune responses impacted and enhanced by FcR targeting of iFT i.n. Specifically, the results of this study demonstrate for the first time that targeting iFT to FcR increases the frequency of activated DCs within the lungs of MAb-iFT-immunized mice subsequent to F. tularensis LVS challenge. In addition, the frequency and number of gamma interferon (IFN-γ)-secreting effector memory (EM) CD4(+) T cells elicited by F. tularensis infection (postimmunization) is increased in an interleukin 12 (IL-12)-dependent manner. In summary, these studies build significantly upon previously published work utilizing this vaccine platform. We have identified a number of additional mechanisms by which this novel, adjuvant-independent, FcR-targeted mucosal vaccine approach enhances immunity and protection against infection, while further validating its potential as a universal vaccine platform against mucosal pathogens.

Transcriptional profiling of recall responses to Francisella live vaccine strain

Global gene expression profile changes were monitored in human peripheral blood mononuclear cells (PBMCs) after challenge with the live vaccine strain (LVS) of Francisella tularensis. Because these PBMCs were from individuals previously immunized with LVS, stimulating these cells with LVS should activate memory responses. The Ingenuity Pathway Analysis tool identified pathways, functions, and networks associated with this in vitro recall response, including novel pathways triggered by the memory response. Dendritic cell (DC) maturation was the most significant among the more than 25 relevant pathways discovered. Interleukin 15, granulocyte-macrophage colony-stimulating factor, and triggering receptor expressed on myeloid cells 1 signaling pathways were also significant. Pathway analysis indicated that Class 1 antigen presentation may not be optimal with LVS vaccination. The top three biological functions were antigen presentation, cell-mediated and humoral immune responses. Network analysis revealed that the top network associated with these functions had IFNγ and TNFα in central interactive positions. Our results suggest that DC maturation is a key factor in the recall responses and that more effective antigen processing and presentation is needed for cytotoxic T lymphocyte responses. Taken together, these considerations are critical for future tularemia vaccine development studies.

Protective immunity against lethal F. tularensis holarctica LVS provided by vaccination with selected novel CD8+ T cell epitopes

Recently we described an unbiased bacterial whole-genome immunoinformatic analysis aimed at selection of potential CTL epitopes located in "hotspots" of predicted MHC-I binders. Applying this approach to the proteome of the facultative intra-cellular pathogen Francisella tularensis resulted in identification of 170 novel CTL epitopes, several of which were shown to elicit highly robust T cell responses. Here we demonstrate that by DNA immunization using a short DNA fragment expressing six of the most prominent identified CTL epitopes a potent and specific CD8+ T cell responses is being induced, to all encoded epitopes, a response not observed in control mice immunized with the DNA vector alone Moreover, this CTL-specific mediated immune response prevented disease development, allowed for a rapid clearance of the bacterial infection and provided complete protection against lethal challenge (10LD50) with F. tularensis holarctica Live Vaccine Strain (LVS) (a total to 30 of 30 immunized mice survived the challenge while all control DNA vector immunized mice succumbed). Furthermore, and in accordance with these results, CD8 deficient mice could not be protected from lethal challenge after immunization with the CTL-polyepitope. Vaccination with the DNA poly-epitope construct could even protect mice (8/10) against the more demanding pulmonary lethal challenge of LVS. Our approach provides a proof-of-principle for selecting and generating a multi-epitpoe CD8 T cell-stimulating vaccine against a model intracellular bacterium.

MAIT cells are critical for optimal mucosal immune responses during in vivo pulmonary bacterial infection

Mucosa-associated invariant T (MAIT) cells are "innate" T cells that express an invariant T-cell receptor α-chain restricted by the nonclassical MHC class I molecule MHC-related protein 1 (MR1). A recent discovery that MR1 presents vitamin B metabolites, presumably from pathogenic and/or commensal bacteria, distinguishes MAIT cells from peptide- or lipid-recognizing αβ T cells in the immune system. MAIT cells are activated by a wide variety of bacterial strains in vitro, but their role in defense against infectious assaults in vivo remains largely unknown. To investigate how MAIT cells contribute to mucosal immunity in vivo, we used a murine model of pulmonary infection by using the live vaccine strain (LVS) of Francisella tularensis. In the early acute phase of infection, MAIT cells expanded robustly in the lungs, where they preferentially accumulated after reaching their peak expansion in the late phase of infection. Throughout the course of infection, MAIT cells produced the critical cytokines IFN-γ, TNF-α, and IL-17A. Mechanistic studies showed that MAIT cells required both MR1 and IL-12 40 kDa subunit (IL-12p40) signals from infected antigen presenting cells to control F. tularensis LVS intracellular growth. Importantly, pulmonary F. tularensis LVS infection of MR1-deficient (MR1(-/-)) mice, which lack MAIT cells, revealed defects in early mucosal cytokine production, timely recruitment of IFN-γ-producing CD4(+) and CD8(+) T cells to the infected lungs, and control of pulmonary F. tularensis LVS growth. This study provides in vivo evidence demonstrating that MAIT cells are an important T-cell subset with activities that influence the innate and adaptive phases of mucosal immunity.

