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

Abstract

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.

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

“…RNA interference studies demonstrate that when cathepsin B transcript levels are suppressed resulting worms show significant growth retardation compared to control parasites ( 9 ). By targeting cathepsin B, reduced egg fitness has been demonstrated by our group ( 10 ), and parasite anti-fecundity has also been seen in other flukes ( 11 ).…”

“…The following day, the eggs present in these tissues were counted by microscopy and adjusted per gram of tissue. Burden reductions were calculated as previously described (12)(13)(14):…”

“…Liver sections in 10% buffered formalin phosphate were stained using hematoxylin and eosin to assess granuloma size and egg morphology. Granuloma area was measured using Zen Blue software (version 2.5.75.0; Zeiss) as previously reported (10,(18)(19)(20)(21). Briefly, while working at 400x magnification, the pointer was used to trace the perimeter of 37-41 granulomas per experimental group with a clearly visible egg which the software converted into an area.…”

T Cell Peptides Derived from Invasive Stages of Schistosoma mansoni as Potential Schistosomiasis Vaccine

“…The sample size was a limitation concerning our results, considering natural variation in worm recovery, egg count and liver lesions. Alternative more powerful delivery systems could improve vaccination results [ 52 ]. The candidates’ immunogenicity was monitored by ELISA and it was observed that SmTNP triggered high specific IgG antibody levels in vaccinated, non-infected mice.…”

Recent Progress in the Development of Liver Fluke and Blood Fluke Vaccines

“…In the past several decades, over 100 schistosome antigens have been advanced as possible vaccines [ 63 , 65 ], and recently developed approaches provide an avenue to identify and test additional candidate molecules for protective efficacy [ 66 , 67 , 68 ]. Indeed, many proteins are exposed at, or released from, the parasite–host interface so many other possibilities remain for potential vaccine exploitation.…”