Live attenuated Francisella novicida vaccine protects against Francisella tularensis pulmonary challenge in rats and non-human primates

Current vaccine strategies and novel approaches to combatting Francisella infection

Tularemia is caused by subspecies of Francisella tularensis and can manifest in a variety of disease states, with the pneumonic presentation resulting in the greatest mortality. Despite decades of research, there are no approved vaccines against F. tularensis in the United States. Traditional vaccination strategies, such as live-attenuated or subunit vaccines, are not favorable due to inadequate protection or safety concerns. Because of this, novel vaccination strategies are needed to combat tularemia. Here we discuss the current state of and challenges to the tularemia vaccine field and suggest novel vaccine approaches going forward that might be better suited for protecting against F. tularensis infection.

Protective potential of outer membrane vesicles derived from a virulent strain of Francisella tularensis

Francisella tularensis secretes tubular outer membrane vesicles (OMVs) that contain a number of immunoreactive proteins as well as virulence factors. We have reported previously that isolated Francisella OMVs enter macrophages, cumulate inside, and induce a strong pro-inflammatory response. In the current article, we present that OMVs treatment of macrophages also enhances phagocytosis of the bacteria and suppresses their intracellular replication. On the other hand, the subsequent infection with Francisella is able to revert to some extent the strong pro-inflammatory effect induced by OMVs in macrophages. Being derived from the bacterial surface, isolated OMVs may be considered a "non-viable mixture of Francisella antigens" and as such, they present a promising protective material. Immunization of mice with OMVs isolated from a virulent F. tularensis subsp. holarctica strain FSC200 prolonged the survival time but did not fully protect against the infection with a lethal dose of the parent strain. However, the sera of the immunized animals revealed unambiguous cytokine and antibody responses and proved to recognize a set of well-known Francisella immunoreactive proteins. For these reasons, Francisella OMVs present an interesting material for future protective studies.

Marmosets as models of infectious diseases

Animal models of infectious disease often serve a crucial purpose in obtaining licensure of therapeutics and medical countermeasures, particularly in situations where human trials are not feasible, i.e., for those diseases that occur infrequently in the human population. The common marmoset (Callithrix jacchus), a Neotropical new-world (platyrrhines) non-human primate, has gained increasing attention as an animal model for a number of diseases given its small size, availability and evolutionary proximity to humans. This review aims to (i) discuss the pros and cons of the common marmoset as an animal model by providing a brief snapshot of how marmosets are currently utilized in biomedical research, (ii) summarize and evaluate relevant aspects of the marmoset immune system to the study of infectious diseases, (iii) provide a historical backdrop, outlining the significance of infectious diseases and the importance of developing reliable animal models to test novel therapeutics, and (iv) provide a summary of infectious diseases for which a marmoset model exists, followed by an in-depth discussion of the marmoset models of two studied bacterial infectious diseases (tularemia and melioidosis) and one viral infectious disease (viral hepatitis C).

The rLVS ΔcapB/iglABC vaccine provides potent protection in Fischer rats against inhalational tularemia caused by various virulent Francisella tularensis strains

Francisella tularensis is one of the several biothreat agents for which a licensed vaccine is needed. To ensure vaccine protection is achieved across a range of virulent F. tularensis strains, we assembled and characterized a panel of F. tularensis isolates to be utilized as challenge strains. A promising tularemia vaccine candidate is rLVS ΔcapB/iglABC (rLVS), in which the vector is the LVS strain with a deletion in the capB gene and which additionally expresses a fusion protein comprising immunodominant epitopes of proteins IglA, IglB, and IglC. Fischer rats were immunized subcutaneously 1-3 times at 3-week intervals with rLVS at various doses. The rats were exposed to a high dose of aerosolized Type A strain Schu S4 (FRAN244), a Type B strain (FRAN255), or a tick derived Type A strain (FRAN254) and monitored for survival. All rLVS vaccination regimens including a single dose of 107 CFU rLVS provided 100% protection against both Type A strains. Against the Type B strain, two doses of 107 CFU rLVS provided 100% protection, and a single dose of 107 CFU provided 87.5% protection. In contrast, all unvaccinated rats succumbed to aerosol challenge with all of the F. tularensis strains. A robust Th1-biased antibody response was induced in all vaccinated rats against all F. tularensis strains. These results demonstrate that rLVS ΔcapB/iglABC provides potent protection against inhalational challenge with either Type A or Type B F. tularensis strains and should be considered for further analysis as a future tularemia vaccine.

