Rebecca E LaFond

Swedish Medical Center Seattle, Seattle, Washington, United States

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Publications (6)33.62 Total impact

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    ABSTRACT: The tprK gene in the syphilis spirochete, Treponema pallidum subsp. pallidum, undergoes antigenic variation in seven variable (V) regions. tprK is highly variable within T. pallidum strains, and a method has been developed to derive clones of T. pallidum that express a single, unique tprK sequence. Rabbits were infected with three different T. pallidum clones or the parent strain from which the clones were derived, and their sera were examined by immunoassay for antibody reactivity against synthetic peptides representing the TprK V regions from each clone. The parent strain expresses many different V region sequences, and infection with this strain induced antibody responses against a wide variety of V regions. In rabbits infected with the Chicago C clone, antibodies developed against all of the V regions except V1, while antibodies developed against only V5, V6, and V7 in Chicago A-infected rabbits. During Chicago B infection, antibodies developed against all of the V regions except V1 and V3. Antibodies were highly specific for the V regions of the infecting clone, and cross-reactivity was rare. The demonstration that the V regions elicit a variant-specific antibody response supports the hypothesis that TprK variants may help organisms to avoid the developing immune response in infected individuals, contributing to the ability of T. pallidum to establish chronic infection.
    Infection and Immunity 12/2006; 74(11):6244-51. · 4.07 Impact Factor
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    ABSTRACT: The tprK gene in Treponema pallidum undergoes antigenic variation. In all T. pallidum isolates examined to date, except the Nichols type strain, heterogeneous tprK sequences have been identified. This heterogeneity is localized to seven variable (V) regions, and tprK sequence diversity accumulates with serial passage in naïve rabbits. The T. pallidum Nichols genome described a single tprK sequence, and after decades of independent passage, only minor tprK sequence diversity is seen among the Nichols strains from different laboratories. We hypothesized that T. pallidum Nichols is capable of only limited tprK diversification. To address this hypothesis, we passaged the T. pallidum Nichols strain in naïve rabbits at the peak of infection (rapid passage) or after the adaptive immune response had cleared most organisms in vivo (slow passage). After 22 rapid passages (9- to 10-day intervals), no tprK V region sequence changes were observed. In contrast, after two slow passages (30- to 35-day intervals), three V regions had sequences that were completely different from that of the original inoculum. New sequences were observed in all seven V regions by the fifth slow passage. In contrast to the rapid-passaged Nichols strain, rapid-passaged Chicago C, a clonal strain isolated from the highly diverse parent Chicago strain, developed significant tprK diversification. These findings suggest that tprK variation can occur, but at a lower rate, in Nichols and that immune pressure may be required for accumulation of bacteria with diverse tprK sequences. Adaptation to growth in rabbits may explain the limited repertoire of V region sequences seen in the Nichols strain.
    Infection and Immunity 04/2006; 74(3):1896-906. · 4.07 Impact Factor
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    Rebecca E Lafond, Sheila A Lukehart
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    ABSTRACT: Syphilis is a chronic sexually transmitted disease caused by Treponema pallidum subsp. pallidum. Clinical manifestations separate the disease into stages; late stages of disease are now uncommon compared to the preantibiotic era. T. pallidum has an unusually small genome and lacks genes that encode many metabolic functions and classical virulence factors. The organism is extremely sensitive to environmental conditions and has not been continuously cultivated in vitro. Nonetheless, T. pallidum is highly infectious and survives for decades in the untreated host. Early syphilis lesions result from the host's immune response to the treponemes. Bacterial clearance and resolution of early lesions results from a delayed hypersensitivity response, although some organisms escape to cause persistent infection. One factor contributing to T. pallidum's chronicity is the paucity of integral outer membrane proteins, rendering intact organisms virtually invisible to the immune system. Antigenic variation of TprK, a putative surface-exposed protein, is likely to contribute to immune evasion. T. pallidum remains exquisitely sensitive to penicillin, but macrolide resistance has recently been identified in a number of geographic regions. The development of a syphilis vaccine, thus far elusive, would have a significant positive impact on global health.
    Clinical Microbiology Reviews 02/2006; 19(1):29-49. · 17.31 Impact Factor
