Human Serum contains a Protease that Protects Against the Cytotoxic Activity of the Bacillus Anthracis Lethal Toxin in vitro

Childrens Hospital at Montefiore/Albert Einstein College of Medicine, Division of Pediatric Infectious Diseases; Albert Einstein College of Medicine, Division of Microbiology and Immunology

^* To whom correspondence should be addressed. Email: dgoldma{at}aecom.yu.edu.

	Abstract

The role of the innate immunity in the host response to Bacillus anthracis is poorly understood. We found that normal human serum contains an anti-toxin mechanism that is capable of protecting macrophages in vitro from B. anthracis lethal toxin-mediated killing. This protective activity was limited to defined amounts of toxin and was lost by heat treatment or serum dilution. Some person to person variation in the protective activity of serum was noted, especially with higher concentrations of lethal toxin. A similar protective activity was found in murine serum, though human serum consistently neutralized more toxin than murine serum. The protective activity of both murine and human serum correlated with cleavage of the protective antigen into 2 fragments with approximate molecular sizes of 20 and 50 kDa that were recognized by the mAbs 7.5G and 10F4 respectively. This pattern of fragmentation is consistent with cleavage at multiple sites, including the furin susceptible site. Cleavage was abolished by heat treatment and calcium chelation. These findings highlight a potential role for serum proteases in protection against the lethal toxin of B. anthracis.