The global effects of tick saliva on host cell function

Hard ticks, such as Dermacentor variabilis, are obligate ectoparasites that remain attached to a host for up to 14 days. To sustain this attachment, ticks must secrete bioactive molecules in their saliva to suppress the host’s immune, inflammatory, wound healing, and hemostatic responses. Previously, we have shown that tick salivary gland extract and saliva have cell-specific effects on the function of NIH3T3-L1 fibroblasts and IC-21 macrophages. Since cell migration is a pre-requisite for tumor invasion and metastasis, we investigated if saliva has global meaning general effects on the migratory, invasive, and signaling activities of Saos-2 osteosarcoma and MDA-MB-231 (MB-231) breast cancer cells. We determined that saliva inhibits Saos-2 and MB-231 migration and invasion. In Saos-2 cells, this inhibition correlated with suppressed epidermal growth factor (EGF) activation of Akt, however, only basal extracellular signal-regulated kinase (ERK) activity was affected in MB-231 cells. Initially, EGF receptor (EGFR) over-expresssion masked the effect of saliva on MB-231 cells, but its ability to inhibit MB-231 migration was comparable to the EGFR inhibitor PD 168393 and MEK inhibitor U0126. Prostaglandin E2 (PGE2), which is found in high concentration in tick saliva, decreases fibroblast migration and increases macrophage migration. Therefore we examined if salivary PGE2 is responsible for the saliva-induced regulation of macrophage and fibroblast migration by using the PGE2 receptor antagonist, AH 6809. Saliva increased platelet-derived growth factor (PDGF)-stimulated macrophage migration, a response reversed by AH 6809. The inhibition of PDGF-stimulated fibroblast migration was also antagonist-sensitive. Saliva induced macrophages to secrete PGE2, and conditioned medium from these cells caused a PGE 2 antagonist-sensitive inhibition of stimulated fibroblast migration. In macrophages, we also showed that saliva decreases the secretion of the pro-inflammatory chemokine CCL5 and tumor necrosis factor-alpha (TNF-α) along with its soluble receptor (sTNFRI) through a PGE2-dependent mechanism mediated by cyclic adenosine monophosphate (cAMP). Our findings indicate that the mechanisms ticks have evolved to regulate wound healing have generalized effects on cell migratory activities. These data also demonstrate how salivary PGE2 is utilized to regulate the activity of macrophages and fibroblasts, cells critically important in wound healing.