Immunogenicity and toxicity of lipopolysaccharide from Francisella tularensis LVS

Abstract Lipopolysaccharide (LPS) from the live vaccine strain of Francisella tularensis ( F . tularensis LVS) was isolated and purified. The LPS did not stimulate lymphocytes from previously tularaemia-vaccinated individuals or lymphocytes from nonprimed individuals. However, serum antibodies from tularaemia vaccines reacted with the LPS whereas virtually no reactivity was found with antibodies from individuals not exposed to F. tularensis LVS. Antibodies of immunoglobulin class M displayed the antibody reactivity predominantly. The LPS failed to induce the mononuclear cell-derived cytokine interleukin-1 and only low levels of tumour necrosis factor were detected. Furthermore, no LPS endotoxin properties were found in galactosamine-treated mice or in the Limulus amoebocyte lysate assay. From these results it can be concluded that F. tularensis LVS possesses a lipopolysaccharide-like molecule, which does not exhibit properties of a classical endotoxin.     

Francisella tularensis selectively induces proinflammatory changes in endothelial cells

Naturally acquired infections with Francisella tularensis, the bacterial agent of tularemia, occur infrequently in humans. However, the high infectivity and lethality of the organism in humans raise concerns that it might be exploited as a weapon of bioterrorism. Despite this potential for illicit use, the pathogenesis of tularemia is not well understood. To examine how F. tularensis interacts with cells of its mammalian hosts, we tested the ability of a live vaccine strain (LVS) to induce proinflammatory changes in cultured HUVEC. Living F. tularensis LVS induced HUVEC to express the adhesion molecules VCAM-1 and ICAM-1, but not E-selectin, and to secrete the chemokine CXCL8, but not CCL2. Stimulation of HUVEC by the living bacteria was partially suppressed by polymyxin B, an inhibitor of LPS, but did not require serum, suggesting that F. tularensis LVS does not stimulate endothelium through the serum-dependent pathway that is typically used by LPS from enteric bacteria. In contrast to the living organisms, suspensions of killed F. tularensis LVS acquired the ability to increase endothelial expression of both E-selectin and CCL2. Up-regulation of E-selectin and CCL2 by the killed bacteria was not inhibited by polymyxin B. Exposure of HUVEC to either live or killed F. tularensis LVS for 24 h promoted the transendothelial migration of subsequently added neutrophils. These data indicate that multiple components of F. tularensis LVS induce proinflammatory changes in endothelial cells in an atypical manner that may contribute to the exceptional infectivity and virulence of this pathogen.     

Innate and adaptive immune responses to an intracellular bacterium,Francisella tularensis live vaccine strain

The immune response to intracellular bacterium, Francisella tularensis, which causes tularemia and is proposed to be a potential bioterrorism pathogen, has been studied in mice using the attenuated live vaccine strain (LVS). Here we review this infection model, which provides a convenient means of studying protective immune mechanisms not only for Francisella, but also for the large and important class of intracellular pathogens.     

A Francisella tularensis live vaccine strain that improves stimulation of antigen-presenting cells does not enhance vaccine efficacy

Vaccination is a proven strategy to mitigate morbidity and mortality of infectious diseases. The methodology of identifying and testing new vaccine candidates could be improved with rational design and in vitro testing prior to animal experimentation. The tularemia vaccine, Francisella tularensis live vaccine strain (LVS), does not elicit complete protection against lethal challenge with a virulent type A Francisella strain. One factor that may contribute to this poor performance is limited stimulation of antigen-presenting cells. In this study, we examined whether the interaction of genetically modified LVS strains with human antigen-presenting cells correlated with effectiveness as tularemia vaccine candidates. Human dendritic cells infected with wild-type LVS secrete low levels of proinflammatory cytokines, fail to upregulate costimulatory molecules, and activate human T cells poorly in vitro. One LVS mutant, strain 13B47, stimulated higher levels of proinflammatory cytokines from dendritic cells and macrophages and increased costimulatory molecule expression on dendritic cells compared to wild type. Additionally, 13B47-infected dendritic cells activated T cells more efficiently than LVS-infected cells. A deletion allele of the same gene in LVS displayed similar in vitro characteristics, but vaccination with this strain did not improve survival after challenge with a virulent Francisella strain. In vivo, this mutant was attenuated for growth and did not stimulate T cell responses in the lung comparable to wild type. Therefore, stimulation of antigen-presenting cells in vitro was improved by genetic modification of LVS, but did not correlate with efficacy against challenge in vivo within this model system.     

