Immune Mechanisms Involved in Schistosoma mansoni-Cathepsin B Vaccine Induced Protection in Mice

Salmonella Typhimurium expressing chromosomally integrated Schistosoma mansoni Cathepsin B protects against schistosomiasis in mice

Schistosomiasis threatens hundreds of millions of people worldwide. The larval stage of Schistosoma mansoni migrates through the lung and adult worms reside adjacent to the colonic mucosa. Several candidate vaccines are in preclinical development, but none is designed to elicit both systemic and mucosal responses. We have repurposed an attenuated Salmonella enterica Typhimurium strain (YS1646) to express Cathepsin B (CatB), a digestive enzyme important for the juvenile and adult stages of the S. mansoni life cycle. Previous studies have demonstrated the prophylactic and therapeutic efficacy of our plasmid-based vaccine. Here, we have generated chromosomally integrated (CI) YS1646 strains that express CatB to produce a viable candidate vaccine for eventual human use (stability, no antibiotic resistance). 6-8-week-old C57BL/6 mice were vaccinated in a multimodal oral (PO) and intramuscular (IM) regimen, and then sacrificed 3 weeks later. The PO + IM group had significantly higher anti-CatB IgG titers with greater avidity and mounted significant intestinal anti-CatB IgA responses compared to PBS control mice (all P < 0.0001). Multimodal vaccination generated balanced T_H1/T_H2 humoral and cellular immune responses. Production of IFNγ by both CD4⁺ and CD8⁺ T cells was confirmed by flow cytometry (P < 0.0001 & P < 0.01). Multimodal vaccination reduced worm burden by 80.4%, hepatic egg counts by 75.2%, and intestinal egg burden by 78.4% (all P < 0.0001). A stable and safe vaccine that has both prophylactic and therapeutic activity would be ideal for use in conjunction with praziquantel mass treatment campaigns.

Pattern recognition receptor signaling and innate immune responses to schistosome infection

Schistosomiasis remains to be a significant public health problem in tropical and subtropical regions. Despite remarkable progress that has been made in the control of the disease over the past decades, its elimination remains a daunting challenge in many countries. This disease is an inflammatory response-driven, and the positive outcome after infection depends on the regulation of immune responses that efficiently clear worms and allow protective immunity to develop. The innate immune responses play a critical role in host defense against schistosome infection and pathogenesis. Initial pro-inflammatory responses are essential for clearing invading parasites by promoting appropriate cell-mediated and humoral immunity. However, elevated and prolonged inflammatory responses against the eggs trapped in the host tissues contribute to disease progression. A better understanding of the molecular mechanisms of innate immune responses is important for developing effective therapies and vaccines. Here, we update the recent advances in the definitive host innate immune response to schistosome infection, especially highlighting the critical roles of pattern recognition receptors and cytokines. The considerations for further research are also provided.

Therapeutic activity of a Salmonella-vectored Schistosoma mansoni vaccine in a mouse model of chronic infection

Schistosomiasis is an important fresh-water-borne parasitic disease caused by trematode worms of the genus Schistosoma. With > 250 million people infected worldwide and approximately 800 million people at risk, the World Health Organization considers schistosomiasis to be the most important human helminth infection. Several prophylactic non-living vaccines are in pre-clinical and clinical development, but only one has been assessed for therapeutic effect in an animal model with modest results. Live attenuated Salmonella have multiple potential advantages as vaccine vectors. We have engineered an attenuated Salmonella enterica Typhimurium strain (YS1646) to produce a vaccine that targets the parasite digestive enzyme Cathepsin B (CatB). A multi-modality immunization schedule was used in chronically infected mice that included three oral (PO) doses of this CatB-bearing YS1646 strain on days one, three, and five as well as an intramuscular (IM) dose of recombinant CatB on day one. Parasite burden (worm count, intestinal and liver egg numbers) were 46.5 - 50.3% lower than in control animals 1 month post-vaccination and relative reductions further increased to 63.9 - 73.3% at 2 months. Serum anti-CatB IgG increased significantly after vaccination with the development of a more balanced T_H1/T_H2 pattern of response (ie: a shift in the IgG1:IgG2c ratio). Compared to control animals, a broad and robust CatB-specific cytokine/chemokine response was seen in splenocytes isolated 1 month post-vaccination. A vaccine that has both prophylactic and therapeutic activity would be ideal for use in conjunction with mass treatment campaigns with praziquantel in schistosome-endemic countries.

