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

The anthelmintic meclonazepam activates a schistosome transient receptor potential channel

Parasitic flatworms cause various clinical and veterinary infections that impart a huge burden worldwide. The most clinically impactful infection is schistosomiasis, a neglected tropical disease caused by parasitic blood flukes. Schistosomiasis is treated with praziquantel (PZQ), an old drug introduced over 40 years ago. New drugs are urgently needed, as while PZQ is broadly effective it suffers from several limitations including poor efficacy against juvenile worms, which may prevent it from being completely curative. An old compound that retains efficacy against juvenile worms is the benzodiazepine meclonazepam (MCLZ). However, host side effects caused by benzodiazepines preclude development of MCLZ as a drug and MCLZ lacks an identified parasite target to catalyze rational drug design for engineering out human host activity. Here, we identify a transient receptor potential ion channel of the melastatin subfamily, named TRPMMCLZ, as a parasite target of MCLZ. MCLZ potently activates Schistosoma mansoni TRPMMCLZ through engagement of a binding pocket within the voltage-sensor-like domain of the ion channel to cause worm paralysis, tissue depolarization, and surface damage. TRPMMCLZ reproduces all known features of MCLZ action on schistosomes, including a lower activity versus Schistosoma japonicum, which is explained by a polymorphism within this voltage-sensor-like domain-binding pocket. TRPMMCLZ is distinct from the TRP channel targeted by PZQ (TRPMPZQ), with both anthelmintic chemotypes targeting unique parasite TRPM paralogs. This advances TRPMMCLZ as a novel druggable target that could circumvent any target-based resistance emerging in response to current mass drug administration campaigns centered on PZQ.

Drug-induced exposure of Schistosoma mansoni antigens SmCD59a and SmKK7

Background

Schistosomiasis is a serious health problem especially in developing countries and affects more than 243 million people. Only few anthelmintic drugs are available up to now. A major obstacle for drug treatment is the different developmental stages and the varying host compartments during worm development. Anthelmintic drugs have been tested mainly on adult schistosomes or freshly transformed cercariae. Knowledge concerning the larval stages is lacking.

Methodology/Principal Findings

In this study, we used in vitro-grown schistosomula (aged between 2 to 14 days) to investigate drug effects of the three anthelmintics praziquantel, artemether, and oxamniquine. Further, we analyzed the antibody accessibility of two exemplary schistosome antigens SmCD59a and SmKK7, before and after drug treatment. Our results demonstrated that praziquantel applied at a concentration of 1 μM inhibited development of all life stages. Application of 10 μM praziquantel led to dramatic morphological changes of all schistosomula. Artemether at 1 and 10 μM had differential effects depending on whether it was applied to 2-day as compared to 7- and 14-day schistosomula. While 2-day schistosomula were not killed but inhibited from further development, severe morphological damage was seen in 7- and 14-day schistosomula. Oxamniquine (1 and 10 μM) led to severe morphological impairment in all life stages. Analyzing the accessibility of the antigens SmCD59a and SmKK7 before drug treatment showed no antibody binding on living intact schistosomula. However, when schistosomula were treated with anthelmintics, both antigens became exposed on the larvae. Oxamniquine turned out to be most effective in promoting antibody binding to all schistosomula stages.

Conclusion

This study has revealed marked differences in anthelmintic drug effects against larvae. Drug treatment increases surface antigen presentation and renders larvae accessible to antibody attack.

Schistosomiasis is one of the major parasitic diseases in developing countries and still causes 200,000 deaths per year. Mass drug administration programs with praziquantel, the drug of choice against schistosomiasis, are currently undertaken in Sub-Saharan Africa. Praziquantel, although efficient against adult worms, fails to cure early infection. The complex developmental stages of schistosomes and migration through varying host compartments with different local drug concentration are a challenge for drug treatment. After infecting their mammalian host, schistosomula traverse through skin and the vasculature of lung, liver and intestines. During their migration, they develop from larvae to paired adults in approximately 4 to 5 weeks. So far, drug effects have been analyzed on adult worms or freshly transformed schistosomula only. Information about the effects on the larval stages is lacking. We were able to transfer the larval development of the first three weeks into the culture dish. This tool can be used for the analysis of drug effects against schistosomula and for investigation of the accessibility, expression and localization of antigens. Rendering the parasite’s larvae vulnerable to the host’s immune system by increasing antigen presentation is an important aspect of drug activity. We demonstrate on in vitro-cultured Schistosoma mansoni larvae, that SmCD59a and SmKK7, as examples for hidden antigens, become accessible to antibodies following drug treatment.