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

Background

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

Methods and Principal Findings

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

Conclusions

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

Clusters versus affinity-based approaches in F. tularensis whole genome search of CTL epitopes

Deciphering the cellular immunome of a bacterial pathogen is challenging due to the enormous number of putative peptidic determinants. State-of-the-art prediction methods developed in recent years enable to significantly reduce the number of peptides to be screened, yet the number of remaining candidates for experimental evaluation is still in the range of ten-thousands, even for a limited coverage of MHC alleles. We have recently established a resource-efficient approach for down selection of candidates and enrichment of true positives, based on selection of predicted MHC binders located in high density “hotspots" of putative epitopes. This cluster-based approach was applied to an unbiased, whole genome search of Francisella tularensis CTL epitopes and was shown to yield a 17–25 fold higher level of responders as compared to randomly selected predicted epitopes tested in Kb/Db C57BL/6 mice. In the present study, we further evaluate the cluster-based approach (down to a lower density range) and compare this approach to the classical affinity-based approach by testing putative CTL epitopes with predicted IC₅₀ values of <10 nM. We demonstrate that while the percent of responders achieved by both approaches is similar, the profile of responders is different, and the predicted binding affinity of most responders in the cluster-based approach is relatively low (geometric mean of 170 nM), rendering the two approaches complimentary. The cluster-based approach is further validated in BALB/c F. tularensis immunized mice belonging to another allelic restriction (Kd/Dd) group. To date, the cluster-based approach yielded over 200 novel F. tularensis peptides eliciting a cellular response, all were verified as MHC class I binders, thereby substantially increasing the F. tularensis dataset of known CTL epitopes. The generality and power of the high density cluster-based approach suggest that it can be a valuable tool for identification of novel CTLs in proteomes of other bacterial pathogens.

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

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

Genetic modification of the O-polysaccharide of Francisella tularensis results in an avirulent live attenuated vaccine

BACKGROUND

Francisella tularensis, the causative agent of tularemia, is a highly virulent microbe. One significant virulence factor of F. tularensis is the O-polysaccharide (O-PS) portion of the organism's lipopolysaccharide.

METHODS

A wzy (O-antigen polymerase) deletion mutant of Ft. live attenuated vaccine strain (Ft.LVS), designated Ft.LVS::Δwzy, was created and evaluated as a live attenuated vaccine. Specifically, the mutant's virulence potential and its protective efficacy against type A and type B strains were investigated by challenge of immunized mice.

RESULTS

F. tularensis LVS::Δwzy expressed only 1 repeating unit of O-PS and yet, upon immunization, induced O-PS-specific antibodies. Compared with Ft.LVS, the mutant was highly sensitive to complement-mediated lysis, significantly attenuated in virulence, and was recovered in much lower numbers from the organs of infected mice. Intranasal immunization with Ft.LVS::Δwzy provided protection against subsequent intranasal infection with the highly virulent type A strain SchuS4 and with Ft.LVS. Immunization with Ft.LVS::Δwzy elicited both humoral and cell-mediated immunity.

CONCLUSIONS

Ft.LVS::Δwzy was avirulent in mice and, despite expressing only 1 repeating unit of the O-PS, induced antibodies to the full-length O-PS. Vaccination with Ft.LVS::Δwzy protected mice against intranasal challenge with both type A and type B strains of F. tularensis and induced functional immunity through both humoral and cellular mechanisms.

Identification of T-cell epitopes in Francisella tularensis using an ordered protein array of serological targets

Francisella tularensis is a Gram-negative intracellular bacterium that is the causative agent of tularaemia. Concerns regarding its use as a bioterrorism agent have led to a renewed interest in the biology of infection, host response and pathogenesis. A robust T-cell response is critical to confer protection against F. tularensis. However, characterization of the cellular immune response has been hindered by the paucity of tools to examine the anti-Francisella immune response at the molecular level. We set out to combine recent advances of genomics with solid-phase antigen delivery coupled with a T-cell functional assay to identify T-cell epitopes. A subset of clones, encoding serological targets, was selected from an F. tularensis SchuS4 ordered genomic library and subcloned into a bacterial expression vector to test the feasibility of this approach. Proteins were expressed and purified individually employing the BioRobot 3000 in a semi-automated purification method. The purified proteins were coupled to beads, delivered to antigen-presenting cells for processing, and screened with Francisella-specific T-cell hybridomas of unknown specificity. We identified cellular reactivity against the pathogenicity protein IglB, and the chaperone proteins GroEL and DnaK. Further analyses using genetic deletions and synthetic peptides were performed to identify the minimal peptide epitopes. Priming with the peptide epitopes before infection with F. tularensis LVS increased the frequency of antigen-specific CD4 T cells as assessed by intracellular interferon-γ staining. These results illustrate the feasibility of screening an arrayed protein library that should be applicable to a variety of pathogens.

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

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

Deletion of the Bacillus anthracis capB homologue in Francisella tularensis subspecies tularensis generates an attenuated strain that protects mice against virulent tularaemia

As there is currently no licensed vaccine against Francisella tularensis, the causative agent of tularaemia, the bacterium is an agent of concern as a potential bioweapon. Although F. tularensis has a low infectious dose and high associated mortality, it possesses few classical virulence factors. An analysis of the F. tularensis subspecies tularensis genome sequence has revealed the presence of a region containing genes with low sequence homology to part of the capBCADE operon of Bacillus anthracis. We have generated an isogenic capB mutant of F. tularensis subspecies tularensis SchuS4 and shown it to be attenuated. Furthermore, using BALB/c mice, we have demonstrated that this capB strain affords protection against significant homologous challenge with the wild-type strain. These data have important implications for the development of a defined and efficacious tularaemia vaccine.

