Anthrax vaccine efficacy in golden Syrian hamsters

Using a Syrian (Golden) Hamster Biological Model for the Evaluation of Recombinant Anthrax Vaccines

In this paper, we demonstrate that a Syrian hamster biological model can be applied to the study of recombinant anthrax vaccines. We show that double vaccination with recombinant proteins, such as protective antigen (PA) and fusion protein LF1PA4, consisting of lethal factor I domain (LF) and PA domain IV, leads to the production of high titers of specific antibodies and to protection from infection with the toxicogenic encapsulated attenuated strain B. anthracis 71/12. In terms of antibody production and protection, Syrian hamsters were much more comparable to guinea pigs than mice. We believe that Syrian hamsters are still underestimated as a biological model for anthrax research, and, in some cases, they can be used as a replacement or at least as a complement to the traditionally used mouse model.

Current Status and Trends in Prophylaxis and Management of Anthrax Disease

Bacillus anthracis has been identified as a potential military and bioterror agent as it is relatively simple to produce, with spores that are highly resilient to degradation in the environment and easily dispersed. These characteristics are important in describing how anthrax could be used as a weapon, but they are also important in understanding and determining appropriate prevention and treatment of anthrax disease. Today, anthrax disease is primarily enzootic and found mostly in the developing world, where it is still associated with considerable mortality and morbidity in humans and livestock. This review article describes the spectrum of disease caused by anthrax and the various prevention and treatment options. Specifically we discuss the following; (1) clinical manifestations of anthrax disease (cutaneous, gastrointestinal, inhalational and intravenous-associated); (2) immunology of the disease; (3) an overview of animal models used in research; (4) the current World Health Organization and U.S. Government guidelines for investigation, management, and prophylaxis; (5) unique regulatory approaches to licensure and approval of anthrax medical countermeasures; (6) the history of vaccination and pre-exposure prophylaxis; (7) post-exposure prophylaxis and disease management; (8) treatment of symptomatic disease through the use of antibiotics and hyperimmune or monoclonal antibody-based antitoxin therapies; and (9) the current landscape of next-generation product candidates under development.

Animal Models for the Pathogenesis, Treatment, and Prevention of Infection by Bacillus anthracis

This article reviews the characteristics of the major animal models utilized for studies on Bacillus anthracis and highlights their contributions to understanding the pathogenesis and host responses to anthrax and its treatment and prevention. Advantages and drawbacks associated with each model, to include the major models (murine, guinea pig, rabbit, nonhuman primate, and rat), and other less frequently utilized models, are discussed. Although the three principal forms of anthrax are addressed, the main focus of this review is on models for inhalational anthrax. The selection of an animal model for study is often not straightforward and is dependent on the specific aims of the research or test. No single animal species provides complete equivalence to humans; however, each species, when used appropriately, can contribute to a more complete understanding of anthrax and its etiologic agent.

Pathology of inhalational anthrax animal models

Anthrax is a lethal disease caused by the bacterium Bacillus anthracis. There are three principal forms of the disease in humans-cutaneous, gastrointestinal, and inhalational-depending on the route of exposure. Of these, inhalational anthrax is the most dangerous; it is rapidly fatal; and it has been used as a deadly biological warfare agent in the last decade. Suitable animal models of inhalational anthrax have been utilized to study pathogenesis of disease, investigate bacterial characteristics such as virulence, and test effectiveness of vaccines and therapeutics. To date, mice, guinea pigs, rabbits, and nonhuman primates are the principal animal species used to study inhalational anthrax. Mice are valuable in studying early pathogenesis and bacterial characteristics. Few pathologic changes occur in the mouse models but may include marked bacteremia and lymphocyte destruction in the spleen and mediastinal lymph nodes. Rabbits and guinea pigs rapidly develop fulminate systemic disease, and pathologic findings often include necrotizing lymphadenitis; splenitis; pneumonia; vasculitis; and hemorrhage, congestion, and edema in multiple tissues. Nonhuman primates consistently develop the full range of classic lesions of human inhalational anthrax, including meningitis; lymphadenitis; splenitis; mediastinitis; pneumonia; vasculitis; and hemorrhage, congestion, and edema in multiple tissues. This review focuses on basic characteristics of the bacterium and its products, key aspects of pathogenesis, and the pathologic changes commonly observed in each animal model species.

Involvement of TLR2 in innate response to Bacillus anthracis infection

The involvement of TLR2 receptor in the innate response to infection with Bacillus anthracis was investigated. We studied the response to virulent or attenuated Vollum strains in either in vitro assays using macrophage cultures, or in an in vivo model comparing the sensitivity of Syrian hamster cells (expressing normal TLR2) to Chinese hamster cells (lacking functional TLR2) to infection by the various B. anthracis strains. Phagocytosis experiments with murine cell cultures or primary macrophages from both hamster strains, using virulent or attenuated Tox+Cap -, Tox-Cap+ or Tox-Cap- spores indicated that the secretion of TNF-α was induced by all the bacterial spores and purified spore antigens. In contrast, capsular antigens induce secretion of TNF-α only by Syrian hamster macrophages indicating the involvement of a functional TLR2 in macrophage activation. Challenge experiments with both hamster strains by intranasal spore inoculation, indicated that, while both strains are equally sensitive to infection with the virulent strain, the Chinese hamster demonstrated a higher sensitivity to infection with the toxinogenic or encapsulated strains. In conclusion, our findings imply that TLR2 has an important role in the attempt of the innate immunity to control B. anthracis infection, although TNF-α secretion was found to be mediated by both TLR2-dependent and TLR2-independent pathways.

