Post-exposure immunization against Francisella tularensis membrane proteins augments protective efficacy of gentamicin in a mouse model of pneumonic tularemia

Development of a multi-epitope vaccine from outer membrane proteins and identification of novel drug targets against Francisella tularensis: an In Silico approach

Background

Francisella tularensis is a category A potential thread agent, making the development of vaccines and countermeasures a high priority. Therefore, identifying new vaccine candidates and novel drug targets is essential for addressing this significant public health concern.

Methods

This study presents an in silico analysis of two strategies against F. tularensis infection: the development of a multi-epitope vaccine (MEV) and the identification of novel drug targets. Outer membrane proteins (OMPs) were predicted using subcellular localization tools and immunogenicity was evaluated using a reverse vaccinology pipeline. Epitopes from these OMPs were combined to create candidate MEV for prophylactic protection. Concurrently, cytoplasmic proteins were subjected to rigorous analysis to identify potential novel drug targets.

Results

Of 1,921 proteins, we identified 12 promising protein vaccine candidates from F. tularensis OMPs and proposed a multi-epitope vaccine (MEV) designed using seven immunodominant epitopes derived from four of these OMPs, including two hypothetical proteins (WP_003026145.1 and WP_003029346.1), an OmpA family protein (WP_003020808.1), and PD40 (WP_003021546.1). In addition, we proposed 10 novel drug targets for F. tularensis: Asp-tRNA (Asn)/Glu-tRNA (Gln) amidotransferase subunit GatC (WP_003017413.1), NAD(P)-binding protein (WP_042522581.1), 30S ribosomal protein S16 (WP_003023081.1), Class I SAM-dependent methyltransferase (WP_003022345.1), haloacid dehalogenase (WP_003014157.1), uroporphyrinogen-III synthase (WP_003022220.1), and four hypothetical proteins (WP_003017784.1, WP_003020080.1, WP_003020066.1, and WP_003022350.1).

Conclusion

This study designed an MEV and proposed novel drug targets to address tularemia, offering broad protection against various F. tularensis strains. MEV, with favorable physicochemical properties, showed strong potential through molecular docking and dynamic simulations. Immune simulations suggest that it may elicit robust responses against pathogens. The identification of novel drug targets can lead to the discovery of new antimicrobial agents. However, further in vitro and in vivo studies are required to validate their efficacy and capability.

Non-vaccinal prophylaxis of tularemia

Tularemia is a re-emerging zoonosis in many endemic countries. It is caused by Francisella tularensis, a gram-negative bacterium and biological threat agent. Humans are infected from the wild animal reservoir, the environmental reservoir or by the bite of arthropod vectors. This infection occurs through the cutaneous, conjunctival, digestive or respiratory routes. Tularemia generally manifests itself as an infection at the site of entry of the bacteria with regional lymphadenopathy, or as a systemic disease, particularly pulmonary. It is often a debilitating condition due to persistent symptoms and sometimes a life-threatening condition. There is effective antibiotic treatment for this disease but no vaccine is currently available for humans or animals. Due to the complexity of the F. tularensis life cycle and multiple modes of human infection, non-vaccine prophylaxis of tularemia is complex and poorly defined. In this review, we summarize the various individual prophylactic measures available against tularemia based on the different risk factors associated with the disease. We also discuss the currently underdeveloped possibilities for collective prophylaxis. Prophylactic measures must be adapted in each tularemia endemic area according to the predominant modes of human and animal infection. They requires a One Health approach to control both animal and environmental reservoirs of F. tularensis, as well as arthropod vectors, to slow the current expansion of endemic areas of this disease in a context of climate change.

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.