Comparative evaluation of protective immunity against Francisella tularensis induced by subunit or adenovirus-vectored vaccines

Tularemia is a highly contagious disease caused by infection with Francisella tularensis (Ft), a pathogenic intracellular gram-negative bacterium that infects a wide range of animals and causes severe disease and death in people, making it a public health concern. Vaccines are the most effective way to prevent tularemia. However, there are no Food and Drug Administration (FDA)-approved Ft vaccines thus far due to safety concerns. Herein, three membrane proteins of Ft, Tul4, OmpA, and FopA, and a molecular chaperone, DnaK, were identified as potential protective antigens using a multifactor protective antigen platform. Moreover, the recombinant DnaK, FopA, and Tul4 protein vaccines elicited a high level of IgG antibodies but did not protect against challenge. In contrast, protective immunity was elicited by a replication-defective human type 5 adenovirus (Ad5) encoding the Tul4, OmpA, FopA, and DnaK proteins (Ad5-Tul4, Ad5-OmpA, Ad5-FopA, and Ad5-DnaK) after a single immunization, and all Ad5-based vaccines stimulated a Th1-biased immune response. Moreover, intramuscular and intranasal vaccination with Ad5-Tul4 using the prime-boost strategy effectively eliminated Ft lung, spleen and liver colonization and provided nearly 80% protection against intranasal challenge with the Ft live vaccine strain (LVS). Only intramuscular, not intranasal vaccination, with Ad5-Tul4 protected mice from intraperitoneal challenge. This study provides a comprehensive comparison of protective immunity against Ft provided by subunit or adenovirus-vectored vaccines and suggests that mucosal vaccination with Ad5-Tul4 may yield desirable protective efficacy against mucosal infection, while intramuscular vaccination offers greater overall protection against intraperitoneal tularemia.

Atomic Structure of IglD Demonstrates Its Role as a Component of the Baseplate Complex of the Francisella Type VI Secretion System

Francisella tularensis, a Tier 1 select agent of bioterrorism, contains a type VI secretion system (T6SS) encoded within the Francisella pathogenicity island (FPI), which is critical for its pathogenesis. Among the 18 proteins encoded by FPI is IglD, which is essential to Francisella’s intracellular growth and virulence, but neither its location within T6SS nor its functional role has been established. Here, we present the cryoEM structure of IglD from Francisella novicida and show that the Francisella IglD forms a homotrimer that is structurally homologous to the T6SS baseplate protein TssK in Escherichia coli. Each IglD monomer consists of an N-terminal β-sandwich domain, a 4-helix bundle domain, and a flexible C-terminal domain. While the overall folds of IglD and TssK are similar, the two structures differ in three aspects: the relative orientation between their β-sandwich and the 4-helix bundle domains; two insertion loops present in TssK’s β-sandwich domain; and, consequently, a lack of subunit-subunit interaction between insertion loops in the IglD trimer. Phylogenetic analysis indicates that IglD is genetically remote from the TssK orthologs in other T6SSs. While the other components of the Francisella baseplate are unknown, we conducted pulldown assays showing IglJ interacts with IglD and IglH, pointing to a model wherein IglD, IglH, and IglJ form the baseplate of the Francisella T6SS. Alanine substitution mutagenesis further established that IglD’s hydrophobic pocket in the N-terminal β-sandwich domain interacts with two loops of IglJ, reminiscent of the TssK-TssG interaction. These results form a framework for understanding the hitherto unexplored Francisella T6SS baseplate.

The O-Ag Antibody Response to Francisella Is Distinct in Rodents and Higher Animals and Can Serve as a Correlate of Protection

Identifying correlates of protection (COPs) for vaccines against lethal human (Hu) pathogens, such as Francisella tularensis (Ft), is problematic, as clinical trials are currently untenable and the relevance of various animal models can be controversial. Previously, Hu trials with the live vaccine strain (LVS) demonstrated ~80% vaccine efficacy against low dose (~50 CFU) challenge; however, protection deteriorated with higher challenge doses (~2000 CFU of SchuS4) and no COPs were established. Here, we describe our efforts to develop clinically relevant, humoral COPs applicable to high-dose, aerosol challenge with S4. First, our serosurvey of LVS-vaccinated Hu and animals revealed that rabbits (Rbs), but not rodents, recapitulate the Hu O-Ag dependent Ab response to Ft. Next, we assayed Rbs immunized with distinct S4-based vaccine candidates (S4ΔclpB, S4ΔguaBA, and S4ΔaroD) and found that, across multiple vaccines, the %O-Ag dep Ab trended with vaccine efficacy. Among S4ΔguaBA-vaccinated Rbs, the %O-Ag dep Ab in pre-challenge plasma was significantly higher in survivors than in non-survivors; a cut-off of >70% O-Ag dep Ab predicted survival with high sensitivity and specificity. Finally, we found this COP in 80% of LVS-vaccinated Hu plasma samples as expected for a vaccine with 80% Hu efficacy. Collectively, the %O-Ag dep Ab response is a bona fide COP for S4ΔguaBA-vaccinated Rb and holds significant promise for guiding vaccine trials with higher animals.