Efficacy of a vaccine based on protective antigen and killed spores against experimental inhalational anthrax

Protective antigen (PA)-based anthrax vaccines acting on toxins are less effective than live attenuated vaccines, suggesting that additional antigens may contribute to protective immunity. Several reports indicate that capsule or spore-associated antigens may enhance the protection afforded by PA. Addition of formaldehyde-inactivated spores (FIS) to PA (PA-FIS) elicits total protection against cutaneous anthrax. Nevertheless, vaccines that are effective against cutaneous anthrax may not be so against inhalational anthrax. The aim of this work was to optimize immunization with PA-FIS and to assess vaccine efficacy against inhalational anthrax. We assessed the immune response to recombinant anthrax PA from Bacillus anthracis (rPA)-FIS administered by various immunization protocols and the protection provided to mice and guinea pigs infected through the respiratory route with spores of a virulent strain of B. anthracis. Combined subcutaneous plus intranasal immunization of mice yielded a mucosal immunoglobulin G response to rPA that was more than 20 times higher than that in lung mucosal secretions after subcutaneous vaccination. The titers of toxin-neutralizing antibody and antispore antibody were also significantly higher: nine and eight times higher, respectively. The optimized immunization elicited total protection of mice intranasally infected with the virulent B. anthracis strain 17JB. Guinea pigs were fully protected, both against an intranasal challenge with 100 50% lethal doses (LD(50)) and against an aerosol with 75 LD(50) of spores of the highly virulent strain 9602. Conversely, immunization with PA alone did not elicit protection. These results demonstrate that the association of PA and spores is very much more effective than PA alone against experimental inhalational anthrax.

Discriminating virulence mechanisms among Bacillus anthracis strains by using a murine subcutaneous infection model

Bacillus anthracis strains harboring virulence plasmid pXO1 that encodes the toxin protein protective antigen (PA), lethal factor, and edema factor and virulence plasmid pXO2 that encodes capsule biosynthetic enzymes exhibit different levels of virulence in certain animal models. In the murine model of pulmonary infection, B. anthracis virulence was capsule dependent but toxin independent. We examined the role of toxins in subcutaneous (s.c.) infections using two different genetically complete (pXO1(+) pXO2(+)) strains of B. anthracis, strains Ames and UT500. Similar to findings for the pulmonary model, toxin was not required for infection by the Ames strain, because the 50% lethal dose (LD(50)) of a PA-deficient (PA(-)) Ames mutant was identical to that of the parent Ames strain. However, PA was required for efficient s.c. infection by the UT500 strain, because the s.c. LD(50) of a UT500 PA(-) mutant was 10,000-fold higher than the LD(50) of the parent UT500 strain. This difference between the Ames strain and the UT500 strain could not be attributed to differences in spore coat properties or the rate of germination, because s.c. inoculation with the capsulated bacillus forms also required toxin synthesis by the UT500 strain to cause lethal infection. The toxin-dependent phenotype of the UT500 strain was host phagocyte dependent, because eliminating Gr-1(+) phagocytes restored virulence to the UT500 PA(-) mutant. These experiments demonstrate that the dominant virulence factors used to establish infection by B. anthracis depend on the route of inoculation and the bacterial strain.

Experimental respiratory anthrax infection in the common marmoset (Callithrix jacchus)

Inhalational anthrax is a rare but potentially fatal infection in man. The common marmoset (Callithrix jacchus) was evaluated as a small non-human primate (NHP) model of inhalational anthrax infection, as an alternative to larger NHP species. The marmoset was found to be susceptible to inhalational exposure to Bacillus anthracis Ames strain. The pathophysiology of infection following inhalational exposure was similar to that previously reported in the rhesus and cynomolgus macaque and humans. The calculated LD(50) for B. anthracis Ames strain in the marmoset was 1.47 x 10(3) colony-forming units, compared with a published LD(50) of 5.5 x 10(4) spores in the rhesus macaque and 4.13 x 10(3) spores in the cynomolgus macaque. This suggests that the common marmoset is an appropriate alternative NHP and will be used for the evaluation of medical countermeasures against respiratory anthrax infection.

Bacillus anthracis: interactions with the host and establishment of inhalational anthrax

Due to its potential as a bioweapon, Bacillus anthracis has received a great deal of attention in recent years, and a significant effort has been devoted to understanding how this organism causes anthrax. There has been a particular focus on the inhalational form of the disease, and studies over the past several years have painted an increasingly complex picture of how B. anthracis enters the mammalian host, survives the host's defense mechanisms, disseminates throughout the body and causes death. This article reviews recent advances in these areas, with a focus on how the bacterium interacts with its host in establishing infection and causing anthrax.

Anthrax vaccine design: strategies to achieve comprehensive protection against spore, bacillus, and toxin

The successful use of Bacillus anthracis as a lethal biological weapon has prompted renewed research interest in the development of more effective vaccines against anthrax. The disease consists of three critical components: spore, bacillus, and toxin, elimination of any of which confers at least partial protection against anthrax. Current remedies rely on postexposure antibiotics to eliminate bacilli and pre- and postexposure vaccination to target primarily toxins. Vaccines effective against toxin have been licensed for human use, but need improvement. Vaccines against bacilli have recently been developed by us and others. Whether effective vaccines will be developed against spores is still an open question. An ideal vaccine would confer simultaneous protection against spores, bacilli, and toxins. One step towards this goal is our dually active vaccine, designed to destroy both bacilli and toxin. Existing and potential strategies towards potent and effective anthrax vaccines are discussed in this review.