The trypsin inhibitor-like domain is required for a serine protease inhibitor of Haemonchus contortus to inhibit host coagulation.

[Display omitted] • Haemonchus contortus protein Hc -SPI-I8A functions as an inhibitor of blood clotting contributed by the TIL domain. • Hc -SPI-I8 is highly expressed in the parasitic stages and at the hypodermis, intestine and gonads of H. contortus. • Hc -SPI-I8 was in the same clade as the homologues, which could inhibit coagulation by interacting with clotting factors. • Hc -SPI-I8A is most likely disturbing the extrinsic coagulation cascade by interacting with Oa TSP1CP through its TIL domain. • Hc -SPI-I8A might also interfering in the intrinsic coagulation cascade by an unknown mechanism. Haemonchus contortus , a blood-feeding nematode, inhibits blood coagulation at the site of infection to facilitate blood-sucking and digesting for successful parasitism. However, the mechanism underlying anti-coagulation at the host-parasite interface is largely unknown. In the current study, Hc-spi-i8 , which has two greatly different transcripts named Hc-spi-i8a and Hc-spi-i8b , respectively, was described. Hc -SPI-I8A was a serine protease inhibitor containing a trypsin inhibitor-like cysteine rich (TIL) domain, while Hc -SPI-I8B was not. Hc -SPI-I8A/B were primarily expressed in the hypodermis, intestines and gonads in the parasitic stages of H. contortus. Hc -SPI-I8A interacted with Ovis aries TSP1-containing protein (Oa TSP1CP), which was determined by yeast two-hybrid, co-immunoprecipitation (Co-IP), pull down and co-localization experiments. The blood clotting time contributed by the TIL domain was prolonged by Hc -SPI-I8A. Hc -SPI-I8A is most likely interfering in the extrinsic coagulation cascade by interacting with Oa TSP1CP through its TIL domain and intrinsic coagulation cascade by an unknown mechanism. These findings depict a crucial point in the host-parasite interaction during H. contortus colonization, which should contribute to drug discovery and vaccine development in fighting against this important parasite worldwide. [ABSTRACT FROM AUTHOR]