Preferential Protection of Borrelia burgdorferi

Background. Ixodes ticks are the main vectors for Borrelia burgdorferi sensu lato. In the United States, B. burg-dorferi is the sole causative agent of Lyme borreliosis and is transmitted by Ixodes scapularis. In Europe, 3 Borrelia species—B. burgdorferi, B. garinii, and B. afzelii—are prevalent, which are transmitted by Ixodes ricinus. The I. scapu-laris salivary protein Salp15 has been shown to bind to B. burgdorferi outer surface protein (Osp) C, protecting the spirochete from antibody-mediated killing. Methods and results. We recently identified a Salp15 homologue in I. ricinus, Salp15 Iric-1. Here, we have demonstrated, by solid-phase overlays, enzyme-linked immunosorbent assay, and surface plasmon resonance, that Salp15 Iric-1 binds to B. burgdorferi OspC. Importantly, this binding protected the spirochete from antibody-mediated killing in vitro and in vivo; immune mice rechallenged with B. burgdorferi preincubated with Salp15 Iric-1 displayed significantly higher Borrelia numbers and more severe carditis, compared with control mice. Furthermore, Salp15 Iric-1 was capable of binding to OspC fromB. garinii andB. afzelii, but theseBorrelia species were not protected from antibody-mediated killing. Conclusions. Salp15 Iric-1 interacts with all European Borrelia species but differentially protects B. burgdorferi from antibody-mediated killing, putatively giving this Borrelia species a survival advantage in nature. Ticks are ectoparasites that, while taking a blood meal