Similar cellular responses after treatment with either praziquantel or oxamniquine in Schistosoma mansoni infection

The effect of treatment with either oxamniquine or praziquantel on S.mansoni specific IFN-gamma, IL-4, IL-5 and IL-10 was compared on PBMC which were collected pretreatment, 6 and 18 weeks post treatment. Using sandwich ELISA on the supernatants harvested from the PBMC stimulation by crude S. mansoni SEA and SWAP antigens after 5 days the levels of PBMC proliferation and cytokine production were similar according to treatment with either praziquantel or oxamniquine. Before treatment, infected groups showed low ratios, of IL-4:IFN-gamma, IL-5:IFNgamma and IL-10:IFN-gamma, indicating that IFN-gamma was high in the infected individuals. The general increase in immuno-modulation was observed post-treatment with elevated immune reactivity and cytokine production in both treatment groups. Treatment induced significant increases in levels of IL-4 (p < 0.05), IL-5 (p < 0.0001) and IL-10 (p < 0.05) cytokines 6 and 18 weeks after treatment. There were no significant differences in the increase in IL-4, IL-5 and IL-10 between children treated with praziquantel or oxamniquine. Pre-treatment IFN-gamma and IL-5 levels were positively correlated with infection (p < 0.001), while post treatment IL-4 cytokine levels were negatively correlated with baseline infection status (p < 0.001). The results suggest that treatment-induced immune responses are similar for both common anti-schistosome drugs praziquantel or oxamniquine having similar and immunizing effect.

Schistosomiasis--a century searching for chemotherapeutic drugs

Schistosomiasis affects 200 million individuals in underdeveloped and developing regions and is a growing concern for travelers worldwide. There has been evidence of resistance to the praziquantel-based therapy and reports of acute-disease manifestation; therefore, other drugs affecting different stages of the schistosome parasites life cycle and alternative therapeutic regimens should be developed and become accessible. The present review results from a comprehensive search in the scientific literature for substances and compounds tested in the past centennial for schistosomiasis therapy. We gathered over 40 drugs providing information on therapeutic action in humans or animal model, toxicity, susceptible Schistosoma stages, species, etc. The drugs were grouped according to their known metabolic effects on the parasite, whether they are on membrane structure and function, carbohydrate metabolism, protein synthesis and function, or on nucleic acid metabolism. We discuss the current knowledge of drug-target interactions, their mechanism of action and possible therapy combinations. Furthermore, based in the literature and in our own experience with large-scale Schistosoma mansoni genome and transcriptome analyses, we put forward several recently described gene products that are promising target candidates for existing or new drugs.

Resistance to oxamniquine of a Schistosoma mansoni strain isolated from patient submitted to repeated treatments

A strain of Schistosoma mansoni (R1) was isolated from patient previously submitted to four treatments with oxamniquine, and to another one with praziquantel. The results obtained with chemotherapeutic test, by using oxamniquine in mice infected with the strains R1 and LE (standard), showed an evident resistance to the drug in worms of the strain R1. Thus, at the dose of 250 mg/kg oxamniquine, all mice (17) infected with the LE strain did not show surviving worms, whereas 12 out of 17 mice infected with the R1 strain presented surviving worms. At the dose of 200 mg/kg, the LE strain showed recovery rates of 1.06% and 20.58%, whereas the R1 strain presented 18.57% and 61.14%, for male and female worms, respectively. At the dose of 100 mg/kg, the recovery of male worms was 2.6% for the LE strain, and 29.9% for the R1 strain. At the same dose, the recovery of females did not show statistically significant differences between the two strains (LE = 76.38%, R1 = 79.12%). Praziquantel showed similar antischistosomal activity against both studied strains, when administered at the dose of 500 mg/kg.

Antischistosomal drugs: past, present ... and future?

