Protective Antigen as a Correlative Marker for Anthrax in Animal Models

Abstract

The most aggressive form of anthrax results from inhalation of airborne spores of Bacillus anthracis and usually progresses unnoticed in the early stages because of unspecific symptoms. The only reliable marker of anthrax is development of bacteremia, which increases with disease progress. Rapid diagnosis of anthrax is imperative for efficient treatment and cure. Herein we demonstrate that the presence and level of a bacterial antigen, the protective antigen (PA), a component of B. anthracis toxins, in host sera can serve as a reliable marker of infection. This was tested in two animal models of inhalation anthrax, rabbits and guinea pigs infected by intranasal instillation of Vollum spores. In both models, we demonstrated qualitative and quantitative correlations between levels of bacteremia and PA concentrations in the sera of sick animals. The average time to death in infected animals was about 16 h after the appearance of bacteremia, leaving a small therapeutic window. As the time required for immunodetection of PA can be very short, the use of this marker will be beneficial for faster diagnosis and treatment of inhalation anthrax.

Expression of either Lethal Toxin or Edema Toxin by Bacillus anthracis Is Sufficient for Virulence in a Rabbit Model of Inhalational Anthrax

“…challenge with equivalent numbers of either the WT or single toxin mutants was similar regardless of whether the organisms produced both toxins, only LT, or only ET. Our results are consistent with previous reports showing that detection of circulating PA levels correlated with bacteremia (32) but that toxin concentrations did not correlate with time to anthracis mutant. The bacterial burdens in the LALN and spleens 72 h after a pulmonary inoculation with PAϪ mutant spores in untreated rabbits and rabbits that had received a 0.75-mg/kg dose of vinblastine (Vin) i.v.…”

“…coinfection were similar to the levels observed when rabbits were infected with either the WT or PAϪ strain alone, suggesting that survival of B. anthracis bacilli was dependent on the ability of each individual organism to produce toxin(s) in order to evade elimination by host effector cells. Our results are consistent with previous reports showing that high plasma levels of toxin are not detected until right before death (32). Instead, the localized effect of toxins may be explained by the recent findings that toxins in blood were associated with capsule (16) and can be packaged as extracellular vesicles which would allow the bacteria to disperse concentrated toxin complexes to act within their immediate vicinity or upon uptake in host phagocytes (63).…”

Association of Bacillus anthracis Capsule with Lethal Toxin during Experimental Infection

“…Adding protease inhibitors would interfere with certain biological assays. Determining which stage of infection an animal is in during the rapidly changing series of terminal events in anthrax infection is also problematic, considering that release of high levels of toxins into the circulation occurs mainly during the final hours before death (17). The data we report here with regard to the absence of PA 83 in the blood and the association of capsule with the toxin complex in three rabbits, three guinea pigs, and four monkeys appear consistent.…”

Characterization of a Therapeutic Model of Inhalational Anthrax Using an Increase in Body Temperature in New Zealand White Rabbits as a Trigger for Treatment

“…The correlation between detection of PA and bacteremia was also demonstrated by Koliber et al by using the ECL assay (12). In the current study, PA levels were variable over time and this may have been due to differences in disease progression for each individual animal.…”