The involvement of IL-17A in the murine response to sub-lethal inhalational infection with Francisella tularensis

Background

Francisella tularensis is an intercellular bacterium often causing fatal disease when inhaled. Previous reports have underlined the role of cell-mediated immunity and IFNγ in the host response to Francisella tularensis infection.

Methodology/Principal Findings

Here we provide evidence for the involvement of IL-17A in host defense to inhalational tularemia, using a mouse model of intranasal infection with the Live Vaccine Strain (LVS). We demonstrate the kinetics of IL-17A production in lavage fluids of infected lungs and identify the IL-17A-producing lymphocytes as pulmonary γδ and Th17 cells. The peak of IL-17A production appears early during sub-lethal infection, it precedes the peak of immune activation and the nadir of the disease, and then subsides subsequently. Exogenous airway administration of IL-17A or of IL-23 had a limited yet consistent effect of delaying the onset of death from a lethal dose of LVS, implying that IL-17A may be involved in restraining the infection. The protective role for IL-17A was directly demonstrated by in vivo neutralization of IL-17A. Administration of anti IL-17A antibodies concomitantly to a sub-lethal airway infection with 0.1×LD₅₀ resulted in a fatal disease.

Conclusion

In summary, these data characterize the involvement and underline the protective key role of the IL-17A axis in the lungs from inhalational tularemia.

Immunoproteomics analysis of the murine antibody response to vaccination with an improved Francisella tularensis live vaccine strain (LVS)

Background

Francisella tularensis subspecies tularensis is the causative agent of a spectrum of diseases collectively known as tularemia. An attenuated live vaccine strain (LVS) has been shown to be efficacious in humans, but safety concerns have prevented its licensure by the FDA. Recently, F. tularensis LVS has been produced under Current Good Manufacturing Practice (CGMP guidelines). Little is known about the immunogenicity of this new vaccine preparation in comparison with extensive studies conducted with laboratory passaged strains of LVS. Thus, the aim of the current work was to evaluate the repertoire of antibodies produced in mouse strains vaccinated with the new LVS vaccine preparation.

Methodology/Principal Findings

In the current study, we used an immunoproteomics approach to examine the repertoire of antibodies induced following successful immunization of BALB/c versus unsuccessful vaccination of C57BL/6 mice with the new preparation of F. tularensis LVS. Successful vaccination of BALB/c mice elicited antibodies to nine identified proteins that were not recognized by antisera from vaccinated but unprotected C57BL/6 mice. In addition, the CGMP formulation of LVS stimulated a greater repertoire of antibodies following vaccination compared to vaccination with laboratory passaged ATCC LVS strain. A total of 15 immunoreactive proteins were identified in both studies, however, 16 immunoreactive proteins were uniquely reactive with sera from the new formulation of LVS.

Conclusions/Significance

This is the first report characterising the antibody based immune response of the new formulation of LVS in the widely used murine model of tularemia. Using two mouse strains, we show that successfully vaccinated mice can be distinguished from unsuccessfully vaccinated mice based upon the repertoire of antibodies generated. This opens the door towards downselection of antigens for incorporation into tularemia subunit vaccines. In addition, this work also highlights differences in the humoral immune response to vaccination with the commonly used laboratory LVS strain and the new vaccine formulation of LVS.

The pathogenesis of acute pulmonary viral and bacterial infections: investigations in animal models

Acute viral and bacterial infections in the lower respiratory tract are major causes of morbidity and mortality worldwide. The proper study of pulmonary infections requires interdisciplinary collaboration among physicians and biomedical scientists to develop rational hypotheses based on clinical studies and to test these hypotheses in relevant animal models. Animal models for common lung infections are essential to understand pathogenic mechanisms and to clarify general mechanisms for host protection in pulmonary infections, as well as to develop vaccines and therapeutics. Animal models for uncommon pulmonary infections, such as those that can be caused by category A biothreat agents, are also very important because the infrequency of these infections in humans limits in-depth clinical studies. This review summarizes our understanding of innate and adaptive immune mechanisms in the lower respiratory tract and discusses how animal models for selected pulmonary pathogens can contribute to our understanding of the pathogenesis of lung infections and to the search for new vaccines and therapies.

Detoxified endotoxin vaccine (J5dLPS/OMP) protects mice against lethal respiratory challenge with Francisella tularensis SchuS4

Francisella tularensis is a category A select agent. J5dLPS/OMP is a novel vaccine construct consisting of detoxified, O-polysaccharide side chain-deficient, lipopolysaccharide non-covalently complexed with the outer membrane protein of N. meningitidis group B. Immunization elicits high-titer polyclonal antibodies specific for the highly-conserved epitopes expressed within the glycolipid core that constitutes gram-negative bacteria (e.g., F. tularensis). Mice immunized intranasally with J5dLPS/OMP exhibited protective immunity to intratracheal challenge with the live vaccine strain, as well as the highly-virulent SchuS4 strain, of F. tularensis. The efficacy of J5dLPS/OMP vaccine suggests its potential utility in immunizing the general population against several different gram-negative select agents concurrently.

