Mouse model of oral infection with virulent type A Francisella tularensis

Case report: tularaemia in a white-handed gibbon (Hylobates lar), Germany

In 2021, a white-handed gibbon (Hylobates lar) succumbed to illness shortly after transfer from one zoo to another in Germany, due to Francisella tularensis subsp. holarctica infection. To determine the source of infection, whole genome sequencing of the gibbon-derived isolate was performed and wild pest rodents (and captive squirrels) from both zoos were screened for F. tularensis. The F. tularensis whole genome sequence obtained from the gibbon was closely related to previous subclade B.281 sequences obtained from hares from Baden-Wuerttemberg, the same region where the gibbon was first housed. However, F. tularensis DNA was detected in one Norway rat from the receiving zoo. Therefore, neither zoo can be excluded as the source of infection.

Rapid Detection of Lipopolysaccharide and Whole Cells of Francisella tularensis Based on Agglutination of Antibody-Coated Gold Nanoparticles and Colorimetric Registration

The paper presents development and characterization of a new bioanalytical test system for rapid detection of lipopolysaccharide (LPS) and whole cells of Francisella tularensis, a causative agent of tularemia, in water samples. Gold nanoparticles (AuNPs) coated by the obtained anti-LPS monoclonal antibodies were used for the assay. Their contact with antigen in tested samples leads to aggregation with a shift of absorption spectra from red to blue. Photometric measurements at 530 nm indicated the analyte presence. Three preparations of AuNPs with different diameters were compared, and the AuNPs having average diameter of 34 nm were found to be optimal. The assay is implemented in 20 min and is characterized by detection limits equal to 40 ng/mL for LPS and 3 × 10⁴ CFU/mL for whole cells of F. tularensis. Thus, the proposed simple one-step assay integrates sensitivity comparable with other immunoassay of microorganisms and rapidity. Selectivity of the assay for different strains of F. tularensis was tested and the possibility to choose its variants with the use of different antibodies to distinguish virulent and non-virulent strains or to detect both kinds of F. tularensis was found. The test system has been successfully implemented to reveal the analyte in natural and tap water samples without the loss of sensitivity.

Mechanisms Affecting the Acquisition, Persistence and Transmission of Francisella tularensis in Ticks

Over 600,000 vector-borne disease cases were reported in the United States (U.S.) in the past 13 years, of which more than three-quarters were tick-borne diseases. Although Lyme disease accounts for the majority of tick-borne disease cases in the U.S., tularemia cases have been increasing over the past decade, with >220 cases reported yearly. However, when comparing Borrelia burgdorferi (causative agent of Lyme disease) and Francisella tularensis (causative agent of tularemia), the low infectious dose (<10 bacteria), high morbidity and mortality rates, and potential transmission of tularemia by multiple tick vectors have raised national concerns about future tularemia outbreaks. Despite these concerns, little is known about how F. tularensis is acquired by, persists in, or is transmitted by ticks. Moreover, the role of one or more tick vectors in transmitting F. tularensis to humans remains a major question. Finally, virtually no studies have examined how F. tularensis adapts to life in the tick (vs. the mammalian host), how tick endosymbionts affect F. tularensis infections, or whether other factors (e.g., tick immunity) impact the ability of F. tularensis to infect ticks. This review will assess our current understanding of each of these issues and will offer a framework for future studies, which could help us better understand tularemia and other tick-borne diseases.

Ecology of Francisella tularensis

Tularemia is a Holarctic zoonosis caused by the gamma proteobacterium Francisella tularensis and is considered to be a vector-borne disease. In many regions, human risk is associated with the bites of flies, mosquitoes, or ticks. But the biology of the agent is such that risk may be fomite related, and large outbreaks can occur due to inhalation or ingestion of contaminated materials. Such well-documented human risk factors suggest a role for these risk factors in the enzootic cycle as well. Many arthropods support the growth or survival of the agent, but whether arthropods (ticks in particular) are obligately required for the perpetuation of F. tularensis remains to be demonstrated. As with most zoonoses, our knowledge of the ecology of F. tularensis has been driven with the objective of understanding human risk. In this review, we focus on the role of the arthropod in maintaining F. tularensis, particularly with respect to long-term enzootic persistence.

