REGIONS OF YERSINIA-PESTIS-V ANTIGEN THAT CONTRIBUTE TO PROTECTION AGAINST PLAGUE IDENTIFIED BY PASSIVE AND ACTIVE IMMUNIZATION

V antigen of Yersinia pestis is a multifunctional protein that has bee n implicated as a protective antigen, a virulence factor, and a regula tory protein. A series of V-antigen truncates expressed as glutathione S-transferase (GST) fusion proteins (GST-V truncates) have been clone d and purified to support immunogenicity and functionality studies of V antigen. Immunization studies with GST-V truncates have identified t wo regions of V antigen that confer protection against Y. pestis 9B (a fully virulent human pneumonic plague isolate) in a mouse model for p lague. A minor protective region is located from amino acids 2 to 135 (region I), and a major protective region is found between amino acids 135 and 275 (region II). In addition, analysis of IgG titers followin g immunization suggested that the major antigenic region of V antigen is located between amino acids 135 and 245. A panel of monoclonal anti bodies raised against recombinant V antigen was characterized by Weste rn blotting against GST-V truncates, and epitopes of most of the monoc lonal antibodies were mapped to region I or II. Monoclonal antibody 7. 3, which recognizes an epitope in region II, passively protected mice against challenge with 12 median lethal doses of Y. pestis GB, indicat ing that region II encodes a protective epitope. This is the first rep ort of a V-antigen-specific monoclonal antibody that will protect mice against a fully virulent strain of Y. pestis. The combined approach o f passive and active immunization has therefore confirmed the importan ce of the central region of the protein for protection and also identi fied a previously unknown protective region at the N terminus of V ant igen.