Infection of mice with Francisella as an immunological model

Francisella tularensis Live Vaccine Strain training of murine alveolar and bone marrow-derived macrophages

Traditionally, successful vaccines rely on specific adaptive immunity by activating lymphocytes with an attenuated pathogen, or pathogen subunit, to elicit heightened responses upon subsequent exposures. However, recent work with Mycobacterium tuberculosis and other pathogens has identified a role for "trained" monocytes in protection through memory-like but non-specific immunity. Here, we used an in vitro co-culture approach to study the potential role of trained macrophages, including lung alveolar macrophages, in immune responses to the Live Vaccine Strain (LVS) of Francisella tularensis. F. tularensis is an intracellular bacterium that replicates within mammalian macrophages and causes respiratory as well as systemic disease. We vaccinated mice with F. tularensis LVS and then obtained lung alveolar macrophages, or derived macrophages from bone marrow. LVS infected and replicated comparably in both types of macrophages, whether naïve or from LVS-vaccinated mice. LVS-infected macrophages were then co-cultured with either naïve splenocytes, splenocytes from mice vaccinated intradermally, or splenocytes from mice vaccinated intravenously. For the first time, we show that immune (but not naïve) splenocytes controlled bacterial replication within alveolar macrophages, similar to previous results using bone marrow-derived macrophage. However, no differences in control of intramacrophage bacterial replication were found between co-cultures with naïve macrophages or macrophages from LVS-vaccinated mice; furthermore, nitric oxide levels and interferon-gamma production in supernatants were largely comparable across all conditions. Thus, in the context of in vitro co-cultures, the data do not support development of trained macrophages in bone marrow or lungs of mice vaccinated with LVS intradermally or intravenously.

IMPORTANCE

The discovery of non-specific "trained immunity" in monocytes has generated substantial excitement. However, to date, training has been studied with relatively few microbes (e.g., Mycobacterium bovis Bacille Calmette-Guérin, a live attenuated intracellular bacterium used as a vaccine) and microbial substances (e.g., LPS), and it remains unclear whether training during infection is common. We previously demonstrated that vaccination of mice with Francisella tularensis Live Vaccine Strain (LVS), another live attenuated intracellular bacterium, protected against challenge with the unrelated bacterium Listeria monocytogenes. The present study therefore tested whether LVS vaccination engenders trained macrophages that contributed to this protection. To do so, we used a previous in vitro co-culture approach with murine bone marrow-derived macrophages to expand and study lung alveolar macrophages. We demonstrated that alveolar macrophages can be productively infected and employed to characterize interactions with LVS-immune lymphocytes. However, we find no evidence that either bone marrow-derived or alveolar macrophages are trained by LVS vaccination.

In vivo and in vitro immune responses against Francisella tularensis vaccines are comparable among Fischer 344 rat substrains

Identifying suitable animal models and standardizing preclinical methods are important for the generation, characterization, and development of new vaccines, including those against Francisella tularensis. Non-human primates represent an important animal model to evaluate tularemia vaccine efficacy, and the use of correlates of vaccine-induced protection may facilitate bridging immune responses from non-human primates to people. However, among small animals, Fischer 344 rats represent a valuable resource for initial studies to evaluate immune responses, to identify correlates of protection, and to screen novel vaccines. In this study, we performed a comparative analysis of three Fischer rat substrains to determine potential differences in immune responses, to evaluate methods used to quantify potential correlates of protection, and to evaluate protection after vaccination. To this end, we took advantage of data previously generated using one of the rat substrains by evaluating two live vaccines, LVS and F. tularensis SchuS4-ΔclpB (ΔclpB). We compared immune responses after primary vaccination, adaptive immune responses upon re-stimulation of leukocytes in vitro, and sensitivity to aerosol challenge. Despite some detectable differences, the results highlight the similarity of immune responses to tularemia vaccines and challenge outcomes between the three substrains, indicating that all offer acceptable and comparable approaches as animal models to study Francisella infection and immunity.

IL-12p40 is essential but not sufficient for Francisella tularensis LVS clearance in chronically infected mice

IL-12p40 plays an important role in F. tularensis Live Vaccine Strain (LVS) clearance that is independent of its functions as a part of the heterodimeric cytokines IL-12p70 or IL-23. In contrast to WT, p35, or p19 knockout (KO) mice, p40 KO mice infected with LVS develop a chronic infection that does not resolve. Here, we further evaluated the role of IL-12p40 in F. tularensis clearance. Despite reduced IFN-γ production, primed splenocytes from p40 KO and p35 KO mice appeared functionally similar to those from WT mice during in vitro co-culture assays of intramacrophage bacterial growth control. Gene expression analysis revealed a subset of genes that were upregulated in re-stimulated WT and p35 KO splenocytes, but not p40 KO splenocytes, and thus are candidates for involvement in F. tularensis clearance. To directly evaluate a potential mechanism for p40 in F. tularensis clearance, we reconstituted protein levels in LVS-infected p40 KO mice using either intermittent injections of p40 homodimer (p80) or treatment with a p40-producing lentivirus construct. Although both delivery strategies yielded readily detectable levels of p40 in sera and spleens, neither treatment had a measurable impact on LVS clearance by p40 KO mice. Taken together, these studies demonstrate that clearance of F. tularensis infection depends on p40, but p40 monomers and/or dimers alone are not sufficient.