Drosophila melanogaster as a model for elucidating the pathogenicity of Francisella tularensis

Drosophila melanogaster is a widely used model organism for research on innate immunity and serves as an experimental model for infectious diseases. The aetiological agent of the zoonotic disease tularaemia, Francisella tularensis, can be transmitted by ticks and mosquitoes and Drosophila might be a useful, genetically amenable model host to elucidate the interactions between the bacterium and its arthropod vectors. We found that the live vaccine strain of F. tularensis was phagocytosed by Drosophila and multiplied in fly haemocytes in vitro and in vivo. Bacteria injected into flies resided both inside haemocytes and extracellularly in the open circulatory system. A continuous activation of the humoral immune response, i.e. production of antimicrobial peptides under control of the imd/Relish signalling pathway, was observed and it may have contributed to the relative resistance to F. tularensis as flies defective in the imd/Relish pathway died rapidly. Importantly, bacterial strains deficient for genes of the F. tularensis intracellular growth locus or the macrophage growth locus were attenuated in D. melanogaster. Our results demonstrate that D. melanogaster is a suitable model for the analysis of interactions between F. tularensis and its arthropod hosts and that it can also be used to identify F. tularensis virulence factors relevant for mammalian hosts.     

Use of acid treatment and a selective medium to enhance the recovery of Francisella tularensis from water

Francisella tularensis has been associated with naturally occurring waterborne outbreaks and is also of interest as a potential biological weapon. Recovery of this pathogen from water using cultural methods is challenging due to the organism's fastidious growth requirements and interference by indigenous bacteria. A 15-min acid treatment procedure prior to culture on a selective agar was evaluated for recovery of F. tularensis from seeded water samples. Mean levels of reduction of virulent strains of F. tularensis subsp. holarctica and subsp. tularensis were less than 20% following acid treatment. The attenuated live vaccine strain (LVS) was less resistant to acid exposure. The acid treatment procedure coupled with plating on cystine heart agar with rabbit blood and antibiotics (CHARBab) allowed the isolation of F. tularensis seeded into five natural water samples.     

Growth and survival of four strains of Francisella tularensis in a rich medium preconditioned with Acanthamoeba palestinensis

The hypothesis of positive interactions between Francisella tularensis LVS (live vaccine strain) and Acanthamoeba palestinensis was tested. Pregrowth of the amoebae, in a rich (autoclaved and filtered) medium, from which they were subsequently removed by filtration, conditioned the medium so that growth of the live vaccine strain of F. tularensis occurred and the growth rate of one other strain was increased.     

Immunologic consequences of Francisella tularensis live vaccine strain infection: role of the innate immune response in infection and immunity

