Native outer membrane proteins protect mice against pulmonary challenge with virulent type A Francisella tularensis

Difference in genes between a high virulence strain G(4) and a low virulence strain G(18) of Flavobacterium columnare by using suppression subtractive hybridization

Flavobacterium columnare is the causative agent of columnaris disease. Different genetic groups of F. columnare show to some extent different degrees of virulence. To identify genetic differences between the high virulence strain G(4) and the low virulence strain G(18) of F. columnare, suppression subtractive hybridization was used. A total of 46 genes were identified from the virulent strain G(4), 35 of which showed some degree of homology with known proteins and can be classified into 11 categories: DNA replication or recombination proteins, inorganic ion transport proteins, outer membrane proteins, enterotoxin, binding proteins, YD repeat proteins, transposase, chaperon, signal transduction-related proteins, regulatory proteins, metabolism-related proteins. Several putative virulence factors identified in other bacteria could also be identified in the virulent strain G(4), such as ferrous iron transport protein, TonB-dependent receptor, transposases, as well as ABC transporter permease protein. The flanking region of a putative transposase ISFclI was analysed, and a putative Rhs element was located at the downstream of the putative transposase. The analysis of isfclI gene in 24 strains of F. columnare isolated in China revealed that 11 strains have isfclI, and all the strains from Zhaoqing, Anhui and Qingjiang have isfclI.

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

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

Francisella virulence: significant advances, ongoing challenges and unmet needs

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

Differential ability of novel attenuated targeted deletion mutants of Francisella tularensis subspecies tularensis strain SCHU S4 to protect mice against aerosol challenge with virulent bacteria: effects of host background and route of immunization

Francisella tularensis subspecies tularensis is a highly virulent facultative intracellular pathogen of humans and a potential biological weapon. A live vaccine strain, F. tularensis LVS, was developed more than 50 years ago by pragmatic attenuation of a strain of the less virulent holarctica subspecies. LVS was demonstrated to be highly effective in human volunteers who were exposed to intradermal challenge with fully virulent subsp. tularensis, but was less effective against aerosol exposure. LVS faces regulatory hurdles that to date have prevented its licensure for general use. Therefore, a better defined and more effective vaccine is being sought. To this end we have created gene deletion mutants in the virulent subsp. tularensis strain and tested them for their ability to elicit a protective immune response against systemic or aerosol challenge with the highly virulent wild-type subsp. tularensis strain, SCHU S4. Both oral and intradermal (ID) primary vaccination routes were assessed in BALB/c and C3H/HeN mice as was oral boosting. One SCHU S4 mutant missing the heat shock gene, clpB, was significantly more attenuated than LVS whereas a double deletion mutant missing genes FTT0918 and capB was as attenuated as LVS. In general mice immunized with SCHU S4DeltaclpB were significantly better protected against aerosol challenge than mice immunized with LVS. A single ID immunization of BALB/c mice with SCHU S4DeltaclpB was at least as effective as any other regimen examined. Mice immunized with SCHU S4Delta0918DeltacapB were generally protected to a similar degree as mice immunized with LVS. A preliminary examination of immune responses to vaccination with LVS, SCHU S4DeltaclpB, or SCHU S4Delta0918DeltacapB provided no obvious correlate to their relative efficacies.

Identification, cloning, expression, and purification of Francisella lpp3: an immunogenic lipoprotein

The severe and fatal human disease, tularemia, results from infection with the Gram-negative pathogen Francisella tularensis. Identification of surface outer membrane proteins, specifically lipoproteins, has been of interest for vaccine development and understanding the initiation of disease. We sought to identify Francisella live vaccine strain lipoproteins that could be a component of a subunit vaccine and have adjuvant properties as TLR2 agonists. We have identified a membrane lipoprotein of Francisella LVS isolated by sarkosyl extraction and gel filtration chromatography that is recognized by sera from LVS-vaccinated individuals and tularemia patients, indicating its potential diagnostic value. Sequencing of the protein by mass spectrometry indicated that it encodes the FTL_0645 open reading frame of F. holarctica LVS, which is 100% identical/homologous to FTT1416c of F. tularensis Schu S4. The predicted 137 amino acid lipoprotein encoded by FTL_0645 ORF, was expressed in Escherichia coli, purified, and demonstrated to be a lipoprotein. This recombinant lipoprotein, named Flpp3, was able to activate TLR2 and induce an immunogenic response in mice, suggesting that the E. coli-expressed Flpp3 is palmitoylated and closely resembles the native protein in structure and immunogenicity. Taken together, these data suggest that Flpp3 could be a candidate for inclusion in a F. tularensis vaccine.

Vaccines against tularemia

Francisella tularensis is a Category A select agent for which vaccine and countermeasure development are a priority. In the past eight years, renewed interest in this pathogen has led to the generation of an enormous amount of new data on both the pathogen itself and its interaction with host cells. This information has fostered the development of various vaccine candidates including acellular subunit, killed whole cell and live attenuated. This review summarizes the progress and promise of these various candidates.

