C-terminal invariable domain of VlsE may not serve as target for protective immune response against Borrelia burgdorferi

VlsE, the variable surface antigen of the Lyme disease spirochete, Borrelia burgdorferi, contains two invariable domains, at the amino and carboxyl te rmini, respectively, which collectively account for approximately one-half of the entire molecule's length and remain unchanged during antigenic varia tion. It is not known if these two invariable domains are exposed at the su rface of either the antigen or the spirochete. If they are exposed at the s pirochete's surface, they may elicit a protective immune response against B . burgdorferi and serve as vaccine candidates. In this study, a 51-mer synt hetic peptide that reproduced the entire sequence of the C-terminal invaria ble domain of VlsE was conjugated to the carrier keyhole limpet hemocyanin and used to immunize mice. Generated mouse antibody was able to immunopreci pitate native VlsE extracted from cultured B. burgdorferi B31 spirochetes, indicating that the C-terminal invariable domain was exposed at the antigen 's surface. However, this domain was inaccessible to antibody binding at th e surface of cultured intact spirochetes, as demonstrated by both an immuno fluorescence experiment and an in vitro killing assay. Mouse antibody to th e C-terminal invariable domain was not able to confer protection against B. burgdorferi infection, indicating that this domain was unlikely exposed at the spirochete's surface in vivo. We concluded that the C-terminal invaria ble domain was exposed at the antigen's surface but not at the surface of e ither cultured or in vivo spirochetes and thus cannot elicit protection aga inst B. burgdorferi infection.