Francisella virulence: significant advances, ongoing challenges and unmet needs

Francisella tularensis, the causative agent of tularemia, is an organism of concern as a potential biowarfare agent. Progress towards understanding the molecular basis of pathogenicity has been hampered by a lack of tools with which to manipulate the pathogen. However, the availability of genome sequence data for a range of strains and the development of a range of plasmids and mutagenesis protocols for use in Francisella has resulted in a huge advance in understanding. No licensed vaccine is yet available. Various approaches towards a new vaccine are being evaluated, but novel adjuvants and delivery systems are needed to induce the complex response required for immunity. Better animal models to more accurately represent human responses to infection are also required.

Working toward the future: insights into Francisella tularensis pathogenesis and vaccine development

Francisella tularensis is a facultative intracellular gram-negative pathogen and the etiological agent of the zoonotic disease tularemia. Recent advances in the field of Francisella genetics have led to a rapid increase in both the generation and subsequent characterization of mutant strains exhibiting altered growth and/or virulence characteristics within various model systems of infection. In this review, we summarize the major properties of several Francisella species, including F. tularensis and F. novicida, and provide an up-to-date synopsis of the genes necessary for pathogenesis by these organisms and the determinants that are currently being targeted for vaccine development.

Epitope-based vaccination against pneumonic tularemia

Francisella tularensis, the etiological agent of tularemia, is one of the most infectious bacterial pathogens known. No vaccine is currently approved for public use. Previously, we identified epitopes recognized specifically by T cells obtained from individuals following infection with F. tularensis. Here, we report that a subunit vaccine constructed based upon these epitopes elicited protective immunity in "humanized" HLA class II (DRB1*0401) transgenic mice. Vaccinated mice challenged intratracheally with a lethal dose of F. tularensis (Live Vaccine Strain) exhibited a rapid increase in pro-inflammatory cytokine production and diminished number of organisms in the lungs, and a concurrent increased rate of survival. These results demonstrate the efficacy of an epitope-based tularemia vaccine and suggest that such an approach might be widely applicable to the development of vaccines specific for intracellular bacterial pathogens.

Identification of a dominant CD4 T cell epitope in the membrane lipoprotein Tul4 from Francisella tularensis LVS

Francisella tularensis is a gram-negative intracellular bacterium that is the causative agent of tularemia. Small mammals such as rodents and rabbits, as well as some biting arthropods, serve as the main vectors for environmental reservoirs of F. tularensis. The low infectious dose, ability to aerosolize the organism, and the possibility of generating antibiotic resistant strains make F. tularensis a prime organism for use in bioterrorism. As a result, some strains of F. tularensis have been placed on the CDC category A select agent list. T cell immune responses are thought to be a critical component in protective immunity to this organism. However, investigation into the immune responses to F. tularensis has been hampered by the lack of molecularly defined epitopes. Here we report the identification of a major CD4(+) T cell epitope in C57Bl/6 (B6) mice. The murine model of F. tularensis infection is relevant as mice are a natural host for F. tularensis LVS and exhibit many of the same features of tularemia seen in humans. Using T cell hybridomas derived from B6 mice that had either been inoculated with F. tularensis and allowed to clear the infection or which had been immunized by conventional means using purified recombinant protein in adjuvant, we have identified amino acids 86-99 of the lipoprotein Tul4 (RLQWQAPEGSKCHD) as an immunodominant CD4 T cell epitope in B6 mice. This epitope is a major component of both the acute and memory responses to F. tularensis infection and can constitute as much as 20% of the responding CD4 T cells in an acute infection. Reactive T cells can also effectively enter the long-term memory T cell pool. The identification of this epitope will greatly aid in monitoring the course of F. tularensis infection and will also aid in the development of effective vaccine strategies for F. tularensis.

An approach to the identification of T cell epitopes in the genomic era: application to Francisella tularensis

The identification and characterization of epitopes is essential for modern immunologic studies. Here, we describe a novel methodology we have developed to identify T cell epitopes exploiting the phenomenon of cross presentation. Particulate antigens, in the form of beads, are very effective in delivering exogenous antigen to both the class I and class II pathways. We will review our efforts to screen entire genomes of pathogens for T cell epitopes taking advantage of the advances in genomics using Francisella tularensis as a model. By automating aspects of this technology we will be able to functionally screen the entire genome of F. tularensis for T cell epitopes. This technology should be applicable not only to F. tularensis, but also to many other pathogens as well.

Cellular and humoral immunity are synergistic in protection against types A and B Francisella tularensis

Herein we report studies with a novel combination vaccine that, when administered to mice, conferred protection against highly virulent strains of Francisella tularensis by stimulating both arms of the immune system. Our earlier studies with Ft.LVS::wbtA, an O-polysaccharide (OPS)-negative mutant derived from the available live vaccine strain of F. tularensis (Ft.LVS), elucidated the role of antibodies to the OPS - a key virulence determinant - in protection against virulent type A organisms. However, when expressed on the organism, the OPS enhances virulence. In contrast, in purified form, the OPS is completely benign. We hypothesized that a novel combination vaccine containing both a component that induces humoral immunity and a component that induces cellular immunity to this intracellular microbe would have an enhanced protective capacity over either component alone and would be much safer than the LVS vaccine. Thus we developed a combination vaccine containing both OPS (supplied in an OPS-tetanus toxoid glycoconjugate) to induce a humoral antibody response and strain Ft.LVS::wbtA (which is markedly attenuated by its lack of OPS) to induce a cell-mediated protective response. This vaccine protected mice against otherwise-lethal intranasal and intradermal challenge with wild-type F. tularensis strains Schu S4 (type A) and FSC 108 (type B). These results represent a significant advance in our understanding of immunity to F. tularensis and provide important insight into the development of a safer vaccine effective against infections caused by clinical type A and B strains of F. tularensis.