The Natural History of Pneumonic Tularemia in Female Fischer 344 Rats after Inhalational Exposure to Aerosolized Francisella tularensis Subspecies tularensis Strain SCHU S4

The inbred Fischer 344 rat is being evaluated for testing novel vaccines and therapeutics against pneumonic tularemia. Although primary pneumonic tularemia in humans typically occurs by inhalation of aerosolized bacteria, the rat model has relied on intratracheal inoculation of organisms because of safety and equipment issues. We now report the natural history of pneumonic tularemia in female Fischer 344 rats after nose-only inhalational exposure to lethal doses of aerosolized Francisella tularensis subspecies tularensis, strain SCHU S4. Our results are consistent with initial uptake of aerosolized SCHU S4 from the nasal cavity, lungs, and possibly the gastrointestinal tract. Bacteremia with hematogenous dissemination was first detected 2 days after exposure. Shortly thereafter, the infected rats exhibited fever, tachypnea, and hypertension that persisted for 24 to 36 hours and then rapidly decreased as animals succumbed to infection between days 5 and 8 after exposure. Tachycardia was observed briefly, but only after the core body temperature and blood pressure began to decrease as the animals were near death. Initial neutrophilic and histiocytic inflammation in affected tissues became progressively more fibrinous and necrotizing over time. At death, as many as 10¹⁰ colony-forming units were found in the lungs, spleen, and liver. Death was attributed to sepsis and disseminated intravascular coagulation. Overall, the pathogenesis of pneumonic tularemia in the female F344 rat model appears to replicate the disease in humans.

Francisella Tularensis Clades B.FTN002-00 and B.13 Are Associated With Distinct Pathology in the European Brown Hare (Lepus europaeus)

Tularemia is a severe disease caused by Francisella tularensis This bacterium has a major pathogenic potential in countless animal species as well as in humans. Despite the relatively significant body of literature available on this microorganism, many questions are still open concerning its biological cycle in the environment, the pathology and pathogenesis of the disease, the possible routes of infection in animals, and the pathologic and ecological relevance of the distinct phylogenetic clusters of F. tularensis In order to address these questions, we have thoroughly characterized the pathology and microbiology of terminally ill European brown hares (Lepus europaeus) infected with F. tularensis subsp. holarctica, collected in Switzerland from 2012 to 2014. F tularensis isolates were typed by defining their phylogenetic clusters. We showed that the pathology associated with F. tularensis subsp. holarctica belonging to the clade B.FTNF002-00 is different from that previously reported to be associated with the clade B.13. In particular, strains of the clade B.FTNF002-00 were almost invariably associated with splenitis and hepatitis and not with the polyserositis affecting pleura, pericardium, and kidney reported in the literature for infections caused by the clade B.13. We describe findings suggesting that the ports of entry for the bacteria might be the respiratory and digestive routes.

Downmodulation of vaccine-induced immunity and protection against the intracellular bacterium Francisella tularensis by the inhibitory receptor FcγRIIB

Fc gamma receptor IIB (FcγRIIB) is the only Fc gamma receptor (FcγR) which negatively regulates the immune response, when engaged by antigen- (Ag-) antibody (Ab) complexes. Thus, the generation of Ag-specific IgG in response to infection or immunization has the potential to downmodulate immune protection against infection. Therefore, we sought to determine the impact of FcγRIIB on immune protection against Francisella tularensis (Ft), a Category A biothreat agent. We utilized inactivated Ft (iFt) as an immunogen. Naïve and iFt-immunized FcγRIIB knockout (KO) or wildtype (WT) mice were challenged with Ft-live vaccine strain (LVS). While no significant difference in survival between naïve FcγRIIB KO versus WT mice was observed, iFt-immunized FcγRIIB KO mice were significantly better protected than iFt-immunized WT mice. Ft-specific IgA in serum and bronchial alveolar lavage, as well as IFN-γ, IL-10, and TNF-α production by splenocytes harvested from iFt-immunized FcγRIIB KO, were also significantly elevated. In addition, iFt-immunized FcγRIIB KO mice exhibited a reduction in proinflammatory cytokine levels in vivo at 5 days after challenge, which correlates with increased survival following Ft-LVS challenge in published studies. Thus, these studies demonstrate for the first time the ability of FcγRIIB to regulate vaccine-induced IgA production and downmodulate immunity and protection. The immune mechanisms behind the above observations and their potential impact on vaccine development are discussed.

