Genome-wide identification and immune response analysis of serine protease inhibitor genes in the blood clam Tegillarca granosa

Serine protease inhibitors (SPIs) are the main regulators of serine protease activities. In this study, we present a genome-wide identification of SPI genes in T. granosa（TgSPI genes）and their expression characteristics in respond to Vibrio stress. A total of 102 TgSPI genes belonging to eight families, including Serpin, TIL (trypsin inhibitor like cysteine rich domain), Kunitz, Kazal, I84, Pacifastin, WAP (whey acidic protein) and A2M (Alpha-2-macroglobulin) were identified, while no genes belonging to Bowman-Birk, amfpi and Antistasin families were identified. The Kazal family has the most TgSPI genes with 38, and 11 TgSPI genes belong to the mollusc-specific I84 family. The TgSPI genes were found to be randomly distributed on 17 chromosomes with 12 tandem duplicate gene pairs. Expression profiles showed that most TgSPI genes were mainly expressed in immune-related tissues such as hepatopancreas, gill and mantle. In the hepatopancreas, most of TgSPI genes were sensitive to Vibrio stress, 28 and 29 TgSPI genes were up-regulated and down-regulated, respectively. Some up-regulated genes with signal peptides, such as the TgSPIs of I84 family, may act as a mechanism to directly prevent Vibrio from invasion. Six Kazal-type TgSPIs (TgSPI29, 45, 49, 50, 51 and 52) were intracellular proteins and their expression was down-regulated in hemocytes after Vibrio stress. This may have boosted protease activity in hemocytes to the point that more hemoglobin derived peptides were produced and secreted into the hemolymph to exert their anti-Vibrio effects. These findings may provide valuable information for further clarifying the roles of SPIs in the immune defense and will benefit future exploration of the immune function of SPIs in molluscs.