The major antischistosomal drugs that have been or still are in use against infections with schistosomes are considered here together with some compounds that have not been in clinical use, but show interesting characteristics. Each individual compound presents aspects that may be enlightening about parasite biochemistry, parasite biology, and host-parasite relationships. Special attention is given to the mechanisms of action, an understanding of which is seen here as a major factor of progress in chemotherapy. Three compounds are currently in use, i.e., metrifonate, oxamniquine, and praziquantel, and all three are included in the World Health Organization list of essential drugs. They are analyzed in some detail, as each one presents advantages and disadvantages in antischistosomal therapy. The reported occurrence of drug-resistant schistosomes after treatment with oxamniquine and praziquantel suggests strict monitoring of such phenomena and encourages renewed efforts toward the development of multiple drugs against this human parasite.

Attenuation of Schistosoma mansoni cercariae with a molluscicide derived from Millettia thonningii

A chloroform extract of molluscicidal plant secondary compounds from the seeds of a West African legume Millettia thonningii was used to attenuate cercariae of Schistosoma mansoni prior 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 min, 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/l Millettia to 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 min after exposure to Millettia extract at concentrations up to 2.5 mg/l were similar to those of control larvae: none was immobile. Exposure to higher concentrations of Millettia progressively reduced swimming activity and increased the proportion of immobile cercariae. After pretreatment with 20 mg/l Millettia the 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/l Millettia. 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 use Millettia-attenuated cercariae to induce useful levels of protection in mice towards further cercarial challenge.

Schistosoma mansoni: evaluation of the activity of oxamniquine on schistosomules, at 24 hours after infection

Mice transcutaneously infected with about 400 cercariae were submitted to treatment with oxamniquine (400 mg/kg), 24 hours after infection. The recovery of schistosomules, at 4, 24, 48 and 72 hours and 35 days after treatment, showed the activity of the drug on the parasites, thus practically preventing their migration from the skin to the lungs. Worm recovery performed in the lungs (96 hours after treatment) showed recovery means of 0.6 worms/mouse in the treated group and 53.8 in the control group (untreated). The perfusion of the portal system carried out at 35 days after treatment clearly showed the elimination of all the parasites in the treated group, whereas a recovery mean of 144.7 worms/mouse was detected in the control group (untreated). These findings confirm the efficacy of oxamniquine at the skin phase of infection, and also show similarity with the immunization method that uses irradiated cercariae. The practical application of these findings in the medical clinic is discussed too.

Biochemistry of irradiated parasite vaccines: Suggested models for their mode of action

Irradiated larvae of a remarkably wide range of parasite species induce potent protective immunity against subsequent challenge of experimental hosts. Ann Wales and John Kusel have studied the irradiated cercariae of Schistosoma mansoni and suggest here that the effects of irradiation on conformation and presentation of parasite antigens may be crucial in enhancing parasite immunogenicity so effectively.

Cloning of a 21.7-kDa vaccine-dominant antigen gene of Schistosoma mansoni reveals an EF hand-like motif

Several cDNA clones encoding a 21.7-kDa antigen (Sm21.7) were detected from a Schistosoma mansoni sporocyst cDNA expression library using irradiated cercaria-vaccinated rabbit serum. The antigen was designated 'vaccine dominant' because parasite-derived Sm21.7 was recognised preferentially by mouse vaccine sera compared with mouse infection sera. The cDNA and corresponding gDNA sequences showed 64% identity at the nucleotide level and 47% identity at the amino acid level with the sequence of a previously described S. mansoni tegumental antigen, sma22.6. Whereas sma22.6 mRNA occurs almost exclusively in the adult worm, Sm21.7 mRNA was equally abundant in the sporocyst, schistosomular and adult stages. Both Sm21.7 and sma22.6 sequences reveal a motif strongly homologous to the EF hand calcium binding domain but lacking the invariant glycine in the calcium binding loop. The disruptive nature of the glutamine which in Sm21.7 replaces the glycine explains why the motif is non-functional, as shown by the inability of Sm21.7 to bind calcium.

Schistosoma mansoni: acquired immunity in mice after the use of oxamniquine at the evolutive skin and pulmonary phases

Mice infected with 350 cercariae of Schistosoma mansoni (LE strain) were treated with oxamniquine, at the dose of 400 mg/kg, 24, 48, 72, and 96 h after infection. Forty days after the treatment, the animals were submitted to a challenge infection with 80 cercariae, through the abdominal and ear skins. The number of immature worms in the animal groups treated 24 and 96 h after the first infection was found to be lower than that in the control group, thus showing that the death of schistosomes by chemotherapy, at the skin and pulmonary phases, causes an acquired resistance state.