IglC and PdpA are important for promoting Francisella invasion and intracellular growth in epithelial cells

The highly infectious bacteria, Francisella tularensis, colonize a variety of organs and replicate within both phagocytic as well as non-phagocytic cells, to cause the disease tularemia. These microbes contain a conserved cluster of important virulence genes referred to as the Francisella Pathogenicity Island (FPI). Two of the most characterized FPI genes, iglC and pdpA, play a central role in bacterial survival and proliferation within phagocytes, but do not influence bacterial internalization. Yet, their involvement in non-phagocytic epithelial cell infections remains unexplored. To examine the functions of IglC and PdpA on bacterial invasion and replication during epithelial cell infections, we infected liver and lung epithelial cells with F. novicida and F. tularensis 'Type B' Live Vaccine Strain (LVS) deletion mutants (ΔiglC and ΔpdpA) as well as their respective gene complements. We found that deletion of either gene significantly reduced their ability to invade and replicate in epithelial cells. Gene complementation of iglC and pdpA partially rescued bacterial invasion and intracellular growth. Additionally, substantial LAMP1-association with both deletion mutants was observed up to 12 h suggesting that the absence of IglC and PdpA caused deficiencies in their ability to dissociate from LAMP1-positive Francisella Containing Vacuoles (FCVs). This work provides the first evidence that IglC and PdpA are important pathogenic factors for invasion and intracellular growth of Francisella in epithelial cells, and further highlights the discrete mechanisms involved in Francisella infections between phagocytic and non-phagocytic cells.

Acid resistance in Francisella tularensis

Francisella tularensis, the etiologic agent of tularemia, can survive under acidic conditions. Tularemia can be acquired by several routes, including by ingestion of contaminated food or water. While acid resistance is usually associated with a low oral infective dose (ID), the ID for gastrointestinal illness is quite high. In this study, four strains of F. tularensis ssp. tularensis (type A) and four strains of F. tularensis ssp. holarctica (type B) were examined for innate acid resistance and the ability to survive in synthetic gastric fluid (SGF) under in vitro conditions similar to passage through the human stomach. Survival for all strains was significantly less in pH 2.5 SGF than in pH 2.5 phosphate-buffered saline and pH 4.0 SGF. Attenuated strains were consistently less resistant. Type B strains are most often associated with waterborne outbreaks and were examined after storage in natural water. Low-nutrient preadaptation resulted in increased resistance. Although F. tularensis can persist under certain acidic conditions, it is sensitive to conditions replicating the fasting human stomach. This may help explain the high ID required for gastrointestinal infections.

Particle size and pathogenicity in the respiratory tract

Particle size dictates where aerosolized pathogens deposit in the respiratory tract, thereafter the pathogens potential to cause disease is influenced by tissue tropism, clearance kinetics and the host immunological response. This interplay brings pathogens into contact with a range of tissues spanning the respiratory tract and associated anatomical structures. In animal models, differential deposition within the respiratory tract influences infection kinetics for numerous select agents. Greater numbers of pathogens are required to infect the upper (URT) compared with the lower respiratory tract (LRT), and in comparison the URT infections are protracted with reduced mortality. Pathogenesis in the URT is characterized by infection of the URT lymphoid tissues, cervical lymphadenopathy and septicemia, closely resembling reported human infections of the URT. The olfactory, gastrointestinal, and ophthalmic systems are also infected in a pathogen-dependent manner. The relevant literature is reviewed with respect to particle size and infection of the URT in animal models and humans.

Use of temperature for standardizing the progression of Francisella tularensis in mice

The study of infectious agents, their pathogenesis, the host response and the evaluation of newly developed countermeasures often requires the use of a living system. Murine models are frequently used to undertake such investigations with the caveat that non-biased measurements to assess the progression of infection are underutilized. Instead, murine models predominantly rely on symptomology exhibited by the animal to evaluate the state of the animal's health and to determine when euthanasia should be performed. In this study, we used subcutaneous temperature as a non-subjective measurement to follow and compare infection in mice inoculated with Francisella tularensis, a Gram-negative pathogen that produces an acute and fatal illness in mice. A reproducible temperature pattern defined by three temperature phases (normal, febrile and hypothermic) was identified in all mice infected with F. tularensis, regardless of the infecting strain. More importantly and for the first time a non-subjective, ethical, and easily determined surrogate endpoint for death based on a temperature, termed drop point, was identified and validated with statistical models. In comparative survival curve analyses for F. tularensis strains with differing virulence, the drop point temperature yielded the same results as those obtained using observed time to death. Incorporation of temperature measurements to evaluate F. tularensis was standardized based on statistical models to provide a new level of robustness for comparative analyses in mice. These findings should be generally applicable to other pathogens that produce acute febrile disease in animal models and offers an important tool for understanding and following the infection process.

