Abstract
BACKGROUND: Aspartyl protease inhibitor (API) was thought to protect intestinal parasitic nematodes from their hostile proteolytic environment. Studies on Ostertagia ostertagi, Ascaris suum and Brugia malayi indicated that aspins might play roles in nematode infection. In a recent study, proteins differentially expressed between free-living third-stage larvae (L3) and activated L3 (xL3) of Haemonchus contortus were identified by 2D-DIGE. API was found downregulated in xL3 when compared with L3. However, there was no report about the functions of H. contortus API in the parasite-host interaction. In this study, the gene encoding API from H. contortus was cloned, expressed, and part of its biological characteristics were studied. RESULTS: A DNA fragment of 681 bp was amplified by RT-PCR. Ninety one percent of the amino acid sequence was similar with that for aspin from O. ostertagi. The recombinant API protein was fusion-expressed with a molecular weight of 48 × 10(3). Results of Western blot showed that the recombinant API could be recognized by serum from goat infected with H. contortus. It was found that API was localized exclusively in the subcutaneous tissue and epithelial cells of the gastrointestinal tract in adult H. contortus. qRT-PCR suggested that the API gene was differentially transcribed in different life-cycle stages, with the lowest level in female adults and the highest in free-living L3 larvae. Enzyme inhibition assay indicated that the recombinant API can inhibit the activity of pepsin significantly, and the optimal reaction pH and temperature were 4.0 and 37-50 °C respectively. In vitro study showed that the recombinant API could induce goat PBMCs to express IFN-γ, IL-4 and IL-10. CONCLUSIONS: A new aspartyl protease inhibitor was cloned from H. contortus and its characteristics were studied for the first time. The results indicate that API may regulate the immune response of the host and play roles in the infection.