The molecular basis of tick-host interactions
Ticks are obligate haematophagous arthropods that represent a major economic drain upon the world's livestock as well being a significant medical and veterinary risk through the transmission of tick-borne pathogens such as Borrelia burgdorferi, the causative agent of Lyme disease. The tick-host relationship is a function of both ecological and physiological factors. Successful feeding requires the effective acquisition and digestion of a bloodmeal by the tick. Acquisition relies upon the ability of the tick to counteract host immune responses induced by the extended feeding periods of ixodid ticks (up to 2 weeks). The host response to tick infestation and the consequent countermeasures employed by the tick, constitute the tick-host interface. The immune response of hosts to Ixodes ricinus infestations was examined through antigenic profiling. The antigens exposed to the host were shown to vary throughout the feeding period and differed between the different development stages of I. ricinus. It was also shown that different host species infested with I. ricinus recognised different antigens. This was true of both natural and non-natural hosts, and even closely related species. Anti-complement activity was investigated in the salivary glands of Ixodes ticks. This activity was shown to inhibit some host species but not others. The pattern of inhibitory activity varied between the tick species tested in a way that was consistent with known tick host-preferences. The mechanisms of anti-complement activity in I. ricinus salivary glands were explored. The alternative but not the classical pathway of complement was inhibited. Activity was present in unfed ticks and throughout the feeding period. Three targets of the complement system were identified as being modulated by the tick. Digestion of the bloodmeal was explored and a haemolytic activity was associated with the salivary glands of I. ricinus ticks. The activity was demonstrated to be Mg2+- dependent. In addition, a subtractive cDNA library enriched for saliva-associated transcripts was successfully produced. Random sampling identified putative differentially expressed genes. The results of this thesis illustrate the complexity of tick-host interactions at the molecular level. It is apparent that the research described poses many more questions than answers.