Working correlates of protection predict SchuS4-derived-vaccine candidates with improved efficacy against an intracellular bacterium, Francisella tularensis

Francisella tularensis, the causative agent of tularemia, is classified as Tier 1 Select Agent with bioterrorism potential. The efficacy of the only available vaccine, LVS, is uncertain and it is not licensed in the U.S. Previously, by using an approach generally applicable to intracellular pathogens, we identified working correlates that predict successful vaccination in rodents. Here, we applied these correlates to evaluate a panel of SchuS4-derived live attenuated vaccines, namely SchuS4-ΔclpB, ΔclpB-ΔfupA, ΔclpB-ΔcapB, and ΔclpB-ΔwbtC. We combined in vitro co-cultures to quantify rodent T-cell functions and multivariate regression analyses to predict relative vaccine strength. The predictions were tested by rat vaccination and challenge studies, which demonstrated a clear relationship between the hierarchy of in vitro measurements and in vivo vaccine protection. Thus, these studies demonstrated the potential power a panel of correlates to screen and predict the efficacy of Francisella vaccine candidates, and in vivo studies in Fischer 344 rats confirmed that SchuS4-ΔclpB and ΔclpB-ΔcapB may be better vaccine candidates than LVS.

Alginate microencapsulation of an attenuated O-antigen mutant of Francisella tularensis LVS as a model for a vaccine delivery vehicle

Francisella tularensis is the etiologic agent of tularemia and a Tier I Select Agent. Subspecies tularensis (Type A) is the most virulent of the four subspecies and inhalation of as few as 10 cells can cause severe disease in humans. Due to its niche as a facultative intracellular pathogen, a successful tularemia vaccine must induce a robust cellular immune response, which is best achieved by a live, attenuated strain. F. tularensis strains lacking lipopolysaccharide (LPS) O-antigen are highly attenuated, but do not persist in the host long enough to induce protective immunity. Increasing the persistence of an O-antigen mutant may help stimulate protective immunity. Alginate encapsulation is frequently used with probiotics to increase persistence of bacteria within the gastrointestinal system, and was used to encapsulate the highly attenuated LVS O-antigen mutant WbtI_G191V. Encapsulation with alginate followed by a poly-L-lysine/alginate coating increased survival of WbtI_G191V in complement-active serum. In addition, BALB/c mice immunized intraperitoneally with encapsulated WbtI_G191V combined with purified LPS survived longer than mock-immunized mice following intranasal challenge. Alginate encapsulation of the bacteria also increased antibody titers compared to non-encapsulated bacteria. These data suggest that alginate encapsulation provides a slow-release vehicle for bacterial deposits, as evidenced by the increased antibody titer and increased persistence in serum compared to freely suspended cells. Survival of mice against high-dose intranasal challenge with the LVS wildtype was similar between mice immunized within alginate capsules or with LVS, possibly due to the low number of animals used, but bacterial loads in the liver and spleen were the lowest in mice immunized with WbtI_G191V and LPS in beads. However, an analysis of the immune response of surviving mice indicated that those vaccinated with the alginate vehicle upregulated cell-mediated immune pathways to a lesser extent than LVS-vaccinated mice. In summary, this vehicle, as formulated, may be more effective for pathogens that require predominately antibody-mediated immunity.

Francisella tularensis induces Th1 like MAIT cells conferring protection against systemic and local infection

Mucosal-associated Invariant T (MAIT) cells are recognized for their antibacterial functions. The protective capacity of MAIT cells has been demonstrated in murine models of local infection, including in the lungs. Here we show that during systemic infection of mice with Francisella tularensis live vaccine strain results in evident MAIT cell expansion in the liver, lungs, kidney and spleen and peripheral blood. The responding MAIT cells manifest a polarised Th1-like MAIT-1 phenotype, including transcription factor and cytokine profile, and confer a critical role in controlling bacterial load. Post resolution of the primary infection, the expanded MAIT cells form stable memory-like MAIT-1 cell populations, suggesting a basis for vaccination. Indeed, a systemic vaccination with synthetic antigen 5-(2-oxopropylideneamino)-6-D-ribitylaminouracil in combination with CpG adjuvant similarly boosts MAIT cells, and results in enhanced protection against both systemic and local infections with different bacteria. Our study highlights the potential utility of targeting MAIT cells to combat a range of bacterial pathogens.

