OspA antibodies inhibit the acquisition of Borrelia burgdorferi by Ixodes ticks.

Ixodes ticks are infected by Borrelia burgdorferi when larvae feed on spirochete-infected mice. We studied the acquisition of B. burgdorferi by larval ticks, characterized the production of outer surface protein A (OspA) by spirochetes entering larvae, and examined the effects of OspA antibodies on the establishment of B. burgdorferi infections in ticks. Most larvae were infected by spirochetes 24 to 48 h after placement on mice. OspA antibodies stained the first spirochetes observed in larvae, suggesting that OspA is synthesized early during the colonization of the vector. When OspA antibodies were administered to B. burgdorferi-infected mice and larvae were then placed on the animals, the severity of larval infection and the number of infected ticks (7 of 16) were decreased compared with that of controls (15 of 16). The inhibitory effects of OspA antibodies were observed with passive antibody transfer as well as active host-generated immunity. The lower larval infection rate observed in the presence of OspA antibodies was exacerbated after the larval molt since only 1 of 12 nymphs was infected, and none of the mice that were fed upon by these nymphs became infected with B. burgdorferi. Therefore, an OspA antibody response in mice altered the reservoir competence of the vertebrate host by inhibiting the movement of B. burgdorferi from the host to the vector.