Acinetobacter baumannii infection inhibits airway eosinophilia and lung pathology in a mouse model of allergic asthma

Allergic asthma is a dysregulation of the immune system which leads to the development of Th2 responses to innocuous antigens (allergens). Some infections and microbial components can re-direct the immune response toward the Th1 response, or induce regulatory T cells to suppress the Th2 response, thereby inhibiting the development of allergic asthma. Since Acinetobacter baumannii infection can modulate lung cellular and cytokine responses, we studied the effect of A. baumannii in modulating airway eosinophilia in a mouse model of allergic asthma. Ovalbumin (OVA)-sensitized mice were treated with live A. baumannii or phosphate buffered saline (PBS), then intranasally challenged with OVA. Compared to PBS, A. baumannii treatment significantly reduced pulmonary Th2 cytokine and chemokine responses to OVA challenge. More importantly, the airway inflammation in A. baumannii-treated mice was strongly suppressed, as seen by the significant reduction of the proportion and the total number of eosinophils in the bronchoalveolar lavage fluid. In addition, A. baumannii-treated mice diminished lung mucus overproduction and pathology. However, A. baumannii treatment did not significantly alter systemic immune responses to OVA. Serum OVA-specific IgE, IgG1 and IgG2a levels were comparable between A. baumannii- and PBS-treated mice, and tracheobronchial lymph node cells from both treatment groups produced similar levels of Th1 and Th2 cytokines in response to in vitro OVA stimulation. Moreover, it appears that TLR-4 and IFN-γ were not directly involved in the A. baumannii-induced suppression of airway eosinophilia. Our results suggest that A. baumannii inhibits allergic airway inflammation by direct suppression of local pulmonary Th2 cytokine responses to the allergen.

Host-pathogen o-methyltransferase similarity and its specific presence in highly virulent strains of Francisella tularensis suggests molecular mimicry

Whole genome comparative studies of many bacterial pathogens have shown an overall high similarity of gene content (>95%) between phylogenetically distinct subspecies. In highly clonal species that share the bulk of their genomes subtle changes in gene content and small-scale polymorphisms, especially those that may alter gene expression and protein-protein interactions, are more likely to have a significant effect on the pathogen's biology. In order to better understand molecular attributes that may mediate the adaptation of virulence in infectious bacteria, a comparative study was done to further analyze the evolution of a gene encoding an o-methyltransferase that was previously identified as a candidate virulence factor due to its conservation specifically in highly pathogenic Francisella tularensis subsp. tularensis strains. The o-methyltransferase gene is located in the genomic neighborhood of a known pathogenicity island and predicted site of rearrangement. Distinct o-methyltransferase subtypes are present in different Francisella tularensis subspecies. Related protein families were identified in several host species as well as species of pathogenic bacteria that are otherwise very distant phylogenetically from Francisella, including species of Mycobacterium. A conserved sequence motif profile is present in the mammalian host and pathogen protein sequences, and sites of non-synonymous variation conserved in Francisella subspecies specific o-methyltransferases map proximally to the predicted active site of the orthologous human protein structure. Altogether, evidence suggests a role of the F. t. subsp. tularensis protein in a mechanism of molecular mimicry, similar perhaps to Legionella and Coxiella. These findings therefore provide insights into the evolution of niche-restriction and virulence in Francisella, and have broader implications regarding the molecular mechanisms that mediate host-pathogen relationships.

Infection of mice with Francisella as an immunological model

This unit describes the utility of various mouse models of infection for studying pathogenesis and adaptive immune responses to the facultative intracellular bacteria pathogen Francisella tularensis. By judicious use of different combinations of mouse and bacterial strains, as well as different routes of infection, murine tularemia models may be used to explore a complete picture of F. tularensis infection and immunity. Moreover, studies using Francisella, particularly the Live Vaccine Strain (LVS), serve as a convenient and tractable model system that appears to be representative of mammalian host responses to intracellular pathogens in general.