Deletion Mutants of Francisella Phagosomal Transporters FptA and FptF Are Highly Attenuated for Virulence and Are Protective Against Lethal Intranasal Francisella LVS Challenge in a Murine Model of Respiratory Tularemia

Francisella tularensis (Ft) is a Gram-negative, facultative intracellular bacterium that is a Tier 1 Select Agent of concern for biodefense for which there is no licensed vaccine. A subfamily of 9 Francisella phagosomal transporter (fpt) genes belonging to the Major Facilitator Superfamily of transporters was identified as critical to pathogenesis and potential targets for attenuation and vaccine development. We evaluated the attenuation and protective capacity of LVS derivatives with deletions of the fptA and fptF genes in the C57BL/6J mouse model of respiratory tularemia. LVSΔfptA and LVSΔfptF were highly attenuated with LD₅₀ values of >20 times that of LVS when administered intranasally and conferred 100% protection against lethal challenge. Immune responses to the fpt mutant strains in mouse lungs on day 6 post-infection were substantially modified compared to LVS and were associated with reduced organ burdens and reduced pathology. The immune responses to LVSΔfptA and LVSΔfptF were characterized by decreased levels of IL-10 and IL-1β in the BALF versus LVS, and increased numbers of B cells, αβ and γδ T cells, NK cells, and DCs versus LVS. These results support a fundamental requirement for FptA and FptF in the pathogenesis of Ft and the modulation of the host immune response.

Tick extracellular vesicles enable arthropod feeding and promote distinct outcomes of bacterial infection

Extracellular vesicles are thought to facilitate pathogen transmission from arthropods to humans and other animals. Here, we reveal that pathogen spreading from arthropods to the mammalian host is multifaceted. Extracellular vesicles from Ixodes scapularis enable tick feeding and promote infection of the mildly virulent rickettsial agent Anaplasma phagocytophilum through the SNARE proteins Vamp33 and Synaptobrevin 2 and dendritic epidermal T cells. However, extracellular vesicles from the tick Dermacentor andersoni mitigate microbial spreading caused by the lethal pathogen Francisella tularensis. Collectively, we establish that tick extracellular vesicles foster distinct outcomes of bacterial infection and assist in vector feeding by acting on skin immunity. Thus, the biology of arthropods should be taken into consideration when developing strategies to control vector-borne diseases.

Finafloxacin Is an Effective Treatment for Inhalational Tularemia and Plague in Mouse Models of Infection

Infection with aerosolized Francisella tularensis or Yersinia pestis can lead to lethal disease in humans if treatment is not initiated promptly. Finafloxacin is a novel fluoroquinolone which has demonstrated broad-spectrum activity against a range of bacterial species in vitro, in vivo, and in humans, activity which is superior in acidic, infection-relevant conditions. Human-equivalent doses of finafloxacin or ciprofloxacin were delivered at 24 h (representing prophylaxis) or at 72 or 38 h (representing treatment) postchallenge with F. tularensis or Y. pestis, respectively, in BALB/c mouse models. In addition, a short course of therapy (3 days) was compared to a longer course (7 days). Both therapies provided a high level of protection against both infections when administered at 24 h postchallenge, irrespective of the length of the dosing regimen; however, differences were observed when therapy was delayed. A benefit was demonstrated with finafloxacin compared to ciprofloxacin in both models when therapy was delivered later in the infection. These studies suggest that finafloxacin is an effective alternative therapeutic for the prophylaxis and treatment of inhalational infections with F. tularensis or Y. pestis.

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

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

Virulence of Francisella tularensis Subspecies holarctica Biovar japonica and Phenotypic Change during Serial Passages on Artificial Media

Francisella tularensis (F. tularensis) is the etiological agent of the zoonotic disease tularemia. F. tularensis subspecies holarctica biovar japonica has rarely been isolated in Japan and is considered to have moderate virulence, although the biological properties of fresh isolates have not been analyzed in detail. Here, we analyzed the virulence of two strains of F. tularensis subspecies holarctica biovar japonica (NVF1 and KU-1) and their phenotypic stability during serial passages in Eugon chocolate agar (ECA) and Chamberlain's chemically defined medium (CDM) based agar (CDMA). C57BL/6 mice intradermally inoculated with 10¹ colony-forming units of NVF1 or KU-1 died within 9 days, with a median time to death of 7.5 and 7 days, respectively. Both NVF1 and KU-1 strains passaged on ECA 10 times had comparable virulence prior to passaging, whereas strains passaged on ECA 20 times and on CDMA 50 times were attenuated. Attenuated strains had decreased viability in 0.01% H₂O₂ and lower intracellular growth rates, suggesting both properties are important for F. tularensis virulence. Additionally, passage on ECA of the KU-1 strains altered lipopolysaccharide antigenicity and bacterial susceptibility to β-lactam antibiotics. Our data demonstrate F. tularensis strain virulence in Japan and contribute to understanding phenotypic differences between natural and laboratory environments.

