Antibodies to anthrax toxin in humans and guinea pigs and their relevance to protective immunity

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

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.

Bacillus anthracis: medical issues of biologic warfare

Recent world events refocused attention on the possibility of nations engaging in biologic warfare, including an attack with Bacillus anthracis. The single available anthrax vaccine in the United States for human use, formerly known as MDPH-PA, has decreased ability to protect laboratory animals against virulent B. anthracis strains, especially compared with new vaccines being developed. Studies with these vaccines, however, have several shortcomings. The pathogenesis, diagnosis, treatment, and prophylaxis of anthrax are discussed, as well as the implications that an attack with B. anthracis would place on the health care system.

Anthrax vaccine. Model of a response to the biologic warfare threat

Anthrax vaccine is being administered to all 2.4 million active duty, reserve, and National Guard troops, as prophylaxis against biologic warfare. The vaccine's effectiveness in this setting may be limited. This article discusses unresolved issues of safety, with an emphasis on the need for careful surveillance of vaccines used by the military, which has sidestepped the commercial process. Also considered are ethical issues related to the development and use of military biologics, as the United States Army advances its Joint Vaccine Acquisition Program, a plan to produce more than ten vaccines specifically for biologic warfare threat, and to administer them to all military servicemembers.

Molecular pathogenesis of Bacillus anthracis infection

This review summarizes the current knowledge pertaining to the pathogenesis of infection with Bacillus anthracis relative to the two exotoxins and the capsule. Emphasis is given to the structure and activities of the individual components of the exotoxins, their interaction with cells, and the response of macrophages to lethal toxin. Finally, results from vaccination studies are reviewed.

Comparative efficacy of experimental anthrax vaccine candidates against inhalation anthrax in rhesus macaques

The authors examined the efficacy of Bacillus anthracis protective antigen (PA) combined with adjuvants as vaccines against an aerosol challenge of virulent anthrax spores in rhesus macaques. Adjuvants tested included i) aluminum hydroxide (Alhydrogel), ii) saponin QS-21 and iii) monophosphoryl lipid A (MPL) in squalene/lecithin/Tween 80 emulsion (SLT). Animals were immunized once with either 50 micrograms of recombinant PA plus adjuvant, or with Anthrax Vaccine Adsorbed (AVA), the licensed human anthrax vaccine. The serological response to PA was measured by enzyme linked immunosorbent assay. Lymphocyte proliferation and serum neutralization of in vitro lethal toxin cytotoxicity were also assayed. In all vaccine groups, anti-PA IgM and IgG titers peaked at 2 weeks and 4-5 weeks postimmunization, respectively. Five weeks postimmunization, animals in all vaccine groups demonstrated PA-specific lymphocyte proliferation and sera that neutralized in vitro cytotoxicity. Six weeks after immunization, the animals were challenged by aerosol with approximately 93 LD50 of virulent anthrax spores. Animals were bled daily for 1 week to monitor bacteremia, and deaths were recorded. Anti-PA ELISA titers in all groups of immunized animals were substantially increased 2 weeks after challenge. One dose of each vaccine provided significant protection (> 90%) against inhalation anthrax in the rhesus macaques.

Anthrax post-vaccinal cell-mediated immunity in humans: kinetics pattern

Seven groups (2596 subjects) were vaccinated with a human live anthrax vaccine (HLAV) by three different routes (scarification, subcutaneous and aerosol). The vaccinees were tested for anthrax cell-mediated immunity using the "Anthraxin" skin test at 7, 15, 30, 90, 180 and 365 days following vaccination. The kinetic pattern obtained from all groups, shows a significant, five-phased curve: phase I (2-6 days post-vaccination) shows a slow increase in positive Anthraxin skin reactions. Phase II (7-15 days post-vaccination) shows an exponential rise to a maximum at day 15. Phase III (16-30 days post-vaccination) shows a decrease to day 30. Phase IV (31-90 days post-vaccination) leads to a relative restoration of the positive skin reactions. During phase V (91-365 days post-vaccination) there is a continuous decrease in positive Anthraxin skin reactions. The loss of the skin test reaction on day 30 is a characteristic feature of post vaccination anthrax cell-mediated immunity. It may be due to a blockade of macrophages by lethal anthrax toxin released by the multiplying vaccine strain. Epidemiological observations of HLAV protective rates correlate with the phases of the skin reaction kinetics.

