Genetic Variation at thevlsELocus ofBorrelia burgdorferiwithin Ticks and Mice over the Course of a Single Transmission Cycle

Abstract

The Lyme disease spirochete, Borrelia burgdorferi, causes a persistent infection in the vertebrate host even though infected animals mount an active immune response against the spirochete. One strategy used by the spirochete to evade vertebrate host immunity is to vary the structure and expression of outer membrane antigens. The vlsE locus represents the best-studied example of antigenic variation in B. burgdorferi. During vertebrate host infection, recombination between the active vlsE locus and silent, partial vlsE copies leads to gene conversion events and the generation of novel alleles at the expression site. In the present study, we followed a population of B. burgdorferi organisms moving through vertebrate host and tick stages to complete one transmission cycle. The major goal of the study was to determine if the vlsE locus was subject to different selective pressure and/or recombination frequency at different stages of the spirochete's life cycle. We report here that the vlsE genetic diversity generated within the rodent host was maintained through the larval and nymphal tick stages. Therefore, naturally infected ticks are likely to transmit spirochete populations with multiple vlsE alleles into naive vertebrate hosts. Although vlsE genetic diversity in mice was maintained through tick stages, the dominant vlsE alleles were different between tick stages as well as between individual ticks. We propose that population-level bottlenecks experienced by spirochetes, especially during the larvalto-nymphal molt, are responsible for individual infected ticks harboring different dominant vlsE alleles. Although vlsE genetic diversity is maintained through tick stages, the VlsE protein is unlikely to be of functional importance in the vector, because the protein was expressed by very few (<1%) bacteria in the vector.

Transcriptional Regulation of the Borrelia burgdorferi Antigenically Variable VlsE Surface Protein

“…Data from this report are consistent with previous studies that examined small parts of the infectious cycle. Ohnishi et al also noticed that the protein was expressed by very few (Ͻ1%) bacteria in midguts of unfed or partially fed nymphs (40). Indest et al observed some VlsE expression in flat nymph midguts but not after feeding commenced (23).…”

“…The spirochete synthesizes little or no VlsE when colonizing the midgut of a vector tick nymph (23,40) but does make that protein during mammalian infection (14,20,21,29,(31)(32)(33)35). However, prior to this study, the complete pattern of VlsE synthesis in vivo had not been examined.…”

“…During mammalian infection, but not during tick colonization or laboratory cultivation, segments of silent vls cassettes randomly recombine into the vlsE expression locus, continually creating novel vlsE gene sequences (23,40,58,59). The resulting variations in the VlsE protein sequence alter its antigenic properties, which apparently allows the bacterium to evade antibody-mediated clearance by the host.…”

Outer Surface Protein C Peptide Derived from Borrelia burgdorferi Sensu Stricto as a Target for Serodiagnosis of Early Lyme Disease

“…An ELISA (IR6) using a peptide derived from the VlsE protein of Borrelia has demonstrated greater specificity in the detection of Lyme disease than whole-cell ELISAs and has been approved for use by the FDA (19,23). However, C6, the peptide derived from VlsE, does not bind IgM particularly well, is derived from an antigen that is expressed only after infection is established (fewer than 1% of bacteria in the tick express VlsE, and transcription of the gene is suppressed prior to transmission of the bacteria), and the IR6 region of VlsE from which the peptide is derived has shown a greater degree of variability than originally thought (19,(23)(24)(25)(26). Though the IR6 assay represents a significant improvement, in terms of specificity, compared to the whole-cell ELISA (27), these concerns have precluded the use of the IR6 assay as a stand-alone diagnostic test for early Lyme disease.…”

Biological Aspects of Lyme Disease Spirochetes: Unique Bacteria of the Borrelia Burgdorferi Species Group

“…Nevertheless signals involved in recombination are unknown. It was found that the number of amino acid changes is signifi cantly higher in immunocompetent C3H mice than in SCID mice and this suggests an infl uence of selective pressure 114,115 .…”