B. J. Andrews

Resistance against Schistosoma mansoni induced by highly irradiated infections: studies on species specificity of immunization and attempts to transfer resistance

Significant levels of resistance against Schistosoma mansoni challenge were developed by mice exposed to highly irradiated (20 krad.) cercariae of the homologous species (53-67%), whereas vaccination with S. bovis, S. haematobium or S. japonicum failed to confer significant levels of resistance (-5-12%), thus confirming the specificity of the immunizing procedure. Attempts to transfer resistance to naive recipients by injection of serum and of spleen or lymph node cells from donor mice vaccinated with highly irradiated cercariae were largely unsuccessful. However, significant levels of resistance could be transferred to mice by injection of serum from rabbits exposed to irradiated cercariae. Comparable levels of resistance were conferred by injection of serum at the time of challenge (34-69%) or 5-6 days later (31-56%). In contrast, sera from rabbits injected with soluble egg antigen or homogenized cercariae failed to confer protection upon recipient mice. Sera from vaccinated mice, vaccinated rabbits and antigen-injected rabbits all caused cell adherence to skin-transformed schistosomula but neither the level of adherence nor the serum titre correlated with the ability to confer protection to mice.

Schistosoma mansoni: characterization of two protective monoclonal antibodies

Summary Two monoclonal antibodies against the surface of S. mansoni schistosomula were found to confer significant passive protection to mice (M7B3A, range 28–70%; M22H12C, range 14–58%). No additive effect was observed when both were transferred together. Neither McAb bound to the cercarial surface but both bound to the surface of in vitro derived schistosomula and schistosomula recovered from mouse skin up to 3 days after infection. The McAbs were species specific, but not S. mansoni strain specific. M22H12C immunoprecipitated an 125I‐labelled surface antigen of relative molecular weight (mol. wt) 32 000. In Western blotting of an NP40 schistosomular extract, M7B3A recognized an antigen smear of 13000–18000 with a dominant band at 16000. This 16000 antigen was recognized by serum from demonstrably immune mice and rats vaccinated with highly irradiated carcariae but not by sera from mice with chronic single sex or bisexual infections.

Further observations on immunization of sheep against Schistosoma mansoni and S. bovis using irradiation-attenuated schistosomula of homologous and heterologous species.

This paper describes further characteristics of the immunization of sheep against schistosomes using live, irradiation schistosomula. Sheep immunized with a non-virulent strain of Schistosoma mattheei were protected against a more virulent strain of the same species for over a year. As there was no evidence that the irradiated parasites were able to persist this long, it was concluded that the vaccine had induced a sterile resistance. Heterologous vaccination, using irradiated S. mattheei schistosomula to immunize against S. bovis or irradiated S. mansoni schistosomula to immunize against S. mattheei, failed to induce any protection.

Resistance following drug attenuation (Ro 11–3128 or oxamniquine) of early Schistosoma mansoni infections in mice

A single dose of Ro 11-3128 was found to be 98-100% effective against Schistosoma mansoni infections at intervals of 3 h to 15 days following infection, and apparently killed the schistosomula stages soon after administration, thus allowing an assessment of the immunizing potential of progressive larval stages. Following infection with 500 unirradiated cercariae, optimum resistance was manifest by groups drug-treated at 48-96 h (60-75%). Significantly lower levels of resistance occurred with early (3 h) or later (6-15 day) treatments. Superimposition of an infection treated at 15 days on a prior infection which had been treated at 2 days did not reduce the level of resistance caused by the latter, indicating that the infection plus delayed treatment had not induced suppression. Thus the peak resistance manifest during the 48-96 h period may reflect optimum expression of protective antigens. Comparison of irradiated (20 krad.) with unirradiated infections showed that, when drug-terminated 24, 48 or 96 h post-infection, irradiated cercariae induced significantly less resistance than unirradiated cercariae, perhaps indicating a delayed appearance of protective antigens following radiation treatment.

Immunisation of baboons against Schistosoma mansoni using irradiated S. mansoni cercariae and schistosomula and non-irradiated S. rodhaini cercariae.

