Intranasal Immunization With Bacillus Anthracis Spore Antigens Sensitizes Mice To Death From Inhalational Anthrax In A B Cell-Dependent Manner

Abstract There is considerable interest in developing an improved Bacillus anthracis (BA) vaccine that would provide broader, more rapid protection of naïve populations. Here, we investigate intranasal (i.n.) immunization as a method of inducing a mucosal immune response against BA spore antigens (BAAs) to facilitate early recognition and clearance. Nanolipoprotein particles (NLPs) were used as a nanoparticle platform for co-localized delivery of multiple spore proteins in conjunction with monophosphoryl lipid A (MPLA) adjuvant via i.n. immunization in mice. Immunization with BAA-MPLA-NLP constructs induced robust IgG and IgA responses in serum and in bronchoalveolar lavage. Surprisingly, when challenged with fully virulent BA, mice immunized with BAA-MPLA-NLPs died significantly faster than unimmunized mice. This hastened time to death following i.n. immunization was not antigen or adjuvant specific, nor related to the NLP platform, as similar results were observed using cholera toxin and CpG as adjuvants co-administered with BAAs in the absence of NLPs. Previous work has shown that B cells can traffic BA spores from the lungs to the lung-draining lymph nodes. Therefore, we investigated a role for B cells in sensitizing immunized mice to BA challenge. B cells in the lung increased following i.n. immunization, and depletion of B cells from immunized mice prior to challenge reversed the sensitized phenotype. Spore binding to B cells was enhanced by pre-incubation of spores with immune sera in a manner that was dependent upon the exosporium protein BclA. These results suggest i.n. immunization is not tenable as a protective strategy against inhalational anthrax due to an unexpected consequence of B cell expansion following immunization.