Immune evasion by tickborne and host-adapted Borrelia burgdorferi.

Immune sera from mice infected with the Lyme disease spirochete, Borrelia burgdorferi, have strong biologic activity against spirochetes cultured in vitro. Recent studies with rodents and ticks infected with B. burgdorferi indicate that spirochetes undergo major changes in protein expression as they adapt to the diverse environments encountered by a vectorborne pathogen. The purpose of this study was to explore the susceptibility of three different adaptive forms of B. burgdorferi (in vitro cultured, host-derived, and tickborne) to immune sera. Passive transfer of immune sera protected mice when they were challenged with spirochetes cultured in vitro. Immune sera did not protect mice from tickborne spirochetes or spirochetes derived from infected mice. These results indicate that spirochetes that have adapted within either the feeding tick or host are relatively invulnerable to the protective effects of immune sera, unlike spirochetes grown in vitro, which are highly susceptible.