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

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.

Improvement of Approaches to the Verification of the Vaccine Strain <i>Francisella tularensis</i> 15 NIIEG during Long-Term Storage

The aim of the study was to improve the methods for verifying the vaccine strain Francisella tularensis 15 NIIEG during long-term storage under current conditions.

Materials and methods. The paper summarizes the results of studying the phenotypic and genetic properties of lyophilized cultures of the vaccine strain F. tularensis 15 NIIEG (1953, 1966, 1969, 1987, 1990, 2003, 2012 and 2013) stored at SCEMAP for a period of one to 60 years.

Results and discussion. Previous studies have revealed that freeze-dried cultures of F. tularensis 15 NIIEG generally had the characteristics of the vaccine strain, with the exception of deviations from the regulatory requirements for residual virulence and specific safety. The stability of preservation of deletions in the pilA and pilE genes (the region of differentiation RD19) and the genes encoding lpp lipoprotein (RD18) in the vaccine strain, which was stored for various periods of time in a lyophilized state, has been confirmed. The vaccine-strain-specific mutation C178404T (by the genome of F. tularensis LVS strain, GenBank NCBI no. CP009694) has been identified, and an approach to determine it has been developed. The data obtained are promising as regards using the above deletions in the RD18/RD19 regions in combination with the C178404T mutation to assess the authenticity of the vaccine strain using molecular genetic methods. Thus, the conducted retrospective analysis of the data on the cultures of tularemia microbe vaccine strain from the 1940s to 2013 and the gathered experimental data, made it possible to supplement the uniform requirements for the manufacture, study, maintenance, storage and movement of F. tularensis 15 NIIEG vaccine strain with new evidence. Based on the results obtained, the authors have drawn a draft methodological recommendations of the federal level “Vaccinal strain Francisella tularensis 15 NIIEG: order of handling”.

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.

Identification of an Attenuated Substrain of Francisella tularensis SCHU S4 by Phenotypic and Genotypic Analyses

Pneumonic tularemia is a highly debilitating and potentially fatal disease caused by inhalation of Francisella tularensis. Most of our current understanding of its pathogenesis is based on the highly virulent F. tularensis subsp. tularensis strain SCHU S4. However, multiple sources of SCHU S4 have been maintained and propagated independently over the years, potentially generating genetic variants with altered virulence. In this study, the virulence of four SCHU S4 stocks (NR-10492, NR-28534, NR-643 from BEI Resources and FTS-635 from Battelle Memorial Institute) along with another virulent subsp. tularensis strain, MA00-2987, were assessed in parallel. In the Fischer 344 rat model of pneumonic tularemia, NR-643 and FTS-635 were found to be highly attenuated compared to NR-10492, NR-28534, and MA00-2987. In the NZW rabbit model of pneumonic tularemia, NR-643 caused morbidity but not mortality even at a dose equivalent to 500x the LD₅₀ for NR-10492. Genetic analyses revealed that NR-10492 and NR-28534 were identical to each other, and nearly identical to the reference SCHU S4 sequence. NR-643 and FTS-635 were identical to each other but were found to have nine regions of difference in the genomic sequence when compared to the published reference SCHU S4 sequence. Given the genetic differences and decreased virulence, NR-643/FTS-635 should be clearly designated as a separate SCHU S4 substrain and no longer utilized in efficacy studies to evaluate potential vaccines and therapeutics against tularemia.

Differential Immune Response Following Intranasal and Intradermal Infection with Francisella tularensis: Implications for Vaccine Development

Francisella tularensis (Ft) is a Gram-negative, facultative intracellular coccobacillus that is the etiological agent of tularemia. Interestingly, the disease tularemia has variable clinical presentations that are dependent upon the route of infection with Ft. Two of the most likely routes of Ft infection include intranasal and intradermal, which result in pneumonic and ulceroglandular tularemia, respectively. While there are several differences between these two forms of tularemia, the most notable disparity is between mortality rates: the mortality rate following pneumonic tularemia is over ten times that of the ulceroglandular disease. Understanding the differences between intradermal and intranasal Ft infections is important not only for clinical diagnoses and treatment but also for the development of a safe and effective vaccine. However, the immune correlates of protection against Ft, especially within the context of infection by disparate routes, are not yet fully understood. Recent advances in different animal models have revealed new insights in the complex interplay of innate and adaptive immune responses, indicating dissimilar patterns in both responses following infection with Ft via different routes. Further investigation of these differences will be crucial to predicting disease outcomes and inducing protective immunity via vaccination or natural infection.

