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Numerous studies have provided suggestive evidence that the loss of plasmids correlates with the loss of infectivity of the Lyme disease spirochetes. In this study we have further investigated this correlation. Clonal populations were obtained from the skin of a mouse infected for 3 months with a clonal population of Borrelia burgdorferi B31MI. The complete plasmid compositions of these populations were determined using a combination of PCR and Southern hybridization. The infectivities of clones differing in plasmid composition were tested using the C3H-HeJ murine model for Lyme disease. While several clones were found to be noninfectious, a correlation between the loss of a specific plasmid and loss of infectivity in the clones analyzed in this report was not observed. While it is clear from recent studies that the loss of some specific plasmids results in attenuated virulence, this study demonstrates that additional mechanisms also contribute to the loss of infectivity.

The ospE gene family of the Lyme disease spirochetes encodes a polymorphic group of immunogenic lipoproteins. The ospE genes are one of several gene families that are flanked by a highly conserved upstream sequence called the upstream homology box, or UHB, element. Earlier analyses in our lab demonstrated that ospE -related genes are characterized by defined hypervariable domains (domains 1 and 2) that are predicted to be hydrophilic, surface exposed, and antigenic. The flanking of hypervariable domain 1 by DNA repeats may indicate that recombination contributes to ospE diversity and thus ultimately to antigenic variation. Using an isogeneic clone of Borrelia burgdorferi B31G (designated B31Gc1), we demonstrate that the ospE -related genes undergo mutation and rearrangement during infection in mice. The mutations that develop during infection resulted in the generation of OspE proteins with altered antigenic characteristics. The data support the hypothesized role of OspE-related proteins in immune system evasion.

It has been postulated that thevlssystem of the Lyme disease spirochetes contributes to immune evasion through antigenic variation. While it is clear thatvlsEundergoes sequence change within its variable regions at a high frequency during the early stages of infection, a definitive role in immune evasion has not been demonstrated. In this report we assessed the murine and human humoral immune response to recombinant (r)-VlsE variants that originally arose during infection in mice. Immunoblot analyses of r-VlsE variants were conducted by using serum samples collected from mice infected withBorrelia burgdorfericlones that carried differentvlsEvariants. All of the r-VlsE variants were recognized by infection sera regardless of the identity of the infecting clone or isolate. In addition, all variants were immunoreactive with a panel of human Lyme disease patient serum samples. It is evident from these analyses that the infection-induced VlsE variants share common epitopes that reside within conserved segments of these proteins. However, preabsorption experiments revealed that the variable regions of the central domain of VlsE, which undergo rapid mutation during infection, also influence the antigenic properties of the protein. A subset of the antibodies elicited againstvlsEvariants that differ in the sequences of their variable regions were found to be variant specific. Hence, in spite of a robust antibody response to conserved segments of VlsE, infection-induced sequence changes within the variable regions alter the antigenicity of VlsE. These results provide the first direct evidence of antigenic variation in the VlsE protein.