MUCOSAL IMMUNIZATION AGAINST PLAGUE

Abstract

Yersinia pestis is the causative agent of plague. There are two forms of the disease, the bubonic and the pneumonic form. The bubonic form can be acquired by humans from the bite of an infected flea which can lead to the pneumonic form where aerosol transmission between humans is highly fatal. There are two vaccines (USP and EV76) currently in use throughout the world and both have serious limitations. The USP vaccine is a formalin-killed whole-cell preparation. Unfortunately this vaccine does not seem to protect against the pneumonic form of the disease. The EV76 preparation is a live attenuated vaccine which has been associated with high reactogenicity. The Low Calcium Response Virulence (LcrV) antigen of Y. pestis is a promising candidate antigen. In this study the LcrV antigen was genetically fused to the N-terminus of cholera toxin A subunit (CtxA) (MTD672) or to the A2 peptide of CtxA (MTD669). These fusion proteins were co-expressed with the B subunit of cholera toxin (CtxB) in Escherichia coli to form a functional chimeric holotoxin. The resulting holotoxins were purified and characterized. Both chimeras retained their ability to bind to Gm ganglioside and to cross react with polyclonal antiserum to LcrV and to Ctx. The chimeras were assessed for their immunogenicity in mice by both the intraperitoneal and intranasal routes of immunization. The results indicate that the chimeras were able to elicit IgG antibody in the serum and the lung by both routes of immunization. Attempts to assess the ability of these vaccine candidates to protect mice against aerosol challenge with Y. pestis were thwarted by the failure of the positive control group to survive challenge.