Subtelomeric expression regions of Borrelia hermsii linear plasmids are highly polymorphic

Abstract

Borrelia hermsii, a relapsing fever agent, undergoes multiphasic antigenic variation to evade its host's immune response. Serotype specificity is determined by variable membrane lipoproteins, Vmps, which are expressed from genes located near the end of a linear plasmid. Using the polymerase chain reaction and primers representing the promoter of the active vmp and a conserved telomeric sequence, we characterized the subtelomeric expression regions of the 25 known serotypes of strain HS1. The distance from the promoter to the telomere fell into three size classes of approximately 1.0, 1.5, and 2.5 kilobases. In the sequenced serotypes the size differences were accounted for by variable lengths of the vmp genes and intervening sequences between 3' end of the vmp gene and the start of a downstream homology block. The degree of nucleotide identity between different vmp genes, or between the different 3' flanking DNA varied from 39-78%. Thus, there is length and sequence variability not only between vmp genes themselves but also between the 3' flanking regions of vmp genes.

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

“…Although the set compositions were similar, we noted that serotype 7, which was the most prevalent serotype in relapses by antibody-typing, was only the fourth most common by PCRtyping. Serotype 7 numbers may be underestimated by PCR for the complete expression site of all mouse serotypes, because the amplification product in this case would be a lengthy 2.3 kb instead of the 1.0-1.5 kb for almost all other serotypes (22,31). To circumvent this possible bias, we developed PCR assays that were specific for serotype 7 and, as a control, serotype 17.…”

“…Serotypes 7 and 19 were cloned by limiting dilution and then propagated in adult female CB17 scid mice (Charles River Laboratories, Wilmington, MA). Serotype identities were confirmed by sequencing of the expression site as described (22). Cells were counted in a Petroff-Hausser counting chamber by phase-contrast microscopy.…”

“…The frequency of new variants in expanding B. hermsii populations either in animals or in culture medium is 10 Ϫ4 to 10 Ϫ3 (19). 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).…”

“…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).…”

Structural Conservation of Neurotropism-associated VspA within the Variable Borrelia Vsp-OspC Lipoprotein Family

“…Vlp and Vsp proteins diverge even more, with 40 to 80% amino acid identities among them (17,28). In contrast to the single plasmid-encoded ospC of B. burgdorferi (29), several archival copies of B. hermsii vsp and vlp genes are maintained on linear plasmids and sequentially expressed from a promoter site after gene conversions or DNA rearrangements (21,30,31). The resulting multiphasic antigenic variation of Vsps and Vlps allows the spirochete to repeatedly evade the host's immune response, which leads to recurrent spirochetemia and the characteristic febrile episodes (16).…”