Lyme disease (LD) caused by Borrelia burgdorferi is among the most important human vector borne diseases for which there is no effective prevention method. Identification of tick saliva transmission factors of the LD agent is needed before the highly advocated tick antigen-based vaccine could be developed. We previously reported the highly conserved Ixodes scapularis ( Ixs ) tick saliva serpin (S) 17 ( Ixs S17) was highly secreted by B . burgdorferi infected nymphs. Here, we show that Ixs S17 promote tick feeding and enhances B . burgdorferi colonization of the host. We show that Ixs S17 is not part of a redundant system, and its functional domain reactive center loop (RCL) is 100% conserved in all tick species... More
Lyme disease (LD) caused by Borrelia burgdorferi is among the most important human vector borne diseases for which there is no effective prevention method. Identification of tick saliva transmission factors of the LD agent is needed before the highly advocated tick antigen-based vaccine could be developed. We previously reported the highly conserved Ixodes scapularis ( Ixs ) tick saliva serpin (S) 17 ( Ixs S17) was highly secreted by B . burgdorferi infected nymphs. Here, we show that Ixs S17 promote tick feeding and enhances B . burgdorferi colonization of the host. We show that Ixs S17 is not part of a redundant system, and its functional domain reactive center loop (RCL) is 100% conserved in all tick species. Yeast expressed recombinant (r) Ixs S17 inhibits effector proteases of inflammation, blood clotting, and complement innate immune systems. Interestingly, differential precipitation analysis revealed novel functional insights that Ixs S17 interacts with both effector proteases and regulatory protease inhibitors. For instance, r Ixs S17 interacted with blood clotting proteases, fXII, fX, fXII, plasmin, and plasma kallikrein alongside blood clotting regulatory serpins (antithrombin III and heparin cofactor II). Similarly, r Ixs S17 interacted with both complement system serine proteases, C1s, C2, and factor I and the regulatory serpin, plasma protease C1 inhibitor. Consistently, we validated that r Ixs S17 dose dependently blocked deposition of the complement membrane attack complex via the lectin complement pathway and protected complement sensitive B . burgdorferi from complement-mediated killing. Likewise, co-inoculating C3H/HeN mice with r Ixs S17 and B . burgdorferi significantly enhanced colonization of mouse heart and skin organs in a reverse dose dependent manner. Taken together, our data suggests an important role for Ixs S17 in tick feeding and B . burgdorferi colonization of the host.Author summaryTicks feed on animals and humans for their survival. During blood meal feeding, ticks inject saliva along with disease causative agents into the hosts. Here, we demonstrate that I . scapularis tick saliva protein, Ixs S17 inhibits host innate immune system proteases and enhances B . burgdorferi colonization of the host. Recombinant Ixs S17 (r Ixs S17) inhibits blood clotting and inflammation systems serine proteases including pancreatic trypsin and trypsin IV (~100%), blood clotting factor Xa and XIa (~60 80%), plasmin and cathepsin G (~50%). Similarly, r Ixs S17 interacts with complement system factors, C1s, C2 and factor I and blocks complement membrane attack complex via the lectin complement pathway by up to 97%. We found that, in the mouse model for Lyme disease, r Ixs S17 significantly increases B . burgdorferi colonization of mouse heart and ear tissues by 5.7 and 2.3 times. Taken together, we conclude that Ixs S17 is a key protein in tick feeding and B . burgdorferi colonization of the host, and thus, a potential target antigen for developing tick antigen-based vaccines against Lyme disease agent transmission.
