Antigenic variation by Borrelia hermsii occurs through recombination between extragenic repetitive elements on linear plasmids

Abstract

SummaryThe relapsing fever agent Borrelia hermsii undergoes multiphasic antigenic variation through gene conversion of a unique expression site on a linear plasmid by an archived variable antigen gene. To further characterize this mechanism we assessed the repertoire and organization of archived variable antigen genes by sequencing ~ 85% of plasmids bearing these genes. Most archived genes shared with the expressed gene a £ 62 nucleotide (nt) region, the upstream homology sequence (UHS), that surrounded the start codon. The 59 archived variable antigen genes were arrayed in clusters with 13 repetitive, 214 nt long downstream homology sequence (DHS) elements distributed among them. A fourteenth DHS element was downstream of the expression locus. Informative nucleotide polymorphisms in UHS regions and DHS elements were applied to the analysis of the expression site of relapse serotypes from 60 infected mice in a prospective study. For most recombinations, the upstream crossover occurred in the UHS's second half, and the downstream crossover was in the DHS's second half. Usually the closest archival DHS element was used, but occasionally a more distant DHS was employed. The downstream extragenic crossover site in B. hermsii contrasts with the downstream extragenic crossover site for antigenic variation in African trypanosomes.

Pathogen escape from host immunity by a genome program for antigenic variation

“…Second, we have shown that UHS and DHS provide the 5Ј and 3Ј sites for recombination between the expression site and a silent locus (23), thus linking our observations on serotype frequencies to specific features of the genome. The demonstration here that homology between the protein coding regions at the expression and silent sites has little effect on switching suggests to us the action of site-specific recombination or an illegitimate recombination process, alone or in combination with general homologous recombination, as may be the case in Trypanosoma brucei (33).…”

“…We inoculated groups of inbred mice with small numbers of one of two serotypes, 7 and 17, which have, respectively, a copy of vlp7 or vlp17 in the expression site (23). That initial serotype proliferated in the blood until day 5 or 6, at which point the host's adaptive immune response rapidly cleared the infecting serotype from the blood (19).…”

“…The DNA sequence-based study of relapses is described in Dai et al (23). In brief, we infected mice with 1-10 spirochetes of serotype 7 or 17.…”

“…The molecular event underlying the majority of switches in surface proteins is a nonreciprocal recombination involving all or nearly all of a variant gene at a single expression site by one of several variant genes at silent loci on linear plasmids (20)(21)(22). We identified most of the silent variants that comprise the antigenic repertoire of B. hermsii, located 59 silent loci on several linear plasmids in the bacterium, and described the 5Ј and 3Ј flanking sequences (23). Fig.…”

“…The vlp genes are further divided into four subfamilies: ␣, ␤, ␥, and ␦ (22,25). Pair-wise differences in amino acid sequences range from 21% to 49% (mean of 37%) for 12 Vsp lipoproteins and 14% to 82% (mean of 65%) for 15 Vlp lipoproteins (23). Most variant genes are arrayed on the same strand in a given plasmid, but some are oppositely oriented.…”