In an attempt to develop a non-pathogenic procedure for immunising baboons against S. mansoni, groups of five baboons were exposed to three doses of 5000 6 Kr-irradiated S. mansoni cercariae or to similar numbers of normal S. rodhaini cercariae and challenged at week 15 with 500 normal S. mansoni cercariae. Faecal egg counts, worm and tissue egg counts, and histopathological examination, showed that neither of the immunising schedules had produced significant protection. In the second experiment baboons were injected by the intramuscular route with 31000 schistosomula of S. mansoni in three doses and the irradiation dose was reduced to near the minimum required for worm sterilisation (2-1--2-4 Kr). Challenge with 3500 normal cercariae of S. mansoni 21 weeks after the first immunising dose again showed no significant protection, although reductions of 20--30% were found in egg and worm counts resulting from the challenge. These results indicate that it may be difficult to develop an effective live vaccine for S. mansoni unless the antigenicity of the immunising larvae can be greatly increased.

Studies on Brugia pahangi . 13. The anthelmintic effect of compounds F151 (Friedheim), HOE 33258 (Hoechst) and their reaction product

F151 was a potent filaricide against adult Brugia pahangi in cats and jirds. HOE 33258 did not kill adult worms in cats but had a marginal effect on adult worms in the peritoneal cavity of jirds. It was not immediately microfilaricidal in cats but the microfilarial counts of treated cats fell within a few weeks of treatment. The reaction product, or mixture, of these two compounds (V5851 = E) was strongly macrofilaricidal in cats and jirds.

Immunization of sheep against Schistosoma bovis using an irradiated schistosomular vaccine.

S. bovis is an economically important parasite of cattle and possibly of sheep and other domestic stock in many African and Mediterranean countries. Control of this infection by chemotherapy and mollusciciding is impractical and this has stimulated interest in vaccination. Previous work in sheep with the Central and South African species S. mattheei has led to the development of an effective vaccine which incorporates irradiated schistosomula produced by artificial transformation of cercariae. In the present experiments a similar S. bovis vaccine has been used in sheep. Fifteen 5 month old Border Leicester × Suffolk crossbred wethers were divided into three groups, two were vaccinated with either 10000 or 20000 6 krad irradiated S. bovis schistosomula, and the third kept as unvaccinated controls. All sheep were challenged with 6700 normal S. bovis cercariae. Worm recoveries and tissue egg counts at necropsy showed that vaccination with 10000 irradiated schistosomula produced a reduction of 71% in the mean worm burden resulting from challenge of the controls and the mean densities of eggs in the tissues were reduced by 75–82%. Immunization with 20000 irradiated schistosomula was not more effective. Histopathological observations showed that both groups of vaccinated sheep had much milder lesions in the intestines than the non-vaccinated controls. There was also a clear reduction in the severity of liver lesions in 4 of the 5 sheep given the lower dose of irradiated schistosomula but the livers of sheep vaccinated with 20000 irradiated schistosomula had severe and widespread lesions due mainly to a massive accumulation of eosinophils and other inflammatory cells around the vessels.

Protection of sheep against Schistosoma bovis using cryopreserved radiation-attenuated schistosomula.

Three sheep were vaccinated with two doses of 3 krad-irradiated cryopreserved Schistosoma bovis schistosomula containing 20,000 and 17,000 organisms respectively, injected intramuscularly 23 days apart after storage in liquid nitrogen for between 9 and 46 days. A challenge of 5360 S. bovis cercariae was administered percutaneously approximately four weeks after the last vaccine dose to these animals and to three controls. Post-challenge the vaccinated animals gained significantly more weight (27% v. 9%), produced fewer eggs in their faeces, showed a smaller reduction in PCV values (-18% v. -27%) and were over-all in better condition than control animals. At perfusion 49.1% fewer adult worms were found in the vaccinated sheep than in controls. The tissue egg burdens were similar in both groups. Histopathologically both groups were similar except that fewer and smaller egg lesions were observed in the livers of vaccinated animals.