Tick-pathogen interactions in bovine anaplasmosis

Abstract

Anaplasma marginale is a tick-borne bacterial pathogen that causes bovine anaplasmosis, one of the most important tick-borne diseases of ruminants worldwide. Despite its importance no satisfactory control methods are available. The development of new vaccine formulations capable of preventing transmission would have significant economic impact. This approach would require a better understanding of mechanisms underlying the complex interactions between A. marginale and its vector ticks. The research presented in this thesis has delineated new molecular interactions that occur at the tick–pathogen interface by identification and characterization tick genes/proteins that facilitate infection and multiplication of A. marginale. During the course of the experiments described in this thesis, a new tick species is tested for the vector competence of A. marginale. It was found that Dermacentor reticulatus male ticks are competent biological vectors of A. marginale by intrastadial transmission (Chapter 2). In Chapter 3 it is demonstrated that A. marginale infection modifies gene expression in Dermacentor variabilis ticks and cultured Ixodes scapularis cells. A functional genomics approach was used to identify and characterize tick genes/proteins that are differentially regulated in response to A. marginale and that facilitate the trafficking of the pathogen from infection of the gut cells through infection and transmission from salivary glands. This study was extended by identifying genes differentially expressed in Rhipicephalus microplus male salivary glands (Chapter 4). In addition, R. microplus derived tick cell line BME26 was used for the first time to study A. marginale-tick interactions (Chapter 4). Four of the identified differentially expressed genes were further analyzed by RNA interference (RNAi) and the effects of silencing on A. marginale development and infection levels in ticks was characterized by means of quantitative PCR and microscopy (Chapter 5). The gene knock down affected the pathogen developmental cycle in D. variabilis ticks, suggesting that the four genes are potential candidates for transmission blocking vaccines. In Chapter 6 the expression of tick protective antigen subolesin in several tick species in response to different pathogens (including A. marginale) is described. The results suggest possible direct and indirect effects of subolesin knockdown on the level of infection of pathogens in ticks. Finally, the molecular response to infection with different obligate intracellular tick–transmitted bacteria is described in Chapter 7. A. marginale and A. phagocytophilum were found by microarray and quantitative RT-PCR to elicit different gene expression responses in cultured tick cells. Overall, the results described in this thesis are expected to contribute to a better understanding of the A. marginale-tick interface and to the development of a new generation of pathogen transmission blocking vaccines designed to prevent transmission and reduce exposure of vertebrate hosts to tick-borne pathogens.