Diverse myeloid and lymphoid cell subpopulations produce gamma interferon during early innate immune responses to Francisella tularensis live vaccine strain

Francisella tularensis, a small gram-negative intracellular bacterium responsible for causing tularemia, is highly pathogenic and classified as a category A agent of bioterrorism. As for other intracellular pathogens, successful protective immune responses to Francisella tularensis require rapid and efficient induction of gamma interferon (IFN-gamma) production. Studies using intracellular bacteria such as Listeria monocytogenes as well as Francisella suggest that natural killer (NK) and T cells are important sources of IFN-gamma. However, comprehensive characterization of specific sources of IFN-gamma produced during Francisella infection in vivo remains incomplete, and depletion of NK cells before infection of mice with the F. tularensis live vaccine strain (LVS) has little impact on the course or outcome of infection. In this study, we determined the cell subpopulations that respond quickly to intradermal F. tularensis LVS infection of mice by producing IFN-gamma within hours to a few days. Splenic and liver lymphocytes were obtained from LVS-infected mice and analyzed for IFN-gamma mRNA by reverse transcription-PCR, for intracellular cytokine expression by multiparameter flow cytometry, and for ex vivo production of IFN-gamma protein by enzyme-linked immunosorbent assay. Cells producing IFN-gamma were readily detectable by day 3 after infection, and numbers progressively increased through days 5 to 7. Importantly, the cell types responsible for IFN-gamma production were much more varied than expected: these included not only NK cells and T cells, which might be predicted, but also other cells, including dendritic cells (DCs), "NK DCs," NK T cells, and neutrophils. Most importantly, since RAG-1 knockout mice appeared to exhibit a frequency of IFN-gamma-producing cells comparable to that of intact wild-type mice, early IFN-gamma production by innate immune cells does not depend on the presence of T or B cells.

CD4+ T cells are required during priming but not the effector phase of antibody-mediated IFN-gamma-dependent protective immunity against pulmonary Francisella novicida infection

We have previously demonstrated the protective efficacy of intranasal vaccination with a defined Francisella tularensis subsp. novicida DeltaiglC mutant (KKF24) against pulmonary F. novicida U112 challenge. In this study, we further characterized the mechanisms of KKF24-induced immunity. Intranasally vaccinated KKF24 C57BL/6 major histocompatibility class (MHC) class II-/- mice produced minimal antigen-specific interferon (IFN)-gamma and serum antibodies and were highly susceptible (0% survival) to F. novicida challenge, compared to MHC class I-/- or wild-type mice (both 100% survival). Protective immunity could be transferred by immune serum into recipient wild type, but not IFN-gamma-/- mice. The protective effect of KKF24 vaccination against the respiratory F. novicida U112 challenge was not abrogated by anti-CD4 neutralizing antibody treatment and was not conferred by adoptive transfer of KKF24-specific CD4+ T cells. The protective effect of antibody was partially dependent upon Fc receptor-mediated clearance. Taken together, our data indicate that CD4+ T cells are required for priming, but not during the effector phase, of anti-KKF24 antibody-mediated IFN-gamma-dependent immunity against pulmonary F. novicida infection.

Immunization with heat-killed Francisella tularensis LVS elicits protective antibody-mediated immunity

Francisella tularensis (FT) has been classified by the CDC as a category A pathogen because of its high virulence and the high mortality rate associated with infection via the aerosol route. Because there is no licensed vaccine available for FT, development of prophylactic and therapeutic regimens for the prevention/treatment of infection is a high priority. In this report, heat-killed FT live vaccine strain (HKLVS) was employed as a vaccine immunogen, either alone or in combination with an adjuvant, and was found to elicit protective immunity against high-dose FT live vaccine strain (FTLVS) challenge. FT-specific antibodies produced in response to immunization with HKLVS alone were subsequently found to completely protect naive mice against high-dose FT challenge in both infection-interference and passive immunization experiments. Additional passive immunization trials employing serum collected from mice immunized with a heat-killed preparation of an O-antigen-deficient transposon mutant of FTLVS (HKLVS-OAg(neg)) yielded similar results. These findings demonstrated that FT-specific antibodies alone can confer immunity against high-dose FTLVS challenge, and they reveal that antibody-mediated protection is not dependent upon production of LPS-specific antibodies.

Immunodominant Francisella tularensis antigens identified using proteome microarray

Stimulation of protective immune responses against intracellular pathogens is difficult to achieve using non-replicating vaccines. BALB/c mice immunized by intramuscular injection with killed Francisella tularensis (live vaccine strain) adjuvanted with preformed immune stimulating complexes admixed with CpG, were protected when systemically challenged with a highly virulent strain of F. tularensis (Schu S4). Serum from immunized mice was used to probe a whole proteome microarray in order to identify immunodominant antigens. Eleven out of the top 12 immunodominant antigens have been previously described as immunoreactive in F. tularensis. However, 31 previously unreported immunoreactive antigens were revealed using this approach. Twenty four (50%) of the ORFs on the immunodominant hit list belonged to the category of surface or membrane associated proteins compared to only 22% of the entire proteome. There were eight hypothetical protein hits and eight hits from proteins associated with different aspects of metabolism. The chip also allowed us to readily determine the IgG subclass bias, towards individual or multiple antigens, in protected and unprotected animals. These data give insight into the protective immune response and have potentially important implications for the rational design of non-living vaccines for tularemia and other intracellular pathogens.

