Role of Anthrax Toxins in Dissemination, Disease Progression, and Induction of Protective Adaptive Immunity in the Mouse Aerosol Challenge Model

Abstract

Anthrax toxins significantly contribute to anthrax disease pathogenesis, and mechanisms by which the toxins affect host cellular responses have been identified with purified toxins. However, the contribution of anthrax toxin proteins to dissemination, disease progression, and subsequent immunity after aerosol infection with spores has not been clearly elucidated. To better understand the role of anthrax toxins in pathogenesis in vivo and to investigate the contribution of antibody to toxin proteins in protection, we completed a series of in vivo experiments using a murine aerosol challenge model and a collection of in-frame deletion mutants lacking toxin components. Our data show that after aerosol exposure to Bacillus anthracis spores, anthrax lethal toxin was required for outgrowth of bacilli in the draining lymph nodes and subsequent progression of infection beyond the lymph nodes to establish disseminated disease. After pulmonary exposure to anthrax spores, toxin expression was required for the development of protective immunity to a subsequent lethal challenge. However, immunoglobulin (immunoglobulin G) titers to toxin proteins, prior to secondary challenge, did not correlate with the protection observed upon secondary challenge with wild-type spores. A correlation was observed between survival after secondary challenge and rapid anamnestic responses directed against toxin proteins. Taken together, these studies indicate that anthrax toxins are required for dissemination of bacteria beyond the draining lymphoid tissue, leading to full virulence in the mouse aerosol challenge model, and that primary and anamnestic immune responses to toxin proteins provide protection against subsequent lethal challenge. These results provide support for the utility of the mouse aerosol challenge model for the study of inhalational anthrax.

Noninvasive Imaging Technologies Reveal Edema Toxin as a Key Virulence Factor in Anthrax

“…Previous reports on the role of ET during infection with either encapsulated 20,21 or nonencapsulated strains 18,19 Figure 5. Secretion of edema and lethal toxins by wild-type B. anthracis elicits a temporal-dependent pattern of lesions in the spleen during cutaneous spore infection (50 hours after inoculation).…”

“…The fine mechanisms of bacterial regulation are thus likely able to operate in the tissue environment that varies in a complex temporal way through the crosstalk between the bacteria and the host control mechanisms. The model system we describe here is therefore complementary to experimental models described in previous studies, using systemic injection of purified ET or LT, 16,17 or infection with nonencapsulated 9,18 or with an encapsulated laboratory B. anthracis. 20,21 These experimental models preferentially explored the long-distance effects of significant amounts of circulating toxins, thus mimicking the terminal stage of anthrax.…”

“…20,21 These experimental models preferentially explored the long-distance effects of significant amounts of circulating toxins, thus mimicking the terminal stage of anthrax. Depending on the presence of the capsule, LT was found to be required 18 or not required 21 for dissemination. Each toxin was found able to induce apoptosis/necrosis, cell depletion, and neutrophil recruitment in both red and white splenic pulp, 16 -18,20,21 which could be related to the significant levels of toxins that reach the spleen on a short time scale.…”

“…All these lesions are considered to be caused primarily by the bacterial toxins, as observed after systemic administration of purified ET and LT. 4,16,17 Infection with nonencapsulated B. anthracis suggests that LT is a key virulence factor in cutaneous and inhalational murine infection. 18,19 Other studies, using an encapsulated laboratory strain, suggest that the toxins do not modify either dissemination or virulence. 20 -22 In the present study, we visualized in real time the effects of concomitant expression of both capsule and toxins, and their respective contribution, on the early and late interactions between wild-type B. anthracis and its host during cutaneous and inhalational infections.…”

Protective-antigen (PA) based anthrax vaccines confer protection against inhalation anthrax by precluding the establishment of a systemic infection

“…In non-human primates and rabbits vaccinated with aluminum salt-adjuvanted PA protein vaccines such as AVA and rPA, the TNA antibodies provide a reliable cross-species prediction of to be dependent on the expression of lethal toxin by vegetative bacilli. 18 It is apparent from Figure 3, that the immune response induced by PA-based vaccines exerts its impact upstream of these impediments of dissemination since the luminescence signal in the NALT of vaccinated mice is much lower than that of unvaccinated mice 24 h post-inhalation spore challenge, the only time point at which we were able to detect any bacillary growth in the vaccinated mice. The true extent of progressive B. anthracis infection in a previously vaccinated immune host following an inhalation exposure to a lethal dose of infective spores has not been explored adequately or demonstrated convincingly.…”