A Family of Tick (Ixodes scapularis) Salivary Proteins that Inhibit Complement

Ixodes scapularis, the blacklegged tick, is an ectoparasitic bloodsucking arthropod that transmits multiple pathogens, including Borrelia burgdorferi, the agent of Lyme disease. I. scapularis ticks secrete numerous salivary anti-hemostatic, anti-inflammatory, and immunosuppressive compounds into the host to inhibit host responses that could interfere with feeding. I. scapularis anti-complement protein (Isac) specifically inhibits the alternative complement pathway by destabilizing the C3 convertase. Here, we report on the identification of several tick proteins that are paralogues of Isac and members of the Isac-like protein (ILP) family, which contains at least 49 unique proteins. In this work we have characterized the biochemical and functional activities of multiple ILPs, their mechanisms of complement inhibition, and their roles in facilitating tick feeding. We expressed recombinant ILPs (rILPs) that possessed several N- and O-linked glycans and inhibited the alternative complement pathway by destabilizing C3 convertases, similar to Isac. rILPs specifically bound properdin, a positive regulator of the alternative complement pathway, causing its removal from C3 convertases and accelerating decay of the convertases. This mechanism of complement regulation is distinct from any characterized negative regulators of the alternative complement pathway, which mediate decay acceleration by interacting with Bb or C3b. As I. scapularis ticks likely co-express multiple ILPs during tick feeding, we evaluated various properties of individual ILPs that possibly justify their co-expression. When multiple rILPs were added together, no synergistic effects were observed, indicating the proteins acted individually. Individual rILPs also inhibited the alternative complement pathway from different animal hosts comparably, indicating rILPs did not display host specificity. We believe that individual I. scapularis ILPs likely display antigenic variation, justifying their co-expression during tick feeding. Expression of the ILP gene family is likely essential for successful tick feeding as ILPs suppress host innate immune responses. RNAi mediated gene silencing of the ILP genes and the generation of antibodies against rILPs in immunized mice had no effect on successful I. scapularis nymphal tick feeding. However, RNAi may have not revealed a phenotype because of redundancy in the ILP genes. Similarly, ILP specific antibodies may have showed no phenotype because of antigenic variation between different ILPs and redundancy in ILP functions