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    ABSTRACT: The tprK gene sequence of Treponema pallidum subspecies pallidum (T. pallidum) is heterogeneous within and among isolates. Heterogeneity in the tprK open reading frame is localized in seven discrete variable (V) regions, and variability results from apparent base changes, insertions or deletions. The TprK V regions are the focus of anti-TprK antibodies arising during infection. To test our hypothesis that V region sequences change during infection and passage, we developed a clonal isolate from the Chicago strain of T. pallidum and confirmed V region diversification during passage of this isolate. We describe the sequence anatomy of the seven V regions of tprK and the identification of putative donor sites for new V region sequences, and we propose a model for generation of new V regions by segmental gene conversion. These findings suggest that antigenic variation of TprK occurs in T. pallidum and may be important in immune evasion and persistence.
    Molecular Microbiology 07/2004; 52(6):1579-96. · 4.96 Impact Factor
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    ABSTRACT: The tprK gene of Treponema pallidum subsp. pallidum, the causative agent of venereal syphilis, belongs to a 12-member gene family and encodes a protein with a predicted cleavable signal sequence and predicted transmembrane domains. Except for the Nichols type strain, all rabbit-propagated isolates of T. pallidum examined thus far are comprised of mixed populations of organisms with heterogeneous tprK sequences. We show that tprK sequences in treponemes obtained directly from syphilis patients are also heterogeneous. Clustering analysis demonstrates that primary chancre tprK sequences are more likely to cluster within a sample than among samples and that tighter clustering is seen within chancre samples than within rabbit-propagated isolates. Closer analysis of tprK sequences from a rabbit-propagated isolate reveals that individual variable regions have different levels of diversity, suggesting that variable regions may have different intrinsic rates of sequence change or may be under different levels of selection. Most variable regions show increased sequence diversity upon passage. We speculate that the diversification of tprK during infection allows organisms to evade the host immune response, contributing to reinfection and persistent infection.
    Journal of Bacteriology 12/2003; 185(21):6262-8. · 3.19 Impact Factor
  • Rebecca E. (Rebecca Elaine) LaFond
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    ABSTRACT: Thesis (Ph. D.)--University of Washington, 2006. The tprK gene of Treponema pallidum subsp, pallidum, the causative agent of venereal syphilis, belongs to a twelve-member gene family and encodes a protein with a predicted cleavable signal sequence and predicted transmembrane domains. All rabbit-propagated strains of T. pallidum examined thus far are comprised of mixed populations of organisms with heterogeneous tprK sequences. Heterogeneity in the tprK ORF is localized in seven discrete variable (V) regions, and variability is due to apparent base changes, insertions, or deletions. Individual variable regions have different levels of diversity, suggesting that variable regions may have different intrinsic rates of sequence change or may be under different levels of selection. As in rabbit-propagated strains, tprK sequences in treponemes obtained directly from syphilis patients are heterogeneous, and tprK sequences within a strain are more related than sequences among strains. We propose a model for generation of new V regions by segmental gene conversion, and show that low-diversity strains of T. pallidum derived from a diverse parent strain gain sequence variability in the tprK V regions when they are serially propagated in naive rabbits. The T. pallidum Nichols type strain that has been propagated in rabbits since the early 1900's and has minimal tprK variability, accumulates a low level of diversity in tprK, but only after treponemes have been exposed to the host immune response. Because the TprK V regions are the focus of anti-TprK antibodies arising during infection, the fact that tprK in Nichols changes only after organisms have been exposed to host immunity suggests that the developing adaptive antibody response selects against organisms expressing the original TprK sequence. Antibodies from rabbits infected with clonal T. pallidum strains expressing unique TprK sequences are highly specific for the V regions of the infecting clone, and antibody cross-reactivity is rare. We conclude from these findings that the tprK gene in T. pallidum undergoes antigenic variation. The diversification of tprK during infection may allow organisms to evade the antibody response mounted by the host, contributing to the ability of T. pallidum to re-infect previously infected individuals and establish persistent infection.

Publication Stats

185 Citations
33.62 Total Impact Points

Institutions

  • 2006
    • Swedish Medical Center Seattle
      Seattle, Washington, United States
  • 2003–2006
    • University of Washington Seattle
      • Department of Medicine
      Seattle, WA, United States