Combating biothreat pathogens: ongoing efforts for countermeasure development and unique challenges

Research to discover and develop antibacterial and antiviral drugs with potent activity against pathogens of biothreat concern presents unique methodological and process-driven challenges. Herein, we review laboratory approaches for finding new antibodies, antibiotics, and antiviral molecules for pathogens of biothreat concern. Using high-throughput screening techniques, molecules that directly inhibit a pathogen’s entry, replication, or growth can be identified. Alternatively, molecules that target host proteins can be interesting targets for development when countering biothreat pathogens, due to the modulation of the host immune response or targeting proteins that interfere with the pathways required by the pathogen for replication. Monoclonal and cocktail antibody therapies approved by the Food and Drug Administration for countering anthrax and under development for treatment of Ebola virus infection are discussed. A comprehensive tabular review of current in vitro, in vivo, pharmacokinetic and efficacy datasets has been presented for biothreat pathogens of greatest concern. Finally, clinical trials and animal rule or traditional drug approval pathways are also reviewed.

Opinions; interpretations; conclusions; and recommendations are those of the authors and are not necessarily endorsed by the US Army.

Therapeutic Antibodies for Biodefense

Diseases can be caused naturally by biological agents such as bacteria, viruses and toxins (natural risk). However, such biological agents can be intentionally disseminated in the environment by a State (military context) or terrorists to cause diseases in a population or livestock, to destabilize a nation by creating a climate of terror, destabilizing the economy and undermining institutions. Biological agents can be classified according to the severity of illness they cause, its mortality and how easily the agent can be spread. The Centers for Diseases Control and Prevention (CDC) classify biological agents in three categories (A, B and C); Category A consists of the six pathogens most suitable for use as bioweapons (Bacillus anthracis, Yersinia pestis, Francisella tularensis, botulinum neurotoxins, smallpox and viral hemorrhagic fevers). Antibodies represent a perfect biomedical countermeasure as they present both prophylactic and therapeutic properties, act fast and are highly specific to the target. This review focuses on the main biological agents that could be used as bioweapons, the history of biowarfare and antibodies that have been developed to neutralize these agents.

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.

Virulence of representative Japanese Francisella tularensis and immunologic consequences of infection in mice

Francisella tularensis, which causes tularemia, is widely distributed in the Northern hemisphere. F. tularensis strains isolated in Japan are genetically unique from non-Japanese strains; however, their phenotypic properties have not been well studied. Thus, mice were infected with representative Japanese strains of F. tularensis and their virulence and mouse immune responses to them assessed. Of four representative Japanese strains, the Ebina, Jap and Tsuchiya strains were susceptible to H2 O2 and did not grow well intracellularly. Only Yama strain grew intracellularly and was lethal to mice. Infection with Yama strain resulted in drastic increases in IFN-γ, CD4 and CD8 double-positive T cells and Th1 cells (CD3, CD4 and Tim3-positive cells), and a decrease in the ratio of CD8-positive CD4-negative T cells in mice. C57BL/6J mice that survived infection produced IgM antibodies to LPS and IgG2c antibodies to 43, 19 and 17 kDa proteinase K-sensitive components. These data are valuable for understanding the phenotypic properties of F. tularensis in Japan.

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.

A rapid real-time quantitative PCR assay to determine the minimal inhibitory extracellular concentration of antibiotics against an intracellular Francisella tularensis Live Vaccine Strain

Francisella tularensis is a highly virulent facultative intracellular bacterium. The lack of a safe and efficient vaccine makes antibiotics the preferred treatment. F. tularensis antibiotic susceptibility tests are based on the in vitro standard CLSI-approved microdilution method for determining the MIC. However, limited data are available regarding the minimal inhibitory extracellular concentration (MIEC) needed to eradicate intracellular bacteria. Here, we evaluated the MIEC values of various WHO-recommended antibiotics and compared the MIEC values to the established MICs. We describe a rapid 3-h quantitative PCR (qPCR) intracellular antibiogram assay, which yields comparable MIEC values to those obtained by the classical 72-h cfu assay. This rapid qPCR assay is highly advantageous in light of the slow growth rates of F. tularensis. Our results showed that the MIECs obtained for doxycycline, chloramphenicol and ciprofloxacin were indicative of intracellular activity. Gentamicin was not effective against intracellular bacteria for at least 32 h post treatment, raising the question of whether slow-penetrating gentamicin should be used for certain stages of the disease. We suggest that the qPCR intracellular antibiogram assay may be used to screen for potentially active antibiotics against intracellular F. tularensis as well as to detect strains with acquired resistance to recommended antibiotics.