Francisella tularensis (Ft), a Gram-negative intracellular bacterium, is the etiologic agent of tularemia. Although attenuated for humans, i.p. infection of mice with <10 Ft live vaccine strain (LVS) organisms causes lethal infection that resembles human tularemia, whereas the LD50 for an intradermal infection is >10(6) organisms. To examine the immunological consequences of Ft LVS infection on the innate immune response, the inflammatory responses of mice infected i.p. or intradermally were compared. Mice infected i.p. displayed greater bacterial burden and increased expression of proinflammatory genes, particularly in the liver. In contrast to most LPS, highly purified Ft LVS LPS (10 microg/ml) was found to be only minimally stimulatory in primary murine macrophages and in HEK293T cells transiently transfected with TLR4/MD-2/CD14, whereas live Ft LVS bacteria were highly stimulatory for macrophages and TLR2-expressing HEK293T cells. Despite the poor stimulatory activity of Ft LVS LPS in vitro, administration of 100 ng of Ft LVS LPS 2 days before Ft LVS challenge severely limited both bacterial burden and cytokine mRNA and protein expression in the absence of detectable Ab at the time of bacterial challenge, yet these mice developed a robust IgM Ab response within 2 days of infection and survived. These data suggest that prior administration of Ft LVS LPS protects the host by diminishing bacterial burden and blunting an otherwise overwhelming inflammatory response, while priming the adaptive immune response for development of a strong Ab response.     

A variety of novel lipid A structures obtained from Francisella tularensis live vaccine strain

F. tularensis is a Gram-negative coccobacillus that causes tularemia. Its LPS has nominal biological activity. Currently, there is controversy regarding the structure of the lipid A obtained from F. tularensis live vaccine strain (LVS). Therefore, to resolve this controversy, the purification and structural identification of this LPS was crucial. To achieve this, LPS from F. tularensis LVS was acid hydrolyzed to obtain crude lipid A that was methylated and purified by HPLC and the fractions were analyzed by MALDI-TOF MS. The structure of the major lipid A species was composed of a glucosamine disaccharide backbone substituted with four fatty acyl groups and a phosphate (1-position) with a molecular mass of 1505. The major lipid A component contained 18:0[3-O(16:0)] in the distal subunit and two 18:0(3-OH) fatty acyl chains at the 2- or 3-positions of the reducing subunit. Additional variations in the lipid A species include: heterogeneity in fatty acyl groups, a phosphate or a phosphoryl galactosamine at the 1-position, and a hexose at the 4' or 6' position, some of which have not been previously described for F. tularensis LVS. This analysis revealed that lipid A from F. tularensis LVS is far more complex than originally believed.     

Uptake of serum-opsonized Francisella tularensis by macrophages can be mediated by class A scavenger receptors

The bacterium Francisella tularensis is highly infective, and this is one of the chief attributes that has led to its development as a bioweapon. Establishment of infection requires efficient uptake of F. tularensis by host macrophages, which provide a safe in vivo environment for F. tularensis replication. Little is known, however, about the cellular entry mechanisms employed by this organism. This report shows that efficient uptake of F. tularensis live vaccine strain (LVS) by macrophages is dependent on a heat-sensitive serum component and is mediated in part by types I and II class A scavenger receptors (SRA), demonstrating for the first time that SRA can act as a receptor for opsonized pathogens. Specifically, uptake of serum-opsonized LVS was partially blocked by general scavenger receptor inhibitors [fucoidan and poly(I)] and was largely inhibited by a specific function-blocking antibody against SRA. A role for SRA in LVS binding was confirmed by showing that ectopic expression of SRA in human embryonic kidney cells conferred the capacity for robust serum-dependent LVS binding. Finally, SRA-/- macrophages ingested significantly fewer LVS than did macrophages from wild-type mice. These findings support a novel role for SRA in innate immunity and suggest a potential therapeutic approach for modulating F. tularensis infection, namely, blocking SRA as a means of hindering F. tularensis access to its intracellular niche.     

Use of transposon-transposase complexes to create stable insertion mutant strains of Francisella tularensis LVS

