Anti-Hemostatic Effects Of A Serpin From The Saliva Of The Tick Ixodes Ricinus

Serpins (serine protease inhibitors) are a large family of structurally related
proteins found in a wide variety of organisms, including hematophagous
arthropods. Protein analyses revealed that Iris, previously described as an
immunomodulator secreted in the tick saliva, is related to the leukocyte elastase
inhibitor and possesses serpin motifs, including the reactive center loop (RCL),
which is involved in the interaction between serpins and serine proteases. Only
serine proteases were inhibited by purified recombinant Iris (rIris), whereas
mutants L339A and A332P were found devoid of any protease inhibitory activity.
The highest Ka was observed with human leukocyte-elastase, suggesting that
elastase-like proteases are the natural targets of Iris. In addition, mutation
M340R completely changed both Iris substrate specificity and affinity. This
likely identified Met-340 as amino acid P1 in the RCL. The effects of rIris and
its mutants were also tested on primary hemostasis, blood clotting, and
fibrinolysis. rIris increased platelet adhesion, the contact phase-activated
pathway of coagulation, and fibrinolysis times in a dose-dependent manner,
whereas rIris mutant L339A affected only platelet adhesion. Taken together, these
results indicate that Iris disrupts coagulation and fibrinolysis via the
anti-proteolytic RCL domain. One or more other domains could be responsible for
primary hemostasis inhibition. To our knowledge, this is the first ectoparasite
serpin that interferes with both hemostasis and the immune response.