Development, Phenotypic Characterization and Genomic Analysis of a Francisella tularensis Panel for Tularemia Vaccine Testing

Francisella tularensis is one of several biothreat agents for which a licensed vaccine is needed to protect against this pathogen. To aid in the development of a vaccine protective against pneumonic tularemia, we generated and characterized a panel of F. tularensis isolates that can be used as challenge strains to assess vaccine efficacy. Our panel consists of both historical and contemporary isolates derived from clinical and environmental sources, including human, tick, and rabbit isolates. Whole genome sequencing was performed to assess the genetic diversity in comparison to the reference genome F. tularensis Schu S4. Average nucleotide identity analysis showed >99% genomic similarity across the strains in our panel, and pan-genome analysis revealed a core genome of 1,707 genes, and an accessory genome of 233 genes. Three of the strains in our panel, FRAN254 (tick-derived), FRAN255 (a type B strain), and FRAN256 (a human isolate) exhibited variation from the other strains. Moreover, we identified several unique mutations within the Francisella Pathogenicity Island across multiple strains in our panel, revealing unexpected diversity in this region. Notably, FRAN031 (Scherm) completely lacked the second pathogenicity island but retained virulence in mice. In contrast, FRAN037 (Coll) was attenuated in a murine pneumonic tularemia model and had mutations in pdpB and iglA which likely led to attenuation. All of the strains, except FRAN037, retained full virulence, indicating their effectiveness as challenge strains for future vaccine testing. Overall, we provide a well-characterized panel of virulent F. tularensis strains that can be utilized in ongoing efforts to develop an effective vaccine against pneumonic tularemia to ensure protection is achieved across a range F. tularensis strains.

Secretory System Components as Potential Prophylactic Targets for Bacterial Pathogens

Bacterial secretory systems are essential for virulence in human pathogens. The systems have become a target of alternative antibacterial strategies based on small molecules and antibodies. Strategies to use components of the systems to design prophylactics have been less publicized despite vaccines being the preferred solution to dealing with bacterial infections. In the current review, strategies to design vaccines against selected pathogens are presented and connected to the biology of the system. The examples are given for Y. pestis, S. enterica, B. anthracis, S. flexneri, and other human pathogens, and discussed in terms of effectiveness and long-term protection.

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

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

A Francisella novicida Mutant, Lacking the Soluble Lytic Transglycosylase Slt, Exhibits Defects in Both Growth and Virulence

Francisella tularensis is the causative agent of tularemia and has gained recent interest as it poses a significant biothreat risk. F. novicida is commonly used as a laboratory surrogate for tularemia research due to genetic similarity and susceptibility of mice to infection. Currently, there is no FDA-approved tularemia vaccine, and identifying therapeutic targets remains a critical gap in strategies for combating this pathogen. Here, we investigate the soluble lytic transglycosylase or Slt in F. novicida, which belongs to a class of peptidoglycan-modifying enzymes known to be involved in cell division. We assess the role of Slt in biology and virulence of the organism as well as the vaccine potential of the slt mutant. We show that the F. novicida slt mutant has a significant growth defect in acidic pH conditions. Further microscopic analysis revealed significantly altered cell morphology compared to wild-type, including larger cell size, extensive membrane protrusions, and cell clumping and fusion, which was partially restored by growth in neutral pH or genetic complementation. Viability of the mutant was also significantly decreased during growth in acidic medium, but not at neutral pH. Furthermore, the slt mutant exhibited significant attenuation in a murine model of intranasal infection and virulence could be restored by genetic complementation. Moreover, we could protect mice using the slt mutant as a live vaccine strain against challenge with the parent strain; however, we were not able to protect against challenge with the fully virulent F. tularensis Schu S4 strain. These studies demonstrate a critical role for the Slt enzyme in maintaining proper cell division and morphology in acidic conditions, as well as replication and virulence in vivo. Our results suggest that although the current vaccination strategy with F. novicida slt mutant would not protect against Schu S4 challenges, the Slt enzyme could be an ideal target for future therapeutic development.

Zoonoses under our noses

One Health is an effective approach for the management of zoonotic disease in humans, animals and environments. Examples of the management of bacterial zoonoses in Europe and across the globe demonstrate that One Health approaches of international surveillance, information-sharing and appropriate intervention methods are required to successfully prevent and control disease outbreaks in both endemic and non-endemic regions. Additionally, a One Health approach enables effective preparation and response to bioterrorism threats.

An O-Antigen Glycoconjugate Vaccine Produced Using Protein Glycan Coupling Technology Is Protective in an Inhalational Rat Model of Tularemia

There is a requirement for an efficacious vaccine to protect people against infection from Francisella tularensis, the etiological agent of tularemia. The lipopolysaccharide (LPS) of F. tularensis is suboptimally protective against a parenteral lethal challenge in mice. To develop a more efficacious subunit vaccine, we have used a novel biosynthetic technique of protein glycan coupling technology (PGCT) that exploits bacterial N-linked glycosylation to recombinantly conjugate F. tularensis O-antigen glycans to the immunogenic carrier protein Pseudomonas aeruginosa exoprotein A (ExoA). Previously, we demonstrated that an ExoA glycoconjugate with two glycosylation sequons was capable of providing significant protection to mice against a challenge with a low-virulence strain of F. tularensis. Here, we have generated a more heavily glycosylated conjugate vaccine and evaluated its efficacy in a Fischer 344 rat model of tularemia. We demonstrate that this glycoconjugate vaccine protected rats against disease and the lethality of an inhalational challenge with F. tularensis Schu S4. Our data highlights the potential of this biosynthetic approach for the creation of next-generation tularemia subunit vaccines.

