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Infection and Immunity, November 2007, p. 5240-5247, Vol. 75, No. 11
0019-9567/07/$08.00+0     doi:10.1128/IAI.00884-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Recombinant Exosporium Protein BclA of Bacillus anthracis Is Effective as a Booster for Mice Primed with Suboptimal Amounts of Protective Antigen

Department of Microbiology and Immunology, Uniformed Services University of the Health Sciences, 4301 Jones Bridge Road, Bethesda, Maryland 20814-4799,¹ Naval Medical Research Center, 503 Robert Grant Ave., Silver Spring, Maryland 20910,² Center for Vaccine Development, Department of Medicine, University of Maryland School of Medicine, University of Maryland, Baltimore, Maryland 21201³

Received 29 June 2007/ Returned for modification 23 July 2007/ Accepted 17 August 2007

Bacillus collagen-like protein of anthracis (BclA) is an immunodominant glycoprotein located on the exosporium of Bacillus anthracis. We hypothesized that antibodies to this spore surface antigen are largely responsible for the augmented immunity to anthrax that has been reported for animals vaccinated with inactivated spores and protective antigen (PA) compared to vaccination with PA alone. To test this theory, we first evaluated the capacity of recombinant, histidine-tagged, nonglycosylated BclA (rBclA) given with adjuvant to protect A/J mice against 10 times the 50% lethal dose of Sterne strain spores introduced subcutaneously. Although the animals elicited anti-rBclA antibodies and showed a slight but statistically significant prolongation in the mean time to death (MTD), none of the mice survived. Similarly, rabbit anti-rBclA immunoglobulin G (IgG) administered intraperitoneally to mice before spore inoculation increased the MTD statistically significantly but afforded protection to only 1 of 10 animals. However, all mice that received suboptimal amounts of recombinant PA and that then received rBclA 2 weeks later survived spore challenge. Additionally, anti-rBclA IgG, compared to anti-PA IgG, promoted a sevenfold-greater uptake of opsonized spores by mouse macrophages and markedly decreased intramacrophage spore germination. Since BclA has some sequence similarity to human collagen, we also tested the extent of binding of anti-rBclA antibodies to human collagen types I, III, and V and found no discernible cross-reactivity. Taken together, these results support the concept of rBclA as being a safe and effective boost for a PA-primed individual against anthrax and further suggest that such rBclA-enhanced protection occurs by the induction of spore-opsonizing and germination-inhibiting antibodies.

Published ahead of print on 4 September 2007.

Editor: R. P. Morrison