Protection afforded against aerosol challenge by systemic immunisation with inactivated Francisella tularensis live vaccine strain (LVS)

BALB/c mice were immunised with inactivated Francisella tularensis live vaccine strain (LVS) and the level of protection afforded against aerosol challenge with virulent strains of F. tularensis ascertained. Intramuscular (IM) injection of inactivated LVS with an aluminium-hydroxide-based adjuvant-stimulated IgG1-biased LVS-specific antibody responses and afforded no protection against aerosol challenge with subspecies holarctica (strain HN63). Conversely, IM injection of inactivated LVS adjuvanted with preformed immune-stimulating complexes (ISCOMS) admixed with immunostimulatory CpG oligonucleotides afforded robust protection against aerosol-initiated infection with HN63. However, despite a significantly extended time-to-death relative to naïve controls, the majority of mice immunised with the most potent vaccine formulation were not protected against a low-dose aerosol challenge with subspecies tularensis (strain Schu S4). These data indicate that parenterally administered non-living vaccines can be used for effective immunisation against aerosol challenges with subspecies holarctica, although not high virulence strains of F. tularensis.

A defined O-antigen polysaccharide mutant of Francisella tularensis live vaccine strain has attenuated virulence while retaining its protective capacity

Francisella tularensis, the causative agent of tularemia, has been designated a CDC category A select agent because of its low infective dose (<10 CFU), its ready transmission by aerosol, and its ability to produce severe morbidity and high mortality. The identification and characterization of this organism's virulence determinants will facilitate the development of a safe and effective vaccine. We report that inactivation of the wbtA-encoded dehydratase of the O-antigen polysaccharide (O-PS) locus of the still-unlicensed live vaccine strain of F. tularensis (LVS) results in a mutant (the LVS wbtA mutant) with remarkably attenuated virulence. Western blot analysis and immune electron microscopy studies associate this loss of virulence with a complete lack of surface O-PS expression. A likely mechanism for attenuation is shown to be the transformation from serum resistance in the wild-type strain to serum sensitivity in the mutant. Despite this significant attenuation in virulence, the LVS wbtA mutant remains immunogenic and confers protective immunity on mice against challenge with an otherwise lethal dose of either F. tularensis LVS or a fully virulent clinical isolate of F. tularensis type B. Recognition and characterization of the pivotal role of O-PS in the virulence of this intracellular bacterial pathogen may have broad implications for the creation of a safe and efficacious vaccine.

Diversity of Francisella tularensis Schu4 antigens recognized by T lymphocytes after natural infections in humans: identification of candidate epitopes for inclusion in a rationally designed tularemia vaccine

The T lymphocyte antigens, which may have a role in protection against tularemia, were predicted by immunoinformatics analysis of Francisella tularensis Schu4. Twenty-seven class II putative promiscuous epitopes and 125 putative class I supertype epitopes were chosen for synthesis; peptides were tested in vitro for their ability to bind HLA and to induce immune responses from PBMCs of 23 previously infected subjects. While the immune responses of individual subjects showed heterogeneity, 95% of the subjects responded strongly to a pool of 27 promiscuous peptides; 25%, 33%, and 44% of subjects responded to pools of 25 A2, A24, and B7 peptides, respectively. These data can aid in the development of novel epitope-based and subunit tularemia vaccines.

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.

Virulence of Francisella spp. in chicken embryos

We examined the utility of infecting chicken embryos as a means of evaluating the virulence of different Francisella sp. strains and mutants. Infection of 7-day-old chicken embryos with a low dose of F. novicida or F. tularensis subsp. holarctica live vaccine strain (LVS) resulted in sustained growth for 6 days. Different doses of these two organisms were used to inoculate chicken embryos to determine the time to death. These experiments showed that wild-type F. novicida was at least 10,000-fold more virulent than the LVS strain. We also examined the virulence of several attenuated mutants of F. novicida, and they were found to have a wide range of virulence in chicken embryos. Fluorescent microscopic examination of infected chicken embryo organs revealed that F. tularensis grew in scattered foci of infections, and in all cases the F. tularensis appeared to be growing intracellularly. These results demonstrate that infection of 7-day-old chicken embryos can be used to evaluate the virulence of attenuated F. tularensis strains.

Immunoproteomic analysis of the murine antibody response to successful and failed immunization with live anti-Francisella vaccines

Francisella tularensis subspecies tularensis is one of the most virulent of bacterial pathogens for humans. Protective immunity against the pathogen can be induced in humans and some, but not all, mouse strains by vaccination with live, but not killed, vaccines. In mice, this protection is mediated predominantly by CD4+ and CD8+ T cells. This is thought to be the case too for humans. Nevertheless, it is possible that successful vaccination elicits antigen-specific antibodies that can serve as correlates of protection. To test this hypothesis we examined the repertoire of antibodies induced following successful immunization of BALB/c and CH3/HeN mice versus unsuccessful vaccination of C57BL/6 and DBA\2 mice with F. tularensis Live Vaccine Strain or following unsuccessful vaccination of BALB/c mice with highly related subspecies, F. novicida. The results showed that successful vaccination elicited antibodies to at least six proteins that were not recognized by antisera from vaccinated but unprotected mice.