Francisella tularensis is a highly virulent zoonotic bacterial pathogen capable of infecting numerous different mammalian species, including humans. Elucidation of the pathogenic mechanisms of F. tularensis has been hampered by a lack of tools to genetically manipulate this organism. Herein we describe the use of transposome complexes to create insertion mutations in the chromosome of the F. tularensis live vaccine strain (LVS). A Tn5-derived transposon encoding kanamycin resistance and lacking a transposase gene was complexed with transposase enzyme and transformed directly into F. tularensis LVS by electroporation. An insertion frequency of 2.6 x 10(-8) +/- 0.87 x 10(-8) per cell was consistently achieved using this method. There are 178 described Tn5 consensus target sites distributed throughout the F. tularensis genome. Twenty-two of 26 transposon insertions analyzed were within known or predicted open reading frames, but none of these insertions was associated with the Tn5 target site. Analysis of the insertions of sequentially passed strains indicated that the transposons were maintained stably at the initial insertion site after more than 270 generations. Therefore, transformation by electroporation of Tn5-based transposon-transposase complexes provided an efficient mechanism for generating random, stable chromosomal insertion mutations in F. tularensis.     

A method for allelic replacement in Francisella tularensis

A vector for mutagenesis of Francisella tularensis was constructed based on the pUC19 plasmid. By inserting the sacB gene of Bacillus subtilis, oriT of plasmid RP4, and a chloramphenicol resistance gene of Shigella flexneri, a vector, pPV, was obtained that allowed specific mutagenesis. A protocol was developed that allowed introduction of the vector into the live vaccine strain, LVS, of F. tularensis by conjugation. As a proof of principle, we aimed to develop a specific mutant defective in expression of a 23-kDa protein (iglC) that we previously have shown to be prominently upregulated during intracellular growth of F. tularensis. A plasmid designated pPV-DeltaiglC was developed that contained only the regions flanking the encoding gene, iglC. By a double crossover event, the chromosomal iglC gene was deleted. However, the resulting strain, denoted DeltaiglC1, still had an intact iglC gene. Southern blot analysis verified that LVS harbors two copies for the iglC gene. The mutagenesis was therefore repeated and a mutant defective in both iglC alleles, designated DeltaiglC1+2, was obtained. The DeltaiglC1+2 strain, in contrast to DeltaiglC1, was shown to display impaired intracellular macrophage growth and to be attenuated for virulence in mice. The developed genetic system has the potential to provide a tool to elucidate virulence mechanisms of F. tularensis and the specific F. tularensis mutant illustrates the critical role of the 23-kDa protein, iglC, for the virulence of F. tularensis LVS.     

Mapping of immunoreactive antigens of Francisella tularensis live vaccine strain

Francisella tularensis subsp. holarctica is the common causal agent of tularemia in Europe. Besides clinical signs, the diagnosis of the disease mostly depends on serological tests. To date, there is a lack of information about the F. tularensis antigens that induce antibody response. Therefore, we have started comprehensive mapping of immunoreactive antigens using the attenuated live vaccine strain of F. tularensis LVS originating from the European virulent strain. For this purpose, the immunoreactivity of sera collected from patients suffering from tularemia, together with the control sera of patients with Lyme disease and healthy blood donors, were examined by means of one-dimensional and two-dimensional immunoblotting. Furthermore, whole cell bacterial lysates, isolated integral membrane proteins and basic proteins were exploited as antigens. By this approach more than 80 different immunorelevant antigens were detected. Most of them came from whole cell bacterial lysate and integral membrane proteins. Conversely, only a negligible reaction was found in the case of basic proteins. Forty-five spots were further selected for mass spectrometric analyses and 22 of them were annotated. Among the spots that provided characteristic reactions with sera from patients with tularemia, 60 kDa and 10 kDa chaperonins that occurred in several charge and mass variants, predominated.     

Galleria mellonella as a model host to study infection by the Francisella tularensis live vaccine strain

We used the killing of Galleria mellonella (Lepidoptera: Pyralidae; the greater wax moth) caterpillar by the live vaccine strain (LVS) of Francisella tularensis to develop an invertebrate host system that can be used to study F. tularensis infection and the in vivo effects of antibacterial compounds on F. tularensis LVS. After injection into the insect hemocoel, F. tularensis LVS, killed caterpillars despite the association of LVS with hemocytes. The rate of killing depended on the number of bacteria injected. Antibiotic therapy with ciprofloxacin, levofloxacin or streptomycin administered before or after inoculation prolonged survival and decreased the tissue burden of F. tularensis in the hemocoel. Delayed drug treatment reduced the efficacy of antibacterials and especially streptomycin. The G. mellonella-F. tularensis LVS system may facilitate the in vivo study of F. tularensis, efficacy with antibacterial agents.     

