Modern Development and Production of a New Live Attenuated Bacterial Vaccine, SCHU S4 ΔclpB, to Prevent Tularemia

Abstract

Inhalation of small numbers of Francisella tularensis subspecies tularensis (Ftt) in the form of small particle aerosols causes severe morbidity and mortality in people and many animal species. For this reason, Ftt was developed into a bona fide biological weapon by the USA, by the former USSR, and their respective allies during the previous century. Although such weapons were never deployed, the 9/11 attack quickly followed by the Amerithrax attack led the U.S. government to seek novel countermeasures against a select group of pathogens, including Ftt. Between 2005–2009, we pursued a novel live vaccine against Ftt by deleting putative virulence genes from a fully virulent strain of the pathogen, SCHU S4. These mutants were screened in a mouse model, in which the vaccine candidates were first administered intradermally (ID) to determine their degree of attenuation. Subsequently, mice that survived a high dose ID inoculation were challenged by aerosol or intranasally (IN) with virulent strains of Ftt. We used the current unlicensed live vaccine strain (LVS), first discovered over 70 years ago, as a comparator in the same model. After screening 60 mutants, we found only one, SCHU S4 ΔclpB, that outperformed LVS in the mouse ID vaccination-respiratory-challenge model. Currently, SCHU S4 ΔclpB has been manufactured under current good manufacturing practice conditions, and tested for safety and efficacy in mice, rats, and macaques. The steps necessary for advancing SCHU S4 ΔclpB to this late stage of development are detailed herein. These include developing a body of data supporting the attenuation of SCHU S4 ΔclpB to a degree sufficient for removal from the U.S. Select Agent list and for human use; optimizing SCHU S4 ΔclpB vaccine production, scale up, and long-term storage; and developing appropriate quality control testing approaches.

Novel Transcriptional and Translational Biomarkers of Tularemia Vaccine Efficacy in a Mouse Inhalation Model: Proof of Concept

“…A finding in keeping with several bioinformatics studies that have shown unexpected CoP for LVS and several vaccines against other infectious diseases [57]. We have recently made a batch lot of ∆clpB under GMP conditions [66] that will be used to conduct clinical trials sometime in 2022 or 2023. Thereafter, we will be able to directly compare human immune responses to vaccination with ∆clpB and LVS that could reveal either a common or unique CoP for these two vaccines.…”

The O-Ag Antibody Response to Francisella Is Distinct in Rodents and Higher Animals and Can Serve as a Correlate of Protection

“…The LVS wbtA mutant similarly used here has been described [50]. The clpB mutant of S4 [10] was provided by the National Research Council-Canada. The RML isolate of LVS was provided by Catharine Bosio.…”

“…We engineered S4 vaccine candidates with unmarked deletions in genes (guaBA and aroD) encoding critical enzymes in biosynthetic pathways; similar mutations in the enteric pathogens Shigella and Salmonella Typhi resulted in promising vaccine strains that are safe, highly attenuated, and immunogenic for Hu in clinical trials [7][8][9]. Independently, the Conlan and Sjostedt groups developed and characterized a S4 vaccine candidate, S4∆clpB, which lacks a chaperone protein required for secretion of virulence factors (see [10] and references therein). All three S4-based vaccine candidates (S4∆clpB, S4∆guaBA, and S4∆aroD) have been tested for attenuation and vaccination efficiency (VE) in one or more rodent models (mice (Mo), guinea pigs, rats (Rt)) [10][11][12].…”

“…Independently, the Conlan and Sjostedt groups developed and characterized a S4 vaccine candidate, S4∆clpB, which lacks a chaperone protein required for secretion of virulence factors (see [10] and references therein). All three S4-based vaccine candidates (S4∆clpB, S4∆guaBA, and S4∆aroD) have been tested for attenuation and vaccination efficiency (VE) in one or more rodent models (mice (Mo), guinea pigs, rats (Rt)) [10][11][12]. Vaccination of rodents with S4∆aroD or S4∆clpB each provided protection superior to LVS against S4 challenge.…”

“…In outbred Rbs, both S4∆aroD and S4∆guaBA provide protection (≥50% VE) superior to LVS when challenged by aerosol with high doses (~2000 CFU) of S4 [13,14]. Results from vaccine trials with S4∆clpB in a non-rodent system are pending [10]. To date, no side-by-side comparison of these three S4-based candidates in an identical system has been reported.…”