Immune lymphocytes halt replication of Francisella tularensis LVS within the cytoplasm of infected macrophages

Francisella tularensis is a highly infectious intracellular bacterium that causes tularemia by invading and replicating in mammalian myeloid cells. Francisella primarily invades host macrophages, where it escapes phagosomes within a few hours and replicates in the cytoplasm. Less is known about how Francisella traffics within macrophages or exits into the extracellular environment for further infection. Immune T lymphocytes control the replication of Francisella within macrophages in vitro by a variety of mechanisms, but nothing is known about intracellular bacterial trafficking in the face of such immune pressure. Here we used a murine model of infection with a Francisella attenuated live vaccine strain (LVS), which is under study as a human vaccine, to evaluate the hypothesis that immune T cells control intramacrophage bacterial growth by re-directing bacteria into toxic intracellular compartments of infected macrophages. We visualized the interactions of lymphocytes and LVS-infected macrophages using confocal microscopy and characterized LVS intramacrophage trafficking when co-cultured with immune lymphocytes. We focused on the late stages of infection after bacteria escape from phagosomes, through bacterial replication and the death of macrophages. We found that the majority of LVS remained cytosolic in the absence of immune pressure, eventually resulting in macrophage death. In contrast, co-culture of LVS-infected macrophages with LVS-immune lymphocytes halted LVS replication and inhibited the spread of LVS infection between macrophages, but bacteria did not return to vacuoles such as lysosomes or autophagosomes and macrophages did not die. Therefore, immune lymphocytes directly limit intracellular bacterial replication within the cytoplasm of infected macrophages.

Innate Immune Recognition: An Issue More Complex Than Expected

Primary interaction of an intracellular bacterium with its host cell is initiated by activation of multiple signaling pathways in response to bacterium recognition itself or as cellular responses to stress induced by the bacterium. The leading molecules in these processes are cell surface membrane receptors as well as cytosolic pattern recognition receptors recognizing pathogen-associated molecular patterns or damage-associated molecular patterns induced by the invading bacterium. In this review, we demonstrate possible sequences of events leading to recognition of Francisella tularensis, present findings on known mechanisms for manipulating cell responses to protect Francisella from being killed, and discuss newly published data from the perspective of early stages of host-pathogen interaction.

A panel of correlates predicts vaccine-induced protection of rats against respiratory challenge with virulent Francisella tularensis

There are no defined correlates of protection for any intracellular pathogen, including the bacterium Francisella tularensis, which causes tularemia. Evaluating vaccine efficacy against sporadic diseases like tularemia using field trials is problematic, and therefore alternative strategies to test vaccine candidates like the Francisella Live Vaccine Strain (LVS), such as testing in animals and applying correlate measurements, are needed. Recently, we described a promising correlate strategy that predicted the degree of vaccine-induced protection in mice given parenteral challenges, primarily when using an attenuated Francisella strain. Here, we demonstrate that using peripheral blood lymphocytes (PBLs) in this approach predicts LVS-mediated protection against respiratory challenge of Fischer 344 rats with fully virulent F. tularensis, with exceptional sensitivity and specificity. Rats were vaccinated with a panel of LVS-derived vaccines and subsequently given lethal respiratory challenges with Type A F. tularensis. In parallel, PBLs from vaccinated rats were evaluated for their functional ability to control intramacrophage Francisella growth in in vitro co-culture assays. PBLs recovered from co-cultures were also evaluated for relative gene expression using a large panel of genes identified in murine studies. In vitro control of LVS intramacrophage replication reflected the hierarchy of protection. Further, despite variability between individuals, 22 genes were significantly more up-regulated in PBLs from rats vaccinated with LVS compared to those from rats vaccinated with the variant LVS-R or heat-killed LVS, which were poorly protective. These genes included IFN-γ, IL-21, NOS2, LTA, T-bet, IL-12rβ2, and CCL5. Most importantly, combining quantifications of intramacrophage growth control with 5-7 gene expression levels using multivariate analyses discriminated protected from non-protected individuals with greater than 95% sensitivity and specificity. The results therefore support translation of this approach to non-human primates and people to evaluate new vaccines against Francisella and other intracellular pathogens.

Increased Resistance to Intradermal Francisella tularensis LVS Infection by Inactivation of the Sts Phosphatases

The Suppressor of TCR signaling proteins (Sts-1 and Sts-2) are two homologous phosphatases that negatively regulate signaling pathways in a number of hematopoietic lineages, including T lymphocytes. Mice lacking Sts expression are characterized by enhanced T cell responses. Additionally, a recent study demonstrated that Sts^-/- mice are profoundly resistant to systemic infection by Candida albicans, with resistance characterized by enhanced survival, more rapid fungal clearance in key peripheral organs, and an altered inflammatory response. To investigate the role of Sts in the primary host response to infection by a bacterial pathogen, we evaluated the response of Sts^-/- mice to infection by a Gram-negative bacterial pathogen. Francisella tularensis is a facultative bacterial pathogen that replicates intracellularly within a variety of cell types and is the causative agent of tularemia. Francisella infections are characterized by a delayed immune response, followed by an intense inflammatory reaction that causes widespread tissue damage and septic shock. Herein, we demonstrate that mice lacking Sts expression are significantly resistant to infection by the live vaccine strain (LVS) of F. tularensis Resistance is characterized by reduced lethality following high-dose intradermal infection, an altered cytokine response in the spleen, and enhanced bacterial clearance in multiple peripheral organs. Sts^-/- bone marrow-derived monocytes and neutrophils, infected with F. tularensis LVS ex vivo, display enhanced restriction of intracellular bacteria. These observations suggest the Sts proteins play an important regulatory role in the host response to bacterial infection, and they underscore a role for Sts in regulating functionally relevant immune response pathways.

