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.