Low Dose Vaccination with Attenuated Francisella tularensis Strain SchuS4 Mutants Protects against Tularemia Independent of the Route of Vaccination

Dedeke Rockx-Brouwer,Robert Child,Tara D Wehrly,Audrey Chong,Catharine M Bosio,Deborah D Crane,Jean Celli,Kim J Hasenkrug

doi:10.1371/journal.pone.0037752

Dedeke Rockx-Brouwer, Robert Child + Show 6 more

Open Access

https://doi.org/10.1371/journal.pone.0037752

Copy DOI

Abstract

Tularemia, caused by the Gram-negative bacterium Francisella tularensis, is a severe, sometimes fatal disease. Interest in tularemia has increased over the last decade due to its history as a biological weapon. In particular, development of novel vaccines directed at protecting against pneumonic tularemia has been an important goal. Previous work has demonstrated that, when delivered at very high inoculums, administration of live, highly attenuated strains of virulent F. tularensis can protect against tularemia. However, lower vaccinating inoculums did not offer similar immunity. One concern of using live vaccines is that the host may develop mild tularemia in response to infection and use of high inoculums may contribute to this issue. Thus, generation of a live vaccine that can efficiently protect against tularemia when delivered in low numbers, e.g. <100 organisms, may address this concern. Herein we describe the ability of three defined, attenuated mutants of F. tularensis SchuS4, deleted for FTT0369c, FTT1676, or FTT0369c and FTT1676, respectively, to engender protective immunity against tularemia when delivered at concentrations of approximately 50 or fewer bacteria. Attenuated strains for use as vaccines were selected by their inability to efficiently replicate in macrophages in vitro and impaired replication and dissemination in vivo. Although all strains were defective for replication in vitro within macrophages, protective efficacy of each attenuated mutant was correlated with their ability to modestly replicate and disseminate in the host. Finally, we demonstrate the parenteral vaccination with these strains offered superior protection against pneumonic tularemia than intranasal vaccination. Together our data provides proof of principle that low dose attenuated vaccines may be a viable goal in development of novel vaccines directed against tularemia.

Highlights

Francisella tularensis is a Gram-negative, facultative intracellular, bacterium, whose species includes subspecies tularensis, subspecies holarctica, and subspecies mediasiatica
The study presented demonstrated that inoculation of low doses of defined, attenuated mutants of F. tularensis strain SchuS4 could protect against parenteral and intranasal challenge with fully virulent wild type SchuS4
We confirmed that inoculation of higher doses of one attenuated mutant, i.e. up to 3log10 higher, did not result in adverse events in the host and offered a similar level of protection against intranasal challenge of up to 50 colony forming units (CFUs) wild type SchuS4

Summary

Introduction

Francisella tularensis is a Gram-negative, facultative intracellular, bacterium, whose species includes subspecies tularensis ( known as Type A), subspecies holarctica ( known as Type B), and subspecies mediasiatica. Following inoculation by a variety of routes, both Type B and Type A subspecies can cause the disease coined tularemia. There are several forms of tularemia, dependent on the route by which the host was exposed to organism. Pneumonic tularemia (acquired following inhalation of the bacterium) mediated by Type A subspecies is widely considered the most dangerous. This is, in part, due to the small numbers of organisms required to cause disease, e.g. 15–20 bacteria [5]. Diagnosis and execution of treatment may be delayed until the organism has disseminated and caused wide spread, potentially septic infection

Objectives

Methods

Results

Conclusion