Rate of trypanosome killing by lectins in midguts of different species and strains of Glossina

The activity of lectins in different species of tsetse was compared in vivo by the time taken to remove all trypanosomes from the midgut following an infective feed and in vitro by agglutination tests. Teneral male Glossina pallidipes Austen, G. austeni Newstead and G. p. palpalis R-D. removed 50% of all Trypanosoma brucei rhodesiense Stephens & Fantham infections within 60 h. A 'refractory' line of G. m. morsitans Westwood took 170 h to kill 50% infections while a 'susceptible' line of the same species failed to kill 50%. Agglutination tests with midgut homogenates showed differences between fly stocks which accorded with differences in rate of trypanosome killing in vivo. Flies fed before an infective feed were able to remove trypanosomes from their midguts more quickly than flies infected as tenerals. Increasing the period of starvation before infection increased the susceptibility to trypanosome infection of non-teneral flies. Teneral flies showed little agglutinating activity in vitro, suggesting that lectin is produced in response to the bloodmeal. Feeding flies before infection also abolished the differences in rate of trypanosome killing found between teneral 'susceptible' and 'refractory' G. m. morsitans, suggesting that maternally inherited susceptibility to trypanosome infection is a phenomenon limited to teneral flies. Electron micrographs of midguts of G. m. morsitans suggest that procyclic trypanosomes are killed by cell lysis, presumably the result of membrane damage caused by lectin action.     

Midgut lectin activity and sugar specificity in teneral and fed tsetse

Abstract. . Midgut infection rates of Trypanosoma congolense in Glossina palpalis palpalis and of Trypanosoma brucei rhodesiense in Glossina pallidipes are potentiated by the addition of D+ glucosamine to the infective feed, but not to the levels of super-infection reported for G. m. morsitans. G. p. palpalis and G.pallidipes are shown to possess two trypanocidal molecules: a glucosyl lectin which can be inhibited by D+ glucosamine and a galactosyl molecule inhibited by D+ galactose. Addition of both D+ glucosamine and D+ galactose to the teneral infective feed promotes super-infection of the midguts of G.p.palpalis. The glucosyl lectin is specific for rabbit erythrocytes and is present in guts of fed G.m.morsitans and G.p.palpalis , titres of lectin activity do not increase substantially after the second bloodmeal. The galactosyl specific molecule does not show any erythrocyte specificity, although haemolytic activity is observed only in G.p.palpalis and not in G.m.morsitans. The presence of two trypanocidal molecules in some species of tsetse may account for the innate refractoriness of these flies to trypanosome infection.
As D+ glucosamine also inhibits the killing of procyclic trypanosomes taken as an infective feed, it is suggested that the midgut lectin is normally responsible for the agglutination of trypanosomes in the fly midgut by binding to the pro-cyclic surface coat, prior to establishment in the ecto-peritrophic space.     

Lectin signalling of maturation of T.congolense infections in tsetse

The process of maturation of Trypanosoma congolense Broden in tsetse has been shown to be initiated by lectin secreted in the fly midgut. In the present study the duration of lectin signal required to induce maturation was determined by the sequential addition or removal of a specific lectin inhibitor (D+glucosamine) to the diet of infected male Glossina morsitans Westwood. An established midgut infection of T.congolense was found to require, at most, 72 h exposure to midgut lectin to begin the process of maturation. Longer exposure to midgut lectin increased the frequency of maturation, suggesting clonal variation in response to lectin stimulation occurs within trypanosome stocks. It is suggested that this variation corresponds to differences in lectin binding sites on the trypanosome surface. Midgut trypanosomes retained their ability to mature throughout their life in the fly; when lectin activity in the midgut was inhibited, the trypanosomes remained as procyclic forms but when this inhibition was removed maturation was able to proceed. This indicates that the process of maturation is dependent upon a signal from the fly and is not predetermined by the trypanosomes undergoing a fixed number of division cycles. The possible role of lectins in the maturation of trypanosomes in vitro is discussed.     

Haemolymph lectin and the maturation of trypanosome infections in tsetse

The tsetse immune system has recently been shown to be involved in trypanosome maturation; lectin secreted in the midgut, normally responsible for preventing the establishment of midgut infections, induces established midgut trypanosomes to mature. We now show that a second lectin, present in tsetse haemolymph, is essential to complete the maturation process. Interactions between tsetse lectins and parasite surface coats probably determine trypanosome transmissibility and may be partly responsible for the distribution of trypanosomiasis in Africa.     

Control of Trypanosoma brucei brucei infections in tsetse, Glossina morsitans

Abstract Numbers of immature Trypanosoma brucei brucei within a tsetse midgut remain remarkably constant after establishment throughout the course of an infection, irrespective of whether the infection eventually matures. These results suggest a system of self regulation of the parasite population in the insect gut based on a form of programmed cell death which would carry advantages for both the parasite and the vector.