Functional characterization of genes of the tick Amblyomma cajennense modulated by infection with Rickettsia rickettsii using RNA interference (RNAi)

Abstract

The etiologic agent of the Rocky Mountain Spotted Fever (RMSF), also known as Brazilian Spotted Fever (BSF) in Brazil, is the bacterium Rickettsia rickettsii. This rickettsiae is transmitted to humans by the bite of various species of ticks. In Brazil, Amblyomma cajennense and A. aureolatum are known as vectors of BSF. The prevalence rates of R. rickettsii infected ticks in RMSF and BSF endemic areas are low, oscillating around 1%. These low prevalence rates seems to be associated with lower reproductive and survival rates of infected ticks, suggesting that R. rickettsii is also pathogenic to its vectors. Experimental infections with R. rickettsii have demonstrated that 80 - 100% of A. aureolatum ticks of a laboratory colony acquire this bacterium, whereas only 10 to 60% of A. cajennense ticks become infected. These results indicate that the responses of these two tick species against infection are different, resulting in different prevalence rates of R. rickettsii. Therefore, the elucidation of the interactions between ticks of the genera Amblyomma and the bacterium R. rickettsii at a molecular level is important to provide information to better understand the mechanisms responsible for the apparent refractoriness of A. cajennense against infection as well as for the elucidation of the virulence mechanisms of R. rickettsii against ticks. The objective of this project is to validate the expression data of A. cajennense genes previously identified as differentially expressed by infection with R. rickettsii using suppression subtractive hybridization (SSH). After validation of SSH data by quantitative polymerase chain reaction followed by reverse transcription (RT-qPCR), two genes induced by infection will be selected for functional characterization using RNA interference (RNAi). The effects of the gene knock-down will be determined by evaluation of the ability of the tick to adhere the host skin and to acquire the blood-meal, the reproductive rate, and capacity to acquire R. rickettsii. In addition to provide information to better understand the interactions between the etiological agent of the FMB and its tick vectors, this study may provide the identification of potential targets for the development of vaccines to control the FMB as well as for the control of R. rickettsii vectors.