The immunomodulatory effect of IrSPI, a tick salivary gland serine protease inhibitor involved in Ixodes ricinus tick feeding

Ticks are strict hematophagous arthropods and are the most important vectors of pathogens affecting both domestic and wild animals worldwide. Moreover, they are second only to mosquitoes as vectors of human pathogens. Hard tick feeding is a slow process—taking up to several days for repletion prior to detachment—and necessitates extended control over the host response. The success of the feeding process depends upon injection of saliva by tick, which not only controls host haemostasis and wound healing, but also subverts the host immune response to avoid tick rejection during this long-lasting process. In turn, the manipulation of the host immune response creates a favourable niche for the survival and propagation of diverse tick-borne pathogens transmitted during feeding. Here, we report on the molecular and biochemical features and functions of IrSPI, an Ixodes ricinus salivary serine protease inhibitor involved in blood meal acquisition. Our results show that IrSPI harbours the typical conformational fold of Kunitz type I serine protease inhibitors and that it functionally inhibits the elastase and, to a lesser extent, chymotrypsin. We also show that IrSPI is injected into the host during feeding. Crucially, we found that IrSPI has no impact on tissue factor pathway-induced coagulation, fibrinolysis, apoptosis, or angiogenesis, but a strong effect on immune cells. IrSPI affects antigen-presenting macrophages by hampering IL-5 production. In addition, IrSPI represses proliferation of mitogen-stimulated CD4+ cells. The inhibition of T cell proliferation was associated with marked reductions in pro-inflammatory cytokine secretion. Our study contributes valuable knowledge to tick-host interactions, and provides insights that could be further exploited to design anti-tick vaccines targeting this immunomodulator implicated in successful I. ricinus tick feeding.