Tick Spit Shines a Light on the Initiation of Coagulation

Abstract

Mammalian blood has numerous essential and well-known functions, including oxygen and nutrient delivery. This elixir is recognized by blood-feeding species of mosquitos, ticks, fleas, lice, leeches, and bats that rely on blood meals for nutrition, life cycle progression, and survival. To obtain these blood meals that require minutes to a week or longer to complete,1 these blood-sucking creatures must thwart endogenous defense systems contained within blood—immune and procoagulant cells and plasma proteins that rapidly clot (within 3–4 minutes) to provide first-line defense against breaches in vascular integrity. In a fascinating display of evolutionary agility, hemovores have adapted elegant mechanisms to evade detection and to prevent blood coagulation by synthesizing an extensive armament of molecules with anesthetic, immunosuppressive, vasodilatory, anticoagulant, and profibrinolytic properties in mammals.1–3 Research characterizing the molecules generated by hemovores to bypass mammalian defense pathways has revealed exciting new mechanisms and, in some cases, novel therapeutic approaches for anticoagulation. Article see p 254 In particular, ticks have received considerable attention for their remarkable evolutionary adaptations to life as obligate hemovores. Briefly, the typical tick life cycle includes 4 stages: egg, 6-legged larva, 8-legged nymph, and adult (Figure). It takes 1 to 3 years to complete this full cycle. Ticks must consume blood at the larval, nymph, and adult stages to survive, and they die if they do not find a host. Interestingly, although neither larva nor nymphs have overt sexual differentiation, adults are fully differentiated into males and females, and compared with males, longer nymph feeding is required for the expression of female characteristics.4 Consequently, tick saliva contains multiple proteins that maintain blood fluidity to enable feeding and therefore tick survival. Figure. Life cycle of a typical tick. Larvae, nymphs, and adults require blood meals for survival and secrete anticomplement (tick salivary lectin pathway inhibitor …