Downregulation of Th1 cytokine production accompanies induction of Th2 responses by a parasitic helminth, Schistosoma mansoni

In the mouse, infection with Schistosoma mansoni results in an egg-producing infection and associated disease, whereas vaccination with attenuated larval stages produces a substantial and specific immunity in the absence of egg-induced pathology. Preliminary data showing enhanced interleukin-5 (IL-5) production by T cells from infected mice and interferon gamma (IFN-gamma) synthesis by cells from vaccinated animals (7), suggested differential CD4+ subset stimulation by the different parasite stimuli. To confirm this hypothesis, lymphocytes from vaccinated or infected animals were compared for their ability to produce IFN-gamma and IL-2 (secreted by Th1 cells) as compared with IL-4 and IL-5 (characteristic Th2 cytokines). After stimulation with specific antigen or mitogen, T cells from vaccinated mice or prepatently infected animals responded primarily with Th1 lymphokines, whereas lymphocytes from patently infected mice instead produced Th2 cytokines. The Th2 response in infected animals was shown to be induced by schistosome eggs and directed largely against egg antigens, whereas the Th1 reactivity in vaccinated mice was triggered primarily by larval antigens. Interestingly, Th1 responses in mice carrying egg-producing infections were found to be profoundly downregulated. Moreover, the injection of eggs into vaccinated mice resulted in a reduction of antigen and mitogen-stimulated Th1 function accompanied by a coincident expression of Th2 responses. Together, the data suggest that coincident with the induction of Th2 responses, murine schistosome infection results in an inhibition of potentially protective Th1 function. This previously unrecognized downregulation of Th1 cytokine production may be an important immunological consequence of helminth infection related to host adaptation.

Induction of immunity against Schistosoma mansoni by drug (Ro11-3128)-terminated infections: analysis of surface antigen recognition

As with 20 krad-irradiated infections in mice, the present study shows that the immunity induced by Ro11-3128 termination of unattenuated infections at the skin stage is species specific, not operating against S. japonicum. Treatment with the drug Ro15-5458, also effective at the skin stage, however, resulted in significantly lower levels of resistance than Ro11-3128. Sera from mice immunized by infection plus Ro11-3128 treatment on days 1 or 2 (Ro11S) coprecipitated essentially the same pattern of 125I-labelled surface antigens as the 20 krad vaccine serum (VMS), viz. Mr 38,000, 32,000, 23,000 and 15,000. However, recognition by Ro11S was markedly stronger. Sera from the infected and Ro15-5458-treated mice (Ro15S) failed to recognize the Mr 23,000 antigen and produced a weaker response than Ro11S or VMS against the Mr 38,000 or 32,000 antigens but a comparable response to VMS against the Mr 15,000 antigen. Ro11S and VMS also recognized the Mr 16,000 surface antigen seen by Western blotting but its recognition by Ro15S was weaker. Compared with sera from animals treated at the skin stage, sera from animals treated at the lung stage (day + 6) showed weaker recognition of the Mr 32,000 and 15,000 antigens and no recognition of the Mr 23,000 antigen. In contrast, sera from mice treated at 15 days recognized both the Mr 32,000 and 23,000 antigens but not the Mr 15,000 antigen. Mice treated at these times show progressively less immunity than at the skin stage. Infected but untreated animals only showed significant recognition of the Mr 32,000 antigen. Thus compared with infections treated with Ro11-3128 on days 1 or 2, treatment at later times or with the drug Ro15-5458 resulted in selective and differential absence or diminution of response against either the Mr 38,000, 32,000, 23,000, 16,000 or 15,000 antigens. In vitro, Ro11-3128, in contrast to Ro15-5458, caused multiple vesicle formation at the surface of skin stage schistosomula but this was progressively less pronounced with lung and liver stage worms. The vesicles were shown to express surface membrane antigens but were apparently not derived from the existing outer leaflet of the surface membrane. It is suggested that this altered antigen expression might explain the optimum immunity induced.