Gut Microbiome Standardization in Control and Experimental Mice

Mouse models are used extensively to study human health and to investigate the mechanisms underlying human disease. In the past, most animal studies were performed without taking into consideration the impact of the microbiota. However, the microbiota that colonizes all body surfaces, including the gastrointestinal tract, respiratory tract, genitourinary tract, and skin, heavily impacts nearly every aspect of host physiology. When performing studies utilizing mouse models it is critical to understand that the microbiome is heavily impacted by environmental factors, including (but not limited to) food, bedding, caging, and temperature. In addition, stochastic changes in the microbiota can occur over time that also play a role in shaping microbial composition. These factors lead to massive variability in the composition of the microbiota between animal facilities and research institutions, and even within a single facility. Lack of experimental reproducibility between research groups has highlighted the necessity for rigorously controlled experimental designs in order to standardize the microbiota between control and experimental animals. Well controlled experiments are mandatory in order to reduce variability and allow correct interpretation of experimental results, not just of host-microbiome studies but of all mouse models of human disease. The protocols presented are aimed to design experiments that control the microbiota composition between different genetic strains of experimental mice within an animal unit. © 2017 by John Wiley & Sons, Inc.

Characterization and Functional Analysis of Mouse Semi-invariant Natural T Cells

Semi-invariant natural killer T (iNKT) cells are CD1d-restricted innate-like lymphocytes that recognize lipid agonists. Activated iNKT cells have immunoregulatory properties. Human and mouse iNKT cell functions elicited by different glycolipid agonists are highly conserved, making the mouse an excellent animal model for understanding iNKT cell biology in vivo. This unit describes basic methods for the characterization and quantification (see Basic Protocol 1) and functional analysis of mouse iNKT cells in vivo or in vitro. This unit also contains protocols that describe enrichment and purification of iNKT cells, generation of CD1d tetramer, and lipid antigen loading onto cell-bound and soluble CD1d for activation of NKT cell hybridomas. © 2017 by John Wiley & Sons, Inc.

Monophosphoryl Lipid A Enhances Efficacy of a Francisella tularensis LVS-Catanionic Nanoparticle Subunit Vaccine against F. tularensis Schu S4 Challenge by Augmenting both Humoral and Cellular Immunity

Francisella tularensis, a bacterial biothreat agent, has no approved vaccine in the United States. Previously, we showed that incorporating lysates from partially attenuated F. tularensis LVS or fully virulent F. tularensis Schu S4 strains into catanionic surfactant vesicle (V) nanoparticles (LVS-V and Schu S4-V, respectively) protected fully against F. tularensis LVS intraperitoneal (i.p.) challenge in mice. However, we achieved only partial protection against F. tularensis Schu S4 intranasal (i.n.) challenge, even when employing heterologous prime-boost immunization strategies. We now extend these findings to show that both LVS-V and Schu S4-V immunization (i.p./i.p.) elicited similarly high titers of anti-F. tularensis IgG and that the titers could be further increased by adding monophosphoryl lipid A (MPL), a nontoxic Toll-like receptor 4 (TLR4) adjuvant that is included in several U.S. FDA-approved vaccines. LVS-V+MPL immune sera also detected more F. tularensis antigens than LVS-V immune sera and, after passive transfer to naive mice, significantly delayed the time to death against F. tularensis Schu S4 subcutaneous (s.c.) but not i.n. challenge. Active immunization with LVS-V+MPL (i.p./i.p.) also increased the frequency of gamma interferon (IFN-γ)-secreting activated helper T cells, IFN-γ production, and the ability of splenocytes to control intramacrophage F. tularensis LVS replication ex vivo Active LVS-V+MPL immunization via heterologous routes (i.p./i.n.) significantly elevated IgA and IgG levels in bronchoalveolar lavage fluid and significantly enhanced protection against i.n. F. tularensis Schu S4 challenge (to ∼60%). These data represent a significant step in the development of a subunit vaccine against the highly virulent type A strains.

