Cercariae là gì? Các công bố khoa học về Cercariae

At present many laboratories throughout the world are studying the chemotherapy and immunology of Schistosoma mansoni in laboratory hosts. Many workers judge the success or failure of their attempts to cure or immunize these hosts from the ratio of the number of living adult worms recovered to the number of infecting cercariae. This ratio is affected, however, not only by the efficacy of any treatment, but also by the methods used to infect the animals and to recover the worms. If these methods result in widely varying worm recoveries amongst the animals in any experimental group, then small but significant effects of treatment might well be missed. Alternatively, such large experimental groups must be used that the work becomes tedious to perform and depends upon the availability of a great deal of technical assistance. This paper describes techniques which are rapid and do not require great skill in their performance. More important, in our hands they have given very consistent results. In this respect, particularly, we believe that these techniques have advantages over others which are currently practised.

The techniques described here are those which were used in other investigations reported in this journal (Smithers & Terry, 1965a, b).

The strain of S. mansoni used throughout this work was isolated in Puerto Rico and was obtained through the courtesy of Dr W. B. DeWitt of the National Institutes of Health. The parasite is maintained in an albino strain of Australorbis glabratus (Newton, 1955). Snails are exposed individually to ten miracidia overnight at 27 °C.

In C57Bl/6 mice vaccinated with a single dose of attenuated cercariae of Schistosoma mansoni, the major site of immune elimination of intact challenge parasites is the lungs. The effector mechanism involves the formation of focal inflammatory responses throughout the pulmonary tissues. These foci are rich in CD4+ T cells, believed to be memory:effector cells of the Th1 type. To investigate the role of IFN-gamma in these inflammatory responses, vaccinated mice were treated with neutralizing mAb. Administration on days 4, 8, 12, and 16 post-challenge, the period over which elimination of challenge parasites takes place in the lungs, gave an average 89.5% abrogation of protective immunity. Analysis of pulmonary cell populations recovered by bronchoalveolar lavage from treated nonimmune mice at day 14 post-challenge revealed a sharp increase in pulmonary eosinophilia, relative to intact vaccinated and challenged animals. The inverse relationship between eosinophilia and protection suggests that eosinophils do not play a vital role in the immune effector mechanism in this model. Pulmonary foci of treated mice were larger, less compact, and of different cellular composition from those of control groups. They contained increased numbers of eosinophils, together with numerous multinucleated giant cells. The effects observed in the anti-IFN-gamma mAb-treated mice, together with the maintenance of MHC class II expression on alveolar macrophages in these animals, could all be explained by the production of IL-4 and other Th2 cytokines. Thus, neutralization of IFN-gamma during challenge responses may shift the Th balance towards domination by the Th2 subset.

The migration of the schistosomula ofSchistosoma mansonilabelled with [⁷⁵Se] has been followed from the skin to the hepatic portal system. Parasites were detected in all mouse tissues by compressed organ autoradiography. Two separate experiments were performed to track parasites in normal mice, and in mice previously vaccinated with irradiated cercariae. In normal mice, the profile of numbers of autoradiographic foci detected in the skin, lungs, systemic and splanchnic organs was described with time post-infection. The distribution of parasites to systemic organs, following exit from the lungs, paralleled the fractional distribution of cardiac output. Accumulation of schistosomula in the hepatic portal system was complete by day 21 post-infection. Only 2–3 passes of parasites around the vascular system would be required to produce the hepatic portal population. No significant decline in total foci was detected in the first 12 days post-infection. The majority of parasite elimination appeared to occur in the lungs as late as day 21, with lesser proportions in the systemic organs and skin infection site. The pattern of migration in vaccinated mice was similar to that in normal animals. One difference observed was the longer duration of stay in the skin; however, the majority of parasites eventually reached the lungs. The systemic phase of migration occurred on a reduced scale, as did accumulation of parasites in the hepatic portal system. The decline in total foci in vaccinated mice commenced approximately 7 days earlier than in normal mice and proceeded to a lower end-point. Again the majority of parasite elimination appeared to occur in the lungs with lesser proportions in the systemic organs and skin infection site. It is suggested that resistance to reinfection in vaccinated mice has two additive components which combine to retard the migration of schistosomula within the vasculature, preventing them from reaching the hepatic portal system.

In C57Bl/6 strain mice vaccinated with attenuated cercariae of Schistosoma mansoni, the major site of immune elimination of normal challenge parasites is the lungs. The immune effector mechanism involves formation of focal inflammatory responses; the abundance of CD4+ T cells and the activation of alveolar macrophages suggests a role for inflammatory cytokines. We report the profile of cytokines produced by cultures of leukocytes recovered by bronchoalveolar lavage (BAL) from the lungs of vaccinated and challenged mice. From 14 days after vaccination, BAL cultures contained infiltrating lymphocytes that produced abundant quantities of IFN-gamma and IL-3 on stimulation with larval Ag. Production declined from day 21 although the infiltrate of lymphocytes persisted. Challenge of vaccinated mice resulted in a second influx of IFN-gamma and IL-3-producing cells, earlier than after vaccination or in the appropriate controls. Ablation studies revealed that CD4+ T cells were essential for the production of IFN-gamma. The timing of cytokine production after vaccination, and challenge was coincident with the phases of macrophage activation previously reported. At no time could lymphocytes in BAL cultures be stimulated to proliferate with either larval Ag or mitogen, in contrast to splenocytes from the same mice. Furthermore, T cell growth factor activity was not detected in BAL cultures stimulated with Ag. We suggest that the lymphocytes recruited to the lungs are memory/effector cells. When Ag released from challenge schistosomula is presented to these cells, they respond by secreting cytokines that mediate the formation of cellular aggregates around the parasites, blocking their onward migration.

Preliminary experiments indicate that mice inoculated with cercariae of Schistosoma mansoni that have been exposed to cobalt-60 radiation in the range of 2500 to 3000 rep develop immunity to reinfection with nonradiated cercariae.

A chloroform extract of molluscicidal plant secondary compounds from the seeds of a West African legumeMillettia thonningiiwas used to attenuate cercariae ofSchistosoma mansoniprior to infection of NMRI female mice. Exposure of cercariae to 0·3, 0·6, 1·3, 2·5, 10 or 20 mg/l concentrations of extract for 30 mm, immediately before standardized infection, was associated with a concentration-dependent decline in worm establishment at 55 days post-infection. The mean numbers of adult worms established declined from about 17 worms/mouse with control cercariae and those exposed to 0·3 mg/lMillettiato 0·1 worms/mouse after 10 mg/l pre-treatment. Mice exposed to cercariae after 20 mg/l pre-treatment had no adult worms at 55 days post-infection. The activities of cercariae 30 mm after exposure toMillettiaextract at concentrations up to 2·5 mg/l were similar to those of control larvae: none was immobile. Exposure to higher concentrations ofMillettiaprogressively reduced swimming activity and increased the proportion of immobile cercariae. After pretreatment with 20 mg/lMillettiathe majority of larvae were immobile. Levels of anti-parasite antibodies (estimated by ELISA assay using an adult worm-based antigen preparation) rose between 21 and 55 days p.i. with control cercariae and those pre-treated with 2·5 mg/lMillettia. After 5 mg/l pre-treatment, cercariae induced a reduced antibody rise, while mice exposed to cercariae after 10 mg/l pre-treatment showed no rise in anti-parasite antibody levels. These results are discussed in the context of protocols which could possibly useMillettia-attenuated cercariae to induce useful levels of protection in mice towards further cercarial challenge.