Schistosoma mansoni cercarial elastase (SmCE): differences in immunogenic properties of native and recombinant forms.

The Schistosoma mansoni cercarial elastase (SmCE) has previously been shown to be poorly immunogenic in mice. However, a minority of mice were able to produce antibodies against SmCE after multiple immunizations with crude preparations containing the enzyme. These mice were partially protected against challenge infections of S. mansoni. In the present study, we show that in contrast to the poor immunogenicity of the enzymatically active native form of SmCE derived from a crude preparation (cercarial transformation fluid), immunization of CBA/Ca mice with two enzymatically inactive forms, namely purified native SmCE or a recombinant SmCE fused to recombinant Schistosoma japonicum glutathione S-transferase (rSmCE-SjGST), after adsorption onto aluminum hydroxide adjuvant, induced specific anti-SmCE immunoglobulin G (IgG) in all mice within 2 weeks of the second immunization. The IgG antibody response to rSmCE-SjGST was mainly of the IgG1 subclass. These results suggest that inactive forms of the antigen could be used to obtain the optimum immunogenic effects as a vaccine candidate against schistosomiasis. Mice immunized with the rSmCE-SjGST on alum had smaller mean worm burdens and lower tissue egg counts when compared with adjuvant alone- and recombinant SjGST-injected controls. The native SmCE was antigenically cross-reactive with homologous enzymes of Schistosoma haematobium and Schistosoma margrebowiei.

Sequence and level of endogenous expression of calcium channel beta subunits in Schistosoma mansoni displaying different susceptibilities to praziquantel.

Kohn et al. [J. Biol. Chem. 276 (2001) 36873] demonstrated that cells expressing the structurally unusual schistosome beta subunit SmCavbeta1 in their voltage-operated calcium channels, exhibit an increased current amplitude in the presence of praziquantel (PZQ). This suggests that the beta subunit is involved in PZQ activity and is consistent with the known pharmacological effects of the drug. If this is so, the low susceptibility to PZQ noted in some Schistosoma mansoni strains could be due to some mutation(s) in the gene coding for this protein. We have sequenced the cDNAs coding for the SmCavbeta1 and SmCavbeta2 subunits of different sensitive and resistant strains and we have not been able to detect any meaningful differences. As an alternative hypothesis, the different sensitivity of schistosomes to PZQ action could be due to the expression of different beta subunits in the parasite. This interpretation could also explain the low PZQ susceptibility of immature worms (28 days). We analyzed Northern blots of various strains and various developmental stages, but we were unable to demonstrate major quantitative differences in the expression of the beta subunits.

Determination of ED50 values for praziquantel in praziquantel-resistant and -susceptible Schistosoma mansoni isolates.

The dose of praziquantel required to kill 50% of adult worms in vivo (i.e. the ED50) was estimated for nine different isolates of Schistosoma mansoni in infected mice. Four of the isolates were selected because they had not knowingly been in contact with the drug (i.e. they were putatively praziquantel-susceptible). Five putatively praziquantel-resistant isolates were chosen because they had been selectively bred for drug-resistance in the laboratory and/or had previously been shown to be relatively resistant to praziquantel in the field. The work was performed in three laboratories in different countries using pre-agreed and comparable experimental protocols. All four praziquantel-susceptible isolates had ED50s estimated to be <100 mg/kg (mean=70+/-7 SD; median=68), while all five putatively praziquantel-resistant isolates had estimated ED50s >100 mg/kg (mean=209+/-48 SD; median=192). Thus, the five praziquantel-resistant isolates, including two that had been subjected to drug pressure during more than 20 passages in mice, had drug ED50s that were approximately three times as great as those of the praziquantel-susceptible isolates. Two of the five isolates in the putatively resistant group had previously been passaged 15 or more times in mice without administration of drug-pressure, but had ED50s consistent with the other three isolates in the group, indicating that the trait of praziquantel-resistance did not necessarily impair biological fitness during laboratory passage. The protocols used here to estimate the praziquantel ED50s of S. mansoni isolates should be useful for establishing and monitoring the drug susceptibility/resistance profiles of parasite isolates freshly obtained from endemic areas, particularly those in which increased usage of the drug is likely to occur.