Monitoring of ELISA-reactive antibodies against anthrax protective antigen (PA), lethal factor (LF), and toxin-neutralising antibodies in serum of individuals vaccinated against anthrax with the PA-based UK anthrax vaccine

Abstract

The human anthrax vaccines currently licensed contain the protective antigen (PA) of Bacillus anthracis as main antigen together with traces of some other bacillus components, e.g. lethal factor (LF). The present study aimed at monitoring the course of specific antibody titres against PA and LF by enzyme linked immunosorbent assays (ELISA), as well as the levels of toxin-neutralising antibodies, in 11 volunteers vaccinated with the human anthrax vaccine UK. After an initial seroconversion in all vaccinees, a significant reduction of both antibody titres against PA and LF, and of neutralising antibodies, was detected just prior to a vaccine boost 6 months after completion of the basic immunisation. Following the booster injection, titres increased again to levels comparable to those after the fourth immunisation. ELISA titres against PA correlated significantly with neutralising antibodies (r = 0.816, p < 0.001). Therefore, the less work- and time-consuming ELISA should be favoured to monitor the efficacy of an anthrax vaccination.

Introduction

The spore-forming bacterium Bacillus anthracis is the causative agent of anthrax, an often fatal zoonosis if the pathogen is internalized via the respiratory route. After germination of the spores in host macrophages, the symptoms are mainly brought about by cytotoxic effects caused by the interaction of three secreted proteins, protective antigen (PA), lethal factor (LF), and edema factor (EF), which form the anthrax lethal and edema toxin [1], [2]. Protection against anthrax toxins requires humoral immune responses [3], [4], [5], [6], [7].

The vaccines currently available and licensed for use in humans are derived from non-virulent strains of B. anthracis which are not encapsulated. The “Anthrax Vaccine UK” and the “US Anthrax Vaccine Adsorbed” (AVA) both elicit specific immune responses which are thought to protect against an infection with B. anthracis[8], [9].

Results of clinical and laboratory trials of experimental recombinant vaccines against anthrax have been published. The variety of approaches includes recombinant PA administered intramuscularly in the presence of different adjuvants [10], [11], adenoviruses encoding PA [12], nasal immunisation with PA [13], plasmid DNA vaccines [14], and dually active vaccines containing a conjugate of the weakly immunogenic capsule and the highly immunogenic PA [15]. So far, no superior vaccine candidate could be identified due to the lack of direct comparison in clinical trials.

The UK human anthrax vaccine is mainly based on PA, but also contains a number of other bacteria- and media-derived proteins [8]. These proteins may contribute to the transient side effects experienced by some individuals, but could also modify or even enhance the vaccine's protective immune response. LF, which is present in traces in the vaccine, is known to augment the immune response against PA and could therefore contribute to protection against the anthrax toxin [3], [16], [17]. The vaccination regimen includes a number of injections to induce an immune response, followed by yearly booster immunisations, which are thought to maintain immunity. With respect to the efficacy and duration of protection induced by the vaccine, very little is known about the quality of the immune response in humans. It would therefore be important to identify in vitro correlates of protection to extrapolate the efficacy of the human anthrax vaccination. Antibody titres against PA can be used as an indicator for a successful immunisation, while, based on results of animal experiments, toxin-neutralising antibodies are proposed as surrogate markers of protection [18], [19], [20].

The aim of the present study is to monitor the course of ELISA titres against PA and LF, as well as toxin-neutralising antibodies over the course of the vaccination schedule with the “Anthrax Vaccine UK”, and to determine whether there is a correlation between these titres.