Interference with LTβR signaling by tick saliva facilitates transmission of Lyme disease spirochetes.

Abstract

Lyme spirochetes have coevolved with ticks to optimize transmission to hosts using tick salivary molecules (TSMs) to counteract host defenses. TSMs modulate various molecular events at the tick-host interface. Lymphotoxin-beta receptor (LTβR) is a vital immune receptor and plays protective roles in host immunity against microbial infections. We found that <i>Ltbr</i> knockout mice were more susceptible to Lyme disease spirochetes, suggesting the involvement of LTβR signaling in tick-borne <i>Borrelia</i> infection. Further investigation showed that a 15-kDa TSM protein from <i>Ixodes persulcatus</i> (<i>I. persulcatus</i> salivary protein; IpSAP) functioned as an immunosuppressant to facilitate the transmission and infection of Lyme disease spirochetes. IpSAP directly interacts with LTβR to block its activation, thus inhibiting the downstream signaling and consequently suppressing immunity. IpSAP immunization provided mice with significant protection against <i>I. persulcatus-</i>mediated <i>Borrelia garinii</i> infection. Notably, the immunization showed considerable cross-protection against other <i>Borrelia</i> infections mediated by other ixodid ticks. One of the IpSAP homologs from other ixodid ticks showed similar effects on Lyme spirochete transmission. Together, our findings suggest that LTβR signaling plays an important role in blocking the transmission and pathogenesis of tick-borne Lyme disease spirochetes, and that IpSAP and its homologs are promising candidates for broad-spectrum vaccine development.