Synopsis The saliva of blood-feeding parasites is a rich source of peptidase inhibitors that help overcome the host’s defense during host-parasite interactions. Using proteomic analysis, the cystatin OmC2 was demonstrated in the saliva of the soft tick Ornithodoros moubata, an important disease-vector transmitting African swine fever virus and the spirochaete Borrelia duttoni. A structural, biochemical and biological characterization of this peptidase inhibitor was undertaken. Recombinant OmC2 was screened against a panel of physiologically relevant peptidases and found to be an effective broad-specificity inhibitor of cysteine cathepsins, including endopeptidases (cathepsins L and S) and exopeptidases (cathepsins B, C and H). The crystal structure of OmC2 was determined at a resolution of 2.45 Å and used to describe the structure-inhibitory activity relationship. The biological impact of OmC2 was demonstrated both in vitro and in vivo. OmC2 affected the function of antigen-presenting mouse dendritic cells by reducing the production of the proinflammatory cytokines TNF-α and IL-12, and proliferation of antigen-specific CD4+ T cells. This suggests that OmC2 may suppress the host’s adaptive immune response. Immunization of mice with OmC2 significantly suppressed the survival of O. moubata in infestation experiments. We conclude that OmC2 is a promising target for the development of a novel anti-tick vaccine to control O. moubata populations and combat the spread of associated diseases.

Two genes coding for cysteine peptidase inhibitors of the cystatin family (Om-cystatin 1 and 2) were isolated from a gut-specific cDNA library of the soft tick Ornithodoros moubata. Both cystatins were clearly down-regulated after a blood meal. Om-cystatin 1 is mainly expressed in the tick gut, while Om-cystatin 2 mRNA was also found in other tick tissues. Authentic Om-cystatin 2 was significantly more abundant than Om-cystatin 1 in the gut contents of fasting ticks and was associated with hemosome-derived residual bodies accumulated in the gut lumen. Om-cystatin 2 was also expressed by type 2 secretory cells in the salivary glands of unfed ticks. The inhibitory specificity of recombinant Om-cystatins 1 and 2 was tested with mammalian cysteine peptidases, as well as endogenous cysteine peptidases present in the tick gut. Both cystatins efficiently inhibited papain-like peptidases, including cathepsin B and H, but differed significantly in their affinity towards cathepsin C and failed to block asparaginyl endopeptidase. Our results suggest that the secreted cystatin isoinhibitors are involved in the regulation of multiple proteolytic targets in the tick digestive system and tick-host interaction.

Ticks are important ectoparasites and vectors of multiple human and animal diseases. The obligatory hemophagy of ticks provides a formidable route for parasite transmission from one host to another. Parasite survival inside the tick relies on the ability of a pathogen to escape or inhibit tick immune defenses, but the molecular interactions between the tick and its pathogens remain poorly understood. Here we report that tick genomes are unique in that they contain all known classes of the α₂-macroglobulin family (α₂M-F) proteins: α₂-macroglobulin pan-protease inhibitors, C3 complement components, and insect thioester-containing and macroglobulin-related proteins. By using RNA interference-mediated gene silencing in the hard tick Ixodes ricinus we demonstrated the central role of a C3-like molecule in the phagocytosis of bacteria and revealed nonredundant functions for α₂M-F proteins. Assessment of α₂M-F functions in a single organism should significantly contribute to the general knowledge on the evolution and function of the complement system. Importantly, understanding the tick immune mechanisms should provide new concepts for efficient transmission blocking of tick-borne diseases.