Functional characterization of components of immune system, saliva, and microbiota of Amblyomma spp. ticks on the establishment of Rickettsia rickettsii infection

Abstract

Ticks are ectoparasitic arthropods that necessarily feed on the blood of vertebrates and versatile vectors of a wide variety of pathogens. Among the pathogens transmitted to humans by ticks, Rickettsia rickettsii, the etiological agent of Rocky Mountain spotted fever, is one of the most lethal. R. rickettsii presents transovarial and transstadial transmissions, so that ticks, in addition of being vectors, are reservoirs of the bacterium in nature. Thus, the functional characterization of the molecular factors involved in the interactions between the pathogen and its vectors is fundamental, and may reveal targets for the development of strategies for controlling the disease. In Brazil, R. rickettsii is transmitted by Amblyomma sculptum and Amblyomma aureolatum, which present remarked differences in relation to susceptibility to infection; A. sculptum is less susceptible than A. aureolatum. Our research group previously reported that A. sculptum genes modulated by the infection are mostly upregulated in the tick gut, including components of the immune system, whereas in A. aureolatum most genes are downregulated. We also showed that the gut of A. aureolatum has a prominent microbiota, consisting basically of bacteria of the genus Francisella. The gut of A. sculptum has an extremely reduced microbiota. Taken together, these data suggest that Francisella, instead of activating, desensitizes the immune system in the gut of A. aureolatum, creating a favorable environment to R. rickettsii. The effects of gene silencing of the antimicrobial peptide microplusin were evaluated, showing an increase in the prevalence of infected A. aureolatum ticks and the bacterial load in the gut. To evade immune system responses and complete their development in host cells, intracellular bacteria use different molecular mechanisms. Interestingly, we have shown that R. rickettsii is able to inhibit apoptosis in tick cells, favoring their proliferation. The transmission of R. rickettsii to the host occurs by tick via saliva. Preliminary immunological tests revealed proteins from A. sculptum saliva capable of stimulating both cellular and humoral immunity in the murine model, being potential targets for blocking transmission. The present research project has as principal objective to deepen the studies of functional characterization of the components of the immune system, saliva and the microbiota of Amblyomma spp. ticks in the establishment of R. rickettsii infection. (AU)