Anticoagulant activity in salivary glands of the insect vector Culicoides variipennis sonorensis by an inhibitor of factor Xa.

Blood feeding by the insect vector Culicoides variipennis sonorensis involves laceration of superficial host tissues, an injury that would be expected to trigger the coagulation cascade. Accordingly, the salivary glands of C.v. sonorensis were examined for the presence of an antihemostatic that prevents blood coagulation. Assays using salivary gland extracts showed a delay in the recalcification time of plasma devoid of platelets, indicating the presence of anticoagulant activity. Retardation in the formation of a fibrin clot was also observed after the addition of tissue factor to plasma that was preincubated with salivary gland extracts. Similarly, an inhibitory effect by salivary gland extracts was detected in assays that included factors of the intrinsic pathway. Inhibition of the catalytic activity of purified factor Xa toward its chromogenic substrate suggested that it was the target of the salivary anticoagulant of C.v. sonorensis. This was corroborated by the coincidence of anticoagulant and anti-FXa activities obtained by reverse-phase HPLC. The depletion of anti-FXa activity from salivary glands during blood feeding suggests that the FXa inhibitor functions as anticoagulant. Molecular sieving HPLC yielded an apparent molecular mass of 28 kDa for the salivary FXa inhibitor of C.v. sonorensis. Preventing the formation of thrombin through the inhibition of FXa likely facilitates blood feeding by maintaining the pool of blood fluid at the feeding site. The salivary FXa inhibitor of C.v. sonorensis could impair the network of host-defense mechanisms in the skin microenvironment by avoiding blood coagulation at the site of feeding.