Aerosol prime-boost vaccination provides strong protection in outbred rabbits against virulent type A Francisella tularensis

Tularemia, also known as rabbit fever, is a severe zoonotic disease in humans caused by the gram-negative bacterium Francisella tularensis (Ft). While there have been a number of attempts to develop a vaccine for Ft, few candidates have advanced beyond experiments in inbred mice. We report here that a prime-boost strategy with aerosol delivery of recombinant live attenuated candidate Ft S4ΔaroD offers significant protection (83% survival) in an outbred animal model, New Zealand White rabbits, against aerosol challenge with 248 cfu (11 LD₅₀) of virulent type A Ft SCHU S4. Surviving rabbits given two doses of the attenuated strains by aerosol did not exhibit substantial post-challenge fevers, changes in erythrocyte sedimentation rate or in complete blood counts. At a higher challenge dose (3,186 cfu; 139 LD₅₀), protection was still good with 66% of S4ΔaroD-vaccinated rabbits surviving while 50% of S4ΔguaBA vaccinated rabbits also survived challenge. Pre-challenge plasma IgG titers against Ft SCHU S4 corresponded with survival time after challenge. Western blot analysis found that plasma antibody shifted from predominantly targeting Ft O-antigen after the prime vaccination to other antigens after the boost. These results demonstrate the superior protection conferred by a live attenuated derivative of virulent F. tularensis, particularly when given in an aerosol prime-boost regimen.

Protection induced by a Francisella tularensis subunit vaccine delivered by glucan particles

Francisella tularensis is an intracellular pathogen causing the disease tularemia, and an organism of concern to biodefence. There is no licensed vaccine available. Subunit approaches have failed to induce protection, which requires both humoral and cellular immune memory responses, and have been hampered by a lack of understanding as to which antigens are immunoprotective. We undertook a preliminary in silico analysis to identify candidate protein antigens. These antigens were then recombinantly expressed and encapsulated into glucan particles (GPs), purified Saccharomyces cerevisiae cell walls composed primarily of β-1,3-glucans. Immunological profiling in the mouse was used to down-selection to seven lead antigens: FTT1043 (Mip), IglC, FTT0814, FTT0438, FTT0071 (GltA), FTT0289, FTT0890 (PilA) prior to transitioning their evaluation to a Fischer 344 rat model for efficacy evaluation. F344 rats were vaccinated with the GP protein antigens co-delivered with GP-loaded with Francisella LPS. Measurement of cell mediated immune responses and computational epitope analysis allowed down-selection to three promising candidates: FTT0438, FTT1043 and FTT0814. Of these, a GP vaccine delivering Francisella LPS and the FTT0814 protein was able to induce protection in rats against an aerosol challenge of F. tularensis SchuS4, and reduced organ colonisation and clinical signs below that which immunisation with a GP-LPS alone vaccine provided. This is the first report of a protein supplementing protection induced by LPS in a Francisella vaccine. This paves the way for developing an effective, safe subunit vaccine for the prevention of inhalational tularemia, and validates the GP platform for vaccine delivery where complex immune responses are required for prevention of infections by intracellular pathogens.

An Improved Tobacco Mosaic Virus (TMV)-Conjugated Multiantigen Subunit Vaccine Against Respiratory Tularemia

Francisella tularensis, the causative agent of the fatal human disease known as tularemia is classified as a Category A Select Agent by the Centers for Disease Control. No licensed vaccine is currently available for prevention of tularemia in the United States. Previously, we published that a tri-antigen tobacco mosaic virus (TMV) vaccine confers 50% protection in immunized mice against respiratory tularemia caused by F. tularensis. In this study, we refined the TMV-vaccine formulation to improve the level of protection in immunized C57BL/6 mice against respiratory tularemia. We developed a tetra-antigen vaccine by conjugating OmpA, DnaK, Tul4, and SucB proteins of Francisella to TMV. CpG was also included in the vaccine formulation as an adjuvant. Primary intranasal (i.n.) immunization followed by two booster immunizations with the tetra-antigen TMV vaccine protected 100% mice against i.n. 10LD₁₀₀ challenges dose of F. tularensis live vaccine strain (LVS). Mice receiving three immunization doses of tetra-antigen TMV vaccine showed only transient body weight loss, cleared the infection rapidly, and showed minimal histopathological lesions in lungs, liver, and spleen following a lethal respiratory challenge with F. tularensis LVS. Mice immunized with the tetra-antigen TMV vaccine also induced strong ex vivo recall responses and were protected against a lethal challenge as late as 163 days post-primary immunization. Three immunization with the tetra-antigen TMV vaccine also induced a stronger humoral immune response predominated by IgG1, IgG2b, and IgG2c antibodies than mice receiving only a single or two immunizations. Remarkably, a single dose protected 40% of mice, while two doses protected 80% of mice from lethal pathogen challenge. Immunization of Interferon-gamma (IFN-γ)-deficient mice with the tetra-antigen TMV vaccine demonstrated an absolute requirement of IFN-γ for the generation of protective immune response against a lethal respiratory challenge with F. tularensis LVS. Collectively, this study further demonstrates the feasibility of TMV as an efficient platform for the delivery of multiple F. tularensis antigens and that tetra-antigen TMV vaccine formulation provides complete protection, and induces long-lasting protective and memory immune responses against respiratory tularemia caused by F. tularensis LVS.