Francisella tularensis LVS surface and membrane proteins as targets of effective post-exposure immunization for tularemia

Francisella tularensis causes disease (tularemia) in a large number of mammals, including man. We previously demonstrated enhanced efficacy of conventional antibiotic therapy for tularemia by postexposure passive transfer of immune sera developed against a F. tularensis LVS membrane protein fraction (MPF). However, the protein composition of this immunogenic fraction was not defined. Proteomic approaches were applied to define the protein composition and identify the immunogens of MPF. MPF consisted of at least 299 proteins and 2-D Western blot analyses using sera from MPF-immunized and F. tularensis LVS-vaccinated mice coupled to liquid chromatography–tandem mass spectrometry identified 24 immunoreactive protein spots containing 45 proteins. A reverse vaccinology approach that applied labeling of F. tularensis LVS surface proteins and bioinformatics was used to reduce the complexity of potential target immunogens. Bioinformatics analyses of the immunoreactive proteins reduced the number of immunogen targets to 32. Direct surface labeling of F. tularensis LVS resulted in the identification of 31 surface proteins. However, only 13 of these were reactive with MPF and/or F. tularensis LVS immune sera. Collectively, this use of orthogonal proteomic approaches reduced the complexity of potential immunogens in MPF by 96% and allowed for prioritization of target immunogens for antibody-based immunotherapies against tularemia.

New therapeutic approaches for treatment of tularaemia: a review

Antibiotic treatment of tularaemia is based on a few drugs, including the fluoroquinolones (e.g., ciprofloxacin), the tetracyclines (e.g., doxycycline), and the aminoglycosides (streptomycin and gentamicin). Because no effective and safe vaccine is currently available, tularaemia prophylaxis following proven exposure to F. tularensis also relies on administration of antibiotics. A number of reasons make it necessary to search for new therapeutic alternatives: the potential toxicity of first-line drugs, especially in children and pregnant women; a high rate of treatment relapses and failures, especially for severe and/or suppurated forms of the disease; and the possible use of antibiotic-resistant strains in the context of a biological threat. This review presents novel therapeutic approaches that have been explored in recent years to improve tularaemia patients' management and prognosis. These new strategies have been evaluated in vitro, in axenic media and cell culture systems and/or in animal models. First, the activities of newly available antibiotic compounds were evaluated against F. tularensis, including tigecycline (a glycylcycline), ketolides (telithromycin and cethromycin), and fluoroquinolones (moxifloxacin, gatifloxacin, trovafloxacin and grepafloxacin). The liposome delivery of some antibiotics was evaluated. The effect of antimicrobial peptides against F. tularensis was also considered. Other drugs were evaluated for their ability to suppress the intracellular multiplication of F. tularensis. The effects of the modulation of the innate immune response (especially via TLR receptors) on the course of F. tularensis infection was characterized. Another approach was the administration of specific antibodies to induce passive resistance to F. tularensis infection. All of these studies highlight the need to develop new therapeutic strategies to improve the management of patients with tularaemia. Many possibilities exist, some unexplored. Moreover, it is likely that new therapeutic alternatives that are effective against this intracellular pathogen could be, at least partially, extrapolated to other human pathogens.

Correlates of immune protection following cutaneous immunization with an attenuated Burkholderia pseudomallei vaccine

Infections with the Gram-negative bacterium Burkholderia pseudomallei (melioidosis) are associated with high mortality, and there is currently no approved vaccine to prevent the development of melioidosis in humans. Infected patients also do not develop protective immunity to reinfection, and some individuals will develop chronic, subclinical infections with B. pseudomallei. At present, our understanding of what constitutes effective protective immunity against B. pseudomallei infection remains incomplete. Therefore, we conducted a study to elucidate immune correlates of vaccine-induced protective immunity against acute B. pseudomallei infection. BALB/c and C57BL/6 mice were immunized subcutaneously with a highly attenuated, Select Agent-excluded purM deletion mutant of B. pseudomallei (strain Bp82) and then subjected to intranasal challenge with virulent B. pseudomallei strain 1026b. Immunization with Bp82 generated significant protection from challenge with B. pseudomallei, and protection was associated with a significant reduction in bacterial burden in lungs, liver, and spleen of immunized mice. Humoral immunity was critically important for vaccine-induced protection, as mice lacking B cells were not protected by immunization and serum from Bp82-vaccinated mice could transfer partial protection to nonvaccinated animals. In contrast, vaccine-induced protective immunity was found to be independent of both CD4 and CD8 T cells. Tracking studies demonstrated uptake of the Bp82 vaccine strain predominately by neutrophils in vaccine-draining lymph nodes and by smaller numbers of dendritic cells (DC) and monocytes. We concluded that protection following cutaneous immunization with a live attenuated Burkholderia vaccine strain was dependent primarily on generation of effective humoral immune responses.