Characterization of Schu S4 aro mutants as live attenuated tularemia vaccine candidates

There is a need for development of an effective vaccine against Francisella tularensis, as this potential bioweapon has a high mortality rate and low infectious dose when delivered via the aerosol route. Moreover, this Tier 1 agent has a history of weaponization. We engineered targeted mutations in the Type A strain F. tularensis subspecies tularensis Schu S4 in aro genes encoding critical enzymes in aromatic amino acid biosynthesis. F. tularensis Schu S4ΔaroC, Schu S4ΔaroD, and Schu S4ΔaroCΔaroD mutant strains were attenuated for intracellular growth in vitro and for virulence in vivo and, conferred protection against pulmonary wild-type (WT) F. tularensis Schu S4 challenge in the C57BL/6 mouse model. F. tularensis Schu S4ΔaroD was identified as the most promising vaccine candidate, demonstrating protection against high-dose intranasal challenge; it protected against 1,000 CFU Schu S4, the highest level of protection tested to date. It also provided complete protection against challenge with 92 CFU of a F. tularensis subspecies holarctica strain (Type B). Mice responded to vaccination with Schu S4ΔaroD with systemic IgM and IgG2c, as well as the production of a functional T cell response as measured in the splenocyte-macrophage co-culture assay. This vaccine was further characterized for dissemination, histopathology, and cytokine/chemokine gene induction at defined time points following intranasal vaccination which confirmed its attenuation compared to WT Schu S4. Cytokine, chemokine, and antibody induction patterns compared to wild-type Schu S4 distinguish protective vs. pathogenic responses to F. tularensis and elucidate correlates of protection associated with vaccination against this agent.

Inhibition of Francisella tularensis phagocytosis using a novel anti-LPS scFv antibody fragment

Francisella tularensis (Ft), the causative agent of lethal tularemia, is classified as a category A biological warfare threat agent. While Ft infection is treatable by antibiotics, many failed antibiotic treatments were reported, highlighting the need for effective new treatments. It has been demonstrated that binding of antibody-coated bacteria to the Fc receptor located on phagocytic cells is a key process needed for efficient protection against Ft. Yet, Ft utilizes the same receptor to enter the phagocytic cells in order to escape the immune system. To address the question whether an anti-Ft LPS antibody lacking the ability to bind the Fc receptor may inhibit the entry of Ft into host cells, a soluble scFv (TL1-scFv) was constructed from an anti Ft-LPS antibody (TL1) that was isolated from an immune single-chain (scFv) phage-display library. Bacterial uptake was assessed upon infection of macrophages with Ft live attenuated strain (LVS) in the presence of either TL1 or TL1-scFv. While incubation of LVS in the presence of TL1 greatly enhanced bacterial uptake, LVS uptake was significantly inhibited in the presence of TL1-scFv. These results prompt further experiments probing the therapeutic efficacy of TL1-scFv, alone or in combination with antibiotic treatment.

Development, Characterization, and Standardization of a Nose-Only Inhalation Exposure System for Exposure of Rabbits to Small-Particle Aerosols Containing Francisella tularensis

Inhalation of Francisella tularensis causes pneumonic tularemia in humans, a severe disease with a 30 to 60% mortality rate. The reproducible delivery of aerosolized virulent bacteria in relevant animal models is essential for evaluating medical countermeasures. Here we developed optimized protocols for infecting New Zealand White (NZW) rabbits with aerosols containing F. tularensis We evaluated the relative humidity, aerosol exposure technique, and bacterial culture conditions to optimize the spray factor (SF), a central metric of aerosolization. This optimization reduced both inter- and intraday variability and was applicable to multiple isolates of F. tularensis Further improvements in the accuracy and precision of the inhaled pathogen dose were achieved through enhanced correlation of the bacterial culture optical density and the number of CFU. Plethysmograph data collected during exposures found that respiratory function varied considerably between rabbits, was not a function of weight, and did not improve with acclimation to the system. Live vaccine strain (LVS)-vaccinated rabbits were challenged via aerosol with human-virulent F. tularensis SCHU S4 that had been cultivated in either Mueller-Hinton broth (MHB) or brain heart infusion (BHI) broth. LVS-vaccinated animals challenged with SCHU S4 that had been cultivated in MHB experienced short febrile periods (median, 3.2 days), limited weight loss (<5%), and longer median survival times (∼18 days) that were significantly different from those for unvaccinated controls. In contrast, LVS-vaccinated rabbits challenged with SCHU S4 that had been cultivated in BHI experienced longer febrile periods (median, 5.5 days) and greater weight loss (>10%) than the unvaccinated controls and median survival times that were not significantly different from those for the unvaccinated controls. These studies highlight the importance of careful characterization and optimization of protocols for aerosol challenge with pathogenic agents.