Live Attenuated Tularemia Vaccines for Protection Against Respiratory Challenge With Virulent F. tularensis subsp. tularensis

Francisella tularensis is the causative agent of tularemia and a Tier I bioterrorism agent. In the 1900s, several vaccines were developed against tularemia including the killed "Foshay" vaccine, subunit vaccines comprising F. tularensis protein(s) or lipoproteins(s) in an adjuvant formulation, and the F. tularensis Live Vaccine Strain (LVS); none were licensed in the U.S.A. or European Union. The LVS vaccine retains toxicity in humans and animals-especially mice-but has demonstrated efficacy in humans, and thus serves as the current gold standard for vaccine efficacy studies. The U.S.A. 2001 anthrax bioterrorism attack spawned renewed interest in vaccines against potential biowarfare agents including F. tularensis. Since live attenuated-but not killed or subunit-vaccines have shown promising efficacy and since vaccine efficacy against respiratory challenge with less virulent subspecies holarctica or F. novicida, or against non-respiratory challenge with virulent subsp. tularensis (Type A) does not reliably predict vaccine efficacy against respiratory challenge with virulent subsp. tularensis, the route of transmission and species of greatest concern in a bioterrorist attack, in this review, we focus on live attenuated tularemia vaccine candidates tested against respiratory challenge with virulent Type A strains, including homologous vaccines derived from mutants of subsp. holarctica, F. novicida, and subsp. tularensis, and heterologous vaccines developed using viral or bacterial vectors to express F. tularensis immunoprotective antigens. We compare the virulence and efficacy of these vaccine candidates with that of LVS and discuss factors that can significantly impact the development and evaluation of live attenuated tularemia vaccines. Several vaccines meet what we would consider the minimum criteria for vaccines to go forward into clinical development-safety greater than LVS and efficacy at least as great as LVS, and of these, several meet the higher standard of having efficacy ≥LVS in the demanding mouse model of tularemia. These latter include LVS with deletions in purMCD, sodBFt , capB or wzy; LVS ΔcapB that also overexpresses Type VI Secretion System (T6SS) proteins; FSC200 with a deletion in clpB; the single deletional purMCD mutant of F. tularensis SCHU S4, and a heterologous prime-boost vaccine comprising LVS ΔcapB and Listeria monocytogenes expressing T6SS proteins.

Single vector platform vaccine protects against lethal respiratory challenge with Tier 1 select agents of anthrax, plague, and tularemia

Bacillus anthracis, Yersinia pestis, and Francisella tularensis are the causative agents of Tier 1 Select Agents anthrax, plague, and tularemia, respectively. Currently, there are no licensed vaccines against plague and tularemia and the licensed anthrax vaccine is suboptimal. Here we report F. tularensis LVS ΔcapB (Live Vaccine Strain with a deletion in capB)- and attenuated multi-deletional Listeria monocytogenes (Lm)-vectored vaccines against all three aforementioned pathogens. We show that LVS ΔcapB- and Lm-vectored vaccines express recombinant B. anthracis, Y. pestis, and F. tularensis immunoprotective antigens in broth and in macrophage-like cells and are non-toxic in mice. Homologous priming-boosting with the LVS ΔcapB-vectored vaccines induces potent antigen-specific humoral and T-cell-mediated immune responses and potent protective immunity against lethal respiratory challenge with all three pathogens. Protection against anthrax was far superior to that obtained with the licensed AVA vaccine and protection against tularemia was comparable to or greater than that obtained with the toxic and unlicensed LVS vaccine. Heterologous priming-boosting with LVS ΔcapB- and Lm-vectored B. anthracis and Y. pestis vaccines also induced potent protective immunity against lethal respiratory challenge with B. anthracis and Y. pestis. The single vaccine platform, especially the LVS ΔcapB-vectored vaccine platform, can be extended readily to other pathogens.

Adaptive Immunity to Francisella tularensis and Considerations for Vaccine Development

Francisella tularensis is an intracellular bacterium that causes the disease tularemia. There are several subspecies of F. tularensis whose ability to cause disease varies in humans. The most virulent subspecies, tularensis, is a Tier One Select Agent and a potential bioweapon. Although considerable effort has made to generate efficacious tularemia vaccines, to date none have been licensed for use in the United States. Despite the lack of a tularemia vaccine, we have learned a great deal about the adaptive immune response the underlies protective immunity. Herein, we detail the animal models commonly used to study tularemia and their recapitulation of human disease, the field's current understanding of vaccine-mediated protection, and discuss the challenges associated with new vaccine development.

