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Inactivation of genes for antigenic variation in the relapsing fever spirochete Borrelia hermsii reduces infectivity in mice and transmission by ticks.

Raffel SJ, Battisti JM, Fischer RJ, Schwan TG - PLoS Pathog. (2014)

Bottom Line: The mutant lacking a Vmp constitutively produced Vtp, was attenuated in mice, produced lower cell densities in blood, and was unable to relapse in animals after its initial spirochetemia.This mutant also colonized ticks and was infectious by tick-bite, but remained attenuated compared to wild-type and reconstituted spirochetes.Thus the ability of B. hermsii to produce Vmps prolonged its survival in blood, while the synthesis of Vtp was essential for mammalian infection by the bite of its tick vector.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Zoonotic Pathogens, Rocky Mountain Laboratories, National Institute of Allergy and Infectious Disease, National Institutes of Health, Hamilton, Montana, United States of America.

ABSTRACT
Borrelia hermsii, a causative agent of relapsing fever of humans in western North America, is maintained in enzootic cycles that include small mammals and the tick vector Ornithodoros hermsi. In mammals, the spirochetes repeatedly evade the host's acquired immune response by undergoing antigenic variation of the variable major proteins (Vmps) produced on their outer surface. This mechanism prolongs spirochete circulation in blood, which increases the potential for acquisition by fast-feeding ticks and therefore perpetuation of the spirochete in nature. Antigenic variation also underlies the relapsing disease observed when humans are infected. However, most spirochetes switch off the bloodstream Vmp and produce a different outer surface protein, the variable tick protein (Vtp), during persistent infection in the tick salivary glands. Thus the production of Vmps in mammalian blood versus Vtp in ticks is a dominant feature of the spirochete's alternating life cycle. We constructed two mutants, one which was unable to produce a Vmp and the other was unable to produce Vtp. The mutant lacking a Vmp constitutively produced Vtp, was attenuated in mice, produced lower cell densities in blood, and was unable to relapse in animals after its initial spirochetemia. This mutant also colonized ticks and was infectious by tick-bite, but remained attenuated compared to wild-type and reconstituted spirochetes. The mutant lacking Vtp also colonized ticks but produced neither Vtp nor a Vmp in tick salivary glands, which rendered the spirochete noninfectious by tick bite. Thus the ability of B. hermsii to produce Vmps prolonged its survival in blood, while the synthesis of Vtp was essential for mammalian infection by the bite of its tick vector.

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Diagram of the vtp locus in the wild-type, Δvtp mutant and reconstituted strain vtp+R.The vtp gene in the wild-type (WT) B. hermsii DAH was inactivated by deleting most of the gene and replacing it with the flgBp-kan cassette by homologous recombination, creating the Δvtp mutant. The reconstituted strain (vtp+R) was constructed by replacing the Δvtp mutation with the flaBp-aacC1 cassette next to an intact vtp gene, also by homologous recombination. Details were described previously [28].
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ppat-1004056-g010: Diagram of the vtp locus in the wild-type, Δvtp mutant and reconstituted strain vtp+R.The vtp gene in the wild-type (WT) B. hermsii DAH was inactivated by deleting most of the gene and replacing it with the flgBp-kan cassette by homologous recombination, creating the Δvtp mutant. The reconstituted strain (vtp+R) was constructed by replacing the Δvtp mutation with the flaBp-aacC1 cassette next to an intact vtp gene, also by homologous recombination. Details were described previously [28].

Mentions: When wild-type spirochetes are acquired by a feeding tick, the bacteria gradually down-regulate bloodstream Vmps and up-regulate Vtp. During persistent infection in the tick salivary glands, the spirochetes exclusively produce Vtp but quickly switch back to the Vmp phenotype when reintroduced to mammalian blood. Given the rapid temporal switch of B. hermsii from Vtp to Vlp7 during mammalian infection by tick-bite [23], and the ongoing production of Vtp by the Vmp− mutant shown above, we tested an isogenic strain of the spirochete in which vtp was deleted (Δvtp)(Fig. 10, modified from Battisti et al.) [28], which rendered the spirochete unable to produce Vtp (Fig. 11). Synthesis of Vtp during persistent infection of the tick salivary glands suggested that this protein might be required by B. hermsii for migration to the salivary glands, stable tick infection, or initial colonization in mammals. To test these hypotheses, we used genetically transformed spirochetes described previously [28], in which the vtp gene was inactivated (Δvtp) in B. hermsii and the mutant reconstituted with the wild-type vtp gene (vtp+R). Battisti and colleagues previously showed that the Δvtp mutant remained infectious in mice by needle inoculation [28], as these spirochetes continued to produce bloodstream Vmps in vitro. In this study we compared these isogenic spirochetes with wild-type B. hermsii in experimental infections with O. hermsi ticks and mice.


