Borrelia miyamotoi in vivo antigenic variation demonstrated by serotype reisolations from infected mice.

Relapsing fever Borrelia (RFB) employs antigenic variation to alter its surface protein structure in response to host immune pressure. This process occurs by the single translocation of archived variable major protein (Vmp) pseudogenes into a vmp expression locus. Borrelia miyamotoi, phylogenetically grouped with RFB, has the genetic makeup for antigenic variation, but it has not been determined whether B. miyamotoi can create new variant serotypes in vivo. We inoculated mice with a non-clonal parental B. miyamotoi CT13-2396 strain with a known Vmp majority serotype with spirochete isolation at various days post-infection. The vmp that determined the reisolated variant serotype was identified by PCR of the expression locus followed by DNA sequencing of the amplified product. For each mouse reisolate, new variants replaced the parent majority serotype. Moreover, some mice produced additional variant reisolates days apart, indicative of the presentation seen in relapsing fever infections. Infection of mice with a clonal population resulted in the elimination of the inoculated serotype and isolation of new variants. Mouse serum obtained following infection revealed IgM antibodies reactive to the parent Vmp serotype, suggesting that the immune response eliminated or greatly reduced the majority population. These results demonstrated that B. miyamotoi reisolated from infected mice exhibited serotype populations differing from the inoculated strain, indicating the spirochetes underwent antigenic variation to evade the host's immune response. However, whether the observed variation occurred by way of outgrowth of minority populations or by translocation of archived pseudogenes to the expression locus creating new variants awaits further study.