Knockout of the Bcmo1 gene results in an inflammatory response in female lung, which is suppressed by dietary beta-carotene

Beta-carotene 15,15'-monooxygenase 1 knockout (Bcmo1 (-/-)) mice accumulate beta-carotene (BC) similarly to humans, whereas wild-type (Bcmo1 (+/+)) mice efficiently cleave BC. Bcmo1 (-/-) mice are therefore suitable to investigate BC-induced alterations in gene expression in lung, assessed by microarray analysis. Bcmo1 (-/-) mice receiving control diet had increased expression of inflammatory genes as compared to BC-supplemented Bcmo1 (-/-) mice and Bcmo1 (+/+) mice that received either control or BC-supplemented diets. Differential gene expression in Bcmo1 (-/-) mice was confirmed by real-time quantitative PCR. Histochemical analysis indeed showed an increase in inflammatory cells in lungs of control Bcmo1 (-/-) mice. Supported by metabolite and gene-expression data, we hypothesize that the increased inflammatory response is due to an altered BC metabolism, resulting in an increased vitamin A requirement in Bcmo1 (-/-) mice. This suggests that effects of BC may depend on inter-individual variations in BC-metabolizing enzymes, such as the frequently occurring human polymorphisms in BCMO1.

Effect of dehydrated storage on the survival of Francisella tularensis in infant formula

Francisella tularensis is a Gram-negative bacterium that can cause gastrointestinal or oropharyngeal tularemia in humans from ingestion of contaminated food or water. Despite the potential for accidental or intentional contamination of foods with F. tularensis, there are few studies on the long-term survivability of this organism in food matrices. Infant formula has previously been implicated as a vehicle for the transmission of a variety of bacterial pathogens in infants. In this study, we investigated the survival of F. tularensis in dehydrated infant formula under various storage conditions. F. tularensis was stored for up to 12 weeks in dehydrated infant formula in an ambient air, dry or nitrogen atmosphere. Viable counts of fresh F. tularensis at 12 weeks in infant formula revealed a 4.15, 3.37 and 3.72-log decrease in ambient air, dry and nitrogen atmosphere, respectively. D-values were calculated (in weeks) as 3.99, 4.68 and 4.47 in air, dry and nitrogen atmosphere, respectively.

Development of a macrophage cell culture method to isolate and enrich Francisella tularensis from food matrices for subsequent detection by real-time PCR

Francisella tularensis is a gram-negative bacterium that can cause gastrointestinal or oropharyngeal tularemia in humans from ingestion of contaminated food or water. Despite the potential for accidental or intentional contamination of foods with F. tularensis, there are no techniques currently available to detect this organism in specific food matrices. In this study, a macrophage cell culture system is combined with real-time PCR to identify F. tularensis in food matrices. The method utilizes a mouse macrophage cell line (RAW 264.7) as host for the isolation and intracellular replication of F. tularensis. Exposure of macrophages to F. tularensis-contaminated food matrices results in uptake and intracellular replication of the bacteria, which can be subsequently detected by real-time PCR analysis of the DNA released from infected macrophage cell lysates. Macrophage monolayers were exposed to infant formula, liquid egg whites, and lettuce contaminated with varying quantities of F. tularensis. As few as 10 CFU/ml (or CFU per gram) F. tularensis was detected in infant formula and lettuce after 5 h postinfection. As few as 10 CFU/ml F. tularensis was detected in liquid egg whites after 18 h postinfection. Intracellular F. tularensis could also be isolated on Mueller-Hinton medium from lysates of macrophages infected with the bacteria in infant formula, liquid egg whites, and lettuce for subsequent confirmatory identification. This method is the first to successfully identify F. tularensis from select food matrices.

Differential requirements for protection against mucosal challenge with Francisella tularensis in the presence versus absence of cholera toxin B and inactivated F. tularensis

Francisella tularensis is a category A biothreat agent for which there is no approved vaccine and the correlates of protection are not well understood. In particular, the relationship between the humoral and cellular immune response to F. tularensis and the relative importance of each in protection is controversial. Yet, understanding this relationship will be crucial to the development of an effective vaccine against this organism. We demonstrate, for the first time, a differential requirement for humoral vs cellular immunity in vaccine-induced protection against F. tularensis infection, and that the requirement for Ab observed in some protection studies, may be overcome through the induction of enhanced cellular immunity. Specifically, following intranasal/mucosal immunization of mice with inactivated F. tularensis organisms plus the cholera toxin B subunit, we observe increased production of IgG2a/2c vs IgG1 Ab, as well as IFN-gamma, indicating induction of a Th1 response. In addition, the requirement for F. tularensis-specific IgA Ab production, observed in studies following immunization with inactivated F. tularensis alone, is eliminated. Thus, these data indicate that enhanced Th1 responses can supersede the requirement for anti-F. tularensis-specific IgA. This observation also has important ramifications for vaccine development against this organism.