Inactivation of genes for antigenic variation in the relapsing fever spirochete Borrelia hermsii reduces infectivity in mice and transmission by ticks.

Raffel SJ, Battisti JM, Fischer RJ, Schwan TG - PLoS Pathog. (2014)

Diagram of the vtp locus in the wild-type, Δvtp mutant and reconstituted strain vtp+R.The vtp gene in the wild-type (WT) B. hermsii DAH was inactivated by deleting most of the gene and replacing it with the flgBp-kan cassette by homologous recombination, creating the Δvtp mutant. The reconstituted strain (vtp+R) was constructed by replacing the Δvtp mutation with the flaBp-aacC1 cassette next to an intact vtp gene, also by homologous recombination. Details were described previously [28].
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3974855&req=5

ppat-1004056-g010: Diagram of the vtp locus in the wild-type, Δvtp mutant and reconstituted strain vtp+R.The vtp gene in the wild-type (WT) B. hermsii DAH was inactivated by deleting most of the gene and replacing it with the flgBp-kan cassette by homologous recombination, creating the Δvtp mutant. The reconstituted strain (vtp+R) was constructed by replacing the Δvtp mutation with the flaBp-aacC1 cassette next to an intact vtp gene, also by homologous recombination. Details were described previously [28].
Mentions: When wild-type spirochetes are acquired by a feeding tick, the bacteria gradually down-regulate bloodstream Vmps and up-regulate Vtp. During persistent infection in the tick salivary glands, the spirochetes exclusively produce Vtp but quickly switch back to the Vmp phenotype when reintroduced to mammalian blood. Given the rapid temporal switch of B. hermsii from Vtp to Vlp7 during mammalian infection by tick-bite [23], and the ongoing production of Vtp by the Vmp− mutant shown above, we tested an isogenic strain of the spirochete in which vtp was deleted (Δvtp)(Fig. 10, modified from Battisti et al.) [28], which rendered the spirochete unable to produce Vtp (Fig. 11). Synthesis of Vtp during persistent infection of the tick salivary glands suggested that this protein might be required by B. hermsii for migration to the salivary glands, stable tick infection, or initial colonization in mammals. To test these hypotheses, we used genetically transformed spirochetes described previously [28], in which the vtp gene was inactivated (Δvtp) in B. hermsii and the mutant reconstituted with the wild-type vtp gene (vtp+R). Battisti and colleagues previously showed that the Δvtp mutant remained infectious in mice by needle inoculation [28], as these spirochetes continued to produce bloodstream Vmps in vitro. In this study we compared these isogenic spirochetes with wild-type B. hermsii in experimental infections with O. hermsi ticks and mice.

Bottom Line: The mutant lacking a Vmp constitutively produced Vtp, was attenuated in mice, produced lower cell densities in blood, and was unable to relapse in animals after its initial spirochetemia.This mutant also colonized ticks and was infectious by tick-bite, but remained attenuated compared to wild-type and reconstituted spirochetes.Thus the ability of B. hermsii to produce Vmps prolonged its survival in blood, while the synthesis of Vtp was essential for mammalian infection by the bite of its tick vector.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Zoonotic Pathogens, Rocky Mountain Laboratories, National Institute of Allergy and Infectious Disease, National Institutes of Health, Hamilton, Montana, United States of America.

ABSTRACT
Borrelia hermsii, a causative agent of relapsing fever of humans in western North America, is maintained in enzootic cycles that include small mammals and the tick vector Ornithodoros hermsi. In mammals, the spirochetes repeatedly evade the host's acquired immune response by undergoing antigenic variation of the variable major proteins (Vmps) produced on their outer surface. This mechanism prolongs spirochete circulation in blood, which increases the potential for acquisition by fast-feeding ticks and therefore perpetuation of the spirochete in nature. Antigenic variation also underlies the relapsing disease observed when humans are infected. However, most spirochetes switch off the bloodstream Vmp and produce a different outer surface protein, the variable tick protein (Vtp), during persistent infection in the tick salivary glands. Thus the production of Vmps in mammalian blood versus Vtp in ticks is a dominant feature of the spirochete's alternating life cycle. We constructed two mutants, one which was unable to produce a Vmp and the other was unable to produce Vtp. The mutant lacking a Vmp constitutively produced Vtp, was attenuated in mice, produced lower cell densities in blood, and was unable to relapse in animals after its initial spirochetemia. This mutant also colonized ticks and was infectious by tick-bite, but remained attenuated compared to wild-type and reconstituted spirochetes. The mutant lacking Vtp also colonized ticks but produced neither Vtp nor a Vmp in tick salivary glands, which rendered the spirochete noninfectious by tick bite. Thus the ability of B. hermsii to produce Vmps prolonged its survival in blood, while the synthesis of Vtp was essential for mammalian infection by the bite of its tick vector.

Show MeSH
Related in: MedlinePlus