Elucidating the pathogen-induced gene expression in tick salivary glands

The Gulf Coast tick (Amblyomma maculatum) is a competent vector of Rickettsia parkeri, a pathogen of public health significance responsible for a Rocky Mountain spotted fever-like illness across the Atlantic to Gulf Coast region of the United States.  In this study, we have established R. parkeri infected, and uninfected A. maculatum tick colonies, and female adult salivary glands were used to study tick-pathogen interactions. We hypothesized that once R. parkeri enters the salivary glands, it exploits the expression of tick’s antioxidant machinery and other important vector defense pathways in order to survive and replicate before transmitted into the host. To test our hypothesis a RNA-Seq approach was utilized to determine the differential gene expression in R. parkeri infected salivary glands. The reads were mapped using A. maculatum reference sialotranscriptome (Karim et al., 2011). We observed significant overexpression of tick salivary protein families importantly lipocalins, proteinase inhibitors, oxidant/antioxidant and immunity related genes. There were 990 transcripts that are 10X overexpressed. Of these, 542 are salivary proteins and 143 are lipocalins, on average 1000 fold overexpressed, while 67 are protease inhibitors, on average 2853X overexpressed. Differential gene expression of the tick’s antioxidant, and immunity related genes were validated using a qRT-PCR. The RNAi assay showed the silencing of antioxidant genes including selenoprotiens significantly decreased the R. parkeri infection level in tick tissues. Our results provide an insight into tick-pathogen interaction and the functional role of tick antioxidants in vector competence.