Francisella tularensis SchuS4 and SchuS4 lipids inhibit IL-12p40 in primary human dendritic cells by inhibition of IRF1 and IRF8

Induction of innate immunity is essential for host survival of infection. Evasion and inhibition of innate immunity constitute a strategy used by pathogens, such as the highly virulent bacterium Francisella tularensis, to ensure their replication and transmission. The mechanism and bacterial components responsible for this suppression of innate immunity by F. tularensis are not defined. In this article, we demonstrate that lipids enriched from virulent F. tularensis strain SchuS4, but not attenuated live vaccine strain, inhibit inflammatory responses in vitro and in vivo. Suppression of inflammatory responses is associated with IκBα-independent inhibition of NF-κBp65 activation and selective inhibition of activation of IFN regulatory factors. Interference with NF-κBp65 and IFN regulatory factors is also observed following infection with viable SchuS4. Together these data provide novel insight into how highly virulent bacteria selectively modulate the host to interfere with innate immune responses required for survival of infection.

Immunotherapy for tularemia

Francisella tularensis is a gram-negative bacterium that causes the zoonotic disease tularemia. Francisella is highly infectious via the respiratory route (~10 CFUs) and pulmonary infections due to type A strains of F. tularensis are highly lethal in untreated patients (>30%). In addition, no vaccines are licensed to prevent tularemia in humans. Due to the high infectivity and mortality of pulmonary tularemia, F. tularensis has been weaponized, including via the introduction of antibiotic resistance, by several countries. Because of the lack of efficacious vaccines, and concerns about F. tularensis acquiring resistance to antibiotics via natural or illicit means, augmentation of host immunity, and humoral immunotherapy have been investigated as countermeasures against tularemia. This manuscript will review advances made and challenges in the field of immunotherapy against tularemia.

Interleukin-17 protects against the Francisella tularensis live vaccine strain but not against a virulent F. tularensis type A strain

Francisella tularensis is a highly infectious intracellular bacterium that causes the zoonotic infection tularemia. While much literature exists on the host response to F. tularensis infection, the vast majority of work has been conducted using attenuated strains of Francisella that do not cause disease in humans. However, emerging data indicate that the protective immune response against attenuated F. tularensis versus F. tularensis type A differs. Several groups have recently reported that interleukin-17 (IL-17) confers protection against the live vaccine strain (LVS) of Francisella. While we too have found that IL-17Rα(-/-) mice are more susceptible to F. tularensis LVS infection, our studies, using a virulent type A strain of F. tularensis (SchuS4), indicate that IL-17Rα(-/-) mice display organ burdens and pulmonary gamma interferon (IFN-γ) responses similar to those of wild-type mice following infection. In addition, oral LVS vaccination conferred equivalent protection against pulmonary challenge with SchuS4 in both IL-17Rα(-/-) and wild-type mice. While IFN-γ was found to be critically important for survival in a convalescent model of SchuS4 infection, IL-17 neutralization from either wild-type or IFN-γ(-/-) mice had no effect on morbidity or mortality in this model. IL-17 protein levels were also higher in the lungs of mice infected with the LVS rather than F. tularensis type A, while IL-23p19 mRNA expression was found to be caspase-1 dependent in macrophages infected with LVS but not SchuS4. Collectively, these results demonstrate that IL-17 is dispensable for host immunity to type A F. tularensis infection, and that induced and protective immunity differs between attenuated and virulent strains of F. tularensis.