Protective immunity induced by Bacillus anthracis toxin-deficient strains.

The two toxins secreted by Bacillus anthracis are composed of binary combinations of three proteins: protective antigen (PA), lethal factor (LF), and edema factor (EF). Six mutant strains that are deficient in the production of one or two of these toxin components have been previously constructed and characterized (C. Pezard, E. Duflot, and M. Mock, J. Gen. Microbiol. 139:2459-2463, 1993). In this work, we examined the antibody response to the in vivo production of PA, LF, and EF in mice immunized with spores of strains producing these proteins. High titers of antibody to PA were observed after immunization with all strains producing PA, while titers of antibodies to EF and LF were weak in animals immunized with strains producing only EF or LF. In contrast, immunization with strains producing either PA and EF or PA and LF resulted in an increased antibody response to EF or LF, respectively. The differing levels of protection from a lethal anthrax challenge afforded to mice immunized with spores of the mutant strains not only confirm the role of PA as the major protective antigen in the humoral response but also indicate a significant contribution of LF and EF to immunoprotection. We observed, however, that PA-deficient strains were also able to provide some protection, thereby suggesting that immune mechanisms other than the humoral response may be involved in immunity to anthrax. Finally, a control strain lacking the toxin-encoding plasmid was unable to provide protection or elicit an antibody response against bacterial antigens, indicating a possible role for pXO1 in the survival of B. anthracis in a host.