Ticks are vectors of a number of important diseases, and with mechanisms for controlling ticks often less effective than those for controlling insects, these arachnids may be more of a health problem for humans and wildlife than insects. The long term goals of the these studies are to aid in the development of mechanisms for preventing transmission of disease-causing microbes from the tick to its next host, and to exploit differences between tick genera to develop control measures for the spread of infectious diseases or to control the microorganisms themselves. This study addresses the fundamental issue of the tick response to bacterial infection and also addresses the question of why some microbes have been able to successfully colonize blood-feeding arthropods and become the agents of vector-borne diseases. Specifically the proposal examines the hypothesis that Dermacentor variabilis (American dog tick) expresses a robust antimicrobial response following bacterial challenge. In contrast Ixodes scapularis (deer tick) and Amblyomma americanum (Lone Star tick) do not appear to express this response, at least to the same microbes. The proposed studies will provide information on how and why these ticks respond differently to assorted microbes. This project will concentrate on one fundamental mechanism by which blood-feeding arthropods resist infection by invading microbes. This is the use of antimicrobial peptides, in particular the defensins, a key component of an organism's innate immune response. The project will use ticks because they harbor and transmit a greater variety of microbes than any other arthropod group. Many arthropods possess a well-developed immune system that rejects most invading microbes, however, little is known about the immune system in ticks. More precise knowledge is needed to understand the limitations in the tick's immune system that have encouraged the ability to harbor such a variety of microorganisms. Ticks of the genus Ixodes tolerate a greater diversity of microbial species than other tick genera. Other genera, including Dermacentor and Amblyomma are noted for their association with one, or at most two specific types of microbe. Most Ixodes spp. that are microbe-tolerant are generalists, feeding on a diverse array of hosts ranging from cold-blooded vertebrates to almost any type of bird or mammal. It is proposed that these tick generalists have a less efficient immune system that facilitates colonization by a diverse array of invading microbes. It is hypothesized that differences in the tick innate immune mechanisms are important determinants of their ability to function as vectors of pathogenic agents. By using the Lyme disease-causing spirochete Borrelia burgdorferi, and a non-pathogenic bacterium, Rickettsia montana, and comparing the response to these microbes in Ixodes scapularis (the vector for B. burgdorferi), Dermacentor variabilis (host for R. montana) and Amblyomma americanum (vector for Ehrlichia), the role of defensin will be examined. The hypothesis will be explored within four aims, concentrating on the tick defensin, varisin. The aims are: (1) Characterize the tick defensin gene (vsnD) from D. variabilis. (2) Determine in which tissues of the adult tick the varisin gene is expressed, and whether the defensin is expressed by nymphs, perhaps giving an indication that defensin is important throughout the various life stages of the tick. (3) Determine whether other tick genera (Ixodes and Amblyomma) have and selectively express a defensin gene in response to different bacteria. (4) Compare the response of the different ticks to Rickettsia montana, an intracellular organism normally found in D. variabilis. Results of these studies will improve our understanding of the role of defensin in the innate immune response of ticks following challenge with various microbes. Elucidation of this critically important defensive system will provide information regarding the reason some of these ticks transmit disease-causing microorganisms.
