Safety, reactogenicity and immunogenicity of Francisella tularensis live vaccine strain in humans

Abstract

We evaluated the safety, reactogenicity and immunogenicity of escalating doses of a new Francisella tularensis Live Vaccine Strain (LVS) lot by scarification (SCAR) or subcutaneously (SQ) in humans. Subjects (N = 10/group) received one dose of LVS via SCAR at 10⁵,10⁷ or 10⁹ cfu/ml or SQ at 10², 10³,10⁴ or 10⁵ cfu/ml; 14 subjects received placebo. All doses/routes were well tolerated. When compared to placebo, vaccination with 10⁷ SCAR and 10⁹ SCAR resulted in significantly higher serologic response frequencies, as measured by ELISA for IgG, IgM, IgA and microagglutination; whereas vaccination with 10⁵ SCAR, 10⁷ SCAR 10⁹ SCAR and 10⁵ SQ elicited a significantly higher interferon-γ response frequency.

Introduction

Francisella tularensis (Ft), the causative agent of tularemia, is one of the most infectious organisms to humans: as few as 25 colony forming units can cause significant respiratory disease [1], [2], [3]. It also displays a very complex ecology, infecting more than 250 species, with amoebas acting as a potential reservoir [4], [5]. The disease epidemiology in humans is characterized by the presence of disease “hotspots” and by the periodic emergence of disease in areas where disease was previously not recognized, mostly mirroring epizoonotic changes [6], [7].

There are 4 subspecies within the Ft species: tularensis, holarctica, mediasiatica and novicida[8]. The organism is transmitted to humans in a myriad of ways: direct contact with infected animals, ingestion of contaminated water, inhalation of aerosolized organisms or via vectors that include mosquitoes, ticks and flies. Clinical manifestations depend on the route of exposure and the Ft subspecies, with a case fatality rate reaching 30% in untreated cases of typhoidal or respiratory disease [9], [10], [11]. Ft subsp. tularensis is the most virulent of the subspecies, causing the most severe disease, albeit with a restricted geographic distribution. The high morbidity and mortality of tularemia, its potential for aerosolization, its low infectious dose and the ease of propagating the organism in vitro have raised concerns about its potential use as a biological weapon. In fact, the USA, USSR and Japan have stockpiled the organism as a weapon in the past, and Ft is classified as a category A select agent by the Centers for Disease Control and Prevention [11], [12], [13]. This recent classification has resulted in renewed interest in tularemia vaccines.

Two tularemia vaccines have been studied in humans in the US: the killed vaccine (Foshay) and the live vaccine strain (LVS). Kadull et al. immunized individuals with the killed vaccine and, in non-controlled trials, showed limited efficacy in preventing the disease and its severity [14]. The live vaccine was developed in the former Soviet Union from a Ft subsp. holarctica strain and was given to millions of individuals to contain outbreaks. In 1956 the Soviet government provided the live vaccine to scientists at Fort Detrick, Maryland. Two colony variants were identified: blue and gray [15]. The blue colony variant was more immunogenic in animals and was designated LVS. The efficacy of Ft LVS was initially evaluated using two routes: inhalation and scarification. The superiority of the Ft LVS over the Foshay vaccine was demonstrated by Saslaw et al. who showed that subjects who received LVS by scarification were less likely to develop signs of tularemia following an aerosol challenge; a protection that was later shown to be overcome with increasing the aerosol challenge dose [1], [3]. Hornick et al. demonstrated that individuals immunized with 10⁸ LVS organisms via the aerosol route were better protected against a high-dose aerosol challenge with Ft than individuals immunized with LVS via scarification or via a lower dose aerosol [16]. However, due to the logistical constraints of aerosolization, the scarification method was adopted thereafter in the US.

Ft LVS was administered under investigational protocols for many years and was shown to be associated with significant reduction in laboratory-acquired tularemia [17], [18]. A correlate of protection for tularemia has not been identified; however, the literature suggests that the high antibody titers that follow vaccination or infection serve as markers of exposure, while the cell-mediated immune response is more closely related to protection [19], [20].

The Ft LVS vaccine was never licensed for use in humans in the US, due to uncertainty about the mechanism of attenuation, concern about reversion to a virulent phenotype and the research-grade production methods. Under a contract from the Joint Vaccine Acquisition Program, Dynport Vaccine Company (DVC) manufactured a new vaccine lot using good manufacturing practices (GMP). Preclinical evaluation of the newly derived lot of Ft LVS in rabbits at escalating dosages of 10⁵–10⁹ cfu by the intradermal, subcutaneous (SQ) routes and by scarification (SCAR) demonstrated its safety and immunogenicity as measured by antibody levels [21]. The findings from the preclinical study provided reassurance to proceed with the evaluation of escalating vaccine doses of the new lot in humans using two routes: SCAR and the more quantitative and convenient SQ route.