Vaccination against the digestive enzyme Cathepsin B using a YS1646Salmonella entericaTyphimurium vector provides almost complete protection againstSchistosoma mansonichallenge in a mouse model

Abstract

24Schistosoma mansoni threatens hundreds of millions of people in >50 countries. Schistosomulae 25 migrate through the lung and adult worms reside adjacent to the intestinal mucosa. None of the 26 candidate vaccines in current development is designed to elicit a mucosal response. We have 27 repurposed an attenuated Salmonella enterica Typhimurium strain (YS1646) to produce such a 28 vaccine targeting Cathepsin B (CatB), a digestive enzyme important for parasite survival. 29 Promoter-Type 3 secretory signal pairs were screened for protein expression in vitro and 30 transfected into YS1646 to generate candidate vaccine strains. Two strains were selected for in 31 vivo evaluation (nirB_SspH1 and SspH1_SspH1). Female C57BL/6 mice were immunized twice, 32 3 weeks apart, using six strategies: i) saline gavage (control), ii) the 'empty' YS1646 vector orally 33 (PO) followed by intramuscular recombinant CatB (20g IM rCatB), iii) two doses of IM rCatB, 34 iv) two PO doses of YS1646-CatB, v) IM rCatB then PO YS1646-CatB and vi) PO YS1646-CatB 35 then IM rCatB. Serum IgG responses to CatB were monitored by ELISA. Three weeks after the 36 second dose, mice were challenged with 150 cercariae and sacrificed 7 weeks later to assess adult 37 worm and egg burden (liver and intestine), granuloma size and egg morphology. CatB-specific 38 IgG antibodies were low/absent in the control and PO only groups but rose substantially in other 39 groups (5898-6766ng/mL). The highest response was in animals that received nirB_SspH1 40 YS1646 PO then IM rCatB. In this group, reductions in worm and intestine/liver egg burden (vs. 41 control) were 93.1% and 79.5%/90.3% respectively (all P<.0001). Granuloma size was reduced in 42 all vaccinated groups (range 32.86-52.83 x10 3 m 2 ) and most significantly in the nirB_SspH1 + 43 CatB IM group (34.74±3.35 x10 3 m 2 vs. 62.22±6.08 x10 3 m 2 : vs. control P<.01). Many eggs in 44 the vaccinated animals had abnormal morphology. Targeting CatB using a multi-modality 45 approach can provide almost complete protection against S. mansoni challenge. 46 Author Summary 47 Schistosomiasis is a parasitic disease that affects over 250 million people worldwide and over 800 48 million are at risk of infection. Of the three main species, Schistosoma mansoni is the most widely 49 distributed and is endemic in the Caribbean, South America, Africa, and the Middle East. It causes 50 a chronic disease with severe negative effects on quality of life. Mass drug administration of 51 praziquantel is the only available course of action due to a current lack of vaccines. However, 52 praziquantel does not protect from reinfection. Therefore, a vaccine would be beneficial as a long-53 term solution to reduce morbidity and transmission of the disease. Our group has repurposed the 54 attenuated YS1646 strain of Salmonella Typhimurium as an oral vaccine vector for the digestive 55 enzyme Cathepsin B of S. mansoni. Oral vaccination followed by an intramuscular dose of 56 recombinant Cathepsin B lead to significant reduction...