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

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

F. novicida-Infected A. castellanii Does Not Enhance Bacterial Virulence in Mice

Francisella tularensis is a facultative intracellular bacterium that causes tularemia in humans and animals. Epidemiology of tularemia worldwide is often associated with water-borne transmission, which includes mosquitoes and amoebae as the potential host reservoirs of the bacteria in water environment. In vitro studies showed intracellular replication of F. tularensis within Acanthamoeba castellanii and Hartmanella vermiformis cells. While infection of amoeba by Legionella pneumophila has been shown to enhance infectivity of L. pneumophila the role of F. tularensis-infected protozoa in the pathogenesis of tularemia is not known. We used 6 h coculture of A. castellanii and F. novicida for investigation of the effect of inhaled amoeba on the pathogenesis of tularemia on in vivo model. Balb/c mice were infected intratracheally with F. novicida or with F. novicida-infected A. castellanii. Surprisingly, infection with F. novicida-infected A. castellanii did not lead to bronchopneumonia in Balb/c mice, and Francisella did not disseminate into the liver and spleen. Upon inhalation, F. novicida infects a variety of host cells, though neutrophils are the predominant cells early during infection in the lung infiltrates of pulmonary tularemia. The numbers of neutrophils in the lungs of Balb/c mice were significantly lower in the infection of mice with F. novicida-infected A. castellanii in comparison to group of mice infected only with F. novicida. These results demonstrate that following inoculation of mice with F. novicida-infected A. castellanii, mice did not develop tularemia.

Contaminated water delivery as a simple and effective method of experimental Salmonella infection

AIM

In most infectious disease models, it is assumed that gavage needle infection is the most reliable means of pathogen delivery to the GI tract. However, this methodology can cause esophageal tearing and induces stress in experimental animals, both of which have the potential to impact early infection and the subsequent immune response.

MATERIALS & METHODS

C57BL/6 mice were orally infected with virulent Salmonella Typhimurium SL1344 either by intragastric gavage preceded by sodium bicarbonate, or by contamination of drinking water.

RESULTS

We demonstrate that water contamination delivery of Salmonella is equivalent to gavage inoculation in providing a consistent model of infection. Furthermore, exposure of mice to contaminated drinking water for as little as 4 h allowed maximal mucosal and systemic infection, suggesting an abbreviated window exists for natural intestinal entry.

CONCLUSION

Together, these data question the need for gavage delivery for infection with oral pathogens.

Border Patrol Gone Awry: Lung NKT Cell Activation by Francisella tularensis Exacerbates Tularemia-Like Disease

The respiratory mucosa is a major site for pathogen invasion and, hence, a site requiring constant immune surveillance. The type I, semi-invariant natural killer T (NKT) cells are enriched within the lung vasculature. Despite optimal positioning, the role of NKT cells in respiratory infectious diseases remains poorly understood. Hence, we assessed their function in a murine model of pulmonary tularemia—because tularemia is a sepsis-like proinflammatory disease and NKT cells are known to control the cellular and humoral responses underlying sepsis. Here we show for the first time that respiratory infection with Francisella tularensis live vaccine strain resulted in rapid accumulation of NKT cells within the lung interstitium. Activated NKT cells produced interferon-γ and promoted both local and systemic proinflammatory responses. Consistent with these results, NKT cell-deficient mice showed reduced inflammatory cytokine and chemokine response yet they survived the infection better than their wild type counterparts. Strikingly, NKT cell-deficient mice had increased lymphocytic infiltration in the lungs that organized into tertiary lymphoid structures resembling induced bronchus-associated lymphoid tissue (iBALT) at the peak of infection. Thus, NKT cell activation by F. tularensis infection hampers iBALT formation and promotes a systemic proinflammatory response, which exacerbates severe pulmonary tularemia-like disease in mice.

NKT cells are innate-like lymphocytes with a demonstrated role in a wide range of diseases. Often cited for their ability to rapidly produce a variety of cytokines upon activation, they have long been appreciated for their ability to “jump-start” the immune system and to shape the quality of both the innate and adaptive response. This understanding of their function has been deduced from in vitro experiments or through the in vivo administration of highly potent, chemically synthesized lipid ligands, which may not necessarily reflect a physiologically relevant response as observed in a natural infection. Using a mouse model of pulmonary tularemia, we report that intranasal infection with the live vaccine strain of F. tularensis rapidly activates NKT cells and promotes systemic inflammation, increased tissue damage, and a dysregulated immune response resulting in increased morbidity and mortality in infected mice. Our data highlight the detrimental effects of NKT cell activation and identify a potential new target for therapies against pulmonary tularemia.

Putting the Jigsaw Together - A Brief Insight Into the Tularemia

Tularemia is a debilitating febrile and potentially fatal zoonotic disease of humans and other vertebrates caused by the Gram-negative bacterium Francisella tularensis. The natural reservoirs are small rodents, hares, and possibly amoebas in water. The etiological agent, Francisella tularensis, is a non-spore forming, encapsulated, facultative intracellular bacterium, a member of the γ-Proteobacteria class of Gram-negative bacteria. Francisella tularensis is capable of invading and replicating within phagocytic as well as non-phagocytic cells and modulate inflammatory response. Infection by the pulmonary, dermal, or oral routes, respectively, results in pneumonic, ulceroglandular, or oropharyngeal tularemia. The highest mortality rates are associated with the pneumonic form of this disease. All members of Francisella tularensis species cause more or less severe disease Due to their abilities to be transmitted to humans via multiple routes and to be disseminated via biological aerosol that can cause the disease after inhalation of even an extremely low infectious dose, Francisella tularensis has been classified as a Category A bioterrorism agent. The current standard of care for tularemia is treatment with antibiotics, as this therapy is highly effective if used soon after infection, although it is not, however, absolutely effective in all cases.

