No abstract

Saliva of the hard tick and Lyme disease vector, Ixodes scapularis, has a repertoire of compounds that counteract host defenses. Following sequencing of an I scapularis salivary gland complementary DNA (cDNA) library, a clone with sequence homology to tissue factor pathway inhibitor (TFPI) was identified. This cDNA codes for a mature protein, herein called Ixolaris, with 140 amino acids containing 10 cysteines and 2 Kunitz-like domains. Recombinant Ixolaris was expressed in insect cells and shown to inhibit factor VIIa (FVIIa)/tissue factor (TF)-induced factor X (FX) activation with an inhibitory concentration of 50% (IC(50)) in the picomolar range. In nondenaturing gel, Ixolaris interacted stoichiometrically with FX and FXa but not FVIIa. Ixolaris behaves as a fast-and-tight ligand of the exosites of FXa and gamma-carboxyglutamic acid domainless FXa (des-Gla-FXa), increasing its amidolytic activity. At high concentration, Ixolaris attenuates the amidolytic activity of FVIIa/TF; however, in the presence of DEGR-FX or DEGR-FXa (but not des-Gla-DEGR-FXa), Ixolaris becomes a tight inhibitor of FVIIa/TF as assessed by recombinant factor IX (BeneFIX) activation assays. This indicates that FX and FXa are scaffolds for Ixolaris in the inhibition of FVIIa/TF and implies that the Gla domain is necessary for FVIIa/TF/Ixolaris/FX(a) complex formation. Additionally, we show that Ixolaris blocks FXa generation by endothelial cells expressing TF. Ixolaris may be a useful tool to study the structural features of FVIIa, FX, and FXa, and an alternative anticoagulant in cardiovascular diseases.

The authors compared the contribution of white-footed mice (Peromyscus leucopus), chipmunks (Tamias striatus), and meadow voles (Microtus pennsylvanicus) to infection of vector ticks with the Lyme disease spirochete, Borrelia burgddorferi. At one Massachusetts location where Lyme disease is endemic, all three species of rodents were found to be infected. Prevalence of infection, however, varied from 90% for mice, and 75% for chipmunks to just 5.5% for meadow voles. Infectivity of these hosts for larval Ixodes dammini also varied, but mice were found to be the most infective, followed by chipmunks and meadow voles. Density estimates of these three hosts, collected between 1981 and 1986 in three coastal Massachusetts locations, revealed that mice were more abundant than the other two rodents in areas in which ticks were abundant. In addition, mice were infested more abundantly with larval I. dammini than the other two host species. Integrating these results, we determined each species' "reservoir potential," a novel term which describes the relative contribution made by a host species to the horizontal infection of a vector population. The authors' findings demonstrate that, at least in coastal Massachusetts, P. leucopus is the most important small mammal reservoir for B. burgdorferi.