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

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          Abstract

          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, sodB Ft , capB or wzy; LVS Δ capB 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 Δ capB and Listeria monocytogenes expressing T6SS proteins.

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          Most cited references 130

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          Tularemia as a biological weapon: medical and public health management.

          The Working Group on Civilian Biodefense has developed consensus-based recommendations for measures to be taken by medical and public health professionals if tularemia is used as a biological weapon against a civilian population. The working group included 25 representatives from academic medical centers, civilian and military governmental agencies, and other public health and emergency management institutions and agencies. MEDLINE databases were searched from January 1966 to October 2000, using the Medical Subject Headings Francisella tularensis, Pasteurella tularensis, biological weapon, biological terrorism, bioterrorism, biological warfare, and biowarfare. Review of these references led to identification of relevant materials published prior to 1966. In addition, participants identified other references and sources. Three formal drafts of the statement that synthesized information obtained in the formal evidence-gathering process were reviewed by members of the working group. Consensus was achieved on the final draft. A weapon using airborne tularemia would likely result 3 to 5 days later in an outbreak of acute, undifferentiated febrile illness with incipient pneumonia, pleuritis, and hilar lymphadenopathy. Specific epidemiological, clinical, and microbiological findings should lead to early suspicion of intentional tularemia in an alert health system; laboratory confirmation of agent could be delayed. Without treatment, the clinical course could progress to respiratory failure, shock, and death. Prompt treatment with streptomycin, gentamicin, doxycycline, or ciprofloxacin is recommended. Prophylactic use of doxycycline or ciprofloxacin may be useful in the early postexposure period.
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            Tularemia: history, epidemiology, pathogen physiology, and clinical manifestations.

            Francisella tularensis has been recognized as a human pathogen for almost 100 years and is the etiological agent of the zoonotic disease tularemia. Soon after its discovery, it became recognized as an important pathogen in several parts of the world, for example, in the United States and Soviet Union. The number of tularemia cases in the two countries peaked in the 1940s and has thereafter steadily declined. Despite this decline, there was still much interest in the pathogen in the 1950s and 1960s since it is highly infectious and transmissible by aerosol, rendering it a potent biothreat agent. In fact, it was one of the agents that was given the highest priority in the offensive programs of the United States and Soviet Union. After termination of the offensive programs in the 1960s, the interest in F. tularensis diminished significantly and little research was carried out for several decades. Outbreaks of tularemia during the last decade in Europe, for example, in Kosovo, Spain, and Scandinavia, led to a renewed public interest in the disease. This, together with a massive increase in the research funding, in particular in the United States since 2001, has resulted in a significant increase in the number of active Francisella researchers. This article summarizes, predominantly with a historical perspective, the epidemiology and clinical manifestations of tularemia and the physiology of F. tularensis.
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              In vivo negative selection screen identifies genes required for Francisella virulence.

              Francisella tularensis subverts the immune system to rapidly grow within mammalian hosts, often causing tularemia, a fatal disease. This pathogen targets the cytosol of macrophages where it replicates by using the genes encoded in the Francisella pathogenicity island. However, the bacteria are recognized in the cytosol by the host's ASC/caspase-1 pathway, which is essential for host defense, and leads to macrophage cell death and proinflammatory cytokine production. We used a microarray-based negative selection screen to identify Francisella genes that contribute to growth and/or survival in mice. The screen identified many known virulence factors including all of the Francisella pathogenicity island genes, LPS O-antigen synthetic genes, and capsule synthetic genes. We also identified 44 previously unidentified genes that were required for Francisella virulence in vivo, indicating that this pathogen may use uncharacterized mechanisms to cause disease. Among these, we discovered a class of Francisella virulence genes that are essential for growth and survival in vivo but do not play a role in intracellular replication within macrophages. Instead, these genes modulate the host ASC/caspase-1 pathway, a previously unidentified mechanism of Francisella pathogenesis. This finding indicates that the elucidation of the molecular mechanisms used by other uncharacterized genes identified in our screen will increase our understanding of the ways in which bacterial pathogens subvert the immune system.
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                Author and article information

                Contributors
                Journal
                Front Cell Infect Microbiol
                Front Cell Infect Microbiol
                Front. Cell. Infect. Microbiol.
                Frontiers in Cellular and Infection Microbiology
                Frontiers Media S.A.
                2235-2988
                15 May 2018
                2018
                : 8
                Affiliations
                Division of Infectious Diseases, Department of Medicine, 37-121 Center for Health Sciences, School of Medicine, University of California, Los Angeles , Los Angeles, CA, United States
                Author notes

                Edited by: Joseph Wayne Conlan, National Research Council Canada (NRC-CNRC), Canada

                Reviewed by: Larry Schlesinger, The Ohio State University, United States; Petra Oyston, Defence Science and Technology Laboratory, United Kingdom

                *Correspondence: Qingmei Jia qjia@ 123456mednet.ucla.edu
                Article
                10.3389/fcimb.2018.00154
                5963219
                Copyright © 2018 Jia and Horwitz.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

                Page count
                Figures: 0, Tables: 5, Equations: 0, References: 136, Pages: 20, Words: 18313
                Funding
                Funded by: National Institutes of Health 10.13039/100000002
                Award ID: AI101189
                Categories
                Microbiology
                Review

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