Versus Tick-Associated Variants of a Relapsing Fever Bacterium
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Bloodstream- Versus Tick-Associated Variants serotype transmitted to the mice (10). Eigh- teen of 95 ticks (19%) transmitted spiro- of a Relapsing Fever Bacterium chetes, and in every infection, the first de- tectable spirochetemia in the mice after tick- Tom G. Schwan* and B. Joseph Hinnebusch bite consisted of the same serotype ingested previously by the ticks, whether this was The relapsing fever spirochete, Borrelia hermsii, alternates infections between a mammal serotype 7 or serotype 8. In addition, poly- and a tick vector. Whether the spirochete changes phenotypically in the different hosts merase chain reaction analysis of the spiro- was examined by allowing the tick vector Ornithodoros hermsi to feed on mice infected chete’s vmp telomeric expression locus in 22 with serotype 7 or serotype 8 of B. hermsii. Upon infection of ticks, the spirochetal infected ticks (11 ticks with serotype 7 and serotype-specific variable major proteins (Vmps) 7 and 8 became undetectable and were 11 ticks with serotype 8) revealed no appar- replaced by Vmp33. This switch from a bloodstream- to tick-associated phenotype could ent DNA rearrangement (11). These obser- be induced in culture by a decrease in temperature. After tick-bite transmission back to vations indicated that the vmp gene present mice, the process was reversed and the spirochetes resumed expression of the same at the telomeric expression site did not Vmp present in the previous infectious blood meal. change during passage through ticks. To determine if spirochetes expressed the telomeric vmp gene in the tick, we infected additional cohorts of ticks with either sero- Relapsing fever was recognized as a human intraplasmidic recombination occurs, some- type 7 or serotype 8 and allowed the ticks to disease back in the time of Hippocrates (1). times augmented by the introduction of molt to the next developmental stage (12). Endemic relapsing fever is caused by numer- point mutations in rearranged vmp genes From 33 to 144 days after infection, we ous species of the spirochete Borrelia and (5, 6). Each population of spirochetes asso- examined 41 ticks, including 23 infected occurs throughout the world in many dis- ciated with a single acute episode is com- with serotype 7 and 18 infected with sero- crete enzootic foci where the spirochetes posed almost entirely of one serotype pro- type 8. Salivary glands from all ticks and a are maintained primarily in rodents and ducing the same Vmp (7). Different sero- lesser number of midgut (n 5 33) and syn- ticks of the genus Ornithodoros (2). In west- types predominate in subsequent popula- ganglion (n 5 22) preparations were exam- ern North America, B. hermsii, which is tions, and this multiphasic antigenic ined by indirect immunofluorescence assay transmitted by the tick O. hermsi, causes variation presumably allows evasion from (IFA) with one of four antibodies (13). Al- human disease that is likely more prevalent the mammalian immune response (8). Yet though all tissues were infected, we were than the number of reported cases (3). Hu- nothing is known about the influence of the unable to detect spirochetes in the salivary man infections result from the bites of these spirochete’s serotype on infection in the glands using antibodies specific for Vmp7 fast-feeding ticks, which usually occur at arthropod host or whether spirochetes (10 examined) (Fig. 1C) or Vmp8 (5 exam- night and unbeknownst to their victims. change phenotypically as they alternate be- ined). Yet, when we examined the other The periodic cycling of acute and afebrile tween ticks and mammals. salivary gland from these same ticks and episodes that led to the naming of relapsing To examine if ticks transmit the same others from the same infected cohort by IFA fever is associated with dramatic changes in serotype of the relapsing fever spirochete as with a monoclonal antibody to Vmp33 the abundance of spirochetes circulating in was acquired during a previous blood meal, (marker of serotype C), a surface protein the patient’s blood (spirochetemia) (1). Ac- we first infected two cohorts of nymphal O. seen previously only in the culture-adapted companying each of these cyclic population hermsi ticks with either serotype 7 or serotype strain HS1 (14, 15), we detected many flu- changes in the number of spirochetes is a 8ofB. hermsii (Fig. 1, A and B) (9). The orescent spirochetes (Fig. 1, D and E). This change in serotype and major immunogenic ticks were allowed to molt and were then fed was true for ticks infected with either sero- lipoprotein associated with the outer surface singly on individual mice to determine the type 7 (n 5 17) or 8 (n 5 12). When of the spirochete. These variable major pro- teins (Vmps) are