Critical role for serum opsonins and complement receptors CR3 (CD11b/CD18) and CR4 (CD11c/CD18) in phagocytosis of Francisella tularensis by human dendritic cells (DC): uptake of Francisella leads to activation of immature DC and intracellular survival of the bacteria

Francisella tularensis is one of the most infectious human pathogens known. Although much has been learned about the immune response of mice using an attenuated live vaccine strain (LVS) derived from F. tularensis subspecies holarctica (Type B), little is known about the responses of human monocyte-derived immature dendritic cells (DC). Here, we show that optimal phagocytosis of LVS by DC is dependent on serum opsonization. We demonstrate that complement factor C3-derived opsonins and the major complement receptors expressed by DC, the integrins CR3 (CD11b/CD18) and CR4 (CD11c/CD18), play a critical role in this adhesion-mediated phagocytosis. LVS induced proinflammatory cytokine production and up-regulation of costimulatory surface proteins (CD40, CD86, and MHC Class II) on DC but resisted killing. Once taken up, LVS grew intracellularly, resulting in DC death. DC maturation and cytokine production were induced by direct contact/phagocytosis of LVS or interaction with soluble products of the bacteria, and enhanced activation was seen when LVS was pretreated with serum. Sonicated LVS and supernatants from LVS cultures were potent activators of DC, but LVS LPS failed to activate DC maturation or cytokine production. Serum-treated LVS rapidly induced (within 6 h) a number of cytokines including IL-10, a potent suppressor of macrophage functions and down-regulator of Th1-like responses and the Th1 response inducer IL-12. These results suggest that the simultaneous production of an activating (IL-12, IL-1beta, and TNF-alpha) and a suppressing (IL-10) cytokine profile could contribute to the immunopathogenesis of tularemia.

Intranasal vaccination with a defined attenuated Francisella novicida strain induces gamma interferon-dependent antibody-mediated protection against tularemia

Francisella tularensis is an intracellular gram-negative bacterium that is the causative agent of tularemia and a potential bioweapon. We have characterized the efficacy of a defined F. novicida mutant (DeltaiglC) as a live attenuated vaccine against subsequent intranasal challenge with the wild-type organism. Animals primed with the F. novicida DeltaiglC (KKF24) mutant induced robust splenic gamma interferon (IFN-gamma) and interleukin-12 (IL-12) recall responses with negligible IL-4 production as well as the production of antigen-specific serum immunoglobulin G1 (IgG1) and IgG2a antibodies. BALB/c mice vaccinated intranasally (i.n.) with KKF24 and subsequently challenged with wild-type F. novicida (100 and 1,000 50% lethal doses) were highly protected (83% and 50% survival, respectively) from the lethal challenges. The protection conferred by KKF24 vaccination was shown to be highly dependent on endogenous IFN-gamma production and also was mediated by antibodies that could be adoptively transferred to naive B-cell-deficient mice by inoculation of immune sera. Collectively, the results demonstrate that i.n. vaccination with KKF24 induces a vigorous Th1-type cytokine and antibody response that is protective against subsequent i.n. challenge with the wild-type strain. This is the first report of a defined live attenuated strain providing protection against the inhalation of F. novicida.

CD4-CD8- T cells control intracellular bacterial infections both in vitro and in vivo

Memory T cells, including the well-known CD4⁺ and CD8⁺ T cells, are central components of the acquired immune system and are the basis for successful vaccination. After infection, CD4⁺ and CD8⁺ T cells expand into effector cells, and then differentiate into long-lived memory cells. We show that a rare population of CD4⁻CD8⁻CD3⁺αβ⁺γδ⁻NK1.1⁻ T cells has similar functions. These cells potently and specifically inhibit the growth of the intracellular bacteria Mycobacterium tuberculosis (M. tb.) or Francisella tularensis Live Vaccine Strain (LVS) in macrophages in vitro, promote survival of mice infected with these organisms in vivo, and adoptively transfer immunity to F. tularensis LVS. Furthermore, these cells expand in the spleens of mice infected with M. tb. or F. tularensis LVS, and then acquire a memory cell phenotype. Thus, CD4⁻CD8⁻ T cells have a role in the control of intracellular infection and may contribute to successful vaccination.

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.

Intranasal interleukin-12 treatment for protection against respiratory infection with the Francisella tularensis live vaccine strain

Francisella tularensis is a gram-negative intracellular bacterium that can induce lethal respiratory infection in humans and rodents. However, little is known about the role of innate or adaptive immunity in protection from respiratory tularemia. In the present study, the role of interleukin-12 (IL-12) in inducing protective immunity in the lungs against intranasal infection of mice with the live vaccine strain (LVS) of F. tularensis was investigated. It was found that gamma interferon (IFN-gamma) and IL-12 were strictly required for protection, since mice deficient in IFN-gamma, IL-12 p35, or IL-12 p40 all succumbed to LVS doses that were sublethal for wild-type mice. Furthermore, exogenous IL-12 treatment 24 h before intranasal infection with a lethal dose of LVS (10,000 CFU) significantly decreased bacterial loads in the lungs, livers, and spleens of wild-type BALB/c and C57BL/6 mice and allowed the animals to survive infection; such protection was not observed in IFN-gamma-deficient mice. The resistance induced by IL-12 to LVS infection was still observed in NK cell-deficient beige mice but not in CD8-/- mice. These results demonstrate that exogenous IL-12 delivered intranasally can prevent respiratory tularemia through a mechanism that is at least partially dependent upon the expression of IFN-gamma and CD8 T cells.

