Deconstructing tick saliva: Non-protein molecules with potent immunomodulatory properties

Dendritic cells (DCs) are powerful initiators of innate and adaptive immune responses. Ticks are blood-sucking ectoparasite arthropods that suppress host immunity by secreting immunomodulatory molecules in their saliva. Here, compounds present in Rhipicephalus sanguineus tick saliva with immunomodulatory effects on DC differentiation, cytokine production, and costimulatory molecule expression were identified. R. sanguineus tick saliva inhibited IL-12p40 and TNF-α while potentiating IL-10 cytokine production by bone marrow-derived DCs stimulated by Toll-like receptor-2, -4, and -9 agonists. To identify the molecules responsible for these effects, we fractionated the saliva through microcon filtration and reversed-phase HPLC and tested each fraction for DC maturation. Fractions with proven effects were analyzed by micro-HPLC tandem mass spectrometry or competition ELISA. Thus, we identified for the first time in tick saliva the purine nucleoside adenosine (concentration of ∼110 pmol/μl) as a potent anti-inflammatory salivary inhibitor of DC cytokine production. We also found prostaglandin E₂ (PGE₂ ∼100 nM) with comparable effects in modulating cytokine production by DCs. Both Ado and PGE₂ inhibited cytokine production by inducing cAMP-PKA signaling in DCs. Additionally, both Ado and PGE₂ were able to inhibit expression of CD40 in mature DCs. Finally, flow cytometry analysis revealed that PGE₂, but not Ado, is the differentiation inhibitor of bone marrow-derived DCs. The presence of non-protein molecules adenosine and PGE ₂ in tick saliva indicates an important evolutionary mechanism used by ticks to subvert host immune cells and allow them to successfully complete their blood meal and life cycle.