composed of two multigene families that have recently been renamed as Fig. 1. Borrelia hermsii switches variable large or small proteins (Vlps and between bloodstream-specific and tick-specific outer surface proteins. Vsps) (4); however, here we refer to them (A) Serotype 7 in mouse blood visu- collectively as Vmps. alized with antibody to Vmp7 (anti- In the HS1 strain of B. hermsii, the single Vmp7). (B) Serotype 8 in mouse vmp gene being expressed at any one time is blood visualized with anti-Vmp8. located in an expression locus near the (C) Spirochetes in a salivary gland telomere of a 28-kb linear plasmid (5). A of a tick that ingested serotype 7 is copy of the expressed gene and all other not detectable with anti-Vmp7. (D) unexpressed vmp genes are present in silent Spirochetes in a salivary gland of a loci on the same or different linear plas- tick that ingested serotype 7 visual- mids. Antigenic switching to a new sero ized with anti-Vmp33. (E) Spiro- - chetes in a salivary gland of a tick type occurs when one of the silent vmp that ingested serotype 8 visualized genes is duplicated and replaces the existing with anti-Vmp33. (F) Serotype 7 in vmp gene in the expression locus, or when mouse blood visualized with anti- Vmp7 after transmission by tick Laboratory of Microbial Structure and Function, Rocky bite. Spirochetes were not detect- Mountain Laboratories, National Institute of Allergy and able in blood with anti-Vmp33. Infectious Diseases, National Institutes of Health, Hamil- 5 m ton, MT 59840, USA. (Bar 25 M). *To whom correspondence should be addressed. E-mail: [email protected] 1938 SCIENCE z VOL. 280 z 19 JUNE 1998 z www.sciencemag.org REPORTS REFERENCES AND NOTES peripheral blood was examined during the identified vmp33 in 23 additional isolates of ___________________________ first spirochetemia in mice only 4 days after B. hermsii from western North America 1. O. Felsenfeld, Borrelia: Strains, Vectors, Human and infection by tick bite, bacteria were visual- (22). Also, genes homologous to vmp33 and Animal Borreliosis (Green, St. Louis, MO, 1971). ized by IFA with anti-Vmp7 (Fig. 1F) and proteins antigenically related to Vmp33 2. T. G. Schwan, W. Burgdorfer, P. A. Rosa, in Manual anti-Vmp8 but not with anti-Vmp33. have been identified in many other species of Clinical Microbiology, P. R. Murray, Ed. (American Society for Microbiology, Washington, DC, 1995), Therefore, as these spirochetes cycle be- of Borrelia, including outer surface protein pp. 626–635. tween ticks and mammals, their outer surface (Osp) C of the Lyme disease spirochete 3. M. S. Dworkin et al., Clin. Infect. Dis. 26, 122 (1998). alternates between bloodstream- and tick- Borrelia burgdorferi (15, 23). Therefore, this 4. P. A. Barstad, J. E. Coligan, M. G. Raum, A. G. Barbour, J. Exp. Med. 161, 1302 (1985); D. Cadavid, associated proteins. family of proteins appears to be conserved P. M. Pennington, T. A. Kerentseva, S. Bergstro¨m, One stimulus for such a switch might be among all members of the genus Borrelia. A. G. Barbour, Infect. Immun. 65, 3352 (1997). the change in temperature as the spiro- The temporal expression of vmp33 and 5. T. Kitten and A. G. Barbour, Proc. Natl. Acad. Sci. U.S.A. 87, 6077 (1990). chetes are transferred from a warm-blooded ospC by B. hermsii and B. burgdorferi, respec- 6. J. T. Meier, M. I. Simon, A. G. Barbour, Cell 41, 403 mammal to a much cooler tick. We tested tively, implicates a common biological func- (1985); R. H. A. Plasterk, M. I. Simon, A. G. Barbour, this hypothesis with blood from a mouse tion for these proteins associated with tick Nature 318, 257 (1985); B. I. Restrepo and A. G. infected with serotype 8 that we cultured at transmission or early colonization in mam- Barbour, Cell 78, 867 (1994); B. I. Restrepo, C. J. Carter, A. G. Barbour, Mol. Microbiol. 13, 287 (1994). 37° or 23°C (16). SDS–polyacrylamide gel mals. Borrelia hermsii produce Vmp33 when 7. H. G. Stoenner, T. Dodd, C. Larsen, J. Exp. Med. electrophoresis (SDS-PAGE) analysis of the temperature cools after their acquisition 156, 1297 (1982). whole-cell lysates (17, 18) indicated that by ticks and continue to produce this protein 8. P. M. Southern and J. P. Sanford, Medicine 48, 129 (1968); A. G. Barbour and S. F. Hayes, Microbiol. growth at 23°C induced expression of during persistent infection of the tick’s sali- Rev. 50, 381 (1986). vmp33, whereas growth at 37°C maintained vary glands until they are rapidly transmitted 9. Ticks were from a spirochete-free colony of O. hermsi the expression of vmp8 (Fig. 2A). Immuno- by Ornithodoros ticks that feed in 15 to 90 maintained at the Rocky Mountain Laboratories. The DAH strain of B. hermsii originated from a relapsing blot analysis with specific antibodies con- min. In contrast, most Lyme disease spiro- fever patient from eastern Washington state.