Attachment of Borrelia burgdorferi within Ixodes scapularis mediated by outer surface protein A

IntroductionLyme disease is the most common arthropod-borne illness in the United States (1, 2).Outer surface protein (Osp) A elicits immunity against Borrelia burgdorferi infection and is being used as a human vaccine (3-6).OspA antibody blocks spirochete transmission to the vertebrate host by binding to B. burgdorferi within the gut of engorging Ixodes scapularis ticks (7-10).Phase III clinical trials demonstrate that vaccination with OspA provides 80-90% protection against Lyme disease, and vaccine failures are generally associated with low antibody responses to OspA in vaccine recipients (5, 6).The temporal expression of ospA during the B. burgdorferi life cycle implies that OspA has a function within the arthropod host.Uninfected larval ticks acquire spirochetes while feeding on infected mice.B. burgdorferi produce OspA soon after leaving the vertebrate host and entering I. scapularis, suggesting an important role for OspA in vector colonization (7-10).Within nymphal ticks, spirochetes abundantly express ospA and remain in close association with the tick gut.When nymphs engorge on a vertebrate host, B. burgdorferi in the gut rapidly multiply, and 70-80% of the organisms clear OspA from their surface (7).B. burgdorferi continues to repress ospA in the mammalian host.ospA mRNA is not readily detectable after tick-borne transmission of spirochetes to mice but can sometimes be found up to 14 days after syringe challenge of mice with OspA-producing B. burgdorferi (11)(12)(13)(14)(15). ospA is also not usually expressed during human infection.However, some individuals may develop humoral and cellular OspA responses in early-stage disease (16-19), and immune responses to OspA have been associated with chronic Lyme arthritis, indicating that this antigen is present in some patients (20,21).The selective expression of ospA when B. burgdorferi goes into and resides within I. scapularis and the subsequent general downregulation of ospA when spirochetes exit the tick gut suggest that OspA may play an important role in B. burgdorferi colonization of the arthropod vector. Methods B. burgdorferi and I. scapularis: culture and cultivationA clonal isolate of B. burgdorferi N40 that is infectious and pathogenic in mice, and represents a prototypic B. burgdorferi sensu stricto organism, was used throughout these studies (22).Spirochetes were cultivated in Barbour-Stoenner-Kelly (BSK) II medium at 33°C (23).B. burgdorferi 46047, a naturally occurring B. burgdorferi sensu stricto isolate that lacks OspA (24), was also used in some spirochete-OspA binding studies.Mated adult female I. scapularis were collected in the field.The egg mass was then laid in the laboratory.Hatched larvae were fed on uninfected C3H mice to produce pathogen-free nymphs.All tick rearing was performed in an incubator at 26°C with 85% relative humidity and a 12-hour light/dark photo period regimen.