IL-23 p19 knockout mice exhibit minimal defects in responses to primary and secondary infection with Francisella tularensis LVS

Our laboratory’s investigations into mechanisms of protective immunity against Francisella tularensis Live Vaccine Strain (LVS) have uncovered mediators important in host defense against primary infection, as well as those correlated with successful vaccination. One such potential correlate was IL-12p40, a pleiotropic cytokine that promotes Th1 T cell function as part of IL-12p70. LVS-infected IL-12p40 deficient knockout (KO) mice maintain a chronic infection, but IL-12p35 KO mice clear LVS infection; thus the role that IL-12p40 plays in immunity to LVS is independent of the IL-12p70 heterodimer. IL-12p40 can also partner with IL-23p19 to create the heterodimeric cytokine IL-23. Here, we directly tested the role of IL-23 in LVS resistance, and found IL-23 to be largely dispensable for immunity to LVS following intradermal or intranasal infection. IL-23p19 KO splenocytes were fully competent in controlling intramacrophage LVS replication in an in vitro overlay assay. Further, antibody responses in IL-23p19 KO mice were similar to those of normal wild type mice after LVS infection. IL-23p19 KO mice or normal wild type mice that survived primary LVS infection survived maximal doses of LVS secondary challenge. Thus p40 has a novel role in clearance of LVS infection that is unrelated to either IL-12 or IL-23.

Monocyte/macrophage inflammatory response pathways to combat Francisella infection: possible therapeutic targets?

Francisella tularensis can bypass and suppress host immune responses, even to the point of manipulating immune cell phenotypes and intercellular inflammatory networks. Strengthening these responses such that immune cells more readily identify and destroy the bacteria is likely to become a viable (and perhaps necessary) strategy for combating infections with Francisella, especially given the likelihood of antibiotic resistance in the foreseeable future. Monocytes and macrophages offer a niche wherein Francisella can invade and replicate, resulting in substantially higher bacterial load that can overcome the host. As such, understanding their responses to Francisella may uncover potential avenues of therapy that could promote a lowering of bacterial burden and clearance of infection. These response pathways include Toll-like Receptor 2 (TLR2), the caspase-1 inflammasome, Interferons, NADPH oxidase, Phosphatidylinositide 3-kinase (PI3K), and the Ras pathway. In this review we summarize the literature pertaining to the roles of these pathways during Francisella infection, with an emphasis on monocyte/macrophage responses. The therapeutic targeting of one or more such pathways may ultimately become a valuable tool for the treatment of tularemia, and several possibilities are discussed.

Novel catanionic surfactant vesicle vaccines protect against Francisella tularensis LVS and confer significant partial protection against F. tularensis Schu S4 strain

Francisella tularensis is a Gram-negative immune-evasive coccobacillus that causes tularemia in humans and animals. A safe and efficacious vaccine that is protective against multiple F. tularensis strains has yet to be developed. In this study, we tested a novel vaccine approach using artificial pathogens, synthetic nanoparticles made from catanionic surfactant vesicles that are functionalized by the incorporation of either F. tularensis type B live vaccine strain (F. tularensis LVS [LVS-V]) or F. tularensis type A Schu S4 strain (F. tularensis Schu S4 [Schu S4-V]) components. The immunization of C57BL/6 mice with "bare" vesicles, which did not express F. tularensis components, partially protected against F. tularensis LVS, presumably through activation of the innate immune response, and yet it failed to protect against the F. tularensis Schu S4 strain. In contrast, immunization with LVS-V fully protected mice against intraperitoneal (i.p.) F. tularensis LVS challenge, while immunization of mice with either LVS-V or Schu S4-V partially protected C57BL/6 mice against an intranasal (i.n.) F. tularensis Schu S4 challenge and significantly increased the mean time to death for nonsurvivors, particularly following the i.n. and heterologous (i.e., i.p./i.n.) routes of immunization. LVS-V immunization, but not immunization with empty vesicles, elicited high levels of IgG against nonlipopolysaccharide (non-LPS) epitopes that were increased after F. tularensis LVS challenge and significantly increased early cytokine production. Antisera from LVS-V-immunized mice conferred passive protection against challenge with F. tularensis LVS. Together, these data indicate that functionalized catanionic surfactant vesicles represent an important and novel tool for the development of a safe and effective F. tularensis subunit vaccine and may be applicable for use with other pathogens.

