Efficacy of a human anthrax vaccine in guinea pigs, rabbits, and rhesus macaques against challenge by Bacillus anthracis isolates of diverse geographical origin

Abstract

The efficacy of a licensed human anthrax vaccine (Anthrax Vaccine Adsorbed (AVA)) was tested in guinea pigs, rabbits, and rhesus macaques against spore challenge by Bacillus anthracis isolates of diverse geographical origin. Initially, groups of Hartley guinea pigs were vaccinated at 0 and 4 weeks with AVA, then challenged intramuscularly at 10 weeks with spores from 33 isolates of B. anthracis. Survival among the vaccinated groups varied from 6 to 100%, although there were no differences in mean time to death among the groups. There was no correlation between isolate virulence and variable number tandem repeat category or protective antigen genotype identified. New Zealand white rabbits were then vaccinated with AVA at 0 and 4 weeks, and challenged at 10 weeks by aerosol with spores from six of the isolates that were highly virulent in vaccinated guinea pigs. AVA completely protected the rabbits from four of the isolates, and protected 90% of the animals from the other two isolates. Subsequently, two of these six isolates were then used to challenge rhesus macaques, previously vaccinated with AVA at 0 and 4 weeks, and challenged at 10 weeks by aerosol. AVA protected 80 and 100% of the animals from these two isolates. These studies demonstrated that, although AVA confers variable protection against different B. anthracis isolates in guinea pigs, it is highly protective against these same isolates in both rabbits and rhesus macaques.

Introduction

The current US human anthrax vaccine, Anthrax Vaccine Adsorbed (AVA), consists of aluminum hydroxide-adsorbed supernatant material, primarily protective antigen (PA), from fermentor cultures of a toxigenic, non-encapsulated isolate of Bacillus anthracis, V770-NP1-R [1], [2]. In humans, vaccination with AVA calls for a series of six doses within 18 months, followed by yearly boosters. Although there are no human clinical data on the efficacy of AVA, a 4-year placebo-controlled study from the 1950s demonstrated that a vaccine similar to AVA afforded a significant degree of protection to humans [3], [4]. Protection studies in different animal species yielded varied results. For example, AVA virtually fails to protect mice [5], [6] or hamsters [7] against a parenteral challenge by virulent B. anthracis spores. Vaccine efficacy studies in guinea pigs showed that AVA only partially protected the animals from parenteral challenge from certain virulent isolates of B. anthracis [8], [9], [10], [11]. Little and Knudson identified nine of 27 B. anthracis challenge isolates that appeared to overcome vaccination of guinea pigs with AVA [10]. Ivins et al. found that vaccination with AVA variably protected guinea pigs against challenge from two isolates of B. anthracis, Ames and Vollum 1B. The Ames isolate, however, was significantly more virulent in the vaccinated animals [11]. Recent vaccine studies demonstrated that rabbits and rhesus macaques are well protected by AVA from an aerosol challenge with the Ames isolate [12]. These data reflect differences in disease pathogenesis, or intrinsic antibody response with respect to the animal model, and the immune response to AVA and anthrax pathogenicity as it relates to humans.

Although human cases of anthrax are relatively rare in the United States, other countries suffer from endemic outbreaks of the disease [13] and there is concern about the possible use of B. anthracis as a weapon [14]. Therefore, it is important to determine whether there are isolates of B. anthracis for which the vaccine is not efficacious. In order to clarify the relationship between AVA efficacy, animal models, and isolate diversity, we systematically compared the efficacy of AVA in three animal models against challenge from a geographically diverse group of B. anthracis isolates. The guinea pig model was initially chosen to screen the isolates for several reasons. Historically, it has been the animal model most often used to test anthrax vaccine efficacy [15], [16], and therefore it is the model for which there are the most data. Furthermore, guinea pigs have been demonstrated to be an appropriate animal model for use in determining differences in isolates of B. anthracis with respect to virulence in an immunized host [8], [9], [10], [11]. The rabbit model was chosen for the second portion of these studies because of its similarity to rhesus macaques with respect to its ability to be highly protected by AVA [12] and the pathology that it demonstrates in experimental inhalational anthrax [17]. The third and final portion of these studies used rhesus macaques because the disease in these animals most closely resembles the infection in humans.