D-099. Infected Mice Produce Antibodies against the Variable Tick Protein of Borrelia hermsii, a Relapsing Fever Agent
Introduction: The relapsing fever agent Borrelia hermsii has two expression sites for major surface proteins: one is active during vertebrate infections, and another, which encodes the Vtp protein, was thought to be active only in the tick vector. However, Vtp proteins differ substantially in sequence between strains, an indication of positive selection. On the basis of this finding we hypothesized that Vtp is expressed during mammalian infection. Methods: To assess this, we determined whether two groups of 5 BALB/c mice infected with either of two serotypes, 7 or 19, of B. hermsii strain HS1 produced antibodies to Vtp. By microscopy, all mice had spirochetes in the blood by day 7. Sera was obtained from mice on day 42 and examined by two different assays at 1:200 dilutions: (1) a microagglutination assay with strain HS1 cells that expressed Vtp, and (2) a dot blot assay against recombinant Vtp proteins of two different strains, CC1 in addition to strain HS1, as well as the conserved GlpQ protein of B. hermsii and a negative control protein. Reactivity against a protein was reported as this index: log-transformed ratio of its scanned intensity value to the value for the negative control protein. Results: We cloned and sequenced the gene for Vtp of strain CC1 and determined that the deduced protein sequences differed by 29%. Infection of all mice was confirmed by antibody reactivity against GlpQ. In comparison to sera from uninfected mice, day 42 sera from all mice infected with either serotype 7 or 19 agglutinated Vtp-expressing B. hermsii as well as Vtp-less cells. Evidence that the agglutination was conferred by anti-Vtp antibodies that had strain-specific characteristics was provided by the blot assays. Mean (95% confidence intervals) antibody indices for the HS1 and CC1 Vtp proteins were 4.5 (2.7-6.4) and 1.8 (1.4-2.3), respectively, for 5 mice infected with serotype 7 and 3.6 (1.9-5.2) and 1.6 (1.2-2.1) for 5 mice infected with serotype 19. Conclusion: The findings indicate that Vtp is expressed during mammalian as well as tick infections and suggest that the high level of Vtp sequence diversity between strains is attributable in part to immune selection.
167/D. Streptococci, Enterococci and Staphylococci
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