A novel toll-like receptor from the pearl oyster Pinctada fucata martensii is induced in response to stress

Graft rejection due to immune incompatibility is a common occurrence in pearl culture, which often cause death to the host oyster. To improve cultured pearl production, host mortality and bead rejection rates must be reduced. Toll-like receptor 4 (TLR4) plays an important role in innate immunity, and may be related to allograft rejection. Here, we cloned the TLR4 cDNA from the pearl oyster Pinctada fucata martensii (PmTLR4). PmTLR4 cDNA was 3138 bp, including a 2625 bp open reading frame encoding 874 amino acids. The predicted PmTLR4 protein was structurally typical of the TLR family. PmTLR4 had relatively high sequence similarity and identity to the TLR4 of the Cyclina sinensis (48.1% and 27.6%, respectively). Multiple alignment of TLR4 sequences across species indicated that the Toll/interleukin-1 (IL-1) receptor domain was conserved among species. PmTLR4 mRNA was expressed in all tissues tested, with the most abundant mRNA expression in hepatopancreas and gill in P. fucata martensii. After being stressed by either lipopolysaccharide (LPS) exposure or the nucleus insertion operation, PmTLR4 mRNA expression increased significantly in the hemocytes as compared to controls. Peak level of PmTLR4 mRNA was observed 6 h after the LPS injection, and 2 d after the nucleus insertion operation. These data suggest that PmTLR4 may play a vital role in the induction of innate immunity and is therefore associated with allograft immunity in the pearl oyster P. fucata martensii.