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.

Aerosol-, but not intradermal-immunization with the live vaccine strain of Francisella tularensis protects mice against subsequent aerosol challenge with a highly virulent type A strain of the pathogen by an αβ T cell- and interferon gamma- dependent mechanism

Francisella tularensis is an extremely virulent facultative intracellular bacterial pathogen of many mammalian species including mice and humans in which it causes a spectrum of disease collectively called tularemia. In humans, intradermal or inhaled inocula of 10cfu or less of the most virulent strains of the pathogen are sufficient to cause severe infection and possible death; in mice similar inocula are routinely lethal. An attenuated live vaccine strain, F. tularensis LVS, was developed almost 50 years ago, and remains the sole prophylactic against virulent strains of the pathogen. Using F. tularensis LVS as a model vaccine, we recently showed that it was possible to systemically immunize various mouse strains and protect them against subsequent massive (2000 cfu) intradermal (i.d.) challenge, but not against low dose (approximately 10 cfu) aerosol challenge, with virulent strains of the pathogen. This is troubling because the latter route is considered an important means of deliberately disseminating F. tularensis in a bioterrorist attack. Others have previously shown that administering LVS to humans, guinea pigs and monkeys as an aerosol enhanced protection against subsequent aerosol challenge with virulent F. tularensis. In the present study, we show the same phenomenon in BALB/c and C3H/HeN mice. In this model, interferon gamma (IFNgamma) and CD4+ and CD8+ T cells are essential for the expression of anti-Francisella immunity in the lungs. Combined this immune response operates by limiting dissemination of the pathogen to susceptible internal organs. Further, understanding of how inhaled LVS elicits local cell-mediated protective immunity will be critical for devising improved vaccines against pulmonary tularemia.

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.

Susceptibility of immunodeficient mice to aerosol and systemic infection with virulent strains of Francisella tularensis

Previous studies have shown that IFN-gamma, TNF-alpha and NOS-2, but not B cells, are crucial for host defense against primary systemic infection with the attenuated live vaccine strain (LVS) of Francisella tularensis. In this study, we examined the importance of these and additional immune components in host resistance against infection with virulent strains of F. tularensis initiated by systemic and airborne routes. Wild-type (WT) mice and mice deficient in IFN-gamma, TNFR1R2, NOS-2, or B cells were equally susceptible to low dose ( approximately 10 colony forming units) aerosol or intradermal challenge with virulent type B F. tularensis, and succumbed to the infection between days 6 and 8 post-inoculation. Quantitative bacteriology showed that IFN-gamma-/- and B cell-/- mice consistently harbored up to one log(10) more bacteria in their lungs, spleens and livers than WT mice at day 5 post aerosol exposure. Surprisingly, however, compared to other strains of KO mice and WT control mice, IFN-gamma-/- mice showed only mild liver damage as assessed by histopathology and liver function tests. Additional experiments established that even mice with broad immunodeficiency (SCID, neutropenic, splenectomized or thymectomized mice and mice treated with corticosteroid) were no more susceptible to aerosol-initiated infection with virulent type B or type A F. tularensis than immunosufficient control mice. Combined, our results indicate that, unlike LVS, normal type A and type B F. tularensis strains are so extremely virulent that even immunocompetent mice are virtually defenseless to low dose aerosol and intradermal challenges with them.

Multiple T cell subsets control Francisella tularensis LVS intracellular growth without stimulation through macrophage interferon gamma receptors

A variety of data suggest that in vivo production of interferon (IFN)-gamma is necessary, but not sufficient, for expression of secondary protective immunity against intracellular pathogens. To discover specific IFN-gamma-independent T cell mediated mechanisms, we took advantage of an in vitro culture system that models in vivo immune responses to the intracellular bacterium Francisella tularensis live vaccine strain (LVS). LVS-immune lymphocytes specifically controlled 99% of the growth of LVS in wild-type murine bone marrow-derived macrophages. Surprisingly, LVS-immune lymphocytes also inhibited LVS intracellular growth by as much as 95% in macrophages derived from IFN-gamma receptor knockout (IFNgammaR KO) mice. CD8+ T cells, and to a lesser degree CD4+ T cells, controlled LVS intracellular growth in both wild-type and IFNgammaR KO macrophages. Further, a unique population of Thy1+alphabeta+CD4-CD8- cells that was previously suggested to operate during secondary immunity to LVS in vivo strongly controlled LVS intracellular growth in vitro. A large proportion of the inhibition of LVS intracellular growth in IFNgammaR KO macrophages by all three T cell subsets could be attributed to tumor necrosis factor (TNF) alpha. Thus, T cell mechanisms exist that control LVS intracellular growth without acting through the IFN-gamma receptor; such control is due in large part to TNF-alpha, and is partially mediated by a unique double negative T cell subpopulation.