Biochemical and immunochemical studies on ticks (Ixodida: Ixodidae)

In developing a vaccine against Rhipicephalus appendiculatus, an important tick ectoparasite of livestock in Africa, a necessary first step is identification of antigens which give protective immunity. Antigen profiles of extracts of unfed immature and adult Rh. appendiculatus ticks and their fractions were compared by immunoblotting with sera raised against tick infestation and against whole or fractionated extracts. Several antigens (51.5, 40, 36.5 and 23kDa) were observed to be absent in extracts of fed or partially fed adult ticks. Antigens of 84, 60 and 40kDa were consistently detected in extract fractions shown in immunisation/tick challenge experiments to be immunogenic. The 60kDa antigen was found both in soluble and membrane fractions, whilst the 84 and 40kDa antigens did not appear to be membrane associated. The 84 and 40kDa species appeared heavily glycosylated with a broad range of carbohydrate moieties being present. The 60kDa antigen did not bind significantly to any of the lectins used, suggesting only minimal glycosylation. Probing extracts of unfed larval ticks of different species with serum raised against an immunogenic fraction of Rh. appendiculatus unfed nymphal extract revealed 60kDa antigen species in each of the five different tick species. Immunostaining of sections of unfed adult female Rh. appendiculatus illustrated marked differences in the distribution of antigens associated with adult tick feed and those associated with immunisation with extracts of unfed nymphal ticks and their fractions. Of the unfed immature Rh. appendiculatus extract fractions used in immunisation and adult tick challenge feed experiments in guinea pigs, SEHPLC fraction 2 and the 45% ammonium sulphate supernate fraction were found to be the most protective. The prospects for developing an anti-Rh. appendiculatus vaccine are discussed, and antigen purification strategies are suggested.