An aspartyl protease inhibitor orthologue expressed by Parelaphostrongylus tenuis is immunogenic in an atypical host.

Parelaphostrongylus tenuis is a neurotropic nematode common in white-tailed deer (Odocoileus virginianus) of eastern North America. This parasite is the causative agent of a debilitating neurologic disease in atypical hosts, including domestic livestock. In order to identify proteins of potential significance in the host-parasite relationship, a cDNA library was produced from adult P. tenuis mRNA. Screening the library with antisera from infected red deer (Cervus elaphus elaphus) and immunized AO strain rats, we identified clones with sequence similarities to aspartyl protease inhibitors from several parasitic nematodes. Antibody that was generated against this recombinant protein of P. tenuis (Pt-API-1) detected the native protein in E/S products, in muscle and gonad, and on the surface of the cuticle of adult male and female P. tenuis. The native protein was detected in internal structures of first-stage (L1) and third-stage (L3) larvae. Reverse transcription-PCR confirmed expression of Pt-api-1 in L1, L3, and adult male and female worms. Expression of Pt-API-1 throughout the life cycle of P. tenuis suggests an essential function. Antibodies specific for recombinant Pt-API-1 were detected by enzyme-linked immunosorbent assay in sera from 12 red deer experimentally infected with P. tenuis. Antibodies were detected within 28 to 56 days postinfection. Responses were sustained or biphasic in animals with patent infections, consistent with expression of Pt-API-1 by L1. Our results are compatible with findings in other parasitic nematodes showing that aspartyl protease inhibitors are highly immunogenic.