Promising Technologies in the Field of Helminth Vaccines

Helminths contribute a larger global burden of disease than both malaria and tuberculosis. These eukaryotes have caused human infections since before our earliest recorded history (i.e.: earlier than 1200 B.C. for Schistosoma spp.). Despite the prevalence and importance of these infections, helminths are considered a neglected tropical disease for which there are no vaccines approved for human use. Similar to other parasites, helminths are complex organisms which employ a plethora of features such as: complex life cycles, chronic infections, and antigenic mimicry to name a few, making them difficult to target by conventional vaccine strategies. With novel vaccine strategies such as viral vectors and genetic elements, numerous constructs are being defined for a wide range of helminth parasites; however, it has yet to be discussed which of these approaches may be the most effective. With human trials being conducted, and a pipeline of potential anti-helminthic antigens, greater understanding of helminth vaccine-induced immunity is necessary for the development of potent vaccine platforms and their optimal design. This review outlines the conventional and the most promising approaches in clinical and preclinical helminth vaccinology.

Adjuvanted Schistosoma mansoni-Cathepsin B With Sulfated Lactosyl Archaeol Archaeosomes or AddaVax™ Provides Protection in a Pre-Clinical Schistosomiasis Model

Schistosomiasis threatens 800 million people worldwide. Chronic pathology manifests as hepatosplenomegaly, and intestinal schistosomiasis caused by Schistosoma mansoni can lead to liver fibrosis, cirrhosis, and blood in the stool. To assist the only FDA-approved drug, praziquantel, in parasite elimination, the development of a vaccine would be of high value. S. mansoni Cathepsin B (SmCB) is a well-documented vaccine target for intestinal schistosomiasis. Herein, we test the increased efficacy and immunogenicity of SmCB when combined with sulfated lactosyl archaeol (SLA) archaeosomes or AddaVax™ (a squalene based oil-in-water emulsion). Both vaccine formulations resulted in robust humoral and cell mediated immune responses. Impressively, both formulations were able to reduce parasite burden greater than 40% (WHO standard), with AddaVax™ reaching 86.8%. Additionally, SmCB with both adjuvants were able to reduce granuloma size and the amount of larval parasite hatched from feces, which would reduce transmission. Our data support SmCB as a target for S. mansoni vaccination; especially when used in an adjuvanted formulation.

Vaccination against the digestive enzyme Cathepsin B using a YS1646 Salmonella enterica Typhimurium vector provides almost complete protection against Schistosoma mansoni challenge in a mouse model

Schistosoma mansoni threatens hundreds of millions of people in >50 countries. Schistosomulae migrate through the lung and adult worms reside in blood vessels adjacent to the intestinal mucosa. Current candidate vaccines aren’t designed to elicit a mucosal response. We have repurposed an attenuated Salmonella enterica Typhimurium strain (YS1646) to produce such a vaccine targeting Cathepsin B (CatB), a digestive enzyme important for parasite survival. Promoter-Type 3 secretory signal pairs were screened for protein expression in vitro and transfected into YS1646 to generate candidate vaccine strains. Two strains were selected for in vivo evaluation (nirB_SspH1 and SspH1_SspH1). Female C57BL/6 mice were immunized twice, 3 weeks apart, using six strategies: i) saline gavage (control), ii) the ‘empty’ YS1646 vector orally (PO) followed by intramuscular (IM) recombinant CatB (20μg IM rCatB), iii) two doses of IM rCatB, iv) two PO doses of YS1646-CatB, v) IM rCatB then PO YS1646-CatB and vi) PO YS1646-CatB then IM rCatB. Serum IgG responses to CatB were monitored by ELISA. Three weeks after the second dose, mice were challenged with 150 cercariae and sacrificed 7 weeks later to assess adult worm and egg burden (liver and intestine), granuloma size and egg morphology. CatB-specific IgG antibodies were low/absent in the control and PO only groups but rose substantially in other groups (5898-6766ng/mL). The highest response was in animals that received nirB_SspH1 YS1646 PO then IM rCatB. In this group, reductions in worm and intestine/liver egg burden (vs. control) were 93.1% and 79.5%/90.3% respectively (all P < .0001). Granuloma size was reduced in all vaccinated groups (range 32.9–52.8 x10³μm²) and most significantly in the nirB_SspH1 + CatB IM group (34.7±3.4 x10³μm²vs. 62.2±6.1 x10³μm²: vs. control P < .01). Many eggs in the vaccinated animals had abnormal morphology. Targeting CatB using a multi-modality approach can provide almost complete protection against S. mansoni challenge.

