The Central Role of Salivary Metalloproteases in Host Acquired Resistance to Tick Feeding
During feeding on vertebrate hosts, ticks secrete saliva composed of a rich cocktail of bioactive molecules modulating host immune responses. Although most of the proteinaceous fraction of tick saliva is of little immunogenicity, repeated feeding of ticks on mammalian hosts may lead to impairment of tick feeding, preventing full engorgement. Here, we challenged rabbits with repeated feeding of both Ixodes ricinus nymphs and adults and observed the formation of specific antibodies against several tick salivary proteins. Repeated feeding of both I. ricinus stages led to a gradual decrease in engorged weights.
To identify the salivary antigens, isolated immunoglobulins from repeatedly infested rabbits were utilized for a protein pull-down from the saliva of pilocarpine-treated ticks. Eluted antigens were first identified by peptide mass fingerprinting with the aid of available I. ricinus salivary gland transcriptomes originating from early phases of tick feeding. To increase the authenticity of immunogens identified, we also performed, for the first time, de novo assembly of the sialome from I. ricinus females fed for six days, a timepoint used for pilocarpine-salivation. The most dominant I. ricinus salivary immunogens identified in our study were zinc-dependent metalloproteases of three different families. To corroborate the role of metalloproteases at the tick/host interface, we fed ticks micro-injected with a zinc metalloprotease inhibitor, phosphoramidon, on a rabbit. These ticks clearly failed to initiate feeding and to engorge. However, neither feeding to ticks immune blood of repeatedly infested rabbits, nor phosphoramidon injection into ticks, prevented their engorgement when fed in vitro on an artificial membrane system. These data show that Zn metalloproteases play a decisive role in the success of tick feeding, mediated by complex molecular interactions between the host immune, inflammatory, and hemostatic processes, which are absent in in vitro feeding. This basic concept warrants further investigation and reconsideration of the current strategies towards the development of an effective “anti-tick” vaccine.
Introduction
Ticks and tick-borne diseases (TBD) represent a growing global burden for both human and animal health (de la Fuente et al., 2008) and are a major constraint for the improvement of livestock industries, particularly in developing countries (Peter et al., 2005). Host immunity-mediated rejection of ticks has potential in the search for a vaccine that offers an effective and environmentally sound approach for controlling ticks and TBDs (de la Fuente et al., 2016).
Ticks salivate proteinaceous saliva into the host during the course of feeding (Francischetti et al., 2009). The individual protein components are secreted into a feeding site at functional concentrations and with adequate affinities for host targets to establish biomolecular associations at the feeding site, thereby modulating host intrinsic hemostatic processes and defence responses (Mans, 2019). To escape immune recognition by the host, tick salivary proteins have evolved low immunogenicity (Chmelar et al., 2016), leading to low or no tick rejection reactions when Ixodes ricinus ticks feed on their natural hosts such as mice, voles, or passerine birds (Dizij and Kurtenbach, 1995; Dusbábek et al., 1995; Heylen et al., 2010). In contrast, tick saliva induces an antibody-mediated response in distinct less adapted hosts (Brossard and Wikel, 2005).
Perner J., Helm D., Haberkant P., Hatalová T., Kropáčková S., Ribeiro J.M., Kopáček P. 2020: The central role of salivary metalloproteases in host acquired resistance to tick feeding. Frontiers in Cellular and Infection Microbiology 10: 563349. doi: 10.3389/fcimb.2020.563349