Intratracheal Inoculation of Fischer 344 Rats with Francisella tularensis

Pulmonary infection with the bacterium Francisella tularensis can lead to the serious and potentially fatal disease, tularemia, in humans. Due to the current lack of an approved tularemia vaccine for humans, research is focused on vaccine development utilizing appropriate animal models. The Fischer 344 rat has emerged as a model that reflects human susceptibility to F. tularensis infection, and thus is an attractive model for tularemia vaccine development. Intratracheal inoculation of the Fischer 344 rat with F. tularensis mimics pulmonary exposure in humans. The successful delivery into the rat trachea is critical for pulmonary delivery. A laryngoscope with illumination is used to properly intubate the tracheae of anesthetized rats; the correct placement within the trachea is determined by a simple device to detect breathing. Following intubation, the F. tularensis culture is delivered in a measured dose via syringe. This technique standardizes pulmonary delivery of F. tularensis within the rat trachea to evaluate vaccine efficacy.

Differential Growth of Francisella tularensis, Which Alters Expression of Virulence Factors, Dominant Antigens, and Surface-Carbohydrate Synthases, Governs the Apparent Virulence of Ft SchuS4 to Immunized Animals

The gram-negative bacterium Francisella tularensis (Ft) is both a potential biological weapon and a naturally occurring microbe that survives in arthropods, fresh water amoeba, and mammals with distinct phenotypes in various environments. Previously, we used a number of measurements to characterize Ft grown in Brain-Heart Infusion (BHI) broth as (1) more similar to infection-derived bacteria, and (2) slightly more virulent in naïve animals, compared to Ft grown in Mueller Hinton Broth (MHB). In these studies we observed that the free amino acids in MHB repress expression of select Ft virulence factors by an unknown mechanism. Here, we tested the hypotheses that Ft grown in BHI (BHI-Ft) accurately displays a full protein composition more similar to that reported for infection-derived Ft and that this similarity would make BHI-Ft more susceptible to pre-existing, vaccine-induced immunity than MHB-Ft. We performed comprehensive proteomic analysis of Ft grown in MHB, BHI, and BHI supplemented with casamino acids (BCA) and compared our findings to published “omics” data derived from Ft grown in vivo. Based on the abundance of ~1,000 proteins, the fingerprint of BHI-Ft is one of nutrient-deprived bacteria that—through induction of a stringent-starvation-like response—have induced the FevR regulon for expression of the bacterium's virulence factors, immuno-dominant antigens, and surface-carbohydrate synthases. To test the notion that increased abundance of dominant antigens expressed by BHI-Ft would render these bacteria more susceptible to pre-existing, vaccine-induced immunity, we employed a battery of LVS-vaccination and S4-challenge protocols using MHB- and BHI-grown Ft S4. Contrary to our hypothesis, these experiments reveal that LVS-immunization provides a barrier to infection that is significantly more effective against an MHB-S4 challenge than a BHI-S4 challenge. The differences in apparent virulence to immunized mice are profoundly greater than those observed with primary infection of naïve mice. Our findings suggest that tularemia vaccination studies should be critically evaluated in regard to the growth conditions of the challenge agent.

Immunoproteomic Analysis of Antibody Response of Rabbit Host Against Heat-Killed Francisella tularensis Live Vaccine Strain

Francisella tularensis, the causative agent of tularemia, has attained the status of one of the high priority agents that could be used in the act of bioterrorism. Currently, there is no licensed vaccine for this highly infectious intracellular pathogen. Being a listed 'Category A' agent of the U.S. Center for Disease Control and Prevention (CDC), vaccines and therapeutics are immediately required against this pathogen. In this study, an immunoproteomic approach based on the techniques of 2-dimensional gel electrophoresis (2DE) and immunoblotting combined with mass spectrometry (MS) was used for elucidation of immunogenic components and putative vaccine candidates. Whole-cell soluble protein extract of F. tularensis LVS (Ft LVS) was separated by 2DE, and immunoblots were developed with sera raised in rabbit after immunization with heat-killed Ft LVS. A total of 28 immunoreactive proteins were identified by tandem mass spectrometry. Rabbit immunoproteome of F. tularensis was compared with those previously reported using sera from human patients and in murine model. Out of 28 immunoreactive proteins identified in this study, 12 and 17 overlapping proteins were recognized by human and murine sera, respectively. Nine proteins were found immunogenic in all the three hosts, while eight new immunogenic proteins were found in this study. Identified immunoreactive proteins may find application in design and development of protein subunit vaccine for tularemia.