Chromosome rearrangement and diversification of Francisella tularensis revealed by the type B (OSU18) genome sequence

The gamma-proteobacterium Francisella tularensis is one of the most infectious human pathogens, and the highly virulent organism F. tularensis subsp. tularensis (type A) and less virulent organism F. tularensis subsp. holarctica (type B) are most commonly associated with significant disease in humans and animals. Here we report the complete genome sequence and annotation for a low-passage type B strain (OSU18) isolated from a dead beaver found near Red Rock, Okla., in 1978. A comparison of the F. tularensis subsp. holarctica sequence with that of F. tularensis subsp. tularensis strain Schu4 (P. Larsson et al., Nat. Genet. 37:153-159, 2005) highlighted genetic differences that may underlie different pathogenicity phenotypes and the evolutionary relationship between type A and type B strains. Despite extensive DNA sequence identity, the most significant difference between type A and type B isolates is the striking amount of genomic rearrangement that exists between the strains. All but two rearrangements can be attributed to homologous recombination occurring between two prominent insertion elements, ISFtu1 and ISFtu2. Numerous pseudogenes have been found in the genomes and are likely contributors to the difference in virulence between the strains. In contrast, no rearrangements have been observed between the OSU18 genome and the genome of the type B live vaccine strain (LVS), and only 448 polymorphisms have been found within non-transposase-coding sequences whose homologs are intact in OSU18. Nonconservative differences between the two strains likely include the LVS attenuating mutation(s).     

Francisella novicida LPS has greater immunobiological activity in mice than F. tularensis LPS,and contributes to F. novicida murine pathogenesis

To further understand the role of LPS in the pathogenesis of Francisella infection, we characterized murine infection with F. novicida, and compared immunobiological activities of F. novicida LPS and the LPS from F. tularensis live vaccine strain (LVS). F. novicida had a lower intradermal LD(50) in BALB/cByJ mice than F. tularensis LVS, and mice given a lethal F. novicida dose intraperitoneally died faster than those given the same lethal F. tularensis LVS dose. However, the pattern of in vivo dissemination was similar, and in vitro growth of both bacteria in bone marrow-derived macrophages was comparable. F. novicida LPS stimulated very modest in vitro proliferation of mouse splenocytes at high doses, but F. tularensis LVS LPS did not. Murine bone marrow macrophages treated in vitro with F. novicida LPS produced IL12 and TNF-alpha, but did not produce detectable interferon-gamma, IL10, or nitric oxide; in contrast, murine macrophages treated with F. tularensis LVS LPS produced none of these mediators. In contrast to clear differences in stimulation of proliferation and especially cytokines, both types of purified LPS stimulated early protection against lethal challenge of mice with F. tularensis LVS, but not against lethal challenge with F. novicida. Thus, although LPS recognition may not be a major factor in engendering protection, the ability of F. novicida LPS to stimulate the production of proinflammatory cytokines including TNF-alpha likely contributes to the increased virulence for mice of F. novicida compared to F. tularensis LVS.     

Tularemia vaccine: past,present and future

Francisella tularensis is a Gram negative intracellular pathogen that causes the highly debilitating or fatal disease tularemia. F. tularensis can infect a wide range of animals and can be transmitted to humans in a variety of ways, the most common being by the bite of an infected insect or arthropod vector. The attenuated F. tularensis live vaccine strain (LVS) has been used previously under investigational new drug status to vaccinate at-risk individuals. However the history of the strain and lack of knowledge regarding the basis of attenuation has so far prevented its licensing. Therefore the focus of current research is on producing a new vaccine against tularemia that would be suitable for licensing.     

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.