Protective immune responses against <i>Schistosoma mansoni</i> infection by immunization with functionally active gut-derived cysteine peptidases alone and in combination with glyceraldehyde 3-phosphate dehydrogenase

<div><p>Background</p><p>Schistosomiasis, a severe disease caused by parasites of the genus <i>Schistosoma</i>, is prevalent in 74 countries, affecting more than 250 million people, particularly children. We have previously shown that the <i>Schistosoma mansoni</i> gut-derived cysteine peptidase, cathepsin B1 (SmCB1), administered without adjuvant, elicits protection (>60%) against challenge infection of <i>S. mansoni</i> or <i>S</i>. <i>haematobium</i> in outbred, CD-1 mice. Here we compare the immunogenicity and protective potential of another gut-derived cysteine peptidase, <i>S</i>. <i>mansoni</i> cathepsin L3 (SmCL3), alone, and in combination with SmCB1. We also examined whether protective responses could be boosted by including a third non-peptidase schistosome secreted molecule, glyceraldehyde 3-phosphate dehydrogenase (SG3PDH), with the two peptidases.</p><p>Methodology/Principal findings</p><p>While adjuvant-free SmCB1 and SmCL3 induced type 2 polarized responses in CD-1 outbred mice those elicited by SmCL3 were far weaker than those induced by SmCB1. Nevertheless, both cysteine peptidases evoked highly significant (<i>P</i> < 0.005) reduction in challenge worm burden (54–65%) as well as worm egg counts and viability. A combination of SmCL3 and SmCB1 did not induce significantly stronger immune responses or higher protection than that achieved using each peptidase alone. However, when the two peptidases were combined with SG3PDH the levels of protection against challenge <i>S</i>. <i>mansoni</i> infection reached 70–76% and were accompanied by highly significant (<i>P</i> < 0.005) decreases in worm egg counts and viability. Similarly, high levels of protection were achieved in hamsters immunized with the cysteine peptidase/SG3PDH-based vaccine.</p><p>Conclusions/Significance</p><p>Gut-derived cysteine peptidases are highly protective against schistosome challenge infection when administered subcutaneously without adjuvant to outbred CD-1 mice and hamsters, and can also act to enhance the efficacy of other schistosome antigens, such as SG3PDH. This cysteine peptidase-based vaccine should now be advanced to experiments in non-human primates and, if shown promise, progressed to Phase 1 safety trials in humans.</p>