Schistosomiasis is a parasitic disease that affects over 250 million people worldwide and over 800 million are at risk of infection. Of the three main species, Schistosoma mansoni is the most widely distributed and is endemic in the Caribbean, South America, and Africa. It causes a chronic disease with severe negative effects on quality of life. Mass drug administration of praziquantel is the only available course of action due to a current lack of vaccines. However, praziquantel does not protect from reinfection. Therefore, a vaccine would be beneficial as a long-term solution to reduce morbidity and transmission of the disease. Our group has repurposed the attenuated YS1646 strain of Salmonella Typhimurium as an oral vaccine vector for the digestive enzyme Cathepsin B of S. mansoni. Oral vaccination followed by an intramuscular dose of recombinant Cathepsin B lead to significant reductions in parasite burden in mice. These animals had the highest titers in serum IgG and intestinal IgA antibodies. This multimodal vaccination approach also elicited both Th1 and Th2 cytokines as seen by the increases in IFNγ and IL-5. Finally, vaccinated mice had reductions in granuloma size along with a higher proportion of morphologically-abnormal eggs. This work demonstrates that a YS1646-based, multimodality, prime-boost immunization schedule can provide nearly complete protection against S. mansoni in a well-established murine model.

Schistosomiasis: Life Cycle, Diagnosis, and Control

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Three main schistosomiasis species can infect humans; S. haematobium, S. japonicum, and S. mansoni.

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The parasites life cycle includes two kind of reproduction; asexual reproduction in snails and sexual reproduction in mammals.

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Multiple diagnostic techniques are used.

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Currently praziquantel is the only drug therapy approved for control of schistosomiasis but other promising candidate drugs (e.g. SpAE, and ruthenium compounds) are being tested.

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A number of vaccine candidates exist including SmCB1, SjAChE, and SmCB.

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Genetic manipulations are being investigated.

Background

Human schistosomiasis is a parasitic disease caused by blood-worms that infect multiple organs, including the liver, intestine, bladder, and urethra. This disease may be eliminated with Praziquantel, vaccines, and gene therapy.

Aims

In this review, the author describes the progress in a study of schistosomiasis that focused on the life cycle, diagnosis, and control.

Methodology

The author searched the PubMed Database at NCBI for articles on schistosomiasis published between 2014 and 2018. All articles were open access and in English.

Results

The life cycle of this parasites involve two hosts: snails and mammals. Manifestations of schistosomiasis can be acute or chronic. Clinical manifestations of acute schistosomiasis can include fever and headache. Symptoms of chronic infections can include dysuria and hyperplasia. Infection can occur in several sites including the bile ducts, intestine, and bladder. The different sites of infection and symptoms seen are related to which of the species involved. Five species can infect humans. The three most commons are S. haematobium, S. japonicum, and S. mansoni. Detection tools for people with schistosomiasis can include the Kato-Katz and PCR. Praziquantel is at present the only effective treatment of this disease. In the future, vaccination or gene therapy may be used.

Conclusion

Kato-Katz and PCR are tools for detecting schistosomiasis on humans. Praziquantel, diagnosis, vaccines, and gene therapy are useful methods for eliminating schistosomiasis.