Here we report the ability of the tick Ixodes scapularis, the main vector of Lyme disease in the United States, to actively and specifically affect the host proteolytic activity in the sites of infestation through the release of a cystatin constituent of its saliva. The cystatin presence in the saliva was verified both biochemically and immunologically. We named the protein sialostatin L because of its inhibitory action against cathepsin L. We also show that the proteases it targets, although limited in number, have a prominent role in the proteolytic cascades that take place in the extracellular and intracellular environment. As a result, sialostatin L displays an antiinflammatory role and inhibits proliferation of cytotoxic T lymphocytes. Beyond unraveling another component accounting for the properties of tick saliva, contributing to feeding success and pathogen transmission, we describe a novel tool for studying the role of papain-like proteases in diverse biologic phenomena and a protein with numerous potential pharmaceutical applications.

Platelet aggregation and acute inflammation are key processes in vertebrate defense to a skin injury. Recent studies uncovered the mediation of 2 serine proteases, cathepsin G and chymase, in both mechanisms. Working with a mouse model of acute inflammation, we revealed that an exogenous salivary protein of Ixodes ricinus, the vector of Lyme disease pathogens in Europe, extensively inhibits edema formation and influx of neutrophils in the inflamed tissue. We named this tick salivary gland secreted effector as I ricinus serpin-2 (IRS-2), and we show that it primarily inhibits cathepsin G and chymase, while in higher molar excess, it affects thrombin activity as well. The inhibitory specificity was explained using the crystal structure, determined at a resolution of 1.8 Å. Moreover, we disclosed the ability of IRS-2 to inhibit cathepsin G-induced and thrombin-induced platelet aggregation. For the first time, an ectoparasite protein is shown to exhibit such pharmacological effects and target specificity. The stringent specificity and biological activities of IRS-2 combined with the knowledge of its structure can be the basis for the development of future pharmaceutical applications.

Ixodes ricinus is a tick that transmits the pathogens of Lyme and several arboviral diseases. Pathogens invade the tick midgut, disseminate through the hemolymph and are transmitted to the vertebrate host via the salivary glands; subverting these processes could be used to interrupt pathogen transfer. Here, we use massive de novo sequencing to characterize the transcriptional dynamics of the salivary and midgut tissues of nymphal and adult I. ricinus at various time points after attachment on the vertebrate host. Members of a number of gene families show stage- and time-specific expression. We hypothesize that gene expression switching may be under epigenetic control and, in support of this, identify 34 candidate proteins that modify histones. I. ricinus-secreted proteins are encoded by genes that have a non-synonymous to synonymous mutation rate even greater than immune-related genes. Midgut transcriptome (mialome) analysis reveals several enzymes associated with protein, carbohydrate and lipid digestion, transporters and channels that might be associated with nutrient uptake and immune-related transcripts including antimicrobial peptides. This publicly available dataset supports the identification of protein and gene targets for biochemical and physiological studies that exploit the transmission lifecycle of this disease vector for preventative and therapeutic purposes.