Introduction: Vaccination of hamsters with Schistosoma mansoni adjuvant-free recombinant cathepsin B1 (SmCB1) and L3 (SmCL3) have been shown to elicit highly significant (P < 0.005) protection against challenge Schistosoma haematobium that was not very superior to that achieved by the cysteine peptidase, papain.  Sterilizing immunity might, however, be induced if hamsters were vaccinated against S. haematobium infection with a homologous cysteine peptidase, i.e., S. haematobium cathepsin L (ShCL).   Methods: Standards methods, techniques, and primers based on the published nucleotide sequence of ShCL were used to clone, amplify and express DNAs encoding the target enzyme in a bacterial expression vector.  Repeat immunization trials were performed using recombinant ShCL alone or in combination with the vaccine candidate S. mansoni recombinant glyceraldehyde 3-phosphate dehydrogenase, in parallel with S. mansoni lecucine aminopeptidase.  Results: The results together indicated that our adjuvant-free, cysteine peptidase-based vaccine elicits highly significant (P < 0.0001) reduction in challenge worm burden and parasite egg viability.  Protection was associated with whole blood cultures release of type 1, type 2, and type 17 cytokines, and modest, yet significant (P < 0.05) humoral response to ShCL.  Conclusion: Sterilizing immunity was, however, not achieved in any trial, likely because of the preponderant role of cysteine peptidases-induced nonspecific factors in amplifying and antagonizing its protective potential.  Experiments are planned in an aim to identify these elusive factors and their exact role.

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