Potential role of lectins in ticks: Rhipicephalus appendiculatus and Rhipicephalus pulchellus (Acari: Ixodidae)

Abstract

The role of lectins in tick vectors was investigated by comparing the presence of lectins in the haemolymph, intestine and salivary glands of 2 tick species from Kenya: (1) Rhipicephalus appendiculatus, the vector of Theileria parva, and (2) Rhipicephalus pulchellus, which is refractory to this protozoan parasite. There was variation in the agglutination titres with erythrocytes from bovine, mouse and rabbit erythrocytes, suggesting different binding affinities and quantities of lectins. The haemolymph from R. pulchellus had the highest agglutination titre with mouse erythrocytes (1024) compared to that obtained with rabbit cells (8). No agglutination was observed with bovine erythrocytes when assayed with the intestinal, salivary gland or haemolymph lectins from either tick species. The intestinal lectin from R. pulchellus also demonstrated high titres (1024) with rabbit cells, but moderate titres with mouse erythrocytes (256). Haemolymph from T. parva-infected R. appendiculatus contained double (512) the levels of lectins compared to uninfected ticks (256), with mouse erythrocytes indicating an increase in the amount of lectin in the presence of infection and implying a role of lectins in the immunity of these arthropods. Lectin from the intestine of R. appendiculatus agglutinated purified piroplasms, a stage in the T. parva life cycle ingested by the tick when it feeds on infected cattle. The anti-lectin antibodies conjugated with fluorescein isothiocyanate (FITC) bound to the piroplasms, suggesting a possible point at which intervention may be developed for the control of theileriosis. The significance of lectins in tick parasite transmission lies in the fact that these molecules play an important part in cell-to-cell adhesion. Defining their binding specificities and quantities in a given tick species may lead to development of a novel type of disease control whose mode of action would be based on competing for the ligands for binding to pathogen receptors or preventing adhesion to host tissues (transmission-blocking), thereby preventing infection. Results indicate that tick-derived antigens in the form of lectins produce antibodies that can recognize the piroplasm and schizont stages of T. parva. With increasing attention on vaccination of cattle against ticks, there is a need to identify and show target antigens within the tick. The study has elucidated three targets, the salivary gland, haemolymph and intestine. It forms one of the first reports on the effect of lectin antibody on the development and transmission of T. parva. This also means that lectins may be used as vaccine antigens that can possibly intervene at the piroplasm stage and the schizont stage.