Borrelia burgdorferi, the spirochetal agent of Lyme disease, is transmitted by Ixodes ticks. A vaccine
based on B. burgdorferi outer surface protein (Osp) A protects mice from spirochete infection.
Here we report on the expression of OspA on spirochetes inside engorging ticks and relate
OspA expression to antispirochetal immunity. Spirochetes in the gut of unfed nymphal
ticks were stained by an OspA antibody, whereas in feeding ticks, the majority of spirochetes in
the gut and salivary glands did not stain with the antibody. Thus, OspA was not expressed on
most spirochetes during transmission from the vector to the vertebrate host. To examine the
mechanism of protection afforded by OspA antibody, mice were passively immunized with
OspA antibody at different times relative to tick attachment. When OspA antibody was administered
to mice before or at the time of tick attachment, spirochetal development events in the
vector, such as growth and salivary gland invasion, were blocked and the mice were protected
from B. burgdorferi infection. When OspA antibody was administered to mice 48 h after tick attachment,
spirochetes persisted in the nymphs and the mice were not protected despite the
presence of circulating antibodies in the host as well as in the tick blood meal. Thus, OspA immunity
appears to be effective only during a narrow window time at the beginning of the
blood meal when antibodies bind to OspA-expressing spirochetes in the tick gut and block
transmission from the vector to the host