Penaeus monodon SERPIN, PmSERPIN6, is implicated in the shrimp innate immunity

Serine proteinase inhibitors (SERPINs or serpins) have been found in a diverse range of organisms. Herein, eight serpin genes, namely PmSERPIN1 – 8, were identified from the Penaeus monodon EST database (http://pmonodon.biotec.or.th/home.jsp). Among those, PmSERPIN6 was selected for further characterization. Tissue distribution analysis revealed that PmSERPIN6 transcripts were expressed in the lymphoid organ, hemocyte, heart and gill, but not in the hepatopancreas. Semi-quantitative RT-PCR analysis at 0–48 h after pathogen challenge demonstrated that the PmSERPIN6 gene transcript expression levels in hemocytes was slightly decreased after systemic white spot syndrome virus (WSSV) injection but remained unchanged upon Vibrio harveyi injection. Interestingly, immunocytochemistry using anti-PmSERPIN6 polyclonal antiserum showed an increase in the number of PmSERPIN6 producing hemocytes at 72 h after both WSSV and V. harveyi injections indicating that the expression of PmSERPIN6 responded to pathogen in the late phase of infection. Our results suggest a likely important function of PmSERPIN6 in the shrimp’s defense against invading pathogens.