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

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

Technical transformation of biodefense vaccines

Biodefense vaccines are developed against a diverse group of pathogens. Vaccines were developed for some of these pathogens a long time ago but they are facing new challenges to move beyond the old manufacturing technologies. New vaccines to be developed against other pathogens have to determine whether to follow traditional vaccination strategies or to seek new approaches. Advances in basic immunology and recombinant DNA technology have fundamentally transformed the process of formulating a vaccine concept, optimizing protective antigens, and selecting the most effective vaccine delivery approach for candidate biodefense vaccines.

A novel role for plasmin-mediated degradation of opsonizing antibody in the evasion of host immunity by virulent, but not attenuated, Francisella tularensis

Opsonization by Abs represents a critical component of the host immune response against many pathogens. The mechanisms by which virulent microbes evade this protective response are not completely understood. In disease mediated by Francisella tularensis, Ab can effectively protect against infections with attenuated strains, for example, LVS, but not virulent strains such as SchuS4. Thus, it is likely that SchuS4 has mechanisms, which are not present in LVS, that allow evasion of opsonization by Ab, dampening the protective effects of these host molecules. Here we demonstrate that evasion of Ab-mediated opsonization and phagocytosis by the highly virulent SchuS4 is associated with its ability to bind the host serine protease plasmin. SchuS4, but not the closely related LVS, bound active plasmin. Plasmin bound SchuS4 degraded exogenous and opsonizing Abs, whereas LVS failed to do so. Furthermore, plasmin-mediated inhibition of Ab opsonization by SchuS4 also inhibited Ab-mediated uptake of this bacterium by macrophages. Ab-mediated uptake of uncoated and opsonized SchuS4 elicited a strong proinflammatory response in infected macrophages. However, plasmin-coated, opsonized SchuS4 poorly elicited production of these protective proinflammatory cytokines. This unique host-pathogen interplay is a novel immune evasion strategy utilized by virulent F. tularensis, and it provides one explanation for the ability of Ab to protect against attenuated, but not virulent, strains of F. tularensis. This mechanism may also represent a more common hereto unrecognized strategy by which virulent bacteria evade detection and clearance by Ig.

Rationally designed tularemia vaccines

Tularemia, caused by the Gram-negative bacterium Francisella tularensis, can be contracted by the bite of an arthropod vector or by inhalation. This disease occurs relatively infrequently but can be severe and even life-threatening if untreated. Until recently, there were few laboratories studying this organism; however, concerns over its potential use as a biological weapon have led to renewed attention to F. tularensis research, particularly in the area of vaccine development. Advances in the ability to genetically manipulate F. tularensis, along with knowledge gained from the creation and refinement of attenuated bacterial vaccines for other diseases, continue to foster significant progress in the development of live-attenuated bacterial vaccines, as well as defined antigen and subunit vaccines.

Vaccination of Fischer 344 rats against pulmonary infections by Francisella tularensis type A strains

Pneumonic tularemia caused by inhalation of the type A strains of Francisella tularensis is associated with high morbidity and mortality in humans. The only vaccine known to protect humans against this disease is the attenuated live vaccine strain (LVS), but it is not currently registered for human use. To develop a new generation of vaccines, multiple animal models are needed that reproduce the human response to F. tularensis infection and vaccination. We examined the potential use of Fischer 344 rat as such a model. Fischer 344 rats were very sensitive to intratracheal infection with the virulent type A strain SCHU S4 and generally succumbed less than 2 weeks after infection. Similar to humans and non-human primates, Fischer 344 rats vaccinated with LVS by subcutaneous or intradermal routes were protected against a greater range of respiratory SCHU S4 challenge doses than has been reported for LVS vaccinated mice. Intratracheal LVS vaccination also induced effective immunity, but it was less protective when the challenge dose exceeded 10(5) SCHU S4. LVS vaccination did not prevent SCHU S4 infection but rather controlled bacterial growth and pathology, leading to the eventual clearance of the bacteria. Our results suggest that the Fischer 344 rat may be a good model for studying pneumonic tularemia and evaluating potential vaccine candidates.

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.

Recombinant attenuated Listeria monocytogenes vaccine expressing Francisella tularensis IglC induces protection in mice against aerosolized Type A F. tularensis

Fransicella tularensis, the causative agent of tularemia, is in the top category (Category A) of potential agents of bioterrorism. To develop a safer vaccine against aerosolized F. tularensis, we have employed an attenuated Listeria monocytogenes, which shares with F. tularensis an intracellular and extraphagosomal lifestyle, as a delivery vehicle for F. tularensis antigens. We constructed recombinant L. monocytogenes (rLm) vaccines stably expressing seven F. tularensis proteins including IglC (rLm/iglC), and tested their immunogenicity and protective efficacy against lethal F. tularensis challenge in mice. Mice immunized intradermally with rLm/iglC developed significant cellular immune responses to F. tularensis IglC as evidenced by lymphocyte proliferation and CD4+ and CD8+ T-cell intracellular expression of interferon gamma. Moreover, mice immunized with rLm/iglC were protected against lethal challenge with F. tularensis LVS administered by the intranasal route, a route chosen to mimic airborne infection, and, most importantly, against aerosol challenge with the highly virulent Type A F. tularensis SchuS4 strain.