Expression of a Phagocytosis Activating Protein (PAP) gene in immunized black tiger shrimp

The ribosomal protein L26 (RPL26) gene, a macrophage activator, isolated from Penaeus monodon infected with the white spot syndrome virus (WSSV) had a deduced amino acid sequence that had 98.6% and a 63.1% similarity to the sequences of RPL26 isolated from Penaeus japonicus and Mus musculus, respectively. As no macrophage has been reported in shrimp, the shrimp macrophage activating protein was named phagocytosis activating protein (PAP). In this study, an intramuscular injection with inactivated Vibrio harveyi (IVH), formalin-inactivated WSSV and fucoidan produced a significantly increased expression (p < 0.05) of the RPL26 gene in P. monodon. Activation was detected by RT-PCR with maximum expression occurring after 72 h (3-fold), 1 week (3-fold) and 2 weeks (4.8-fold) post-injection of the immunostimulants, respectively. The survival rates of shrimps challenged with a 7 × 10− 6 and 9 × 10− 6 dilution of WSSV, 72 h after an intramuscular injection of IVH were 60.00% and 53.33%, respectively. An oral administration of IVH at 225, 300 and 375 mg/kg of body weight/day also protected shrimps from WSSV infection (survival rates were 33.33%, 80% and 40%, respectively). In addition, the recombinant phagocytosis activator protein (rGST-PAP) showed a 2 fold increased percentage of haemocyte phagocytosis compared with a non activated or rGST activated haemocyte. Therefore, the results indicate that immuno-stimulation with IVH will induce the PAP gene and facilitate the protective defenses against WSSV infection. Moreover, the expression level of the PAP gene might act as an indicator of the immune response of cultured shrimp.