MyD88-dependent signaling prolongs survival and reduces bacterial burden during pulmonary infection with virulent Francisella tularensis

Francisella tularensis is the causative agent of the debilitating febrile illness tularemia. The severe morbidity associated with F. tularensis infections is attributed to its ability to evade the host immune response. Innate immune activation is undetectable until more than 48 hours after infection. The ensuing inflammatory response is considered pathological, eliciting a septic-like state characterized by hypercytokinemia and cell death. To investigate potential pathological consequences of the innate immune response, mice deficient in a key innate immune signaling molecule, MyD88, were studied. MyD88 knockout (KO) mice were infected with the prototypical virulent F. tularensis strain, Schu S4. MyD88 KO mice succumbed to infection more rapidly than wild-type mice. The enhanced pathogenicity of Schu S4 in MyD88 KO mice was associated with greater bacterial burdens in lungs and distal organs, and the absence of IFN-γ in the lungs, spleens, and sera. Cellular infiltrates were not observed on histological evaluation of the lungs, livers, or spleens of MyD88 KO mice, the first KO mouse described with this phenotype to our knowledge. Despite the absence of cellular infiltration, there was more cell death in the lungs of MyD88 KO mice. Thus, the host proinflammatory response is beneficial, and MyD88 signaling is required to limit bacterial burden and prolong survival during pulmonary infection by virulent F. tularensis.

Generation of protection against Francisella novicida in mice depends on the pathogenicity protein PdpA, but not PdpC or PdpD

Previous results suggest that mutations in most genes in the Francisella pathogenicity island (FPI) attenuate the bacterium. Using a mouse model, here we determined the impact of mutations in pdpA, pdpC, and pdpD in Francisella novicida on in vitro replication in macrophages, and in vivo immunogenicity. In contrast to most FPI genes, deletion of pdpC (FnΔpdpC) and pdpD (FnΔpdpD) from F. novicida did not impact growth in mouse bone-marrow derived macrophages. Nonetheless, both FnΔpdpC and FnΔpdpD were highly attenuated when administered intradermally. Infected mice produced relatively normal anti-F. novicida serum antibodies. Further, splenocytes from infected mice controlled intramacrophage Francisella replication, indicating T cell priming, and mice immunized by infection with FnΔpdpC or FnΔpdpD survived secondary lethal parenteral challenge with either F. novicida or Francisella tularensis LVS. In contrast, deletion of pdpA (FnΔpdpA) ablated growth in macrophages in vitro. FnΔpdpA disseminated and replicated poorly in infected mice, accompanied by development of some anti-F. novicida serum antibodies. However, primed Th1 cells were not detected, and vaccinated mice did not survive even low dose challenge with either F. novicida or LVS. Taken together, these results suggest that successful priming of Th1 cells, and protection against lethal challenge, depends on expression of PdpA.

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

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

Development of functional and molecular correlates of vaccine-induced protection for a model intracellular pathogen, F. tularensis LVS

In contrast with common human infections for which vaccine efficacy can be evaluated directly in field studies, alternative strategies are needed to evaluate efficacy for slowly developing or sporadic diseases like tularemia. For diseases such as these caused by intracellular bacteria, serological measures of antibodies are generally not predictive. Here, we used vaccines varying in efficacy to explore development of clinically useful correlates of protection for intracellular bacteria, using Francisella tularensis as an experimental model. F. tularensis is an intracellular bacterium classified as Category A bioterrorism agent which causes tularemia. The primary vaccine candidate in the U.S., called Live Vaccine Strain (LVS), has been the subject of ongoing clinical studies; however, safety and efficacy are not well established, and LVS is not licensed by the U.S. FDA. Using a mouse model, we compared the in vivo efficacy of a panel of qualitatively different Francisella vaccine candidates, the in vitro functional activity of immune lymphocytes derived from vaccinated mice, and relative gene expression in immune lymphocytes. Integrated analyses showed that the hierarchy of protection in vivo engendered by qualitatively different vaccines was reflected by the degree of lymphocytes' in vitro activity in controlling the intramacrophage growth of Francisella. Thus, this assay may be a functional correlate. Further, the strength of protection was significantly related to the degree of up-regulation of expression of a panel of genes in cells recovered from the assay. These included IFN-γ, IL-6, IL-12Rβ2, T-bet, SOCS-1, and IL-18bp. Taken together, the results indicate that an in vitro assay that detects control of bacterial growth, and/or a selected panel of mediators, may ultimately be developed to predict the outcome of vaccine efficacy and to complement clinical trials. The overall approach may be applicable to intracellular pathogens in general.

Diseases such as tuberculosis (caused by Mycobacterium tuberculosis) or tularemia (caused by Francisella tularensis) result from infections by microbes that live within cells of a person's body. New vaccines are being developed against such intracellular pathogens, but some will be difficult to test, because disease takes a long time to develop (e.g., tuberculosis) or because outbreaks are unpredictable (e.g., tularemia). Usually such infections are controlled by activities of T cells. However, there are no accepted measures of T cell function that reliably predict vaccine-induced protection. We studied two new ways to do so. We used a group of vaccine candidates against tularemia that stimulated good, fair, or poor protection of mice against Francisella challenge. We then measured whether Francisella–immune cells from vaccinated mice controlled the growth of bacteria inside cells, and/or whether the expression of immune genes in Francisella–immune cells was increased. We found that the degree of protection was matched by the degree of the cells' function in controlling intramacrophage bacterial growth. Further, the degree was predicted by relative amounts of gene expression for several immune mediators. Thus the two new options explored here may help predict protection, without waiting for the onset of disease.