Antigen localization and the induction of resistance in mice vaccinated with irradiated cercariae of Schistosoma mansoni

The fate of 75Se-labelled parasites and their released pre-synthesized macromolecules has been followed in three murine infection models. Parasite numbers in specific tissues were determined by autoradiography, and released material was estimated by gamma-counting of tissues, with adjustment for the presence of parasite-associated radiolabel. Marked differences were found between the three models. The pattern of migration of normal schistosomula was similar to that previously reported. In addition we have described the transit of parasites through the lymph nodes draining the infection site. Significant quantities of released material were detected in the skin, draining lymph nodes, bloodstream and liver. The circulating material was of parasite origin, macromolecular, and hence potentially antigenic. In comparison to the normal infection, radiation-attenuated parasites (inducing a high level of resistance to challenge) persisted in the skin, draining lymph nodes and lungs, releasing a proportionally greater amount of material in the nodes. In mice exposed to attenuated parasites and treated with the compound RO11-3128 at 24 h (inducing a low level of resistance) there was an early death and rapid clearance of the parasites whilst still in the skin. This situation resulted in the highest levels of released material in the skin, bloodstream and liver, but negligible levels in the draining lymph nodes. We suggest that the persistence of radiation-attenuated parasites in the skin and draining lymph nodes, together with the prolonged release of antigen in the latter site, compared to the normal situation, are major factors in the induction of resistance.

Comparison of the live vaccine potential of different geographic isolates of Schistosoma mansoni

We have investigated the capacity of irradiated larvae of each of six different isolates of Schistosoma mansoni to protect mice against challenge infections from the homologous or a different isolate. The isolates differed in respect of their geographic origins, and in the lengths of time they had been maintained in the laboratory. All isolates induced significant protection against unattenuated challenge infections, and neither the degree of geographic disimilarity between vaccinating and challenge parasites, nor the numbers of laboratory cycles, appeared to influence the degree of protection induced by irradiation-attenuated larvae.

Schistosoma mansoni: chemotherapy of infections of different ages

Mice were treated with potassium antimony tartrate, hycanthone, oxamniquine, niridazole, or praziquantel at different times after infection with Schistosoma mansoni. The rate of cure was assessed by perfusion of surviving worms approximately 4 weeks after treatment, and the percentage reduction in worm burden was estimated relative to the number of adult worms perfused from control mice, comparably infected but untreated. All six drugs were relatively inactive against S. mansoni between 3 and 4 weeks after infection when compared with treatment at 5 to 6 weeks. However, the drugs differed in the patterns of cure they achieved in the 2-week period after administration of cercariae and in the period around the onset of patency. Worms that had been subjected to amoscanate or hycanthone in the third week after infection showed evidence of this as adults in having a reduced fecundity. Factors such as worm or host physiology, or host immune status may have had roles in the outcome of chemotherapy at different stages of maturation of S. mansoni.

Schistosoma mansoni: an ultrastructural examination of skin migration in the hamster cheek pouch

The hamster cheek pouch has been used to investigate the skin migration of schistosomula at the ultrastructural level. Parasites have been observed up to 72 h post-infection. Despite rapidly reaching the epidermal basement membrane schistosomula remain in an epidermal location for at least 40 h post-infection, by which time the acetabular glands and their ducts have been lost and the transformation from cercaria to schistosomulum completed. Entry into the dermis and exit from the skin via the blood vessels is therefore by mechanical means or by lytic secretions emanating from a source other than the acetabular glands. The head gland which persists in the dermal schistosomula is a probable source of such secretions. The observations suggest that the apical area and musculature of the head capsule are important in gaining access to the lumen of blood vessels.

Induction of resistance to Schistosoma mansoni in mice by chemotherapy: migration of schistosomula in primary and challenge infections

The fate of 75Se-labelled schistosomula in mice treated at 24 h post-infection with either Ro 11-3128 or oxamniquine, compared to untreated controls, was followed by compressed organ autoradiography. No difference in the total percentage of schistosomula detected as foci was found between the three groups at each sampling time. However, the distribution of schistosomula was altered. In oxamniquine-treated mice there was a delayed migration from the skin relative to controls and fewer parasites in total appeared to reach the lungs. In Ro 11-3128 treated mice very few parasites left the skin. Ro 11-3128 treatment induces resistance to challenge whereas oxamniquine does not. The fate of challenge schistosomula in previously infected Ro 11-3128 treated mice was also compared to that in control mice. There was evidence for delayed or reduced migration from the skin of Ro 11-3128 treated mice. Significant death of challenge parasites occurred in the lungs, or post-lung sites in both control and treated mice. It was not possible to conclude that any of the late attrition was immune-dependent in the treated mice and the results appeared to indicate that a significant fraction of parasite death occurred at the skin stage.