POPULATION-STRUCTURE OF THE RELAPSING FEVER SPIROCHETE BORRELIA-HERMSII AS INDICATED BY POLYMORPHISM OF 2 MULTIGENE FAMILIES THAT ENCODE IMMUNOGENIC OUTER SURFACE LIPOPROTEINS

The tick-borne relapsing fever spirochete Borrelia hermsii evades the mammalian immune system by periodically switching expression among mem bers of two multigene families that encode immunogenic, antigenically distinct outer surface proteins, The type strain, B. hermsii HS1, has at least 40 complete genes and pseudogenes that participate in this mu ltiphasic antigenic variation, Originally termed vmp (for variable maj or protein) genes, they have been reclassified as vsp (for variable sm all protein) and vip (for variable large protein) genes, based on size and amino acid sequence similarities, To date, antigenic variation in B. hermsii has been studied only in the type strain, HS1, Nucleotide sequence comparisons of 23 B. hermsii HS1 genes revealed five distinct groups, the vsp gene family and four subfamilies of vip genes, We use d PCR with family-and subfamily-specific primers, followed by restrict ion fragment length polymorphism analysis, to compare the vsp and vip repertoires of HS1 and seven other B. hermsii isolates from Washington , Idaho, and California. This analysis, together with pulsed-field gel electrophoresis genome profiles, revealed that the eight isolates for med three distinct groups, which likely represent clonal lineages, Mem bers of the three groups coexisted in the same geographic area, but th ey could also be isolated across large geographical distances, This po pulation structure may result from immune selection by the host, as ha s been proposed for other pathogens with polymorphic antigens.