Murine survival of infection with Francisella novicida and protection against secondary challenge is critically dependent on B lymphocytes

Respiratory infection of mice with Francisella novicida has recently been used as a model for the highly virulent human pathogen Francisella tularensis. Similar to F. tularensis, even small doses of F. novicida administered by respiratory routes are lethal for inbred laboratory mice. This feature obviously limits study of infection-induced immunity. Parenteral sublethal infections of mice with F. novicida are feasible, but the resulting immune responses are incompletely characterized. Here we use parenteral intradermal (i.d.) and intraperitoneal (i.p.) F. novicida infections of C57BL/6J mice to determine the role of B cells in controlling primary and secondary F. novicida infections. Despite developing comparable levels of F. novicida-primed T cells, B cell knockout mice were much more susceptible to both primary i.d. infection and secondary i.p. challenge than wild type (normal) C57BL/6J mice. Transfer of F. novicida-immune sera to either wild type C57BL/6J mice or to B cell knockout mice did not appreciably impact survival of subsequent lethal F. novicida challenge. However, F. novicida-immune mice that were depleted of T cells after priming but just before challenge survived and cleared secondary i.p. F. novicida challenge. Collectively these results indicate that B cells, if not serum antibodies, play a major role in controlling F. novicida infections in mice.

Cathepsin B modulates lysosomal biogenesis and host defense against Francisella novicida infection

Kanneganti and collaborators propose that the lysosomal protease cathepsin B provides a checkpoint for activation of the transcription factor TFEB and lysosomal biogenesis and explore the impact of this pathway on host defense against bacterial infection.

Lysosomal cathepsins regulate an exquisite range of biological functions, and their deregulation is associated with inflammatory, metabolic, and degenerative diseases in humans. In this study, we identified a key cell-intrinsic role for cathepsin B as a negative feedback regulator of lysosomal biogenesis and autophagy. Mice and macrophages lacking cathepsin B activity had increased resistance to the cytosolic bacterial pathogen Francisella novicida. Genetic deletion or pharmacological inhibition of cathepsin B down-regulated mechanistic target of rapamycin activity and prevented cleavage of the lysosomal calcium channel TRPML1. These events drove transcription of lysosomal and autophagy genes via transcription factor EB, which increased lysosomal biogenesis and activation of autophagy initiation kinase ULK1 for clearance of the bacteria. Our results identified a fundamental biological function of cathepsin B in providing a checkpoint for homeostatic maintenance of lysosome populations and basic recycling functions in the cell.

Respiratory and oral vaccination improves protection conferred by the live vaccine strain against pneumonic tularemia in the rabbit model

Tularemia is a severe, zoonotic disease caused by a gram-negative bacterium, Francisella tularensis We have previously shown that rabbits are a good model of human pneumonic tularemia when exposed to aerosols containing a virulent, type A strain, SCHU S4. We further demonstrated that the live vaccine strain (LVS), an attenuated type B strain, extended time to death when given by scarification. Oral or aerosol vaccination has been previously shown in humans to offer superior protection to parenteral vaccination against respiratory tularemia challenge. Both oral and aerosol vaccination with LVS were well tolerated in the rabbit with only minimal fever and no weight loss after inoculation. Plasma antibody titers against F. tularensis were higher in rabbits that were vaccinated by either oral or aerosol routes compared to scarification. Thirty days after vaccination, all rabbits were challenged with aerosolized SCHU S4. LVS given by scarification extended time to death compared to mock-vaccinated controls. One orally vaccinated rabbit did survive aerosol challenge, however, only aerosol vaccination extended time to death significantly compared to scarification. These results further demonstrate the utility of the rabbit model of pneumonic tularemia in replicating what has been reported in humans and macaques as well as demonstrating the utility of vaccination by oral and respiratory routes against an aerosol tularemia challenge.

Tularemia vaccine development: paralysis or progress?

Francisella tularensis (Ft) is a gram-negative intercellular pathogen and category A biothreat agent. However, despite 15 years of strong government investment and intense research focused on the development of a US Food and Drug Administration-approved vaccine against Ft, the primary goal remains elusive. This article reviews research efforts focused on developing an Ft vaccine, as well as a number of important factors, some only recently recognized as such, which can significantly impact the development and evaluation of Ft vaccine efficacy. Finally, an assessment is provided as to whether a US Food and Drug Administration-approved Ft vaccine is likely to be forthcoming and the potential means by which this might be achieved.

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.

Pore-Forming Toxins Induce Macrophage Necroptosis during Acute Bacterial Pneumonia

Necroptosis is a highly pro-inflammatory mode of cell death regulated by RIP (or RIPK)1 and RIP3 kinases and mediated by the effector MLKL. We report that diverse bacterial pathogens that produce a pore-forming toxin (PFT) induce necroptosis of macrophages and this can be blocked for protection against Serratia marcescens hemorrhagic pneumonia. Following challenge with S. marcescens, Staphylococcus aureus, Streptococcus pneumoniae, Listeria monocytogenes, uropathogenic Escherichia coli (UPEC), and purified recombinant pneumolysin, macrophages pretreated with inhibitors of RIP1, RIP3, and MLKL were protected against death. Alveolar macrophages in MLKL KO mice were also protected during S. marcescens pneumonia. Inhibition of caspases had no impact on macrophage death and caspase-1 and -3/7 were determined to be inactive following challenge despite the detection of IL-1β in supernatants. Bone marrow-derived macrophages from RIP3 KO, but not caspase-1/11 KO or caspase-3 KO mice, were resistant to PFT-induced death. We explored the mechanisms for PFT-induced necroptosis and determined that loss of ion homeostasis at the plasma membrane, mitochondrial damage, ATP depletion, and the generation of reactive oxygen species were together responsible. Treatment of mice with necrostatin-5, an inhibitor of RIP1; GW806742X, an inhibitor of MLKL; and necrostatin-5 along with co-enzyme Q₁₀ (N5/C₁₀)_, which enhances ATP production; reduced the severity of S. marcescens pneumonia in a mouse intratracheal challenge model. N5/C₁₀ protected alveolar macrophages, reduced bacterial burden, and lessened hemorrhage in the lungs. We conclude that necroptosis is the major cell death pathway evoked by PFTs in macrophages and the necroptosis pathway can be targeted for disease intervention.