Preparation for emergency relief after biological warfare

Upon invitation by the World Health Organization during the Gulf War, a task force "Scorpio" independent from the nations involved in the armed conflict was formed whose task was to determine whether, which and to what extent biological warfare agents had been used. risk assessment concluded that anthrax and Clostridium botulinum toxin were the major risks. The 21 civilian experts had rapidly to decide on the doctrine of operation, to assemble material which could be used and to be immunized or protected otherwise against the potential risks. Biological warfare agents may be used anywhere any time, be it by terrorists or during open or clandestine hostilities. The general population cannot rely on the military to take care of civilian relief, thus international and national organizations may wish to establish similar task forces basing on the "Scorpio" model on a national or regional basis.

Efficacy of a standard human anthrax vaccine against Bacillus anthracis spore challenge in guinea-pigs

The efficacy of an anthrax vaccine licensed for human use, MDPH-PA, was tested in guinea-pigs intramuscularly challenged with 10, 100 or 1000 LD50 of spores from two virulent strains of Bacillus anthracis, Vollum 1B and Ames. As demonstrated in other investigations, immunization with MDPH-PA provided better protection against challenge with the Vollum 1B strain than with the Ames strain, although vaccine efficacy against the Ames strain was better than previously reported. Enzyme-linked immunosorbent assay of serum antibody titres to B. anthracis protective antigen showed that there was no significant correlation between survival and antibody titres.

Serology and anthrax in humans, livestock and Etosha National Park wildlife

Results are presented from a number of epidemiological studies using enzyme immunoassays (EIA) based on the purified anthrax toxin antigens, protective antigen, lethal factor and oedema factor. Studies on sera from a group of 62 human anthrax patients in Turkey and from cattle in Britain following two unrelated outbreaks of anthrax show that EIA using protective antigen can be a useful diagnostic aid and will detect subclinical infections in appropriate circumstances. A serological survey on wildlife in the Etosha National Park, Namibia, where anthrax is endemic, showed that naturally acquired anthrax-specific antibodies are rare in herbivores but common in carnivores; in carnivores, titres appear to reflect the prevalence of anthrax in their ranges. Problems, as yet unresolved, were encountered in studies on sera from pigs following an outbreak of anthrax on a farm in Wales. Clinical details, including treatment, of the human and one of the bovine outbreaks are summarized and discussed in relation to the serological findings.

Bacillus anthracis but not always anthrax

Gram-positive bacilli isolated during epidemiological investigations which, on the basis of conventional tests, resemble Bacillus anthracis but which fail to produce the capsule or to induce anthrax in test animals have long been dismissed in clinical and veterinary laboratories as B. cereus or simply as unidentified Bacillus spp. and thereupon discarded as inconsequential. In this study, the application of newly available DNA probe, polymerase chain reaction and specific toxin antigen detection technology has revealed that a proportion of such strains are B. anthracis which lack the plasmid carrying the capsule gene (pXO2). While these techniques cannot, of course, be used to confirm the identities of strains resembling B. anthracis but which also lack the plasmid carrying the toxin genes (pXO1), the likelihood that these also are bona fide B. anthracis becomes more acceptable. (As yet no naturally occurring pXO1-/2+ strains have been found.) At this point, the significance of the presence of such avirulent forms of B. anthracis in specimens can only be a subject for speculation, but the possibility that they may be indicators of virulent parents somewhere in the system being examined must be considered.