Lymphotoxin-alpha plays only a minor role in host resistance to respiratory infection with virulent type A Francisella tularensis in mice

This study examined the role of lymphotoxin (LT)-α in host defense against airborne infection with Francisella tularensis, a gram-negative facultative intracellular bacterium and the causative agent of tularemia. Following a low-dose aerosol infection with the highly virulent type A strain of F. tularensis, mice deficient in LTα (LTα−/−) consistently harbored approximately 10-fold fewer bacteria in their spleens at day 2 and 10-fold more bacteria in their lungs at day 4 than LTα+/+ mice. However, the mortality and median time to death were indistinguishable between the two mouse strains. In addition, the inflammatory responses to the infection, as reflected by the cytokine levels and leukocyte influx in the bronchoalveolar lavage fluid and histopathological analysis, were generally similar between LTα−/− and LTα+/+ mice. These data suggest that although LTα does not contribute significantly to the resistance and host responses of mice to airborne type A F. tularensis infection, it does play a subtle role in the multiplication/dissemination of F. tularensis.

Inhibition of airway eosinophilia and pulmonary pathology in a mouse model of allergic asthma by the live vaccine strain of Francisella tularensis

BACKGROUND

It has been suggested that exposure to certain microbes and their products, particularly during neonatal and early childhood periods, may shift the immune response towards a T-helper cell (Th) 1 phenotype and thereby prevent the development of and/or alleviate the clinical symptoms of allergic airway diseases.

OBJECTIVE

We evaluated the ability of the live vaccine strain (LVS) of Francisella tularensis to suppress airway eosinophilia and pulmonary pathology in a murine model of allergic airway disease.

METHODS

C57BL/6 mice were sensitized by intraperitoneal injection of ovalbumin (OVA) on days 1 and 14, and challenged intranasally (i.n.) with OVA on day 21 or thereafter. Some sensitized mice were i.n. treated with live LVS or its cell-free sonicate extract (CFSE) before i.n. OVA challenge. Bronchoalveolar lavage fluid, regional lymph node cells, lung tissues and serum samples were collected 3-7 days after the i.n. challenge.

RESULTS

Intranasal and, to a lesser degree, intradermal immunization of OVA-sensitized mice with LVS suppressed the development of airway eosinophilia and associated pulmonary pathology induced by i.n. OVA challenge. Moreover, CFSE prepared from LVS showed a similar inhibitory effect whereas neither LPS nor DNA purified from F. tularensis LVS had such an effect. The inhibition was associated with the reduction in mRNA expression and protein levels of Th2 cytokines IL-5 and IL-13 in the lungs and the enhanced production of OVA-induced IFN-gamma by local draining lymph node cells, but not with the serum levels of OVA-specific IgG1 or IgE.

CONCLUSION

F. tularensis LVS is capable of suppressing allergic airway inflammation probably through a Th1-mediated suppression of an ongoing Th2 response mechanism, and raises the possibility of exploring LVS and its components as potential therapeutic modalities for human allergic asthma.

Identification of immunologic and pathologic parameters of death versus survival in respiratory tularemia

Francisella tularensis can cause severe disseminated disease after respiratory infection. The identification of factors involved in mortality or recovery following induction of tularemia in the mouse will improve our understanding of the natural history of this disease and facilitate future evaluation of vaccine candidate preparations. BALB/c mice were infected intranasally with the live vaccine strain (LVS) of F. tularensis subsp. holarctica and euthanized at different stages of disease to analyze the induction of immune molecules, gross anatomical features of organs, bacterial burdens, and progression of the histopathological changes in lung and spleen. Tissue-specific interleukin-6 (IL-6), macrophage inflammatory protein 2, and monocyte chemotactic protein 1 were immune markers of mortality, while anti-LVS immunoglobulin M and IL-1beta were associated with survival. Moribund mice had enlarged spleens and lungs, while surviving mice had even more prominent splenomegaly and normal-appearing lungs. Histopathology of the spleens of severely ill mice was characterized by disrupted lymphoid follicles and fragmented nuclei, while the spleens of survivors appeared healthy but with increased numbers of megakaryocytes and erythrocytes. Histopathology of the lungs of severely ill mice indicated severe pneumonia. Lungs of survivors at early time points showed increased inflammation, while at late times they appeared healthy with peribronchial lymphoid aggregates. Our results suggest that host immune factors are able to affect bacterial dissemination after respiratory tularemia, provide new insights regarding the pathological characteristics of pulmonary tularemia leading to systemic disease, and potentially identify immune markers associated with recovery from the disease.