BORRELIA-BURGDORFERI OSPA IS AN ARTHROPOD-SPECIFIC TRANSMISSION-BLOCKING LYME-DISEASE VACCINE

Borrelia burgdorferi, the spirochetal agent of Lyme disease, is transm itted by Ixodes ticks. A vaccine based on B. burgdorferi outer surface protein (Osp) A protects mice from spirochete infection. Here we repo rt on the expression of OspA on spirochetes inside engorging ticks and relate OspA expression to antispirochetal immunity. Spirochetes in th e 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 vertebr ate host. To examine the mechanism of protection afforded by OspA anti body, mice were passively immunized with OspA antibody at different ti mes relative to tick attachment. When OspA antibody was administered t o mice before or at the time of tick attachment, spirochetal developme nt events in the vector, such as growth and salivary gland invasion, w ere blocked and the mice were protected from B. burgdorferi infection. When OspA antibody was administered to mice 48 h after tick attachmen t, 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 o nly during a narrow window time at the beginning of the blood meal whe n antibodies bind to OspA-expressing spirochetes in the tick gut and b lock transmission from the vector to the host.