Necroptosis is a pro-inflammatory mode of programmed cell death that is marked by the intentional disruption of host membranes and the release of pro-inflammatory cytosolic components into the milieu. Until just recently necroptosis was not appreciated to play a role during infectious disease. Herein, we demonstrate that alveolar macrophages exposed to the nosocomial pathogen Serratia marcescens undergo necroptosis and this leads to enhanced disease severity. We subsequently demonstrate that necroptosis is the principle mode of cell death experienced by macrophages following their exposure to bacteria that produce pore-forming toxins (PFTs). We dissect the molecular mechanisms by which PFTs induce necroptosis and demonstrate that loss of ion homeostasis at the cell membrane and mitochondrial damage result in ATP depletion and ROS generation that together are responsible. Finally, we demonstrate that inhibition of necroptosis by various means is protective against hemorrhagic pneumonia caused by S. marcescens.

Characterization of Francisella tularensis Schu S4 defined mutants as live-attenuated vaccine candidates

Francisella tularensis (Ft), the etiological agent of tularemia and a Tier 1 select agent, has been previously weaponized and remains a high priority for vaccine development. Ft tularensis (type A) and Ft holarctica (type B) cause most human disease. We selected six attenuating genes from the live vaccine strain (LVS; type B), F. novicida and other intracellular bacteria: FTT0507, FTT0584, FTT0742, FTT1019c (guaA), FTT1043 (mip) and FTT1317c (guaB) and created unmarked deletion mutants of each in the highly human virulent Ft strain Schu S4 (Type A) background. FTT0507, FTT0584, FTT0742 and FTT1043 Schu S4 mutants were not attenuated for virulence in vitro or in vivo. In contrast, Schu S4 gua mutants were unable to replicate in murine macrophages and were attenuated in vivo, with an i.n. LD₅₀ > 10⁵ CFU in C57BL/6 mice. However, the gua mutants failed to protect mice against lethal challenge with WT Schu S4, despite demonstrating partial protection in rabbits in a previous study. These results contrast with the highly protective capacity of LVS gua mutants against a lethal LVS challenge in mice, and underscore differences between these strains and the animal models in which they are evaluated, and therefore have important implications for vaccine development.

Mutations in guanine biosynthesis genes, but not in four other hypothetical virulence factors in highly virulent Francisella tularensis strain Schu S4 resulted in attenuation in macrophage replication and mouse virulence.

Preclinical testing of a vaccine candidate against tularemia

Tularemia is caused by a gram-negative, intracellular bacterial pathogen, Francisella tularensis (Ft). The history weaponization of Ft in the past has elevated concerns that it could be used as a bioweapon or an agent of bioterrorism. Since the discovery of Ft, three broad approaches adopted for tularemia vaccine development have included inactivated, live attenuated, or subunit vaccines. Shortcomings in each of these approaches have hampered the development of a suitable vaccine for prevention of tularemia. Recently, we reported an oxidant sensitive mutant of Ft LVS in putative EmrA1 (FTL_0687) secretion protein. The emrA1 mutant is highly sensitive to oxidants, attenuated for intramacrophage growth and virulence in mice. We reported that EmrA1 contributes to oxidant resistance by affecting the secretion of antioxidant enzymes SodB and KatG. This study investigated the vaccine potential of the emrA1 mutant in prevention of respiratory tularemia caused by Ft LVS and the virulent SchuS4 strain in C57BL/6 mice. We report that emrA1 mutant is safe and can be used at an intranasal (i. n.) immunization dose as high as 1x106 CFU without causing any adverse effects in immunized mice. The emrA1 mutant is cleared by vaccinated mice by day 14-21 post-immunization, induces minimal histopathological lesions in lungs, liver and spleen and a strong humoral immune response. The emrA1 mutant vaccinated mice are protected against 1000-10,000LD100 doses of i.n. Ft LVS challenge. Such a high degree of protection has not been reported earlier against respiratory challenge with Ft LVS using a single immunization dose with an attenuated mutant generated on Ft LVS background. The emrA1 mutant also provides partial protection against i.n. challenge with virulent Ft SchuS4 strain in vaccinated C57BL/6 mice. Collectively, our results further support the notion that antioxidants of Ft may serve as potential targets for development of effective vaccines for prevention of tularemia.